Rare earth elements and other critical elements in the magnetic fraction of fly ash from several Polish power plants

Fly ash is a promising alternative source of many critical raw materials, including rare earth elements. According to the EU report, the demand for these metals is set to grow due to the economic growth of the developing countries, along with the development of new innovative technologies. A geochemical analysis showed that the overall content of critical and excessive elements in fly ash from several Polish power plants was higher than in the magnetic fraction. However, the outlook coefficient values for magnetic concentrates were higher than for ashes. Ashes and the magnetic fraction from brown coals seem to be the most promising source of REY recovery. The separation of fine-size grades from the analyzed ashes does not have a significant impact on REY concentration in the obtained products. A microanalysis showed that iron spinels were enriched in Co, Cu, Ni, Mn, Mo, and Zn. The concentration of these metals in magnetic concentrates insignificantly exceeded the world average value of fly ash. The sample with the highest heavy metal enrichment factor also had the highest Fe enrichment factor. The phase composition of the magnetic fraction was dominated by: magnetite, hematite, magnesioferrite, and other iron spinels with Mn and Al substitutions. The chemical composition of ferrospheres was determined using energy dispersive spectroscopy (EDS) together with the SEM technique, and was then linked to their morphology. It was concluded that the impact on the enrichment efficiency and thus the composition of the magnetic fraction had: the diversity of morphological structures of iron spinels, the presence of AlSi glass interlayers, the presence of substitutions in the magnetite crystal structure as well as the ash particle size distribution.