Effects of oxidant and particle size on uranium leaching from coal ash

Migration behavior and leaching ability of radioactive uranium during incineration of uranium-containing strippable coating wastes

It is currently thought that the incineration approach is an effective method to minimize the volume of radioactive wastes. In this paper, we used an incinerator to burn uranium-containing strippable coating waste. The migration behavior of radioactive uranium during the incineration process were investigated based on hierarchical sampling and mass spectrometry. Results shows that the radioactive uranium is more easily to adhere to the particles with smaller size. The leaching abilities of radioactive uranium in the bottom ash and the fly ash were analyzed. The leaching rate of the uranium from the fly ash and bottom ash were 1% and 6%, respectively, indicating that most of the radioactive uranium was fixed in the ash and the same storage/disposal methods can be used for both the fly ash and bottom ash. According to x-ray spectrometry and SEM-EDS, mineral compositions of the original uranium ore and the bottom ash were mostly the same. Calcium plays an important role in uranium fixation during incineration. The potential mechanism of the uranium special transformation during uranium-containing strippable coating waste combustion was revealed. Our research results can provide technical support for nuclear emergency waste treatment and disposal.

Critical insight and indication on particle size effects towards uranium release from uranium mill tailings: Geochemical and mineralogical aspects

Uranium (U) is both chemically toxic and radioactive. Uranium mill tailings (UMTs) are one of the most important sources of U contamination in the environment, wherein the mechanisms that control U release from UMTs with different granularities have not yet been well understood. Herein, the release behaviours and underlying release mechanisms of U from UMTs with five different particle size fractions (<0.45, 0.45-0.9, 0.9-2, 2-6 and 6-10 mm) were studied with a well-defined leaching test (ANS 16.1) combined with geochemical and mineralogical characterizations. The results showed that the most remarkable U release unexpectedly emerged from UMT_{2-6 mm}; in contrast, the smallest particle size UMT_{<0.45 mm} contributed to the least U release. The predominant mechanism of U release from UMT_{2-6 mm} was the oxidative dissolution of U-bearing sulfides, while abundant gypsum present in UMT_{<0.45 mm} inhibited U release. The study highlights the importance of combined geochemical and mineralogical investigation when performing leaching tests of mineral-containing hazardous materials such as UMTs with consideration of particle size effects. The findings also indicate that elevating the content of gypsum and avoiding the oxidation of sulfides can effectively help immobilize and minimize the residual U release from the UMTs.

Water-leaching characteristic of valuable trace metals (U, V, and Ga) from (NH4)2SO4-treated coal ash: A coprecipitation behavior at high temperature

Coal ash (CA) becomes the most significant industrial solid waste and attracts much attention due to its potential environmental risk and reuse as the supplementary material. In this study, experiments were conducted to investigate the mode of occurrence and the leaching behavior of valuable trace metals (U, V, and Ga) from CA and (NH₄)₂SO₄-treated CA (NCA), based on the recovery of aluminum. Integrations of Fe- and K-oxide with Si-Al glass increased the ash strength and obstructed the activation of NH₄⁺ on amorphous Al-bearing phases, resulting in a limited improvement in the leaching efficiency of trace metals. On the other side, a higher liquidus temperature, contributing to the dissolutions of Al³⁺ and Ca²⁺, could promote the leaching of U from NCA as well, whereas the water-leaching behaviors of V and Ga involved a sophisticated trend with temperature > 40℃. Water-leached V/Ga tended to transfer into Fe-Mn oxide-bound and residual V/Ga owing to the noticeable hydrolysis of Fe and Ti ions that facilitated the formation of coprecipitation. However, 0.1 M H₂SO₄ could re-dissolve that coprecipitation, and thus leaching efficiencies of U, V, and Ga were 1.9, 1.3, and 5.0 times higher than those by directly leaching CA, respectively.

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

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.