Gibbsite Crystals Research Articles

A dynamic light scattering investigation of nucleation and growth in supersaturated alkaline sodium aluminate solutions (synthetic Bayer liquors)

Dynamic light scattering was used to characterise gibbsite particle nucleation and growth in supersaturated alkaline sodium aluminate solutions. The supersaturation and temperature conditions investigated include those experienced in the crystallisation step of Bayer processing. Nuclei growth rates, which are constant for a period of several hours, are dependent on the square of supersaturation and on temperature through an Arrhenius relationship. The activation energy for nuclei growth has been determined to be 51±5 kJ mol −1. Comparisons are drawn between nuclei growth behaviour in the nanometer size range and the growth of micrometer sized gibbsite crystals under equivalent conditions. Similarities and differences are discussed with respect to the rate determining steps and growth mechanisms during gibbsite crystallisation. For nuclei less than 200 nm the growth mechanism is dominated by agglomeration or Ostwald ripening, whereas for larger nuclei, single particle growth is the dominant mechanism.

Interparticle forces, interfacial structure development and agglomeration of gibbsite particles in synthetic Bayer liquors

Surface force, rheological and crystallization techniques have been used to investigate interparticle forces and their link to interfacial structuring and agglomeration of gibbsite crystals in pure, supersaturated sodium and potassium aluminate solutions (synthetic Bayer liquors). Initially repulsive interparticle forces exist; these attenuate with time, and are followed by the emergence and growth of adhesive forces upon further aging. The adhesive forces were smaller at the gibbsite (001) crystal face compared with other faces and their rate of growth greater in sodium aluminate than in potassium aluminate solutions over similar aging periods. The rheological data correlated well with the interparticle force determinations. Both the thixotropic structure and yield stresses increased with time and were greater in sodium-based than in potassium-based liquors. SEM and crystal size analyses of the crystallized product showed that agglomeration was more pronounced in sodium than in potassium-based Bayer liquors and was more prevalent between non-(001) crystal faces. For the size enlargement of sub-micron sized gibbsite crystals, agglomeration, rather than crystal growth, was the predominant mechanism and is directly controlled by particle interaction forces.

Interfacial Charging Phenomena of Aluminum (Hydr)oxides

The interfacial charging of Al(OH)3 (gibbsite and bayerite) and Al2O3 has been studied. For Al(OH)3 it can be shown that the very strong variation in charging behavior for different preparations is related to the relative presence of differently reacting crystal planes. The edge faces of the hexagonal gibbsite crystals are proton reactive over the whole pH range, in contrast to the 001 plane, which is mainly uncharged below pH = 10. On this 001 face only doubly coordinated surface groups are found, in contrast to the edges which also have singly coordinated surface groups. The results are fully in agreement with the predictions of the Multi site complexation (MUSIC) model. The proton adsorption, electrolyte ion adsorption, and shift of the IEP of gibbsite and aluminum oxide have been modeled simultaneously. For gibbsite, the ion pair formation of Na is larger than that of Cl, as is evidenced by modeling the experimentally observed upward shift of the IEP and charge reversal at high electrolyte concentrati...

Growth rates of gibbsite single crystals determined using in situ optical microscopy

Although the crystallization of gibbsite via the Bayer process is an established industry, the mechanism by which gibbsite crystallizes is not well understood. In the microscopic investigation of gibbsite crystal growth, the phenomenon of growth rate dispersion was observed. The study of the growth rate dependence on supersaturation showed that growth of the prismatic faces is occurring via a spiral growth mechanism. In the case of the basal face, above a relative supersaturation of 0.67, birth and spread is the principal mechanism operating, while spiral growth is the major mechanism operating below a relative supersaturation of 0.67.

The impact of twinning on the morphology of γ-Al(OH) 3 crystals

Gibbsite, γ-Al(OH) 3, is an important technical product, used mostly as an intermediate in the production of aluminium. According to the literature, the morphology of natural and industrial gibbsite crystals is pseudohexagonal with {0 0 1} basal, and {1 0 0} and {1 1 0} side faces. Laboratory experiments on the growth of γ-Al(OH) 3 crystals from unseeded sodium aluminate/sodium hydroxide solutions result in crystals (and agglomerates) with various morphologies, such as lozenge-shaped or hexagonal plates, as well as prisms. Optical polarisation microscopy shows many of the crystals to be twinned. Three kinds of reflection twin with both mirror and composition planes {1 1 0}, {0 0 1} and {1 0 0} were identified. In addition, [1 3 0] rotation twins with {0 0 1} as composition plane were found. The effect of various kinds of twinning on the growth morphology and growth kinetics of γ-Al(OH) 3 crystals is studied.

