gibbsite

Infrared emission spectroscopy has been used to study the dehydroxylation behavior over the temperature range from 200 to 750 degrees Celsius of three major Al-minerals in bauxite: gibbsite (synthetic and natural), boehmite (synthetic and natural) and diaspore. A good agreement is found with the thermal analysis and differential thermal analysis curves of these minerals. Loss in intensity of especially the hydroxyl-stretching modes of gibbsite, boehmite and diaspore as function of temperature correspond well with the observed changes in the TGA/DTA patterns. The DTA pattern of gibbsite clearly indicates the formation of boehmite as an intermediate shown by a endotherm around 500 degrees Celsius. Dehydroxylation of gibbsite is followed by a loss of intensity of the 3620 and 3351 cm-1 OH-stretching bands and the corresponding deformation band around 1024 cm-1. Dehydroxylation starts around 220 degrees Celsius and is complete around 350 degrees Celsius. Similar observations were made for boehmite and diaspore. For boehmite dehydroxylation was observed to commence around 250 degrees Celsius and could be followed by especially the loss in intensity of the bands around 3319 and 3129 cm-1. The DTA pattern of diaspore is more complex with overlapping endotherms around 622 and 650 degrees Celsius. The dehydroxylation can be followed by the decrease in intensity of the OH-stretching bands around 3667, 3215 and 2972 cm-1. Above 550 degrees Celsius only a single band is observed that disappears after heating above 600 degrees Celsius corresponding to the two endotherms around 622 and 650 degrees Celsius in the DTA

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.

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.

The aging of the aluminum tridecamer in acidified aluminum sec-butoxide is studied with 27Al nuclear magnetic resonance. At 20°C the tridecamer content passes through a maximum of 19.7% after 133 h of aging to decline subsequently to a final content of approximately 12%. At 90°C the tridecamer disappears within 21 h of aging due to clustering. After 91 h colloidal gibbsite forms, while the pH drops continuously from 4.3 to 3.5. Simultaneously the monomer concentration increases to approximately 6.6%. The increase is possibly due to liberation of the central fourfold-coordinated Al from the tridecamer that rearranges to hexameric rings, and to dissolution of some gibbsite in the acid solution. The hexameric rings are considered to be the precursor for the crystallization of gibbsite and boehmite. Increase of the preparation temperature from 20°C to 80°C has no effect on the amount of tridecamer, although at 90°C the amount increases from approximately 9% to 14.1%. An increase of the preparation temperature leads to changes in the second-order quadrupole effects, which results in a line sharpening from 19.5 to 9.8 Hz for the central fourfold-coordinated Al signal of the tridecamer at 63.3 ppm.

The infrared absorption, infrared emission, and Fourier transform (FT)-Raman spectra of a series of gibbsites well defined by X-ray diffraction have been obtained. Hydroxyl stretching frequencies were found ∼ at 3670, 3620, 3524, 3452, 3395, 3375, and ∼ 3300 cm−1. Hydroxyl deformation vibrations were observed at 1059, 1023, 969, 938, and 915 cm−1. Hydroxyl stretching bands were observed in the Raman spectra at 3524, 3436, and 3365 cm−1 and correspond well with the three infrared bands. These bands are both Raman and infrared active. The bands at ∼ 3670, 3620, and 3395 cm−1 are infrared active only. The hydroxyl stretching frequencies show a pronounced blue shift, while the hydroxyl deformation modes show a pronounced red shift. Infrared absorption bands were observed at 3413, 3283, and 3096 cm−1 for the hydroxyl stretching frequencies and at 1024, 969, and 914 cm−1 for the hydroxyl deformation frequencies. Low-frequency infrared absorption vibrations were found at ∼ 860, 838, 800, 747, 666, ∼ 625, 585, 560, 522, 452, and 423 cm−1 and infrared emission bands at 834, 778, 728, 652, 640, 609, 580, 512, and 489 cm−1. The infrared emission low-frequency bands moved to higher frequencies upon thermal treatment. The dehydroxylation of gibbsite was followed by the combination of infrared emission spectroscopy and differential thermal analysis over the 200 to 750 °C temperature ranges. Dehydroxylation is followed by the loss of intensity of the hydroxyl stretching frequencies observed at 3620 and 3351 cm−1 and by the loss of intensity of the hydroxyl deformation modes at 1024 cm−1. Dehydroxylation starts at 220 °C and is complete by 350 °C. Some variation in the gibbsite endotherms was found between the synthetic and natural gibbsite dehydroxylations. Spectral changes in the low-frequency bands confirm that dehydroxylation commenced at 220 °C.