# Things which may be of interest to members



## Deano

I have been involved with gold processing in Australia on a commercial scale for over 40 years.
There are many facets of processing which I have developed over that time and which may be of interest to members, I am retiring from commercial activities and would not like to see this knowledge wasted.
Please understand that I have access to all the equipment and use it on a routine basis, this includes Atomic Adsorption Spectrometry (AAS), pH meters and redox meters for measuring Eh.
I mention the above not for any reason but that having the equipment makes the processing easier and in some cases it is necessary to have the equipment to efficiently carry out the processing.
I will need to put the information in a series of posts due to the sheer scale of the information. I will attempt to keep each post on a single topic.
Often I will be on a remote sits and will be not contactable for days at a time.
I felt that the chemical processes site was the most suitable forum for these posts even though some of them are about smelting processing.
If it is felt that these posts are not suitable for any forum I will not be upset about it.

Topic 1

*Chlorine leaching*

Equipment required - pH meter, Eh meter

Textbooks say that gold leaching solution requires an Eh of 1000mv to leach the gold.
This is not quite correct, what the solution requires an Eh of 1000mv for is to retain the gold in solution. A lower Eh will still dissolve gold but there will be an equilibrium established such that as more gold is leached a matching amount of gold precipitates out of solution.
If all the gold is dissolved and the solution is filtered then the gold will generally stay in solution for at least hours and depending on how clean the solution is it may be several days.

What the above means is that you want a high Eh for leaching and removal of the leach liquor from any residual solids by filtration when the leaching is finished.

The major problem with chlorine leaching is the outgassing of chlorine from the leach liquor. This will lower the Eh of the leach solution below the 1000mv required to keep the gold in solution.
If the pH of the leach solution is kept between 3.0 and 3.5 the outgassing of the chlorine is kept to a minimum, an odour of chlorine is noticeable but not really strong.
This is not to be taken as an invitation to do this type of leaching where there is no fume removal system in place, even low levels of chlorine will cause health problems.

What you end up doing with the leach is a continual sequence of adding HCl to adjust the pH and then adding hypochlorite to adjust the Eh, hence the need for the meters.
Some form of agitation is required to ensure complete leaching, for large scale a peristaltic pump with suitable hosing can be used, for small scale a magnetic stirrer is used.
A thick walled rotating plastic pressure vessel can be used for batch operation, as there is no outgassing of chlorine in this format there is no need to continually check the leach conditions after they have been initially set.

Note that you should always have a catch tray under all of your processing equipment.

Next post

recovery of gold from acid solutions

Deano


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## Geo

Deano? From Australia? That's just too much. 

Are you goofing us Deano?


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## WEEE Ben

What's wrong with Deano? it's a very common nick name for someone called Dean  

I'm listening Deano, not that I understand anything of what your saying as i'm new to refining, But I appreciate you wanting to pass your knowledge on.


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## g_axelsson

Hi Deano and welcome to the forum. (Although you have been a member since February.)
I'm for one finds it interesting to hear about gold refining and reclamation from a little more scientific point of view. Looking forward to the coming postings.

Is chlorine leaching used commercially anywhere today?

Now we have one Deano from New zealand (NoIdea) and one from Australia. 8) 

Göran


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## solar_plasma

> I am retiring from commercial activities and would not like to see this knowledge wasted.



Very honorable, I like this attitude! Please post more!


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## Deano

*Chlorine leaching*

A couple of things I accidentally left out of the last post.

If you are wanting to dissolve a substantial quantity of gold in a chlorine leach you should add 4% sodium chloride to the leach liquor. 
This ensures that there are enough chloride ions available to form high levels of gold chloride and also speeds up the leach reaction.

Most gold leaching reactions are a trade off between reagent concentration, temperature and time, more of the first two will speed up the leaching rate.
Generally the leaching rate doubles for each 100 Celcius increase in temperature.
Provided you do not run out of reagent or freeze the leach then you will get all the gold in solution in time.

Each leaching batch is looked at from the point of what is most suitable to the operator.
Reagents and heat cost money, if you are time poor then your time also costs money.
Work out for yourself what is the cheapest leach type for what you are doing, generally small batches of less than 100 grams of gold are run hot with stronger reagent concentrations.

Deano


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## Deano

Goran

Chlorine leaching is used commercially only for specific leaches, generally where the gold is distributed throughout a metal matrix and the whole matrix must be dissolved to access the gold.

I started this thread with chlorine leaching as a lot of the posts I read used chlorine leaching for base metal removal ( hydrogen peroxide - hydrochloric acid leaching).
If you are looking for a good way to dissolve gold and have it reprecipitate as ultra fine particles then the H2O2 - HCl leach is what you use.

Note that these fine particles are so fine that they will pass through micro filters, they will not be caught on any paper filters.

Another version of this type of leaching occurs when you have a leach liquor containing HCl and iron chloride, formed when you contact iron with HCl.
The Eh of the leach will be about 600 mv, enough to dissolve gold but not enough to retain the gold in solution.
Worse still, a leach consisting of copper chloride in HCl will also form these micro particles of gold.

What the above means is that if you use a HCl based leach for base metal recovery you will be putting some of your gold into a form where the gold can only be recovered by leaching with stronger reagents such as chlorine or nitric acid in the leach liquor.
Usually chlorine as hypochlorite is cheap and readily available so why would you risk gold losses without carrying out this leaching step.
You do not need to do all of your leaching using hypochlorite in the HCl leach, just a final step at the end of the base metal leaching stage.

Deano


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## Deano

Recovery of gold from acid solutions

Gold in acid solution is in the form of gold chloride for the most common forms of leaching.
It can be in other forms but you need to have a strong need to leach the gold in these other forms to bother doing so.
Generally leaching gold with other than straight chloride reagents is either more expensive or more toxic than the chlorides.

There are two main methods for recovering gold from acid solutions. Both have their strengths and weaknesses.

The first method is precipitation with a reducing agent.
This method is very simple, it has two steps.

Step 1 is to get the pH of the solution between 1 and 2. I usually run the precipitation at pH 1.5.

Step 2 is to add the cheapest sulfite chemical available as the calculated weight + 20% and allow to precipitate overnight.

Step 1 methods.
If the pH of the solution is greater than 1.5 I bring the pH down to 1.5 with HCl. This is usually needed for a chlorine leach liquor where you have leached at pH 3.
If the pH of the solution is less than 1.5 I bring the pH up to 1.5 with urea and only urea. This is usually needed for an aqua regia leach liquor.
The urea used must be clean, this cuts out most garden and agricultural versions. I use Harnstoff technical grade urea from Germany, the cost is not significant when set against the gold recovered.
Using this method also forces you to keep your leach liquor volumes as small as possible so that the amounts of pH adjusting chemicals are the least possible.

Step 2 methods
I use sodium metabisulfite to get the greatest bang for my buck, you may be able to access cheap batches of sodium sulfite or sodium bisulfite. Food grade as used to clean out beer or wine fermentation vats is OK.
Allowing precipitation overnight is very important if you do not have much gold in the leach liquor (this is called low tenor gold in solution).
The gold precipitated at the start when there is a high gold level in solution will form large particles on the bottom of the beaker.
As the gold level drops the gold precipitated will form smaller particles, towards the end the gold particles are extremely fine.
This means that you will need to recover the gold on a microfilter, I use a 0.45 micron cellulose filter in a tall form micro filter cup when doing larger quantities of gold.
This allows me to filter off about 1000 grams of gold at a time.

An alternative to microfiltration is to use a floc agent to bind the gold particles together to allow use of much larger filter papers.
The best floc agent is Cytec N300 superfloc. This is a food grade neutral floc agent which works beautifully in the strong acid pH range.
It needs to be premixed at the rate of 1 gram per litre of water. Use hot water and a magnetic stirrer, the mixing takes a long time.
I use 100 ml of the premix per kilo of gold on the bottom of the beaker. Add the premix to the beaker with the gold in and use a plastic stirrer such as a paint stirrer in a drill to beat the absolute pants off it for 10 seconds.
The liquor goes all brown for a short time from the gold in suspension but then becomes clear as the gold flocs precipitate.
These flocs can be filtered on any 15 micron or finer filter paper, eg Whatman no 1.

What can go wrong when using a floc agent.
You try to use a beaker which is too small to contain the leach liquor and floc premix under strong agitation.
You think that you will make the premix stronger than 1 gram per liter. Two reasons why this is not a good idea.
1 If you make it a lot stronger you will not be able to stir it and dissolve the N300 in the water.
2 The dilution from adding the premix helps in the floc action.
All the information about using floc agents says to gently stir for a long time to form large flocs. You think that this is what the product experts recommend so you try it. It does not work well.

With all the above methods in place you will get 999 gold from the smelt

Organic extraction of gold from acid liquors

I use di butyl carbitol as the extractant, the full name is di ethylene glycol di butyl ether. Also has a trade name of Ansul E444.
Why do I use this instead of the other extractants available?

At room temperature.
1 I can use it on undiluted aqua regia after gold dissolution.
2 I can load it to the point where it sinks in the liquor, this makes it easier to use in a separatory funnel.
3 When used with the normal ventilation requirements there are no side effects on health after 30 years of use.
4 recyclable indefinitely with oxalic acid reduction of the gold or vacuum distillation recovery of the gold.
5 It is not objectionable to use from an odour perspective.

Disadvantages of using it.
There is a low solubility in the leach liquor, this requires a scavenge cycle contact to recover any gold values associated with this dissolved organic.
This is not a great disadvantage as I always carry out a triple contact sequence to scavenge any gold values.
If the gold loading in the organic is low then the gold recovery from oxalic acid will be poor and difficult to recover as films on walls etc.
There is a low solubility of the organic in the liquor which means reagent loss. Note that the solubility figures quoted for this reagent are not usually achieved in plant practice.

How do you use organic extraction

Have a filtered solution of gold chlorides
Add what is theoretically the required organic volume + 10% and agitate strongly with a plastic stirrer such as a paint stirrer in a drill for 1 minute.
Remove the stirrer and place all the liquor in a separatory funnel.
Allow to sit for about 1 hour for full phase separation.
Unless you really screwed up on your volumes you should have an orange layer on top of the liquor.
If you have a yellow layer rather than an orange layer you have used way too much organic. A yellow layer will cause problems at the oxalic acid stage as the gold grade is way low.
The only way to get out of the yellow layer problem is to contact the organic with more gold chloride solution until an orange layer is achieved.
If you have an orange layer at the bottom of the liquor you have used too small a volume of organic. The upside of this is that you will have easy separation of the organic and high recovery of the gold at the oxalic acid step.
Either way you then put another lot of organic in the leach liquor after the separation step has been completed and repeat the cycle.
The organics from the second and third cycles are kept to be used as the first and second contact organics on the next batch of leach liquor
The first cycle organics are contacted with 50% HCl for base metal recovery and again separated in the separatory funnel before the organic phase is placed on the oxalic acid solution.
The oxalic acid solution is made up as a concentrated solution and the organic is left on it overnight at room temperature (20C)
Next day the organic layer is recovered in a separatory funnel ready to be the third contact organic on the next batch of liquor.
The gold is filtered and smelted
Done properly this will give you 999 gold.

Deano


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## Geo

Thank you Deano. I sincerely hope you did not take offense to my earlier remark. We have a member on the forum with the username : NoIdea and he calls himself Deano. I was a bit confused as to whether he was just messing with us. 

This is some great information. I really appreciate you sharing with us.


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## nh6886

Thanks for sharing Deano,

I look forward to reading your contributions. 
I have a question for you or anyone who cared to answer. What I have to work with at this time is,
HM Digital ORP-200 Waterproof ORP Meter, -999 to +1000 mV ORP Range, 1 mV Resolution, +/- 0.5% 
I was not familiar with "Eh" before I read this, so my question is whether this meter is sufficient for this process.

All the Best,

John


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## Deano

John

Eh is like chemists shorthand for a specific ORP (which is an acronym for Oxidation Reduction Potential).

Eh means that the potential is measured against the standard hydrogen electrode, this is the international reference standard.

There are several other standards still being used, in particular in the US the Ag/AgCl reference electrodes still get an outing.
I can only presume that this is so because the users of these electrodes have used them for a long time and are reluctant to update to international standards.

Usually an ORP meter calibrated in mv is reading Eh values, any(rare) which are not reading Eh will have a prominent announcement on the literature.

The bottom of the actual gold chloride stability range is 1000 mv, generally in a chlorine leach you will have an upper limit of 1300 mv, usually in the mid 1200s.
Your meter will tell you if the Eh is high enough for gold leaching but will not tell you how your Eh is varying in the leaching range.
It is not a good idea to be have to wait until the Eh has dropped below 1000 mv so that you can read it before taking action to get the Eh back up over the 1000 mv mark.

I would say to get a meter which reads at least to 1300 mv to make chlorine leaching easier.
The meter does not have to be an expensive model, there are some very good cheap ones around.

When using the meter in a leach do not leave the electrode in the leach solution permanently, you will shorten the life of the electrode a lot.
I usually have the pH and Eh electrodes in a beaker of water along side the leaching vessel and only place the electrodes in the leach when I want a leach reading.
After doing a couple of leaches you get a good idea how long the intervals should be between readings.
The reading intervals can be affected by impurities in the water, however these impurities will not affect the actual leaching as long as the Eh and Ph conditions are maintained.

Deano


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## Deano

A further missed point on the recovery of gold from acid solutions

If you have a commercial quantity of base metal such as copper in your solution then you can electrowin the metal after the gold recovery step. 
It is difficult to carry out individual metal electrowin runs even with all of the right equipment, it is easiest to carry out a bulk electrowin of all metals.
Usually not much iron comes out in these electrowin runs as it tends to bounce between the valence states of +3 and +2 at the electrodes rather than plate out on the cathode.

The best form of EW cell for small scale electrowinning uses carbon felt as the cathode and a carbon rod as the anode. The carbon felt is sold as soundproofing material in large rolls.

Cell design

Get a piece of plastic pipe about 100 mm in diameter and about 300mm long. Glue in place one end cap.
Get someone to drill as many 3mm holes as possible in the pipe and wrap a layer of carbon felt around the outside making sure that at the end with no end cap you extend the felt past the end of the pipe by 30mm to allow for electrical connection.
Hold the felt in place with teflon thread tape.
Note that you cannot use this trick with gold cyanide solutions as the gold cyanide will dissolve the teflon.

Get a second end cap and cut out a hole in the centre of the cap which will just fit a 25mm diameter carbon rod which is long enough to go from the glued end plate to about 50mm outside the not glued end plate.
The diameter of the rod does not need to be 25mm, it is just what I use. Any diameter between 20 and 50mm will do, depends on what you can get cheaply.
Drill a second hole near the carbon rod hole of a diameter suitable to take a small pump suction hose. 
I use peristaltic pumps so there is no contact between the acid liquor and the pump. These pumps are self priming, it won't hurt them to run dry.
There are some cheap peristaltic pumps available with built in speed control, the AQUA brand from Italy has been reliable.

Stand the piece of pipe upright in a container which is about the same height as the pipe and about 100mm greater in diameter than the pipe.
This container can be made from a piece of 200mm diameter plastic pipe with one glued end as the bottom. Standard blue pipe glue works fine.
Drill a 12mm hole in the wall of the outer pipe as close to the top of the wall as possible and glue a short piece of 12mm OD plastic pipe in the hole so that the pipe projects at least 30mm outside the wall.
Use a length of 12mm ID plastic hose placed over the plastic pipe to direct overflow from the large pipe back to a plastic container which holds the leach solution which is to be stripped.

What we now have is a plastic bucket or whatever containing the leach solution which is to be stripped in the cell.
A peristaltic pump pumps the liquor from this plastic bucket into the leaching cell between the two pieces of pipe, a wooden clothes peg holds the discharge line in place.
As the leaching cell fills with the leach liquor some of the liquor will pass through the carbon felt into the inside perforated pipe.
When the leaching cell is full of leach liquor the liquor will start to return to the plastic bucket via the 12mm overflow hose .
This means that the leaching cell cannot overflow without some creative stupidity being applied.

A second peristaltic pump set to run slightly slower than the first pump is used to suck the liquor from between the perforated cell wall and the carbon rod and discharge it into a tailings bucket.

The carbon rod is used as the anode and the carbon felt is the cathode, current is supplied from a constant voltage power source. Usually around 2.5 volts but depends on the actual cell dimensions and the liquor conditions.
Bubbling from the electrodes escapes from the hole that the peristaltic pump suction line passes through.
I use standard spring clamp connections on the anode and cathode. I then connect the power leads to these clamp connections with the clamp connections attached to the power leads.
This way when the inevitable happens and the leach liquor wicks its way up the felt and the carbon rod it can only contact the first clamps, treat these clamps as consumables and get the cheapest available.

When starting a run through the cell, the cell is first filled with an acid solution at pH near that of the leach solution to be stripped.

The power to the electrodes is turned on and then the peristaltic pumps are started.

The cell is run until until the liquor bucket is empty and the liquor level in the cell is down to near bottom.
At this stage a couple of litres of acid water , same as the start water, is added to the bucket to flush the last of the liquor through the cell.

When the flush water has passed through the cell a couple of litres of plain water are run through to remove the acid from the cathode carbon felt.
Now turn off the power and stop the pumps.
Undo the teflon tape and remove the felt containing the metals from the leach solution.

Depending on your recycler you may be able to sell the loaded felt as is or you can use it as an anode in a flat electrode electrowinning cell with stainless steel or other metal cathode.

This form of cell is incredibly efficient and will remove most metals from solution. Iron is the major exception to the effective removal. It can be removed but high overvoltage is required.

When using the cell for the first few times there are some things you need to find out for your solutions.
I usually run pretty standard solutions through the cell and I know what actual voltage and liquor flow rate through the cell I need for these solutions.
When I am about to run a liquor where I am not sure of the metal levels I have the luxury of running the solutions through AAS and adjusting the cell flow rates to suit the levels.
If you do not have this option and most people don't then you have to find out the maximum flow rate of the liquor through the cell by trial and error.

Start running the cell with a flow rate of 1 litre per hour. Run a batch all through making sure to save all the liquor.
Remove the felt and replace with a new piece. Check the level of metal deposit on the felt.
Now rerun the saved liquor, save once again.
Remove the felt and check for metal deposits, compare with the first felt.
If there are metals on the felt you need to slow down the rate of liquor through the cell and rerun the test with fresh leach liquor.
If there are no metals on the second felt then you can speed up the rate of liquor through the cell and rerun the test with fresh liquor.

Generally I can tell how the electrowinning is going by the amps the cell is drawing.
This is much harder to do when you are running batches where the acid concentration and metal levels in solution vary wildly.
In this case I would err strongly on the side of caution and run a much slower flow rate of liquor through the cell. Remember that the power draw for the whole setup is very low, doubling the electrowinning time is not going to hit you finnancially.
In my case where I am running large batches of liquor containing high levels of metal the cost of doubling my cell time would hurt, this is why I use the ASS. It is just a very useful cost effective tool.

If you don't change your felt often enough or are running solutions with a lot of metal in them then you can actually load the felt with so much metal that the liquor cannot flow through the felt.
You generally find this out when you go to check if the processing is complete and you find your second pump is sucking air because no liquor is getting through the felt.
When this happens take the felt off and wash in water to neutralise any acid residual. Give it several rinses in fresh batches of water. All of this wash and rinse water will have to be put through the cell when the new felt has been fitted.

Things that sooner or later will go wrong.

You will clog the felt with metal as above.

You look at the felt after a run and there is little or no metal on it. 
Usually you are running a solution with little or no metal in it to start with.
Other causes are poor electrical connections to the electrodes, running the solution too quickly through the cell and having a liquor where in the leaching step you have used up all the acid, this acid carries the current in the cell. No acid, no electrowinning.
Always treat the electrode connectors and felt as consumables, it is false economy to try and get a few more runs out of dodgy materials.

Note that the dimensions given above are not set in stone. You can vary them to suit your own circumstances.
If you are running small batches the pipe can be shorter and of smaller diameter but make sure that there is a minimum gap of 20mm between the carbon rod and the pipe so that you are not drawing liquor through the felt in one area. 
The carbon rod can be of lesser diameter but the erosion rate will be much greater, not a concern if you have access to a cheap batch of them.

Deano


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## g_axelsson

Very interesting. One advantage of electrowinning that I see is to remove metal salts as a step of treating waste water. Even if it isn't worth a lot in metal value, just making it easier to process the waste is worth a lot.

A lot of what we do in refining is in chloride based solutions. Is there a lot of chlorine given off during electrowinning?
Have you tested using platinized titanium instead of a carbon rod?

