Supersaturated Sodium Aluminate Solutions Research Articles

Preparation of Near-Spherical α-Al2O3 Particles by Enhanced Precipitation of Gibbsite from Sodium Aluminate Solution Using Highly Dispersed Al2O3 as Seeds.

The near-spherical α-Al2O3 particles (∼200 nm) were prepared for the first time through the precipitation of gibbsite from a supersaturated sodium aluminate solution, with the addition of an acidic solution containing highly dispersed nanosized Al2O3 seeds. The near-spherical α-Al2O3 were achieved by employing seeds at 1100 °C, with the initial nucleation temperature being only 900 °C, due to the highly homogeneous dispersion of the seeds into gibbsite formation by in situ growth that can effectively enhance the role of the seeds in the phase transition. Moreover, the precipitation rate of sodium aluminate solution was significantly accelerated by 17-fold. This acceleration was due to the slightly acidic solution breaking the stability of sodium aluminate solution through a neutralization reaction on the seed surface, leading to the rapid formation of gibbsite nuclei on the seed surface. Our findings provide a feasible approach for the fabrication of well-dispersed α-Al2O3 particles and make the industrial production of well-dispersed α-Al2O3 from sodium aluminate solution a commercial reality.

Preparation of CaCO3/Al(OH)3 Composites via Heterogeneous Nucleation.

As one of the most widely used inorganic fine powder fillers, calcium carbonate is cheap. However, considering its poor light transmittance, it is not suitable to be added to resin matrix composites that require high light transmittance. Aluminum hydroxide has good light transmission and flame retardancy, but it is more expensive than calcium carbonate. CaCO3/Al(OH)3 composites with a core-shell structure that showed a trend toward the performance of aluminum hydroxide not only improved the surface properties of CaCO3, but also increased the added value of CaCO3. In the present paper, CaCO3/Al(OH)3 composites were successfully prepared in sodium aluminate solution via heterogeneous nucleation. Four types of calcium sources, including calcite-type precipitated calcium carbonate, vaterite-type precipitated calcium carbonate, ground calcium carbonate with two different particle sizes as the precursors and supersaturated sodium aluminate solution as the substrate, have been deeply investigated in terms of their influence on the preparation of CaCO3/Al(OH)3 composites. Results showed that the calcium carbonate precursor greatly affected the formation of CaCO3/Al(OH)3 composites. Both the precipitated calcium carbonate and the small particle ground calcium carbonate are likely to undergo anti-causticization and a complexation reaction with it to generate 3CaO·Al2O3·6H2O and 3CaO·Al2O3·CaCO3·11H2O, which go against the coating of calcium carbonate with aluminum hydroxide. Within the experimental range, the use of ground calcium carbonate with a particle size of 400-500 mesh is more suitable as a precursor for the preparation of core-shell CaCO3/Al(OH)3 composites.

The effect of mechanical and thermal treatments on the dissolution kinetics of kaolinite in alkaline sodium aluminate solution under conditions typical to Bayer desilication

The kinetics of dissolution of kaolinite in industrial spent liquor followed by the precipitation of the desilication product was studied by using calcined and dimethyl sulfoxide intercalated Eckalite1 as starting material. These studies were aimed at understanding the influence of the combination of different pre-treatments on the variations in the maximum silica concentrations and shift (or in some cases, the disappearance) of the trigger point in the Bayer desilication process. In addition to the quantitative characterization of the dissolution kinetics, a variety of physico-chemical parameters of the kaolinites, like the specific surface area, Lewis acidity, hydrodynamic diameter, zeta potential, optical band gap and thermogravimetric behaviour was also determined. From the experimental data, it can be concluded that the thermal/mechanochemical dehydroxylation and the formation of metakaolinite/mechanochemically amorphized kaolinite are responsible for the increase in the concentration of the dissolved silica in the supersaturated sodium aluminate solution. The variation in the position of the trigger point could be connected to the deformation, fragmentation/delamination, and amorphization of the kaolinite structure evolved during the mechanochemical pre-treatment. The joined application of pre-calcination and pre-milling verified the conclusions drawn. In addition, the combined treatment can be optimized to end up with a dissolution process that is carried out in a time and energy saving manner. The extent of the desilication products obtained at the end of the dissolution tests showed a considerable dependence on the applied pre-treatments: the formation of the metakaolinite phases was found to facilitate the evolution of the ball of yarn-type lepispherical structures with sizes ranging from 600 to 3000 nm.

