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Kinetics of boehmite precipitation from supersaturated sodium aluminate solutions
Hydrometallurgy 68 (2003) 57 – 68 www.elsevier.com/locate/hydromet
Kinetics of boehmite precipitation from supersaturated sodium aluminate solutions C. Skoufadis, D. Panias *, I. Paspaliaris School of Mining and Metallurgical Engineering, National Technical University of Athens, Laboratory of Metallurgy Zografos Campus, Athens 15780, Greece Received 20 August 2002; received in revised form 15 October 2002; accepted 15 October 2002
Abstract This work presents the effect of the most important parameters, the precipitation temperature, the sodium hydroxide concentration and the initial seed ratio (SR) in the solution, on the boehmite precipitation from supersaturated sodium aluminate solutions. A kinetic model that describes the experimental data was developed. According to that model, boehmite precipitation follows second order reaction kinetics and has activation energy of 89 kJ/mol. The orders of the precipitation reaction with respect to the initial sodium hydroxide concentration and the initial seed ratio are estimated to be 1.8 and 0.54, respectively. The most important result is that the boehmite precipitation reaches an apparent equilibrium stage at which the alumina concentration is much higher compared to the value of the boehmite solubility under the same experimental conditions. The results reveal that the boehmite precipitation is a self-decelerated process and the observed kinetic inhibitions are related to the sodium hydroxide concentration in the supersaturated solution. Finally, a mechanistic interpretation of the experimental data is presented based on the concept of the surface precipitation of boehmite on the active sites of the boehmite seed. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Boehmite precipitation; Kinetics; Aluminate solutions
1. Introduction Boehmite (a-AlOOH) is one of the well-known aluminium oxide – hydroxides. It usually occurs in nature as a constituent of bauxitic ores that are almost exclusively used for the production of smelter grade alumina by the Bayer process and primary aluminium by the Hall – Heroult process. Boehmite is the base for the production of various activated aluminas (Sleppy et al., 1991), which find applications as catalysts in * Corresponding author. E-mail address: [email protected] (D. Panias).
the chemical industry. Moreover, boehmite can be used for the production of specialty-calcined aluminas (Sleppy et al., 1991) that are commercially produced by heating the Bayer alumina trihydrates. Critical factors for the above applications are the availability and the price of boehmite. Boehmite can be produced (Misra, 1986) with hydrothermal conversion of gibbsite, which is an intermediate product of the Bayer process, at elevated temperatures in autoclaves. It is produced also as a byproduct of the production of linear alcohols with the Ziegler process. Finally, fibrous boehmite is prepared with hydrolysis of aluminium salts in the presence of
0304-386X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X ( 0 2 ) 0 0 1 6 5 - 2
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acetic acid. Boehmite production is based on gibbsite or other more expensive aluminium salts, and therefore, it is an expensive material with small availability in the market. A new process for producing boehmite (Panias et al., 2001a) has been developed in the Laboratory of Metallurgy of the National Technical University of Athens. The process comprises a modification of the Bayer process precipitation stage. According to the new process, crystalline monohydrate alumina (boehmite, Al2O3H2O) is precipitated from supersaturated sodium aluminate solutions under conditions which are similar to those of the Bayer process. The new process is highly innovative and can be used not only for the production of smelter grade alumina, but also for the production of activated and specialty-calcined aluminas. In the present work, the effect of the most important parameters, such as temperature, sodium hydroxide concentration and initial boehmite seed ratio on boehmite precipitation is studied and a model describing the kinetics of boehmite precipitation from supersaturated sodium aluminate solutions is presented.
2. Experimental procedures and apparatus The kinetic experiments were performed in an Inconel autoclave. The experimental apparatus consisted of the following items: (a) the reactor that is a pressure vessel with a maximum capacity of 600 mL, (b) the head of the pressure vessel equipped with a safety valve, a gas inlet, a cooling loop, a magnetic drive for the mechanical stirrer and a Pt thermocouple for the temperature control, (c) the electrical furnace for the heating of the pressure vessel, (d) the proportional temperature controller. The apparatus was additionally equipped with a specially designed tube sampler that permitted the sampling from the reactor. The tube sampler was installed at the head of the pressure vessel and had a ceramic microfilter at its bottom, with the ability to filter all solid particles with size higher than 1 Am. A constant overpressure was created artificially in the
reaction vessel with the insertion of pure nitrogen in order to allow sampling under the experimental conditions. The autoclave was loaded with 420 mL supersaturated sodium aluminate solution with pre-adjusted Al2O3 and Na2O concentrations and the appropriate amount of boehmite seed. The pulp was rapidly heated up to the required temperature and the experiment was performed under isothermal conditions. Liquid samples were taken from the reactor periodically and were analyzed for their Al2O3 content. At the end of the experiments, the solids were characterized mineralogically with thermogravimetric analysis and X-ray diffractometry in order to verify that the precipitated solids were pure crystalline boehmite. The alumina content of the liquid samples was determined according to the following procedure. The aluminate solution, which contained 10 –100 mg of aluminium, was boiled with 50 mL of 0.1 M EDTA solution for an approximate time of 5 min. After cooling, 0.2 g of xylenol orange-indicator and drops of HCl 1 M solution were added up to the point where the solution became yellow. In the next stage, the solution pH was adjusted with ammonium acetate/ acetic acid buffer solution to a value of about 5– 6 and the excess of EDTA was titrated with 0.1 M zinc sulfate solution up to the point where the color changed from yellow to red. According to the reaction stoichiometry, 2.698 mg of dissolved Al was complexed with 1 mL of 0.1 M EDTA solution. The crystalline boehmite seed was produced according to the following procedure. A volume of 400 mL of pulp, with an alumina trihydrate concentration of 375 g/L, was heated in the autoclave at 300 jC for 4 h with an agitation rate of 600 min 1. The pulp was cooled rapidly down to 70 jC after the end of the experiment, filtered and the cake from the solid/liquid separation was dried for 24 h at 100 jC. The supersaturated sodium aluminate solutions were prepared according to the following procedure. Weighed amounts of pure alumina trihydrate and sodium hydroxide were inserted into an Inconel autoclave that contained 300 mL of distilled water. The pulp was heated to 160 jC for 1 h and the solids were totally dissolved. After rapid cooling, the liquor was diluted to a final volume of 500 mL. The chemicals that were used for the production of the boehmite seed and the preparation of the super-
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saturated sodium aluminate solutions were pure hydrargillite (Al2O33H2O) and sodium hydroxide commercially available from Merck.
3. Results The most important parameters that affect the boehmite precipitation from the supersaturated sodium aluminate solutions are the precipitation temperature, the initial sodium hydroxide concentration in the solution and the initial boehmite seed ratio in the solution. 3.1. Effect of temperature The effect of temperature on the boehmite precipitation was studied in the temperature range of 90– 120 jC. In the present work, the sodium hydroxide concentration in the solution was expressed as Na2O and was 120 g/L. The initial boehmite seed ratio in the solution (SR) had a value of 1.76 and the agitation rate was kept constant at 300 min 1. Seed ratio (SR) was defined as the mass of boehmite added in the solution divided by the mass of alumina contained in the
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solution. The alumina (Al2O3) content of the supersaturated aluminate solutions was 132 g/L and thus, the mass ratio (MR) that was expressed as gAl2O3/ gNa2O in the solution was 1.1. The results are shown in Fig. 1 where the alumina content in the solution is plotted versus temperature and precipitation time. As shown in Fig. 1, the precipitation curves consist of three distinct parts that represent different stages of the boehmite precipitation from the aluminate solution. The first stage, which is positively affected by temperature, is characterized by very fast precipitation and has duration of about 9 h. This stage is followed by a transition stage during which a gradual but substantial decrease of the precipitation rate takes place. In the final stage, the boehmite precipitation rate is very low. Practically, it seems that the boehmite precipitation has reached an apparent equilibrium stage in which the alumina concentration in the solution is higher than the boehmite solubility under the same conditions as also shown in Fig. 1. This behavior indicates that the boehmite precipitation process is a self-decelerated process as long as there are no external actions that inhibit the process. The values of boehmite solubility shown in Fig. 1 have been calculated from a theoretical model (Panias
Fig. 1. Effect of temperature on boehmite precipitation from supersaturated sodium aluminate solutions and comparison between real and apparent equilibrium stages (132 g/L Al2O3, 120 g/L Na2O, SR = 1.76, 300 min 1).
