Available on line at Association of the Chemical Engineers AChE www.ache.org.yu/CICEQ Chemical Industry & Chemical Engineering Quarterly 14 (1) 39–45 (2008)
IRENA NIKOLIĆ DRAGOLJUB BLEČIĆ NADA BLAGOJEVIĆ University of Montenegro, Faculty of Metallurgy and Technology, Podgorica, Montenegro SCIENTIFIC PAPER UDC 547.476.3:661.343:546.62.36:548.2
CI&CEQ
THE INFLUENCE OF TARTARIC ACID ON THE PHENOMENA OF Al(OH)3 CRYSTALLIZATION FROM THE CAUSTIC SODA SOLUTION Crystallization of Al(OH)3 from the caustic soda solution is the most important part of the Bayer process for alumina production because several phenomena which determine the properties of precipitated Al(OH)3 occur simultaneously. These processes are nucleation, agglomeration and Al(OH)3 growth. In this paper we have investigated the influence of tartaric acid on agglomeration, nucleation and Al(OH)3 crystal growth processes. The results have shown the inhibitory influence of these impurities on the phenomena mentioned above. Key words: crystallization, caustic soda solutions, Al(OH)3, tartaric acid, agglomeration, nucleation, crystal growth.
The caustic soda solution consists of Al2O3, Na2O and H2O, and this solution is used for the alumina production by the Bayer process. Crystallization of Al(OH)3 is one part of the Bayer process resulting in precipitation of Al(OH)3 particles. During the crystallization of Al(OH)3 from the caustic soda solution, several phenomena occur simultaneously and they all influence the physicochemical characteristic of precipitated Al(OH)3. These phenomena are nucleation, agglomeration and crystal growth of Al(OH)3 crystals. Because of the fact that the crystallization of Al(OH)3 from caustic soda solutions occurs in the presence of the previously precipitated seed (Al(OH)3 particles, the new nuclei are mainly formed by the secondary nucleation. Agglomeration is conglomeration of small Al(OH)3 particles into the bigger agglomerates, which becomes more compact with regard to the process of crystal growth. Nowdays, specialy atention is given to the research of the Al(OH)3 crystal growth, more exactly to the research of the mechanism of the Al(OH)3 crystal growth. Aggravation is the fact that Al(OH)3 crystalized in the form of agglomerates, what comlicates this research. So it is not posible to investigate the crystal growth of certain crystal faces, but the research is limited to the consideration of the overall crystal growth rate [1,2]. Corresponding author: Irena Nikolić, University of Montenegro, Faculty of Metallurgy and Technology, Cetinjski put bb, 81000 Podgorica, Montenegro. E-mail:
[email protected] Paper received: 22 September 2007. Paper revised: 6 March 2008. Paper accepted: 10 March 2008.
All the processes mentioned are influenced by many parameters such as temperature, caustic soda concentrations, seed ratio and the presence of impurities as well. The investigation of the temperature, caustic soda concentrations and seed ratio is widely investigated in the past [3-16]. Nowadays, the researches are focused on the investigation of the influence of impurities on the Al(OH)3 crystallization, especially on the influence of organic impurities: polyols, alditols and hydroxycarboxylic acids [17-19]. Organic compounds have an inhibitory influence on the Al(OH)3 crystallization process. It depends on several factors like the carbon chain length, the number and stereochemistry of hydroxyl groups. The aim of this research was the investigation of carboxyl acid – tartaric acid – on the crystallization of Al(OH)3 from the caustic soda solution. Tartaric acid is not naturally present in the caustic soda solution. But, another carboxyl acid – oxalic acid - has the industrial importance because the presence of oxalate in the caustic soda solution has been confirmed. So this paper is the attempt to contribute to the investigation of the influence of carboxyl acids on the phenomena of the crystallization process. EXPERIMENTAL The crystallization process of Al(OH)3 was carried out from the synthetic caustic soda solutions, which had been prepared by dissolving of pure Al, purity of 99.999 %, (Alfa Aeser, USA) in a hot solution of NaOH. The experiments were caried out using the
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I. NIKOLIĆ, D. BLEČIĆ, N. BLAGOJEVIĆ: THE INFLUENCE OF TARTARIC ACID…
