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.
