Common problems - and progress towards solutions - in the process mineralogy of rare earths

Abstract

The geochemistry and mineralogy of rare earth element (REE) deposits is diverse and ranges from carbonatite-related deposits and alkaline rocks to mineral sands, ion adsorption clays, marine crusts, nodules and clays, by-products of phosphate and bauxite, and re-use of waste materials. Despite the large number of recent exploration projects, very little additional REE production has started. An in-depth understanding of the mineralogy is essential for process design and all of the deposit types have mineralogical advantages and challenges, which will be reviewed and explained. For example, the deposits with the best established processing routes are monazite-bearing mineral sands but monazite radioactivity renders most unusable. Ion adsorption clays are easily leachable but deposits are low grade and shallow so new environmentally-friendly leaching techniques are needed. The diverse mineralogy of alkaline rocks has required development of processing routes for rare minerals such as steenstrupine and eudialyte. Carbonatites tend to have high proportions of the least valuable, lightest REE in REE fluorcarbonates or monazite. A deposit with two ore minerals, REE fluorcarbonate and apatite, that combine to give a REE profile close to that required by industry has an advantage if the ore minerals can be recovered efficiently.