The work presented in this dissertation is an investigation of-the ferric leaching of zinc from sphalerite. A further aspect of this study was an investigation of the influence of pyrite on the dissolution rate of sphalerite due to possible galvanic interactions. This study is one component of a larger study of the sub-processes involved in the bioleaching of sulphide minerals in which the ferric leaching of the sulphide mineral is assumed to be a chemical step with the bacteria oxidising ferrous iron to ferric iron and elemental sulphur, if formed, to sulphate. The literature showed that two types of model have been used to describe the ferric leaching of the sphalerite. The first type was a shrinking-particle model in which the reaction was described by first order kinetics or an electrochemical mechanism. The second type included a mass transfer resistance in terms of a shrinking-core model described by half-order kinetics or a decaying diffusion coefficient. All four of these models were tested for their ability to predict published data for the ferric leaching of sphalerite. It was found that the models fitted the data for the initial period of a leach up to conversions of about 50%. However, no one of the models was found to be successful in predicting the data for prolonged leaching to high conversions. Because of this, a new model was developed based on the assumption of deactivation of the sphalerite surface due to precipitating products. This surface area de~ctivation model for the rate of conversion has the form: where the kcR(1-xr2'3 term is based on the shrinking-particle mechanism and the surface area deactivation term is exp (k1 t). When tested against ferric leach data from the literature it was found to be satisfactory for both the initial leach period and high conversion data at prolonged leach times. In the experimental study the following factors were investigated: the effect of temperature between 30 °C and 55 °C; the size of the mineral particles in the range 106+90 μm to -45+38 μm; initial ferric iron concentration between 0.06 M to 0.5 M. Tests were run under conditions where the redox potential was controlled at fixed values ranging from 673 mVto 443 mV (vs Ag/AgCI) by the addition of 3% H202. In other tests the redox potential was not controlled but dropped during the course of the leach as the ferric iron was consumed. The Surface Area Deactivation Model was found to be a satisfactory fit for all of the leach data obtained in this study. Using Arrhenius plots, the temperature dependence of the chemical reaction rate constant was calculated to be 47 kJ.mor1 and the activation energy calculated from the surface area deactivation rate constant was 43 kJ.mor1 . In both cases, these values are in the range where the chemical reaction at the mineral surface is thought to control. The surface area specific conversion rate decreased at the same rate for each particle size. This suggests a constant chemical rate per unit surface area and a constant specific rate of surface deactivation, in both cases independent of particle size. Examination of the leach residue by scanning electron micrography showed that at long leach times there was a layer of elemental sulphur attached to the surface of the sphalerite leading to its deactivation. From the data obtained under controlled redox potential conditions, it was found that the reaction rate constants were a linear function of the redox potential. However, the gradient of the relationship changed at a redox potential of approximately 520 mV (vs Ag/AgCI). These results are consistent with the work of Crundwell (1988b) where the chemical reaction was shown to be dependent on the solution equilibrium of the iron species and the ionic species responsible for transfer of charge. From the data where the redox potential was not controlled, the reaction was limited by the amount of ferric iron available for the dissolution reaction. The effect on the leach kinetics of possible galvanic interactions between pyrite and sphaleri~e were tested in two ways: the addition of pyrite concentrate to the sphalerite concentrate, and leaching of hand-picked samples of ore containing particles of combined unliberated pyrite and sphalerite in direct contact. Where the pyrite concentrate was added to the sphalerite concentrate, no increase in the rate of zinc leaching was measured for pyrite/sphalerite mass ratios of 0.01, 0.1 and 1.0. On the other hand, in the ferric leaching of the particles of combined unliberated pyrite-sphalerite, the rate of zinc leaching was found to be greater than for sphalerite on its own under the same conditions. This suggests that for galvanic interactions to improve the rate leaching, the minerals have to be in intimate contact.

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