At Lüttich, where the calamine of Altenberg, near Aix-la-Chapelle, is smelted, a reduction furnace, containing long horizontal earthen tubes, is employed. The roasted calamine is finely ground, and mixed with from one-third to two-thirds its volume of coke or charcoal, broken to pieces the size of nuts.
[Fig. 1233.] represents this zinc furnace in elevation; and [fig. 1234.] in a vertical section through the middle. From the hearth to the bottom of the chimney it is 9 feet high, and the chimney itself is 18 or 20 feet high. a, is the ash-pit; b, the grate; c, the fireplace; d, the hearth; e, e, the laboratory; f, the upper arch, which closes in the laboratory; f, the second arch, which forms the hood-cap of the furnace; g, the chimney; h, the fire-wall, which rests against a supporting wall of the smelting-house. Through the vaulted hearth the flame of the fire draws through ten flues i, i, two placed in one line; betwixt these 5 pairs of draught openings, upon the sole of the hearth, the undermost earthen tubes k, immediately rest. The second and third rows of tubes k, k, lie in a parallel direction over each other, at about one inch apart; in the sixth row there are only two tubes; so that there are 22 tubes altogether in one furnace. At their two ends these tubes rest upon fire-tiles, which form, with the side-walls, a kind of checquer-work l, l. The tubes are 4 feet long, 4 to 5 inches in diameter within, 5⁄4 of an inch thick. The fire, which arrives at the laboratory through the flues i, i, plays round the tubes, and passes off through the apertures m, m, in both arches of the furnace, into the chimney. n, is an opening in the front wall between the two arches, which serves to modify the draught, by admitting more or less of the external air.
The two slender side walls o, o, of the furnace, are a foot distant from the chequer-work, so that on the horizontal iron bars q, q, supported by the hooks p, p, the iron receivers r, r, may have room to rest at their fore part. These receivers are conical pipes of cast iron, 11⁄2 foot long, posteriorly 11⁄2 inch, and anteriorly 1 inch wide at the utmost. After the earthen tubes have been filled with the ore to be smelted, these conical pipes are luted to them in a slightly slanting position. These cones last no more than three weeks; and are generally lengthened with narrow-mouthed wrought-iron tubes, to prevent the combustion of the zinc, by contact of air. When the furnace is in activity, a blue flame is to be seen at the mouths of all these pipes. Every two hours the liquefied metal is raked out into a shovel placed beneath; and in 12 hours the charge is distilled; after which the tubes are cleared out, and re-charged. 100 pounds of metallic zinc are the product of one operation. It is remelted at a loss of 10 per cent., and cast into moulds for sale.
[Fig. 1235.] is a longitudinal section of the furnace for calcining calamine in Upper Silesia; [fig. 1236.] is a ground-plan of the furnace. a, is the orifice in the vault or dome, for the introduction of the ore; b, b, apertures in the side-walls, shut with doors, through which the matter may be turned over; c, the chimney; d, the fire-bridge; e, the grate; f, the feed opening of the fire, the fuel being pitcoal. The calamine is stirred about every hour; and after being well calcined during 5 or 6 hours, it is withdrawn; and a new charge is put in. These Silesian furnaces admit of 30 cwt. at a time; and for roasting every 100 cwt. 15 Prussian bushels of fuel, equal to 23 English bushels, are employed. These calcining furnaces are sometimes built alongside of the zinc smelting-furnaces, and are heated by the waste flame of the latter. The roasting is performed in the Netherlands in shafts, like small blast iron-furnaces, called schachtofen.