The influence of additives on the crystal habit of gibbsite

Crystallization of gibbsite (Al(OH) 3) is an important stage of the Bayer process, production of alumina from bauxite ores. In both pure or industrial supersaturated sodium aluminate solutions, gibbsite crystals are always agglomerated. In the present paper, we present results of a study concerning the influence of different polycarboxylic acids as crystal habit modifier for gibbsite. In pure solution, agglomerated hexagonal plates are observed. Whereas acicular and tabular morphologies are found in the presence of different additives. These results are discussed referring to the crystallographic structure of gibbsite. It is found that only oxygen atoms are present on gibbsite surface. This observation leads us to propose an additive way of acting by formation of a molecular complex between the growth unit and the carboxylic groups of the additive.

A non-immersed induction conductivity system for controlling supersaturation in corrosive media: the case of gibbsite crystals agglomeration in Bayer liquors

Agglomeration of gibbsite Al(OH) 3 crystallites is an important stage of the Bayer process, aiming at increasing the initial size of the particles. In the present work, a semi-continuous crystallizer working at constant and imposed supersaturation, and equipped with an automatic withdrawal system was developed to study the agglomeration of gibbsite crystals in supersaturated Bayer liquors. The liquor conductivity was measured using an induction conductivity system placed around the crystallizer, the conductivity regulation being used to work at constant supersaturation. Using this system allowed one to work with both a small crystallizer and a highly corrosive and abrasive suspension of gibbsite in a five molar caustic soda solution at 70°C. Analyses of the withdrawals were carried out with an Elzone particle counter, in order to draw N(t) N(0) = f(t) plots, representing the decrease of crystal number with time, due to agglomeration.

The effects of model organic compounds on gibbsite crystallization from alkaline aluminate solutions: polyols

The crystallization of gibbsite from synthetic Bayer liquors is known to be inhibited by compounds containing adjacent hydroxyl groups such as the polyhydric alcohols or polyols. Complementary studies examining the inhibition of gibbsite crystallization by polyols, and the interaction of the same compounds with aluminate ions in solution (by 13C NMR and Raman spectroscopy) have shown that the strongest inhibitors also complex aluminate under Bayer liquor conditions. By examining the most likely conformations of metal-ligand complexes in solution, we are able to make suggestions for the mechanism by which these compounds adsorb onto gibbsite seed and hence cause inhibition of crystallization. Specifically, it has been found that the strongest polyol inhibitors are able to form solution complexes with a single aluminium atom through a particular conformation of three oxo bonds. It is proposed that the inhibiting strength of polyols is related to the ease with which similar connections are formed with aluminium sites on the surface of the gibbsite seed.

The tridecameric aluminum complex as an appropriate precursor for fibrous boehmite: A 27Al NMR study on the partial hydrolysis of aluminum sec-butoxide

The aging of the aluminum tridecamer in acidified aluminum sec-butoxide is studied with 27Al nuclear magnetic resonance. At 20°C the tridecamer content passes through a maximum of 19.7% after 133 h of aging to decline subsequently to a final content of approximately 12%. At 90°C the tridecamer disappears within 21 h of aging due to clustering. After 91 h colloidal gibbsite forms, while the pH drops continuously from 4.3 to 3.5. Simultaneously the monomer concentration increases to approximately 6.6%. The increase is possibly due to liberation of the central fourfold-coordinated Al from the tridecamer that rearranges to hexameric rings, and to dissolution of some gibbsite in the acid solution. The hexameric rings are considered to be the precursor for the crystallization of gibbsite and boehmite. Increase of the preparation temperature from 20°C to 80°C has no effect on the amount of tridecamer, although at 90°C the amount increases from approximately 9% to 14.1%. An increase of the preparation temperature leads to changes in the second-order quadrupole effects, which results in a line sharpening from 19.5 to 9.8 Hz for the central fourfold-coordinated Al signal of the tridecamer at 63.3 ppm.

