Characterisation of bauxite and seawater neutralised bauxite residue using XRD and vibrational spectroscopic techniques

Bauxite refinery residues are derived from the Bayer process by the digestion of crushed bauxite in concentrated caustic at elevated temperatures. Chemically, it comprises, in varying amounts (depending upon the composition of the starting bauxite), oxides of iron and titanium, residual alumina, sodalite, silica, and minor quantities of other metal oxides. Bauxite residues are being neutralised by seawater in recent years to reduce the alkalinity in bauxite residue, through the precipitation of hydrotalcite-like compounds and some other Mg, Ca, and Al hydroxide and carbonate minerals. A combination of X-ray diffraction (XRD) and vibrational spectroscopy techniques, including mid-infrared (IR), Raman, near-infrared (NIR), and UV–Visible, have been used to characterise bauxite residue and seawater neutralised bauxite residue. The ferric (Fe3+) ions within bauxite residue can be identified by their characteristic NIR bands, where ferric ions produce two strong absorption bands at 25,000 and 14,300 cm−1. The presence of adsorbed carbonate and hydroxide anions can be identified at around 5,200 and 7,000 cm−1, respectively, attributed to the 2nd overtone of the 1st fundamental overtones observed in the mid-IR spectra. The complex bands in the Raman and mid-IR spectra around 3,500 cm−1 are assigned to the OH-stretching vibrations of the various oxides present in bauxite residue, and water. The combination of carbonate and hydroxyl units and their fundamental overtones give rise to many of the features of the NIR spectra.