Separation of Gold metal from Potassium Gold Cyanide salt

Dear sir,
if you need good purity then you go for aquaregia process.

A. Hi, Gorgolino. I'm not confident that I fully understand your question, but you can electroplate some of the gold out of the solution in your standard plating tank. When the gold level drops to where the process is inefficient, you can use a specialty electrolytic cell or ion exchange. Good luck.

Regards,

A. Gold Potassium Cyanide is significantly more valuable than the gold it contains due to the rather specialised process required to manufacture it. If you have GPC, sell it to a plating company and use the cash to buy gold if you need to.

A. About the only people I can think of that would legitimately want to recover gold from PGC are refiners. However, I am going to assume that you have honest intentions in wanting to do this.

In its purist form, which is usually used in acid gold baths, the PGC will contain about 68.3% gold. If it is used for cyanide baths, it might also contain some potassium cyanide, which would lower the gold percentage. If it is used for acid baths, it could also contain a little nickel or cobalt and will have a green or pink color. The presence of these won't interfere with the recovery of the gold.

(1) Dissolve the PGC in water. At room temperature, you can dissolve about 143 grams of PGC in a liter of water. This would contain about 3 troy oz of gold, if it is the 68.3% variety. Since the solubility of PGC varies considerably, based on temperature, I would only dissolve about 2/3 of that in the water or, about 2 oz of gold per liter.

(2) Too prevent the very dangerous evolution of hydrogen cyanide gas in the following steps, first adjust the pH to 12 with potassium hydroxide. Try not to exceed this pH, since too much hydroxide will increase the amount of zinc needed in the step below.

(3) If the PGC is of the 68.3% variety, it would help the gold precipitation to also dissolve about 15 grams of potassium cyanide per liter in the solution. Without the extra cyanide, you may or may not have a problem dropping the gold. The gold may finally drop, but it will be slower. It is best to add the extra cyanide.

(4) With constant stirring, add 325 mesh zinc powder until all the gold has dropped out. This should take from 1 to 2 oz of zinc per oz of gold. The gold will drop out and appear as a brown powder. To insure all the gold has dropped out, you'll need a slight excess of zinc, which is of a gray color. Add the zinc in increments, allow it to work for a couple of minutes, and observe the color. When the observed powder is of a uniform gray color, all the gold should be out.

The zinc powder tends to clump when stored and must be broken up before adding. The easiest way to break it up is to weigh out what you think you'll need and place it in one of those small flour sifters that has a squeeze handle on it. Add the zinc to the solution directly from the sifter.

(5) Allow the powder to settle overnight. To prevent any gold from re-dissolving, hang a couple of zinc bars in the solution.

(6) Either siphon or dip off most of the solution without disturbing the settled solids.

(7) Filter the solids and rinse several times with hot water in order to remove any cyanide solution.

(8) Remove all the solids from the paper and put them into a plastic bucket. Keep the paper(s) for future incineration.

(9) Cover the solids with water and stir a bit to break up the solids and create a slurry.

(10) UNDER A FUME HOOD!, add a small amount of concentrated nitric acid. You should see an immediate reaction. When the reaction subsides, give it a stir and add a little more nitric. Repeat until an addition of nitric produces no reaction. You MUST use a fume hood for this, just in case you didn't rinse out all of the cyanide. The combination of acid and cyanide will produce deadly hydrogen cyanide gas

(11) Transfer the solids to a beaker [beakers on eBay or Amazon [affil links] , cover with water, and heat to a little below boiling. Add some nitric and heat for about 15 minutes.

(12) Filter the solids and rinse several times with hot water. Remove the solids from the paper and slowly dry.

(13) At this point, you should have 99+% gold powder. To purify it further, you can use the standard aqua regia method.

NOTES:
(1) Wear rubber gloves and a full face shield for all operations.
(2) In the above, I suggested using Potassium hydroxide and Potassium cyanide because the solution is Potassium to start with. I think it would work as well using Sodium hydroxide and Sodium cyanide instead.

A few additional notes on the process I outlined above.

The purpose of the nitric acid, in steps (10) and (11) is to leach out all of the excess zinc. Also, if any Ni or Co is present, it will also be leached out with the nitric. Step (11) is done to insure all the zinc, etc. is leached out.

