Effects of Inorganic Anions on the Morphology of Sodium Oxalate

Mar 5, 2014 - The University of Queensland, School of Chemical Engineering, Level 3, Chemical Engineering Building (74), College Road, St. Lucia ...
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Effects of Inorganic Anions on the Morphology of Sodium Oxalate Crystallized from Highly 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: The crystallization of sodium oxalate (Na2C2O4) crystals was studied in the presence of different inorganic anions of the Bayer liquor in the alumina industry, including OH−, CI−, Al(OH)4−, CO32−, and SO42−. The morphology of the obtained Na2C2O4 crystals depends on the type and concentration of anion species under similar oxalate supersaturation. The presence of divalent anions (CO32−, SO42−) has a greater influence on the morphological variability, including cyclic twins, spherulites, and double-leaf structures, through the branching growth process. The specific surface areas of Na2C2O4 crystals grown in the presence of divalent anions (CO32−, SO42−) are more than 2 times higher than those in the presence of monovalent anions (OH−, CI−, Al(OH)4−). X-ray photoelectron spectra results indicate that the inorganic anions (CI−, Al(OH)4−, CO32−, SO42−) are adsorbed onto the surface of Na2C2O4 crystals, thereby inducing internal stress (defects) of crystals and modifying the crystal morphology.

1. INTRODUCTION Bauxite is the main ore type from which aluminum is sourced for the primary production of aluminum metal. Australia is the world’s largest producer of bauxite, representing 32% of global production in 2011.1 Unfortunately, most Australian bauxites contain high levels of organic matter, approximately 0.15−0.3% (w/w) total organic carbons, which is considerably higher than that found in bauxite ore bodies elsewhere in the world.2 More than 85% of the bauxite mined globally passes through the Bayer process to produce metallurgical grade alumina (Al2O3), which is the precursor of aluminum.1 In the Bayer process, alumina is extracted from bauxite using sodium hydroxide solution at high temperature in digestion vessels, while the majority of the organic carbon present in the bauxite is also extracted into the process liquor (named as Bayer liquor), where it contributes to numerous operational, maintenance, and production problems. One estimate of the impact of this on production costs places the annual cost of organics to the Australian alumina industry alone to be over A$500 million (US$450 million).3 Of many organic compounds present in Bayer liquor, sodium oxalate (Na2C2O4) is known to have one of the most detrimental effects on the alumina crystallization process.4 Sodium oxalate coprecipitates with the product, aluminum hydroxide (also named gibbsite, Al(OH)3), in the cooler end of the crystallization circuit of the Bayer process according to eq 1. 2Na +(aq) + C2O4 2 −(aq) = Na 2C2O4(s) © 2014 American Chemical Society

This is because the equilibrium solubility of Na2C2O4 can be substantially decreased by reducing the temperature.5 Such Na2C2O4 crystallization, in turn, results in numerous industrial problems, such as increasing the content of gibbsite fines,6 decreasing gibbsite agglomeration efficiency,7 increasing the formation rate of oxalate−gibbsite scale,8 and increasing the residual soda in alumina products.9 Most of these problems can be attributed to the morphological variability of Na2C2O4 crystals. Although oxalate tends to form monoclinic crystals, the morphology of Na2C2O4 crystals grown under industrial Bayer conditions is highly variable. Sodium oxalate crystals grown from aqueous solution were characterized by twinning along the {2 0 0} plane with a prismatic shape, well-developed {110} faces, and poorly developed {001} faces,10,11 as illustrated in Figure 1. Strom et al.11 studied Na2C2O4 crystals grown from pure aqueous solutions at two different supersaturations. High supersaturation (σ = 0.351) yielded small crystals (0.2 × 1.3 mm) elongated along the c axis, which tended to form agglomerates. In most cases, these crystals showed small {200} faces. Low supersaturation (σ = 0.054) yielded larger crystals (1.0 × 2.0 mm) without {200} faces and no elongation. Fu and Vaughan,12 who investigated Na2C2O4 crystals grown in aqueous sodium hydroxide solution, found that high concentrations of sodium hydroxide solution had a dramatic effect on Received: January 18, 2014 Revised: February 27, 2014 Published: March 5, 2014

