Borik asit büyüme kinetiğine elektriksel alan ve safsızlıkların etkisi
Effect of impurities and electrical field on the kinetics of the crystallization of boric acid
- Tez No: 14409
- Danışmanlar: DOÇ.DR. A. NUSRET BULUTÇU
- Tez Türü: Yüksek Lisans
- Konular: Kimya Mühendisliği, Chemical Engineering
- Anahtar Kelimeler: Borik asit, Elektrik alanları, Kristalleşme, Safsızlık, Boric acid, Electric fields, Crystallization, Impurity
- Yıl: 1991
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Bu çalışmada, borik asitin kristalizayon kinetiğine etki edebi leceği düşünülen safsızlıkların ve elektriksel alanın etkileri ince lenmiştir. Bununla ilgili denemeler laboratuvar ölçüsünde akışkan ya taklı bir kristali zörde gerçekleştirildi. Borik asitin üretiminde kar şılaşılan bir başka problem de dentiritik büyümedir. Bu yüzden denti- ritik büyüme çalışmamızda devamlı gözönünde tutularak, safsızlıkların ve elektriksel alanın dentiritik büyüme üzerine etkileri mikroskopik ölçüde gözlemlenmiştir. Çalışmamız sonucunda, saf ortamda -0.1<AC<0 bölgesinde çözün menin eğri sel bir görüntü verdiği ve yine saf ortamda -0.02<AC<0.08 aralığında büyüme ve çözünmenin olmadığı bir bölgenin varlığı görülmüş tür. Borik asitin 100-1000 mV bölgesindeki doğru akım alanında daha yüksek çözünürlük hızına sahip olduğu, buna karşılık büyüme hızının bastırıldığı tespit edilmiştir. Doğru akımın yönünün değiştirilmesi bir farklılık göstermemektedir. Buna karşılık elektriksel alan gerili mi arttıkça metastabil bölge ve saf ortamda borik asitin çözünme-b'üyü- me görülmeyen bölgesi daralmaktadır. 15000-48000 mV gerilim altında elektriksel alanın etkisi azalmakta, hemen hemen ortadan kalkmaktadır. Doğru akım gerilimi altındaki büyümelerin de dentiritik olduğu görül müştür. Borik asitin mannitollü ortamda büyütülmesi nde büyüme hızının 1000 ppm ve 5000 ppm mannitol konsantrasyon bölgesinde, konsantrasyon dan bağımsız olarak arttığı belirlenmiştir. Buna karşın çözünme hızı etkilenmemektedir. Kristalizasyon ortamındaki mannitolun varlığı den tiritik büyümeyi önleyememektedir. Mannitollü ortamda, saf ortamda görülen büyüme ve. çözünmenin olduğu bölge ortadan kalkmaktadır. %10 civarında anyoniklik derecesine sahip poliakrilamidli ortam da borik asitin büyüme ve çözünme hızlan saf ortamdaki ile tamamen aynı olmak tadır. Poliakrilamidin varlığı büyümede görülen dentiritleri zayıf latmakta ve dentiritik büyümeyi geriletmektedir. iv
Özet (Çeviri)
EFFECT OF IMPURITIES AND ELECTRICAL FIELD ON THE KINETICS OF THE CRYSTALLIZATION OF BORIC ACID SUMMARY In order to solve the problems encountered in the production of boric acid from kolemanite mineral the crystallization of boric acid and the impurity effects on the crystallization, have been investigated The main problem encountered in the crystallization of boric acid is the formation of (fentritical crystal growth. This causes some indus trial problems such as a broad range of crystal size distribution, low washing characteristics, difficulty in drying and caking tendency. Therefore in this study the rate of growth and dissolution is deter mined and then crystal habit is controlled in pure boric acid solution, in the presence of some impurities such as d-mannit and polyacryl amide (PAA) and in the different level of electrical fields. The main reason for trying electrical field, a charged chemical such as PAA and a chemical such as d-mannit which can cause surface character is the different behaviour seen in boric acid crystals. These crystals adhese to the any surface very strongly. This phenome na shows that boric acid crystals have surface charge. Changing the charge either by supressing or depressing can cause some different effect on crystal growth. In this study, effects of impurities and electrical fields on the kinetics of the crystallization of boric acid are investigated. A laboratory scale fluidized bed crystal! izer, have been used in the experiments as it is shown in Figure 5-1 and 5-2. Crystal! izer which has a curculating flow system, is made of glass and has a total capa city of approximetly 10 liters. It is observed that temperature changing during any particular experiment was less than *0.1°C. The height of bed is 20 cm. and its diameter is 1.6 cm. All fluidized experiments were carried out by using of 3 grs seed crystals and for a period of 12 mins. The particle size used are 500-600 ym and 425-500 ym. At the end of the experiments crystal habits were also observed under the microscope if there are any changing in pure and impure medium. As the impurities, d-mannit (1000 ppm, 5000 ppm) and polyacryl- amide (1 ppm, 5 ppm, 100 ppm) is fed directly to the solution. To obtain the electrical fields in the system, DC supply has been used. The electrical charges were 100 mV, 1000 mV, 15000 mV and 48000 mV. In 1000 mV experiments, (+) and (-) sides has been changed to obtain different directions of current. At the end of the experiments, boric acid crystals which can readly be grown by the cooling of supersaturated solutions are easily seperated from the mother liquor by vacuum filtration and washed bydi ethyl ether and then these crystals are dried at the room tempera ture. Growth rate is determined by following equation for each ex periments, L2-h =_h_ (A)17'.! t t Gl Where; L,, L~ : Crystal sizes, at the beginning and at the end of the experiment respectively. G-|, Go : Crystal we'ighths, at the beginning and at the end of the experiment respectively. At the end of the experiments, the following results had been found: -For the 500-600 urn, dissolution rate is written as follows, D=3. 