Boraks pentahidratın transisyon sıcaklığı altında kristalizasyon mekanizmasının araştırılması
Boraks pentahidratın transisyon sıcaklığı altında kristalizasyon mekanizmasının araştırılması
Dosyalar
Tarih
1997
Yazarlar
Gözmen, Tülin
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Institute of Science and Technology
Özet
Bu çalışmada boraks pentahidratın Na20-B203-H20 sisteminin çözünürlük- sıcaklık ilişkisinde varolan 60.8°C civarındaki transisyon noktasının altında kristal izasyon koşulları incelenmiştir. ön deneyler Etibank'ın Kırka tesisinde üretilen boraks pentahidrat kullanılarak yapılmış, ancak bu deneyler boraks pentahidratın yüksek Ca, Mg içeriği nedeniyle başarılı olmamıştır. Daha sonra laboratuvarda saf boraks dekahidrat hazırlanmış, bu hazırlanan boraks dekahidrat ile 90-95°C da doygun boraks çözeltileri hazırlanmış ve bu çözeltiler belirli soğutma hızları ile soğutularak boraks pentahidratın kristal izasyon koşulları incelenmiştir. Bu amaçla değişik boyutlarda kristalizör ve değişik tipte karıştırıcılar denenmiş, optimum olarak belirlenen cihaz sistemi ile 100-200-300-400 devir/dakika karıştırma hızlarında ve 10-20-30 °C/saat soğutma hızlarında deneyler yapılmış ve optimum proses koşulları deneysel kısımda tanımlanan kristalizör sisteminde, 200 devir/dakika karıştırma hızı ve 20-30 °C/saat soğutma hızı olarak bulunmuştur. Ayrıca sistem üzerine etkisini görmek için bazı polielektrolitler, borik asit ve sodyum hidroksit ilavesiyle deneyler yapılmıştır, incelenen polielektrolitlerin boraks pentahidratın düşük sıcaklıklarda elde edilmesine yardımcı olmadığı görülmüştür. Borik asit ilavesi boraks pentahidratın nükleasyonunu ve kristal izasyonunu önlemiş ve düşük sıcaklıklarda boraks dekahidrat kristallenmiştir. Sodyum hidroksit ilavesi ise boraks pentahidrat nükleasyon ve kristal izasyonunu hızlandırmış ve 50-60 °C arasında boraks dekahidrat kristallenmiştir. Endüstriye uygulanabilirliği açısından çalışılan kesikli kristalizör tipi dışında MSMPR ve boru tipli kristalizörlerde aynı amaçla denenmiştir. MSMPR tipi kristalizör ile yapılan deneyler bu tip kristalizörün amaca uygun olmadığını göstermiştir. Boru tipi kristalizör deneylerinde laboratuvarda sağlanan koşullarda sistem ancak 47 °C'ta inilebilmesine izin vermiş ve bu sıcaklıkta elde edilen ürün boraks pentahidrat olmuştur. Ayrıca bu tip kristalizörde yüksek kristal büyüme hızları elde edilmiştir.
