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Nov 21, 2014 - The Rio Tinto Alcan Queensland Research and Development Centre (QRDC), Pullenvale, Queensland 4069, Australia. ABSTRACT: To mimic ...
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Aspects of the Mechanism of Nucleation and Intergrowth of Gibbsite Crystals on Sodium Oxalate Surfaces in Concentrated Alkaline Solutions Weng Fu,*,† James Vaughan,*,† and Alistair Gillespie‡ †

The University of Queensland, School of Chemical Engineering, Level 3, Chemical Engineering Building (74), College Road, St Lucia, Queensland 4072, Australia ‡ The Rio Tinto Alcan Queensland Research and Development Centre (QRDC), Pullenvale, Queensland 4069, Australia ABSTRACT: To mimic the process of gibbsite nucleation on sodium oxalate crystals and their intergrowth in alumina Bayer process, such a coprecipitation process has been broken down into three distinct stages and studied by in situ AFM and ex situ SEM under static conditions. In the first stage, a solution saturated in aluminate and supersaturated in oxalate has been used for investigating the oxalate growth on its {110} face, which is controlled by a layer-by-layer mechanism. In the second stage, in a solution saturated in oxalate and supersaturated in aluminate, gibbsite secondary nucleation was preferred along macrosteps of sodium oxalate {110} faces and appeared as two-dimensional (2D) islands with pisolitic shapes. There does not appear to be a preferred epitaxial relationship at the heterointerface between gibbsite crystals and sodium oxalate substrates, indicating that the magnitude of the surface free energy for nucleation is greater than that associated with strain energy caused by lattice mismatch. In the third stage following gibbsite secondary nucleation, in a solution saturated in aluminate and supersaturated in oxalate, the inclusion of gibbsite crystallites into the bulk of sodium oxalate crystals through step growth on {110} faces of sodium oxalate was observed.

1. INTRODUCTION The Bayer process is the principal industrial means of producing alumina (Al2O3). The process was developed by the Austrian chemist Karl Bayer1 and patented in 1889. Today, it produces nearly all the world’s alumina supply as an intermediate in aluminum production. The process dissolves the aluminum component of bauxite ore in hot sodium hydroxide (caustic soda, NaOH), removes impurities from the solution, and precipitates aluminum hydroxide2,3 (also named gibbsite, γ-Al(OH)3), which is finally calcined to alumina. At the cooler end of the precipitation circuit of the Bayer process, the organic impurity, sodium oxalate (Na2C2O4) coprecipitates with the productgibbsite (γ-Al(OH)3), according to eqs 1 and 2. This is because the equilibrium solubility of both Al(OH)3 and Na2C2O4 can be substantially decreased by reducing the temperature.4 − Al(OH)4 −(aq) = Al(OH)3(solid) ↓ + OH(aq)

(1)

+ 2Na(aq) + C2O4 2 −(aq) = Na 2C2O4(solid)↓

(2)

precipitated oxalate crystals are ideal sites for gibbsite secondary nucleation promoting the formation of gibbsite fines. Fines may also be generated by a reduction in gibbsite agglomeration efficiency. Two possible mechanisms have been proposed for fines generation: (1) The large amount of fine oxalate physically interferes in the agglomeration of hydrate particles.7 (2) In plant operation, if gibbsite contaminated with oxalate enters the agglomeration tanks, a substantial part of the supersaturation will be relieved by gibbsite nucleation on the oxalate needles. Particles such as these will not agglomerate effectively.8 Sodium oxalate can decrease the efficiency of particle size classification. The attachment of oxalate to gibbsite interferes with particle size classification by elutriation which depends upon segregation of particles by free settling according to Stokes’ Law. The presence of oxalate tends to prevent gibbsite particles from behaving as individual entities, by physically attaching them together.9 Sodium oxalate can also increase the rate of oxalate-gibbsite scale formation. Precipitated oxalate has a tendency to attach to metal surfaces, where it can then form nucleation sites for hydrate. Mature hydrate particles also have a tendency to attach to oxalate crystals. All of these attachments

This coprecipitation process can result in numerous operational, maintenance, and production problems. The most common problem caused by sodium oxalate is the generation of gibbsite fines,5,6 disrupting the required particle size distribution in the gibbsite product. The surfaces of © XXXX American Chemical Society

Received: October 1, 2014 Revised: November 12, 2014

A

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Table 1. Summary of in Situ AFM Experimental Conditions [NaCl], mol/L

[NaOH], mol/L

[C2O42−], mol/L

[Al(OH)4−], mol/L

[Al(OH)4−] supersaturation, σAl

[C2O42−] supersaturation, σOx

4 4

1 1

0.01067 0.00485

0.14 0.30

0 1.14

1.2 0

Test Liquor 1 (oxalate growth) Test Liquor 2 (gibbsite nucleation)

Both of test liquors have a fixed pH of 13.4. bThe formula for relative supersaturation is σ = (Cinitial − Cequilibrium)/Cequilibrium. cAll test liquors have a sodium concentration of 5.0 M.

