LEACHING THE CALCINED ORE.

GENERAL METHOD USED IN TESTS.

After the best conditions for roasting the zinc in the ores had been determined the next step was to run larger leaching tests. As 750° C. had been found the best temperature, the experiments were made by roasting the ores at that heat and then leaching the calcines in 2.5-liter bottles. The calcines, before leaching, were ground to pass a 60-mesh sieve, in order that all lumps might be broken. Five leaches were made on the calcines of each roast, as follows: (1) Using the theoretical amount of sulphuric acid needed to dissolve the zinc and agitating the mixture for 1.5 hours; (2) using a slight excess of sulphuric acid over the theoretical amount and agitating 1.5 hours; (3) using a large excess of acid and agitating for 1.5 hours; (4) using a small excess of acid and leaching for 3 hours; (5) using a large excess of acid and leaching for 3 hours. This series was to determine whether an excess of acid was necessary for extracting all of the zinc and whether 3 hours' agitation would give any better extraction than did 1.5 hours. Previous work in the laboratory had shown that as a rule most of the zinc went into solution during the first few minutes of agitation.

The acid efficiency was calculated from the analyses of the solutions and the assay of the tailing. The percentage of zinc recovery was calculated from either the analysis of the solution or the assay of the tailing, and often as a check on the work from both. The recoveries as calculated by these two methods should serve as a check without the running of duplicate analyses.

In many of the tests the tailing from the acid leaches was again leached with saturated brine in order to remove the lead sulphate that had formed during the roasting or during the leaching of the zinc. Thus it was possible to determine whether an ore of mixed lead and zinc sulphides might be roasted, and the zinc and the lead leached separately.

TESTS OF ORE FROM DALY-JUDGE MINE.

The first ore treated was the Daly-Judge No. 1, previously described. The results of the leaching tests of this ore are given in Table 16. The first five tests seemed to indicate a slight advantage in treating the ore for 3 hours instead of for 1 hours, but this is not true of the next five tests. In any event the difference is slight. The ore used up all of the acid when only the amount theoretically required to dissolve the zinc was added. Of course, some of the zinc sulphide had been roasted to sulphate, but the amount was small as determined a number of times by water leaches of the calcines. The temperature was high enough and the time of roasting long enough

TABLE 16.-Results of leaching Daly Judge calcines.

[Calcines from Daily Judge, No. 1, roasted at 750° C.]

ROASTED AT 750° C. FOR 4 HOURS IN CLAY DISHES.

ROASTED AT 700° C. FOR 6 HOURS IN CLAY DISHES.

a Not agitated.

so that usually most of the zinc sulphate formed was broken up. With this ore the recovery of zinc possible by the method of leaching used is apparently about 85 per cent, and the acid efficiency varies from about 65 to 75 per cent. About 80 per cent of the lead can be recovered from the tailing by leaching with saturated brine for two hours.

TESTS OF ORE FROM BULLION COALITION MINE.

The Daly-Judge ore was roasted on clay roasting dishes. In testing the Bullion Coalition No. 5 ore, roasting both in dishes and on the bottom of the muffle was tried. The results of these two tests, presented in Table 17, show that roasting on the bottom of the muffle was fairly successful, whereas roasting in dishes gave low recoveries, both of the zinc and of the lead. As it was thought that the temperature of roasting had been too high, another roast was made (see Table 17) at 700° C. and for a longer period (6 hours). The zinc recovery was better, but the lead recovery was somewhat low. This ore is difficult to roast on account of the large content of pyrite; seemingly the best recovery possible is about 75 per cent of the zinc and about 80 per cent of the lead, with about 80 per cent acid efficiency. For removing zinc from the calcines a 3-hour leach seems to show little improvement over a 14-hour leach.

EFFECTS OF ROASTING WITH SODIUM CHLORIDE.

As some success was had in the removal of lead from such mixed sulphide ores by roasting with sodium chloride in tests reported under the subject of "Lead in complex sulphides," one roast was made in this series of tests in which 10 per cent of common salt was added to the ore. As a result of adding the salt (see Table 18), only a small proportion of the zinc sulphide was converted to soluble form, but the lead sulphide was rendered almost completely soluble. This result is much like that obtained in the work on lead, except that with a blast roaster the lead can be almost entirely converted to soluble form in the presence of zinc sulphide, whereas usually only 4 to 10 per cent of the zinc sulphide is converted. On that account salt is evidently not a desirable addition product in reverberatory roasting of sphalerite.

TREATMENT OF LOW-LEAD MILL PRODUCTS.

Next a series of roasts were made on mill products that did not contain lead in important amounts. Two samples of zinc concentrates of relatively high grade, one from Joplin, Mo., and the other from Midvale, Utah, were roasted and leached, to determine what recoveries of zinc might be expected under the conditions in the laboratory roaster. Also, a sample of zinc-iron middling from the