Turkey has one of the largest boron minerals reserves in the world. Two of the most commercially important boron compounds are borax decahydrate and borax pentahydrate. Production of these two boron compounds are handled by Etibank in Turkey. In Etibank's Kırka plant tincal mineral (raw borax pentahydrate) is dissolved at 90-95°C and separated from its clay and mother liquor is then crystallized. The slurry leaves the crystallizer at 66°C and is centrifuged to separate the crystals from the solution. Mother liquor which has a high concentration of borax is then recycled back to the tincal dissolution tank. This decreases the production rate of the whole process. An investigation of the solubility-temperature relationship of Na20-B203-H20 system, one can see that there is a possibility for crystallization of borax pentahydrate below the transition point of 60.8°C. In this study first Kırka plant's borax pentahydrate was dissolved at 90-95°C, and cooled down at different rates, but high amounts of Ca and Mg cause Ca, and Mg borate precipitation and these precipitates act as nucleation centers which enhance borax decahydrate crystallization. Because of this foreign ion effect, Kirka's borax pentahydrate was first dissolved at 90-95°C, Ca and Mg carbonates were precipitated by the additon of sodium carbonate. The slurry was filtered and recrystallized twice. Two batches of borax decahydrate were prepared in this manner to be used in all experiments. The first batch of borax decahydrate contained 2.5 ppm Ca, 2.5 ppm Mg, 52 ppm total organic carbon and 520 ppm Si whereas the second batch of borax decahydrate contained 5.6 ppm Ca, 3.8 ppm Mg, 50 ppm total organic carbon and 520 ppm Si. Ca and Mg analyses were performed by HITACHI model 180-80 atomic absorption spectrophotometer, total organic carbon analyses were performed by SCHIMADZU model TOC- 500 total organic carbon analyzer and Si analyses were performed by THERMO JARELL ASH model Atomscan sequential plasma type ICP spectrophotometer. By using recrystallized borax decahydrate, saturated borax solutions were prepared at 90 °C and a number of experiments were carried out. Saturated solutions of borax were cooled down by certain cooling rates. Several sized laboratory glass crystallizers and different shaped stirrers were tried. Optimum conditions were obtained with 1 It glass reactor of Wertheim Normschliff and 5 cm diameter paddle type stirrer. With this crystallizer system mixing rates of 100, 200, 300, and 400 rpm and cooling rates of 10, 20, and 30 °C/hourwere studied. XUl During dissolution of borax, temperatures were raised slowly and saturation concentrations at the beginning of the experiments were determined. During cooling, starting from 60 CC, for every 10 °C decrease samples were taken from crystals and solutions and B203 and Na20 analysis were carried out for both crystals and solutions. Na2B407 concentrations of the solutions were shown on the solubility temperature curve of the hydrates of borax. By this way a path followed by the solution's concentrations was determined. With 100 rpm and 20 °C/hour borax decahydrate crystallised out at 6.5 °C. But this mixing rate was not sufficient enough to hold the crystals in suspension. Crystals were classified in the crystallizer as larger crystals at the bottom and fines are at upper levels of the crystallizer. Larger crystals plugged out the outlet of the crystallizer and to take samples to follow up the process was not possible. For this reason, in spite of obtaining positive results, other experiments were not carried out at this mixing rate. Instantaneous borax decahydrate crystallization were detected easily by the instantaneous temperature rise of the crystallizer due to the highly exothermic heat of crystallization of borax decahydrate. In the experiments at 200, 300, and 400 rpm and cooling rate of 10 °C/hour, and also at 300, 400 rpm and 10, 20, and 30 °C/hour, below transition point, between 50-55 °C, borax decahydrate crystallized. Only at 300 rpm and 30 °C\hour borax decahydrate crystallization was obtained between 30-40 CC. These results can be expected from the theory of crystallization, higher mixing rates and lower cooling rates decrease the width of the metastable region. The best results were obtained by 100 - 200 rpm and cooling rate of 20 - 30 °C/hour. At these conditions borax decahydrate crystallization temperature was as low as 2 °C. And even at this temperature, before borax decahydrate crystallization, the crystals obtained were still borax pentahydrate. The composition of the solutions always followed a path quite above the borax pentahydrate saturation curve showing