Optimization of uranium alkaline leaching from the solid waste sample containing high fluoride and nitrate by response surface methodology experimental design

Carbonate solution is used for the alkaline leaching of uranium from solid waste. Time, temperature, stirring speed (independent parameters) and total carbonate concentration, carbonate to bicarbonate ratio, and the ratio of liquid eluent volume to solid waste weight (L/S) (dependent parameters) are effective parameters in the leaching process. Independent parameters were optimized using the one variant at time (OVAT) method and the dependent parameters were evaluated simultaneously with the use of MINITAB software and using the face central composite design (FCCD) method. The solid waste powder was analyzed by EDAX, SEM, and XRD, and elemental analysis of samples was performed by ICP-OES after complete dissolution. Duration time of 0.5 hours, the stirrer rotation speed of 150 rpm, room temperature, concentrations of 0.09, 0.01, and 0.1 M for sodium carbonate, bicarbonate, and total carbonate, respectively, with a ratio of 9.1 and 20 for the ratio of sodium carbonate to sodium bicarbonate and L/S, respectively, were optimal conditions. Reproducibility of solid waste leaching was obtained by performing 4 experiments and the values of 94.2±2 and 4.15±1.6 were obtained for the average percentage of leach and solid residue, respectively. The average uranium grade was 2.09±0.02.