Gibbsite Growth Research Articles

The Dimensional Effect and Crystal Morphological Peculiarities of Gibbsite in a Bauxite-Bearing Weathering Crust

The correlation between the crystal morphological peculiarities of gibbsite and its position in a bauxite-bearing weathering profile is revealed from the example of the Tsentral’noe deposit of the Chadobets Uplift. The free space favorable for the growth of gibbsite increases from bottom to top in various parts of the profile as a result of infiltration metasomatosis and various physicochemical conditions. The higher the position of the rock, the higher its porosity. The size of the gibbsite crystals thus increases, and their morphology becomes more complex upward through the section. The results of thermal analysis showed that the higher the sizes of the gibbsite crystals, the higher the amount of boehmite forms upon its heating. These conclusions will help the technologists to choose the most feasible reworking scheme of bauxites.

Boehmite Preparation via Alditols-Interacting Transformation of Metastable Intermediates in Al–H2O Reaction Crystallization

Boehmite was prepared efficiently by adding alditols to Al–H2O reaction crystallization in moderate condition. We demonstrate that it is the interaction between alditols and metastable intermediates that inhibits the decomposition of aluminate ions into gibbsite and favors the formation of boehmite rather than the blocking of gibbsite growth sites as reported in the previous investigations. The analysis of the crystal formation process shows that the alditols play a role before the nucleation. The interaction simulated by density functional theory and the local hard and soft, acids and bases theory suggests that the metastable intermediates act as soft bases, while the alditols play the role of acids in the interaction, and the higher the alditols’ softness, the stronger the inhibiting effect. The configuration of the interacting metastable intermediates and alditols was also verified by the comparison of both experimental and computational Fourier transform infrared spectra.

In situ AFM investigation of gibbsite growth in high ionic strength, highly alkaline, aqueous media

In situ atomic force microscopy (AFM) was used to study the growth mechanism of the gibbsite basal (001) face and prismatic (100) face in high ionic strength (4M NaCl+1M NaOH) and highly alkaline solution (pH=13.4). The height of the steps on the gibbsite (001) faces can vary from less than one nanometre (elementary steps) to several nanometres (microsteps) and finally to dozens of nanometres (macrosteps). The lateral advancement rates of micro/macrosteps are related to the height of steps. Step coalescence occurs during lateral advancement, driven by Ostwald ripening. There are three possible pathways for the formation of micro/macrosteps on basal (001) face, all of which are controlled by layer-by-layer growth mechanism. Gibbsite structure suggests that the frequent and random formation of the elementary steps on basal (001) face can be attributed to weak inter-layer hydrogen bonds along the c-axis of gibbsite unit cell. The prismatic (100) faces are characterised by microstep trains along gibbsite [001] direction. These microsteps grow higher along gibbsite [100] direction, advance laterally and step coalescence also occurs. Two dimensional islands often form on the terrace adjacent to the edges of micro/macrosteps, suggesting that the step edge should be a preferred site for island nucleation due to the Ehrlich–Schwoebel effect.

Precipitates of Al(III), Sc(III), and La(III) at the muscovite-water interface.

The interaction of Al(III), Sc(III), and La(III) with muscovite-water interfaces was studied at pH 4 and 10 mM NaCl using second harmonic generation (SHG) and X-ray photoelectron spectroscopy (XPS). SHG data for Sc(III) and La(III) suggest complete and/or partial irreversible adsorption that is attributed by XPS to the growth of Sc(III) and La(III) hydroxides/oxides on the muscovite surface. Al(III) adsorption appears to coincide with the growth of gibbsite (Al(OH)3) deposits on the muscovite surface, as indicated by the magnitude of the interfacial potential computed from the SHG data. This interpretation of the data is consistent with previous studies reporting the epitaxial growth of gibbsite on the muscovite surface under similar conditions. The implication of our findings is that the surface charge density of mica may change (and in the case of Al(III), even flip sign from negative (mica) to positive (gibbsite)) when Al(III), Sc(III), or La(III) is present in aqueous phases in contact with heterogeneous geochemical media rich in mica-class minerals, even at subsaturation conditions.

