A mixture is made of "spongy iron" (see p. 19) and pitch. This mixture is heated to redness in order to decompose the pitch, which consists of compounds of carbon and hydrogen. These compounds are decomposed, and a part of the carbon is left mixed with the spongy iron, while the hydrogen escapes in combination with the rest of the carbon. To this mixture, placed in an iron crucible, caustic soda is added; the lid of the crucible, which is furnished with a curved tube sloping downwards to a condenser, is fixed in place, and the crucible is heated in a furnace to bright redness. The carbon removes oxygen both from the hydrogen and the sodium, and sodium and hydrogen pass over into the condenser along with carbon monoxide, the sodium alone condensing, for the others are gaseous and escape. The equation is: 2NaOH + 2C = 2CO + H2 + 2Na. The condenser consists of a flat hollow copper vessel; the sodium is raked out as it accumulates.
The chief ore of zinc is the sulphide. To convert it into the oxide, it is roasted on a flat hearth in a current of air : 2ZnS + 3O2 = 2ZnO + 2SO2. The oxide is mixed with small coal (slack) and placed in cylindrical retorts of fireclay. These retorts have pipes of rolled sheet-iron luted to the open ends with fireclay; they are packed into a furnace in tiers, and the temperature is raised to bright redness. The coal distils first, giving off coal-gas, which expels air from the retorts. When the temperature exceeds 1000% the zinc distils and condenses in the iron pipes. It happens that almost all zinc ores contain cadmium sulphide, which, like zinc sulphide, is converted into oxide by roasting; and on distillation, the cadmium, which is the more volatile metal, distils over first and condenses in the outer portion of the tubes. These are untwisted and the metal removed with a chisel.
The chief ores of iron are the carbonate and the oxide. The former is practically always mixed with clay (clayband) or with coal (blackband), and generally contains sulphur and phosphorus in the form of calcium sulphate, CaSO4, and calcium phosphate, Ca3(PO4)2. The sulphur is sometimes present in the form of iron pyrites, FeS2. The ore is roasted to expel carbon dioxide, thus: 4FeCO3 + O2 = 2Fe2O3 + 4CO2. If it were then in its impure state smelted with coal, the iron would not flow, but would remain mixed with the clay. However, this process, if the ore is pure and charcoal is used as fuel, yields a mass of iron sponge, which can be heated and welded by hammering into a coherent mass.
The process is still used by Africans, and was at one time universal. On the large scale, however, it is necessary to add lime in order to form a flux with the clay. Clay consists of a compound of silica, SiO2, and alumina, Al2O3, and with lime it melts to a glassy slag. Alternate layers of coal, lime, and the roasted ore are fed in at the top of a blast-furnace, a tall conical erection of firebrick, strengthened by being bound with iron hoops; at the bottom there is a 11 crucible," or receptacle for the molten iron, which can be discharged when required by forcing a hole in its side with an iron bar. There are also holes which admit water-jacketed tubes or "tuyeres," which convey a blast of air heated to about 6oo° to increase the temperature of combustion of the coal. Here the reduction takes place in the upper part of the furnace, owing to the carbon monoxide formed by the combustion of the coal in the lower part of the heated mass; it acts on the oxide of iron thus : Fe2O3 + 3CO = 2Fe + 3CO2. As the iron passes down the furnace it melts, and is met by the fused slag; it then coheres and runs into the crucible, whence it is drawn off from time to time.
Carbon unites with molten iron, forming a carbide; hence the product of the blast-furnace is not pure iron, but a mixture of iron with its carbide, and also with its sulphide and phosphide, if the ore has contained sulphates or phosphates. When such impure iron is brought in contact with oxygen in a molten or semi-molten condition, the carbon, sulphur, and phosphorus are oxidised mostly before the iron. If lime be present, sulphate and phosphate of calcium are formed. The modern process of removing these impurities is to pour the molten metal into a pear-shaped iron vessel lined with bricks made of magnesia; while it is molten, air is blown through the metal, and the carbon burns to carbon dioxide; the sulphur and phosphorus are likewise oxidised and combine with lime, a layer of which floats on the surface of the molten metal. When these impurities have thus been removed in the " Bessemer converter," the metal is poured into a mould. Steel is a mixture of iron with a trace of its carbide, and it is produced by mixing with the blown iron, before it is poured, a quantity of iron containing carbon and manganese (a metal which confers valuable properties on iron). The quantity of carbon in steel may vary between 0.6 and 1.5 per cent.; with the content of carbon varies also the quality of the steel; that with a small proportion is soft, with a high proportion hard.
Oxygen is used in Deacon's process to liberate chlorine from hydrogen chloride. The latter gas, mixed with air, is passed through a chamber kept between the limits of temperature 375°-400°, containing bricks soaked with cupric chloride, CuCl2. At this temperature the cupric chloride decomposes into cuprous chloride, CuCl, and free chlorine, but the cuprous chloride is reconverted into cupric chloride at the expense of the chlorine produced by the interaction of the hydrogen chloride and the air, thus: 4HCI + O2 = 2H2O + 2C12. The cupric chloride is again decomposed. This kind of action, where a limited quantity of a substance, itself not permanently changed, causes an apparently unlimited change in other reacting bodies, is termed " surface action," for its rate is dependent on the extent of the surface of the agent; and the name "catalysis" is sometimes given to such an action. The action would take place independently of the catalytic agent, but at a very slow rate; the presence of the catalyser has the effect of greatly increasing the rate at which the change takes place. The chlorine thus prepared is not pure, but mixed with the nitrogen and argon of the air, but it serves for some purposes. The rate of such action of oxygen in displacing bromine or iodine from their compounds with hydrogen is much greater, and at a high temperature the elements could be formed thus, but they are not usually produced in this way.