Yüzey yükünün kristalizasyon kinetiğine etkisi
Yüzey yükünün kristalizasyon kinetiğine etkisi
Dosyalar
Tarih
1998
Yazarlar
Şahin, Ömer
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Institute of Science and Technology
Özet
Bir yüzey prosesi olan kristalizasyon işlemi, aşırı doygun bir çözeltiden kristal yüzeyine maddenin difüzyonu (difüzyon kademesi) ve yüzeye adsorbe olan maddenin kristal şebekesine entegrasyonu (reaksiyon kademesi) kademelerinden oluşur. Bu adımların her biri farklı kuvvetlerin etkisi altında olduğundan şu anki bilgilerle kristalizasyonda her olayı açıklamak oldukça güçtür. Açıklanamayan bu olaylardan bir tanesi de aynı aşın doygunlukta ve aynı partikül boyutundaki kristallerin farklı hızlarda büyümesi sonucu oluşan saçılmadır. Bu çalışmada ortaya konan temel kavram, büyüme ve çözünmedeki saçılmaların nedeninin partiküllerin taşıdıkları yüzey yükü olduğudur. Spesifik iyon adsorpsiyonu ile oluşan yüzey yükü çözünürlüğün değişmesine ve aynı çözeltideki değişik yüzey yüklü partiküllerin farklı aşın doygunluklara maruz kalmasına neden olmaktadır. Çalışma için seçilen maddelerin hepsi kristal büyümesi sırasında saçınım verdiği bilinen maddelerdir. Bu amaçla potasyum sülfat, amonyum alüminyum sülfat, sodyum perborat tetrahidrat ve borik asit kullanılmıştır. Maddelerin yüzey yüklerine göre aynlması bir elektrostatik ayıncı yardımıyla yapılmış ve her maddenin kendine özgü yüzey yük dağılımı gösterdiği tespit edilmiştir. Yüzey yüklerine göre sınıflandınlan bu maddelerin düşük ve yüksek yüzey yüklü olanlannın büyüme ve çözünme kinetikleri tek kristal hücresinde ve akışkan yataklı ölçüm hücresinde incelenmiştir. Deneysel veriler, incelenen tüm maddeler için yüksek yüzey yüklü partiküllerin büyüme hızının düşük yüzey yüklü olanlardan düşük, çözünme hızının ise yüksek olduğunu göstermiştir. Kristallerin yüzey yükü nedeniyle oluşturduklan elektriksel çift tabakaya, iyonik gücü artıncı madde ve elektriksel alanın etkileri incelenerek temel düşüncenin doğruluğu irdelenmiştir. 100 mV doğru akım gerilimi altında yürütülen sürekli kristalizasyon deneyinde, sodyum perborat tetrahidratın kristalizasyonunun yönlendirilebileceği gösterilmiştir. Ortaya konan teori ve elde edilen deneysel bilgiler ışığında, incelenen maddelerin çözünürlük, kristal büyüme ve çözünmesindeki belirsizlikler açıklanabilmektedir.
