Ion Adsorption Mineralisation in Regolith-Hosted REE Deposits

Abstract

Supergene deposits are an attractive source of rare earth elements (REE) because of the potential for reduced energy impact of mining relative to hard-rock deposits, for grade enrichment during the breakdown of rock-forming minerals, and for the potential separation of REE from deleterious radionuclides in the weathering environment. Ion adsorption deposits are a subclass of regolith-hosted REE deposits with the lowest cut-off grade of any currently producing REE deposit type (ca. 0.1 wt. % total rare earth oxides). They are formed via the breakdown of REE-bearing igneous, metamorphic and, less commonly, sedimentary rocks during supergene weathering, which releases the REE from rock-forming and accessory mineral phases. REEs are then carried within soil water, with the depth of maximum REE deposition commonly being localised by a sharp vertical gradient in soil water pH or redox, for example, where acidic surface-derived water interacts with higher pH groundwater. Here, REEs are adsorbed onto the surface of clay minerals, most commonly kaolinite and halloysite. A major advantage of ion adsorption deposits is the ease of REE extraction, with all currently active operations making use of relatively simple ammonium sulfate leach solutions. In situ, and heap or tank leaching approaches have been used. Today, extraction from ion adsorption deposits has a similar energy use and carbon emission potential per mass REE to operations at the hard-rock carbonatite REE deposits Mountain Pass and Bayan Obo. However, the 2D nature of IADs and low cut-off grade can result in greater risks of negative environmental impacts, such as slope instability, vegetation loss and groundwater contamination. Extraction from higher-grade IADs formed on REE-rich protoliths and the use of novel bioleaching cultures have the potential to increase recovery and significantly reduce environmental risk. Several prospects outside of China have now progressed to a detailed characterisation stage. It is, therefore, anticipated that ion adsorption deposits will continue to make important contributions to global REE production over the coming decade.