Removal of uranium and gross radioactivity from coal bottom ash by CaCl2 roasting followed by HNO3 leaching.

A roast-leach method using CaCl2 and HNO3 to remove uranium and gross radioactivity in coal bottom ash was investigated. Heat treatment of the ash with 100% CaCl2 (900°C, 2h) significantly enhanced uranium leachability (>95%) compared with direct acid-leaching (22.6-25.5%). The removal efficiency of uranium and gross radioactivity increased steeply with increasing CaCl2 content, from 10% to 50%, and a HNO3 leaching time from 5 min to 1h, but remained nearly constant or decreased slightly with increasing CaCl2 dosage >50% or acid-leaching time >1h. The majority of the uranium (87.3%), gross α (92.9%) and gross β (84.9%) were removed under the optimized roast-leach conditions (50% CaCl2, 1M HNO3 leaching for 1h). The mineralogical characteristics of roasted clinker indicated that molten CaCl2 promoted the incorporation of Ca into silica and silicates and resulted in its progressive susceptibility to acid attack. Uranium and other radionuclides, most likely present in the form of silicates or in association with miscellaneous silicates in the highest density fraction (>2.5g mL(-1)), were probably leached out as the result of the acid decomposition of newly formed "gelatinizing silicates".