Abstract
Heavy Rare Earths (HRE) are found in numerous applications related to energy transition and defense and are subject to strong geopolitical tensions. Today, they are mostly mined from Ion-Adsorption Clay (IAC) deposits in which Rare Earths Elements (REE) dissolved over geological times by weathering of REE-bearing minerals are adsorbed onto the layered structure of clay minerals. Total REE content in such deposits is typically low (500 – 5000 ppm) but REE can be recovered by simple ion exchange using reagents such as (NH4)2SO4. This IX process can be implemented in tank leaching, heap leaching or in-situ leaching. In the first two cases, it generates intensive mining activities as millions of tons of ore need to be processed yearly to have an industrial operation. In the third case, there is no mining involved but in situ leaching of IAC deposits as implemented in southern China or Myanmar did result in disastrous environmental effects such as pollution of rivers or aquifers and landslides due to uncontrolled injection of the leaching solution in the ground. Such consequences are mostly at the origin of the current image suffered by the REE mining industry in general.
As all REE deposits, IAC deposits also contain U and Th but with relatively low TREO contents, they tend to have higher U3O8/TREO and ThO2/TREO ratios. This leads to the presence of radionuclides from the naturally occurring radioelements decay chains in the pregnant leach solution, very often at levels that are higher than acceptable for RE refineries.
With several industrial and institutional partners, Carester has worked on a process to mitigate environmental impact of REE extraction from IAC deposits by in-situ leaching. This process involves:
- Onsite geological, geophysical and hydrological data acquisition to build a hydrogeological model and foresee underground flow of leaching solution,
- Implementation of monitoring techniques and washing stages to guarantee the quantitative recovery of leaching reagents,
- Implementation of a patented solvent extraction process to eliminate and sequestrate radionuclides from the PLS.
This process will help responsible sourcing of HRE-rich concentrates for Caremag REE refinery, scheduled to start-up by the end of 2026.
Ongoing work on in-situ leaching as well as recent development in elimination of radionuclides by solvent extraction and its implementation in the case of in-situ leaching operations will be discussed.