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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 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.

This work reports the semiquantitative laboratory chemical procedures developed for some Rare Earth Elements (REE) extraction and concentration from coal bottom ash (BA), including recycling the recovered valuable components. This preliminary assessment applied in case of 25 coal BA samples of Ceplea Valley landfill of Romania describes the chemical methods by using digestion with nitric acid coupled with microemulsion assisted extraction procedure. The enrichment of REE by concentrated nitric acid followed by water washing proved to be an adequate approach since water recovers 29.42% of metals comparing to that extracted by the total phase (ac + aq). The REE concentration process achieved by precipitation with NaOH, at different pH values, shows to be almost completely in the fraction at pH ≈ 0 and 3. The statistical outlook coefficient Coutl shows that evaluation of REE concentration in acid/aqueous solution falls into the “promising” field for their recovery by chemical procedures. Optimal microextraction was obtained by coupling microemulsion Winsor II ternary system consisting in Brij 30/AcOEt:AcOBu/metal ions solution, with citric acid as complexing agent. Results confirm that recovery of lanthanides as nanoparticles from microemulsion system yields over 85%. Optical characterization of oxide nanoparticles suggested applications such as for energy efficient buildings, photo-catalysts for water remediation or thin films for photonic optical devices. The ash residue can be also recycled as an alternative raw material for building materials production.

An investigation has been carried out into the changes occurring in the organic and mineral matter of lignite during combustion and the effect of bottom ash characteristics on making fired-ceramic composites and lightweight heat-resistant concretes. In addition to examination of physico-chemical properties using XRD, XRF, SEM/EDS and Raman microspectroscopy, detailed petrographic studies were performed, including texture and microstructure analysis of feed coal and bottom ash, with special reference to a novel approach for bottom ash reuse in the production of ceramic composites. The results of the petrographic study show correlation of the low rank coal composition with bottom ash optical texture, providing information on the active surface gasified by oxygen, as well as the origin of char particles of woody origin that participated in the combustion process. The bottom ash microstructure is directly related to the reactivity of the porous residue and the properties of the ceramic composites produced, which include density, absorbency, porosity and firing compression strength. In particular, the reduction in density indicates an increased potential of thermal insulation of fired-ceramic composites containing up to 50% bottom ash, and lightweight heat-resistant concretes containing up to 75% bottom ash. The results of the study indicate the positive contribution that can be made by using petrography as a complementary method to study the organic phases, and SEM/EDS and Raman microspectroscopy to analyse the chemical composition of the inorganic phases, when studying bottom ash utilization, and the promising potential for bottom ash to be recycled in making ceramic composites.