Crushing rolls at the Pembroke mines

—Waggons, moved on a railway by an endle 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 pa es 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 pa es through that sieve, forms the heap S . The coarser portion is to ed 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 finene . S ′′ is ground again, being mixed, in the uppermost hopper T , along with the lumps from the waggons. IMG:4147767755307473660_illo1246b.png:Crusher roll 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 hardne of the ores of tin or copper; and can grind about 50 tons of rich copper ore in 12 hours; but le of the poorer sort. The next proce is the calcination in the burning-house ; which includes several reverberatory furnaces. At the mine of Poldice, they are 4 or 5 yards long, by from 2 1 ⁄ 2 to 3 yards wide. Their hearth is horizontal; the elevation, about 26 inches high near the fireplace, sinks slightly towards the chimney. There is but one opening, which is in the front; it is closed by a plate-iron door, turning on hinges. Above the door there is a chimney, to let the sulphureous and arsenical vapours fly off, which escape out of the hearth, without annoying the workmen. This chimney leads to horizontal flues, in which the arsenious acid is condensed. Six hundred weight of ore are introduced; the calcination of which takes from 12 to 18 hours, according to the quantity of pyrites contained in the ore. At the beginning of the operation, a moderate heat is applied, after which it is pushed to a dull red, and kept so during several hours. The door is shut; the materials are stirred from time to time with an iron rake, to expose new surfaces, and prevent them from agglutinating or kerning , as the workmen say. The more pyrites is present, the more turning is nece ary. Should the ore contain black oxide of iron, it becomes peroxidized, and is then easily removed by a subsequent washing. IMG:4147767755307473660_illo1246c.png:Roasting furnace Figs. 1149 , 1150. represent the furnace employed at Altenberg, in Saxony, for roasting tin ores. a is the grate; b , the sole of the roasting hearth; c , an opening in the arched roof for introducing the dried schlich (the ground and elutriated ore); d , is the smoke-mantle or chimney-hood, at the end of the furnace, under which the workmen turn over the spread schlich, with long iron rods bent at their ends; e , is the poison vent, which conducts the arsenical vapours to the poison chamber ( gifthaus ) of condensation. When the ore is sufficiently calcined, as is shown by its ceasing to exhale vapours, it is taken out, and exposed for some days to the action of the air, which decomposes the sulphurets, or changes them into sulphates. The ore is next put into a tub filled with water, stirred up with a wooden rake, and left to settle; by which means the sulphate of copper that may have been formed, is di olved out. After some time, this water is drawn off into a large tank, and its copper recovered by precipitation with pieces of old iron. In this way, almost all the copper contained in the tin ore is extracted. The calcined ore is sifted, and treated again on the racks, as above described. The pure schlich, called black tin , is sold under this name to the smelters; and that which collects on the middle part of the inclined wash-tables, being much mixed with wolfram, is called mock lead . This is pa ed once more through the stamps, and washed; when it also is sold as black tin . Stream tin is dre ed by similar methods: 1. by washing in a trunking-box, of such dimensions that the workman stands upon it in thick boots, and makes a skilful use of the rake; 2. by separating the larger conglomerate pebbles from the smaller pure ones; picking, stamping, and washing, on a kind of sleeping-tables . See Metallurgy , figs. 677 , 678 . The tin ores of Cornwall and Devons hire are all reduced within the counties where they are mined, as the laws prohibit their exportation out of them. Private interests suffer no injury from this prohibition; because the ve els which bring the fuel from Wales, for smelting these ores, return to Swansea and Neath loaded with copper ores. The smelting-works belong in general to individuals who po e no tin mines, but who purchase at the cheapest rate the ores from the mining proprietors. The ores are appraised according to their contents in metal, and its finene ; conditions which they determine by the following mode of a ay. When a certain number of bags of ore, of nearly the same quality, are brought to the works, a small sample is taken from each bag, and the whole are well blended. Two ounces of this average ore are mixed with about 4 per cent. of ground coal, put into an open earthen crucible, and heated in an air furnace (in area about 10 inches square) till reduction takes place. As the furnace is very hot when the crucible is introduced, the a ay is finished in about a quarter of an hour. The metal thus revived, is poured into a mould, and what remains in the crucible is pounded in a mortar, that the grains of tin may be added to the ingot. This method, though imperfect in a chemical point of view, serves the smelter’s purpose, as it affords him a similar result to what he would get on the great scale. A more exact a ay would be obtained by fusing, in a crucible lined with hard-rammed charcoal, the ore mixed with 5 per cent. of