interpreting the results of some of the tests described in the following pages, when the acid efficiencies are stated, for finding the weight of acid necessary to recover a pound of zinc from the ore in question.

LEACHING AGENTS.

SULPHURIC ACID.

As the foregoing tests had indicated that the strength of acid solution used had no effect either on the acid efficiency or on the percentage of zinc recovered, further tests were made of a wide

17

NUMBER OF POUNDS OF ACID CONSUMED PER POUND OF ZINC RECOVERED.

10 20 30 40 50 60 70 80 90 100 PERCENTAGE OF ACID EFFICIENCY.

FIGURE 23.-Curves for computing acid efficiency when weights

of acids consumed and zinc recovered are known.

variety of oxidized zinc ores in order to confirm these conclusions and to study the various impurities that might be found in the solutions from different ores.

The first series of leaches was with samples representing different types of oxidized ores of zinc from various mining districts. The composition and the screen analysis of each of these samples are shown in Table 41, following:

TABLE 41.-Compositions of oxidized zinc ores used in leaching tests with sulphuric acid.

Most of these ores were tested with dilute acetic acid to dissolve zinc carbonate and with dilute sulphuric acid to dissolve zinc carbonate and zinc silicate. The results of the tests, shown in Table 42, give a rough idea of the total percentage of oxidized zinc present as carbonate.

TABLE 42.-Results of trial leaches on 1-gram samples of oxidized ores.
[Samples ground to pass through 100-mesh sieve.]

The ores were then tested for acid consumption and zinc recovery by treating 100-gram lots, ground to pass a 20-mesh sieve, in sulphuric acid solutions of about 10 per cent strength. The ore and solution was placed in 2.5-liter acid bottles and agitated on a set of agitating rolls. The principal results of each set of tests are reported in Table 43 following. Most of the zinc can be removed from the average oxidized zinc ore by the use of sulphuric acid solutions, but the efficiency of the acid used in leaching varies greatly for different ores, being naturally dependent on the amounts of lime, magnesia, and other acid-consuming constituents of the ore.

TABLE 43.-Results of leaching oxidized zinc ores with sulphuric acid, showing zinc recovery and acid efficiency.

[100-gram sample, ground to pass 20-mesh sieve, used in each test.]

Such tests are of fundamental importance in determining whether enough zinc can be recovered from a definite ore and whether the consumption of acid will be excessive, but an even more important point is the properties of the solutions made, because the purity of the finished product depends on the character of the solutions. As the modern methods of purifying zinc sulphate solutions depend upon reducing the solution to a neutral condition, analysis of the

solutions from such ores, when only sufficient acid is used to dissolve the zinc and to leave the solution neutral, is of prime importance. On that account, a sample of each of the ores was leached with sulphuric acid solution of 10 per cent strength for 1 hours. Then enough concentrated acid was added to bring the acidity back to 10 per cent, and the solution was passed over fresh ore until no free acid remained. The analyses of the resulting solutions are shown in Table 44. In almost every test it was noted that when the pulp was neutral there was some difficulty in filtering the solution. Neutral solutions from Scranton No. 2 and No. 3 ores were almost impossible to filter and finally set as gels, on account of the high percentage of silicic acid present in them. Some of the gel that formed in one test was washed and dried, and gave the following analysis: Zn, 2 per cent; Fe, 2 per cent; CaO, 0.7 per cent, and SiO2, 93.6 per cent.

TABLE 44.-Results of analyses of neutral solution of zinc sulphate and impurities.

As it was thought probable that electrolytic recovery of the zinc from such solutions might be attempted in some instances, the effects of silicic acid gel were further investigated. It was found that silicic acid caused serious trouble only when the solutions were neutral, and that the acid solutions were much more easily filtered. Many of the pulps, when allowed to stand over-night in the neutral condition could not be filtered the next morning and in many cases the whole solution has set to a gel. The lowest percentage of silica ever observed in one of these gels was 0.6 per cent and this gel took about two days to set. Electrolytic precipitation of the zinc would not remove the silica from the solutions, therefore the silica would tend to build up. The analyses of the solutions indicated that with continued use the silicic acid would soon build up to a point where it would entirely prevent further operation. Cyclic leaching of a number of the ores tested proved that the silica had such an effect. As a rule when the silica content of the solution reached more than 2 per cent, filtering became very

difficult and could be done only when the neutral liquor was acidified and used for treating fresh ore without removal of the zinc. As is shown subsequently, removal of this colloidal silica from solutions contaminated with it was almost impossible. Therefore, in treating oxidized zinc ores containing silica, the preparation of zinc sulphate solutions in such a way that the silica would not be present is necessary. Silicic acid can be dessicated to form "insoluble silica" and water by heating it to more than 125° C. in the open air. Consequently, an attempt was made to prepare a solution free of silicic acid by treating the ore with just enough strong sulphuric acid to moisten it, heating the mixture of sulphuric acid and ore to about 150° C., or higher, and leaching the calcined material with water.

It was found that concentrated sulphuric acid would not moisten the ore sufficiently, so the necessary acid for converting the zinc into zinc sulphate was added as a 50 per cent solution, with satisfactory results. The laboratory roasts were made in roasting dishes in a gas muffle furnace. A large proportion of the iron and the alumina are converted into the corresponding sulphates by this treatment, unless the temperature of the roast is maintained at about 600° C. or higher. Iron sulphate breaks up rather slowly at 600° C., requiring at least two hours' roasting to prevent the subsequent appearance of iron in the solutions. At 700° C. the necessary time for obtaining the same result varies from one-half to one hour. At 800° C. some of the zinc sulphate is broken up by the heat, and as a result leaching with water does not obtain as much zinc. The zinc sulphate formed tends to cake the material together, and in most furnaces this caking would probably cause mechanical difficulty in handling such material. Possibly some type of horizontal kiln would be necessary in order to avoid trouble from this source.

Practically all the oxidized ores tested yielded to this treatment, the zinc recoveries being about the same as those obtained by direct leaching with solutions of sulphuric acid, except that when a charge was overheated some of the zinc sulphate was broken down into zinc oxide or into basic zinc sulphate. Under these conditions sulphuric acid had to be added to the leaching water in order to get good recoveries. In almost every test it was necessary to use enough sulphuric acid to satisfy all the zinc, lead, lime, and magnesia in the ore before satisfactory recovery of the zinc could be made. The amounts of iron usually entering the solution varied from 10 to 20 milligrams per liter. Manganese was not so easy to keep out of the solution, because its sulphate does not seem to decompose as easily as that of iron. Consequently there was usually a considerable amount of manganese in the solutions prepared from these ores. Silica was rarely present in more than traces.