that solutions remain supersaturated. To change the compositions of borax solutions through the boric acid or metaborate side still remaining in the region of borax decahydrate, 0.3, 0.5, 1 % boric acid addition or 0.2, 0.5, 1 % sodium hydroxide addition into the solutions were investigated. Experiments showed that by the addition of boric acid, instantaneous borax decahydrate crystallization was obtained, without borax pentahydrate nucleation and crystallization or with borax pentahydrate nucleation without crystallization. So that small amounts of boric acid addition inhibits the nucleation and crystallization of borax pentahydrate. In contrast to this result the addition of small amounts of sodium hydroxide promotes the nucleation and crystal I izaton of borax pentahydrate and around 50 °C borax decahydrate crystallizes. XIV During the batch crystallization experiments, sometimes B203 content of borax pentahydrate crystals were found to be more than 49%. Borax ion schematically can be shown as given below. r OH I o-^o |2- ho^Ia I ^oh ^ OH J The unexpected analytical results and this structure gave an idea that there may be polymerization of borax ion as a mixture of dimers and trimers which stays metastable in the supersaturated solutions. Otherwise the literature value of 4.67 moles of water of crystallization is not the only form and also some borax tetrahydrate can be present. The X-ray analysis showed some minor deviations, but these deviations were not good enough to prove this. It was thought that higher temperatures may promote the polymerization. So that experiments at pressurized vessel (PARR, Pressure Reaction Apparatus, Model Type A 3039-71) were performed. Experiments were made at two different temperatures, 115 and 125 °C, and borax solution was saturated at approximatey 90 °C. After removing the pressure, the temperature of the solution was 85-86 °C and solution contained nuclei. During cooling period, samples were withdrawn at 66 and 56 °C. Borax decahydrate crystallization took place at 53 °C in all the experiments. Solution concentrations had exactly the same values with borax pentahydrate saturation curve. These results were opposite of what was expected and analysis of crystals gave B203 values which did not support the idea of the polymerization. Some experiments were made with the addition of small amounts of polyelectrolytes to see if there is a positive effect on metasable crystallization of borax pentahydrate. For this aim cationic polyelectrolytes FO 4115 and FO 4650 by SNF, anionic polyelectrolyte Superfloc A-130 and nonionic polyelectrolyte Superfloc N-100 by Cyanamid were investigated. Polyelectrolytes were added into the solution as 50 ppm in final solution. This solution prepared at 90-95 °C cooled down with 20 °C\hour at 200 rpm and borax pentahydrate nucleation and borax decahydrate spontaneous crystallization temperatures were determined. No specific effect of the polyelectrolytes investigated has been found on metastable crystallization of borax pentahydrate. XV Batch crystallization experiments showed that metastable crystallization of borax pentahydrate is possible. To see if continuous metastable crystallization of borax pentahydrate is possible MSMPR (mixed suspension mixed product removal) and pipe type crystallization experiments were carried out. MSMPR type crystallizer used in the experiments was made of plexiglas and had 2 liters volume. A cooling coil placed at the center of crystallizer played also a role as a draft tube, its diameter was 75 and height was 70 mm and an impeller with four blade and 60 mm diameter had been used as a stirrer. At pre-experiments several mixing rates were observed and most appropriate flowlines were obtained with 560 rpm. Feed solution was prepared in the 70 liters jacketed stainless steel reactor. This feed tank temperature kept at approximately 90 °C. At this type crystallizer feeding and product removal rates must be continuous and constant. Feeding had been done with peristaltic pump. However laboratory sized crystallizer is small in volume so that suspension between 95-105 % of total volume has been withdrawn intermittently for product removal. Feeding rate was 4lt/hour so that retention time was 0.5 hours. 