Association characteristic study and preliminary recovery investigation of rare earth elements from Fire Clay seam coal middlings

Rare earth element (REE) recovery from coal and coal byproducts using physical and/or chemical methods has been the focus of recent investigations due to concerns over worldwide supply. In the current study, the association characteristics of REEs and the recovery of REEs from the organic and inorganic components were systematically studied for a Fire Clay seam coal source. The coal sample was ground in a stirred ball mill and treated by froth flotation in stages to obtain clean coal concentrates and tailing samples. The clean coal concentrate from the first stage was re-ground to liberate more micro-dispersed mineral matter followed by flotation. After six grinding and recovery stages, the cumulative grinding time was 110 min, which reduced the particle size from 90% passing 84.3 μm to 4.5 μm. Analyses conducted on the tailing samples collected from each stage revealed that the REEs have a strong association with micro-dispersed kaolinite. The REE content in the tailings material increased from around 580 ppm to 1200 ppm on an ash basis. The ratio of heavy-to-light REEs in both the clean coal concentrates and tailing samples increased, which indicated a stronger affinity between the heavy REEs and the finest mineral matter grains in the organic matrix. Based on these findings, REE concentration tests were performed using froth flotation, magnetic separation and acid leaching. A product containing nearly 7000 ppm of REEs was achieved from a feed containing 500 ppm REEs using a batch WHIMS unit. The elemental content indicated that the rare earth mineral in the concentrate was primarily monazite. Leaching the high ash content fractions generated from the sequential grinding and flotation steps using 1 M nitric acid resulted in recovering more than 80% of total REEs with the exception of scandium. A lower recovery value obtained for scandium indicated a unique association with the mineral matter relative to the other REEs.