Sodyum perbosat tetrahidrat kristalizasyonunun kristal yüzey şarjının kontrolü ile yönlendirilmesi

Abstract

ÖZET Sodyum perborat tetrahidrat, sodyum metabarat vs hidrojen peroksit çözeltileri kullanılarak reakaiyonlu kristalizasyonla üretilmektedir. Sodyum perborat tetrahidrat üreticileri, hem deterjan hammaddesine hem de sodyum per borat monohidrat üretimine uygun ürün elde etmek zorundadır. Perborat üreticileri için çözülmesi gereken bir sorun da, kristalizördeki kabuk oluşumudur. Bu çalışmada, pilot ölçekte akışkan yataklı kristalizör kullanılarak, kristalizasyon ana çözeltisinde sodyum metabarat fazlalığının, reaktanların karıştırma şeklinin, hidrodinamik şartların ve kristal yüzey şarjını modifiye edici polielektrolitlerin, ürün özelliklerine ve kabuk oluşumuna etkisi incelenmiştir. Araştırma sonucunda, kristalizasyon ortamındaki sodyum metaborat konsantrasyonunun artışı ile ürünün döküm yoğunluğunun arttığı saptanmıştır. Ayrıca, düzgün kristal büyümesi sağlanabilmesi için bölgesel hidrojen peroksit fazlalığına izin verilmemesi bu nedenle akışkan yataklı kristalizörler için, hidrojen peroksit beslemesinin, kristalizasyon bölgesine girişten önce yapılması gerektiği saptanmıştır. Kristalizasyon sırasında kristallerin yüzey şarjını nötrolize etmeye yeterli doğru akım geriliminin uygulanmasının, kristalizasyon kinetiğini ve aglomerasyon hızını etkilediği, yine kristal yüzey şarjını nötralize etmeye elverişli polielektrolit katkısı ile, kristalizasyon kinetiğini ve ürün özelliklerinin yönlendirilebileceği görülmüştür. Yine doğru akım gerilimi uygulanması ve polielekt rolit katkısı ile kristalizördeki kabuk oluşumunun şarj nötralizasyonu mekanizması üzerinden kontrol edilebileceği saptanmıştır.

