Jean-Paul Franzidis

Heap Leaching Technology—Current State, Innovations, and Future Directions: A Review

ABSTRACT Heap leaching is a well-established extractive metallurgical technology enabling the economical processing of various kinds of low-grade ores, which could not otherwise be exploited. However, despite much progress since it was first applied in recent times, the process remains limited by low recoveries and long extraction times. It is becoming increasingly clear that the choice of heap leaching as a suitable technology to process a particular mineral resource, which is both environmentally sound and economically viable, very much depends on having a comprehensive understanding of the underlying fundamental mechanisms of the processes and how they interact with the particular mineralogy of the ore body under consideration. This paper provides an introduction to the theoretical background of various heap leach processes, offers a scientific and patent literature overview on technology developments in commercial heap leaching operations around the world, identifies factors that drive the selection of heap leaching as a processing technology, describes challenges to exploiting these innovations, and concludes with a discussion on the future of heap leaching.

An electrochemical study of the dissolution of chalcopyrite in ammonia–ammonium sulphate solutions

Ammoniacal solutions are an effective lixiviant for the oxidative dissolution of some mineral sulphides. A study of the anodic dissolution of chalcopyrite in ammonium sulphate-ammonium hydroxide solutions has been carried out using cyclic voltammetry, chrono-amperometry and rest-potential measurements. The role of the copper(II)/copper(I) redox couple in the oxidation process has been evaluated. Rest potentials have been found not to be affected by oxygen but to increase with an increase in initial copper(II) concentration. This trend remains more or less unchanged with increasing total ammonia (NH3+NH4+) concentration. Cyclic voltammetry analysis shows an oxidation peak/shoulder when anodically polarising the chalcopyrite after attaining rest potential in the presence of copper(II) ions. All reverse sweep curves are more or less identical, but the Tafel slope decreases from around 150 mV/decade at 1M total ammonia concentration to around 120 mV/decade at higher ammonia concentrations. Pseudo-steady-state anodic current densities measured in the absence of copper(II) ions at the rest potentials obtained in the presence of copper(II) ions increase with increasing concentration of copper(II) ions, confirming the positive effect of copper(II) ions on the rate of dissolution of the mineral. Cyclic voltametric and chrono-amperometric data show that in the absence of initial copper(II) ions, the rate of dissolution in the presence of dissolved oxygen is significantly lower. These results confirm that the effective oxidant for the mineral under the conditions of the study is copper(II) and not dissolved oxygen. Coulometric measurements have been used to establish the stoichiometry of the anodic reaction at different potentials as involving approximately seven electrons per mole of chalcopyrite, suggesting the formation of thiosulphate, although thiosulphate ions have not been tested for or identified in solution. SEM and energy dispersive X-ray (EDX) analysis of the mineral surface left to equilibrate over time shows a copper-depleted surface, rich in iron but relatively low in sulphur. The iron in the surface layer can easily be removed by short contact with concentrated acid, which is consistent with the formation of a secondary iron-based precipitation layer rather than an altered mineral phase.

The Development of a Simplified System for Measuring the Passage of Particles on and Through Moving Screen Surfaces Using DEM

Screening is the practice of separating granular materials into multiple size fractions, and is employed in most mineral processing plants. Currently, the design and scale-up of screens relies on rules of thumb and empirical methods. To go beyond the current state-of-the-art in screen modelling, DEM was employed to study particle motion along a dynamic (vibrating) inclined screen. Granular flow on vibrating screens exhibits complex phenomena such as segregation, percolation and flow of oversize material over the separating medium. In this work, a unique granular rheology is established for particles moving on a vibrating screen. DEM was used to provide key data (velocity, volume concentration, shear rate, bed depth) for the development, testing and calibrating the granular flow models. A binary mixture of glass beads flowing on an inclined vibrating screen was simulated. The subsequent continuum analysis of the flowing layer revealed a co-existence of three flow regimes—a dense quasi-static regime, an intermediate liquid regime, and a gaseous regime—which are based on the measured volume concentration. The appropriate constitutive shear stresses were then used to derive a new rheology that captures all phases of the flow transition points observed in the simulation.