Occurrence and characteristics of abundant fine included mineral particles in Collie coal of Western Australia

Collie coal is the only coal being mined in Western Australia and plays an important role in supplying cheap energy to the State's energy mix. This study reports a systematic investigation into the occurrence and characteristics of the fine mineral matter in Collie coal that is crushed to the size of <212 µm and density separated. The results show that 94.1 wt% of the mineral matter in the whole Collie coal is present in the three density fractions of 1.2-1.4, 1.4-1.6 and ≥2.0, including 26.8 and 58.6 wt% as included mineral matter in two density fractions with specific gravities of 1.2-1.4 and 1.4-1.6, and 8.7 wt% as excluded mineral matter in the density fraction of ≥2.0. Further analysis of these key Collie coal density fractions using computer-controlled scanning electron microscopy (CCSEM) shows that 54.13 wt% of the total mineral matter in the whole Collie coal are fine mineral particles of sizes <10 μm. These included fine mineral particles (<10 μm) are distributed as 20%:80% in the two coal density fractions of 1.2-1.4 and 1.4-1.6, which contribute to 10.64 and 43.49 wt% of the total mineral matter in the whole coal, respectively. CCSEM analysis also shows that the key minerals in the fine mineral particles are quartz, kaolinite, K-, Ca- or Fe-Al silicates and pyrite-related minerals. However, large proportions of these included fine mineral particles have “unclassified” mineral phases. Such unclassified particles contribute to 43.88 and 36.84 wt% of the fine mineral particles with size <10 µm present in the coal density fractions of 1.2-1.4 and 1.4-1.6 (equivalent to 4.67 and 16.02 wt% of the total mineral matter in the whole coal), respectively. The chemistry data show that these individual fine mineral particles (<10 µm) contain mixed compositions of Si + Al, Fe + Ti and other elements such as Ca, K, S and P, suggesting that one single fine mineral particle contains multiple mineral phases.