Gold is occasionally deprived of the baser metalF by the use of sulphuret of antimony, sulphur having a stronger attraction For the other metals than for anti mony, while it does not combine with gold. To pu rify gold by this means, it is necessary to employ cru cibles which are not porous. To render them so, they arc scaked in oil, and then besmeared in the inside Nvitli pounded borax. The gold alloy is fused in these, and about twice its weight of sulphuret of antimony is added in successive portions, taking care that the mix ture does not swell and flow out of the crucible. If the quantity of gold be very small, the sulphuret• must be previously fused, with an additional quantity of sul phur, so that too much antimony may not be used. When the whole of the mixture is in a state of fusion, and when it sparkles, it is poured into greased cones, which must be struck gently as the matter consolidates, to cause the gold to fall to the bottom. When cold, the gold is found united with the antimony, and also with a little of the alloying metals ; it must, therefore, be subjected to the same process, with an additional quantity of sulphuret, a second and a third time, by which the whole of these are removed. The next part of the process is to separate the gold and antimony. For this purpose the alloy, reduced to fragments, is fused with one-fourth of its weight of sulphur, in a covered crucible, by which the greater part of the an timony quits the gold, and unites with the inflamma ble body. The fused matter is then poured into greased cones ; and when cold, the gold still retaining a little antimony, is collected at the bottom. To free it from this it must be fused, and a stream of air made to pi•ss over it, by which the antimony is oxidated, and dissi pated by the heat, and the gold is left. If not quite purified by this process, it must be fused with a little nitre, which will oxidate the antimony, and leave the gold pure. The sulphuret of antimony, formed by the fusion of the alloy and sulphur, contains a small quan tity of gold. To obtain this from it, it must be fused with about one fifth of its weight of iron filings, which will unite with the sulphur ; the antimony will combine with the gold, and may be separated by the method above described.
The process of purifying gold by cementation, is still practised in sonic particular cases. It consists in beat ing the alloy to thin plates, and placing these in alternate layers, with a cement composed of 1 of nitre, 2 of cal. clued green vitriol, and 2 of pounded tiles, made into a paste will) urine, taking care that there be cement at the top and bottom of the vessels. These are placed in a furnace, and kept at a heat, below the inching point of gold, for 24 hours. The gold is then removed, and boiled, first in water, and afterwards in nitric acid, by which the foreign matter is washed off. The plates are repeatedly subjected to a similar operation ; and in this way a great deal of the alloying metals arc removed by the nitric acid, set at liberty from the nitre by the ac tion of the acid of the salt of iron on it. The superiori ty of this process depends on the nitric acid, at the temperature to which it is exposed, acting on silver and copper, though in small quantity, which is not the case when the liquid acid is boiled on the alloy ; it does not, however, afford the gold pure, it is therefore practised only when this metal is not required very line, as in the formation of trinkets.
Assay of Gold Ores.--As gold ores contain in general but few ingredients, their assay is easily performed. For this purpose the ore is reduced to powder, and mixed with 1 of fused borax, and 4 of sub-carbonato of potassa, and subjected to heat in a crucible. When fused, it is poured out, and if any remain in the vessel, it must he removed by muriatic acid. This, with the fused matter reduced to powder, is digested in muriatic acid, to which from time to time a little nitric acid is added. When the action ceases, the solution is poured off, and the re sidue is well washed with water, the washings being mixed with the solution. To the solution, neutralized i by a fixed alkali, green sulphate of iron is added, as long as it causes any precipitation; the precipitate, after being washed, is fused in a crucible with a little nitre ; and affords gold in a state of purity. If the part of the
ore not acted on by the acid become black by expo sure to light, it contains silver. To procure this, it inns! be fused with about thrice its weight of sub-car bonate of potassa, by which the silver is reduced and is mixed with the earthy matter. By washing the pro duct in muriatic acid, the latter is separated, and the sil ver is left.
Iron pyrites containing gold is analysed by digest ing the ore in muriatic acid, to which a little nitric acid is added. What is left must be roasted to drive off the sulphur, and again digested in acid. The solu tions obtained must be neutralized by potassa, and pro to-nitrate of mercury is added. The precipitate thrown down is then fused with nitre, by which the gold is procured. If silver be also contained in the ore, it is obtained by treating that part of it not acted On by the acid with sub•carbonate of potassa and muriatic acid, as above described. Galena, containing gold, is ana lysed by digesting it in nitro•muriatic acid, evaporat ing the solution, and afterwards adding proto•nitrate of mercury. The insoluble part, when treated as above, will yield the silver, if any exist in the ore.
The assay of gold, as performed on its alloys, will he given when treating of assay of silver.
Silver is obtained net only from the proper ores of silver, but likewise from sonic of the ores which aye wrought to yield other metals. Of these the principal are the ores of lead.
Fled:talon of Ores.
Two methods are practised for procuring silver, the one is by amalgamation, the other is by fusion. The former is followed when the ore is rich in silver, the latter when it contains little of the precious metal.
Amalgamation. The ores which are subjected to the process of amalgamation contain, in general, besides lead and copper, a small quantity of iron pyrites ; if they do not, some of this is usually mixed with them, by which the extraction of the silver is facilitated. The ore, after being freed as much as possible from its stony ingredients, is mixed with common salt, in the proportion of 8 or 9 per cent. it the ore contain 8 oz. per quintal ; and 10 or 12 per cent. if more silver be contained in it. It is then exposed for some hours on the floor of a reverberatory, till the vapour cease to come off; the mixture being frequently stirred to ex pose the whole of it to the flame. If, when removed, too little or too much of the salt have been added, (which the workman by experience easily knows,) the proportions must be adjusted, by adding either more of the salt or of the ore, and the mixture is again roast ed in the reverberatory. During this part of the ope ration, the sulphur of the ore is partly dissipated in the form of sulphurous acid, part of it in the state of acid combines with the alkali of the salt, and the mnriatic acid set free unites with the silver. When the matter is cold, it is reduced to powder in a mill, and then amalgamated with about an equal quantity of mercury. The amalgamation is performed in barrels, which are made to revolve, or in tubs which contain an appara tus for keeping the mixture in constant agitation. The mixture is put into these vessels, and made into a thickish paste with water, and kept agitated for about two days, and after the amalgam has fallen to the bottom, it is withdrawn through an aperture in the lower part of the vessel. What remains is washed, by which more of the amalgam is got from it, and the re sidual matter, if the ore contained a great deal of silver, is again roasted with about 3 per cent. of sulphur, and subjected to a similar process, by which the whole of the metal is extracted. The amalgam procured by these different operations is freed of its superfluous mercury, and is then subjected to distillation, by which the whole of the mercury is expelled and the silver is left ; retaining, however, a little copper, which is remov ed by cupellation.