# How to Recover Gold from the Gold Stripper



## studentmetallurgy (Sep 8, 2016)

I have stripped the gold from gold plated pins of brass with a gold stripper with the following ingredients:
Zinc tetracyanide
Meta-nitrobenzene Sulphonic acid sodium salt
Sodium Hydroxide

Please suggest what would be the best chemical or chemicals to precipitate elemental gold from the above mentioned gold stripper.
Thanks!


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## goldsilverpro (Sep 8, 2016)

studentmetallurgy said:


> I have stripped the gold from gold plated pins of brass with a gold stripper with the following ingredients:
> Zinc tetracyanide
> Meta-nitrobenzene Sulphonic acid sodium salt
> Sodium Hydroxide
> ...


First of all, zinc tetracyanide is an ion in solution and not a compound. As a compound, it is probably potassium or sodium zinc tetracyanide.

Second, I see no way that the formulation you indicated will strip gold. The ingredients in your stripper are probably:
Sodium or Potassium cyanide
Meta-nitrobenzene Sulphonic acid sodium salt (often called m-NBSS)
Sodium Hydroxide

The standard method of precipitating gold from this solution is by using 1 to 2 grams of zinc powder (325 mesh "zinc dust" is best) per gram of gold, with a lot of stirring. I would first test this on a small amount of solution - say, 100ml. Normally you use enough zinc to drop out all the gold plus a little extra zinc. Any silver or copper will drop with the gold. Then you filter, rinse very well, and treat with an acid to remove the excess zinc. I prefer 50/50 nitric because it will also dissolve the silver and copper. Finally, filter, rinse well, dissolve the gold in AR, precipitate with SMB, filter, rinse well, dry, and melt.


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## studentmetallurgy (Sep 8, 2016)

Thanks for your reply.
Yes it is sodium zinc tetracyanide
Potassium cyanide
m-NBSS
Sodium Hydroxide
I stripped gold with this solution and tried to recover it with zinc dust. But it takes a lot of time. And the recovery is not hundred percent. I have heard there are some chemicals also with which you can recover gold from this stripper.
Please advise.


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## anachronism (Sep 8, 2016)

Your problem is twofold.

You have MNBSS in your leach solution which combined with the free cyanide is fighting with the zinc for the gold. That's a very "lay" way of putting it however in essence you are doing the same thing as trying to precipitate the gold from a pregnant AR solution that still contains free Nitric. 

You precipitate gold, and the leach solution re-dissolves it. 

The reason I say your problem is twofold is down to the two problems you have. Free cyanide, and oxidiser. You need to get rid of one or both to get all your gold. How do you know for certain you have gold cyanide still in your solution - are you using an AAS that is telling you how many ppm are present? 

There are other ways to recover gold from these solutions without solving the above conundrum. Namely activated carbon and an electrowin cell. Neither requires you to have a non active leach. Both of these are searchable on the forum. 

I hope that helps.

Edit: Something else. What's the purpose of the NaOH, is it to convert the original compound to free cyanide? I have no experience of that however I do know that your NaOH will also be reacting with your zinc so it can be added to the points above.


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## goldsilverpro (Sep 8, 2016)

I have zinced gold out of a NaCN + m-NBSS solution many times. If you start with too much m-NBSS, it can cause problems. I wouldn't use over 10 g/l with about 30g/l NaCN, at, say, 50C. The sodium zinc tetracyanide was probably added to inhibit the attack on the copper, but it causes zincing problems and I would prefer not to use it. The formula you're using is probably designed for stripping/re-plating situations, where any attack on the copper would be objectionable, and not for stripping/refining. I would use only enough NaOH to raise the pH to 12 and no higher. It's in there to prevent the formation of HCN, but a pH of 12 is plenty high for that. 

The only zinc I recommend is 325 mesh "zinc dust" without any added silica (often found in commercial 200 mesh zinc) to prevent clumping. With 325 mesh zinc dust, there will be clumping. Put it through a fine sieve before using.


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## nickvc (Sep 9, 2016)

As has been pointed out you have a problem which will be a lot worse if you do not have access to assays for the solution as there is no easy way to be sure you have removed all the gold from your solution.
You could try heating the solution to as hot as possible and then use zinc dust but you will need to agitate the solution and possibly repeat the process several times, the best way forward as anachronism pointed out is to use activated carbon which will remove all metals and leave a solution which can be used again to strip more product.


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## studentmetallurgy (Sep 9, 2016)

All of you have really been very kind. Thank you all for your suggestions!


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## styphy (May 18, 2019)

I had to process about 10,000 litres of gold stripper containing anywhere between 0.5 and 3 g/l.
It was the result of 30+ years in the gold plating industry, we had tons of gold bearing copper wire, the easiest way to remove the gold was using one of those cyanide/oxidiser gold strippers.
In the early days we plated it out, used aluminium and zinc, all had some success but always
had difficulty removing all the gold and produced a horrible orange precipitate that made gold
recovery harder.
Ironically I recieved a method to process gold strippers 20 years earlier but never tried it, it sat in a folder
for 20 years forgotten about until one day recently I was looking for something else and came across it.
I had been trying to process some stripper and was finding it hard to establish a reliable method of extraction.
The problem with cyanide strippers as I have seen in other articles is the oxidising agent, it has a strong affinity for the gold, when adding zinc dust the zinc and the oxidiser are fighting one another for the gold, the gold may drop out but then can redissolve. The key to these strippers is to weaken the oxidisers grip on the gold. 

