3 io SCIENCE PROGRESS. 



small quantity of oxygen (amounting to only about 0*4 per 

 cent, by volume in good solutions in the Transvaal) in the 

 neighbourhood of a particle of gold would soon be exhausted, 

 long before the cyanide could be saturated with gold. 



It is not necessary to take special means to add oxygen 

 to cyanide solutions when ores, such as Transvaal tailings, 

 poor in gold and free from reducing agents, are in course 

 of treatment. Enough air is entangled in the ore or dis- 

 solved in the solution for all practical purposes. When 

 pyritic ores are treated the supply of oxygen is exhausted 

 before the whole of the gold is dissolved, and it has been 

 found desirable to resort to the "double treatment," as for 

 example at the Primrose Works at Johannesburg, where 

 pyritic tailings are leached in two vats in succession, the 

 process of draining dry and transferring the ore being chiefly 

 beneficial on account of the aeration that is thus effected. 

 When concentrates rich in sulphides came to be treated, 

 the difficulties of supplying sufficient oxygen were found 

 to be still greater. The gold dissolves with such extreme 

 slowness that treatment occupied two or three weeks, even 

 if the ore was drained dry at intervals and stirred up. 



It has long been remarked that a small percentage of a 

 soluble sulphide present in the cyanide solution greatly 

 delays the . dissolution of gold. Doubtless this is partly 

 owing to the abstraction of oxygen from the solution by the 

 sulphide, for gold sulphide is freely soluble in KCy so that 

 the surface of the metal is kept free from sulphide if the 

 cyanide is not too dilute. Bettel however points out 1 that 

 silver sulphide is far less soluble than gold sulphide, and 

 that if native gold is alloyed with 20 per cent, of silver, no 

 uncommon occurrence, a film almost insoluble in dilute 

 cyanide solutions may be formed. It is certain that some 

 specimens of gold leaf dissolve with great difficulty if they 

 have been previously dipped in sulphide solutions, or if 

 traces of soluble sulphides or sulpho-cyanides are present in 

 the solution. The difficulty disappears if the sulphides are 

 removed, either by being precipitated with lead salts, or by 

 the action of certain oxidisers. 



1 South African Mitring Journal, 8th May, 1897. 



