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Kinetics of crystal growth of mirabilite in aqueous supersaturated solutions
Journal of Crystal Growth 338 (2012) 189–194
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Kinetics of crystal growth of mirabilite in aqueous supersaturated solutions A.I. Vavouraki a,n, P.G. Koutsoukos b a b
Department of Chemical Engineering, University of Patras, University Campus, Karatheodori 1, GR 26500 Patras, Greece Institute of Chemical Engineering and High Temperature Chemical Processes, Stadiou Str., Platani, P.O. Box 1414, GR-26504 Patras, Greece
a r t i c l e i n f o
abstract
Article history: Received 23 February 2011 Received in revised form 7 September 2011 Accepted 3 November 2011 Communicated by B.A. Korgel Available online 12 November 2011
The crystallization of sodium sulfate decahydrate (Na2SO4 10H2O, mirabilite) from supersaturated solutions was investigated using stable supersaturated solutions seeded with mirabilite seed crystals. The experiments were done in batch, stirred reactors in which the supersaturated solutions were prepared either by dissolution of sodium sulfate anhydrous at 32 1C followed by cooling to 18 or 20 1C or by mixing equal volumes of equimolar ammonium sulfate and sodium hydroxide solutions at 20 1C. Inoculation of the solutions supersaturated only with respect to mirabilite with seed crystals was accompanied with temperature increase of the thermostated solution. Despite the fact that crystal growth was initiated with seed crystals, the process started past the lapse of induction times inversely proportional to the solution supersaturation. The rates of crystal growth were measured both from the temperature rise and from the concentration–time profiles, which were linearly correlated. The measured crystal growth rates showed a parabolic dependence on supersaturation at low supersaturations. For higher values this dependence changed to linear, a behavior consistent with the BCF spiral crystal growth model. The morphology of the crystals growing at 20 1C showed typical prismatic habit, while at 18 1C when crystallized from cooled sodium sulfate solutions changes in the crystal habit to a leaf like morphology were observed. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Supersaturated solutions A2. Crystal growth rates A2. Growth from solutions A3. Seed crystals B1. Mirabilite B1. Sodium sulfate decahydrate
1. Introduction The crystallization of sodium sulfate decahydrate (Na2SO4 10H2O, mirabilite) in aqueous solutions is of concern mainly because of the damaging effects on building materials and also because of its significant industrial interest. The latter, stems from the fact that sodium sulfate is used in home laundry detergents as a filler, in the Kraft process of paper pulping, as a fining agent in glasses for bubble removal in molten glass, in the manufacture of textiles where it improves application of dyes because of charge reduction of the fabric fibers, etc. [1,2]. Despite the extensive research done concerning the crystallization conditions, understanding of the factors governing the kinetics of formation of sodium sulfate hydrates and the underlying mechanisms are still limited [3]. Considerable interest on the crystallization of sodium sulfate hydrates has originated from the possibility of use in energy storage applications [4]. Sodium sulfate is produced globally either from mineral resources or to a large extent through recovery from a number of processes where evaporative or cooling crystallization is used [5–8]. Five forms of sodium sulfate have been reported [9] but at ambient temperatures the decahydrate
n
Corresponding author. Tel.: þ30 2610997579. E-mail addresses: [email protected], [email protected] (A.I. Vavouraki). 0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.11.007
(Na2SO4 10H2O, mirabilite) and the anhydrous (Na2SO4, thenardite) salts are encountered, while the transient heptahydrate salt (Na2SO4 7H2O) may be formed depending on the solution conditions (temperature-supersaturation) [10,11]. During the crystallization of sodium sulfate from supersaturated solutions a number of transient, metastable crystalline phases differing in the extent of hydration have been recently reported to form [10,12]. The crystallization of sodium sulfate has been investigated in undercooled solutions [2], by salting out processes in MSMPR crystallizers [13] by evaporation crystallization through solution conductivity measurements [14], by differential scanning calorimetry [15] and by growth in capillaries [16]. A number of investigations have been concerned with the crystallization of mirabilite in capillaries and/or pores in which crystallization takes place due to local supersaturation development [17–19]. The investigation of crystal growth kinetics of mirabilite has been rather limited and mechanistic information is based either on the measurement of the crystal growth of single crystals in undercooled solutions by weight changes [2] or in MSMPR by cooling crystallization using particle size distribution measurements [20]. The limit for the labile region, where spontaneous precipitation of mirabilite takes place at 20 1C, has been reported to be at concentrations of sodium sulfate equal to 3.7 m [21]. The mechanism of crystal growth of mirabilite however in supersaturated solutions, which are practically stable for very
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long time periods (kinetic stabilization) has not yet clearly been described. This supersaturation domain allows for the performance of careful kinetics measurements and for the precise determination of the effect that additives may have on the crystal growth process. Clarification of the mechanism at specific operational conditions is of particular importance for the design of the appropriate strategies to control the crystal growth of mirabilite in aqueous media. It is the aim of this study to obtain mechanistic information on the crystal growth of mirabilite from supersaturated solutions using the seeded growth technique as the most appropriate for relatively low supersaturations. The results of measurements of the crystallization kinetics of mirabilite in aqueous supersaturated solutions, stable for very long time periods, were obtained at temperatures of 18 and 20 1C. The two temperature values selected were sufficient not only for the development of differences in the solution supersaturation but most important could provide indications concerning changes in the morphology of the mirabilite crystals, which grow out of the supersaturated solutions. The investigation of the crystal growth kinetics at different temperatures was limited to small differences due to the large variation of mirabilite solubility with temperature [22]. Two different methods of preparation of the supersaturated solutions were tested: dissolution of sodium sulfate at higher temperature and mixing ammonium sulfate and sodium hydroxide solutions. The stability domain of the system was defined through a series of preliminary experiments. Subsequent crystal growth experiments were done in the stable domain of the supersaturated solutions using mirabilite seed crystals. 2. Experimental All experiments were done in a double walled jacketed reactor, thermostated with circulating water from a programmable waterbath (Cole-Parmer, Polyscience) equipped with heater and refrigerating unit ( 70.1 1C). The experimental setup is shown in Fig. 1. The temperature of the water bath was initially set at 32 1C and the reactor was thermostatted pumping the thermostatted water through the walls of the glass jacket. The working solutions were prepared in the reactor at this temperature (at which they were undersaturated with respect to mirabilite) either by the dissolution of anhydrous sodium sulfate (thenardite, Merck, Puriss) in a carefully measured mass of water ( 70.01 g) or by
5
4 6 1
3
2
Fig. 1. Experimental set-up for the investigation of the crystallization of mirabilite from aqueous supersaturated solutions. 1: heated–cooled circulating thermostat; 2: magnetic stirrer; 3: double walled water jacketed glass reactor; 4: thermocouple temperature sensor; 5: interface for data acquisition and transfer and 6: computer for data collection.
