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Crystallization of potash alum: effect of power ultrasound
Ultrasonics Sonochemistry 8 (2001) 265±270
www.elsevier.nl/locate/ultsonch
Crystallization of potash alum: eect of power ultrasound Nacera Amara, Berthe Ratsimba, Anne-Marie Wilhelm *, Henri Delmas Laboratoire de Genie Chimique (UMR CNRS 5503 INPT/UPS), Ecole Nationale Superieure d'Ingenieurs de Genie Chimique (ENSIGC-INPT), 18 chemin de la Loge, 31078 Toulouse Cedex, France
Abstract The in¯uence of power ultrasound on the crystallization of potash alum was investigated. Experiments have been carried out in a batch stirred vessel. It was found that ultrasonic waves decrease the supersaturation limits and modify the morphology of the crystals produced. The average crystal size decreases with an increase of ultrasonic power. To investigate also the action of ultrasound on already existing crystals, crystals produced in silent conditions were suspended in saturated potash alum solution at various ultrasonic powers. The results show that ultrasound has also an abrasive eect on potash alum crystals for high power inputs. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Crystallization; Potash alum; Power ultrasound; Supersaturation; Size distribution
1. Introduction The physicochemical phenomena occurring during crystallization are particularly complex. For instance the phase of nucleation i.e. crystal birth, being very fast, is very dicult to control. A possible improvement of the crystallization process is to use power ultrasound but this requires an appreciation of all the phenomena occurring during sonocrystallization. Crystallization with ultrasound was the object of some studies published previously. But most of these studies were aimed at proving the feasibility of the process, and the parameters of ultrasound (for instance: ultrasonic power) were rarely controlled and still less systematically evaluated. However, some main results can be reported. It has been observed that ultrasonic waves have an in¯uence on the growth rate and the crystal size distribution during crystallization in saturated solutions [1]. The nucleation rate [2] of the crystals of CaSO4 has been found to be multiplied by more than 10 with ultrasound. The induction time of nucleation is strongly reduced in presence of ultrasonic waves for dierent systems [3]. The growth of sugar crystals has been found to be faster under ultrasound compared with mechanical ag-
itation [4]. The crystal growth rate depends on ultrasound frequency and intensity [1]. Several authors have noted this acceleration of the growth of crystals under ultrasound for dierent solutes [5] but today the mechanism of the ultrasound action is not yet well understood. It has been found also that ultrasound can reduce or modify the agglomeration and improves the product handling [6], as it is possible that the ultrasonic wave increases the probability of collision between the particles as in the case of the primary nucleation. So, to conclude, we can say that the eects of ultrasound on crystallization are very diverse, mostly positive, but that they are numerous and dicult to analyze separately. The purpose of this work is to study the in¯uence of ultrasonic waves on the behavior of a model crystallization from a solution by cooling. The parameters studied were the nucleation temperature, the mass, the shape and the size distribution of the crystals.
2. Experimental 2.1. Material
*
Corresponding author. Tel.: +33-5-62-25-23-00; fax: +33-5-62-2523-18. E-mail address: [email protected] (A.M. Wilhelm).
The compound chosen for these experiments is potassium aluminum sulfate hydrate, (KAl(SO4 )2 12H2 O), or alum. It is soluble in dilute acid and in water.
1350-4177/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 1 7 7 ( 0 1 ) 0 0 0 8 7 - 6
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N. Amara et al. / Ultrasonics Sonochemistry 8 (2001) 265±270
Fig. 1. Experimental setup.
The solubility of this salt versus temperature has been determined by Mulin [7] and its value is 11.4 g per 100 ml H2 O at 20°C. 2.2. Experimental device and procedures Preliminary experiments of crystallization have been carried out in a stirred vessel, in the presence of ultra-
sound, with dierent power (10, 30, 100 W), and in silent conditions. The experimental device is shown in Fig. 1; it involves a double-jacketed glass cylinder (10 cm in inner diameter). The ultrasound emitter (4.5 cm in diameter) is located at the bottom of the reactor; it is connected to the 20 kHz generator (VIBRACELL). The homogenization of the solution is ensured by a propeller and the stirring velocity is 1000 r.p.m.
Fig. 2. Cooling curves.
