gibbsite

High‐quality Raman spectra were used for the characterization of alumina phases of gibbsite, bayerite, diaspore and boehmite. The Raman spectrum of gibbsite shows four strong, sharp bands at 3617, 3522, 3433 and 3364 cm−1 in the hydroxyl stretching region. The spectrum of bayerite shows seven bands at 3664, 3652, 3552, 3542, 3450, 3438 and 3420 cm−1. Five broad bands at 3445, 3363, 3226, 3119 and 2936 cm−1 and four broad and weak bands at 3371, 3220, 3085 and 2989 cm−1 are present in the Raman spectrum of the hydroxyl stretching region of diaspore and boehmite. The hydroxyl stretching bands are related to the surface structure of the minerals. The Raman spectra of bayerite, gibbsite and diaspore are complex whereas the Raman spectrum of boehmite shows only four bands in the low‐wavenumber region. These bands are assigned to deformation and translational modes of the alumina phases. A comparison of the Raman spectrum of bauxite with those of boehmite and gibbsite showed the possibility of using Raman spectroscopy for on‐line processing of bauxites that contain a mixture of alumina phases.

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−1, attributed to the first overtone of the hydroxyl stretching mode, and (2) the 4000–4800 cm−1 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−1. Gibbsite shows five major bands at 7151, 7052, 6958, 6898, and 6845 cm−1. Boehmite displays three near-IR bands at 7152, 7065, and 6960 cm−1. Diaspore shows a prominent band at around 7176 cm−1. 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.

Fourier transform infrared photoacoustic spectroscopy (FTIR.-PAS) has advantages in distinguishing hydroxyl surface spectra as a function of depth. FTIR-PAS spectra of kaolinite were recorded at mirror velocities of 0.05, 0.1, and 0.2 cm s·1 , and compared to gibbsite spectra recorded at mirror velocity of 0.2 cm s·1 • Four strong bands at 3694, 3670, 3654 and 3621 cm·1, and a weak band at 3435 cm·1 in the hydroxyl stretching region were observed for kaolinite. The intensity of these bands increased as mirror velocity increased. Because an increase in mirror velocity means a shallower depth of the surface is measured, the increasing intensity of the hydroxyl stretching and bending vibrations is a consequence of more hydroxyl units on the outer surface than on the inner surface. The intensity of the kaolinite band at 103 7 cm·1 was
greater than that at 1010 cm·1 at 0.2 cm s-1 velocity but this order changed when mirror velocity was reduced to 0.05 cm s·1 in the hydroxyl deformation and water bending region. For the Si-0-Al modes of kaolinite in the low frequency region, one band at 473 cm·1 is observed at 0.2
cm s·1 velocity, while this band is split into two at 477 and 467 cm·1 at 0.05 cm s·1 velocity. Some vibrations of gibbsite resemble those of kaolinite.