The effect of calcination on the occurrence and leaching of rare earth elements in coal refuse

Coal refuse, as one of the solid wastes with huge yield, is a promising source of rare earth elements (REE) given the demand-supply gap, zero-cost construction and policy-related subsides. In this paper, the mechanism and effects of calcination on the occurrence and leaching of REE in coal refuse were examined by sequential chemical extraction procedure, characteristic analysis, and acid leaching test. The results showed that the calcination promotes the transformation of rare earth from silicate & aluminosilicate form to metal oxide form, and then, turns back with the continuous increase of temperature. The characteristic analysis indicated that the sample calcined at 400 ℃ began to undergo a dehydration reaction, and kaolinite began to transform into metakaolin. The transformation reaction may be completed at 600 ℃ when REE was released from the mineral lattice. Metakaolin was transformed to the amorphous phase as the temperature continued to rise to 900 ℃. The transition of kaolinite is one of the most important reasons for the variation of REE occurrence, resulting in the change of leaching rate. Leaching of REE in coal refuse took advantage of the proper calcination and reached up to 98 % at the optimal temperature ranging from 600 ℃ to 900 ℃. This theoretical work gives an implication for low-temperature combustion assist to REE recovery from coal refuse of which the calorific value meets the requirements of the fluidized bed. • Calcination results in the transformation of rare earth elements in aluminosilicate and metal oxide form. • The change of clay mineral properties by calcination decides the variation of rare earth occurrence. • Rare earth elements (98 %) leach out at optimal solution conditions from coal refuse calcined at 600 ℃.