A promising process for transformation of hematite to magnetite with simultaneous dissolution of alumina from red mud in alkaline medium

The Bayer bauxite residue (red mud, RM) is environmentally hostile and hazardous to human health. Due to a high content of iron, RM is utilized here by hydrothermal digestion in an original one-stage method of hematite into magnetite conversion during recovery of alumina. The changes in the content of alumina, hematite, and magnetite are confirmed using XRD, SEM, chemical analysis and magnetometry. The yield of alumina reaches 75.4% and more during digestion with addition of lime and FeSO4. The formation of magnetite involves the interaction between ferrous ions generated from FeSO4 and dissolved from hematite of RM in the Bayer liquor and hematite reduction by ions-reductants from initial RM during repeated digestion. The conversion efficiency of hematite reaches 52% through the Bayer digestion at the molar ratio Fe(II)/Fetot in RM 0.17 with following separation of magnetic fraction of RM (MFRM) containing 33% Fe3O4. The saturation magnetization of MFRM grows to 29.5 emu/g compared with 2.90 emu/g for parent RM without noticeable amount of magnetite. The yield of the magnetic fraction increases with increasing iron(II) dosage. The magnetite concentrate isolated from parent RM by the enrichment method with 10.5% Fetot extraction and with mass yield of this concentrate of only 6% has the highest value, 65.5 emu/g, with 34% Fe3O4 content almost similarly to that in MFRM. The particle aggregation is found to be the same in all samples and may not critically affect their magnetic properties. On the whole, we have demonstrated the feasibility of RM digestion for the recovery of alumina and synthesis of magnetite to improve the enrichment procedure, as well as the use of wastes as a technogenic feedstock.