Ind. Eng. Chem. Res. 2001, 40, 5901-5907
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MATERIALS AND INTERFACES Poisoning of Aluminum Hydroxide Precipitation by High-Molecular-Weight Fractions of Bayer Organics Damian E. Smeulders,† Michael A. Wilson,*,† and Lyndon Armstrong‡ Department of Chemistry, Materials and Forensic Science, University of Technology, Sydney, P.O. Box 123, Broadway, NSW 2007 Australia, and Queensland Alumina Limited, P.O. Box 1, Gladstone, Qld 4680 Australia
This work studies the effects on the Bayer process of various high-molecular-weight fractions of organic matter (1200 to >300 000 Da) derived from the high-temperature digestion of Weipa bauxite. This preliminary study shows the impacts on precipitation of aluminum hydroxide (gibbsite) and sodium oxalate under typical Bayer precipitation conditions. Different molecular weight fractions of the Bayer-degraded humic substances were found to have different detrimental effects on gibbsite precipitation yields and on crystal sizing and surface area. It is clear that certain high-molecular-weight organics, although present in low concentrations in Bayer process liquor, can have strong affects on the dynamics of gibbsite precipitation, adversely affecting yields but also lowering particle surface areas. These are significant new findings and emphasize the specific effects of Bayer organics according to molecular weight. These highmolecular-weight organics were also found to stabilize sodium oxalate in solution by significant amounts, exhibiting an increasing trend with molecular weight. Introduction The Bayer process is used for the industrial-scale production of alumina (aluminum oxide) from bauxite ore. In the Bayer process, bauxite is subjected to a hightemperature digestion (140-250 °C) in concentrated caustic solution (∼3 M NaOH) in high-pressure reactors. The product liquor, termed pregnant or green liquor because it is supersaturated in sodium aluminate, is filtered to remove insoluble residues and then seeded to precipitate aluminum hydroxide (gibbsite). A final calcination step converts the gibbsite to aluminum oxide. Typical concentrations of organics in Bayer liquors that are extracted from the bauxite into the Bayer process liquor during digestion range from a few grams per liter up to 40 g/L1 (expressed as carbon). They have molecular weights from less than 100 Da to greater than 300 000 Da (300 kDa).2 This geo-organic matter causes various problems in the operation of alumina refineries.3,4 For example, some of the compounds inhibit gibbsite precipitation by reducing the kinetics, and therefore the yield and desired particle size, and by increasing the amount of soda impurity in the product crystals.1,5 It is known that plant gibbsite seeds themselves contain adsorbed Bayer organics that can diminish the seed activity. Interestingly, Mitchell et al.6 discovered that Bayer precipitation yields could be improved rather than just diminished if poisoning organic species were adsorbed onto certain calcined aluminas. The presence of organics also affects the * Author to whom correspondence should be addressed. † University of Technology, Sydney. ‡ Queensland Alumina Limited.
precipitation of sodium oxalate, which is a degradation byproduct of the process and also needs to be removed for operating efficiency. The compositions of organic materials obtained from a low-temperature Bayer refinery (145-150 °C) and a high-temperature refinery (250-255 °C) have been studied in our laboratory.2,7 These studies characterized the organic materials by infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, pyrolysis gas chromatography/mass spectrometry (py-GC/ MS), thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). Exposure of the organic matter in the bauxite and in the Bayer liquor to high temperatures, pressure and strong sodium hydroxide concentrations in an oxidizing environment degrades the complex organic species to low-molecular-weight (e.g., 300 kDa) of organic matter recovered from a high-temperature (250-255 °C) Bayer alumina refinery on the precipitation of aluminum hydroxide and sodium oxalate from caustic aluminate Bayer liquor. These tests were performed to determine which high-molecular-weight organics are particularly detrimental to the operation of the Bayer precipitation process. The present work uses typical gibbsite seeds from a Bayer plant washed with only hot water prior to testing.