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2. EXPERIMENTAL SECTION 2.1. Materials. Sodium oxalate was purchased from Alfa Aesar. Sodium hydroxide, sodium chloride, sodium carbonate, sodium sulfate, and aluminum hydroxide were obtained from Sigma-Aldrich. 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) were cleaned and rinsed with Millipore water. The stock solutions of 5 M NaOH, 4 M NaOH + 1 M NaCl, 3 M NaOH + 1 MNa2CO3, and 3 M NaOH + 1 M Na2SO4 were prepared by dissolving analytical grade chemicals in Millipore water, while the stock solutions of 4 M NaOH + 1 M NaAl(OH)4 and the synthetic Bayer liquor, 2.6 M NaOH + 1 M NaAl(OH)4 + 0.32 M Na2CO3 + 0.1 M Na2SO4 + 0.56 M NaCl), were prepared in an autoclave at 150 °C for 1 h. All of the crystallization experiments were carried out in aqueous solutions with a constant sodium concentration of 5.0 M as sodium has a direct influence on sodium oxalate supersaturation. 2.2. Measurements of Apparent Equilibrium Concentrations of Sodium Oxalate. To study the effects of different kinds of inorganic anions on the morphological variation of Na2C2O4 crystals under synthetic Bayer conditions, the supersaturation of Na2C2O4 in solutions containing different kinds of inorganic anions has to be controlled at very similar levels because supersaturation is the driving force of the crystallization process, playing an important role in the growth habits of crystals. Therefore, it is necessary to measure the apparent equilibrium Na2C2O4 concentrations under different synthetic Bayer conditions. The procedure for measurements of apparent equilibrium Na2C2O4 concentrations is as follows: Approximately 250 mL of synthetic solutions, the compositions of which are shown in Table 1, was placed inside the conical flask, and an excess amount of Na2C2O4 powders was added. Then, the above mixed solution was stirred and heated at 50 °C for 2 h, dissolving Na2C2O4 powder into the solution. The resulting solution was sealed and allowed to stand under static condition in an air-bath oven at 30 ± 1 °C for 3 months. Before sampling, each solution was filtered through a 0.2 μm pore size Nylon membrane filter. The saturated solutions were analyzed for oxalate anion concentration by ion chromatography (Dionex ICS-2000). The analytical results are shown in Table 1. 2.3. Crystallization of Sodium Oxalate. In a typical crystallization experiment, 1.32 g of Na2C2O4 was added into 1 L of 5 M NaOH solution in a conical flask. 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 filtered solution was sealed and allowed to stand under static condition in an air-bath oven at 30 °C for 7 days. After that, about 0.4 g of Na2C2O4 crystals was filtered, washed with 5 mL of 200 g/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 initial Na2C2O4 concentration was varied according to the compositions of synthetic solutions, as illustrated in Table 1. 2.4. Characterization. The morphology of Na2C2O4 crystals was examined by scanning electron microscopy (SEM, Philips XL30) at an accelerating voltage of 5 kV. X-ray powder diffraction (XRD) was carried out on a Bruker D8 ADVANCE X-ray diffractometer with Cu Kα irradiation (λ = 1.5406 Ǻ ) at a scanning speed of 0.025°/s over the 2θ range of 10−70°. XPS measurements were performed to analyze

Figure 1. Crystalline habits of Na2C2O4 grown from pure aqueous solution.

reducing the size of Na2C2O4 crystals during the crystallization process. Atomic force microscopy (AFM) analysis indicated that the surface roughness of {110} faces of sodium oxalate crystals grown in sodium hydroxide solution is higher than those grown in water, which is attributed to a high density of steps on {110} faces.12 Reyhani et al.7 reported the morphology of typical Na2C2O4 crystals grown from the industrial Bayer liquor, in which a wide range of strange morphologies were observed, such as fanlike, dumbbell, bow-tie, and even spherical shapes. Here, the branching resulting from intrinsic faults in crystals is responsible for the observed macroscopic morphologies. However, the origins of this morphological variability are still poorly understood because the composition of industrial Bayer solution is very complex and many components, particularly inorganic and organic impurities that can substantially influence the morphology of Na2C2O4 crystals. In this work, the relationship between morphological variability of Na2C2O4 crystals and inorganic components of the Bayer liquor, including NaOH, NaCl, NaAl(OH)4, Na2CO3, and Na2SO4, is established. We realized that the morphology of Na2C2O4 crystals could be made more manageable by adjusting the inorganic components and their concentrations in the Bayer liquor, which may have potential applications in the alumina industry. On the experimental side, unlike the agitated crystallization process in the alumina industry, the crystal growth process in this study was investigated under static conditions as the shear stress from mechanical agitators could damage the intrinsic morphology of Na2C2O4 crystals. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopic (XPS), and nitrogen adsorption techniques were used to characterize Na2C2O4 crystals produced under different crystallization conditions.

