Hematite (α-Fe₂O₃) - A potential Ce⁴⁺ carrier in red mud

Area of Science:

• Materials Science
• Geochemistry
• Environmental Science

Background:

• Red mud, a waste product from alumina production, contains abundant rare earth elements (REEs), particularly cerium (Ce).
• The speciation and carrier phases of Ce within red mud are poorly understood, hindering effective resource recovery and management.

Purpose of the Study:

• To investigate the speciation and primary carrier phases of cerium in red mud.
• To explore the incorporation mechanism of Ce into hematite, a major component of red mud.

Main Methods:

• Synthesis of Ce-doped hematite and characterization using X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy.
• Analysis of red mud samples from Lauta, Germany, using XRD, inductively coupled plasma mass spectrometry (ICP-MS), electron microprobe analysis (EMPA), and X-ray absorption near-edge structure (XANES) spectroscopy.

Main Results:

• Synthetic hematite incorporates up to 1.70±0.01 wt% Ce, causing systematic increases in unit cell parameters.
• EXAFS and XANES analyses suggest Ce⁴⁺ incorporation into hematite via a novel atomic arrangement, distinct from Fe³⁺.
• Red mud samples contain significant amounts of hematite (34-58 wt%) and Ce (average 464±3 mg/kg), with the Fe-rich fine-grained fraction identified as the main Ce carrier.

Conclusions:

• Iron oxides, specifically hematite, are the predominant carriers of cerium in red mud.
• Cerium exists primarily as Ce⁴⁺ in red mud, incorporated into hematite through a unique structural mechanism.
• Understanding Ce speciation in red mud is crucial for developing strategies for REE recovery and waste valorization.