Theory of the above operations.—At Holywell, Grassington, and in Cornwall, the result of the first graduated roasting heat, is a mixture of undecomposed sulphuret of lead, with sulphate and oxide of lead, in proportions which vary with the degree of care bestowed upon the process. After the roasting, the heat is raised to convert the sludge into a pasty mass; in which the oxide and sulphate re-act upon the sulphuret, so as to produce a sub-sulphuret, which parts with the metal by liquation. The cooling of the furnace facilitates the liquation every time that the sub-sulphuret is formed, and the ore has passed by increase of temperature from the pasty into the liquid state. Cooling brings back the sludge to the pasty condition, and is therefore necessary for the due separation of the different bodies. The drying up of the thin slags by lime is intended to liberate the oxide of lead, and allow it to re-act upon any sulphuret which may have resisted roasting or decomposition. It is also useful as a thickener, in a mechanical point of view. The iron of the tools, which wear away very fast, is also serviceable in reducing the sulphuret of lead. The small coal added along with the lime at Grassington, and also sometimes at Holywell, aids in reducing the oxide of lead, and in transforming the sulphate into sulphuret.
3. The smelting furnace or ore hearth.—This furnace, called by the French écossais, is from 22 to 24 inches in height and 1 foot by 11⁄2 in area inside; but its horizontal section, always rectangular, varies much in its dimensions at different levels, as shown in [fig. 634.]
The hearth and the sides are of cast iron; the sole-plate A B is also of cast iron, 21⁄2 inches thick, having on its back and two sides an upright ledge, A C, 21⁄2 inches thick, and 41⁄4 high. In front of the hearth there is another cast iron plate M N, called the work-stone, surrounded on every side excepting towards the sole of the furnace, by a ledge one inch in thickness and height. The plate slopes from behind forwards, and its posterior ledge, which is about 41⁄2 inches above the surface of the hearth, is separated from it by a void space q, which is filled with a mixture of bone ash and galena, both in fine powder, moistened and pressed down together. The melted lead cannot penetrate into this body, but after filling the basin at the bottom of the furnace, flows naturally out by the gutter (nearly an inch deep) through a groove in the work-stone; and then passes into a cauldron of reception P, styled the melting-pot, placed below the front edge of the work-stone.
The posterior ledge of the sole is surmounted by a piece of cast iron C D, called the back-stone, 28 inches long, and 61⁄2 high; on which the tuyère or blast-pipe is placed. It supports another piece of cast iron E, called pipe-stone, scooped out at its under part, in the middle of its length, for the passage of the tuyère. This piece advances 2 inches into the interior of the furnace, the back wall of which is finally crowned by another piece of cast iron E H, called also back-stone.
On the ledges of the two sides of the sole, are placed two pieces of cast iron, called bearers, each of which is 5 inches in breadth and height, and 26 inches long. They advance an inch or two above the posterior and highest edge of the work-stone, and contribute effectually to fix it solidly in its place. These bearers support, through the intervention of several ranges of fire-bricks, a piece of cast iron called a fore-stone, which has the same dimensions as the piece called the back-stone, on which the base of the blowing-machine rests. This piece is in contact, at each of its extremities, with another mass of cast iron, 6 inches cube, called the key-stone, supported on the masonry. Lastly, the void spaces left between the two key-stones and the back part of the furnace are filled up with two masses of cast iron exactly like the key-stones.
The front of the furnace is open for about 12 inches from the lower part of the front cross-piece called fore-stone, up to the superior part of the work-stone. It is through this opening that the smelter operates.
The gaseous products of the combustion, on escaping from this ore-hearth, are frequently made to pass through a long flue, sloped very slightly upwards, in which they deposit all the particles of ore that they may have swept along; these flues, whose length is sometimes more than 100 yards, are usually 5 feet high and 3 feet wide in the inside, and always terminate in a chimney stalk. The matters deposited near the commencement of the flue require to be washed; but not the other dusty deposits. The whole may then be carried back to the roasting furnace, to be calcined and re-agglutinated, or introduced without any preparation into the slag-hearth.