mixing equal volumes (100 mL each) of equimolar solutions ammonium sulfate and sodium hydroxide, prepared from the respective stock solutions. Solutions were stirred magnetically throughout the crystal growth experiments. The stock solutions were prepared from the respective crystalline solids (Merck, Pro Analisi) using triply distilled water followed by filtration through membrane filters (0.22 mm Millipore). The sodium hydroxide solutions were standardized by titrations of potassium hydrogen phthalate. The solutions were left in the reactor under magnetic stirring (ca. 200 rpm) for 30 min at 32 1C. Next, the solution was cooled at constant rate (0.5 1/min) to 18 and/or 20 1 depending on the experimental conditions in each case. The temperature reduction was sufficient for the development of supersaturation with respect to mirabilite. The solutions were allowed to equilibrate at the new temperature for at least 2 h under stirring. The crystallization of mirabilite was initiated by inoculation of the supersaturated solutions with mirabilite (ca. 250 mg, 71 mg) seed crystals (mirabilite, 99þ %, Merck, A.C.S. reagent). The stability of the seed crystals, kept at 4 1C at all times, was regularly checked both with respect to water content and the X-ray diffraction pattern to ensure that no dehydration took place. Following the addition of the seed crystals, crystal growth of mirabilite started past the lapse of very small yet reproducibly (725 s) measured induction time. The crystal growth of mirabilite, is exothermal at ambient pressure [23,24] and the process was therefore followed through the temperature variation as a function of time using a thermocouple connected to a computer with the appropriate interface and recording software. In all experiments of the present work, the crystal growth of mirabilite on the seed crystals was rather fast (duration 2 min). During the course of the crystallization, samples were withdrawn, filtered through membranes filters (0.22 mm Millipore) in order to separate the liquid from the solid. Solids on the filter were characterized and the filtrates were analyzed for sodium sulfate. Solids were examined by scanning electron microscopy (SEM) and characterized by powder X-ray diffraction (XRD, Philips 1830/40). Both SEM and XRD analyses were done as fast as possible to avoid dehydration of mirabilite. In SEM, due to the high vacuum, the onset of dehydration could be clearly seen and at this point morphology analysis was terminated. The sodium sulfate concentration analysis, during the course of crystal growth experiments was done by evaporation of the liquid (water) in weighing crystal vials at 80 1C. The respective mass was measured until constant weight. Duplicate analyses were done by conductivity titrations of the filtrates with standard barium nitrate solutions. Since mirabilite crystals used to seed the solutions were obtained commercially, their solubility was determined, since it was possible that the presence of impurities, which could not be detected by our analytical methods could yield differences in the solubility thus leading to erroneous calculations of the solution supersaturation. The solubility of mirabilite crystals used to seed the supersaturated solutions in the present work was measured at the experimental temperatures 18 and 20 1C. The measurements were done approaching equilibrium from both under- and supersaturated solutions. In the former case seed crystals were introduced in undersaturated solutions and were allowed to reach equilibrium, while in the latter mirabilite was allowed to reach equilibrium from supersaturated solutions past seeded growth. More specifically, accurately weighted amounts of anhydrous sodium sulfate were dissolved in 200 mL distilled water by successive solid additions, until no more salt could be dissolved. In the case of supersaturated solutions mirabilite (250 mg) was introduced in stable supersaturated solutions close to the estimated (literature values) equilibrium. The mirabilite crystals, with a high surface to volume ratio, required only a short time to reach equilibrium. Samples were withdrawn over a time period
A.I. Vavouraki, P.G. Koutsoukos / Journal of Crystal Growth 338 (2012) 189–194
of 24 h in order to confirm the constancy of the solute concentration. In both cases, the precise concentration of sodium sulfate in the bulk solutions was measured by gravimetric analysis and the solubility of sodium sulfate was found to be equal to 1.03 at 18 1C ( 70.03) and 1.23 ( 70.05) mol kg 1 H2O at 20 1C. Solubility values were in good agreement with solubility values reported earlier [22,25]. The temperature– time and concentration–time profiles were used for the calculation of the crystal growth rates of mirabilite at different supersaturations (RT and RC, respectively). The reported values are the average of experiments done in triplicate. The supersaturations investigated in the experiments of the present work with respect to mirabilite ranged between 0.24–0.66 at 18 1C and 0.18–0.54 at 20 1C.
3. Results and discussion The thermodynamic driving force for the crystallization of mirabilite is the solution supersaturation, S. Assuming that at relatively small deviations from equilibrium (as in the present work) activity coefficients in the supersaturated solutions and at equilibrium are the same, S may be defined as S¼
ms m1
prepared mixing equimolar solutions of ammonium sulfate and sodium hydroxide as described in the experimental section. Following the introduction of seed crystals in the supersaturated solutions and past a short induction time the solution temperature increased and after reaching a maximum decreased to return to the initial temperature of the thermostated reactor. Sampling and analysis for the concentration of the remaining in solution sodium sulfate during the time corresponding to the temperature changes, showed that this time period corresponded to the desupersaturation of the solution, which returned to a point close but not exactly to equilibrium. The temperature and concentration changes of the sodium sulfate concentration in solution for a typical experiment are shown in Fig. 3. The temperature increase, Dy, in the solution following the addition of seed crystals in the supersaturated solutions was attributed to the heat of crystallization. In all cases the temperature rise was proportional to the solution supersaturation, as may be seen in Fig. 4. The experimental conditions (i.e. temperature and concentrations) were such that heat transfer effects in the process of crystal growth may be neglected [26]. The induction times preceding the
ð1Þ
3.0
where ms and mN are the solute molal concentrations in the supersaturated solution and at equilibrium, respectively. The relative supersaturation, s, is
2.5 2.0
cs c1 ¼ S1 c1
ð2Þ
The supersaturated solutions were prepared using the information of the solubility of the various hydrate salts, so that the working solutions were supersaturated exclusively with respect to mirabilite to avoid the potential formation of transient phases. The domain of the supersaturation range of the experiments done in the present work is shown in Fig. 2. As may be seen, the only sodium sulfate possible thermodynamically to form in our experimental conditions was the decahydrate. The relative supersaturation range for this salt was between s ¼0.24–0.66 for 18 1C and 0.18–0.54 at 20 1C for solutions prepared by dissolution of anhydrous sodium sulfate at 32 1C, and between 0.12 and 0.46 for supersaturated solutions
Δθ/°C
s¼
191
1.5 1.0 0.5 0.0 5
0
10
15 t / min
20
25
30
2.0
4.0 1.8
3.0
m Na2SO4/mol Kg-1
Solubility / mol Kg-1
Thenardite
Heptahydrate
2.0
Mirabilite
1.6
1.4
1.2
1.0
1.0
0.0
10
15
20 25 Temperature /°C
30
35
Fig. 2. Solubility isotherms for sodium sulfate. Solubility data for mirabilite and thenardite from Ref. [23] and for heptahydrate from Ref. [13]; (J) solubility measurements of the present work; (): crystal growth experiments of mirabilite at 18 1C (Table 1); () crystal growth experiments of mirabilite at 20 1C (Table 1).