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et al., 2001b) that predicts the solubility of boehmite in 2 –4.5 M sodium hydroxide solutions as a function of temperature in the range 30 –150 jC. 3.2. Effect of the initial sodium hydroxide concentration The effect of the initial sodium hydroxide concentration on the boehmite precipitation was studied in the range of 70– 120 g/L Na2O. The mass ratio (MR) was constant and had a value of 1.1, and thus, the initial alumina concentration varied in the range of 77– 132 g/L. The experiments were performed at 110 jC, the initial seed ratio (SR) was 1.76 and the agitation rate was 300 min 1. Fig. 2 shows the alumina content in the solution versus the initial sodium hydroxide concentration and precipitation time. The results reveal again the strong kinetic inhibitions that take place as pure crystalline boehmite is precipitated from the supersaturated sodium alumi-
nate solutions. The system has reached an apparent equilibrium stage at which the alumina concentration is far away from the real value of boehmite solubility under the same conditions. Moreover, it is observed that the achieved apparent equilibrium shifts closer to the real equilibrium as the initial sodium hydroxide concentration decreases from 120 to 70 g/L. Indeed, the %yield of the boehmite precipitation process, which is defined as the percent amount of the maximum alumina that could be precipitated from the aluminate solution, decreases from 67.14% to 60.26% when the sodium hydroxide concentration increases from 70 to 120 g/L. The above results show that there is a strong interrelation between the observed kinetic inhibitions and the sodium hydroxide concentration in the solution. 3.3. Effect of the initial boehmite seed ratio The rate of boehmite precipitation process is accelerated by the presence of boehmite seed material in
Fig. 2. Effect of sodium hydroxide concentration on boehmite precipitation from supersaturated sodium aluminate solutions (110 jC, MR = 1.1, SR = 1.76, 300 min 1).
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the aluminate solution. Moreover, the addition of seed is absolutely essential in the case that the precipitation of boehmite is performed under atmospheric conditions, namely temperatures lower than 100 jC and atmospheric pressure, otherwise, boehmite cannot be precipitated from the solution. Therefore, the study of the effect of seed ratio on the rate of boehmite precipitation process is crucial in order to understand the phenomena related to the boehmite precipitation mechanism. The experiments were performed at 110 jC with an aluminate solution that contained 100 g/L Na2O and had a mass ratio (MR) value of 1.1. The initial boehmite seed ratio (SR) ranged from 0.4 to 1.76 and the agitation rate was kept constant at 300 min 1. The results are shown in Fig. 3. The results confirm the positive effect of the seed ratio on the boehmite precipitation rate and once again reveal the strong kinetic inhibitions that occur as pure crystalline boehmite precipitates. The most important observation is that the attained apparent equilibrium shifts closer to the real equilibrium as the initial seed ratio increases. This result indicates that the boehmite seed plays a critical role during the boehmite precipitation from supersaturated sodium
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aluminate solutions, thus directly affecting the precipitation mechanism.
4. Development of kinetic model The rate of boehmite precipitation from the supersaturated sodium aluminate solutions depends on the reaction’s ‘‘driving force’’ and the rate constant as in the case of any chemical reaction. The driving force of any precipitation reaction is the supersaturation degree (S) that is defined as the difference between the actual alumina (Al2O3) concentration of a supersaturated solution and the equilibrium concentration at the same conditions. In the specific case of boehmite precipitation where an apparent equilibrium is attained instead of a real equilibrium, the term ‘‘equilibrium’’ is substituted by the term ‘‘apparent equilibrium’’ as shown in the Eq. (4.1). S ¼ C Ceapp
ð4:1Þ
Therefore, the rate of boehmite precipitation is given by the following equation: dCpr ¼ kðC Ceapp Þn dt
ð4:2Þ
where Cpr is the amount of precipitated boehmite expressed as equivalent alumina per solution volume (gAl 2 O 3 /L), C eapp is the alumina concentration (gAl2O3/L) at the apparent equilibrium stage, C is the alumina concentration at time t (gAl2O3/L), k is the rate constant, n is the reaction order. The amount of precipitated boehmite (Cpr) is a function of the initial Al2O3 concentration of the supersaturated aluminate solution (Ci) and is given by the following equation: Cpr ¼ Ci C
ð4:3Þ
Thus, dCpr dðCi CÞ dC ¼ ¼ dt dt dt
ð4:4Þ
The combination of the Eqs. (4.2) and (4.4) results in the following rate equation: Fig. 3. Effect of the initial seed ratio value on boehmite precipitation from supersaturated sodium aluminate solutions (110 jC, MR = 1.1, 100 g/L Na2O, 300 min 1).
dC ¼ kðC Ceapp Þn dt
ð4:5Þ
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Fig. 4. Plots of 1/Cpr versus 1/t in the temperature range 90 – 120 jC (experimental conditions: 132 g/L Al2O3, 120 g/L Na2O, SR = 1.76, 300 min 1).
The integration of the above rate equation for n>1 results in the following solution: 1 C ¼ Ceapp þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð4:6Þ ðn1Þ ðCi Ceapp Þðn1Þ þ ðn 1Þkt The above solution is transformed to Eq. (4.7) by the subtraction of Ci from both sides of the above equality: Cpr ¼ Si
1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðn1Þ ðn1Þ Si þ ðn 1Þkt
ð4:7Þ
In Eq. (4.7), the term Si = Ci C eapp denotes the initial supersaturation degree. Eq. (4.7) gives the amount of precipitated boehmite at any time t as a function of the initial supersaturation degree (Si), the reaction order (n) and the rate constant (k). The rate constant (k) is a temperature-dependent parameter that is calculated from the Arrhenius equation: Ea
k ¼ A*e RT
ð4:8Þ
where, Ea = activation energy; R = ideal gas constant; T = absolute temperature; A* = pre-exponential factor. The pre-exponential factor (A*) is considered to be a function of the initial sodium hydroxide concen-
tration (CNa2O)and the initial boehmite seed ratio and thus is written as: A* ¼ AðCNa2 O Þa ðSRÞb
ð4:9Þ
The values of the above parameters A, a, b as well as the activation energy Ea and the reaction order can be experimentally determined. However, in the case of boehmite precipitation, the system reaches an apparent equilibrium stage at which the alumina concentration (C eapp) is an unknown parameter. Therefore, Eq. (4.7) cannot be used for the determination of the unknown parameters and has to be simplified accordingly. A useful simplification arises from the assumption that the boehmite precipitation follows second order reaction kinetics in analogy to the well-known
Table 1 Apparent equilibrium concentrations (C app e ) and rate constants (k) for boehmite precipitation at 90, 100, 110 and 120 jC Temperature, jC
1/Si
1/Si2k
k
C eapp
90 100 110 120
1.45E 2 1.75E 2 1.87E 2 1.99E 2
31.04E 2 20.31E 2 10.21E 2 6.26E 2
6.77E 04 1.51E 03 3.42E 03 6.33E 03
63.00 75.43 78.92 82.60
Conditions: 132 g/L Al2O3, 120 g/L Na2O, SR = 1.76, 300 min 1.