Göran


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## kurtak

g_axelsson said:


> A lot of what we do in refining is in chloride based solutions. Is there a lot of chlorine given off during electrowinning?
> Have you tested using platinized titanium instead of a carbon rod?
> 
> Göran



I had the same questions

I tried running an experiment winning cell to win the copper from a copper nitrate solution (after cementing silver from silver nitrate) & the carbon anode deteriorated before the winning was done

As a test it was a small cell (1 gallon) with a 6' long X 1" diameter carbon rod as the anode & a 1" diameter copper pipe as the cathode --- it did win some metal before the anode was destroyed 

I was then going to try using a titanium anode but read where it needs to be platinum plated or it would end up forming a titanium oxide layer thereby hindering &/or stopping the winning - So :?: 

The chlorine question is a good question as well 

Kurt


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## Deano

Goran

There is always evolution of chlorine when electrowinning from a chloride matrix.

I can only repeat my earlier warning about always carrying out leaching and recovery processing in a well ventilated area.

I use either a fume cupboard or a room with excess extraction capacity and make sure that the extractors are running.

Platinised titanium is the best anode to use, it is also the most expensive anode to buy but in the long term when you are doing a lot of electrowinning it is cheaper than having to keep buying carbon rods.

There are a lot of different grades of carbon available as carbon rod, unless you specify that you are using the carbon rod as an anode you will probably be sold the cheapest grade which is also the softest grade and the fastest to fall apart.

If you can get a really good deal on softer carbon rod and you do not do a lot of electrowinning then these would be my preference.

Using any metal rod as an anode apart from the platinised titanium will either form a non-conductive layer on the surface or consume the anode. 
Keep in mind that the anode metal in the latter case will now be in your cell discharge stream so your metal removal efficiency is lowered.
It is possible to configure the cell so that the liquor is discharged through the cathode felt and any metals dissolved from the anode are removed at the cathode, apart from the iron.
I was trying to keep the cell at its simplest and cheapest form, it is a much more expensive build and requires more expensive pumps and a platinised titanium anode in sealed format if you want to use it as cathode discharge type.

I automatically run leach solutions through a cell after gold recovery just to remove the base metals and make the solutions ready for final clean up.

I take the tailings liquor from the cell discharge, check the base metal levels on the AAS to make sure I removed them all and then neutralise the solution with the cheapest alkali I have available to pH 7.
The solution will go brown as all the ferric ions are converted to insoluble ferric hydroxide. 
Use the N 300 flocculant solution with the vigorous agitation step to bring the ferric hydroxide into a layer at the bottom of your reaction vessel ( bucket or whatever).
You can filter these flocs out on any paper filter.

As pure ferric hydroxide these flocs can be dried and disposed of in landfill, at least here in Australia.
The solution remaining is neutral water with salt and can be disposed of in the sewer, at least in Australia.

Deano


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## g_axelsson

Thanks for the answers Deano.

Actually there is at least one electrowinning process that can be run without chlorine evolution, at least theoretically. 2 CuCl -> CuCl2 + Cu and the resulting copper(II) chloride can be used to leach copper. It might be hard to develop this into a practical leach method but we discussed it on the forum a little over a week ago.
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=34&t=21454&start=20#p221877
One problem with this method is that you can't increase the voltage too high or you would start to release chlorine. That would put a limit on how fast you can run the cell I guess.

Good to hear about the platinized titanium, as I bought one this autumn but haven't had time to test it yet. I will probably count my production of copper in kilos at most, but since it's a hobby for me, slow is okay.

It is good that you keep the cell design simple so it will be practical for most people. I think I know a few of our members that will test your design so we will probably be seeing reports in a while. At least I hope so. 8) 

Göran


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## solar_plasma

Even though I also tried high overvoltage, I couldn't smell any chlorine (though I could have been cheated by the fume hood) and there was no visible gas evolving at the anode, only at the cathode, if I remember correctly. I hope I get some time to try it once again soon and let it run for a longer time.

Setting, as far as I remember:
saturated CuCl2 solution
saturated the CuCl2 solution with copper
cathode: copper
anode: gold plated non-magnetic base metal alloy (assumed as white copper alloy)
voltages tried: 0-24V, left for some minutes at low voltages 0-2V

In that short time the brown (CuCl) electrolyte did not become green (CuCl2).
There were visible fluid turbulences near the anode.
Gold plating got dissolved (probably got reduced back somewhere) and visible erosion at the cathode occured.
Gas evolved at the cathode, which I assumed to be H2.
Copper metal sponge formed at the cathode.


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## solar_plasma

@Deano
If you could draw a fast sketch of your cell, it would be easier to understand all details for those of us, for whom English isn't the first language. Thank you for all the great information!

@all
Does anybody know a source for carbon felt in Germany?


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## Deano

Bjorn

I hope that the attachments work, let me know if they do not and I will try another method.

Deano


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## FrugalRefiner

They worked for me. Thanks!

Dave


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## Deano

Bjorn

The carbon felt is very light. It would cost very little in postage to have someone in the US mail you a couple of square metres of it.
It is widely and cheaply available in the US where it is used as a soundproofing material.

The reasons for using the carbon felt as a cathode are that the felt is not attacked by the liquors and that the surface area per unit area is immense so the layer of felt is very efficient at recovering the metals from solution.

Often there will be no or little current flow through the cell, this is generally caused by bad electrical connectors or the solution pH is close to 7 and so there are few current carrying ions if the metal level in the liquor is low. Make sure that there is good current flow at the end of the run to ensure full metal recovery, you may need to add acid to get the current flow.

I have deliberately not given current flows as they are dependent on metal in solution levels, pH levels and cell shapes.
If someone is using a cell which is half the height of the size I gave then the current will be halved. 
Similarly the current will vary depending on the distance between the electrodes.
I tried to use pipe sizes which are standard in Australia and should be approximately available elsewhere.

Note that when you perform the neutralising step for iron removal you will co-precipitate any residual metals that you may have failed to electrowin.
This means that if you have used stainless rod as the anode the final liquor will have some chromium and nickel in solution. 
These metals will contaminate the iron hydroxide during the precipitation step and may make the precipitates so contaminated that they cannot be disposed of in landfill.
A good reason for using either platinised or carbon electrodes.
Note that both the above will be subject to erosion if high overvoltages are used. 
You would only use high overvoltages if you want to recover the iron during the electrowin and it is much cheaper to use the neutralisation trick.

Deano


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## Deano

*Cyanide leaching of gold*

There are a lot of myths about the use of cyanide.
These are usually along the line that the vapours will kill anything within a couple of kilometres.

The fact is that cyanide is a toxic poison the same as a whole lot of other toxic poisons which people use daily without any thought or concern.
The use of cyanide is safe provided the recommended operating procedures are strictly adhered to.
Problems are usually caused by familiarity breeding contempt, laziness or not having an understanding of the proper procedures.
Treat cyanide as a useful tool requiring careful handling. Use gloves and eye protection when handling cyanide pellets or solutions.

The single most important thing is to have really good ventilation. If you look at a gold mine with large cyanide leach tanks you will see workers walking on walkways across the tops of these tanks. What you will not see is workers lying dead on these walkways.
Providing the pH of a cyanide leaching solution is kept above 10.5 there is very little outgassing of hydrogen cyanide gas. Most plants are run at pH 11 to provide a safety buffer.
The above is not recommending that you use a cyanide leach at pH 11 in an enclosed area without full extraction equipment operating.

Most labs carry out cyanide leach tests in enclosed areas but they make sure that the ventilation equipment is of sufficient capacity and operating.

If I was faced with a choice of low level cyanide fumes or low level chlorine fumes I would take the cyanide. That said, I take great care not to have to make such a choice.

Cyanide leaches used in gold mines usually run 1gram per litre of sodium or calcium cyanide in water with lime added to pH 11.
The above has an actual cyanide level of around 1/2 gram per litre or 500 parts per million (ppm)
That level of cyanide will, without adding lime, give a protective alkalinity of about pH 10.5. Lime is added to increase the pH safety level to around 11.
It is not necessary to use lime for the pH adjustment step, you can use caustic soda or similar. Lime is usually the cheapest.

If the ore has sulfates present then the use of lime may cause precipitation of gypsum (calcium sulfate). If there is a lot of gypsum formed you may be forced into using caustic soda.

If there is a lot of copper present in the ore the leach can be made more selective for gold by lowering the cyanide level. This also prevents consuming a lot of cyanide as copper cyanide. The pH is kept at 11 no matter what the cyanide level is. Check the pH regularly during the leach.

If you are leaching gold from the outside of electrical components you can easily see when leaching is completed.
Remember that cyanide is a slow leach and you expect the leaching to take hours if not days depending on the cyanide level, amount of agitation and temperature.

Cyanide needs to have oxygen dissolved in the leach liquor in order to dissolve the gold. Pumping the leach solution through your bath of components will keep the oxygen level up if you have a small drop from the pump discharge line to the bath liquor level. Don't have a large drop which will splash the solution.

When the cyanide leach has finished the cyanide liquor is transferred into a bucket for gold recovery, Make sure the leached components are well rinsed before disposing of them.

At this stage I run the leach solution through a small electrowin cell to get the gold as a metal. You absolutely must have full air extraction operating during this step.
Remember that you are treating an alkaline solution so the starting liquor you put in the cell is pH 11 water.
After you have treated all of the liquor in the electrowin cell and run a pH 11 rinse through the cell you can remove the felt from the perforated pipe and dissolve the gold from the felt with aqua regia.

Recovery of the gold from the aqua regia is done by sulfite precipitation at pH 1.5 as per a previous epistle.

Things to remember

Sooner or later you will somehow accidentally add acid to your cyanide solution. This is not the immediate disaster you imagine, the hydrogen cyanide gas does not come roaring out of the liquor and kill you in 5 seconds.

The outgassing of hydrogen cyanide gas is relatively slow at low cyanide concentrations, you have time to add caustic soda to the solution to get back to pH 11 provided you have caustic soda nearby in a prepared place ready for such a situation. Make sure you have this ready and do not use it for anything else.


Deano


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## solar_plasma

> If there is a lot of copper present in the ore the leach can be made more selective for gold by lowering the cyanide level.



Now it gets me wondering, if this also works for other gold leaching methods like iodine, iodine/iodide or thiosulfate based. Would they become more selective at lower concentrations? I think I'll reread all documents I've found, especially the charts, but maybe someone can answer this easily.


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## Deano

Bjorn

The reagent dilution trick does not work to a notable degree with thiosulfate leaching of copper and gold ores.
Thiosulfate leaches usually use copper in solution as the oxidant for the gold leaching stage.

I have no direct knowledge of lower tenor leach liquors being more gold specific for halide or sulfide leaches.

Certainly for chloride leaches all I got was more rapid consumption of the chlorine and the need for more frequent Eh / pH checks.
Not what you want for ease of processing.

Generally there are really good reasons why some leaches are used commercially and others are not.

Cyanide is the gold industry's leach agent of choice because of it's cheap price, robust performance and the ease of recovery of the gold complexes.
Toxicity is the main reason for non use in some areas.
If you use the Electrowinning cell I described in the post on cyanide leaching you can recycle the liquor with a cyanide addition, just filter out any carbon particles from the anode before doing so. If you are using a platinised anode you do not need to carry out the filtration step.
Gold recovery from cyanide leaches is simple, it loads readily and with high loadings onto activated carbon, it can be direct electrowon if the gold tenor is high enough or it can be zinced out.
Note that the use of zinc fell out of favour when the carbon in pulp process was introduced.
This was due to cost factors, not just the zinc but the conditioning steps required to get a clean liquor in the optimum condition for zincing. Filtration is expensive for finely milled ores.

Of the halide leaches the only one which is economical for use in some large scale processing is chlorine/chloride.
This leach either has the problem of continuous chlorine evolution in open style leaching (there are many clever ways to minimise this chlorine evolution, none really suitable for large scale processing) or if used in sealed reaction vessels it can only be run as a batch process and still has the problem of chlorine release when the vessel is opened.

The upside of halide leaching is the ease of recovery of the gold chlorides, they load rapidly and to high levels on carbon or resin after the Eh is dropped at the end of the leach.
They can also be precipitated out of solution easily with reducing agents or zinc displacement.

The downside of halide leaching is reagent cost and consumption.
Those halides which must be run acid will have a lot of ores where the ore will react with the acid and consume the acid.
Those halides which can be run at neutral pH will generally react with a lot of the ore components and thus consume the expensive reagents.

Thiosulfate leaching suffers from a major defect in that there are no adsorbents which will load the gold thiosulfate complexes to the ready high levels achieved by gold cyanide.
At regular intervals there are announcements that some or other researchers have made a new adsorbent which is the all singing all dancing product to advance thiosulfate leaching.
Note that thiosulfate leaching has not displaced cyanide leaching in the gold industry.
The most used recovery method for gold thiosulfate is still zinc displacement, this is the method which was discarded on cost grounds by the cyanide process.

Deano


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## Deano

Smelting of gold concentrates

A lot of research has gone into the fluxes used in fire assaying, the work done by people in the 1800s and early 1900s is impressive.

A general assumption has been made that what works for fire assaying as a flux should also work for concentrate smelting of metals.

There is a major difference between fire assay flux requirements and concentrate smelting flux requirements.
When carrying out a fire assay you are wanting to get all of the ore components into a liquid form so that all parts of the ore can be accessed for metal recovery.
When carrying out a metal concentrate smelt you are effectively wanting to form all of the metal values into a single molten unit, it is more of a metal melt than anything.
If you can get any base metal values to go into the molten flux and not be present in the molten precious metal this is a large bonus.

Effectively smelting is carried out in two types of crucibles.
These are straight clay crucibles and carbon containing crucibles.
The carbon containing crucibles generally are two types, silicon carbide and graphite.

The carbon containing crucibles are less subject to attack from liquid borax than the straight clay crucibles.
Because of this factor they are usually favoured for gold smelting for two reasons.
1 They can be reused many times.
2 They are less likely to rupture and put your metal values at the bottom of your furnace.

The carbon removed from the crucible by the borax mostly remains in the slag and can be seen as a coating on top of the slag after pouring.

What is generally not known is that the carbon type crucibles will remove much less base metals from a smelt than will the clay type crucibles, the carbon in the smelt slag interferes with the base metal removal.

If you have low base metal levels in the smelt concentrate feed the carbon type crucibles will remove very little of these base metals and they will report to the gold bar giving you a lesser purity than hoped.

I have been unable to improve in a significant way the purity and recovery % of precious metals when using a carbon type crucible.

Clay crucibles suffer from one major defect, this is the rapid attack on the crucible by molten borax. These crucibles should always be treated as single use crucibles which are then discarded.

These crucibles do have one major upside which is the recovery of higher levels and purities of gold in a smelt.

This is achieved in the following manner.

For a smelt of less than 100 grams of metal a 30 gram crucible is used.
This crucible is placed inside a crucible which is the next size up and will take the smaller crucible inside it. The smaller crucible does have to slide down to the base of the larger crucible, there must be a large enough gap between the small and large crucible so that your tongs can remove just the smaller crucible for the final pour.
The larger crucible acts as a catch vessel if the borax in the smaller crucible eats its way through the smaller crucible walls.
I presume that a lot of crucible suppliers will have crucibles of varying quality, wall thickness and size.
Try what you have available for sizes etc.

Always use face protection and thermal gloves when smelting, a full length leather or thermal apron should also be worn.
Heat the crucibles to 12500C if you have a temperature readout, if not heat until white hot.
Take the crucible combo from the furnace and fill the smaller crucible with borax which has been pre-dried in a steel tray in the heat from the furnace. This pre-drying is very important as you are putting the borax in a high temperature crucible, any moisture will cause a violent eruption of molten borax in your direction.

Having safely put the dried borax in the crucible you replace the crucible in the furnace and continue heating.
The borax is slow to melt, depending on how much the crucible cooled down and how strong a heat source you have it will take from 15 to 30 minutes to fluidise all of the borax.
The molten borax is a darkish brown colour when first fully melted.
When the temperature of the molten borax nears 12500C there is a noticeable colour change to a light brown colour.
Give the heating another 5 minutes and then remove the crucible combo from the furnace and pour the concentrates you wish to smelt into the molten borax, takes about 10 seconds for a 100 gram charge. Once again the concentrates must be pre-dried before adding to the crucible.

Place the crucible combo back in the furnace for another 15 minutes.
Take out and pour into a heated angle sided mould which has had the inner walls covered in a layer of white chalk, swirl the crucible before pouring.

There are three things which you must heat to absolute dryness before using as you are going to contact these three things with extremely hot material.
These are the borax, the concentrates and the pouring mould.

What can go wrong

If for any reason you do not absolutely dry your components you will be grateful you are using a head covering full face mask, long arm gloves and body apron. Molten borax is incredibly corrosive but with all the safety gear on you will survive even if scarred. This applies to any smelting operation, not just the one above.

If you go to pour the smelted gold and find that the small crucible has ruptured and most of the borax is in the larger crucible this is not a problem. Usually the rupture level is well above the base of the small crucible so the gold is still retained in the small crucible. Do not stand there looking at the crucible and wondering what happened, just do your pour before the crucible cools and will not release all of the gold.
If there is no metal or borax in the smaller crucible then the rupture occurred at the small crucible base and all of your values are in the larger crucible. Immediately pour the contents of the larger crucible into the mould before cooling occurs.

Do not reuse any crucible which has come into contact with molten borax. You will be inviting crucible failure.

If all of your crucibles are rupturing either you or your supplier have had them on the shelf for too long or the crucibles are not of reasonable quality, change your supplier.


The dried concentrates which are poured into the molten borax must be finely divided or as very thin films. Precipitates from chloride leaching are ideal feedstock provided you have not left them clumped together from the filtration step.
Generally you can expect to add another 9 on your purity by smelting as above.
Concentrates which might run say 95 % gold in a carbon crucible smelt will run 99 +%, Cons which might run say 99% gold in a carbon crucible smelt will run 999+%.

The above has really no commercial effect here as pretty well all gold produced is sold to refiners and you will pay the same refining fee if your bars are 60% gold or 9999 gold.
I just developed the method out of curiosity.


Deano


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## Deano

One of the more enduring myths in gold processing is that any gold reporting to the slag in a gold smelt of the cathode material from electrowinning the gold strip solution of the carbon used in a CIP circuit can be successfully processed by throwing the slag into the mill and leaching the resulting product in the cyanide circuit.

If the slag is passed through a hammer mill and the crushed slag is then passed over a wilfley table a disconcertingly large amount of fine gold particles report to the table concentrate.

If this concentrate is leached in cyanide or aqua regia only a fraction of the gold dissolves. This does not fit in with the reprocessing of slag approach used by the gold industry.

It appears that the surface passivation on the gold particles not only prevents leaching but also is the reason for these particles not entering the main body of metal in the smelt.

The only way in which these gold particles may be successfully processed is to use these particles as the feed in the previous post " Smelting of gold concentrates".

This was the method I developed for recovery of the slagged gold, the fact that it improved the gold purity was a bonus.

Deano


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## Lou

What do you mean surface passivisation of gold? Under what conditions???


Regarding the cathode from cyanide plating--can usually get down to 10-15 ppm Au with steel wool cathode, stainless anode, pH 12+; run until much O2 fizzing. 
Incinerate the cathode (it will be brown like gold coming out) and redissolve in aqua regia; filter to remove AgCl; precipitate, wash gold, 9995%+

Lou


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## Deano

Lou

A lot of the material I work with is the product of mine CIP carbon stripping circuits.

Basically the electrowin is very similar to what you wrote but we always have some copper and lead on the cathode wool. Depending on the mine there may be up to 20 kg of gold per strip cycle. Not surprisingly the mine management want to get this fairly portable gold on wool product into a single less portable bar as quickly as possible. As soon as it is dried after aciding out the residual steel wool it is smelted.

All gold mined in Australia is sold to refineries such as the Perth Mint. There will be a refining charge for each bar sent to the refiner along with various other batch and sales charges.

There is the same charge per ounce for refining if the precious metal levels are 80% or if they are 9999. No mine is going to attempt higher purity in the bars when it will cost more and there is no discount on the refining charge.

The surface passivation on the gold beads is caused by a coating of lead minerals. This coating is only a few hundred angstroms thick and is not detectable by optical methods, I had to use XRF.

The coating is not soluble in any solvent in the classical chemistry line and believe me I tried them all. You get a little depressed when you boil the treated beads in aqua regia or cyanide and nothing happens. A few hundred times.

The only way I found to recover the gold from these beads is with the borax smelt in clay crucible trick.

