A quantitative evaluation of uranium mobility and potential environment risk in coal ash with SiO₂-Al₂O₃-Fe₂O₃-CaO...

Uranium-enriched coal ash (CA) receives a significant attention as a supplementary nuclear resource also due to its potential environmental risk. Combining with CA, the changing trend of uranium occurrence in synthetic coal ash (SCA) was described at CaO-Al₂O₃-Fe₂O₃ ternary phase diagrams with a fixed SiO₂ (wt. %, 30%, 50% and 70%) and Na₂O (2%) content. This study reveals that the mode of uranium occurrence proposes a three-stage changing process during coal combustion including uranium oxidation, combination and encapsulation. Furthermore, a high frequency of encapsulated uranium resulted from the complicated interactions among major components with a medium SiO₂ content, whereas the degree was higher at a higher SiO₂, resulting in the decrease of uranium mobility. Uranium was encapsulated by Si-Al matrix and Fe-Si depletion if provided the high Al₂O₃ and Fe₂O₃ but low CaO contents. However, with the development of calcium looping and biomass co-firing process, the change of element mobility should be considered in industry owing to the extra-added alkaline metals. As the activation of Ca²⁺ induces a significant susceptibility of acid attack, cautions must be paid in CA with a higher Ca/Si ratio to avoid its utilization as constructive materials for the potential environmental risk.