Yang, and 9 co-authors, reported in 2020 that uranium-enriched coal ash (CA) has received significant attention as a supplementary nuclear resource albeit 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. The authors reveal 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.
The authors concluded that as the CAs with different components show a diversity in physico-chemical properties, it is necessary to investigate the uranium mobility considering the various utilizations and treatments of ash. In this study, the mode of uranium occurrence in CA and SCA were systematically investigated based on the SiO2-Al2O3-Fe2O3-CaO system, and a reasonable ratio of major component of coal itself was proposed to regulate the uranium mobility followed the coal combustion. The results lead to conclusions as follows:
1) Mode of uranium occurrence involves a three-stage changing process during coal combustion. Exchangeable and organics-bound uranium in coal are firstly oxidized to U-oxides, which tend to interact with Ca2+ to form stable uranates. However, U-oxides and uranates transfer to integrate with Fe-oxides and aluminosilicates due to the decomposition of carbonate minerals and the formation of low-melting-point eutectic once the temperature exceeds 700°C. Finally, the compact Si-Al matrix generated at 1100°C can encapsulate U-bearing fractions, leading to a low chemical activity.
2) Although uranium mobility in SCA can be significantly improved with increasing CaO content, uranium, incorporated by low-melting point eutectics, tends to be extracted as the form of S3 rather than S2 if provided excess CaO.
3) Without the activation of Ca2+ on bed minerals, the sufficient amounts of Fe2O3 contributes to the formation of Fe-Si depletion and promotes the encapsulated behavior of uranium, resulting in a larger proportion of residual uranium in SCA.
4) The effect of Al2O3 on uranium mobility is between CaO and Fe2O3, while the competition of uranium occurrence in Si-Al amorphous phase cannot be ignored due to its largest distribution in ash.
5) SiO2 content plays a critical role on ash sintering as well as uranium mobility. A high frequency of encapsulated uranium ascribes to the inhomogeneous composition at a medium SiO2 content, whereas the degree of encapsulated behavior is continuously strengthened with rising SiO2 content leading to the decrease of uranium mobility.
6) Increasing SiO2 content and regulating Ca/Si, Al/Si and Fe/Si mole ratio ＜0.5, ＞1.25 or ＞0.75 before coal combustion can benefit from decreasing the environment risk of uranium in CA. On the other hand, it is effective enough to improve the extraction efficiency of uranium when the Ca/Si ratio is 1.0 once the U-enriched CA is served as the potential nuclear resource in the future.
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