(633g) Hydrometallurgical Circuits for the Recovery of RARE Earth Elements from Coal Sources

Rare earth elements (REEs) contained in coal and associated mineral matter exist in several forms including micro-dispersed minerals, ion adsorption onto clay surfaces, ion substituted in the inner layers of hydrated clays and chemically bound in minerals and the organic matter. An extensive test program found that the REEs can be effectively recovered from certain components of several coal sources by leaching under mild conditions followed by solvent extraction.

Individual core and channel samples collected from three coal seams were segregated into different sections based on their lithology. Each segment was evaluated for their REE content and further studied in batch leaching tests to assess the ability for REE recovery. The REE distribution was found to be enriched in certain segments such as specific partings and immediate floor material while other segments were low in REE content. The ability to recover the REEs by leaching also varied significantly which explained the results obtained on samples collected from process streams of active processing plants. REE recovery by leaching was found to be more favorable when treating mineral matter produced from the middle density fractions of a given coal as opposed to the coarse and fine reject process streams which were comprised of a composite of materials from roof, floor and partings.

A range of pH values were evaluated at a solution temperature of 25^OC and 75^OC using 1.2 M sulfuric acid. Nearly 85% recovery of the REEs was extracted from the middlings material collected from a plant treating coal from the Fire Clay seam. Acid leaching of a second middling source from a plant treating West Kentucky No. 13 material resulted in 60% of the total REEs being recovered under the same leaching conditions.

The leachates produced from coal sources contain relatively low concentrations of REEs in the range of 50 -100 ppm while contaminant ions such as iron, aluminum, magnesium and calcium were present in concentrations measured in the thousands of ppm. The enrichment of the REEs was be achieved using either solvent extraction or selective precipitation. After either pathway, oxalic acid was added to the enriched REE solution which selectively precipitated the REEs from solution. The REE precipitates were roasted to produce a final RE oxide mixed concentrate.

For solvent extraction, the leachates were mixed into kerosene containing 1 M D2EHPA at an organic:aqueous ratio of 1:1 prior to a phase separation. A loading, scrubbing and stripping sequence resulted in a mixed concentrate containing around 50% REEs. Using the alternative approach involving selective precipitation provided a concentrate of 80% REEs or a 97% RE oxide mix. A pilot-scale plant is being construct to evaluate the processes and circuits when operated continuously.