ground gla of borax. To the crucible a gentle heat should be applied during the first hour, then a strong heat during the second hour, and, lastly, an intense heat for a quarter of an hour. This proce brings out from 4 to 5 per cent. more tin than the other; but it has the inconvenience of reducing the iron, should any be present; which by subsequent solution in nitric acid will be readily shown. This a ay would be too tedious for the smelter, who may have occasion to try a great many samples in one day. The smelting of tin ores is effected by two different methods :— In the first, a mixture of the ore with charcoal is exposed to heat on the hearth of a reverberatory furnace fired with coal. In the second, the tin ore is fused in a blast furnace, called a blowing-house, supplied with wood charcoal. This method is practised in only a few works, in order to obtain a very pure quality of tin, called grain tin in England, and étain en larmes in France; a metal required for certain arts, as dyeing, . This method is applied merely to stream tin. In the smelting-houses , where the tin is worked in reverberatories, two kinds of furnaces are employed; the reduction and the refining furnaces. IMG:4147767755307473660_illo1247.png:Smelting furnaces Figs. 1151 , 1152. represent the furnaces for smelting tin at St. Austle, in Cornwall; the former being a longitudinal section, the latter a ground plan, a is the fire-door, through which pitcoal is laid upon the grate b ; c is the fire-bridge; d , the door for introducing the ore; e , the door through which the ore is worked upon the hearth f ; g , the stoke-hole; h , an aperture in the vault or roof, which is opened at the discharge of the waste schlich, to secure the free escape of the fumes up the chimney; i , i , air channels, for admitting cold air under the fire-bridge and the sole of the hearth, with the view of protecting them from injury by the intensity of the heat above. k , k , are basins into which the melted tin is drawn off; l , the flue; m , the chimney, from 35 to 50 feet high. The roasted and washed schlich is mixed with small coal or culm, along with a little slaked lime, or fluor spar, as a flux; each charge of ore amounts to from 15 to 24 cwt., and contains from 60 to 70 per cent. of metal. IMG:4147767755307473660_illo1248.png:Smelting furnace Fig. 1153. represents in a vertical section through the tuyère, and fig. 1154. in a horizontal section, in the dotted line x , x , of fig. 1153. , the furnace employed for smelting tin at the Erzgebirge mines, in Saxony. a , are the furnace pillars, of gnei ; b , b , are shrouding or casing walls; c , the tuyère wall; d , front wall, both of granite; as also the tuyère e . f , the sole-stone, of granite, hewn out basin-shaped; g , the eye , through which the tin and slag are drawn off into the fore-hearth h ; i , the stoke-hearth; k , k , the light ash chambers; l , the arch of the tuyère; m , m , the common flue, which is placed under the furnace and the hearths, and has its outlet under the vault of the tuyère. In the smelting furnaces at Geyer the following dimensions are preferred:—Length of the tuyère wall, 11 inches; of the breast wall, 11 inches; depth of the furnace, 17 inches. High chimney-stalks are advantageous where a great quantity of ores is to be reduced, but not otherwise. The refining furnaces are similar to those which serve for reducing the ore; only, instead of a basin of reception, they have a refining basin placed alongside, into which the tin is run. This basin is about 4 feet in diameter, and 32 inches deep; it consists of an iron pan, placed over a grate, in which a fire may be kindled. Above this pan there is a turning gib, by means of which a billet of wood may be thrust down into the bath of metal, and kept there by wheeling the gibbet over it, lowering a rod, and fixing it in that position. The works in which the blast furnaces are employed, are called blowing-houses . The smelting furnaces are 6 feet high, from the bottom of the crucible (concave hearth) to the throat, which is placed at the origin of a long and narrow chimney, interrupted by a chamber, where the metallic dust, carried off by the blast, is deposited. This chamber is not placed vertically over the furnace; but the lower portion of the chimney has an oblique direction from it. The furnace is lined with an upright cylinder of cast iron, coated internally with loam, with an opening in it for the blast. This opening, which corresponds to the lateral face opposite to the charging side, receives a tuyère , in which the nozzles of two cylinder single bellows, driven by a water-wheel, are planted. The tuyère opens at a small height above the sole of the furnace. On a level with the sole, the iron cylinder presents a slope, below which is the hemispherical basin of reception, set partly beneath the interior space of the furnace, and partly without. Near the corner of the building there is a second basin of reception, larger than the first, which can discharge itself into the former by a sloping gutter. Near this basin there is another, for the refining operation. These are all made either of brick or cast iron. The quality of the average ground-tin ore prepared for smelting is such, that 20 parts of it yield from 12 1 ⁄ 2 to 13 of metallic tin, (62 1 ⁄ 2 to 65 per cent.) The treatment consists of two operations, smelting and refining .