Gibbsite Crystals Research Articles

Understanding Trace Iron and Chromium Incorporation During Gibbsite Crystallization and Effects on Mineral Dissolution.

Incorporation of pollutants, e.g., heavy metals, or critical elements, e.g., lithium, as impurities in mineral phases can significantly affect their mobility or sequestration in the environment. Even when present at low concentrations, impurities can alter the solubility and reactivity of the host mineral. In this study, we investigate the incorporation of trace amounts of iron (Fe3+) and chromium (Cr3+) during the crystal growth of the aluminum (Al3+) hydroxide, gibbsite, a major component of bauxite ores, an important soil mineral, and a dominant mineral phase in stored radioactive wastes. Using a comprehensive suite of analytical techniques, we show that both Cr3+ and Fe3+ can be incorporated into the gibbsite lattice during coprecipitation by replacing Al3+ in octahedral sites. These small amounts are consistent with limited to no structural isomorphism shared between Al3+ and Cr3+/Fe3+ hydroxide precipitates, nor room temperature miscibility of their isostructural M2O3 oxide forms, in contrast with oxyhydroxide forms where Al3+ and Fe3+ share similar structural topologies. Despite the limited uptake of Cr3+/Fe3+, we show that these impurities have significant implications for gibbsite dissolution behavior. The limited uptake of Cr3+/Fe3+ (e.g. 0.43% Cr3+ and 0.4% Fe3+), we show that these impurities have significant implications for gibbsite dissolution behavior and subsequent reactivity in complex environments.

Mechanism of catalyst on morphological evolution of Al(OH)3 during Al–H2O reaction

The morphologies of Al(OH)3 crystals prepared via the Al–H2O reactions with various catalysts and the growth mechanisms of these crystals were investigated. The reaction was carried out with NaOH, KOH, tetramethylguanidine (TMG), and tetramethylammonium hydroxide (TMAH) as catalysts. The evolution of the obtained product was observed in situ, and the products obtained after 120 min of reaction have the shapes of the hexagonal prism, long-hexagonal prism, long rod, and irregular rod, respectively. In the NaOH and KOH systems, the products are gibbsite, and the gibbsite crystals grow along the (110), (001), and (100) faces in the NaOH system, and the (100), (102), and (110) faces in the KOH system. However, in the TMG and TMAH systems, the products are bayerite, and the bayerite crystals grow along the (110), (111), and (001) faces. It is worth mentioning that gibbsite has more Al—O bonds than bayerite, giving rise to the formation of columnar crystals.

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.

DIMENSIONAL EFFECT AND CRYSTALLOMORPHOLOGICAL FEATURES OF GIBBSITE IN THE BAUXITE-BEARING WEATHERING CRUST

Using the example of the Central Chadobets uplift deposit, the relationship of the crystallomorphological features of gibbsite with its position in the bauxite-bearing profile of the weathering crust is revealed. From the down up, in various parts of the profile, as a result of infiltration metasomatosis and various physico-chemical conditions, there is an increase in free space, contributing to the growth of gibbsite crystals. The higher the rock lies, the more porous it is. Thus, in the down-up section, the size of gibbsite crystals increases, its morphology becomes more complicated. By the method of thermal analysis, it was found that the larger the size of the gibbsite crystals, the more boehmite is formed when it is heated. These conclusions will help technologists to choose the most rational scheme for processing bauxite.

DECOMPOSITION OF GIBBSITE FROM ALUMINATE SOLUTIONS IN LOW-INTENSITY ULTRASONIC FIELDS

The results of laboratory studies of the practical application of the reagentless method of strain-pulse regulation in the industrial method of obtaining gibbsite from aluminate solutions using the Bayer method are presented. During decomposition in regulatory regimes, the size and uniformity of crystals increases, and there is an increase in the rates of dissolution of the seed and crystallization of gibbsite. Quantitative characteristics of the degree of transformations are given and the parameters of gibbsite crystals obtained by the traditional method and in different modes of regulation are determined. The results of X-ray phase analysis and granulometric analysis of images obtained using a scanning electron microscope are described.