8 times of retention time accepted sufficient to reach equilibrium conditions. At the end of this period a characteristic sample was withdrawn, crystals were washed with acetone saturated with borax, dried in air and sieve analysis were made, and population balance theory was applied. In the initial experiments main tank was kept at 90 °C and MSMPR was kept at 62.5 ± 0.5 °C to remain at borax pentahydrate side to see whether MSMPR conditions were satisfied or not and the experiments showed that MSMPR conditions were satisfied well. In the following experiments main tank was kept at 90 °C and MSMPR kept at 55 ± 0.5 °C to remain below transition point where borax decahydrate is stable. In these experiments always approximately in half an hour borax decahydrate crystallized out. These experiments showed that at MSMPR conditions created in the laboratory, borax pentahydrate metastable crystallization cannot be obtained and below transition point where borax decahydrate is stable phase, equilibrium conditions has been reached. However, batch crystallizer experiments showed that metastable crystallization of borax pentahydrate was possible and even at very low temperatures, borax pentahydrate was obtained. According to these results, during the crystallization where borax pentahydrate seed crystals exist, there are two forces effecting the crystallization. The first one is the force which is necessary for crystal growth of borax pentahydrate and it depends on the difference between the borax concentration of the solution and the metastable saturation concentration of borax pentahydrate. However for the crystallization of borax decahydrate, first decahydrate must nucleate and then these nuclei must grow. The force for this two step process is the difference between the solution's borax concentration and saturation concentration of borax decahydrate at that temperature. Another mechanism for borax decahydrate crystallization is the conversion of metastable borax XVI pentahydrate crystals to stable borax decahydrate crystals. Batch reactor experiments indicated that this transformation is not easy. To see that if industrial application of the obtained results is possible, continuous crystallization experiments with a pipe type crystallizer were done. The crystallizer is a jacketed pipe which is cooled countercurrently with cooling medium to give the desired temperature profile and cooling rate. With this type crystallizer, if the system permits, it is possible to work at every degree of supersaturation. To model this system two factors has been considered. The first one is to give the system a certain supercooling. According to previous data, this cooling rate must be over 20 °C\hour. The second factor is to keep seed and growing crystals in suspension. This rate which prevents deposition of crystals must be over 2 m\sec. To obtain this rate laboratory crystallizer's pipe diameter must be quite small and the length of the pipe must be sufficient for necessary cooling of the solution. However, at very thin pipes seed crystals plug the pipe and the outlet of the pipe. So that at the initial experiments, pipes with 16 mm diameter and 1.40 m length were arranged with angles having total height of 3.5 m. At this system up to 30 Itthour feeding rate crystal moved from the surfaces by rolling over and sticked to surfaces on which are relatively cool. So that at the following experiments two pipes with 16 mm diameter placed vertically having 2.80 m total height. Even at these experiments deposition at cool surfaces and plugging of the pipe outlet were not prevented. The feed solution which contained approximately 18% Na2B407 and 3% borax pentahydrate seed crystals with particule size smaller than 150 u fed to the pipe with peristaltic pump at a certain rate. System permitted to work down to 47 °C, at this temperature the outlet of the pipe had been plugged. The product obtained at this temperature was borax pentahydrate. The experiment for 30 lt\hour feed rate average particle size has been grown to 160 u level showing that there was an important particuler growth in the system. For this experiment crystal growth rates at 51 °C was 6.2 * 10"7, and at 47 °C with time difference were 1* 10"6 and 1.6 ^O"6 m\sec respectively. These rates obtained for metastable crystallization of borax pentahydrate were quite higher than the rates of 10"8 found by Sayan [42] for the crystallization of borax pentahydrate above transition point where pentahydrate is stable phase. The crystal growth rates at feeding rates of 15 lt\h and 5 lt\h were 2.4 * 10"7and 9.2 * 10"8 m\sec respectively. According to results obtained from these experiments pipe type crystallizer for metastable borax pentahydrate crystallization must work under the following conditions: a) The retention time must be maximum 7 minutes. b) The solution flow rate must be over 2 m\sec. xvu c) When seed crystals exist and nucleation has been prevented product crystals average particular size can be obtained from following equation: mi\ m0 = ( U\ U)3 Here m0 is the