26 BULLETIN 12, UNITED STATES NATIONAL MUSEUM. 



SILVER-LEAD-SMELTING ORES. 



These ores are simply lead ores containing a valuable amount of sil- 

 ver. In most cases there are no physical characters to distinguish the 

 argentiferous from the nonargentiferous lead ores; and since the pro- 

 cess of smelting the lead is the same in both cases, they will be shown 

 in the lead series. 



Processes of Extracting Silver. 



the extraction of silver from base ores by roasting and 



MILLING. 



The ore as delivered at the mills is first passed over a " grizzly," 

 which is a screen of parallel bars, to separate the coarse from the fine. 

 The coarse material is then passed through rock breakers to reduce it 

 to a suitable uniform size. The tine ore from the grizzly and that from 

 the breaker are passed on to some kind of a drying apparatus. After 

 being thoroughly dried the ore is crushed, the fineness of the crushing 

 depending on the character of the ore. The stamped ore is generally 

 mixed with a certain proportion of dry stamped salt ; in some cases, 

 however, the salt is mixed with the ore previous to stamping. The 

 amount of salt added to the ore varies according to circumstances, de- 

 pending both upon the amount of silver in the ore and also upon the 

 character of the gangue, rich ores requiring more salt and ores con- 

 taining manganese requiring large amounts of salt. 



The mixed ore and salt are next passed on to some kind of a roasting 

 furnace, the object of the roasting being to transform the silver into 

 chloride by means of the salt. Osving to the fineness of the material a 

 considerable amount is carried out of the furnace by the draft, and this 

 is collected as far as possible in dust chambers and flues, the material 

 so collected being added from time to time to the product of the furnace. 



After roasting the material is withdrawn from the furnace and al- 

 lowed to remain in heaps m order to complete the (;hloridizing of the 

 silver, after which it is transferred to the pans. In the pans a suita- 

 ble amount of water is added to make a thin paste and sufficient 

 mercury to gather the silver into an amalgam. Sometimes chemicals 

 are also added in the pan, but this should be avoided as far as pos- 

 sible, and generally can be, by giving sufficient attention to the roast- 

 ing. A thoroughly roasted ore should require no treatment in the pan. 

 The use of a small amount of zinc in the pan is quite common and 

 serves to protect the iron of which the pans are made. 



By suitable arrangements the pulp in the pans is kept agitated, in 

 order to bring the mercury into the closest contact possible with the 

 ore ; at the same time the ore is ground still finer. This operation is 

 continued until as much as possibleof the silver has combined with the 

 mercury. 



