Numerical modelling study on the quantification of in-situ leaching (ISL) of copper in porphyry rock.

Abstract

This study aims to assess the various aspects of quantifying in-situ leaching (ISL) of copper in porphyry rock through numerical modelling. The first part involves a detailed review of physical and chemical processes that are expected to affect the leaching rate of copper. For this purpose, chalcopyrite was selected as representative leachable copper sulphide. Three alteration phases of hypogene copper deposits and supergene enriched porphyry rock have the highest potential to be economical ore bodies. Various lixiviant systems are discussed of which hydrochloric acid (HCL) was selected to be most suitable leaching agent for the purpose of the initial development of a modelling framework. Transport of the injected lixiviant relies on sufficiently permeable fractures, due the naturally low permeability of crystalline rock. Fracture stimulation is considered essential to establish a connected fracture network which would also develop an increased reaction surface area between the lixiviant and the target mineral. The second part of the study integrates all relevant hydrodynamic and chemical processes and parameters into a reactive transport modelling framework. A range of conceptual modelling scenarios were developed and translated into illustrative numerical models. The considered cases were guided by the analysis of mineral maps that were developed from high resolution chemical analyses of samples for porphyry rock deposits. Simulations were performed at the cm-scale under consideration of physical and chemical heterogeneity. The impact of a hydraulic fracture was considered in which predominant solute transport occurs through advection. In contrast solute migration in the adjacent matrix was controlled by diffusion. Model variants were defined to investigate the sensitivity of copper extraction rates to selected physical parameters, such as different orientation of the hydraulic fracture in relation to the vein and porosity variations according porosity measurements on similar samples. In other scenarios the reaction network was varied by the addition of secondary minerals and the composition of the lixiviant and/or groundwater solution was varied. The orientation of the hydraulic fracture, the pH of the leaching solution and porosity variations showed to be most important factors for the copper extraction rate. The study provides a basis for developing suitable upscaling-approaches that will allow realistic reactive transport model simulations at the field scale.