Holistic approach to dissolution kinetics : linking direction-specific microscopic fluxes, local mass transport effects and global macroscopic rates from gypsum etch pit analysis

Dissolution processes at single crystal surfaces often involve the initial formation and expansion of localized, characteristic (faceted) etch-pits at defects, in an otherwise comparatively unreactive surface. Using natural gypsum single crystal as an example, a simple but powerful morphological analysis of these characteristic etch pit features is proposed that allows important questions concerning dissolution kinetics to be addressed. Significantly, quantitative mass transport associated with reactive microscale interfaces in quiescent solution (well known in the field of electrochemistry at ultramicroelectrodes) allows the relative importance of diffusion compared to surface kinetics to be assessed. Furthermore, because such mass transport rates are high, much faster surface kinetics can be determined than with existing dissolution methods. For the case of gypsum, surface processes are found to dominate the kinetics at early stages of the dissolution process (small etch pits) on the cleaved (010) surface. However, the contribution from mass transport becomes more important with time due to the increased area of the reactive zones and associated decrease in mass transport rate. Significantly, spatial heterogeneities in both surface kinetics and mass transport effects are identified, and the morphology of the characteristic etch features reveal direction-dependent dissolution kinetics that can be quantified. Effective dissolution velocities normal to the main basal (010) face are determined, along with velocities for the movement of [001] and [100] oriented steps. Inert electrolyte enhances dissolution velocities in all directions (salting in), but a striking new observation is that the effect is direction-dependent. Studies of common ion effects reveal that Ca2+ has a much greater impact in reducing dissolution rates compared to SO42−. With this approach, the new microscopic observations can be further analysed to obtain macroscopic dissolution rates, which are found to be wholly consistent with previous bulk measurements. The studies are thus important in bridging the gap between microscopic phenomena and macroscopic measurements

The Effectof polyelectrolytes on sodium perborate crystallization

Tez (Doktora)-- İTÜ Fen Bil. Enst., 1997Sodyum metaborat ve hidrojen peroksit çözeltilerinden reaksiyonla kristalizasyon ile üretilen sodyum perborat tetrahidrat, ağarncı özelliğinden dolayı deterjan üretiminin önemli girdilerinden birisidir. Sodyum perborat tetrahidrattan beklenen fiziksel özellikler, kristalizasyon koşullanma, ortam stokiometrisinin ve katkıların etkisi ile yönlendirilebilmektedir. Aynca, sodyum perborat tetrahidratta olduğu gibi, yüzey yükü gösteren tüm tuzların kristalizasyonuna yüzey yükünün de etkisi olmalıdır. Bu nedenle, bu çalışmada, sodyum perborat tetrahidratin ürün kalitesinin, kullanılan polielektrolitierle ne şekilde değiştirilebildiği ve uygun polielektrolitin geliştirilmesi amaç edinilmiştir. Polielektrolitier seçilirken aynı yapıda, farklı yük yoğunluğuna sahip katyonik, anyonik ve non iyonik polielektrolitier obuasına dikkat edilmiştir. Kullanılan deney sistemleri, akışkan yataklı kristalizör ve MSMPR tipi kristalizördür. MSMPR tipi kristalizörde elde edilen ürün üzerinde ise, tane dağılımı, döküm yoğunluğu ve aşınma dereceleri tayin edilmiştir. Aynca hem akışkan yataklı kristalizörde ölçülen büyüme ve çözünme hızlan sonucunda hücreden alınan kristallerin, hem de MSMPR tipi kristalizörde yapılan sürekli kristalizasyon sonucunda üretilen ürün perboratın her fraksiyonun mikroskop fotoğraflan çekilmiştir. Deneysel çalışmaların değerlendirilmesi sonucunda, sodyum perborat tetrahidrat kristallerinin kendi doygun çözeltilerinde negatif yüzey yükü gösterdiği, sodyum metaborat fazlalığındaki artışın ise sodyum perborat tetrahidratin yüzey yükünü hem şiddet hem de işaret olarak değiştirdiği görülmüştür. Sodyum perborat tetrahidrat kristallerinin büyümesi, birinci dereceden bir fonksiyonla yüzey reaksiyon kontrollü olup, büyüme yüzey yüklü noktalardan olmaktadır. %2.5 NaBC>2 fazlalığında yürütülen deneyler sonucunda, bu şartlarda, ürün özelliklerini düzeltmek amacıyla anyonik veya katyonik polielektrolitlerin kullanılabileceği görülmüştür. Ancak, kullanılan polielektrolitin anyonik veya katyonik oluşuna ve konsantrasyonuna bağlı olarak, olayda ya adsorpsiyon ya da yüzey yükünün cinsi veya her ikisi birden etkin olabilmektedir. %2.5 NaBOa fazlalığında sodyum perborat tetrahidrat kristalleri pozitif yük taşıdıklarından, adsorpsiyon etkisi yanında, katyonik polielektrolit kullanımında itme kuvvetleri, anyonik polielektrolit kullanımında ise yük nötralizasyonu görülmektedir. %2.5 NaB02 fazlalığı içeren ortamda yürütülen sodyum perborat tetrahidrat kristalizasyonunda, yüzey yükü nedeni ile oluşan dentritik görünümlü kristal büyümesi, düşük anyonik veya katyoniklik derecesine sahip polielektrolitlerin katkısıyla engellenebilir. Ancak bu durumda etki şekli polielektrolit konsantrasyonuna bağlıdır. Yüksek anyoniklik derecesine sahip polielektrolit kullanımında ise, dentritik görünüm engellenmesi ve