0.5
1.0 t / min
1.5
2.0
Fig. 3. Temperature– and concentration–time profiles for the seeded growth of sodium sulfate decahydrate from supersaturated solutions; 20 1C, s ¼ 0.38. (a) Temperature–time profile following the introduction of mirabilite seed crystals. Dashed line indicates the slope of the rising part, which was used for the calculation of the rates of crystal growth; (b) concentration–time profile of the sodium sulfate decahydrate concentration remaining in the solution for the experiment corresponding to (a).
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4.0
15
3.5 1
3.0
2 3
10 RT / °C min-1
Δθ/°C
2.5
4 5
2.0
6
1.5
7
5
8
1.0
9 10
0.5 0.0 0
10
5
15 t / min
20
25
0
30
0
30
60
90
G / ms-1
Fig. 4. Crystal growth of sodium sulfate decahydrate following seeding in supersaturated solutions with mirabilite seed crystals at 20 1C, at different values of relative supersaturation, s; 1:0.54; 2:0.50; 3:0.46; 4:0.42; 5:0.38; 6:0.34; 7:0.30; 8:0.26; 9:0.22 and 10:0.18.
3.0
Fig. 6. Seeded growth of sodium sulfate decahydrate from supersaturated solutions: correlation between the rate measured from the respective rate of the temperature rise, following the addition of mirabilite crystals, RT, and the linear growth rate, G, calculated from the decrease of the solute concentration as a function of time; supersaturated solutions prepared by the dissolution of sodium sulfate at 32 1C: () 18 1C; (~) 20 1C and (J) 20 1C, solutions prepared mixing ammonium sulfate and sodium hydroxide.
2.5
of sodium sulfate decahydrate. For the same temperature, the linear relationship is valid regardless of the method of preparation of the supersaturated solutions. The initial experimental conditions and the crystal growth rates measured from the concentration–time profiles are summarized in Tables 1 and 2. The linear growth rates, G, were calculated from the respective mass rates, RC, using Eq. (3) [28]
Δθ/°C
2.0 1.5 1.0 0.5
τ
RC ¼
S
3a
b
rmirabilite G
ð3Þ
where 3a/b is a shape factor equal to 1 (cubes and spheres) and rmirabilite is the density of the mirabilite crystals.
0.0
0.0
0.5
1.0
1.5
2.0
t / min Fig. 5. Crystal growth of mirabilite from supersaturated solutions prepared from sodium sulfate solutions, on mirabilite seed crystals at 20 1C, s ¼ 0.38. Plot of the change of solution temperature as a function of time; S: addition of seed crystals and t: induction time preceding crystallization.
onset of crystal growth and the rates of crystal growth were measured from the temperature–time profile variation for each experiment as shown in Fig. 5. Induction times preceding crystal growth in seeded growth of sparingly soluble salts has been reported in the literature and has been used as a tool for the investigation of the mechanism of crystal growth, in combination with results from experiments in which the precipitation was spontaneous [27]. The crystal growth rates were doubled upon doubling the amount of the inoculating seed crystals, suggesting that it was the seed crystals, which grew and that secondary nucleation was absent. Measurements of the rates of crystal growth done measuring both the rate of temperature rise, RT, and the rate of concentration decrease of sodium sulfate in the supersaturated solutions, RC (or the corresponding linear growth rate defined in Eq. (3)) showed a linear relationship, as may be seen in Fig. 6. This linear relationship, entirely empirical, suggested that it is possible, at least for the experimental conditions of the present work (temperature and supersaturation range) to calculate mass crystal growth rates from the rate of temperature rise as a result of the crystallization
The plots of the linear growth rates as a function of the relative supersaturation, shown in Fig. 7, suggested that there is a parabolic dependence at the low supersaturation range followed by linear at higher supersaturations in agreement with the prediction of the Burton–Cabrera–Frank model of spiral growth [29]. This complex dependence of the growth rates on the relative supersaturation for both systems investigated, suggested that the mechanism is surface diffusion controlled. It is interesting to note that large differences in the measured rates were observed at the different temperatures measured, apart only by 2 1C. These differences implied large values of the corresponding apparent activation energy. The small temperature difference investigated was justified by the fact that the solubility of mirabilite is strongly affected by temperature. The rate determining step in the crystal growth of mirabilite from supersaturated solutions is surface diffusion, suggesting the possibility to control kinetics by changes of the surface of mirabilite, which may be done e.g. by the adsorption of compounds capable of electrostatic or other type of interactions at the solid/water interface [30]. The strong temperature dependence (high apparent activation energy) corroborated further the mechanistic conclusions drawn from the kinetics measurements. Finally, it is also interesting to note that the source of the crystallizing mirabilite lattice ions did not seem to play an important role at low supersaturations, while at higher values the rates measured were lower exceeding the experimental error710%. The magnitude of the crystal growth rates of mirabilite measured in our experiments were comparable with the rates
A.I. Vavouraki, P.G. Koutsoukos / Journal of Crystal Growth 338 (2012) 189–194
r
s (s) Dh (1C) RT (1C min 1) RC (mol g 1 min 1) G ( 10 6 m s 1)
18 1C 0.24 0.30 0.36 0.42 0.48 0.54 0.60 0.66
90 30 24 12 7 6 o6 o6
0.28 0.64 1.02 1.35 1.92 2.32 2.68 2.95
0.11 0.39 0.85 1.13 2.00 3.88 5.59 8.21
0.038 0.042 0.050 0.062 0.080 0.100 0.114 0.140
1.2 1.3 1.6 2.0 2.6 3.2 3.6 4.5
20 1C 0.18 0.22 0.26 0.30 0.34 0.38 0.42 0.46 0.50 0.54
70 42 36 36 30 28 24 22 12 10
0.48 0.76 1.10 1.34 1.70 1.98 2.36 2.63 3.00 3.29
0.19 0.97 1.60 1.75 2.65 3.80 7.50 9.00 12.00 13.00
0.110 0.300 0.400 0.550 0.750 1.200 1.500 1.800 2.300 2.600
3.5 9.6 12.8 17.6 23.9 38.4 48.0 57.5 73.5 83.1
8.0x10-5 Growth Rate / m s-1
Table 1 Seeded crystal growth experiments of mirabilite (250 mg mirabilite seed crystals), from supersaturated solutions prepared by dissolution of thenardite at 32 1C at different relative supersaturation values, s, induction times, t, maximum change in temperature due to crystallization, Dy, rate of crystal growth measured from temperature–time profiles, RT and rate of crystal growth measured from the concentration–time profiles, RC.