Gibbsite Crystallinity and Morphology in Ferralsols and Bauxites

Gibbsite is a common mineral, present in several soil classes around the globe, especially in the tropical belt. It is also a key mineral in bauxite, the main natural source of aluminum. The occurrence of gibbsite in the soil greatly influences its physical and chemical characteristics. However, little is known about the important structural properties of this mineral, which determines how these characteristics will be expressed. In this work, we investigated the variation in the morphology and crystallinity of naturally occurring gibbsites from soils and bauxites. The crystallinity of the gibbsite was assessed using full width at half maximum (FWHM), mean crystallite dimensions (MCD), and deoxydylation temperature (DT) measurements. The results of these analyses were convergent, indicating a better crystallinity for gibbsites from the evaluated Amazon soils and bauxites, and worse crystallinity for the remaining soil samples. SEM analysis was used to assess the gibbsite morphology, and X-ray diffraction data were used to propose an orientation index for this mineral. The results indicated that there is little to no isomorphic substitution of Al by Fe in natural gibbsites, there is a prevalence of larger crystals, with a more laminar morphology, with orientation capability for the gibbsites of Amazon soils and bauxites, and smaller crystals tending to spherical shape for the other samples. These results suggest that natural gibbsite crystals of good crystallinity have large crystals with a laminar shape, while crystals of lower crystallinity present a smaller size and approximately an isodiametric shape.

Effects of Methanol on the Seeded Precipitation of Al(OH)3 from Potassium Aluminate Solution

AbstractThe effects of methanol on the seeded precipitation of Al(OH)3 from potassium aluminate solution are investigated. The solubility of Al(OH)3 decreases in the presence of methanol, leading to a faster crystallization and a higher precipitation ratio than the normal seeded precipitation process. Under the optional precipitation conditions of temperature 323.15 K, K2O concentration 170 g L−1, and addition of an equal volume of methanol to potassium aluminate solution, the precipitation ratio can exceed 85% within 7 h. Characterization of the Al(OH)3 products using X‐ray diffraction, scanning electron microscopy, and particle size distribution shows a typical gibbsite crystal phase composed of sheet‐like agglomerates with an average particle size of 74 µm. An interaction model between methanol molecules and Al(OH)3 crystal face is established and calculated by using the molecular dynamics method, which indicates the morphology transition mechanism of the Al(OH)3 crystal. The result indicates that methanol molecules would hinder the growth of the (002) crystal face, so that the (002) crystal face is eventually retained and becomes the main crystal face for the Al(OH)3 product. Infrared radiation and Raman spectra analyses suggest that the added methanol captures water molecules from the aluminate ion hydration sphere and thus catalyzes the crystallization process.

Sol–gel synthesis and characterization of lithium aluminate (L–A–H) and lithium aluminosilicate (L–A–S–H) gels

AbstractHydrous lithium aluminosilicate (L–A–S–H) and lithium aluminate (L–A–H) gels are candidate precursors for glass‐ceramics and ceramics with potential advantages over conventional processing routes. However, their structure before calcination remained largely unknown, despite the importance of precursor structure on the properties of the resulting materials. In the present study, it is demonstrated that L–A–S–H and L–A–H gels with Li/Al ≤ 1 can be produced via an organic steric entrapment route, while higher Li/Al ratios lead to crystallization of gibbsite or nordstrandite. The composition and the structure of the gels was studied by thermogravimetric analysis, X‐ray diffraction, 27Al and 29Si magic‐angle spinning nuclear magnetic resonance, and Raman spectroscopy. Aluminium was found to be almost exclusively in six‐fold coordination in both the L–A–H and the L–A–S–H gels. Silicon in the L–A–S–H gels was mainly in Q4 sites and to a lesser extent in Q3 sites (four‐fold coordination with no Si–O–Al bonds). The results thus indicate that silica‐rich and aluminium‐rich domains formed in these gels.

