Microstructure evolution of chalcopyrite agglomerates during leaching – A synchrotron-based X-ray CT approach combined with a data-constrained modelling (DCM)

Highlights

• Synchrotron X-ray CT and DCM revealed the mineral agglomerates structure.

• Decomposition and structure evolution during leaching can be quantified with XCT/DCM.

• Column leaching results proved that sufficient binding capacity is essential.

• Effective pore network structure within agglomerate enhanced the leaching kinetics.

Abstract

Agglomeration provides substantial advantages on heap leaching, such as creating a porous heap to improve low-permeability ore leaching efficiency, as well as building an environmentally friendly heap by reducing metal releases from waste rock and tailings. In this study, we employed synchrotron X-ray computed tomography (X-ray CT) combined with a data-constrained modelling (DCM) approach to investigate the properties and evolution of pore networks of chalcopyrite-dominant agglomerates during leaching. In particular, these agglomerates, with and without prior CaCl₂ addition to produce binding reagent gypsum, were subjected to column leaching tests for a period of 37 days. The copper recovery is found to be highly dependent on interior structures of agglomerates: oxidative dissolution of sulphide minerals within agglomerates was considerably promoted by high CaCl₂ addition, through more interior connected pore network structure, resulting in effective migration and diffusion of lixiviant solution. The leaching data clearly show that sufficient binding capacity is essential for the maintenance of agglomerate structure by improving its mechanical resistance. Synchrotron X-ray CT data reveals that the decomposition of agglomerate has a performance of dispersing sulphide grains within the agglomerate particle and improved intra-particle porosity. This study demonstrates that combined synchrotron X-ray CT and DCM approach is a powerful tool for understanding the characteristics and evolution of valuable minerals during leaching of agglomerates.

Introduction

Heap leaching is considered as a green and economical option to treat complex copper, zinc, nickel, gold, and uranium ores (Petersen, 2016). Compared to conventional leaching processes typically operated at high temperature and/or pressure, heap leaching offers many techno-economic benefits, such as low cost, suitability for low grade ores, feasibility and flexibility of small to large deposits, and diversified process conditions (Dhawan et al., 2012). Although the protocol of heap leaching varies because of geological settings, grade, mineralogy, process chemistry and/or operational conditions, common practices such as agglomeration are likely to have a large effect on the leaching performance. Precise assessment and prediction of agglomeration process is essential for the performance of the heap (Lin et al., 2016). To date, the heap leaching practice has been lacking fundamental understanding of these inter-related processes primarily due to ore variability and limited characterisation tools.

The agglomeration process provides substantial advantages on heap leaching, such as creating a porous heap to improve low-permeability ore leaching efficiency, as well as building an environmentally friendly heap by reducing metal releases from waste rock and tailings (Bouffard, 2005). The agglomeration aims to adhere fine particles with coarse particles and binding fine particles together with which to limit the particle size variation, prevent the migration of fine particles and simultaneously improve the heap permeability (Dhawan et al., 2013). The moisture, binders and curing time are main conditions that impact the quality of agglomeration process (Dhawan et al., 2012). A proper moisture is essential to promote a complete mixing of particles in various sizes and avoid disintegration of the agglomerates (Bouffard, 2008). The binding agent is used to prevent fine particles from clogging the void within the heap and thus lead to a better heap permeability and an even leaching solution distribution (Dhawan et al., 2013). A longer curing time results in a stronger bond between the coarse particles and the fine clays. This also helps to produce a homogeneous acid distribution in the ore bed, thereby prompting mineral dissolution (Velásquez-Yévenes and Quezada-Reyes, 2018).

Since the leaching of oxide and sulphide ores generally consumes oxygen as oxidant, the proportion of air-filled void spaces in a heap is more useful than bulk density or moisture content alone. To better describe air-filled voids in the heap rather than use dry proportion, a term ‘wet bulk density’ is introduced as defined to be the sum of the dry-stacking ores and the amount of moisture retained per unit volume (Bouffard, 2008). The characteristics and evolution of the solid, solution and pores in heap leaching systems and their quantitative description, wet bulk density, were little explored, mostly due to lacking characterisation tools for investigations of interior solid-solution interactions and the resultant changes in void spaces.

X-ray computed tomography (X-ray CT) allows non-destructive imaging of an object's interiors and its microstructure features can be visualised and reconstructed through absorption induced X-ray attenuation for characteristic and treatment purposes (Maire and Withers, 2014). Unlike most polychromatic X-ray sources, synchrotron radiation offers high-flux tuneable, monochromatic X-rays which enables imaging with high spatial resolution and phase contrast. Taking advantage of these unique features, synchrotron-based quantitative multi-energy X-ray CT imaging showed an excellent capacity in mineral micro-structure analysis (Scarlett et al., 2016a, Scarlett et al., 2016b). In order to analyse monochromatic multi-energy X-ray CT data, data-constrained modelling (DCM) analysis was developed for quantitative characterisation and modelling of material microstructures (Yang et al., 2015; Yang et al., 2007). DCM uses mathematical optimisation to reconstruct and generate 3D compositional map against X-ray CT datasets and X-ray attenuation of each component at each photon energy using statistic physics principles(Yang et al., 2020; Yang et al., 2013).

So far, definition of collective behaviour of individual grains in heap leaching systems, even qualitative, has been sparse. Combination of X-ray micro CT and DCM is a powerful approach which can be applied for leaching mechanism study to reveal microstructural features of ores and process materials, leading to a better understanding of comminution and agglomeration processes of individual mineral grains and the heap leaching systems. On the basis of that, the leaching kinetics on the individual particle surface and their inner-particles mass transfer behaviour can be consolidated that give the overall apparent particle scale kinetics.

Our recent study on the chloride leaching of low-grade chalcopyrite ores has demonstrated that combined X-ray CT and DCM method is a powerful tool to provide quantitative information on void generation and microstructure evolution over leaching processes (Yang et al., 2019). In this study, we extend the boundaries of this advanced characterisation tool to identify preferred conditions of agglomeration for improved heap leaching performances on a particular ore under varied leaching conditions. Additional 10 wt% chalcopyrite was added into low-grade ore for observation of particle structures and improving contrast before and after leaching. Synchrotron X-ray CT experiments has been conducted to investigate the effect of varying calcium chloride (CaCl₂) concentration added during agglomeration on the pore structure of agglomerated grains. We aim to understand the characteristics and evolution of sulphide minerals within agglomerates during agglomeration and leaching by using synchrotron X-ray CT technique and column leaching tests, which underlay the fundamental knowledge to develop more adaptable design protocols and operation techniques for complex ores.