Agglomeration during precipitation: I. Tracer crystals for Al(OH)3 precipitation

AbstractTracer crystals are developed to study the agglomeration, growth and residence time behavior of gibbsite crystals, an industrially important polymorph of Al(OH)3, in continuous and batch precipitation. Satisfactory tracer crystals contain detectable amounts of Zn and have the crystal structure of gibbsite. The tracer crystals behave in the same manner during precipitation as the nontracer Al(OH)3 crystals. The application of the tracer crystals for investigating, both batch and continuous, seeded Al(OH)3 precipitation is demonstrated. The tracer experiments provide information on the growth and agglomeration kinetics of different size fractions. The tracers are a useful tool for the quantitative investigation of solid‐phase residence time distributions within a crystallizer.

27Al and 23Na MAS NMR and Powder X-ray Diffraction Studies of Sodium Aluminate Speciation and the Mechanistics of Aluminum Hydroxide Precipitation upon Acid Hydrolysis

{sup 27}Al and {sup 23}Na MAS NMR, powder X-ray diffraction, and infrared spectroscopic investigations of freeze-dried sodium aluminates and aluminum hydroxides formed through acid hydrolysis have been undertaken, with OH/Al hydrolysis ratios between 5.3 and 2.8 being analyzed. Numerous {sup 27}AlNMR resonances were observed, the intensities of which vary as a function of OH/Al ratio, and these have been assigned to four-, five-, and six-coordinate aluminum species constituting a variety of structural moieties. The dominant species at an OH/Al ratio above 4.4 appears to be a Q{sup o}Na[Al(OH);{sub 4}] salt, as indicated by a {sup 27}Al resonance at 86.6 ppm. In addition, a second, broader resonance at 71.3 ppm demonstrates the simultaneous existence of further four-coordinate aluminum species linked thorough oxo bonds to other four-coordinate aluminums (e.g., Q{sup 2} [Al(OH);{sub 2}(OAl){sub 2}];{sup x-}). At an OH/Al ratio between 4.4 and 4.1, a water-soluble phase forms that contains both four- and six-coordinate aluminum. At OH/Al ratios fo 4.0 and below, a water-soluble phase forms that contains both four-and six-coordinate aluminum. AT OH/Al ratios of 4.0 and below, a water-insoluble phase exists possessing four-, five-, and six-coordinate aluminum. At OH/Al{le}3.9 range exhibits {sup 27}Al chemical shifts similar to those reported for transitionalmore » aluminas such as {gamma}-, {eta}-, and 0-Al{sub 2}O{sub 3} and an infrared spectrum similar to pseudo-spinel gels, suggesting that a pseudo-spinel intermediate is the first phase involved in the crystallization of gibbsite. The resonance assigned to five-coordinate aluminum probably results from species involved in the transformation of the pseudo-spinal phase to pseudo-boehmite. The formation of gibbssite on the acid hydrolysis of alkaline sodium aluminate solutions thus appears to follow the pathway pseudo-spinel {r_arrow} pseudo-boehmite {r_arrow} bayerite {r_arrow} gibbsite. 82 refs., 7 figs., 3 tabs.« less

Phosphate sorption as related to mineralogy of a hydrosequence of soils from the Cerrado region (Brazil)

Phosphate sorption by minerals having surface FeOH or AlOH groups is an important process in highly weathered tropical soils. In this study, P sorption in 12 samples of soils of a typical hydrosequence of the Cerrado region (Brazil) was predicted by means of combining data on: (1) the amount and properties of gibbsite, goethite, hematite and kaolinite, and (2) the P-sorption characteristics of these minerals, according to existing data in the literature. The concentration, crystal morphology and surface area of these four minerals were studied by X-ray diffraction (XRD), thermogravimetry, transmission electron microscopy (TEM) and N 2-adsorption. Gibbsite crystals were subrounded with its longest axis between 50 and 100 nm. Kaolinite platelets had a diameter of 40–200 nm and frequently had euhedral faces. According to XRD data, the mean crystallite dimension (MCD) perpendicular to the (110) plane of goethite was about 30 nm; hematite crystals appeared to be platy (i.e., MCD 110 > MCD 104) and slightly larger than the goethite crystals. For most samples, the P sorption, measured at an equilibrium concentration of 10 −5M, was markedly lower than P sorption obtained by the sum of the sorptions estimated for each of the four minerals. The difference between measured and predicted P sorption became smaller when the soils were treated with H 2O 2. Moreover, the increase in P sorption with this treatment was positively correlated with the organic matter content. These results suggest that organic compounds, by probably competing for sorption sites, can substantially hinder P sorption of tropical soils. For soils low in organic matter, P sorption can be reasonably well predicted through the mineralogical characteristics of the soil.

