Metallurgic treatment of gold

—The gold found in the sands of rivers, or in auriferous soils, needs not be subjected to any metallurgic proce , properly speaking. The Orpaillers, separate it from the sands, by washing them first upon inclined tables, sometimes covered with a cloth, and then by hand in wooden bowls of a particular form. Amalgamation is employed to carry off from the sand, the minuter particles of gold they may contain. The people called Bohemians, Cigans, or Tehinganes, who wash the auriferous sands in Hungary, employ a plank with 24 transverse grooves cut in its surface. They hold this plank in an inclined position, and put the sand to be washed in the first groove; they then throw water on it, when the gold mixed with a little sand collects usually towards the lowest furrow. They remove this mixture into a flat wooden basin, and by a peculiar sleight of hand, separate the gold entirely from the sand. The richest of the auriferous ores consist of the native gold quite visible, di eminated in a gangue, but the veins are seldom continuous for any length. The other ores are auriferous metallic sulphurets, such as sulphurets of copper, silver, arsenic, ., and, particularly iron. The stony ores are first ground in the stamping mill, and then washed in hand-basins, or on wooden tables. The auriferous sulphurets are much more common, but much poorer than the former ores; some contain only one 200,000th part of gold, and yet they may be worked with advantage, when treated with skill and economy. The gold of these ores is separated by two different proce es; namely, by fusion and amalgamation. The auriferous metallic sulphurets are first roasted; then melted into mattes , which are roasted anew; next fused with lead, whence an auriferous lead is obtained, which may be refined by the proce of cupellation. When the gold ores are very rich, they are melted directly with lead, without preliminary calcination or fusion. These proce es are however little practised, because they are le economical and certain than amalgamation, especially when the gold ores are very poor. If these ores consist of copper pyrites, and if their treatment has been pushed to the point of obtaining auriferous rose copper, or even black copper including gold, the precious metal cannot be separated by the proce of liquation, because the gold having more affinity for copper than for lead, can be but partially run off by the latter metal. For these reasons the proce of amalgamation is far preferable. This proce being the same for silver, I shall reserve its description for this metal. The rich ores in which the native gold is apparent, and merely di eminated in a stony gangue, are directly triturated with quicksilver, without any preparatory operation. As to the poor ores, in which the gold seems lost amid a great ma of iron, sulphuret of copper, ., they are subjected to a roasting before being amalgamated. This proce seems requisite to lay bare the gold enveloped in the sulphurets. The quicksilver with which the ore is now ground, seizes the whole of its gold, in however small quantity this metal may be present. The gold procured by the refining proce with lead, is free from copper and lead, but it may contain iron, tin, or silver. It cannot be separated from iron and tin without great difficulty, and expense, if the proportion of gold be too small to admit of the employment of muriatic acid. By cupellation with lead, gold may be deprived of any antimony united with it. Tin gives gold a remarkable hardne and brittlene ; a piece of gold, exposed for some time over a bath of red hot tin, becomes brittle. The same thing happens more readily over antimony, from the volatility of this metal. A two thousandth part of antimony, bismuth, or lead, destroys the ductility of gold. The tin may be got rid of by throwing some corrosive sublimate or nitre into a crucible, containing the melted alloy. By the first agent, perchloride of tin is volatilized; by the second, stannate of potash forms, which is carried off in the resulting alkaline scoriæ. Gold treated by the proce of amalgamation, contains commonly nothing but a little silver. The silver is di olved out by nitric acid, which leaves the gold untouched; but to make this parting with succe and economy on the great scale, several precautions must be observed. If the gold do not contain fully two thirds of its weight of silver, this metal being thoroughly enveloped by the gold, is partially screened from the action of the acid. Whenever, therefore, it is known by a trial on a small scale, that the silver is much below this proportion, we must bring the alloy of gold and silver to that standard by adding the requisite quantity of the latter metal. This proce is called quartation. This alloy is then granulated or laminated; and from twice to thrice its weight of sulphuric or nitric acid is to be boiled upon it; and when it is judged that the solution has been pushed as far as po ible by this first acid, it is decanted, and new acid is poured on. Lastly, after having washed the gold, some sulphuric acid is to be boiled over it, which carries off a two or three thousandth part of silver, which nitric acid alone could not di olve. Thus perfectly pure gold is obtained. The silver held in solution by the sulphuric or nitric acid is precipitated in the metallic state by copper, or in the state of chloride by sea-salt. See