462 



SILVER 



lodite, lodyrite, lodargyrite, Iodide of Stiver, 

 is a comparatively rare silver ore, occurring in a 

 few of the Mexican and South American mining 

 districts ; also in Arizona and Spain. 



Arqueritt is a native silver amalgam containing 

 86} percent, of silver and 13J of mercury. It is 

 the principal ore of the mi in- in the Arqneros 

 district near Coquimbo in Chili, but it is also 

 found with alluvial gold in British Columbia. 

 Other silver amalgams are found in some European 

 mines an well as elsewhere. 



MKTALLURGICAL PROCESSES. Owing to ite 

 high value and the many different ways in which 

 silver occurs in nature, the metallurgical processes 

 in use for the extraction of the metal from its ores 

 are perhaps more varied than tlm-e employed in 

 the case of any other metal. If it were obtained 

 only or chiefly from comparatively rich or pure ores 

 the state of matters would lie different, but silver 

 is most largely extracted from ores in which the 

 metal in some form is minutely and sparsely dis- 

 seminated through an earthy or rocky matrix, or 

 from ores of common metals in which it is present 

 in only very small proportion, as in the case of 

 argentiferous lead and copper ores. More than 

 one-half of all the silver produced is obtained from 

 lead ores. The scarcity or abundance of fuel is 

 an important question in determining what process 

 can be most economically employed. The three 

 important methods of separating silver from its 

 ores are (1) by forming an amalgam of silver 

 with mercury, " from which the latter is after- 

 wards separated by distillation ; (2) by converting 

 the silver in sulphuretted ore or regulus into 

 a soluble salt, and from its solution precipitat- 

 ing the silver by metallic copper or iron ; (3) by 

 forming a rich alloy with lead, as through the 

 smelting of silver ores with galena or other lead 

 ore, from which the silver is afterwards separated 

 by cupellation. 



Amalgamation Processes. The Mexican or Patio 

 amalgamation process, invented by -Bartholome 

 Medina about the middle of the 16th century, has 

 been practised with but little alteration down to 

 the present time. Silver is one of the metals which 

 mercury unites with at common temperatures, but 

 the amalgamation proceeds more quickly with the 

 mercury at or near ite boiling-point. Mexican 

 silver ores consist of sulphides, chloride, chloro- 

 bromide, and native silver generally disseminated 

 through rocky gangue or other minerals. The ore, 

 first broken into small pieces by stamps or edge 

 stone mills, is most frequently ground in Mexico 

 in a circular trough (arrastre), in some cases 9 

 feet in diameter and 1 foot deep, with a flat bottom 

 formed of hard stones. In trie centre revolves a 

 vertical shaft on which are fixed four arms, each 

 carrying a heavy stone by which the ore is reduced 

 to a fine powder, or rather, as water is used, to a 

 fine mini. 



In the Patio process, which is best suited for 

 sulphuretted silver ores, the materials used besides 

 the ground ore are magistral (sulphide of copper 

 roasted, so as to produce as much sulphate as 

 possible), common salt, and mercury. The ore, at 

 first a fine mud, is partially dried and worked into 

 flat circular heaps, which, though sometimes 

 smaller, generally contain from thirty to sixty 

 tons. Salt-earth is then added in such quantity 

 as will yield chloride of sodium amounting to fully 

 six percent, of the ore, a limited quantity of water 

 being used. After a mixture of this salt with the 

 ore is made, the heap is made circular and troddrn 

 by horses for two hours, the mixture being turned 

 over by men in the middle of the operation. The 

 next stop is to add about two per cent, of magistral, 

 which is spread over the ore and incorporated with 

 it also by the feet of horses. If the ore under 



treatment contains about sixty ounces of silver per 

 ton, then mercury to the extent of 24 Ib. per 

 tun i* spread over the heap in small globules by 

 pressing it through linen bags. Only about two- 

 thirds of this mercury is, however, added in the 

 first instance, and the mass again trodden for two 

 hours by horses. The ore in the heap, now called 

 a tortn, is next turned over by men, and horses 

 once more trot over it for a rather longer time. 

 Alter this the mixed mass of ore, mercury, and 

 other materials is left to rest for a day, when 

 horses again travel over the torta. It is now time 

 to add the second portion of mercury, and again to 

 turn over the ore mixture. Further treading by 

 horses and turning over by men follow at intervals 

 of days, the completion of the amalgamation pro- 

 cess taking about a fortnight in summer and con- 

 siderably longer in winter. The dry amalgam pro- 

 duced by the above operations now receives an 

 addition of mercury to render it liquid enough to 

 admit of ite being separated from the slime by 

 washing. 



For what is called the Cazo or Caldron procett 

 i.e. the hot amalgamation method as practised 

 to some extent in Mexico, but more generally in 

 South America^ the suitable ores are those rmi- 

 taining native silver or such as consist of silver in 

 union with chlorine, bromine, or iodine. It i> 

 unsuited for sulphuretted ores. After having been 

 ground, dressed, and washed, the ore is put into a 

 caldron with a thick copper bottom, along with 

 water to form a thin mud. A fire is then fighted 

 beneath it, and just before the liquid stuff begins to 

 boil common salt to the amount of about one-sixth 

 part of the weight of the ore is added, and enough 

 mercury put in, but not all at once, to form a soft 

 amalgam. The liquid is constantly stirred by 

 revolving arms on an upright shaft with copper 

 blocks attached to them. In about six hours the 

 operation is completed, and the amalgam is then 

 separated from impurities. Care is necessary in 

 this process not to use excess of mercury, which 

 should not be more than four times that of the 

 silver by weight. A greater quantity produces 

 adhesion of the silver amalgam to tin- cornier, and 

 sometimes a copper amalgam is formed if the 

 blocks rotate too slowly. 



The amalgam, after "being subjected to filtration 

 through canvas, is distilled in an iron bell -shaped 

 vessel or cylindrical retort, by which the mercury 

 is expelled as vapour and the fumes condensed in 

 a trough containing water. The silver remains in 

 the retort, and is afterwards cast into bars. 



The great silver-producing region of the United 

 States was only discovered so recently as 1869. 

 The lodes occur either in the Kocky Mountains or 

 to the west of them, in the states of Montana, 

 Idaho, Nevada, California, Colorado, Arizona, and 

 in Utah and New Mexico. There is great variety 

 both in the kind of ores which are found and in 

 their richness in silver. Much of the ore raised 

 contains both gold and silver. 



Argentiferous lead ores consisting of earthy car- 

 bonates and sulphates as well as galena are found 

 largely in Utah. The amount of silver in these 

 ranges from 10 to 150 ounces per ton, but in some 

 places the minimum quantity is 70 ounces. These 

 ores are smelted at various works, and the resulting 

 argentiferous lead is desilverised by the zinc pro- 



- -! I.I MI . Sulphuretted -liver MM Jiievail 



in the United States, and these require to be 

 roasted in furnaces along with the common salt to 

 drive off the sulphur and to form the silver into a 

 chloride. Such ores are previously crushed by 

 stamps or rollers in the dry state, but the more 

 easily decomposed silver ores are crushed wet. 

 What is called the Pan Amalgamation Process is 

 that most largely practised in the silver-works of 