a

Table 2. Summery of Ex Situ Crystallization Conditions Test Liquor 3 at 50 °C, for 8 h Test Liquor 4 at 80 °C,for 48 h Test Liquor 5 at 50 °C, for 8 h

A/C

[C2O42−], mol/L

[Al(OH)4−], mol/L

[Al(OH)4−] supersaturation, σAl

[C2O42−] supersaturation, σOx

0.65 0.65 0.19

0.01134 0.02127 0.03037

2.43 2.43 0.71

2.42 0.71 0

0 0 1.2

The formula for relative supersaturation is as follows: σ = (Cinitial − Cequilibrium)/Cequilibrium. bAll of test liquors have a constant sodium concentration of 5.0 M. a

Preparation of Test Liquor 1. The appropriate amount of commercial Na2C2O4 was added into 200 mL of Stock Solution in a conical flask, to produce solutions with the C2O42− supersaturations shown in Table 1. Then, the above mixed solution was stirred and heated at 90 °C for 20 min, dissolving Na2C2O4 powder into the solution. The resulting hot solution was filtered through a 0.2 μm pore size Nylon membrane filter (Merck Millipore). The test liquor supersaturated in oxalate after filtration was sealed and allowed to cool in an air-bath oven at 25 °C for 15 min. Preparation of Test Liquor 2. A certain amount of commercial Al(OH)3 was added into 5 mL of Stock Solution in six TFM-PTFE pressure vessels, according to Al(OH)4− supersaturation shown in Table 1. The mixed liquor was digested in a speed wave 4 microwave digester produced by Berghof GmbH Co. The microwave digestion was carried out in three stages. In the first stage, the reactor contents in each vessel were heated to 150 °C for 5 min with a ramp up time 3 min followed by heating at 180 °C for 5 min with ramp up time of 3 min. In the third stage, the solutions were heated at 200 °C for 5 min with a ramp up time of 5 min. After the digestion, the test liquor was filtered through a 0.2 μm pore size Nylon membrane filter (Merck Millipore). 2.2. In Situ AFM Observations. AFM images were captured in a closed fluid cell which can hold the Test Liquors 1−2 statically without fluid exchange at 25 ± 1 °C. This implies that the relative supersaturation is only well-defined at the beginning of each run. In situ AFM observations were made by contact mode using a MFP-3DBIO (Asylum Research) equipped with a piezoelectric scanner capable of scan areas to a maximum of 90 × 90 μm2. Surfaces were imaged using commercial Si3N4 cantilevers that have triangular tips with a length of 200 μm, and a force constant of approximately 0.06 N/m. To reduce the possibility of artifactual changes in microtopography by scanning tip−surface interactions, the contact force was carefully minimized. To clarify the interaction mechanisms between gibbsite and sodium oxalate, the experiments for growth and nucleation were broken down into three distinct stages: Stage 1 − Na2C2O4 growth on its {110} faces in Test Liquor 1. It is important to understand how oxalate grows on its own faces, before allowing foreign species (Al(OH)4−) to precipitate on Na2C2O4 surfaces. Na2C2O4 crystals used as seed crystals in this experiment has been precipitated from synthetic Bayer liquor (4 M NaOH + 1 M NaCl), the procedure for which is described elsewhere.13 Stage 2 − Gibbsite nucleation on {110} faces of Na2C2O4 crystals in Test Liquor 2. The same type of Na2C2O4 crystals has been employed in this experiment for gibbsite secondary nucleation. Stage 3 − Gibbsite inclusion in Na2C2O4 crystals in Test Liquor 1. After formed on Na2C2O4 surface, inclusion of gibbsite into the relatively fast-growing Na2C2O4 crystals is likely. Thus, Na2C2O4 crystals covered with gibbsite crystallites prepared in Test Liquor 3 at 50 °C for 8 h have been used in this experiment.

appear to be made via surface active organic contaminants which readily adsorb to the various available surfaces.9 Most of these industrial problems can be attributed to one fundamental phenomenon of gibbsite secondary nucleation on sodium oxalate crystals and their intergrowth. This research contributes to the understanding of the interactions between gibbsite and sodium oxalate which has been studied using in situ atomic force microscopy (AFM) at 25 °C. Considering that the glass window of the AFM cantilever holder cannot resist highly caustic Bayer solution (∼5 M NaOH), the test liquor used for in situ AFM investigation contains 4 M NaCl and 1 M NaOH, supersaturated in sodium oxalate or aluminate. Ex situ scanning electron microscopy (SEM) was also used to characterize gibbsite and sodium oxalate crystals precipitated from synthetic Bayer liquor under static conditions.

