Characterization of Aluminum Trihydroxide Crystals Precipitated from

Chapter 12

Characterization of Aluminum Trihydroxide Crystals Precipitated from Caustic Solutions Mei-yin Lee , Gordon M . Parkinson , Peter G. Smith , Frank J. Lincoln , and Manijeh M . Reyhani

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1997 | doi: 10.1021/bk-1997-0667.ch012

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Research Centre for Advanced Mineral and Materials Processing, Department of Chemistry, University of Western Australia, Nedlands, Western Australia A . J. Parker Cooperative Research Centre for Hydrometallurgy, School of Applied Chemistry, Curtin University of Technology, GPO Box U 1987, Perth 6001, Western Australia A . J. Parker Cooperative Research Centre for Hydrometallurgy, Division of Minerals, CSIRO, P.O. Box 90, Bentley, Western Australia

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Gibbsite is one of the polymorphs of aluminium trihydroxide and is produced commercially by the Bayer Process through its crystallization from sodium aluminate solutions. This work demonstrates that there is a complex interplay of factors which affect the polymorphism and morphology of aluminium trihydroxide crystals precipitated from concentrated aluminate solutions. The formation of gibbsite is favoured at higher temperatures, while bayerite is formed predominantly at room temperature. The nature of the alkali metal ion present in the aluminate solution has a substantial influence on the morphology of the single crystals of gibbsite formed, with hexagonal plates resulting from sodium aluminate solutions and elongated hexagonal prisms from potassium aluminate solutions. The conversion of bauxite ore into alumina, via the Bayer process, is a well established major industry. However, the rate of precipitation of alumimum trihydroxide as gibbsite is an extremely slow process and is not well understood. Moreover, difficulties are encountered in regulating particle growth to achieve desirable characteristics of purity, strength, morphology and size distribution. A s part of an overall programme to understand the mechanism and the rate of crystal growth from solution, the process of crystal growth on different faces of synthetic gibbsite is being studied by in-situ and ex-situ microscope techniques. The work reported here concentrates on the initial characterization of different of duminium trihydroxide crystals produced by precipitation from alurninate solutions. There are three polymorphs of aluminium trihydroxide: gibbsite, bayerite and nordstrandite. The difference between the three polymorphs is in the stacking order © 1997 American Chemical Society

In Separation and Purification by Crystallization; Botsaris, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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of their [Al (OH) ] double layers, and the inclusion of impurities may affect this stacking sequence. Bayerite has been reported to form as somatoids, with an "hour glass", cone or spindle shape (somatoids are defined to be "bodies" of uniform shape that are not enclosed by crystal faces), while gibbsite forms as agglomerates of tabular and hexagonal shaped crystals (1). A study by Misra and White (2) showed that single crystals of up to 40μτη can be produced from potassium aluminate solutions (rather than sodium alurninate solutions) under conventional Bayer process type conditions. Wefers (3) suggested that the inclusion of potassium ions results in the formation of elongated crystals that are morphologically quite perfect, whilst with the inclusion of sodium ions, less well formed crystals with growth distortions result. It is believed these cations are incorporated by substituting for the hydrogen atom of a hydroxyl group, and as a result of the different sites of the cation incorporation, different crystal morphologies can result (4). Carbonation of potassium aluminate solutions at elevated temperatures (7080°C) is reported to result in the formation of large (-80 μπι) single crystals of gibbsite (Rosenberg, S.P. private communication, 1993). A study by Wojcik and Pyzalski (5) however, showed bayerite to be the predominant species formed in the carbonation process of potassium aluminate and sodium aluminate solutions between 50 - 80°C. In this work, several methods of growing crystals were followed, using both sodium aluminate and potassium aluminate solutions, at either room temperature or 70°C and the precipitates were characterised using scanning electron microscopy to examine the morphology of the crystals, and powder X-ray diffraction to identify the major phases present. 2