Crystallization is a very complex operation. The main reason for this complexity is the number of mass transfer steps involved in the process. In the supersaturated solution, first step is the creation of a new surface by nucleation, then diffusion of solute to the surface, following the adsorption of solute on the surface and integration of solute to the crystal structure. All these steps are governed by different physical laws. Present state of science is not able to explain every phenomenon in crystallization. One of the phenomena which is difficult to explain is the growth and dissolution rate dispersion. This dispersion results from the different growth and dissolution rates of different particles during the growth and dissolution in the same solution. Many materials show this kind of property, given in Table 2. 1. Explanation of this dispersion is based on BCF theory developed by Burton, Cabrera and Frank. This theory suggests that the distribution of dislocation points on the crystal surface may be the reason of different growth rates. Crystallization is a surface phenomenon, but surface properties were often omitted. Especially in dissolution, no other step than diffusion was considered. As it was shown by Fabian and Ulrich [39] dissolution rate of K2SO4 showed dissolution rate dispersion which is not possible to explain by the present theories. Therefore, other surface properties should be taken into account to overcome these explanation difficulties. The present work emphasizes the importance of surface potential, which causes electrical double layer formation, on the crystal growth and dissolution mechanism. At the beginning of this work, two main assumptions are considered. These are; 1) Each crystal has a different surface potential. For this reason, there is a surface potential distribution for a group of crystals in the same particle size range. 2) Surface potential has a definite effect on crystallization. Each of these assumptions should be based on the physical reality and supported by the experiments. Unfortunately there is no detailed experimental research on the surface charge (potential) determination in soluble salts. This can be made by zeta potential determination, but high conductivity causes some measurement problems. Only some hints can be obtained from colloid chemistry. xvi Effect of surface potential on the crystallization kinetics was based on the Knapp effect. Knapp[5] made some corrections for the effect of particle size on the solubility, originally developed by Ostwald[3] and Freundlich[4]. Original theory predicted the solubility as a function of particle size as given in eq.(2. 1). Unrealistic results of this theory were corrected by Knapp, by considering the importance of surface electrical effect, as given in eq.(2.2). But Knapp assumed that this surface effect was valid in very low particle size. This is the main point where we don't agree with Knapp theory. In our theory, we accept that particles, even in the large particle size range, have surface potential originating from specific ion adsorption or desorption. This specific adsorption or desorption causes to form electrical double layer. Since any ion which should be incorporated to the crystal lattice in some order, should pass this electrical double layer. This layer gives extra resistance to crystallization. Some scientists do not agree with this surface potential effect. They suppose that high ionic strength depresses this electrical double layer. Other scientists agree with this depression. They accept that even at the surface charge depression, there is still some charge on the surface. This residual charge has a definite effect on the crystallization. If the formation of electrical double layer is accepted, external effects on this layer and on the crystallization kinetics can be explained more easily. For example, if the ionic strength is increased, electrical double layer is compressed. Therefore any admixtures having this effect should have influence on the crystallization kinetics. If there is any heavy metal ion having a strongly adsorption property on the surface should have a very deep effect. Polyelectrolytes have the biggest effect on the electrical double layer. Therefore they should have a specific effect on the crystallization, depending on how the surface charge is affected by increasing, decreasing or reversing the sign of the charge. This effect was already shown by Sayan[40] and Titiz[41] for boric acid and sodium perborate tetrahydrate crystallization, respectively. If the effect of electrical double layer is accepted, effect of electrical field which directly effects this double layer is inevitable. Therefore this theory explains many unknown or unpredictable effects on the crystallization. In the experimental part of this work, four chemicals, potassium sulfate, ammonium alum, boric acid and sodium perborate tetrahydrate were chosen for investigation. All these chemicals are different with respect to physical and crystallographic properties. In the experimental work, three different measurement techniques are used. These are single crystal growth cell [stagnant (Figure 5.1) or flow type (Figure 5.2)], fluidized bed crystal growth cell (Figure 5.3) and MSMPR type continuous crystallization (Figure 5.5) Classification of closely sized crystals in electrostatic separator at varying separation voltages are given in Figure 6. 