A RATE EQUATION FOR CRYSTALLIZATION OF SODIUM OXALATE UNDER BAYER CONDITIONS

Anthony Mckinnon2, Gordon Parkinson and Kevin Beckham1

A.J. Parker Cooperative Research Centre for Hydrometallurgy

Curtin University of Technology

GPO Box U 1987, Perth, Western Australia 6845

1Alcoa of Australia, Kwinana Alumina Refinery, Cockburn Road, Kwinana,

Western Australia 6167

2Now located at Alcoa of Australia

ABSTRACT

During the digestion of bauxite in caustic, and subsequent operations in a Bayer plant, entrained organic matter is degraded, much of it to the relatively stable oxalate anion. There is consequently a continuous build up of sodium oxalate in the Bayer liquor, and this must be removed in order to prevent unwanted crystallization during hydrate precipitation.

In other industries, rate equations have been employed to predict the extent of crystal growth. Although rate equations have been developed for the crystallization of sodium oxalate in aqueous systems, no work has been published on crystallization under Bayer conditions. This work will describe the development of a rate equation to predict sodium oxalate crystallization in a synthetic liquor system. The effect of supersaturation, seed charge, stirring rate and temperature on rate of crystal growth will be discussed. From activation energy calculations it has been discovered that the crystallization process is predominantly controlled by surface-integration and not bulk diffusion.

A comparison of growth on blocky and acicular oxalate seed has shown that the rate of crystallization is not a simple function of available surface area, which suggests that some surfaces are more active than others. This will be demonstrated with aid of SEM pictures.

KEY WORDS:

sodium oxalate, rate equation, crystallization, kinetics, Bayer

A RATE EQUATION FOR CRYSTALLIZATION OF SODIUM

OXALATE UNDER BAYER CONDITIONS

Anthony Mckinnon, Gordon Parkinson and Kevin Beckham

1.0 INTRODUCTION

During the digestion of bauxite in caustic, and subsequent operations in a Bayer plant, entrained organic matter is degraded, much of it to the relatively stable oxalate anion. If the sodium oxalate concentration is allowed to continually increase in the process then it can coprecipitate with gibbsite. Coprecipitation leads to a number of gibbsite product quality problems, including an increase in the level of fine particles and an increase in sodium levels in the final product. It is therefore essential that the concentration of sodium oxalate in the process stream be tightly controlled. Although there are a number of possible ways of removing oxalate from Bayer liquors (Gnyra, 1979), Alcoa employs a separate sidestream oxalate crystallization removal process. Thus, the crystallization of sodium oxalate is very important to the operation of Alcoa’s Bayer process.

The development of a rate equation for the crystallization of sodium oxalate is an important step in understanding the factors which affect the rate at which sodium oxalate can be removed from Bayer liquors. Once developed, rate equations can be employed to evaluate how potential physical changes to the process will effect sodium oxalate crystallization and how the addition of various compounds to Bayer liquor will influence crystallization. The crystallization kinetics of sodium oxalate have been reported in aqueous solutions (McGregor, 1995 and Xu, 1993) but to date no work has been published on crystallization under Bayer conditions. Bayer liquors contain a multitude of components, a number of which are known to inhibit sodium oxalate crystallization (Lever, 1983). The presence of these components means that it is impractical to develop a fundamental model for the crystallization kinetics of sodium oxalate using process liquor. Thus, in this study, a synthetic liquor solution was used in the development of a kinetic expression for the crystallization of sodium oxalate.

2.0 EXPERIMENTAL

A synthetic liquor was prepared which reflected the major characteristics of plant liquor. The final liquor composition is TA = 280 g/L, TC/TA = 0.84, Al2O3/TC = 0.28, NaCl = 40.0 g/L, Na2SO4 = 10.0 g/L, SiO2 = 0.5 g/L, P2O5 = 0.3 g/L, Sodium Formate = 15.0 g/L, Sodium Acetate = 20.0 g/L, Sodium Malonate = 5.0 g/L, Sodium Succinate = 15.0 g/L, Sodium Oxalate = 0.8 g/L (at 60oC) total soda (TS) = 364 g/L, and a TS/TA = 1.3.

Crystallization experiments were carried out in a 4 L stainless steel water jacketed reaction vessel. Typically, a 1 g/L seed charge of synthetic acicular sodium oxalate was added to a concentrated synthetic liquor solution previously saturated with sodium oxalate and the resulting solution was equilibrated at 60oC for 2 hours with stirring at 200 RPM. Sufficient aqueous sodium oxalate solution (from a 35 g/L stock) was added to supersaturate the liquor to the desired oxalate concentration. The resulting precipitation was monitored by sampling at regular intervals and analysing for soluble oxalate by GC/MS.

