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carbon dioxide seemingly does not impair the solubility of the zinc oxide and other oxidized compounds of zinc. Ammonium zincate is a compound which is often assumed to result from the reaction of the solutions on the zinc minerals. Whether the zinc oxide actually combines with the ammonia, or whether it is merely dispersed and taken into colloidal solution, is immaterial, as long as the zinc can be dissolved.

A lengthy series of tests with solutions of ammonia and solutions of ammoniacal ammonium sulphite was made in the effort to determine under just what conditions the zinc could be most easily leached. It was found that the solutions containing ammoniacal ammonium sulphite would dissolve twice as much zinc from the ore as the ammonia solutions. A solution containing about 5 per cent. free NH, and about 3 per cent (NH),SO, gave as good recoveries as stronger solutions. The amount of zine recovered was, then, roughly proportional to the free ammonia present, being about 0.29 gram of zinc per gram of free NH,. This latter ratio shows that 14 molecules of NH, are required to extract 1 atom of zn. Such leaching solutions dissolved slightly more zinc than would dilute acetic acid, when applied to the leaching of an oxidized zine ore. Evidently the ammonial solutions attack only the zine carbonate, leaving the zinc silicate minerals almost unaffected. On leaching a roasted sphalerite ore it was found that all the zinc oxide, zinc sulphate, and zinc sulphite were dissolved. Zinc sulphide and zinc ferrate were seemingly unaffected.

The large amount of ammonia necessary to dissolve the zinc makes such a process of doubtful value, because either the proportion of leaching solution to ore must be large or else strong solutions of ammonia must be used, in which event only about a 5 per cent solution of zinc can be prepared. Neither of these conditions is desirable.

Therefore, the conclusion may be made that ammoniacal leaching solutions are not adapted to complete recovery of zinc from either roasted sulphide ores or oxidized zinc ores, except certain sulphide. ores that can be roasted without the formation of ferrites, or oxide ores that contain no zinc silicate. In any event the mechanical difficulties in handling the leaching solutions are serious.

PURIFICATION AND PRECIPITATION OF SOLUTIONS.

The solutions of zinc prepared from oxidized ores by various leaching processes will not differ greatly from those prepared from the sulphide ores, except in a few particulars. As a rule, a greater proportion of impurities will be taken up from the oxidized ores, hence the solutions will require more careful purification.

Silicic acid was the most prominent impurity noted in the work with oxidized ores. Acid leaching solutions attack the zinc silicate, which is almost invariably present in the oxidized ores, forming silicic acid which tended to set to a gel as soon as the solutions became neutral. As most of the iron, alumina, and arsenic are removed from the zinc sulphate or zinc chloride solutions while in neutral condition, this formation of silicic acid introduced a serious difficulty. Attempts to precipitate the silicic acid by the use of zinc oxide, lime, and glue, resulted in a light fluffy precipitate suspended in the solution which was difficult to filter and retained too much zinc. No satisfactory method of purifying the solution of silicic acid could be found. This resulted in the adoption of methods of ore treatment that would convert the zinc to a soluble form while breaking up the silicic acid by desiccation.

Limonite and similar compounds in oxidized zinc ores are readily attacked by sulphuric acid solutions, with the result that much iron is dissolved and must be removed before either satisfactory electrolytic or chemical precipitation can take place. The removal of iron from such solutions, however, is not difficult and has been discussed sufficiently in the part of this bulletin on the hydrometallurgy of zinc sulphide ore. The same statement applies to the manganese

in these ores.

The precipitation of the various leaching solutions has been hinted at during the discussion of the leaching. Precipitation of these solutions is no different from the precipitation of the solutions prepared from sulphide ores, hence need not be discussed at this point except where the treatment of oxidized ores may introduce some new factor not previously mentioned.

There are no commercial plants in operation at present for treating oxidized zinc ores by leaching and precipitation. As sulphuric acid is practically the only leaching agent used in the United States, and electrolytic precipitation is the only method in common use, seemingly some plant for treating oxidized zinc ores by a combination of these methods would have been established if the method were feasible. The writer's experience in leaching raw oxidized zinc ores has been that the difficulty of removing the silicic acid makes purification of the solutions for satisfactory electrolytic precipitation impossible. The only feasible course was to treat the ore with strong sulphuric acid solutions, and heat it to about 600° C. in order to break up silicic acid, iron sulphates, and similar unde-irable impurities. The leaching of this product with water would give a good zinc sulphate solution, but after electrolytic precipitation the dilute sulphuric acid solution, containing some zinc sulphate, would not be sufficiently concentrated to treat the ore for the first

step of the process. Hence sulphuric acid leaching and electrolytic precipitation do not seem to be adaptable to oxidized zinc ores.

If the ores are treated with sulphuric acid without electrolytic precipitation and recovery of the solvent, there may be a possibility of leaching such ores. This necessarily involves chemical precipitation of the zinc sulphate solutions. The lime precipitation method, in which the zinc sulphate solution is converted into zinc chloride solution, has been described under the discussion of sulphide ores. Also, sodium hydrate or carbonate might be used as precipitants, except that the precipitate is rather difficult to settle and the cost of such reagents would usually be excessive. The use of ammonia as a precipitant for zinc sulphate solutions may have some possibilities as the ammonia could later be distilled from the precipitated solutions after adding an excess of lime to them. This method has not yet been tested by us but will later receive attention. The most doubtful point is the percentage of ammonia that would be recoverable for reuse.

The precipitation of bisulphite solutions of zinc has likewise been discussed under sulphide ores, and as regards precipitation the necessary considerations are well outlined in that part of this bulletin. With oxidized zinc ores the solutions contain greater percentages of iron and similar impurities, and the purification of sulphite solutions containing these is not well understood; more work is necessary before definite conclusions can be drawn.