Interaction of gibbsite with oleic acid: Surface energetics and modelling

We examined the surface interaction energetics of gibbsite with oleic acid by employing both experimental techniques and quantum chemical calculations. The infrared frequencies computed for the pure oleic acid as well as for the sodium–oleate complex using the Amsterdam Density Functional program show a shift to lower wavenumbers associated with the formation of a metal-carboxylate fragment. Our MSINDO calculations indicate that the oleic acid-surface complex may form and influence the growth of gibbsite at a pH range of 5 to 9 as compared to more alkaline pH, however adsorption is most favoured at defect and dopant sites. Comparison of the theoretical results with experimental findings imply the possible role of intermediate species such as sodium–oleate at high pH. The morphological importance of the [110] prismatic face is shown by the more favourable adsorption energies for oleic acid (−201kJ/mol) and sodium–oleate (−173kJ/mol) evaluated for this face.

In situ embedment and growth of anhydrous and hydrated aluminum oxide particles on polyvinylidene fluoride (PVDF) membranes

Surface modification of polyvinylidene fluoride (PVDF) membranes was successfully performed by doping anhydrous and hydrated aluminum oxide particles (γ-alumina, boehmite and gibbsite) through in situ particle embedment and subsequent crystal growth under a hydrothermal environment. The approach of particle embedment involves the dispersion of anhydrous or hydrated aluminum oxide particles in the water phase during PVDF membrane precipitation, and the result was found to strongly depend on the change of free energy in particle–membrane interaction. The embedment of γ-alumina nanoparticles is thermodynamically favorable, but the embedment of boehmite or gibbsite particles is less favorable, especially when using hydrophilic polyvinylpyrrolidone (PVP) additive within the membrane cast solution. The approach of 105°C hydrothermal treatment initiates the growth of gibbsite on embedded γ-alumina nanoparticles, and the treatment duration determines the coverage of gibbsite particles on PVDF membrane. The overall hydrophilicity of membranes increases with the embedment of γ-alumina and the growth of gibbsite. An increase of membrane resistance was not observed with γ-alumina embedment, but the excessive growth of gibbsite particles may reduce membrane permeability. The Escherichia coli attachment result shows that the surface doping of aluminum oxide particles can greatly reduce E. coli adhesion to the membrane surface, suggesting the positive effect on the reduction of bio-fouling. The success of this PVDF membrane modification method has also provided a promising strategy to manipulate the surface property of other polymer membranes by inorganic materials.

Atomic force microscopy study of the growth mechanism of gibbsite crystals

A combination of atomic force microscopy (AFM) and scanning electron microscopy (SEM) has been used to investigate the crystal growth mechanism of gibbsite (aluminium trihydroxide) in pure solutions. Under the conditions studied, the growth on the basal face of synthetic gibbsite prepared from sodium aluminate solution proceeds by a continuous birth and spread mechanism. The nuclei formed on the surface of the basal face of gibbsite grow both laterally and vertically, with lateral growth being much faster than vertical growth. Moreover, a remarkable cyclical, smooth→rough→smooth→rough process has been directly observed. The effect of alkali ions on the crystal growth of gibbsite has been investigated. Curved features and a growth hillock were observed for the first time on the basal face of gibbsite prepared from potassium aluminate solution; whereas, steps terminating within the plane and hexagonal shaped features tilted at an angle and related to the symmetry of the face were imaged on the basal faces of gibbsite prepared from sodium aluminate solution. The results suggest faster growth on the basal face of gibbsite prepared from potassium aluminate compared to that prepared from sodium aluminate, leading to the observed elongated morphology.