Table 1. Initial and Apparent Equilibrium Concentrations of Sodium Oxalate in Different Synthetic Solutions at 30 °C composition of synthetic liquors

initial Na2C2O4 concentration, g/L

equilibrium Na2C2O4 concentration, g/L

relative supersaturation, (Cinitial − Cequilibrium)/Cequilibrium

5 M NaOH 4 M NaOH + 1 M NaCl 4 M NaOH + 1 M NaAl(OH)4 3 M NaOH + 1 M Na2CO3 3 M NaOH + 1 M Na2SO4 synthetic Bayer liquor

1.32 1.21 1.86 2.50 2.41 1.90

0.88 0.81 1.23 1.65 1.60 1.27

0.50 0.49 0.51 0.52 0.51 0.50

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the “rod-to-bundle-to-flower-like structure” upon growth. As shown in Figure 3(a,f,k), the oxalate rods are about 40−60 μm in diameter and 500−800 μm in length growing along the [001] direction. The morphological development of oxalate bundles begins with oxalate rods on which new layers develop by branching along the growth direction in Figure 3(g,k). These layers form new oxalate rods, and the ends of the structure start to branch. The cross-shaped bundle was also observed (Figure 3b). The SEM image (see Figure 3b, inset in the top right corner) clearly indicates that it arises from a mutually penetrated growth process of two individual crystals, leading to so-called penetration twins.15 When crystals join to form a twin during nucleation, they should develop to become equal in size, which is observed in this system. The flower-like structures in Figure 3(c,h,m) and oxalate bundle in Figure 3(i) are constructed by the rodlike crystalline units with different widths and lengths. The growth orientation between each rodlike unit is not so ordered. Moreover, the oxalate rods with different sizes randomly aggregated together at the root of the flower-like structures in the presence of 5 M NaOH and 4 M NaOH + 1 M NaCl, shown in Figure 3d,i, respectively. The enlarged SEM images of Figure 3e,j show the details of such aggregates in Figure 3d,i. Considering the steric crystallization conditions, the effect of mechanical stirring or shaking on the formation of the aggregates can be excluded. The oxalate secondary nucleation could be more favorable at the root of the flower-like structures since the crystal faces here are less developed and very rough, which provide potential sites for oxalate secondary nucleation. In contrast, the root of the flower-like structures in the presence of Al(OH)4− in Figure 3(n,o) shows well-organized radial growth from the core. The SEM images shown in Figure 4 demonstrate the morphological evolution of Na2C2O4 crystals obtained from the caustic solution in the presence of Na2CO3 (Figure 4a−e), Na2SO4 (Figure 4f−j), and synthetic Bayer liquor (Figure 4k− o) for 7 days at 30 °C. It clearly indicates that oxalate crystals obtained in the mixed solution of NaOH and Na2CO3 are composed of spherulites (Figure 4e) as well as a few intermediates, such as bundles (Figure 4a), cross-shaped bundles (Figure 4b), six-arm bundles (Figure 4c), and lessdeveloped spherulites (Figure 4d). A bundle of oxalate rods is shown in Figure 4a, with the top and bottom fanning out while the middle remaining thin. The individual oxalate rods have an average diameter of ∼10 μm, and the bundles are ∼300 μm in length. Compared with cross-shaped bundles grown in 5 M NaOH solution (Figure 3b), cross-shaped bundles in Figure 4b show the three-dimensional hierarchical structures that are produced by multistep-branching growth from the symmetric core of the bundles. These are called penetration twins. When this twinning relation is repeated on {110} faces, a six-arm bundle (Figure 4c) is formed, which is referred to as cyclic twins.15 The six-arm structure exhibits a symmetric structure: arm lengths are all nearly equal, and angles between major arms are roughly 60°. To form a twin that is in a higher energy state, an additional driving force is required to overcome this excess energy. A high driving force may result not only from a highsupersaturation condition but also from the presence of one or more impurities.15 Considering the similar supersaturation in six different crystallization systems in Table 1, CO32− might play an important role in the formation of cyclic twinning structures. The arms of such cyclic twinning structures kept fanning out perpendicular and parallel to the twinning plane,

