Tests were made of a sample of ore from the Bullion Coalition mine. The results of an ultimate analysis of this ore are given in the following table, the mineralogic composition, as calculated from this analysis, being included. Composition of ore from Bullion Coalition mine. In appearance the yellow ocherous limonite predominated and seemed to be plastered over the particles of zinc carbonate. The latter were occasionally visible as large white specks, but in the greater part of the ore were indistinguishable. The material was crushed to pass a 20-mesh screen, mixed with 5 per cent of coal dust, and heated in a muffle furnace out of contact with the air for two hours. The weight of the resulting calcine was only 85.5 per cent of the original weight. Of this calcine 40 per cent was separated on a low-intensity Campbell magnetic separator. The magnetic material analyzed 22.3 per cent zinc. The other 45.5 per cent of the ore contained 38.2 per cent zinc, a product which could easily be sold, as prices for oxidized zinc ore are figured on a basis of 35 per cent zinc. Normally a 30 per cent zinc ore can not be shipped from Utah to the "gas belt" at a profit on account of freight costs. With prewar conditions and 5-cent spelter, the net value in Salt Lake City of 1 ton of "carbonate" ore containing 30 per cent zinc would vary from nothing to $5, whereas that of a ton of 38 per cent zinc ore under the same conditions would be about $12.50. As mining costs have to be paid out of this net return and the higher grade ore has a sure market, while the price paid for the lower grade ore fluctuates according to the conditions of the stock piles at the zinc smelters, such ore could easily be treated at a profit by magnetic separation, although the reject contains 22.3 per cent zinc-30 per cent of the total zinc in the ore. The low recovery of zinc, however, would normally condemn such a procedure. Many small-scale tests were made in order to determine the effects of various percentages of reducing agent, different lengths of time, and different temperatures of reduction. The averages of the best results showed that on nearly all different sizes it was possible to obtain a nonmagnetic residue containing 40 per cent zinc and weighing about 42 per cent of the original ore, and a magnetic product containing 21.8 per cent zinc and also weighing about 42 per cent of the original ore. This latter work was done with a sample containing only 24.82 per cent zinc. None of the zinc in the magnetic product could be separated, on account of the intimate crystallization of the zinc with the iron minerals. The ore used is representative of much of the oxidized brown "carbonate" ore of the Rocky Mountain mining districts. CALCINING OXIDIZED ZINC ORES.a Calcining oxidized zinc ores as a method of concentration has been repeatedly proposed. The fundamental principle of this treatment is that the water of hydration and the combined carbon dioxide in the ore can be driven off, leaving all the zinc and the other metals in a product of much less weight and consequently higher grade. There is no doubt that calcining any zinc ore which is normally of shipping grade, but which is at some distance from the zinc smelters, will pay. For instance, with the oxidized zinc ores of Nevada, the freight rates to the "gas belt" are $8 to $11 per ton; hence a loss in weight by the ore amounting to 25 or 30 per cent effects an important saving. A number of such ores were calcined in the laboratory by roasting 100-gram samples in dishes of 5-inch diameter at 800° C. for one to two hours. The roasted samples, on standing in the laboratory for a number of days, did not take up moisture and carbon dioxide to any serious extent, as shown by the figures in Table 36, but in some samples the lime slaked vigorously when they were wetted with water. Therefore such calcined ore would have to be shipped in closed cars to keep it dry and prevent its increasing in weight again. As the normal condition of transportation is such that the railroads prefer loading eastbound zinc ore in box cars, this would be no disadvantage. The figures in the table show that few nonshipping ores would be raised to shipping grade, but that with the shipping ores important savings in freight would be effected and a slight concentration of the zinc value, which on some contracts would more than pay for the cost of calcining. The analyses of these ores are found in Table 3 (p. 21). • Experimenters: H. J. Morgan, H. C. Neeld, and W. R. Sholes. By igneous concentration of oxidized zinc ores is meant that process in which the ore is mixed with an excess of fuel and heated under forced draft so that zinc distills out of the ore and reoxidizes in the air to form zinc oxide fume, which is later filtered from the flue gases. Most of the commercial zinc oxide in the United States is made in this manner, although the mechanical equipment used for the process varies widely. The increased use of zinc oxide in paints, for filling rubber, and other purposes, make this process of importance. It is said that some automobile tires contain 60 per cent ZnO. Further, the process seems to offer a solution of the problem of treating low-grade oxidized ores of zinc which are amenable to no other concentration process. WETHERILL GRATE FURNACE. The best known furnace in which zinc oxide is made from oxidized ores is the Wetherill' grate furnace, which is built much like the fire box under a large stationary boiler. The mixed ore and fuel (coke breeze or anthracite coal slack) is dropped on the grates in a bed 4 inches to 12 inches deep. The air is blown in under the grates and thus keeps them