Further things which may be of interest to members

[Greenhorn]

Deano, in your cyanide heap leach, you said you set up three activated charcoal filters to catch the gold in solution, how do you test to see when the first filter is saturated and know when to pull it for refining? I'm assuming there is a tool for this, or is it a simple test for the presence of gold in solution? (stannous paper?)

Also, the forum talks about testing concentration of cyanide in order to maintain safe and effect recovery, is there a tool to do this as well?

Thanks,
Greg

 

[Deano]

If you are running a heap leach the liquors are usually tested by AAS to see when the carbons need stripping, after a few cycles you will learn the approximate time it takes to load the first column so you really only need to start testing the liquor shortly before this time.

Usually you would also check the loadings on the second column at the same time, after a few cycles the gold missed by the first column can start to load up appreciably on the second column.

The AAS readings are usually done by direct aspiration of the liquor, no organic extraction is necessary. This testing can be done quickly and simply by any one with an AAS, a local lab should, depending on instrument availability, be able to do the test in a few minutes.

For safety you keep the pH at around 11, use a small handheld meter for spot checks.

For efficient leaching you are interested in free cyanide levels, these are checked using a cyanide meter with cyanide probe.

Both the above meters are purchased online, most are of similar quality so buy on price.

Deano

 

[Greenhorn]

The probe, I did find one listed as a photometer, readings in mg/l (ppm) what range of meter do I need to get, or in other words, what ppm should this solution be.

In your photo's of the heap leach, you show a guy walking under the bird netting to set/adjust the water spray hoses, these open heap leachs, if the ph is kept around 11, is there almost no cyanide released through evaporation, mostly a safety question because caution should always be kept, but on a hot day with no wind, is there a potential of injury to anyone/animal walking near these vats?

I know you mentioned that one of a couple problems with gravity separation is loosing gold to what is referred to as the slims as they are a whole different animal. For a small operator, will these slims cause problems in these smaller vats? And is there any justifiable reason to wash these slims out of the crushed ore and treat them separately (I understand it's a cost/time thing) but can it have a benefit. I ask because you had mentioned that you can have areas in the heap that will develop water returns to bottom that may bypass some parts of the ore and that maintaining the whole head ore is better then trying to concentrate it.

I want to express my own thanks to you for sharing this information and also answering questions about it. The previous owner of the mine I have turned 90 last year, he said he would love to be on site and help teach what he knows, but he's since moved to warmer/drier climate and his wife fears that travel may end his days and I'd rather that not be the case. I've reached out to my local GPAA, but they all seem to enjoy panning in the rivers, getting a couple of grams of gold over the weekend. I was told specifically by one person that if he can hear birds singing above him then he feels he's in the wrong place. I'm sure in my ventures I may find someone as helpful as you, your skills are very valuable to those like myself.

Thanks,
Greg

 

[Deano]

The standard cyanide level for heap leaching ranges from 50 to 500 ppm, the higher level is used for a fairly clean ore and the lower level is used where there is a high cyanide soluble copper level in the ore. So you are wanting a meter reading up to 500 ppm cyanide for direct readings but one with a lower maximum can be used if the cyanide solution is diluted with clean cyanide free water to get the readings into the meter's operating range.

Note that the meter will read free cyanide in solution, this is approximately half the ppm level of sodium cyanide itself. So if you add 1 gram of sodium cyanide to one litre of water you have made a 1,000 ppm solution of sodium cyanide but the free cyanide level will be approximately 500 ppm.

The main reason for HCN off gassing from a heap is when you use sprinklers to spray the cyanide solution over the heap, the surface area of exposed liquid surface is dramatically increased as is the resulting HCN off gassing.

If the solution is applied as a flow into a paddock there is virtually no surface area increase and so the HCN off gassing is minimised.

The above is similar to a CIP tank where the solution is pumped into the tank rather than sprayed, you do not see people on the walkways above the tanks suffering from HCN poisoning. The above is an OHS detail in which a lot of care is taken to keep safe, apart from not wanting to cause injury to operators you would void insurance cover if not operating in a safe manner.

If running a vat leach the milling is done to a coarser size than for tank leaching such as CIP. Each ore will have a minimum milling size which, if over milled, the higher production of fines will plug the heap either entirely or in zones.

Tank leaching is done to a sizing where gold leaching is finished within 24 hours whereas vat leaching can run for months.