2. EXPERIMENTAL SECTION 2.1. Materials and Solution Preparation. Sodium oxalate was purchased from Alfa Aesar. Sodium hydroxide, sodium chloride, and aluminum trihydrate were obtained from Sigma−Aldrich (USA). The water was purified via a Millipore system with a resistivity of 18 MΩ/ cm. All glassware (glass beakers and small pieces of glass substrates) was cleaned and rinsed with Millipore water. Given that the glass window of the AFM cantilever holder cannot resist highly caustic Bayer solution (∼5 M NaOH), the test liquor used for in situ AFM contains 4 M NaCl and 1 M NaOH, shown in Table 1. The reason this kind of test liquor has been chosen to mimic industrial Bayer liquor is that both liquors provide high ionic strength and highly alkaline aqueous media for sodium oxalate and gibbsite crystallization, in which the predominant anion species for oxalate and aluminum10 are C2O42− and Al(OH)4−, respectively. The similar methodology has been applied to use FTIR-ATR to measure the adsorption of organic molecules on sodium oxalate crystals under highly alkaline (pH = 12), high ionic strength (5 M NaCl) conditions to mimic industrial Bayer conditions.11,12 In such test liquors, the supersaturations which are the most important factor for nucleation and growth of oxalate and gibbsite have been reasonably controlled in Table 1, compared with the supersaturations in synthetic Bayer liquor in Table 2. Preparation of Stock Solution. 500 mL of solution containing 4 M NaCl and 1 M NaOH, and excess amounts of commercial Al(OH)3 and Na2C2O4 powders were introduced to a 1 L conical flask with magnetic stirrer and rubber stopper and stirred for three months at 25 ± 1 °C. After that, the solution was filtered through a 0.2 μm pore size Nylon membrane filter. The apparent equilibrium C2O42− and Al(OH)4− concentrations in Stock Solution were measured by ion chromatography and ICP-OES, respectively, the results of which are shown in Table 1. B

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2.3. Ex Situ Observations Using SEM and AFM. The composition of synthetic Bayer liquor for ex situ experiments is as follows: 3.6 M NaOH + 0.32 M Na2CO3 + 0.1 M Na2SO4 + 0.56 M NaCl + x M Al(OH)3 + y M Na2C2O4, where the amount of Al(OH)3 and Na2C2O4 is dependent on crystallization conditions shown in Table 2. The equilibrium solubilities of Al(OH)4− and C2O42− in synthetic Bayer liquor at different temperature have been calculated, based on the publications of S. P. Rosenberg et al.14 and K. R. Beckham et al.,4 respectively. The synthetic Bayer liquors were prepared in an autoclave at 150 °C for 1 h. The aluminate-to-caustic ratio A/C for this synthetic Bayer liquor is shown in Table 2, where A represents the aluminate concentration expressed in grams per liter Al2O3, and C represents caustic concentration expressed in grams per liter Na2CO3. In a typical crystallization experiment, Na2C2O4 crystals used as seed crystals in this experiment has been precipitated from synthetic Bayer liquor (4 M NaOH + 1 M NaCl), the procedure of which is described elsewhere.13 1.0 g of such Na2C2O4 crystals was added into 50 mL of synthetic Bayer liquor (Test Liquor 3) in a conical flask. The above mixed solution was sealed and allowed to stand under static condition in a water bath at 50 °C for 8 h. After that, about 0.8 g of Na2C2O4 crystals were filtered, washed with 5 mL of 200g/L NaOH solution and 5 mL of absolute ethanol in a Swinnex filter holder, and then dried at room temperature. This experimental procedure has been repeated, in which the synthetic Bayer liquor has been changed to Test Liquor 4 at 80 °C for 48 h, as illustrated in Table 2. In order to test if the gibbsite crystallites can be occluded into the bulk of Na2C2O4 crystals, Na2C2O4 crystals covered with gibbsite crystallites obtained from Test Liquor 4 at 80 °C for 48 h have been used for the further growth of Na2C2O4 {110} faces. 1.0 g of such Na2C2O4 crystals bounded to gibbsite crystallites was added into 50 mL of Test Liquor 5 in a conical flask. The above mixed solution was sealed and allowed to stand under static condition in a water bath at 50 °C for 8 h. After that, the crystals were filtered and washed using the same procedure for the crystals obtained from Test Liquor 3. The morphology and distribution of gibbsite crystallite grown on Na2C2O4 crystals was also examined by scanning electronic microscopy (SEM, Philips XL30) at an accelerating voltage of 5 kV. Ex situ AFM observations were made by Contact Mode using a MFP3D-BIO (Asylum Research).

Figure 1. AFM deflection images of growth sequence of macro and microsteps on sodium oxalate {110} faces in Test Liquor 1 at 25 °C. The scan areas for the images (a−c) are 1.5 × 1.5 μm2. (d) Height profiles along the blue dotted lines with an arrowhead pointed to [11̅0] direction in images (a−c), curve (1) 0 min, (2) 60 min, (3) 120 min.

