Abstract

Author(s): Deepak M. Kirpalani*, Emily Chang, Nancy Perreault and Dave Kingston

Chrysotile, a product from the serpentinization of ultramafic rocks, is present in low-grade serpentinites from low-grade metamorphic ophiolite massifs in Canada. Critical metals of interest from serpentine ores and tailings include: nickel, cobalt, manganese, magnesium and silicon. The overall process for extracting critical metals from serpentinite usually starts with acid leaching. The introduction of hybridization before leaching process emerged due to low recovery of critical metals. Thermal activation via calcination prior to leaching is reported to increase magnesium and other serpentine critical mineral dissolution with the use of less aggressive leaching agents and lowered energy operating parameters. Other physical activation approaches include milling of serpentinites. Fine-grained milling of serpentinite, characterized by serpentine activation or destruction of 85-95% fibres, have shown the highest reactivity. However, fine milling produces environmental risks in dry asbestos-rich materials processing. Alternatives to dry milling such as wet milling or fluidized bed grinding have emerged with limited success in serpentine activation. 

Nickel/cobalt extraction relies on the attachment of nickel and other transition metals to the silicate layer in saprolitic ore through pyrometallurgy, while nickel as an oxide/hydroxide (limonite) is treated with hydrometallurgy followed by the Caron process for Ni recovery. Both pyrometallurgy and hydrometallurgy constitute high capital investment and operating costs. Recent, thermochemical approaches such as sulfation-roasting-leaching hybridization have emerged as cost-effective alternative. A sequential extraction approach has also been suggested with a combination of inorganic and organic acids to obtain critical metals of interest. Microbially enhanced carbonate precipitation, has potential applications in stabilizing serpentine tailings into carbonate lattices for metals recovery.

Overall, the challenges in serpentine activation can be summarized by the diffusion limiting SiO₂ layer that limits critical metal recovery and the energy considerations in new approaches to promote further critical metal dissolution. Ultrasound milling and microwave activation prior to acid leaching, in bench studies, show success through de-skeletisation of silica sub-layers to access inner critical metals. pH swing carbonation to release destabilized silica and form metal carbonates is also suggested. Microbial extraction, a potential innovation for the mining industry, can expedite the carbonation process and offset the carbon footprint for critical minerals recovery upon serpentine activation.

Pathways to exploit the metals recovery and develop large scale economically process strategies through concurrent critical metal recovery and carbon mineralization outlining challenges in process implementation will be discussed.