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assumption of the second order boehmite precipitation kinetics is correct.
5. Estimation of the model parameters The model parameters can be estimated from the slope and the intercept of the 1/Cpr versus 1/t plots. The linearity of these plots will confirm the validity of the kinetic model. 5.1. Arrhenius plot Fig. 5. Arrhenius plot for the boehmite precipitation from supersaturated sodium aluminate solutions.
gibbsite precipitation kinetics (Audet and Larocque, 1989). With this assumption, Eq. (4.7) takes the following form: 1 1 1 1 ¼ 2 ð4:10Þ þ Cpr Si k t Si According to Eq. (4.10), the inverse Cpr is linearly dependent on the inverse time provided that the
The plots of 1/Cpr versus 1/t for the experiments regarding the effect of temperature on the boehmite precipitation are given in Fig. 4. From Fig. 4, it is clearly observed that the inverse Cpr is a linear function of the inverse time. Thus, the experimental results confirm the validity of the kinetic model. The rate constants, as well as the apparent equilibrium concentrations of alumina at various temperatures, are calculated from the slopes and intercepts of 1/Cpr versus 1/t plots and are presented in Table 1.
Fig. 6. Plots of 1/Cpr versus 1/t for the various initial sodium hydroxide concentrations in the range 70 – 120 g/L (experimental conditions: 110 jC, MR = 1.1, SR = 1.76, 300 min 1).
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Table 2 Apparent equilibrium concentrations (C eapp) and rate constants (k) for boehmite precipitation at 70, 90, 100 and 120 g/L initial sodium hydroxide concentrations [Na2O], g/L
1/Si
1/Si2k
k
C eapp
120 100 90 70
1.87E 2 2.05E 2 2.31E 2 2.95E 2
10.21E 2 7.59E 2 7.68E 2 9.50E 2
3.42E 03 5.54E 03 6.95E 03 9.16E 03
78.92 60.91 56.40 43.19
Conditions: T = 110 jC, MR = 1.1, SR = 1.76, agitation rate = 300 min 1.
The Arrhenius plot given in Fig. 5 leads to the conclusion that the boehmite precipitation follows the Arrhenius equation. As seen from Eq. (5.1), the slope of the Arrhenius plot is the Ea/R factor, while the intercept is the logarithm of the pre-exponential factor (lnA*). lnk ¼ lnA*
Ea R
1 T
ð5:1Þ
The estimated value for the activation energy is 89 kJ/ mol and the value of the pre-exponential factor A* is given in Eq. (5.2): A* ¼ AðCNa2 O Þa ðSRÞb ¼ 4:89 109
ð5:2Þ
5.2. Determination of the reaction order with respect to the initial sodium hydroxide concentration (exponential parameter a)
Taking into consideration that the initial seed ratio and the precipitation temperature are kept constant during the experimental series that study the effect of the initial sodium hydroxide concentration, the logarithm of Eq. (5.3) results in the following Eq. (5.4): h i Ea ð5:4Þ lnk ¼ ln AðSRÞb e RT þ alnðCNa2 O Þ Therefore, the reaction order ‘a’ with respect to the initial sodium hydroxide concentration can be determined from the slope of the plot of lnk versus ln(CNa2O) that is presented in Fig. 7. It is concluded from Fig. 7 that the value of the exponential parameter ‘a’ is 1.8. Moreover, the quantity ln[A(SR)be (Ea)/(RT)] can be estimated from the intercept of the above plot and thus the following Eq (5.5) is valid. Ea
AðSRÞb e RT ¼ 20:68
ð5:5Þ
5.3. Determination of the reaction order with respect to the initial seed ratio (exponential parameter b) The plots of 1/Cpr versus 1/t for the experiments regarding the effect of the initial seed ratio on the boehmite precipitation are given in Fig. 8. Once again the experimental data confirm the linearity of the relation between 1/Cpr and 1/t. The
The plots of 1/Cpr versus 1/t for the experiments regarding the effect of the initial sodium hydroxide concentration on the boehmite precipitation are given in Fig. 6. Furthermore, from Fig. 6, it is observed that 1/Cpr is linearly related to 1/t confirming again the validity of the kinetic model. The rate constant values and the apparent equilibrium concentrations of alumina at various initial sodium hydroxide concentrations are given in Table 2. The rate constant of the precipitation reaction is given in Eq. (5.3) as a function of the process parameters CNa2O, SR and temperature. Ea
k ¼ AðCNa2 O Þa ðSRÞb e RT
ð5:3Þ
Fig. 7. lnk versus ln(CNa2O) for the boehmite precipitation from supersaturated sodium aluminate solutions at 110 jC and SR = 1.1.
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Fig. 8. Plots of 1/Cpr versus 1/t for the various initial seed ratio values in the range 0.4 – 1.76 (experimental conditions: 110 jC, MR = 1.1, 100 g/L Na2O, 300 min 1).
predicted rate constants at various initial boehmite seed ratio values are given in the Table 3. In this experimental series, the initial sodium hydroxide concentration and the precipitation temperature were kept constant. Therefore, the logarithm of Eq. (5.3) leads to the following Eq. (5.6): h i Ea lnk ¼ ln AðCNa2 O Þa e RT þ blnðSRÞ
from the slope and intercept of the plot of lnk versus ln(SR) that is presented in Fig. 9. The estimated value of the exponential parameter b is 0.54, while the value of the quantity ln[A(CNa2O)a
ð5:6Þ
As observed from Eq. (5.6), the reaction order b with respect to the initial boehmite seed ratio and the quantity ln[A(CNa2O)ae (Ea)/(RT)] can be determined Table 3 Calculated rate constants (k) for boehmite precipitation as a function of initial seed ratio values SR
1/Si
1/Si2k
k
0.40 0.80 1.76 2.50
3.25E 2 2.62E 2 2.05E 2 2.17E 2
42.89E 2 17.27E 2 7.59E 2 6.53E 2
2.46E 03 3.98E 03 5.54E 03 7.21E 03
Conditions: 110 jC, MR = 1.1, 100 g/L Na2O, 300 min 1.
Fig. 9. lnk versus ln(SR) for the boehmite precipitation from supersaturated sodium aluminate solutions at 110 jC, MR = 1.1 and 100 g/L Na2O.
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Table 4 Determination of the model parameter A Parameters
[Na2O] = 120 g/L, SR = 1.76
110 jC, SR = 1.76
110 jC, [Na2O] = 100 g/L
a = 1.8, b = 0.54, Ea = 89.33 kJ/mol Aestimated Amean
A(CNa2O)a(SR)b = 4.89 109 1.99 1013 2.29 1013
A(SR)be (Ea)/(RT) = 20.68 2.33 1013
A(CNa2O)ae (Ea)/(RT) = 4.20 10 3 2.55 1013
e (Ea)/(RT)] is found to be 5.47, and thus, the following equation is valid: Ea
AðCNa2 O Þa e RT ¼ 4:20 103
ð5:7Þ
5.4. Determination of the model parameter A The constant parameter A of the pre-exponential factor A* can be estimated from Eqs. (5.2), (5.5) and (5.7). The results are summarized in Table 4. As shown in Table 4, the mean calculated value for the model parameter A is 2.29 1013. 5.5. Rate equation By combining all the previous results, the rate of boehmite precipitation from supersaturated sodium aluminate solutions can be described by the following equation: dCpr ¼ 2:29 1013 ðCNa2 O Þ1:8 dt ðSRÞ0:54 e
10;750 T
ðC Ceapp Þ2
ð5:8Þ
In Eq. (5.8), the terms Cpr, Ceapp and C are expressed as grammes of Al2O3 per liter of solution. The time t is measured in hours and the initial sodium hydroxide concentration CNa2O is expressed as grammes Na2O per liter of solution. Finally, T is the absolute temperature measured in K.