Influences of New Synthesized Active Seeds and Industrial Seed on the Aluminum Hydroxide Precipitation from Sodium Aluminate Solution

The aluminum hydroxide precipitation process from sodium aluminate solution was studied by adding the new synthesized active seeds and industrial seed separately. The results show that the precipitation rate reached 47, 81, and 68% at 60 h by adding industrial seed, seed A, and seed B (active seeds), respectively. Specific surface area, composition, morphology, and particle size distribution of seeds/precipitated products from supersaturated sodium aluminate solutions were investigated by Brunauer, Emmett, and Teller (BET) analysis, X-ray crystallography (XRD) analysis, Scanning electron microscope (SEM) images, and Power-spectral-density (PSD) analysis. Also, to study and understand the functional groups on the surface of seed/precipitated products and precipitation mechanisms, Fourier-transform infrared spectroscopy (FTIR) analysis was recorded. The result showed that the precipitation rate of the active seeds is higher than that of the industrial seed owing to the higher surface area, more active sites, crystal defects, and finer particle size that cause rise to nucleation for gibbsite. Also, improved precipitation of aluminum hydroxide from sodium aluminate solution may be due to the predominant phase crystallized of bayerite in the composition of active seeds. The FTIR analysis shows that the decline of the precipitation process in the presence of industrial seed may be due to oxalate–Al bonds and the impurity of carbonate.

Precipitation of spherical boehmite from concentrated sodium aluminate solution by adding gibbsite as seed

The precipitation of spherical boehmite was studied by surface energy calculations, measurements of precipitation ratios, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The surface energy calculation results show that the (001) and (112) planes of gibbsite surfaces are remarkably stable because of their low surface energies. In addition, the (010) plane of boehmite grows preferentially during precipitation because of its low surface energy. Thus, we propose a method to precipitate spherical boehmite from a supersaturated sodium aluminate solution by adding gibbsite as seed in a heterogeneous system. In this method, gibbsite acts as the preliminary seed and saturation modifier. The results show that the fine boehmite first nucleates on the (001) and (112) planes of gibbsite and then grows vertically on the (001) and (112) basal planes of gibbsite via self-assembly, thereby forming spherical boehmite. Simultaneously, gibbsite is dissolved into the aluminate solution to maintain the saturation for the precipitation of boehmite. The precipitation ratio fluctuates (forming an M-shaped curve) because of gibbsite dissolution and boehmite precipitation. The mechanism of boehmite precipitation was further discussed on the basis of the differences in surface energy and solubility between gibbsite and boehmite. This study provides an environmentally friendly and economical method to prepare specific boehmite in a heterogeneous system.

Kinetics investigation on the decomposition process of supersaturated sodium aluminate solution using UV spectroscopy

The kinetics of the spontaneous decomposition process of supersaturated sodium aluminate solution was investigated by UV spectroscopy. The UV spectra show that the absorbance increased with time at the initial decomposition stage. At 369nm, the concentration of the intermediate reached the maximum after decomposition for 9h. The spontaneous decomposition process from sodium aluminate solution was considered as the consecutive complicated reaction and the kinetic model was established. The rate constants of the two elementary reactions were calculated as k1=0.02447h−1 and k2=0.3046h−1 respectively. The concentration of Al-containing species with time was also predicted in the decomposition process of supersaturated sodium aluminate solution.

Controllable Synthesis of Homogeneous Mesoporous Alumina by Reaction Crystallization

Homogeneous mesoporous alumina (HMA) was synthesized by reaction–crystallization from supersaturated sodium aluminate solution and sodium bicarbonate solution, using methylcellulose as a template. The novel HMA material showed properties of mesoporous alumina and sandy alumina. Effects of synthesis conditions on the structure and characteristics of the HMA material were investigated. The mechanism of the synthesis process for HMA material was revealed.