kristal görünümünün düzeltilebilmesi, polielektrolit konsantrasyonundan bağımsız olarak elde edilebilmektedir. Sonuç olarak, ağırlıkça %2.5 NaBO2 fazlalığı kullanılarak yürütülen sodyum perborat tetrahidrat kristalizasyonunda, kristal şeklini düzeltmek, mekanik mukavemetini arttırmak için, oldukça yüksek anyonikliğe sahip polielektrolit kullanıhnalıdır. in this study, the effect of polyelectrolytes on the sodiıım perborate crystallization which is produced by reaction crystallization of sodium metaborate and hydrogen peımide solutions, was investigated. Factors affecting the crystallization of sodium perborate tetrahydrate are numerous. Besides supersaturation ratio, mixing intensity, crystallization temperature and effects of impurities which are normally encountered in ali crystallization processes, ratio of reactants in the following equation: NaBO2+ H2O2+ 3 H20 -» NaB03.4H20 are also very important. in addition to this main equation, preparation of sodium metaborate from sodium hydroxide and borax by the following equation: Na2B4O7 + 2 NaOH -> 4 NaB02 + H20 impose some additional parameters such as excess of borax ör sodium hydroxide in metaborate solution. Excess of some reactants may result 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 stabüise the sodium perborate tetrahydrate affect the phenomena depending on the concentration of magnesium silicate and excess öne of the components. Sodium perborate tetrahydrate crystals have surface charge in its saturated solutions and it is possible to obtain a product with desired properties by the changing of surface charge of sodium perborate crystals. The effects of additives on crystallization can take place by different mechanisms. However, in ali these mechanisms the connection of these additives by chemical and physical forces on the crystal surface is the main stage. The usage of additives from the same group to prevent the effects of different adsorption mechanisms, is the best way to investigate the importance of surface charge. in this respect, polyelectrolytes are good type of additives. The polyelectrolytes which were used in the experiments5 are anionic, cationic and nonionic polyelectrolytes which have different charge density. in this study, two different measurement techniques for the growth rate are used. They are fluidised bed and MSMPR type crystallizers. The fluidised bed xxii crystallizer is shown in Figüre 5.1. The system is made of glass and it has 20 liter solution storage tank. The solution is circulated through the system continuously by a magnetic drive pump. The fluidisation celi has 200 mm height and 18 mm inner diameter. 3 g ±10 mg sodium perborate tetrahydrate seed crystals were put into celi for each experiment. The growth and solubility experiments were lasted 12 minutes. Supersaturation which is necessary for the experiments was obtained by keeping the temperature of solution which passed through the celi lower than the temperature of saturation temperature of the solution. At the end of the experiments, the crystals, which were taken from the celi, filtered, dried in the air and weighed. The growth and dissolution rates were calculated depending on the equation 5.5. in order to investigate the effect of particle size in stoichiometric condition in fluidised bed crystallizer, sodium perborate tetrahydrate with three different particle size was used. 0.9%, 2.2% and 4.23% (w/w) excess sodium metaborate were used in the fluidised bed experiments to observe the effect of excess sodium metaborate in the growth rate. At the end of the experiments, it was decided that the most appropriate working condition was to use 2.5% (w/w) excess sodium metaborate. Therefore, the experiments in which the effect of polyelectrolytes on the growth rate were investigated were taken place in this condition. in the experiments, which the effect of polyelectrolytes on the grovvth rate were investigated, anionic, cationic and non ionic polyelectrolytes which have different charge density were used. The properties of the polyelectrolytes which were used in the experiments are shown in Table 6. l. The other technique for the measurement of growth rate is MSMPR type crystalHzer. This system is made of plexiglass and has 2 liter active volume. in order to provide a homogeneous mixture, three baffles and a draft-tube were established into the crystallizer. Moreover, the bottom of the crystallizer was designed in W- shape for the same purpose. in order to prevent the decomposition of sodium perborate, metallic parts were not used in the system. The heat of reaction which released during crystallization was taken away by a glass heat exchanger. So that the temperature inside of the crystallizer was kept at 20°C. Stirring was provided by a stirer with three wings. This stirrer was established into the heat exchanger which has glass spirals. Sodium metaborate and hydrogen peroxide solutions were fed by a peristaltic pump. Polyelectrolyte solutions were fed continuously from a different channel. The products were taken out by a vacuum pump. in each experiment, eight fold of residence time in the crystalHzer was used in order to reach the steady state. At the end of the experiment, the characteristic suspension sample was taken out. The residence time was calculated by the equation 5.13. The feeding solutions consisted of metaborate, hydrogen peroxide and polyelectrolyte solutions. The crystal products, which were obtained after the filtration of suspension sample, were washed with ether saturated by perborate and then they were left at room temperature to dry. The sieve analysis was applied to crystals after drying. The volume of the solution which was obtained after filtration was measured in order to calculate the suspension density and then chemical and physical analysis were applied. 