The impact of chloride ion on the leaching of sulphide mineral in acidic solution

Acid mine drainage (AMD) is one of the most significant environmental problems affecting natural watercourses related to the mining industry. Containing soluble chemical metals, AMD is mainly produced by the dissolution and oxidation of sulphide minerals present in mine waste and tailings. In sulphide mineral leaching processes, artesian water which contains high concentrations of naturally occurring chloride ions were increasingly used in Australian mines and globally, due to the scarcity of freshwater resources. The presence of chloride ions has significant effect on sulphide dissolution process which has not been clearly revealed. Knowledge of sulphide dissolution with varying chloride additions in chemical and bioleaching is of considerable interest to facilitate better prediction of its behaviour at the process scale.

In this work, the impact of chloride ions on batch leaching performance was investigated on three typical sulphide minerals, and a series of column leaching tests was conducted to understand their leaching behaviour:

- Pyrrhotite is known to be one of the most reactive sulphide minerals and highly prone to oxidation and its oxidative dissolution often leads to the formation of fairly complex and fine grained secondary mineral assemblage as the major forms of oxidised sulphur. The dissolution of pyrrhotite with Acidithiobacillus ferrooxidans in acidic Fe2(SO4)3 lixiviant with addition of various chloride ions has been investigated at a controlled temperature of 25 °C.

- Arsenopyrite, one of the most common arsenic (As) minerals, occurs in a variety of hydrothermal deposits and processing settings. Its oxidative dissolution releases As, iron (Fe) and sulphur (S) to the environment, causing well-known severe environmental problems to water system and surrounding ecosystems. A series of batch leaching experiments of arsenopyrite at 25 and 48 °C, were conducted to investigate the effect of varied chloride additions on arsenopyrite dissolution in both the presence and absence of the mesophilic or moderate thermophilic microorganisms in acidic Fe2(SO4)3 lixiviant.

- Chalcopyrite occurs in most sulphide mineral deposits globally and has been the most important ore of copper. In this study, the effect of chloride ions on chalcopyrite bioleaching and chemical leaching was investigated at 25 °C in acidic Fe2(SO4)3 lixiviant.

- 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. A synchrotron X-ray computed tomography (CT) combined with a data-constraining modelling (DCM) approach was employed to investigate the properties and evolution of pore networks of chalcopyrite-dominant agglomerates during leaching. These agglomerates, with and without prior CaCl2 addition to produce binding reagent gypsum, were subjected to column leaching tests at 25 °C.

Several analytical methods, including synchrotron-based X-ray absorption near edge structure (XANES) and X-ray CT, have been employed in this study to provide an important control on sulphide mineral reactivity in the natural environment and its environmental remediation strategies. The combination of XANES and X-ray diffraction studies aimed to improve understanding on oxidation products of sulphides, such as elemental S, jarosite and As species, under site relevant conditions in particular in the presence of naturally occurring chloride. These characterisation techniques, combined with thermodynamic simulation of leaching reactions, provide comprehensive approaches to fully define bulk, surface, and solution species involved in the leaching process for improved understanding of leaching mechanism and kinetics in industrial processes.

In general, the results of batch leaching experiments of aforementioned sulphide minerals indicated that the presence of chloride ions showed an inhibition effect on both chemical and bacterial leaching performance at 25 and 48 °C. More specifically,

- For the leaching of pyrrhotite, the presence of chloride ions showed a promotion effect on the initial two weeks of both chemical and bacterial leaching of pyrrhotite, due to the effect of chloride ions on formation of a crystalline and porous sulphur layer. In the later stage of chemical leaching system when dissolved oxygen became the main oxidant, the presence of chloride ions inhibited the reaction process by behaving as aggressive anions and prohibiting diffusion of oxygen on the mineral surface and subsurface. In addition, the existence of 6 g L-1 chloride ion exhibited toxic effect on bacteria activity, which may result in the inhibition of oxidation of elemental sulphur during the later stage of bioleaching system. This work indicated that the existence of bacteria improved the pyrrhotite dissolution. In chemical leaching system, goethite was formed in addition to jarosite, as pH remained at about 3.5, while jarosite was dominant iron precipitate after bioleaching at pH of approximately 2.2 after 24 days of leaching.

- For the leaching of arsenopyrite, solution data showed that excessive chloride addition might prohibit arsenopyrite dissolution at higher temperature for both chemical and bioleaching systems. S K-edge XANES analysis suggested that predominant S species remained in unreacted arsenopyrite, but considerable amounts of elemental sulphur and jarosite were also formed as secondary products resulting from arsenopyrite dissolution. On the other hand, As K-edge XANES analysis indicated that the presence of microorganisms appeared to prohibit arsenopyrite dissolution resulting in the reduced formation of jarosite. These results also revealed that the release of As (V) was slower at room temperature compared to 48 °C and increased with rising temperature and increasing chloride addition.

- For the leaching of chalcopyrite, the results showed that the existence of sodium chloride inhibited chalcopyrite dissolution under both chemical leaching and bioleaching conditions at 25 °C. In chemical leaching, the solution redox potential in the experiment without Cl- stayed in a range more suitable for chalcopyrite dissolution compared with experiments with Cl-. In the case of bioleaching, the bacteria growth was suppressed by Cl-, which also slowed the leaching kinetics. The characterisation using XANES and SEM-EDX of leaching residues confirmed the formation of potassium jarosite, S and covellite on the mineral surface, while no sodium jarosite precipitated at the temperature of 25 °C.

- In addition, from the investigation on agglomerate leaching, the copper recovery was found to be highly dependent on interior structures of agglomerate: oxidative dissolution of sulphide minerals within agglomerate was considerably promoted by 10 wt % CaCl2 addition, which represented a more connected porous network structure within agglomerate, resulting in effective migration and diffusion of lixiviant solution. The leaching data clearly showed that sufficient binding capacity is essential for the maintenance of agglomerate structure by improving its mechanical resistance. Synchrotron X-ray CT data revealed that the decomposition of agglomerate has a performance of dispersing sulphide grains within the agglomerate particle and improved intra-particle porosity.