A few tests were made with the precipitation of the leaching solutions obtained by treating an oxidized zinc ore with strong caustic soda. The results of these tests are given in connection with the data on leaching in Table 46 (p. 133). Dilution of the leaching solution to ten times its original volume still left about one-third of the zinc in the solution, whereas the other two-thirds was precipitated as zinc hydroxide through hydrolysis of the sodium zincate solution. Obviously the dilution of a solution to this extent would make the expense of evaporating the precipitated solution, in order to obtain the caustic soda for reuse, too high.

Zinc can be precipitated from sodium zincate solutions by electrolysis as sponge zinc. This sponge can be dried in the open air over a hot plate without danger of much oxidation and has a high silver precipitation efficiency for cyanide solution when used as zinc dust. If all the details could be worked out with respect to filtration of the solutions and recovery of the caustic soda from the wash water, this fact could easily be made the basis of a process for the preparation of zinc dust which would be of high efficiency and hence of special value for cyanide plants.

a Morgan, H. J., and Ralston, O. C., Electrolytic zinc dust: Trans. Am. Electrochem. Soc., vol. 30, Sept. 30, 1916, pp. 231-241.

Ammoniacal solutions of zinc would have to be boiled to remove the ammonia and precipitate the zinc. The cost of fuel might easily be excessive, as the leaching solutions would probably not contain more than 2 per cent of zinc, necessitating the boiling of large quantities of solution. In order to save fuel expensive heat exhangers would be necessary, and the troubles of operating such a plant might easily condemn the process. The use of ammoniacal solutions for leaching oxidized copper ores is now meeting with some favor, but it must be remembered that a pound of zinc requires the use of more ammonia than a pound of copper and is worth only a third as much. Careful engineering would be necessary to overcome the disparity between the two metals.

SUMMARY OF HYDROMETALLURGY OF OXIDIZED ZINC ORES.

Serious, although not insuperable, difficulties are to be met in the leaching of most oxidized zinc ores by either acid or basic solvents. Nearly all such ores contain rather large amounts of acid-consuming minerals other than the zinc minerals. Leaching with sulphuric acid is difficult, on account of the tendency for the silicic acid formed to jelly when the solution approaches the neutral condition during purification. This makes necessary the "fuming" of the ore with sulphuric acid in order to break up the silicic acid. Sulphurous acid leaching is also difficult, as the solutions must be filtered from the ore while acid and hence carry large amounts of impurities; also, the purification of such solutions is still to be accomplished. Caustic soda and ammoniacal solutions do not seem to attack the zinc silicates, which are present in almost every oxidized ore of zinc, and precipitation of both solutions promises to be difficult, or at least expensive.

PUBLICATIONS ON METALLURGY.

A limited supply of the following publications of the Bureau of Mines has been printed and is available for free distribution until the edition is exhausted. Requests for all publications can not be granted, and to insure equitable distribution applicants are requested to limit their selection to publications that may be of especial interest to them. Requests for publications should be addressed to the Director, Bureau of Mines.

The Bureau of Mines issues a list showing all its publications available for free distribution as well as those obtainable only from the Superintendent of Documents, Government Printing Office, on payment of the price of printing. Interested persons should apply to the Director, Bureau of Mines, for a copy of the latest list.

PUBLICATIONS AVAILABLE FOR FREE DISTRIBUTION.

BULLETIN 16. The uses of peat for fuel and other purposes, by C. A. Davis. 1911. 214 pp., 1 pl., 1 fig.

BULLETIN 67. Electric furnaces for making iron and steel, by D. A. Lyon and R. M. Keeney. 1914. 142 pp., 36 figs.

BULLETIN 70. A preliminary report on uranium, radium, and vanadium, by R. B. Moore and K. L. Kithil. 1914. 114 pp., 4 pls., 2 figs.

BULLETIN 73. Brass furnace practice in the United States, by H. W. Gillett. 1914. 298 pp., 2 pls., 23 figs.

BULLETIN 77. The electric furnace in metallurgical work, by D. A. Lyon, R. M. Keeney, and J. F. Cullen. 1914. 216 pp., 56 figs.

BULLETIN 84. Metallurgical smoke, by C. H. Fulton. 1915. 94 pp., 6 pls., 15 figs. BULLETIN 85. Analyses of mine and car samples of coal collected in the fiscal years 1911 to 1913, by A. C. Fieldner, H. I. Smith, A. H. Fay, and Samuel Sanford.

444 pp., 2 figs.

1914.

BULLETIN 97. Sampling and analysis of flue gases, by Henry Kreisinger and F. K. Ovitz. 1915. 68 pp., 1 pl., 37 figs.

BULLETIN 100. Manufacture and uses of alloy steels, by H. D. Hibbard. 1915.

78 pp.

BULLETIN 150. Electrolysis of cyanide solutions, by S. B. Christy. 1918. 170 pp., 8 pls., 41 figs.

BULLETIN 154. Lead and zinc mining and milling, by C. A. Wright. 1918. 134 pp., 17 pls., 13 figs.

BULLETIN 157. Innovations in the metallurgy of lead, by O. C. Ralston. 1918. 126 pp., 13 figs.

BULLETIN 171. Tests of a rocking electric brass furnace, by H. W. Gillett and A. F. Rhoads. 1917. 131 pp., pls., figs.

TECHNICAL PAPER 8. Methods of analyzing coal and coke, by F. M. Stanton and A. C. Fieldner. 1913. 42 pp., 12 figs.

TECHNICAL PAPER 50. Metallurgical coke, by A. W. Belden. 1913. 48 pp., 1 pl., 23 figs.

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