This process can also be used for any acid or cyanide gold baths, already in solution form, that contain PGC. Also, it can be used for cyanide/H2O2 or most cyanide/m-NBSS gold stripping solutions. In these cases, just start at step (2). For cyanide baths or gold strippers, eliminate step (3).

The process will NOT work for baths containing sodium gold sulfite. These sulfite baths are rarer than PGC baths, but there's a lot of them out there.

Hi, Chris. Thanks for the terrific exposition. I'll only reinforce your warning that these instructions are for trained chemists who fully understand cyanide chemistry and toxicity. The internet is a giant one-room schoolhouse where people will overhear instructions for operations that they are not trained, equipped, or situated to safely do.

While a library is a similar one-room schoolhouse, people tend to read the book not just open books to a random page and proceed :-)

Regards,

A. Hi, Gareth. As you see, we appended your inquiry to a thread which should answer it for you: 1) The GPC may be worth more that the gold it contains, per Geoff Smith's posting. 2) The gold can be recovered, per Chris Owen's posting, but only by trained chemists with deep knowledge of cyanide chemistry.

Best of luck.

Regards,

A. Hi cousin Akash.

Chris Owen has advised us that GPC is 68.3% gold. You will find that you can verify this number in the "Metal Contents of Common Plating Salts" appendix in the Metal Finishing Guidebook, rather than having to rely on an internet posting.

If you'd like to learn how to fish rather than where to buy a fish, the formula for GPC is KAu(CN)₂, and the respective atomic weights are --
K: 39.096, Au: 197.2, C: 12.01, N: 14.008.  So, %Au = 197.2 / 288.332

Luck and Regards,

A. Hi Rich. The big question is whether this bottle was ever opened. If not, it presumably has a troy ounce of gold in it, so should be worth over $1000 and worth a plating shop's time to consider (assuming they are not doing ISO or other spec work which demands traceability). If your calls to plating shops haven't worked, you might list it on eBay or Craigslist.

If it has ever been opened,there's no telling what's in it . . . maybe a solution so exhausted that the previous owner didn't want to pay the disposal fees. You might try one of those "We buy gold" stores that are on every town street corner these days. Good luck.

Regards,

"Recovery and Refining Of precious Metals"
by C.W. Ammen

on AbeBooks

or eBay or

Amazon

(affil links)

A. Hi Vivek. Theoretically, yes, gold sulfide is precipitated. But there are probably practical reasons why the Merrill-Crowe process, electrowinning, and carbon in pulp are the principal methods instead. I don't know much about the subject, but it seems to me that precipitating a gold sulfide this way is going backwards from the desired direction of refining the gold :-)
Have you consulted any books for practical process design of gold recovery?

Regards,

A. Hi Kamalesh. We appended your question to a page where it's already answered. Good luck.

Regards,

A. Hi Arshad. This is a job for a refinery, not a pharmaceutical lab. But if you are saying that you are a student with a student lab assignment, you could try putting two stainless steel electrodes into the solution, and apply a very low voltage (like 0.1 volts) and the deposit should be primarily gold.

As a matter for science class, what you would then do is dissolve the metal you have plated and plate it out again. The second time you would be starting with a solution that was richer in gold than the first time, so the second low voltage deposit would be purer than the first. You could continue that process until you had pure gold. Presumably you are versed in the serious dangers of working with cyanide. Good luck.

Regards,

Hi again. Sorry, I have no experience in refining and cannot suggest what steps can be improved. My initial answer was simply based on my electroplating experience, thinking you might be a student doing a science project.

Regards,

A. I'd follow Ted's advice about plating out the gold at very low voltage. Then, I'd oxidize the cyanide with hypochlorite or hydrogen peroxide, cautiously acidify with nitric acid (fume hood a must for this) then add sodium chloride. That will precipitate the silver as the chloride, from which it may be recovered by roasting with charcoal. If you wish to recover the copper, add some zinc dust ⇦this on eBay or Amazon [affil links] , it'll precipitate.