Deano


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## Deano

Lou

I was rushed on the last reply and left out a broader description of carbon stripping, my apologies.

Most mines in Australia use a hot caustic cyanide strip on their carbon from the CIP/CIL circuit. If a slight overpressure is used in the strip columns with a pressure drop chamber at the entrance to the EW box then the strip time can be substantially reduced as the temperature of the leach can be run slightly above 1000C. The change can be as much as going from a 24 hour strip to a 6 hour strip. This effect is used to increase the capacity of a strip circuit without having to install greater power rectifiers and more EW cells etc.

We usually do a HCl contact of the carbon before the cyanide strip, this gets rid of a lot of the base metals and improves the gold recovery from the carbon.

In a lot of mines we have difficulty getting the gold on carbon level below 100 ppm. This is usually due to high levels of base metals in the ore.

Usually we are trying to get the gold on carbon level down to less than 50 ppm, preferably less than 30ppm.

In most stripping circuits the gold on carbon levels have been well established as a function of stripping current, there is a definite drop in current as the gold level drops.

There is not a great emphasis on getting all of the gold from the strip liquor as the used liquor is later used as make up liquor for the leach circuit. Provided the gold on carbon level is about right and the strip liquor tenor is less than 20ppm gold then most mines are happy.

If you put the recovery into context, you are getting say $500,000 of gold from a strip. Actually carrying out this strip might cost you $ 1,000. There is not a great incentive to be more efficient in the strip process.

Usually a fair amount of the gold will fall off the cathode and form a sludge layer on the bottom of the cell. This gold has to be drained from the cell, filtered and dried before being added to the other gold before smelting.

The steel wool is acided out from the gold cons with HCl. An incredible amount of gold is dissolved in this step, the acid solution and rinse solutions are usually poured through a plastic drum full of carbon for recovery of this gold.

The gold cons after aciding are dried, chopped up finely with a steel scoop and are ready for smelting.

There are as many methods of actually doing the smelt as there are mines.
Some mines heat the crucible to temperature before adding the flux, they get the flux up to temperature and then add the gold.
Other mines will heat the crucible to temperature and then add the flux and gold as a single charge.
Still others will put the flux and gold charge in the crucible before starting heating.
Any variation of the above is being used somewhere.

Similarly every mine has a pet flux formula, usually set by the first gold room superintendent and not changed as an article of faith ever since.
If the gold charge is split into two before smelting there is usually no difference in gold recovery between the formula flux and straight borax. Takes a bit of the mystique out of it all.

After the carbon has been stripped and a sample retained for analysis the carbon is usually run through the regeneration kiln to restore adsorption efficiency. A lot of times it doesn't need the full regeneration but is improved by a quick clean in the kiln.

Deano


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## g_axelsson

Deano said:


> The steel wool is acided out from the gold cons with HCl. An incredible amount of gold is dissolved in this step, the acid solution and rinse solutions are usually poured through a plastic drum full of carbon for recovery of this gold.


Have you ever tried to precipitate this gold with SO2 (or sodium meta bisulfite)? Afterwards the really low level gold bearing fluid could be treated in a "stock pot" as being described in many places on this forum or in Hookes book, then discarded as waste (properly).

As I'm not familiar with the running of a CIP facility, I suspect there is a (unknown to me) reason to do it your way. 

Göran


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## Deano

Goran

Think of processing in a gold room as industrial scale but carried out by people with little understanding of the processes involved. Would you really want someone like this carrying out a precipitation and filtration stage with the tying up of the equipment when the liquor can just be poured into a drum of carbon.

This carbon is stripped with the other CIP circuit carbon when the gold level is high enough, if you have gold metal showing on the carbon you have left it too long before stripping.

The liquor from the carbon adsorption drum has effectively no gold in it and is disposed of in the tailings dam, 100 litres in 10,000 tons of water does not constitute a hazard.

The major problems in the gold room relate to processing and theft.

The processing problems are overcome by following a standard set of simple procedures, no intellectual input required.

The theft problems are minimised by having the gold in a portable form for the shortest time possible, having the gold on carbon does not rate as portable gold.

Always remember that high purity of the bullion is nice to achieve but not an economic requirement at this scale.

Deano


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## g_axelsson

Ahhh... the people factor. :mrgreen: 

Even Hoke has a section on losses due to dishonesty.

Göran


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## Deano

Water sourcing

Many mines use water pumped from an underground aquifer for make-up water. This water is usually stored in a dam until required. There are often two problems with doing this.

One is the growth of algae in the dam water to the point where it interferes with the use of the water.
The algae may be removed by two methods.
The first method is to stock the dam with carp at a rate of one fish to about 200 tons of water, this rate is dependent on the nutrient load in the water, temperature and surface to volume relationship. Make sure that only one sex of fish is used if more than one fish is required.
Many authorities will not allow dams to be stocked with carp so the second method has to be used.

This involves spraying the surface of the dam with a ferric salt solution, usually ferric chloride. This must be done using all plastic equipment including pumps and spray nozzles. You are aiming to cover the surface of the water only, not turn the dam into an acid bath.
When the ferric chloride turns into ferric hydroxide in the dam water there are short lived hydroxyl radicals formed.
These radicals are toxic to algae, in fact to most small organisms including bacteria.
The spraying must be done in daylight so that the algae are present in the upper layers of the water.

The second problem with the dam water is that many aquifers have substantial levels of ferrous salts present. When the water is pumped into the dam a slow reaction with dissolved oxygen will convert the ferrous salts into ferric salts.
These salts will precipitate out slowly forming a coarse sand. Unfortunately the precipitation occurs not only in the dam itself but also in the pipework from the dam to the mine plant.
This causes blockages in the pipework and valves, a surprising amount of this sandy material is formed.

To stop the sand formation the aquifer water should be sprayed into the dam in a fan jet so that the water is saturated with oxygen from the air. This causes the ferrous to ferric reaction to happen much faster and cuts out the slow sand formation.

The above is applicable to farm situations as well as mines.

Deano


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## Deano

Thiourea leaching

Many people try different methods of leaching gold out of curiosity and general interest. These efforts are to be applauded, even failures add to the sum of knowledge.

Effectively there are two types of gold leachants, organic and non-organic.
The non-organic leachants comprise the halide complexes and some of the more esoteric leachants such as selenic acid.

These non-organic leachants all suffer from one or more of the following defects.
1 They must be run under acid conditions.
2 They have serious health and safety issues.
3 They are expensive to purchase let alone use.
4 Complex chemistry.

The organic leachants suffer from one or more of the following defects.
1 Toxicity.
2 Reagent degradation.
3 Complex chemistry.

When thiourea became of greater interest as a precious metal leachant back in the 1980s a lot of work was done by many groups to optimise operating conditions.
Unfortunately all of these groups failed to recognise that the major cause of leach difficulty and high reagent consumption was reagent degradation.

Thiourea or its oxidised form called formamadine disulfide, when in soultion, is attacked by ultraviolet light and degrades to elemental sulfur.
The degradation rate increases as the solution Eh decreases and/or the Ph increases.

Thiourea as such will not complex with gold, it requires an oxidant to form formamadine disulfide (FDS). This is what actually complexes with the gold.
FDS is formed when an oxidant with an Eh of greater than 300mv, preferably greater than 350mv, is added to a solution of thiourea at levels so that the Eh is maintained in the solution.

Most testwork carried out with thiourea has used ferric salts under acid conditions as the oxidising agents. All sorts of clever Eh balancing acts have been used to minimise the losses of thiourea in the leach solution, what no one has done is to carry out the experiments in the absence of uv light.

Thiourea can be used under alkaline leach conditions with oxidisers such as hypochlorite, however the thiourea is very sensitive to uv degradation at these higher Ph ranges.
If you want to run thiourea under alkaline conditions you must do so in the absence of uv.

Keep in mind that thiourea is rated as a carcinogen and as such is not recommended for general use outside of specialist labs.
The gold thiourea complex also suffers from poor loading levels on carbon and resin, it needs to be electrowon or zinced out of solution.

Thiosulfate and other organic form leaches can be made more robust and employ simpler oxidants if used under low or no uv conditions, remember that they also suffer from extraction difficulties.

Deano


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## kurtak

This is some very interesting reading --- thanks for posting it Deano --- & please keep posting as I look forward to continued reading 8) :!: 

Kurt


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## Lou

Thiourea cannot be used under basic conditions at all. 

I used to elute PGM and Au thiourea complexes off of a macrocylic PS resin functionalized with thiouronium groups (Purolite S920 IIRC).

The options at that point were to use borohyride and save your thiourea, or if really concentrated, you could bring up the pH and boil and destroy your thiourea. Precious metal sulfides precipitate in quantitative yield and may then be worked up in aqua regia.


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## Deano

Lou

The standard elution for gold from strong base resins is thiourea in sulfuric acid solution where the sulfuric acid supplies the Eh required to complex the gold from the cyanide or chloride form into the gold thiourea complex. This is usually carried out as either a multi-stage series of contacts or as an extended flow through contact. Either way you get a strong acid solution of gold thiourea which, as you pointed out, can have the gold extracted in a multitude of ways.

If you raise the pH you destroy the Eh as the sulfuric acid is neutralised. At this stage the thiourea complex is very sensitive to degradation by uv and will precipitate elemental sulfur in minutes. If the solution is not accessed by uv there is no precipitation of sulfur.

An alkaline solution of thiourea in alkaline hypochlorite solution and exposed to uv is stable only for a few minutes but will leach gold in that time. If the uv is not allowed to contact the solution then the solution is stable as a gold leachant for a long time.

I have kept some of these solutions in a uv free area for over a year, they are crystal clear when they are then exposed to uv and last only minutes before elemental sulfur is precipitated.

None of the above means that I use thiourea as a leachant for gold, the health concerns and difficulty in recovering the gold values as a concentrate on an adsorbent see to that.

Deano


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## Deano

Gold complex adsorbents

In industry there are two adsorbents used for collecting and concentrating dissolved gold values.
These are activated carbon and ion exchange resin.

Activated carbon is the gold industry's adsorbent of choice in that it is cheap and easily handled through the loading and stripping cycles.
The downside of carbon is that the gold loadings from mine leach tanks are relatively low. This means that the carbon must be recycled through the load and strip cycles at a fair rate to recover all the gold available, you do not want to leave the carbon in the circuit for a long time in an attempt to raise the gold loading on the carbon as the attrition losses will increase.
Carbon will also adsorb most metals in the leach solution, especially copper and lead, these can give problems requiring extra steps in the stripping cycle as well as lowering the number of loading sites for gold in the adsorbence stages.

The elution of gold from carbon in a mine situation will always leave some low level gold irretrievably locked in the carbon, this gold can only be recovered by ashing, leaching the ash with cyanide and loading the gold onto fresh carbon.

Problems with using carbon relate to attrition of the carbon resulting in the generation of fines which carry gold values into the tailings dam, the time it takes to strip the carbon and the need for regeneration of the carbon in a kiln on a regular basis. Generally the industry uses a hot caustic cyanide strip solution for carbon but many variations are available.

Ion exchange resin is really only used widely in Russia and neighbouring countries.
The resin can load to higher gold levels than carbon and generally does so more quickly than carbon.
A lot of resin manufacturers have worked on providing a resin which is gold specific and thus will not load appreciable quantities of base metals. Generally they have been relatively unsuccessful, there appears that there is a trade off with ease and level of gold eluted in the strip cycle.

Most commercial resins used for gold cyanide recovery are functionalised with quaternary ammonium groups, these are called strong base resins and are commercially stripped with a sulfuric acid /thiourea solution.
Purolite have a gold resin which uses mixed non quaternary groups, this can be eluted with caustic cyanide solution (yay).

The pros of using resin are the faster and higher gold loadings obtained and the speed and ease of the strip cycle.

The cons of using resin are the high initial cost, the need for finer screens in the gold plant, the level of gold lockup in the resins and the effect of osmotic shock on the resins.
After a number of cycles the gold level locked up in the resin is so high that this inaccessible gold represents a substantial proportion of the gold inventory in the circuit.
This locked up gold also greatly lessens the amount of gold which can be adsorbed and desorbed per cycle as there are less adsorbance sites available.
Like carbon, this locked gold can only be recovered by ashing the resin and leaching the ash.

Generally the resin beads are less subject to wear than carbon particles. Unfortunately they suffer from osmotic shock which carbon does not suffer from.
Osmotic shock is the swelling and contraction which the beads go through when the liquor in which they are immersed changes from acid to alkali or reverse.
Going from strong alkali to very strong acid conditions and back again will ensure that the beads suffer osmotic shock and will start to physically fall apart into smaller particles.
This means that resin and thus entrapped gold losses are substantial even with extra screening.


Deano


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## Deano

Organics destruction

There are often requirements for the removal of organics from a water matrix. This can vary from cyanide destruction to dye oxidation and a general removal of a large number of of other organics which are wanted to be removed for health or other reasons.

The destruction of these organics can be achieved by reversing polarity of the electrowin cell detailed earlier. This means that the carbon felt will now be the anode and it will be generating very high levels of hydroxyl radicals. These radicals are short lived but very highly oxidising, the passing of the solution through the felt ensures that the organic particles will come into contact with these radicals.

There are two controls to the process, one is the flow rate of the liquor through the cell and the other is the rate of generation of the hydroxyl radicals.

There must be current flowing through the call for the radicals to be generated. This current can be carried by by acid or alkali in the liquid or by a neutral, non active salt such as sodium sulfate in which case the current is carried by water splitting at the electrodes.

Generally as long as the electrodes have gas evolution from them then there is enough current flowing to generate the hydroxyl radicals.

Due to the sheer number of unknowns in any solution such as organic type, concentration etc. I cannot give a general current level for any liquor.
I have always adjusted pH and/or salt in solution so that I had gas evolved at the electrodes, easiest seen at the now cathode in the centre of the cell.
I then adjusted the liquor flow rate through the cell so that all of the organics were destroyed in a single pass through the cell. You can use low gas evolution with slow flow rate or high gas evolution with faster flow rate.

Deano


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## Deano

Organics destruction

Once again some things I left out.

The felt cell run as per previous instructions will, by by removing organics from water, remove offensive odours.

It will also sterilise the water but the sterilising process is not an extended effect. The water is sterile only until it exits the cell. At this stage it can be recontaminated.

Deano


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## Deano

It appears, based on the advice posted on this forum, that members are burning filter papers so that they can recover gold from solution or suspension which has been adsorbed onto these papers. 

It has been fairly common knowledge that burning filter papers will lead to metal losses both from dissolved and particulate gold.

Out of curiosity I have run a series of tests to quantify the extent of these losses.

I loaded gold chloride as a solution onto Whatman 90mm diameter No 1 and No 42 ashless papers.
The loadings were calculated at 10 micrograms and 50 micrograms on each paper. The papers were then dried at 80C.

A straight digest of these papers in aqua regia with DIBK/aliquat 336 extraction from the liquor and flame AAS analysis gave recoveries of 9.6 and 48.5 micrograms for both paper types and gold levels.

When the filter papers were burnt with flame the recoveries were 3 and 21 micrograms respectively for the #1 papers and 2.9 and 5.2 micrograms for the #42 papers.

A repeat of the above but with ashing in an electric furnace for 2 hours at 380C and no flame gave recoveries of 3.7 and 24 micrograms for the #1 papers and 8.8 and 9.6 micrograms for the #42 papers.

I then crumpled the papers into a tight ball before placing the gold solution onto them and drying as before.

Recoveries for the # 1 papers burnt with flame were 3.7 and 22.9 micrograms and for the #42 papers 4.4 and 16.1 micrograms.

The repeat of the crumpled papers ashed in the furnace gave 8.0 and 43 micrograms for the #1 papers and 9.5 and 42.6 micrograms for the #42 papers.

A further set of tests were conducted where the papers were not pre-dried bur were put wet into the furnace.

The #1 papers gave 8.1 and 45.6 micrograms, the # 42 papers gave 8.8 and 45.6 micrograms.

The crumpled wet #1 papers gave 9.7 and 46.3 micrograms, the crumpled wet #42 papers gave 9.8 and 44.9 micrograms.

The results indicate that if you want to minimise gold losses when ashing filter papers you should conduct the ashing in an electric furnce at 380C with the filter papers tightly crumpled.
Having the papers wet going into the furnace may slightly improve the recovery further again.

All ashings were conducted in 100 mm diameter porcelain dishes.

Deano


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## butcher

I am not surprised at all by your experiment, gold and a chloride salt will put gold into a vapor form the finer the gold I suspect more of the loss involved, the faster the temperature is raised and not controlled the more gold I would expect to vapor off.

Even table salt NaCl and Gold will vapor off the gold, when hot enough, it is also hard to get to a temperature that the chloride would vapor off without taking gold with it, as the boiling point of the salt is so close to the vapor point of the gold chloride.

As an mass of different base metal chloride powders and gold are heated, it is my belief that as one base metal chloride gives up its chloride ion as gas, another base metal in the mass, or even a more noble metal can take up the newly formed chlorine gas, and if the temperature was hot enough, and raised fast enough there your gold goes up in smoke.

It would be extremely hard to control the temperature to drive off chlorides and not have the gold (or silver) follow the (chloride vapors, chlorine or HCl fumes).
I think it may be possible in a controlled lab experiment, if the powders could be heated to just the right temperature and held there long enough, where just the chloride would vapor off without taking the gold with it, but in reality we do not have this kind of control of temperature with what we do.

Harold speaks of a purple trailer parked where his exhaust fumes had colored it in his early years of learning recovery and refining. He also spoke of the gold mine from his fume hoods as a nice little piggy bank once cleaned out. 

Gold can be dissolved in chlorine gas, and carried along with fumes (normally around 800 degrees).
Vaporization of gold chloride, from your filters is no surprise, like silver chloride which is also volatile at higher temperatures.

Gold chloride I would expect high lose to vaporization of the gold, the more control of the temperature rise in the electric furnace may help somewhat, but I do not believe it will solve the problem.

(I can not think of where I would incinerate filters that were saturated with gold chloride solution, as these are normally rinsed with water, so the soluble gold chloride passes on through the filter material), although you can have elemental gold powders with a base metal chloride salt, (or silver chloride), in the filter, which if incinerated would lead to the gold becoming volatile in the chloride fumes at a high temperature...

I normally wash my filters, or impure gold and base metal chlorides powders,  in a sodium hydroxide solution, before incineration, in the hopes of rinsing out the NaCl (that forms from the sodium hydroxide and the involved chloride metals), with the following water washes, converting many of the base metal chlorides, and silver chlorides to oxides, (and some possible hydroxides depending on base metal involved), to remove as much of the chloride ions as possible before incinerating.


(Precipitated or powdered gold is elemental metal and not a chloride), so removing the salt or chloride base metals, that may be mixed with the fine gold.
It is my thought, or theory, I would have less lose of gold because of chloride salts, because I remove them as much as possible with pretreatment.
I also raise heat slowly at the beginning portion of the incineration process, normally with the hot plate prior to using the torch to finish the incineration process, to drive off any chloride the previous treatment of hydroxide and washing may have missed, heating the powders to drive off water and fumes of the acids, before raising the temperature of the crushed powders to the red hot and oxidizing them further with air or oxygen.

I have not done any controlled experiment's to verify this theory that the hydroxide wash will do as much as I believe it to in this regard.

But would be interested in the results of a controlled lab test.

I


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## justinhcase

Thank you Deano for taking the time to help improve our recovery process.
I have a good sized bin bag of filter papers to process and have held off until I built a flue baffle to catch P.M.'s in the fumes.
I will try to work your findings into my practices' had been salting my wet papers with ferrous sulphate crystals to minimise loss through heating gold chloride. 
you and the more experienced members of the forum continue to amaze me with the amount of time and effort you put into improving the craft for others.
Much thanks
J


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## g_axelsson

Interesting topic! Again! 8) 

I have always thought about the volatility of gold chloride but never very hard. Now I have been thinking for a while...

If we lose gold while incinerating and it isn't as dust then it has to be in the form of a vapor, a chloride vapor. But gold(III) chloride breaks down from heat into gold(I) chloride and chlorine gas at a quite modest temperature of 160 °C. Gold(I) chloride in it's turn breaks down into gold and chlorine gas at 198 °C. But if the temperature is raised above 420 °C then gold(I) chloride transforms into gold and gold(III) chloride which is stable at this elevated temperature. The equilibrium is controlled by the partial pressure of chlorine gas so we want to get rid of it.

What if we raise the temperature of the filters to about 200 °C and keep it there for a longer time and slowly exchange the air in the chamber. For the hill billy refiner it can be done with an electrical stove out in the yard. The chlorine gas should be removed over time and we end up with metallic gold in the filters and no gold chloride that can evaporate.