The rule of thumb when milling for vat leaching is that if a damp sample of the milled ore is formed into a hard squeezed sausage around 30mm diameter and held in a hand with around 50mm of the sausage protruding from the thumb/1st finger circle, if the sausage slumps like you would expect from clay then the milling is too fine and the grind is coarsened until the sausage will break off rather than slumping.

A coarse screened hammer mill or rolls mill will usually provide a milling profile which is suitable for a vat leach, it is very dependent on the ore type and the milling profile of that ore as to what milling regime is used.

If there is a high level of sulfides present in the ore it may be better to run a series of jaw crushers to do the milling, this will minimise fines production as sulfides tend to slime in high impact type milling.

If you wash out the fines you will need to treat these fines in a tank leach so you will end up with two leach circuits and the associated extra costs.

If the fines have a high sulphide level then it adds extra costs to breakdown the sulfides to allow the leach to contact the gold locked inside the sulfides. It is much simpler and cheaper to mill in a manner which minimises slimes formation and use the extended vat or heap leaching time to breakdown the sulfides naturally in the vat or heap.

Deano

 

[Greenhorn]

Thanks again Deano, but got a couple questions for clarification.
You talk about heap leach, and I know that that is, but you also had been mentioning vat leach as well, I've been assuming your using vat as it's a smaller version of acre sized heap leaching, but your last comment made me wonder if the vat is somewhere between open heap leach and a tank.
In regards to the tank, I thought you were more for heap leaching for effectiveness, I like the idea of only leaching for 24 hours, but didn't think from what I've read on the forum that this wasn't even achievable.
I live in Idaho, and am surrounded by dairy farms and grain silo's. So you when your talking above, I was visioning a large round container, 3+ meters in diameter, and how ever much high as needed, with a steel punch plate at the bottom covered in a filter/mess to prevent the ore from going through and only fluids can drain below that into a cone that then feeds into a recycling system like you outlined with the heap leach in an open field.
Would this be the tank system your referring to, or is more of a vat?
And your definition of coarse material for this system, are you talking something like 6-8 mm or more like something in the 20 mesh, not sure where to start with the sagging vs breaking of the 30 mm cylinder of ore.
Is the use of ordinary steel work with cyanide? Does it react with it? Or does it need to be layered with a plastic film like you talked about in the heap leach?

The ore (that I still need to get permitting to open the shaft to even get a sample) as I understand it is gold running with iron pyrite, he gave me a 25-30 mm diameter piece of high grade that has both gold grains and the pyrite cube structures. He told me that he was only chasing the gold and discarded any other materials found. So I'm not sure of any copper or silver contents until I have an assay done. When he was operating it back in the 80's, they removed the vein material, crushed to around a 200 mesh, ran on a shaker table to remove freemill gold and then sent the rest off for processing. At the time I didn't know about running head ore vs just concentrates. I was also under the impression that they took the ore offsite to process, so only thing remaining next to the shaft is the quartz monzonite pile of waste. I really haven't look through it much, as the forest has slowly reclaiming where it's been dumped.

 

[snoman701]

Greenhorn...you should download a copy of Rose's Chemistry of Gold. I think it will help fill in some gaps between what you already know, and what Deano is posting. Some things have changed a lot in 100+ years, but some haven't changed much.

 

[Deano]

CIP tanks are made of ordinary mild steel, usually with cathodic protection to minimise rusting. Because cyanide leaching is run at an alkaline pH there is no attack on the steel by the pulp components.

If the previous operator was separating the sulfides from the rest of the ore then I would recommend that you look at doing the same.

Some sulfides will cyanide well but many will be refractory and need some other approach, there is usually a good reason why a particular option is used.

If you have a mix of free gold and gold in sulfides then you have three options for treatment.

The first is to mill and vat or heap leach, if you are in a farming area then you are not likely to get permitting for cyanide use so look at the next options.

The second option is to sell any mined ore to a nearby operator and let them have the worry of treating it. The gold grade of the ore will determine the distance it is profitable to haul the ore to an operator.

The third option is to separate the sulfides and free gold from the ore and sell these concentrates to a smelter or toll treater.

Realistically there are two methods of separation which will give you all of your gold values in the one concentrate, these are flotation and gravity concentration.

Flotation is not a process you learn from books, it is best learned hands on in an operating plant with a good operator to teach you. The upside is that you are not looking to be an all round flotation expert but to just be a competent operator on a single ore type.

