Article pubs.acs.org/crystal
Growth Mechanisms and Kinetics of Gibbsite Crystallization: Experimental and Quantum Chemical Study Jun Li,† Jonas Addai-Mensah,‡ Alagu Thilagam,*,† and Andrea R. Gerson*,† †
Minerals and Materials Science & Technology, Mawson Institute, Division of ITEE, and ‡Ian Wark Research Institute, Mawson Lakes Campus, University of South Australia, South Australia 5095 ABSTRACT: 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.
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tion growth mechanism.11−13 This surface growth was observed to happen readily at high supersaturation at specific surface sites where the nucleation barrier is lowered by adsorbed impurities, thus resulting in reduced free energy.14 The crystallization kinetics were seen to be strongly temperature dependent with the high nucleation activation energies ranging from 87 to 160 kJ mol−1.12,15,16 Unlike the nucleation kinetics, the gibbsite crystal growth rate has been widely reported to have second-order relative supersaturation dependence,5,11,12,17−19 and is associated with a slow gibbsite growth rate at high supersaturation5,6,11,20 and high growth activation energies of 51−141 kJ mol−1.11,12,17,21,22 The growth of different crystal faces was noted to proceed via various mechanisms;20,23 for instance, the gibbsite prismatic faces grew via a screw dislocation, while the basal faces grew via a screw dislocation mechanism at smaller supersaturation, and via two-dimensional nucleation growth mechanisms at greater supersaturation.23 Considerable dispersion in crystal growth rate was demonstrated using in situ optical and atomic force microscopy investigations of various crystal faces.20 These variations in the growth rate increased at greater temperatures, possibly influenced by variations in dislocation characteristics,
INTRODUCTION Gibbsite (γ-Al(OH)3) precipitation from supersaturated sodium aluminate solution is the rate-determining step in the production of alumina (Al2O3) from bauxite by the Bayer process.1−4 The rate of growth of gibbsite crystals is very slow under Bayer precipitation conditions, with typical growth rates of less than 5 μm/h at 60−80 °C in highly supersaturated caustic aluminate Bayer liquors,5,6 and, as a consequence, the production of the desired size range of gibbsite crystals takes over 50% of the total production time in a Bayer refinery. Hence, methodologies for increasing the rate of crystallization have been an important area of investigation for many years. A well-known approach that has been extensively used within the alumina industry involves the use of specifically developed chemical additives or crystal growth modifiers (CGMs). CGMs are used during the Bayer process to enhance the average gibbsite particle size and to improve the particle size distribution by minimizing the formation of very fine gibbsite particles.7,8 CGMs are also known to inhibit the coprecipitation of organic materials such as sodium oxalate.7 The rate of crystal growth is influenced by factors such as Al(III) supersaturation, seed surface area, temperature, caustic concentration, and the presence of organic impurities,9 which can decrease the rate of crystallization and also alter the morphology of the crystalline product.10 The nucleation kinetics under Bayer conditions has been shown to follow a fourth-order dependence upon Al(III) relative supersaturation via a two-dimensional surface nuclea© 2012 American Chemical Society
Received: March 1, 2012 Revised: April 25, 2012 Published: May 17, 2012 3096
dx.doi.org/10.1021/cg3003004 | Cryst. Growth Des. 2012, 12, 3096−3103
Crystal Growth & Design
Article
step sources at the crystal surface, the overall lattice strain of crystals and impurities. A recent study24 has shown the structure of aluminumbearing species present in synthetic Bayer liquors to be hydrated, alkali metal ion-paried, monomeric Al(OH)−4 ions cooexsisting with minor species such as Al2O(OH)−6 and polymeric aluminates. The existence of Al(III) in both tetragonal and octahedral coordination has been discussed earlier. 25−29 However, there appears to be no clear comprehension of the mechanism by which the solution phase tetrahydroxo Al(OH)−4 transform into an octahedrally coordinated Al(OH)3 crystalline phase during nucleation and growth,30 nor is it understood how the addition of CGM may perturb these transformation processes. It is also not known if the transformation of the solution tetrahydroxo species to the octahedrally coordinated crystalline phase involves a minor solution species or whether it takes place on the surface of the gibbsite during crystallization. In this regard, there still exists incomplete understanding of both the kinetics of precipitation of gibbsite and the actual atomic-scale growth mechanism. Hence, there is ample scope for further investigation of the underlying mechanisms through which CGMs act to enhance the product quality of crystals despite the available knowledge7−9,20,24,31,32 in this field. In this work, we present experimental and molecular modeling studies of the influence of an industrially applied CGM on the gibbsite crystallization mechanism and kinetics. The aim of the experimental work was to examine secondary nucleation, crystal growth and kinetics, as well as agglomeration properties for seeded, pure synthetic, and real plant Bayer liquors in the absence and presence of a CGM. In conjunction with the experimental outcomes, we use semiempirical quantum-chemical methods to compute the energies of reactions between the oleate ion, a component of the CGM, and various aluminum species believed to be present in Bayer liquors.
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Figure 1. SEM photomicrographs of gibbsite seed crystals.
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CRYSTAL GROWTH AND NUCLEATION EXPERIMENT A baffled, well-sealed, 2.5 dm3, stainless steel vessel was used for the isothermal batch crystallization experiments at selected temperatures (60, 65, and 70 °C), with and without the presence of the CGM. A central, 4 blade, 45°, pitch turbine impeller driven by a 70 W, multispeed motor provided a constant agitation speed of 400 rpm and a fully developed axial flow with a high degree of suspension uniformity in the crystallizer. A thermocouple sensor and a conductivity probe were fitted through the lid of the crystallizer, which was submerged in a 15.0 dm3, thermostatically controlled oil bath, maintaining a constant temperature to within ±0.05 °C. A fresh, 2 dm3 optically clear sodium aluminate solution was used for each crystallization experiment. As soon as the experimental temperature was reached, preheated seeds were added and conductivity measurements were commenced. Two 30 cm3 samples were taken periodically. These were filtered through a 0.2 μm membrane. One of the solid residues was washed with Milli-Q water and then dispersed in water for size determination using a Malvern particle sizer, while the second solid residue was washed and dried in a desiccator for crystal mass content determination and SEM and BET characterization. The filtrate was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) for Al(III) concentration. Each crystallization experiment was carried out for 24 h. Where required, CGM was added to the suspension in the crystallizer as soon as the experimental temperature was reached. After 3 min, the seeds were added and the measurements commenced.
EXPERIMENTAL DESIGN
The main objective of the experimental work was to examine the influence of a NALCO industrial CGM on gibbsite crystallization kinetics and growth mechanisms under industrially relevant conditions. The growth mechanisms and kinetics of gibbsite crystallization from seeded, pure synthetic Bayer liquors containing 4.0 M NaOH and 2.55−3.20 M Al(III) at 60, 65, and 70 °C were examined. The measurements in the absence of CGM serve as a benchmark for testing the role played by the CGM. The pure, synthetic, supersaturated sodium aluminate solutions were prepared from analytical grade and high-purity reagents using (a) gibbsite (C31 grade, 0.01% SiO2, 0.004% Fe2O3; ALCOA, AR), (b) sodium hydroxide (99.0% pure, 0.01% Si, Merck, Australia), (c) Milli-Q water (surface tension 72.8 N m−1 at 20 °C, specific conductivity
