A comparative study on the association and extractability of rare earth elements from laboratory ash, bottom ash, fly ash: A perspective on Indian coals

• Published in: Minerals engineering 2024-08, Vol.213, p.108745, Article 108745
• Main Authors: Banerjee, Riya, Chakladar, Saswati, Chattopadhyay, Shyamal Kumar, Chakravarty, Sanchita
• Format: Article
• Language: English
• Subjects: Alkali roasting; Coal ash; Leaching; Magnetic separation; Rare earth element; Solvent extraction
• ISSN: 0892-6875; 1872-9444
• DOI: 10.1016/j.mineng.2024.108745

•Coal ash from multiple sources was compared for extractability of REEs.•Optimization of suitable pre-treatment to enhance extractability was performed.•Morphological differences between coal ash samples were established.•A holistic flow sheet was developed for enhanced rare earth elements extraction. The conditions under which coal ash is formed largely dictates the association of valuables within its matrix. An optimum utilization of coal waste in commercial level is only feasible upon performing an in-depth comparative study of fly ash (FA), bottom ash (BA) and laboratory ash (LA). In the present study, two coal ash samples were acquired from a power plant in India (1200–1300 °C), whereas the one under laboratory conditions was prepared at 815 °C. The total concentrations of rare earth elements (hereafter abbreviated as REEs) were- LA (873 ppm), FA (509 ppm) and BA (373 ppm) and their respective outlook coefficients were-LA (0.78), FA (0.87) and BA (0.88). Sequential extraction revealed that the chosen LA possessed 40 % of the total REE in extractable form, whereas both FA and BA possessed only 10 % of the total REE in extractable form. Magnetically isolated Fe-containing matrix with 50 % Fe2O3 was observed in case of FA. Alkaline pre-treatment to enhance the extractability of REEs was performed using both NaOH and Na2CO3, under varying temperature from 400–850 °C. The optimum condition was deduced to be using NaOH at 400 °C. Sodium silicate was eliminated during water leaching, from which Si was precipitated at neutral pH. The leaching parameters were optimized on these alkali roasted coal ash samples, and 0.5 M tartaric acid at 90 °C stirred for 60 min demonstrated 75–80 % increase in leaching of REEs as compared to baseline leaching of 27 % (LA), 5 % (FA), 12 % (BA). Significant morphological change was observed post alkaline pre-treatment and acid leaching. NaOH roasting resulted in cracks and fractures on the surface of spherical BA and FA samples followed by development of pores during the leaching process. Solvent extraction was performed with 10 % (v/v) D2EHPA (di-(2-ethylhexyl) phosphoric acid) in kerosene with an additional scrubbing stage (5 % oxalic acid solution) to enhance the purity of REE extraction.