The disadvantage of getting the zinc into solution in this manner is that only strong solutions of sulphuric acid can be used, whereas electrolytic precipitation of a zinc sulphate solution would regenerate only a weak solution of sulphuric acid which could not be used for sulphating the ore. Consequently this method of preparing pure solutions involves chemical precipitation of the zinc from the solution and loss of the sulphuric acid. Such a procedure would naturally be limited to localities where sulphuric acid was cheaply available in large amounts.

Niter cake, the impure bisulphate of sodium (NaHSO), which is usually available as a waste product from nitric acid plants, powder plants, sulphuric acid plants, etc., has an available acidity of about 30 per cent in the usual commercial article, and melts at a low temperature. Niter cake was tried as a substitute for sulphuric acid in sulphating ores at high temperatures. The efficiency in recovering zinc was about the same as with sulphuric acid, but the sodium and zinc sulphates resulting caked the ore harder than when only sulphuric acid was used. Furthermore, with some ores the amount of niter cake necessary to satisfy the acid requirements of the ore was excessive, and the resulting burden of sodium sulphate in the water leaches made recovery of the zinc from the solution doubtful. Therefore, sodium sulphate is an undesirable diluent of the acid used in preparing the ore.

HYDROCHLORIC ACID.

The preceding considerations of the adaptability of sulphating at a high temperature followed by chemical precipitation for recovering zinc recalls the fact that no acceptable method of precipitating zinc from its sulphate solutions by any commercially available chemicals has been devised. Lime is the cheapest base available, but it forms calcium sulphate which will mix with the zinc hydroxide precipitate. Hence the preparation of chloride solutions of zinc is desirable, if this can be done, because they would be better adapted to precipitation by lime. As was shown in an earlier part of this paper, hydrochloric acid is equally efficient with sulphuric acid, by molecular weights, in dissolving zinc from its oxidized ores, and wherever cheaply available could easily be used for leaching such ores. The station has contemplated making some experiments on the preparation of hydrochloric acid by the interaction of sodium chloride with silica and steam at high temperatures. The reaction is known to take place, but development work needs to be done on the mechanical application of the reactions. If hydrochloric acid can be prepared cheaply in this way, application to the leaching of zinc ores will then be easy.

86198°-19-Bull. 168- -9

SULPHUROUS ACID.

The use of sulphur dioxide as a leaching agent has always appealed to the hydrometallurgist, but to date there have been practically no successful commercial installations of sulphurous-acid leaching plants for either zinc or copper ores. Many attempts have been made to absorb sulphur dioxide with water or steam and use the resulting sulphurous acid for leaching either oxidized zinc or copper minerals, but the mechanical difficulties of applying the process have caused all such enterprises to fail. The asphyxiating character of the fumes rising from the solutions necessitate the use of closed vessels, whereas difficulties from reprecipitation and crystallization of salts in pipes and other closed vessels have made it almost necessary to work with open vessels. Furthermore, some of the proposed methods have looked toward recovery of the sulphur dioxide from the sulphite solutions, whereas practice has proved that much of the zinc sulphite is oxidized to higher oxides, from which it was impossible to recover sulphur dioxide.

In most of the intermountain districts the recovery of sulphur dioxide is not vitally necessary, and any satisfactory method for the leaching of the zinc or the copper and the ultimate wasting of the sulphur dioxide would usually be acceptable. On that account further investigation of the possibility of sulphurous acid leaching of oxidized zinc ores was deemed advisable.

The first factor determined was the approximate solubility of zinc bisulphite, the soluble compound of zinc formed by the use of excess of sulphurous acid. The monosulphite of zinc is insoluble, and most of the proposed processes for sulphurous acid leaching involve leaching of the zinc as a bisulphite solution with subsequent precipitation of insoluble zinc monosulphite. By suspending an excess of zinc oxide in water and then passing sulphur dioxide from a cylinder of the liquified gas until the solution smelled strongly of excess sulphur dioxide, it was found that at the ordinary temperature of the laboratory 6.97 per cent of zinc was in solution and the total sulphites present were equivalent to 16.08 per cent SO2. Thus about 2.36 per cent of SO, was uncombined. Most of the values in the literature on the subject for solubility of zinc sulphite agree reasonably well with these figures, although in some patent specifications mention is made that some of the zinc often oxidizes to the sulphate form, when the total zinc content of the sulphite leaching solution can approach 10 per cent.

A number of tests were made with several ores, to determine the leaching efficiency of sulphurous acid as compared with sulphuric acid. The results of these leaches are given in Table 45. The zinc recoveries, even with very dilute solutions, are fully as good as those

possible with sulphuric acid solutions. The acid efficiency obtainable is likewise comparable with the acid efficiency of the sulphuric acid solutions. The figures in the column for acid efficiency represent the apparent efficiency, which is twice the actual efficiency, because in titrating the leach solution for acidity the zinc bisulphite reacts. with the acid until completely neutralized, forming zinc monosulphite. Hence the figures in the column of acid efficiencies should be divided by two.

TABLE 45.-Results of sulphurous acid leaching of oxidized zinc ores.

The impurities taken into solution by sulphite solutions are lime, alumina, iron, copper, and small amounts of silica. Lead, silver, and gold do not dissolve to any extent.

The mechanical requirements for successful leaching of zinc ores by the bisulphite method have already been considered in the part of this bulletin dealing with the hydrometallurgy of the sulphide ores (pp. 81-84). Except for the tendency of silicic acid to enter the solutions in the leaching of oxidized ores and later contaminate the precipitate of zinc monosulphite, there is little difference between the problem of the treatment of oxidized and of sulphide ores by sulphurous acid. As the zinc bisulphite solutions must be acid for filtering, no filtration difficulty is experienced, whereas zinc sulphate solutions are usually filtered in the neutral condition where the silicic acid causes trouble.

