Enrichment of critical elements from coal fly ash by the combination of physical separations

•The occurrence pattern of critical elements in coal fly ash (CFA) was studied.•Particle size, density and magnetic separations were investigated.•Critical elements can be maximally enriched by combination of physical fractions.•The process flowsheet of recovering critical elements from CFA was designed. The increasing demand for critical elements and the depletion of conventional ores have stimulated the recovery of critical elements from secondary resources. The concentration of critical elements in coal fly ash (CFA) are significant, which makes it a potential alternative source for the recovery of critical elements. Preconcentration treatment is an important step to recover critical elements, which not only directly affects the subsequent leaching and extraction efficiency, but also determines the reagent and energy consumption. The distribution and enrichment characteristics of critical elements in different physical separation fractions of CFA are worthy of further elucidation. In this paper, the physicochemical characteristics and the modes of occurrence of critical elements (Li, Be, Rb, Sr, Zr, Nb, Cs, Hf, Ta, Ga, Ge, Se, Cd, Tl, In, Te, and Re) of CFA from Pingwei power generation plant in Huainan, Anhui Province, China were investigated, with emphasis on several physical (particle size, density, magnetic) separation methods and the enrichment characteristics of critical elements in each physical fraction. The density and magnetic separation of CFA after particle size classification was proposed, and different sorting combinations were adopted for various types of critical elements, to achieve efficient enrichment of critical elements. The results show that the critical elements in CFA are mainly enriched in the residual state, which consists of aluminosilicate minerals and amorphous glass. The CFA with greater recovery potential exists in smaller particle size. Higher Rb, Sr, Zr, and Cs concentrations were found in the low-density fractions of fine particle size CFA, while Li, Be, and Ga are mainly enriched in the non-magnetic fractions of fine particle size CFA. Some critical elements could be further enriched by density separation or magnetic separation of CFA after particle size classification. The combination of different physically separated fractions is considered as an effective way to maximize the enrichment of critical elements from CFA.