Abstract
The rare earth elements (REEs) are part of the key critical minerals identified globally as being essential for modern technology, clean energy solutions and national security but they are also vulnerable to supply chain disruption. Therefore, there has been significant effort in both exploration and process development for new rare earth deposits in recent years, in particular for clay deposits similar to those historically processed in China.
The most sought after deposits are ionic adsorption deposits (IADs), which have been historically processed in China, and more recently in Myanmar. In IADs, the rare earths are ionically adsorbed and enriched on clay surfaces during migration of REE bearing solutions following chemical weathering of the rare earth rich host rocks. The adsorbed REEs are then readily extracted by desorption with a salt solution (e.g. (NH4)2SO4 or NaCl) via an ion-exchange process at pH 4-5.
However, not all deposits which contain clay minerals and REEs are of the IAD type. “Non-IAD” deposits may contain a small portion of desorbable REE content, but the REEs are mainly present in other mineral phases including colloidal sediments, carbonate minerals, oxide/hydroxide phases or refractory mineral phases. These clay-hosted RE deposits require moderate acidity or aggressive leach conditions for rare earth extraction, depending on the mineralogy present.
ANSTO (Australian Nuclear Science and Technology Organisation) has been closely involved with the development of a number of these new IAD and clay-hosted REE projects, in both Australia and South America. From experimental laboratory-scale test work through to continuous piloting operations, ANSTO has supported industry clients in establishing flow sheets for REE extraction, purification, product precipitation and reagent recycle/water treatment.
Unlike most hard rock/primary REE deposits (e.g. monazite), the IAD/clay-hosted deposits contain very low concentrations of uranium and thorium. However, because of the low concentrations of REEs in the clay deposits, the radioactivity associated with uranium/thorium and their radioactive daughter products needs to be controlled/removed in some areas in order to meet strict rare earth product specifications. ANSTO has applied its’ specific expertise in this area during the development of clay-processing flowsheets.
This paper will provide an overview of REE processing options from clay deposits, including example flowsheets for processing IADs via tank leach (e.g. South American projects) and processing clay-hosted RE deposits via heap leach (e.g. Australian clay-hosted projects). The paper will also include the radioactivity aspects associated with REE clay processing, including limits for handling and transportation.