Data analysis was carried out using the kinetic analysis program, Cryskin, developed by Kevin Beckham (Beckham, 1994). This program fits desupersaturation data and produces a growth order and rate constant of best fit. It requires input of the seed charge with a defined surface area for each experiment and automatically adjusts the seed area as crystal growth occurs. In order to achieve this, it requires that the seed be well characterised and that the morphology of the seed remains constant throughout the experiment. Well defined acicular sodium oxalate seed crystals were prepared from an aqueous solution of sodium hydroxide and characterised by SEM. A comparison of seed crystals prior to, and during a crystallization experiment has established that the aspect ratio remains constant throughout the desupersaturation experiments.

3.0 RESULTS AND DISCUSSION

Rates of crystallization (R) were calculated from the experimental curves (oxalate concentration versus time) by use of the following equation:

R = -dC/dt = k A (1)

where k is the rate constant, A is the surface area of the crystal, C is the oxalate concentration in the solution, Cs is the equilibrium saturation concentration and g is the growth order. To develop a kinetic expression for the crystallization of sodium oxalate, the values for the order of growth (g) and the rate constant (k) must be determined experimentally. The major variables which can influence the rate equation are the supersaturation and seed surface area and these will be examined in detail. The effect of temperature and stirring rate will also be investigated.

3.1 Effect of Supersaturation on Oxalate Crystallization

Investigations into the effect of supersaturation on oxalate growth were carried out by varying the initial oxalate driving force (?C) whilst maintaining a fixed seed charge (1 g/L acicular). Desupersaturation curves corresponding to initial supersaturations from 0.75 to 2.85 g/L are shown in Figure 1.

Figure 1

The effect of varying the initial supersaturation on the desupersaturation

of sodium oxalate in synthetic liquor at 60oC.

The desupersaturation data shown in Figure 1 have been analysed by Cryskin, and the resulting rate constants and growth orders are displayed in Table 1. As the value of the growth rate is strongly dependent upon the value chosen for the growth order, it is necessary to fix the value of the growth order to compare rates under different conditions. Tests 4 - 7, which cover a supersaturation range of 1.65 to 2.55 g/L (oxalate concentration: 2.3 - 3.2 g/L), were used to calculate an overall growth order and rate constant. A value of 2.45 was obtained for the overall growth order and 8.21 x 10-7 kg m-2s-1 for the overall rate constant. A growth order of 2.45 was fixed and the rate constant for each set of data recalculated. This yields a close agreement between the rate constants over the supersaturation range of 1.05 to 2.55 g/L. The rate constants obtained at both the extreme high and low range of driving force (?C = 2.85 & 0.75 g/L) are also acceptable.

Table 1

Overall rate constants and growth exponents for the desupersaturation of a solution of sodium oxalate in synthetic liquor with varying initial supersaturations at a constant seed charge of 1 g/L acicular oxalate at 60oC

Test

Initial Supersaturation
?C

Growth Order

Rate constant k

(kg m-2s-1)

k given g = 2.45

(kg m-2s-1)

1

0.75 g/L

3.59

8.86 x 10-7

7.02 x 10-7

2

1.05 g/L

2.78

8.70 x 10-7

8.81 x 10-7

3

1.35 g/L

2.63

8.07 x 10-7

8.15 x 10-7

4

1.65 g/L

2.42

8.06 x 10-7

8.01 x 10-7

5

1.95 g/L

2.42

8.66 x 10-7

8.84 x 10-7

6

2.25 g/L

2.44

8.80 x 10-7

8.54 x 10-7

7

2.55 g/L

2.44

8.34 x 10-7

8.51 x 10-7

8

2.85 g/L

2.36

10.43 x 10-7

9.74 x 10-7

4-7

 

2.45

8.21 x 10-7

 

3.2 Effect of Acicular Seed Charge on Oxalate Crystallization

The effect of acicular seed charge on oxalate growth was investigated by varying the acicular seed charge while maintaining a fixed sodium oxalate concentration of 2.3 g/L. Desupersaturation curves corresponding to a range of seed charge from 0.5 to 3 g/L are shown in Figure 2.

