(4) Hunt And Douglas's
'This process is based on the difference of solubility of the chlorides, and consists of the following operations.. The mineral, after being submitted to ordinary chlo-rination, is put into wooden vats furnished with stirring-arms, and a solution of iron and sodium chlorides, called " Hunt and Douglas liquor," is kept in contact with it for 4 or 5 hours, the whole mass being meantime heated by steam and subjected to an atmosphere of sulphurous acid. Under the influence of the various chlorides introduced or formed, the silver salts which have resisted the first chlorination process are converted into chloride; after roasting, the liquid portion is drawn off, and the residue is washed with hot water, by which the iron and copper chlorides are removed. Then, in the same vessels, the silver chloride is dissolved by adding lime hyposulphite, the temperature being raised to 100° F. (38° C), and the whole is stirred for about 3 hours. After washing the residues, nearly all the silver originally in the ore will be found in the more or less concentrated liquors, and may be precipitated as sulphide by adding calcium sulphide.
It remains to wash the precipitate, dry it, and roast it at a low temperature in a small reverberatory furnace, to remove accidental impurities, which will be minimized by conducting the hot-water washing with great care. The products employed undergo revivification: the liquor drawn from the vats is filtered, and its copper is recovered by adding metallic iron. The lime hyposulphite liquor, after precipitation of the silver, always contains a slight excess of the calcium sulphide employed: by passing a current of sulphurous acid, the sul-phide is transformed into hyposulphite.
This is described under Lead, pp. 350-3.
(6) Percy And Patera's
This process is suitable only for ores containing 2 Per cent. and upwards of silver, though the presence of antimony, arsenic, bismuth, cobalt, copper, iron, lead, nickel, and sulphur does not interfere with it. The preliminary step is roasting the comminuted ore with 6 to 12 Per cent. common salt and 2 to 3 Per cent. iron sulphate in a furnace to which steam is admitted. The mass is next washed with hot water for 6 hours, by which the chlorides and sulphates of the metals other than silver are carried away in solution. The silver chloride deposit in the washing-vats is treated with cold dilute sodium hyposulphite solution, which dissolves the silver. From this solution the silver is precipitated by means of a solution consisting chiefly of sodium pentasulphide with a little sodium hyposulphite, obtained by fusing soda-ash with sulphur and boiling the aqueous solution of the product with excess of sulphur. The silver sulphide is deposited in about 6 hours as a slimy mud; this is strained through canvas, washed, dried, heated to redness in presence of free air, and fused in graphite crucibles with addition of metallic iron; the scum thus produced is removed and used as a substitute for the iron sulphate in the first roasting, and finally the application of a little bone-ash and wood-ashes cleanses the surface of the metallic silver, and leaves it ready for casting.
(7) Sulphuric Acid
Nearly all the silver and gold contained in copper mattes may be extracted by treatment with hot dilute sulphuric acid, when the copper forms a solution of sulphate. The process is applicable to materials containing antimony, arsenic, and lead, but not iron. The finely granulated metal (preferably black copper) is moistened with the acid and exposed to the air; fresh supplies of the acid are added to dissolve the sulphate formed, and this solution is allowed to run through a long leaden trough, in which the cooling liquor deposits imperfect crystals of copper sulphate and a muddy sediment containing the silver and gold. The strongly acid liquor flowing away clear is re-used on the copper, while the accumulated matters in the trough are removed and dissolved, when the copper sulphate forms almost pure crystals,' and the silver-gold mud is liberated. This latter is smelted with litharge so as to afford a silver-lead alloy containing 2 Per cent. of the more valuable metal; the alloy is cupelled, and yields a silver holding about 1 1/2 to 1 3/4 per cent. gold.
The copper sulphate produced in the process is sold in that form.
The basis of this process is the fact that on roasting argentiferous copper- and iron-pyrites (sulphides), the iron is converted successively into ferrous sulphate, ferric sulphate, and ferric oxide; the copper into cupric sulphate and cupric oxide, the sulphuric anhydride being expelled; and the silver into sulphate. On lix-iviation, the silver sulphate passes into solution, from which it can be precipitated in the metallic form. The process is well suited for treating copper mattes containing 80 Per cent. copper subsulphide, 11 of iron sulphide, and 0.4 of silver sulphide, without appreciable quantities of antimony, arsenic, lead, and zinc. The material to be operated on is granulated, ground fine, sifted, and roasted in a reverbera-tory furnace in contact with fuel for about 10 hours, care being taken to stir it frequently. The roasted mass is taken to rats and there treated with hot water containing a little sulphuric acid, till common salt gives no precipitate in the liquor running away. The subsequent treatment of the silver sulphate solution, first with copper to throw down the silver, and then with iron to recover the copper, presents no novel feature.
The whole success of the process depends upon the roasting in the reverberatory furnace, which is a delicate and difficult operation. At the same time, in dealing with such materials as mentioned before, the process is said to be the most effective and least costly of all.
Commercial silver is never pure, traces of copper and gold being universally present. For coinage purposes, these impurities are not detrimental; but for analytical applications, greater purity is necessary. This may be attained by boiling well-washed silver chloride with caustic potash and sugar; when the metal is completely reduced (as ascertained by a well-washed sample dissolving completely in pure dilute nitric acid), it is melted with pure sodium carbonate in a crucible. To obtain absolutely pure silver, the following plans are in use:-(1) The ordinary metal is dissolved in pure nitric acid, leaving a residue of gold; the solution is evaporated to dryness, and the residue is fused to eliminate accidental traces of platinum nitrate, and then dissolved in dilute ammonia solution; the solution is filtered, and the filtrate is diluted till the silver present amounts to 2 per cent.; enough normal ammonium sulphite solution is added to destroy all colour on heating, and the mixture is left for 24 hours in sealed bottles; about 33 per cent. of the silver crystallizes out, and the remainder is thrown down on heating the solution to 140° to 158° F. (60° to 70° C); the metal is washed with water, and left in contact with concentrated ammonia for some days, to get rid of any remaining copper; after another washing and drying, it is melted with 5 Per cent. each of sodium nitrate and fused borax in an unglazed porcelain crucible, and then cast in moulds coated with a mixture of burnt and un-burnt china clay; the bars are cleaned with sand, heated with potash to destroy attached silicate, and washed with water. (Roscoe and Schorlemmer.) (2) Commercial fine silver is dissolved in dilute nitric acid, the solution is evaporated to dryness, and the residue is fused till red fumes cease to come off, dissolved in water, filtered, and diluted with 30 parts filtered rain-water to each one of silver; the silver is precipitated as chloride by adding pure hydrochloric acid, washed with dilute hydrochloric acid and with water, dried, triturated in a mortar, repeatedly digested with aqua-regia, well washed, and reduced by boiling with pure dilute caustic potash in presence of milk sugar; finally the metal is washed with dilute sulphuric acid and with water, and then dried and fused. (Stas.)