Na2C2O4 surfaces to identify the chemical species produced on the surface. Data were acquired using a Kratos Axis ULTRA X-ray photoelectron spectrometer incorporating a 165 mm hemispherical electron energy analyzer. The incident radiation was monochromatic Al Kα X-rays (1486.6 eV) at 150 W (15 kV, 15 mA). Survey scans were carried out over a 1200−0 eV binding energy (BE) range with 1.0 eV steps and a dwell time of 100 ms. Narrow high-resolution scans were run with 0.05 eV steps and a 250 ms dwell time. Spectra analysis and peak fitting were carried out using CasaXPS software (Version 2.3.14). Spectra have been charge corrected using the main line of the carbon 1s spectrum produced by surface deposition of adventitious carbon (AC) with an assigned BE of 284.6 eV as an internal reference. AC is predominantly formed from adsorbed hydrocarbon polymers.13 The sources of AC could include the constituents of the ambient air to which the sample is exposed before introduction into the XPS spectrometer, as well as the possibility of contamination through sample handling or exposure to low partial pressures of contaminants in the vacuum system of XPS.14 The surface area of sodium oxalate crystals was measured by nitrogen gas adsorption. Samples were first degassed overnight to pressures of ∼2 Pa at 200 °C in a Micromiretics VacPrep061, prior to analysis with a Micromeritics Tristar 3000 gas adsorption instrument.

3. RESULTS AND DISCUSSION The white sodium oxalate crystals were characterized using Xray diffraction (XRD) to verify their structures. All of the

Figure 2. XRD patterns of Na2C2O4 crystals obtained in the presence of different kinds of inorganic impurities: 5 M NaOH (a), 4 M NaOH + 1 M NaCl (b), 4 M NaOH + 1 M NaAl(OH)4 (c), 3 M NaOH + 1 M Na2CO3 (d), 3 M NaOH + 1 M Na2SO4 (e), synthetic Bayer liquor: 2.6 M NaOH + 1 M NaAl(OH)4 + 0.32 M Na2CO3 + 0.1 M Na2SO4 + 0.56 M NaCl (f).

diffraction peaks, as shown in Figure 2, could be indexed to the monoclinic phase (space group: P21/c) of Na2C2O4 with calculated lattice constants a = 10.426 Å, b = 5.255 Å, c = 3.479 Å (JCPDS 01-075-1816). No peaks of other impurities were detected in the experimental error range, indicating the formation of pure products. The morphologies of Na2C2O4 crystals produced in the presence of different kinds of inorganic impurities for 7 days at 30 °C were studied by scanning electron microscopy (SEM), and the results are shown in Figures 3 and 4. A coexisting family of rods, bundles, and flower-like structures appear shown in Figure 3(a,b,f,g,k−o), suggesting a morphology transition of 1974

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Figure 3. SEM images of Na2C2O4 obtained in the presence of different kinds of inorganic impurities including 5 M NaOH (a−e), 4 M NaOH + 1 M NaCl (f−j), 4 M NaOH + 1 M NaAl(OH)4 (k−o).

take place both in the middle of the bundles and at their ends, which is more flexible than the branching growth only from the symmetric core of the bundles, as shown in Figure 4a−d. Such more flexible branching growth helps the bundles keep fanning out, producing the two-anchor structures (Figure 4g) and twohemisphere structures (Figure 4h). Then, each fluke in one two-anchor structure (Figure 4g) started to connect with the

leading to the less-developed spherulites (Figure 4d) and ultimately well-developed spherulites (Figure 4e). A very different growth scenario was found for Na2C2O4 crystals grown in the presence of Na2SO4. The oxalate rods within the bundles (Figure 4f) and their surfaces are more compact than that in the presence of Na2CO3 (Figure 4a). Multistep-branching growth in such bundles (Figure 4f) could 1975

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Figure 4. SEM images of Na2C2O4 obtained in the presence of different kinds of inorganic impurities including 3 M NaOH + 1 M Na2CO3 (a−e), 3 M NaOH + 1 M Na2SO4 (f−j), synthetic Bayer liquor: 2.6 M NaOH + 1 M NaAl(OH)4 + 0.32 M Na2CO3 + 0.1 M Na2SO4 + 0.56 M NaCl (k−o).