Supersaturation and temperature dependency of gibbsite growth in laminar and turbulent flows

The temperature and supersaturation dependencies of gibbsite crystal growth rates in caustic aluminate solutions were experimentally investigated in both the laminar (Taylor–Couette precipitator) and turbulent (stirred tank) mixing regimes. The gibbsite precipitation experiments were conducted batchwise in a temperature range from 55°C to 90°C, but all with the same initial liquor composition and seed type. At low supersaturation it was found that gibbsite crystal growth could be described satisfactorily by a screw dislocation growth mechanism over the complete temperature range, irrespective of the flow regime. At higher supersaturation and lower temperatures, i.e. 55°C and 60°C, it is proposed that two-dimensional polynuclear growth is the dominant growth mechanism in turbulent flows. Any differences observed in the measured growth rate estimates from the laminar and turbulent systems were explained by error analysis.

The effect of isotopic substitution of deuterium for hydrogen on the morphology of products precipitated from synthetic Bayer solutions

In the production of alumina (Al 2O 3), the precipitation of gibbsite (Al(OH) 3) is the slowest step in the Bayer process. Gibbsite growth rates are generally in the vicinity of microns per hour. Thus, research is focussed on increasing productivity (gibbsite precipitation rates) without compromising product quality. Fundamental to this is an improved understanding of gibbsite growth mechanisms. Isotopic substitution of deuterium for hydrogen in synthetic Bayer (sodium aluminate) solutions was used primarily to investigate deprotonation and reprotonation of the surface hydroxyl groups. Gibbsite precipitates in a highly alkaline solution. The hydroxyl groups situated on the surfaces of the growing crystals are likely to be deprotonated because the pH of the Bayer liquor is >14 and the pK a for the surface protons is in the range of 9–10. The rate of gibbsite growth may therefore depend on the deprotonation and reprotonation of the hydroxyl groups, as the gibbsite crystal lattice consists of hydrogen bonded layers of Al(OH) 3. However, the gibbsite precipitation rates from equivalent sodium aluminate solutions (NaAl(OH) 3 and NaAl(OD) 3) were very similar, indicating that the deprotonation and reprotonation of the hydroxyl groups are not rate-limiting. Nonetheless, the isotopic substitution of deuterium for hydrogen in synthetic Bayer solutions did have effects on the gibbsite solubility, crystallization growth process, product phase and morphology. This paper focuses on the changes in product morphology. Isotopic substitution resulted in changes in the orientation of nuclei formed on seed surfaces as well as the nuclei morphology. This is the first time that isotopic substitution has been used to investigate gibbsite precipitation in synthetic Bayer solutions.

Effect of temperature, pH and ageing time on hydration of rho alumina by studying phase composition and surface properties of transition alumina obtained after thermal dehydration

Crystallization of alumina hydrates from rho alumina at different hydration conditions has been studied by using combined loss on ignition (LOI) and XRD technique. Extreme pH conditions during hydration of rho alumina favoured gibbsite growth while higher temperature favoured crystallization rate. Alumina hydrates obtained at different pH and temperature conditions were calcined at 450°C and evaluated for surface properties for comparison with those of pure transition alumina phases. Hydrated product obtained under normal hydration conditions exhibited closely spaced bimodal pore size distribution. The distribution resolved further with increase in temperature whereas pore widening was observed with increase in pH. The results of this study will help to manipulate the required surface properties and phase mixture of transition alumina by exercising control during hydration of rho alumina.

Apparent gibbsite growth ages for regolith in the Georgia Piedmont

–Carbon bound in gibbsite, collected from a residual weathering profile developed on a Paleozoic granite in the Georgia Piedmont, was examined for its 14C content and found to be geologically young. The study site, located at the Panola Mountain Research Watershed, has developed a granite–saprolite–soil regolith in which 14C-gibbsite model ages deep within the profile (C-horizon) average about 8000 years. Near the surface (A- and B-horizon) 14C-gibbsite model ages range from 2100 to 4200 years. Quartz has acquired 26Al and 10Be inventories suggesting a near-surface residence time of at least 90,000 years. This age disparity supports the notion that secondary minerals undergo significant recrystallization as weathering fronts propagate into the landscape. Combining the results of 14C, 26Al, and 10Be analyses offers the potential to assess differential rates of chemical weathering and continental denudation to understand better the links between rates of silicate rock weathering, climate, and soil residence times.