10.1021/ie010216p CCC: $20.00 © 2001 American Chemical Society Published on Web 11/16/2001
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Table 1. Yields, Acidity, and Elemental Analysis of the Bayer Humic Substance Fractionsa,b
fraction
molecular weight fraction (kDa)
%C
%H
%N
1 2 3 4 5 6 7 8
300
50.79 48.25 54.32 53.87 55.58 48.85 49.13 54.52
4.22 3.78 4.23 3.67 5.41 4.05 4.85 3.78
0.56 1.87 3.22 1.39 3.69 4.92 5.92 2.05
a
Dry ash free basis. b From Smeulders et al.2
c
%S
%O difference
O/C
H/C
N/C
0.17 0.32 0.60 0.44 0.42 0.68 0.99 0.42
44.26 45.78 37.62 40.62 34.90 41.50 39.11 39.24
0.65 0.71 0.52 0.57 0.47 0.64 0.60 0.54
0.99 0.93 0.93 0.81 1.16 0.99 1.18 0.83
0.0095 0.033 0.051 0.022 0.057 0.086 0.10 0.032
S/C
pH at 5 g/L in water
mass yield (%)c
0.0013 0.0025 0.0041 0.0031 0.0028 0.0052 0.0075 0.0029
2.48 2.96 3.39 2.90 3.79 3.99 4.64 4.36
87.0 3.3 0.6 1.6 1.0 3.8 0.7 2.0
The weight of fraction over total weight of material recovered.
These seeds still contain certain adsorbed organics from the original plant liquor, which, of course, contained the full molecular weight range of bauxite-degraded organics. This study therefore mimics the usual competition for the available growth sites by any organics in liquor, in this case the high-molecular-weight organic fractions in the test solution, and simulates, in that aspect, the precipitation dynamics encountered in the Bayer process. Experimental Section Extraction of Humic Substances from Bayer Liquors. The organic matter from the Bayer process was isolated using the method described by Smeulders et al.2 The methodology is repeated briefly here for completeness. It is noted that the refinery from which these liquors were isolated operates at a higher temperature (250-255 °C, 3500 kPa steam pressure) and produces different organic materials than are produced by refineries operating at lower temperatures with different feed bauxites.7 Recovered organics from pregnant (or green) liquor (i.e., prior to plant precipitation of gibbsite) and spent liquor (i.e., after plant precipitation of gibbsite) were isolated, but only organic matter from spent liquor was further separated into molecular weight fractions for the present work. The organic compounds present in the Bayer process liquor were acidified and separated from the inorganic material present in the Bayer liquor using adsorption chromatography on Amberlite XAD-7 resin (2 cm × 60 cm column, 200 g of resin). The organic compounds contained in the eluant from the XAD-7 column were protonated using a column of the cation-exchange resin IR-120(H+) (BDH, 2 cm × 60 cm). The protonated humic substances obtained from the above procedure were concentrated by rotary evaporation and then freeze-dried. This procedure was performed on 10-L batches of each of the spent and pregnant Bayer liquors. The pregnant liquor gave 5.26 g/L, and the spent liquor yielded 5.10 g/L of organic matter in the acid form. This represents only 11% of the Bayer organic matter (on a dissolved organic carbon basis) because volatiles are lost during the evaporation steps of this separation procedure. The volatiles include methanol, phenols, and simple carboxylic acids such as formic and acetic acid,2 which are significant byproducts of bauxite digestion, particularly at high temperature. Therefore, only the high-molecular-weight fractions (>1.2 kDa) were deemed to be recovered sufficiently intact from this procedure to be evaluated for their impact on precipitation. The protonated organics from the spent Bayer liquor were separated into their molecular weight fractions
using Spectra/Por aqueous extracted molecular porous dialysis membranes constructed from cellulose and cellulose ester. These membranes had nominal molecular weight cutoffs (MWCOs) of 1.2, 6, 12, 25, 50, 100, and 300 kDa. Dialysis was performed on each of the seven different membranes, isolating eight different molecular weight fractions of the humic substances. As each fraction was isolated, it was freeze-dried at 200 mTorr and -70 °C. The freeze-dried samples were stored in a desiccator over silica gel in a cool dark cupboard. In total, 29.59 g of the whole organic fraction obtained from the spent Bayer liquor was fractionated by dialysis. The yields of each of the fractions recovered from the process organics, their elemental compositions, and their acidities are detailed in Table 1, which is repeated from our earlier paper.2 Seed Preparation. Plant gibbsite seeds were sampled from the high-temperature Bayer alumina refinery as typical fine-seed and coarse-seed slurries from tertiary and secondary gravity classifiers, respectively. Slurry samples were filtered under vacuum and prepared by washing with hot distilled water followed by drying in ambient air. To the secondary seed was added the equivalent of 0.2% (w/w) sodium oxalate (all units for sodium oxalate are expressed in this paper as Na2CO3 equivalents, an