Mineralogy and leaching characteristics of rare earths from the micro-dispersed mineral matter in the organic matrix of Fire Clay coal

Abstract Previous studies have shown that rare earth elements (REEs) in the micro-dispersed mineral matter of Fire Clay coal exhibit high leachability. To better understand the fundamental mechanisms underlying this behavior, pH-dependent leaching tests and comprehensive SEM-EDS analyses were conducted. At a pH of 3.3, around 10% of light REEs (LREEs) and 40% of heavy REEs (HREEs) were leached. Reducing the pH to −0.05 significantly increased the leaching efficiency of LREEs relative to HREEs. The leaching efficiencies of the individual REEs at high pH values followed an order similar to that of the binding constants of REEs with organic acids, suggesting that the recoveries at pH 3.3 were primarily due to organically associated REEs in the coal. Numerous REE-bearing particles were identified by SEM-EDS analyses. Most of the particles identified in the raw material had irregular shapes and were formed by the intergrowth of small spheres and/or flakes, indicating that the rare earth minerals were of low-crystallinity. SEM-EDS analyses of the leaching solid residue at pH −0.05 identified fewer LREE-containing particles than HREE-bearing particles. This finding agrees with the higher leaching efficiency of LREEs at pH −0.05 and the greater increase in the leaching efficiency of LREEs as the solution pH decreased. The enhanced leaching of LREEs is primarily attributed to the dissolution of low-crystallinity rare earth phosphate minerals.