seed crystals amount, mi is the seed crystals plus product amount, L0 is an average size of seed crystals, U is an average size of product crystals. Similar calculations can be made by using Mc Cabe's AL law, if seed crystals sieve analysis is known. d) The lowest crystallization temperature is restricted by the pipe geometry, the solution flow rate and cooling medium temperature. e) There must not be other surfaces rather than borax pentahydrate seed crystals in crystallizing surfaces. The results of this study are summarized below: - Etibank's Kırka plant's borax pentahydrate was not appropriate for metastable crystallisation of borax pentahydrate, due to high content of Ca and Mg. - During the cooling period the solution follows a path quite above the solubility curve cited in the literature and remains supersaturated. - During the experiments at optimum conditions mentioned above borax pentahydrate remains metastable to borax decahydrate. - In the appropriate crystalliser system optimum conditions for the lowest borax decahydrate crystallization temperature when borax pentahydrate crystals were present, were found to be at 200 rpm and 20 - 30 °C/hour cooling rate. - At optimum conditions crystals obtained at 2°C were still borax pentahydrate. - Polyelectrolytes investigated have no specific effect on this metastable crystallization. - With boric acid present decahydrate crystals were still obtained at low temperature. The nucleation and crystallisation of borax pentahydrate, however, were greatly inhibited which is not acceptable for our purpose. - A small amount of sodium hydroxide addition, however, favour the crystallization of decahydrate which occurs immediately below the transition point. Will - At some analysis over 49% of B203 were found in borax pentahydrate, showing that possibly 4.67% of water of crystallisation cited in the literature is not the only form and probably there is also some formation of borax tetrahydrate in different amounts. - MSMPR type crystallizer conditions created at the laboratory were not appropriate for the metastable crystallization of borax pentahydrate. - With the pipe type crystallizer working at conditions cited above it is possible to perform metastable borax pentahydrate crystallization.
Turkey has one of the largest boron minerals reserves in the world. Two of the most commercially important boron compounds are borax decahydrate and borax pentahydrate. Production of these two boron compounds are handled by Etibank in Turkey. In Etibank's Kırka plant tincal mineral (raw borax pentahydrate) is dissolved at 90-95°C and separated from its clay and mother liquor is then crystallized. The slurry leaves the crystallizer at 66°C and is centrifuged to separate the crystals from the solution. Mother liquor which has a high concentration of borax is then recycled back to the tincal dissolution tank. This decreases the production rate of the whole process. An investigation of the solubility-temperature relationship of Na20-B203-H20 system, one can see that there is a possibility for crystallization of borax pentahydrate below the transition point of 60.8°C. In this study first Kırka plant's borax pentahydrate was dissolved at 90-95°C, and cooled down at different rates, but high amounts of Ca and Mg cause Ca, and Mg borate precipitation and these precipitates act as nucleation centers which enhance borax decahydrate crystallization. Because of this foreign ion effect, Kirka's borax pentahydrate was first dissolved at 90-95°C, Ca and Mg carbonates were precipitated by the additon of sodium carbonate. The slurry was filtered and recrystallized twice. Two batches of borax decahydrate were prepared in this manner to be used in all experiments. The first batch of borax decahydrate contained 2.5 ppm Ca, 2.5 ppm Mg, 52 ppm total organic carbon and 520 ppm Si whereas the second batch of borax decahydrate contained 5.6 ppm Ca, 3.8 ppm Mg, 50 ppm total organic carbon and 520 ppm Si. Ca and Mg analyses were performed by HITACHI model 180-80 atomic absorption spectrophotometer, total organic carbon analyses were performed by SCHIMADZU model TOC- 500 total organic carbon analyzer and Si analyses were performed by THERMO JARELL ASH model Atomscan sequential plasma type ICP spectrophotometer. By using recrystallized borax decahydrate, saturated borax solutions were prepared at 90 °C and a number of experiments were carried out. Saturated solutions of borax were cooled down by certain cooling rates. Several sized laboratory glass crystallizers and different shaped stirrers were tried. Optimum conditions were obtained with 1 It glass reactor of Wertheim Normschliff and 5 cm