industry convention). The sodium oxalate was prepared by precipitation from sodium hydroxide solution under conditions designed to give a crystal size, morphology (acicular), and aspect ratio similar to those of the seed from a Bayer plant.15 Precipitation Experiments. Precipitation experiments were performed with equal concentrations (measured in g/L) of each organic fraction added to a synthetic caustic aluminate liquor with the hot-waterwashed seed. Synthetic Bayer liquor was prepared using gibbsite from a low-organics plant, Alcoa C31, 263.1 g/L (C31 defines a company product), and sodium hydroxide pellets, 177.1 g/L (Ajax Chemicals AR-grade). This is equivalent to 235 g/L caustic (caustic, C, by convention is the sum of sodium hydroxide and sodium aluminate, expressed as Na2CO3) and 172 g/L alumina (A, aluminate, expressed as Al2O3), giving a 0.732 A/C ratio. The impurities sodium carbonate (34.4 g/L), sodium chloride (10.0 g/L), and sodium oxalate (3.0 g/L) were added to the synthetic liquor as Ajax AR-grade chemicals, and the mixture was then filtered through double glass filters (Whatman GF/C). The precipitation test used a water bath with endover-end rotation of test bottles. Liquor and seed samples (200-mL total slurry volume) were contained in 250-mL Nalgene polypropylene bottles. The bath was
Ind. Eng. Chem. Res., Vol. 40, No. 25, 2001 5903 Table 2. Agglomeration Yield and Sizing Results with Degraded Humic Molecular Weight Fractions at 0.1 g/L
a
organic fraction
organic dose (g/L)
difference in alumina yield (g/L)a
% 20 µm
% 300 kDa
0 0.1 0.1 0.1 0.1
0 0.0 -0.2 0.1 0.5
4.5 3.3 3.3 3.3 3.5
27.4 22.8 23.3 24.4 26.1
0.167 0.157 0.151 0.153 0.152
Relative to control. b Determined by Mastersizer.
Table 3. Yield and Sizing Results from Growth Precipitation Conditions with Degraded Humic Molecular Weight Fractions at 0.1 g/L
a
organic fraction
organic dose (g/L)
difference in alumina yield (g/L)a
% 20 µm
% 300 kDa
0 0.1 0.1 0.1 0.1
0 -0.3 -0.8 -0.7 -1.7
6.5 3.8 3.6 3.6 4.8
13.7 10.6 10.5 10.4 12.5
0.165 0.120 0.117 0.118 0.131
Relative to control. b Determined by Mastersizer.
Figure 1. Impact on agglomeration yield of aluminum hydroxide by high-molecular-weight Bayer organic fractions.
programmed to simulate typical time-temperature precipitation conditions encountered during the Bayer process: for agglomeration, 75 °C for 2 h; for growth simulation, and initial temperature of 75 °C for 8 h, a sudden decrease to 68 °C, and then gradual cooling to a final temperature of 60 °C over the remaining 14 h of the 22-h total run time. One-liter aliquots of the synthetic Bayer liquor were heated to 65 °C with stirring in stainless steel beakers. To each beaker was added a sample of the organic molecular weight fraction at a 0.1 g/L dose. This mixture was filtered using double Whatman GF/C filters. Four 200-mL samples of each of these solutions were then placed into individual 250-mL bottles in the water bath at 75 °C. Two duplicate bottles from each sample set were charged with preheated secondary seed (300 g/L seed charge) containing 0.2% (w/w) sodium oxalate to monitor the impacts on precipitation of both gibbsite and oxalate under notional Bayer growth conditions. The other two bottles from each sample set were used to monitor impacts on agglomeration. These bottles were charged with preheated tertiary seed (150 g/L). The degree of agglomeration was tested after 2 h at 75 °C. Precipitation growth yields were tested after 22
h. To perform these evaluations the solids were allowed to settle, and then 10-mL aliquots of solution were removed and syringe filtered through a Millipore (MillexHN 0.45-µm nylon) syringe filter to prevent further precipitation. The filtered solutions were analyzed for alumina, caustic, soda and oxalate. The remaining slurries from each agglomeration bottle and each precipitation growth bottle were vacuum filtered and hot water washed. The precipitated material was oven dried at 105 °C, and particle sizing was performed on each of the duplicates. The impact of each of the organic fractions was measured with respect to a control experiment with no organic added. The changes in the yield of alumina, the sizing, and the surface area are shown in Tables 2 and 3 and Figures 1 and 2. Precipitation dynamics were measured using a Metrohm autotitrator. This instrument determines the alumina, caustic, and soda concentrations of the liquor based on the method developed by Connop.8 The particle size distributions of the crystalline product were measured with a Malvern Mastersizer S instrument, which employs a laser diffraction technique to determine the sizes of product crystals dispersed in water in an agitated vessel. This technique determines the mass percent distribution of
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Figure 2. Impact on growth yield of aluminum hydroxide by high-molecular-weight Bayer organic fractions.