6. Discussion and conclusions The experimental results prove that pure crystalline boehmite can be precipitated from pure supersaturated sodium aluminate solutions at temperatures as low as 90 jC in the presence of boehmite seed. The results are in opposition to the current practice according to
which crystalline boehmite is formed at temperatures higher than 100 jC (Linsen, 1970; Panias and Paspaliaris, 1999). Moreover, the results are in partial accordance with the thermodynamic data that predict the boehmite formation at temperatures as low as 50 jC (Panias and Paspaliaris, 1999). The key for understanding the deviation between the experimental data and the predictions of the thermodynamic analysis lies in the kinetic analysis of the boehmite precipitation that reveals serious kinetic inhibitions that occur during the corresponding chemical reactions. The kinetic analysis reveals that the activation energy of boehmite precipitation from supersaturated sodium aluminate solutions is 89 kJ/mol. The activation energy of the boehmite precipitation is much higher compared to that of the gibbsite precipitation, which typically varies between 50 and 59 kJ/mol (Audet and Larocque, 1989). This observation indicates that boehmite formation is kinetically favoured at elevated temperatures, while the gibbsite formation is kinetically favoured at lower temperatures. Therefore, the experimental difficulty for boehmite precipitation at temperatures lower than 90 jC can be attributed to the high activation energy of the precipitation reaction.
Table 5 Calculated apparent equilibrium Al2O3 concentrations and theoretical boehmite solubility values Initial sodium hydroxide concentration, g/L Na2O
Temperature, jC
C eapp, g/L Al2O3
Boehmite solubility, g/L Al2O3
120 120 120 120 100 90 70
90 100 110 120 110 110 110
63.00 75.43 78.92 82.60 60.91 56.40 43.19
34.83 39.27 44.03 48.98 36.17 32.79 25.46
C. Skoufadis et al. / Hydrometallurgy 68 (2003) 57–68 Table 6 Percent process efficiency as a function of seed ratio SR
Boehmite solubility, g/L Al2O3
C eapp, g/L Al2O3
Process efficiency, %
0.40 0.80 1.76
36.17 36.17 36.17
79.43 72.55 60.91
41.56 51.20 66.35
Conditions: T = 110 jC, MR = 1.1, [Na2O] = 100 g/L, agitation rate = 300 min 1.
The kinetic analysis reveals also that the boehmite precipitation reaches an apparent equilibrium stage at which the alumina concentration in the solution is 1.7– 1.9 times higher than the value of boehmite solubility under the same experimental conditions as shown in Table 5. The results indicate that after an initial fast precipitation stage, a strong inhibition that pushes the system at an apparent equilibrium stage is observed while the remaining supersaturation degree in the solution is very high. This is an unusual situation, which is probably related to the sodium hydroxide concentration in the solution. The order of the precipitation reaction with respect to the initial sodium hydroxide concentration is minus 1.8. This result indicates that the rate of reaction decreases as the sodium hydroxide concentration increases, and thus, the negative effect of the sodium hydroxide on the boehmite precipitation process is revealed. Moreover, as observed from the experimental results, the achieved apparent equilibrium shifts closer to the real equilibrium as the initial sodium hydroxide concentration decreases from 120 to 70 g/L, indicating the negative effect not only on reaction rate but also on the process efficiency.
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The boehmite precipitation reaction rate and the process efficiency are improved when the seed ratio, the amount of initially added boehmite seed, is increased. The order of reaction with respect to the seed ratio (SR) is plus 0.54. The system reaches closer to the real equilibrium as the SR increases. As observed in Table 6, the alumina concentration at the apparent equilibrium stage decreases as the SR increases, and thus, the process efficiency increases from 42% to 66%. This is an unusual result as long as the seed ratio is known to affect the rate at which the precipitation reaction reaches equilibrium and not the equilibrium level. This result indicates that the boehmite seed plays an important role during precipitation and also affects the phenomena that are responsible for the selfdeceleration of the precipitation process. Fig. 10 shows the typical morphology of the boehmite seed grains at the end of precipitation experiments. The formation of nuclei on the surface of the boehmite particles is clearly observed. This is typical for the surface precipitation mechanism that prevails in the case of the boehmite precipitation from supersaturated sodium aluminate solutions (Panias et al., 2001c). According to this mechanism, the aluminate ions (Al(OH)4) are adsorbed on the surface aluminol groups (>AlOH0) of the boehmite particles and form the surface nuclei through surface precipitation. The self-deceleration of the precipitation process is probably attributable to the blocking of the surface aluminol groups of boehmite seed from the free sodium hydroxide that is liberated during the precipitation process. As seen in Fig. 11, the final free sodium hydroxide concentration in the solution is always 1.8 – 1.9 times higher than the initial one. As the free sodium hydroxide concentration increases, the
Fig. 10. Surface precipitation of boehmite on active sites of boehmite seeds.
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droxide concentration increases. The process will be inevitably inhibited but in this case, the attained equilibrium will be closer to the real equilibrium, and thus, the process efficiency will be higher. As a conclusion, the kinetic analysis of boehmite precipitation from supersaturated sodium aluminate solutions shows that the precipitation reaction has high activation energy and follows second order kinetics. Moreover, this work proves that serious kinetic inhibitions take place during the boehmite precipitation and reveals the role of the sodium hydroxide as a process inhibitor as well as the role of the boehmite seed ratio as a process accelerator.
Acknowledgements
Fig. 11. Concentration of free sodium hydroxide as a function of precipitation time and initial total sodium hydroxide concentration in the solution.
equilibrium of the reaction (6.1) shifts to the right, and therefore, the surface aluminol groups become negatively charged. > AlOH0 þ OH ¼> AlO þ H2 O
ð6:1Þ
For that reason, strong repulsive forces are developed between the negatively charged aluminate ions and the negatively charged surface of the boehmite seed particles. Thus, the energy that is necessary for an aluminate ion to approach the surface of the seed increases and therefore the self-deceleration of the precipitation process takes place and finally the precipitation process is totally inhibited. An increase of the seed ratio in the solution has as a direct result, the increase of the number of the surface aluminol groups, and thus, the decrease of the negative surface charge. The kinetic inhibitions are attenuated from an energetic point of view and the boehmite precipitation process is accelerated. The phenomena that are responsible for the self-deceleration of the precipitation process will appear again as the precipitation proceeds and the free sodium hy-
The financial support of the European Commission within the frameworks of the Non-Nuclear Energy Programme JOULE III (Contract No. JOE3/CT95/ 0003) and Industrial and Materials Technologies Programme Brite-Euram III (Contract No.BRPR/ CT98/0728) is gratefully acknowledged.