shown at the top right corners of Figure 4f−j as an aid to understanding a possible path of the morphological evolution. The morphologies of Na2C2O4 crystals grown from synthetic Bayer liquor in Figure 4k−o show traits attributed to different anions. As shown in Figure 4k, bundle-like and cross-shaped morphologies are based on subindividual oxalate rods that are more compact than that in the presence of monovalent anions (OH−, CI−, Al(OH)4−), but less compact than that in the

opposite one, inducing the formation of two voids in the spherical center of the final forms (Figure 4i), which are referred to as double-leaf structures.16 The meeting of the opposite ends at the equator of the two-hemisphere structure results in well-developed spherulites shown in Figure 4j. Schematic illustration of the morphology of Na2C2O4 crystals obtained from the caustic solution in the presence of Na2SO4 is 1976

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Figure 5. SEM images of subindividual oxalate rods within flower-like/spherulite morphologies in the presence of different kinds of inorganic anions including 5 M NaOH (a), 4 M NaOH + 1 M NaCl (b), 4 M NaOH + 1 M NaAl(OH)4 (c), 3 M NaOH + 1 M Na2CO3 (d), 3 M NaOH + 1 M Na2SO4 (e), synthetic Bayer liquor: 2.6 M NaOH + 1 M NaAl(OH)4 + 0.32 M Na2CO3 + 0.1 M Na2SO4 + 0.56 M NaCl (f).

presence of divalent anions (CO32−, SO42−). Similar to the flexible branching growth in the presence of Na2SO4 shown in Figure 4f−h, the multistep-branching growth for oxalate crystals produced from synthetic Bayer liquor increases the total misorientation between the original nucleus and the last generation of subindividual oxalate rods in Figure 4i−m, leading to the less-developed spherulites (Figure 4n) and welldeveloped spherulites (Figure 4e), although SO42− has the lowest the concentration among all the anion species in synthetic Bayer liquor. Another evidence to support the effect of SO42− on the branching growth is that, similar to the doubleleaf structure in Figure 4i, a one-leaf structure is formed in synthetic Bayer liquor (see Figure 4o, the inset in the top right corner). The other leaf might be overlapped by the subindividual oxalate rods in this structure. These results suggest that divalent anions exhibit a more significant effect on morphological variability of Na2C2O4 crystals than monovalent anions. Similarly, Ye et al.,17 who studied the crystallization of Na2CO3, found that the presence of SO42− affected the morphology of the Na2CO3 crystals, whereas NO3− and CI− had no apparent effect on the crystalline products. On the basis of our experimental observations concerning morphology, the branching growth may be discussed within the context of the so-called “noncrystallographic” crystal branching model.17 This model is generally associated with fast crystal growth and is caused by high supersaturation of the surrounding medium, internal crystal stress (defects), and a

Figure 6. Partial XRD patterns of Na2C2O4 crystals obtained in the presence of different kinds of inorganic impurities: 5 M NaOH (a), 4 M NaOH + 1 M NaCl (b), 4 M NaOH + 1 M NaAl(OH)4 (c), 3 M NaOH + 1 M Na2CO3 (d), 3 M NaOH + 1 M Na2SO4 (e), synthetic Bayer liquor: 2.6 M NaOH + 1 M NaAl(OH)4 + 0.32 M Na2CO3 + 0.1 M Na2SO4 + 0.56 M NaCl (f).

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Figure 7. (1) XPS spectra of Na2C2O4 crystals in the presence of different kinds of inorganic anions including 5 M NaOH (a), 4 M NaOH + 1 M NaCl (b), 4 M NaOH + 1 M NaAl(OH)4 (c), 3 M NaOH + 1 M Na2CO3 (d), 3 M NaOH + 1 M Na2SO4 (e), synthetic Bayer liquor: 2.6 M NaOH + 1 M NaAl(OH)4 + 0.32 M Na2CO3 + 0.1 M Na2SO4 + 0.56 M NaCl (f); (2−4) High-resolution XPS spectra of carbon 1s peaks for 5 M NaOH (a), 3 M NaOH + 1 M Na2CO3 (d), and synthetic Bayer liquor (e), respectively.

