V.A. Chanand H.M. Ang

† School of Chemical Engineering, Curtin University of Technology,

GPO Box U1987, Perth, W.A. 6001

‡ Kaiser Engineers Pty Ltd, 6th Floor, 250 St Georges Tce, Perth, W.A. 6000


Due to the requirement of high purity crystalline products, an understanding of the effects of impurities on the crystallization process is necessary. Trace amounts of impurities present in a system have long been recognised for their ability to induce striking changes in all kinds of crystallization characteristics such as crystal morphology and growth rates, nucleation rates and tendencies to agglomerate. The effect of impurities on crystallization kinetics is very unpredictable and cannot be explained by any general rule. In common with the majority of industrial crystallization processes, the green liquor which aluminium hydroxide crystallizes in the Bayer process is by no means pure, but contains many impurities. The presence of impurities in the alumina industry (inorganic and organic substances from the bauxite and Na2CO3 through atmospheric uptake) is known to adversely affect the Bayer precipitation process. From batch experiments investigating the effect of various common impurities on the kinetics of crystallization, sodium humate was identified to have the most significant effect on the aluminium hydroxide crystallization. A continuous laboratory crystallizer was developed, enabling crystallization studies under steady state conditions. Subsequent crystallization runs using a continuous laboratory crystallization setup were conducted to study the effect of humate on crystallization kinetics: nucleation rate, mass and linear growth rates and yield. The results of both batch and continuous experiments showed that humate impurity adversely affected the kinetics of aluminium hydroxide crystallization.


continuous; crystallization; impurities; kinetics; aluminium hydroxide; alumina


V.A. Chan and H.M. Ang


The objective of this study was to investigate the influence of sodium humate on the batch and continuous crystallisation of aluminium hydroxide from caustic aluminate solutions.

Crystallization features such as kinetics of nucleation and growth, macrostep formation, agglomeration or dispersion and uptake of foreign ions in the crystal structure can be greatly affected by the presence of impurities in the crystallizing solution. Depending on the type of impurity, its influence on the crystallization process can be controlled by various mechanisms.

There are inherent impurities, particularly organics, present in the Bayer Process which inevitably affect all stages of the Bayer Process. The presence of organics in the aluminium hydroxide precipitation stage results in lower than acceptable quality product, and also attributes to other general operating difficuties such as colouring of the liquor and aluminium hydroxide, lowering of the red mud settling rates and loss of caustic.

The majority of experimental laboratory scale work on aluminium hydroxide crystallization are conducted in batch crystallizers, as they are easier to operate and do not require such large quantities of raw material compared to continuous experiments. Among the workers who investigated the continuous crystallization of aluminium hydroxide are Sang (1986), Ilievski (1991) and Veesler and Boistelle (1994).

Several workers including Lever (1978), Bird et al. (1983), Power and Tichbon (1990), Ang and West (1992) and Patra (1993), studied the effect of various common impurities inherent in the Bayer Process, on the batch crystallization of aluminium hydroxide. Among the impurities studied, sodium carbonate, as well as organic compounds: sodium humate and oxalate, have received much attention.

The precipitation of aluminium hydroxide in the refining of alumina from bauxite ore is usually performed in a series of continuous crystallizers that are either mechanically or air agitated. A continuous laboratory crystallizer was used in this reported work to study the crystallization process under steady state conditions.



A continuous crystallization system, depicted in Figure 1, was developed for the crystallization studies. It consisted of a 1.5 litre flat-bottom, mechanically agitated crystallizer, with separate seed and liquor feeding mechanisms.


  • Figure 1

    Laboratory Continuous Crystallization System

    The same 1.5 litre crystallizer was also used for batch runs. A polystyrene float level controller maintained the level in the crystallizer. A detailed description of the equipment is found in (Chan, 1997). The inlet liquor stream consisted of concentrated caustic aluminate liquor, which mixed with the seed-water slurry stream as they both entered the crystallizer. Aluminium hydroxide seed crystals were sieved to a size range of 53 to 90 m m. An earlier study (Chan et al., 1994) confirmed that attrition of the seed crystals during storage and transport from the seed tank was minimal. The system was designed to handle residence times in the range of 40 minutes to 1 hour; seed charges of between 50 to 200 g/L; and temperatures up to 80C. A study of the particle dynamics and the solid and liquid residence time distributions showed that the set-up represents a normal continuous mixed suspension mixed product removal (CMSMPR) system (Chan and Ang, 1996).



    3.1 Effect of Humate on the Batch Crystallization of Aluminium Hydroxide

    All batch crystallization experiments were performed at 70C, with a seed slurry density of 200 g/L comprising of aluminium hydroxide seed agglomerate crystals in the size range of 53 to 90 m m. An initial alumina concentration of A/C = 0.6 (based on 200 g/L sodium carbonate) was used. Sufficient synthetic liquor for one run was prepared by digestion on a hot plate. Sodium humate impurity (5 g/L) was added during digestion. Once digested, the liquor was filtered through a 0.45 m m membrane pressure filter to remove any undissolved material.

    The prepared liquor was transferred to the crystallizer and mechanically agitated. The synthetic liquor, in the absence of seed crystals, was stable at room temperature, and will not precipitate out for extended periods of time. When the liquor in the crystallizer had reached the operating temperature, the preweighed and sized dry alumina trihydrate seeds were added to the crystallizer.

    Batch runs were conducted for 24 hours. Samples were taken hourly for the first four hours, then every 2 to 3 hours for the next 6 hours and the last sample was taken after 24 hours. Two samples were taken and analysed for alumina concentration by a standard alumina industry technique (Watts and Utely, 1956) and particle size determination using a Coulter Counter Industrial Model D.

