Rare earth elements (REEs) comprise 14 out of 15 elements from the lanthanide series, excluding the man-made promethium, and including scandium and yttrium. These elements have unique chemical and physical properties that make them vital for the chemical industry and, more recently, for green technology. The heavier (H) REEs are used in wind turbines and electrical vehicles. This has led to the U.S. Department of Energy classifying those elements which are important to this sustainable industry, and at risk of supply, as ‘critical’ REEs. The supply of REEs has been dominated over the past two decades by China, which holds 50 % of the world’s REE reserves and is responsible for 90 % of global production. Increased export quotas, however, have seen an increase in the research of REEs outside of China to reduce its monopoly on this valuable commodity.
The unique chemical and physical properties of REEs make them difficult to mine and beneficiate, and separate from another. Increased costs associated with dealing with radioactive tailings as well as pyrometallurgical costs has motivated the search for alternative REE sources. Low grade clay deposits (0.05 – 0.3 wt % REE content) have been shown to contain a higher proportion of HREEs, and to be more economically attractive ores, as 90 % of TREO can be recovered using simple monovalent salt lixiviants at ambient temperatures. This hydrometallurgical process is applied to in-situ conditions to reduce capital investment in the process.
Resent research has shown that ammonium sulphate is the ideal lixiviant because of its improved recovery rates over sodium chloride. The use of this salt solution has led to environmental issues as it leaches important salts from the soil. The current research is focussed on improving the performance of ammonium sulphate by adding organic agents to reduce lixiviant consumption as well as investigating alternative solutions such as magnesium sulphate.