EECENT COPPEE-EXTEACTII^'G PROCESSES. 141 



state. At tliis stage gold, if present, makes its first appearance, 

 having been dissolved in tlie first instance mtli tlie otlier chlor- 

 ides, precipitated by iodide of potassium, and converted into tlie 

 metallic state by zinc. The precipitate tlien consists of silver, 

 with a small quantity of gold, lead (about 60 per cent.), oxides 

 of iron and zinc, and a small quantity of lime and copper ; there 

 is. no iodine present, as it has combined with the zinc in solution, 

 from which it is recovered for further use. The copper solution, 

 which has been drawn off clear from the silver precipitating tank, 

 is passed into another tank containing metallic iron, which pre- 

 cipitates it at the cost of the iron. 



At the Widness works the value of the precious metals con- 

 tained in the ore is only (2s. lOd.) two shillings and tenpence ; 

 they consist of half an ounce of silver and one and a half grains 

 of gold ; the total cost of extraction is lOd. per ton of ore treated, 

 and the profit 2s. per ton, on 30,000 tons treated primarily for 

 copper. The method was first introduced in 1871, and in the 

 first year 16,300 tons of burnt pyrites were operated upon ; the 

 additional expense connected with the extraction of the precious 

 metals was £416, while the value of the gold and silver after 

 deduction of the cost of melting and refining was £3,232. The 

 process has been very largely employed since. In Cornwall there 

 are mines producing large quantities of poor ores which have 

 hitherto been treated in the dry way, for copper only, but have 

 been found to contain more silver than the Spanish pyrites ; 

 these have recently been treated profitably by the process. In 

 the first part of the process sulphate of soda of great purity is 

 obtained, and the iron of the pyrites being very free from extra- 

 neous matter is recovered in a state of very fine division, used for 

 polishing looking-glasses, and sold in large quantities to the iron 

 manufacturers for " fettling" their puddling furnaces. 



The Mindeleff Peocess 



is comparatively new, though the chemical theories involved are 

 well known. The mode of applying these principles is certainl}^ 

 new. The inventor, a If-ussian metallurgist, has mtroduced his 

 process into America, whence we derive details of the modus 

 operandi ; it consists of a new mode of applying light carbur- 

 etted hydrogen as a reducing agent for oxides, sulphides, arsen- 

 ides, and carbonates. The chemistry of the process has been 

 long known, and various attempts have been made to utilize it, 

 but hitherto without success. In California some experiments 

 were made on copper ores, which were placed in a retort and 

 heated to expel moisture ; when sufficient heat was attained the 

 gas was admitted under pressure, the escape pipe being adjusted 

 to admit of the slow escape of the gases evolved. It is claimed 

 that the process is a perfect success, the ore being thoroughly 