N. Amara et al. / Ultrasonics Sonochemistry 8 (2001) 265±270
267
The solution volume is 500 ml including 212.05 g of the alum and 403.45 g distilled water, so the concentration of potash alum hydrate is 23.1%. The crystals (ACROS ORGANICS, purity 99.5%) are dissolved into distilled water and heated to 72°C, and then the solution is slowly cooled down to 20°C by circulating cooling liquid in the jacket. The cooling time varies between 224 and 240 min. The crystals produced are immediately ®ltered, washed and dried.
3. Results and discussion 3.1. Eect of ultrasound on supersaturation Fig. 2 shows pro®les of temperature in silent condition and with ultrasound (10 W). Our results show that ultrasound decreases the limit of supersaturation because the nucleation temperature increases in the presence of ultrasonic waves. This phenomenon is a well-known eect of vibrations on the supersaturation limit. During nucleation (Fig. 3), it can be noticed that with ultrasound, the cooling rate remains approximately constant whereas in silent conditions a temperature increase can be observed. After nucleation, a decrease of the cooling rate was observed when ultrasonic power was increased. Two opposite eects can be distinguished: cooling is decelerated due to crystallization heat but heat exchange is improved by ultrasound. 3.2. Eect of ultrasound on the amount of crystals and on the temperature of nucleation In Fig. 4, we can notice clearly a signi®cant increase of the amount of crystals recovered as soon as ultrasonic waves are applied. Then, when ultrasonic power is in-
Fig. 4. Eect of ultrasound on recover mass.
Fig. 5. Eect of ultrasound on temperature of nucleation.
creased, only a slight increase in the recovered mass was found. Two phenomena could explain these results. The ®rst one is the birth of more numerous nuclei that leads to a greater surface area for the growth. The second one is an improvement of the growth rate of some faces. The same result has been obtained for nucleation temperature (Fig. 5): the nucleation temperature increases as soon as ultrasound is applied and then an increase of power has only a small eect. 3.3. Eect of ultrasound on crystal shape and size
Fig. 3. Phase of nucleation.
The crystals produced have been examined using an electronic scanning microscope (ESM). Sieving and laser diractometry (MALVERN MasterSizer S.) have been also used to determine their size distributions. Two phenomena have been clearly found by the analyses. First, the crystals obtained in the presence of ultrasound are smaller than those obtained without ultrasound, and their mean size decreases with an increase
268
N. Amara et al. / Ultrasonics Sonochemistry 8 (2001) 265±270
of ultrasonic power. We noticed that the size distribution is narrower when we applied ultrasound waves (Fig. 6). Secondly, the crystal shape is modi®ed by ultra-
Fig. 6. Size distribution of crystal recover.
sound: a potash alum crystal has an octahedral shape without ultrasound, but if it is born and grows in an ultrasonic ®eld, its shape becomes a decahedron (Fig. 7). It can be deduced from the average size and the mass of the crystals that the number of nuclei is greater with ultrasound even with the lowest power, but that an increase of power has no notable eect. The narrow size distribution obtained with ultrasound shows that the nuclei appear during a shorter time under ultrasound than in silent conditions. The modi®cation of the crystal shape results from an increase of the growth rate of the new faces. At this moment it is not yet possible to reach any conclusions as to the mechanism of the ultrasound action on the faces growth rate, because these rates depend on limiting phenomenon: (i) the diusion of ions from the solution to the crystal surface through a limit ®lm or (ii) the integration of ions to the crystal lattice. 3.4. Eect of ultrasound on abrasion To better understand the mechanisms of the ultrasound action, we wanted to evaluate their abrasive eect. Crystals resulting from previous experiments without ultrasound were suspended in the crystallizer, at 20°C, with mechanical agitation, in potash alum solutions saturated at 20°C. Then various ultrasonic powers (0, 10, 100 W) have been applied for 3 h. Conductivity measurements have shown that there was neither dissolution, nor crystallization. But the ESM revealed that the shape of the crystals had changed due to erosion (Fig. 8). Moreover the crystal size was shown to decrease with an increase of ultrasonic power. This result was con®rmed by size analysis (Fig. 9), which showed the appearance of small particles when ultrasound waves are applied. The amount of smaller crystals is less at low power (10 W) and increases dramatically with an increase of ultrasonic power. 4. Conclusion
Fig. 7. Shape of crystals produced with or without ultrasound: (a) silent condition, (b) 30 W, and (c) 100 W.