References Audet, D.R., Larocque, J.E., 1989. Development of a model for prediction of productivity of alumina hydrate precipitation. Light Metals, 21 – 26. Linsen, B.G., 1970. Physical and Chemical Aspects of Adsorbents and Catalysts. Academic Press, London. Misra, C., 1986. Industrial alumina chemicals. ACS Monograph, vol. 184. American Chemical Society, Washington, DC. Chapter 3. Panias, D., Paspaliaris, I., 1999. Thermodynamic determination of the stability area of boehmite in Al2 O3 – Na 2 O – H 2 O and Al2O3 – H2O systems. Erzmetall 52 (11), 585 – 595. Panias, D., Paspaliaris, I., Amanatidis, A., Schmidt, H.W., Hollnagel, A., 2001a. Boehmite process: an alternative technology in alumina production. Light Metals, 97 – 103. Panias, D., Asimidis, P., Paspaliaris, I., 2001b. Solubility of boehmite in concentrated sodium hydroxide solutions. Model development and assessment. Hydrometallurgy 59, 15 – 29. Panias, D., Skoufadis, C., Paspaliaris, I., 2001c. Boehmite process: an innovative variation of the Bayer process. Proceedings of European Metallurgical Conference EMC 2001, vol. 2, pp. 371 – 384. Friedrichshafen, Germany. Sleppy, W.C., Pearson, A., Misra, C., MacZura, G., 1991. Nonmetallurgical use of alumina and bauxite. Light Metals, 117 – 124.
Kinetics of boehmite precipitation from supersaturated sodium aluminate solutions C. Skoufadis, D. Panias *, I. Paspaliaris School of Mining and Metallurgical Engineering, National Technical University of Athens, Laboratory of Metallurgy Zografos Campus, Athens 15780, Greece Received 20 August 2002; received in revised form 15 October 2002; accepted 15 October 2002
Abstract This work presents the effect of the most important parameters, the precipitation temperature, the sodium hydroxide concentration and the initial seed ratio (SR) in the solution, on the boehmite precipitation from supersaturated sodium aluminate solutions. A kinetic model that describes the experimental data was developed. According to that model, boehmite precipitation follows second order reaction kinetics and has activation energy of 89 kJ/mol. The orders of the precipitation reaction with respect to the initial sodium hydroxide concentration and the initial seed ratio are estimated to be 1.8 and 0.54, respectively. The most important result is that the boehmite precipitation reaches an apparent equilibrium stage at which the alumina concentration is much higher compared to the value of the boehmite solubility under the same experimental conditions. The results reveal that the boehmite precipitation is a self-decelerated process and the observed kinetic inhibitions are related to the sodium hydroxide concentration in the supersaturated solution. Finally, a mechanistic interpretation of the experimental data is presented based on the concept of the surface precipitation of boehmite on the active sites of the boehmite seed. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Boehmite precipitation; Kinetics; Aluminate solutions
1. Introduction Boehmite (a-AlOOH) is one of the well-known aluminium oxide – hydroxides. It usually occurs in nature as a constituent of bauxitic ores that are almost exclusively used for the production of smelter grade alumina by the Bayer process and primary aluminium by the Hall – Heroult process. Boehmite is the base for the production of various activated aluminas (Sleppy et al., 1991), which find applications as catalysts in * Corresponding author. E-mail address: [email protected] (D. Panias).
the chemical industry. Moreover, boehmite can be used for the production of specialty-calcined aluminas (Sleppy et al., 1991) that are commercially produced by heating the Bayer alumina trihydrates. Critical factors for the above applications are the availability and the price of boehmite. Boehmite can be produced (Misra, 1986) with hydrothermal conversion of gibbsite, which is an intermediate product of the Bayer process, at elevated temperatures in autoclaves. It is produced also as a byproduct of the production of linear alcohols with the Ziegler process. Finally, fibrous boehmite is prepared with hydrolysis of aluminium salts in the presence of
0304-386X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X ( 0 2 ) 0 0 1 6 5 - 2
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acetic acid. Boehmite production is based on gibbsite or other more expensive aluminium salts, and therefore, it is an expensive material with small availability in the market. A new process for producing boehmite (Panias et al., 2001a) has been developed in the Laboratory of Metallurgy of the National Technical University of Athens. The process comprises a modification of the Bayer process precipitation stage. According to the new process, crystalline monohydrate alumina (boehmite, Al2O3H2O) is precipitated from supersaturated sodium aluminate solutions under conditions which are similar to those of the Bayer process. The new process is highly innovative and can be used not only for the production of smelter grade alumina, but also for the production of activated and specialty-calcined aluminas. In the present work, the effect of the most important parameters, such as temperature, sodium hydroxide concentration and initial boehmite seed ratio on boehmite precipitation is studied and a model describing the kinetics of boehmite precipitation from supersaturated sodium aluminate solutions is presented.
2. Experimental procedures and apparatus The kinetic experiments were performed in an Inconel autoclave. The experimental apparatus consisted of the following items: (a) the reactor that is a pressure vessel with a maximum capacity of 600 mL, (b) the head of the pressure vessel equipped with a safety valve, a gas inlet, a cooling loop, a magnetic drive for the mechanical stirrer and a Pt thermocouple for the temperature control, (c) the electrical furnace for the heating of the pressure vessel, (d) the proportional temperature controller. The apparatus was additionally equipped with a specially designed tube sampler that permitted the sampling from the reactor. The tube sampler was installed at the head of the pressure vessel and had a ceramic microfilter at its bottom, with the ability to filter all solid particles with size higher than 1 Am. A constant overpressure was created artificially in the
reaction vessel with the insertion of pure nitrogen in order to allow sampling under the experimental conditions. The autoclave was loaded with 420 mL supersaturated sodium aluminate solution with pre-adjusted Al2O3 and Na2O concentrations and the appropriate amount of boehmite seed. The pulp was rapidly heated up to the required temperature and the experiment was performed under isothermal conditions. Liquid samples were taken from the reactor periodically and were analyzed for their Al2O3 content. At the end of the experiments, the solids were characterized mineralogically with thermogravimetric analysis and X-ray diffractometry in order to verify that the precipitated solids were pure crystalline boehmite. The alumina content of the liquid samples was determined according to the following procedure. The aluminate solution, which contained 10 –100 mg of aluminium, was boiled with 50 mL of 0.1 M EDTA solution for an approximate time of 5 min. After cooling, 0.2 g of xylenol orange-indicator and drops of HCl 1 M solution were added up to the point where the solution became yellow. In the next stage, the solution pH was adjusted with ammonium acetate/ acetic acid buffer solution to a value of about 5– 6 and the excess of EDTA was titrated with 0.1 M zinc sulfate solution up to the point where the color changed from yellow to red. According to the reaction stoichiometry, 2.698 mg of dissolved Al was complexed with 1 mL of 0.1 M EDTA solution. The crystalline boehmite seed was produced according to the following procedure. A volume of 400 mL of pulp, with an alumina trihydrate concentration of 375 g/L, was heated in the autoclave at 300 jC for 4 h with an agitation rate of 600 min 1. The pulp was cooled rapidly down to 70 jC after the end of the experiment, filtered and the cake from the solid/liquid separation was dried for 24 h at 100 jC. The supersaturated sodium aluminate solutions were prepared according to the following procedure. Weighed amounts of pure alumina trihydrate and sodium hydroxide were inserted into an Inconel autoclave that contained 300 mL of distilled water. The pulp was heated to 160 jC for 1 h and the solids were totally dissolved. After rapid cooling, the liquor was diluted to a final volume of 500 mL. The chemicals that were used for the production of the boehmite seed and the preparation of the super-
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saturated sodium aluminate solutions were pure hydrargillite (Al2O33H2O) and sodium hydroxide commercially available from Merck.
