SILVER 



463 



these western states. This method is founded on 

 the Cazo process above described, an iron pan being 

 used instead of a copper vessel. These pans, which 

 were introduced soon after the silver ores of 

 Nevada and adjoining states began to be worked, 

 are tubs 5 or 6 feet in diameter, and generally made 

 entirely of iron. Each has a vertical shaft passing 

 up through the centre, upon which is fitted a re- 

 volving muller with iron shoes, which come almost 

 close to dies fixed to the bottom, and between which 

 the pulp passes. This shaft is kept in motion by 

 strong spur gearing, and there is usually a double 

 bottom to the pan for heating by steam. Different 

 pans are in use, but they vary more in detail than 

 in principle. Fig. 1, from Egleston's Metallurgy, 

 shows the Horn pan in section. These vessels are 

 charged half-full of crushed ore and water, and 

 they are designed with the intention of uniting 

 as tar as possible a grinding with a stirring sur- 

 face. Common salt, sulphate of copper, and some- 

 times other chemicals are added to the charge. A 

 pulp is produced just thick enough to allow the 



Fig. 1. Amalgamating Pan : 



a, muller ; b, shoe* fixed to muller ; r, die* Died to bottom of 

 pan ; d, scraper ; e, driving wheels ; /, wings to prevent pulp 

 rising too high ; g t steam space. 



muller to rotate, and the temperature of the charge 

 is raised by ateam to from 160 to 200 F. The 

 necessary quantity of mercury is added, and the 

 pulp containing the amalgam is ready to be re- 

 moved after the charge has been in the pan five 

 hours. The amalgam is then cleaned and strained, 

 and the mercury separated from the silver by 

 distillation in a cast-iron retort. 



Freiberg Processes. The Barrel Amalgamation 

 Process, formerly practised with much success at 

 Freiberg in Saxony, is or very recently was in use 

 at some silver-extraction works in America. The 

 Freiberg ores are complex. Certain lodes contain 

 various sulphuretted silver ores in quartz vein-stuff, 

 but other lodes yield along with these argentiferous 

 galena, zinc-ble*nde, copper and iron pyrites, and 

 other metalliferous minerals in a vein-stuff of 

 baryta, spathic carbonates, &c. These ores were 

 pulverised and so mixed that the poorer class con- 

 tained 30 or 40 ounces of silver per ton, and the 

 richer class three or four times as much. An ore of 

 thin nature is roasted with common salt to convert 

 the sulphide of silver into chloride, after which it 

 is finely ground and placed in the amalgamating 

 casks along with water, scrap-iron, and mercury. 

 The iron reduces the silver to the metallic state, 



and it then combines with the mercury to form an 

 amalgam which, when separated from the resi- 

 dues, is distilled in iron retorts. This is a more 

 perfect system than the Mexican method, but there 

 is a considerable expense for fuel. Fig. 2 shows 



Fig. 2. Vertical Section of Amalgamating Apparatus t 



C, C, oak barrels ; 8, toothed wheel for setting the barrels In 



motion ; A, A, troughs for amalgam; T, T, troughs for residues. 



the arrangement of rotating barrels used iu this 

 process. 



At the present time silver is obtained at the 

 Freiberg works by the smelting of argentiferous 

 lead and copper ores in a Pilz blast-furnace, an 

 illustration of which is given in the article LEAD. 

 The charge is composed of lead ores, zinc-blende, 

 and pyritic ores, some of which are highly siliceous. 

 This ore-mixture contains the following metals, 

 the average percentage of. each being lead 23, 

 zinc 13, copper 3, and silver - 08 to -60, together with 

 sulphur and quartz gangue. The ore is first roasted 

 in a reverberatory furnace, and then smelted in the 

 Pilz furnace along with slags and various residues 

 rich in silver. Coke is used as fuel, and the pro- 

 ducts of the furnace are lead containing a variable 

 amount of silver, a regulus of lead and copper with 

 from O'l to 0*2 per cent, of silver, and a slag con- 

 taining lead with sometimes "04 per cent, of silver. 

 The lead in the regulus is separated from the 

 copper by another smelting. Most of the silver is 

 retained by the lead, and is recovered from it by 

 Pattinson's desilverising process and subsequent 

 cupellation. See LEAD. 



Aiujustin Process. This and the next are wet 

 processes for the extraction of silver. When cer- 

 tain argentiferous ores or products are roasted with 

 common salt the chloride of silver is formed. This 

 chloride is soluble in a strong solution of common 

 salt, and from the solution so formed metallic silver 

 is precipitated by means of copper. An equivalent 

 quantity of copper chloride is at the same time 

 formed, from which metallic copper is in turn pre- 

 cipitated by iron. Sulphuretted ores or compounds 

 are used in this process. 



Ziervogel Process. No simpler process is known 

 for the extraction of silver from argentiferous 

 copper pyrites (which nearly always contains iron) 

 or of an ore in which it is an important constituent. 

 The method is based upon the fact that sulphide of 

 silver contained in the ore, or matt derived from 

 it, is converted into a soluble sulphate by very 

 careful roasting. By the reactions in the furnace 

 sulphate of iron is first formed, then sulphate of 

 copper, and finally, by the decomposition of these, 

 sulphate of silver, which is readily dissolved out of 

 the roasted ore by hot water. The silver is then 

 precipitated from the solution by copper. 



Smelting of Silver. At Kongsberg in Norway, 

 and at Wvandotte in Michigan, silver ores, consist- 

 ing chiefly of native silver with other minerals 

 in a rocky matrix, are smelted in furnaces. At the 

 former locality, by the addition of iron pyrites to 