Kinetics of silicate reaction with gibbsite

The reaction of silicate with gibbsite in sodium aluminate solution was studied. Sorption kinetics indicated that sorption of silicate was surface-induced. Silicate is believed to have adsorbed and precipitated as sodium aluminum silicate (SAS) on gibbsite by a three-step reaction; a first fast step, a lag period step, and a slow step. The fast and slow steps conformed to first-order rate laws with respect to silicate concentration. The extent of sorption depended on silicate concentration, suspension concentration and temperature. Further, silicate increased the equilibrium solubility of gibbsite in sodium aluminate solution. For a silicate concentration of 500 μgSiO 2 /cm 3 , adsorption/precipitation accounted for 86% of the reduction in gibbsite crystallization, while the remaining 14% was due to increased solubility of gibbsite. Thus, our adsorption—precipitation observations confirm the postulate that the reduction in gibbsite crystallization in the Bayer process is due mainly to the adsorption of silicate on gibbsite seed crystals.

Monomeric versus mixed monomeric-polymeric models for aqueous aluminium species: Constraints from low-temperature natural waters in equilibrium with gibbsite under temperate and tropical climate

Spring waters from Fougères (Armorican Massif), Margeride and Mont Lozère (Central Massif, France), groundwaters, flood waters and recession waters from Sinnamary (French Guyana) were collected in small watersheds on granitic or metamorphic rocks. Leaching experiments were also conducted on soils from Fougères. Waters were analyzed for major elements and Al, in order to check the control on dissolved Al by gibbsite, kaolinite, imogolite and other possible secondary minerals, and of different models concerning the speciation of Al in aqueous solution. The results show that: (1) the single monomeric model hitherto widely used leads to large supersaturations with respect to gibbsite, kaolinite and even in some cases to amorphous Al-hydroxide, and to a great scattering of data; (2) the polymeric-monomeric model leads to values of IAP (ionic activity product) in agreement with control by gibbsite; (3) total dissolved Al decreases at constant pH and temperature and gets closer to the solubility curve of gibbsite in deeper springs in Fougères and in recession waters in Guyana as contrary to shallower springs in Fougères and flood waters in Guyana. This allows us to propose a new reaction scheme for Al precipitation: as pH increases to > 5, in a first step a polymeric species forms, and in a second step gibbsite crystallization from monomeric species proceeds, coupled with depolymerization. This picture is consistent with classical data, if one considers no more polymerization to occur by an edge-to-edge mechanism, but by a face-to-face mechanism. The polymeric form thus appears to constitute a pool buffering Al activity. IAP of gibbsite is well correlated with temperature, but uncertainty on the solubility product of gibbsite is amplified by the polymer-monomer relationship, which limits the use of the relation obtained as a low-temperature geothermometer.

Mechanisms of Gibbsite Crystallization from Partially Neutralized Aluminum Chloride Solutions

AbstractTo interpret the erratic conditions, rates, and extent of gibbsite crystallization from partially neutralized A1C13 solution, the following hypothesis is proposed: The initial OH-A1 polymers in the freshly prepared solutions were probably unstable and transformed into either gibbsite or stable OH-A1 polymers via two different reaction paths. In the presence of nuclei, the OH-A1 polymers dissociated into monomeric ions, which then deposited onto nuclei to form gibbsite. In the absence of nuclei, the unstable polymers slowly converted to stable polymers. The erratic stability of OH-Al solutions and gibbsite crystallization are therefore attributed to the relative magnitudes of these two reaction paths which, in turn, are attributed to two key factors: (1) the distribution of unstable vs. stable OH-Al polymers; and (2) the presence or absence of nuclei. The duration of aging of the parent solution governed the distribution of unstable vs. stable polymers. The rate of neutralization resulted in varying localized high alkalinity in OH-Al solution preparation and thus varying development of nuclei.

Micromorphological and analytical studies of the fine matrix of an Australian humus iron podzol

SUMMARYDetailed micromorphological observations and associated electron probe microanalyses are used to examine the proposition that kaolinite and gibbsite in the B horizon of a humus iron podzol could arise by transformation of allophanic deposits. The B horizon of this podzol is highly complex. In the upper B horizon, this complexity is due to alternating episodes of the influx of organic matter and the deposition of allophanic material; evidence for the separate deposition of allophane and organic matter is presented. Material within the B horizon that has gibbsiporphyric fabric appears to have arisen by crystallization of gibbsite from aluminium‐rich gels deposited before the main influx of allophanic material; there is no convincing evidence that allophanic material has transformed to kaolinite or gibbsite.

