Precip. of gold from sulfite plating solution

[inspector071]

I've been assigned with reclaiming some gold from my university's microelectronics lab. I'm pretty new to reclaiming metals, but have always been very interested in chemistry. I've been doing some reading on the forums before making an account, but haven't found much on these sulfite based solutions.

My lab has some old cyanide and sulfite based plating solutions. I believe I understand how to precipitate the gold from the cyanide solutions, but the sulfite ones are giving me trouble. What's the best way to precipitate gold from the sulfite solutions? Will sodium metabisulfite work? Do I need to adjust the pH?

As for the cyanide solutions, they are already basic (pH 11), so my understanding is that all I need to do is add a little excess of powdered zinc. The gold should fall out of solution and the cyanide then complexes with the zinc. A rinse with a strong acid then dissolves any leftover zinc. I did some reading on this, the Merrill-Crowe process, and saw a step mentioning deoxygenating the solution. Is this necessary in a small, lab based setup, or is it solely important in industry?

 

[Lou]

You can get a great recovery just plating onto a steel wool cathode.

Stainless is the anode. 3-4 V, as many amps as wont't give a foam.

 

[goldsilverpro]

I assume the pH of the sulfite is 8-10. First try just a small sample, say 25-50ml (measured). Acidify with 10-20% (v/v) sulfuric aid. I don't know what pH to shoot for - a guess would be 1-2. In the sample, I would keep it stirring and add the sulfuric dropwise (keep track of what it took) until all the gold is out. Unless the modern sulfite baths are totally different than the older ones, all the gold should precipitate as a brown powder with simple acidification. Of course, don't try this with the cyanide bath.

 

[inspector071]

Thanks for the help! The sulfuric acid did the trick. The only issue is that the addition of sulfuric acid clouds the solution so I've got to let it settle before adding more. Then I'll rinse it well of the sulfite, dry it, an melt it down.

Does my cyanide process sound spot on?

 

[niteliteone]

Does my cyanide process sound spot on?

I haven't worked with cyanide solutions, but I would think some cyanide will will be present in the dropped gold powder. I am of the understanding that cyanide and acid do not play nice if the solution goes acidic and creates cyanide gas.

Maybe incinerate the dropped gold powder to remove traces of cyanide before adding any acids to remove the excess zink.

Hopefully one of our experienced members will come in and verify this or correct it.

 

[samuel-a]

Just lower the pH of the sulfite solution to below 4-5. All of the gold will precipitate.
I have recovered gold from dozens of gallons of sulfite solutions over the years and found the best acidifying agent to be HCl 30-32%. It gives packed and adherent precipitate which is easily washed.

 

[inspector071]

It appears all of the gold from the sulfite solutions has precipitated into nice little clumps. I'll filter it off tomorrow and see how much I've got. Is water the best rinsing agent to try and remove all of the sulfite, or would an acidic solution work better?

My cyanide solutions are looking a little strange now. I'm working with several gallons of varying concentrations of gold (1/16 troy oz per liter all the way up to 1.125 troy oz per liter), so I was going to evaporate off some of the water so I could work with a smaller total amount of solutions. I put a few liters of the more concentrated solutions into a crystallizing dish over a hot plate stirrer in a fume hood. I have kept the temperature around 60 celsius for the past 24 hours or so. The solution had remained slightly yellow/clear, but when I checked on it a few minutes ago it had turned an opaque brown. I shined a light through the solution and noticed tiny particles. I assume it is gold, but why has it fallen out of solution as the metal?

I put some of the less concentrated solutions into a pyrex casserole dish over another hot plate stirrer, also at 60 celsius. This solution has started to form a film of crystals across the top of it. Are these likely potassium gold cyanide crystals? Here are some photos of the two solutions.

Should I continue trying to reduce the volume of these solutions, or stop here and try to precipitate using zinc in a 5 gallon bucket?