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Experimental Liquor Preparation. The aluminate solutions (synthetic Bayer liquors) were prepared using gibbsite (C31; Alcoa Chemical Division, Arkansas), sodium hydroxide pellets (AR Grade) and sodium carbonate (AR Grade) for sodium aluminate; or with potassium hydroxide (AR Grade) and potassium carbonate (AR Grade) for potassium aluminate solutions. A mixture of gibbsite, caustic and deionised water was heated, with stirring, in a stainless steel vessel until all the gibbsite had dissolved. This was then added to pre-dissolved carbonate. The solutions were allowed to cool and made to volume with deionised water to obtain a final concentration of 140g/L A 1 0 , 200g/L total caustic (expressed as N a C 0 ) and 240g/L total alkali (expressed as N a C 0 ) , in line with the North American alumina industry terminology. The solutions were filtered through a 0.45μιη membrane prior to use. 2

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Solid Preparation. The precipitate from each experiment was collected by filtration through a 0.45μιη membrane, washed with hot de-ionised water and air dried at room temperature. The precipitates were characterised using powder X-ray diffraction and scarming electron microscopy.


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Precipitation Conditions. The amount of alurriinium dissolved in the synthetic Bayer liquors studied here is in excess of the solubility of the Al(OH) polymorphs, and solid will precipitate out if the solutions are simply left for a sufficiently long period of time, referred to as ageing (4 - 48 hours, depending on conditions). The ageing period included the crystallization induction time and some precipitation, but the rate of precipitation can be increased by partially neutralising the alkali hydroxide by the addition of carbon dioxide or acid, or by the addition of seed crystals.


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Ageing of Alurninate Solution. The alurninate solutions were allowed to age for 48 hours at room temperature or for 4 hours at 70°C in the absence of any seed crystals or the addition of acid or C 0 . 2

Addition of Carbon Dioxide Gas (Carbonation). Bubbling carbon dioxide through a tube into synthetic Bayer liquors at moderate rates results in the exit holes for the gas becoming blocked by precipitated gibbsite. To try to overcome this problem, and hence achieve reproducible rates of precipitation, two approaches were adopted. First, carbon dioxide was bubbled through a bubbler ring with many holes into a stirred sodium or potassium alurninate solution at room temperature or 70°C. Secondly, carbon dioxide was bubbled through a gas chromatography syringe (0.5mm) into a potassium alurninate solution at 70°C, with no stirring. Addition of Acid. Hydrochloric acid (0.5moles/L) was added dropwise via a peristaltic pump to a stirred sodium or potassium alurninate solution at room temperature or 70°C, at a rate of 0.5mL/rnin. The volume of acid added completely neutralised the caustic solution. Addition of Seed. Sodium or potassium alurninate solutions were seeded with lOg/L of gibbsite (C31) in Nalgene bottles and tumbled end over end at 70°C for 2 hours. Results For all conditions, bayerite was the predominant species formed at room temperature, while the formation of gibbsite was favoured at 70°C. Bayerite Formation. Ageing of Alurninate Solution. The initial products from ageing sodium alurninate solution at room temperature (after 16 hours) were mainly calcium containing compounds (CaC0 and 3CaO.Al2O3.CaCO3.HH2O) and FeO(OH), and some bayerite. Calcium and iron are present as impurities in the reagents used for liquor preparation. The morphology of these precipitates consisted of rounded aggregates of plates. Bayerite with a "woolball" morphology was observed after 48 hours (Figure 1). 3



SEPARATION AND PURIFICATION BY C R Y S T A L L I Z A T I O N

Figure 1 : Bayerite with "woolball" morphology from agein sodium aluiriinate solutions

Figure 2: Bayerite from ageing potassium aluminate solutions


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The morphology of bayerite formed by ageing potassium aluminate solutions consisted of agglomerates of hexagons and "diamonds' as well as single crystals of elongated hexagonal prisms (Figure 2) These elongated prisms are typical of the morphology of crystals grown from potassium aluminate solutions. Addition of Carbon Dioxide Gas (Carbonation). The bayerite crystals produced by the carbonation of sodium aluminate consisted of "web-like" and "frond" shaped crystals (Figure 3), while from potassium aluminate solutions, bayerite consisted of agglomerates of fine crystals, with some "web-like" and "frond" shaped crystals. These were quite unlike the somatoid shapes described in the literature. 1

