Sequential alkaline-organic acid leaching process to enhance the recovery of rare earth elements from Indonesian coal fly ash

Abstract

In this decade, coal fly ash (CFA) is considered a potential secondary source of rare earth elements (REEs). However, most REEs in coal fly ash are encapsulated in aluminosilicate glass, making it challenging to recover them through acid leaching. In this study, a sequential alkaline–organic acid leaching was developed for the recovery of REEs from CFA. The effect of alkaline leaching using NaOH solution on the destructive ability of aluminosilicate glass, as well as the mineralogy and morphology changes of the resulting coal fly ash, was first studied. Furthermore, the effectiveness of alkaline leaching on the recovery ability of REEs through organic acid leaching was evaluated. The results show that the maximum leaching efficiency for Si and Al, which was obtained at the optimum alkaline leaching conditions, namely NaOH concentration of 10 mol/L, reaction temperature of 65 °C, liquid/solid (L/S) ratio of 10 mL/g, and reaction time of 90 min, is 28% and 32%, respectively. The digestion reaction with NaOH lixiviants also causes coal fly ash to become more porous, making it advantageous in the organic acid-leaching process at the REEs recovery stage. The utilization of the desilicated residue produced from the digestion process in acid leaching effectively increases the overall REEs recovery from 32.2% to 77.6%.