Page:The American Cyclopædia (1879) Volume XVI.djvu/851

 ZINC 821 to grains of one to two millimetres diameter, though a preliminary calcination of fragments may be performed in open heaps or kilns ; and there are a few varieties of blende (like some light-colored, massive blendes of the Khine provinces) which, once ignited, burn of them- selves, and may hence be easily roasted by the ruder method. Usually, however, even the finely crushed blende yields its sulphur with difficulty, because the sulphate which is formed decomposes at white heat only. The best prac- tice, under ordinary circumstances, leaves 1 to 2 per cent, of sulphur in the charge, since to remove this remnant would involve a dispro- portionate consumption of fuel and an increased loss of zinc in roasting. The sulphur remains either in undecomposed sulphides or as sul- phate. At the Lehigh works, Bethlehem, Pa., 40 per cent, of coal dust is mixed with the blende in roasting, but 1 or 2 per cent, of sul- phur still remains. The chief inferiority of zinc blende as an ore to the carbonate, silicate, or red oxide lies in the greater cost of the roasting as compared with the calcination of the latter ores. About nine times as much fuel is required per unit of zinc subsequently produced. Moreover, the usually greater im- purity of blende, and the higher temperature required in its treatment, cause the presence of a larger amount of metallic oxides (iron, lead, manganese, &c.), which both attack destructively the retorts and yield a final product of inferiof quality. This is not the case with exceptionally pure blendes. The choice of apparatus for roasting blende is determined to some extent by the utilization of the sulphurous acid vapors. If these are to be transformed into sulphuric acid, muffle fur- naces, or the inclined furnace of Hasenclever, or the upright terrace furnace of Gerstenhofer (Stetefeldt's showering furnace has also been recommended), may be employed. The Hasen- clever furnace, in its latest form, with which the name of Hilbig also is associated, is used at several European works. In the accompanying sectional sketch, * is the hopper into which the ore is charged ; s, an inclined channel, depressed 43 from horizontal, T8 metre wide, 0'5 metre high, and 9 metres long, heated from below by the flame in the flue d from around the muffle furnace &/ A, A, 50 partitions, which stop short several centimetres above the inclined floor, forcing the ore to descend in a thin layer, while the gases from the muffle, &, passing through openings placed zigzag in these partitions, are made to traverse the surface of the ore for a long distance, and finally allowed to escape at, loaded with sulphurous acid. The inclined channel and the flues are accessible through side doors. At g is a hollow, air-cooled, revolv- ing feeder, operated periodically by a water power, at each turn of which a certain amount of ore is taken from the bottom of the incline and pushed into the muffle, while the layer of ore in the incline slips downward. Every two hours the ore is spread out in the muffle by hand, through working doors, and gradually pushed to the back, where it falls through an opening upon the hearth a, heated by direct flame. Here it is completely roasted, the last FIQ. 1. Hasenclever Furnace. portions of sulphurous acid escaping with the gases of combustion through e, c, d, and m into* the stack. The Boetius gas producer Z, and the air supplied at n, give an economical heat to the hearth, the working door of which is at f. It will be seen that this arrangement keeps the flame gases, smoke, &c., separate from the charge till the roasting is nearly complete, and thus furnishes sulphurous acid gas of greater purity (escaping at ) for the manufacture of sulphuric acid. The gas passes from s first into a cooling chamber, on the iron top of which ore is dried, thus completing the utilization of the heat. Even blende poor in sulphur (which is the hardest to roast) can be successfully treated in this apparatus. A blende contain- ing 20 per cent, of sulphur when charged was found to contain at g 10 per cent., at the back side of J 6-4 per cent., and at the fire bridge of a (just before withdrawal) 1'2 per cent. The further dimensions of the furnace are, in me- tres: muffle, 5, 6-5 long, 1-8 wide, 0'4 high, with five working doors on one side ; hearth, a, 6-T long, 0-4 high; generator, I, 1'6 high, 0-5 broad at the bottom, 1-4 at the arch. This furnace is a good example of the modern im- provement of combining the muffle and the reverberatory, neither of which is perfectly satisfactory alone, since the former furnishes usually a dilute gas for the sulphuric acid cham- bers, with a relatively large consumption of fuel, and effects an incomplete roasting, while the latter contaminates the sulphurous acid with flame gases and smoke. Nevertheless, the reverberatory is still largely employed, especially with two hearths disposed one over the other, or at different levels side by side, to permit the more complete utilization of heat and the gradual advance of the ore to hotter zones. Very long reverberatories are some-