Kinetics of biochemical leaching of sphalerite concentrate
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I.
INTRODUCTION
THEpyrometaUurgical treatment of sulfides generates SO2 and thus causes atmospheric pollution. An alternative to such technique could be bacterial leaching of sulfide concentrates which would recover the sulfur values as different sulfates and also partially as elemental sulfur. Generally bacterial leaching is commercially well exploited to recover metal values from low-grade ores, 12 but recently attempts have also been taken to recover metal values from concentrate.3'4 Extensive works 5-12 have been carried out on sphalerite concentrate. Torma et al. 5 correlated the solubility product of the sulfide of Zn with the extraction rate. The dissolution of Zn from the sulfide has also been correlated through galvanic interaction. 6'7 CO2 also been observed 8 to be the rate limiting factor for the dissolution of sphalerite concentrate. Torma et al. 9 correlated the dissolution of Zn vis-a-vis the particle size. Gormley et al. 10 modeled the kinetics of a bio-leached sphalerite concentrate in a continuous stirred tank reactor. The rate of sphalerite leaching was found to be dependent on the various strains of thiobacillus ~ isolated from different sources. Duncan et al. L2 described a feasible process for the extraction of metal values through vat leaching. The kinetics of bacterial leaching are often affected by product layers like jarosite and elemental sulfur. The literature 4'~3 does not provide much information regarding such kinetics. In order to achieve a better understanding of , the leaching process, attempts are made in this paper to interpret some leaching data kinetically.
flasks with different sieve fractions. The leaching curves were drawn taking the average of duplicate runs. In all cases, unless specified, the leaching studies were conducted at 1 pct pulp density and at a pH of 2.5. In each set of experiments uninoculated controls were also run where HgC12 was added as a bacterlocide.~6 Suitable amounts of samples were withdrawn at regular intervals from each flask for the estimation of various metal ions as well as the concentration of microorganisms. Concentrations of metals were determined with the help of a 372-Perkin Elmer atomic absorption spectrophotometer. Esttmations of bacterial concentrations were made on solid agar medium as described by Mishra et al. ~v III.
RESULTS AND DISCUSSION
A set of experiments in duplicate was conducted with different sieve fractions, - 100 + 150, - 150 +200, - 2 0 0 +300, and - 3 0 0 +350 B.S.S. at an initial bacterial concentration of 103 cells/ml. The bacterial concentration increased to 107 cells/ml after 31 days of leaching as shown in Figure 1. The initial rate of sphalerite concentrate leaching was slow, but as the bacterial population increased the leaching rate showed a steady increase as shown in Figures 2, 3, and 4. In all these three figures the leaching of uninoculated controls for only one particle size is shown as the variation of the same had very little effect on the leaching. With all the sieve fractions the Cu, Zn, and Cd were
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