Cobalt: A Critical Element in Short Supply

Cobalt is a transition metal with unique physical properties. It has been in use for various purposes for thousands of years, with applications including ancient and modern pigments, additives to improve strength in various materials, medicines, batteries, magnets, and radiochemistry. Cobalt is relatively scarce in Earth's crust however, with 79 % of the global budget hosted in the seafloor. It remains a secondary product in mining processes, and recovery from its ores and other Co-bearing minerals is still challenging. Most of the cobalt produced for commercial use is sourced from Africa, and very little is recovered in the US. The recent increase demand resulting from the battery industry has brought the need for greater domestic production of cobalt into sharper focus. There is a dearth of thermodynamic data for Co-bearing silicates and oxides; and even for the sulfides, which are the most well-studied cobalt minerals, little data are available. Although cobalt is a critical material, the thermodynamic stability of minerals from which cobalt is derived is largely unknown. Where thermodynamic data are available, the phases are limited to mineral end-member compositions, and cobalt end-members within any major mineral group are vanishingly rare, cobalt is almost exclusively present in solid solution with other elements, especially, Ni, Cu, and Fe. Cobalt partitioning in natural silicate systems, for example, is dependent not only on availability of cobalt, but prevailing environmental conditions of its formation. Understanding the solid solution partitioning therefore enables use of composition to make inferences about formation conditions of host minerals. Cobalt properties also affect recovery processes, much of which are via hydrometallurgy or pyrometallurgy. For these recovery methods, in addition to thermodynamic properties, mineralogy, oxidation state, and relative abundance of cobalt are the most important properties to study. We present a compilation and systematization of the current thermodynamic data on cobalt mineral solid solutions and end member phases. This may generate new ideas for cobalt recovery in the context of critical materials for the economy as well open up new lines of scientific inquiry.