12% (w/w) püre sodium metaborate, 15% (w/w) hydrogen peroxide and 0.05% (w/w) polyelectrolyte solutions were used in the experiments. The bulk density and the attrition degree which show the qualification of sodium perborate were determined for the dried crystal products. After the sieve analysis, the volume shape factors of each fraction were determined. The population xxiii density values are obtained when ali the results are applied to equation 5.21. The slope of the straight line which is obtained from Ln n-L graph gives the growth rate and the intercept gives the nuclei density. General results obtained from experiments are shown as follows: 1. The change of growth rate of sodium perborate tetrahydrate with respect to supersaturation gives first order fimction and growth of sodium perborate crystals is controlled by surface reaction. 2. The dissolution rate of sodium perborate tetrahydrate does not change with the partide size. But as the partide size increases the growth rate decreases. 3. The presence of excess sodium metaborate more than 2.2% (w/w) in the sodium perborate tetrahydrate crystaUization media deppress the growth and dissolution rate comparing to püre media. 4. The growth rate of sodium perborate tetrahydrate in the presence of 2.5% (w/w) excess sodium metaborate, changes linearly with supersaturation in the supersaturation region of AÇ >0. l g salt/100 g saturated solution. 5. Sodium perborate tetrahydrate crystals show negative surface charge in its own saturated solution. 6. Sodium perborate tetrahydrate grows from the surface charged points. in the presence of 2.5% (w/w) excess sodium metaborate, the surface charge causes to shift the growth and dissolution rates about 0.292* l O"8 m/s. 7. The growth rate of sodium perborate tetrahydrate shows a decreasing tendency as the amount of excess sodium metaborate increases upto 1% (w/w). And then the deppresing in the growth rate of sodium perborate tetrahydrate begins to decrease as the amount of excess sodium metaborate increases upto 2.2% (w/w). Concentration of excess sodium metaborate higher than 2.5% (w/w) leads to an increase in the growth and dissolution rates. Therefore, the increasing in the concentration of free sodium metaborate causes to change the surface charge of sodium perborate both in magnitude and sign. 8. For the negative values of the surface charge of sodium perborate tetrahydrate, the metastable region width becomes narrover and for the positive values it becomes larger. 9. As polyelectrolytes are water soluble organic materials, they change the viscosity of the solution. it was observed that as the concentration of polyelectrolyte increases, the viscosity of the solution increases. The effect of cationic polyelectrolytes into the viscosity of solution is less effective with respect to anionic polyelectrolytes. Cationic and anionic polyelectrolytes whose concentrations are in between 10-100 ppm change the viscosity of solutions in between 1.04-1.08 cSt and 1.05-1.40 cSt, respectively. 10. The increasing in the ionic charge of the anionic and cationic polyelectrolytes, which have same molecular weight, does not lead to any appreciable changing in the viscosity of the sodium perborate solution. 11. Although the adsorptive forces in the presence of both 2.5% (w/w) excess sodium metaborate and low concentration of polyelectrolytes which have low degree of cationicity (2%), partial adsorption takes place because repelling forces are also low. As the non ionic parts of polyelectrolytes are greater, these parts of polyelectrolytes cover many areas including charge centers. At higher concentrations of polyelectrolytes repelling forces becomes dominant. 12. As the ionic charge of cationic polyelectrolytes increases, adsorption becomes to take place at higher concentrations of polyelectrolytes. xxiv 13. The tendency of adsorption decreases as the degree of cationicity of polyelectrolyte increases and it increases as the concentration of the polyelectrolyte increases. 14. The adsorption is extremely effective when 2.5% (w/w) excess sodium metaborate and anionic polyelectrolyte, which has low degree of anionicity (3%), are used in the crystallization media. During this adsorption, surface charge of particle decreases because of the opposite charge of polyelectrolyte. As the concentration of polyelectrolyte increases, adsorption layer becomes thicker and therefore charge neutralization becomes less effective. 15. The adsorption is effective when the polyelectrolyte used has low degree of anionicity. The effect of adsorption decreases as the degree of anionicity of polyelectrolyte increases, but the effect of neutralization increases. The surface charge can be removed when a polyelectrolyte that has 50% anionic charge is used. 16. Adsorption and charge neutralization compete to each other when anionic polyelectrolyte is used in the presence of 2.5 %(w/w) sodium metaborate. 