Geochemistry of a spheroidal weathering profile in a tropical mountainous landscape, Rio de Janeiro, Brazil

Boulders are an important material in debris flow and their source is coupled with spheroidal weathering profiles that produce corestones. The goal of this work was to establish the geochemical transformations that produced corestones and distinguished them from the surrounding grus in two tropical granite weathering profiles (P1 and P2). Sampling was not performed in a vertical profile; instead, we gathered 13 (P1) to 16 (P2) samples displaying different weathering degrees (corestone and saprolite) and spatial positions in the profiles. We conducted the geochemistry (EDXRF/EDX and INAA) and mineralogy (petrography and XRD) of the samples. The CIA values ranged from 46 (corestones) to 93 (saprolite). Granite spheroidal weathering under a tropical mountainous slope develops mostly due to feldspar weathering (foremost plagioclase) in the following sequence: porosity growth, kaolinite, and gibbsite crystallization. Zircon weathering stability and its probable mobility as grain along the weathering profile play an important role in REE concentration. Spheroidal weathering is mainly a lixiviation process, yet specific locations (below the individualized corestones) presented REE enrichment due to translocation. They are hosted mainly by clay minerals and, to some extent, by amorphous Fe oxyhydroxide. The evolution of spheroidal weathering results in a vertical patchy weathering profile.

Machine Learning Approach to Simulation of Continuous Seeded Crystallization of Gibbsite

Continuous seeded crystallization is characterized by oscillations of particle size distribution (PSD) and liquor productivity. To describe these oscillations using analytical methods is a complicated task due to non-linearity and slow response of the process. This paper uses a statistical approach to the preparation of initial data, determination of the significant factors and arrangement of the said factors by their impact on the dynamics of crystal population development. Various methods of machine learning were analyzed to develop a model capable of forecasting the time series of particle size distribution and composition of the final solution. The paper proposes to use deep learning methods for predicting the distribution of crystals by grades and liquor productivity. Such approach has never been used for these purposes before. The study shows that models based on long short-term memory (LSTM) cells provide for better accuracy with less trainable parameters as compared with other multilayer neural networks. Training of the models and the assessment of their quality are performed using the historical data collected in the hydrate crystallization area at the operating alumina refinery

Molecular-level understanding of gibbsite particle aggregation in water

Using molecular dynamics simulations, we investigate the molecular scale origin of crystal face selectivity when one gibbsite particle attaches to another in water. A comparison of the free energy per unit surface area of particle–particle attachment indicates that particle attachment through edge surfaces, where the edge surfaces are either (1 0 0) or (1 1 0) crystal faces, is more energetically favorable compared to attachment between two basal surfaces (i.e., (0 0 1) crystal faces) or between the basal surface of one particle and the edge surface of another. This result suggests that gibbsite crystals with low basal/edge surface area ratio will preferentially attach through edge surfaces, potentially helping the crystals grow laterally. However, for larger gibbsite particles (high basal/edge surface area ratio) the total free energy, not normalized by surface area, of particle attachment through the basal surfaces is lower (more negative) than attachment through the edge surfaces, indicating that larger gibbsite particles will preferentially aggregate through basal surface attachments. The short-range electrostatic interactions including the interparticle hydrogen bonds from surface –OH groups drive particle attachment, and the dominant contribution to the free energy minimum is enthalpic rather than entropic. However, the enthalpy of basal-edge attachment is significantly offset by the entropy leading to a higher free energy (less negative) compared to that of basal-basal attachment. Study of the free energy for a few imperfect attachments of two particles indicates a higher free energy (i.e., less negative, less stable), compared to a perfect attachment

Effect of oxalate on seed precipitation of gibbsite from sodium aluminate solution

The precipitation performance and kinetics of gibbsite from sodium aluminate solution with different sodium oxalate concentrations as well as the corresponding influence mechanism of oxalate during the seed precipitation process were systematically investigated by physicochemical properties test, using SEM and Raman spectra. As the concentration of sodium oxalate increases, both the precipitation rate and particle size of gibbsite decrease. The presence of sodium oxalate not only increases the viscosity of sodium aluminate solution, but also promotes the transformation of Al(OH)4- to Al2O(OH)62. The overall reaction rate constant decreases and the apparent activation energy of gibbsite increases with the increasing sodium oxalate concentration, the rate controlling step of which is chemical reaction. The needle-like sodium oxalate precipitates on the gibbsite crystals and covers the active Al(OH)3 seed sites, which leads to the lower precipitation rate and the finer particle size of gibbsite during the seed precipitation process.