crystal diameter, along with calculations of the surface area and particle numbers. The oxalate concentration in each solution was determined using ion chromatography. For these determinations, 50-µL of each sample solution was added to 5 mL of a 1000-ppm solution of potassium fluoride, and then each solution was accurately diluted to 500 mL with water. These solutions were then chromatographed on a Dionex DX-100 ion chromatograph, and the oxalate concentrations were determined from peak areas compared to a standard calibration. Results for the 1.2-6, 12-12, 50-100, and >300 kDa fractions are reported here. Other fractions are not reported because of commercial sensitivity. Results and Discussion In the discussion below, we term alkali-soluble material in Bayer plant liquor as “Bayer-degraded humic substances”. The definition of humic substances varies somewhat from one research group to another, as well as inside and outside the alumina industry. The description of Bayer organics within the industry mostly follows the breakdown classification first proposed by Lever16,18 as low-, medium-, and high-molecular-weight fractions. The last grouping is generally called “humates”, and they have traditionally been blamed for most of the deleterious impact by Bayer organics on the process.1,3,4,16,18-20 Precipitation Testing. The chemical compositions of the high-molecular-weight organic fractions (>1.2 kDa) are detailed elsewhere,2,9 but information relevant to this paper is outlined in Table 1. Previous characterization showed that the 12-25 and 25-50 kDa fractions appear to resemble material more akin to kerogen, whereas the highest-molecular-weight organic material (>300 kDa) behaves as a soluble char.2 Table 1 shows that the acidities of the high-molecularweight organic fractions are highly variable. The most acidic are the 1.2-6 and 12-25 kDa fractions; otherwise, the pH of fractions generally increases with increasing molecular weight. The acidity of the fractions can be an important consideration for precipitation testing. When the organic fractions are added in their acid form, the ionizable compounds react with the free caustic in the starting liquor to modify the starting
caustic concentration and, therefore, the starting alumina supersaturation, a major factor in determining precipitation output. However, in this work, where fractions were dosed only at 0.1 g/L, this effect is insignificant (and calculations indicate that it is also insignificant up to at least a dose of 5 g/L of any fraction). The mass percentages of these high-molecular-weight fractions in the original Bayer spent liquor are also shown in Table 1. The levels are low, as has been noted previously, and they are even lower from a hightemperature (250-255 °C) digest process relative to a low-temperature (∼150 °C) process.2,6 Material at 300 kDa fraction. These results need to be discussed in relation to the impact of the combined organic fractions in the original liquor. The Bayer-degraded humic materials can interact with each other in solution to produce the net precipitation effects, or they can interact in tandem with the gibbsite surface. Research on natural systems suggests that the poisoning mechanism of humic material is a sequential one.10 Binding sites exist on the surface for competition by both aluminate species and organic material. Initially, ligand exchange of water can occur at the surface, followed by a fast process where low-molecular-weight species occupy binding sites. These molecules can then be displaced by irreversibly adsorbing macromolecular organic species. This proposed mechanism would suggest that all of the molecular weight fractions play a role in the poisoning process, with the larger-molecular-weight materials having later roles. It also implies the possibility that the sum of the test results from the molecular weight fractions might not give the same result as the combined fractions in the original liquor because of specific interactions between different organic fractions with the surface. The characterization study on these molecular weight fractions showed a host-guest aggregation,2,9 indicating that an interactive association is possibly involved in the mechanism of the poisoning of gibbsite precipitation. Sodium Oxalate. Sodium oxalate plays an important role in most Bayer plant precipitation operations, as well as influencing sodium impurity and particle fines contents of alumina.1,5 The presence of organic matter in the Bayer liquor is known to stabilize the sodium oxalate in solution, decreasing or delaying its precipitation.4,15,16,18,19 This can result in “oxalate showers” during the precipitation of gibbsite, where oxalate crystal nuclei form suddenly in the liquor. Gibbsite itself readily nucleates on the oxalate surface, leading to a gibbsite fines imbalance, classification issues, and, in some cases, poor-quality alumina.1,5,19 In the present work, the growth precipitation test used gibbsite seed containing 0.2% (w/w) sodium oxalate