193
6.0x10-5
4.0x10-5
2.0x10-5
0.0
0.1 0.2 0.3 Relative supersaturation / σmirabilite
0.4
Fig. 7. Plots of the rate of crystal growth of sodium sulfate decahydrate as a function of the relative solution supersaturation. Seeded crystal growth from: () supersaturated solutions prepared from the dissolution of sodium sulfate at 20 1C; (m) supersaturated solutions made mixing equimolar quantities of (NH4)2SO4 and NaOH at 20 1C and (~) supersaturated solutions prepared from the dissolution of sodium sulfate at 18 1C.
2000 3,83
r
s (s)
Dh(1C) RT (1C min 1) RC (mol g 1 min 1) G ( 10 6m s 1)
20 1C 0.12 0.15 0.18 0.20 0.22 0.24 0.26 0.28 0.32 0.34 0.41 0.46
192 114 48 48 18 18 12 o 12 o 12 o 12 o 12 o 12
0.08 0.17 0.33 0.39 0.72 0.87 1.04 1.20 1.34 1.60 2.26 2.60
0.01 0.04 0.18 0.21 0.43 1.07 1.50 1.76 2.20 2.69 6.37 7.97
0.01 0.03 0.09 0.15 0.23 0.29 0.40 0.50 0.70 0.74 1.40 1.70
0.2 1.1 2.9 4.7 7.5 9.4 12.8 16.0 22.4 23.7 44.7 54.3
Intensity / a.u.
1600 Table 2 Seeded crystal growth experiments of mirabilite (250 mg mirabilite seed crystals), from supersaturated solutions prepared by mixing equimolar (NH4)2SO4 and NaOH solutions at various relative supersaturation values, s with respect to sodium sulfate salt at 20 1C, induction times, t, maximum change in temperature due to crystallization, Dy, rate of crystal growth measured from temperature–time profiles, RT and rate of crystal growth measured from the concentration–time profiles, RC.
1200
1
3ðkTÞ3 ðlog SÞ2
3.11
0 10
20
30
40
50
60
70
80
2θ/° Fig. 8. Powder X-Ray Diffraction pattern for mirabilite collected following seeded ˚ crystal growth in supersaturated solutions at 20 1C. d spacing values in A.
temperature. The molecular volume was calculated from Eq. (5):
of crystal growth of sodium sulfate decahydrate measured in cooling crystallization from supersaturated solutions [2,31] and in MSMPR crystallizer [1]. The dependence of the induction time on the solution supersaturation showed a typical profile as expected from the classical nucleation theory [28,32] according to which the relationship between the logarithm of the induction time and the inverse of the square of the logarithm of the relative supersaturation is linear: 16pu2m g3s
2,52
800
400
um ¼
log t ¼ A þ
5.49
ð4Þ
In Eq. (4), A is a constant, 16p/3 is a shape factor corresponding to spherical shape of the new nuclei, um the molecular volume of mirabilite, gs is the surface tension of the new nuclei developing on the surface, k Boltzmann’s constant and T the absolute
FW Mirabilite NA rmirabilite n
ð5Þ
where FWMirabilite and rMirabilite are the formula weight and the density of the solid (322.20 and 1.49 g cm 3, respectively), NA Avogadro’s number and n the number of ions in the formula of mirabilite [33]. The values calculated from the linear fit of the experimental data according to Eq. (4) yielded values for the surface energy of 3.9 and 2.1 mJ m 2 for 18 and 20 1C, respectively. The low values were anticipated since the nucleation is mainly heterogeneous on the inoculating seed crystals. The trend however of decreasing surface energy with increasing temperature is both expected and rather large for a temperature variation of only 2 1C. The crystalline solid was identified as mirabilite by powder X ray diffraction (XRD) as may be seen in the spectrum shown in Fig. 8, where the reflections corresponding to d ¼5.49, 3.83, 3.48 and 3.11 A˚ typical of synthetic mirabilite (ICDD 11-647). The XRD patterns obtained were in agreement with the results of the
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A.I. Vavouraki, P.G. Koutsoukos / Journal of Crystal Growth 338 (2012) 189–194
surface diffusion controlled mechanism, consistent with the spiral growth model. The method of preparation of the supersaturated solutions did not affect the kinetics at low supersaturations. The effect of temperature on the rate constants was significant.
Acknowledgments Financial support from the EC, FP6 Programme Contract no. SSP1-CT-2003-501571-SALT CONTROL is acknowledged. References
Fig. 9. Scanning electron micrographs of mirabilite crystallized on mirabilite seed crystals grown at (a) 20 1C and (b) 18 1C.
analysis of the crystalline solids by Correcher et al. [34] during the thermal transformation of mirabilite to thenardite. The inconsistency of the relative intensities observed and the extra reflections may be attributed to phase changes in the specimen holder during the recording of the XRD patterns. The morphology of the crystallized mirabilite showed the typical prismatic habit [9] at 20 1C as may be seen in Fig. 9a, while at 18 1C a different rose-like crystal habit was seen (Fig. 9b).
