Sodyum perborat kristalizasyonunun stokimetrik ve stokimetrik olmayan şartlarda incelenmesi

Abstract

Sodyum metaborat ve hidrojen peroksit hammaddeleri kullanılarak kimyasal yöntemle sodyum perborat tetrahidrat üretiminde, kristalizasyon şartlarına hammadde oranlarının etkisini inceleyen her hangi bir bilimsel yayın literatürde mevcut değildir. Bu çalışmada bu eksikliği gidermek için, stokiometrik şartlardaki ve, metaborat ve boraks fazlalığı olacak şekilde stokiometrik olmayan şartlarda ki kristal izasyonlar incelenerek en uygun teknolojik üretim şartları bilimsel yöntemlerle açıklanmaya çalışılmıştır. Çalışmada, sürekli karıştırmalı ve sürekli ürün çekişli kristalizasyon deneyleri gerçekleştirilerek, endüstriyel üretimle benzeşim sağlanmaya çalışılmıştır. Kristalizasyonlardan elde edilen ürünlerin elek analizi sonuçları sayı yoğunluğu teorisi kullanılarak değerlendirilmiş ve bu sonuçlar, nükleasyon hız ölçümleri ile desteklenmiştir. Bu kristalizasyonlarda, döküm yoğunluğu, mekanik mukavemet gibi endüstri açısından önemli olan fiziksel özellikler de incelenmiştir. Bu araştırma sonucunda, stokiometrik ortamdaki kristalizasyonun nükleasyona yatkın olduğu, kristalizasyon çözeltisindeki metaborat fazlalığının artışının kinetik mertebede azalmaya, boraks miktarındaki artışın ise kinetik mertebedeki artışa neden olduğu tespit edilmiştir. Kinetik mertebenin azalması, ürünün ortalama partikül boyutunu 'artırdığı için sodyum perborat üretiminde meta borat fazlalığının gerekliliği ispatlanmıştır. Ortalama partikül boyutu, partikül şekli ve dağılımı, döküm yoğunluğu ve mekanik mukavemet gibi ürün kalitesini belirleyen tüm faktörler gözönüne alındığında, kristalizasyon ana çözeltisinde 27 gram/litre civarında NaB02 ve 5 gram/litre civarında Na2B40y içeren ve pH'm 10.40'da tutulduğu kristal izasyonun üretim için en uygun durumu sağladığı tespit edilmiştir.