The simplest gravity system is a table but you would need to run at least two size fractions of your ore in separate streams in order to get reasonable recovery of the gold values.

Sulfides are notorious for sliming when milled so you are looking for the milling method which minimises sliming.

In small scale mining you can run a series of jaw crushers with intermediate screens, this will get you down to 300 microns particle size maximum but the capacity is limited. having said that, there are some very cheap and efficient jaw crushers coming out of China which will let you set up an efficient milling circuit at a reasonable cost.

It is probably worth talking to the previous owner to see who he sent the ore to, was he sending the entire ore less any free gold or was he sending just concentrates less the free gold. this may give you a better idea of what processing is possible financially.

Deano

 

[Deano]

Occasionally there is a need for a bottle to be cut so that the bottom section can be used as a beaker or similar.

There are many various methods used to achieve this but the only one I know of which allows you to get a sloping cut is the oil method.

The bottle is filled with oil, usually used engine oil, to the level where the cut is to be made.

The bottle can be leaned over at any angle which does not have the oil come out the spout.

If you are wanting a square cut you just place the bottle on its base, but angled cuts are done with the bottle lying on a sloping bed of sand.

A piece of rebar which will fit through the spout of the bottle is heated to red hot either with an oxy set or in a really hot fire.

The rebar is then inserted through the bottle spout and into the oil, keep it there until the glass cuts.

The glass will cut cleanly along the oil level, if it does not cut you did not have the rebar hot enough.

Deano

 

[SamW]

Table optimising

Many years ago I was involved with a project to maximise gold recovery from a milled ore by using a small (3') wilfley table to clean up concentrates and middlings from a bank of larger tables.

Due to environmental constraints chemical processing was not allowed on site.

A short period of operation revealed that running the table in its standard format was leading to losses of fine gold where the particle size was less than 50 micron screen size.

The gold particles were flattened from the milling and appreciable quantities were also lost from the plus 100 micron screen sizes.

Despite doing all of the standard optimisings of adjusting feed size screening, feed rate onto the table, table side tilt and water flow I could not get a major recovery improvement.

I went into the literature and found that the largest number and most useful papers were those from the British tin industry.

Basically they said that keeping a tight sizing on the feed was vital, I was already doing that so OK there.

Making sure that the table was level on the longitudinal axis was a fundamental which I was also doing.

Feed rate was best when a loose bed was set up along the table, don't put too little feed on the table nor try to put too much feed on the table.

Keeping the feed rate constant was also very important, I was feeding from a wet sump with a screw feeder so OK there also.

Side tilt was to be such that clay fraction particles were washed over the side of the table but the tilt was to be little enough that a middlings product could be readily separated at the end of the table.

Even when I had the table set up to cover all of the above parameters I was still losing fine gold.

The only area where I did not have full control was the side water coming onto the table, no matter how much I tried I could not get a perfectly even flow across the table.

I decided that I needed to improve the delivery of water to the table.

I did so by running a length of 3/4" copper pipe suspended about 4" above the side of the table where the water exited the original flow boxes.

The pipe was blanked off at the table exit end and was connected to a hose and ball valve at the feed supply end.

Ever 1" along the pipe was drilled a 1/8" hole.

When the water was turned on a curtain of water sprayed down onto the top edge of the table and delivered an even flow of water which could be easily adjusted for flow rate with the ball valve.

Even this change only partly improved the recovery, it was evident that there needed to be a difference in the water flow rate supplied to various parts of the table.

After a lot of testwork I settled on having the water delivery pipe as two pipes.

The holes remained the same size and spacing but the delivery was split into two parts.

The first part was as above but only extended 2/3 of the table.

The second part covered the last 1/3 of the table, each part was as a separate length of pipe so that adjustments could be made to either part without affecting the other part's flow rate.

In order to keep the water holes at the 1" spacing the pipe for the last 1/3 of the table was fed from the bottom of the table and the pipe ends almost touched.

Each pipe length had its own ball valve for separate flow adjustment.

The side wash water pipes were fed from an overflow overhead tank so that a constant head was maintained.

This was important on a mine site where valves were being opened and closed in other parts of the circuit, this would affect the pressure to the wash pipes.

This setup allowed recovery of free gold down to 25 microns, the disappointing part was the low weight of the 25 to 50 micron gold recovered, it looked a lot as a sheet like paint on the table but weighed far less.

On the plus side there was a substantial improvement in the plus 50 micron gold which did weigh well.