diameter paddle type stirrer. With this crystallizer system mixing rates of 100, 200, 300, and 400 rpm and cooling rates of 10, 20, and 30 °C/hourwere studied. XUl During dissolution of borax, temperatures were raised slowly and saturation concentrations at the beginning of the experiments were determined. During cooling, starting from 60 CC, for every 10 °C decrease samples were taken from crystals and solutions and B203 and Na20 analysis were carried out for both crystals and solutions. Na2B407 concentrations of the solutions were shown on the solubility temperature curve of the hydrates of borax. By this way a path followed by the solution's concentrations was determined. With 100 rpm and 20 °C/hour borax decahydrate crystallised out at 6.5 °C. But this mixing rate was not sufficient enough to hold the crystals in suspension. Crystals were classified in the crystallizer as larger crystals at the bottom and fines are at upper levels of the crystallizer. Larger crystals plugged out the outlet of the crystallizer and to take samples to follow up the process was not possible. For this reason, in spite of obtaining positive results, other experiments were not carried out at this mixing rate. Instantaneous borax decahydrate crystallization were detected easily by the instantaneous temperature rise of the crystallizer due to the highly exothermic heat of crystallization of borax decahydrate. In the experiments at 200, 300, and 400 rpm and cooling rate of 10 °C/hour, and also at 300, 400 rpm and 10, 20, and 30 °C/hour, below transition point, between 50-55 °C, borax decahydrate crystallized. Only at 300 rpm and 30 °C\hour borax decahydrate crystallization was obtained between 30-40 CC. These results can be expected from the theory of crystallization, higher mixing rates and lower cooling rates decrease the width of the metastable region. The best results were obtained by 100 - 200 rpm and cooling rate of 20 - 30 °C/hour. At these conditions borax decahydrate crystallization temperature was as low as 2 °C. And even at this temperature, before borax decahydrate crystallization, the crystals obtained were still borax pentahydrate. The composition of the solutions always followed a path quite above the borax pentahydrate saturation curve showing that solutions remain supersaturated. To change the compositions of borax solutions through the boric acid or metaborate side still remaining in the region of borax decahydrate, 0.3, 0.5, 1 % boric acid addition or 0.2, 0.5, 1 % sodium hydroxide addition into the solutions were investigated. Experiments showed that by the addition of boric acid, instantaneous borax decahydrate crystallization was obtained, without borax pentahydrate nucleation and crystallization or with borax pentahydrate nucleation without crystallization. So that small amounts of boric acid addition inhibits the nucleation and crystallization of borax pentahydrate. In contrast to this result the addition of small amounts of sodium hydroxide promotes the nucleation and crystal I izaton of borax pentahydrate and around 50 °C borax decahydrate crystallizes. XIV During the batch crystallization experiments, sometimes B203 content of borax pentahydrate crystals were found to be more than 49%. Borax ion schematically can be shown as given below. r OH I o-^o |2- ho^Ia I ^oh ^ OH J The unexpected analytical results and this structure gave an idea that there may be polymerization of borax ion as a mixture of dimers and trimers which stays metastable in the supersaturated solutions. Otherwise the literature value of 4.67 moles of water of crystallization is not the only form and also some borax tetrahydrate can be present. The X-ray analysis showed some minor deviations, but these deviations were not good enough to prove this. It was thought that higher temperatures may promote the polymerization. So that experiments at pressurized vessel (PARR, Pressure Reaction Apparatus, Model Type A 3039-71) were performed. Experiments were made at two different temperatures, 115 and 125 °C, and borax solution was saturated at approximatey 90 °C. After removing the pressure, the temperature of the solution was 85-86 °C and solution contained nuclei. During cooling period, samples were withdrawn at 66 and 56 °C. Borax decahydrate crystallization took place at 53 °C in all the experiments. Solution concentrations had exactly the same values with borax pentahydrate saturation curve. These results were opposite of what was expected and analysis of crystals gave B203 values which did not support the idea of the polymerization. Some experiments were made with the addition of small amounts of polyelectrolytes to see if