Figure 2

The effect of acicular seed charge on the desupersaturation of sodium oxalate in synthetic liquor at 60oC

The desupersaturation data shown in Figure 2 have been analysed by Cryskin and the resulting rate constants and growth orders are displayed in Table 2. These results show an overall growth order of 2.44 and an overall rate constant of 8.43 x 10-7 kg m-2s-1. The rate constants for the data in Figure 2 were then recalculated using a growth order of 2.45, which was obtained from the variation in initial supersaturation experiments (Table 1) and these are shown in the last column in Table 2. The agreement between the overall growth order obtained from the variation in supersaturation experiments (2.45) and that obtained from the variation in acicular seed charge (2.44) is very good.

Table 2

Overall rate constants and growth exponents for the desupersaturation of a 2.3 g/L sodium oxalate in synthetic liquor solution with varying initial seed charges of acicular oxalate using growth orders calculated from the variation in seed charge experiments (Figure 2) and from the variation in initial supersaturation experiments (Figure 1)

Test

Acicular seed charge
(g/L)

Growth Order

Rate constant - k

(kg m-2 s-1)

k given g=2.44

(kg m-2 s-1)

k given g=2.45

(kg m-2 s-1)

1

0.5

2.23

9.23 x 10-7

7.97 x 10-7

7.92 x 10-7

2

0.75

2.43

7.51 x 10-7

7.72 x 10-7

7.71 x 10-7

3

1

2.29

10.50 x 10-7

10.15 x 10-7

10.14 x 10-7

4

2

2.85

7.37 x 10-7

7.49 x 10-7

7.39 x 10-7

5

3

2.67

8.93 x 10-7

9.56 x 10-7

9.42 x 10-7

1-5

 

2.44

8.43 x 10-7

   

3.3 The effect of Stirring Rate on Oxalate Crystallization

The effect of stirring rate on the desupersaturation of sodium oxalate in synthetic liquor was determined by carrying out crystallization experiments containing 2.3 g/L sodium oxalate with a 1 g/L acicular seed charge at stirring speeds of 200, 400 and 600 RPM. Rate constants were calculated from the data obtained, using a fixed growth order of 2.45. The results displayed in Table 3 show that increasing the stirring speed has only a negligible effect on the rate of oxalate growth and therefore demonstrates that the growth of sodium oxalate is not diffusion controlled. This is confirmed by the growth order calculations, which show that the growth of sodium oxalate follows a second order reaction, as the value of g is approximately equal to 2 (Mullin, 1993). The slight decrease in rate constant, with increased stirring rates, is possibly due to the attrition of some particles which results in a slight increase in the overall surface area.

Table 3

Rate constants for the desupersaturation of a solution of 2.3 g/L sodium oxalate in synthetic liquor with varying stirring rates, given a growth order of 2.45

Stirring Speed (rpm)

Rate constant given g=2.45 (kg m-2 s-1)

200

8.01 x 10-7

400

7.93 x 10-7

600

7.75 x 10-7

3.4 The Effect of Temperature on Oxalate Crystallization

It is expected that if the mechanism by which growth occurs is constant over a range of temperatures, then the apparent rate constant for crystal growth will increase with increasing temperature. Rates of crystallization were measured over a temperature range of 25 to 80oC, while attempting to maintain a fixed initial supersaturation (?C = 1.65 g/L) and seed charge (1 g/L acicular). As predicted, the apparent rate constant for oxalate growth increases with increasing temperature (Table 4).

Table 4

Overall rate constants and growth exponents for the desupersaturation of a solution of sodium oxalate in synthetic liquor, undertaken at various temperatures with a constant initial supersaturation and seed charge of 1 g/L acicular oxalate

Temperature (oC)

Initial Supersaturation ?C

Growth Order

G

Rate constant k
(kg m-2 s-1)

k given g = 2.45
(kg m-2 s-1)

25

1.79 g/L

3.16

0.022 x 10-7

0.0809 x 10-7

32

1.58 g/L

3.03

0.081 x 10-7

0.193 x 10-7

38

1.66 g/L

2.62

0.74 x 10-7

0.893 x 10-7

44

1.64 g/L

2.60

1.52 x 10-7

1.73 x 10-7

50

1.52 g/L

2.28

4.51 x 10-7

4.17 x 10-7

55

1.74 g/L

2.73

4.48 x 10-7

5.22 x 10-7

60

1.56 g/L

2.42

8.06 x 10-7

8.01 x 10-7

65

1.35 g/L

1.98

25.2 x 10-7

25.0 x 10-7

70

1.68 g/L

2.86

17.8 x 10-7

19.4 x 10-7

75

1.29 g/L

2.30

42.9 x 10-7

46.4 x 10-7

80

1.26 g/L

2.16

65.5 x 10-7

75.4 x 10-7

 

The activation energy for the growth of sodium oxalate in synthetic liquor can be calculated from the slope of a plot of ln (K) versus 1/T (Figure 3). An activation energy of 106 kJ/mol was obtained, which is further evidence that the crystallization of sodium oxalate is predominantly a process controlled by surface-integration (denoted as an activation energy above 40 kJ/mol) (Mullin, 1993), and not bulk diffusion. The linearity of the Arrhenius plot indicates that the mechanism of crystal growth is constant over the temperature range of 25 to 80oC.

