The leaching of galena in ferric sulfate media
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B. Leaching Procedures
Because of the low solubility of PbSO4 in sulfate solutions, the course of the galena leaching reaction was monitored by following the amount of cogenerated ferrous ion. A 1- or 2-g sample of sized galena was agitated at 200 min -~ in 2.7 L of Fe(SOn)l.5-H2SO4 solution heated in a thermostated water bath. Samples were taken periodically, either by pipette or by an automatic sampiing device connected to a fraction collector. The samples were then titrated for Fe z§ using ceric sulfate. It should be noted that the titration of low concentrations of Fe 2+ is analytically difficult. The stoichiometry of the leaching reaction was established by slurrying 4-g samples of sized galena in the ferric sulfate solution for a fixed period and then filtering and washing the contents. The solids were immediately releached in 4 M NaC1 at 60 ~ to 70 ~ to ascertain the amount of PbSO4 generated. The brine-leached solids were then air-dried prior to treatment with CS2 in a Soxhlet reactor to recover the elemental sulfur. All parts of the leaching apparatus were rinsed with CS2 to recover any sulfur adhering to the vessel walls, stirrer, etc. Finally, the leach solution was brought to volume and was titrated for ferrous ion. C. Mineralogical Studies
Supporting mineralogical studies were carried out on both massive and sized galena which had been leached for various times and under different conditions. Both the as-leached surfaces and cleaved cross sections were examined using the scanning electron microscope (SEM) equipped with an energy-dispersive X-ray (EDX) analyzer. X-ray diffraction analyses were used to confirm the phases present. Details of all the mineralogical techniques have been presented previously. 1191
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Fig. 1 - - L e a c h i n g curves realized for the dissolution of mounted cubes of galena in ferric sulfate-sulfuric acid and ferric chloridehydrochloric acid media.
(open triangles) obtained when a similar mounted cube of galena was leached at 90 ~ in 0.3 M FeC13-0.3 M HC1-3 M NaC1 solution. The leaching rate is at least an order of magnitude more rapid in the chloride system despite the similar iron concentrations and Eh values of the two media. These variations could be due to the significant differences in the solubilities of the PbC12 and PbSO4 reaction products or to the strong complexation of the Pb 2+ ion in chloride media. The leaching curve obtained in the sulfate solution is clearly nonlinear, and in fact, it has a generally parabolic form. Accordingly, when the data are replotted on a parabolic basis, that is, as (mg Fe2+/cm2) 2 vs time, a straight-line relationship is obtained as indicated by the dashed line and open circles. The implication is that the leaching of galena in ferric sulfate media obeys the parabolic rate law, at least approximately. Because the very small amounts of Fe 2+ produced during the leaching of the
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