there is a positive effect on metasable crystallization of borax pentahydrate. For this aim cationic polyelectrolytes FO 4115 and FO 4650 by SNF, anionic polyelectrolyte Superfloc A-130 and nonionic polyelectrolyte Superfloc N-100 by Cyanamid were investigated. Polyelectrolytes were added into the solution as 50 ppm in final solution. This solution prepared at 90-95 °C cooled down with 20 °C\hour at 200 rpm and borax pentahydrate nucleation and borax decahydrate spontaneous crystallization temperatures were determined. No specific effect of the polyelectrolytes investigated has been found on metastable crystallization of borax pentahydrate. XV Batch crystallization experiments showed that metastable crystallization of borax pentahydrate is possible. To see if continuous metastable crystallization of borax pentahydrate is possible MSMPR (mixed suspension mixed product removal) and pipe type crystallization experiments were carried out. MSMPR type crystallizer used in the experiments was made of plexiglas and had 2 liters volume. A cooling coil placed at the center of crystallizer played also a role as a draft tube, its diameter was 75 and height was 70 mm and an impeller with four blade and 60 mm diameter had been used as a stirrer. At pre-experiments several mixing rates were observed and most appropriate flowlines were obtained with 560 rpm. Feed solution was prepared in the 70 liters jacketed stainless steel reactor. This feed tank temperature kept at approximately 90 °C. At this type crystallizer feeding and product removal rates must be continuous and constant. Feeding had been done with peristaltic pump. However laboratory sized crystallizer is small in volume so that suspension between 95-105 % of total volume has been withdrawn intermittently for product removal. Feeding rate was 4lt/hour so that retention time was 0.5 hours. 8 times of retention time accepted sufficient to reach equilibrium conditions. At the end of this period a characteristic sample was withdrawn, crystals were washed with acetone saturated with borax, dried in air and sieve analysis were made, and population balance theory was applied. In the initial experiments main tank was kept at 90 °C and MSMPR was kept at 62.5 ± 0.5 °C to remain at borax pentahydrate side to see whether MSMPR conditions were satisfied or not and the experiments showed that MSMPR conditions were satisfied well. In the following experiments main tank was kept at 90 °C and MSMPR kept at 55 ± 0.5 °C to remain below transition point where borax decahydrate is stable. In these experiments always approximately in half an hour borax decahydrate crystallized out. These experiments showed that at MSMPR conditions created in the laboratory, borax pentahydrate metastable crystallization cannot be obtained and below transition point where borax decahydrate is stable phase, equilibrium conditions has been reached. However, batch crystallizer experiments showed that metastable crystallization of borax pentahydrate was possible and even at very low temperatures, borax pentahydrate was obtained. According to these results, during the crystallization where borax pentahydrate seed crystals exist, there are two forces effecting the crystallization. The first one is the force which is necessary for crystal growth of borax pentahydrate and it depends on the difference between the borax concentration of the solution and the metastable saturation concentration of borax pentahydrate. However for the crystallization of borax decahydrate, first decahydrate must nucleate and then these nuclei must grow. The force for this two step process is the difference between the solution's borax concentration and saturation concentration of borax decahydrate at that temperature. Another mechanism for borax decahydrate crystallization is the conversion of metastable borax XVI pentahydrate crystals to stable borax decahydrate crystals. Batch reactor experiments indicated that this transformation is not easy. To see that if industrial application of the obtained results is possible, continuous crystallization experiments with a pipe type crystallizer were done. The crystallizer is a jacketed pipe which is cooled countercurrently with cooling medium to give the desired temperature profile and cooling rate. With this type crystallizer, if the system permits, it is possible to work at every degree of supersaturation. To model this system two factors has been considered. The first one is to give the system a certain supercooling. According to previous data, this cooling rate must be over 20 °C\hour. The second factor is to keep seed and growing crystals in suspension. This rate which prevents