Addition of Acid. The addition of acid (HC1) to the aluminate solutions at room temperature resulted in the formation of two distinct morphologies of bayerite. In both sodium aluminate and potassium aluminate, the bayerite consisted of agglomerates of elongated triangular prisms that showed radial growth characteristics (Figure 4). From potassium aluminate solutions, bayerite also took the form of single crystals (~40 mm), and sometimes with "zig-zag' edges (Figure 5). Gibbsite Formation. Ageing of Aluminate Solution. Ageing both sodium aluminate and potassium aluminate solutions at 70°C resulted in the formation of gibbsite and nordstrandite. Trace amounts of bayerite were also observed in aged sodium alurninate solutions. The morphology of gibbsite formed by ageing aluminate solutions is similar to that of natural gibbsite. Agglomerates of hexagonal tablets as well as "diamond" shaped crystals were observed from both sodium aluminate and potassium aluminate solutions, with the finer crystals from sodium alurninate solutions. Single crystals were also formed in both cases, elongated hexagonal prisms from potassium aluminate and hexagonal tablets from sodium alurriinate solutions. Figure 6 is a micrograph of the crystals collected from potassium aluminate solutions. Addition of Carbon Dioxide Gas (Carbonation). Addition of carbon dioxide through a bubbler ring into a stirred aluminate solution at 70°C resulted in the formation of gibbsite and bayerite, while carbonation of potassium aluminate though a single inlet resulted in the formation of relatively large, single crystals of gibbsite (Figure 7). The morphology of the bayerite/gibbsite crystals consisted of "web-like" structures with protruding hexagonal prisms (Figure 8). Addition of Acid. The crystals formed by acid addition to aluniinate solution were agglomerates of hexagons, diamonds and elongated prisms. Again, coarser crystals resulted from potassium aluminate solutions (Figure 9).




Figure 3: Bayerite from carbonation of sodium alurninate solutions

Figure 4: Bayerite from acid addition to potassium alurninate



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Characterization ofAluminum Trihydroxide Crystah

Figure 5: Single crystals of bayerite with "zig-zag" edges from acid addition to potassium alurninate solutions

Figure 6: Gibbsite from ageing potassium alurninate solution


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Figure 7: Single crystal of gibbsite from carbonation of potassium alurninate solutions

Figure 8: GibbshWbayerite from carbonation of sodium alurninate



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Figure 9: Gibbsite from acid addition to potassium alurninate solutions



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Figure 10: Industrially produced gibbsite (C31)

Figure 11 : Gibbsite from seeding potassium aluminate solutions


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Addition of Seed. Seeding sodium aluminate solutions with gibbsite (C31) at 70°C resulted in agglomerates of hexagons and diamonds, as well as single crystals of hexagonal plates. The morphology of these crystals is similar to that of industrially produced gibbsite (Figure 10). Seeding potassium aluminate solutions with gibbsite (C31) resulted in the formation of crystals typical for this system - elongated hexagonal prisms of gibbsite (Figure 11).


Conclusions • Bayerite formation is favoured at room temperature while gibbsite formation is favoured at 70°C. • The morphologies of bayerite are not necessarily somatoids and will depend on the method of preparation. • Although the morphology of gibbsite varies with the crystallization conditions, the crystals are generally agglomerates of hexagons and/or diamonds or single crystals of hexagonal plates from sodium aluminate, and single crystals of elongated hexagonal prisms from potassium aluminate solutions. • The different morphologies of gibbsite crystals grown from sodium and potassium duminate solutions, under otherwise identical conditions, suggest that the alkali metal plays an important role in the crystallization mechanism. Acknowledgments This work has been supported under the Australian Government's Cooperative Research Centres programme, and by the Australian Mineral Industries Research Association and this support is gratefully acknowledged. The support of the Centre for Microscopy and Microanalyses, University of Western Australia is also acknowledged. References 1. 2. 3. 4.

Wefers, K.; Misra, C. Alcoa Technical Paper No 19, 1987 Revised Misra, C.; White, E.T. J Crystal Growth, 1971, 8, 172 Wefers, K. Die Naturwissenschaften, 1962, 49, 1 Lee, M.; Rohl, A . L . ; Gale, J.D.; Parkinson, G . M . ; Lincoln, F.J. Trans IChemE, 1996, 74A, 739 5. Wojcik, M.; Pyzalski, M. Light Metals, 1990, 161


Characterization of Aluminum Trihydroxide Crystals Precipitated from

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