Rare earth elements (REEs) recovery from coal waste of the Western Kentucky No. 13 and Fire Clay seams. Part II: Re-investigation on the effect of calcination

In previous studies, the authors found that the leaching recovery of rare earth elements (REEs) from coal waste can be improved by calcination without adding any additives. Two potential mechanisms were proposed to explain the enhanced recovery: (1) decomposition of difficult-to-dissolve REE-bearing minerals into soluble forms; and (2) liberation of REE-bearing minerals encapsulated in clays after calcination due to the dehydration and disintegration of the layered clay structure. Unfortunately, no direct evidence was obtained to confirm the mechanisms. In the present study, the mineralogy of REEs existing in the leaching solid residue of raw and calcined coal wastes of the Western Kentucky No. 13 and Fire Clay seams was systematically characterized through scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis. Characterization results showed that crandallite-group minerals still existed in the leaching solid residue of the raw coal wastes but disappeared in the leaching solid residue of the calcined products. Therefore, calcination decomposed crandallite-group minerals into easy-to-leach forms and thereby enhanced the leaching recovery of REEs. Moreover, the numbers of REE-bearing particles (N) found per area under SEM were calculated. Larger N values were obtained from the leaching solid residue of the calcined coal wastes relative to the raw coal wastes and the leaching solid residue of the raw coal wastes. This phenomenon confirmed that calcination promoted the liberation of REE-bearing particles. Overall, findings from the present study confirmed the two mechanisms that explain the positive effect of calcination on the recovery of REEs from coal waste.