1 to 6.4. Same kind of classification is carried out for citric acid, now ever no further experiments are conducted for this material. xvu Experimental results show that all interested material which are known to have growth rate dispersion properties, have surface charge distribution. The shape of distribution is the property of the material. It is observed that the smaller the particle size is, the higher the surface charge. This indicates that surface charge is a function of particle size. In citric acid no detectable surface charge is observed for the particles bigger than 300 urn. Single crystal growth cell experiments for all kind of material show that particles having the higher surface charge have always slower growth rates and quicker dissolution rates than the particles having the lower surface charges, by accepting that the saturation point is constant for charged and uncharged particles. Growth rate versus supersaturation graphs are given in Figure 6.9 and 6.10 for K2S04, in Figure 6.15 and 6.16 for ammonium alum, in Figure 6.20 for sodium perborate, and in Figure 6.25 for boric acid. During the growth of individual particles, it is observed that each crystal grows at constant rate. This result shows that CCG (constant crystal growth) model is perfectly true as it is shown in Figure 6.6, 6.7, 6.1 1, 6.12, 6.17, 6.21 and 6.22 To the contrary of single crystal growth /dissolution experiment, fluidized bed crystal growth/dissolution rate measurements determine the average rate for a group of crystals. It is estimated that 40.000 particles can be put to the measurement cell in the fluidized bed experiments and therefore safer data can be obtained. Obtained data from these experiments are very similar to single crystal measurements. Results of fluidized bed measurements for sodium perborate tetrahydrate are shown in Figure 6.26. Growth/dissolution rates for highly charged particles is lower than the lower charged ones. But the difference is small compared to the single crystal measurements. Disagreement in the two different measurement techniques is attributed to the breakage of dentrites in fluidization condition. In order to understand the way of charge gaining of the particles and effect the surface quality, two parallel fluidized bed experiments were carried out. In the first group of experiments, seed crystals in the sieve range of -425+355 um directly used in the measurements. In the second group of experiments, seed crystals are prepared by dissolving the bigger crystals to the same size range. In this way, surface quality is increased to obtain almost perfect surface. Figure 6.27 gives the obtained results for these two groups. Results show that perfect crystals have a higher growth rate than the ordinary crystals. These results indicate that surface quality plays an important role in specific ion adsorption. One of the ways to observe the effect of surface charge is to increase this charge deliberately by applying direct or alternative current to the measurement cell during the growth or dissolution. Figure 6.28 shows the results of two sets of experiments for ordinary crystals under 100 mV direct current and without any voltage. The results are very similar with the ones obtained in single crystal cell for highly charged and lower charged particles. Same results are obtained for other voltages up to 20 V. (Figure 6.29 and 6.30). Alternatiny current applications at different frequency also give the similar results as it is shown in Figure 6.3 1. xvui Similar experiments are carried out for sodium perborate tetrahydrate in the presence of 2.5% sodium metaborate. This condition is known as almost non-charged state for sodium perborate tetrahydrate crystals as it was shown by Titiz[41]. As it is seen in Figure 6.32, in this case there is almost no effect of electrical field in crystal growth and dissolution. It is therefore concluded that electrical field can only have an effect on crystallization in the case of charged or chargeable surface. For this specific case, increasing sodium ion concentration by sodium metaborate addition causes to decrease the anion adsorption and therefore competing effect of this two ion gives non-charged surface. Crystal growth and dissolution rates of boric acid are studied in fluidized bed growth cell for higher and lower charged particles. Figure 6.34 shows the experimental results, which is confident with the results obtained in single crystal cell, by showing that highly charged particles have higher dissolution rate and lower growth rate than lower charged particles. In order to see the effect of sodium chloride concentration which does not contain any common ion, on the growth/dissolution rates of boric acid, experiments are carried out in the presence of 65 to 665 ppm NaCl and results are shown in Figure 6.35. NaCl causes to increase the ionic strength of the solution and to compress the electrical double layer. As it is seen in Figure 6.35 double layer compression causes to decrease the growth rate, but has a little effect on dissolution rate. Effect of electrical field on the growth/dissolution rates of boric acid, measured in fluidized bed system, are shown in Figure 6.36 to 6.38. Obtained results are completely similar with the ones obtained for sodium perborate tetrahydrate. In continuos crystallization experiments, only crystallization kinetics of sodium perborate tetrahydrate is studied in the absence and in the presence of electrical field. MSMPR experiments are carried out at 0.75h retention time and experimental conditions for two experiments were completely identical, except the presence of electrical field. Crystal samples taken at the steady state conditions are observed under the microscope and following differences are detected: a) Nuclei formed in the absence of electrical field are in the shape of rod. In the presence of 100 mV direct current, length/diameter ratio of rod decreases and they become spherical. b) Twinning observed at the particles greater than 106 |im is less effective in the presence of electrical field and crystals are more compact in this condition. Table 6. 