Rare Earth Elements Extraction from Coal Fly Ash and the application to the South African situation

Abstract

The global demand for rare earth elements (REEs) has been increasing significantly due to their applications in various high-technology industries, including renewable energy, electronics, and defence. This rising demand has necessitated the exploration of secondary sources of REEs, among which coal and its combustion residues have emerged as viable alternatives. Coal fly ash (CFA), the byproduct of coal combustion, has garnered particular interest due to its widespread availability and relatively high concentrations of REEs. South Africa, where coal serves as the primary energy source, operates numerous coal-fired power plants that generate substantial quantities of CFA. This has prompted extensive research into the potential of CFA as a secondary source of REEs. While previous studies have largely focused on CFA from Mpumalanga's Eskom power stations, the present study extends the investigation to CFA from Free-State, Sasolburg’s Sasol Steam Stations 1 and 2. This research builds upon the findings of Seleka (2021) and Mokoena (2021), who examined the influence of particle size on REE enrichment and the optimization of hydrochloric acid (HCl) leaching concentrations for improved REE recovery, respectively. However, both studies reported relatively low REE recovery rates, which were attributed primarily to the inadequate pre-treatment of CFA. South African CFA is classified as Class F, characterized by a low calcium oxide (CaO) content and a significant proportion of acid-resistant aluminosilicate glass. The presence of this glass phase poses a challenge for effective REE extraction, necessitating pre-treatment strategies to enhance REE recovery. The primary objective of this study was to optimize the pre-treatment of CFA to enhance its porosity and facilitate the subsequent leaching process. Two pre-treatment methods were explored: (1) the addition of sodium hydroxide (NaOH) to CFA, wherein varying CFA-to NaOH ratios were investigated while maintaining a constant CFA mass of 10 g, and (2) the variation of NaOH molar concentrations to determine the optimal conditions for REE extraction. The results indicated that increasing the NaOH molar concentration was a more effective approach, with the highest REE recovery observed at a NaOH concentration of 12 M, MREEs showing a high recovery of 94%. Further analysis of the final leached residues was conducted to quantify the remaining REEs post-leaching. The findings revealed that the 12 M NaOH-treated residue retained only 93.49 ppm of REEs from an initial total of 471.89 ppm, demonstrating a significant improvement in REE extraction efficiency. These results underscore the critical role of pre-treatment in overcoming the challenges posed by the glass phase in South African CFA and highlight the potential of alkaline treatment as a viable strategy for enhancing REE recovery from CFA.