If run from a municipal water supply the overhead header tank may not be necessary depending on the vagaries of the particular supply.


Deano

Deano! what a wealth of information in this thread!

Can I ask what sort of overall gold recovery rate you were achieving with the large tables? I'm currently trialing a full size Wilfley on my gold ore and yet to find someone with this specific experience. Would be great to know what sort of recovery I should expect before spending too much time on something that may not be achievable.


Cheers
Sam

 

[Deano]

Gold recovery on tables is dependent on the size of the gold put onto the table, the shape of the gold particles, where in the ore the gold is located and how the table is operated.
Many ores have some or all of the gold associated with sulfides, you are then carrying out a sulphide recovery rather than a gold recovery. The milling required for gravity recovery of sulfides is always aimed at a coarser grind, but some sulfides are very fine grained and need a finer grind for liberation, this makes the recovery of the sulfides by gravity a much more difficult process. Usually at this point the sulfides are recovered by flotation rather than by gravity. Many flotation reagents will give recovery of free gold as well as sulfides.

If you carry out fraction assays and find that the sulfides are not viable to float then you are looking at gravity separation. Tests must be run to find the minimum grind size which liberates the gold. Milling finer will flatten the gold particles and make gravity separation more difficult.

The maximum gold recovery gotten by gravity separation is dependent on a mix of all of the above factors, there is no all encompassing number for % recovery unless the system is optimised for the above factors.

The project I wrote about was run by a large mining company, so all of the test work above was done and the best size for milling was established.

Table recovery of total gold was around 80%, it was not viable to chase the remainder which was associated with sulfides and other lower grade ore particles.

To get this recovery % we had to fit the water spray bars on the primary tables in a similar configuration to the ones on the clean up table. This increased the recovery from 50% to 80%, no viable grades of free gold were found in the table tailings for either the full size primary or smaller secondary tables.

Deano

 

[jobinyt]

I suggest that knowing what someone else got, someplace else, is of little value. However, you asked the right question "What should you expect?" What you are doing is density separation. For that you need well classified material - sized just above and just below the material you want to recover. Take a large sample - screened to size of what you plan to run. Weigh and volume measure a sample to be run; watch closely, and see what you get. Then split the run material in half - bigger and smaller - and run each again. Then, take 2 more samples from the original lot: one the next mesh size up, the other the next mesh size down and run those across your table. Again, watch closely, and see what you get. Sometimes your target will vary in size enough to make more than 1 classification and run necessary. At any rate, you should now have a good idea of what you can do with your material. The key is classification - tight size ranges allow separating particles of about the same physical size but very different densities. Only through experiment will you learn what you can recover from your materiel.

 

[SamW]

Thanks Deano and jobinyt. That gives me some good information to work with. I plan on sharing my progress in a new thread soon!

 

[jobinyt]

I look forward to it.

 

[goldshark]

Just a quick note to say that surface tension of the water is a big part of fine gold recovery. Using a recirculated water source with Jet Dry type additives, helps to relieve the problem. the water can get pretty heavy with fine silts, so a rather large settling pond helps.

 

[Yggdrasil]

I came over a text regarding one of these "green" gold leachants.
In the text there was a recommendation on mixing the leachant with HypoChlorite leach.
Since the original "green" solution is made up of SodiumCyanate, I wondered how these two play together.
You can push the HypoChlorite leach, to say, pH 9 and then pull the Cyanate leach down to the same.
But one end up with two leaches in not ideal zones and in the end even may destroy each others.

Are there any credit to this kind of process?
Aren't it better to alternate them?

Any thoughts from our experts here?

 

[Deano]

In any pH form of hypochlorite leaching there is always some free chlorine in the leach solution, you can smell the chlorine evolved from the leach. The greater the agitation applied to the leach solution the greater the rate of evolution of the free chlorine.
Free chlorine in solution likes nothing better than to oxidise soluble organic complexes such as cyanide and its derivatives.
If you run a mix of cyanide and hypochlorite in a leach solution you will end up with a solution containing chloride complexes from the hypochlorite and oxidation products such as carbon dioxide from the cyanide.
Not the cheapest or most sensible form of leaching solution.
I have always had high regard for people who try new things but a little research would say that this mixture is not going to be a winner either as a gold leach or as a cheap gold leach.
Deano

 

[Yggdrasil]

In any pH form of hypochlorite leaching there is always some free chlorine in the leach solution, you can smell the chlorine evolved from the leach. The greater the agitation applied to the leach solution the greater the rate of evolution of the free chlorine.
Free chlorine in solution likes nothing better than to oxidise soluble organic complexes such as cyanide and its derivatives.
If you run a mix of cyanide and hypochlorite in a leach solution you will end up with a solution containing chloride complexes from the hypochlorite and oxidation products such as carbon dioxide from the cyanide.
Not the cheapest or most sensible form of leaching solution.
I have always had high regard for people who try new things but a little research would say that this mixture is not going to be a winner either as a gold leach or as a cheap gold leach.
Deano

Excellent, you confirmed my suspicion.
I found this recommendation odd and wanted a second opinion.