    3.2 Effect of Humate on the Continuous Crystallization of Aluminium Hydroxide

    The effect of different humate concentrations on the crystallization kinetics was investigated at a constant residence time. The following experimental variables were used: a solution concentration of A/C = 0.6; seed slurry density = 200 g/l seeds in the range of 53 to 90 m m; crystallization temperature = 70C. The continuous crystallization system, illustrated in Figure 1 was used in this study. Further details on the experimental preparation such as liquor digestion, seed slurry preparation and sampling technique can be found in (Chan, 1997). The system was operated in batch mode for one residence time, then changed to continuous operation. Samples were taken at every one to two residence times for crystal size distribution analysis (CSD), slurry density determination and solution concentration determination.

    The continuous crystallization system reached steady state when the solid CSD, solid density and liquor concentration were constant with time. The time required to reach steady state was between 10 to 12 residence times. It was found that the solid CSD required the longest time to achieve steady state conditions. For a more detailed discussion on this area, refer to (Chan and Ang, 1996).


    4.1 Effect of Humate on the Batch Crystallization of Aluminium Hydroxide

    Of all the individual impurities investigated, humate most significantly affected the crystallization process. The effect of other impurities on the batch crystallization of aluminium hydroxide was presented in another paper by the same authors (Chan and Ang, 1995). This present paper focuses on the influence of humate in both batch and continuous crystallization processes.

    The nucleation rates were determined from the total number of particles of size 1.1 m m per ml in the crystallizer versus time plots. The slope of the curve at a certain time was determined as the effective nucleation rate for the corresponding supersaturation.

    The growth rates were determined from the cumulative distributions where the increase in crystal size was assumed to be purely due to growth. To ensure that the increase was not due to agglomeration, only crystals larger than 20 m m were taken into account for the growth rate determinations.

    The results for the nucleation and growth rates are shown in Figures 2 and 3 respectively.

    Figure 2

    The Effect of Humate on Nucleation Rate in a Batch Crystallization System

    The maximum nucleation rate occurred at a lower supersaturation ratio (c/c*=1.2) compared to that of the control run (c/c*=1.5). A significantly decreased nucleation rate was evident in the presence of humate. This could be due to a number of reasons. The large humate molecules may have blocked the seed surface responsible for nuclei formation or altered the surface characteristics of the aluminate molecules, resulting in a more stable liquor than the liquor with no impurities, at the same supersaturation. It was reported by Gnyra and Lever (1979), that humates are responsible for increased liquor stability with respect to alumina and oxalate solubilities. The growth rate of aluminium hydroxide was significantly retarded by the presence of humate.

    Figure 3

    The Effect of Humate on Growth Rate in a Batch Crystallization System


    4.2 Effect of Humate on the Continuous Crystallization of Aluminium Hydroxide

    Various experimental runs were conducted at a residence time of 40 minutes to investigate the impact of humate concentrations on the aluminium hydroxide crystallization kinetics. The concentrations of humate tested were 0 g/L, 0.033 g/L and 3.3 g/L. A summary of the findings is listed in Table 1.

    Table 1

    Results for the Continuous Crystallization Runs with Humate Impurity









    Al2O3 Yield (g/L)




    Mass Deposition Rate (g/m2 seed.hr)




    Linear Growth Rate (m m/hr)




    Nucleation Rate (#/m3.hr)

    5.16 E+14

    4.19 E+14

    6.16 E+13

    The Al2O3 yield was observed to decrease with increasing humate concentration. This trend was attributed to the final steady state supersaturation, where the presence of humate reduced the amount of solution desupersaturation and thereby reduced the product yield. From Table 1, it can be seen that the presence of 0.033 g/L of humate reduced the Al2O3 yield from 27.6 g/L for no impurities, to 13.2 g/L; that is more than 50% reduction in yield resulted with only 0.033 g/L` of humate impurity. This reduction in the percentage yield suggested that humate imposed a significant impact on the crystallization process, resulting in reduced productivity.

    Increasing the humate concentration reduced the mass deposition rate. It is important to note that the presence of 0.033 g/L humate is enough to significantly reduce the mass deposition rate, from 2.232 g/m2seed.hr for no impurities to 0.545 g/m2seed.hr. The higher humate concentration of 3.3 g/L further decreased the mass deposition rate further, yielding 0.222 g/m2seed.hr (ie a 59% decrease compared to the value given at 0.033 g/L humate).

    In contrast to the yield and mass deposition rate, the linear growth rate only decreased slightly as the humate impurity concentration increased. The linear growth rate dropped from 4.2 m m/hr to 3.9 m m/hr for humate concentrations of 0 g/L and 3.3 g/Ll respectively.

    In line with the effects of humate on the yield, mass deposition and linear growth rates, the presence of humate impurity in a continuous Al2O3 crystallization system also reduced the nucleation rate. The higher the humate concentration, the more significant was its effect on the nucleation rate. Higher concentrations of humate could be responsible for binding the alumina clusters of molecules and significantly hindering nucleation.

    Product crystals from the runs with humate were pale brown in colour compared to the bright white colour of the seed crystals, which suggested that the humate formed a surface layer over the seed crystals, thereby reducing the seed activity and thus the productivity and crystallization kinetics.


    The effects of sodium humate on the crystallization of aluminium hydroxide was investigated for both batch and continuous crystallization systems. It was identified that the presence of humate adversely affects both batch and continuous crystallization kinetics.

    The presence of larger molecules such as humate resulted in reduced nucleation and growth rates in the crystallization runs. The mass deposition rates, growth rates and nucleation rates reduced with increasing humate concentrations. Product crystals from the runs with humate were pale brown in colour. This suggested that the humate molecule formed a surface layer over the seed crystals, which may be responsible for reduced seed activity, thereby resulting in lower productivity and crystallization kinetics.


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