A. Here's how I would do it:

(1) Precipitate all the gold, silver, and copper with 325 mesh zinc dust ⇦this on eBay or Amazon [affil links] . Since it is best to use a slight excess of zinc, there will also be some undissolved zinc mixed with the other metals.
(2) Filter and rinse the solids very well in order to remove the cyanide solution. I prefer hot water rinses.
(3) Transfer the solids along with the filter paper to a large beaker. Place on a hot plate under a fume hood.
(4) Cover the solids with distilled water and heat to about 50 °C. Add 70% nitric in very small increments. With each addition, allow the reaction to subside before making another addition. Repeat. When an addition of nitric produces no further visible reaction, the zinc, silver, and copper have dissolved, leaving the gold as a solid.
(5) Dilute with about an equal amount of distilled water. Filter out the gold powder (plus the pulped filter paper) and rinse it well with distilled water. Dissolve the gold with aqua regia, filter, rinse well, and precipitate the gold with your favorite precipitant (most commonly, ferrous sulfate ⇦this on eBay or Amazon [affil links] or sodium metabisulfite).
(6) The dilute nitric now contains zinc, silver, and copper. Cement the silver selectively, as a solid, by suspending copper bars in the solution until all the silver has been removed. The solution can be tested for silver by removing a drop, placing it in a spot plate or a plastic spoon, and adding a drop of table salt solution or hydrochloric acid (HCl) to it. A white cloud indicates that there is still silver in solution. In that case, leave the copper bars in the solution until the solution tests negative for silver.
(7) Filter out the silver powder and rinse well. At this point, zinc and copper remain in the solution. The copper is then cemented out of the solution with scrap iron. If this goes too slow, add a few drops of HCl to the solution. Once all the copper has cemented, the only thing left in the solution is zinc. Also, the green or blue color (from the copper) will be gone and the solution will be close to colorless.

Dear Chris,

It was simply marvelous you defined. Thanks.

Kind Regards,
Arshad

A. Hi cousin Waqas. The problem, of course, is that "an uneducated boy" should certainly not be working with cyanide at all -- even more so a boy whose health has already been compromised by it! But here in the USA, poor uneducated boys often deal illegal drugs, which is probably equally dangerous ... what can either of us do except try our best to build a better world where boys don't do such things. We clearly cannot teach uneducated children how to deal drugs slightly more safely, and we certainly should not try to teach sickly, uneducated children how to recover gold from cyanide wastes slightly more safely.

As you see, we appended your question to a thread which discusses it. Best of luck.

Regards,

A. Hi Zhakeer. Chris Owen already explained one way, but it may be better to take it to a skilled and trusted plating shop or jewelry plater; they will be able to electroplate it out as gold metal or hopefully even just buy it from you for its gold value. Good luck.

Regards,

A. zhakeer,
Before the gold is plated out or zinc'd out (see my procedure above), you would have to dissolve it in either a sodium cyanide or potassium cyanide solution. In both plating and zinc-ing, you should use a little extra cyanide, maybe about 15 g/l extra, beyond what it takes to dissolve the AuCN.

For sodium cyanide, it will theoretically take 49 g in solution to dissolve 223 g of AuCN. For potassium cyanide, 65 g for 223 g of AuCN.

A. Ted's idea is by far the best. Even if you are skilled in lab work you will not get 100% recovery; and if not it could be far less.

A. Hi Zakeer ,
You please add sodium hydroxide to increase pH, then add zinc dust into your AuCN solution, be careful, slowly add Zn because of vigorous reaction under fume hood; kept for whole night. Collect residue, wash it by hot water, dry it in furnace, then treat for aqua regia treatment.

A. Hi Erinda,
Solution obtained during bombing process, increase pH level of the solution and then add base metal for reduction process. You will get cemented silver as well as gold.

A. Bombing solution gold recovery:

Raise pH of solution to 13 with sodium hydroxide or caustic liquid.

Agitate the solution to raise all of the insolubles into the solution.

Add approx. 4 ounces of Sodium Hydro-sulfite for every ounce of gold expected in the solution. Sodium Hydro-sulfite is also known as Sodium dithionite, it is a white crystalline powder which may have a yellowish coloration. It has a slight sulfur smell.

Agitate well and allow the gold to settle.

Test the remaining solution for complete precipitation with additional sodium hydro-sulfite. This is easiest to see if a small half pint sample is placed in a clear beaker. The presence of gold is indicated by a black or brown color which is the remaining gold precipitating.