The presence of ultrasound decreases the supersaturation limit, increases the number of nuclei and modi®es the growth rate of some crystal faces. The consequences of these eects are a decrease of the average size of the crystals, an increase of the recovered mass and a modi®cation of the crystal shape. These effects increase slightly with an increase of ultrasonic power but are clearly noticeable as soon as ultrasound is added. At high power, ultrasound has also an eect of abrasion on the crystals of potash alum. This last result illustrates the need for a deeper understanding of all the (positive or negative) eects of ultrasound on crystalli-
N. Amara et al. / Ultrasonics Sonochemistry 8 (2001) 265±270
269
Fig. 8. Eect of ultrasound on the abrasion of the potash alum crystals: (a) seed crystals, (b) crystals suspended without US, (c) crystals suspended with US (10 W), and (d) crystals suspended with US (100 W).
Fig. 9. Eect of ultrasound on size distribution.
zation, in order to be able to favor nucleation and growth, while keeping abrasion to its minimum. But the present work already shows the potential of using low ultrasonic power to improve the control of a crystallization process.
References [1] A. Le Bras, Action des ultrasons sur les processus physicochimiques, Revue de chimie minerale 4 (1967) 283±315. [2] V.V. Yaminsky, K.B. Yaminskaya, A.V. Pertsov, E.D. Shchukin, Nucleation and coagulation eects of ultrasound during
270
N. Amara et al. / Ultrasonics Sonochemistry 8 (2001) 265±270
precipitation in solution, Kolloidnyi Zurnal 53 (1) (1991) 100± 104. [3] Zhang Qiang, Huang Weijie, Shi Lei, Huang Rongbin, Zheng Lansun, In¯uence of sonic ®eld on kinetics of solution, Huaxue Tongbao, 1 (1997) 44±46. [4] A.V. Kortnev, N.V. Martynovskaya, Eect of ultrasound on the latent period of crystallization from supersaturated solutions, Sb. Mosk. Inst. Stali. Splavov. 77 (1974) 98±100.
[5] B. Ratsimba, A.M. Wilhelm, H. Delmas, Proceedings of the First European Congress of Chemical Engineering, Florence, 1997. [6] U. Kunz, C. Binder, U. Homann, Preparation of ®ne particles as catalysts and catalysts precursors by the use of ultrasound during precipitation, Stud. Surf. Sci. Catal. 91 (7) (1995) 869±879. [7] J.W. Mulin, Crystallization, second ed., Butter Worth N, London, 1972.
www.elsevier.nl/locate/ultsonch
Crystallization of potash alum: eect of power ultrasound Nacera Amara, Berthe Ratsimba, Anne-Marie Wilhelm *, Henri Delmas Laboratoire de Genie Chimique (UMR CNRS 5503 INPT/UPS), Ecole Nationale Superieure d'Ingenieurs de Genie Chimique (ENSIGC-INPT), 18 chemin de la Loge, 31078 Toulouse Cedex, France
Abstract The in¯uence of power ultrasound on the crystallization of potash alum was investigated. Experiments have been carried out in a batch stirred vessel. It was found that ultrasonic waves decrease the supersaturation limits and modify the morphology of the crystals produced. The average crystal size decreases with an increase of ultrasonic power. To investigate also the action of ultrasound on already existing crystals, crystals produced in silent conditions were suspended in saturated potash alum solution at various ultrasonic powers. The results show that ultrasound has also an abrasive eect on potash alum crystals for high power inputs. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Crystallization; Potash alum; Power ultrasound; Supersaturation; Size distribution
1. Introduction The physicochemical phenomena occurring during crystallization are particularly complex. For instance the phase of nucleation i.e. crystal birth, being very fast, is very dicult to control. A possible improvement of the crystallization process is to use power ultrasound but this requires an appreciation of all the phenomena occurring during sonocrystallization. Crystallization with ultrasound was the object of some studies published previously. But most of these studies were aimed at proving the feasibility of the process, and the parameters of ultrasound (for instance: ultrasonic power) were rarely controlled and still less systematically evaluated. However, some main results can be reported. It has been observed that ultrasonic waves have an in¯uence on the growth rate and the crystal size distribution during crystallization in saturated solutions [1]. The nucleation rate [2] of the crystals of CaSO4 has been found to be multiplied by more than 10 with ultrasound. The induction time of nucleation is strongly reduced in presence of ultrasonic waves for dierent systems [3]. The growth of sugar crystals has been found to be faster under ultrasound compared with mechanical ag-
itation [4]. The crystal growth rate depends on ultrasound frequency and intensity [1]. Several authors have noted this acceleration of the growth of crystals under ultrasound for dierent solutes [5] but today the mechanism of the ultrasound action is not yet well understood. It has been found also that ultrasound can reduce or modify the agglomeration and improves the product handling [6], as it is possible that the ultrasonic wave increases the probability of collision between the particles as in the case of the primary nucleation. So, to conclude, we can say that the eects of ultrasound on crystallization are very diverse, mostly positive, but that they are numerous and dicult to analyze separately. The purpose of this work is to study the in¯uence of ultrasonic waves on the behavior of a model crystallization from a solution by cooling. The parameters studied were the nucleation temperature, the mass, the shape and the size distribution of the crystals.
