Mechanical preparation of the copper ores in Cornwall

—The ore receives a first sorting, either within the mine itself, or at its mouth, the object of which is to separate all the pieces larger than a walnut. These are then reduced by the hammer to a smaller size; after which the whole are sorted into four lots, according to their relative richne . The fragments of poor ore are pounded in the stamps so that the metallic portion may be separated by washing. The rich ore is broken into small bits, of the size of a nut, with a flat beater, formed of a piece of iron 6 inches square and 1 inch thick, adapted to a wooden handle. The ore to be broken is placed upon plates of cast-iron; each about 16 inches square and 1 1 ⁄ 2 inch thick. These iron plates are set towards the edge of a small mound about a yard high, constructed with dry stones rammed with earth. The upper surface of this mound is a little inclined from behind forwards. The work is performed by women, each furnished with a beater; the ore is placed in front of them beyond the plates; they break it, and strew it at their feet, whence it is lifted and disposed of to the smelting-houses. Inferior ores, containing a notable proportion of stony matters, are also broken with the beater, and the rich parts are separated by riddling and washing from the usele matters. The smaller ore is washed on a sieve by shaking it in a stream of water, which carries away the lighter stony pieces, and leaves the denser metalliferous. They are then sorted by hand. Thus by beating, stamping, and riddling in water, the stony substances are in a great measure separated. The finer ground matter is washed on a plane table, over which a current of water is made to flow. Finally, the ore nearly fine is put into a large tub with water, and briskly stirred about with a shovel, after which it settles in the order of richne , the pure metallic ore being nearest the bottom. The stamps used for copper ore in Cornwall are the same as those used for tin ores, of which we shall speak in treating of the latter metal, as well as of the boxes for washing the fine powder or slime . These in fact do not differ e entially from the stamping mills and washing apparatus described in the article Metallurgy . Crushing rolls are of late years much employed. See Lead and Tin . Cornwall being destitute of coal, the whole copper ore which this county produces is sent for smelting to South Wales. Here are 15 copper works upon the Swansea and Neath, which pursue a nearly uniform and much improved proce , consisting in a series of calcinations, fusions, and roastings, executed upon the ores and the matters resulting from them. The furnaces are of the reverberatory construction; they vary in their dimensions and in the number of their openings, according to the operations for which they were intended. There are 5 of them:—1. The calcining furnace or calciner; 2. The melting furnace; 3. The roasting furnace or roaster; 4. The refining furnace; 5. The heating or igniting furnace. IMG:4147767755307473660_illo0318a.png:Calcining furnace 1. The calcining furnace rests upon a vault, C , into which the ore is raked down after being calcined; it is built of bricks, and bound with iron bars, as shown in the elevation, fig. 296. The hearth, B B , figs. 296. and 298. is placed upon a level with the lower horizontal binding bar, and has nearly the form of an ellipse, truncated at the two extremities of its great axis. It is horizontal, bedded with fire-bricks set on edge, so that it may be removed and repaired without disturbing the arch upon which it reposes. Holes, not visible in the figure, are left in the shelves before each door, c c , through which the roasted ore is let fall into the subjacent vault. The dimensions of the hearth B B are immense, being from 17 to 19 feet in length, and from 14 to 16 in breadth. The fire-place, A , fig. 298. , is from 4 1 ⁄ 2 to 5 feet long, and 3 feet wide. The bridge or low wall, b , fig. 302. , which separates the fire-place from the hearth, is 2 feet thick; and in Mr. Vivian’s smelting-works is hollow, as shown in the figure, and communicates at its two ends with the atmosphere, in order to conduct a supply of fresh air to the hearth of the furnace. This judicious contrivance will be described in explaining the roasting operation . The arched roof of the furnace slopes down from the bridge to the beginning of the chimney, f , fig. 296 , 298. , its height above the hearth being at the first point about 26 inches, and from 8 to 12 at the second. Such great calcining furnaces have 4 or 5 doors, c c c c , fig. 298. , one for the fire-place, as shown at the right hand in fig. 297. , and 3 or 4 others for working the ore upon the reverberatory hearth. If there be 3, 2 of them are placed between the vertical binding bars upon one side, and a third upon the opposite side of the furnace; if there be 4, 2 are placed upon each side, facing one another. These openings are 12 inches square, and are bound with iron frames. The chimney is about 22 feet high, and is placed at one angle of the hearth, as at f , fig. 298. , being joined by an inclined flue to the furnace. For charging it with ore there is usually placed above the upper part of the vault 2 hoppers, E E , in a line with the doors; they are formed of 4 plates of iron, supported in an iron frame. Beneath each of them there is an orifice for letting the ore down into the hearth. These furnaces serve for calcining the ore, and the matts or crude coppers : for the latter purpose, indeed, furnaces of two stories are sometimes employed, as represented in fig. 301. The dimensions of each floor in this case are a little le than the preceding. Two doors, c c , correspond to each hearth, and the workmen, while employed at the upper story, stand upon a raised movable platform. IMG:4147767755307473660_illo0319a.png:Melting furnace 2. Melting furnace , figs. 299 and 300. —The form of the hearth is also elliptical, but the dimensions are smaller than in the calcining