to simulate an oxalate-gibbsite coprecipitation Bayer process. (All units for sodium oxalate are expressed as Na2CO3 equivalent, an industry convention.) The sodium oxalate was prepared by precipitation from sodium hydroxide solution using conditions to give a crystal size, morphology (acicular), and aspect ratio similar to those of seed from a Bayer plant.15 However, because the oxalate surface is free of adsorbed organics, this sodium oxalate will be more active in promoting oxalate crystallization and gibbsite nucleation than oxalate seed from a Bayer plant. The initial oxalate level in the starting liquor was 2.37 g/L. The finishing oxalate concentration was measured in each case, and the actual yields of sodium oxalate were determined for these precipitation tests. These results are plotted in Figure 3. No oxalate stabilization occurs with the 1.2-6 kDa fraction: it shows the same oxalate yield from the precipitation as the undosed control liquor. Oxalate stabilization was apparent for the other higher-molecular-weight fractions and displayed an increasing trend with molecular weight. The finishing oxalate concentration was increased by 16% for the largest-molecular-
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Figure 3. Sodium oxalate yields from coprecipitation under growth conditions with high-molecular-weight Bayer organic fractions.
weight fraction (>300 kDa), which is a significant stabilization of oxalate in Bayer liquor at these test conditions. To our knowledge, these are the first results showing a relationship between different molecular weight fractions of Bayer organics and oxalate stabilization. Previous work has established that certain organics, generally termed humates, impact oxalate crystallization.16,18,19 The removal of colored organics from Bayer liquor by using adsorbants, or by treating Bayer liquors with high-molecular-weight sequesterants or polyelectrolytes, tends to destabilize oxalate in solution, sometimes with marked impacts on plant operations.19 With the benefit of some organics characterization work, but also by implication, these types of organics have been assumed in the industry to all be of high molecular weight, although there is some evidence that lower-molecularweight species are also removed by these methods.20 This work demonstrates a differentiation in effect of these organics on oxalate stability, with an increasing trend in oxalate stabilization only as the molecular weight increases above 12 kDa. Relationship between Poisoning and Chemical Composition. Studies with model compounds have shown that many small hydroxy organic compounds act as poisons to the Bayer process.5,11-14 Model hydroxybenzene carboxylic acid compounds, similar in structure to those isolated in the lowest-molecular-weight fraction (300 kDa fractions would need to bind with some of the remaining free precipitation sites on the seed. Conclusions 1. All higher-molecular-weight fractions from 1.2 to >300 kDa separated from bauxite-degraded organic matter affect the precipitation yields and particle sizing of the product aluminum hydroxide in both agglomeration and growth test simulations. 2. High-molecular-weight fractions were detrimental to precipitation growth yields, with the greatest impact from the >300 kDa fraction. However, these fractions also decreased the particle sizing and surface area of the product aluminum hydroxide, which, in itself, contributes partly to the yield impact. 3. Oxalate stabilization by 12 to >300 kDa fractions occurs during oxalate-gibbsite coprecipitation, with the greatest impact (16% decrease in oxalate yield) by the highest-molecular-weight fraction (>300 kDa). Literature Cited (1) Grocott, S. C.; Rosenberg, P. R. Soda in alumina. Possible mechanisms for soda incorporation. In Proceedings of the International Alumina Quality Workshop, Gladstone, Australia, 1988; p 271. (2) Smeulders, D. E.; Wilson, M. A.; Patney, H. K.; Armstrong, L. Structure of molecular weight fractions of Bayer humic substances II. High-temperature products. Ind. Eng. Chem. Res. 2000, 39, 3631. (3) Atkins, P.; Grocott, S. C. The impact of organic impurities on the production of refined alumina. In Proceedings of the Science, Technology and Utilisation of Humic Acids; CSIRO Division of Coal and Energy Technology: Sydney, Australia, 1988; pp 8594. (4) Grocott, S. C. Bayer liquor impurities: Measurement of organic carbon, oxalate and carbonate extraction from bauxite digestion. Light Metals 1988, 833. (5) Armstrong, L. Bound soda incorporation during hydrate precipitation. In Proceedings of the Third International Alumina Quality Workshop, Hunter Valley, Australia, 1993; p 282. (6) Mitchell, C. H.; Alamdari, A.; Wainwright, M. S.; Raper, J. A. Improvement of batch precipitation of alumina trihydrate from Bayer pretreated with calcined aluminas. In Proceedings of the Nineteenth Australasian Chemical Engineering Conference (Chemeca 91), Newcastle, Australia, 1991; The Institute of Engineers: Sydney, Australia, 1991; p 114.