Homogeneous nucleation of Al(OH)3 crystals from supersaturated sodium aluminate solution investigated by in situ conductivity

The homogeneous nucleation process of Al(OH)3 crystals from supersaturated sodium aluminate solution was investigated by a homemade online conductance testing system. Combined with the turbidity and equivalent circuit of conductometry, the conductance curve was divided into five regions, where the charge and discharge region, adsorption region and ionic polymerization region were identified as induction period in the decomposition process of sodium aluminate solution. According to the homogenous nucleation theory, the nucleation kinetics during the decomposition process was studied with the intermediate concentration solution at 333K. The results showed that the solid–liquid interfacial energy was 26.1mJ·m−2, critical nucleation radius was 2.6–3.8nm and the apparent reaction order was 2.9. Meanwhile, UV–vis spectra of sodium aluminate solution in homogeneous process were investigated with decomposition time, where three peaks were observed at 220, 260, and 370nm, respectively. Combined with the conductance curve, the ionic structure change and nucleation mechanism have been discussed.

Reactive extraction of sodium hydroxide from alkali solutions for the separation of sodium and aluminum—Part II. Effect of diluent

Enhancing the precipitation of Al(OH)3 from supersaturated sodium aluminate solution has been a challenge for the alumina industry for decades in its effort to increase the productivity. However, limited by the thermodynamic equilibrium of alumina solubility in caustic solutions, the processes that are available to date for promoting the precipitation of Al(OH)3 are unsatisfactory. An acid–base reactive solvent extraction method is proposed to enhance the separation of sodium hydroxide from sodium aluminate mother liquor by creating a condition for re-precipitation. The extraction process has been reported in Part I of this study by the present authors. In this paper the effects of diluents on sodium extraction when using 2-tert-butyl-4-methylphenol as the main organic extractant were systematically investigated. According to the classification of diluents, the capabilities for extracting sodium in different types of diluents were compared, and the protonic polar diluents were identified as being the most effective diluent. In the absence of alkyl phenol organic extractant, the sodium extraction equilibrium in 1-octanol and 2-ethyl hexanol, and the co-extraction of water in each of them were compared. In the presence of alkyl phenol, the extraction capabilities for sodium in different C8 alcohols, including 1-octanol, 2-ethyl hexanol and 2-octanol, were compared and analyzed. The results showed that 1-octanol is the most effective diluent among the three that are studied in this project. Furthermore, inert diluents including kerosene, octane and heptane, were used to dilute the pure 1-octanol solvent in different proportions in order to decrease the viscosity of the organic phase. The effects of the inert diluent on sodium extraction and water co-extraction were studied. Finally, the particle size distributions in the equilibrated organic phases and the Margules ternary 2-suffix equations were introduced to discuss the mechanisms of sodium extraction by 1-octanol and 2-ethyl hexanol. The results of this systematic research are expected to provide valuable guidance for the selection of diluents.

Growth Mechanisms and Kinetics of Gibbsite Crystallization: Experimental and Quantum Chemical Study

We present experimental and molecular modeling studies, which examine the influence of crystal growth modifiers (CGMs) on the crystallization mechanism and kinetics of gibbsite crystallization in supersaturated sodium aluminate solutions. Seeded, pure caustic sodium aluminate synthetic liquors and real industrial Bayer liquor containing significant amounts of organic and inorganic impurities, in the absence and presence of a CGM containing oleate ions, were investigated. Our results show massive growth of colloidal sized protrusions on the seed particle surfaces on addition of the CGM in the pure synthetic caustic aluminate liquors. Although the activation energies for both secondary nucleation and growth were found to be increased, the overall crystallization rate was surprisingly also increased by addition of CGM. This behavior is attributed to the increased pre-exponential factor possibly arising from higher probability of effective collisions due to more effective species orientation facilitating the crystallization process. The crystallization rate was significantly slower in the industrial Bayer liquor, most likely due to the presence of impurities; nevertheless, a greater degree of surface protrusions on crystal growth was noted on addition of the CGM. In conjunction with the experimental results, we use semiempirical quantum-chemical calculations to examine the underlying processes that result in the enhanced growth due to the presence of the CGM. Our quantum-chemical modeling results suggest that the increased crystallization rate may be due to a mechanism by which Al(III)-containing dimers are produced due to intervention by the oleate ion.

Quartz crystal microbalance as a tool for scale deposition monitoring in Bayer process conditions

This paper describes the study of bayerite or gibbsite scale formation and growth from supersaturated sodium aluminate solutions using a quartz crystal microbalance. Analysis of frequency vs time curves and scanning electron microscopy images allowed us to propose a surface nucleation mechanism that leads to cementation of particles produced by catastrophic secondary nucleation. We also highlighted the influence of the filtration of the supersaturated sodium aluminate solution step on the scale nucleation kinetics. It was observed that use of filter with porosity below 2.5μm delays the formation of scale.

