Iron ores are generally carbonates, hydrates, or oxides of the metal, the latter being the best.
British iron is obtained from ores found in several strata, but chiefly in those of the coal-bearing or carboniferous series, in which they are most conveniently interspersed with the fuel (coal) and the flux (limestone) necessary for their reduction.
The following are the principal British iron ores: -
Clay Ironstone is a carbonate of iron of clay-like appearance. This is a very impure ore, containing not only clay, but pyrites and sulphur, and producing in some cases as little as 20 per cent of iron. However, on account of the large quantities in which it is found, and in consequence of its being near coal and limestone, it is the most important iron ore worked in Great Britain.
It occurs chiefly in the coal measures of Derbyshire, Staffordshire, Shropshire, Yorkshire, Warwickshire, and South Wales; also in the Lias formations of Yorkshire (Cleveland). The ores vary greatly in quality, having a yield of iron which ranges from 20 to 40 per cent.
Blackband is clay ironstone darkened by from 10 to 25 per cent of bituminous and carbonaceous matter, which makes it cheaper to smelt. It is found chiefly in Lanarkshire and Ayrshire, where it yields about 40 per cent of iron; also in Staffordshire, Durham, North Wales, where the yield varies considerably, being generally less than in Scotland.
This is the richest British iron ore, the chief impurity being silica; it yields from 50 to 60 per cent of iron.
This ore is found in the carboniferous limestone of Cumberland (Cleator Moor, Whitehaven), Lancashire (Ulverston), and in Glamorganshire.
Some of these ores are greatly in demand for making Bessemer steel.
Brown Haematite is also an oxide of iron (hydrated), and of a brown colour. It contains some 60 per cent of iron, and is found in Gloucestershire (Forest of Dean), Cumberland (Alston Moor), in Durham, Devonshire, Northamptonshire, and on the Continent.
Magnetic Iron Ore is seldom found in this country. A little occurs in Devonshire, but it exists in large quantities in Sweden and Norway.
Spathic Ore is a crystallised carbonate of iron, generally mixed with lime, found in Durham (Weardale), Devonshire (Exmoor), and Somersetshire (Brendon Hills). It yields about 37 per cent of iron.
Such of these ores as are rich in manganese are used for the manufacture of Spieg-eleisen (see p. 304).
Iron ores are not sufficiently valuable to pay for their being wasted and dressed.
Those that occur in large - masses, such as clay ironstone, are roasted to drive off the carbonic acid, and to render them more easy to break up.
The extraction of the metal from the ore is effected in a large upright furnace lined with firebrick.
Into this furnace a strong blast of air is forced.
In former years the air for the blast was supplied at its ordinary temperature. This is still done in some few instances, the process being called the "cold blast" and the resulting material cold-blast iron.
Some years ago the hot-blast process was introduced. In this the air is raised to a temperature of some 800° or 900° Fahr. (sometimes to 1200° or 1400°) before being forced into the furnace. By this a very great saving of fuel is effected, and a greater heat obtained. Moreover, calcining may sometimes be dispensed with, coal may be used instead of coke, and altogether the process is far more economical.
The object of smelting is to free the metal from its combinations, and to get (as far as possible) all impurities out of the ore in. the form of a fusible slag.
To effect this a flux is added of a nature suited to combine with the impurities or "gangue " in the ore.
If the gangue is chiefly clay, as it often is in this country; limestone is added as a flux. If the gangue is chiefly quartz, an argillaceous iron ore and limestone are added. If the gangue itself is limestone, clay or clayey ores are added.
The furnace is filled to a certain height with fuel. When this is burning, ore mixed with flux is introduced from the top, and then layers of fuel and ore, with flux, alternately.
When the furnace is fully heated, the molten iron sinks to the ■ bottom, being covered by the lighter and more fusible impurities in the form of "slag."
A furnace once lighted is not allowed to go out until it requires thorough repair, but is continually replenished with fuel and ore at the top.
When a considerable quantity of molten iron has collected, the furnace is tapped, and the iron is run into a long channel formed in sand, having branches on each side, called the sow and her pigs - hence the bars produced are called "pig-iron."
The very high temperature produced by the hot blast enables many of the impurities in the ore to be reduced to a molten state, and run out with the metal.
If this is taken advantage of, the impurities, are retained in the resulting metal, instead of being got rid of in the smelting process, and a very weak inferior iron is produced.
It is evident, then, that the hot blast may be used to produce a very inferior material, and this for some time brought it into disrepute.
Many specifications still direct that cold-blast iron is to be used.
It has, however, been shown by experience that the temperature of the blast has, in itself, but little effect upon the iron produced, and that, with the same care in the selection of materials and conduct of the process, iron may be produced by the hot blast of as good quality, and as reliable, as that from a cold-blast furnace.
After a great many experiments on the relative strength of hot-blast and cold-blast iron, Sir William Fairbairn came to the following conclusion :-
"From the evidence here brought forward it is rendered exceedingly probable that the introduction of a heated blast in the manufacture of cast irons has injured the softer irons, while it has frequently mollified and improved those of a harder nature; and considering the small deterioration that the irons of quality No. 21 have sustained, and the apparent benefit to those of No. 3, together with the great saving effected by the heated blast, there seems some reason for the process becoming general, as it has done." 2
There are but few cold-blast furnaces now in the country. Among them may be mentioned those at the celebrated Lowmoor and Bowling works; also some at Blaenavon in South Wales.
1 See page 249.
2 Iron Manufacture, etc., by Sir William Fairbairn.