A NOTE ON THE PROCESSES IN 1867
Iron, the chief constituent of all kinds of steel, is not found pure in nature in commercially valuable quantities, but mixed or combined with oxygen, silicon, alumina, lime, phosphorus, sulphur and other materials. Hence though old iron and steel can be used in steel- making, the separation of iron from newly won iron-ore is a necessary initial process in any steel industry whose output is to be sustained, let alone expanded.
By 1867 this separation had for long been effected by smelting in a blast furnace. A mixture of iron-ore and coke (with limestone as a flux) was "charged" (which in practice meant "emptied out of wheelbarrows") into the top of a tall hollow firebrick "stack" encased in iron plates--in form a cross between a cone and a cylinder--at whose base, the "hearth" of the furnace, a fire had been kindled. The newest furnaces were eighty feet high. Combustion was maintained by blowing hot air through a number of pipes or "tuyeres" near the hearth, and in this region, where the heat was greatest, the ore melted, and the bulk of the non-ferrous "impurities" floated as a scum or "slag" on top of the iron. It was a simple matter to draw off or "tap" the slag and the iron separately; and when this was done more ore and coke descended to the melting zone, and more were charged at the top. The process was cyclical and continuous, a furnace remaining alight normally until the firebrick lining was burnt out. During the process a great volume of combustible gas was produced (its most important constituent being CO from the combustion of the coke), and this was utilised in the newer furnaces to raise steam and to heat the blast. The newer furnaces also had a new type of hot-blast stove. Hitherto the blast had passed through cast-iron pipes immersed in flames. The new stoves were composed of a network of firebricks encased in plates. This network alternately absorbed heat generated by the burning of the furnace gases, and transmitted the heat so absorbed to the blast. More than one stove was, of course, required for each stack.