2. Experimental 2.1. Material
*
Corresponding author. Tel.: +33-5-62-25-23-00; fax: +33-5-62-2523-18. E-mail address: [email protected] (A.M. Wilhelm).
The compound chosen for these experiments is potassium aluminum sulfate hydrate, (KAl(SO4 )2 12H2 O), or alum. It is soluble in dilute acid and in water.
1350-4177/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 1 7 7 ( 0 1 ) 0 0 0 8 7 - 6
266
N. Amara et al. / Ultrasonics Sonochemistry 8 (2001) 265±270
Fig. 1. Experimental setup.
The solubility of this salt versus temperature has been determined by Mulin [7] and its value is 11.4 g per 100 ml H2 O at 20°C. 2.2. Experimental device and procedures Preliminary experiments of crystallization have been carried out in a stirred vessel, in the presence of ultra-
sound, with dierent power (10, 30, 100 W), and in silent conditions. The experimental device is shown in Fig. 1; it involves a double-jacketed glass cylinder (10 cm in inner diameter). The ultrasound emitter (4.5 cm in diameter) is located at the bottom of the reactor; it is connected to the 20 kHz generator (VIBRACELL). The homogenization of the solution is ensured by a propeller and the stirring velocity is 1000 r.p.m.
Fig. 2. Cooling curves.
N. Amara et al. / Ultrasonics Sonochemistry 8 (2001) 265±270
267
The solution volume is 500 ml including 212.05 g of the alum and 403.45 g distilled water, so the concentration of potash alum hydrate is 23.1%. The crystals (ACROS ORGANICS, purity 99.5%) are dissolved into distilled water and heated to 72°C, and then the solution is slowly cooled down to 20°C by circulating cooling liquid in the jacket. The cooling time varies between 224 and 240 min. The crystals produced are immediately ®ltered, washed and dried.
3. Results and discussion 3.1. Eect of ultrasound on supersaturation Fig. 2 shows pro®les of temperature in silent condition and with ultrasound (10 W). Our results show that ultrasound decreases the limit of supersaturation because the nucleation temperature increases in the presence of ultrasonic waves. This phenomenon is a well-known eect of vibrations on the supersaturation limit. During nucleation (Fig. 3), it can be noticed that with ultrasound, the cooling rate remains approximately constant whereas in silent conditions a temperature increase can be observed. After nucleation, a decrease of the cooling rate was observed when ultrasonic power was increased. Two opposite eects can be distinguished: cooling is decelerated due to crystallization heat but heat exchange is improved by ultrasound. 3.2. Eect of ultrasound on the amount of crystals and on the temperature of nucleation In Fig. 4, we can notice clearly a signi®cant increase of the amount of crystals recovered as soon as ultrasonic waves are applied. Then, when ultrasonic power is in-
Fig. 4. Eect of ultrasound on recover mass.
Fig. 5. Eect of ultrasound on temperature of nucleation.
creased, only a slight increase in the recovered mass was found. Two phenomena could explain these results. The ®rst one is the birth of more numerous nuclei that leads to a greater surface area for the growth. The second one is an improvement of the growth rate of some faces. The same result has been obtained for nucleation temperature (Fig. 5): the nucleation temperature increases as soon as ultrasound is applied and then an increase of power has only a small eect. 3.3. Eect of ultrasound on crystal shape and size
Fig. 3. Phase of nucleation.