One problem I see with this way to process the filters is that there may be other chlorides in the filter papers that doesn't break down and can emit chlorine that attacks the gold at higher temperatures, like when gold ore were roasted with sodium chloride to make it easier to dissolve the gold. (A procedure used before cyanide leaching was invented.)

Göran


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## Deano

Goran

I actually ran a much longer series of tests trying to find out what was the minimum temperature that the filter papers would ash at without flaming. I started at 200C and went up in 20C increments holding each temperature for 2 hours. They required a temperature of 380C to completely ash both types of paper.

The test series were also extended to temperatures up to 660C. There were no differences in results between the 380C ashings and the 660C ashings.
All the losses had occurred at the 380C level.

There was no difference in recoveries if the furnace and contained paper was slowly brought up to temperature over a 2 hour period prior to the temperature being held at 380C for a further 2 hours or if the papers were placed straight into a furnace at 380C.

For me the interesting part of the results was the high recoveries for the crumpled papers (around 93%) and the low recoveries for the uncrumpled papers (around 50%). It appears that the gold is lost in proportion to the surface area of the papers during the ashing step.
If the crumpling could be better performed to give, say, a hard paper pellet I would expect the losses to be even less than I recorded.

Butcher

I have always found that a boiling solution of gold chloride will have some carry over of gold into the vapour phase.
Tests I carried out many years ago indicated that the low levels of gold chloride in an aqua regia digest of an ore sample had such a low carry over level that losses were difficult to quantify.
However when working with concentrates there was a substantial carry over gold loss.

This is a separate issue to the ashing losses.

You are correct that you would not normally ash filter papers with high gold levels in the papers but the results indicate that you may be able to be less diligent than usual in the rinsing stage if you can minimise the surface area with an ashing of 380C.

I will run a caustic test in the following manner.

Put the gold chloride onto the paper as per previous tests.
Add caustic solution to the paper such that the paper is saturated but not losing liquid to the dish.
Use caustic levels of 1 and 10% solutions.
Dry the papers after a 30 minute contact with the caustic solution.
Ash at 380C with both preheated furnace and a slowly ramped furnace with papers inside.

If there are any steps I have missed out let me know and I will run further tests to incorporate them.
Let me know if I have totally misunderstood what you were interested in.

Deano


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## butcher

If the test was to reduce the gold, and to wash out the chloride salts, And I was going to try and do it with hydroxide, I would go for a stronger caustic solution, and more solution, letting the solution settle with the paper, decant and filter the gold and paper and any settled powder through another filter, rinsing it with water to wash out as much of the soluble sodium chloride as possible to remove the sodium chloride from the gold powders and the filter papers...

Again I cannot think of where I would be incinerating a paper soaked with gold chloride solution.
If for some odd reason I had some to do, I would most likely soak them in a minimum amount of liquid of concentrated solution of SMB, or sodium sulfite before hand and filter the clumps of filters. 

SMB or some sodium sulfite, would reduce the gold chloride in the paper, working better than a hydroxide solution...

The hydroxide wash would be better where chloride base metal salts where involved in the filtered powders...

I do not know how to explain my thinking here.
Carbon in the paper may hold some of the answer here.

The paper forming carbon (slowly heating the waded paper which will naturally have less exposure to the air inside the tight clump of the paper, the paper can form carbon better), the carbon which may help to capture the gold, of the gold chloride fumes, the carbon capturing gold in the papers carbon, giving the chloride anion a chance to fume off leaving the gold Cation in the carbon, before the paper incinerates to ashes, giving the chloride ion time to form vapor leaving more of the reduced gold in the carbonized paper. (something like using carbon in a gold leach to capture gold).

This I believe would work better with slowly bringing up the heat, in a controlled manner (reducing the oxygen at least for the first of the process, to carbonize the paper to carbon).

And not just flashing the paper to fumes with the hot flame of the torch, where incinerating the paper almost before the fumes of the chloride have a chance to be driven off from the gold, and to carry the gold with them in the fumes.

The tightly waded paper would not only heat up more slowly, but would act more as pyrolysis of the paper to carbon easier, (where the carbon can act to absorb the gold fumes), than say it could with a faster burning of the open paper exposed to plenty of air, or the filter open (not into a wad), where the paper in the open air with the carbon of the paper possibly incinerating or burning off the carbon almost faster than the gold chloride has a chance to vapor off in the fumes.



It is with base metals and chloride salts that I am more concerned with lose of fine gold or silver powders in an incineration processes. Where I would be more interested in doing controlled experiments.

It has been some time since I have had time to do any work out in the lab, with this job, and my back and hip giving me so much trouble, this is one project that I think would be fun and interesting to work on.

I have been doing some work on paper, getting ready to try some experiments whenever I get a chance to get back out to my little lab, which is making nitric acid out of some sodium nitrite, and doing some work with the little bit of silver I have been collecting.


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## justinhcase

Would Hoke's recommendation of using an excelerant or liquid fuel to incinerate paper's make a little more sense when look at with this new data.
I know it is no longer recommended but a flame burning on a wick like structure like filter paper would keep the main body of the material relatively cool.
Then it would increase in temperature as the fuel runs out and the ends of the "wick" turns to carbon before being fully consumed .
It would be interesting to compare results from the traditional method and the latest advice. 
Regards
Justin


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## Deano

Butcher

I will try three test methods.

The first will be as described previously but using 20% NaOH solution. As all liquids will be retained in the paper it will show any losses by ashing gold metal in the presence of chlorides.

Secondly I will add an excess of the 20% NaOH solution to the filter paper, again allow to contact it for 30 minutes and then filter with five rinse cycles of water using 200 ml per cycle.
This will show any losses from ashing in the absence of chlorides.

Thirdly I will contact the paper with a saturated solution of sodium sulfite and filter and five times rinse with water.
This will show any losses from ashing in the absence of chlorides but comparing NaOH with sodium sulfite for gold chloride reduction to gold metal.

I expect that there will be some losses of gold through the filter papers during the filtration steps, all of the solutions will be analysed for gold values.

Let me know if I have missed anything.

I can have a look at base metals and gold in chloride matrices for you if you can lay out what you want done.

Deano


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## g_axelsson

Deano said:


> Goran
> 
> I actually ran a much longer series of tests trying to find out what was the minimum temperature that the filter papers would ash at without flaming. I started at 200C and went up in 20C increments holding each temperature for 2 hours. They required a temperature of 380C to completely ash both types of paper.
> 
> The test series were also extended to temperatures up to 660C. There were no differences in results between the 380C ashings and the 660C ashings.
> All the losses had occurred at the 380C level.



Hi Deano and thanks for sharing your results.

Göran


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## Deano

Finally completed the filter paper ashing tests.

The standard addition of gold chloride for each test was 47.0 micrograms.

Straight aqua regia digest of the filter paper returned 47.1 micrograms.

NaOH tests using 20% NaOH solution

Whatman #1 papers

Ashed flat using flame returned 38.8 micrograms, ashed crumpled using flame returned 40.0 micrograms.

Ashed flat in furnace at 380C returned 43.8 micrograms, ashed crumpled in furnace returned 45 micrograms

Pre processed using 5 cycles of rinse water after NaOH contact.

Ashed flat using flame returned 29.2 micrograms with 0.8 micrograms in the rinse water. Ashed crumpled using flame returned 32.5 micrograms with 1.0 micrograms in the rinse water.

Ashed flat in furnace returned 35.8 micrograms with 0.7 micrograms in the rinse water. Ashed crumpled in furnace returned 39.2 micrograms with 0.7 micrograms in the rinse water.


Whatman #42 papers

Ashed flat using flame returned 34.6 micrograms, ashed crumpled using flame returned 40.4 micrograms.

Ashed flat in furnace returned 42.9 micrograms, ashed crumpled in furnace returned 42.9 micrograms.

Pre processed using 5 cycles of rinse water after NaOH contact.

Ashed flat using flame returned 34.6 micrograms with 0.5 micrograms in the rinse water. Ashed crumpled with flame returned 38.8 micrograms with 0.8 micrograms in the rinse water.

Ashed flat in furnace returned 36.7 micrograms with 1.0 micrograms in the rinse water. Ashed crumpled in the furnace returned 38.8 micrograms with 0.8 micrograms in the rinse water.

It was very noticeable that the NaOH formed what could best be described as a skin over the filter papers which appeared to offer some protection to the gold during the ashing processes. NaOH is difficult to wash out and even after 5 rinse cycles there was still some caustic skin on the papers during ashing.

Repeat of the above tests using saturated sodium sulfite solution instead of NaOH solution.

Whatman #1 papers

Ashed flat using flame returned 30.2 micrograms, ashed crumpled returned 32.2 micrograms.

Ashed flat in furnace returned 33 micrograms, ashed crumpled in furnace returned 41.3 micrograms.

Whatman #42 papers.

Ashed flat using flame returned 18.6 micrograms, ashed crumpled using flame returned 24.3 micrograms.

Ashed flat in furnace returned 26 micrograms, ashed crumpled in furnace returned 40.3 micrograms.


Pre-processed using 5 cycles of rinse water after sodium sulfite contact.

Whatman #1 papers

Ashed flat using flame returned 26.1 micrograms with 0.8 micrograms in rinse water, ashed crumpled using flame returned 31.1 micrograms with 0.7 micrograms in the rinse water.

Ashed flat using furnace returned 33.9 micrograms with 0.6 micrograms in the rinse water, ashed crumpled in furnace returned 38.2 micrograms with 0.9 micrograms in the rinse water.

Whatman #42 papers.

Ashed flat using flame returned 27.1 micrograms with 0.8 micrograms in rinse water, ashed crumpled using flame returned 33.1 micrograms with 0.8 micrograms in the rinse water.

Ashed flat using furnace returned 32.2 micrograms with 0.7 micrograms in the rinse water, ashed crumpled in furnace returned 36.4 micrograms with 0.6 micrograms in the rinse water.


In summary the NaOH appeared to have a protective effect on the gold even after the rinse cycles.

The sodium sulfite treated papers gave less losses of gold than the untreated papers but still allowed substantial losses during ashing.

Low levels of gold were rinsed out of the papers for both reagent types and paper types.

The lowest level of losses were from the caustic only contacted papers even though these papers were the most difficult to maintain in a crumpled shape during ashing, the crumpling appeared to have a minimal effect for these papers when furnaced.

The use of sodium sulfite improved the gold recovery from the ashing process but the results were inferior to the use of NaOH.

The rinsing out of chlorides in the sodium sulfite tests improved the gold recovery in the poorer performing test modes but had a lesser effect in the better performing test modes.

The rinsing out of chlorides had a negative effect on gold recovery for the NaOH tests, this was attributed to loss of a protective NaOH layer on the papers.


Deano


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## g_axelsson

Then the question is... does the missing gold go off as metallic gold or as gold chloride? Where does it go?

Göran


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## Deano

Goran

As part of the test program I fitted a stainless steel condenser plate, water cooled, inside the electric furnace just under the exhaust hole.
This condenser can be quickly removed and placed in a cyanide bath to check on the level of adhering gold, if any.

I still have most of the cyanide strip solutions to analyse but those which I have looked at show that nearly all of the missing gold is recovered on the condenser.
Recoveries are better than 90% of the missing gold.

Judging from the prior test results I consider that the sulfite treated chloride rinsed free filter papers had the gold volatilise from particulate metal.
The greater volatilisation levels from the untreated papers indicates that the gold chloride form of the gold is, as expected, more readily volatilised than the particulate metal.
The median gold volatilisation levels from the sulfite treated but not chloride rinsed papers indicates that the presence of chlorides adjacent to particulate gold enhances the volatilisation of the gold but not to the level of the untreated gold chlorides.


Deano


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## butcher

Nice work, on the experiments.
Thank you for taking the time and going through the trouble do this work to help us better understand
what may be happening.

I would still find it interesting the results of incineration or of melting impure gold powder, brought to red heat to drive off chlorides, or melted with base metal chlorides.
And how well using NaOH to convert the base metal chlorides into NaCl, and base metal oxide hydroxides, (then rinsing out the salt water) will work to keep the volatility of the gold to a minimum.
But I believe your experiment's results help to show it should be at least somewhat helpful.

Good work thanks.


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## kurtak

This is awesome :!: 

Thank you Deano for taking your time & trouble in posting not just the filter/incineration experiments but the whole thread in general 8) 

Kurt


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## colchis

Deano - Thanks for a informative process practice. There's one problem about the pH - eH balancing for best leaching- The description states: "What you end up doing with the leach is a continual sequence of adding HCl to adjust the pH and then adding hypochlorite to adjust the Eh, hence the need for the meters." The problem is this: In using Clorox, it contains sodium hydroxide, and will tend to increase the pH. Does this mean that it is necessary to use expensive technical grade sodium hypochlorite? About Clorox- according to the MSDS, the "germicidal" branded clorox contains only 2 ingredients- hypochlorite and hydroxide- the other types of clorox contain a long list of other chemicals in addition. What to do?


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## Deano

Butcher

I can fairly readily do some furnacing tests using impure gold electrowinning cons containing around 10% silver and 10% copper. 
I can add copper chloride at various rates to see what effect this has on gold volatilisation levels at various temperatures.
I can also add NaOH solution to the above blend and run the rinsed and unrinsed products at the various temperatures.
Gold volatilisation would be measured by the levels recovered from the condensing plate in the furnace.
Keep in mind that the cons have the gold in a fairly finely divided form, it is not like a sheet or as grains.

Let me know what copper chloride rates and temperature levels you would like looked at.

Deano


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## Deano

Colchis

It is not necessary to use the expensive tech grade hypochlorite, I use pool granular hypochlorite if I am performing the leach in larger quantities.

The liquid hypochlorite is easier to use but it degrades fairly quickly and if it has been sitting on a supermarket shelf for a while it may be fairly weak. Usually the supermarket varieties will have at least one which is fairly pure hypochlorite with caustic present as a stabiliser, this will usually be the no name house brand which is also usually the cheapest.

I would never use hypochlorite which has other chemicals added, you will get enough garbage in solution from the leach without starting off with something which you have no idea what effect it will have on the leach.

The amount of HCl required to keep the pH in the best zone is fairly minimal, for leaches using just a couple of litres of solution I use a plastic 3ml pipette for the acid additions and a separate pipette for the hypochlorite additions. pH 3 is a fairly weak acid level, you do not need much HCl to get there.

Once you have done a couple of leaches you get to know fairly well the times and quantities for the reagent additions.

Deano


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## Deano

I was interested to read about leaching gold from stainless steel using 97% of 50% HNO3 with 3% HCl as a leach.

I do not have access to a source of this type of material but I was interested in what effect this leach may have on an ore.

I have access to samples of a gold ore of grade 1.15 ppm gold by fire assay and by aqua regia digest. Cyanide recovery was 1.0 ppm gold.
The above grades were from 7 replicates with a confidence interval greater than 95%.

This ore contains around 70% quartz and 30% iron as haematite.
The sample was milled to 100% minus 150 microns and 25 gram splits were riffled out.

Leaches were carried out in beakers using 200 ml liquor to the 25 gram ore samples. All leaches were carried out for 24 hours without agitation.

Baseline leaches using the starting leach formula above gave a recovered gold grade of 0.75 ppm gold.
The gold could not be read directly or by organic extraction from the leach liquor by AAS.
The liquor had to be converted to full strength aqua regia before the readings could be reasonably accurately done.

This effect sparked my curiosity and I spiked blank leaches with gold chloride to see what level of hydrochloric acid was required in the leach before it could be reasonably accurately read on AAS.
It appears that any HCl level less than 50% at the spiking stage causes problems at the AAS analysis stage.
I then contacted the spiked liquors with activated carbon, dried and ashed the carbon and analysed the residues.
The results were identical to the AAS readings for the spiked liquors.

Overall the analyses of the spiked liquors showed only 30% of the added gold.
When the spiked liquors were converted into full strength aqua regia I could read 75% of the added gold.

It appears that using this leach type formulation leads to the formation of some gold complexes which neither report to an organic extractant (1% aliquat 336 in DIBK) or are adsorbed onto activated carbon. Why this is I do not know.

Further testing of the ore showed that the level of iron dissolved in the leach was dependent on the temperature of the leach and the HCl %.

If the leach was run at 25C with 3% HCl then very little iron was solubilised and very low levels of colour were seen in the leach liquor.
Running the leach at 50C, 75C and 97C gave greatly increasing levels of iron in solution even with leach times of less than 1 hour.

Lowering the HCL level to 1% in the leach lessened the iron dissolution level but had no effect on the gold level leached from the ore or the difficulty with AAS analysis.

Raising the HCl level in the leach to 5, 10, 20 and 30% greatly increased the iron dissolution levels but had no effect on the AAS analytical difficulties or gold analysed as dissolved from the ore samples.


It appears that the use of 3% HCl in the leach at room temperature is about the optimum condition for gold leaching with minimal iron dissolution.

The fact that there is little iron dissolved in the above leach in the presence of stainless steel is probably less due to any passivating effect of the stainless steel and more due to the low levels of iron dissolution under the conditions of the leach.

It should be noted that there are gold losses in solution associated with the use of such a leach, I do not know if these losses are low level losses associated with low level spiked solutions or if they are proportional through all gold levels.


Deano


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## Lou

Certainly not proportional. One can have very high gold loadings with this leach but it is in appropriate to ore due to cost. Your use in the lab for high Fe matrices is interesting. It is a passivation phenomena--even low chloride content will corrode Fe, but less so for its oxides.

Using this leach proficiently is difficult both in achieving optimum leach conditions and most critically, maintaining proper ORP and ligand content to prevent loss of passivation, a wholesale nightmare.


FYI, these solutions analyze fine for Au w/ Flame AAS using standard addition with acceptable recoveries.


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## Deano

Alternative gold leach

There is little information on Thiocyanate leaching of gold which would be useful to a small scale user.

Here is what I have found to be a realistic method of using thiocyanate for gold leaching.

First note that thiocyanate complexes of silver have only low solubility so thiocyanate leaching of a mixed gold/silver piece may run into passivation problems.

The most important part of thiocyanate leaching is that thiocyanate leaches are not considered to be toxic, the Australian MSDS for them has no restrictions on transport or storage.

Sodium thiocyanate is preferable to ammonium thiocyanate purely because the ammonium thiocyanate is deliquescent and will turn into a solution if exposed to moisture in the air.

Apart from the toxicity aspect thiocyanate has another advantage over cyanide and that is a much cheaper price.


For a thiocyanate leach to dissolve gold it requires an oxidant to be present in solution.

The standard oxidant used is ferric ions in acid solution. This has the downside of the ferric ions attacking and solubilising many base metals.

Other oxidants which can be used are hypochlorite and chromic salts.

Hypochlorite is tricky to use as it requires constant monitoring to maintain the free chlorine levels below the point where the chlorine attacks the thiocyanate, not recommended.

Chromic salts are a health hazard and are not recommended for use outside a specialist processing facility.


However sulfuric acid can be used as an oxidant for thiocyanate solution. The sulfuric acid levels referred to are dilutions of concentrated sulfuric acid 98%.

Battery acid of 35% or so can be used with the dilutions adjusted for the lower starting strength.

A useful starting level is 5 grams of sodium thiocyanate in a litre of 5% sulfuric acid, mix by adding the acid to the water and then adding the thiocyanate.

The sulfuric acid level will start gold leaching at 1% sulfuric solution, the more sulfuric added the faster the leach rate.

The ability to trade off gold leaching rate against sulfuric level allows the user to have a leach which they are comfortable with from a user safety aspect. This does not mean that all usual safety precautions are not needed. Eye protection is a must, gloves can be of a lower acid rating than for nitric acid etc.

The trade off above also allows the user to refine the process in that the sulfuric acid levels can be adjusted so the the acid attack on any base metals present is minimised.


Deano


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## Lou

Think it will work on gold plated material?


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## Deano

When developing this leach the standard test material was gold foil cut to size so that a direct comparison could be made between different leach conditions.

If it digests gold foil it would take off plated gold, there may be some interaction with base metals present.

This is not the all singing all dancing method for gold leaching, it is another tool in the box. 
There will certainly be cases where this method is not suitable under certain conditions, I was just wanting to get the method out so that people could do their own testwork and develop a list of suitable and unsuitable conditions.
I could do leach tests for weeks and still not include all the test conditions that members use.

Please note that sodium thiosulfate is also deliquescent, it does so at a much slower rate than ammonium thiosulfate, I did not make this clear previously.
Moral is to keep thiosulfate containers properly capped, do not have open containers left around.


