MET'ALLIIRGY is the art of extracting metals from their ores. The operations are• partly mechanical and partly chemical. Those processes which depend principally on chemical reactions for their results have reference chiefly to the roasting and smelting of ores, and are described under the heads of the different metals. But there are certain preliminary operationa of a mechanical kind which metallic Ores Undergo, such as crush ing, jigging, washing, etc., which we shall describe here, as they are essentially the same for the ore,s of lead, copper, tin, zinc, and indeed most of the metals. (For see that head.) Ores are first broken up with hammers into pieces of a convenient size for crushing or stamping. Waste material, such as pieces of rock, spar, etc., which always accom pany ore, are as far as possible picked out by hand, and the ore itself arranged in sorts according to its purity. Various kinds of apparatus, such as riddles, sieves, etc., are then used for separating it into different sizes, in order to secure a uniforrn strain on the crushing machinery.
In one of the most approved forms of crushing-mills the ore is raised by means of small wagons to a platform, where it is ready to be supplied to the crushing-rollers through an opening. These rollers are mounted in a strong iron frame, held together by wrought-iron bars, and bolted to strong beams. Their distance apart is regulated by means of a lever to which a weight is attached. The bearings of the rollers slide in grooves, so that when any extra pressure is put upon them by a. large or hard piece of ore, the lever rises, and allows the space between the rollers to widen. The crushed ore falls upon a series of sieves, which are made to vibrate. These have meshes incrqasing in fineness as they descend; and the upper two are so wide that pieces of ore too large to pass through them are conducted into the lower part of the bucket-wheel and raised again to the platform to be recrushed. The lower four sieves separate the remaining portion of the crushed ore into different degrees of fineness, which is collected in pits.
Instead of crushing-rollers, sometimes a stamping-mill is -used, especially for tin ores, which require to be reduced to a fine powder. The stamping-mill consists of a series of
upright shafts with a weighty piece of iron at the bottom of each. They are raised by means of an axle with projecting cams, and then, falling by their own weight, act like hammers.
After being crushed, the ore is washed and sifted on a jigging sieve. In one of its simplest forms the ore is placed on a table from which a sieve is filled. It is then immersed in a tub of water and a jigging motion communicated to it by a workman alternately raising and lowerinig a handle. This effects two purposes—it washes the ore, and separates the material into two layers: the upper consists of the lighter spar and other impurities, which are raked off; and the lower consists of the heavier and purer portions of the ore, which are now ready for the roasting furnace.
It will be apparent that in the bottom of the tub there must be a quantity of more or less valuable ore, which, from its fineness, has fallen through the sieve. This is called sludge or slime; and the minute particles of ore it contains are recovered either by simply forming an incline on the ground, and washing it with a current of water, or by using an inclined table called a sleeping-table. Ore which has been reduced to powder at the stamping-mill, as well as slime, is washed by this apparatus. The material is put into a chest which is placed in a sloping position, and is supplied with water on turning a stop-cock. The current carries the contents of the chest through an opening at the bottom, and spreads it, with the aid of a series of stops, or small bits of wood, over the surface of the table. A. stream of water is then kept flowing over the table till the earthy impurities are all carried down into a trough, the pure particles of the ore remain ing, by reason of their greater specific gravity, near the top of the table, whence they are removed to be smelted. Sometines the table is suspended by chains, and receives a succession of blows at the top from a buffer, moved by cams on the same principle as the stamping-mill. This arrangement is found of great advantage in dressing very poor ores.