The Dissolution of Sphalerite in Ferric Chloride Solutions
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The kinetics of dissolution of both sintered sphalerite disks and untreated sphalerite particles in ferric chloride-hydrochloric acid solutions have been investigated. Over the temperature interval 25 to 100°C, the dissolution occurred according to a linear rate law and with an associated apparent activation energy of about 10 kcal/jnole. Most of the oxidized sulfide ion reported as elemental sulfur in the leach residues. The leaching rate was independent of the disk rotation speed and this fact, together with various hydrodynamic calculations, indicated that the reaction was chemically controlled. The dissolution rate increased as the 0.36 power of the ferric chloride concentration and it also increased substantially in the presence of dissolved Cu Cls. The accumulation of the fer-rous chloride reaction product severely retarded the leaching reaction, but the presence of dissolved zinc chloride only slightly impeded it. The leaching rate was relatively insensitive to low levels of HCI (" the rate decreased when Fe Cl; was present in the leaching solution. An apparent activation energy of about 11 kcal/mole was reported over the temperature range 44 to 87°C. As can be seen from the discussion above, there is little really firm data available concerning the kinetics of dissolution of sphalerite in ferric chloride solutions in spite of the potential commercial significance of this method. Accordingly, the present study was undertaken to try to de lineate the important factors affecting the dissolution reaction in ferric chloride media; some thoughts on possible rate controlling mechanisms have also been offered. The bulk of the work was done using sintered disks prepared from natural high-purity sphalerite although important variables were tested using both carefully sized and cleaned sphalerite as well as lumps derived from sphalerite crystals. EXPERIMENTAL All work in the current experimental program was based on sphalerite crystal fragments from Santander, Spain. This material was supplied as yellow-orange, clear cleavage fragments free from visible gangue or other sulfide minerals; the mean crystal size was 544-VOLUME 9B, DECEMBER 1978
several centimeters. For some of the tests "lumps" about 1 em" were sawn from the crystal fragments, cast into araldite cylinders, polished flat and then fixed to a stirring rod for the leaching experiments. Other tests were done using - 65, +100 mesh sphalerite prepared by careful wet screening of the crushed crystal fragments; this material was repeatedly washed to remove all fines. The bulk of the test work was, however, done using disks of sphalerite prepared from the ground crystal fragments. The sphalerite was crushed to minus 150 mesh, pelletized at 80,000 psi and then sintered for 5 to 6 h at 950°C; higher sintering temperatures or longer sintering times were not advantageous because of the sublimation of ZnS. The disks were cooled to 750°C and held at that temperature for 24 h to ensure completion of the wurtzite-sphalerite transition and, finally, were slowly cooled to r
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