As part of the AMIRA sponsored "Fundamentals of Alumina Precipitation" project, a systematic study of sodium aluminate solutions (synthetic Bayer liquors) has been carried out using FTIR and FT-Raman spectroscopic techniques. Consistent changes in spectra resulting from increasing concentration include both the appearance of new vibrational bands as well as increased intensities of those bands associated with aluminate ions.
It is concluded from the data obtained in this study and from that reported in the literature, that the predominant aluminate ion in caustic aluminate solutions is the tetrahedral Al(OH)4- ion. For solutions of higher aluminium and caustic concentrations, there is evidence for both the tetrahedral ion and one or more as yet undetermined species. It is suggested that these other species could be polymeric anions in which aluminium atoms are either tetrahedral or octahedral in coordination and are linked by either oxo- or hydroxo-bridges. Accordingly, it is concluded that the structures of the anionic species cannot be determined on the basis of evidence from Raman and infrared spectroscopy alone.