189+48. 86. AC (AC<-0.1) -In growing area, the growth rate of crystals is not linear with super saturation. Figure 6-4 shows this situation for the 425-500 ym fraction. The dentritic growth is observed in pure solution. The photographs of crystals are shown in Figure 6-6 and Figure 6-7. -In<-0.1<AC<(harea, dissolution rate is not linear function of concentration driving force. It means that dissolving is not only controlled by diffusion phenomena. This behaviour should be as a resul t.of surface reaction, but it is not clear. -Between -0.02<AC<0.08 concentration area, there is no growing and dissolving for 500-600 ym fraction in pure solution. -Boric acid has the highest diffusion rates on the electri cal field area between 100-1000 mV as it is seen from the disso lution rate curve. Although the growth rate is suppressed. -Changing of the direction of direct current has not any effect on crystal rates of growth and dissolution. -In the field of electricity the supersaturation which is given to the solution can not be increased so much comparing to VIthe pure solutions. So that electrical field decreases the maximum super saturation level and probably increases the rate of nucleation. Figure 6-10 shows the growing crystals on the 1000 mV electrical charge. In both figures the dentrical growth are seen easily. A growing crystal in a pure solution. -The rate of growth and dissolution of boric acid at 15000 mV and 48000 mV electrical field are shown in Figure 6-14 and 6-15. When 15000 and 48000 mV electrical charges applied, it is seen that there are no effect on the crystal growth rates of boric acid. But increasing the intensity of electrical field causes to narrow the metastable zone and to shorten the non-growing section seen in pure solution. -In Figure 6-16 and 6-17, some crystals growing on 15000 and 48000 mV electrical fields are given. We can say that growing vncrystals have a strong dentrical structure. -Figure 6-18 and 6-19 show the crystal growth and dissol i-, i. tion rates for the 425-500 ym fraction from the solution containing 1000 ppm and 5000 ppm d-manhit. -Increasing the concentration of d- mannit, the dissolution temperature of boric acid increases regularly. In d-mannit solutions, the growth rate increases although the disso lution section does not change. -Figure 6-21 and 6-22 show the growing crystals in 1000 ppm and 5000 ppm d-mannit solutions. The presence of d-mannit on crystal lisation system can not suppress dentritical growth. In d-mannit solution, non-growing section at low supersaturation levels seen in pure solution disappeared.. -Figure 6-23, 6-24 and 6-25 show the experimantal results for 1 ppm polyacryl amide (PAA), 5 ppm polyacryl amide and 100 ppm polyacryl amide solution. As the results, the growth and dissolution rates for boric acid are exactly the same with the pure solution. In these experiments Cyanamide A100 flocculant which is an anionic polyacryl amide are used. -Figure 6-27, 6-28 and 6-32 show the growing crystals on 1 ppm polyacrylamide, 5 ppm polyacryl amide and 100 ppm polyacryl amide solution respectively. As it is seen from these figures, polyacryl amide depress the dentritical growth. All the experimental results are given in appendix as tables. In these tables, headings are given as experiment number, temperature in fluidized bed crystal growth cell, saturation concentration at par ticulate situation, period of experiment in minute, weight of crystal taken from crystal cell after experiment, real concentration in flu idized bed, supersaturation, grQwht rate anc' medium condition respec tively. All the experimental results are evaluated with respect to crystal growth-dissolution kinetics and crystal habit. And it is con cluded that d-mannit gives more promising effect with respect to crystal growth kinetics. But from the industrial point of view, the most important factor should be taken into consideration is to project the growth from dentrit formation, since it causes very important problems in manufacturing process such as difficulty in drying, in creasing caking tendency, broad range of particle size distrubution. Therefore habit modification by impurity effect is taken as fundamental parameter. As a general result, it is concluded that polyacryl ami dâ has the most important effect on growing crystal since it depress the viiidentritical growth. Therefore more research should be done with this kind of material by using different kind of polyacryl amide in molecu lar weight and charge density. IX
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