industrial seed (from Alumina Plant in Podgorica), grain size of below 32 µm and between 75 and 63 µm, keeping the seed ratio constant at the value of 1. The seed ratio is a mass ratio of Al2O3 in the seed to Al2O3 in the solution. The experiments related to the investigation of the agglomeration process were carried out with the seed ganulation below 32 µm while those related to the investigation of nucleation and crystal growth processes were carried out with the seed grain size between 75 and 63 µm. The crystallization process was carried out in the glass vessel with double walls through which the water warmed at the temperature of crystallization is circulated. The seed charge was poured into the solution and stirring was ensured by using the air agitation which was just sufficient to prevent the settling of the seed. At the end of each experiment the slury was filtered and the resulted filter cake was rinsed with distilled water and dried at the temperature of 105 °C within 1 h. Afterwards, the grain size analysis, chemical and microstructural analysis of the obtained samples were carried out. The grain size analysis was carried out on the particle grain size analyser ″MALVERN 3600″. The chemical analysis was carried out on the atomic spectrophotometer SMITH HIEFTJE and the microstructural analysis on the Electron Microprobe JEOL JCXA-733. The experiments were carried out in batch conditions, from pure caustic soda solutions and in the presence of oxalic acid, at the temperature of 60, 65, 70 and 75 °C, while the caustic soda concentrations were 130, 140 and 150 g/l. These parameters correspond to the values of supersaturation of caustic soda solutions which are given in Table 1. Table 1. The change of supersaturation β as a function of temperature and the caustic soda concentration (c(Na2Oc)) t / °C
c(Na2Oc) / g l-1 130
140
150
60
2.41
2.33
2.26
65
2.17
2.10
2.04
70
1.96
1.9
1.84
75
1.78
1.72
1.67
Supersaturation of the caustic soda solution is expressed as the ratio between the actual and the equilibrium concentration of Al(OH)3:
β =
c ( Al2 O3 ) c 0 ( Al2 O3 )
(1)
where the c(Al2O3) and c0(Al2O3) are actual and equilibrium concentrations of Al2O3, respectively [9]. The equilibrium concentration of Al2O3 is calculated using the relation:
40
CI&CEQ 14 (1) 39–45 (2008)
6 .211 − c 0 ( Al 2 O 3 ) =e c 0 (Na 2 O (c ) )
2486 .7 + 1 .09 c 0 (Na 2 O ( c ) )
T
(2)
where c0(Al2O3) and c0(Na2O(c)) are equilibrium concentrations in g l-1, and T is the temperature in K [5]. Mass ratio (ratio of c(Al2O3) to c(Na2O(c)) in caustic soda solution) was 1.05. The influence of oxalic acid on the Al(OH)3 crystal growth during the crystallization process from caustic solutions was investigated at the concentration of 20, 30 and 40 mM of oxalic acid. The overall crystal growth of Al(OH)3 is determined from the data obtained by the grain size analysis by the method of Randolph and Larson [1,20]. RESULTS AND DISCUSSION The influence of the investigated parameters on the agglomeration process The change of the agglomeration process with the change of the investigated parameters is expresed by the change of the agglomeration degree (A, %) which is calculated by:
A = 100 (1 −
N ) N0
(3)
where N and N0 are the number of particles in the precipitated Al(OH)3 and the seed, respectively [15]. The number of particles in precipitated Al(OH)3 and in a seed have been estimated by using a grain size analyser. Ratio N/N0 is a relative number of Al(OH)3 particles. When the relative number of particles is less then 1, the agglomeration process occurs, while when it is more than 1, the nucleation process is dominant. The change of a relative number of particles of Al(OH)3 and the agglomeration degree during the crystallization from the pure caustic caustic soda solution and in the presence of tartaric acid is shown in Figure 1. It is evident that the change of a relative number of particles and the agglomeration degree versus temperature has the same character in both cases. The relative number of particles decreases, and the agglomeration degree increases with the increase of the crystallization temperature. So, the rate of agglomeration process increases with the increase of the temperature. But, it is also evident, that the agglomeration degree is less in the presence of tartaric acid than in the case of the pure caustic soda solution, which indicates an inhibitory influence of this acid on the agglomeration process. Besides, the decrease of the caustic soda concentration at the constant temperature results in the increase of the agglomeration degree of Al(OH)3 in both cases of the pure caustic soda solution and in the presence of tartaric acid (Fi-
I. NIKOLIĆ, D. BLEČIĆ, N. BLAGOJEVIĆ: THE INFLUENCE OF TARTARIC ACID…
gure 2) and this can be explained by the increase of supersaturation of the caustic soda solutions discussed earlier [15]. The results obtained by the investigation of the influence of tartaric acid on the total
CI&CEQ 14 (1) 39–45 (2008)
soda content in precipitated Al(OH)3 show the increase of the total soda content with the increase of the temperature (Figure 3).