small kinetic coefficient of the growing crystal faces (usually characterized by the slowest growth rate). Supersaturation is the driving force for crystallization and is the most important parameter controlling noncrystallographic branching. Considering the similar supersaturation for six different crystallization systems in our experimental design, shown in Table 1, branching growth could be attributed to impurity anions present in the crystallization systems, the mechanism of which is that impurity anions can be incorporated into the growing crystal, thereby inducing internal stress (defects) and modifying the anisotropy of crystal growth. A direct relationship between impurity concentration, internal stress, and degree of branching has been established for several crystals.16 In particular, the size, shape, and structure of hematite (α-Fe2O3) have been controlled systematically by adsorption of impurity anions (Cl−, SO42−, PO43−, OH−) on the crystal surface during the crystallization of hematite.18,19

The morphologies of subindividual oxalate rods within flower-like/spherulite structures in the presence of different kinds of impurity anions are shown in Figure 5a−f. It can be inferred from the examples given in Figure 5a,b that {110} faces of oxalate rods contain a high density of steps parallel to the [001] direction, particularly for oxalate rods grown in the presence of NaCl, indicating that the growth of {110} faces takes place in terms of a layer-by-layer growth mechanism. Partial XRD patterns in Figure 6 (curves a and b) demonstrate very strong (110) peaks for these two kinds of oxalate rods, while the strong (200) peaks can also be attributed to the high density of steps that could be made of adjacent (110) and (200) planes, as illustrated in Figure 1c. The ends of oxalate rods in Figure 5a,b have rounded forms with several growth hillocks, indicating that the growth of (001) faces does not dominate. This is supported by the very weak (001) peaks in Figure 6 (curves a and b) as well. For oxalate rods grown in the presence of NaAl(OH)4 shown in Figure 5c, the pseudohex1978

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the amounts of each impurity anion species incorporated, XPS spectra of oxalate crystals from six different crystallization systems were obtained. The results indicate that impurity anions, including Cl−, Al(OH)4−, and SO42−, are adsorbed onto the surface of Na2C2O4 crystals in Figure 7 (1). High-resolution photoelectron spectra of carbon 1s peaks have been used to test if CO32− have been adsorbed onto Na2C2O4 crystals in Figure 7 (2−4). It is important to note that no CO32− was detected on the oxalate sample grown in 5 M NaOH solution, as shown in Figure 7 (2), in which C−C and C−O bonding situated at 284.8 and 286.3 eV, respectively, can be attributed to adventitious carbon depositing on the surface of almost all air-exposed materials.14 As shown in Figure 7 (3−4), CO bonding of the carbonate group (289.0 eV) was separated from CO bonding of the oxalate group (288.3 eV) in Na2C2O4 crystals.13 The atomic percentages of the carbonate group on Na2C2O4 crystals grown in 3 M NaOH + 1 M Na2CO3 solution and synthetic Bayer liquor are 5.2% and 1.7%, respectively, the ratio of which is equal to 3.06 (5.2%/1.7% ≈ 3.06). This ratio corresponds to the ratio of Na2CO3 concentrations in such two kinds of solutions (1 M/0.32 M ≈ 3.13), suggesting that the amount of adsorbed CO32− on Na2C2O4 crystals can rise with the increase of Na2CO3 concentration in the solution. To quantify the effect of inorganic anions on the morphological variability of Na2C2O4 crystals, BET specific surface areas for six different crystallization systems are obtained in Figure 8. Sodium oxalate crystals grown in the presence of divalent anions (CO32−, SO42−) exhibit much higher specific surface areas than oxalate crystals produced in the presence of monovalent anions (OH−, Cl−, Al(OH)4−) and in the synthetic Bayer liquor. This large amount of additional surface area could provide more potential sites for secondary nucleation of gibbsite,7 thereby increasing fine particles of gibbsite in the Bayer process and changing the final particle size distribution in gibbsite products. As correct particle size of gibbsite products is very important to smelter operations for aluminum production, divalent anions CO32− and SO42− are needed to be carefully controlled, particularly SO42−, which could be the most detrimental inorganic anion in the Bayer liquor in this respect.

