Assessment of bottom ash landfilled at Ceplea Valley (Romania) as a source of rare earth elements

Abstract

This study aims to demonstrate, by a detailed fundamental analysis, an alternative for the production of rare earth elements, including yttrium and scandium (REY + Sc), from the historic bottom ash deposit at Ceplea Valley (Romania).Twenty-five samples representing 20% of this deposit were studied using a combination of chemical (proximate and elemental analysis), mineralogical and petrographic techniques. This analysis also included, for the first time, a scanning electron microscopy with X-ray microanalysis SEM/EDS point-count method for tracking REE-bearing minerals.The results showed that the bulk REY + Sc concentration is consistent throughout the landfill, and a bottom ash composite sample was made to assess the chemical composition of several size fractions.Based on Seredin-Dai's criteria (Seredin and Dai, 2012), the plot of the outlook coefficient of REY ores (Coutl) versus the percentage of critical REY of the deposit samples (278 ppm in average) and of the composite sample size fractions shows that this ash is a potentially promising source of REY. However, Gd seems to be the most-promising element, especially in the 0.090–0.125 mm and < 0.063 mm size fractions.Finally, the SEM/EDS results show that the only REE-bearing minerals (except one case with Y, Gd, Er, Dy) are micrometre-sized phosphate grains with Ce, La and Nd, with an average diameter of 4.85 μm, which do not entirely account for the total REE determined by ICP-MS and indicates that a fraction of these elements must be dispersed in the glassy aluminosilicate matrix.