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

Cerium is the most abundant rare earth element (REE) within the waste product of alumina production (red mud), but its speciation in this complex material is still barely understood. Previous studies showed evidence for a correlation between Ce and the main constituent of red mud, iron oxides, which led us to investigate the most abundant iron oxide in red mud, hematite, as possible carrier phase for Ce. Synthetic hematite can incorporate up to 1.70±0.01wt% Ce, which leads to a systematical increase of all unit cell parameters. Investigations by extended X-ray absorption fine structure spectroscopy suggest an incorporation of Ce⁴⁺O₆ into the hematite structure by a novel atomic arrangement, fundamentally different from the close-range order around Fe³⁺ in hematite. Samples of red mud were taken in Lauta (Saxony), Germany and analyzed by powder X-ray diffraction, inductively coupled plasma mass and optical emission spectrometry, electron microprobe analysis and X-ray absorption near-edge structure spectroscopy. Red mud samples consist of hematite (Fe₂O₃) (34-58wt%), sodalite (Na₈Al₆Si₆O₂₄Cl₂) (4-30wt%), gibbsite (Al(OH)₃) (0-25wt%), goethite (FeOOH) (10-23wt%), böhmite (AlOOH) (0-11wt%), rutile (TiO₂) (4-8wt%), cancrinite (Na₆Ca₂Al₆Si₆O₂₄(CO₃)₂) (0-5wt%), nordstrandite (Al(OH)₃) (0-5wt%) and quartz (SiO₂) (0-4wt%). While the main elemental composition is Fe>Al>Na>Ti>Ca (Si not included), the average concentration of REE is 1109±6mg/kg with an average Ce concentration of 464±3mg/kg. The main carrier of Ce was located in the Fe-rich fine-grained fraction of red mud (0.10wt% Ce₂O₃), while other potential Ce carriers like monazite, lead oxides, secondary Ce-minerals and particles of potentially anthropogenic origin are of subordinated relevance. Cerium in red mud occurs predominantly as Ce⁴⁺, which further excludes Ce³⁺ minerals as relevant sources.