The importance of reaction mechanisms and coupled dissolution with reprecipitation (CDR) reactions when modelling copper leaching in heap systems

Abstract

Chalcopyrite is an abundant source of copper, which can be extracted from low-grade ores in heaps. The dissolution of chalcopyrite is relatively slow partly because of the formation of secondary minerals at the chalcopyrite surface also referred to as surface ‘passivation’. Although the passivation mechanism has been extensively studied, present models do not adequately account for it. Hence, a surface-passivate model (SPM) was used in a reaction path model (RPM) to examine and illustrate how passivation limits copper recovery from chalcopyrite. In addition, the role of different gangue minerals and different chalcopyrite dissolution mechanisms were assessed by incorporating rate equations from Kimball et al. (2010) and Rimstidt et al. (1994) into the RPM, and the state of saturation was determined for various minerals to constrain limiting conditions for copper recovery. RPM with different rate laws describing proton-promoted, ferric-iron promoted, and combined ferric-iron-proton promoted chalcopyrite dissolution in the presence of gangue minerals in chloride system were used. Ferric-iron promoted chalcopyrite dissolution was the fastest reaction mechanism leading to the highest copper mobilization. However, copper mobilization was limited by the formation of iron-hydroxy sulphates (e.g., jarosite), and iron oxide (e.g., hematite) and different gangue minerals, pointing to the importance of accurate representation of primary and secondary reactions, their co-location, and their reaction kinetics. The SPM was capable to simulate surface coverage of the chalcopyrite surface by jarosite, thereby lowering the reactive surface area and consequently suppressing chalcopyrite dissolution. Conversely, the SPM failed to model the incongruent dissolution of chalcopyrite leading to a copper sulfide layer deficient in iron in the form of covellite because covellite was continuously undersaturated in trial models. Further, the presence of gangue minerals like hematite and gypsum may have a positive effect on chalcopyrite dissolution, whereas the presence of silicates (e.g., feldspar and muscovite) have a negative influence. The findings of this study are important as it provides new insights into concurrent reactions controlling the recovery of copper in heaps and how these reactions can be best modelled.