Beside the ores named above, a good deal of the silver of commerce is obtained from mixed ores, that is. the ores of other metals are frequently found to contain it. In many cases, the =aunt of silver falls greatly short of one per cent. These ores are for the most part sulphurets of lead, arsenic, copper, zinc, and iron.
In the reduction of silver ores, the processes followed arc based Upon the fact, that both lead and mercury have a strong nffinity for silver. A more recent process depends upon the solubility of chloride of silver in a hot solution of common salt, and its separa tion again on cooling.
The simplest pocess is ordinary smelting, and is only applied to the richest ores. These are crushed, mixed with old slag, lend in some form, and a little iron ore and lime. The mixture is then heated in a furnace with charcoal, which brings down the silver and lead together as an alloy. The silver is afterward easily separated by colic:I I:ohm, the principle of which is described in the article ASSAY; but on the large scale, instend of a small bone ash cope), a cupellotion furnace, say 6 feet in diameter, is used. Here the alloy is melted, bellows are used to remove the lead as litharge, or oxide of lend, and a cake of silver isleft on the cupel forming the bottom of the furnace.
It happens that not many even of the richer ores are pure enough to be treated with advantage by simply roasting them with lead; necordingly, another plan, called the amllgtination process, is more commonly adopted. The following is an outline of the way in which this is practiced at Freiberg in Saxony. The vein stuff (largely silica), containing a mixed ore of lead, copper, zinc, etc., as sulphurets, and only from 3 to 3} oz. of silver per cwt., is ground to powder as described under MarrAw.tomy; but some sulphuret of iron is also present, or must be added. About ten per cent of common salt is then mixed with the ore, and the mixture heated in are verberatory furnace (q.v.) ton temperature sufficient to expel water, and in part arsenic, zinc, and antimony. After two hours, the sulphur of the sulphurets bikes fire, and is burned off as sulphurous acid, or converted into sulphuric acid, so that the metals become oxides and sulphates. The temperatnre of the furnace is now raised, when the chlorine of the common salt forms volatile chlorides with zinc, antimony, and iron, and a fixed chloride with silver.
During the roasting, the contents of the furnace are continually stirred, so that they ultimately form a coarse powder.
The product of the roasting furnace, after being ground to a fine powder, is mixed in the proportion of 10 cwt. with 3 cwt. of water and 1 Cwt. of iron in fragments; the mixture being effected in oak casks, which are then made to revolve for two hours on their axes. During the operation the iron decomposes the metallic chlorides in the roasted ore, forming chloride of iron, while the- copper is partly reduced to subcbloride and partly to metallic copper. if there is not enough iron present to convert the copper into subchloride, then mercury will be wasted in the next stage by conversion into its subehloride. Quicksilver to the amount of 5 cwt. is next run into each of the casks, which are then set in motion, and continue for 22 hours at the rate of 12 revolutiousper minute. The result of this is, that the silver being precipitate by the presence of metallic copper, is then dissolved by the mercury, but the amalgam so formed is usually a complex one.
In order to separate the amalgam from the earthy matters and the sulphates and chlorides, the bands, which were Hitherto only two-thirds full, arc now tilled xvith water (the dilution throwing down any chloride of silver held in solution by the sea-salt), and kept revolving for two hours; after which, by means of a stop-cock, the amalgam is allowed to flow into the amalgam chamber, and the rest of the contents, except the iron frag:nents, into a wash tun. The superfluous quieligilver has next to be separated from the amalgam. This is done in bags of ticking, through which the mercury at first flows readily by its own weight, and is afterward squeezen out on a fiat surface. The result of this operation is, that the amalgam of mercury, silver, copper, etc., is left in the bags: its actual composition being nearly 85 per cent of mercury, 10 per cent of silver, and 5 of copper, lead, and antimony. Finally, the quicksilver of the amalgam itself is separated by heat in a distilfing furnace, Here the amalgam is put into a row of iron pots, which go into a large retort. When heat is applied the quicksilver volatilizes, and is condensed in a pipe attached to the wort, from which it is collected in a trough. The Impure silver left in the retort is refined by fusion and subsequent cupellation.