3. Results The most important parameters that affect the boehmite precipitation from the supersaturated sodium aluminate solutions are the precipitation temperature, the initial sodium hydroxide concentration in the solution and the initial boehmite seed ratio in the solution. 3.1. Effect of temperature The effect of temperature on the boehmite precipitation was studied in the temperature range of 90– 120 jC. In the present work, the sodium hydroxide concentration in the solution was expressed as Na2O and was 120 g/L. The initial boehmite seed ratio in the solution (SR) had a value of 1.76 and the agitation rate was kept constant at 300 min 1. Seed ratio (SR) was defined as the mass of boehmite added in the solution divided by the mass of alumina contained in the
59
solution. The alumina (Al2O3) content of the supersaturated aluminate solutions was 132 g/L and thus, the mass ratio (MR) that was expressed as gAl2O3/ gNa2O in the solution was 1.1. The results are shown in Fig. 1 where the alumina content in the solution is plotted versus temperature and precipitation time. As shown in Fig. 1, the precipitation curves consist of three distinct parts that represent different stages of the boehmite precipitation from the aluminate solution. The first stage, which is positively affected by temperature, is characterized by very fast precipitation and has duration of about 9 h. This stage is followed by a transition stage during which a gradual but substantial decrease of the precipitation rate takes place. In the final stage, the boehmite precipitation rate is very low. Practically, it seems that the boehmite precipitation has reached an apparent equilibrium stage in which the alumina concentration in the solution is higher than the boehmite solubility under the same conditions as also shown in Fig. 1. This behavior indicates that the boehmite precipitation process is a self-decelerated process as long as there are no external actions that inhibit the process. The values of boehmite solubility shown in Fig. 1 have been calculated from a theoretical model (Panias
Fig. 1. Effect of temperature on boehmite precipitation from supersaturated sodium aluminate solutions and comparison between real and apparent equilibrium stages (132 g/L Al2O3, 120 g/L Na2O, SR = 1.76, 300 min 1).
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et al., 2001b) that predicts the solubility of boehmite in 2 –4.5 M sodium hydroxide solutions as a function of temperature in the range 30 –150 jC. 3.2. Effect of the initial sodium hydroxide concentration The effect of the initial sodium hydroxide concentration on the boehmite precipitation was studied in the range of 70– 120 g/L Na2O. The mass ratio (MR) was constant and had a value of 1.1, and thus, the initial alumina concentration varied in the range of 77– 132 g/L. The experiments were performed at 110 jC, the initial seed ratio (SR) was 1.76 and the agitation rate was 300 min 1. Fig. 2 shows the alumina content in the solution versus the initial sodium hydroxide concentration and precipitation time. The results reveal again the strong kinetic inhibitions that take place as pure crystalline boehmite is precipitated from the supersaturated sodium alumi-
nate solutions. The system has reached an apparent equilibrium stage at which the alumina concentration is far away from the real value of boehmite solubility under the same conditions. Moreover, it is observed that the achieved apparent equilibrium shifts closer to the real equilibrium as the initial sodium hydroxide concentration decreases from 120 to 70 g/L. Indeed, the %yield of the boehmite precipitation process, which is defined as the percent amount of the maximum alumina that could be precipitated from the aluminate solution, decreases from 67.14% to 60.26% when the sodium hydroxide concentration increases from 70 to 120 g/L. The above results show that there is a strong interrelation between the observed kinetic inhibitions and the sodium hydroxide concentration in the solution. 3.3. Effect of the initial boehmite seed ratio The rate of boehmite precipitation process is accelerated by the presence of boehmite seed material in
Fig. 2. Effect of sodium hydroxide concentration on boehmite precipitation from supersaturated sodium aluminate solutions (110 jC, MR = 1.1, SR = 1.76, 300 min 1).
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the aluminate solution. Moreover, the addition of seed is absolutely essential in the case that the precipitation of boehmite is performed under atmospheric conditions, namely temperatures lower than 100 jC and atmospheric pressure, otherwise, boehmite cannot be precipitated from the solution. Therefore, the study of the effect of seed ratio on the rate of boehmite precipitation process is crucial in order to understand the phenomena related to the boehmite precipitation mechanism. The experiments were performed at 110 jC with an aluminate solution that contained 100 g/L Na2O and had a mass ratio (MR) value of 1.1. The initial boehmite seed ratio (SR) ranged from 0.4 to 1.76 and the agitation rate was kept constant at 300 min 1. The results are shown in Fig. 3. The results confirm the positive effect of the seed ratio on the boehmite precipitation rate and once again reveal the strong kinetic inhibitions that occur as pure crystalline boehmite precipitates. The most important observation is that the attained apparent equilibrium shifts closer to the real equilibrium as the initial seed ratio increases. This result indicates that the boehmite seed plays a critical role during the boehmite precipitation from supersaturated sodium
61
aluminate solutions, thus directly affecting the precipitation mechanism.
4. Development of kinetic model The rate of boehmite precipitation from the supersaturated sodium aluminate solutions depends on the reaction’s ‘‘driving force’’ and the rate constant as in the case of any chemical reaction. The driving force of any precipitation reaction is the supersaturation degree (S) that is defined as the difference between the actual alumina (Al2O3) concentration of a supersaturated solution and the equilibrium concentration at the same conditions. In the specific case of boehmite precipitation where an apparent equilibrium is attained instead of a real equilibrium, the term ‘‘equilibrium’’ is substituted by the term ‘‘apparent equilibrium’’ as shown in the Eq. (4.1). S ¼ C Ceapp
ð4:1Þ
Therefore, the rate of boehmite precipitation is given by the following equation: dCpr ¼ kðC Ceapp Þn dt
ð4:2Þ
where Cpr is the amount of precipitated boehmite expressed as equivalent alumina per solution volume (gAl 2 O 3 /L), C eapp is the alumina concentration (gAl2O3/L) at the apparent equilibrium stage, C is the alumina concentration at time t (gAl2O3/L), k is the rate constant, n is the reaction order. The amount of precipitated boehmite (Cpr) is a function of the initial Al2O3 concentration of the supersaturated aluminate solution (Ci) and is given by the following equation: Cpr ¼ Ci C
ð4:3Þ
Thus, dCpr dðCi CÞ dC ¼ ¼ dt dt dt
ð4:4Þ
The combination of the Eqs. (4.2) and (4.4) results in the following rate equation: Fig. 3. Effect of the initial seed ratio value on boehmite precipitation from supersaturated sodium aluminate solutions (110 jC, MR = 1.1, 100 g/L Na2O, 300 min 1).
dC ¼ kðC Ceapp Þn dt
ð4:5Þ
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Fig. 4. Plots of 1/Cpr versus 1/t in the temperature range 90 – 120 jC (experimental conditions: 132 g/L Al2O3, 120 g/L Na2O, SR = 1.76, 300 min 1).
The integration of the above rate equation for n>1 results in the following solution: 1 C ¼ Ceapp þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð4:6Þ ðn1Þ ðCi Ceapp Þðn1Þ þ ðn 1Þkt The above solution is transformed to Eq. (4.7) by the subtraction of Ci from both sides of the above equality: Cpr ¼ Si
1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðn1Þ ðn1Þ Si þ ðn 1Þkt
ð4:7Þ
In Eq. (4.7), the term Si = Ci C eapp denotes the initial supersaturation degree. Eq. (4.7) gives the amount of precipitated boehmite at any time t as a function of the initial supersaturation degree (Si), the reaction order (n) and the rate constant (k). The rate constant (k) is a temperature-dependent parameter that is calculated from the Arrhenius equation: Ea
k ¼ A*e RT
ð4:8Þ
where, Ea = activation energy; R = ideal gas constant; T = absolute temperature; A* = pre-exponential factor. The pre-exponential factor (A*) is considered to be a function of the initial sodium hydroxide concen-
tration (CNa2O)and the initial boehmite seed ratio and thus is written as: A* ¼ AðCNa2 O Þa ðSRÞb
ð4:9Þ
The values of the above parameters A, a, b as well as the activation energy Ea and the reaction order can be experimentally determined. However, in the case of boehmite precipitation, the system reaches an apparent equilibrium stage at which the alumina concentration (C eapp) is an unknown parameter. Therefore, Eq. (4.7) cannot be used for the determination of the unknown parameters and has to be simplified accordingly. A useful simplification arises from the assumption that the boehmite precipitation follows second order reaction kinetics in analogy to the well-known
Table 1 Apparent equilibrium concentrations (C app e ) and rate constants (k) for boehmite precipitation at 90, 100, 110 and 120 jC Temperature, jC
1/Si
1/Si2k
k
C eapp
90 100 110 120
1.45E 2 1.75E 2 1.87E 2 1.99E 2
31.04E 2 20.31E 2 10.21E 2 6.26E 2
6.77E 04 1.51E 03 3.42E 03 6.33E 03
63.00 75.43 78.92 82.60
Conditions: 132 g/L Al2O3, 120 g/L Na2O, SR = 1.76, 300 min 1.
