of the puddling of iron, a process which is conducted in the presence of a layer of ferruginous cinder. Lead oxide is an energetic oxidising agent on account of its being readily reducible to the metallic state. It is, however, expensive, and yields but 7 per cent. of oxygen. Lead oxide and lead sulphide, when heated together, give metallic lead— 2PbO + PbS = 3Pb + SO2 Oxides of copper and antimony serve as oxidisers in the treatment of these metals. In heating together the sulphide and oxide of copper, copper is obtained. Potassium and sodium nitrates are employed in refining silver and antimony, and in the "Heaton" process sodium nitrate is employed to convert phosphorus and vanadium in certain varieties of pig-iron into sodium phosphate and vanadate. Metallic sulphates are often used as oxidising agents, the sulphate oxidising the sulphide and reducing the metal. In the case of iron in the blast furnace, carbonic anhydride, as is shown in the following chapter, also acts as an oxidiser. Reducing Agents. When a metal is separated from a state of chemical combination, it is said to be reduced, and the process of separation is termed reduction. The agents employed for this purpose are mainly carbon and hydrogen, or their compounds. Occasionally, however, the metallurgist makes use of iron, manganese, lead, or of sulphides and arsenides. The efficacy of a reducing agent depends on the absence of oxidised or inert elements. Thus, on account of the water and oxidised compounds they contain, wood, peat, and lignite are less energetic as reducing agents than coal. Similarly, coals rich in carbon are more valuable as reducing agents than coals rich in oxygen. Carbon can take up 1rd or 23rds of its weight of oxygen according to the oxide in whose presence it is used, to the temperature at which the reduction is effected, and to the relative proportion between the oxygen to be removed and the carbon employed. In the first case carbonic oxide is formed, and in the second carbonic anhydride. The reduction of metals, of which the oxides are easily reduced, such as lead and copper, is complete even if carbonic anhydride alone is formed. In the case of iron, however, total reduction is not possible unless carbonic oxide is present in excess, on account of the oxidising action of carbonic anhydride. Indeed, a mixture of equal volumes of carbonic oxide and carbonic anhydride will not reduce iron oxides below ferrous oxide. This is also true of manganese oxides. Reduction by solid carbon is slow, and is effected merely by cementation, that is, by a gradual transmitting action, if carbonic oxide is not formed. This gas is the most important reducing agent. It penetrates to the centre of the oxidised substance, absorbs its oxygen, and is converted into carbonic anhydride. In this way I part by weight of carbonic oxide gives 1.57 parts of carbonic anhydride. Besides this, Gruner* and Sir Lowthian Bell † have shown that, even at low temperatures, carbon, in the presence of iron, separates out from carbonic oxide, 2CO = CO, + C, and has a powerful reducing action. It continuously converts carbonic anhydride into the lower oxide. Reference is made to this subject in the description of the blast furnace given in the following chapter. In addition to the above-mentioned reducing agents, metals and metalloids are sometimes used to remove oxygen. Thus, iron deoxidises the salts and oxides of copper, of lead, and of mercury, and sodium liberates the metals magnesium and aluminium, from their haloid salts. Sulphides and arsenides are also employed. Thus, sulphides of iron and zinc separate copper from certain slags. Silicates of iron and zinc are formed, while sulphur retains the copper in the regulus. Chemical Agents.-Certain sulphurising agents are employed in metallurgical processes. These are specially useful in the treatment of silver and copper. The agents are iron or copper pyrites, barium or calcium sulphates, and, less frequently, metallic sulphides. Nickel and cobalt behave with regard to arsenic as silver and copper do in the case of sulphur. These metals may be protected, by means of their affinity for arsenic, from the scorifying action of silicates. ores. Chlorine is largely used in the treatment of gold and silver It is employed in the gaseous state or in the form of alkaline hypochlorites. The perchlorides of iron, copper, and mercury act as chloridising agents by being converted into lower chlorides. They are obtained usually by the direct action of hydrochloric acid on peroxides. Iodine is used in the metallurgy of silver, and bromine in that of gold. The agents employed for effecting the solution of metallic substances are very varied. The most important solvent is water, which is used for dissolving sulphates of iron, copper, and zinc. Other salts are dissolved by salt