Mechanisms and kinetics of atmospheric sphalerite oxidative and non-oxidative leaching

ENGLISH SUMMARY: A kinetic study of the non-oxidative and oxidative leaching of sphalerite concentrates, under elevated temperatures (75 – 95 °C) and atmospheric pressure is presented in this dissertation. Sphalerite, a zinc sulphide ore, is commonly associated with impurities and other sulphides (i.e. chalcopyrite, galena, pyrite etc.). The mineralogical nature of sphalerite concentrates is complex and the chemistry of iron-containing reactive systems is generally poorly understood, especially under aggressive hydrometallurgical conditions. The aim of this work was the development of an engineering model capable of describing the rate and extent of sphalerite leaching in non-ferric and ferric containing systems. The mathematical framework presented in this thesis consists of various objectives, each addressing thermodynamic and kinetic aspects of the primary leach process. Comprehensive literature investigations are presented which constitutes the mechanisms and rate models, supplemented by phenomenological data obtained from batch experimentation. The different objectives are each covered in a chapter of this dissertation, and include the following: i) a solution thermodynamic framework, ii) intrinsic oxidation mechanisms and rate expressions and iii) quantification and validation of the intrinsic rate expression. Thermodynamic considerations provided a rigorous framework for the interpretation of the solution chemistry, with the explicit recognition of the important solution species. Speciation measurements from various literature sources were utilised to construct the Pitzer model for the various subsystems of the ZnSO4 – Fe2(SO4)3 – FeSO4 – H2SO4 – H2O system. The model gave accurate speciation trends up to concentrations of 1.5 M ZnSO4, 1.5 M FeSO4, 1.5 M Fe2(SO4)3 and 2 M H2SO4. The model distinguishes between inner- and outer-sphere complexes, which was achieved through the inclusion of Raman spectroscopic stability constants. Contact ion pair (CIP) formations was predicted by the Pitzer model and shown results with suitable accuracy for the application in modelling the ionic aqueous solution relevant to this metallurgical kinetic study. A detailed investigation into the electrochemical and mineralogical nature of natural sphalerite gave insights to the leaching mechanism. Iron impurity was found to be integral to sphalerites dissolution mechanism, with the electron exchange at the mineral surface limiting reaction rate. Polarization of sphalerite particle surface by the electrolytic solution caused surface states (barriers) that limits the rate of movement of charge carriers (i.e. electrons). A mechanism was proposed based on the assumption that the first electron or proton transfer step are the rate-limiting step of the non-oxidative and oxidative leaching mechanism. The resulting electrochemical half reactions from the mechanism was used to define the activation polarisation relationships, the Butler-Volmer equations. Through application of the mixed potential theory of metallic corrosion, rate expression for the non-oxidative and oxidative leaching of sphalerite were derived. Experimental batch data obtained from Dr JDT Steyl (1996) were used to quantify and validate the rate parameters of the derived rate expressions. The shrinking core model was applied within a batch reactor model to predict the leaching extents of sphalerite under various initial conditions. The rate parameter regression followed a two-fold strategy whereby the model was first linearized and regressed using a linear regression technique, and obtaining preliminary kinetic constants at average solution compositions. The second strategy consisted of a detailed differential batch reactor model including the solution speciation model and concentrate characteristics, which was used to quantify the intrinsic rate parameters using a non-linear regression technique. A sphalerite leaching mechanism and intrinsic reaction rate model was proposed in this study and the model was quantified using phenomenological batch data. The model was found to be able to predict the leaching rate of sphalerite.