Characterization and acid leaching of rare earth elements in coal gangue using pretreatment of selective grinding, tailings discarding and alkali roasting

Co-associated rare earth elements (lanthanide and yttrium, REY) in coal and its by-products have been considered important potential nontraditional rare earth sources. In this study, a coal gangue sample collected from a coal processing plant in Jinsha County of Guizhou Province, southwest China, was used as the research object. The content, modes of occurrence, and extraction (acid leaching after pretreatment of selective grinding, tailings discarding, and alkali roasting) of REY from the sample were analyzed. The result shows that the content of REY (1038.26 μg/g) in pyrite and quartz is low but mainly enriched in kaolinite. Under the following conditions of a filling ratio of 40% (grinding media steel ball) and grinding time of 8 min, selective grinding pretreatment is applied to achieve 176.95 μg/g (yield 24.08%) and 1104.93 μg/g (yield 75.92%) of REY in +2 mm and −2 mm fractions, respectively. Thus, the −2 mm coal gangue fraction is selected, used as the feed, and roasted and leached with HCl. When Na2CO3 and NaCl are separately used as roasting activators, the REY leaching ratios are 91.41% and 68.88%, respectively, under the optimum conditions. The contents of REY in the final leachate are 1010.02 and 761.08 μg/g when Na2CO3 and NaCl are used, respectively. The two REY contents are relatively higher than the impurity ions in the leachate, which facilitates further REY separation. The mechanism study reveals that high-temperature roasting increases the pore size and the total pore area of the gangue, which promotes leachate penetration and improves reaction efficiency. In addition, roasting facilitates the reaction between the sodium salt activator and kaolinite and other aluminosilicate minerals in the coal gangue to generate soluble salts, thus releasing REY into the solution. The appropriate roasting temperature transforms the activator into a molten state. Thus, the reaction between coal gangue and activator is a solid–liquid reaction rather than a solid–solid reaction, which improves the efficiency of the chemical reaction.