The crystals produced have been examined using an electronic scanning microscope (ESM). Sieving and laser diractometry (MALVERN MasterSizer S.) have been also used to determine their size distributions. Two phenomena have been clearly found by the analyses. First, the crystals obtained in the presence of ultrasound are smaller than those obtained without ultrasound, and their mean size decreases with an increase
268
N. Amara et al. / Ultrasonics Sonochemistry 8 (2001) 265±270
of ultrasonic power. We noticed that the size distribution is narrower when we applied ultrasound waves (Fig. 6). Secondly, the crystal shape is modi®ed by ultra-
Fig. 6. Size distribution of crystal recover.
sound: a potash alum crystal has an octahedral shape without ultrasound, but if it is born and grows in an ultrasonic ®eld, its shape becomes a decahedron (Fig. 7). It can be deduced from the average size and the mass of the crystals that the number of nuclei is greater with ultrasound even with the lowest power, but that an increase of power has no notable eect. The narrow size distribution obtained with ultrasound shows that the nuclei appear during a shorter time under ultrasound than in silent conditions. The modi®cation of the crystal shape results from an increase of the growth rate of the new faces. At this moment it is not yet possible to reach any conclusions as to the mechanism of the ultrasound action on the faces growth rate, because these rates depend on limiting phenomenon: (i) the diusion of ions from the solution to the crystal surface through a limit ®lm or (ii) the integration of ions to the crystal lattice. 3.4. Eect of ultrasound on abrasion To better understand the mechanisms of the ultrasound action, we wanted to evaluate their abrasive eect. Crystals resulting from previous experiments without ultrasound were suspended in the crystallizer, at 20°C, with mechanical agitation, in potash alum solutions saturated at 20°C. Then various ultrasonic powers (0, 10, 100 W) have been applied for 3 h. Conductivity measurements have shown that there was neither dissolution, nor crystallization. But the ESM revealed that the shape of the crystals had changed due to erosion (Fig. 8). Moreover the crystal size was shown to decrease with an increase of ultrasonic power. This result was con®rmed by size analysis (Fig. 9), which showed the appearance of small particles when ultrasound waves are applied. The amount of smaller crystals is less at low power (10 W) and increases dramatically with an increase of ultrasonic power. 4. Conclusion
Fig. 7. Shape of crystals produced with or without ultrasound: (a) silent condition, (b) 30 W, and (c) 100 W.
The presence of ultrasound decreases the supersaturation limit, increases the number of nuclei and modi®es the growth rate of some crystal faces. The consequences of these eects are a decrease of the average size of the crystals, an increase of the recovered mass and a modi®cation of the crystal shape. These effects increase slightly with an increase of ultrasonic power but are clearly noticeable as soon as ultrasound is added. At high power, ultrasound has also an eect of abrasion on the crystals of potash alum. This last result illustrates the need for a deeper understanding of all the (positive or negative) eects of ultrasound on crystalli-
N. Amara et al. / Ultrasonics Sonochemistry 8 (2001) 265±270
269
Fig. 8. Eect of ultrasound on the abrasion of the potash alum crystals: (a) seed crystals, (b) crystals suspended without US, (c) crystals suspended with US (10 W), and (d) crystals suspended with US (100 W).
Fig. 9. Eect of ultrasound on size distribution.
zation, in order to be able to favor nucleation and growth, while keeping abrasion to its minimum. But the present work already shows the potential of using low ultrasonic power to improve the control of a crystallization process.
References [1] A. Le Bras, Action des ultrasons sur les processus physicochimiques, Revue de chimie minerale 4 (1967) 283±315. [2] V.V. Yaminsky, K.B. Yaminskaya, A.V. Pertsov, E.D. Shchukin, Nucleation and coagulation eects of ultrasound during
270
N. Amara et al. / Ultrasonics Sonochemistry 8 (2001) 265±270
precipitation in solution, Kolloidnyi Zurnal 53 (1) (1991) 100± 104. [3] Zhang Qiang, Huang Weijie, Shi Lei, Huang Rongbin, Zheng Lansun, In¯uence of sonic ®eld on kinetics of solution, Huaxue Tongbao, 1 (1997) 44±46. [4] A.V. Kortnev, N.V. Martynovskaya, Eect of ultrasound on the latent period of crystallization from supersaturated solutions, Sb. Mosk. Inst. Stali. Splavov. 77 (1974) 98±100.
[5] B. Ratsimba, A.M. Wilhelm, H. Delmas, Proceedings of the First European Congress of Chemical Engineering, Florence, 1997. [6] U. Kunz, C. Binder, U. Homann, Preparation of ®ne particles as catalysts and catalysts precursors by the use of ultrasound during precipitation, Stud. Surf. Sci. Catal. 91 (7) (1995) 869±879. [7] J.W. Mulin, Crystallization, second ed., Butter Worth N, London, 1972.