Investigation of enhancement of rare earth elements from thermal power plant fly ash

This study evaluates fly ash from thermal power plants as a secondary source of rare earth elements (REEs). Fly ash samples were collected from six different thermal power plants. Characterization analyses were conducted, and the samples were processed to achieve particle sizes suitable for experimental studies. The procurement of bacteria, application of nutrient medium, and monitoring of their growth stages were ensured. Experimental sets were designed using the Taguchi method. Based on the results of the characterization analyses, the most suitable fly ash sample for bioleaching studies was selected, and bioleaching experiments were performed. The reliability of the Taguchi experimental design was tested in terms of pH and REE enrichment ratios, with ANOVA analysis confirming the design's reliability. The analysis results indicated that the experiment's most influential parameter was the amount of fly ash (solid ratio). To enhance the REE yield, chemical leaching and bioleaching methods were applied together using the experimental parameters that yielded the highest REE recovery. As a result of the bioleaching experiments, the highest REE enrichment yield was obtained at 42.1%, with yields exceeding 60% for Pr and Gd. The yield for heavy rare earth elements (HREEs) was calculated at 50.43%, while the yield for light rare earth elements (LREEs) was 38.61%. The combined use of chemical leaching and bioleaching methods resulted in a yield of 56.5%, with the enrichment yield for Pr being determined at 99%. The results demonstrated that the bioleaching method is effective in enriching REEs. It was also determined that the combination of chemical leaching and bioleaching enhanced the enrichment performance and holds potential for large-scale applications. Additionally, the study emphasizes that fly ash should be considered a recoverable resource, contributing to a sustainable circular economy approach. Keywords: Rare Earth Element, Fly Ash, Bioleach, Mining Waste Management, Acidithiobacillus Ferrooxidans