Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/772

Rh 7-18 the adoption of the more simple forms, in which the grind- ing is effected between horizontal ﬂat surfaces instead of curved or conical bottoms, and in the pans now usually employed these ﬂat grinding surfaces form an annular ﬂoor round a central cone through which a vertical shaft passes. The Knox pan, fig. 7, may be considered to be fairly typical. It is of cast-iron, -1- feet in diameter and 14 inches deep. It has a false bottom to form a hollow annular space through which steam can be introduced. The centre of the yoke cl attached to the muller m, is keyed to a vertical wrought-iron shaft S,
 * 2 inches in diameter,

which can be brought in connexion with the driving gear G. The blocks r, r are of wood. In working the pan 100 ll) of skinnnings are introduced, and water added until the pulp will just adhere to a stick. After three hours grinding the pulp is heated with steam. About 5 lb of mercury are atlded for every charge, together with a cupful of equal parts of saltpetre and sal ammoniac. After three hours further working, water with a little caustic lime is added, and the pulp is discharged first through an upper and then through a lower hole. One of the greatest difficulties in the treatment of gold by amalgamation, and more particularly in the treatment of pyrites, arises from the socalled sickening or flouring of the mercury ; that is, the particles, losing their bright metallic surfaces, are no longer capable of coalescing with or taking up other metals. Of the numerous remedies pro- posed the most eﬂicacious is perhaps sodium amalgam. It appears that amalgamation is often impeded by the tarnish found on the surface of the gold when it is associated with sulphur, arsenic, bismuth, antimony, or tellnrium Vurtz 1 in America (1864) and Crookes in England (1865) made independently the discovery that, by the addition of a small quantity of sodium to the mercury, the operation is much facilitated. It is also stated that sodium prevents both the “sickening ” and the “ﬂouring” of the mercury which is produced by certain associated minerals. Cosmo N ewberry has investigated with much care the action of certain metals in impeding amalgam- ation.2 Wurtz recommends two amalgams, one containing 2 and the other4 per cent. of sodium,and in practice 1 per cent. or less of these is added to the mercury in the amalgamator. Crookes employs three kinds, which he calls A, B, and C amalgams ; each contains 3 per cent. of mercury. but the B variety has, in addition to the sodium, 20 per cent. of zinc, and C is mixed with 10 per cent. of zinc and 10 per cent. of tin. The addition of cyanide of potassium has been suggested to assist the amalgamation and to prevent “ ﬂouring,” but Skey3 has shown that its use is attended with loss of gold. Separatixm of Gold from the Amal_r/am.—The amalgam is first pressed in wetted canvas or buckskin in order to re- move excess of mercury. According to Rittinger, mercury will dissolve from 0'05 to 0'08 per cent. of native gold of standard 650 to 850 without loss of ﬂuidity, the solubility of the gold increasing with its ﬁneness; and until the point of saturation is reached, no separation of solid amalgam is possible. Lumps of the solid amalgam, about 2 inches in FIG. 7. —Knox Pan. 1 American Journal of Science and Arts, vol. xli., March 1866. '-' Ure's Dictirmary of Arts, supplement to 7th ed.. p. 412 3 Transactions of the New Zealand Instﬂule, ]‘7G. GOLD diameter, are introduced into an iron vessel lined with a paste of fire-clay and wood ashes, and provided with an iron tube that dips below the surface of water. The dis- tillation is then effected by heating, care being taken that the retort does not become visibly red in daylight. The amalgam yields about 30 to 40 per cent. of gold. In California the amalgam is retorted in cast-iron pans placed in ca.st—iron cylinders 11 inches in diameter, 4 feet 6 inches long, supported on brick work. The bullion left in the retorts is then melted in black-lead crucibles, with the addition of small quantities of suitable fluxes. The extraction of gold from auriferous minerals by fusion, except as an incident in their treatment for other metals, is very rarely practised. It was at one time proposed to treat the concentrated black iron obtained in the Ural gold washings, which consists chiefly of magnetite, as an iron ore, by smelting it with charcoal for auriferous pig-iron, the latter metal possessing the property of dissolving gold in considerable quantity. By subsequent treatment with sulphuric acid the gold could be recovered. Experiments on this point were made by Anossow in 1835, but they have never been followed in practice. Gold in galena or other lead ores is invariably recovered in the refining or treatment of the lead and silver obtained. Pyritic ores containing copper are treated by 1l1€tlH)(l.~= analogous to those of the copper smelter. This is exten- sively done. In Colorado the pyritic ores containing gold and silver in association with copper are smelted in re- verberatory furnaces for regulus, which, when desilverized by Ziervogel’s method, leaves a residue containing 20 or 30 ounces of gold per ton. This is smelted with rich gold ores, notably those containing tellurium for white metal or regulus; and by a following process of partial reduction analogous to that of selecting in copper smelting, “ bottoms "’ of impure copper are obtained in which practically all the gold is concentrated. By continuing the treatment of these in the ordinary way of refining, poling, and granulat- ing, all the foreign matters other than gold, copper, and silver are removed, and, by exposing the granulated metal to ahigh oxidizing heat for a considerable time, the copper may be completely oxidized while the precious metals are unaltered. Subsequent treatment with sulphuric acid renders the copper soluble in water as sulphate, and the final residue contains only gold and silver, which is parted or refined in the ordinary way. This method of separating gold from copper, by converting the latter into oxide and sulphate, is also used at Olcer in the Harz. Clzlorizzation Process.—Plattner suggested that the resi - dues from certain mines at Reichenstein, in Silesia, should be treated with chlorine after the arsenical products had been extracted by roasting. The process, which depends upon the fact that chlorine acts rapidly upon gold, but does not attack ferric oxide, is. now adopted in Grass Valley, California, where the waste mine- rals, principally pyrites from tail- ings, have been worked for a considerable time by amalgama- tion. The roasting is conducted at a low temperature in some form of reverberatory furnace. Salt is added in the roasting to convert all the metals present, except iron, into chlorides. The auric chlorideis,however,decom- _ _ posed at the elevated temperature into finely-divided metallic gold, which is then readily attacked by the chlo-- rine gas. The roasted mineral, slightly moistened, 1S next introduced into a wooden vat, pitched inside, and furnished with a double bottom, as is shown in fig. 8. Chlorine.