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Recovering rare-earth elements from coal byproducts

By Mary Page Bailey |

A team from the University of Kentucky (Lexington, Ky.; www.uky.edu) and Virginia Polytechnic Institute (Blacksburg, Va.; www.vt.edu) has set up a novel pilot plant to recover rare-earth elements (REEs) from coal-based sources, aiming to produce high-grade REE concentrates from coal-based leachate. Byproducts of coal mining are a promising source for highly in-demand REEs, says Rick Honaker, professor of mining engineering at the University of Kentucky. “All of the REEs, including scandium and yttrium, are present in coal, whereas most other sources contain only a select number of REEs,” he explains.

Leaching conditions from coal sources require milder sulfuric-acid concentrations than leaching from other mineral sources, but the low REE composition in the leachate (<100 parts per million; ppm), along with the high concentration of contaminant ions, including iron, magnesium, aluminum, calcium and others, requires a rigorous solvent-extraction and scrubbing circuit designed by the team specifically for handling coal-based resources. Prior to solvent extraction, the leachate is chemically reduced to convert Fe3+ to Fe2+ ions using ascorbic acid, which enables effective rejection of iron ions in the solvent-extraction circuit.rare-earth elements

The pilot plant utilizes a sorting technology based on dual X-ray transmission to effectively extract higher-grade REE content. “The material extracted by the sorter is more leachable than other portions of the material,” explains Honaker. The pilot plant has produced concentrates containing greater than 99% total REEs from coarse coal refuse material, coal-based mine acid water and precipitate waste material generated from the treatment of mine acid water.

The 0.25-ton/h pilot plant is currently the world’s largest unit focused on REE recovery and concentration from coal sources, and with funding from the U.S. Department of Energy (DOE), the team is currently designing a 20-ton/h commercial plant utilizing this technology. According to Honaker, this commercial plant would likely produce three different REE concentrations: 99% scandium; a mixture of neodymium, praseodymium and dysprosium (Nd-Pr-Dy); and an yttrium product.

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