Borik asitin kristalizasyonu üzerine safsızlıkların etkisi

Abstract

Dünya'nın ikinci büyük bir üreticisi ve bor ürünlerinin Avrupa'daki en büyük satıcısı Türkiye, 1970 ' lerden beri bor ürünlerinin en önemli bileşiği borik.asit'i Etibank'ın Bandırma ve Borax ve Borik asit fabrikaların da üretmektedir. Üretim teknolojisinin basit olmasına karşılık, kristalize üründen beklenen özellikler, borik asit'in kristalizasyon kinetiğini gerçekten çok iyi bir şekilde bilmemizi zorunlu kılmaktadır. Çalışmamızda, borik asit üretiminde karşımıza çıkan yüksek miktarlardaki Safsızlıkların kristalizasyon kinetiği üzerine etkilerini incelemek ilk amacımız olmuştur. Bu amaçla seçilen safsızlıkların, nükleasyon hızı ve büyüme hızı üzerine etkileri, nükleasyon hızı ölçüm deneyleri, sürekli karıştırmalı sürekli ürün çekişli kristalizör deneyleri ve akışkan yatakta büyüme hızı ölçüm de neyleri sonrasında değerlendirilmiştir. Borik asitin üretiminde karşılaşılan bir problemde dentritik büyümedir. Bu olay üretimde pek çok sorunu beraberinde getirmektedir. Bu nedenle dentritik büyüme çalışmamızda devamlı göz önünde tutularak, safsızlıkların dentritik bü yüme üzerine etkileri gözlemlenmiştir. Deneylerimiz borik asit üretiminde ikincil (sekonder) nükleasyonun önemini ortaya çıkarmış kırılma sonucu ortaya çıkacak borik asit kristallerinin düzgün kristallere nazaran büyüme hız farklılıkları yine akışkan yataklı kristalizörde ölçülmüştür. Bu çalışmanın neticesinde, sülfat iyonunun nükleasyon hızını arttırdığı dentritik büyümeyi bastırdığı ve büyüme hızı üzerine negatif etkisi bulunduğu ve MgSO, 'm sülfat iyonuna benzer etkisi tespit edilmiştir. Ayrıca sülfat iyonunun sürekli karıştırmalı sürekli ürün çekişli kristalizör sonuçları partikül boyutuna bağlı olacak şekilde, amprik bir denklem türetilmiştir. Ca i- yonunun büyümeye pozitif etkisi deneyler sonucunda görülmüştür Kırık ve mükemmel kristallerle yapılan deneyler sonucunda ise, özellikle düşük aşırı doygunlukta kırık kristallerin mükemmel kristallere nazaran daha yüksek büyüme hızına sahip olduğu bulunmuştur.

