578 MINING INDUSTRY. 



brought. Its successful performance demands much practical skill and expe- 

 rience, but its general features are familiar to all metallurgists, and are de- 

 scribed in various works that treat of such subjects. 



An outline of the method of operations in the smelting establishment 

 here referred to will suffice for the purpose of this chapter, while for a more 

 minute description of the processes involved, the reader is referred to metal- 

 lurgical treatises. 



The ores, as they ordinarily occur, disregarding the accessory or associ- 

 ated minerals peculiar to some veins, which sometimes require modifications 

 of the common course of treatment, consist essentially of iron and copper 

 pyrites with a siliceous gangue, carrying from 3 or 4 to 10 or 12 ounces of 

 fine gold to the ton, and silver in more variable quantities, though, usually, in 

 the proportion of two ounces of silver for one of gold, and sometimes very 

 much more. 



In the smelting process, the purpose of which is to separate the copper, 

 and with it the gold and silver, from the earthy gangue, the sulphur plays an 

 important part, since the desired ultimate result of the successive chemical 

 reactions in the furnace is to obtain the copper in the form of sulphide, while 

 a portion of the iron and other foreign elements are removed in the form of 

 slag. An excess of sulphur in the melted charge is to be avoided, because, in 

 such case, the matt produced will be made poor by the presence of too much 

 iron. A lack of sulphur, on the other hand, involves a loss of copper, since 

 in the absence, of a sufficient proportion of sulphur the copper becomes scori- 

 fied or taken up in the slag. The first step, therefore, in the treatment of the 

 ores is to subject them to a roasting process, by which means a portion of 

 the sulphur is expelled and a partial oxidation of the metals effected. If the 

 roasting should be carried too far, the evil can be remedied by the addition 

 of raw ore in mixing the charge for the smelting furnace. 



The proper mixture of the charge demands much practical knowledge 

 of the processes involved and an intimate acquaintance with the character of 

 the material to be treated, in order to adjust the proportions of the various 

 elements to each other so as to effect the desired combinations and to insure 

 other conditions, such as the formation of a liquid slag that may be distinctly 

 separated from the heavier matt; since if the slag be thick, or not sufficiently 