the higher density of steps on {110} faces than those grown in water, increasing the surface roughness of {110} faces and thus providing more potential sites for gibbsite secondary nucleation. After injecting Test Liquor 1 into the AFM fluid cell, the step train began to advance laterally along the oxalate [11̅0] direction, as illustrated in Figure 1d. Such lateral advancement of step trains indicate that step growth on {110} faces is controlled by layer-by-layer growth mechanism.18 Another typical macrostep on the oxalate {110} faces is composed of relatively wide terrace and smooth step riser, which is the area surrounded by edges ① and ② (white dashed lines) in Figure 2a,b. When the macrostep moved laterally on the surface along the oxalate [11̅0] direction and came into contact with many small dislocations (less than 0.5 nm in height) at the higher right side of edge ① in Figure 2a, it swept over the dislocation outcrop on the surface, relieving the interfacial strain energy on the initial surface. Near the completion of the first 30 min, a train of elementary steps entered at the lower left side of edge ③, shown in Figure 2b, making the beginning of propagation of elementary steps on a macrostep terrace. The advancement of these elementary steps was oriented along the oxalate [11̅0] direction as well. The height for most elementary steps in the step train is in the range of 0.2−0.5 nm, which corresponds to the interplanar spacing (d(110) = 0.47 nm) on the oxalate {110} faces. The interplanar spacing (d(110)) has been calculated according to the Bragg equation (d = λ/2 sin θ), where d is the spacing between the planes in the atomic lattice, λ is the wavelength of Cu Kα irradiation (λ = 1.5406 Å) and θ is the angle between the incident X-ray and the scattering planes. The angle θ which corresponds to oxalate {110} faces is 9.48°, based on XRD results of Na2C2O4 crystals by Fu et al.16 When the macrostep moved in contact with microsteps (more than 1.0 nm in height) in Figure 3a−d, the macrostep cannot sweep over the microsteps without leaving any trace.

3. RESULTS AND DISCUSSION 3.1. Step Growth on {110} Faces of Sodium Oxalate Crystals in Test Liquor 1 at 25 °C: In Situ AFM Observations. From the stage 1 experiment, sodium oxalate crystals grown from aqueous solution were characterized by twinning along the {200} plane with a prismatic shape, well developed {110} faces, and poorly developed {001} faces.15−17 The crystal habit of sodium oxalate is determined by the three faces, {200}, {110}, and {001}. The order of morphological importance (MI) of these faces could be as follows: MI(110) > MI(200) > MI(001). In particular, the growth rate of {200} faces under high ionic strength conditions can be faster than that of {110} faces, resulting in the large {110} faces and the disappearance of {200} faces. Moreover, {110} faces are usually characterized by striations running parallel to the oxalate [001] direction (the c-axis). Therefore, the different types of step growth on {110} faces of Na2C2O4 crystals have been investigated. The height of the steps on the oxalate {110} faces can vary in general from less than one nanometre (elementary steps) to several nm (microsteps) and finally to dozens of nm (macrosteps). The step train made by macro and microsteps in Figure 1a−c is one of typical surface microtopographs of oxalate {110} faces. Fu et al.13 who investigated Na2C2O4 crystals grown in aqueous sodium hydroxide solution found that high ionic strength and high alkalinity solution can lead to C

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area surrounded by ①, ②, and ③, resulting in the diffusioncontrolled growth within the triangle area and thus promoting the formation of a deep valley at 90 min. Similar to the kink sites on crystal surface, such deep valley formation becomes an energetically favorable point at which a growth unit can more easily incorporate into lattices, leading to the surface integration-controlled growth and thus promoting the formation of a smoother surface at 200 min. Moreover, the reduced bulk concentration of sodium oxalate after 90 min decreased the growth rate of the vertical height between ① and ②, contributing to formation of the smoother surface indirectly. 3.2. Gibbsite Nucleation on {110} Faces of Sodium Oxalate Crystals. 3.2.1. In Situ AFM Observations at 25 °C. In the second stage where the oxalate crystal is exposed to supersaturated aluminate solution, in situ AFM images in Figure 4a−c show that a pisolitic-shaped gibbsite crystallite which is a two-dimensional (2D) island formed on the terrace adjacent to the edge of a Na2C2O4 macrostep, overlapping a Na2C2O4 microstep, highlighted in Figure 4d. The location of this gibbsite crystallite suggests that the step edge could be a preferred site for island nucleation. According to the Ehrlich− Schwoebel (ES) effect,19,20 step edges act as an additional diffusion barrier for adatoms/surface species which migrate from upper terrace sides to a lower step. This effect results in locally accumulated concentration of adatoms/surface species at step edges. When the concentration of adatoms/surface species exceeds a critical value, 2D islands form at step edges. After 240 min, the 2D island of gibbsite crystallite reached a significant height of ∼22 nm along the [110] direction, a width of ∼280 nm along the [11̅0] direction, and a length of ∼350 nm in the oxalate [001] direction. Two dimensional nucleation also took place on the surface of the gibbsite crystallite highlighted by blue arrows in Figure 4b, suggesting that the growth of the gibbsite crystallite is controlled by the layer-bylayer growth mechanism.18,21 The height of these 2D nuclei on the surface of the gibbsite crystallite is in the range of 0.4−1 nm corresponding to the gibbsite unit lattice height (a = 8.684 Å, b = 5.078 Å, c = 9.736 Å),22 so that a 2D island has a thickness of 1−2 gibbsite molecules.

Figure 2. AFM deflection images of growth sequence of macro and elementary steps on sodium oxalate {110} faces in Test Liquor 1 at 25 °C. The scan areas for the images (a−d) are 1.5 × 1.5 μm2.

