Efforts have been made to improve the efficiency of the blast furnace by treating the air which is blown into it. The first step, which has long since passed into universal practice, is the pre heating of the blast. More recently it has been suggested to re move the moisture from the air of the blast. Extensive trials, especially in America, have shown that the furnace works better with dry air, but the methods of drying available some years ago—mainly by refrigeration—were too expensive. More re cently the remarkable drying powers of "silica gel" which is ob tained by drying gelatinous precipitated silica, have been utilised and it seems probable that the application of this drying process to blast furnaces may find large-scale application. Another im provement can be effected by enriching the air of the blast with oxygen. Undoubted advantages can be obtained by this means, but its economy depends upon the cost of oxygen.
Since large volumes of air are blown into the blast furnace, corresponding volumes of gas must pass through the top. Form erly these were allowed to burn at the top of the furnace. This waste of valuable fuel is now avoided by closing the top of the furnace. The gas is collected and used in a variety of ways. It supplies the heat required for pre-heating the blast for the fur nace and it may be burnt under steam boilers to generate the power required for the blowing engines that produce the blast while the surplus may produce electric power for other purposes.
Certain difficulties arise in the utilisation of blast-furnace gas from the fact that the gas carries with it a large volume of dust. Even if the gas is to be burnt almost immediately under a steam boiler, the bulk of this dust must be removed, otherwise the boiler and its flues rapidly become choked. If the gas is to be used for burning in large internal combustion (gas) engines then still more complete cleaning from dust is essential. Gas cleaning appliances are installed in many blast-furnace plants. They either operate by passing the gas at fairly low speeds through a large number of sacks of finely-woven material (Hal berg-Beth system) or else the dust is removed by the Lodge Cottrell method of electrical precipitation. This method of pre cipitating dust from fumes is used in a number of other metal lurgical processes. It depends upon the fact that fine particles of dust or moisture suspended in air, when electrified rapidly coalesce into particles which are heavy enough to settle. Electri
fication is produced by a high-tension discharge from a point. This requires special appliances, but the process is now widely used. In the case of blast-furnace gas, however, it only appears to become economical where the degree of cleaning which can be achieved by sack filtration is insufficient.
The quality of the pig-iron produced by the blast furnace depends upon the character of the ore with which the furnace is fed and also on the way in which the furnace is run. The pig iron always contains a considerable amount of carbon—of the order of 3 or 4 per cent. and different amounts, according to circumstances, of silicon, sulphur and phosphorus, as well as other impurities. The silicon of the iron is mainly derived from the siliceous "gangue" material which is always present, more or less, in the ore. Sulphur is derived, usually, from pyrites (iron sulphide) in the ore and phosphorus from phosphate minerals in the ore. The coke also contributes to the impurities of the iron, principally to the sulphur and for that reason iron made with wood-charcoal, as fuel in Sweden, is of higher purity than coke made iron, although the high purity of the Swedish ores has much to do with the result.
The composition of the pig iron determines the uses to which it can be put. These are, broadly, of four kinds :—the production of iron castings in the foundry ; the production of wrought iron by the puddling process and the third and fourth are the produc tion of steel by the acid and basic processes respectively, although further distinctions exist between irons suited for Bessemer as distinct from open-hearth steel-making. In iron founding—i.e., the production of iron castings, the requisite composition is ob tained by a judicious blending of different varieties of pig-irons of different composition. The cast-iron must have a sufficient content of silicon to render it grey and soft by the formation of graphite, and frequently the presence of a considerable amount of phosphorus is considered desirable in order to render the iron easily fusible and fluid. Formerly the scientific aspects of iron founding were much neglected, but a good deal of research work on the subject has been done recently, particularly in Germany where methods for the production of special and superior qualities of cast iron have been worked out. In England the subject has also received attention from the British Cast Iron Research Association.