364 METALS AND THP:1R COMBINATIONS. 



other metals, sometimes also in combination with selenium and tellu- 

 rium. This impure gold is separated from most of the adhering 

 sand and rock by a mechanical process of washing, in which advan- 

 tage is taken of the high specific gravity of the metallic masses. The 

 remaining mixture of heavy material is treated with mercury, which 

 dissolves gold and silver, leaving behind most other impurities. The 

 gold amalgam is placed in a retort and heated, when the mercury 

 distils over, while the gold is left behind. 



From ores containing but little gold the metal is now extracted 

 largely by treating the finely powdered material with a solution of 

 potassium cyanide, which forms a soluble double cyanide of gold and 

 potassium, AuK(CN) 2 . From the solution gold is precipitated elec- 

 trolytically or by adding metallic zinc. 



Refining gold. Gold obtained by either of the above processes is not pure, 

 but has to be purified or refined by methods which differ according to the nature 

 or quantity of the impurities present, or according to the use to be made of the 

 gold. The methods employed may be divided into two classes, viz., dry and 

 wet processes. In the dry or crucible methods the operation is conducted at a 

 temperature sufficiently high to melt the gold, while in the wet processes the 

 dissolving action of acids is made use of. 



Of dry methods may be mentioned the following : The gold is fused in a 

 clay or graphite crucible which has been glazed on the inside with borax, and 

 a stream of chlorine is passed through the molten mass. The chlorides of zinc, 

 bismuth, arsenic, and antimony, when present, are volatilized, while the chlo- 

 rides of silver and copper rise to the top, forming, with some of the borax, a 

 layer over the purified gold. Another method consists in melting the gold in 

 a crucible, prepared as before mentioned, and adding gradually a mixture of 

 potassium nitrate and carbonate with borax. All base metals are converted 

 into oxides which become dissolved in the borax; but silver is not eliminated 

 by this method. It may, however, be gotten rid of by heating to a temperature 

 just below fusion, the granulated gold with about one-sixth of its weight of 

 sulphur. The mixture should be protected with a layer of fine charcoal. 

 Silver sulphide is formed during the operation and the gold, after being fused, 

 may be cast into an ingot mold. 



Another dry method, used, however, more for assaying gold ores or gold alloys 

 than for purifying gold on a large scale, is the cupellation process. It depends 

 on the solubility of gold in molten lead and the readiness with which lead takes 

 up oxygen when heated in an oxidizing flame. In carrying out the process the 

 material to be operated on is fused with a quantity of lead amply sufficient to 

 dissolve the metals present. The resulting alloy, called the lead button, is then 

 submitted to fusion on a very porous support, made of bone-ash, and called a 

 cupel. All metals except gold and silver are oxidized ; the lead oxide, which is 

 fusible, takes up all other oxides, and the whole of this mass is absorbed by the 

 porous bone-ash, on the surf ace of which is finally left a button of gold and silver. 



Of wet processes for the separation of gold and silver is to be mentioned the 

 method known as parting. It depends on the extraction of the silver (and cop- 