Deano


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## Platdigger

So which is it you are talking about here Deano?
Thiocyanate? Or Thiosulfate?


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## Deano

My embarassment

I was referring to thiocyanate, trying to hold a couple of conversations about thiosulfate at the same time as typing, got side tracked and mistyped.

Deano


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## Deano

Further to the posts on thiocyanate leaching, it is possible to have silver thiocyanate in solution if the molarity of the thiocyanate solution is greater than 0.1 molar.

An example is ammonium thiocyanate which has a molecular weight of 76. 
A 1 molar solution is approximately 76 g per litre, a 0.1 molar solution is approximately 7.6 g/l.

Thus if you have say 10 g/l of ammonium thiocyanate in solution you are safely over the minimum requirement for having silver thiocyanate in solution.

Keep in mind that if you are dissolving metals with such a solution you must remember that for the silver to stay in solution you must maintain the uncomplexed 10 g/l level of thiocyanate in solution.

This means that if you are carrying out a leach which will complex say 5 grams of thiocyanate with metals then you will need to start the leach with your 10 grams plus the extra 5 grams of thiocyanate in solution.

Think of the 10 g/l as just going along for the ride but not getting used.

The above applies for all thiocyanate leaching irrespective of whether the oxidant used is ferric salt at pH 1 to 2 or sulfuric acid at around 5% or other oxidants.

This means that if you are leaching material which contains both gold and silver you can dissolve both metals with the one leach.

The downside is that if metallic copper is present it too will stay in solution rather than form a passivating layer which prevents further complexation. This can use up your safety margin of thiocyanate fairly quickly and precipitation of copper and silver thiocyanate will occur.

Thiocyanate is cheap and non toxic so it is easy to use a leach solution with a large excess of thiocyanate if you do not know what the consumption of thiocyanate will be for a particular leach.


Deano


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## Anonymous

Deano

I've been reading this thread with interest and even more so the later developments regarding Thiocyanate leaching. Strangely enough it is both simple and cheap to get Ammonia Thiocyanate in the UK in liquid form but Sodium Thiocyanate is extremely expensive. I also note that Sodium Thiocyanate can be made using Ammonia Thiocyanate and Sodium Hydroxide. I'm no chemist so please correct me if I am wrong, but how simple would the process be? 

I would also be extremely interesting in working out a process "end to end" as I get a lot of plated material and I have a hormonal aversion to cyanide leeching (i.e. I like to run away from it) and would like to see where this could be taken.

Assuming the success of deplating an item, what process would you recommend for precipitating? 

Regards

Jon


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## Deano

Jon

It makes no difference to the leaching if ammonium or sodium or potassium thiocyanate is used.

By using liquid ammonium form you get around the worry of keeping the thiocyanate, in whichever form you want to use, in a dry state.

The thiocyanate part is the important bit for forming metal complexes, the sodium or ammonium just goes along for the ride.

How you use this leach depends on what you are wanting to leach the precious metal from and what precious metals are present.

If you have the simplest combination of just gold plated on iron base ( this includes most forms of steel ) then run the leach at pH 1 to 2 using sulfuric or hydrochloric acid. Under these conditions the leach will need some iron in solution to act as an oxidiser before the gold will go into solution.

This leach form is very forgiving so there is no optimal iron level at which the leach must be run. Having less than theoretical iron levels just means that the leach is a bit slower, not that it will not work. Pretty well as long as you have iron colouring in the leach it will work.

Once some of the gold has been stripped the acid in the leach will start to attack the iron in the substrate. This will increase the iron level in the leach but will also raise the pH of the leach solution, this needs to be watched and more acid added if necessary. Keep in mind that generally 1% HCl solution will have a pH around 1 as will a 0.1% H2SO4 solution depending on what impurities are present in your water. This means that pH adjustment acid additions are usually very small, use a 1 to 5 ml disposable plastic pipette.

Hydrochloric acid will attack the iron much faster than sulfuric acid so if you are running large batches you want to use sulfuric acid so that you are not making a lot of iron complexes in your leach which you will have to deal with later. Also sulfuric acid is always readily available as battery acid at a cheaper price than hydrochloric acid.

I usually use around 5 grams of sodium thiocyanate per litre to ensure a reasonable leach rate. This gives a leach rate between cyanide and aqua regia. If you want it faster use more thiocyanate and dissolved iron.

If the gold is plated on a copper type base use a leach of maximum 5 g/l thiocyanate with just 5% sulfuric acid. Under these conditions the copper will form an insoluble copper thiocyanate layer which passivates the copper surface thus stopping acid attack on the copper.

If silver is also present in commercial quantities or at levels which will passivate the precious metal surface then you need to have the thiocyanate levels at 10 g/l of free sodium thiocyanate. Note that sodium and ammonium thiocyanate are interchangeable in this context.

This means that if you run this leach where the substrate is copper there will be no protective passivation of the copper when the precious metal layer is at least partly removed.

When using these leaches I always recover all dissolved metals with a carbon felt electrowin cell. No electrowin cell will strip all iron from a leach solution but if all other metals have been removed then bringing the residual solution to pH 7 will precipitate out the iron as an uncontaminated product which can be legally disposed of in landfill. The leftover solution of thiocyanate acts as a really good fertiliser. Thiocyanate is cheap enough that it is not worth trying to guess how much is left in the solution for re-use.

I dissolve the metals from the carbon felt in aqua regia and do a dibutyl ether recovery with oxalic acid finish if just gold is the precious metal I have leached.

If I have leached for low level silver as well as gold then I dissolve the metals from the carbon felt in aqua regia but adjust the liquor volume to keep all of the silver chloride in solution. An addition of pure table salt, not iodised, will cut down on the liquor volume needed. I adjust the pH to 1.5 with technical grade urea and precipitate the precious metals with sodium meta bisulfite.

Any contaminant metals in the residual aqua regia after the precipitation are removed by running it through another electrowin pass with the felt being reused multiple times until it is choked with base metals and it is then disposed of in an appropriate facility.

The above is easier than trying to play electrowinning games with voltages and current levels for clean precious metal recovery, it is not a system that is easy to control right through the process.

One obvious question is why do I use dibutyl ether instead of just a precipitation step when I have gold without silver.

It is cheaper to do the extraction method rather than the precipitation, the only chemical cost is the exchange of oxalic acid for gold.

You can drop metals directly from the leach solution with meta bisulfite but you will consume a lot of metabisulfite trying to knock out the iron in solution or the redox from the sulfuric acid. Way cheaper to do the electrowin step.


Deano


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## Platdigger

Will this leach any of the pgms?


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## Anonymous

Hi Deano

For reference my base material tends to be IT equipment, so gold plating tends to be copper based with a nickel layer between. There are some items which have gold direct onto copper and these tend to be heavier units and since you mentioned the passivation of the copper base within this process I think it certainly deserves some research. There is no iron at all in the product I will be processing, once prepared correctly. 

Also most of the material has either silver or gold so there is very very little crossover. 

As such this method could be an extremely practical alternative. Thank you.

Also Ammonium Thiocyanate is sold in the UK in 2.5l bottles at a concentration of 0.1M. Is that suitable?


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## Deano

The only reference I can find to leaching of PGMs with thiocyanate says that only palladium is leached by thiocyanate at room temperature. No reference is made to leach conditions apart from the temperature.

According to the literature nickel thiocyanate can only be formed as a reaction between salts in solution. If this is correct then there should be no attack on the nickel layer by a thiocyanate leach assuming that the reagents in the leach apart from the thiocyanate do not strongly attack the nickel. I cannot find any data for solubility of nickel thiocyanate in water.

0.1M ammonium thiocyanate solution is right on the cusp of silver solubility. Think of it as around 7.6 g/l solution.

If you were to dilute it to 5g/l the solution will not dissolve silver but will just take up gold.

If you want to dissolve silver then you will want to raise the level of the ammonium thiocyanate so that you always have 10 g/l ammonium in solution after dissolving the silver.

You can increase the ammonium thiocyanate level in solution by boiling off some of the water from your solution. Keep in mind that ammonium thiocyanate is only stable up to 170C, so keep your heat source on low or use a water bath and accept the extra time involved.

Deano


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## sprigsie

Hullo there Deano,

Thank-you for your very interesting information.

Do you know if the thiocyanate leach is as selective as the thiourea leach as I have ore that is 30% Antimony (as stibnite), chalcopyrite, arsenopyrite and better than 5oz gold and 10oz silver per ton.

My idea is to crush the ore using a jaw crusher and ball mill and then float off the stibnite using flotation tanks and then put the remains left in the bottom of the tanks over a vibration table to separate and treat the free gold and silver and then roast the sulphides to release their values using a suitable leach.

I understand that the antimony will interfere and destroy most luxivants, but thiourea is not effected by this effect.

I was planning to use thiourea but I also understand that there is problems with finding a precipitant that is reasonably well priced and easily available.

Cheers Mate,
Allan.


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## Deano

Generally you are going to float all the sulfides at once along with any free gold, you might want to reconsider your flowsheet.

Thiourea is not used much because gold thiourea complexes are not easily concentrated on an adsorbent medium such as activated carbon. This means that you are restricted to either direct electrowinning from the leach solution and the related destruction of the thiourea or chemical precipitation from the leach solution.

Both of the above involve a filtration stage.

Personally I don't use thiourea because it is a known carcinogen.

That said, if the thiourea leach is totally kept away from UV then it is a very good leach for use with stibnite.

Personally I would look at doing a bulk float and selling the float cons or having them toll treated at a smelter. You can then test the float tails to see if they are worth further treatment.

I have no information on the use of thiocyanate with stibnite.

Deano


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## kurtak

Deano

Good to hear from you again - its been a couple months since you last posted - I enjoy reading the info you post here

Kurt


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## Eamonn

Just saw this thread. 
Brilliant info thanks Deano.

sprigsie 
It was interesting to me that you mention thiourea and that it was carcinogenic because it is in Goodards Silver dip available in the UK. It was the only thing that removed the silver nitrate stains from my worktop which had been there for about six months when I started silver refining. I tried everthing else I had, to remove those stains. Thought they would be there for ever.


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## sprigsie

Hullo there Eamonn,
Yes, that's correct that theourea is a known carcinogen, but I don't know just at what concentrations that it is actually carcinogenic because currently the main use for theourea is in municipal city water supplies throughout the world to clarify the water, so I ask, is this chemical really that dangerous.

Cheers Mate,
Allan.


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## Deano

I would be interested in where you got the information that thiourea is used in municipal water supplies. I can find no record of thiourea being used for this purpose anywhere.

Deano


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## necromancer

Deano said:


> I would be interested in where you got the information that thiourea is used in municipal water supplies. I can find no record of thiourea being used for this purpose anywhere.
> 
> Deano




this is the only worthy document i can find about "thiourea in our water" in canada.
it says nothing about it being used in any "municipal fresh water supplies" only as a environmental toxin from Industry.

Source: (Environment Canada) see section (5.1)

PDF version
http://www.ec.gc.ca/ese-ees/EC008C1F-D726-423A-B1E4-07B00BAD338C/batch2_62-56-6_rm_en.pdf


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## Deano

Further to the previous thiocyanate testwork I was interested in the recovery of gold thiocyanate from leach solution.

There is a lot of literature which says that gold thiocyanate will load onto activated carbon from a pulp. There is not a lot of literature which compares the loading rates and levels of gold thiocyanate onto carbon with gold cyanide or gold chloride.

I ran three liquors with the same gold tenor of 1ppm gold as a chloride at pH 2 (pH adjusted with HCl), gold as a cyanide at pH 11 (500 ppm cyanide in solution), gold as a thiocyanate at pH 2 (5 grams per litre thiocyanate and 5 grams per litre iron as ferric chloride).

Each liquor was contacted with activated carbon at the same rate of 5 grams per litre carbon in liquor.

After 1 hour the gold levels in the liquors were chloride 0.21ppm, cyanide 0.32ppm, thiocyanate 0.06ppm.

After 2hours the gold levels in the liquors were chloride 0.11ppm, cyanide 0.16ppm, thiocyanate 0.01ppm.

After 3 hours the gold levels in the liquors were chloride 0.07ppm, cyanide 0.12ppm, thiocyanate 0.00ppm.

After 3 hours the carbons were removed from the liquors and dried at 90C overnight.

The dried carbons were then ashed in open top ceramic dishes in an electric furnace at 600C.

The ash was digested in aqua regia and the recovered gold was calculated from AAS analysis.

For gold chloride 93 micrograms of gold loaded onto activated carbon returned 91.3 micrograms

For gold cyanide 88 micrograms of gold loaded onto activated carbon returned 86.5 micrograms.

For gold thiocyanate 100 micrograms of gold loaded onto activated carbon returned 98.6 micrograms.

Effectively the three types of gold complexes recovered similar similar percentages of gold from carbon within the accuracy of the analyses.

It should be noted that the gold thiocyanate loaded faster and more completely than the gold chloride or gold cyanide under the conditions described.

The above indicates that activated carbon is a suitable adsorbent for gold thiocyanate complexes at pH 2 where iron is used as an oxidant. 

Deano


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## Lou

Always appreciate such a study, and I compliment you on you your work--always good stuff from you. Seems like the recoveries by AA might mean even better news. Sometimes AA runs low.

You should test a poly acrylic ester resin for gold sorptiin. XAD-7 (Dow) is supposed to be good. Gold loves esters/ethers/ketones. Imagine if it took gold out of reverse aqua regia. I theorize it should. Reckon it elutes with water. Should be oxidatively stable.


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## solar_plasma

This thread should be copied to the library. Informatively and captivating, understandible to us kinda laypeople, it shows the beauty of scientific work.


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## FrugalRefiner

That's an excellent suggestion Björn. Your wish is my command.

Dave


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## Deano

Testwork carried out on the ashing of carbon with low levels of adsorbed gold

Over the years I have carried out many ashings of activated carbon which had been used to adsorb gold from a liquor.

Early tests I did many years ago indicated that when using an electric furnace with the door just cracked for air access a temperature of 600C was suitable.

The carbon was always ashed in porcelain dishes 100mm diameter and 40mm deep, claimed capacity of 150 ml.

I thought that it was time to run a series of tests and define the best conditions for ashing carbon in these crucibles.

I loaded 10 micrograms of gold as gold chloride onto replicate batches of dust free virgin activated carbon of various brands.

The batches varied in quantity from 1 to 20 grams in weight with the 20 gram samples having a bed depth of 15mm.

Due to the difficulty of getting complete ashing using carbon which had been used to adsorbed gold from solutions containing large quantities of base metals I have always used a maximum of 20g of carbon giving the bed depth of 15mm.

If larger quantities of carbon are used in these dishes then several episodes of rabbling the carbon during the ashing will be needed to achieve full ashing.

I ran the batches of carbon for 8 hours at temperatures of 400C, 500C, 600C, 700C and 800C.

All of the 400C batches failed to ash completely.

The 500C batches containing less than 5 grams of carbon ashed completely but the 10, 15 and 20g batches failed to ash completely unless rabbled several times.

All of the 600C, 700C and 800C batches ashed completely without rabbling.

All of the 600C batches returned the full 10 micrograms of gold.

All of the 700C batches returned 9 micrograms of gold.

All of the 800C batches returned between 6 and 7 micrograms of gold.

The above results indicate that although the temperatures in the furnace did not reach even the temperature at which gold melts there is still substantial volatilization of the finely divided gold at 800C.

Overall ashing of carbon should be conducted at 600C to minimize gold losses but still be done in an 8 hour time frame.


Deano


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## Research135

Maybe carbon ashing with concentrated sulphuric acid will work better and faster?. 8 hours is a full day's work. Losses should be less than 1/10000 (0.01%). It is a beautiful solution for used filters. Much better than incineration. 

edit: I failed to read that you loaded Gold Chloride, instead of gold. Please disregard my post. IMHO it's the gold chloride that volatilizes, not the gold. It happens even at 80 degC.


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## g_axelsson

You didn't say which form of gold chloride you used to load the samples. I assume you used HAuCl4 as that would be what we get when leaching gold.

Then I suggest that the reactions while ashing the carbon would be...

1. When heated (haven't found the temperature yet) HCl is removed.


> 2 HAuCl4 → Au2Cl6 + 2 HCl



2. Further heating breaks down the gold tri chloride above 160 °C and takes away 2/3 of the remaining chloride.


> AuCl3 → AuCl + Cl2 (>160 °C)



3. At yet higher temperatures (>420 °C) the remaining gold mono chloride reacts and creates metallic gold and gold tri chloride, which probably goes off as a gas.


> 3 AuCl → AuCl3 + 2 Au (>420 °C)



This could explain why you see about 2/3 of the gold remaining in the ash and 1/3 going off as gas.

Ref:
Wikipedia : Chloroauric acid
Wikipedia : Gold(III) chloride

Göran


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## Deano

Gold was in the form of tetrachloroauric acid standard from Merck, Germany.

The standard interpretation of reactions during the ashing of the carbon is as you have shown and should lead to gold losses at all of the temperatures used in the test regimes.

The reason I posted the results is that there were no losses at all until the temperature reached 700C.

The indication is that the conventional expectations for gold loss do not occur in the expected manner and that further testwork is required to ascertain what is actually happening.

This is a secondary result separate from the fact that 600C is probably near optimal for carbon ashing.


Deano


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## Deano

Further on the ashing of carbon.

I re-ran the ashing tests under both cold start furnace and steady temperature furnace conditions for all tests.

All results were identical with the previously reported numbers.

This indicates that furnace temperature ramp up conditions play no part in the gold losses.

I then took dishes containing carbon which had been ashed at 600C and subjected half of them to furnacing at 700C and the other half at 800C for 8 hours.

I also subjected some of these 600C ashings to aqua regia digest to confirm that the gold levels were indeed the same as previously reported, they were, no losses.

The ashings subjected to the higher temperature furnacings returned identical results to the original higher temperature ashings.

This indicates that the losses are solely due to the finely divided gold being kept in the higher temperature furnacing conditions.

They are not due to volatilisation of gold chlorides as such.


Deano


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## Deano

By accident I placed some activated carbon blanks in a furnace which has become contaminated with gold from smelts.

Normally this furnace is now only used for smelts, certainly not for precise analytical work.

To my surprise the carbon blanks all registered high levels of gold.

Repeating this test at various temperatures from 600C to 900C showed gold from all samples, the lowest temperature samples registering the highest gold values.

I repeated the tests but only placed empty dishes in the furnace, no carbon.

There were no gold values registered from the empty dishes at any temperatures.

I took dishes containing ash from carbon blanks which had been ashed in a clean furnace and placed half of them in the contaminated furnace at the various temperatures.

All of the samples registered no gold, thus the ash plays no part in the adsorption of gold in the contaminated furnace.

In summary, a dish containing blank carbon ashed in the contaminated furnace registered gold values.

An empty dish placed in the contaminated furnace registered no gold values.

A dish containing ash from a previous ashing in a clean furnace registered no gold values after being subject to the same temperature regimes in the contaminated furnace.

All furnacings were carried out for 8 hours.

Unless activated carbon is present in the dishes in the contaminated furnace there are no gold values registered from these dishes.

It appears that the activated carbon is capable of adsorbing volatilised gold values from the furnace atmosphere until such time as the activated carbon is completely ashed.

This most likely is the mechanism by which gold losses as gold chloride during carbon ashing are eliminated.

It appears that any gold volatilised as gold chloride is adsorbed on the carbon, this effect is repeated until the furnace temperature is high enough to convert the gold chloride to metallic gold.

At this stage there is still enough un-ashed carbon present to stop any volatilisation losses.

Once the gold chloride has been converted to gold particulates then the volatilisation of these particulates will continue in the closed cycle until all of the carbon present has been ashed.

At this stage any volatilisation of the particulate gold will lead to gold losses.

As the 800C ashing is completed in a much shorter time than the 700C ashing there is a longer time for volatilisation to occur when all ashings are conducted for 8 hours.

The 600C ashing is completed just short of the 8 hours and so very little time is available for free volatilisation of the particulate gold. 

All of the above fits with there being no gold losses for the 600C ashings, a small loss for the 700C ashings and a larger loss for the 800C ashings.

If you wish to minimise any losses you should keep a close watch on any ashings and turn the heat off as soon as ashing is complete.

Deano


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## mkamalahmadi

My name is Mahmoud Ahmadi, and I am a PhD student in Chemical engineering at State University of New York (SUNY) at Buffalo.we developed a new technology (patent app in process) for tuning a metal scavenging chemistry to cost effectively capture high value metals in low ppm and ppb concentrations from aqueous solution (gold, palladium). 
To validate our technology we need to get input from industry experts. Since gold electroplating and refinery cause gradual loss of gold in waste effluent , I wanted to ask if it would be possible for experts to get back to me at [email protected] to share their thoughts and advice regarding where our technology might fit in. 