4. Conclusions The kinetics of crystal growth of sodium sulfate decahydrate at 18 and 20 1C from stable supersaturated solutions were investigated with the highly reproducible, seeded crystal growth methodology. The kinetics of crystal growth were estimated by the temperature rise in an isothermal reactor monitoring carefully the temperature variation with time. The dependence of the crystal growth kinetics measured on the respective solutions supersaturation, suggested a
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Kinetics of crystal growth of mirabilite in aqueous supersaturated solutions A.I. Vavouraki a,n, P.G. Koutsoukos b a b
Department of Chemical Engineering, University of Patras, University Campus, Karatheodori 1, GR 26500 Patras, Greece Institute of Chemical Engineering and High Temperature Chemical Processes, Stadiou Str., Platani, P.O. Box 1414, GR-26504 Patras, Greece
a r t i c l e i n f o
abstract
Article history: Received 23 February 2011 Received in revised form 7 September 2011 Accepted 3 November 2011 Communicated by B.A. Korgel Available online 12 November 2011
The crystallization of sodium sulfate decahydrate (Na2SO4 10H2O, mirabilite) from supersaturated solutions was investigated using stable supersaturated solutions seeded with mirabilite seed crystals. The experiments were done in batch, stirred reactors in which the supersaturated solutions were prepared either by dissolution of sodium sulfate anhydrous at 32 1C followed by cooling to 18 or 20 1C or by mixing equal volumes of equimolar ammonium sulfate and sodium hydroxide solutions at 20 1C. Inoculation of the solutions supersaturated only with respect to mirabilite with seed crystals was accompanied with temperature increase of the thermostated solution. Despite the fact that crystal growth was initiated with seed crystals, the process started past the lapse of induction times inversely proportional to the solution supersaturation. The rates of crystal growth were measured both from the temperature rise and from the concentration–time profiles, which were linearly correlated. The measured crystal growth rates showed a parabolic dependence on supersaturation at low supersaturations. For higher values this dependence changed to linear, a behavior consistent with the BCF spiral crystal growth model. The morphology of the crystals growing at 20 1C showed typical prismatic habit, while at 18 1C when crystallized from cooled sodium sulfate solutions changes in the crystal habit to a leaf like morphology were observed. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Supersaturated solutions A2. Crystal growth rates A2. Growth from solutions A3. Seed crystals B1. Mirabilite B1. Sodium sulfate decahydrate
1. Introduction The crystallization of sodium sulfate decahydrate (Na2SO4 10H2O, mirabilite) in aqueous solutions is of concern mainly because of the damaging effects on building materials and also because of its significant industrial interest. The latter, stems from the fact that sodium sulfate is used in home laundry detergents as a filler, in the Kraft process of paper pulping, as a fining agent in glasses for bubble removal in molten glass, in the manufacture of textiles where it improves application of dyes because of charge reduction of the fabric fibers, etc. [1,2]. Despite the extensive research done concerning the crystallization conditions, understanding of the factors governing the kinetics of formation of sodium sulfate hydrates and the underlying mechanisms are still limited [3]. Considerable interest on the crystallization of sodium sulfate hydrates has originated from the possibility of use in energy storage applications [4]. Sodium sulfate is produced globally either from mineral resources or to a large extent through recovery from a number of processes where evaporative or cooling crystallization is used [5–8]. Five forms of sodium sulfate have been reported [9] but at ambient temperatures the decahydrate
n
Corresponding author. Tel.: þ30 2610997579. E-mail addresses: [email protected], [email protected] (A.I. Vavouraki). 0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.11.007
(Na2SO4 10H2O, mirabilite) and the anhydrous (Na2SO4, thenardite) salts are encountered, while the transient heptahydrate salt (Na2SO4 7H2O) may be formed depending on the solution conditions (temperature-supersaturation) [10,11]. During the crystallization of sodium sulfate from supersaturated solutions a number of transient, metastable crystalline phases differing in the extent of hydration have been recently reported to form [10,12]. The crystallization of sodium sulfate has been investigated in undercooled solutions [2], by salting out processes in MSMPR crystallizers [13] by evaporation crystallization through solution conductivity measurements [14], by differential scanning calorimetry [15] and by growth in capillaries [16]. A number of investigations have been concerned with the crystallization of mirabilite in capillaries and/or pores in which crystallization takes place due to local supersaturation development [17–19]. The investigation of crystal growth kinetics of mirabilite has been rather limited and mechanistic information is based either on the measurement of the crystal growth of single crystals in undercooled solutions by weight changes [2] or in MSMPR by cooling crystallization using particle size distribution measurements [20]. The limit for the labile region, where spontaneous precipitation of mirabilite takes place at 20 1C, has been reported to be at concentrations of sodium sulfate equal to 3.7 m [21]. The mechanism of crystal growth of mirabilite however in supersaturated solutions, which are practically stable for very
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long time periods (kinetic stabilization) has not yet clearly been described. This supersaturation domain allows for the performance of careful kinetics measurements and for the precise determination of the effect that additives may have on the crystal growth process. Clarification of the mechanism at specific operational conditions is of particular importance for the design of the appropriate strategies to control the crystal growth of mirabilite in aqueous media. It is the aim of this study to obtain mechanistic information on the crystal growth of mirabilite from supersaturated solutions using the seeded growth technique as the most appropriate for relatively low supersaturations. The results of measurements of the crystallization kinetics of mirabilite in aqueous supersaturated solutions, stable for very long time periods, were obtained at temperatures of 18 and 20 1C. The two temperature values selected were sufficient not only for the development of differences in the solution supersaturation but most important could provide indications concerning changes in the morphology of the mirabilite crystals, which grow out of the supersaturated solutions. The investigation of the crystal growth kinetics at different temperatures was limited to small differences due to the large variation of mirabilite solubility with temperature [22]. Two different methods of preparation of the supersaturated solutions were tested: dissolution of sodium sulfate at higher temperature and mixing ammonium sulfate and sodium hydroxide solutions. The stability domain of the system was defined through a series of preliminary experiments. Subsequent crystal growth experiments were done in the stable domain of the supersaturated solutions using mirabilite seed crystals. 2. Experimental All experiments were done in a double walled jacketed reactor, thermostated with circulating water from a programmable waterbath (Cole-Parmer, Polyscience) equipped with heater and refrigerating unit ( 70.1 1C). The experimental setup is shown in Fig. 1. The temperature of the water bath was initially set at 32 1C and the reactor was thermostatted pumping the thermostatted water through the walls of the glass jacket. The working solutions were prepared in the reactor at this temperature (at which they were undersaturated with respect to mirabilite) either by the dissolution of anhydrous sodium sulfate (thenardite, Merck, Puriss) in a carefully measured mass of water ( 70.01 g) or by
5
4 6 1
3
2
Fig. 1. Experimental set-up for the investigation of the crystallization of mirabilite from aqueous supersaturated solutions. 1: heated–cooled circulating thermostat; 2: magnetic stirrer; 3: double walled water jacketed glass reactor; 4: thermocouple temperature sensor; 5: interface for data acquisition and transfer and 6: computer for data collection.
mixing equal volumes (100 mL each) of equimolar solutions ammonium sulfate and sodium hydroxide, prepared from the respective stock solutions. Solutions were stirred magnetically throughout the crystal growth experiments. The stock solutions were prepared from the respective crystalline solids (Merck, Pro Analisi) using triply distilled water followed by filtration through membrane filters (0.22 mm Millipore). The sodium hydroxide solutions were standardized by titrations of potassium hydrogen phthalate. The solutions were left in the reactor under magnetic stirring (ca. 200 rpm) for 30 min at 32 1C. Next, the solution was cooled at constant rate (0.5 1/min) to 18 and/or 20 1 depending on the experimental conditions in each case. The temperature reduction was sufficient for the development of supersaturation with respect to mirabilite. The solutions were allowed to equilibrate at the new temperature for at least 2 h under stirring. The crystallization of mirabilite was initiated by inoculation of the supersaturated solutions with mirabilite (ca. 250 mg, 71 mg) seed crystals (mirabilite, 99þ %, Merck, A.C.S. reagent). The stability of the seed crystals, kept at 4 1C at all times, was regularly checked both with respect to water content and the X-ray diffraction pattern to ensure that no dehydration took place. Following the addition of the seed crystals, crystal growth of mirabilite started past the lapse of very small yet reproducibly (725 s) measured induction time. The crystal growth of mirabilite, is exothermal at ambient pressure [23,24] and the process was therefore followed through the temperature variation as a function of time using a thermocouple connected to a computer with the appropriate interface and recording software. In all experiments of the present work, the crystal growth of mirabilite on the seed crystals was rather fast (duration 2 min). During the course of the crystallization, samples were withdrawn, filtered through membranes filters (0.22 mm Millipore) in order to separate the liquid from the solid. Solids on the filter were characterized and the filtrates were analyzed for sodium sulfate. Solids were examined by scanning electron microscopy (SEM) and characterized by powder X-ray diffraction (XRD, Philips 1830/40). Both SEM and XRD analyses were done as fast as possible to avoid dehydration of mirabilite. In SEM, due to the high vacuum, the onset of dehydration could be clearly seen and at this point morphology analysis was terminated. The sodium sulfate concentration analysis, during the course of crystal growth experiments was done by evaporation of the liquid (water) in weighing crystal vials at 80 1C. The respective mass was measured until constant weight. Duplicate analyses were done by conductivity titrations of the filtrates with standard barium nitrate solutions. Since mirabilite crystals used to seed the solutions were obtained commercially, their solubility was determined, since it was possible that the presence of impurities, which could not be detected by our analytical methods could yield differences in the solubility thus leading to erroneous calculations of the solution supersaturation. The solubility of mirabilite crystals used to seed the supersaturated solutions in the present work was measured at the experimental temperatures 18 and 20 1C. The measurements were done approaching equilibrium from both under- and supersaturated solutions. In the former case seed crystals were introduced in undersaturated solutions and were allowed to reach equilibrium, while in the latter mirabilite was allowed to reach equilibrium from supersaturated solutions past seeded growth. More specifically, accurately weighted amounts of anhydrous sodium sulfate were dissolved in 200 mL distilled water by successive solid additions, until no more salt could be dissolved. In the case of supersaturated solutions mirabilite (250 mg) was introduced in stable supersaturated solutions close to the estimated (literature values) equilibrium. The mirabilite crystals, with a high surface to volume ratio, required only a short time to reach equilibrium. Samples were withdrawn over a time period
A.I. Vavouraki, P.G. Koutsoukos / Journal of Crystal Growth 338 (2012) 189–194
of 24 h in order to confirm the constancy of the solute concentration. In both cases, the precise concentration of sodium sulfate in the bulk solutions was measured by gravimetric analysis and the solubility of sodium sulfate was found to be equal to 1.03 at 18 1C ( 70.03) and 1.23 ( 70.05) mol kg 1 H2O at 20 1C. Solubility values were in good agreement with solubility values reported earlier [22,25]. The temperature– time and concentration–time profiles were used for the calculation of the crystal growth rates of mirabilite at different supersaturations (RT and RC, respectively). The reported values are the average of experiments done in triplicate. The supersaturations investigated in the experiments of the present work with respect to mirabilite ranged between 0.24–0.66 at 18 1C and 0.18–0.54 at 20 1C.
3. Results and discussion The thermodynamic driving force for the crystallization of mirabilite is the solution supersaturation, S. Assuming that at relatively small deviations from equilibrium (as in the present work) activity coefficients in the supersaturated solutions and at equilibrium are the same, S may be defined as S¼
ms m1
prepared mixing equimolar solutions of ammonium sulfate and sodium hydroxide as described in the experimental section. Following the introduction of seed crystals in the supersaturated solutions and past a short induction time the solution temperature increased and after reaching a maximum decreased to return to the initial temperature of the thermostated reactor. Sampling and analysis for the concentration of the remaining in solution sodium sulfate during the time corresponding to the temperature changes, showed that this time period corresponded to the desupersaturation of the solution, which returned to a point close but not exactly to equilibrium. The temperature and concentration changes of the sodium sulfate concentration in solution for a typical experiment are shown in Fig. 3. The temperature increase, Dy, in the solution following the addition of seed crystals in the supersaturated solutions was attributed to the heat of crystallization. In all cases the temperature rise was proportional to the solution supersaturation, as may be seen in Fig. 4. The experimental conditions (i.e. temperature and concentrations) were such that heat transfer effects in the process of crystal growth may be neglected [26]. The induction times preceding the
ð1Þ
3.0
where ms and mN are the solute molal concentrations in the supersaturated solution and at equilibrium, respectively. The relative supersaturation, s, is
2.5 2.0
cs c1 ¼ S1 c1
ð2Þ
The supersaturated solutions were prepared using the information of the solubility of the various hydrate salts, so that the working solutions were supersaturated exclusively with respect to mirabilite to avoid the potential formation of transient phases. The domain of the supersaturation range of the experiments done in the present work is shown in Fig. 2. As may be seen, the only sodium sulfate possible thermodynamically to form in our experimental conditions was the decahydrate. The relative supersaturation range for this salt was between s ¼0.24–0.66 for 18 1C and 0.18–0.54 at 20 1C for solutions prepared by dissolution of anhydrous sodium sulfate at 32 1C, and between 0.12 and 0.46 for supersaturated solutions
Δθ/°C
s¼
191
1.5 1.0 0.5 0.0 5
0
10
15 t / min
20
25
30
2.0
4.0 1.8
3.0
m Na2SO4/mol Kg-1
Solubility / mol Kg-1
Thenardite
Heptahydrate
2.0
Mirabilite
1.6
1.4
1.2
1.0
1.0
0.0
10
15
20 25 Temperature /°C
30
35
Fig. 2. Solubility isotherms for sodium sulfate. Solubility data for mirabilite and thenardite from Ref. [23] and for heptahydrate from Ref. [13]; (J) solubility measurements of the present work; (): crystal growth experiments of mirabilite at 18 1C (Table 1); () crystal growth experiments of mirabilite at 20 1C (Table 1).
