Gold is one of the most widely distributed metals, and occurs almost always in the native state under 3 separate conditions - (1) as free gold in ancient and modern alluvial deposits; (2) encased in non - metalliferous rocks, chiefly quartz; and (3) associated with the sulphides and arsenides of other metals, principally iron and copper, more rarely lead and antimony. These 3 conditions of occurrence determine the mode of extraction. In the case of (1), advantage is taken of the great sp. gr. of the metal to separate it by more or less complicated washing processes in water; in the case of (2), the rock requires to be crushed to a gritty or even pulpy consistence before the washing operation can be entered upon; while in the case of (3), both the crushing and washing are brought into requisition in order to liberate the gold - holding part of the mass of ore, and this comparatively small proportion of the whole is subjected to chemical or metallurgical treatment for the purpose of isolating its metallic contents, with which the gold is associated. This last - mentioned treatment varies with the nature of the metalliferous mass, and will be found described under the heads of the various other metals concerned.
When the gold is by these means rendered independent of its former associated baser metals, it is in a condition which admits of its recovery by agitating the mass with mercury, which has a great affinity for gold, and rapidly forms an amalgam with it. The mercury is then distilled off for re - use, and the gold thus obtained is sent into the market. It always contains more or less silver, and has to be subjected to an operation known as "parting," which, however, is only done in the refinery where the metal is worked up. The proportion of silver present is ascertained by assay, and governs the market price per oz, of the commercial metal.
The object sought is the removal from the gold of every particle of foreign metals, such as silver, copper, lead, antimony, which may be present - The process depends upon the solubility of these metals in certain hot acids which have no solvent action upon gold. When the operation is conducted on a large scale, with plenty of space for the requisite plant, concentrated sulphuric acid (oil of vitriol) is the acid employed; but wider other conditions nitric acid is used. At some establishments, both the sulphuric acid and the nitric add processes are need, according to circum - cumstances, but generally only one or the other of them. Neither of these acids, however, will by any length of boiling remove from the gold the silver which it contains in small proportion; to enable the acid to dissolve it, the proportion of silver has to be greatly increased. The first thing, therefore, done with the gold is to alloy it with 3 times its weight of silver, which contains a limited proportion of copper. This process is technically termed "quartation," and is effected by melt ing the silver and gold together in a coke furnace. The alloy thus made is " granulated," by Lulling it out of the melting - pots and pouring it into a vessel of water.
The alloy or "parting metal " is then in a physical condition favourable for the action of the acid upon it. The acid portion of the operation is technically termed " parting."
When sulphuric acid is used as the solvent, the plant employed is that represented in Figs. 130,131. The granu - 8 lated alloy is introduced into a hemispherical cast - iron pot a, about 2 or 2 1/2 ft. in diameter, which is heated by a fire beneath, and provided with a flat cover of sheet - lead b, having 2 openings near its front part (the side nearest the workman's platform c). The smaller of these openings, provided for the introduction of a siphon, has usually a removable cover; the other, a larger opening used for the introduction of the materials, is usually furnished with a R hinged cover. There is also, proceeding from the back part of the leaden covering, a pipe d, made either of lead or platinum, the use of which is to convey away the vapours generated in the process. The alloy having been thrown in, oil of vitriol is added in some excess of the quantity known to be requisite for the solution of the metals in the alloy, and heat is applied. The sulphuric acid is heated to boiling, and after about 1 hour's heating, the solution is usually complete, the foreign metals are dissolved, and the gold lies at the bottom of the pot in a powdery condition.
The reaction which takes place is this: half the sulphuric acid necessary for the solution of the metals is decomposed, giving 1 of its 3 atoms of oxygen to them for their oxidation, thus reducing this proportion of the acid to the condition of sulphurous acid, which is evolved in the gaseous state; the other half of the acid necessary for the solution combines with the metallic oxides thus formed to produce silver, copper, lead, etc, sulphates. The acid used being in excess, the liquor is still acid. Abundant and dense white acid fumes are evolved; they consist of steam, sulphuric acid vapour, and sulphurous acid. When the solution is complete, and the gold has subsided to the bottom of the iron pot, the liquor is ladled, or, by means of a platinum siphon, drawn off into water contained in a lead - lined tank e, which is furnished with a wooden cover /, in some works completely, and at other works imperfectly. In the former case, a hole in the cover is provided, through which the long arm of the siphon is introduced. The temperature is now raised by the injection of steam through the pipes g. In this tank the diluted solution remains for about 3 hours.
Being left to rest for a time, a deposit of lead sulphate, etc, occurs, and then the clear solution is drawn off by a tap near the bottom into another vessel or tank A, lined with lead, and containing sheets of copper; this tank is uncovered. The liquor, as it runs from the tank e, is of a blue colour, due to the copper sulphate it contains. The object of the copper is to decompose the silver sulphate in solution, the result of the action being the solution of the copper with the formation of copper sulphate, and the deposition of the silver of the silver sulphate in a powdery condition. The solution of copper sulphate is run off into appropriate tanks, and the silver, being removed, washed with water, and pressed in a hydraulic press, is melted into ingots of pure silver. The gold taken out of the iron pot is not, however, even now absolutely free from silver. Hence, to remove the last traces of this metal, the gold in the powdery condition is boiled again in a smaller pot with more oil of vitriol, the heating being continued for 2 hours. At small works, this boiling is effected in the same pot as the first boiling. The gold is then washed with hot rain or distilled water, fused with soda bicarbonate in a graphite crucible, and subsequently melted into ingots.