furnace. The length does not exceed 11 or 11 1 ⁄ 2 feet, and the breadth varies from 7 to 8. The fire-place is however larger in proportion, its length being from 3 1 ⁄ 2 feet to 4, and its breadth from 3 to 3 1 ⁄ 2 ; this size being requisite to produce the higher temperature of this furnace. It has fewer openings, there being commonly three; one to the fire-place at D , a second one, O , in the side, kept generally shut, and used only when incrustations need to be scraped off the hearth, or when the furnace is to be entered for repairs; and the third or working-door, G , placed on the front of the furnace beneath the chimney. Through it the scoriæ are raked out, and the melted matters are stirred and puddled, . The hearth is bedded with infusible sand, and slopes slightly towards the side door, to facilitate the discharge of the metal. Above this door there is a hole in the wall of the chimney ( fig. 300. ) for letting the metal escape. An iron gutter, O , leads it into a pit, K , bottomed with an iron receiving-pot, which may be lifted out by a crane. The pit M is filled with water, and the metal becomes granulated as it falls into the receiver. The melting furnaces are surmounted by a hopper, L , as shown in fig. 299. IMG:4147767755307473660_illo0319b.png:Melting cum calcination furnace Melting furnaces are sometimes used also for calcination. There are some such near Swansea, which serve this double purpose; they are composed of 3 floors ( fig. 301. ) The floor A is destined for melting the calcined ore; the other two, B C , serve for calcination. The heat being le powerful, upon the upper sole C , the ore gets dried upon it, and begins to be calcined—a proce completed on the next floor. Square holes, d , left in the hearths B and C , put them in communication with each other, and with the lower one A ; these perforations are shut during the operation by a sheet of iron, removable at pleasure. The hearths b and c are made of bricks; they are horizontal at top and slightly vaulted beneath; they are 2 bricks thick, and their dimensions are larger than those of the inferior hearths, as they extend above the fire-place. On the floors destined for calcination the furnace has two doors on one of its sides: on the lower story there are also two; but they are differently collocated. The first, being in the front of the furnace, serves for drawing off the scoriæ, for working the metal, .; and the second, upon the side, admits workmen to make nece ary repairs. Below this door the discharge or tap-hole A is placed, which communicates by a cast-iron gutter with a pit filled with water. The dimensions of this furnace in length and breadth are nearly the same as those of the melting furnace above described; the total height is nearly 12 feet. It is charged by means of one or two hoppers. 3. Roasting furnace. —The furnaces employed for this purpose are in general analogous to the calcining ones; but in the smelting works of Hafod, the property of Me rs. Vivian, these furnaces, alluded to above, present a peculiar construction, for the purpose of introducing a continuous current of air upon the metal, in order to facilitate its oxidize ment. This proce was originally invented by Mr. Sheffield, who disposed of his patent right to Me rs. Vivian. IMG:4147767755307473660_illo0318b.png:Bridge between hearth and fire-place The air is admitted by a channel, c c , through the middle of the fire-bridge, fig. 302 , and extending all its length; it communicates with the atmosphere at its two ends c c ; square holes, b b , left at right angles to this channel, conduct the air into the furnace. This very simple construction produces a powerful effect in the roasting operation. It not only promotes the oxidize ment of the metals, but burns the smoke, and a ists in the vaporisation of the sulphur; while by keeping the bridge cool it preserves it from wasting, and secures uniformity of temperature to the hearth. 4. Refining furnace. —In this, as in the melting furnace, the sole slopes towards the door in front, instead of towards the side doors, because in the refining furnace the copper collects into a cavity formed in the hearth towards the front door, from which it is lifted out by ladles; whereas, in the melting furnaces, the metal is run out by a tap-hole in the side. The hearth sole is laid with sand; but the roof is higher than in the melting furnace, being from 32 to 36 inches. If the top arch were too much depre ed, there might be produced upon the surface of the metal a layer of oxide very prejudicial to the quality of the copper. When the metal in that case is run out, its surface solidifies and cracks, while the melted copper beneath breaks through and spreads irregularly over the cake. This accident, called the rising of the copper , hinders it from being laminated, and requires it to be exposed to a fresh refining proce , when lead must be added to di olve the oxide of copper. This is the only occasion upon which the addition of lead is proper in refining copper. When the metal to be refined is mixed with others, particularly with tin, as in extracting copper from old bells, then very wide furnaces must be employed, to expose the metallic bath upon a great surface, and in a thin stratum, to the oxidizing action of the air. The door G , fig. 300. , upon the side of the refining furnace, is very large, and is shut with a framed brick door, balanced by a counter-weight. This door being open during the refining proce , the heat is stronger at B than at A ( figs. 299 , 300. ) 5. Heating furnaces , being destined to heat the pigs or bars of copper to be laminated, as well as the copper sheets themselves, are made much longer in proportion to their breadth. Their hearth is horizontal, the vault not much depre ed; they have only one door, placed upon the side, but which extends nearly the whole length of the furnace: this door may be raised by means of a counter-weight, in the same way as in the furnaces for the fabrication of sheet-iron and bra .