The mechanism study on the integrated process of NaOH treatment and citric acid leaching for rare earth elements recovery from coal fly ash

Rare earth elements and yttrium (REY) have vital functions in the global economy because of their numerous applications. Coal fly ash (CFA) is considered a potential alternative resource for REY. In this paper, sequential chemical extraction procedure, NaOH treatment, and citric acid leaching were used to study modes of occur-rence, enrichment, and recovery of REY in CFA after physical separation. The proportion of REY (84%) bond to aluminosilicate phase in the separated sample is higher than that in the raw CFA. Monazite, apatite, scheelite, and aluminosilicate phase were proved to be REY carriers via SEM-EDS observation. REY in CFA was enrichment by NaOH treatment, and the glassy phase dissolution conformed to shrinking core model. The dissolution was first controlled by a chemical reaction and then inner diffusion, with activation energies of 70.89 and 3.21 kJ/ mol and reaction orders of 1.02 and 0.73, respectively. The gel conjunction and zeolite generation limited CFA dissolution, concluded from the chemical reaction verification, SEM observation, and XRD analysis. The REY content after multi-step NaOH treatment increased to 2135 & mu;g/g with an enriched factor of 2.08. Citric acid (0.05 mol/L) leaching was investigated and REY recovery is 55%. A conceptual integrated process of NaOH treatment and citric acid leaching for the CFA utilization was designed. This paper will provide a theoretical fundamental and technological support for REY recovery.