In Situ 27Al NMR Spectroscopy of Aluminate in Sodium Hydroxide Solutions above and below Saturation with Respect to Gibbsite.

Aluminum hydroxide (Al(OH)3, gibbsite) dissolution and precipitation processes in alkaline environments play a commanding role in aluminum refining and nuclear waste processing, yet mechanistic aspects underlying sluggish kinetics during crystallization have remained obscured due to a lack of in situ probes capable of isolating incipient ion pairs. At a molecular level Al is cycling between tetrahedral ( T d) coordination in solution to octahedral ( O h) in the solid. We explored dissolution of Al(OH)3 that was used to produce variably saturated aluminate (Al(OH)4-)-containing solutions under alkaline conditions (pH >13) with in situ 27Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopy, and interrogated the results with ab initio molecular dynamics (AIMD) simulations complemented with chemical shift calculations. The collective results highlight the overall stability of the solvation structure for T d Al in the Al(OH)4- oxyanion as a function of both temperature and Al concentration. The observed chemical shift did not change significantly even when the Al concentration in solution became supersaturated upon cooling and limited precipitation of the octahedral Al(OH)3 phase occurred. However, subtle changes in Al(OH)4- speciation correlated with the dissolution/precipitation reaction were found. AIMD-informed chemical shift calculations indicate that measurable perturbations should begin when the Al(OH)4-···Na+ distance is less than 6 Å, increasing dramatically at shorter distances, coinciding with appreciable changes to the electrostatic interaction and reorganization of the Al(OH)4- solvation shell. The integrated findings thus suggest that, under conditions incipient to and concurrent with gibbsite crystallization, nominally expected contact ion pairs are insignificant and instead medium-range (4-6 Å) solvent-separated Al(OH)4-···Na+ pairs predominate. Moreover, the fact that these medium-range interactions bear directly on resulting gibbsite characteristics was demonstrated by detailed microscopic and X-ray diffraction analysis and by progressive changes in the fwhm of the O h resonance, as measured by in situ NMR. Sluggish gibbsite crystallization may arise from the activation energy associated with disrupting this robust medium-range ion pair interaction.

Hydrothermal process of synthetic gibbsite and the characteristics of Na in gibbsite crystal

The presence of Na in synthetic gibbsite was studied by several modern testing methods such as X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and inductively coupled plasma. Results show that Na is located between gibbsite crystals and is not present as sodium aluminosilicate hydrate in synthetic gibbsite. Synthetic gibbsite crystal can be transformed into boehmite under the hydrothermal condition of 210 °C for 60 min. During the reaction process, synthetic gibbsite crystal particles break up into small fragments and form boehmite. During the process, impurities are released and the Na can be removed by washing. Over 90% of Na can be removed during the hydrothermal transformation process. This technology can be applied in the production process of high-purity alumina.

Mineral structure and crystal morphologies of high-iron hydrargillite

Various characterization methods, including scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, Brunauer–Emmett–Teller surface-area measurements, thermogravimetry–differential scanning calorimetry, X-ray diffraction, and infrared spectroscopy, were used to study the mineral structure and surface characteristics of high-iron hydrargillite. Gibbsite, goethite, and hematite were found to be the main mineral components of hydrargillite, whereas the goethite and hematite were closely clad to the surface of the multilayer gibbsite crystals. Compared with the synthetic gibbsite, the hydrargillite contained more structural micropores generated by the mineral evolution during the mineralization process. The gibbsite in hydrargillite contained less crystal water compared with the synthetic gibbsite, and it was a typical polymorphic structure. The isomorphous substitution of Al and Fe was observed in goethite. The dissolution-controlling step of hydrargillite was the ionic diffusion speed because of the goethite and hematite that closely covered and encapsulated the gibbsite crystals.