Parting . Not only has the ratio between the value of gold and silver varied much in different ages of the world; but the ratio between these metals and the commodities they represent, has undergone variations, owing to the circumstances in which their mines have been succe ively placed; since they have always poured a greater quantity of the metals into the market than has been absorbed by use. This quantity has greatly increased since the discovery of America, a period of little more than 300 years. The mines of that continent, rich, numerous, and easily worked, by augmenting the ma of gold and silver, nece arily le ened the value of these metals compared with that of the objects of commerce represented by them, so that every thing else being equal, there is now required for purchasing the same quantity of commodities, much more gold or silver than was nece ary in the reign of Henry VII., before the discovery of America. This productivene of the American mines has had an influence on those of the ancient continent; many of whose silver and gold mines have been abandoned, not because the veins or auriferous sands are le rich than they were; but because their product no longer represents the value of human labour, and of the goods to be furnished in return for their exploitation. In the 3d. vol. of the Mining Journal, p. 331., we have the following statement as to the produce of the precious metals.—“In 40 years, from 1790 to 1830, Mexico produced 6,436,453 l. worth of gold, and 139,818,032 l. of silver. Chile, 2,768,488 l. of gold, and 1,822,924 l. of silver. Buenos Ayres, 4,024,895 l. of gold, and 27,182,673 l. of silver. Ru ia, 3,703,743 l. of gold, and 1,502,981 l. of silver. Total, 1880 millions sterling, or 47 millions per annum.” The following table shews what proportion the product of the mines of America bears to that of the mines of the ancient continent. Table of the Quantities of Gold which may be considered as having been brought into the European Market, every Year on an Average, from 1790 to 1802. | Continents. | Gold. | | Ancient Continent. | lbs. Avoir. | | Asia: | | | Siberia | 3740 | | Africa | 3300 | | Europe: | | | Hungary | 1430 | | Salzbourg | 165 | | Austrian States | | - | 165 | | Hartz and He ia | | Saxony | | Norway | | Sweden | | France | | Spain, . | | Total of the Ancient Continent | 8800 | | New Continent. | | | North America | 2860 | | South America: | | | Spanish dominions | 22,000 | | Brazil | 15,400 | | Total of the New Continent | 40,260 | The mines of America have sent into Europe three and a half times more gold, and twelve times more silver, than those of the ancient continent. The total quantity of silver was to that of gold in the ratio of 55 to 1; a very different ratio from that which holds really in the value of these two metals, which is in Europe as 1 to 15. This difference depends upon several causes, which cannot be investigated here at length; but it may be stated that gold, by its rarity and price, being much le employed in the arts than silver, the demand for it is also much le ; and this cause is sufficient to lower its price much beneath what it would have been, if it had followed the ratio of its quantity compared to that of silver. Thus also bismuth, tin, ., though much rarer than silver, are, neverthele , very inferior in price to it. Before the discovery of America, the value of gold was not so distant from that of silver, because since that era silver has been distributed in Europe in a far greater proportion than gold. In Asia the proportion is now actually only 1 to 11 or 12; the product of the gold mines in that quarter, being not so much below that of the silver mines as in the rest of the world. The total annual production of Gold at present has been estimated as follows. | From | the ancient Spanish colonies of America | 10,400 | kilogrammes | | Brazil | 600 | | | Europe and Asiatic Ru ia | 6,200 | | | The Indian Archipelago | 4,700 | | | Africa | 14,000 |? | | 35,900 | = 36 tons nearly | without taking into account the quantity of gold now extracted from silver. Gold has le affinity for oxygen than any other metal. When alone, it cannot be oxidized by any degree of heat with contact of air, although in combination with other oxidized bodies, it may pa into the state of an oxide, and be even vitrified. The purple smoke into which gold leaf is converted by an electric discharge is not an oxide, for it is equally formed when the discharge is made through it in hydrogen gas. There are two oxides of gold; the first or protoxide is a green powder, which may be obtained by pouring, in the cold, a solution of potash into a solution of the metallic chloride. It is not durable, but soon changes in the menstruum into metallic gold, and peroxide. Its constituents are 96·13 metal, and 3·87 oxygen. The peroxide is best prepared by adding magnesia to a solution of the metallic chloride; washing the precipitate with water till this no longer takes a yellow tint from muriatic acid; then digesting strong nitric acid upon the residuum, which removes the magnesia, and leaves the peroxide in the form of a black or dark brown powder, which seems to partake more of the properties of a metallic acid than a base. It contains 10·77 per cent. of oxygen. For the curious combination of gold and tin, called the Purple Precipitate of Ca ius , see this article, and Pigments Vitrifiable .