88 ECONOMIC GEOLOGY 



The zinc may be used in the form of zinc dust, shavings, granu- 

 lated zinc or sheet zinc, but the zinc dust is generally preferred 

 because it exposes a larger percentage of the surface to the 

 action of the solution. The following equation shows the re- 

 action that might take place. 



2KAu (CN) 2 +Zn = K 2 Zn(CN) 4 +2Au. 



The process is applicable to certain free milling ores, to refrac- 

 tory ores, but was designed especially for the treatment of tail- 

 ings which were allowed to flow to waste for many years. Many 

 western plants now have their cyanide plant in connection with 

 their amalgamation plant. 



Chlorination Process. This is not applicable to free milling 

 ores carrying nuggets, but to sulphides carrying large quantities 

 of free gold. The ore is crushed, roasted, weighed and then 

 charged into barrels with 18 tons capacity, 6| ft. in diameter and 

 15 ft. long. Nascent chlorine is the solvent. The solution from 

 the barrels passes to a filter tank for the removal of the sand. 

 From the filter tank it passes to a settling tank for the removal 

 of the fine particles held in suspension; from the settling tank it 

 passes to the precipitation tank, in which is placed zinc ribbons, 

 scrap zinc, or zinc dust. Hydrogen sulphide is sometimes 

 passed into these tanks, and the resulting gold is reasonably pure, 

 but charcoal is the most efficient reducing agent, and the gold ob- 

 tained is 0.995 fine. 



2AuCl 3 +6FeS0 4 = 2Au+Fe 2 Cl 6 +2Fe 2 (30 4 )3. 



Reduction by Sodium Thiosulphate. A solution of Na 2 S 2 3 

 for the extraction of gold and silver has far greater solvent power 

 than potassium cyanide and is non-toxic in its physiological effect. 

 It can be prepared in large quantities at low price according to 

 the following formula : 



2 parts. Na 2 S 2 3 ; 2 parts. CH 3 C0 2 Na; 3/4 part. FeCl 3 ; add 10 

 times the volume of H 2 0. This will dissolve from 15 to 20 times 

 as much gold in 10 hours as a 2 per cent, solution of potassium 

 cyanide, 2 per cent, being the maximum strength allowable in 

 cyanide solutions. The gold can be recovered from the solution 

 by zinc shavings, zinc dust, zinc ribbons, and by electrolysis. 

 The cost of treatment by this process is estimated at $2.75 per ton. 



Reduction by Electrolysis. Gold is readily separated by elec- 

 trolysis from its various solutions, and in this method of treat- 



