Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning 2017-03-23T09:55:18+00:00
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Buildup on Anode (3 replies)

3 weeks ago
CA 3 weeks ago

We've been successfully running a small scale alcohol (isopropyl) zadra stripping process for a while now.  

We've begun a carbon stripping service for clients and today ran into something we have never seen.  During the electrowinning process, a black scale formed on our stainless steel punch plate anodes.  The anodes were completely pitch black. 

Meanwhile, the cathodes which are stainless screens also loaded with a Cu/Ag/Au mix that was so difficult to take off that in the end, we had to reverse plate them.  This is also not normal for us because we usually clean off the screens with a toothbrush and a steel wire brush in a matter of a few minutes.

During the reverse plating, the black element dropped off of the punch plate and loaded onto the screens, which were then technically the anodes.

The elution solution makeup was the same as we always use except that we used ethanol instead of isopropyl alchohol.  The vots/amps were also the same as normal.

The only other variable that I can think of was that the leaching agent used by our clients was not cyanide, but rather another leaching product that is on the market with the following make up: 

Sodium oxide = 35 - 50%

Nitrogen= 12 -20%

Ammonia = 7-12%

Calcium = 1-5%

Iron = 1-5%

"water insoluble substances": = 3-8%


Does anybody have an idea as to what loaded onto the anodes?  Thanks for your comments.  

3 weeks ago
Deano 3 weeks ago
2 likes by Colette and David

The leach sounds like one of the Eco-goldex leaches, effectively a cyanide leach with added extra complexants and oxidisers. This leach will complex many metals and is a pain to strip from carbon as you have found out.

You are also finding out why the leachant is not particularly good for commercial use, the user has shifted the recovery problem to your stage.

You can improve the strip by giving the carbon an extended contact with conc HCl and a good rinse before stripping.

Use an XRF unit to find out the composition of the deposits.

3 weeks ago
CA 3 weeks ago

Thanks Deano.  The leaching agent can be found here for reference; it is a Peruvian product:  

Yes, after reading MSDS of Eco-goldex, the products look very similar.  I will try the extended HCl washing and see if that improves the recovery.  

We finally finished cleaning off the cathodes, but we had to bathe them in nitric acid as the reverse plating was taking way too long.  Thanks for the info.  


Jorge Ganoza
3 weeks ago
Jorge Ganoza 3 weeks ago

I think the leaching agent is responsible to form the black deposit on the anodes. The oxidants and electrowinning process can form a black deposit on the stainless steel. There are several secret recipes used to color stainless steel. The parameters and reagents used in the process help to produce the deposit.

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