Troubles with mix solution of copper and gold

[holocron]

Hello everyone,

Even if I'm studying the art of metal recovery for couple of years now, I have to deal with an issue quite strange. In my process, I get a solution of copper (9000ppm), gold (8ppm) and palladium (10ppm) in aqua regia.

So basically, I tried to put a bunch of SMB to neutralize the nitric acid, decrease the redox potential to precipitate the gold and palladium. But the solution turns red and nothing else.

So, I tried cementation with copper: a nice piece of copper in the solution for many hours and not a single particle at the bottom.

More than a solution (greatly appreciated), I would like to get an understanding of this situation.
Thank you

 

[anachronism]

How many litres do you have?

8ppm gold is 8 grammes per thousand litres of liquid.

 

[holocron]

I have 20L - and made the analysis with a friend of mine (ICP machine)

 

[Dr.xyz]

I feel like you don't even understand what "ppm" means. You have virtually nothing there.

 

[galenrog]

You have, in your 20 liter solution, it your analysis is correct, approximately 0.16 grams gold, and 0.20 grams of palladium. Ask yourself if this is worth the trouble and expense of recovery and refining.

Time for more coffee.

 

[FrugalRefiner]

So, I tried cementation with copper: a nice piece of copper in the solution for many hours and not a single particle at the bottom.

Cementation is the right approach, but it depends on surface area of the copper, circulation of the solution, and time. With a weak solutions, a small piece of copper, and a few hours in a still solution, the values may barely coat the copper.

Dave

 

[Lino1406]

"SMB to neutralize nitric acid" not acceptable. But if you de-nox firstly, then you may be more lucky, at least by enriching the assay, since nature acts in stages where copper is the last.

 

[holocron]

Thank you for your comments.

"SMB to neutralize nitric acid not acceptable" : why? If it works, it isn't a bad idea? Unlikely to urea, there is no safety concerns?

I know my solution is very low concentrated, so yes, 1 ppm is only 1 mg/L. But it's more for curiosity and understanding of the chemistry.

About the cementation, the gold will stick to the copper or it will drop? Because, if it drops, it will be dissolved by aqua regia or the redox potential of the solution is low as far as there is metal copper?
If it is complete low potential solution, why would it be necessary to get a large surface of copper?

 

[g_axelsson]

If you stick a piece of copper in aqua regia you would get a lot of brown NOx fumes. If you don't get that you don't have any nitric left. Gold powder will redissolve as long as you have nitric and hydrochloric acid still in solution.

0.16 gram of gold is such a tiny amount that I wouldn't count on seeing it in 20 liters of solution. It can be suspended as a fine powder, taking days or even weeks to settle. Some of it will be stuck to your copper.
That kind of gold level is what I would put in my stock pot. It will collect on the bottom if you are doing more refining and add more liquids over time.

20 liters... buckets, not beakers? Plastic buckets are porous at the surface and the gold could easily stick to the side of whatever you are using.

Göran