Supersaturated Aluminate Solutions Research Articles

Experimental investigation and CFD simulation of liquid–solid–solid dispersion in a stirred reactor

For understanding the monosodium aluminate hydrate crystallization from the supersaturated aluminate solution containing red mud as the leaching liquor of bauxite, the liquid–solid–solid dispersion of a simulant system, i.e. glycerite, red mud and sand, in a stirred reactor has been experimentally investigated as well as simulated using computational fluid dynamics model (CFD) for the first time. The computational model is based on the Eulerian multi-fluid model along with RNG k– ε turbulence model, where Syamlal–obrien-symmetric drag force model ( Syamlal, 1987) of the inter-phase momentum transfer between two dispersed solid phases is taken into account. A good agreement is obtained between the experimental data of solid distributions and the simulation results in the flow fields of liquid–solid–solid as well as liquid–solid systems. The solid suspension qualities of both liquid–solid and liquid–solid–solid systems in the stirred reactors with and without draft tube were also studied in detail based on mixing time, the standard deviation of solid concentration proposed by Bohnet and Niesmak (1980), the flow pattern and power number. The influence of the interaction between two dispersed solid phases on the suspension of red mud is found significantly greater than that of sand. The holdup of sand below the impeller is considerably larger than that above the impeller and the red mud dispersion approaches homogeneous in the reactor. The mixing time of liquid–solid–solid suspension is longer than that of liquid–solid suspension under the same conditions, and the mixing times of both systems in the stirred reactor with draft tube are longer than that in the reactor without draft tube. Furthermore, the distributions of sand and red mud in the reactor with draft tube were found less homogeneous than those without draft tube in most cases.

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.

Effects of temperature and initial molar ratio of Na2O to Al2O3 on agglomeration of fine Al(OH)3 seed in synthetic Bayer solution

Fine Al(OH)3 crystals were aggregated from supersaturated aluminate solution in the batch reaction tanks. By means of laser particle size analyzer and scanning electron microscopy, the influences of temperature and initial molar ratio of Na2O to Al2O3 (α K) on agglomeration of fine seed in Bayer process were investigated. The results show that agglomeration is almost finished in 8 h, and seeds with size less than 2 μm are easily aggregated together, and almost disappear in 8 h under the optimal process conditions. In the aluminate solution with the same moderate initial α K, when the reaction temperature reaches 75 °C, the secondary nucleation does not occur, and the effect of agglomeration is better. And at the same reaction temperature, when the initial α K is 1.62, the initial supersaturation of aluminate solution is moderate, the binders on the surfaces of the seed are enough to maintain the agglomeration process, and the agglomeration degree is better. From SEM images, agglomeration mainly occurs in the fine particles, the combinations among the fine particles are loose and the new formed coarse crystal shapes are irregular.

The synthesis by deposition–precipitation of porous γ-alumina catalyst supports on glass substrates compatible with microreactor geometries

A coating technique based on the deposition–precipitation of aluminium hydroxide from supersaturated aluminate solutions was applied to the preparation of catalyst supports for application in a glass microreactor. The porous alumina layer obtained after calcination of the deposited hydroxide was characterised by BET, SEM and XRD techniques. Pt/Al 2O 3 catalysts were prepared using the deposited porous alumina as support. The microreactor coating obtained demonstrated the adhesion to the glass substrate compatible with liquid/liquid liquid/gas application and good compatibility with the complex microreactor shape. The phase composition and specific surface area of the coating are very similar to a more conventional powdered γ-alumina catalyst support.

Supersolubility and induction of aluminosilicate nucleation from clear solution

The supersolubility of silica in supersaturated aluminate solution and the sodium aluminosilicate precipitation nucleation mechanism from the clear solution were investigated by experiment. The obtained semi-empirical expression of the induction (t(ind)) of sodium aluminosilicate precipitation from clear solution is significant for zeolite A membranes synthesis, desilication and. more important for valuating the stability of the supersaturated SiO2 in the aluminate solution during the. hydrolysis operation in the - most used alkaline alumina manufacture. The semi-empirical expressions of the silica supersolubility in clear supersaturated sodium aluminate solution for both homogeneous and heterogeneous nucleation were originally conducted. The homogeneous nucleation predominated with the supersaturation (S) above 6.0, conversely, heterogeneous nucleation with S below 4.0. The inferred activation energy (Ea) of the homogeneous nucleated reaction was 14.9 kJ/mol, and that of the heterogeneous 50.5 kJ/mol. The interfacial tension (gamma(s)) between the precipitate of sodium aluminosilicate and the surrounding aqueous solution was obtained at 15.1 mJ/m(2), negligibly decreased with increasing temperature. It was much larger than the value of 12.4 mJ/m(2) in pure water (J. Phys. Chem. B 105 (2001) 5383), but very much smaller than the estimated value of 38.64-50mJ/m(2) in the solution for traditional zeolite synthesis. (C) 2003 Elsevier Science B.V. All rights reserved.

ChemInform Abstract: Synthesis of Lithium Dialuminate by Salt Imbibition.

