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 less powerful, upon the upper sole C, the ore gets dried upon it, and begins to be calcined—a process 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, &c.; and the second, upon the side, admits workmen to make necessary 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 Messrs. 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 oxidizement. This process was originally invented by Mr. Sheffield, who disposed of his patent right to Messrs. Vivian.
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 oxidizement of the metals, but burns the smoke, and assists 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 depressed, 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 process, when lead must be added to dissolve 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 process, the heat is stronger at B than at A ([figs. 299], [300.])