In this study, crystallization kinetics of sodium perborate tetrahydrate which is produced by reaction crystallization from so dium metaborate and hydrogen peroxide, is studied in stoichiometric and non-stoichiometric conditions. For this purpose mixed suspension mixed product removal (MSMPR) crystallizer is used and products from this crystallizer are examined not only from the point of crystal size distribution but also from the point of some industrially impor tant parameters such as particle shape, bulk density and mechanical' resistance to abrasion and breakage. Excess of sodium metaborate and sodium tetra borate are used during the reaction crystallization in the non-stoichiometric part of this study. Factors affecting the crystallization of sodium perborate tet rahydrate are numerous. Besides supers aturation ratio, mixing inten sity, crystallization temperature and effects of imparities which are normally encountered in all crystallization processes, ratio of reac- tants in the following equation: NaB02 + H202 + 3H20.> NaB03.4H2O are also very important. In addition to this main equation, prepara tion of sodium metaborate from NaOH and borax by the following equa tion: i Na2B4Û7 +. 2Na0H ?*? 4NaB02 + H20 impose some additional parameters such as excess of borax or sodium hydroxide in metaborate solution. Excess of some reactants may re sult in different nucleation and crystal growth mechanism which in turn may influence the crystal size distribution and other physical properties. Besides some impurities which may exist in reactants, some chemicals are added to hydrogen peroxide as stabiliser and inhibitors and these also may affect the crystallization kinetics depending on the kind and concentration. Similarly adding magnesium sulfate and sodium silicate to produce magnesium silicate in the crystallizer in order to stabilise the sodium perborate tetrahydrate affect the phe nomena depending on the concentration of magnesium silicate and ex cess of one of the components. Literature survey shows that present information about this subject is mainly in form of patent literature. Each patent deals with only one property of the product taking only one parameter, disregarding the other parameters. As it can be expected many viii contradictory results exist in the literature because of the comp lexity of the system. No systematic research is performed up to the present time in order to quantify the different effects. The reason for this is the fact that no scientific base for this system has been established before. In order to construct main base, in this study crystallization of sodium perborate is performed by using ex tra pure components. In this pure media only stoichiometric condi tions are changed to see the effect. After the establishment of this base, different impurities may be added deliberately to the system to see the final effect. From the literature and from the industrial experiments, excess of hydrogen peroxide is known to cause porous crystals which have low bulk density. Since this type of structure is not prefer red, excess of hydrogen peroxide was not used at the present work. Particle size distribution of products from MSMPR reaction crystallizer are used to determine nuclei population density and the growth rate as it is shown in figure 3-2 using the well-known popu lation balance theory. Results from three different retention times at the same stoichiometry permit the evaluation of the kinetic order i.e. the ratio of the order of nucleation to the order of crystal growth as shown in figure 3-3. Figure 4-1 shows the MSMPR crystallizer system. Crystallizer has a 2.5 liter active volume and is equipped with four flow breakers. Spiral made of glass in the crystallizer acts both as a heat exchan ger and also as an internal draft tube. Temperature inside the crystallizer is kept at 20 + 0.1°C using a magnetic vabsre, a contact thermometer and a cryostat combination. A three-blade stirrer rota ted at 720 rpm is used to maintain a homogeneous suspension in the crystallizer. 20%, Ww sodium metaborate solution is fed conti nuously at constant rate by using a diaphragm pump which is ©quipped with a pulsation dampener and working against the 6 bar back pres sure in order not to change the feed rate by solution level at the feed tank. 30%, w/w H2O2 is fed from two channels. First feeding is done by using a diaphragm pump similar to the one explained above, in such a way that greater part of the H2O2 (almost 90%) necessary for any stoichiometric condition is conducted from this channel. The rest of H2O2 is fed from the second channel in controlled manner by using a pH-controlled pump which takes signal from the combined glass electrode in the crystallizer. Keeping pH constant in the crystallizer serves to control of stoichiometric ratio of the reactant. Contrary to the continuous feeding, suspension withdrawal is made discontinuously in order to ensure isokinetic withdrawal and not to cause particle classification in.the outlet. For this rea son, 10% increase in the volume of crystallizer is permitted and this excess is sucked at high velocity by using vacuum. In order to attain steady state in the crystallizer, system is run for a period of 8 residence times and then the characteristic sample is fed to a graduated flask. After measuring the volume of ix suspension, crystals are filtered and chemical analyses of the mot her liquor are performed immediately. Crystals on the filter are washed by diethylether saturated with sodium perborate and then air dried. Sieving and physical tests to determine bulk density, volume shape factor, shape of the crystals and other factors are performed on this sample. Pure NaBC>2.4H20 is prepared from technical Na2B407.5H2Û and NaOH by double recrystallization. Pure H2O2 is prepared from tech nical 50% H2O2 in two step distillation in all-glass apparatus. In the first step, H2O2 is totally distilled under vacuum to be freed from non-volatile matter. In the second step fractional distillation is carried out at 58 - 60 mm Hg and 96 - 100°C to separate the vola tile matter. By this method H2O2 in pure state is obtained. This is diluted to 30% before being used. Volume shape factors of the different sieve fractions are determined by the method proposed by Harris [72]. Apparatus used in the determination of bulk density is shown in figure 4-3. Shape of the particles is observed by microscope. MSMPR results are shown in Tables A.l to A. 17 for different pH values which control the stoichiometric conditions. In these tables composition, density, refractive index and pH of the mother liquor and suspension density, Mj are shown at the headings. Para meters which are necessary to calculate the population density figu res are clearly indicated in these tables, where AL = Size increment, cm L = Mean crystal size, cm AW = Weight fraction of crystals appearing in size incremenet AL a = Volume shape factor n = Crystal population density, number/ cm. (ml clear filtrate) Figures A.l to A. 17 show the particle size distribution in RRS diagram for respective population balance tables. Figures B.l to B.17 show the variation of.volume shape factors with mean particle size. Figures C.l to C.17 show the population balance plots for different pH's and different retention times respectively. In these plots, least square regression method is used to obtain a linear re lationship between population density and mean particle size. Cal culated nuclei density, n° and crystal growth rate, G from these figures are used to evaluate the kinetic order in the specific stoichiometric conditions as they are shown in figures 5.1 to 5.6. In order to support the- MSMPR results, rate of nucleations are measured in the mother liquor obtained from MSMPR experiments using the experimental system shown in figure 4-2 to fit the values to equations 4.2 and 4.3- Cooling rate (b), dissolution temperature, nucleation temperature and maximum supercooling (ATmax) values obtained for each sample are listed respectively in Table 5.1 determination of nucleation order (£) and rate cons tant (kfl) is shown in figures 5-7 to 5-10 and is given in Table 5-2. From the experimental results listed in Table 6.1 the following conclusions can be obtained: a) In real stoichiometric condition; kinetic order' is 2.94 which implies that dominant particle size decreases with increasing supers a tur at ion (or retention time) b) Free metaborate in the mother liquor causes the kinetic order (i) to decrease. c) Free borax in the mother liquor causes the kinetic order to increase. d) The most suitable crystallization condition is obtained when mother liquor contains about 27 g/Z NaBC^ and about 5 g/£ Na2B4G7 at pH = 10.40. e) In pure media all crystals has the form of prismatic structu res. Twin and triple crystals are obtained when particle size increases. The crystals became spherical in shape as a result öf this. f) Growth dispersion is the dominant factor in the crystallization. Particles between 150 - 250 um in size have the highest growth rate. Particles smaller and bigger than this size range have smaller growth rate. For smaller sizes size-dependent growth rate is observed. g) Shape factors are size dependent because of gradual, changing of the crystal shape from prismatic to spherical. xx h) Homogeneous nucleation rate decreases with increasing metaborate concentration. Increasing borax concentration shows the revers effect. This observation is in good agreement with MSMPR results i) The most proper control parameter in crystallization is pH which shows? the stoichiometric ratio. In addition to this density or refractive index should be checked in order to control the super- saturation level.