1 shows the attrition ratio and bulk density of the products obtained in these experiments. It is clear from this table that technological properties of the product is better in the presence of electrical field. The main reason for these differences results from the non-dentritical growth of the crystals in the presence of electrical field. Non- dentritical growth causes to decrease the breakage and attrition of the crystals and therefore apparent growth rate becomes higher and nucleation rate becomes lower. xix These results can be seen from the RRS diagram shown in Figure 6.39, from the population diagram shown in Figure 6.40 and from Table 6.2. From all the experiments, following general conclusions are drawn: 1- All investigated crystals which are known to have growth rate dispersion properties, have surface charges and these charges are the properties of material. 2- Distribution of surface charge is a function of particle size. Decreasing the particle size causes to increase the magnitude of this potential. 3- The higher the surface charge the lower the growth rate and the higher the dissolution rate. 4- Main reason for growth and dissolution rate dispersions is the charge distribution. 5- There is no random fluctuation in crystal growth as it is proposed random fluctuating (RF) theory. Each crystal has its own growth and dissolution rates at constant values. This indicates that assumption of CCG theory is time. 6- Crystal growth or dissolution rates may be changed by effecting the electrical double layer resulting from the surface charge. 7- Electrical double layer may be influenced by increasing ionic strength of solution and by applying electrical field in the form of direct or alternating current. 8- All of the investigated crystals have negative surface charge. This implies that surface potential results from specific anion adsorption. 9- By the proposed hypothesis it is possible to explain all peculiar solubility and growth/dissolution rates behaviors which are not possible to explain by the previous knowledge.
Crystallization is a very complex operation. The main reason for this complexity is the number of mass transfer steps involved in the process. In the supersaturated solution, first step is the creation of a new surface by nucleation, then diffusion of solute to the surface, following the adsorption of solute on the surface and integration of solute to the crystal structure. All these steps are governed by different physical laws. Present state of science is not able to explain every phenomenon in crystallization. One of the phenomena which is difficult to explain is the growth and dissolution rate dispersion. This dispersion results from the different growth and dissolution rates of different particles during the growth and dissolution in the same solution. Many materials show this kind of property, given in Table 2. 1. Explanation of this dispersion is based on BCF theory developed by Burton, Cabrera and Frank. This theory suggests that the distribution of dislocation points on the crystal surface may be the reason of different growth rates. Crystallization is a surface phenomenon, but surface properties were often omitted. Especially in dissolution, no other step than diffusion was considered. As it was shown by Fabian and Ulrich [39] dissolution rate of K2SO4 showed dissolution rate dispersion which is not possible to explain by the present theories. Therefore, other surface properties should be taken into account to overcome these explanation difficulties. The present work emphasizes the importance of surface potential, which causes electrical double layer formation, on the crystal growth and dissolution mechanism. At the beginning of this work, two main assumptions are considered. These are; 1) Each crystal has a different surface potential. For this reason, there is a surface potential distribution for a group of crystals in the same particle size range. 2) Surface potential has a definite effect on crystallization. Each of these assumptions should be based on the physical reality and supported by the experiments. Unfortunately there is no detailed experimental research on the surface charge (potential) determination in soluble salts. This can be made by zeta potential determination, but high conductivity causes some measurement problems. Only some hints can be obtained from colloid chemistry. xvi Effect of surface potential on the crystallization kinetics was based on the Knapp effect. Knapp[5] made some corrections for the effect of particle size on the solubility, originally developed by Ostwald[3] and Freundlich[4]. Original theory predicted the solubility as a function of particle size as given in eq.(2. 1). Unrealistic results of this theory were corrected by Knapp, by considering the importance of surface electrical effect, as given in eq.