deposition of crystals must be over 2 m\sec. To obtain this rate laboratory crystallizer's pipe diameter must be quite small and the length of the pipe must be sufficient for necessary cooling of the solution. However, at very thin pipes seed crystals plug the pipe and the outlet of the pipe. So that at the initial experiments, pipes with 16 mm diameter and 1.40 m length were arranged with angles having total height of 3.5 m. At this system up to 30 Itthour feeding rate crystal moved from the surfaces by rolling over and sticked to surfaces on which are relatively cool. So that at the following experiments two pipes with 16 mm diameter placed vertically having 2.80 m total height. Even at these experiments deposition at cool surfaces and plugging of the pipe outlet were not prevented. The feed solution which contained approximately 18% Na2B407 and 3% borax pentahydrate seed crystals with particule size smaller than 150 u fed to the pipe with peristaltic pump at a certain rate. System permitted to work down to 47 °C, at this temperature the outlet of the pipe had been plugged. The product obtained at this temperature was borax pentahydrate. The experiment for 30 lt\hour feed rate average particle size has been grown to 160 u level showing that there was an important particuler growth in the system. For this experiment crystal growth rates at 51 °C was 6.2 * 10"7, and at 47 °C with time difference were 1* 10"6 and 1.6 ^O"6 m\sec respectively. These rates obtained for metastable crystallization of borax pentahydrate were quite higher than the rates of 10"8 found by Sayan [42] for the crystallization of borax pentahydrate above transition point where pentahydrate is stable phase. The crystal growth rates at feeding rates of 15 lt\h and 5 lt\h were 2.4 * 10"7and 9.2 * 10"8 m\sec respectively. According to results obtained from these experiments pipe type crystallizer for metastable borax pentahydrate crystallization must work under the following conditions: a) The retention time must be maximum 7 minutes. b) The solution flow rate must be over 2 m\sec. xvu c) When seed crystals exist and nucleation has been prevented product crystals average particular size can be obtained from following equation: mi\ m0 = ( U\ U)3 Here m0 is the seed crystals amount, mi is the seed crystals plus product amount, L0 is an average size of seed crystals, U is an average size of product crystals. Similar calculations can be made by using Mc Cabe's AL law, if seed crystals sieve analysis is known. d) The lowest crystallization temperature is restricted by the pipe geometry, the solution flow rate and cooling medium temperature. e) There must not be other surfaces rather than borax pentahydrate seed crystals in crystallizing surfaces. The results of this study are summarized below: - Etibank's Kırka plant's borax pentahydrate was not appropriate for metastable crystallisation of borax pentahydrate, due to high content of Ca and Mg. - During the cooling period the solution follows a path quite above the solubility curve cited in the literature and remains supersaturated. - During the experiments at optimum conditions mentioned above borax pentahydrate remains metastable to borax decahydrate. - In the appropriate crystalliser system optimum conditions for the lowest borax decahydrate crystallization temperature when borax pentahydrate crystals were present, were found to be at 200 rpm and 20 - 30 °C/hour cooling rate. - At optimum conditions crystals obtained at 2°C were still borax pentahydrate. - Polyelectrolytes investigated have no specific effect on this metastable crystallization. - With boric acid present decahydrate crystals were still obtained at low temperature. The nucleation and crystallisation of borax pentahydrate, however, were greatly inhibited which is not acceptable for our purpose. - A small amount of sodium hydroxide addition, however, favour the crystallization of decahydrate which occurs immediately below the transition point. Will - At some analysis over 49% of B203 were found in borax pentahydrate, showing that possibly 4.67% of water of crystallisation cited in the literature is not the only form and probably there is also some formation of borax tetrahydrate in different amounts. - MSMPR type crystallizer conditions created at the laboratory were not appropriate for the metastable crystallization of borax pentahydrate. - With the pipe type crystallizer working at conditions cited above it is possible to perform metastable borax pentahydrate crystallization.
Açıklama
Tez (Doktora)-- İTÜ Fen Bil. Enst., 1997.
Anahtar kelimeler
Boraks pentahidrat,
Kristalleşme,
Borax pentahydrate,
Crystallization