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63
assumption of the second order boehmite precipitation kinetics is correct.
5. Estimation of the model parameters The model parameters can be estimated from the slope and the intercept of the 1/Cpr versus 1/t plots. The linearity of these plots will confirm the validity of the kinetic model. 5.1. Arrhenius plot Fig. 5. Arrhenius plot for the boehmite precipitation from supersaturated sodium aluminate solutions.
gibbsite precipitation kinetics (Audet and Larocque, 1989). With this assumption, Eq. (4.7) takes the following form: 1 1 1 1 ¼ 2 ð4:10Þ þ Cpr Si k t Si According to Eq. (4.10), the inverse Cpr is linearly dependent on the inverse time provided that the
The plots of 1/Cpr versus 1/t for the experiments regarding the effect of temperature on the boehmite precipitation are given in Fig. 4. From Fig. 4, it is clearly observed that the inverse Cpr is a linear function of the inverse time. Thus, the experimental results confirm the validity of the kinetic model. The rate constants, as well as the apparent equilibrium concentrations of alumina at various temperatures, are calculated from the slopes and intercepts of 1/Cpr versus 1/t plots and are presented in Table 1.
Fig. 6. Plots of 1/Cpr versus 1/t for the various initial sodium hydroxide concentrations in the range 70 – 120 g/L (experimental conditions: 110 jC, MR = 1.1, SR = 1.76, 300 min 1).
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Table 2 Apparent equilibrium concentrations (C eapp) and rate constants (k) for boehmite precipitation at 70, 90, 100 and 120 g/L initial sodium hydroxide concentrations [Na2O], g/L
1/Si
1/Si2k
k
C eapp
120 100 90 70
1.87E 2 2.05E 2 2.31E 2 2.95E 2
10.21E 2 7.59E 2 7.68E 2 9.50E 2
3.42E 03 5.54E 03 6.95E 03 9.16E 03
78.92 60.91 56.40 43.19
Conditions: T = 110 jC, MR = 1.1, SR = 1.76, agitation rate = 300 min 1.
The Arrhenius plot given in Fig. 5 leads to the conclusion that the boehmite precipitation follows the Arrhenius equation. As seen from Eq. (5.1), the slope of the Arrhenius plot is the Ea/R factor, while the intercept is the logarithm of the pre-exponential factor (lnA*). lnk ¼ lnA*
Ea R
1 T
ð5:1Þ
The estimated value for the activation energy is 89 kJ/ mol and the value of the pre-exponential factor A* is given in Eq. (5.2): A* ¼ AðCNa2 O Þa ðSRÞb ¼ 4:89 109
ð5:2Þ
5.2. Determination of the reaction order with respect to the initial sodium hydroxide concentration (exponential parameter a)
Taking into consideration that the initial seed ratio and the precipitation temperature are kept constant during the experimental series that study the effect of the initial sodium hydroxide concentration, the logarithm of Eq. (5.3) results in the following Eq. (5.4): h i Ea ð5:4Þ lnk ¼ ln AðSRÞb e RT þ alnðCNa2 O Þ Therefore, the reaction order ‘a’ with respect to the initial sodium hydroxide concentration can be determined from the slope of the plot of lnk versus ln(CNa2O) that is presented in Fig. 7. It is concluded from Fig. 7 that the value of the exponential parameter ‘a’ is 1.8. Moreover, the quantity ln[A(SR)be (Ea)/(RT)] can be estimated from the intercept of the above plot and thus the following Eq (5.5) is valid. Ea
AðSRÞb e RT ¼ 20:68
ð5:5Þ
5.3. Determination of the reaction order with respect to the initial seed ratio (exponential parameter b) The plots of 1/Cpr versus 1/t for the experiments regarding the effect of the initial seed ratio on the boehmite precipitation are given in Fig. 8. Once again the experimental data confirm the linearity of the relation between 1/Cpr and 1/t. The
The plots of 1/Cpr versus 1/t for the experiments regarding the effect of the initial sodium hydroxide concentration on the boehmite precipitation are given in Fig. 6. Furthermore, from Fig. 6, it is observed that 1/Cpr is linearly related to 1/t confirming again the validity of the kinetic model. The rate constant values and the apparent equilibrium concentrations of alumina at various initial sodium hydroxide concentrations are given in Table 2. The rate constant of the precipitation reaction is given in Eq. (5.3) as a function of the process parameters CNa2O, SR and temperature. Ea
k ¼ AðCNa2 O Þa ðSRÞb e RT
ð5:3Þ
Fig. 7. lnk versus ln(CNa2O) for the boehmite precipitation from supersaturated sodium aluminate solutions at 110 jC and SR = 1.1.
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Fig. 8. Plots of 1/Cpr versus 1/t for the various initial seed ratio values in the range 0.4 – 1.76 (experimental conditions: 110 jC, MR = 1.1, 100 g/L Na2O, 300 min 1).
predicted rate constants at various initial boehmite seed ratio values are given in the Table 3. In this experimental series, the initial sodium hydroxide concentration and the precipitation temperature were kept constant. Therefore, the logarithm of Eq. (5.3) leads to the following Eq. (5.6): h i Ea lnk ¼ ln AðCNa2 O Þa e RT þ blnðSRÞ
from the slope and intercept of the plot of lnk versus ln(SR) that is presented in Fig. 9. The estimated value of the exponential parameter b is 0.54, while the value of the quantity ln[A(CNa2O)a
ð5:6Þ
As observed from Eq. (5.6), the reaction order b with respect to the initial boehmite seed ratio and the quantity ln[A(CNa2O)ae (Ea)/(RT)] can be determined Table 3 Calculated rate constants (k) for boehmite precipitation as a function of initial seed ratio values SR
1/Si
1/Si2k
k
0.40 0.80 1.76 2.50
3.25E 2 2.62E 2 2.05E 2 2.17E 2
42.89E 2 17.27E 2 7.59E 2 6.53E 2
2.46E 03 3.98E 03 5.54E 03 7.21E 03
Conditions: 110 jC, MR = 1.1, 100 g/L Na2O, 300 min 1.
Fig. 9. lnk versus ln(SR) for the boehmite precipitation from supersaturated sodium aluminate solutions at 110 jC, MR = 1.1 and 100 g/L Na2O.
