A kinetic study of the dissolution of natural and synthetic sphalerite in aqueous sulphuric acid and in acidic ferric suplhate media.

Abstract

Four sphalerites (synthetic, high grade natural, moderately impure flotation concentrate and highly impure flotation concentrate) were leached in acid sulphate media without and with ferric ions present under the following conditions :- Case (i) [Fe3+]o : [H2S04]o = 0,0 Case ( ii) [Fe3+]o : [H2S04]o = 1,8 Case (iii) [Fe3+]o : [H2S04]o = 0,1 Extensive data for leaching under these conditions are tabulated. Kinetic mechanisms based on Langmuir-Hinschelwood adsorption theories were proposed, and leaching models were developed for different assumed rate limiting steps. The initial rate and overall forms of the models were tested using experimental data.Leaching under case (i) conditions Non-oxidative dissolution took place with Zn2+ and H2S the predominant reaction products. The H2S partial pressure was monitored continuously and solution samples were taken for analysis at discrete time intervals. Vibratory (i.e. attrition) milling eliminated very large differences observed in the leaching characteristics of course size fractions of the natural sphalerites. The initial rate form of a model based on a dual site reaction mechanism and on either H+ adsorption or reaction product desorption rate control was found to fit the data for the synthetic and vibratory milled forms of sphalerite. The most impure vibratory milled sphalerite adsorbed Zn2+ and H2S very strongly, and this resulted inproduct desorption rate control. Vibratory milled forms of the high grade natural sphalerite and the moderately impure flotation concentrate, exhibited virtually identical initial rate dissolution kinetics, despite large differences in their chemical compositions. Leaching under case (ii) conditions Oxidative dissolution took place with Zn2+ and elemental sulphur the predominant reaction products. Scanning electron microscope photographs of leached and unleached particles showed the sulphur present on the particle surface. These photographs, and optical microscope photographs of etched polished sections, showed that dissolution took place in a complex way. A model based on ferric ion adsorption as the rate limiting step was proposed and confirmed experimentally. The model demonstrated a proportional dependency of the rate on the area and ferricion concentration, and an inverse dependency on the hydrogen ion concentration. For a -90,0 + 63,0 um size fraction, the three natural sphalerites exhibited virtually identical dissolution rates per unit area. The effect of ball milling or vibratory milling the sphalerites fine, was to increase the rate per unit area for the most impure natural sphalerite but decrease the rate per unit area for the high grade natural sphalerite.It was shown that for course size fractions of sphalerite, the most impure sphalerite which leached slowest under case (i) conditions (i.e. adsorbed H+ poorly) leached fastest under case (ii) conditions (i.e. adsorbed Fe3+ strongly). The reverse was true for the high grade natural sphalerite. Except in the case of synthetic sphalerite leaching under case (i) conditions, no correlation was shown to exist between the way the B.E.T. measured area changed, and the way the calculated active area changed during leaching. Leaching under case (iii) conditions Oxidative and non-oxidative dissolution, as well as H2S oxidation by Fe3+ occured simultaneously. The extents to which oxidative or non-oxidative dissolution occured could be explained in terms of the hydrogen ion and ferric ion adsorption characteristics of the sphalerites.The ferric ion oxidation of H2S was studied in the absence and presence of solids, and the presence of sphalerite or activated charcoal catalysed this reaction. No advantage was gained by leaching in the presence of activated charcoal with or without Fe3+ present, unless conditions were such that H2S was formed as a product of reaction.