Abstract
Experiments were carried out to test the amenabilities of mineral deposits that contained cobalt deported in arseno-sulfide (cobaltite) and arsenide (skutterudite) minerals, to oxidative bioleaching at mesophilic temperatures and low pH. An ore sample from the Iron Mask deposit (Canada) and a mineral concentrate from a working mine (Bou Azzer, Morocco) were thoroughly characterised, both prior to and following bio-processing. A “top down” approach, using microbial consortia including (initially) 13 species of mineral-degrading acidophiles was used to bioleach the ore and concentrate in shake flasks and bioreactors. Cobalt was successfully liberated from both materials tested (up to 93% from the ore, and 49% from the concentrate), though the chemistries of the leach liquors were very different, with redox potentials being >200 mV lower, and concentrations of soluble arsenic about 7-fold greater, with the concentrate. Addition of pyrite to the arsenide concentrate was found to promote the biomineralisation of scorodite (ferric arsenate), which was detected by both XRD and SEM-EDX, but was not found in bioleached residues of the arseno-sulfide ore. A model was proposed wherein pyrite had three critical roles in facilitating the genesis of scorodite: (i) providing the catalytic surface to promote the oxidation of As (III) to As (V); (ii) acting as a putative “seed” for scorodite crystallisation; (iii) being a secondary source of iron, since the molar ratios of iron:arsenic in the concentrate itself (0.19:1) was well below that required for effective removal of soluble arsenic as scorodite (1:1). This work provided proof of concept that cobalt arseno-sulfide and arsenide ores and concentrates are amenable to bio-processing, and also that it is possible to induce concurrent solubilisation of arsenic from primary minerals and immobilisation in a secondary mineral, scorodite.
Highlights
The global demand for cobalt has been increasing greatly in recent decades, due to its use in super-alloys, rechargeable batteries and catalysts, and cobalt is currently widely considered to be a “critical” metal (e.g. European Commission, 2017)
Cobaltite was identified with both X-ray powder diffraction (XRD) and scanning electron microscope (SEM)-EDX as the major mineral within which cobalt and arsenic were deported (Supplementary Fig. S1)
The Iron Mask ore contained ~13 wt% iron, no primary Fe-rich phase was identified with XRD, while SEM-EDX studies revealed that iron was associated with silicates and cobaltite
Summary
The global demand for cobalt has been increasing greatly in recent decades, due to its use in super-alloys, rechargeable batteries and catalysts, and cobalt is currently widely considered to be a “critical” metal (e.g. European Commission, 2017). Cobalt is currently sourced as the major metal produced from primary ores in only one location (the Bou Azzer mines in Morocco; Petavratzi et al, 2019). New sources of cobalt are being sought, together with identifying means by which the metal can be extracted from primary ores and waste materials while minimising how this impacts the environment. Cobalt exhibits both chalcophilic and siderophilic characteristics, bonds preferentially with sulfur, and is associated with iron, copper and nickel in a variety of sulfide and arseno-sulfide phases (Roberts and Gunn, 2014). Sulfide minerals found in reduced ores include cobaltite (Co,Fe)AsS, carrollite (CuCo2S4), and linnaeite (Co,Ni)3S4, while skutterudite (chemical formula (Co,Ni)As3-x; empirical formula Co0.75Ni0.25As2.5) is the most commonly encountered cobalt arsenide
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