The topographical change of microsteps is quite different from that of small dislocations (less than 0.5 nm in height) in contact with macrostep shown in Figure 2a,b. The zigzag-front of macrostep (highlighted by dash green line) in Figure 3a moved laterally along the blue dotted lines with an arrowhead pointed to the [11̅0] direction. In order to quantify the topographical change of the microstep, the height sequence of the microstep closest to the zigzag-front of the macrostep in Figure 3a has been highlighted by symbols (①, ②, ③) in Figure 3e. The vertical height between ① and ② has increased from 1.6 nm at 0 min to 18.8 nm at 90 min, and then reduced to 5.8 nm at 200 min. The reason for this phenomenon might be as follows: compared with other sites on crystal surface, it is relatively difficult for solute to diffuse toward the surface in the triangle

Figure 3. AFM deflection images of growth sequence of macrosteps on sodium oxalate {110} faces in Test Liquor 1 at 25 °C. The scan areas for the images (a−d) are 1.0 × 1.0 μm2. (d) Height profiles along the blue dotted lines with an arrowhead pointed to the [110̅ ] direction in images (a−d). D

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experiments were therefore conducted at 25 °C using Test Liquor 2 which is significantly different from industrial Bayer solutions and conditions. Therefore, crystallization studies using AFM and SEM under conditions closer to industrial practice in terms of solution concentrations and temperature (but not agitation) can only be done ex situ. In order to promote gibbsite nucleation on Na2C2O4 crystals, high aluminum supersaturation (σ = 2.42), low temperature (50 °C), and short residence time (8 h) are necessary for the birth of nuclei by secondary nucleation. Unlike the agitated crystallization process in the alumina industry, the gibbsite nucleation process in this study was investigated under static conditions without the influence of shear stress which led to agglomeration or nuclei breakage from their original deposited sites, ensuring that the gibbsite secondary nuclei formed all came from the Na2C2O4 seed crystals. A coexisting family of gibbsite crystallites with different morphologies and sizes on sodium oxalate {110} faces has been shown in Figure 5a, suggesting a morphology transition from 2D islands to 3D aggregates. It has been found that gibbsite 2D islands with pisolitic shapes align along macrosteps of sodium oxalate {110} faces in Figure 5d, indicating that Na2C2O4 macrosteps could be preferred nucleation sites, as the step edges play a role in blocking adatoms/surface species from migrating from upper terrace sides to a lower step. Pisolitic-shaped 2D islands are the predominant morphology of gibbsite secondary nuclei on oxalate {110} faces, which is similar to the morphology of 2D islands observed by using in situ AFM shown in Figure 4, but they lack any apparent crystallographic symmetry which has been further confirmed by the topographical image in Figure 5f.

Figure 4. AFM deflection images of gibbsite nucleation on macro steps of sodium oxalate {110} faces in Test Liquor 2 at 25 °C. The scan areas for images (a−c) are 1.5 × 1.5 μm2. (d) Height profiles along the blue dotted lines with an arrowhead pointed in the [110̅ ] direction in images (a, c).

3.2.2. Ex Situ SEM Observations. The use of in situ AFM is limited to low temperature and low caustic strengths because the glass window of the AFM cantilever holder and silicon nitride cantilever cannot resist highly caustic Bayer solution and high crystallization temperature (50−80 °C). In situ AFM

Figure 5. SEM images of gibbsite nucleation on sodium oxalate {110} faces in Test Liquor 3 at 50 °C for 8 h (a−e). (b) Enlarged image of gibbsite aggregates, corresponding to the yellow dotted rectangle in (a). (d) Enlarged image of gibbsite crystallites on macrosteps of oxalate {110} faces, corresponding to the light blue dashed rectangle in (a). (f) Ex situ AFM image of gibbsite crystallite on a macrostep of oxalate {110} faces under the same conditions. E

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Figure 6. SEM images of gibbsite growth on sodium oxalate {110} faces in Test Liquor 4 at 80 °C for 48 h (a−k). The crystal face notations (Miller Indices) and direction for gibbsite and sodium oxalate have been labeled by yellow and light blue integers, respectively, in (a−d,f,g). (j) Enlarged image of gibbsite 2D islands on a {001} face of gibbsite aggregates, corresponding to the red dotted rectangle in image (i). (k) Enlarged image of gibbsite 2D islands on a {001} face of gibbsite aggregates, corresponding to the green dashed rectangle in image (i).

the top of the original one. The surfaces of these newly formed gibbsite crystallites, in turn, provide more potential sites for gibbsite 2D nucleation, promoting the formation of the next generation of gibbsite crystallites. Several generations of gibbsite crystallites stack randomly in the normal direction, eventually forming the 3D gibbsite aggregates in Figure 5b,c. Compared with the surface of the sodium oxalate {110} faces, the surface of gibbsite crystallites is more energetically favorable for gibbsite nucleation, since gibbsite nucleation on Na2C2O4 crystals is a process of heteroepitaxial nucleation, the difficulty of which arises from the fact that a lattice mismatch between gibbsite and sodium oxalate leads to the strain energy stored in the gibbsite nuclei grown on a mismatched Na2C2O4 substrate. In order to promote gibbsite growth on Na2C2O4 crystals, low aluminum supersaturation (0.71), high temperature (80 °C), and long residence time (48 h) are necessary for the