Adsorption of oxalate on gibbsite

The adsorption of oxalate ion on gibbsite was measured using a radiotracer. For oxalate concentrations of 2 to 18 mol/m 3, Na 2O of 0.1 to 0.125 g/cm 3 and Al 2O 3/Na 2O of 0.21 to 0.32 by weight, the adsorption appears to about 0.02 μmol/m 2, or less than 0.01 monolayer. However, this is probably enough adsorption to be responsible for the observed retardation of gibbsite crystallization by oxalate.

The nature and stability of aluminum hydroxide precipitated on wyoming montmorillonite

A freeze—fracture technique was used to prepare replicas of interlayer surfaces of a hydroxy-aluminum montmorillonite (OH-Al-SWy-1), synthesized by titrating an Al-clay with NaOH. Aqueous suspensions containing 0.01 M NaClO 4 and 4% OH-Al-SWy-1 were frozen rapidly with Freon 22 to vitrify the solution phase and stored in liquid N 2. The frozen samples were fractured at 10 −6 Torr and 158K and C-Pt replicas were made of the exposed surface. Transmission electron microscopy (TEM) of the replicas showed a randomly distributed surface precipitate. The precipitate was flat and angular, resembling gibbsite morphologically. Gibbsite crystals were observed outside the interlayer when the suspension was air-dried on microscope grids, underscoring the value of the freeze—fracture technique in minimizing alteration of the OH-Al-SWy-1 complex. Using the mass of Al(OH) 3 fixed by the clay and the average particle perimeter observed by TEM (49.3 nm), we estimated the maximum Al(OH) 3 surface charge density and site density available for phosphate adsorption. The range of surface charge density for the Al(OH) 3, estimated from crystallographic data, was very close to the range of surface charge density for montmorillonite, suggesting that the latter may influence the maximum particle size of the precipitate. The total charge and edge [-Al(OH)(H 2O)] site density were less than the observed CEC reduction on SWy-1 and maximum phosphate retention on OH-Al-SWy-1 at pH 5.3, respectively. Interlayer Al(OH) 3 is known to be unstable with respect to gibbsite and it is possible that a relatively high specific surface area was responsible for the high solubility.

Feldspar Weathering in Lateritic Saprolite

AbstractFeldspars in granitic saprolite in southwestern Australia have altered to halloysite, kaolinite, and gibbsite with no evidence of noncrystalline material. The secondary minerals are commonly present as intimate mixtures within altered feldspar grains, but discrete zones of gibbsite or halloysite-kaolinite also are present. Variations in the chemical microenvironment within micrometer-size zones in grains apparently controlled the type and distribution of secondary minerals. The alteration of both plagioclase and alkali feldspars involved congruent dissolution by soil solution and crystallization of halloysite, kaolinite, and gibbsite from this solution. Highly altered feldspar grains consist of etched feldspar fragments embedded within a highly porous framework of subhedral to euhedral platy crystals of kaolinite and gibbsite, or of spherical and felted aggregates of halloysite.

Cu2+ Interaction with Microcrystalline Gibbsite. Evidence for Oriented Chemisorbed Copper Ions

AbstractThe ability of a high surface-area gibbsite to adsorb Cu2+ was studied using a Cu2+ ion-selective electrode, electron spin resonance, infrared spectroscopy, and electron microscopy. The gibbsite chemi-sorbed small amounts of monomelic Cu2+ (<0.5 mmole/100 g) which was oriented with its z-axis perpendicular to the (001) plane of the mineral. The proposed chemisorption sites are at gibbsite crystal “steps” observed by electron microscopy. Although Cu2+ adsorption on gibbsite as a function of pH was largely reversible, exposure of the chemisorbed Cu2+ to NH3 did not result in desorption from the surface despite the displacement of several OH− or H2O ligands by NH3. The results indicate that at least one Cu-O-Al bond is formed in the process of chemisorption.At pH > 5, the gibbsite appeared to promote the hydrolysis and polymerization of Cu2+, with further adsorption at the surfaces. Infrared spectroscopy revealed no effect of the adsorption on the (001) surface hydroxyl groups, although the anisotropic diffusion of protons in the gibbsite structure was verified from deuteration studies.