Influence of Ti, Fe, and Ca on the Enrichment of Tantalum in Engineered Artificial Mineral (EnAM) Phases in Solidified Synthetic Silicate Melts

The recovery of oxygen-affine elements such as tantalum (Ta) using pyrometallurgical routes is difficult because this element cannot easily be enriched in a metal alloy, as is the case with battery recycling for the more noble metals Co, Ni, and Cu. A promising procedure, on the other hand, is to enrich this element in simple oxide compounds formed in a silicate melt. This enrichment in tailored mineral compounds is also known as the “Engineered Artificial Minerals” (EnAM) approach. Currently, the Technological Readiness Level (TRL) of this approach is relatively low and limited to understanding the mechanisms involved in the incorporation of target elements and the search for suitable compounds with a high enrichment factor, favorable morphology, and early crystallization during solidification in order to achieve maximum recovery yield of the selected compound (element). Due to its high ion charge (high field strength) and small ion radius for a heavy element, it is plausible that Ta behaves similarly to the abundant element titanium (Ti), whose chemistry is much better known. Ti minerals such as ulvospinel, perovskite, ilmenite, and pseudobrookite are therefore suitable candidates in the search for a suitable tantalum EnAM. A comparison of the solidification of synthetic silicate melts dominated by iron and calcium with Ti as an additive show that Ta is not incorporated into ulvospinel formed in olivine-containing Fe-rich silicate melts (base composition with 57 wt.% FeO). In contrast, the perovskites formed in silicate melts dominated by calcium-alumosilicate (max. 10 wt.% FeO addition) do incorporate Ta. Crystal size and Ta content increase with increasing iron content (up to a maximum of about 10 wt.%). The results indicate a possible solid solution with the well-known compounds CaTiO3 and FeTiO3 and the virtual compounds Ca0.8TiO3 and Fe0.8TiO3.