AbstractLiAl2(OH)7·2 H2O (I), prepared by reaction of solid lithium hydroxide, polycrystalline bayerite, and water vapor at 20 °C appears to be identical with that prepared by precipitation from supersaturated aluminate solutions or by reaction of lithium salts with freshly precipitated aluminum hydroxide in aqueous solution (TGA, IR, XRD).

Precipitation from supersaturated aluminate solutions. IV. Influence of citrate ions

The influence of citrate on the precipitation in unseeded and seeded supersaturated aluminate solutions is reported. At high Cit/Al ratios (>0. 1) extensive complex formation takes place but already at low ratios (10 −4–10 −2), where this effect hardly changes the supersaturation, the precipitation rate is strongly retarded. The formation of bayerite appears to be more sensitive to citrate concentration than that of pseudoboehmite and can even be prevented at rather low Cit/Al ratios (∼0.01). Transformation of pseudoboehmite into bayerite is also inhibited at these low Cit/Al ratios. Citrate may therefore be used to selectively precipitate pseudoboehmite. A simple adsorption model is introduced to explain the role of citrate in retarding growth. Evidence is presented to show the dual role played by citrate in the nucleation of bayerite as revealed by long time lags in relaxation experiments. More nuclei are generated with increasing citrate concentration but the growth of these particles is slowed down.

Precipitation from supersaturated aluminate solutions. III. Influence of alkali ions with special reference to Li +

The precipitation of Al(OH) 3 from supersaturated aluminate solutions ( C Al = 4 × 10 −3 M) in the presence of Li +, Na +, and K + is compared. In the presence of Na + and K + at 25°C the same precipitation behavior is observed. The precipitation sequence, amorphous phase → pseudo-boehmite → bayerite, is dictated by the supersaturation (pH + pAl) and the growth rate of bayerite is found to depend on the available surface area and on the square of the supersaturation. In the presence of Li + ions a selective and drastic acceleration of the precipitation rate of bayerite is observed at 25 and 50°C. Pseudo-boehmite plays a minor role in the overall precipitation process because lithium not only selectively accelerate bayerite formation but also increase the rate of transformation of pseudoboehmite to bayerite. The discontinuous nature of the acceleration of bayerite formation strongly suggests that a fast nucleating lithium aluminate phase acts as a precursor for the precipitation and growth of bayerite. From an analysis of the measured relaxation times a variable composition of the precursor phase is deduced with a Li Al ratio ranging from 0.2 to 0.4. At high solution Li Al ratios and low supersaturations a distinct phase of composition LiA1 2(OH) 7 · 2H 2O is observed.

Precipitation from supersaturated aluminate solutions. II. Role of temperature

The effect of temperature on the precipitation of aluminum hydroxide from dilute potassium aluminate solutions ( C Al(OH) 4 = 4 × 10 −3 M) was studied in acid titration and pH-stat experiments. The precipitation sequence is largely dictated by the supersaturation (II) and follows the Ostwald rule of stages: amorphous → pseudoboehmite → bayerite → gibbsite. The precipitation boundaries, -log a Al(OH) 4 − a OH − , of the amorphous phase (−1.9) and pseudoboehmite (−1.2) evaluated by interpretations of relaxation time curves are temperature insensitive. The interpretation of relaxation curves in the supersaturation region where pure bayerite forms is based on a surface nucleation mechanism and allows estimates of the free energy of formation of the critical nucleus (∼20 kJ/mole), interfacial tension and two activation energies (−30 kJ/mole, +95 kJ/mole). The preferred formation of pseudoboehmite at high pOH is shown to be the result of a lower interfacial energy and the retarding effect it exerts at all temperatures on the competing growth of bayerite crystals. The growth rate of bayerite is shown to be proportional to the available surface and the square of the supersaturation and is characterized by an activation energy of about 60–80 kJ/mole.

Precipitation from supersaturated aluminate solutions: I. Nucleation and growth of solid phases at room temperature

The precipitation of aluminum hydroxide from dilute potassium aluminate solutions (10−4 M < C Al < 16 × 10 −4 M) was studied in acid titration and pH-stat experiments. In the relatively fast titrations a pseudo-equilibrium is obtained by formation of an amorphous phase. The precipitation behavior in pH-stat relaxation experiments is largely dictated by the supersaturation (II). At high supersaturations the precipitation sequence is amorphous-pseudoboehmite-bayerite and at low II only bayerite forms. Measurements of relaxation times reveal the precipitation boundary (log α H+ · α Al(OH) − 4 ) for the amorphous phase (−12.1 ± 0.05) and for pseudoboehmite (−12.7 ± 0.1). The remarkable extrema in the relaxation time vs supersaturation curves are shown to indicate the retardation of the nucleation and growth of bayerite by heterogeneous nucleation of pseudoboehmite on the bayerite surface. A simple theoretical analysis of the dual role of pH, in determining both the supersaturation and the interfacial tension, shows that drastic changes in nucleation behavior can occur at pH values about 4 units away from the point-of-zero charge. The growth rate of bayerite in seeded and unseeded pH stat experiments is found to be second order in the aluminate concentration and somewhat less than second order in the hydrogen ion concentration. A brief discussion is given of a number of growth models which are consistent with the experimental growth rate expression.