Cleaner synthesis of mesoporous alumina from sodium aluminate solution

Mesoporous alumina (MA) has attracted much attention because of its potential industrial applications as catalyst and adsorbent. The synthesis of MA is performed commonly from organic aluminium alkoxide in non-aqueous media such as alcohol, although this process is more expensive than its inorganic synthesis. However, to date, MA inorganic synthesis processes have been environmentally unfriendly and generate large masses of worthless salt by-product. A cleaner process for the synthesis of MA from supersaturated sodium aluminate solution by the neutralization of sodium bicarbonate, using Cetyltrimethyl Ammonium Bromide (CTAB) as a template, is proposed in this research. The sodium carbonate by-product from this process can be readily recycled by carbonation with carbon dioxide to prepare the sodium bicarbonate reagent solution. This novel process can be integrated into the commonly and widely used sintering process in the alumina industry, i.e., part of the sodium carbonate by-product can be used to extract alumina from bauxite to make sodium alumina solution through sintering. The prepared MA was characterized as hydrothermally stable and short-range ordered mesoporous γ-alumina with controllable pore size and qualified modality. The cleaner and economical synthesis of MA from inorganic sources can significantly promote its application in industry.

Adjustment on gibbsite and boehmite co-precipitation from supersaturated sodium aluminate solutions

Gibbsite is the usual precipitation product from alumina refineries with either Bayer or sintering process. However, the advantage of boehmite precipitation over gibbsite precipitation is the significant energy saving in the subsequent calcination step. The current investigation takes a pragmatic approach to measure precipitation ratios, determine product phase, morphology and particle size distribution, and assess the impacts and adjustment capability of main parameters such as seed, temperature, ethanol medium, and supersaturation on the precipitation kinetics and alumina hydrate type during co-precipitation process. The results clarify that gibbsite and boehmite both can be precipitated from supersaturated sodium aluminate solutions simultaneously, and the competitive formation between Al(OH) 3 and γ-AlOOH determines the main precipitate phases from pregnant liquor. Boehmite seeds, high temperature and ethanol addition can promote the boehmite precipitation and improve the mass fraction of boehmite in products. Co-precipitation changes the multimodal distribution of seeds to a normal and well distribution of products, and the particle size is more than several times that of seeds.

Nucleation and Morphology of Monosodium Aluminate Hydrate from Concentrated Sodium Aluminate Solutions

The induction time for monosodium aluminate hydrate (MAH) crystallization from supersaturated solution at 333 to 373 K is systematically investigated by experiment and the primary nucleation is also identified according to the classical nucleation theory. The truncated quadrangular orthopyramid and the twin/composite truncated pyramid crystals of MAH except for the tabular crystals and octagonal platelets obtained in the research are presented for the first time; furthermore, the supersaturation and temperature of the nucleation are experimentally studied. The interfacial free energies γ of truncated pyramid and octagonal platelet MAH crystals in the supersaturated sodium aluminate solution are conducted, and then the octagonal platelet MAH crystal is considered more stable in the solution than the truncated pyramid crystals. The two-dimensional (2D) mediated growth mechanism for MAH crystallization is suggested based on the regression of experimental data of nucleation.

Precipitation of Al(OH) 3 crystals from supersaturated sodium aluminate solution irradiated with ultrasonic sound

The influence of ultrasonic irradiation on precipitation of Al(OH) 3 crystals from supersaturated aluminate solution has been investigated under batch condition of isothermal crystallization. The results indicated that, the precipitation rate was greatly increased by ultrasonic irradiation. Compared with general process, the time of precipitation ratio from 0% to 45% was reduced about 15 h under the irradiation of 20 kHz, and also 33 kHz. The effect of ultrasound on the precipitation process varies with ultrasound frequency. Irradiation of 33 kHz ultrasound shortens the induction period and increases the second nucleation, while 20 kHz enhance agglomeration and growth of Al(OH) 3 which result in the increasing of the average diameter of final product by around 5 μm. Raman spectra and UV-absorbance analysis indicated that ultrasound is beneficial to the formation of sodium–aluminate ion pair, which will increase the decomposition rate of supersaturated aluminate solution.

