Delivered at the Crystal Engineering to Crystal Growth: Design and Function Symposium, ACS 223rd National Meeting, Orlando, Florida, April 7-11, 2002
CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 5 371-374
Influence of Additives on the Width of the Metastable Zone S. Titiz-Sargut† and J. Ulrich*,‡ Istanbul Technical University, Faculty of chemical and Metallurgical Engineering, Chemical Engineering Department, Istanbul, Turkey, and Martin-Luther-Universita¨ t Halle-Wittenberg, FB Ingenieurwissenschaften, Institut fu¨ r Verfahrenstechnik/TVT, Halle, Germany Received April 6, 2002
ABSTRACT: The width of metastable zone of unseeded potassium alum solutions was determined for different conditions and in the presence of additives. The measurements were taken at three different cooling rates and for three different saturation temperatures at a cooling rate of 10 K/h by means of an ultrasonic measuring technique. The influence of Cr3+, Cu2+, and Mg2+ ions on the width of metastable zone of a potassium alum solution was investigated at a cooling rate of 5 K/h. The influence of Mg2+ ions on the width of metastable zone of potassium chloride solutions was also investigated at a cooling rate of 10 K/h. Cr3+, Cu2+, and Mg2+ ions show both enlargement and suppression effects on the width of metastable zone of potassium alum and potassium chloride solutions depending on their concentrations. Cr3+, Cu2+, and Mg2+ ions also change the saturation temperature of potassium alum solutions depending on their concentrations. Introduction Supersaturation is the driving force for both the nucleation and the crystal growth step in a crystallization process. Every solution has a maximum supersaturation limit that is defined as metastable zone width (MZW). The knowledge of MZW is very important in terms of designing crystallization processes and obtaining desired crystal sizes, shapes, and purities. For this reason, the supersaturation level should be controlled and the crystallizer has to be operated at an optimum supersaturation level that is approximately half of the width of the metastable zone. However, the supersaturation limit in contrast to the saturation limit is thermodynamically not defined. It depends on temperature, cooling rate, stirring rate, the presence of impurities, type of measuring techniques, etc. The influence of impurities on MZW is poorly understood and cannot be clearly predicted to date. It has long been known, for example, that the presence of small amounts of colloidal substances such as gelatin can suppress nucleation in aqueous solutions, and certain surface-active agents also exert a strong inhibiting effect. Traces of foreign ions, especially Cr3+ and Fe3+ can have a similar action on inorganic salts.1 The presence of soluble impurities can also affect the MZW, but it is virtually impossible to predict the effect. Effects of soluble impurities may be caused by changing the equilibrium solubility or the solution structure, by adsorption or chemisorption on nuclei or heteronuclei, † ‡
Istanbul Technical University. Martin-Luther-Universita¨t Halle-Wittenberg.
and by chemical reaction or complex formation in the solution. The effects of insoluble impurities are also unpredictable so far.1 The type of measuring technique is very important on determination of MZW. The most widely applied method proposed by Nyvlt is based on the measurement of the temperature of the first crystal formation in the solution.2 According to this technique, the first nuclei formation is observed visually. However, since this method is dependent on the subjectivity of the observer, the illumination and the transparency of the solution, discrepancies on the MZW can occur. For this reason, the change of temperature dependence on some physical properties such as conductivity, density, refractive index, etc. has been determined to measure MZW recently. Nyvlt et al. determined the supersaturation of aqueous solutions of potassium alum during a batch crystallization experiment by means of conductometry and refractometry.3 Dunuwila and Berglund have used ATR FTIR spectroscopy for in situ measurement of supersaturation.4 The ultrasonic measuring technique proposed by Omar and Ulrich is one of the methods that enables to determine supersaturation and metastable range.5 They have reported that this technique is a new and ideal method for in situ determination of the crystallization parameters and more preferable because of being independent of the transparency of the medium, reliable, robust and simple in application. In this contribution, the ultrasonic measuring technique was tested for potassium alum (KAl(SO4)2‚ 12H2O). The influence of cooling rate, saturation temperature and impurities on the MZW of potassium alum
10.1021/cg020011z CCC: $22.00 © 2002 American Chemical Society Published on Web 08/08/2002
372
Crystal Growth & Design, Vol. 2, No. 5, 2002
Titiz-Sargut and Ulrich
Figure 1. Influence of the cooling rate on the width of the metastable zone of potassium alum.