When all of the gold is precipitated and settled decant the cyanide liquid and set it aside for cyanide destruction.

Filter the sludge into a Buchner funnel and suck it dry.

Add the filtered liquid to the collection of cyanide solution awaiting treatment.

Burn the paper and heat the solids with a torch until they crumble and glow red. This incineration process is important to destroy any cyanide and prepare for acid digestion.

If the jewelry bombed had significant solder it is very likely you will have tin in the incinerated residues. If you do and you put the incinerated residues directly into Aqua Regia, the resulting liquid will filter VERY slowly. If this happens you know why and next time you will add the next 2 steps to remedy this.

If you suspect tin;

1) Place the incinerated residues in a beaker and cover with a few inches of hydrochloric acid until the material is covered a few inches with the acid. Heat the beaker with the residues and acid on a hot plate until it is just about to boil and keep it hot for half an hour.

2) Again filter sludge into a Buchner funnel. Rinse well, the tin will have dissolved in the hydrochloric acid and is now rinsed free of the material you need to refine.

( I do not know why this happens but it has been my experience that jewelers who use bombing to polish are consistent. A jeweler that has solder in their solution always seems to have it and one that doesn't have any never seems to get any tin causing problems. I have seen this in different cities all over the US. My suggestion is if you feel you want to skip the steps, do it and see if the customers lot filters slowly and if it does, next time you know. I just find it easier to incinerate and leach every time. )

Now the material is put in aqua regia. It will dissolve and all of the insolubles (which may contain silver) are on the paper when you filter it so place it in the burnables for recovery bucket.

From here on out it is refining the usual aqua regia way.

A few safety precautions;

Cyanide and acid do not mix, in fact it is deadly. It is best to do this process in a room where you have no acid in use.

To legally deal with cyanide from Jewelers you need an EPA Hazardous waste transporters license although if it is delivered to your facility you can treat it.

If you are lucky enough to have an incinerator you can just evaporate the liquids and destroy the cyanide with heat.

Cyanide can also be destroyed with bleach ⇦ bleach/sodium hypochlorite in bulk on eBay or Amazon [affil links] . Additions of Sodium Hypochlorite to a strongly alkaline waste will destroy cyanide. Adding the bleach with mixing and testing with chloride test papers until the papers indicate a presence of chlorine tells you it is done. Mix it one more time and retest in 1/2 hour. If it still shows chlorine on the test papers it is OK to lower the pH to 7 and discharge if it meets city discharge standards.

I have never seen simply destroying the cyanide make the waste suitable for discharge anywhere in the US. There is often copper and silver and zinc enough to be an issue. At the neutral point, after chlorination it is safe to evaporate to a sludge which can go with your burnable wastes but not for discharge without a lab approval that it meets standards.

When figuring what you can charge for refining bombing waste and treating bombing water, keep in mind the waste you have to deal with on the tail end.

A. Hi Chad
Traditional fire assay by cupellation is accurate but requires a furnace etc. and a lot of prep work. It is usually only applied to solid samples.
The common gravimetric analysis involves boiling a sample with concentrated nitric and sulfuric acids to destroy the complex and weighing the residual gold on a tared sintered glass filter. A fume cupboard is essential as cyanide gas is evolved and boiling concentrated acids should not be considered without adequate training so I will give no more details.
However, I cannot understand your objection to using AA. Without considerable lab skills AA results should be as good as any other method and much safer. It is almost certainly the method that the reclaim lab will use.
If you don't trust the reclaimer (or even if you do)you should retain a sample of each batch to resolve any future dispute.
But make sure that the sample is truly representative and make sure that the reclaimer knows that you are doing this. Don't expect that the gold return will always be exactly what you have predicted as there are many variables like how precicely did you measure the volume submitted for reclaim, did you allow for any solids etc..

A. Hi William. If the bottle is still sealed you may be able to find a local gold plating shop who will buy it from you. If not, you can probably send it to a refiner for recovery.

If it is open it may be nothing but a hazardous waste. If you have no instrumentation, and no haz-op training and equipment, it can be hazardous as it is probably a cyanide-based product. Actually converting it to solid gold is a many step process. But you can search the site for "recover gold from plating solution" and you'll see more than a half-dozen threads on the topic if you don't want to send it to a refiner. Good luck.

Luck & Regards,