Thermal Decomposition of Bauxite Minerals: Infrared Emission Spectroscopy of Gibbsite, Boehmite and Diaspore

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 DT

Spectroscopic studies of nano-structures of AI and Fe phases, bauxite and their thermally activated products

This thesis is made as it is submitted as a sum of published papers by the candidate. Aluminium hydroxides including gibbsite, boehmite and diaspore, are the major components, while iron hydroxides/oxides and kaolinite are the major impurities in bauxite. The dehydroxylation pathways during thermal activation of bauxite have been debated for decades. Phase transformation during thermal activation or calcination of bauxite to achieve high yields of alumina has been an important goal for the refining industry. This study deals with natural and synthetic aluminium and iron hydroxides using vibrational spectroscopy in conjunction with X-ray diffraction and electron microscopy, followed by the characterisation of the phase transformation in activated bauxite. In the Raman spectra, gibbsite shows four bands at 3617, 3522, 3433 and 3364 cm-1, and bayerite shows seven bands at 3664, 3652, 3552, 3542, 3450, 3438 and 3420 cm-1 in the hydroxyl stretching region. Five bands at 3445, 3363, 3226, 3119 and 2936 cm-1 for diaspore and four at 3371, 3220, 3085 and 2989 cm-1 for boehmite are present. The far infrared spectrum of boehmite resembles that of diaspore in the 300-400 cm-1 region. Boehmite has two characteristic bands at 366 and 323 cm-1 while diaspore has five at 354, 331, 250, 199 and 158 cm-1. The far infrared spectrum of gibbsite resembles that of bayerite in the 230-300 cm-1 region. Gibbsite shows three characteristic bands at 371, 279 and 246 cm-1 whereas bayerite shows six at 383, 345, 326, 296, 252 and 62 cm-1. The far infrared spectra are in-harmony with the FT-Raman spectra, allowing the study and differentiation of the stretching of AlO4 units to characterize these four alumina phases. The surface properties of kaolinite and gibbsite are studied using Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS). The FTIR-PAS spectra of kaolinite are recorded at mirror velocities of 0.05, 0.1, and 0.2 cm s-1, and compared to the gibbsite spectra recorded at mirror velocity of 0.2 cm s-1. It is found that the hydroxyl surface spectra are a function of depth. For the FTIR spectroscopy of thermal dehydroxylation of goethite to form hematite, the intensity of hydroxyl stretching and bending vibrations decreased with the extent of dehydroxylation of goethite. Infrared absorption bands clearly show the phase transformation between goethite and hematite, in particular the migration of excess hydroxyl units from goethite to hematite. Data from the band component analysis of FT-IR spectra indicate that the hydroxyl units mainly affect the a- plane in goethite and the equivalent c- plane in hematite. A larger amount of non-stoichiometric hydroxyl unit is found to be associated with a higher aluminium substitution. A shift to a higher wavenumber of bending and hydroxyl stretching vibrations is attributed to the effects of aluminium substitution associated with non-stoichiometric hydroxyl units on the a-b plane relative to the b-c plane of goethite. The dehydroxylation pathways of both the aluminium hydroxides and the impurities are intensively studied. Gibbsite completely decomposed at 250 °C, followed by boehmite and kaolinite at 500 °C. No phase transformations were observed for hematite, anatase, rutile or quartz up to 800 °C. Small amounts of gibbsite transformed to boehmite but the majority transformed to chi (?) alumina, a disordered transition alumina phase, after dehydroxylation at 250 °C. The dehydroxylation pathways of crystalline gibbsite follow the orders: (a) gibbsite (&lt250 °C) to boehmite (250-450 °C) to gamma alumina (?) (500-800 °C); or (b) gibbsite (&lt250 °C) to chi alumina (?) (250-800 °C) to chi (?) + kappa alumina (?) (700-800 °C). Boehmite completely altered to gamma alumina (?), while kaolinite altered to metakaolinite at 500 °C. The vibrational spectroscopy including FT-IR and FT-Raman, is a rapid, accurate and non-destructive technique in characterising both single and mixed mineral phases. In particular, the vibrational spectroscopy has shown its advantages over other techniques in terms of its sensitivity to hydroxyl groups. Future work on the simulation of bauxite dehydroxylation with emphasis on the studies of transition aluminas is proposed. The application of the advanced technique synchrotron x-ray spectroscopy, in addition to those techniques used in the present study, is recommended

