Page:The American Cyclopædia (1879) Volume XVI.djvu/854

 824 ZINC flame makes its appearance through these, the sheet-iron prolongs are added. The liquid zinc is drawn from the condensers at intervals into ladles, skimmed, and cast into ingots. "When the distillation in each retort is complete, the condenser is removed and cleaned, and the resi- dues are scraped out of the retort. A row can be emptied in 10 to 15 minutes. The residues from the uppermost are the richest. The re- tort is then patched or replaced, if damaged. The destruction of the furnace ends the cam- paign. The following examples show the working of the process : 1. Engis. .New dou- ble furnace, 92 retorts; day charge, 400 kilos ore (calamine and blende), 72 kilos rich scraps, and 166 kilos coal for reduction; night charge (the furnace being hotter), 500 kilos ore. Charging from 6 to 9.15 A. M. First tap at 1 P. M., second at 4.30, third at 6. Length of campaign, 150 to 180 days ; production, 82,910 kilos zinc from 285,520 of ore, containing 40'82 per cent, zinc; loss, 11 '28 per cent. ; consump- tion for 100 kilos zinc produced, 5*8 hectolitres coal for fuel, 1*8 hectolitres coal for reduction ; cost of treatment, 38'97 francs. 2. Other Bel- gian works. Furnaces with 70 retorts, taking 1,300 kilos of charge per 24 hours (ore contain- ing 47 to 48 per cent, zinc), use 700 kilos of coal for reduction and 2,000 as fuel, and pro- duce 470 to 480 kilos of zinc ; say 18 per cent, gross loss, which is reduced by subsequent treatment of the residues. New furnaces with 164 retorts produce in 24 hours 1,050 kilos of zinc with 4,000 kilos of fuel. Furnaces which are heated with gas producers yield the same quantity of zinc with 3,000 kilos of fuel. As compared with the Silesian process, the Belgian requires less fuel, but a longer flame, owing to the greater distance of the retorts from the fire; a larger quantity and more refractory quality of clay; a finely crushed charge ; richer ores, on account of the expense for fuel and clay ; and greater skill and endu- rance of workmen. On the other hand, it fur- nishes a more rapid and complete extraction. The mines worked by the Lehigh zinc company are in Lehigh co., Pa., in a valley from two to three miles broad, and four miles south of Bethlehem. The north and south boundaries of the valley are syenitic hills, and between these a dolomitic limestone, underlaid by slate, Potsdam-sandstone, and syenite, in downward order, is the outcropping rock. In this lime- stone the zinc deposits occur, in nearly vertical bodies, along an anticlinal axis. They comprise a massive light-colored blende of waxy lustre, and smithsonite and calamine, in pure, earthy, and argillaceous varieties, up to a compact, unc- tuous clay, containing from 26 to 32 per cent, of zinc. The first discovery was made by Prof. W. T. Eoepper in 1845, and the property was bought by the present company in 1861, since when it has been regularly worked. About 20,000 tons of ore are extracted annually. The chief difficulty of mining is the large quantity of water, which flows underground in rivers into the mines, all the wells and surface streams of the neighborhood having been drained. At intervals since 1854 pumping engines of in- creased power have been erected, until in 1871 the climax was reached with the largest mine- pumping engine in the world, the diameter of the steam cylinder being 110 in. and the stroke 10 ft., the power at 60 Ibs. steam 3,000 horse power, and the ordinary capacity at consider- ably lower pressure 17,000 gallons of water lifted per minute from a depth of 300 ft. The mechanical and metallurgical treatment of the ore is described in a paper in vol. i. of the "Transactions of the American Institute of Mining Engineers." The following is a brief outline. The blende is crushed in a Blake's crusher, and passed over a sieve with one mesh to the inch. The larger pieces, passing over the sieve, are separated by hand from gangue, and sent directly to the furnaces, or roasted in heaps of 50 tons. The silicates and carbonates are washed and sized, and the larger pieces sent to the furnaces, the fine stuff being con- centrated in buddies or jigs. For the manufac- ture of spelter, the heap-roasted and also the screened blende are thoroughly roasted in re- verberatories, with the addition of 40 per cent, of coal dust. The silicates and carbonates are calcined in kilns, and then crushed with 40 per cent, of coal and sifted, before charging into the retorts. The ordinary Belgian process of distillation is employed. Each furnace con- tains 56 retorts, in seven rows, besides a bot- tom row of six protectors or " cannons," which are not charged. The retorts are 42 in. long, 9 in. exterior and 6 in. interior diameter, made of a mixture of fresh fire clay and ground frag- ments of old retorts, and provided with con- densers or nozzles, 16 in. long, 6 in. diameter at the largest and 3 in. at the smallest end. The furnaces are in groups of four, with inter- nal walls of fire brick and external of common brick lined with fire brick. The slant of the retorts varies with the quality of the ore, but is generally from 8 to 6 in. from the rear to the front. Of the mixture of ore (47 per cent, zinc) and coal 40 Ibs. is charged every 12 hours into each retort, except the. upper row, which are charged every 24 hours with skimmings, &c., containing from 60 to C5 per cent, of zinc. As soon as zinc vapors are seen burning at tho ends of the condensers, prolongs or conical caps of sheet iron are put on. Every 12 hours the zinc is collected in ladles and poured into ingot moulds, 7 by 24 by 1 in., containing 40 to 45 Ibs. The chemical reactions are described as follows. The air introduced with the charge causes an imperfect combustion of a portion of the carbon present, forming carbonic oxide, which reduces oxide of zinc to a metallic state, and forms carbonic acid. There are then pres- ent in the retort, at a high temperature, oxide of zinc, carbon, and carbonic oxide. By the reduction of two equivalents of the former, one equivalent of carbonic oxide is converted into carbonic acid, and one of carbon into car-