HYDROFLUOSILICIC ACID.

As the writers had been informed that zinc fluosilicate solutions could be easily electrolyzed to give good cathode spelter, a few hydrofluosilicic acid leaches of oxidized zinc ores were tried in order to see whether the acid would be an acceptable leaching agent. Leaching the raw ore with hydrofluosilicic acid gave an acid efficiency of more than 100 per cent. The theoretical weight of acid which should be consumed is 2.2 pounds per pound of zinc, whereas the observed consumption was only 1.9 pounds. This lower rate was attributed to the presence of some hydrofluoric acid in the hydrofluosilicic acid,

which, on account of the lower molecular weight of the former, would make the hydrofluosilicic acid appear more than 100 per cent efficient. On leaching part of the same sample of ore with the acid after calcination, the acid consumption amounted to only 0.52 pound per pound of zinc. On the assumption that the zinc oxide from the roasting caused hydrolysis of the hydrofluosilicic acid with resultant formation of zinc fluoride, in place of zinc fluosilicate, only 0.615 pound of the acid would be required to leach 1 pound of zinc. Thus, the results would seem to indicate that the following reactions took place in the two tests:

H2SiF+ZnCO3=ZnSiF ̧+H2O+CO2
H2SiF ̧+3ZnO =3ZnF2 +H2O+SiO2

This shows that hydrofluosilicic acid might be used for the leaching of zinc carbonate ores, but that it would be unsuitable for the treatment of any type of roasted products. No further work was done with it.

ALKALINE AGENTS.

The fact that zinc oxide and zinc carbonate will combine with solutions of caustic soda or of ammonia to form zincates is the basis of many proposed processes for leaching oxidized zinc ores. In many instances these processes would have an especial advantage over acid leaching, because most of the oxidized zinc ores contain important amounts of calcium and magnesium carbonates, and other acid consuming minerals which would not affect the alkaline leaching agents. Hence much better chemical efficiency of the leaching agent should be possible. With this in mind, two leaching agents were tested-caustic soda solutions and ammoniacal solutions.

CAUSTIC SODA.

Caustic soda, as a solvent for zinc ores, has been proposed by many inventors, but has never been applied commercially with any success." The available literature gives little information as to the causes for the failure of caustic soda as a leaching agent. However, the writers soon found that, although many patents claim the application of caustic soda solutions to oxidized ores, satisfactory recoveries of the zinc from these ores by the use of solutions containing 10 to 25 per cent NaOH was almost impossible.

The results of six leaches of the May Day No. 2 ore with caustic soda solutions are recorded in Table 46. In each of these leaches

a A list of the principal United States patents covering the use of caustic soda leaching solutions is as follows: Ketchum, E. C., No. 592055, Nov. 21, 1896; Ryan, Thomas, jr., and Hughes, Newton, No. 647989, Dec. 26, 1899; Strzoda, Wilhelm, No. 656305, Jan. 20, 1899; Sadtler, S. S., No. 700563, July 10, 1900; Barton, Thomas, and McGlue, T. B., 689,275, July 30, 1901; Ranson, Charles, No. 1023964, July 8, 1910; Bretherton, S. E., No. 1204843, Dec. 23, 1912; Asef, Waldemar, No. 1165743, Oct. 26, 1914; Snyder, E. H., No. 1184585, Mar. 31, 1915.

33.3 grams of ore ground to pass an 80-mesh sieve was agitated in 100 cc. of a 22.44 per cent NaOH solution. With cold solutions the recovery was very low. Even with hot solutions only about twothirds of the zinc was obtained and the solution had to be maintained at 97°C. for four hours for this result. The zinc oxide is supposed to be precipitated from the solution on dilution, hence enough water was added to make 1,000 cc., which, of course, reduced the strength of the solution to about 2 per cent. Under these conditions a precipitate of zinc hydroxide was obtained containing about 40 per cent of the zinc originally in the ore, as shown by the last column in Table 46. The remainder of the zinc stayed in solution.

TABLE 46.-Results of leaching oxidized zinc ore with caustic soda solution.

On account of the difficulty of dissolving the zinc, to say nothing of the practical difficulties involved in dilution of the solution followed by evaporation in order to recover the sodium hydroxide, no further work was done with this process.

The consumption of alkali, determined in each of the tests shown in Table 46, was about 80 to 100 pounds of sodium hydroxide per ton of ore. The sodium is supposedly fixed as sulphate or chloride, or possibly even as silicate, but the results had been so discouraging that this latter point was not determined.

AMMONIA.

Ammoniacal leaching solutions have received much more favorable consideration than caustic soda from metallurgists in times past, although no commercial plants using such solutions have been built.a

Most of the methods proposed for leaching zinc ores with ammonia involve the use of ammoniacal ammonium-carbonate solutions, because carbon dioxide tends to accumulate in the solutions as ammonium carbonate, and also because the presence of combined

• United States patents relating to ammoniacal leaching of zinc ores are as follows: Burghardt, C. A., and Rigg, Gilbert, 585355, June 1, 1896; Koehler, W. J., 611917, Nov. 7, 1896; Howard, Henry, 623154, May 14, 1898; Rigg, Gilbert, 654804, May 6, 1899; Hirsching, Henry, 735512, Mar. 19, 1902; Bermont, Victor, 751712, May 6, 1902; Joseph, T. B., 780293, May 18, 1904.