Gibbsite crystallization inhibition: 2. Comparative effects of selected alditols and hydroxycarboxylic acids

The effects of eight alditols and six related hydroxycarboxylates on the nucleation, agglomeration and growth of gibbsite from caustic aluminate solutions (synthetic Bayer liquors) were studied under controlled but variable conditions, with the aim of elucidating the relationships between crystallization inhibition and sodium incorporation. Results obtained for pure solutions were consistent with the current understanding of gibbsite crystallization. The observed inhibitory effects of the selected model compounds differ only in the extent to which they (i) decrease precipitation, (ii) confirm that the number and stereochemistry of hydroxyl groups determines the inhibition strength of the alditols and (iii) show that tridentate binding and/or terminal carboxylate increases that strength. Inhibitors are thought to adsorb at specific sites, mainly on the basal faces, and reduce the number of available sites to which aluminate ions can attach. The effects of intermediate-strength inhibitors on gibbsite morphologies and sodium incorporation form the basis of the proposed inhibition mechanisms, in which both organic molecules and sodium ions become incorporated between gibbsite double layers. Their effect is to disrupt “birth and spread” growth on the basal faces and to weaken the bonding between double layers. “Spiral” growth continues on the prismatic faces leading to the formation of large, thin hexagons.

Gibbsite growth kinetics on gibbsite, kaolinite, and muscovite substrates: atomic force microscopy evidence for epitaxy and an assessment of reactive surface area

New experimental data for gibbsite growth on powdered kaolinite and single crystal muscovite and published data for gibbsite growth on gibbsite powders at 80°C in pH3 solutions show that all growth rates obey the same linear function of saturation state provided that reactive surface area is evaluated for each mineral substrate. Growth rate (mol m −2 s −1) is expressed by Rate ppt = (1.9 ± 0.2) × 10 −10|Δ G r |/ RT (0.90±0.01), which applies to the range of saturation states from Δ G r = 0 to 8.8 kJ mol −1, where Δ G r = RT[ln( Q/ K)] for the reaction Al 3+ + 3H 2O = Al(OH) 3 + 3H +, and equilibrium defined as Δ G r = 0 was previously determined. Identification of the growth phase as gibbsite was confirmed by rotating anode powder x-ray diffraction. Rates on kaolinite were determined using steady-state measured changes between inlet and outlet solutions in single-pass stirred-flow experiments. Rates on muscovite were determined by measuring the volume of precipitated crystals in images obtained by Tapping Mode™ atomic force microscopy (TMAFM). In deriving the single growth rate law, reactive surface area was evaluated for each substrate mineral. Total BET surface area was used for normalizing rates of gibbsite growth onto powdered gibbsite. Eight percent of the BET surface area, representing the approximate amount occupied by the aluminum octahedral sheet exposed at crystal edges, was used for powdered kaolinite. The x - y area of the TMAFM images of the basal surface was used for single crystal muscovite. Linearity of growth rates with saturation state suggests that the dominant nucleation and growth mechanisms are two dimensional. Such mechanisms are supported by observations of the morphologies of gibbsite crystals grown on muscovite at Δ G r = 8.8 kJ mol −1. The morphologies include (1) apparent epitaxial films as determined by hexagonal outlines of edges and thicknesses of 30 to 40 Å, (2) elongate crystals 30 to 40 Å thick aligned with the structure of the distorted Si-tetrahedral sheet of the 2 M 1 muscovite, and (3) micrometer-scale three-dimensional clumps of intergrown crystals. Reactive surface area as defined now for heterogeneous crystal growth in reactive-transport models must be modified to include substrates other than that of the growing mineral and to account for the role of structural and chemical controls on epitaxial nucleation and growth.