Mineralogical and geochemical characteristics of the Permian coal from the Qinshui Basin, northern China, with emphasis on lithium enrichment

Comprehensive information about the concentrations, distribution, and modes of occurrence of elements in coal are important from the environmental and economic point of view. Although a great number of previous studies have investigated the geology of coalbed methane in the Qinshui Basin, only a few studies focused on the inorganic constituents in coal. More specifically, the mode of occurrences of valuable element Li in the No. 3 Coal is still unclear, although Li was found enriched. In this study, we present mineral characteristics, as well as multi-element data on the Permian No. 3 Coal from the Sucun and Gaohe Mines, Changzhi City, southern Qinshui Basin. The studied coals are characterized by low- to medium-ash yield (Ad = 5.72%-28.18%, 12.34% on average), low volatile matter yield (Vdaf = 8.49-15.17%, 10.96% on average), suggesting a low volatile bituminous coal to semi-anthracite. NH4-illite and kaolinite are the main minerals in the coals detected by XRD, and trace amount of minerals calcite, dolomite, quartz, pyrite and diaspore can also be found. The major elements of the studied No. 3 coals are dominated by SiO2 and Al2O3, ranging 2.49-16.45 wt% and 2.13-12.9 wt% (on a whole-coal basis), respectively. Li is enriched in the No. 3 coals (5 < CC < 10). Li in the studied coals is positively correlated with Al2O3, SiO2 and ash yield, with the correlation coefficients (r) of 0.86, 0.8, and 0.78, respectively. The sequential leaching results show that 96.6%-99.4% of Li in the coals was removed by HF. The proportion of Li present as the ion-exchange form and water soluble form accounts for < 1.1% and < 0.29% of the total Li respectively in the coals. Li in the studied coals are probably bound to clay minerals kaolinite and illite, residing mainly in non-exchangeable positions, either in the octahedral or pseudo-hexagonal sites.