Recovery of Rare Earth Elements from Coal Fly Ash with High Calcium and Iron Contents Using Alkaline and Acid Leaching

This research aimed to recover rare earth elements (REEs) from Indonesian coal fly ash (CFA) with high calcium and iron contents using the sequential alkaline-acid leaching method. There were two steps, namely the destruction of the amorphous glass phase in CFA via alkaline leaching, followed by the extraction of REEs from the desilicated residue resulting from the first step via the acid leaching. As a comparison, the extraction of REEs from the original CFA was also carried out through the direct acid leaching method. The results showed that the optimum conditions for the alkaline leaching process were a NaOH concentration of 9.5 M, a temperature of 75 °C, a liquid-to-solid ratio of 10 mL/g, and a reaction time of 90 min. The alkaline process can dissolve up to 70.99% of Si and 72.17% of Al at the optimum conditions. Recovery of REEs using the sequential alkaline-acid leaching method resulted in an efficiency value of 87.91%. However, recovery of REEs using the direct acid leaching method resulted in a higher efficiency value, namely 90.51%. Statistically, there was no significant difference between sequential alkaline-acid leaching and direct acid leaching in REE extraction. The sequential alkaline-acid leaching process formed silica gel and mineral by-products such as calcium silicates, sodalite, katoite, and srebrodolskite in the alkaline leaching residues that need more HCl lixiviant to extract the REEs. Therefore, to minimize the occurrence of this competitive reaction, REE extraction from CFA with high levels of Ca and Fe should be carried out via direct acid leaching.