solutions; thus, chloride of silver is dissolved by an aqueous solution of sodium chloride. * Ann. de Phys. et de Chim., 1872; Traité de Métallurgie, vol. i. (1875), p. 172. +Chemical Phenomena of Iron Smelting. Calcium chloride, sodium hyposulphite, and ammonium carbonate are also used. Compounds of sulphur, oxygen, and calcium are now largely used in the metallurgy of silver. Metallic oxides are dissolved by acids, gold and platinum are dissolved by aqua regia, and in the amalgamation of gold and silver, mercury is the solvent employed. In metallurgical processes there are scarcely any limits to the use of ordinary chemical reagents beyond those imposed by the price of the material. M CHAPTER VII. FURNACES. Materials used in the Construction of Furnaces. In addition to the ordinary building materials used for the exterior portions of furnaces, refractory bricks and materials are required for the interior where a high temperature and the scouring action of metallic oxides have to be resisted. These materials may be used either in the natural state or as bricks. Of the natural materials, sandstones are most largely employed, the best varieties being those in which the quartz grains are cemented by a siliceous material. In the form of quartz, silica is able to resist all temperatures except that of the oxy-hydrogen blowpipe. Coarse-grained sandstones, such as millstone-grit, are frequently advantageously used. The Dinas rock found in the Vale of Neath, South Wales, is an example of this type. It usually contains 98 per cent. of silica. The pulverised rock is mixed with a little lime or clay to make it cohere, and is pressed into bricks. These resist a very high temperature, and are especially useful for the arches of reverberatory furnaces, as they expand with heat. Their composition, however, does not enable them to resist the action of metallic oxides. In steel-melting furnaces, where Dinas bricks are used, the tie-rods must be slackened as the heat increases, and tightened when the furnace subsequently cools. Silica bricks should be set as hot as possible, and the temperature of the furnace gradually raised. Ganister is a siliceous material, somewhat similar to the Dinas stone, found in the lower coalmeasures of Yorkshire. There are three classes of refractory materials : 1. Acid, such as Dinas stone and ganister. 2. Neutral, such as graphite, chrome-iron ore, and fire-clay. Basic materials are now largely used. Bauxite is used for resisting metallic oxides, and for that reason it is used for lining Siemens furnaces. It is first calcined with 3 per cent. of argillaceous clay containing 6 per cent. of graphite. It is essentially a hydrated ferric aluminate, and has the following com The use of basic refractory materials has been rendered necessary by the extension of the basic Bessemer process. Formerly the Bessemer converter, capable of holding 5 tons of metal, was invariably lined with ganister or Dinas stone. With such a lining, however, it was impossible to get rid of the phosphorus in the iron, whilst with a basic lining this is easily accomplished. Lime is one of the most refractory substances known, but it cannot be used on account of its readiness to become hydrated, and consequently to disintegrate. Magnesia, too, does not appear to answer well; but magnesite gives excellent bricks. Its great cost, however, precludes its general adoption. The mixtures of lime and magnesia, obtained by heating dolomite, give the most satisfactory results. Coated with tar, they are easily protected from moisture, and the proportion of silica they contain is considered advantageous. A refractory material of this kind containing is largely used. The slag obtained from basic refractory materials. in the Bessemer converter is being largely used as a fertiliser for agricultural purposes on account of the phosphorus it contains. Some 400,000 tons of it are annually used in Germany. Fire-bricks are mostly made of fire-clay, mixed with quartz, burnt clay, or pulverised refractory materials that have previously been used. The admixture of graphite is not so usual for firebricks as for the refractory crucibles in which metals and alloys. are melted. Fire-clays consist essentially of hydrated aluminium silicates, having the following composition :— SiO2. Al2O3. 30 to 35 H20. 10 to 15 When lime, magnesia, potash, or soda are present in quantities. exceeding 1 per cent. the clay becomes fusible. In aluminous clays less than 0.7 per cent. of these oxides does not depreciate their refractory value. As a rule, a small amount of these oxides or of ferrous oxide is sufficient to condemn a clay. Thus ordinary shale fuses at a comparatively low temperature, on account of its large percentage of alkaline oxides and ferrous oxide. The plasticity of clays depends upon the fineness of the particles |