Figure 3

Arrhenius plot of the logarithm of crystallization rate constant as a function of the reciprocal of the temperature for the crystallization of sodium oxalate in synthetic liquor. Initial oxalate supersaturation of approximately 1.65 g/L over a temperature range of 25 -80oC.

3.5 Growth from Blocky Seed in Synthetic Liquor

The rate of growth of blocky seed in synthetic liquor was compared to that of acicular seed to establish whether the morphology of seed crystals influences the rate of growth or whether growth is simply related to total surface area. If the morphology of seed crystals is irrelevant to the rate of oxalate growth, then desupersaturation experiments carried out with blocky and acicular seed at a constant oxalate supersaturation and temperature should only be dependent on the surface area of the seed. The BET surface areas of acicular and blocky seed were found to be 1.04 and 0.21 m2/g, respectively. If identical desupersaturation experiments are carried out with 1 g/L acicular seed (total surface area 1.04 m2/L) and 2 g/L blocky seed (total surface area 0.42 m2/L), then it is expected that the desupersaturation experiment containing the acicular seed would proceed faster as the total surface area is greater. The results displayed in Figure 4 show that this is not the case, as the blocky seed desupersaturation experiment is quicker. If the blocky seed is examined by SEM, both prior to and during crystallization (Figure 5), it is seen that the relatively smooth faces of the control seeds show pronounced dendritic growth from the end faces (001) after growth in synthetic liquor. Surface area analysis of the blocky crystals at the completion of the experiment reveals a large increase in surface area (0.96 m2/L). Thus, if blocky crystals are placed in supersaturated synthetic liquor, dendritic growth occurs leading to an acicular like morphology. This demonstrates that the end faces (001) of sodium oxalate grow faster in synthetic Bayer liquor than the side faces.

Figure 4

A comparison of the effect of seed type on the desupersaturation of sodium oxalate in synthetic liquor at 60oC, with a constant initial oxalate concentration of 2.3 g/L

 

(a) (b)

Figure 5

SEM pictures of the crystals obtained from a desupersaturation experiment with a 1 g/L blocky seed charge and an initial oxalate concentration of 2.3 g/L: (a) initial blocky seed (Hayashi, full scale = 97 ?m); (b) after 40 mins (full scale = 165 ?m)

Calculations of the total amount of surface area of the (001) faces available on the blocky and acicular seed shows that the total amount of (001) face surface area is higher for the 2 g/L blocky seed charge than for the 1 g/L acicular seed charge. This explains why blocky seed has a higher growth rate than acicular seed (Figure 4).

4.0 CONCLUSIONS

A rate equation to predict sodium oxalate crystallization in a synthetic liquor system has successfully been developed. The rate of growth of sodium oxalate can be described by the following equation in the synthetic liquor system used in this study.

R = 8.21 x 10-7 A

 

Investigations into the effect of temperature on oxalate crystallization show that, with a fixed growth order, that the apparent rate constant increases with increasing temperature. An activation energy of 106 kJ/mol was calculated for the growth of sodium oxalate in synthetic liquor. This demonstrates that the crystallization of sodium oxalate is predominantly a process controlled by surface-integration and not bulk diffusion.

A comparison of rate of growth of oxalate seed with different morphologies has shown that the rate of growth is not a simple function of available surface area and that some crystal faces are more active than others.

ACKNOWLEDGMENTS

This research was supported by a grant from MERIWA.

REFERENCES

Beckham, K. (1994). Unpublished data.

Gnyra, B. and Lever, G. (1981). Review of Bayer organics-oxalate control process. Light Metals. pp. 151-161.

Lever, G. (1983). Some aspects of the chemistry of bauxite organic matter on the Bayer process: the sodium oxalate-humate interaction. Travaux.13 pp. 335-347.

McGregor, A. (1995). Sodium oxalate crystallisation kinetics. Thesis (Honours) University of Queensland.

Mullin, J.W. (1993). Crystallization, 3rd ed., Oxford, Butterworth-Heinemann.

Xu, B.A.; Giles, D. and Ritchie, I.M. (1993). Report on the crystallization of sodium oxalate. A.J. Parker Cooperative Research Centre for Hydrometallurgy.