Things which may be of interest to members

[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

 

[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

 

[g_axelsson]

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

Göran

 

[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

 

[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.

 

[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?

 

[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

 

[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

 

[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

 

[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.

 

[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

 

[Lou]

Think it will work on gold plated material?

 

[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

 

[Platdigger]

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

 

[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

 

[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

 

[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

 

[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

 

[Platdigger]

Will this leach any of the pgms?

 

[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?