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LEACHING THE CALCINE.

As there was some doubt as to the relative value of sulphuric acid and acetic acid for recovering the zinc, and of acid ammonium acetate and acid brine for recovering the lead, comparative leaches were made on a calcine roasted at 700° C. of material ground to pass 80 mesh. The full-line curves in figure 3 give the recoveries with the acetic acid solvents, and the dotted lines, those for sulphuric acid and acid brine solvents, respectively. It can be seen that much more zinc is dissolved by the sulphuric acid solution than by the acetic acid solution, and that for practical purposes the leaches with acetic acid give low results. For this reason sulphuric acid is the leaching agent usually used in mills. In the same way a saturated brine, acidified with sulphuric acid, is a better solvent for the lead. Incidentally a roast at 775° C., the upper allowable limit of temperature, was made, with the result that practically all of the zine was rendered soluble in sulphuric acid in less than four hours. Few of the commercial roasters will do such rapid work, and it would seem that roasters adapted to the use of very shallow beds of ore and to the use of finely divided ore might well be devised in order to obtain rapid roasting.

ROASTING OF A HIGH-IRON ORE FROM HONORINE MINE.

After roasting tests had been made with several sizes of this lowiron ore at different temperatures and for different lengths of time it was thought best to perform similar tests with an ore that was high in iron. Such an ore was available in the Honorine mine of the Bullion Coalition Mining Co., at Stockton, Utah; the sample obtained analyzed as follows: 13.1 per cent Zn, 10.56 per cent Pb, 22.2 per cent Fe, 30.32 per cent S, 18 per cent insoluble, and 2.42 per cent As.

The portions of this sample that were used for the tests to determine the effect of temperature and of length of roasting period gave the following screen analysis after preparation by crushing.

Results of screen analysis of ore from Honorine mine after crushing.

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EFFECTS OF TEMPERATURE AND TIME OF ROASTING.

The lowest temperature investigated in this series of tests was 700° C., as the lower temperatures had been shown to be unsatisfactory for roasting zinc and lead minerals. Figure 4 shows that during the first two hours more soluble zinc is formed by roasting at 800° C.

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0 1 2 3 4 5 6 7 8 9 10 11 12 13
TIME OF ROASTING, HOURS.

14 15 16 17

FIGURE 4.-Curves showing effects of temperature on speed of roasting; Bullion Coalition, No. 5 sample, 97.4 per cent passed 20-mesh.

than by roasting at 700° C., owing to the more rapid roasting, but that after two hours the 800° roast contains less soluble zinc than the 700° roast, whereas the solubility of the zinc is very low after a 900° roast. This decrease in solubility was assumed to be due to the formation of ferrites, although the lead might be the cause of some of the trouble. However, as the lead did not cause any trouble in the first ore tested, this latter assumption is not regarded as probable. The

curves show that it is not safe to heat the ore much above 700° C. during the roasting. Moreover, some of the lead is rendered insoluble on long roasting at high temperatures and the lead recovery curves show a decided drop with increasing time. Practically all of the zinc is rendered soluble in about six hours' roasting at 700°, whereas nearly 10 hours' roasting seemed to be necessary for maximum solubility of the lead.

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FIGURE 5.-Curves showing effects of size of ore on speed of roasting; Bullion Coalition, No. 5 sample, roasted at 700° C., 20-mesh and 100-mesh sizes.

EFFECTS OF FINENESS OF GRINDING.

Figure 5 shows the time-extraction curves for two roasts on material of different sizes, 20-mesh and 100-mesh. It is surprising how small a difference the size of the particles makes in the roasting of

sphalerite. The material ground to pass a 100-mesh screen showed the gradual formation of ferrite during the last eight hours of the roast. This seems reasonable, as the ore is more finely ground and there is a better chance for the sphalerite being in contact with pyrite during the roasting, with the resultant combination of iron oxide with zinc oxide as time goes on. As regards the galena, the lead recoveries from the 100-mesh material were markedly higher, after a given length of time, than the corresponding ones for 20-mesh material. The maximum percentage of lead rendered soluble by roasting 20-mesh material was about 80 per cent, whereas nearly 90 per cent was rendered coluble in the roasting of the 100-mesh material.

EXPERIMENTS TO DETERMINE EFFECTS OF LEAD.

In these tests it was suspected that some of the lead after roasting tended to form lead silicate. To obtain light on this point, a finely

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650°C.

750°C

750°C.

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ground slime from the dump of the Horn Silver mine at Frisco, Utah, was roasted. This material had the following analysis: 6.3 per cent Zn, 7.4 per cent Pb, 53.6 per cent insoluble, 4.4 per cent Fe, 2.2 per cent CaO, 6.13 per cent S, and 8.5 per cent Al2O3. The results of the two roasts made are plotted in figure 6. It can be seen that roasting at 650° C. renders practically all of the lead soluble in a very short time. Eighty per cent of the lead in this ore was oxidized before roasting and only 20 per cent of the zinc was oxidized. When the ore was roasted at 750° C., the solubility of the lead seemed to be less, although the recovery of zinc was much better, than from roasting at 650° C. However, the formation of ferrite of zinc at 750° C. seems to be quite plainly indicated by the fall in the last two-thirds of the zinc curve for that temperature.

ZINC ROASTED, PER CENT.

0

0 1 2 3 4 5 6 7 8 9
TIME OF ROASTING HOURS.

FIGURE 6.-Curves showing effects of temperature on solubility of zinc and lead in slime from Horn Silver dump, Frisco, Utah, 650 and 750° C.

Similar curves for the roasting of a series of ores all of which contained both lead and zinc, at 750° C., are shown in figure 7. The analyses of these ores are given in Table 15. Bullion Coalition No. 1 sample is representative of a large tailing dump from a concentration

mill in which most of the zinc and lead sulphides had been removed from the pyritiferous gangue by gravity concentration. Bullion Coalition No. 2 sample is representative of a zinc-iron-lead middling product, made at this mill, which is run into the tailing. The American Flag No. 2 sample is representative of the partly oxidized ores of the Park City district of Utah, and the DalyJudge No. 2 sample is representative of the deeper sulphide ores of that district when iron is present in larger amounts. The Ophir Hill No. 2 sample is of the slime tailing of the Ophir Hill mill at Ophir, Utah, not far from Stockton, where the Bullion Coalition mines are situated.

In general, the tests showed that for all of these ores the most favorable roasting temperature was 750° C. At this temperature the maximum proportion of soluble zinc obtained is formed in about four hours, if the roasting bed is not more than an inch deep. The same remark applies to the lead in these ores.

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0 1 2 3 4 5 6 7 8 TIME OF ROASTING, HOURS. FIGURE 7.-Curves showing effects of roasting on solu bility of zinc and lead in various ores, 750°C.: a, Bul lion Coalition, No. 1 sample; b, Bullion Coalition, No. 2 sample; c, American Flag, No. 2 sample; d, Daly Judge, No. 2 sample; e, Ophir Hill, No. 2 sample.

TABLE 15.-Results of analyses of zinc-lead sulphide ores.

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