relatively cool. The ore bed is heated to 1,100° to 1,300° C. by the combustion of part of the fuel; at this temperature the zinc is reduced and distilled from the ore particles but reoxidizes in the excess of air which is blown into the furnace. After partial cooling of the gases they are filtered in a bag house to remove the fume of zinc oxide. The grates differ somewhat in a Experimenters: H. C. Neeld, J. F. Cullen, R. H. Bradford, and O. C. Ralston. design, some being hollow and filled with holes from which the air is blown into the mass, and others being like ordinary boiler grates. In nearly all of them the air openings are tapered, with the largest diameter at the bottom in order to prevent blinding. The grates are charged and discharged by hand, so that the cost of labor involved is rather large. In addition the grates tend to buckle and have to be removed periodically from the hot furnaces and replaced by fresh grates. The buckled members are heated uniformly in a special furnace and then straightened. Another type of furnace used is very similar to the ordinary zinc retort furnace except that an air pipe is led to the back of each muffle and zinc oxide is driven off instead of metallic zinc. This furnace likewise requires much hand labor, and, being a retort furnace, has a rather high fuel consumption. A rotary cement kiln is reported to have been used as a furnace at a plant in Mexico. The ore was fed continuously into the cool end and discharged at the hot end, and a reducing atmosphere was maintained. This distilled all the zinc, which reoxidized in the flue gases. The fuel consumption was said to amount to about 15 per cent of fuel oil based on the weight of the charge. This method has the advantage over the two previously described methods that all the fuel is used in the flame that heats the furnace, so that a reducing atmosphere is easily obtained. The writers have not been informed whether the zinc oxide product was sufficiently pure for use as pigment. However, evidently air could be mixed with the hot gases in a combustion chamber at the head of the kiln in order that all carbon might be burned and the zinc oxidized completely. Rotary kilns often make much dust, but most of this could be caught in a settling chamber, so that the zinc oxide product need contain no more dust than that made on Wetherill grates. For making pigment the kiln would have to be fired with either oil or gas and not with powdered coal, because the ash would contaminate the zinc oxide. Ash contamination might not be serious if the purpose was to make a high-grade concentrate for the zinc smelter rather than pigment. The only disadvantage of this type of furnace for igneous concentration of zinc ores is in the power necessary to turn a heavy kiln. A blast furnace would be a much better type for continuous mechanical feeding and firing, and would use most of the power for the blast. A number of attempts have been made to blow zinc oxide in blast furnaces, but one great drawback to the method has been that a slag has to be formed to facilitate removal of the residue. When the usual types of slags were made the zinc oxide tended to Ingalls, W. R., Zinc burning as a metallurgical process: Bull. 129, Am. Inst. Min. Eng., September, 1917, pp. 1229-1234. enter the slag instead of entering the fume. One of the latest attempts was that of R. D. Divine and B. F. Hedges to recover zinc in a lead blast furnace at the old South Chicago plant of the American Smelting & Refining Co." The material treated was a zinciferous lead-smelting slag. Only 60 per cent of the zinc could be blown before the slag melted and ran out of the furnace. A similar plant, making similar recoveries, is now in operation at Florence, Colo. With these results in mind, some experiments in the blast furnace treatment of such ores were made at the Salt Lake City station, with the object of blowing more of the zinc into the fume, if possible. A small "homemade" blast furnace was constructed from fire brick and sheet iron, as shown in figure 10. The three tuyères were made of iron pipe, and as they ་ ད་་ burned off they were pushed farther into the furnace. The pipes leading to the tuyères from the air main were made of 2-inch canvas hose, so that they could be pinched down till the right amount of air was being used. The firebrick lining of the furnace melted away several times during the course of about 25 runs, but was easily replaced and served the purpose, although a standard water-jacketed lead blast furnace would be needed for large-scale work. The inside dimensions of this furnace were about 5 feet in height, 1 foot in diameter at the top, and 18 inches in diameter at the bottom to prevent hanging. After heating the furnace with any convenient type of fuel the charges were added with various percentages of coke fuel. In each test the residue was drained out of the furnace before another charge of different composition was used. The slag analysis served as an index of the completeness of volatilization of the zinc from the ore. Under these conditions such a furnace could easily melt 2 or 3 tons of ore daily, although it was not run continuously for more than a few hours, a ་་་་་ FIGURE 10.-Laboratory blast furnace used for zinc-oxide experiments. a, Fire-brick lining; b, sheet-iron casing; c, 14-inch iron pipe; d, 2-inch canvas hose; e, slag notch; ƒ, fire-clay plug. a Pulsifer, H. G., Zinc oxide from lead blast-furnace slag, South Chicago: Met. and Chem. Eng., vol. 13, Nov. 1, 1915, pp. 783-785. b Hall, R. G., Some economic factors in the production of electrolytic zinc: Bull. 129, Am. Inst. Min. Eng., September, 1917, pp. 1287-1302. |