17. The growth rate and nuclei density were found as 3.076* 10*8 m/s and 8.523* 1014 respectively when the crystallization of sodium perborate tetrahydrate was carried out by using 12% (w/w) sodium metaborate, 15% (w/w) hydrogen peroxide and 2.5% (w/w) sodium metaborate. In addition, the attrition degree and bulk density which determine the quality of the product were found 17.62% and 658 kg/m3, respectively. 18. In the crystallization in which canonic polyelectrolyte are used, sodium perborate tetrahydrate crystals which have positive surface charge are affected by the polyelectrolytes at the low degree of cationicity where the adsorption is possible. This effect is disappeared when a polyelectrolyte that has high degree of cationicity (55%) is used. 19. The total effect of growth and nucleation rates in the continuous crystallization increases the average particle size in the region where the growth rate is high and the nucleation rate is low. The average particle size decreases as the charge effect is being increased and it reaches to the original size at the high degree of cationicity where the adsorption is not possible. 20. The increase in surface charge causes to accelerate the so-called surface nucleation phenomena, decreases the bulk density and increases the attrition degree. 21. In the crystallization in which anionic polyelectrolyte are used, the strength of the crystal increases as the degree of anionicity of the anionic polyelectrolyte is increased upto 10%. As a result, the nuclei density becomes lower and the average particle size increases. 22. The structure of the product which is obtained from the continuous crystallization weakens when the polyelectrolyte that has 30% anionic charge degree is used. Therefore, the nuclei density increases and the overall effective growth rate decreases as the seconder nucleation accelerates. 23. The strength of the crystal increases as the degree of anionicity of polyelectrolyte increases more than 50%. Therefore, visible growth rate and average particle size increase as a result of slower secondary nucleation. 24. The dentritical growth is observed in the product which is obtained in the presence of polyelectrolyte that has 30% anionic charge. At the higher anionic charge values, more compact crystals, which has not dentritic growth, are obtained. 25. The change in the molecular weight of anionic polyelectrolyte which is used in the continuous crystallization does not affect the bulk density of the product. XXV 26. As the molecular weight of the anionic polyelectrolyte increases, the strength of the crystals increases and therefore the attrition degree decreases. 27. As the molecular weight of the anionic polyelectrolyte increases, the charge neutralisation becomes better. The recommendable working conditions for the crystallization of sodium perborate tetrahydrate are summarized below depending on the results which were obtained from both fluidised bed and continuous crystallization: a) In the crystallization of sodium perborate tetrahydrate in the presence of 2.5% excess sodium metaborate, the growth of dentritic crystal can be prevented by the help of polyelectrolytes which have low cationic or anionic degree. However, in this situation the type of effect depends on the concentration of polyelectrolyte. b) In the crystallization of sodium perborate tetrahydrate in the presence of 2.5% (w/w) excess sodium metaborate, the prevention of the dentritic appearance and the reforming of the crystal appearance are obtained independently from the polyelectrolyte concentration when a polyelectrolyte which has high anionic degree is used. c) As a result, in the crystallization of sodium perborate tetrahydrate in the presence of 2.5% excess sodium metaborate, a polyelectrolyte which has considerably high anionic must be used to reform the shape and to increase the mechanical strength of the crystals.DoktoraPh.D

Effect of trace metals on reactive crystallization of gypsum

The effect of Fe2+, Fe3+, and Cr3+ ions on crystallization of calcium sulfate dihydrate (gypsum) produced by the reaction between calcium hydroxide suspension and sulphuric acid solution was investigated at 3.5 pH and 65 degrees C in the absence and presence of 2500 ppm citric acid concentration. Crystal size distributions, filtration rates, and morphology of gypsum were determined and discussed as a function of ion concentration. Average particle size of gypsum was not affected significantly by the presence of Fe2+, Fe3+, and Cr3+ ions individually. Variation of gypsum morphology depending on ion concentration affected the filtration characteristics. The presence of Fe3+ or Cr3+ ions besides 2500 ppm citric acid influenced both average particle size and filtration characteristics. The effect of citric acid on gypsum morphology was suppressed at high Fe3+ and Cr3+ ion concentrations. The change of morphology is related to the complex formation between Fe3+ or Cr3+ ions and citric acid at high ion concentrations. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim