Page:The American Cyclopædia (1879) Volume XVI.djvu/853

 ZINC 823 This arrangement more completely utilizes the fuel, and reduces the consumption of coal to from six to eight parts for one part of zinc produced. The production is also increased, and the annoying smoke is removed. The long flame permits an increase of the number of muffles to 40 ; and by the use of gas as fuel (see below) as many as 136 have been heated successfully in a single furnace. The nozzles are made like those in the Belgian retort fur- naces, so that the zinc, instead of issuing in drops, and requiring to be remelted afterward, flows at once into moulds, with less loss by oxi- dation. What zinc powder is formed is mostly caught in balloons attached to the extremity of the nozzles. The residues fall, when the fur- nace is cleaned out, through slits into ash pits, from which their vapors are conducted away through flues. A great improvement in this variety of the Silesian process has been effected by the use of gas producers instead of ordinary grates. The Siemens regenerative gas furnace has been adapted for the purpose, with the advantages of a great gain in fuel, and an in- creased durability of the muffles, which carries with it a diminution both in the cost of repairs and in the leakage of vapors. On the other hand, this apparatus requires large expendi- ture in construction, careful adjustments, and skill and faithfulness in attendance. Zinc ox- ide, which accumulates in the regenerators, requires to be removed. The Siemens appa- ratus, invaluable for metallurgical operations requiring the very highest temperatures, can be advantageously replaced in the present case by the simpler gas producer of Boetius, shown above in connection with the Hasenclever fur- nace. The general operations of distillation by the Belgian-Silesian method resemble those of the old Silesian, except as to the liquid zinc, mentioned above. The following exam- ples will show the economy of the process: 1. Furnace with ordinary grate (requiring a long-flame coal) : 24 muffles ; daily charge, 580 kilos of calamine, containing 50 per cent, zinc, of which 39'2 per cent, is obtained ; con- sumption of coal for reduction, 150 kilos; for fuel, 1,808 kilos; 0'4 muffle and 2 nozzles de- stroyed daily. The consumption of fuel is to the product of zinc about as 8 : 1. 2. Furnace with Siemens regenerators : 72 muffles ; dura- tion of campaign, 2,798 days; daily destruc- tion of muffles, 1-1 ; ore treated daily per muf- fle, 28'24 kilos; zinc produced, 11'18 kilos; percentage of zinc in ore, 48 ; of yield, about 42 per cent. (i. e., a loss of about 13 per cent, of the zinc in the ore) ; consumption of fuel per unit of zinc, 5'34 units. 3. Furnace with Boetius producer: 136 muffles; ores contain- ing 50 per cent, yield 41 per cent, of zinc; proportion of fuel to zinc, 5 to 6 : 1. The Belgian system of distillation is conducted in inclined cylindrical retorts, disposed in rows above the fireplace, and provided with fire- clay nozzles or condensers, over the outer ends of which conical tubes (balloons, caps, or "pro- FIG. 2. Section of Belgian Furnace. longs") of sheet iron are placed during the operation. The ordinary form is shown in fig. 2, which presents a section from front to rear. In this furnace the eight retorts, a', of the lowest row are left empty, to serve as "protectors" and regulators of the tem- perature, by means of openings in them, through which the flame may be drawn at will. Above them are 61 useful retorts, a. A very impor- tant matter in this process is the manu- facture of the retorts, which are formed from a carefully pre- pared mass of clay, chamotte (dried or burnt clay in sharp fragments), and wa- ter, kneaded, and af- ter some weeks re- kneaded, &c. Coke powder may be add- ed, to give solidity, imperviousness to zinc vapors, and smooth surface. A good compo- sition, used at Engis, is 30 raw clay, 27 cha- motte, 18 coke, 15 old retort material, 10 sand. From this mass the retorts are formed either by machinery (a core being pressed into a mould filled with the material, or the clay being pressed over a mandrel, as in the manufacture of lead pipes, or massive cylin- ders of clay being bored out), or by hand, in various ways, the best of which, at least for some materials, appears to be the building up of the retort, like a circular tower, by winding spirally around its upper edge a succession of long rolls of plastic clay. This is the method in the Lehigh works at Bethlehem, Pa. The proper drying of the retorts requires months, the longer the better. It is performed in large heated chambers. They are highly heated just before use, and are introduced glowing into the furnace. Eecently more silicious materials (up to 94 per cent, silica) have been employed in Belgium, with gain in durability and saving of cost. The stages of the distillation are as follows: The furnace is gradually heated for two days, the retorts being empty. (The ma- terial of the retorts first put in, at the begin- ning of a campaign, should contain no coke or anthracite, since this would burn up while the retorts were thus heated empty.) The charges are then gradually introduced by means of semi-cylindrical spoons, the nozzles being re- moved for this purpose. The lower rows of retorts are most heavily charged with ore. The furnace attains its full productive capacity on the 13th or 14th day. After charging, the condensers are attached, and when the zino