The Behavior of Hydroxyl Units of Synthetic Goethite and its Dehydroxylated Product Hematite

The behavior of the hydroxyl units of synthetic goethite and its dehydroxylated product hematite was characterized using a combination of Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) during the thermal transformation over a temperature range of 180-270 degrees C. Hematite was detected at temperatures above 200 degrees C by XRD while goethite was not observed above 230 degrees C. Five intense OH vibrations at 3212-3194, 1687-1674, 1643-1640, 888-884 and 800-798 cm(-1), and a H2O vibration at 3450-3445 cm(-1) were observed for goethite. The intensity of hydroxyl stretching and bending vibrations decreased with the extent of dehydroxylation of goethite. Infrared absorption bands clearly show the phase transformation between goethite and hematite: in particular. the migration of excess hydroxyl units from goethite to hematite. Two bands at 536-533 and 454-452 cm(-1) are the low wavenumber vibrations of Fe-O in the hematite structure. Band component analysis data of FTIR spectra support the fact that the hydroxyl units mainly affect the a plane in goethite and the equivalent c plane in hematite

Near-Infrared Spectroscopic Study of Basic Aluminum Sulfate and Nitrate

The tridecameric Al-polymer [AlO4Al12(OH)24(H2O)12]7+ was prepared by forced hydrolysis of Al3+ up to an OH/Al molar ratio of 2.2. Under slow evaporation crystals were formed of Al13-nitrate. Upon addition of sulfate the tridecamer crystallised as the monoclinic Al13-sulfate. These crystals have been studied using near-infrared spectroscopy and compared to Al2(SO4)3.16H2O. Although the near-infrared spectra of the Al13-sulfate and nitrate are very similar indicating similar crystal structures, there are minor differences related to the strength with which the crystal water molecules are bonded to the salt groups. The interaction between crystal water and nitrate is stronger than with the sulfate as reflected by the shift of the crystal water band positions from 6213, 4874 and 4553 cm–1 for the Al13 sulfate towards 5925, 4848 and 4532 cm–1 for the nitrate. A reversed shift from 5079 and 5037 cm–1 for the sulfate towards 5238 and 5040 cm–1 for the nitrate for the water molecules in the Al13 indicate that the nitrate-Al13 bond is weakened due to the influence of the crystal water on the nitrate. The Al-OH bond in the Al13 complex is not influenced by changing the salt group due to the shielding by the water molecules of the Al13 complex

Near-infrared spectroscopic study of [AlO4Al12(OH)23(H2O)12]7+-O-Si(OH)3 nitrate crystals formed by forced hydrolysis of Al3+ in the presence of TEOS

The polymer [AlOAl(OH)(HO)]-O-Si(OH) was prepared by forced hydrolysis of Al up to an OH/Al molar ratio of 2.0 in the presence of monomeric orthosilicic acid. Crystalline material was obtained by slow evaporation. Although the near-infrared spectra of the Al-sulfate and Al-O-Si(OH) are very similar, there are differences related to the bonding of the -O-Si(OH) group to the Al-unit. The strong complex of bands around 7000 cm associated with the overtones and combination bands of the OH-stretching modes for Al-sulfate is much weaker for Al-O-Si(OH) and the opposite is true for the complex of bands around 5000 cm associated with the water overtone and combination modes, suggesting that the outer OH-groups of the Al-unit are involved in the formation of the new Al-O-Si(OH) units. A weak band around 7370-7631 cm is interpreted as the overtone of the Si-OH stretching vibration around 3740 cm. A low intensity band, absent for Al-sulfate and -nitrate is observed around 5550-5570 cm and is interpreted as the overtone of the OH-stretching mode of the OH-groups in the vicinity of the central AlO in the Al-unit around 2890-2935 cm. The interaction between the -O-Si(OH) group and the Al-unit has a small influence on other bands like the combination modes of water in the 4400-4800 cm region, which show a small shift towards higher wavenumbers. The internal OH-groups in the Al-complex are relatively shielded by the water molecules and therefore do not reflect the influence of the -O-Si(OH) in their band positions

