Kinetics of silver leaching from manganese-silver associated ores in sulfuric acid solution in the presence of hydrogen
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MANGANESE-SILVER associated or manganiferous silver ores occur in many parts of the world.[1,2,3] A number of manganese-silver ore mines have been found in China.[4,5] Generally, most of silver occurs within the crystal lattices of manganese minerals as tiny particles and is difficult to be extracted by conventional silver leaching methods. A pretreatment or pre-extraction of manganese by roasting or leaching is usually required to recover silver from the ores.[5,6,7] The authors of this article have investigated the simultaneous leaching process of manganese and silver in the sulfuric acid solution in the presence of H2O2 from manganese-silver associated ores, and an extraction of 97 pct for manganese and 85 pct for silver has been achieved at room temperature.[8] A literature survey shows that few articles reported on the leaching kinetics of silver in sulfuric acid solution, although there are many investigations on the kinetics of silver leaching in cyanide,[9–12] thiourea,[13,14,15] halide,[16,17,18] nitric acid,[19] ammonia,[20,21] and thiosulfate[22] systems. This article investigates the kinetic behaviors of silver leaching in sulfuric acid solution in the presence of H2O2 from manganese-silver associated ores. II. MATERIALS AND METHODS The ore sample was obtained from Dixin Manganese Mine (Guangxi, China). It analyzed 0.30 pct S, 1 pct Pb, 4.74 pct Fe, 3.24 pct Al, 0.04 pct Cu, 0.06 pct Zn, 0.1 pct Ca, 0.2 pct Mg, 12.2 pct Mn, and 1850 g/t Ag. Scanning electron microscope analysis of the ore revealed that the manganese occurs mainly as MnO2 (95.97 pct) in the ore. About 57.85
pct of silver is dispersed in the matrix of manganese minerals as tiny particles, 34.62 pct of silver occur as native silver, and 5.1 pct as Ag2S in the ore. The sample used in leaching tests was prepared by crushing, wet grinding, sizing, and drying. The sulfuric acid and hydrogen peroxide used in experiments are of analytical grade. Experiments were carried out in a 1000 mL glass reactor. The ore sample was first added to the agitated sulfuric acid solution at the required temperature; then, hydrogen peroxide was added to the solution. The ratio of solid to liquid was maintained constant at 1/20, and the stirring speed was kept at 950 rpm. At selected time intervals, a sample of 5 mL solution was taken using a glass-sampling device, and the solids in the sample were discarded. The sample solutions were analyzed for determination of silver content, from which the extraction fraction of silver was calculated. III. RESULTS AND DISCUSSIONS A. Determination of Kinetic Model Figure 1 shows the kinetic curves of silver leaching obtained under the conditions of 0.9 M H2O2, 0.9 M H2SO4, 0.074 to 0.053 mm particle size, 950 rpm, and 25 ⬚C. The upper line is silver fraction reacted (x) against time (t), the middle is the relation between 1 ⫺ (1 ⫺ x)1/3 and t, and the lower responds to the relation between 1 ⫺ 2x/3 ⫺ (1 ⫺ x)2/3 and t. It is clearly shown that the leaching rate of silver in the sulfuric acid solution in the presence of H2O2
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