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

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 (C-outl) 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 mu m, 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.