The saving effected by this apparatus in preserving a quantity of valuable matter, which would otherwise, as heretofore, escape to the iniury of the neighbourhood, would of itself amount in one year, where four furnaces are employed (as described in the plan) to a sum equal to the entire cost of the improved apparatus; that is, the upper part of the tower, with its roof, cap, vane, shutter, and appendages" which we shall next proceed to describe.
Scale of forty feet.
Fig. 1, in the preceding engraving, represents a vertical section of a lofty and capacious tower, placed in the centre of four smelting furnaces, and receiving, by distinct flues, the smoke and vapour from each of them. The drawing being a central section, but two of the furnaces are brought into view, which are marked a a, their flues b b opening into separate chimneys c c in the tower, which they ascend for twenty or more feet; then, by lateral passages at ddd, they respectively enter the central shaft ee; here the vapours come in contact with a powerful ascending current of cold air, and are likewise checked in their upward progress by striking against a dome or cap of iron f, which is suspended over the throat of the central shaft e. The ascending vapours thus intercepted and acted upon are, for the most part, immediately condensed, and the metallic particles are precipitated upon a floor g, called the lodge floor. A plan of this floor, and the cap f, are given in a separate figure (2), which is a transverse horizontal section of the tower just above the cap; another advantage resulting from this arrangement consists in the effect produced in the furnaces below, where it is found that the carbonaceous matter is more completely consumed than by the former disposition of things; such portion of the heavy particles that do not fall upon the lodge floor are precipitated to the bottom of the central shaft The cap f is suspended by a vertical rod h, which is connected to a transverse beam by means of a sort of stirrup-iron i, through which the upper extremity of the rod is screwed, and by the turning of a nut upon this screw the height of the cap above the throat of the central shaft is regulated.
The cap is steadied in its movements and preserved in its position by several upright bars passing through it, two of which are brought into view; these are perforated with holes, through which keys or bolts are put to lock the cap securely in its place. The lower part of the cap or dome is circumscribed by a broad hoop; by the action of regulating screws, this hoop is shifted up or down over the periphery of the cap, and the passage for the vapours is thus more readily adjusted at pleasure. The more volatile portion of the vapours pass from under the dome, and ascend to the top of the tower, which, being covered with a roof nearly flat, the heaviest particles are driven hack, and fall condensed also upon the lodge floor, while the lightest and least pernicious escape into the atmosphere at the lateral openings k k. There are a regular series of vent holes all round this part of the tower, one half of which (those that happen to be windward) are always closed by a shutter m. The lower extremity of the shaft, upon which the vane I is fixed, turns in a bearing upon the cross beam; and the arms of the circular shutter being also attached to this shaft, when the wind turns this vane the shutter is, consequently, in like manner turned against it.
Fig. 3 is a transverse section of the tower immediately under the roof, by which the circular frame of the shutter is shown, as closing one half of the apertures, or those to windward of it. When the deposition from the condensed vapour has become considerable, it is removed from the lodge floor at a time when the smelting furnaces are not at work; this is done by a man ascending a narrow circular staircase, constructed in the masonry of the tower, up to the lodge floor, where he throws down the accumulated deposition with a shovel to the bottom of the shaft; from thence it is harrowed out, and carried to a roasting furnace. When any one of the furnaces is not at work, communication with the tower is to be cut off by means of a damper, as those shown at o o. In the drawings attached to the specification, a general plan of a smelting work is delineated. The area is inclosed by a quadrangular wall, with a smelting furnace on each side, the chimneys of which are conducted into the central tower. The corners of the quadrangle are occupied by the other buildings required in such establishments. The spaces between the angles of the several flues, the patentee states, may be conveniently occupied by small furnaces for tests and experimental purposes.
