GOLD EXTRACTION PROCESSES. 495 



After a day or two the liquid is filtered off, and the 

 gold precipitated by ferrous sulphate or sulphuretted 

 hydrogen. 



The problem of the extraction of gold contained in 

 pyrites and complex minerals was partially solved by the 

 chlorination process, but the cost of roasting is a stumbling- 

 block in many cases, and it was the desirability of avoiding 

 this which led to the introduction of the use of cyanide 

 solutions for leaching, probably the most important event in 

 the history of the metallurgy of gold since the first applica- 

 tion of mercury to gold extraction. The extraordinary 

 properties of very dilute solutions of cyanide of potassium 

 were unmarked and in great part unknown until quite 

 recently. That metallic gold is soluble in alkaline cyanides 

 unaided by an electric current remained an interesting but 

 useless fact until it was found that a solution containing 

 only one per cent, of potassium cyanide dissolves gold at 

 least as rapidly as much stronger solutions, although it 

 has a very slow and partial action on most sulphides and 

 other minerals occurring in gold ores. The presence of 

 free oxygen is necessary for the dissolution of the gold, 

 which takes place according to the equation — 



4A11 + SKCy + Oo + 2 H 2 = 4KAuCy 2 + 4KHO. 



The oxygen is supplied from the air entangled in the porous 

 ore, or dissolved in the solutions, in which MacLaurin * has 

 shown that it can be retained in the presence of alkaline 

 cyanides in spite of its rapid absorption by the latter with 

 the formation of cyanates. The gold can be recovered by 

 precipitation on zinc and subsequent melting, or by electro- 

 deposition on suitable cathodes, of which lead only has been 

 largely used. 



The development of the cyanide process proceeded 

 apace as soon as it had been introduced. The mechanical 

 improvements devised were numerous, the most important 

 being the enlargement of the size of the leaching vats, until 

 the largest now hold 600 tons of ore. The chief chemical 

 improvements have been the introduction of the use of 



x Jvur. Chem. Soc, vol. lxiii., p. 724 (1893). 



