Morphology, mineralogical composition, and heavy metal enrichment characteristics of magnetic fractions in coal fly ash

Magnetic fractions (MFs) are a group of mineral fractions in coal fly ashes (CFAs) from coal-fired power plants, in the form of magnetite, hematite, and iron-bearing silicates. There is an enrichment of potentially harmful metals in MFs, thus posing a risk to the environment and human health. The MFs in CFAs were separated by wet magnetic separation with the yield range of 5.96–8.42 wt% and characterized by Fourier transform infrared spectrometer (FTIR), scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM–EDS), X-ray diffraction, inductively coupled plasma-mass spectrometry (ICP-MS) and Mössbauer spectroscopy. The presence of Si–O–Si, Fe–O, aromatic rings and aliphatic groups in CFAs and MFs were observed by FTIR analysis, especially rich in Si–O–Si and Fe–O. The SEM images displayed numerous iron spheres with rough surfaces in MFs. The crystallized iron in the magnetic particles is primarily in the form of magnetite, mixed with hematite in various proportions. Mössbauer spectra of the MFs reveal that the iron in the magnetic material is present mainly as Fe 3 O 4 , Fe 2+ , and Fe 3+ in aluminosilicate glasses and γ-Fe 2 O 3 . Fe, Na, and Mn are highly enriched in the MFs, while Mg and P have relatively low enrichment factors. Trace elements including Ni, Co, Cu, Zn, Pb, and Cr, are enriched in the MFs. A deep understanding of the characteristics of MFs in CFAs and the enrichment of heavy metals in MFs is of great importance for environmental protection and efficient utilization of CFAs.