Another improvement of the patentee deserves mentioning: he directs that the tapping sides of the contiguous furnaces be made "opposite" to each other; by which is meant that they may both face the area which lies between them, in order that the fluid metel from the pans of each surface may be run into pigs, or conveyed into one receiver, and thence into moulds, so as to be formed into thick sheets, ready for milling or rolling, by which arrangement of the furnaces it is considered an important saving of labour and expense will be effected, and the waste by remelting the lead avoided. In the process of smelting, the ore is spread upon the concave hearth, so that the flame may act upon it, and release the sulphur. When the sulphur has escaped, the lead combines with the oxygen, and the oxide of lead thus formed combines with and reduces the earthy matter to a liquid, which floats upon the surface of the metal, and, for the remainder of the operation, protects it from the action of the oxygen. The temperature of the furnace is now considerably raised, to separate as quickly as possible the lead from the liquid scoria; after which a considerable portion of the scoria is tapped off, leaving only so much behind as is necessary to protect the metal from the action of the oxygen, The fire is now slackened, and a quantity of slack or refuse pit-coal thrown into the furnace, which serves to diminish the heat, and to concrete the melted scoria, which effect is promoted by the addition of powdered lime; the scoria thus consolidated is broken into pieces with a rake, and thrust to the opposite side of the furnace, where it is taken through the apertures already mentioned.
The lead is now tapped in a manner similar to that described in the manufacture of iron, and allowed to run into a capacious iron pan, whence it is ladled into moulds to cast it into pigs. When the ores abound with blend, or black-jack, or with the sulphate of iron, fluate of lime is added as a flux. The scoria last mentioned contains a portion of lead, besides that which is in the state of oxide; it is therefore exposed to the heat of another furnace, being a species of blast furnace, and called a slag-hearth, which fuses the scoria and causes the metal to penetrate through it and fall into a cavity, where it is protected from the agency of the blast, and from whence it is taken and cast into pigs. All lead ores contain some portion of silver, which is extracted when it is in sufficient quantity to afford a recompense for the operation; the method adopted in France is very simple and efficacious, and is thus described in Rees's Cyclopcedia: - "A shallow vessel, or cupal, is filled with prepared fern-ashes, well rammed down, and a concavity cut out for the reception of the lead, with an opening on one side for the mouth of the bellows, through which the air is forcibly driven during the process.
The French smelters cover the surface of the ashes with hay, and arrange symmetrically the pieces of lead upon it; when the fire is lighted, and the lead is in a state of fusion from the reverberation of the flame, the blast from the bellows is made to play forcibly on the surface, and, in a short time, a crust of yellow oxide of lead or litharge is formed, and driven to the side of the cupel opposite to the mouth of the bellows, where a shallow side or aperture is made for it to pass over; another crust of litharge is formed, and driven off. The operation continues about forty hours, when the complete separation of the lead is indicated by a brilliant lustre on the convex surface of the melted mass in the cupel, which is occasioned by the removal of the last crust of litharge that covered the silver. The French introduce water through a tube into the cupel to cool the silver rapidly and prevent its spirting out, which it does when the refrigeration is gradual, owing, probably, to its tendency to crystallize. In England the silver is left to cool in the cupel, and some inconvenience is caused by the spirting, which might be avoided by the former mode.
The silver thus extracted is not sufficiently pure; it is again refined in a rever-beratory furnace, being placed in a cupel, lined with bone ashes, and exposed to greater heat; the lead, which has escaped oxydation by the first process, is converted into litharge and absorbed by the ashes of the cupel. The last portions of litharge in the first process are again refined for silver, of which it contains a part which was driven off with it. The litharge is converted into lead again, by heating it with charcoal; part is sometimes sold for pigment, or converted into red-lead. The loss of lead by this process differs considerably, according to the quality of the lead. The litharge commonly obtained from three tons of lead amounts to fifty-eight hundred weight; but when it is again reduced to a metallic state, it seldom contains more than fifty-two hundred weight of lead, the loss on three tons being eight hundred weight. The Dutch are said to extract the silver from the same quantity of lead with only the loss of six hundred weight." - See Separation.