Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning 2017-04-21T02:33:06+00:00
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Gold removal from cathodes (6 replies and 1 comment)

9 months ago
Charlie 9 months ago

I am having a problem of gold not coming off from the steel knitted mesh in the cathodes in the electro-winning cell. I tried putting the steel knitted mesh in nitric acid before washing it off with a high pressure wap machine but still doesn't come off completely, what could be the cause gold to just stick and not let itself be removed?

9 months ago
ThabisoKwenane 9 months ago

What are the other present ions in your electrolyte besides the typical ones; CN-, Au(CN)2-, Na+, OH- etc? 

9 months ago
Charlie 9 months ago

Hi! I have not looked at that in details.  What would be my options/possibilities depending on what other ions are in electrolyte?

9 months ago
ThabisoKwenane 9 months ago
2 likes by David and Oberfuhrer

I am thinking that you might have used too much cathodic potential to favour plating of other undesired metals thereby entraping the gold on the cathode. Some metals require mechanical stripping once they are plated unlike gold that is simple to strip using high pressurized water. Remember, based on the thermodynamics (Refer to Pourbaix diagram for Au-CN), Gold is favoured to plate first, but once too much potential is applied some metals can be plated considering the kinetics of the system. They might  have initially plated due to mass transport control but once the potential is sufficient it will be by kinetic control. I think you must re-check your applied potential in your cells. 

Well with removing the gold that is left on that cathode, You can use dilute Aqua Regia (3:1 Molar, HCL:HNO3). I am not sure if whether the prepared solution would dissolve the cathode. If it would, I dont think it would at a fast rate as the gold. In that regard, try to immerse your cathode in the prepared solution starting with 30 sec and introduce it to high pressured water and check if it can work. if it does'nt, immerse for another 30 sec, then check. If it works do 1 minute for all those cathodes. Play around with time intervals until you get it right. You can still start by 10 seconds or even 5 seconds since the aim is just to destabilize that plated gold.

9 months ago
Oberfuhrer 9 months ago
1 like by David

I had this problem and simply used very inexpensive steel wool. After the metals (removed from the carbon) were plated onto it, I rinsed the loaded wool with water, then placed it into 72 Liter round bottom glass reactors, sealed the reactors with their lids and clamps, then hooked one of the necks to our urea and NaOH scrubbers, the center neck to the agitation motor, and the other side neck to pressure equalized addition funnels to feed aqua-regia into the reactor. All the steel wool and copper quickly dissolves and generates enough heat to rapidly also dissolve the gold, silver (becomes silver chloride precipitate) and base metals.

Follow the dissolution with chilling and filtration to remove the solid chlorides of silver, lead and mercury, then precipitate pure 999.9 Fine gold with SO2 after you eliminate any excess nitric acid.

Also check your copper concentration (and possibly other base metals) in your pregnant feed and barren. You possibly have a high copper co-deposition rate. Also check that you are not running high cell current, the recommended range is between 30 to 50 Amps per cell (preferably on the high range).

9 months ago
ManUtd67 9 months ago

When you say steel knitted mesh, do you mean mild steel or stainless steel?

9 months ago

very inexpensive steel wool=mild steel

Greg Henderson
9 months ago
Greg Henderson 9 months ago
1 like by David

I agree with Herr Oberfuhrer, check for copper on your loaded activated carbon, it can cause gold to stick to cathodes.  Geologists need educating about copper in gold plants!

If you have appreciable copper present, do a cold cyanide wash step prior to elution (make sure the acid is washed out of the column well first!).   Some mines use nitric acid instead of hydrochloric acid for the wash step which will remove Ca, Mg and Cu all in one go (it might pull off some of your silver too though). 

As for the gold on your cathodes you could try switching them over as anodes to redissolve the gold and redeposit on fresh cathodes at a lower current level.  If copper is not present you can run at a higher current density to make the gold sludge off rather than depositing on the cathodes but first you need to figure out why the gold is adhering.  Some detective work needed!     

I agree with the causes given above Cu is the most common cause and usually plates as a red colour. High current densities are also needed to plate as a sludge. Usual causes of strongly adhering plating if it isn't Cu is passivation of the anodes through not enough NaOH or high hydrogen overvoltage. Check the anodes are clean along with all connections on the bus bars.

Using the plated cathodes as anodes is one way to get the gold off. Another is to circulate cyanide solution with Leachwell or LeachAid through the cell without current as in an Acacia Reactor. Typically Acacia electrowinning circuits have heaters to 70 Deg. C to neutralise excess Leachwell. Turning on the power and the heater should then re-plate the dissolved gold into hopefully a non adherent sludge that can be washed off if the original problem has been rectified.

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