Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

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black substance formed during nitric acid refining (2 replies and 2 comments)

J
John Urban
6 years ago
John Urban 6 years ago

I made a mistake, and started off using nitric acid that was too concentrated. When I refine scrap jewellery from a customer, I calculate how much gold there is, and then melt in enough copper to bring it down to 4 kt. I pour the melt into a bucket of icewater, and it breaks up into small pieces, giving it much more surface area. Refining this should be started with very dilute nitric, and gradually increasing the concentration all the way up to pure nitric. What happened was all the metal turned black, and the refining process was stalled.  I drained it, and experimented by adding liquid ammonia from the hardware store. It immediately reacted with the black mess, turning bright blue, and the black mess then went back to the coppery colour. Then, after rinsing, I went back to refining with dilute, and increasingly concentrated nitric, all the way up to pure, and the refining continued to a successful completion.

My question is: what was the black substance?

 

T
Todd H
6 years ago
Todd H 6 years ago
Like
1 like by David

Likely you made a silver compound like silver nitrate that can be black as it drops silver out of solution.  The blue color is the return of copper nitrate 

The black color is due to the decomposition of the Ag+ ion into Ag, silver. Silver nitrate will slowly decompose with the silver ion reverting to elemental silver. The microscopic particles of silver are so small that they absorb light instead of reflecting it, and so appear black, instead of silver. - One possibility the other is t could be a sulfide of silver if there is sulfur present.

Regards

Todd Harvey - Global Resource Engineering http://www.global-resource-eng.com

J
John Urban
6 years ago

Thanks, Todd. Since there was very little silver in this batch of gold scrap, I think Dave's answer is more likely.

D
Dave Tahija
6 years ago
Dave Tahija 6 years ago
Like
1 like by David

My guess is that is synthetic tenorite (CuO) as that is black and its fast dissolution in ammonia fits the blue solution. As Todd notes, though, very fine particles of silver (or any metal) can be black as well.

J
John Urban
6 years ago

Thanks, Dave. I think your answer is the most likely.


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