Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning 2017-04-21T02:33:06+00:00
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Refining of an alloy of gold and silver (5 replies and 2 comments)

mine master
2 years ago
mine master 2 years ago

Hi, everybody

I want to separate an alloy of gold and silver with gold content between 25% and 92% Au.

As you know, in the alloy of gold and silver with gold content of less than 25%, Ag can be dissolved with sulfuric Acid or Nitric Acid.

In the alloy of gold and silver with gold content of more than 92%, gold can be dissolved with Aqua regia but when silver is more than 8%, the gold cannot be dissolved (because a layer of AgCl is formed around the gold).

The summary is given in the table.

Process Alloy Compound
Au (%) Ag (%)
Dissolve Ag with two Acids 0
25
100
75
low Au grade
??? 30
50
75
90
70
50
25
10
(26 – 91) % Au
Dissolve Au with Aqua regia 92
100
8
0
High Au grade

Now what is your suggestion?

David
2 years ago
David 2 years ago

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David
2 years ago

Have you searched and asked on http://goldrefiningforum.com/phpBB3/

David
2 years ago
David 2 years ago

Hi, I’m guessing you wants to know what to do with the stuff in the middle. You could do one or more of the following:

  • Chlorinate if he has that capability (Miller Process
  • Crush and grind the material to increase surface area of gold, reducing the possibility of AgCl passivation and then dissolve 
  • Electro refine it twice (very expensive) 
  • Dissolve Ag with nitric first then gold (I would do this after crushing)

Also see http://www.slideshare.net/MGSRefining/how-to-make-pure-gold and http://goldminingandprospecting.blogspot.ca/2012/02/miller-process-for-refining-gold.html

Also the original Miller Chlorination Process Patent https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US2997508.pdf of Wax!

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mine master
2 years ago
mine master 2 years ago

Thank you so much, my friend. I appreciate your help.

But the problem is that Ag is not dissolved with acids when silver is less than 75% !

Can adding silver be a solution?

David
2 years ago

i have not idea.

David
2 years ago
David 2 years ago

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David
2 years ago
David 2 years ago

The Miller process consists of injecting chlorine into the charge through a clay tube connected to silver tubing that is connected to the chlorine source. About 0.2 kg (0.5 Ib) of borax glass is sprinkled on top of the bullion to prevent volatilization. The chlorides begin to float on top of the charge. With a No. 10 assay crucible, the chlorides are skimmed off and borax replaced on the charge and the operation continues. Usually after the second or third skim a brown gas will be noticeable, indicating the chlorine is not being consumed and the process is near completion. The charge is skimmed and borax sprinkled on top and 0.2 kg (0.5 lb) of sodium chloride added. The crucible is covered with a clay plate containing a slot for the clay tube. Chlorine is slowly bubbled through the bullion for one hour and the borax and salt are skimmed off. A pinch of bone ash is added to absorb any remaining chlorides and the bar is poured into a graphite mold. Bar sizes range from 15.6-17.1 kg (500-550 oz) and will be 997-999 fine. These bars are cooled, cleaned and recharged into 128 kg (281 lb) capacity induction furnace. They are remelted and poured into anodes. The anodes, weighing 6.2-7.7 kg (200-250 oz), are sent to them electrolytic mom for Wohlwill processing. The anodes are 150 mrn x 190 mm (6 in. X 7.5 in.). They are placed in Coors stoneware electrolytic tanks containing 80-100 g/L (10.6-13.3 oz per gal) gold chloride solution. The solution also contains 15% excess HCL for proper dissolution and deposition. Cathode starter strips are rolled from previously produced fine gold and are 100 x 305 x 0.5 mm (4 X 12 X 0.02 in.). Direct current is supplied by Udylite rectifier at 0.5 V and 100 amps. A small amount of air is bubbled through each cell to give a stirring action and prevent short circuiting. The finished cathodes are washed, cleaned of any flakes of mechanically deposited gold, dried, melted in an induction furnace, and poured into good delivery bars. Bar fineness is 999.9. 

Refining Silver Cathodes: At the Creede plant the silver plated steel wool cathodes are removed from the electrowinning circuit when they contain 47 kg (1,500 oz) of silver. A considerable amount of silver sludge is present in the bottom of the cathode baskets. The sludge and half of the steel wool cathode is placed in a Denver fire clay tilting furnace with a No. 150 Starbide crucible. If even mixing of the fluxes is not accomplished there is a possibility sodium nitrate will oxidize the base metals, but there is not enough borax and silica to carry the base metals to the top of the melt. When the melt is poured, there will be nodular pieces of base metals and silver together on the interface between the precious metal bullion and the slag. The flux consists of 13.6 kg (30 lbs) of sodium nitrate, 13.6 kg (30 lbs) of anhydrous borax and 13.6 kg (30 lbs) of silica sand.

The tilting furnace is maintained at 109S°C (2000°F) for one hour. The slag is poured into a cone mold and the silver poured into a cast iron mold. Each charge is handled in the same way. The silver bullion at this point is 950 fine and the slag contains 343 g/t (10 oz per st) of silver. The slag from the melting furnaces are returned to the grinding circuit. 

http://www.onemine.org/document/abstract.cfm?docid=7930

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