Abstract

Near-infrared (IR) spectroscopy has been used to distinguish between alumina oxo and hydroxy phases. Two near-IR spectral regions are identified for this function: (1) the high-frequency region between 6400 and 7400 cm<sup>-1</sup>, attributed to the first overtone of the hydroxyl stretching mode, and (2) the 4000-4800 cm<sup>-1</sup> region attributed to the combination of the stretching and deformation modes of the AlOH units. Near-IR spectroscopy allows the study and differentiation of the hydroxy and oxo(hydroxy) alumina phases, since each phase has its own characteristic spectrum. The spectrum of bayerite resembles that of gibbsite, whereas the spectrum of boehmite is similar to that of diaspore. Bayerite has four characteristic near-IR bands at 7218, 7128, 6996, and 6895 cm<sup>-1</sup>. Gibbsite shows five major bands at 7151, 7052, 6958, 6898, and 6845 cm<sup>-1</sup>. Boehmite displays three near-IR bands at 7152, 7065, and 6960 cm<sup>-1</sup>. Diaspore shows a prominent band at around 7176 cm<sup>-1</sup>. The use of near-IR reflectance spectroscopy to study alumina surfaces has a wide application, particularly with thin films and surfaces. The technique is rapid and accurate. Near-IR, because of its sensitivity, can be used in reflectance mode for the on-line processing of bauxitic minerals.

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