The lead in the bath gradually becomes enriched in silver, and when it reaches a value of 70/. to 80/. a ton, it is removed to the cupellation furnace, already described.

(C) Wet Methods

This term is applied to those processes in which the metals present in the ore are brought into solution in the form of salts, and are reprecipitated separately. They are mainly as follows.

(1) Augustin's

This process is adapted for treating speisses and mattes containing 50 to 70 Per cent. copper, and free from antimony, arsenic, lead, and zinc. It is divided into 3 stages:-(1) roasting the matte to form sulphates of the metals; (2) roasting with salt to form silver chloride; (3) lixiviating and precipitating the metals. The finely ground and sifted matte is roasted for 10 hours in a reverberatory furnace, when a sample should give a faintly blue solution with hot water, and a precipitate of silver chloride on addition of salt. The charge is then drawn, cooled, ground fine, sifted, mixed with 3 to 5 Per cent. common salt, and roasted for 2 or 3 hours at a low temperature in the same furnace. The chloridized ore is transferred to a series of lixiviating-tanks provided with linen strainers supported on a layer of twigs; into these is run a solution of common salt, previously heated by steam-pipes, which dissolves the silver chloride present and carries it off to another series of tubs, where the silver is precipitated in the metallic state by adding cement copper The cupreous liquors are next treated with scrap iron, which, causes the metallic copper to go down; and the brine solution, thus completely freed from both silver and copper, is pumped up for re-use on a fresh lot of chloridized ore.

By the addition of chlorine water to the brine solution, any gold which may be present is dissolved at the same time with the silver. The silver obtained is washed with hydrochloric acid to remove traces of copper, again with water to remove any adhering acid, and is then made into small balls; these are dried and fused. The copper matte, when exhausted of its silver, is washed with water, and smelted in the ordinary copper furnace. The copper precipitated by the iron is used again instead of cement copper for throwing down the silver. Usually 8 to 12 Per cent. of the silver in the ore is lost; and even less satisfactory results are obtained with speisses than with mattes.

(2) Claudet's

This process was devised to effect the recovery of the precious metals (mainly silver) contained in the liquors resulting from the wet method of extracting copper from Spanish and Portuguese pyrites. The first 3 washings contain 95 Per cent. of the silver yielded by the wash-waters, and these alone are treated. The silver is extracted as an iodide, in the following manner. The liquor is allowed to settle for a time in a capacious vat, and is then run into another of somewhat greater capacity, where it receives an addition of a soluble iodide in just sufficient quantity to precipitate the silver ascertained to be present, and about -1/10 of the volume of the liquor of water; the whole is stirred thoroughly during the filling, and. settled for 48 hours. The clear liquor is then run off, and the accumulated sediment is collected once a fortnight. The latter consists essentially of lead (sulphate and chloride), silver (iodide), and copper (subsalts); the copper salts are removed by washing with water containing hydrochloric acid, and the precipitate is decomposed by metallic zinc, from which result (1) a precipitate rich in silver and containing some gold, and (2) a zinc iodide which is used for precipitating fresh quantities of silver.

The chief components of the precipitate are 56 Per cent. lead, 15 Per cent. zinc, about 1450 oz. silver per ton, and 15 to 20 oz. gold per ton; or, on a working scale, over 1/2 oz. silver and S gr. gold are recovered from each ton of pyrites, at a total cost of 8d. per ton. (Phillips.)

(3) Cumenge's

Ores containing large quantities of antimony, arsenic, copper, and silver are successfully treated by Cumenge's process, which is a combination of dry and wet methods. It is based on the oxidation of antimony and arsenic by injection of steam during the preliminary roasting, and on the fact that this oxidation is limited by the presence of the hydrogen set at liberty; this latter combines with the antimony, the arsenic, and the sulphur, effecting at once the elimination of these troublesome bodies in the form of volatile products. But this plan presents serious inconveniences in the shape of the long duration of the operation and the cost of the steam; these have been largely overcome by introducing a maximum of 5 Per cent. of soda nitrate into the furnace, which renders the oxidation much more active, and causes an increased production of sulphuric acid from the sulphur present, with consequent formation of soluble double salts. Nevertheless the reducing influences still have sufficient effect to prevent the iron present being oxidized to the utmost; while the antimony and arsenic are almost completely removed by volatilization. The silver is mainly sulphatized, but a certain portion is reduced to the metallic state during or after the action of the iron subsalts.

All these reactions are clean and easily realized, but it is essential that the ore be first submitted to smelting in a cupola furnace, to remove great part of the earthy matters; it is the matte thus obtained, weighing only 1/10 to -1 /10 of the original mass of ore, that is powdered and roasted. The sulphatized silver is dissolved in 5 .or 6 times its weight of hot water, and precipitated by copper, iron being then used to recover the copper. The roasting with the soda nitrate is conducted in fireclay vessels resembling gas retorts, the pulverized mineral being spread about 1/2 in. thick and constantly stirred to prevent agglomeration. The roasting is continued for 8 hours with steam alone (superheated by preference); the soda nitrate is then added; next follows another roasting for 4 hours, the nitrate being thus introduced towards the end of the operation to prevent formation of silver and soda antimonates and arsenates; finally, 4 hours' roasting effects the sulphatization of the silver. The heat of the retort should be carefully maintained between dark-redness and cherry-redness.