Transitions in Al Coordination during Gibbsite Crystallization Using High-Field 27Al and 23Na MAS NMR Spectroscopy

Mechanisms of nucleation and growth of Al hydroxides such as gibbsite from aqueous solution, particularly in highly alkaline conditions, remain poorly understood. In this work, quantitative 27Al and 23Na MAS NMR experiments were conducted on solid samples extracted from the crystallization of gibbsite from an amorphous aluminum hydroxide gel precursor. The use of a high magnetic field and a moderate sample spinning rate of 20 kHz allowed transitional tetrahedral (AlT) and pentahedral (AlP) aluminum species to be observed along with the octahedral aluminum (AlO) that dominates the gibbsite product. Low-coordinated Al species could be detected at concentrations as low as 0.1% of the total Al sites. The following results have been established: (a) AlT and AlP coexist on the surface of growing gibbsites even with a combined percentage over the total Al sites of less than 1%. (b) Different synthesis methods generate gibbsite with varying amounts of low-coordinated Al. (c) The amorphous gel precursor contains a...

PGA Fiber Reinforced Calcium Aluminate Cement for Orthopaedic Application

Calcium aluminate cement (CAC) is a penitential candidate for bone replacements with good bioactivity but relative lower strength. In this study, biodegradable PGA fiber was incorporated into the CAC paste in order to improve the strength of the material. And MC3T3 cells were seeded on the surface of CAC and CAC/fiber to study their in vitro biocompatibility. The results indicate that the PGA fiber can improve the compressive strength of CAC without changing the crystalline phases and micromorphology. Calcium aluminate oxide hydrate, katoite and Gibbsite crystals were detected by XRD. Plate-like crystals can be observed under FESEM. The MC3T3 cells were attached well on both CAC and CAC/fiber composite, indicating their good in vitro biocompatibility. In summary, fiber reinforcement can be an effective way to improve the properties of calcium aluminate cement for orthopaedic application.

Crystallization of gibbsite from synthetic chromate leaching solution in sub-molten salt process

For the clean and economical production of chromium compounds, it is crucial to remove aluminates from chromate alkali solutions and utilize aluminum-containing compounds. In this work, carbonization was used to remove aluminates from a synthetic chromate leaching solution containing a high K2O/Al2O3 mole ratio. The influence of reaction temperature, carbonization time, flow rate of carbon dioxide, and seed ratio on the precipitation of Al was investigated. The optimal output was obtained under the following experimental conditions: a reaction temperature of 50 °C, a carbonization time of 100 min, a carbon dioxide flow rate of 0.1 L/min, and a seed ratio of 1.0. Gibbsite was obtained following carbonization. The structure and morphology of the gibbsite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and laser particle size analyzer. The particle size distribution and morphology of the gibbsite were significantly influenced by the experimental conditions. The gibbsite had a mean particle size (d50) of 16.72 μm. The thermal decomposition of the gibbsite was analyzed by XRD and the decomposition path was determined. The obtained coarse α-Al2O3 precipitate, which contains 0.08% Cr2O3 and 0.10% K2O, was suitable for subsequent utilization.

Specific features of the phase transition of gibbsite into boehmite under hydrothermal treatment of floccules in an aqueous suspension

Effect of the conditions in which floccules of gibbsite are hydrothermally treated on the phase composition of products of its dehydration products, obtained at T = 180–210°C and P = 1.0–1.9 MPa was studied. The phase transition of gibbsite into boehmite occurs by the dissolution–deposition mechanism upon delamination of gibbsite crystals along the (001) plane to give a multitude of layers with thicknesses of 20–100 nm and cracks in between, with widths of 10–50 nm. In the dissolution of gibbsite, [Al(OH)4]– anions pass into solution and react with protons of hydroxy groups on the (001) planes of gibbsite, with the subsequent nucleation of boehmite and growth of its crystals. The crystallization of coarse boehmite particles favors formation of nonporous floccules. Boehmite particles form no strong crystallization bonds with each other, which impairs their abrasion resistance.

Features of phase formation and phase distribution in the dehydration of coarse gibbsite floccules

A complex study of the phase composition and phase distribution in coarse floccules of the dehydration products of gibbsite thermally treated at atmospheric pressure in air at 250–500°C was performed. The dehydration of gibbsite floccules generates coarsely and finely crystalline boehmite phases. Finely crystalline boehmite is formed by partial “fragmentation” of gibbsite crystals. The layer of coarse boehmite crystals surrounds a finely crystalline boehmite core. χ-Al2O3 predominantly crystallizes on the outer surface of gibbsite particles that contacts with the environment.