0.5
1.0 t / min
1.5
2.0
Fig. 3. Temperature– and concentration–time profiles for the seeded growth of sodium sulfate decahydrate from supersaturated solutions; 20 1C, s ¼ 0.38. (a) Temperature–time profile following the introduction of mirabilite seed crystals. Dashed line indicates the slope of the rising part, which was used for the calculation of the rates of crystal growth; (b) concentration–time profile of the sodium sulfate decahydrate concentration remaining in the solution for the experiment corresponding to (a).
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A.I. Vavouraki, P.G. Koutsoukos / Journal of Crystal Growth 338 (2012) 189–194
4.0
15
3.5 1
3.0
2 3
10 RT / °C min-1
Δθ/°C
2.5
4 5
2.0
6
1.5
7
5
8
1.0
9 10
0.5 0.0 0
10
5
15 t / min
20
25
0
30
0
30
60
90
G / ms-1
Fig. 4. Crystal growth of sodium sulfate decahydrate following seeding in supersaturated solutions with mirabilite seed crystals at 20 1C, at different values of relative supersaturation, s; 1:0.54; 2:0.50; 3:0.46; 4:0.42; 5:0.38; 6:0.34; 7:0.30; 8:0.26; 9:0.22 and 10:0.18.
3.0
Fig. 6. Seeded growth of sodium sulfate decahydrate from supersaturated solutions: correlation between the rate measured from the respective rate of the temperature rise, following the addition of mirabilite crystals, RT, and the linear growth rate, G, calculated from the decrease of the solute concentration as a function of time; supersaturated solutions prepared by the dissolution of sodium sulfate at 32 1C: () 18 1C; (~) 20 1C and (J) 20 1C, solutions prepared mixing ammonium sulfate and sodium hydroxide.
2.5
of sodium sulfate decahydrate. For the same temperature, the linear relationship is valid regardless of the method of preparation of the supersaturated solutions. The initial experimental conditions and the crystal growth rates measured from the concentration–time profiles are summarized in Tables 1 and 2. The linear growth rates, G, were calculated from the respective mass rates, RC, using Eq. (3) [28]
Δθ/°C
2.0 1.5 1.0 0.5
τ
RC ¼
S
3a
b
rmirabilite G
ð3Þ
where 3a/b is a shape factor equal to 1 (cubes and spheres) and rmirabilite is the density of the mirabilite crystals.
0.0
0.0
0.5
1.0
1.5
2.0
t / min Fig. 5. Crystal growth of mirabilite from supersaturated solutions prepared from sodium sulfate solutions, on mirabilite seed crystals at 20 1C, s ¼ 0.38. Plot of the change of solution temperature as a function of time; S: addition of seed crystals and t: induction time preceding crystallization.
onset of crystal growth and the rates of crystal growth were measured from the temperature–time profile variation for each experiment as shown in Fig. 5. Induction times preceding crystal growth in seeded growth of sparingly soluble salts has been reported in the literature and has been used as a tool for the investigation of the mechanism of crystal growth, in combination with results from experiments in which the precipitation was spontaneous [27]. The crystal growth rates were doubled upon doubling the amount of the inoculating seed crystals, suggesting that it was the seed crystals, which grew and that secondary nucleation was absent. Measurements of the rates of crystal growth done measuring both the rate of temperature rise, RT, and the rate of concentration decrease of sodium sulfate in the supersaturated solutions, RC (or the corresponding linear growth rate defined in Eq. (3)) showed a linear relationship, as may be seen in Fig. 6. This linear relationship, entirely empirical, suggested that it is possible, at least for the experimental conditions of the present work (temperature and supersaturation range) to calculate mass crystal growth rates from the rate of temperature rise as a result of the crystallization
The plots of the linear growth rates as a function of the relative supersaturation, shown in Fig. 7, suggested that there is a parabolic dependence at the low supersaturation range followed by linear at higher supersaturations in agreement with the prediction of the Burton–Cabrera–Frank model of spiral growth [29]. This complex dependence of the growth rates on the relative supersaturation for both systems investigated, suggested that the mechanism is surface diffusion controlled. It is interesting to note that large differences in the measured rates were observed at the different temperatures measured, apart only by 2 1C. These differences implied large values of the corresponding apparent activation energy. The small temperature difference investigated was justified by the fact that the solubility of mirabilite is strongly affected by temperature. The rate determining step in the crystal growth of mirabilite from supersaturated solutions is surface diffusion, suggesting the possibility to control kinetics by changes of the surface of mirabilite, which may be done e.g. by the adsorption of compounds capable of electrostatic or other type of interactions at the solid/water interface [30]. The strong temperature dependence (high apparent activation energy) corroborated further the mechanistic conclusions drawn from the kinetics measurements. Finally, it is also interesting to note that the source of the crystallizing mirabilite lattice ions did not seem to play an important role at low supersaturations, while at higher values the rates measured were lower exceeding the experimental error710%. The magnitude of the crystal growth rates of mirabilite measured in our experiments were comparable with the rates
A.I. Vavouraki, P.G. Koutsoukos / Journal of Crystal Growth 338 (2012) 189–194
r
s (s) Dh (1C) RT (1C min 1) RC (mol g 1 min 1) G ( 10 6 m s 1)
18 1C 0.24 0.30 0.36 0.42 0.48 0.54 0.60 0.66
90 30 24 12 7 6 o6 o6
0.28 0.64 1.02 1.35 1.92 2.32 2.68 2.95
0.11 0.39 0.85 1.13 2.00 3.88 5.59 8.21
0.038 0.042 0.050 0.062 0.080 0.100 0.114 0.140
1.2 1.3 1.6 2.0 2.6 3.2 3.6 4.5
20 1C 0.18 0.22 0.26 0.30 0.34 0.38 0.42 0.46 0.50 0.54
70 42 36 36 30 28 24 22 12 10
0.48 0.76 1.10 1.34 1.70 1.98 2.36 2.63 3.00 3.29
0.19 0.97 1.60 1.75 2.65 3.80 7.50 9.00 12.00 13.00
0.110 0.300 0.400 0.550 0.750 1.200 1.500 1.800 2.300 2.600
3.5 9.6 12.8 17.6 23.9 38.4 48.0 57.5 73.5 83.1
8.0x10-5 Growth Rate / m s-1
Table 1 Seeded crystal growth experiments of mirabilite (250 mg mirabilite seed crystals), from supersaturated solutions prepared by dissolution of thenardite at 32 1C at different relative supersaturation values, s, induction times, t, maximum change in temperature due to crystallization, Dy, rate of crystal growth measured from temperature–time profiles, RT and rate of crystal growth measured from the concentration–time profiles, RC.
