Recovery of critical metals from carbonatite-type mineral wastes: Geochemical modeling investigation of (bio)hydrometallurgical leaching of REEs

Rare earth elements (REEs) are typically found in low concentrations within natural rocks that make up mine tailings, such as carbonates in association with silicates within carbonatite igneous rocks, so it is of interest to develop (bio)hydrometallurgical ways to liberate them from the silicate matrix. This work investigated, through geochemical modeling, the extraction of europium and ytterbium carbonates from rocks containing one of four silicates (chrysotile, forsterite, montmorillonite, and phlogopite) via chemical (mineral acid) or biological (organic acid) leaching. The results indicated conditions that led to either congruent or incongruent dissolution of the mineral phases and the formation of transient mineral phases. Chemical leaching models suggest that REE carbonates are recoverable in one-step leaching from forsterite and chrysotile rocks, while they are recoverable in a secondary leaching step from montmorillonite and phlogopite rocks. Gibbsite as a transient phase is shown to complicate REE recovery, potentially requiring reactive extraction. REEs have the potential to be recovered from silicate rocks via chemoorganotrophic bioleaching, but the process configuration would differ depending on the predominant minerals that make up the rock, and the type of REE present in it.