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Table 4 Determination of the model parameter A Parameters
[Na2O] = 120 g/L, SR = 1.76
110 jC, SR = 1.76
110 jC, [Na2O] = 100 g/L
a = 1.8, b = 0.54, Ea = 89.33 kJ/mol Aestimated Amean
A(CNa2O)a(SR)b = 4.89 109 1.99 1013 2.29 1013
A(SR)be (Ea)/(RT) = 20.68 2.33 1013
A(CNa2O)ae (Ea)/(RT) = 4.20 10 3 2.55 1013
e (Ea)/(RT)] is found to be 5.47, and thus, the following equation is valid: Ea
AðCNa2 O Þa e RT ¼ 4:20 103
ð5:7Þ
5.4. Determination of the model parameter A The constant parameter A of the pre-exponential factor A* can be estimated from Eqs. (5.2), (5.5) and (5.7). The results are summarized in Table 4. As shown in Table 4, the mean calculated value for the model parameter A is 2.29 1013. 5.5. Rate equation By combining all the previous results, the rate of boehmite precipitation from supersaturated sodium aluminate solutions can be described by the following equation: dCpr ¼ 2:29 1013 ðCNa2 O Þ1:8 dt ðSRÞ0:54 e
10;750 T
ðC Ceapp Þ2
ð5:8Þ
In Eq. (5.8), the terms Cpr, Ceapp and C are expressed as grammes of Al2O3 per liter of solution. The time t is measured in hours and the initial sodium hydroxide concentration CNa2O is expressed as grammes Na2O per liter of solution. Finally, T is the absolute temperature measured in K.
6. Discussion and conclusions The experimental results prove that pure crystalline boehmite can be precipitated from pure supersaturated sodium aluminate solutions at temperatures as low as 90 jC in the presence of boehmite seed. The results are in opposition to the current practice according to
which crystalline boehmite is formed at temperatures higher than 100 jC (Linsen, 1970; Panias and Paspaliaris, 1999). Moreover, the results are in partial accordance with the thermodynamic data that predict the boehmite formation at temperatures as low as 50 jC (Panias and Paspaliaris, 1999). The key for understanding the deviation between the experimental data and the predictions of the thermodynamic analysis lies in the kinetic analysis of the boehmite precipitation that reveals serious kinetic inhibitions that occur during the corresponding chemical reactions. The kinetic analysis reveals that the activation energy of boehmite precipitation from supersaturated sodium aluminate solutions is 89 kJ/mol. The activation energy of the boehmite precipitation is much higher compared to that of the gibbsite precipitation, which typically varies between 50 and 59 kJ/mol (Audet and Larocque, 1989). This observation indicates that boehmite formation is kinetically favoured at elevated temperatures, while the gibbsite formation is kinetically favoured at lower temperatures. Therefore, the experimental difficulty for boehmite precipitation at temperatures lower than 90 jC can be attributed to the high activation energy of the precipitation reaction.
Table 5 Calculated apparent equilibrium Al2O3 concentrations and theoretical boehmite solubility values Initial sodium hydroxide concentration, g/L Na2O
Temperature, jC
C eapp, g/L Al2O3
Boehmite solubility, g/L Al2O3
120 120 120 120 100 90 70
90 100 110 120 110 110 110
63.00 75.43 78.92 82.60 60.91 56.40 43.19
34.83 39.27 44.03 48.98 36.17 32.79 25.46
C. Skoufadis et al. / Hydrometallurgy 68 (2003) 57–68 Table 6 Percent process efficiency as a function of seed ratio SR
Boehmite solubility, g/L Al2O3
C eapp, g/L Al2O3
Process efficiency, %
0.40 0.80 1.76
36.17 36.17 36.17
79.43 72.55 60.91
41.56 51.20 66.35
Conditions: T = 110 jC, MR = 1.1, [Na2O] = 100 g/L, agitation rate = 300 min 1.
The kinetic analysis reveals also that the boehmite precipitation reaches an apparent equilibrium stage at which the alumina concentration in the solution is 1.7– 1.9 times higher than the value of boehmite solubility under the same experimental conditions as shown in Table 5. The results indicate that after an initial fast precipitation stage, a strong inhibition that pushes the system at an apparent equilibrium stage is observed while the remaining supersaturation degree in the solution is very high. This is an unusual situation, which is probably related to the sodium hydroxide concentration in the solution. The order of the precipitation reaction with respect to the initial sodium hydroxide concentration is minus 1.8. This result indicates that the rate of reaction decreases as the sodium hydroxide concentration increases, and thus, the negative effect of the sodium hydroxide on the boehmite precipitation process is revealed. Moreover, as observed from the experimental results, the achieved apparent equilibrium shifts closer to the real equilibrium as the initial sodium hydroxide concentration decreases from 120 to 70 g/L, indicating the negative effect not only on reaction rate but also on the process efficiency.
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The boehmite precipitation reaction rate and the process efficiency are improved when the seed ratio, the amount of initially added boehmite seed, is increased. The order of reaction with respect to the seed ratio (SR) is plus 0.54. The system reaches closer to the real equilibrium as the SR increases. As observed in Table 6, the alumina concentration at the apparent equilibrium stage decreases as the SR increases, and thus, the process efficiency increases from 42% to 66%. This is an unusual result as long as the seed ratio is known to affect the rate at which the precipitation reaction reaches equilibrium and not the equilibrium level. This result indicates that the boehmite seed plays an important role during precipitation and also affects the phenomena that are responsible for the selfdeceleration of the precipitation process. Fig. 10 shows the typical morphology of the boehmite seed grains at the end of precipitation experiments. The formation of nuclei on the surface of the boehmite particles is clearly observed. This is typical for the surface precipitation mechanism that prevails in the case of the boehmite precipitation from supersaturated sodium aluminate solutions (Panias et al., 2001c). According to this mechanism, the aluminate ions (Al(OH)4) are adsorbed on the surface aluminol groups (>AlOH0) of the boehmite particles and form the surface nuclei through surface precipitation. The self-deceleration of the precipitation process is probably attributable to the blocking of the surface aluminol groups of boehmite seed from the free sodium hydroxide that is liberated during the precipitation process. As seen in Fig. 11, the final free sodium hydroxide concentration in the solution is always 1.8 – 1.9 times higher than the initial one. As the free sodium hydroxide concentration increases, the
Fig. 10. Surface precipitation of boehmite on active sites of boehmite seeds.
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droxide concentration increases. The process will be inevitably inhibited but in this case, the attained equilibrium will be closer to the real equilibrium, and thus, the process efficiency will be higher. As a conclusion, the kinetic analysis of boehmite precipitation from supersaturated sodium aluminate solutions shows that the precipitation reaction has high activation energy and follows second order kinetics. Moreover, this work proves that serious kinetic inhibitions take place during the boehmite precipitation and reveals the role of the sodium hydroxide as a process inhibitor as well as the role of the boehmite seed ratio as a process accelerator.
Acknowledgements
Fig. 11. Concentration of free sodium hydroxide as a function of precipitation time and initial total sodium hydroxide concentration in the solution.
equilibrium of the reaction (6.1) shifts to the right, and therefore, the surface aluminol groups become negatively charged. > AlOH0 þ OH ¼> AlO þ H2 O
ð6:1Þ
For that reason, strong repulsive forces are developed between the negatively charged aluminate ions and the negatively charged surface of the boehmite seed particles. Thus, the energy that is necessary for an aluminate ion to approach the surface of the seed increases and therefore the self-deceleration of the precipitation process takes place and finally the precipitation process is totally inhibited. An increase of the seed ratio in the solution has as a direct result, the increase of the number of the surface aluminol groups, and thus, the decrease of the negative surface charge. The kinetic inhibitions are attenuated from an energetic point of view and the boehmite precipitation process is accelerated. The phenomena that are responsible for the self-deceleration of the precipitation process will appear again as the precipitation proceeds and the free sodium hy-
The financial support of the European Commission within the frameworks of the Non-Nuclear Energy Programme JOULE III (Contract No. JOE3/CT95/ 0003) and Industrial and Materials Technologies Programme Brite-Euram III (Contract No.BRPR/ CT98/0728) is gratefully acknowledged.
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