Page:The American Cyclopædia (1879) Volume XI.djvu/438

 426 METALLURGY copper ores which may contain 5 per cent, or less of metal are worked by a process known as matte smelting, whereby the copper is raised to 70 or 80 per cent, by the removal of earthy ingredients and iron. In this process the cop- per in the ore is collected in a matte which is mainly a combination of iron, copper, and sul- phur. The iron is gradually removed in suc- cessive operations, until only copper and sul- phur are left. From this enriched product the copper is separated by the roasting-reaction process. (See COPPER.) Nickel ores are like- wise first smelted to a matte. The furnaces employed for metallurgical operations may be divided into the shaft or blast furnace, the gas or reverberatory furnace, and the crucible. In the shaft furnace the ore or other metal- lic compound is charged alternately with the fuel, and a reducing atmosphere, that is, one in which carbonic oxide predominates, always ex- ists. In a reverberatory furnace the fuel does not come in contact with the hearth of the furnace, on which the substance to be heated is placed. The atmosphere here is generally oxidizing, owing to the practical impossibility of obtaining a high temperature without the admission of a slight access of air. In gas fur- naces the access of air can be better regulated than with the ordinary reverberatory, but it must not be overlooked that carbonic acid, the product of combustion, gives off oxygen to some metals (as iron) at a red heat. Where a low temperature only is required, a more or less reducing atmosphere may be maintained by limiting the supply of air. But when an active reducing action is required on the hearth of a reverberatory furnace, the substance to be treated is intimately mixed with coal. A cov- ered crucible or retort heated from without is entirely independent of the source and character of the heat, and consequently the nature of cru- cible smelting depends solely on the substances employed. The earthy matters associated with the metals in ores are removed in ordinary smelt- ing operations in the form of a fusible com- pound, which when solidified is generally hard and stony and is called slag or cinder. In iron smelting the slag consists mainly of silica, lime, and alumina. It is rarely the case that an iron ore contains these substances in the proper proportion to form a fusible cinder, and conse- quently the substance which is deficient must be added. As most iron ores are silicious, limestone is usually added as flux. In copper and lead smelting, the cinder usually contains, besides the earthy bases, a considerable amount of iron in the form of ferrous oxide. The cinder from copper matte smelting, is almost exclusively a ferrous silicate. Mill cinder pro- duced in the operation of puddling pig iron is also a ferrous silicate, but the iron contained in it is the incidental and unavoidable result of the oxidizing atmosphere in the furnace neces- sary for the removal of the silicon and carbon ; it does not therefore represent an enriched product, but a waste of iron. In order to facilitate the extraction of the metal, it is often necessary or desirable to change the physical or chemical constitution of the ores. This is effected by roasting or calcination. Roasting in its simplest form consists merely in the exposure of the ore to heat, in order to render it friable and porous and thereby more readily reducible. Compact iron ores are often thus treated. Again, an ore may contain volatile in- gredients which can be driven off by heat. It becomes thereby enriched, besides being render- ed porous. The spathic ores of iron (carbonates) and the brown hematites (hydrates) when heat- ed part with their carbonic acid and water, and are converted, the former into porous magnetic oxide, and the latter into red oxide. This kind of roasting, generally called calcination, is usu- ally effected in low-shaft furnaces or kilns, the heat being generated by fuel charged with the ore, or by the use of gas. The sulphides and arsenides of the heavier metals are often roast- ed in order to break up existing combinations and to form others which are more suscepti- ble to treatment. This roasting may be either oxidizing or chloridizing. Oxidizing roasting consists in subjecting the ore, or other metallic combination, as matte, in the form of lumps or powder, to the action of heat, with free access of air. It is sometimes conducted in heaps in the open air by piling up lumps of ore or matte with layers of fuel. When suffi- cient sulphur is present, the pile when once ignited continues to burn without the aid of fuel. This method is always tedious, and gen- erally imperfect. The ordinary method of fur- nace roasting consists in exposing the ore in the form of powder to the action of heat and air on the hearth of a reverberatory furnace. The ore must be frequently turned and rab- bled, so that the oxidation shall proceed uni- formly. It is also necessary to avoid a tem- perature which would sinter or fuse the mass, and thus hinder the complete exposure of the small particles to the air. To obviate the ne- cessity of hand labor in turning the charge, and also to hasten the roasting, mechanical ap- pliances have been employed, such as revolving chambers and hearths. It has also been found that showering the ore or matte in fine pow- der into a heated chamber or stack is a very expeditious method of roasting. Gerstenhofer was the first to introduce this practice. His furnaces are rectangular chambers provided with iron bars of triangular section arranged at regular intervals, base uppermost. The time of exposure in falling is thus somewhat pro- longed. The sulphides of the different metals behave very differently when roasted. In gen- eral the metal and sulphur are both oxidized , part of the latter passes off as sulphurous acid, which under favorable conditions can be util- ized in the production of oil of vitriol; and part is converted into sulphuric acid, which combines with the metallic oxide. At a high temperature this sulphuric acid may be driven off, either wholly or in part, and the oxide left.