Only a few 2D islands appear to the pseudohexagonally symmetric in Figure 5e. N. Brown23 who studied gibbsite secondary nucleation on industrial gibbsite crystals also found that the secondary nuclei were not crystallographically welldefined and the pseudohexagonal symmetry only became apparent later in their morphological development. Considering the static crystallization conditions, the formation of 3D gibbsite aggregates shown in Figure 5b,c cannot go through the agglomeration mechanism by which many small particles collide and adhere, eventually forming a new large particle with the help of agitation. The layer-by-layer growth mechanism which is also named the two-dimensional (2D) nucleation mechanism could be responsible for the morphology of 3D gibbsite aggregates. As described in Figure 4b, several 2D nuclei appear and grow simultaneously on the surface of the gibbsite crystallite, finally forming new gibbsite crystallites on F

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Figure 7. AFM deflection images of gibbsite and oxalate inclusion through the growth sequence of macro and elementary steps on sodium oxalate {110} faces in Test Liquor 1 at 25 °C. The scan areas for the images (a−f) are 1.5 × 1.5 μm2. Height profiles along the blue and yellow dotted lines in images (b, d, f, and h) have been analyzed in Figure 8.

relative to {110} faces. If the difference in growth rate between these prismatic faces is large enough under suitable conditions, the {100} faces will disappear and gibbsite hexagons develop into star-like crystals and lozenges bounded by {110} faces, shown in Figure 6c and f, respectively. Compared with individual crystals, the formation of gibbsite crystal aggregates on oxalate {110} faces are more common in Figure 6e−i. In the early stages of aggregation, several individual hexagons and lozenges formed unordered clusters shown in Figure 6e,f. This process could be through the 2D nucleation mechanism under the static crystallization conditions. Then, the subsequent intergrowth among individual crystals within one aggregate take place, cementing the initial formed loose clusters to firm aggregates in Figure 6g−i. The morphology of randomly oriented aggregates suggest that there might not be a specific pattern to achieve the oriented selfassembly among individual crystals. It can however be seen that the flat basal {001} faces seem to play a role in the predominantly exposed planes of gibbsite aggregate, particularly in Figure 6f,h,i, because the growth adhesive forces were greater at the non-{001} faces in comparison with the {001} faces.26 As shown in Figure 6j,k, the pseudohexagonally shaped protrusions which share the same orientation with respect to their underlying mother gibbsite crystals further confirm that the formation of gibbsite aggregates can be attributed to 2D nucleation mechanism and subsequent crystal growth.

development of the apparent crystallographic symmetry. Our results indicate that the gibbsite growth on sodium oxalate {110} faces results in crystals with various morphologies, including hexagons in Figure 6d, lozenges in Figure 6f, blocks in Figure 6a (crystal ①) and Figure 6(b,c), and random aggregates in Figure 6(e−i). According to the literature,24,25 the typical morphology of nature and industrial gibbsite crystals is pseudohexagonal with {001} basal, {100} and {110} prismatic faces, as shown in Figure 6b. The appearance of {101} chamfered faces in Figure 6b indicates that this crystal experienced long-term growth after its nucleation on oxalate {110} faces, since the occurrence of these faces agreed with the equilibrium morphology of gibbsite at low driving forces (low aluminum supersaturation). Low driving forces can promote spiral growth on {001} faces as seen occasionally, for example, in Figure 6d, even though all of the gibbsite faces usually grow by a layer-by-layer growth mechanism or a 2D nucleation mechanism. On the other hand, the high driving forces (high aluminum supersaturation) obtained at the beginning of the gibbsite crystallization process can result in flat {001} faces with a rounded morphology as seen in Figure 6a (crystal ③) as all prismatic faces grow kinetically rough.25 C. Sweegers et al.24 who calculated the edge (free) energies of nuclei on prismatic {100} and {110} faces using Monte Carlo algorithm suggested that the edge energy of {100} faces is much lower than that of {110} faces, and thus the {100} faces are the fast growing faces G

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Figure 8. Height analysis of gibbsite and sodium oxalate crystallites based on the in situ AFM observation. (a) Sequence of height profiles for gibbsite crystallites highlighted by blue dotted lines in Figure 7. (b) Sequence of height profiles for oxalate crystallites highlighted by yellow dotted lines in Figure 7.

