# Recovery of gold from PGC.



## goldsilverpro (Nov 14, 2009)

Recently, I wrote up this process, in another forum, for the recovery of gold from PGC (potassium gold cyanide) salts.
http://www.finishing.com/535/11.shtml

In case you're interested, being an old plater, I have made quite a few posts on that forum. Some involve plating and others involve PM recovery. Actually, there are quite a few other threads on there that involve PM recovery. To find my stuff, go here. http://www.finishing.com/ At the bottom of this page, click on Search Engine. In the slot that says, Search the forums & articles, type in Chris Owen. For other articles, type in something like: refine gold. Finishing.com is the premier plating forum on the internet. There's lots of good stuff on there, much of which is written by professionals. 

I have outlined this process before, but I thought a little repetition wouldn't hurt. PGC is the most common gold plating salt. It is used in both acid gold baths (pH usually about 4.3 - 6.0) and cyanide gold baths (pH of usually 10.3 or higher). Most all gold plating on electronics is done in acid baths containing PGC, which also often contain a little nickel or cobalt as a hardener. The cyanide baths are usually used in the jewelry industry, although they also use acid baths.

This process can also be used for any acid or cyanide gold baths, already in solution form, that contain PGC. Before starting, however, make sure that any solids are dissolved. This can usually be done by heating and stirring. Solids that have formed over time usually contain gold. Also, it can be used for cyanide/H2O2 or 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. I discussed the sulfite baths here:
http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=34&t=4363&hilit=sulfite+plating+bath
http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=34&t=4641&p=43665&hilit=sulfite+plating+bath#p43665 

My statement in the first paragraph was made because of the possibility that the PGC was stolen, usually by employees, from companies that did gold plating. This happens more often than you would think.

*"*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 purest 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) To 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, 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.*"*
(3) 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 was present, it will also be leached out with the nitric. Step (11) is done to insure all the zinc is leached out.

Chris Owen


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## goldnugget77 (Nov 14, 2009)

Hi Chris
Thanks for that helpful post
This is worth saving in my files
Good for you.


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## Irons (Nov 14, 2009)

Could you treat the residual Cyanide solution with Ferrous Sulfate to make Potassium Ferrocyanide. This could be treated with the addition of Ferric Ion to precipitate Prussian Blue.

http://www.answers.com/topic/ferrocyanide

"The [Fe(CN)6]4− ion is very stable, with the CN− ligands adopting the octahedral geometry typical of FeII complexes. The free acid hydrogen hexacyanoferrate(II), H4[Fe(CN)6], is soluble in water and is isolated as a white powder by ether precipitation of the ion in strongly acidic solutions. The salts of sodium, Na4Fe(CN)6, and potassium, K4Fe(CN)6, are prepared by heating an aqueous mixture of sodium cyanide (NaCN), iron(II) sulfate (FeSO4), and other salts. The complexes are substitutionaly inert (in contrast to ferricyanide, "

This would have to be done before the addition of Nitric acid, as ferrocyanide is a powerful reducing agent and reaction with oxidzers such as metal Nitrate, Nitrite, Chlorate or Perchlorate could cause an explosion.

http://en.wikipedia.org/wiki/Potassium_ferrocyanide

Interestingly, Potassium Ferrocyanide can reduce copper, so it most likely can reduce other precious metals as well.

I wonder if adding Ferrous Sulfate directly to the PGC solution would precipitate the Gold directly.


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## Lino1406 (Nov 20, 2009)

In my opinion Ferrous sulphate will
precipitate KAu(CN)2 by eliminating KCN


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## Irons (Nov 20, 2009)

Here's an ancient Gold plating bath using Ferrocyanide:

http://www.finishing.com/233/01.shtml

Ferrocyanide gold bath(late 19.century):
2,65 gm gold chloride
15 gm potassium ferrocyanide
15 gm sodium carbonate
1 lit water
Stainless steel anode,2-6 V/o,5 A/dm2


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## nickvc (Nov 22, 2009)

GSP ive always used those same methods but used hydrochloric instead of nitric,would you say nitric is a better acid to use?


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## goldsilverpro (Nov 23, 2009)

About the only thing readily soluble in HCl is the excess zinc. HNO3 is more of a universal solvent. If other metals, such as Cu, Ni, or Ag were present in the cyanide solution, they would have been precipitated by the zinc, along with the gold. Nitric does a better job of removing them away from the gold. 

With pure PGC (no Ni or Co), since gold and zinc are the only 2 metals involved, it makes no difference what you use - HNO3, HCl, weak H2SO4. 

With stripping solutions, there is bound to be copper and nickel mixed with the gold and, therefore, HNO3 would be the best choice.


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## nickvc (Nov 24, 2009)

Thanks GSP just checking that i hadnt been given the wrong advice,and i agree that with anything but pure GPC it would be far better treated with nitric because as you stated its going to have all sorts of metals in the mix.Hope that your new venture is going well , all your free advice and insight im sure has been imvaluable to most of the forum members.


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## Franciz (Feb 21, 2012)

Dear Chris Owen,

After I have read your post, I have dissolved 48grams of KCN in 400ml of dilute water. I then slowly put in about 3Lb of misc gold plated scrap and give it a good stirred and soak for awhile after about 10 minute, All gold was gone left only copper and stainless steel base. The solution turn Yellow i filtered those misc item using a stainless flour sifters and slowly rinse those misc item with 100ml of dilute water to get any yellow solution down to the breaker together with the 400ml solution. So I have a total of 500ml of gold solution.

This is the first time i am going to precipitate gold from KCN so i think I need your help and your advices before i continue to prevent creating a mess...lol...

From what i understand from your useful post;

1.To prevent the very dangerous evolution of hydrogen cyanide gas, First i 
must first adjust the pH not less than 12 and also not too much over 12 with potassium hydroxide. Could you let me know should i use the potassium hydroxide in pallet forms or in solution form and how many % the potassium hydroxide must be?

2.Before i precipitation the gold, I may put in some extra potassium cyanide into the solution to make the gold precipitate more faster and easier.

3.For precipitation i must first stir the solution very very well before adding the zinc, With constanting stirring add sift Zinc 325 mesh using a flour sifters. I should be adding in the zinc in small volume if i do not know how much gold will i get in the solution and observe the color and add in abit excess zinc which is harmless and i should allow the zinc to work until i saw a grey precipitaion. If after couple minutes i still not yet saw some grey precipitaion means i may slowly add the zinc in until i saw a grey precipitaion.

4.After filtered out all the solid precipitaion, I have to make sure i have carefully rinse and removed any cyanide solution to prevent any toxic gas in next step when purifying.

I am trying to let you know what i understand from your post and i may be wrong. Please advice me if any of the part you think i am wrong or need to improved.
By the way, May i know what is the percentages of the gold that can be recovered by using this method if done properly?

Thanks for spending your time and effort to share this wonderful post.
Update that i miss out above, I have also add in 10g of gold stripper powder to the solution together with KCN.

Franciz


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