Page:EB1911 - Volume 18.djvu/223

 General Sequence of Operations.—Occasionally, but rarely, metallic ores occur as practically pure compact masses, from which the accompanying matrix or “gangue” can be detached by hand and hammer. In most cases the “ore” (see ; ), as it comes out of the mine or quarry, is simply a mixture of ore proper and gangue, in which the latter not unfrequently predominates. Hence it is generally necessary to purify the ore before the liberation of the metal is attempted. Most metallic ores are specifically heavier than the accompanying impurities and their purification is generally effected by reducing the crude ore to a fine enough powder to detach the metallic from the earthy part, and then washing away the latter by a current of water, as far as possible (see ).

The majority of ores being chemical compounds, the extraction of their metals demands chemical treatment. The chemical operations involved may be classified as follows:—

1. Fiery Operations.—The ore, generally with some “flux,” is exposed to the action of fire. The fire in most cases has a chemical, in addition to its physical, function. Moreover the (q.v.) is designed so as to facilitate the action of the heat and furnace gases in the desired direction. It is intended either to burn away certain components of the ore—in which case it must be so regulated as to contain a sufficient excess of unburned oxygen; or it is meant to deoxidize (“reduce”) the ore, when the draught must be restricted so as to keep the ore constantly wrapped up in combustible flame gases (carbon monoxide, hydrogen, marsh-gas, &c.). The majority of the chemical operations of metallurgy fall into this category, and in these processes other metal-reducing agents than those naturally contained in the fire (or blast) are only exceptionally employed.

2. Amalgamation.—The ore by itself (if it is a reguline one), or with certain reagents (if it is not), is worked up, with mercury so that the metal is obtained as an amalgam, which can be separated mechanically from the dross. The purified amalgam is distilled, when the mercury is recovered as a distillate while the metal remains.

3. Wet Processes.—Strictly speaking, certain amalgamation methods fall under this head; but, in its ordinary acceptance, the term refers to processes in which the metal is extracted either from the natural ore, or from the ore after roasting or other preliminary treatment, by an acid or salt solution, and from this solution precipitated—generally in the reguline form—by some suitable reagent.

Few methods of metal extraction at once yield a pure product. What as a rule is obtained is a more or less impure metal, which requires to be “refined” to become fit for the market.

Chemical Operations.—Amalgamation and wet-way processes have limited applications, being practically confined to copper, gold and silver. We therefore here confine ourselves, in the main, to pyro-chemical operations.

The method to be adopted for the extraction of a metal from its ore is determined chiefly, though not entirely, by the nature of the non-metallic component with which the metal is combined. The simplest case is that of the reguline ores where there is no nonmetallic element. The important case is that of gold.

Oxides, Hydrates, Carbonates and Silicates.—All iron and tin ores proper fall under this heading, which, besides, comprises certain ores of copper, of lead and of zinc. The first step consists in subjecting the crude ore to a roasting or calcining process, the object of which is to remove the water and carbon dioxide, and burn away, to some extent at least, the sulphur, arsenic or organic matter. The residue consists of an impure oxide of the respective metal, which in all cases is reduced by treatment with fuel at a. high temperature. Should the metal be present as a silicate, lime must be added in the smelting to remove the silica and liberate the oxide.

The temperature required for the reduction of zinc lies above the boiling point of the metal; hence the mixture of ore and reducing agent (charcoal is generally used) must be heated in a retort combined with condensing apparatus. In all the other cases the reduction is effected in the fire itself, a tower-shaped blast furnace being preferably used. The furnace is charged with alternate layers of fuel and ore (or rather ore and flux, see below), and the whole kindled from below. The metallic oxide, partly by the direct action .of the carbon with which it is in contact, but principally by that of the carbon monoxide produced in the lower strata from the oxygen of the blast and the hot carbon there, is reduced to the metallic state; the metal fuses and runs down, with the slag, to the bottom of the furnace, whence both are withdrawn by opening plug-holes.

Sulphides.—Iron, copper, lead, zinc, mercury, silver and antimony very frequently present themselves in this state of combination, as components of, a family of ores which may be divided into two sections: (1) such as substantially consist of simple sulphides, as iron pyrites (FeS2), galena (PbS), zinc blende (ZnS), cinnabar (HgS); and (2) complex sulphides, such as the various kinds of sulphureous copper ores (all substantially compounds or mixtures of sulphides of copper and iron); bournonite, a complex sulphide of lead, antimony and copper; rothgiltigerz, sulphide of silver, antimony and arsenic; fahlerz, sulphides of arsenic and antimony, combined with sulphides of copper, silver, iron, zinc, mercury, silver; and mixtures of these and other sulphides with one another.

