Spinel Ferrite Transformation for an Efficient Fe Removal from Circulating Fluidized Bed Fly Ash by Carbothermal...

Alumina (Al₂O₃) extraction from circulating fluidized bed (CFB) fly ash (CFBFA) is one of the most important pathways for value-added utilization. However, in CFBFA, impurity iron (Fe) normally coexists, resulting in complicated separation processes, low Al₂O₃ extraction efficiency, and substandard Al₂O₃-based products. How to remove Fe impurity effectively from CFBFA has become an important issue. For an effective Fe removal from CFBFA, spinel ferrite transformation by carbothermal reduction at a low temperature was discussed in the paper. The effects of the reduction temperature and reduction time on the removal efficiency of Fe and the recovery of aluminum (Al) as well as the removal of other metals were systematically investigated, and the transformation mechanisms of Fe-containing phases were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and a scanning electron microscope-energy dispersive spectrometer. The results showed that Fe in CFBFA was present in the form of weakly magnetic α-Fe₂O₃, leading to a Fe removal of about 17.1% after magnetic separation; however, the recovery efficiency of Al reached 97.4%. Weakly magnetic hematite (α-Fe₂O₃) could be converted to strongly magnetic spinel-type ferrite (MFe₂O₄) after carbothermal reduction at 700 °C for 60 min, and the Fe removal efficiency could reach 62.8% after magnetic separation; however, the recovery of Al was 81.2%, which was decreased compared to the recovery of Al under the condition without carbothermal reduction treatment. However, the carbothermal reduction-magnetic separation process did not have a major effect on the existing form and leaching behavior of Al, Li, and Ga. Simultaneously, it could be observed that some transition metal elements such as Mn, Cr, and so forth could be enriched in spinel-type MFe₂O₄ and removed after magnetic separation, which also provided a way for transition metal enrichment and extraction of transition metals from other tailings.