Gibbsite growth on sodium oxalate {110} faces is generally one kind of heteroepitaxial growth since gibbsite crystal and Na2C2O4 substrate do not exhibit the same crystal structure. As illustrated in Figure 6a−f, gibbsite {001} faces seem to be preferentially grown on sodium oxalate {110} substrates ({001}Gibbsite∥{110}Oxalate). However, there is no preferred inplane orientation relationship between these two faces. For example, the [100]Gibbsite direction on the {001} face of crystal ③ shown in Figure 6a is almost parallel to the [001]Oxalate direction on the {110} faces of Na2C2O4 substrate, whereas the [100]Gibbsite direction on the {001} face of hexagonal gibbsite in Figure 6d is almost perpendicular to the [001]Oxalate direction on the {110} faces of Na2C2O4 substrate. More importantly, gibbsite crystals tend to nucleate and grow at the edge of Na2C2O4 substrates in Figure 6c,g,h; in some cases, the contact area between gibbsite and the Na2C2O4 substrate involves two Na2C2O4 faces shown in Figure 6g,h. This phenomenon has also been observed by in situ AFM observations shown in Figure 4d, where a gibbsite crystallite overlapped a Na2C2O4 microstep and grew adjacent to the edge of a Na2C2O4 macrostep. Ex situ SEM observation for gibbsite nucleation in Figure 5d,f has further demonstrated that gibbsite 2D islands align along macrosteps of sodium oxalate {110} faces. Unlike atomically flat substrates for conventional epitaxial growth,27,28 the sodium oxalate {110} faces characterized by the high density of steps parallel to [001]Oxalate direction are rough substrate, providing energetically ideal sites for gibbsite secondary nucleation and growth. Aluminum molecules that attach to Na2C2O4 steps make more bonds to neighboring molecules than the ones that attach to flat substrate, decreasing the surface free energy for gibbsite nucleation. Therefore, in competition with surface free energy for nucleation, the strain energy of epitaxial growth arising from lattice mismatch cannot be the predominant factor on rough Na2C2O4 substrates, because it fails to regulate the preferred orientation relationship between gibbsite and Na2C2O4 substrate. 3.3. Inclusion of Gibbsite and Sodium Oxalate Crystallites into {110} Faces of Sodium Oxalate Substrates. 3.3.1. In Situ AFM Observations at 25 °C. During industrial crystallization, as gibbsite nucleates on Na2C2O4 crystals, the growth of steps on sodium oxalate {110} faces takes place as well. This intergrowth process could result in the inclusion of gibbsite crystallites into the bulk of Na2C2O4 crystals. To mimic this intergrowth process, Na2C2O4

crystals covered with gibbsite crystallites shown in Figure 5a have been used for the further growth of steps on sodium oxalate {110} faces. As shown in Figure 6a, there are two crystallites with different shapes on sodium oxalate {110} faces, one of which in a rounded shape is a gibbsite crystallite labeled by white arrow ①. The surface of this gibbsite crystallite could be made up of many gibbsite 2D nuclei held together by weak interparticle forces. When the lateral force originating from the AFM cantilever tip exceeded the threshold of interparticle forces, some of these 2D nuclei were manipulated by the cantilever tip and moved uncontrollably during every scan line, leading to this blurry image of gibbsite crystallites. The other crystallite, with a needle-shaped Na2C2O4 labeled by white arrow ② in Figure 6a, was a foreign crystal rather than an original crystal nucleated on sodium oxalate {110} substrates. During the step growth on sodium oxalate {110} substrates, these two crystallites experienced very different situations. In the first 75 min, the growth of {110} faces is dominated by propagation of elementary steps in Figure 7a−e. The advancement of these elementary steps gradually covered the surface of the gibbsite crystallite, forming a cavity around residual gibbsite crystallite at 45 min. The height analysis in Figure 8a also confirms that the vertical height of gibbsite crystallite (between ① and ③) decreased from 72.5 nm at 15 min to 33.2 nm at 45 min, and the vertical depth of the cavity reached 16.3 nm (between ① and ②) at 45 min. The strain energy of epitaxial growth arising from the lattice mismatch between gibbsite and sodium oxalate leads to the relatively slow advancement for the first layer of elementary steps which directly contacts with the surface of gibbsite crystallite. The subsequent layers of elementary steps which grow on the top of the first layer without any strain energy still keep a relatively fast advancement rate, and catch up with the forepart of the first layer, stacking along oxalate [110] direction (Figure 8a) and finally forming this cavity structure. Near the completion of first 60 min, a macrostep (indicated by dark purple color) entered at the upper right side in Figure 7e. The advancement of this macrostep is oriented along the oxalate [11̅0] direction. At 75 min, this macrostep has also been labeled by blue color in Figure 8a. After 105 min, this macrostep swept over the residual of the gibbsite crystallite, only leaving a tiny little cavity shown in Figure 8a. The {200} face of the foreign Na2C2O4 crystallite (white arrow ② in Figure 7a) is parallel to {110} faces of Na2C2O4 H

dx.doi.org/10.1021/cg501465v | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 9. SEM images of gibbsite inclusion in sodium oxalate {110} faces in Test Liquor 5 at 50 °C for 8 h (a). (b) Enlarged image of gibbsite inclusion in sodium oxalate {110} faces, corresponding to the yellow dotted rectangle in image (a). (c) Enlarged image of gibbsite inclusion in sodium oxalate {110} faces, corresponding to the light blue dashed rectangle in image (a).