In treating a sulphureous ore, the first step as a rule is to subject it to oxidation by roasting it in a reverberatory or other furnace, which leads to the burning away of at least part of the arsenic and part of the sulphur. The effect on the several individual metallic sulphides (supposing only one of these to be present) is as follows:—

1. Those of silver (Ag2S) and mercury (HgS) yield sulphur dioxide gas and metal; in the case of silver, sulphate is formed at low temperatures. Metallic mercury, in the circumstances, goes off as a vapour, which is collected and condensed; silver remains as a. regulus, but pure sulphide of silver is hardly ever worked.

2. Sulphides of iron and zinc yield the oxides Fe2O3 and ZnO as final products, some basic sulphate being formed at the earlier stages, especially in the case of zinc. The oxides can be reduced by carbon.

3. The sulphides of lead and copper yield, the former a mixture of oxide and normal sulphate, the latter one of oxide and basic sulphate. Sulphate of lead is stable at a red heat; sulphate of copper breaks up into oxide, sulphur dioxide and oxygen. In practice, neither oxidation process is ever pushed to the end; it is stopped as soon as the mixture of roasting-product and unchanged sulphide contains oxygen and sulphur in the ratio of O2&#x202f;:&#x202f;S. The access of air is then stopped and the whole heated to a higher temperature, when the whole of the sulphur and oxygen is eliminated. This method is largely utilized in the smelting of lead from galena and of copper from copper pyrites.

4. Sulphide of antimony, when roasted in air, is converted into a kind of alloy of sulphide and oxide; the same holds for iron, only its oxysulphide is quite readily converted into the pure oxide Fe2O3 by further roasting. Oxysulphide of antimony, by suitable processes can be reduced to metal, but these processes are rarely used, because the same end is far more easily obtained by “precipitation,” i.e. withdrawing the sulphur by fusion with metallic iron, forming metallic antimony and sulphide of iron. Both products fuse, but readily part, because fused antimony is far heavier than fused sulphide of iron. A precisely similar method is used occasionally for the reduction of lead from galena. Sulphide of lead, when fused together with metallic iron in the proportion of 2Fe&#x202f;:&#x202f;1PbS yields a regulus (＝1Pb) and a “mat” Fe2S, which, however, on cooling, decomposes into the ordinary sulphide FeS, and finely divided iron. What we have been explaining are special cases of a more general metallurgic proposition: Any one of the metals, copper, iron, tin, zinc, lead, silver, antimony, arsenic, in general, is capable of desulphurizing (at least partially) any of the others that follows it in the series just given, and it does so the more readily and completely the greater the number of intervening terms. Hence, supposing a complete mixture of these metals to be melted down under circumstances admitting of only a partial sulphuration of the whole, the copper has the best chance of passing into the “mat,” while the arsenic is the first to be eliminated as such, or, in the presence of oxidants, as oxide.

Arsenides.—Although arsenides are amongst the commonest impurities of ores generally, ores consisting essentially of arsenides are comparatively rare. The most important are certain double arsenides of cobalt and nickel, which in practice are always contaminated with the arsenides or other compounds of foreign metals, such as iron, manganese, &c. The general mode of working these ores is as follows. The ore is first roasted by itself, when a part of the arsenic goes off as such and as oxide, while a complex of lower arsenides remains. This residue is now subjected to careful oxidizing fusion in the presence of some solvent for metallic bases. The effect is that the several metals are oxidized away and pass into the slag (as silicates) in the following order—manganese, iron, cobalt, nickel; and at any stage the as yet unoxidized residue of arsenide assumes the form of a fused regulus, which sinks down through. the slag as a “speis.” (This term has the same meaning in reference to arsenides as “mat” has in regard to sulphides.) By stopping the process at the right moment, we can produce a speis which contains only cobalt and nickel, and if at this stage also the flux is renewed we can further produce a speis which contains only nickel and a slag which substantially is one of cobalt only. The composition of the speises generally varies from AsMe3/2 to AsMe2, where “Me” means one atomic weight of metal in toto, so that in general 1Me＝xFe + yCo + zNi, where x + y + z＝1. The siliceous