Investigation on the Value-Added Production of Silicon Dioxide and Synthesizing Zeolites as well as Extraction of Rare Earth Elements from Fly Ash

Abstract

Coal fly ash is one of the most promising secondary sources for extracting high value-added rare earth elements. Nevertheless, the majority of rare earth elements in coal fly ash are associated with the aluminosilicate glassy phase, hindering their solubility during the acid leaching process and resulting in the traditional rare earth elements extraction method, which is unavoidably complex in operation and poor in the economy. In this study, prior to the conventional acid leaching, the realization of the coal fly ash activation was considered. This consisted of two steps involving the coal fly ash calcination at the elevated temperatures using recyclable Na2CO3 and the water and alkali washing. It helped in developing the pore structures in coal fly ash, facilitating the leaching solution to rare earth elements, and reducing the acid consumption of rare earth elements leaching. Simultaneously, the generated aqueous solutions could precipitate two new valuable products, the purified silica oxide powder (257.58 g·kg−1, 338.1 m2·g−1 BET, 40 nm grain size, 93.43% purity) and porous zeolites (410.3 g·kg−1). The residual rare earth elements in the pretreated solid residue can be easily extracted, with an extraction efficiency of 91.24% and an acid saving rate of 74.5%. Therefore, a multiple of value-added products can be obtained by this new extraction method with great economic significance.