As described before, one of most common problems caused by such coprecipitation is the generation of gibbsite fine particles on Na2C2O4 crystals through a 2D nucleation mechanism, which can be easily broken away from the Na2C2O4 surface by shear stress in agitated precipitation tanks and thus increase fine particles in the gibbsite product. On the other hand, the inclusion of gibbsite crystallites into the bulk of sodium oxalate {110} faces might have an influence on reducing the number of fine gibbsite crystals to some extent.

substrate or perpendicular to the [110] direction of Na2C2O4 substrate, which has been highlighted by the green color in Figure 7c. During the first 75 min, compared with the gibbsite crystallite, the foreign Na2C2O4 crystallite remained almost constant in height as shown in Figure 8b, suggesting that both the {200} face of the foreign Na2C2O4 crystallite and {110} faces of Na2C2O4 substrate share a similar growth rate along the [110] direction of Na2C2O4 substrate. Moreover, the width of the {200} face of the foreign Na2C2O4 crystallite reduced from 52.8 nm at 15 min to 29.3 nm at 75 min, while the {110} face of the foreign Na2C2O4 crystallite adjacent to its {200} face became much wider in the first 75 min, which has been highlighted by light blue color in Figure 7g. This phenomenon further reinforces the morphological importance of sodium oxalate {110} faces under these conditions. Meanwhile, this foreign Na2C2O4 crystallite also grow longer along its own [001] direction (the direction of crystallite length), from 620 nm at 15 min to 670 nm at 75 min. When the macrostep moved in contact with this foreign Na2C2O4 crystallite, it began to sink into the growing layer of Na2C2O4 substrate at 105 min, forming a long cavity merely on the left side of the forgein crystallite, shown in Figure 7h and Figure 8b. This indicates that the foreign Na2C2O4 crystallite was high enough to block the first wave of macrostep advancement which tried to entrap it into the bulk of the Na2C2O4 substrate. 3.3.2. Ex Situ SEM Observations. To further demonstrate the formation of gibbsite inclusion into sodium oxalate {110} faces, Na2C2O4 crystals covered with big gibbsite crystals obtained from the second stage gibbsite nucleation on oxalate substrate have been employed to test gibbsite inclusion using Test Liquor 5 in the third stage. As shown in Figure 9a, gibbsite crystals tend to form randomly oriented large aggregates on the {110} faces of Na2C2O4 rods through the 2D nucleation mechanism and there is no preferred in-plane orientation relationship between gibbsite crystals and Na2C2O4 substrates. The blurry edges between gibbsite and Na2C2O4 crystals in Figure 9b,c indicate that many gibbsite crystals have been embedded into Na2C2O4 substrates, but the size of gibbsite crystals is too large to be fully enclosed in the bulk of Na2C2O4 crystals.

4. CONCLUSION The mechanism of gibbsite nucleation on sodium oxalate crystals and their intergrowth were studied under static conditions. To mimic the industrial Bayer liquor and overcome the limitation of AFM instrument, the test liquors containing 4 M NaCl and 1 M NaOH for in situ AFM study has been employed to investigate the step growth on {110} faces of sodium oxalate crystals, gibbsite nucleation, and inclusion on {110} faces of sodium oxalate crystals at 25 °C. The lateral advancement for oxalate steps was oriented along oxalate [11̅0] direction, controlled by layer-by-layer growth mechanism. The growth of steps on sodium oxalate {110} faces results in the inclusion of gibbsite crystallites into the bulk of sodium oxalate crystals. In situ AFM images also indicated that a pisoliticshaped gibbsite crystallite which is a two-dimensional (2D) island formed on the terrace adjacent to the edge of an oxalate macrostep, overlapping an oxalate microstep. To link the results of in situ AFM with industrial precipitation process, the synthetic Bayer liquor mainly containing 3.6 M NaOH + 0.32 M Na2CO3 + 0.1 M Na2SO4 + 0.56 M NaCl for ex situ SEM study has been used to demonstrate the gibbsite nucleation and inclusion on oxalate substrate. As shown in ex situ SEM images, the morphology of gibbsite 2D crystallites aligned along macrosteps of sodium oxalate {110} faces is consistent with that obtained by in situ AFM. More importantly, ex situ SEM images illustrate that there does not appear to be a preferred epitaxial relationship at the heterointerface between gibbsite crystals and sodium oxalate substrates, indicating that the magnitude of the surface free energy for nucleation is greater I

dx.doi.org/10.1021/cg501465v | Cryst. Growth Des. XXXX, XXX, XXX−XXX

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than that associated with strain energy caused by lattice mismatch.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the industrial sponsors, Rio Tinto Alcan and Queensland Alumina Limited and acknowledge the guidance provided by the industry advisor, Mr. Andrew Denton. The authors acknowledge the facilities, and the scientific and technical assistance, of the Australian Microscopy & Microanalysis Research Facility at The University of Queensland. The atomic force microscope (AFM) part of this work was performed at the Queensland node of the Australian National Fabrication Facility (ANFF).

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REFERENCES

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dx.doi.org/10.1021/cg501465v | Cryst. Growth Des. XXXX, XXX, XXX−XXX