Thank you for the time and help,

Regards,
Mahmoud


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## 4metals

Mahmoud,

This is an public forum, we discuss topics openly and get input from the broad spectrum of our members. We welcome you to discuss your process openly on the forum but to seek private email conversation does little to benefit the forum. This is why your post was reported as it is seen as spamming the board. 

If your process is worthy, it should be aired publicly.


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## Deano

There has previously been substantial posting regarding hypochlorite leaching.

This post is an attempt to pull all of the previous work into a single post and also to incorporate further work not mentioned in the earlier threads.

Hypochlorite in solution in its generic sense exists in three forms depending on solution pH.

Below pH 3.5 it exists as chlorine gas.

Between pH 3.5 and pH 7.5 it exists as hypochlorous acid.

Above pH 7.5 it exists as the hypochlorite ion.

At all pH levels there will be some of all of the above forms present.

Thus at pH 10 there will not only be hypochlorite ions present but there will be some hypochlorous acid and some gaseous chlorine present.

In the above example the main form present will be the hypochlorite ion with minor levels of the other forms. You can smell the free chlorine gas present when you open a bottle of bleach even though the pH is above 10.

The pH only controls what dominant species will be present, it does not allow only that species to be present.


If leaching is carried out as a chlorine leach at a pH below 3.5 the chlorine will offgas and be lost to the process unless the reaction is carried out in a sealed vessel.

It is only when the sealed vessel is pressurised that enough of the chlorine will stay in solution and be available for leaching with.

If leaching is carried out as a hypochlorous acid leach it is most efficiently carried out at a pH between 6 and 7.

This is because you are operating in the part of the hypochlorous acid leaching zone which is furtherest away from the free chlorine gas zone and so there is less loss of free chlorine gas.

If leaching is carried out as a hypochlorite leach it is best carries out at pH near 8.

This is because it is operating close to the hypochlorous acid leach zone and will have a fairly large component of hypochlorous acid present.


How can you improve the operation of a hypochlorite system.

Very simply, add salt.

Gold chloride is very enthusiastic about adsorbing onto quartz and sulfides as well as other ore components.

This adsorption can be minimised by adding salt, sodium chloride, to the leach.

About 20% by weight is a good working level of salt addition.

Technically, apart from minimising adsorption of gold chloride complexes onto ore components the presence of the salt stabilises the gold chloride complexes and allows leaching to occur at higher pH levels and lower Eh levels.

Generally pH levels should be kept under 9 even with the salt additions.

The maximum salt addition level theoretically is around 36%, practically it is around 30%.

Care must be taken that evaporation does not cause the salt level to rise to the level where salt starts to crystallise in the leach solution, apart from the operational problems this can cause there will be gold losses from co-precipitation.


What are the benefits of leaching with hypochlorite.

Near neutral pH conditions.

Very little attack on iron complexes at these pH's.

Rapid adsorption of gold complexes onto activated carbon.

Rapid leaching.

Will attack sulfides thus allowing leaching of gold values from sulfides.


Disadvantages of leaching with hypochlorite

Health and safety, chlorine is a cumulative poison and should only be used where all safety gear is being used.

If used around pH 7 the offgassing of chlorine is minimised but is not stopped.

Always keep in mind that chlorine is effective enough as a chemical warfare agent to have been used as such in world war 1.

Plant must be constructed of non-metallic parts, a slow attack on iron complexes does not mean no attack.

A close check must be kept on carbon loadings, if loadings are allowed to get too high there will be loss of gold by attrition from the carbon.

This loss also indicates that the gold should be stripped in the loading canister rather than be transferred to a specialist stripping column, there will be losses during the transfer.

It also indicates that gold loadings during CIP operations should be closely monitored so as to not overload the carbon.

Attack of sulfides by the leach will cause a drop in pH, this needs to be closely monitored during the leach cycle.


Best conditions for running a hypochlorite leach.

My preference is for pH 7, 200 grams per litre plain salt, calcium hypochlorite starting at 1 gram per litre.

The lower you can keep the hypochlorite levels the less attack on metals apart from gold.

This also makes a good silver leach if the solution is circulated through a cannister of activated carbon so that the silver level in solution is always kept low.

The silver will load on the carbon to the point where it can actually be burnished, similarly to gold.

Recover the precious metals by ashing the carbon after drying.


Deano


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## g_axelsson

Good write up. I hope I will get an opportunity to test this leach some day.

Is temperature affecting the leach in any major way? I guess higher temperature gives higher leach rates but also increases evaporation.

What is typical leach times? I understand that it depends a lot of ore chemistry and how fine the ore has been milled, but are we talking hours, days or weeks here?

I has probably been mentioned before but what is the maximum loading of gold on carbon?

Göran


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## Deano

The hypochlorite leach rate is between cyanide and aqua regia.

For an ore comparison, generally hypochlorite is around 3 times as fast as cyanide but can be sped up by having more hypochlorite present.

This usually means leach times in hours rather than days.

The leach rate is controlled by the rate at which the oxidant can access the gold so some form of agitation is usual.

The agitation can be applied by stirring or by pumping the liquor, the former for ores and the latter for component leaching.

As you stated, the leach rate increases with temperature as does the evaporation rate.

The maximum loading of gold onto carbon is dependent on both the form of the gold complexes and the tenor of the gold liquor( think concentration of gold in the liquor).

The higher the gold tenor the greater the loading possible.

Gold chloride will load to a higher level and do so faster than gold cyanide.

The gold cyanide loading is an adsorption system whereas the gold chloride is mainly a reduction mechanism.

This means that gold chloride will reduce to gold metal on activated carbon and will do so to levels where the gold particles will fall off the carbon. Great care must be taken with carbon loaded from gold chloride not to get to this stage.

The maximum gold loading levels onto carbon are also very dependent on the levels of other metal species present in the leach liquors as these other metals will take up sites on the carbon and prevent gold from loading on these sites.

Temperature and pH are also among the major loading controllers.

In CIP circuits the gold loadings are anywhere from 2,000 to 20,000 ppm on the carbon depending on the mine conditions.

In artificial laboratory conditions under acid pH and low temperature gold loadings over 200,000 ppm have been achieved.

Note that gold cyanide complexes are stable under acid conditions, it is only when the acid conditions are strongly oxidising that the cyanide complex is degraded.

Deano


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## solar_plasma

While reading discussions about recovering gold from carbon, I often wondered, if nickel will be of greater concern, since finely devided nickel reacts with CO at 50-100 °C to nickel tetracarbonyl ("liquid death" or "Nickel carbonyl is one of the most toxic substances encountered in industrial processes"), - though it will be decomposed at 180-200°C.

At 100°C or below the reaction of carbon to CO is minimal, even though it exists. So, is it correct to assume, that we are save (regarding nickel tetracarbonyl), when heating let say 100g nickel contaminated carbon to 600°C in a muffel furnace?


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## Deano

Regarding the ashing of carbon containing nickel I ran a quick series of ashing tests using carbon loaded with nickel chloride, carbon loaded with nickel cyanide and carbon loaded with nickel chloride reduced to nickel metal with hydrazine.

A fourth dish had only the reduced nickel present, no carbon.

After ashing the ash was digested in aqua regia and the nickel losses were calculated.

There were no nickel losses for any of the dishes where carbon was present.

There was a loss of 20 micrograms of nickel from 100 micrograms from the dish where no carbon was present.

The above indicates that the carbon acts as an adsorbent for any nickel volatilised during the ashing stage, similarly to how it acted for gold.

It is only when all of the carbon has been ashed that volatilisation of the nickel and associated losses occurs.

It appears that the nickel carbonyl does not present a threat during the ashing of carbon.


Deano


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## solar_plasma

wow! Thanks a lot!


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## Anonymous

Well......

Having invested a lot of time and a fair bit of money building the cell Deano recommended, I can confirm that it does exactly what Deano said that it would and I would strongly recommend that people look at building one. 

Thanks Dean. 

Jon


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## 4metals

Jon,

Might you post some photo's of your system?


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## Anonymous

Sure.

Since taking this picture I swapped the input pipe with the overflow pipe. The overflow pipe runs to the leech solution to keep a steady level in the cell.


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## nickvc

Looking good mate have you had it on trial ?


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## Anonymous

Oh yes.

Strips like a dream. That cell is too big for a home refiner though. The felt will take many Kg of gold and I don't have that much at a time so I'll be cutting the cell down by at least 3/4 and building in lots of improvements. I loaded around half an ounce on to the bottom quarter of the felt and apart from a visible colour change it was patently apparent that much much larger quantities would be needed to even make a dent in the capacity of the cathode.

This was very much a proof of concept build. By that I mean proof of concept to me as opposed to proving that Dean's theory worked!

Gentlemen I would strongly suggest and recommend that you read Dean's offerings again, and again. There are nuggets (sorry I couldn't resist) of information in his posts that can be used by everyone who is prepared to apply them. On this project alone I have learned so much it's incredible, and although sometimes frustrating I have benefited greatly from the experience.


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## Yggdrasil

Thanks Deano.

This is an excellent and informative thread

I have a couple of questions.
Can the leach be reused after running through the carbon felt cell?
If so, a miniature cell in a loop with leach wold ensure the leach never get saturated.

Do anyone have a link to where one can buy carbon felt?


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## Anonymous

I'm not sure whether Deano can reply at the moment however I'm now able to answer your questions. 8) 

Yes the leech can be reused after stripping.

A cell of the size I built can hold many KG of gold so a smaller one will still hold a lot. Unless you are stripping from very large quantities of base product - i.e. in the many hundreds of grammes your chances of saturating the carbon felt are minimal. You can tell if you do because no fluid comes out of the centre of the cell because it simply cannot get through the felt any more. Edit: I didn't answer your direct question. Yes you can ensure that your leach liquor never saturates although given the quantity of gold that 200g of cyanide in 20 litres can hold it may not be a problem unless you are likely to add more than half a kilo of gold at a time. I believe it will hold roughly 4 times it's raw weight in metals (Chris/4metals/Lou may correct me) and you would have to also factor the weight of other metals stripped, into this amount. 

Carbon felt is for some reason not so easy to come by. I had to to a lot of googling to find a supplier who didn't want to charge me £300 per square meter. I have a supplier per "cell sheet" I managed to pay around £14 for 34cm x 30cm . Given how much gold that holds- it's hardly a big issue. 

I'd happily supply you with one of my sheets for the price I paid for it plus the shipping cost if that helps? 

Jon


----------



## nickvc

Jon the one advantage of a larger cell is that you can run solutions to be stripped in larger volume and so recover the values quicker or is my thinking off here :shock:


----------



## kurtak

spaceships said:


> Carbon felt is for some reason not so easy to come by. I had to to a lot of googling to find a supplier who didn't want to charge me £300 per square meter. I have a supplier per "cell sheet" I managed to pay around £14 for 34cm x 30cm . Given how much gold that holds- it's hardly a big issue.
> 
> Jon



Jon 

I have run into the same problem (high cost for the carbon felt) I have everything else needed to build this cell including a 4"X4"X16" piece of high density graphite (for the anode) which I can cut into (4) 2'' anodes

I want to set mine up for winning the copper (& other base metals) from my copper nitrate solutions & then re-using the nitric for base metal digestion again but I am having a hard time justifying the high cost of the carbon felt for a base metal winning application --- do you have a link to the supplier you are getting the better priced per "cell sheets" from :?:

Also I have another question - you are using yours to win your gold (&/or PMs) back from solution - but if it is being won back in the matrix of the carbon felt don't you have to then incinerate the carbon felt to turn it to ash & then smelt it (flux to slag the ash off) or - incinerate to re-leach it (the PMs) from the ash --- or am I missing something here :?: 

Kurt


----------



## Yggdrasil

> spaceships wrote:
> 
> 
> Carbon felt is for some reason not so easy to come by. I had to to a lot of googling to find a supplier who didn't want to charge me £300 per square meter. I have a supplier per "cell sheet" I managed to pay around £14 for 34cm x 30cm . Given how much gold that holds- it's hardly a big issue.
> 
> Jon
> 
> 
> Jon
> 
> I have run into the same problem (high cost for the carbon felt) I have everything else needed to build this cell including a 4"X4"X16" piece of high density graphite (for the anode) which I can cut into (4) 2'' anodes
> 
> I want to set mine up for winning the copper (& other base metals) from my copper nitrate solutions & then re-using the nitric for base metal digestion again but I am having a hard time justifying the high cost of the carbon felt for a base metal winning application --- do you have a link to the supplier you are getting the better priced per "cell sheets" from :?:
> 
> Also I have another question - you are using yours to win your gold (&/or PMs) back from solution - but if it is being won back in the matrix of the carbon felt don't you have to then incinerate the carbon felt to turn it to ash & then smelt it (flux to slag the ash off) or - incinerate to re-leach it (the PMs) from the ash --- or am I missing something here :?:
> 
> Kurt



Hi 
I'm curious about what voltage that are used for the cell?
I guess it will create chlorine if it is too high, but if that is fed back to the leach it could be beneficial wouldn't it?

Regarding the felt is it possible to use the same method as they use to make carbon fiber?

Take a sheet or form made from loose felt/wool/cotton.
Evaporate everything else than carbon in vacuum or anaerobe atmosphere in an oven, I can't actually remember temps just now.
I have no idea if it is economically viable, but for the ones that already have furnaces it should be technically possible though.
And one could create whatever form one needs.

How would the felt and/or graphite electrode behave in sulfuric acid ref to Deanos tiocyanate leach?


----------



## nickvc

Kurt it depends on what type of solutions you are running in the cell and what metals you have in solution, it will do an excellent job of recovering values from base solutions which can be melted and sold as is or refined if that is your preference , with acidic solutions I believe by varying the voltage/ ampage it can be made selective to a degree or so I believe, trials were been conducted by another firm into how it could be done but assays are needed to check what is been stripped or left.


----------



## Anonymous

Kurt/Nick

The secret is to have a DC PSU with variable voltage. The maximum current needs to be at a level whereby the requirements never exceed the maximum available current. If you do that you reach an equilibrium in the cell whereby the metals are plating out and redissolving and therefore you have no resulting metal.

As you increase the voltage the current draw will increase. Also, and importantly the level of metal in solution affects the current drawn too. I made the mistake of running the cell the first time at far too low a voltage- I was at 2V with a current draw of around 1A. Not a lot happened. I increased the voltage until there was a definite fizzing in the cell and gas was visibly being produced. It is important to note that I was stripping metals from a non acidic leech solution so I was not producing Chlorine gas, which you will produce from any HCl based solution. In order to get the cell operating for the particular solution I was running the voltage was around 10V with a current draw of around 13A.

Do note that dimensions of your cell i.e. diameter and distance from cathode to anode, along with volume of liquid AND saturation of metals in solution all affect the current drawn. Dean explains this far better than me I am afraid, but safe to say you need to adjust for the conditions you are operating within. 

My DC power supply runs a max of 30V and 60A - not a cheap piece of kit. Dean regularly runs at 200 amps.


----------



## Barren Realms 007

spaceships said:


> Kurt/Nick
> 
> The secret is to have a DC PSU with variable voltage. The maximum current needs to be at a level whereby the requirements never exceed the maximum available current. If you do that you reach an equilibrium in the cell whereby the metals are plating out and redissolving and therefore you have no resulting metal.
> 
> As you increase the voltage the current draw will increase. Also, and importantly the level of metal in solution affects the current drawn too. I made the mistake of running the cell the first time at far too low a voltage- I was at 2V with a current draw of around 1A. Not a lot happened. I increased the voltage until there was a definite fizzing in the cell and gas was visibly being produced. It is important to note that I was stripping metals from a non acidic leech solution so I was not producing Chlorine gas, which you will produce from any HCl based solution. In order to get the cell operating for the particular solution I was running the voltage was around 10V with a current draw of around 13A.
> 
> Do note that dimensions of your cell i.e. diameter and distance from cathode to anode, along with volume of liquid AND saturation of metals in solution all affect the current drawn. Dean explains this far better than me I am afraid, but safe to say you need to adjust for the conditions you are operating within.
> 
> My DC power supply runs a max of 30V and 60A - not a cheap piece of kit. Dean regularly runs at 200 amps.



The surface areas of your anode and cathode heavily come into play on this as well Jon.


----------



## 4metals

http://www.preciousmetals-pmpc.com/GoldBug.pdf

Would these copper foam cathodes work as well for you Jon?


----------



## Anonymous

Actually 4metals no they wouldn't. One of the main features of the carbon is the sheer surface area that's available. That increases the efficiency enormously, and that just isn't there in the copper product on the link. 

Thanks though.


----------



## 4metals

They work getting plating solutions down to 1 ppm and have been around for 30 years. They have a platinized titanium anode. Maybe it wouldn't grab everything first pass but they melt nice.


----------



## Anonymous

Oh yeah I get that. The way that Dean set this up though it's designed to get it all out on the first pass.


----------



## 4metals

What is your flow rate through the cell. I like those little peristaltics you have mounted on the surrounding enclosure.


----------



## solar_plasma

If used for pretreating chloride waste solutions, what would be the best and safest way to capture the chlorine gas? Of course I have some ideas, but those are pure theory and assuming (like using Ca(OH)2, NaOH (in the cold) or oxidizing FeCl2 to FeCl3 - as said assuming without knowing how well it works). Is there an easy and definite answer to this?


----------



## Anonymous

4metals said:


> What is your flow rate through the cell. I like those little peristaltics you have mounted on the surrounding enclosure.



The important part of the flow rate is the outflow and the setting I am currently using is just under 1 litre per hour. Even though that is slow it is because it "safe" to strip all the metals out at that flow rate according to Dean, and I can increase this once my AAS arrives and I can sample the outflow and adjust accordingly. An AAS isn't essential when using this cell but having one allows you to use higher flow rates because you can monitor and sample the liquor.

The input side is set to over 1 litre per hour because the return pipe that you can see limits the volume of liquor in the cell and having the input greater than the output, with it's self regulating return prevents the cell from running dry.

On the MK2 vervsion there will be a switch on the input that shuts off the flow automatically is there is no current flowing through the cell thereby introducing a safety mechanism that allows the cell to be run unattended.

I'll also be using more expensive peristaltic pumps that are more accurate, rather than these chinese fleabay ones. 

Does that answer your question properly 4metals? Let me know if you need any more info.

Jon


----------



## Anonymous

solar_plasma said:


> If used for pretreating chloride waste solutions, what would be the best and safest way to capture the chlorine gas? Of course I have some ideas, but those are pure theory and assuming (like using Ca(OH)2, NaOH (in the cold) or oxidizing FeCl2 to FeCl3 - as said assuming without knowing how well it works). Is there an easy and definite answer to this?



To be honest Bjorn it's not something that I am experienced with and I'd love to know the answer too. 

Jon


----------



## 4metals

Jon,

Thanks for the response, at 1 liter/hr your thru put for a large volume can be quite some time, I am weighing in my mind the benefits of slow flow one pass on carbon cathode vs a batch in the tank multi pass carbon foam cathode. Other than the obvious need for analytics on the AA to determine when stripping is complete.


----------



## Anonymous

I quite agree with your thinking. 

I process in batches of approx 20l at a time when doing a "leach" so a time factor of less than a day really doesn't create a major problem. Obviously there's also no harm in setting up two cells in parallel and doubling the throughput, if required or alternatively simply building a bigger unit because the cell scales. 

Dean's cell is much larger, and processes a lot more per hour. 

My own setup will eventually have two cells in series- the second cell to oxidise the leach liquor to water with no waste or hazardous substances.


----------



## Yggdrasil

Hi Jon!
What size do your cell have, half of the carbon felt size? 
170 x 300 mm? 
Seems the dimensions is approximately 60-80 mm diameter and some 200 mm tall?

Have you tried running in reverse polarity?
Do you expect degradation/consumption of the felt when running with reverse polarity?


----------



## Anonymous

Running in reverse polarity is the actual basis of how the cell was developed. It destroys organics completely in one pass. Yes there is naturally degradation of the carbon felt in this mode however the rate of degradation vs the sheer surface area available means it can destroy an awful lot of organics before it's "spent." 8) 

Jon

I must note that credit for this cell goes to Deano not me. I'm merely the first person to build and use it on this forum. His advice is excellent and if you read his thread deeply there are a number of other things that members will benefit from greatly.


----------



## kurtak

nickvc said:


> Kurt it depends on what type of solutions you are running in the cell and what metals you have in solution, it will do an excellent job of recovering values from base solutions which can be melted and sold as is or refined if that is your preference , with acidic solutions I believe by varying the voltage/ ampage it can be made selective to a degree or so I believe, trials were been conducted by another firm into how it could be done but assays are needed to check what is been stripped or left.