Figure 8. BET specific surface areas of Na2C2O4 crystals obtained in the presence of different kinds of inorganic impurities: 5 M NaOH (a), 4 M NaOH + 1 M NaCl (b), 4 M NaOH + 1 M NaAl(OH)4 (c), 3 M NaOH + 1 M Na2CO3 (d), 3 M NaOH + 1 M Na2SO4 (e), synthetic Bayer liquor: 2.6 M NaOH + 1 M NaAl(OH)4 + 0.32 M Na2CO3 + 0.1 M Na2SO4 + 0.56 M NaCl (f).

agonal (001) faces have been formed and divided into two parts by the {200} planes, which is the twinning plane for oxalate crystals grown in pure aqueous solution. Reyhani et al.,20 who studied the morphologies of Na2C2O4 grown in pure aqueous solution, indicated that the growth is similar on both sides of the {200} twinning plane, and there is evidence of chamfering between two of the opposing faces in Figure 1a,b. Unlike oxalate grown in pure aqueous solution, the growth of (001) faces is interrupted by the {200} plane into two parts with different heights, indicating that (001) faces can grow via a twodimensional nucleation mechanism. The partial XRD pattern in Figure 6 (curve c) shows the strongest (200) peak within six different oxalate rods, as {200} planes play an important part in the formation of pseudohexagonal (001) faces. Oxalate rods grown in the presence of Na2CO3, illustrated in Figure 5d, exhibit pseudohexagonal (001) faces on which multiple nucleation occurs and spreads across the crystal face, suggesting that the two-dimensional nucleation mechanism controls the face growth. Polygonal spiral growth hillocks with single summits are observed on the lozenge-shaped (001) faces of oxalate rods produced in the presence of Na2SO4 (Figure 5e), indicating that this face grows by the spiral growth mechanism. Oxalate rods grown in synthetic Bayer liquor inherit many traits from different anions, with lozenge-shaped (001) faces that are interrupted by the (200) plane. Two-dimensional nuclei on (001) faces (Figure 5f) indicate that the growth of (001) faces is controlled by a birth and spread mechanism. The lozengeshaped (001) faces for these two kinds of oxalate rods correspond to the very weak (200) peaks within six different oxalate rods in Figure 6 (curves e and f). These results suggest that impurity anions not only change the morphologies of oxalate crystals but also affect the shapes of subindividual oxalate rods within the complex flower-like/spherulite structures. The impurity anions present in the six different crystallization systems studied can be adsorbed on the surface of Na2C2O4 crystals, thereby inducing internal stress (defects) and modifying the morphology of oxalate crystals. To investigate

4. CONCLUSION The Na2C2O4 crystallization was studied in synthetic solutions in the presence of compounds that are generally among the most prevalent inorganic components of Bayer liquor, namely, sodium hydroxide, sodium chloride, sodium aluminate, sodium carbonate, and sodium sulfate. These were studied individually and in a combination typical of Bayer liquor. Under the similar oxalate supersaturations, the morphology of the oxalate crystals obtained was shown to depend on the nature and concentrations of the inorganic anion species present. The presence of monovalent anions (OH−, Cl−, Al(OH)4−) produces the flower-like oxalate crystals constructed by the rodlike units with different widths and lengths. {110} faces of oxalate rods in the presence of NsCl contain a high density of steps parallel to the [001] direction, indicating that the growth of {110} faces occurs by a layer-by-layer growth mechanism. For oxalate rods grown in the presence of NaAl(OH)4, pseudohexagonal (001) faces have been formed and divided by the {200} planes into two parts with different heights. The divalent anions (CO32−, SO42−) exhibit a more significant effect on morphological variability of Na2C2O4 crystals than monovalent anions. The presence of these divalent anions 1979

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Crystal Growth & Design

Article

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leads to many different oxalate structures, including cyclic twins, spherulites, and double-leaf structures, through a branching growth process. It is proposed that the mechanism for the morphological variability of Na2C2O4 crystals involves the adsorption of CI−, Al(OH)4−, CO32−, and SO42− onto the surface of Na2C2O4 crystals, inducing internal stress (defects) of crystals and thus modifying the crystal morphology. This work on the Na2C2O4 crystallization by different inorganic anions provides useful information for morphology control of Na2C2O4 crystals, suggesting that divalent anions CO32− and SO42− in the Bayer liquor can largely increase the specific surface area of Na2C2O4 crystals. This large amount of surface area could provide more potential sites for secondary nucleation of gibbsite, increasing gibbsite fines. Therefore, Na2C2O4 crystals with different morphologies and specific surface areas prepared in this study can be potentially used to study the gibbsite nucleation mechanisms on the surface of Na2C2O4 crystals in the precipitation circuit of the alumina industry.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.F.). *E-mail: [email protected] (J.V.). 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 Centre for Microscopy and Microanalysis at The University of Queensland.

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REFERENCES

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dx.doi.org/10.1021/cg5000952 | Cryst. Growth Des. 2014, 14, 1972−1980