To do this you add Hydrochloric acid to a PH of 4. You MUST do this under extraction!! 
Heat the stripper to 50 degrees Celcius first and keep it at 50 degrees. Add the HCL slowly, there is a point where it reacts strongly and produces a foam head, adding too quickly during this period and the reaction could overflow the solution and produce a mess as well as potentially losing some gold. I lost some up the extraction system before I realised my mistake. The HCL can be added faster at the beginning and at the end, but there is a period when the PH is getting somewhere in the 7 oto 9 range where it has to go in drip by drip. I used a 20 litre container with a tap, I was doing 500 litres at a time and had to add about 30-40 litres of 33% HCL. I did it over a 24-48 hour period. Smaller batches could probably be done much quicker.
Once the PH is 4 and stably so you can start adding zinc dust. Temperature 50' Celcius.
Add 1 g/l at a time with air or mechanical agition leaving it to react for 1 hour between additions. Re-adjust the ph to 4 again with HCL, the HCL can be added quite quickly now. Add another 1 g/l zinc and keep repeating untill 10 g/l has been added. I found it best to take the time with the zinc additions, adding too quickly didnt precipitate all the gold. Also I added the zinc through a sieve and sprinkled it in slowly. It took a further 20 litres of HCL in total to adjust the PH between zinc additions for a 500 litre batch.
Let the solution settle.
Once 10 g/l of zinc dust has been added you can test for complete removal of gold, Use an AAS if you have access to one.
Another way to do it is to pipette 20ml of the settled stripper into a boiling flask, add 300ml concentrated
Sulphuric acid and 5 ml Nitric acid, boil for 1-2 hours, if there is gold you will see it floating around with gas evolving from it, if there is no gold it will be clear with no precipitate whatsoever. 
Everytime I did a batch and tested after the procedure I have explained above I had zero gold left, the only times I had gold left was in the first batches before I realised I had to add the zinc slower and keep re-adjusting the PH to 4 between additions of zinc.
Once you have established all the gold is out then turn off the heat and allow the solution to cool and settle. Pump or syphon off the top clear liquid being careful not to suck up the sludge and refill with clean water, stir and settle again. Repeat untill you have flushed the sludge well and the PH is 6-7, I did it about 10 times washing it twice a day. You will have to treat the washings, raise the PH to 11 or 12, add sodium hypochlorite untill your Potassium iodide starch paper turns blue and keep adding untill the starch paper stays blue for 48 hours after the last addition of sodium hypochlorite. It will take some sodium hypochlorite to do this, also the first and second washings will heat up, keep the solution cool otherwise you will waste your chlorine. Make sure it stays blue for 48 hours! 
The sludge should look pinky/purplish and be heavy, it can almost be confused for copper, certainly thats what I thought the first time, that is the gold, there should be some grey sludge which is the excess zinc confirming that you have added an excess and removed all the gold.
Add HCL to the clean washed sludge to dissole the excess zinc and flush again as above. Untill your PH is 6 or 7.
After the 2nd flushing you have a couple of options. Send it off to the refiner as is for further refining and payout, OR if you have the skills refine it yourself.

I hope this helps someone out there that is looking for a method that is quite easy, performed as above it will do the job of removing all the gold. It is time consuming but what refining method isn't? The payout at the end makes it well worth the effort! Adding acid to cyanide and the rest of the process has to be done under fume extraction untill the sludge has been washed thoroughly. Wear all safety gear including a respirator.


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## platinumgold1776 (Dec 11, 2019)

Hi guys, I hope you're doing well.

Do you what would be the chemical equations of these reactions?


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## Lino1406 (Dec 12, 2019)

When you add acid to cyanide solution you produce HCN gas, so this should be bubbled through hypochlorite solution, within a filled column and be controlled. On safety grounds it is preferable to use aluminum/zinc on the alkaline solution, thus minimizing the HCN issue to the precipitated part, despite the "mess". The bulk of remaining cyanide solution should go to an authorized site. As for the formulation of gold cyanidation, one option is
2Au + 4CN- + 1/2 O2 + H2O = 2Au(CN)2- + 2OH-


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## platinumgold1776 (Dec 20, 2019)

Sorry, but I can't understand all the process. Could you explain it step by step? On the other side what does 1/2 O2 mean? Thanks for reading.


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## butcher (Dec 21, 2019)

what does 1/2 O2 mean? 
Half a mole of oxygen gas.

Next question what is a mole of oxygen?


http://chemistry.bd.psu.edu/jircitano/mole.html


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## platinumgold1776 (Dec 30, 2019)

Thanks, butcher, now I understand what I thought as nonsense. But I can't understand the steps in the process.


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## Lino1406 (Dec 31, 2019)

If you mean the forming of soluble gold potassium cyanide, the gold bearing material is covered with alkaline cyanide solution and air is bubbled through


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## platinumgold1776 (Jan 12, 2020)

Thanks, Lino.


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