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Fig. 6 - - P a r a b o l i c leaching curves realized for mounted galena cubes leached in 0.3 M FeCI3 media containing various HCI concentrations.

Fig. 8 - - T y p i c a l leaching curves obtained when mounted galena cubes were leached in HCI media in the absence of ferric ion. The linear kinetics should be noted.

rate by only --20 pet. Above 3 M HC1, by constrast, the rate increases rapidly and H2S w a s evident above the leach solutions, indicating direct acid dissolution of the PbS. Parabolic rate constants were evaluated for all curves, including those at high acid levels where the parabolic law was not exactly obeyed, and these data are compared in Figure 7. In the presence of 0.3 M FeCI3, the dissolution rate is essentially independent of acid concentration to - 3 M HCI, but thereafter the rate increases very rapidly. Above - 5 M HC1 the leaching kinetics were nearly linear; parabolic rate constants deduced from such curves for comparison purposes are very high ( > 4 0 mg2/cm 4 9 h), but are not particularly accurate. Figure 7 indicates that ferric ion attack dominates at acid concentrations < 3 M, and that acid dissolution, in the presence of 0.3 M FeC13, becomes significant only above 3 M HCI. Arai et a l . 7 previously demonstrated that acid concentrations < 0 . 5 M HC1 had no effect on the rate of galena dissolution in ferric ion media.

The dissolution of mounted cubes of galena was also investigated at various acid concentrations but in the absence of ferric ion, and some typical leaching curves from these experiments are illustrated in Figure 8. Unlike the parabolic kinetics observed in the presence of FeC13, the simple acid leaching curves are linear in agreement with previously published studies. ~5'16 For low acid concentrations, the rate is low and increases only gradually with increasing HCI concentrations in the range 0 to 2 M. Above about 2 M HCI, however, the rates rise significantly and very high rapid attack is evident. The evolution of H2S suggests the reaction:

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PbS + 2HCI ~ PbCI2 + H2S

Linear rate constants were deduced for the leaching curves realized in the absence of ferric chloride, and these kinetic data are summarized in Figure 9 (solid line). The direct acid leaching rate increases gradually to - 2 M HC1 and then rises very rapidly in agreement with published data. ~6 Whereas the rate constant observed at 80 ~ in ! M

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