(2.2). But Knapp assumed that this surface effect was valid in very low particle size. This is the main point where we don't agree with Knapp theory. In our theory, we accept that particles, even in the large particle size range, have surface potential originating from specific ion adsorption or desorption. This specific adsorption or desorption causes to form electrical double layer. Since any ion which should be incorporated to the crystal lattice in some order, should pass this electrical double layer. This layer gives extra resistance to crystallization. Some scientists do not agree with this surface potential effect. They suppose that high ionic strength depresses this electrical double layer. Other scientists agree with this depression. They accept that even at the surface charge depression, there is still some charge on the surface. This residual charge has a definite effect on the crystallization. If the formation of electrical double layer is accepted, external effects on this layer and on the crystallization kinetics can be explained more easily. For example, if the ionic strength is increased, electrical double layer is compressed. Therefore any admixtures having this effect should have influence on the crystallization kinetics. If there is any heavy metal ion having a strongly adsorption property on the surface should have a very deep effect. Polyelectrolytes have the biggest effect on the electrical double layer. Therefore they should have a specific effect on the crystallization, depending on how the surface charge is affected by increasing, decreasing or reversing the sign of the charge. This effect was already shown by Sayan[40] and Titiz[41] for boric acid and sodium perborate tetrahydrate crystallization, respectively. If the effect of electrical double layer is accepted, effect of electrical field which directly effects this double layer is inevitable. Therefore this theory explains many unknown or unpredictable effects on the crystallization. In the experimental part of this work, four chemicals, potassium sulfate, ammonium alum, boric acid and sodium perborate tetrahydrate were chosen for investigation. All these chemicals are different with respect to physical and crystallographic properties. In the experimental work, three different measurement techniques are used. These are single crystal growth cell [stagnant (Figure 5.1) or flow type (Figure 5.2)], fluidized bed crystal growth cell (Figure 5.3) and MSMPR type continuous crystallization (Figure 5.5) Classification of closely sized crystals in electrostatic separator at varying separation voltages are given in Figure 6. 1 to 6.4. Same kind of classification is carried out for citric acid, now ever no further experiments are conducted for this material. xvu Experimental results show that all interested material which are known to have growth rate dispersion properties, have surface charge distribution. The shape of distribution is the property of the material. It is observed that the smaller the particle size is, the higher the surface charge. This indicates that surface charge is a function of particle size. In citric acid no detectable surface charge is observed for the particles bigger than 300 urn. Single crystal growth cell experiments for all kind of material show that particles having the higher surface charge have always slower growth rates and quicker dissolution rates than the particles having the lower surface charges, by accepting that the saturation point is constant for charged and uncharged particles. Growth rate versus supersaturation graphs are given in Figure 6.9 and 6.10 for K2S04, in Figure 6.15 and 6.16 for ammonium alum, in Figure 6.20 for sodium perborate, and in Figure 6.25 for boric acid. During the growth of individual particles, it is observed that each crystal grows at constant rate. This result shows that CCG (constant crystal growth) model is perfectly true as it is shown in Figure 6.6, 6.7, 6.1 1, 6.12, 6.17, 6.21 and 6.22 To the contrary of single crystal growth /dissolution experiment, fluidized bed crystal growth/dissolution rate measurements determine the average rate for a group of crystals. It is estimated that 40.000 particles can be put to the measurement cell in the fluidized bed experiments and therefore safer data can be obtained. Obtained data from these experiments are very similar to single crystal measurements. Results of fluidized bed measurements for sodium perborate tetrahydrate are shown in Figure 6.26. Growth/dissolution rates for highly charged particles is lower than the lower charged ones. But the difference is small compared to the single crystal measurements. Disagreement in the two different measurement techniques is attributed to the breakage of dentrites in fluidization condition. In order to understand the way of charge gaining of the particles and effect the surface quality, two parallel fluidized bed experiments were carried out. In the first group of experiments, seed crystals in the sieve range of -425+355 um directly used in the measurements. In the second group of experiments, seed crystals are prepared by dissolving the bigger crystals to the same size range. In this way, surface quality is increased to obtain almost perfect surface. Figure 6.27 gives the obtained results for these two groups. Results show that perfect crystals have a higher growth rate than the ordinary crystals. These results indicate that surface quality plays an important role in specific ion