Thanks Deano

 

[Deano]

I have been asked a few times why I use urea as a reactant in aqua regia leaches after filtration and before precipitating gold.
In Australia most gold is sold through refining companies such as the Perth Mint.
The major contaminant in unrefined gold is usually copper, provided that the copper level is less than 30% there are no penalties for having copper present in the dore.
The major cost elements in aqua regia treatment are chemical inputs and labour.
This means that if you can minimise labour costs you have minimised one of the two greatest costs of treatment.
It also means that employing quick and dirty recovery methods is not only a labour cost saving but does not lead to penalties from the refiners.
If you are not running your product through a refiner you will be looking for a treatment method which gives you a cleaner product at the cheapest cost.
In aqua regia leaching you have costs associated with the leaching stage and the precipitation stage.
In small stage leaching you have the cost of nitric acid, this is minimised by careful staged additions of nitric acid into the leach to ensure that you do not have excessive surplus nitric acid in the leach solution when all of the gold has dissolved.
There are many methods detailed in this forum which will minimise nitric acid additions. These are good methods and are employed by members but will add costs to the processing, mainly by increasing labour costs. These increased labour costs have substantially increased with the latter increased cost of labour in the general labour market.
These increased costs generally are minimal for small scale processors such as members of this forum who do not really price their time into the cost ledger.
In larger operations methods are used to minimise the labour costs as this will more than compensate for any small chemical cost increases due to the cruder processing methods employed.
Generally this means that a quick calculation is made as to the amount of aqua regia needed to leach the amount of material being treated.
The leach is run hot and fast to minimise time and thus labour costs.
You will end up with a leach solution which has a mixture of gold and silver chlorides and nitrates along with base metal chlorides and nitrates and some free nitric acid.
You have two practical options to clean out the free nitric acid.
You can use chemical reactants to destroy the free nitric acid in the solution or you can simmer the solution until the solution fumes change from orange to white.
I have always costed the simmering option as being cheaper than chemical processing but locally you have to compare the costs for the two methods.
It also helps that I usually work in a lab with good extraction systems so I do not have to factor in the cost of installing these systems.
So you now have a liquor which has been denoxxed but which still has base metal nitrates and chlorides present.
If you add a precipitant at this stage you will find that the copper nitrates will reduce from copper three to copper two but only after some of the gold has reduced to gold zero.
The copper three will act as an oxidant to redissolve the finely dispersed gold zero back to gold three in solution.
This sequence will continue until all of the copper has been reduced to copper two.
Unfortunately when the copper has been reduced there will be an excess of nitrate groups in solution, these groups will oxidise some of the copper two back to copper three to continue the roundabout until all of the excess nitrates have been consumed.
The above sequence will cause you to have to add substantially more precipitant than would be needed if the copper was not present.
I am using copper as an example in the above sequence because it is nearly always present in these leaches but any divalent base metal will do the same thing.
The cheapest method I have found to destroy the nitrates is by addition of urea, for me it is cheaper and quicker than adding precipitant. Once again it depends on the local chemical costs and time required for the cheapest method to be used be worked out.
A further advantage gained from the urea addition is that the urea will raise the pH of the liquor.
I usually add urea until the instrumental pH is 1.5, this corresponds to actual pH 2.5.
If precipitant is added at this pH then the gold drop will be much less subject to copper contamination.
Urea does not denox solutions but will destroy complexed nitrates in the solutions, most reactants for free nitric will not destroy these complexed nitrates.
Deano

 

[snoman701]

Why does urea in aqua regia with free nitric remaining not form urea nitrate? (explosive)

 

[snoman701]

I guess a better way to ask, if a solution of AR was lacking copper, or another metal that can bounce between oxidation states (Palladium?), would it be conceivable to accidentally form an explosive compound?