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

Abstract

The conditions under which coal ash is formed largely dictate the association of valuables within its matrix. Optimum utilization of coal waste at the 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 degrees C), whereas the one under laboratory conditions was prepared at 815 degrees 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 the case of FA. Alkaline pre-treatment to enhance the extractability of REEs was performed using both NaOH and Na2CO3, under varying temperatures from 400-850 degrees C. The optimum condition was deduced to be using NaOH at 400 degrees 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 degrees C stirred for 60 min demonstrated a 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 the 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.