Infrared spectroscopy of geothite dehydroxylation. II Effect of aluminium substitution on the behaviour of hydroxyl units

Dehydroxylation of goethite as affected by aluminium substitution was investigated using Fourier transform infrared spectroscopy (FT-IR) in conjunction with X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA). The band intensities of hydroxyl vibrations were indicative of the degree of dehydroxylation and the changes in band parameters due to aluminium substitution were observed. The effect of aluminium substitution on band parameters of FT-IR spectra of goethite and its partially and fully dehydroxylated products, the mixture of goethite/hematite and hematite, were interpreted. The results of this study have confirmed that aluminium substituted goethite is thermally more stable than non-substituted goethite and is in harmony with the results of XRD and DTGA. A larger amount of non-stoichiometric hydroxyl units is associated with a higher aluminium substitution. A shift to a higher wavenumber of bending and hydroxyl stretching vibrations is attributed to the effects of aluminium substitution associated with non-stoichiometric hydroxyl units on the a-b plane relative to the b-c plane of goethite. The results provide information for the characterisation of activated bauxite containing hematite and goethite

FT-IR and Raman Microscopic Study at 293 K and 77 K of Celestine, SrSO4, from the Middle Triassic Limestone (Muschelkalk) in Winterswijk, The Netherlands

This paper describes the Raman and infrared spectroscopy of SrSO or celestine from the Muschelkalk of Winterswijk, The Netherlands. The infrared absorption spectrum is characterised by the SO modes v at 991 cm, v at 1201, 1138 and 1091 cm, and v at 643 and 611 cm. An unidentified band is observed at 1248 cm. In the Raman spectrum at 293 K the v mode is found at 1000 cm and is split in two bands at 1001 and 1003 cm upon cooling to 77 K. The v mode, not observed in the infrared spectrum, is observed as a doublet at 460 and 453 cm. The v mode is represented by four bands in the Raman spectrum at 1187, 1158, 1110 and 1093 cm and the v mode as three bands at 656, 638 and 620 cm. Cooling to 77 K results in a general decrease in bandwidth and a minor shift in frequencies. A decrease in intensities is observed upon cooling to 77 K due to movement of the Sr atom towards one or more of the oxygen atoms in the sulfate group

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

Infrared spectroscopy of geothite dehydroxylation:III FT-IR microscopy of in situ study of the thermal transformation of geothite to hematite

Fourier transform infrared microscopy has been used to investigate in situ dehydroxylation of goethite to form hematite. The characterisation was based on the behaviour of hydroxyl units, which were observed in the hydroxyl stretching and hydroxyl deformation and water bending regions, and the Fe–O vibrations of the newly formed hematite during the thermal dehydroxylation process. Two hydroxyl stretching modes (ν1 and ν2), and three bending (νbending-1, 2, 3) and two deformation (νdeformation-1, 2) modes were observed for goethite. The characteristic vibration at 916 cm−1 was observed together with the residuals of the ν1 and ν2 bands in hematite spectrum. The structural transformation between goethite and hematite through thermal dehydroxylation was interpreted in order to provide criteria that can be used for the characterisation of thermally activated bauxite and their conversion to activated alumina phases