The high-temperature pressure oxidation of a Witwatersrand pyrite concentrate

[摘要] ENGLISH ABSTRACT: The oxidation of the pyrite present in gold and uranium ores is often desirable, to either liberate the gold present in the pyrite, or to generate iron(III) and acid for uranium extraction. The current study aimed to define a rate equation to describe the pressure oxidation (POX) kinetics of a Witwatersrand pyrite concentrate. Iron(III) and sulphuric acid, which is produced during POX of pyrite, may be used to leach uranium, which is also present in Witwatersrand ores. In addition, gold recovery from the POX residue may be improved significantly, especially if the gold ore is partially refractory, as is the case in many Witwatersrand tailings dumps. A rate equation to describe the pyrite oxidation kinetics will, thus, be important for reactor design purposes, as well as for operating and capital expenditure estimations to evaluate the feasibility of flow sheets incorporating POX.The pyrite POX kinetics was investigated in terms of temperature (180 to 210°C), oxygen partial pressure (480 to 1100 kPa), acid concentration (10 to 50 g/L) and particle size (+38 to 150 μm) by using a batch 2 gallon Parr autoclave. A batch model was subsequently developed in MATLAB and employed to confirm the observed rate dependencies. The oxygen gas-liquid mass transfer coefficient was also measured independently during the study, to enable a quantitative description of the dissolved oxygen concentration during modelling of the batch oxidation process.The activation energy was calculated as ~120 kJ/mol in relation to dissolved oxygen concentration, which indicated that the reaction was controlled by a chemical reaction at the surface of pyrite particles and that no diffusional limitations applied. The oxidation rate decreased with increasing acid concentration with a reaction rate order in acid concentration, ranging between -0.2 and -0.3. The oxidation kinetics was found to be relatively insensitive to particle size at oxygen partial pressures lower than 1000 kPa.For all practical purposes, the pyrite oxidation rate was found to be first order in dissolved oxygen concentration; however, this assumption led to poor prediction of the iron(II) and iron(III) solution concentrations during modelling of the batch oxidation tests. Accurate quantification of the iron(II) and iron(III) solution concentrations would also be important to consider for reactor design purposes, as it will dictate the maximum quantities of iron(III) and acid that can be produced during POX.Simulations showed that improved predictions of the iron(II) and iron(III) concentrations are obtained when a direct reaction between pyrite and iron(III) was allowed for. This means that both dissolved oxygen and iron(III) are responsible for the oxidation of pyrite at typical POX conditions, i.e., the pyrite could have a dual rate dependency on the oxygen and iron(III) concentrations in two additive rate-determining steps. Regression indicated that the experimental data may be represented by two additive rate equations with orders of ~0.6-1.0 in dissolved oxygen concentration and half-order in iron(III) concentration. The relative contribution of the two reactions to the overall rate appears to be influenced by the slurry density, particle size, and oxygen partial pressure.It is proposed that a follow-up study should be conducted to quantify the rate dependency of pyrite in solutions of varying iron(III) concentrations, at the temperatures employed during this study and in the absence of dissolved oxygen, that is, to provide an independently measured rate equation to the batch POX model. The homogenous iron(II) to iron(III) oxidation rate should also be measured independently to confirm whether the employed rate equation was correct. Furthermore, the possible effect of secondary minerals, in this case, pyrophyllite, should be clarified by conducting experimental work at higher slurry densities.

[发布机构] Stellenbosch University