in the marmatite tends to unite with the zinc to form ferrite of zinc. Fortunately in most blendes such high percentages of "combined" iron are rare. Marmatite, being a solid solution of iron sulphide in zinc sulphide, has variable proportions of iron, but even the purest pieces rarely contain as much as 10 per cent. However, as all "black blendes" will tend to form ferrites of iron during roasting, the advisability of trying to leach such ores is doubtful. The only argument in favor of leaching ores of this type would be that the maximum zinc loss would not, as a rule, be more than about 10 per cent, although it might be higher for an ore, like the Wasa, having an unusually high content of combined iron. MAGNETIC ROASTING. By magnetic roasting of zinc ores is meant the process of heating zinc-iron sulphide mixtures to a sufficient degree to partly break up the pyrite into pyrrhotite and elemental sulphur. The lower sulphides of iron are magnetic, and sphalerite is nonmagnetic, hence magnetic roasting puts the ore into such a condition that it is capable of magnetic separation, provided the particles of magnetic iron sulphide and nonmagnetic zinc sulphide are mechanically separable. The data on some laboratory experiments along this line made at the Salt Lake City station are given under "Dry concentration processes" (pp. 45 to 53). HYDROMETALLURGY OF ZINC SULPHIDE ORES. Hydrometallurgical treatment of zinc sulphide ores has been proposed in many different forms during the past half century, but has in general failed to be adopted, on account of the peculiar chemical properties of zinc and the low value of the products that can be made. Zine requires more chemical or electrical energy, in comparison with its value after recovery, than almost any other major metal on the market; and the high consumption of chemicals or of electric energy involved in its recovery has usually made hydrometallurgical processes of questionable value. Zinc sulphide resists most ordinary solvents, not being dissolved with sufficient rapidity by any of the commercial acids, hence requires roasting before the majority of the proposed hydrometallurgical processes can be applied. Sulphide of zinc is one of the most refractory sulphides to roast, as is mentioned in the part of this bulletin on roasting (p. 53), and on that account the cost of zinc roasting is higher than that for any other major metal. The lowest reported costs of oxide roasting in zinc smelters are $1.50 per ton of green ore, •Experimenters: O. L. Larson, A. E. Gartside, W. J. Woolf, R. W. Johnson, C. E. Sims, M. J. Udy, 0. E. Young, A. J. McChrystal, and J. F. Cullen. and in the older smelters $4 was not an uncommon cost. Chlorine gas is the only commercial medium that will attack zinc sulphide in the cold, and on this fact are based many of the "chloride" processes later described, their principal value being that the zinc sulphide need not be roasted. SULPHATE PROCESSES. In order to get the zinc of the ore into solution as zinc sulphate a number of alternative processes are possible. The ore may be given a sulphating roast, or it may be given an oxidizing roast and then leached with sulphuric acid, or it can be mixed with strong sulphuric acid, without previous roasting, and heated in a reverberatory roaster until SO, fumes are evolved. This latter method has been tested in the laboratories at Anaconda and Great Falls, Mont. A sulphating roast is usually rather difficult of accomplishment and the cost is high. It is reported that C. A. Hansen at Bully Hill, Cal., was able to sulphate as much as 90 per cent of the zinc in the ore from the Bully Hill mine. At Anaconda not more than 50 per cent is sulphated, and with many ores it is not possible to sulphate even 25 per cent of the zinc during roasting. Each ore seems to act differently and the free milling "rosin jack" ores do not sulphate as well as do the "black jack" ores. On the other hand, it is just as hard to roast the sulphide completely to zinc oxide, leaving no zinc sulphate in the ore, even by heating as highly as 1000° C. When a sulphate solution is to be made, the most desirable roast may be that which will either drive off all the sulphur or leave it as zinc sulphate, unless the consumption of sulphuric acid in a process is so large that the maximum sulphating effect is desired more than the formation of the maximum amount of soluble zinc. Roasting of zinc sulphide ores, followed by leaching with solutions of sulphuric acid, has already been considered (pp. 53 to 79). In general the roasting should be so conducted as to convert all the zinc sulphide to soluble forms, yet the ore must not be heated above 700° C. on account of the tendency for the oxides of iron and of zinc to unite as ferrites of zinc. This makes the roasting a slow process; in spite of all precautions it is impossible with some ores to obtain good recoveries of the zinc by leaching. Thus the leaching plants at Anaconda, Bully Hill, and others have often been unable to get recoveries higher than 60 to 70 per cent. For a time the low recoveries with the sulphuric acid leaching process seemed to threaten its life, but supplementation of the process by igneous concentration of the zinc in the residue, as at Anaconda, seems to give it a better chance of survival. If the gangue of the ore contains any acid-consuming substances, such as calcium carbonate or calcium oxide, or magnesia compounds or aluminum compounds soluble in sulphuric acid, the efficiency of the acid for dissolving zinc falls off. As enough acid must be used to supply the requirements of these compounds in addition to that of the zinc minerals, it does