193
6.0x10-5
4.0x10-5
2.0x10-5
0.0
0.1 0.2 0.3 Relative supersaturation / σmirabilite
0.4
Fig. 7. Plots of the rate of crystal growth of sodium sulfate decahydrate as a function of the relative solution supersaturation. Seeded crystal growth from: () supersaturated solutions prepared from the dissolution of sodium sulfate at 20 1C; (m) supersaturated solutions made mixing equimolar quantities of (NH4)2SO4 and NaOH at 20 1C and (~) supersaturated solutions prepared from the dissolution of sodium sulfate at 18 1C.
2000 3,83
r
s (s)
Dh(1C) RT (1C min 1) RC (mol g 1 min 1) G ( 10 6m s 1)
20 1C 0.12 0.15 0.18 0.20 0.22 0.24 0.26 0.28 0.32 0.34 0.41 0.46
192 114 48 48 18 18 12 o 12 o 12 o 12 o 12 o 12
0.08 0.17 0.33 0.39 0.72 0.87 1.04 1.20 1.34 1.60 2.26 2.60
0.01 0.04 0.18 0.21 0.43 1.07 1.50 1.76 2.20 2.69 6.37 7.97
0.01 0.03 0.09 0.15 0.23 0.29 0.40 0.50 0.70 0.74 1.40 1.70
0.2 1.1 2.9 4.7 7.5 9.4 12.8 16.0 22.4 23.7 44.7 54.3
Intensity / a.u.
1600 Table 2 Seeded crystal growth experiments of mirabilite (250 mg mirabilite seed crystals), from supersaturated solutions prepared by mixing equimolar (NH4)2SO4 and NaOH solutions at various relative supersaturation values, s with respect to sodium sulfate salt at 20 1C, induction times, t, maximum change in temperature due to crystallization, Dy, rate of crystal growth measured from temperature–time profiles, RT and rate of crystal growth measured from the concentration–time profiles, RC.
1200
1
3ðkTÞ3 ðlog SÞ2
3.11
0 10
20
30
40
50
60
70
80
2θ/° Fig. 8. Powder X-Ray Diffraction pattern for mirabilite collected following seeded ˚ crystal growth in supersaturated solutions at 20 1C. d spacing values in A.
temperature. The molecular volume was calculated from Eq. (5):
of crystal growth of sodium sulfate decahydrate measured in cooling crystallization from supersaturated solutions [2,31] and in MSMPR crystallizer [1]. The dependence of the induction time on the solution supersaturation showed a typical profile as expected from the classical nucleation theory [28,32] according to which the relationship between the logarithm of the induction time and the inverse of the square of the logarithm of the relative supersaturation is linear: 16pu2m g3s
2,52
800
400
um ¼
log t ¼ A þ
5.49
ð4Þ
In Eq. (4), A is a constant, 16p/3 is a shape factor corresponding to spherical shape of the new nuclei, um the molecular volume of mirabilite, gs is the surface tension of the new nuclei developing on the surface, k Boltzmann’s constant and T the absolute
FW Mirabilite NA rmirabilite n
ð5Þ
where FWMirabilite and rMirabilite are the formula weight and the density of the solid (322.20 and 1.49 g cm 3, respectively), NA Avogadro’s number and n the number of ions in the formula of mirabilite [33]. The values calculated from the linear fit of the experimental data according to Eq. (4) yielded values for the surface energy of 3.9 and 2.1 mJ m 2 for 18 and 20 1C, respectively. The low values were anticipated since the nucleation is mainly heterogeneous on the inoculating seed crystals. The trend however of decreasing surface energy with increasing temperature is both expected and rather large for a temperature variation of only 2 1C. The crystalline solid was identified as mirabilite by powder X ray diffraction (XRD) as may be seen in the spectrum shown in Fig. 8, where the reflections corresponding to d ¼5.49, 3.83, 3.48 and 3.11 A˚ typical of synthetic mirabilite (ICDD 11-647). The XRD patterns obtained were in agreement with the results of the
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surface diffusion controlled mechanism, consistent with the spiral growth model. The method of preparation of the supersaturated solutions did not affect the kinetics at low supersaturations. The effect of temperature on the rate constants was significant.
Acknowledgments Financial support from the EC, FP6 Programme Contract no. SSP1-CT-2003-501571-SALT CONTROL is acknowledged. References
Fig. 9. Scanning electron micrographs of mirabilite crystallized on mirabilite seed crystals grown at (a) 20 1C and (b) 18 1C.
analysis of the crystalline solids by Correcher et al. [34] during the thermal transformation of mirabilite to thenardite. The inconsistency of the relative intensities observed and the extra reflections may be attributed to phase changes in the specimen holder during the recording of the XRD patterns. The morphology of the crystallized mirabilite showed the typical prismatic habit [9] at 20 1C as may be seen in Fig. 9a, while at 18 1C a different rose-like crystal habit was seen (Fig. 9b).
4. Conclusions The kinetics of crystal growth of sodium sulfate decahydrate at 18 and 20 1C from stable supersaturated solutions were investigated with the highly reproducible, seeded crystal growth methodology. The kinetics of crystal growth were estimated by the temperature rise in an isothermal reactor monitoring carefully the temperature variation with time. The dependence of the crystal growth kinetics measured on the respective solutions supersaturation, suggested a
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