Nick 

I get all of that --- concerning the underlined above I also understand that Jon is using it to "recover" his gold from his cyanide stripping solution so it seems to me that because the gold is being deposited at the cathode (carbon felt) the gold is then tied up with in the matrix of the felt & therefore you would then need to recover the gold from the felt

Being carbon it seems to me that you would therefore need to incinerate the felt to turn it to ash so that you could then ether smelt it to slag off the ash (& depending on the ash to gold ratio maybe even add a collector metal) or incinerate the felt to turn it to ash to re-leach the gold from the ash

Or am I missing something here ?

Kurt


----------



## Anonymous

Kurt it's perfectly fine to put your felt in AR and dissolve the gold from it.

Edit of course you need to wash it first if you're using a cyanide leach.


----------



## kurtak

spaceships said:


> Kurt it's perfectly fine to put your felt in AR and dissolve the gold from it.
> 
> Edit of course you need to wash it first if you're using a cyanide leach.



That doesn't make sense to me - as we know "carbon loading" is another method of recovery used to recover low PPM from acidic solutions --- that's why we ALWAYS advise that when leaching gold bond wires from incinerated IC chip that you make sure the epoxy is incinerated to "complete" ash - other wise if there is carbon in the ash you will suffer some degree of your gold due the gold chloride loading on the carbon --- not ?

Kurt


----------



## kurtak

The other option would be to sell the felt (once it is loaded with gold) to another refiner - but I would assume you would take a bit of a hit on price - which is not always a bad thing --- kind of like selling dore metal from a smelt - you take a bit of a hit on what you get paid for the PMs - BUT - you save on the chems needed & the time spent & the waste created trying to recover the PMs from the dore yourself

Kurt


----------



## Yggdrasil

Hi!

The way I understand it, you can AR the felt and extract most of the values.
Then attach the felt again and continue stripping with it until a point where you decide the felt is spent.
At that time incineration/fusion/smelting or similar methods has to be applied to completely recover whats in the felt?
Correct me if I'm wrong but the felt could be fine until its too degraded to retain its form.
Do anyone know how a felt like this, would hold up in weak or even strong sulfuric acid?


----------



## g_axelsson

If it's loaded with gold you should be able to melt it directly, if you are using an oxidizing flame (or environment / flux) the carbon should just burn away. Just like melting gold in a filter paper that some members do regularly.

Another solution would be to run the cell in reverse, using the felt as anode and deposit the gold on a massive carbon cathode or a metallic cathode, preferably of pure gold. This could be made as a second smaller loop. If it's possible to increase the amperage with a more concentrated electrolyte it would take shorter time to run the "unload" phase.

Göran


----------



## kurtak

Yggdrasil said:


> Do anyone know how a felt like this, would hold up in weak or even strong sulfuric acid?



Probably not well - it would most likely ash the felt - its called wet ashing

Kurt


----------



## Yggdrasil

Thanks Kurt.
So its wise to keep the sulfuric out of the leach if one plan to run this cell :lol:


----------



## kurtak

g_axelsson said:


> If it's loaded with gold you should be able to melt it directly, if you are using an oxidizing flame (or environment / flux) the carbon should just burn away. Just like melting gold in a filter paper that some members do regularly.
> 
> Göran



Yes - doable - however not quite as simple as putting a paper filter in a melting dish & ashing it - paper readily ashes - not all carbon readily ashes (keep in mind diamonds are carbon)

Example - here is carbon fiber felt used for welding blanket insulation that withstands 1800 degrees (I assume that is 1800 F) :arrow: http://www.autobodytoolmart.com/velvet-shield-carbon-fiber-welding-blankets-p-39772.aspx?utm_source=bing&utm_medium=cpc&utm_campaign=Bing%20Shopping&utm_term=1100001935516&utm_content=All%20Products

That's close to the melt temp of gold (1947.52 F) so its not going to readily ash - & the felt cathode from the cell is quite large so its not going to happen in a dish with a torch 

You could load the felt in a crucible & then "first" ash it in the furnace - then add your flux --- however I am not sure you would get complete ashing because the gold is loaded "on" the fiber - therefore the gold is going to be somewhat of a passive layer preventing oxygen from getting to the carbon to completely ash it - so you would likely need to add an oxidizer to the flux ( like potassium nitrate - very hard on crucibles) --- if the gold to ash ratio is not high enough (in gold) you would also need a collector metal (silver would be preferred though copper would work) to prevent ball metal hanging up in the slag - in which case you then would need to part the gold from the collector metal 



> Another solution would be to run the cell in reverse, using the felt as anode and deposit the gold on a massive carbon cathode or a metallic cathode, preferably of pure gold. This could be made as a second smaller loop. If it's possible to increase the amperage with a more concentrated electrolyte it would take shorter time to run the "unload" phase.



This is also doable - I guess my point is that with recovery by "winning" with this cell involves added steps before your final product of a melted bar/button --- unless I am missing something 

The positive side to the winning cell is that you recover your leach solution so that it can be re-used thereby significantly reducing your waste 

Edit to add; - by the way - I think I just found a more affordable source for the carbon felt in the above link - that price is way lower then other prices I have seem

Kurt


----------



## Yggdrasil

Hi!
A couple of thoughts.

If it is hard to ash it in an oven/kiln/furnace etc.



> Yggdrasil wrote:
> Do anyone know how a felt like this, would hold up in weak or even strong sulfuric acid?
> 
> 
> 
> 
> 
> Probably not well - it would most likely ash the felt - its called wet ashing
Click to expand...


Is it possible to wet ash it as mentioned above? (It has after all relatively low carbon volume)
Or would a passive layer come into play even then?

And if so, how would one treat it after, to recover the values?

I found the same welding mats, but guessed that they were made of a different quality carbon and disregarded them..
Since it is made from"carbon fiber" I was of the idea that each fiber is stronger slicker and has a more compact form.
The carbon felt for sound proofing would be better off, if they were made of more porous graphite style fibers.
If this is correct this carbon felt, may have a significantly lower ashing temperature.
Anyone here with some actual information to set me straight?


----------



## Anonymous

kurtak said:


> spaceships said:
> 
> 
> 
> Kurt it's perfectly fine to put your felt in AR and dissolve the gold from it.
> 
> Edit of course you need to wash it first if you're using a cyanide leach.
> 
> 
> 
> 
> That doesn't make sense to me - as we know "carbon loading" is another method of recovery used to recover low PPM from acidic solutions --- that's why we ALWAYS advise that when leaching gold bond wires from incinerated IC chip that you make sure the epoxy is incinerated to "complete" ash - other wise if there is carbon in the ash you will suffer some degree of your gold due the gold chloride loading on the carbon --- not ?
> 
> Kurt
Click to expand...



Are you sure you're not talking about activated carbon?


----------



## Anonymous

g_axelsson said:


> If it's loaded with gold you should be able to melt it directly, if you are using an oxidizing flame (or environment / flux) the carbon should just burn away. Just like melting gold in a filter paper that some members do regularly.
> 
> *Another solution would be to run the cell in reverse, using the felt as anode and deposit the gold on a massive carbon cathode or a metallic cathode, preferably of pure gold.* This could be made as a second smaller loop. If it's possible to increase the amperage with a more concentrated electrolyte it would take shorter time to run the "unload" phase.
> 
> Göran



No that defeats the whole object of the cell Goran. Think surface area and a single path of liquid from outside to inside. The liquor outside has values, inside it does not.


----------



## g_axelsson

spaceships said:


> g_axelsson said:
> 
> 
> 
> If it's loaded with gold you should be able to melt it directly, if you are using an oxidizing flame (or environment / flux) the carbon should just burn away. Just like melting gold in a filter paper that some members do regularly.
> 
> *Another solution would be to run the cell in reverse, using the felt as anode and deposit the gold on a massive carbon cathode or a metallic cathode, preferably of pure gold.* This could be made as a second smaller loop. If it's possible to increase the amperage with a more concentrated electrolyte it would take shorter time to run the "unload" phase.
> 
> Göran
> 
> 
> 
> 
> No that defeats the whole object of the cell Goran. Think surface area and a single path of liquid from outside to inside. The liquor outside has values, inside it does not.
Click to expand...

You have to see it like a silver cell, in this case we would disconnect the carbon felt cell from the original leach tank and replace the weak cyanide solution with something more concentrated, like a gold plating solution.
If we are using the felt with the gold deposit as an anode then we could just put a cathode in the center of the cell and have the gold deposited on it.

But it all comes down to the price of the carbon felt, if it is cheaper and more convenient to just replace it

Göran


----------



## Anonymous

g_axelsson said:


> spaceships said:
> 
> 
> 
> 
> 
> g_axelsson said:
> 
> 
> 
> If it's loaded with gold you should be able to melt it directly, if you are using an oxidizing flame (or environment / flux) the carbon should just burn away. Just like melting gold in a filter paper that some members do regularly.
> 
> *Another solution would be to run the cell in reverse, using the felt as anode and deposit the gold on a massive carbon cathode or a metallic cathode, preferably of pure gold.* This could be made as a second smaller loop. If it's possible to increase the amperage with a more concentrated electrolyte it would take shorter time to run the "unload" phase.
> 
> Göran
> 
> 
> 
> 
> No that defeats the whole object of the cell Goran. Think surface area and a single path of liquid from outside to inside. The liquor outside has values, inside it does not.
> 
> Click to expand...
> 
> You have to see it like a silver cell, in this case we would disconnect the carbon felt cell from the original leach tank and replace the weak cyanide solution with something more concentrated, like a gold plating solution.
> If we are using the felt with the gold deposit as an anode then we could just put a cathode in the center of the cell and have the gold deposited on it.
> 
> But it all comes down to the price of the carbon felt, if it is cheaper and more convenient to just replace it
> 
> Göran
Click to expand...


No I don't have to see it like a silver cell. Goran this is a practical item that works. You can theory craft as much as you like but you know sometimes you actually have to .. do something? You know? Go out there and build one but until you have, then enough of theoretical improvements that effectively break the principle of something that works in practice.

If you built one you would understand I can promise you.


----------



## kurtak

spaceships said:


> kurtak said:
> 
> 
> 
> 
> 
> spaceships said:
> 
> 
> 
> Kurt it's perfectly fine to put your felt in AR and dissolve the gold from it.
> 
> Edit of course you need to wash it first if you're using a cyanide leach.
> 
> 
> 
> 
> That doesn't make sense to me - as we know "carbon loading" is another method of recovery used to recover low PPM from acidic solutions --- that's why we ALWAYS advise that when leaching gold bond wires from incinerated IC chip that you make sure the epoxy is incinerated to "complete" ash - other wise if there is carbon in the ash you will suffer some degree of your gold due the gold chloride loading on the carbon --- not ?
> 
> Kurt
> 
> Click to expand...
> 
> 
> 
> Are you sure you're not talking about activated carbon?
Click to expand...


Yes - activated carbon is what's normally used for gold carbon loading - I am also aware that there are different types of carbon - graphite is a type of carbon - very high temp carbon which is why its used for crucibles & molds & I don't "think" it would load like activated carbon --- concerning the carbon felt I was unsure so had to ask mate :mrgreen: (which is why I kept say - "unless I am missing something") --- after all "your the man" actually running this cell 8) 8) 

Looks like I am going to have to buy that 4'X6' welding blanket I posted a link to - $100 is not a bad price 

Kurt


----------



## kurtak

After doing a little more looking around I also found this one :arrow: http://www.mscdirect.com/product/details/93215853?src=pla&cid=PLA-Bing-PLA+-+Test&CS_003=61497716&CS_010=93215853&cid=PLA-Google-Shopping+Campaign+Test%28BSC%29_swzaS38Ch|dc_PLA__11490253749_c_S&mkwid=swzaS38Ch|dc&pcrid=11490253749&src=pla

its a bigger piece 6'X8' instead of 4'X6' for $283 so a bit more expensive - but if you look at the description it gives 1800 F as the max heat temp but right under that it also says 200 F "constant" max temp so "assume" it would ash & then smelt if smelting would be the choice of recovery --- & I would assume this would be true with any of the carbon felt welding blankets 

Kurt


----------



## Anonymous

4metals said:


> They work getting plating solutions down to 1 ppm and have been around for 30 years. They have a platinized titanium anode. Maybe it wouldn't grab everything first pass but they melt nice.



I missed the second part of your post earlier 4metals. Yes a platinised Titanium Anode is great for this cell. The main consideration though for the proof of concept unit that I initially built was cost. For most home users too I would imagine the same would apply. A graphite rod is way cheaper, but for sustained, high volume throughputs I will certainly invest in the anode you suggested. 

This in turn raises another point. If you are looking to scale the diameters of the cell down, then you need to replace the single internal return (barren liquor) with at least 4 because it provides a flow from multiple points within the cell and prevents a build up on one point of the cathode. This can be achieved simply by using T pieces on the pipe to provide multiple ends. 

Jon


----------



## hadriansluicer

Hi. Is my first post on the forum ,after a lot of reading. English is not my first language so I m apology for mistakes. Firs I want to thank you for the huge amount of knowledge I ghatered here. I understud the rules of board,and when something was not clear I read again and again,until all is settle nice. Reading Deano post about ferocyanate leaching,wanted to try it.i go to eBay to get some ferocyanate and read msds sheet. They wright there -in contact with acids is releasing toxic gases(one of msds speak of hydrogen cyanide :shock: . Deano use is leach at ph 1-3,with addition of sulfuric acid. My question -it is safe? Sorry for taking your time but msds was little disturbing,so I must to ask. Don't want to kill anyone of my neighbors ,thought I like it a lot :lol:


----------



## hadriansluicer

Thyocyanate. Sorry


----------



## hadriansluicer

http://www.labchem.com/tools/msds/msds/75510.pdf thiocyanate,sorry.


----------



## goldmet

OOOPS the concept should be the same


----------



## goldmet

Deano said:


> The hypochlorite leach rate is between cyanide and aqua regia.
> 
> For an ore comparison, generally hypochlorite is around 3 times as fast as cyanide but can be sped up by having more hypochlorite present.
> 
> This usually means leach times in hours rather than days.
> 
> The leach rate is controlled by the rate at which the oxidant can access the gold so some form of agitation is usual.
> 
> The agitation can be applied by stirring or by pumping the liquor, the former for ores and the latter for component leaching.
> 
> As you stated, the leach rate increases with temperature as does the evaporation rate.
> 
> The maximum loading of gold onto carbon is dependent on both the form of the gold complexes and the tenor of the gold liquor( think concentration of gold in the liquor).
> 
> The higher the gold tenor the greater the loading possible.
> 
> Gold chloride will load to a higher level and do so faster than gold cyanide.
> 
> The gold cyanide loading is an adsorption system whereas the gold chloride is mainly a reduction mechanism.
> 
> This means that gold chloride will reduce to gold metal on activated carbon and will do so to levels where the gold particles will fall off the carbon. Great care must be taken with carbon loaded from gold chloride not to get to this stage.
> 
> The maximum gold loading levels onto carbon are also very dependent on the levels of other metal species present in the leach liquors as these other metals will take up sites on the carbon and prevent gold from loading on these sites.
> 
> Temperature and pH are also among the major loading controllers.
> 
> In CIP circuits the gold loadings are anywhere from 2,000 to 20,000 ppm on the carbon depending on the mine conditions.
> 
> In artificial laboratory conditions under acid pH and low temperature gold loadings over 200,000 ppm have been achieved.
> 
> Note that gold cyanide complexes are stable under acid conditions, it is only when the acid conditions are strongly oxidising that the cyanide complex is degraded.
> 
> Deano


Agree


----------



## labalkeny

The party begins here gentlemen!
Deano,
At the hypochlorous acid leach "zone", confrim a high density plastic tank (air tight tanl) can be opened for ph checks, with out interfering with the leach process.
if u were out prospecting hundreds of miles away in gold land and realize ur ph meter battery is dead. and the leach is already in water, a bag full of litmus paper is in the car together with its color chart coresponding to each ph scale from 1 till 14. how would u use litmus paper here?


----------



## FrugalRefiner

labalkeny said:


> if u were out prospecting hundreds of miles away in gold land and realize ur ph meter battery is dead. and the leach is already in water, a bag full of litmus paper is in the car together with its color chart coresponding to each ph scale from 1 till 14. how would u use litmus paper here?


labalkeny, please do not use text lingo on the forum. Many of our members have to use translation software to read, and things like "u" and "ur" do not translate well.

Dave


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## labalkeny

Frugal ,sorry about those 'letter words' am using a terrible phone. But i will try avoid that next time.


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## FrugalRefiner

Thank you. We all appreciate the effort. 8) 

Dave


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## Deano

A pH 7 saline hypochlorite leach evolves very little free chlorine, the amount is so small that it will not affect the leach if this chlorine is lost.

Try making up a leach solution with 20% salt and 5 grams per litre calcium hypochlorite at pH 7- 8 (equals pH 8-9 on a meter), sniff it carefully and see what level of chlorine there is in the air. Usually you have to try fairly hard to smell any chlorine.

Dip the litmus paper in the solution and read the pH from the colour chart. You need to be quick, the leach will bleach the paper colour.

This gives you a direct comparison between meter pH and paper pH.

Deano


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## labalkeny

Thanks Deano. 
If I may ask. how is leaching at a specific pH, 7&8 different from
" Simultaneous sulfide oxidation and gold leaching of a refractory gold ore, by chloride hypochlorite solution, an Article in mineral engineering's September report.
is there no simultaneous oxidation and leach at ph7&8? this guys start there Leach from pH 11 down to around 8.

And how does one tell all the Ag and Au has been leached by the HYPOCHLORITE system and in how much time?


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## labalkeny

forgot something.
leaching in a large concrete tank. 
is it necessary to separate the pregnant solution from the rest of the ore and silt prior to contacting this solution with activated carbon?
recovery in the cynide system with carbon is an adsorption process while in the gold and silver chloride system its a reduction process.
if the mud/ silt is not separated from the solution. for the HYPOCHLORITE system , won't values drop back into the silt and ore?


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## FrugalRefiner

Spending a couple of minutes with the Preview button would save everyone else a lot of time trying to parse that.


labalkeny said:


> Thanks Deano.
> If I may ask. how is leaching at a specific pH 7& 8 different from
> " Simultaneous sulfide oxidation and gold leaching of a refractory gold concentrate by chloride–hypochlorite solution Article in Minerals Engineering · September 2013'
> is there no simultaneous oxidation and leach at pH 7 & 8? this guys start there Leach from pH 11 down to around 8.
> 
> And how does one tell all the Ag and Au has been leached by the HYPOCHLORITE system and in how much time?


Dave


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## labalkeny

thanks Dave!
the volume of document here is colossal and can take one quite some time to discover an important point.
Any links?


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## kurtak

labalkeny said:


> thanks Dave!



I think what Dave was trying to tell you is please take the time to proof read your post (by using the preview button) before submitting your post - that way you can fix/edit any problems in your post "before" submitting it - such as the problem of words run together like the following



> Thanks Deano.
> If I may ask. how is leaching at a specific pH 7&8 different from
> " Simultaneoussulfideoxidationandgold leachingofarefractorygoldconcentrateby chloride–hypochloritesolution Article in MineralsEngineering·September2013'
> is there no simultaneous oxidation and leach at ph7&8? this guys start there Leach from pH 11 down to around 8.



I had to "struggle" reading that "before" scrolling down to see that Dave had edited it to make it readable

Please go back & "edit" your original post labalkeny so others don't have to "try" reading it "before" figuring out that Dave edited it to make it readable 

Kurt


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## labalkeny

ok. Kurt , I didn't understand what Dave meant .
I shall do that. thanks once again.


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## labalkeny

Frugal I appreciate the correction and I have also done some correction on that post. I should also say I really spent a lot of time trying to fix such errors but I realized. when a lot of time is spent typing, the whole text body can't be submitted.
I am using a phone and out in the Field.Not sure if this only happens to me .
But trust me I typed several texts many many times and it just can't be submitted when you spend lots of time going through what is written. For even this may not submit.


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## FrugalRefiner

Labalkeny, I appreciate your efforts to fix your previous post and to get it right to begin with. I don't use a phone to post, so I can't really relate to the problems it might create when posting. Since we have members from around the world (like you), we ask that members make their best efforts. Even small errors can result in great difficulties when using translation software.

I type my posts on a laptop. I proofread what I've typed in the edit box. Then I always use the preview button and read it again. I almost always end up correcting something or adding some clarification. Then, once I submit the post, I read through it again. I know that's extreme, but it's just how I am.  