not take much of these compounds to make the consumption of acid excessive. Even with 100 per cent efficiency of the acid, 1.5 pounds are required to dissolve one pound of zinc. Therefore, if a zinc sulphide ore does not have a large content of acid-soluble gangue minerals, and roasting will leave no unoxidized blende, insoluble ferrites, basic sulphates, or other compounds, in the calcine, roasting and sulphuric acid leaching to recover the zinc is permissible. If the ore contains any considerable proportion of acid-consuming minerals any hydrometallurgical process of this kind will have to be preceded by mechanical concentration of the ore in order to remove such impurities, together with some of the iron sulphides that are not desirable. SULPHITE PROCESSES. A number of processes have been proposed involving the leaching of zinc calcines with a solution of sulphurous acid to form zinc bisulphite, which is sufficiently soluble in water to make such a method a commercial possibility. The original methods involved precipitation of the zinc as monosulphite of zinc, which is insoluble, by warming the solution of bisulphite. In heating the solution some sulphur dioxide would be liberated, and this could be caught and used again with that obtained from thermal decomposition of the zinc monosulphite in a furnace heated to a sufficiently high temperature. DIFFICULTIES OF APPLYING PROCESS. In practice some difficulties in the application of this process were met. In roasting zinc sulphide ores it is possible to obtain roaster gases containing about 5 per cent SO,. The strength of the solution that can be prepared from this gas varies from 0.5 to 1.5 per cent SO2. On the assumption that the solution will average 1 per cent SO, and that the ore to be treated contains 30 per cent zinc, 60 tons of this 1 per cent solution will be required to leach the zinc completely from 1 ton of ore. This solution ratio is too large for economical work in any hydrometallurgical process. Consequently the roaster gas would have to be applied directly to a pulp of the ore, rather than being dissolved by water in an absorption tower. Pulp can not be treated in a tower as it would soon choke the openings, hence some type of mechanical agitator for applying the gas to the pulp would be necessary. As far as the authors can learn, no such machine has ever been tried. The solubility of zinc bisulphite is such that about 6 per cent zinc can be easily obtained in 86198-19--Bull, 168- -6 solution, hence a 30 per cent ore would require 5 tons of solution per ton of ore, a volume which is very easily handled in the machines now available. Possibly an apparatus comprising a rotating drum, in which the gases and the pulp would pass in countercurrent, would be acceptable for this work, the zinc oxide of the ore combining with the dissolved sulphur dioxide fairly rapidly, so that more sulphur dioxide could dissolve in the water until all the zinc oxide had been converted to monosulphite of zinc, after which the addition of sulphur dioxide would form bisulphite of zinc until all the zinc had gone into solution. In order to design such a machine properly, the rate of reaction of sulphur dioxide solutions of various strengths on roasted zinc sulphide ore, and the rate of absorption of sulphur dioxide gas by water in a tumbling barrel of this type would have to be determined. It is known that the rate of absorption of gases in liquids, when the liquids are showered down through the gases, is extremely rapid and depends on the effectiveness with which the water is sprayed. Nothing was known as to the rate of reaction between the sulphurous acid solutions and the ordinary zinc oxide calcine. EXPERIMENTS. Therefore, a series of tests were made on such a calcine with different strengths of sulphur dioxide solution for different time intervals, in order to obtain an idea of the reaction velocity. A sample of the zinc sulphide concentrate from the Daly-Judge concentrating mill, at Park City, Utah, was roasted and the calcine was treated with measured quantities of sulphurous acid solutions of known strength. Treatment was in 2.5-liter bottles placed on their sides on a rolling agitator to give much the same motion as that given to pulp in a tube mill or similar rotating barrel arrangement. The sample analyzed 46.6 per cent Zn, 7.4 per cent Fe, 1.86 per cent Pb, 1.54 per cent Cu, and 30.44 per cent S. The calcine contained 49.6 per cent Zn. The screen analysis of the calcine was as follows: In the calcine 10.8 per cent of the zinc was present as zinc sulphate. The results of the tests are plotted in figure 9. In each test the theoretical amount of sulphur dioxide needed to render all the zinc soluble was used. Consequently, as 10.8 per cent of the zinc was present in the calcine as sulphate, about 10 per cent excess SO, over that necessary to leach the remainder of the zinc was present. From the curves it can be seen that even with 0.2 per cent solutions of SO,, all the soluble zinc was dissolved in about 30 minutes. Only 96 per cent of the total zinc was soluble, the other 4 per cent presumably being present as unaltered sulphide or as ferrite. In a FIGURE 9.-Curves showing effect of strength of sulphurous acid on velocity of solution of rotating barrel, with continuous feed and discharge through the trunnions, and countercurrent application of the roaster gases, the tendency would be for the strongest solution of sulphur dioxide to form at the point where the pulp is discharged, the pulp traversing the barrel in solution of gradually increasing strength. With a 1 per cent solution, 91.5 per cent of the zinc is dissolved in 10 minutes |