In order to solve the problems encountered in the production of boric acid from Kolemanite mineral, + he crystallization of boric acid and the impurity effects on the crystallization, have been investigated, Boric acid from the kolemanite can be produced by the following equation: Ca2 B6 0^. 5H20 + 2^50^ + 6 HgD - * SCaSO^. 2H20 + 6H3B03 The saturated solution which is fed to crystallizer contains many impurities from the raw material and sulp hate ion from the sulfuric acid. These impurities affect quite a lot the crystallization of boric acid. For this reason in our study sulphate, calcium and magnesium ions have been choosed. The solutions which have different amounts of impurity concentrations have been compared to see the effects against the experiments in pure medium. In this study three experimental set-ups havebeen used. These are: * The apparatus of measuring metastable limits in agitated solution. * The mixed suspension mixed product removal (MSMPR) crystallizer. * A laboratory scale fluidized bed crystallizer. IX Nucleation rates are generally measured within the system studied and fitted to an empirical expression, The metastable limits uias measured by the visual method» The solution contained in a 100 ml which has a coding jacket Erlenmeyer flask. The solution agitated by a magnetic stirrer. The temperature of the solution is programmed to decrease until the first crystals were observed, the temperature is then increased slowly until the crystals were again observed to go into solution. The difference between these temperatures AT is known maximum undercooling and the experiments nave re peated untill tD find the repeated points. The effect of the sulphate ion concentrations (1000- 2000 ppm) on the nucleation was measured with this ap paratus by choosing 2D.1, 10.2, 7.6 C/h cooling rates. Fig 5.1. shows the apparatus of measuring metas table limits in aqitated solutions. After the nucleation experiments these results can be written as a conclusion: * The metastable limits of the solution is getting narrow the exist of the sulphate ion. * * The metastable zone windth is getting narrow the increase of the hydrodynamic conditions. The saturation temperature of the solution increase by increasing of the sulphate ion con centration in the solution. Figure 5.2. shows the MSMPR crystallizer system. Plexiglass crystallizer has a 3 liters active volume, and is equipped with flow breakers. The"spiral made of glass in the crystallizer acts both as a heat exchanger and also as an internal draft tube. * The solution which has saturated at 60 C, crystallized at 30DC. The_Tem- perature inside the crystallizer is kept at 30DC +0,1DC using a magnetic valve, a contact thermometer and a cryostat combination. A three -blade stirrer rotated to maintain a homogeneous suspension in the crystallizer. Boric acid solution [l3g/100 g ss] is fed continously at three different constant rate by using a diaphragm pump. The suspension withdrawal is made discontinuously in order tD maintain isokinetic withdrawal and not to cause particle classification in the outlet. For this reason the increase in the volume of crystallizer is permitted har< this excess is suched at a high velocity by using vacuum. In order to attain steady state in the crystallizer, system is run for a -period of 6 residence times and then the characteristic sample was taken for the measure crystal size distrubution. For the measure of the crystal size distrubution, Laser difraction method has been used which is shown in part 5.4. The experiments was done three different residence time for pure solution and then they were repeated for 1DDD ppm sulphate ions. The cumulative distrubution of the crystals and their population density plot of the experiment? are shown Fig 6.5. and Fig 6.6., respectively. The results of all MSMPR experiments are given in Table : 1"6 appendix. The important results of these experiments can be stated: * The medium particle size increases by the exis tence Df the sulfate ion in the solution. * The growth rate of boric acid is a f un nt.ion of particle size distrubution and can be stated as follow ing equation for pure medium: G= 9D.199a|l+7,3.10"3(L-9D)|D'55 XI Similary for the exist of the 1000 ppm sulphate ions in the solution. G= 10S,55| 1 + 6,06.10-3(|_-90)|D'6 A laboratory-scale fluidized bed crystallizer have been used for the measure of the overall growth rates n f boric acid crystals in pure and impure solutions which is shown in Fig. 5.3. The crystallizer which has a circulating flow sys tem is made of glass and has a total capacity of app roximately 20 liters. The crystallization section consists of a glasstube which has an internal diameter of 1cm. At the end of the experiments, boric acid crystals which can readily be grown by the cooling of supersatu rated solutions, are easly separated from the mother liquor which is on the crystal surface and dried. The growth rate is determined by following for each experi ments t^C 2) 1/3 - 1] where ; L^.Lr,: Crystal sizes, at the begining and at the end of the experiment respectively. E.jEr,: Crystal weights, at the begining and at the end of the experiment respectively. The effect of the impurities (sulfate, calcium and magnesium sulfate) and their concentrations on the growth rate of boric acid was measured for three sieve fractions, as follow: 300 400 500 itOO 500 600 P r m xn All fluidized bed experiments were carried out at the canditions of 3 grs seed crystals and 12 rnins. In this study, dentritic grouing was also observed in pure and impure medium. Typical dentric.growths of boric acid shown in Fig. 6.11.Some of the results from the laboratory scale fluidized bed experiments on the crystallization of boric acid for the sulphate ion can be stated as: * There is no effect on the growth rate of boric acid until 5DD ppm sulphate ion concentration * After this concentration, the temperature at solubility raises and the growth rate of boric acid crystals starts to decrease. * Sulphate on depresses the dentritic growth. The results of the experiments with boric acid solutions which contain different amounts of MgSD, ion was nearly the same as the results of the experiments in boric acid solutions which have sulphate ion. For investigate the effect of the calcium ion, cal cium was added into the solution as CaCOH),, and also some amounts boric acid was added to protect the big changes on the saturation temperature, the experiments are as follow: The results of * The maximum supersaturation in pure solution becames two times bigger when there is existence of calcium ion. * The calcium ion affects the growth rate of boric acid positively. * There is no effect on the dentritic growth. xni The perfect and hurt crystals were measured with a camara which has two growing cells an fluidized bed to investigate the differences between their growth rates. In this cells, the hydrodynamic conditions and all the physical properties were the same. The perfect and hurt crystals were growing in the same conditions and at the end of the experiment the growth rates of the crystals were calculated by weight difference. * The growth rates and the dissolution rates of the hurt crystals are bigger than the perfect ones and this effect especially much bigger in low supersaturations. * The growth sate ratio for the perfect and hurt crystals is decreasing in time.