GOLD FROM DEPOSITS AND LODES. 129 



and the residual mercury with gold being then retorted to 

 leave only the gold behind. 



Auriferous ore with sulphides may firstly be roasted and 

 then treated with mercury. At the same time such a pro- 

 cedure is not always economically satisfactory, especially 

 when there is a coating of any foreign metallic compound 

 around the gold. 



The following summary will afford an idea of some of the 

 methods applicable : 



a Chlorination process.* 

 Cyanide 



Pyrites and refractory 



y Amalgamation after roasting. 

 8 Concentration in metallic 



materials. -, ^ n. 



lead, metallic copper, or in 



a mixed regulus or in iron 

 sulphide. 



In the Chlorination process, chlorine gas, which has a 

 great affinity for gold, is generated and unites with the 

 gold to form a chloride of gold, which is then dissolved by 

 water, and the precious metal precipitated by means of 

 sulphate of iron or other agent. There is no occasion to 

 enter fully into an account of the process, suffice it to know 

 that whether black oxide of manganese with salt and 

 sulphuric acid, or chloride of lime and sulphuric acid, or 

 chloride of lime with another agent, not a free acid, sea salt 

 or salt water and lime, salt and caustic lime, are used, the 

 object is to attain the most efficient way of obtaining the 

 gold as a chloride. 



In the cyanide process, much used in the Transvaal, &c., 

 for obtaining the gold from tailings, a solution of cyanide 

 of potassium is used for the purpose of seizing the gold, to 

 form a cyanide of gold compound, which is passed over 

 metallic zinc, or otherwise treated, whereby the gold is 

 deposited. 



It must be borne in mind, that any new chemical process, 

 before being used, should be well investigated as to its 

 suitability. One may not be advisable, if the gold is in too 



* This process is mucli used in the Transvaal for the treatment of 

 fulphide concentrates. 