DIRECTED CRYSTALLIZATION DF SDDIUM PERBGRATE TETRAHYDRATE BY SURFACE CHARGE CONTRDL SUMMARY It is very important to produce refined boron com pounds from the raw materials which were found to be abundant in Turkey. One of this product sodium perborate tetrahydrate. Because of the several factors affecting the production of this product, it is not produced with high quality and the share of our country in the market is very low depending on these problems. Sodium perborate tetrahydrate (5PT, NaBO,.4H`0, M=153.8, q=1 731 kg/cm3) is produced from aqueous solution By reac tion crystallization of sodium metaborats (SMB, NaBD`)and hydrogen peroxide (H?G`). The reaction involved in 5PT production takes place in two steps as shown in the fol lowing equations: Na2B4D? + 2NaOH - 4NaBD2 + H2D (1 ) NaB02 + H202 + 3H20 - NaBD3.4H2D (2) Excess of one or two reactants in both steps can be used to control some of the physical proporties of the crystals produced. Product proporties also can be impro ved by using excess of magnesium sulphate or sodium sili cate additives to form magnesium silicate stabilizer in the crystallizer. The factors that affecting the sodium perborate tetrahydrate crystal for the consumption in others in dustries such as in detergent production are, bulk den sity, mechanical strenght, attrition resistance and etc. Although these factors are depend on the stoichiometric of.the crystallization medium, kind of the stabilizer and thestabilizer and the impurities that present in the reac tants, they are also depend on other several physical factors such as feeding conditions of reactants and hydrodynamic conditions. Crystallization step conventionally conducted at 20 C by treating 20 % (by ueight) aqueous sodium meta- borate solution with 70 % (by weight) aqueous hydrogen peroxide. Temperature increase by reaction and crystal lization was prevented by using cooler provided next '^to crystallizator. In sodium perborate tetrahydrate production, fluidi- zed bed crystallizators were frequently used. Fluidiza- tion conditions were controlled by the ratio of circula tion rate of mother liquour to feed solutions. Slurry, taken from the crystallizator was passed through thickener in order to increase the suspension density up to §0 % and than centrifuged. Crystalls taken from centrifuge, dried in fluldized drier and crystalls having the particle size ranging 1mm-100um were obtained. The fraction smaller than 1 00 um are f eeded to crystallizator as a template. Another problem that encountered during the sodium perborate tetrahydrate, production is scale formation on the walls of crystallizator. Although the scale was continuously scrubed by scruber, its formation can not be prevented and therefore the production was stopped for washing. It was though that the reason of the scale formation is the negative charge of the sodium perborate tetrahydrate crystal surface which were measured as -Zkm/l and rate of scale formation was reduced by controlling this charge. The scope of this study is to investigate the affects of the presence of excess sodium metaborate, mixing of the reactants, hydrodynamic conditions and the effects of polyelectrolytes and direct current applied to crystalli zator which were used to modifie the surface charge of the crystals. In order to prepgre a reasonable pure sodium metabora te, borax pentahydrate and sodium hydroxide were treated by distilled water in the presence of sodium carbonate in 50 1 stainless steel reactors. By this way, it is possible to obtain the solution having metaborate content saturated at 50°C. Mixing this solution were conducted VIat 90DC-100DC for jtuia hours, fallowed by filtration and Eooling to room temperature to crystallize sodium metaborats tetrahydrate. These crystalls mere used to prepare the solution having 15 % (by weight) metaborate by dissolving in distilled water. Hydrogen peroxide solution is prepared from 7D % solution using approprate water in order to obtain 15% solution. In order to determine the content of mother liqour alkalinity, boron and active oxygen analysis were conduc ted and the fallowing equations were used. g/1 NaBD3 = 4D.9 S^/V g/1 NaB02 = 32.9(2S1 N,j -S3N3/V = 32.9(2S2N2-S3N3)/// in this /l = Sample volume in titration S. = [/L, normality of HC1 5`= Njp, normality of NaOH 53= N3, normality of KMnO, Crystal quality was checked by determination of bulk density, sieve analysis and suspension density. Experiments were conducted by the following experi mental set-up which was designed according to the one used in industrial production.. Experimental set-up is shown in Figure 4.4. Effect of surface charge is not well-underatoad in crystallization theory. In spite of this, many applica tion were known in precipitation, eventhough they were not systematic. Surface charge of the sparingly soluble salts were easily measured and therefore this effect may easily be evaluated in precipitation. Measurement of the surface charge of readily soluble salts are not easy in their saturated solution and it requires special measure ment technics. This is the main reason why the surface charge effect was omitted in crystallization. Sayan showed that crystallization of boric acid can be modified and directed to any desired direction by controlling the surface charge by means of Zeta potential Vllmeasurements, İt is still not known haw his results have the general character. Dne of the main aim of this work is to see the same effect in sodium perborate tet- rahydrate crystallization which is different than Sayan's cooling crystallization. Polyelectrolytes effect on the many step of cryst- tallization. These steps are nuclei formation, nuclea- tion rate, crystal growth rate, agglomeration rate, scale formation etc. Sub-nuclei particles which have surface charge can not grow to the critical size and therefore7 highly supersaturation can be obtained. This electrical forces may be decreased or increases by means of poly electrolytes having different charge density. Similarly, non-charged particles may be charged by the same kind of aditives. Crystal growth rate may be directed, i.e. decreased or increased at the vicinity by giving excessive surface charges. Surface charge of crystals may cause; increased scale formation rate, if the crystallizer body gained the opposite charge in the electrolytic media. Since the all step of crystallization is effected by surface charge, modification of this charge by paly- electrolytes should give different way of crystallization, Polyelectrolytes used in the experiments are listed in table 4.1. Surface charge may also be controlled by the elect rical field applied on the crystallizer body. DC current giving on the crystallizer is expected to charge the way of crystallization, similar to polyelectrolyte effect. In order not to cause the electrolysis in the crystallizer, voltage applied to the crystallizer should be below approximately 1000 mU. Therefore in the experiments DC current in the range of 50-500 ml/ were used. During the experiments feeding way of reactants, Sampling technic in this pilot plant crystallizer were evaluated by the trial and error method during the ex periments. Experiments showed that one of the important point is the place where hydrogen peroxide was injected. Solution velocity which effect to fluidization condition and therefore to the average particle size of the product taken from crystallizer were evaluated by experimentally in the suspension density valid in fluidi zation zone of crystallizer. This velocity is standar dized by a float-type flowmeter and the results are shown in table 5i1. VlllGeneral results obtained during the experiments are given beloui. 1- Bulk density of the crystals which characterize the quality of the product is the function of free sodium metaborate concentration in the reaction media. Increas ing the sodium metaborate concentration up to 3 % causes to increase the bulk density, as it is given in figure 5.2. 2- Feeding of hydrogen peroxide should be different than practiced in industry. In order to create perfect mixing of this reactant, it should be fed before entering the crystallization zone. 3- Application of DC voltage to the crystallizer body caused to charge the crystallization kinetics and agglomeration rate. k- Polyelectrolytes at 6 ppm level causes to charge the crystallization as expected. 5- Scale formation in the crystallizer were control led by either application of DC voltage on crystallizer body orbyadding polyelectrolytes. These results showed that one of the main reason for scale formation is the surface charge of the particles.