The slimes are freed from the lighter mud in the trunking-box, [figs. 1144], [1145.]; which is from 7 to 8 feet long. Being accumulated at M, the workman pushes them back with a shovel from a towards b. The metallic portion is carried off, and deposited by the stream of water upon the table; but the earthy matters are floated along into a basin beyond it. The product collected in the chest is divided into two portions; the one of which is washed once, and the other twice, upon the rack, [fig. 1146.] This is composed of a frame C, which carries a sloping board or table, susceptible of turning round to the right or left upon two pivots, K, K. The head of the table is the inclined plane T. A small board P, which is attached by a band of leather L, forms the communication with the lower table C, whose slope is generally 5 inches in its whole length of 9 feet; but this may vary with the nature of the ore, being somewhat less when it is finely pulverized. The ore is thrown upon T, in small portions of 20 or 25 lbs. A woman spreads it with a rake, while a stream of water sweeps a part of it upon the table, where it gets washed. The fine mud falls through a cross slit near the lower end, into a basin B. After working for a few minutes, should the schlich seem tolerably rich, the operative turns the table round its axis K, K, so as to tumble it into the boxes below. The mud is in B; an impure schlich in B′, which must be washed again upon the rack; and a schlich fit for roasting in B′′.
The slope of the rack-table for washing the roasted tin ore, is 73⁄4 inches in the 9 feet.
Crushing rolls at the Pembroke mines.—Waggons, moved on a railway by an endless rope, bring the ore to be crushed, immediately over the rolls, as shown in [fig. 1147.] A trap being opened in the side of the waggon, the ore falls into the hopper T, whence it passes directly between the twin cylinders C, C, and next upon the sieve D, which receives a seesaw motion horizontally, by means of the rod L, and the crank of the upright turning-shaft. The finer portion of ore, which passes through that sieve, forms the heap S. The coarser portion is tossed over the edge of the sieve, and falls between the cylinders C′ C′, upon a lower level, and forms the second heap S′ of sifted, and S′′ of unsifted, ore.
The holes of the sieves D, D′, being of the same size, the products S, S′, are of the same fineness. S′′ is ground again, being mixed, in the uppermost hopper T, along with the lumps from the waggons.
The diameter and length of the under rolls (see [fig. 1148.]) are each 16 inches. a b, is the square end of the gudgeon t, which prevents the shaft shifting laterally out of its place. The diameter of the upper rolls is 18 inches, but their length is the same. Both are made of white cast iron, chilled or case-hardened by being cast in iron moulds instead of sand; and they last a month, at least, when of good quality. They make from 10 to 15 turns in a minute, according to the hardness of the ores of tin or copper; and can grind about 50 tons of rich copper ore in 12 hours; but less of the poorer sort.