Modelling jarosite precipitation in isothermal chalcopyrite bioleaching columns

Abstract

Modelling chalcopyrite leaching involves accounting for the precipitation of jarosite and other iron hydroxide minerals; the difficulty in modelling these processes arises from uncertainty in the precipitation rate, and its dependence on the relevant variables (such as pH, Fe 3+ concentration). Furthermore, an added complexity is accounting for the clogging of the macro and micro pore space, and its effects on leaching rate and liquid flow. There has been a lack of modelling of such processes in the literature, and in this work we focus on the inclusion of a basic model of jarosite precipitation with the associated removal of ferric ions from solution. The solubility criterion is formulated from experimental data of the log of ferric concentration versus pH: a linear slope in log Fe versus pH is apparent. This rule forms the basis for the solubility of ferric, and if the rule is met a first order dependence of the overall precipitation rate on the ferric is used. A rate constant for the precipitation rate is needed and a value chosen to provide reasonable model behaviour i.e. a reasonable amount of ferric is removed from solution: comparison with experimental data is needed to ensure appropriate model parameters are chosen. Four cases are considered: a base case, a case without the effects of jarosite precipitation, a higher inlet pH, and a reduced gangue acid consumption case. The base model results show jarosite precipitates after a distance of around 0.8 m below the inlet, at a position where the pH has risen sufficiently from the (lower) inlet value. In the higher inlet pH case, the spatial position where the onset of jarosite precipitation begins is further up in height in the column: the position is higher up because the jarosite criteria is met sooner, due to the pH being higher. Consequently, copper extraction is worse and perhaps non-intuitively, only a small percentage (1%) more overall jarosite precipitates than in the base case. In the reduced gangue acid consumption case, the pH is lower due to reduced acid consumption, and spatial position where the onset of jarosite precipitates begins is lower in the column; overall extraction is far better (89%) and jarosite precipitation is far less (67% less) than the base case.