Extractability and occurrence of rare earth elements in bottom ash through sequential leaching and pre-treatment

Abstract

Utilization of coal bottom ash (BA) produced by thermal power plants has been a national issue. Insufficient knowledge on the association of rare earth elements (REEs) with the parent aluminosilicate matrix of bottom ash limits its optimum utilization in commercial level. In the present study, we analyzed the distribution and occurrence modes of REEs within the bottom ash sample acquired from a power plant in India. The concentrations of major oxides were in the order of SiO2 > Al2O3 > Fe2O3 > TiO2 > K2O > CaO similar to MgO. The total concentration of REEs in the bottom ash sample was found to be 383 ppm where the concentration were in the order of Ce > Nd > La > Sc (for LREE) and Y > Dy > Er > Ho (for HREE). The total concentration of critical REE was 117 ppm and C-outl was deduced to be 0.84. Sequential extraction confirmed that about 80% of total REE remained embedded in the residual fraction. X-ray diffraction (XRD) analyses showed the presence of anatase/rutile/, hematite, mullite, quartz, calcite, and berlinite as major mineral phases. A comparative study on effects of multiple pre-treatments such as alkaline roasting (1:1 NaOH, 400 degrees C) and acid baking (1:1 h(2)SO(4) at 180 degrees C) as well as particle size alteration (75-250 mu m) was ascertained, where alkaline treatment was concluded to be optimum. The novelty of the present work lies in delineation of optimum pre-treatment of industrial bottom ash through sequential extraction aiming at improving the leaching potency of REEs using environment friendly organic acids.