a
b
c
d
e
f
Figure 1. The change of the relative number of Al(OH)3 particles (a, c and e) and agglomeration degree (b, d, f) versus the temperature at the caustic soda cocncentration of 130 (a, b), 140 (c, d) and 150 g l-1 (e, f) in the case of crystallization of the pure caustic solution ( ) and in the presence of tartaric acid (c), using the seed grain size below 32 µm
Figure 2. The change of the agglomeration degree of Al(OH)3 particles as a function of the caustic soda concentration in the presence of tartaric acid at the temperature of 60 ( ), 65 (•), 70 () and 75 °C ()
a
b
c
Figure 3. The change of the total soda content in precipitated Al(OH)3 as a function of the temperature at different caustic soda contents (a) 130, (b) 140 and (c) 150 g l-1 in the case of crystallization of the pure caustic solution ( ) and in the presence of 20 mM tartaric acid (c)
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I. NIKOLIĆ, D. BLEČIĆ, N. BLAGOJEVIĆ: THE INFLUENCE OF TARTARIC ACID…
CI&CEQ 14 (1) 39–45 (2008)
The influence of the investigated parameters on the nucleation process
The investigation of the mechanism and kinetics of Al(OH)3 crystlas growth
The change of the relative number of Al(OH)3 particles and the agglomeration degree in dependence on the temperature at different caustic soda concentrations are shown in Figure 4. The relative number of particles is more than 1 in all investigated cases, so the nucleation process is dominant. Moreover, the nucleation process is more intensive if the temparature is lower. But, the relative number of particles is less in the presence of tartaric acid than in the case of the pure caustic soda solution which indicates an inhibitory influence of tartaric acid on the nucleation process.
The microstructural investigation has shown only the presence of hexagons during the crystallization of Al(OH)3 in both investigated cases, from the pure caustic soda solutions and in the presence of tartaric acid (Figures 5a and 1b). Also, it is evident that the growth of Al(OH)3 follows the B+S model (birth and spread), that is to say, the simultaneous formation of multiple nuclei before the complete spread of the existing nuclei and the formation of nuclei on the top of the existing ones as it can be seen in the frames of Figure 5.
a
b
c
Figure 4. The change of the relative number of Al(OH)3 particles and the agglomeration degree at different caustic soda concentrations: (a) 130, (b) 140 and (c) 150 g l-1in the case of crystallization of the pure caustic solution ( ) and in the presence of 20 mM tartaric acid (c)using the seed granulation between 75 and 63 µm a
b
Figure 5. Microstructure of Al(OH)3 particles precipitated from the pure caustic soda solution (a) and in the presence of tartaric acid (b)
The results obtained by the investigation of the kinetics of Al(OH)3 crystal growth during the crystallization from the caustic soda solution are shown in Figure 6. It is evident that the overall crystal growth is increased with the increase of the temperature in both
42
cases, in the case of crystallization from the pure caustic soda solution and in the presence of tartaric acid. Besides, the values of the overall crystal growth are less in the presence of tartaric acid than in the case of crystallization from the pure caustic soda
I. NIKOLIĆ, D. BLEČIĆ, N. BLAGOJEVIĆ: THE INFLUENCE OF TARTARIC ACID…
concentrations of Al2O3, respectively), and K and K2 are complex constants explained by Lui and coworkers [22]. To test whether such a growth mechanism (B+S) is plausible, it is usual to plot ln(G/σ5/6) versus 1/σ. If the crystal growth follows the B+S model, the mentioned function is linear. This is not the case when the crystal growth follows a BCF (Barton, Frank and Cabrera) model, where the crystal growth is promoted by the presence of screw dislocations [23]. The function ln(G/σ5/6) versus 1/σ obtained is linear and it is shown in Figures 7 and 8. It is evident that the function ln(G/σ5/6) versus 1/σ can be approximated by a straight line at all temperatures and therefore the Al(OH)3 crystal growth follows the B+S model ( a polynuclear process), in both cases (in the case of decomposition of the pure caustic soda solution and in the presence of tartaric acid).