adsorption. One of the ways to observe the effect of surface charge is to increase this charge deliberately by applying direct or alternative current to the measurement cell during the growth or dissolution. Figure 6.28 shows the results of two sets of experiments for ordinary crystals under 100 mV direct current and without any voltage. The results are very similar with the ones obtained in single crystal cell for highly charged and lower charged particles. Same results are obtained for other voltages up to 20 V. (Figure 6.29 and 6.30). Alternatiny current applications at different frequency also give the similar results as it is shown in Figure 6.3 1. xvui Similar experiments are carried out for sodium perborate tetrahydrate in the presence of 2.5% sodium metaborate. This condition is known as almost non-charged state for sodium perborate tetrahydrate crystals as it was shown by Titiz[41]. As it is seen in Figure 6.32, in this case there is almost no effect of electrical field in crystal growth and dissolution. It is therefore concluded that electrical field can only have an effect on crystallization in the case of charged or chargeable surface. For this specific case, increasing sodium ion concentration by sodium metaborate addition causes to decrease the anion adsorption and therefore competing effect of this two ion gives non-charged surface. Crystal growth and dissolution rates of boric acid are studied in fluidized bed growth cell for higher and lower charged particles. Figure 6.34 shows the experimental results, which is confident with the results obtained in single crystal cell, by showing that highly charged particles have higher dissolution rate and lower growth rate than lower charged particles. In order to see the effect of sodium chloride concentration which does not contain any common ion, on the growth/dissolution rates of boric acid, experiments are carried out in the presence of 65 to 665 ppm NaCl and results are shown in Figure 6.35. NaCl causes to increase the ionic strength of the solution and to compress the electrical double layer. As it is seen in Figure 6.35 double layer compression causes to decrease the growth rate, but has a little effect on dissolution rate. Effect of electrical field on the growth/dissolution rates of boric acid, measured in fluidized bed system, are shown in Figure 6.36 to 6.38. Obtained results are completely similar with the ones obtained for sodium perborate tetrahydrate. In continuos crystallization experiments, only crystallization kinetics of sodium perborate tetrahydrate is studied in the absence and in the presence of electrical field. MSMPR experiments are carried out at 0.75h retention time and experimental conditions for two experiments were completely identical, except the presence of electrical field. Crystal samples taken at the steady state conditions are observed under the microscope and following differences are detected: a) Nuclei formed in the absence of electrical field are in the shape of rod. In the presence of 100 mV direct current, length/diameter ratio of rod decreases and they become spherical. b) Twinning observed at the particles greater than 106 |im is less effective in the presence of electrical field and crystals are more compact in this condition. Table 6. 1 shows the attrition ratio and bulk density of the products obtained in these experiments. It is clear from this table that technological properties of the product is better in the presence of electrical field. The main reason for these differences results from the non-dentritical growth of the crystals in the presence of electrical field. Non- dentritical growth causes to decrease the breakage and attrition of the crystals and therefore apparent growth rate becomes higher and nucleation rate becomes lower. xix These results can be seen from the RRS diagram shown in Figure 6.39, from the population diagram shown in Figure 6.40 and from Table 6.2. From all the experiments, following general conclusions are drawn: 1- All investigated crystals which are known to have growth rate dispersion properties, have surface charges and these charges are the properties of material. 2- Distribution of surface charge is a function of particle size. Decreasing the particle size causes to increase the magnitude of this potential. 3- The higher the surface charge the lower the growth rate and the higher the dissolution rate. 4- Main reason for growth and dissolution rate dispersions is the charge distribution. 5- There is no random fluctuation in crystal growth as it is proposed random fluctuating (RF) theory. Each crystal has its own growth and dissolution rates at constant values. This indicates that assumption of CCG theory is time. 6- Crystal growth or dissolution rates may be changed by effecting the electrical double layer resulting from the surface charge. 7- Electrical double layer may be influenced by increasing ionic strength of solution and by applying electrical field in the form of direct or alternating current. 8- All of the investigated crystals have negative surface charge. This implies that surface potential results from specific anion adsorption. 9- By the proposed hypothesis it is possible to explain all peculiar solubility and growth/dissolution rates behaviors which are not possible to explain by the previous knowledge.
Açıklama
Tez (Doktora)--İTÜ Fen Bil. Enst., 1999.
Anahtar kelimeler
Difüzyon,
Kristalleşme,
Yüzey yükü,
Diffusion,
Crystallization,
Surface charge