Ind. Eng. Chem. Res., Vol. 40, No. 25, 2001 5907 (7) Wilson, M. A.; Ellis, A. V.; Lee, G. S. H.; Rose, H. R.; Xiaoqiao, L.; Young, B. R. Structure of molecular weight fractions of Bayer humic substances. 1. Low-temperature products. Ind. Eng. Chem. Res. 1999, 38, 4663. (8) Connop, W. A new procedure for the determination of alumina, caustic and carbonate in Bayer liquor. In Proceedings of the Fourth International Alumina Quality Workshop, Darwin, Australia, 1996; p 321. (9) Smeulders, D. E.; Wilson, M. A.; Kannangara, G. S. K. Host-guest interactions in humic materials. Org. Geochem., in press. (10) Ochs, M.; Cosovic, B.; Stumm, W. Coordinative and hydrophobic interaction of humic substances with hydrophilic Al2O3 and hydrophobic mercury surfaces. Geochim. Cosmochim. Acta 1994, 58, 639. (11) Coyne, J. F.; Wainwright, M. S.; Cant, N. W.; Grocott, S. C. Adsorption of hydroxy organic compounds on alumina trihydrate. Light Metals 1994, 39. (12) Alamdari, A.; Raper, J. A.; Wainwright, M. S. Poisoning of the precipitation of alumina trihydrate by mannitol. Light Metals 1993, 143. (13) The, P. J. The effect of glusoisosaccharinate on the Bayer precipitation of alumina trihydrate. Light Metals 1980, 119. (14) Watling, H. R.; Smith, P. G. Loh, J.; Crew, P.; Shaw, M. Comparative effects of model organic compounds on gibbsite crystallisation. In Proceedings of the Fourth International Alumina Quality Workshop, Darwin, Australia, 1996; p 553. (15) Reyhani, M. M.; Dwyer, A.; Parkinson, G. M.; Rosenberg,
S. P.; Healy, S. J.; Armstrong, L.; Soirat, A.; Rowe, S. Gibbsite Nucleation at Sodium Oxalate Surfaces. In Proceedings of Fifth International Alumina Quality Workshop, Bunbury, Australia, Mar 21-26, 1999; pp 181-191. (16) Gnyra, B.; Lever, G. Review of Bayer Organics-Oxalate Control Processes. Light Metals 1979, 151-161 and references therein. (17) Cornell, R.; Pannett, D.; Sullivan, N. Clarke, P.; Bailey, C. Precipitation of gibbsite: Development of a new rate equation. In Proceedings of Fifth International Alumina Quality Workshop, Bunbury, Australia, Mar 21-26, 1999; pp 153-161. (18) Lever, G. Some aspects of the chemistry of bauxite organic matter on the Bayer process: The sodium oxalate-humate interaction. Travaux 1983, 13, 335-344 and references therein. (19) Power, G. P.; Tichbon, W. Sodium oxalate in the Bayer process: Its origin and effects. In Proceedings of Second International Alumina Quality Workshop, Perth, Australia, 1990; pp 99115. (20) Moody, G. M.; Adkins, S. J.; Lee, A. An alternative approach to organics removal in the Bayer process. In Proceedings of Third International Alumina Quality Workshop, Darwin, Australia, 1996; pp 402-412.
Received for review March 5, 2001 Revised manuscript received August 13, 2001 Accepted September 13, 2001 IE010216P