solution. So, it could be said that tartaric acid has the inhibitory influence on the growth process. The values of the crystal growth rate constant at different crystallization conditions are shown in Table 2. Additionaly, the energy of the activation (∆Ea) can be determined by using the values of the crystal growth rate constants (Table 3). The obtained results indicate the inhibitory influence of tartaric acid, as well. The results obtained by a kinetic investigation of Al(OH)3 crystal growth during the decomposition of the caustic soda solutions [21] can be used as a confirmation of the mentioned mechanism of Al(OH)3 crystal growth according to which the growth rate of Al(OH)3 crystal growth can be described by the Equation:
G=Kσ5/6exp(K2/σ)
CI&CEQ 14 (1) 39–45 (2008)
(4)
where G is the overall Al(OH)3 crystal growth rate in µm h-1, σ is supersaturation of the solution expressed as σ =(c-c0)/c0 (c and c0 are the actual and equilibrum a
b
c
20 22
18 18
20
16 16
16 14
14 14
12
G / µm h–1
G / µm h–1
G / µm h–1
18
12
12
10
10
8
8
6
10 8 6 4
58
60
62
64
66
68
70
72
74
76
58
60
62
64
66
68
70
72
74
76
58
t / °C
t / °C
60
62
64
66
68
70
72
74
76
t / °C
Figure 6. The change of the overall Al(OH)3 crystal growth in a function of the temperature during the crystallization from the pure caustic solution ( ) and in the presence of tartaric acid (c) at different caustic soda concentrations, (a) 130, (b) 140 and (c) 150 g l-1 Table 2. The values of constant of Al(OH)3 crystal growth rate, k, in dependence on the temperature, caustic soda concentration and the presence of tartaric acid kx10-3 / h-1 Solution
c(Na2Oc) / g l-1
t / °C 130
140
150
Pure
60 65 70 75
2.30 2.88 4.64 6.15
1.96 2.46 3.88 5.06
1.65 2.15 3.38 4.42
20 mM tartaric acid
60 65 70 75
1.21 1.78 3.01 4.42
0.81 1.32 2.56 3.82
0.68 1.19 2.27 3.22
Table 3. The values of the activation energy ∆Ea ( KJ/mol) obtained during the crystallization from the pure caustic soda solution and in the presence of tartaric acid -1
∆Ea / kJ mol
c(Na2Oc) / g l-1
Solution Pure 20 mM tartaric acid
130
140
150
65.84 84.51
63.11 101.94
64.8 102.48
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I. NIKOLIĆ, D. BLEČIĆ, N. BLAGOJEVIĆ: THE INFLUENCE OF TARTARIC ACID…
CI&CEQ 14 (1) 39–45 (2008)
a
b
3.0
2.9 2.9
2.8
ln(G/σ5/6)
ln(G/σ5/6)
2.8 2.7 2.6
2.7 2.6 2.5
2.5 2.4
2.4
2.3
2.3
0.70
0.72
0.74
0.76
0.78
0.84
0.80
0.86
0.88
0.90
0.92
c
0.96
d
3.0
2.9
2.9
2.8
ln(G/σ5/6)
2.8
ln(G/σ5/6)
0.94
1/σ
1/σ
2.7 2.6
2.7 2.6 2.5
2.5 2.4
2.4
2.3
2.3 1.04
1.06
1.08
1.10
1.12
1.25
1.30
1.35
1.40
1.45
1.50
1/σ
1/σ
Figure 7. The plot ln(G/σ5/6) versus 1/σ for the case of the pure caustic soda solution at different temperatures: (a) 60, (b) 65, (c) 70 and (d) 75 °C. a
b
2.4
2.4 2.3
2.3
ln(G/σ5/6)
ln(G/σ5/6)
2.2
2.1
2.0
2.2
2.1 1.9
1.8 0.70
0.72
0.74
0.76
0.78
2.0 0.84
0.80
0.86
0.88
0.90
0.92
0.94
0.96
1/σ
1/σ
c
d
2.60
2.60 2.55
2.55
ln(G/σ5/6)
2.50
ln(G/σ5/6)
2.50
2.45
2.45 2.40 2.35 2.30
2.40
2.25
2.35 1.04
1.06
1.08
1/σ
1.10
1.12
2.20 1.25
1.30
1.35
1.40
1.45
1.50
1/σ
Figure 8. The plot ln(G/σ5/6) versus 1/σ in the presence of 20 mM of tartaric acid at different temperatures: a) 60, b) 65, c) 70 and d) 75 °C
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I. NIKOLIĆ, D. BLEČIĆ, N. BLAGOJEVIĆ: THE INFLUENCE OF TARTARIC ACID…
CONCLUSIONS According to the results obtained it can be concluded that the presence of tartaric acid in the caustic soda solution shows the inhibitory influence on the crystallization process, i.e., on the phenomena of the crystallization process: agglomeration, nucleation and crystal growth of Al(OH)3 particles. The presence of tartaric acid has no influence on the character of the dependance of the mentioned phenomena of crystallization parameters, i.e., agglomeration, nucleation and crystal growth of Al(OH)3 are changed in the same way with the change of working parameters as in the case of the pure caustic soda solution. So,
CI&CEQ 14 (1) 39–45 (2008)
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[12]
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[14]
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– the agglomeration process is more intensive if
the crystallization temperature is higher and the caustic soda concentration is lower;
Acknowledgements The SEM microstructure investigations were done at the National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, University of California at Berkeley, which is supported by the U.S Department of Energy under Contract # DE-AC02-05CH11231.
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J.S.C.Loh, H.R.Watling, G.M.Parkinson, Proceedings of
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S. Freij, M.Lee, M. Reyhani, G. Parkinson, Proceedings of the Fifth International Alumina Quality Workshop, Bunbury, Western Australia, (1999), p.38.
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