Dave


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## anachronism

I know we're helping this new chap here with forum posting and it's a great thing to do however is there a way we can avoid clogging up this brilliant thread with this Dave?


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## FrugalRefiner

anachronism said:


> I know we're helping this new chap here with forum posting and it's a great thing to do however is there a way we can avoid clogging up this brilliant thread with this Dave?


Jon, this thread has already been cleaned up and archived in the Library. From time to time, as new posts add to the content, and time allows, one of the moderators will undertake to copy the thread to an area where we can work on it, move the new, deserving posts to the Library copy of the thread, then clean up the detritus of the process.

This is an open forum. Every member is free to post wherever they choose. We do ask that they try to choose an appropriate section of the forum when creating a new thread. We also ask that if they study the forum and if they have questions about a thread, they post their questions in that thread rather than starting a new one which simply adds to the fragmentation of information.

When we add a thread to the Library, we provide a link to the original thread and Encourage members to add to the original thread.

This is not the first time you have felt the need to question the way I, or other moderators, post or moderate this forum. If you have a problem with the way I post or carry out my responsibilities as a moderator, I suggest you contact the board administrator, Noxx, and voice your complaints to him. 

Dave


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## anachronism

Good heavens where did all that come from? I was asking for your help and advice not having a go at you Dave. I'm really not sure how you managed to read my post in such a negative light because being negative wasn't the intention.


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## Deano

The article on sulfide oxidation and gold leaching is a research article, not a how to actually do it best article.

All hypochlorite leaches will attack sulfides.

At high pH values the hypochlorite is in the form of the hypochlorite complex and will attack the sulfides at the slowest rate of any of the hypochlorite forms.

Around pH 8 the predominant form of the hypochlorite is as the hypochlorous complex, this will attack the sulfides at the fastest rate of any of the hypochlorite forms depending on the agitation supplied.

Around pH 3 and below, the predominant form of the hypochlorite complex is as free chlorine, not recommended for any leaching process on safety grounds.

The higher the pH for any hypochlorite solution, the less free chlorine that is evolved, but always some will be evolved.

At pH 8 you can smell weak levels of chlorine, at pH 11 the smell is very weak but still present.

Note that with the 20% level of salt present the true pH levels will read about 1 unit lower on a meter, so true pH 8 will read pH 7 on a meter.

If you are running a saline hypochlorite leach at any pH where there are sulfides present you will have a lowering of the pulp pH caused by the formation of sulfuric acid during the oxidation of the sulfides.

Any variation in the sulfide levels will cause a corresponding variation in the final leach pH.

The best process route is to monitor the pH during the leach procedure and adjust the pH to around 8 with caustic soda on a continuous basis.

At the same time you are monitoring the Eh of the pulp to ensure that you have not run out of hypochlorite in the leach.

The sulfides will be a major consumer of the hypochlorite so you could be adjusting the pH to 8 but have no hypochlorite left to do any further leaching.

Good luck if you would prefer to hope that the pH and eH levels will miraculously end up at pH 8 with all gold in solution, just because they had a result does not mean that this will always occur.

The gold and silver in solution levels are read on AAS, this is the only easy way to do this.

Leaching is continued until there is no rise in the gold in solution levels, at this stage consult your clock for leach times.

Deano


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## Yggdrasil

Just a question regarding the Hypochlorite leach.
I found references that states it can leach both Gold and Silver.
Do anyone here have a explanation on how this can be done, and why the silver do not form AgCl immediately?

How about PGM's ?
I have seen a video from one of the biggest PGM mines in South Africa and they used HCl/Cl in pressure tanks but will it leach PGms as a simple Hypochlorite leach?


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## fishaholic5

Silver will only stay in solution in any quantity in a concentrated leach solution. Once it is diluted the silver chloride will drop.
Open leaches will dissolve fine PGM's in ore. 
I've used both open leaches and pressurised leach systems for this using HCl/ Hypochlorite


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## Yggdrasil

Thanks a lot.
So in a high silver situation, Cyanide based or SSN will be favourable?

The ThioCyanate Deano was telling about seem really interesting, but there was no information on how much silver it can hold.
Maybe do some tests 

Bye the way good to see result from your bacterias, treat them well.
Always interesting to hear from you.


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## fishaholic5

I have no experience with SSN, cyanide or thiourea leaches. Dean is the one to talk to there.
My thoughts are that SSN will produce Silver Chloride too. Much of what I do contains high quantities of Silver as Tellurides, Sulfides or Silver Halides.
Using an agitated or rotating leach system stops the Silver Halides passivating the other metals in the ore during leaching.
The ore residue can be treated with NaOH and the Silver recovered by smelting or leaching afterwards.

Cheers Wal


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## Yggdrasil

FrugalRefiner said:


> Jon, this thread has already been cleaned up and archived in the Library. From time to time, as new posts add to the content, and time allows, one of the moderators will undertake to copy the thread to an area where we can work on it, move the new, deserving posts to the Library copy of the thread, then clean up the detritus of the process.
> 
> This is an open forum. Every member is free to post wherever they choose. We do ask that they try to choose an appropriate section of the forum when creating a new thread. We also ask that if they study the forum and if they have questions about a thread, they post their questions in that thread rather than starting a new one which simply adds to the fragmentation of information.
> 
> When we add a thread to the Library, we provide a link to the original thread and Encourage members to add to the original thread.
> 
> This is not the first time you have felt the need to question the way I, or other moderators, post or moderate this forum. If you have a problem with the way I post or carry out my responsibilities as a moderator, I suggest you contact the board administrator, Noxx, and voice your complaints to him.
> 
> Dave


I've been looking for the library in here a couple of times, but so far it has eluded me. I'm not able to comfortably navigate the new forum yet. It is more action/sosial media centric than the old forum that had information as its focal point.
If anyone knows how to find it directly I'd be grateful.


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## Yggdrasil

fishaholic5 said:


> I have no experience with SSN, cyanide or thiourea leaches. Dean is the one to talk to there.
> My thoughts are that SSN will produce Silver Chloride too. Much of what I do contains high quantities of Silver as Tellurides, Sulfides or Silver Halides.
> Using an agitated or rotating leach system stops the Silver Halides passivating the other metals in the ore during leaching.
> The ore residue can be treated with NaOH and the Silver recovered by smelting or leaching afterwards.
> 
> Cheers Wal


Thanks Wal.
I was pondering your reply and revisited.
So you tank leach the ore with mechanical agitation?
And smelt after leaching. From that I assume your tanks are not that massive, 200-1000 liter, smelting 20-50 kgs at a time?

Have you tried agitation by compressor and air?
Am I getting something wrong here?

Edit.
Just saw smelting or leaching, so the NaOH to convert AgCl to Silver oxide so it can be leached or smelted directly?


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## fishaholic5

Yggdrasil said:


> Thanks Wal.
> I was pondering your reply and revisited.
> So you tank leach the ore with mechanical agitation?
> And smelt after leaching. From that I assume your tanks are not that massive, 200-1000 liter, smelting 20-50 kgs at a time?
> 
> Have you tried agitation by compressor and air?
> Am I getting something wrong here?
> 
> Edit.
> Just saw smelting or leaching, so the NaOH to convert AgCl to Silver oxide so it can be leached or smelted directly?



Yes, tank leaching with mechanical agitation, to stop the Silver Halide from passivating the other metals it must be continually abraded off them. Angled rotating systems work best and I often use marbles in the mix to help.
The scale can be from bottles ( I use 1.5 litre PET milk bottles for sample runs of 200g) buckets or cement mixers ( coat with epoxy or modify to take a 120 or 200 litre poly drum)
For a larger scale the same thing can be done with a cement truck.

Using air for agitation is not as effective.

Most of my leaching is relatively small scale, using 20L buckets and approx. 5L of ore per bucket.
This is mostly for ease of manual handling and because the focus of my work is now on the use of microorganisms rather than making money.
My vessels for cementation, precipitation or bacterial recovery from the leach solutions are 20-1000 Litre.

Yes, the Sodium Hydroxide is to convert the Silver Halide to Silver Oxide so that the ore residue can be leached or smelted directly.
I've used this system on ores that contained over 10% Silver by weight.

Cheers Wal


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## Yggdrasil

Thanks, and treat your bacterias well.
I'm looking forward for the next update.


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## Yggdrasil

Just a question, how low initial gold concentration does these leaches take?

Cyanide species can leach from _as an example only, say 500ppm to 0.01 ppm_ 
ThioCyanate and saline from ?? to ??
Saline Hypochlorite from ?? to ??
Of course there are a plethora of factors involved, but there may be a general rule/hunch.
Anyone with a tip here?


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## kurtak

Yggdrasil said:


> I've been looking for the library in here a couple of times, but so far it has eluded me.


Yaggdrasil - sorry but didn't see this till now 

For what ever reason (that doesn't make a lot of sense to me) the library is no longer on the front page (index page) 

It is now in Tutorials, Calculators & Data 

Here is a link









LIBRARY INDEX THREAD


How could our library be complete without a copy of Hoke's book? Here are the links for the book. Printer friendly copy of Refining Precious Metal Wastes Screen readable copy of Refining Precious Metal Wastes For those of you who prefer to have their books in the "dead tree format" the...




goldrefiningforum.com





Kurt


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## Yggdrasil

kurtak said:


> Yaggdrasil - sorry but didn't see this till now
> 
> For what ever reason (that doesn't make a lot of sense to me) the library is no longer on the front page (index page)
> 
> It is now in Tutorials, Calculators & Data
> 
> Here is a link
> 
> 
> 
> 
> 
> 
> 
> 
> 
> LIBRARY INDEX THREAD
> 
> 
> How could our library be complete without a copy of Hoke's book? Here are the links for the book. Printer friendly copy of Refining Precious Metal Wastes Screen readable copy of Refining Precious Metal Wastes For those of you who prefer to have their books in the "dead tree format" the...
> 
> 
> 
> 
> goldrefiningforum.com
> 
> 
> 
> 
> 
> Kurt


Thanks mate. 
I got an answer in PM already.


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## Yggdrasil

Deano. 
Thanks for a beautiful thread.

We have been trying the Hypochlorite leach, since we have no experience with it, 
we don't know if there will be color change or anything like that when the leach pick up gold.
I don't even know a small thing like if it will evolve bubbles during leach.

We do suspect it has leached some, but the results are inconclusive.
Will stannous work, how about SMB, will that drop what is in the test leach?

Will Carbon or Resin be best for production leach?
We struggle and wonder if the salt may be the issue, we tried a electrolytic cell, but I guess the gold concentration may be too low.

Regards Per-Ove


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## Deano

Hypochlorite leaches have a leach rate between cyanide and aqua regia, it is unlikely that you will see bubbling from gold but if you have a lot of base metals in the ore you may get some bubbling from these as they dissolve.
All leaches will dissolve all of the available gold in an ore, they will not dissolve occluded gold in quartz etc as they cannot access it.
Hypochlorite leaches will dissolve sulfides and make gold occluded in these sulfides available for leaching, the leaching rate of these sulfides is slow as the particle size is usually larger than the gold particles in them.
If you have a lot of sulfides in the ore there is a good chance that you will run out of hypochlorite before you have leached all of the gold.
Use a handheld ORP meter to check when you need to add more hypochlorite on the first few runs then add hypochlorite on a time basis for later runs, checking before additions are made.
The hypochlorite additions will raise the solution pH, use a handheld pH meter to check pH and adjust with HCl as necessary.
The chloride ions will lower the actual pH readings on your meter, if you are reading pH7 on the meter you will have a true reading between 6 and 7 depending on the chloride ion level in solution.
Usually you try for a meter reading of 7 so that you are working in the safe range of 6 to 7.
There is always some free chlorine in the leach but by working in the pH range 6 to 7 you minimise the free chlorine level to where it is not a health hazard.
Always only put the meter probes into the leach solution when you want to get a reading, after reading you rinse the probes and leave them halfway in a container of water.
Metal recovery is always done with carbon, the leach will destroy resins.
Metals will be reduced to the native form on carbon so you do not want to run the carbon as CIP, you will attrition the gold etc from the ore.
Best method of recovery is to pack the carbon in canisters and flow the leach liquor through them without disturbing the carbon.
Always keep in mind that all wetted surfaces must be made of nonmetallic materials, the leach will dissolve metallic surfaces.
The only realistic way to check for gold levels in the leach liquor is by AAS, you will have such a high level of oxidant that reduction testing by stannous or similar is not conclusive.
Recovery of the gold from the carbon is either by cyanide stripping or ashing of the carbon.
Deano


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## nickvc

Having tried this leach with Deanos supervision I can tell you it works but needs almost constant monitoring and attention much more so than cyanide and even AR , you cannot set up and walk away for more than about 15 minutes in my experience.


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## Yggdrasil

Deano said:


> Hypochlorite leaches have a leach rate between cyanide and aqua regia, it is unlikely that you will see bubbling from gold but if you have a lot of base metals in the ore you may get some bubbling from these as they dissolve.
> All leaches will dissolve all of the available gold in an ore, they will not dissolve occluded gold in quartz etc as they cannot access it.
> Hypochlorite leaches will dissolve sulfides and make gold occluded in these sulfides available for leaching, the leaching rate of these sulfides is slow as the particle size is usually larger than the gold particles in them.
> If you have a lot of sulfides in the ore there is a good chance that you will run out of hypochlorite before you have leached all of the gold.
> Use a handheld ORP meter to check when you need to add more hypochlorite on the first few runs then add hypochlorite on a time basis for later runs, checking before additions are made.
> The hypochlorite additions will raise the solution pH, use a handheld pH meter to check pH and adjust with HCl as necessary.
> The chloride ions will lower the actual pH readings on your meter, if you are reading pH7 on the meter you will have a true reading between 6 and 7 depending on the chloride ion level in solution.
> Usually you try for a meter reading of 7 so that you are working in the safe range of 6 to 7.
> There is always some free chlorine in the leach but by working in the pH range 6 to 7 you minimise the free chlorine level to where it is not a health hazard.
> Always only put the meter probes into the leach solution when you want to get a reading, after reading you rinse the probes and leave them halfway in a container of water.
> Metal recovery is always done with carbon, the leach will destroy resins.
> Metals will be reduced to the native form on carbon so you do not want to run the carbon as CIP, you will attrition the gold etc from the ore.
> Best method of recovery is to pack the carbon in canisters and flow the leach liquor through them without disturbing the carbon.
> Always keep in mind that all wetted surfaces must be made of nonmetallic materials, the leach will dissolve metallic surfaces.
> The only realistic way to check for gold levels in the leach liquor is by AAS, you will have such a high level of oxidant that reduction testing by stannous or similar is not conclusive.
> Recovery of the gold from the carbon is either by cyanide stripping or ashing of the carbon.
> Deano


Since we are new to this and the ore is somewhat refractory/Special we are doing a lot of trials.
This is what we have notice so far:
Stannous do not work as expected as per information. It gives a precipitate in stead of colloidal color change.
There are no color change.
Stannous tests show less return after long time 7 hours seem half of two hours.
All this is per information, so we will proceed to carbon column in circuit.
Is it best to let the leach finish and then pump it throuhgh, or constant pumping?

Regards Per-Ove


----------



## Deano

Gold chloride will happily adsorb onto any iron oxides which are part of the ore as well as adsorbing onto some quartz types, it will do this to a greater degree than gold cyanide will.
In order to minimise the adsorption effect it is best to run a continuous bleed stream of liquor through a carbon column in order to keep the gold tenor in the pulp to a minimum, this low tenor will minimise adsorption effects.
The bleed stream is usually monitored before and after the carbon column by AAS to ensure that there is enough active carbon in the column to keep the pulp gold tenor low.
The weight of carbon in the column is around one tenth the weight of liquor passed through the column in an hour, you use AAS to establish if, for your particular ore, you have enough carbon present in the column to remove the gold values.
You also check the liquor to see when breakthrough of gold in the column occurs, this is when you change the carbon for fresh carbon.
Deano


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## Yggdrasil

Deano said:


> Gold chloride will happily adsorb onto any iron oxides which are part of the ore as well as adsorbing onto some quartz types, it will do this to a greater degree than gold cyanide will.
> In order to minimise the adsorption effect it is best to run a continuous bleed stream of liquor through a carbon column in order to keep the gold tenor in the pulp to a minimum, this low tenor will minimise adsorption effects.
> The bleed stream is usually monitored before and after the carbon column by AAS to ensure that there is enough active carbon in the column to keep the pulp gold tenor low.
> The weight of carbon in the column is around one tenth the weight of liquor passed through the column in an hour, you use AAS to establish if, for your particular ore, you have enough carbon present in the column to remove the gold values.
> You also check the liquor to see when breakthrough of gold in the column occurs, this is when you change the carbon for fresh carbon.
> Deano


Thanks Deano


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## Yggdrasil

What would be a suitable rotation for such a system? 3-10 volumes per hour? Or is that too slow/fast?


----------



## Deano

If you have one ton of carbon in your container then you would start with a flow rate of 10 tons of liquor per hour through the carbon, scale this back to whatever size operation you are running.
You adjust the flow rate up or down depending on what the assay numbers tell you about the adsorption level in the column.
If you have high grade areas of gold in your ore then you will have to have a second carbon column in series with the first column, this has two functions. 
The first is to recover gold from surges in the gold tenor in solution when first leaching the ore sample.
The second is to use a bypass line to run your liquor through the second column when the first is off line for stripping the carbon.
Deano


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## Yggdrasil

Thanks again Deano.

How much metals are normal to load up on AC with this leach system?
Single digit grams or double digit grams per kilo?
All are variables depending on the actual situation, I know, just as an expectation for scaling.
Regards Per-Ove


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## Deano

You usually would expect low to high single digits grams of gold per kilo of carbon, not only ore type dependent but carbon activity dependent.
The gold chloride is reduced to metal on the carbon so if the gold comes as an initial flush most of it will just load on the surface of the carbon and clog access to the high storage pores.
Deano


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## Yggdrasil

Thanks again Deano.


----------



## Yggdrasil

Hi I'm going to do an unconventional stunt this time.
I'll post a PDF document since it have pictures and text together and I felt it would behave better in a PDF. 

Anyways I hope someone has some views and information regarding the content in the PDF.

Regards Per-Ove


----------



## PeterM

Yggdrasil said:


> Hi I'm going to do an unconventional stunt this time.
> I'll post a PDF document since it have pictures and text together and I felt it would behave better in a PDF.
> 
> Anyways I hope someone has some views and information regarding the content in the PDF.
> 
> Regards Per-Ove


Filter through a tight filter, or wash into a porcelain spoon dry, or extract most of the liquid with a small piece of tissue paper, then dry, anneal then scope it... then show me.... if it's gold it will be Crystalline.


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## Yggdrasil

The Ore owner has prepared a electrolytic cell with tin cathode as per recommendation from “Extraction of Gold Ore" 1905, Dickey

The funny thing is that when he adds approximate a ml leach to the spoon and then more or less the same with Stannous. Both crystal clear. This black precipitate collects in a point in the bottom. Deano has already said that Hypochlorite leach do not fare well with stannous due to high amount of oxidiser and salt.
But it still is a bit cool how the powder just emerges out of nothing.


----------



## Punterr

Yggdrasil said:


> Deano.
> Thanks for a beautiful thread.
> 
> We have been trying the Hypochlorite leach, since we have no experience with it,
> we don't know if there will be color change or anything like that when the leach pick up gold.
> I don't even know a small thing like if it will evolve bubbles during leach.
> 
> We do suspect it has leached some, but the results are inconclusive.
> Will stannous work, how about SMB, will that drop what is in the test leach?
> 
> Will Carbon or Resin be best for production leach?
> We struggle and wonder if the salt may be the issue, we tried a electrolytic cell, but I guess the gold concentration may be too low.
> 
> Regards Per-Ove





Yggdrasil said:


> Since we are new to this and the ore is somewhat refractory/Special we are doing a lot of trials.
> This is what we have notice so far:
> Stannous do not work as expected as per information. It gives a precipitate in stead of colloidal color change.
> There are no color change.
> Stannous tests show less return after long time 7 hours seem half of two hours.
> All this is per information, so we will proceed to carbon column in circuit.
> Is it best to let the leach finish and then pump it throuhgh, or constant pumping?
> 
> Regards Per-Ove


Hi Per, I didnt realise you posted images of the spoon tests, anyway, attached are images of the the spoon test particles (reduced/precipitated by the stannous) as seen through the microscope at 100x, DF, no filters. Sorry for the late posting. The cubic crystals are salt from the hypo leach. Testing is still on going as you know .


----------

