Alkaline leaching characteristics of uranium from Lincang coal: Correlation with the dissolution of coal humic substances

Abstract

The characteristics involving the leachability of uranium from coal by utilizing sodium carbonate-bicarbonate mixtures and the role of coal humic substances dissolution in the leaching procedure were investigated. Alkaline leaching is an effective method to extract uranium from coal without reducing the usability of coal. Under the optimum leaching condition, viz. liquid/solid (L/S) ratio = 20: 1 and [CO32−]/[HCO3−] ratio = 2: 1, the leaching efficiency increases from 80.50 ± 0.24% at 10 °C to 96.29 ± 0.70% at 90 °C with the leaching agent concentration of 0.6 mol/L. As the primary form of uranium in the coal, the organic-bound uranium decreases by 64.95 ± 3.25 mg/kg after leaching at ambient temperature of 25 °C, and is the main source of the alkaline-leached uranium. Besides, the uranium is linearly and significantly correlated with the dissolved organic carbon (DOC) in the leachate (R2 = 0.916, p < 0.01) before the leaching equilibrium. The fluorescence excitation-emission matrix (EEM) analysis, coupled with parallel factor (PARAFAC), peak-picking and fluorescence regional integration (FRI) analysis, shows that the leaching concentration of uranium is closely correlated with the fluorescence components represented humic acid (HA)-like substances in leachate, with R2 values around 0.8 and p < 0.05. Thus, we hypothesize that the dissolution of HA-like substances can be the key factor for the alkaline leaching of uranium from the coal.