Precipitating sandy aluminium hydroxide from sodium aluminate solution by the neutralization of sodium bicarbonate

In the sinter process for alumina production, it is still difficult to produce sandy aluminium hydroxide by the carbonation of sodium aluminate solution. Therefore, a precisely controllable and feasible process for the precipitation of sandy aluminium hydroxide from the supersaturated sodium aluminate solution neutralized by sodium bicarbonate aqueous solution is proposed for the first time, in which the neutralization reagent (NaHCO 3) can be efficiently recycled by the carbonation of the produced sodium carbonate aqueous solution. The influencing factors of the precipitation, i.e. the pH value, the initial NaOH concentration, the temperature and the addition rate of neutralization agent, were investigated. All the aluminium hydroxides precipitated in the research were identified as gibbsite type. The gibbsite products had regular globular morphology and narrow size distribution whose minimum residue on a 44 µm mesh was 92–94%.

Preparation of a nano-sized α-Al 2O 3 powder from a supersaturated sodium aluminate solution

Nano-sized α-Al 2O 3 particles with an average size of 30–40 nm were prepared successfully from supersaturated sodium aluminate solution with liquid-attached aluminum hydroxide and nano-sized α-Al 2O 3 as seeds as well as a reasonable amount of PEG20000 as surfactant. The powders were characterized by differential scanning calorimetry (DSC)/thermogravimetry (TG), scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that liquid-attached Al(OH) 3 and nano-sized α-Al 2O 3 seeds can accelerate the precipitating process and reduce the α-Al 2O 3 particles size.

Investigation of the Homogeneous Nucleation of Al(OH)<SUB>3</SUB> Crystals from Supersaturated Sodium Aluminate Solution by Conductance Probe

Investigation of the Homogeneous Nucleation of Al(OH)<SUB>3</SUB> Crystals from Supersaturated Sodium Aluminate Solution by Conductance Probe

Effect of temperature and alumina/caustic ratio on precipitation of boehmite in synthetic sodium aluminate liquor

In Bayer process gibbsite (alumina trihydrate, Al 2O 3·3H 2O) is precipitated from sodium aluminate liquor for production of alumina by calcination. Precipitation of boehmite or alumina monohydrate (Al 2O 3·H 2O) is an alternative method for production of alumina. Boehmite is also a good precursor material for production of different activated aluminas. At present this boehmite precipitation process is in the early stage of development. This process if developed would reduce the energy consumption for making alumina. The present work is aimed to carry out boehmite precipitation under atmospheric pressure conditions. Parameters such as alumina/caustic (A/C) ratio and temperature are varied to precipitate out boehmite from synthetically prepared sodium aluminate liquor. It was found that boehmite precipitation could be possible at normal pressure only when boehmite seed was added to the supersaturated sodium aluminate solution keeping the alumina/caustic ratio at either 1.1 or 1.0 and temperature at ≥ 85 °C. By decreasing the A/C ratio to 0.95 and below, it was possible to precipitate out boehmite at 60 °C.

Selective electrochemical reactions of an alumina hydrate crystallization inhibitor

Selective reactions of catechol have been studied in regard to its inhibition properties towards alumina hydrate crystallization. Electrochemical reactions of the inhibitor leading to products without inhibition behaviour have been obtained using different conditions. First, a pure sodium hydroxide solution (1.0 M NaOH) was used to electrochemically react the catechol. The inhibitor reactions were then carried out in both saturated and supersaturated sodium aluminate solutions (NaAl(OH)4). Electrocatalytic hydrogenation (ECH) of the aromatic ring over rhodium-based catalysts leads to the saturated non-inhibiting molecule, 1,2-cyclohexanediol. Temperature shows no significant influence on the ECH process within the temperature range where the alumina hydrate crystallization is normally carried out. The presence of aluminate ions in solution has, however, a detrimental effect on the current efficiency. Electro-oxidation of the catechol produces muconic acid by cleavage of the carbon-carbon bound. It also produces other oxidation products and, eventually, may lead to total mineralization of catechol into carbonate, which is a less inhibiting molecule. A detrimental effect of the presence of aluminate on the electro-oxidation is also observed. Electrochemical techniques are proposed as new pathways for organic control in caustic medium.