Figure 2. Determination of the limits of the metastable zone of potassium alum by means of the ultrasonic measuring technique at a cooling rate of 10 K/h.
was investigated. Cr3+, Cu2+, and Mg2+ ions were chosen as impurities. The influence of Mg2+ ions on the MZW of potassium chloride was also investigated using the same technique.
Table 1. Saturation and Nucleation Temperatures Determined by Means of the Ultrasonic Measuring Technique and Optically Observations
Experimental Procedure The ultrasonic velocity and the temperature of the prepared solutions were measured using a immersed LiquiSonic 30 sensor developed by SensoTech GmbH, Magdeburg, Germany. The precision of this system for ultrasonic velocity and for temperature is ( 0.01 m/s and ( 0.01 °C, respectively. Measurements of MZW were performed in a 0.5 L jacketed glass crystallizer. Temperature control of the solution in reactor was maintained by a programmable thermostatic bath. Stirring was accomplished by a magnetic stirrer and the stirring rate at each experiment was kept constant to avoid an effect of stirring rate on MZW. The different experimental solutions were prepared at desired saturation temperatures by adding analytical grade potassium alum or potassium chloride to deionized water according to the solubility data.6 Then the solution was heated 5 °C above the saturation temperature and kept there for 30 min. The solution was cooled at a constant rate until nucleation occurred. Thereafter, the solution was heated at the same rate until all crystals were dissolved. The response of the ultrasonic velocity during this cycle can be used to determine the nucleation and saturation temperature as described by Strege et al.7 The difference between saturation and nucleation temperatures was taken as MZW at arbitrary conditions.
Results and Discussions To assess the applicability of the ultrasonic measuring technique to determine the MZW of potassium alum, the experiments were carried out using solutions saturated at 30 °C for three different cooling rates. The measured change in the ultrasonic velocity of potassium alum solutions was shown in Figure 1. As can be seen in Figure 1, the ultrasonic velocity decreases linearly with decreasing temperature. When nucleation happens, a sharp decrease in ultrasonic velocity can be observed. During the following heating cycle, the ultrasonic velocity increases and intercepts the cooling curve when all crystals are dissolved. The temperature of this intersection shows the saturation point. A characterization of the whole MZW as a function of the measured variables (ultrasonic velocity and temperature) is essential to obtain the desired quality of product and to achieve a good reproducibility. For this purpose, the MZW should be characterized at different equilibrium temperature ranges due to changes in the solution
C (g/100 g of water)
Tsat (optically) (°C)
Tsat (ultrasonics) (°C)
Tnuc (optically) (°C)
Tnuc (ultrasonics) (°C)
11.4 14.14 16.58
20.6 27.3 31.2
20.2 26.3 30.4
9.5 17.2 21.4
9.0 16.2 21.0
concentration during of crystal growth. The metastable zone width is not the same at all temperature levels. The experiments were carried out with potassium alum solutions saturated at three different temperatures at a cooling rate of 10 K/h. The results are shown in Figure 2. Such measurements obtained from the measured ultrasonic velocity and temperature of the supersaturated solution can be used to determine the level of supersaturation at which a crystallizer has to be operated. The saturation and nucleation temperatures obtained from these experiments by means of the ultrasonic measuring technique were compared to those of an optically observation. The results are given in Table 1. As can be seen from Table 1, there is a systematic difference of ( 0.5-1.0 °C in the saturation and nucleation temperatures determined by means of ultrasonics compared to the optically obtained ones. These differences depend on the sensitivity of the sensor. Because the sensor needs a certain number of nuclei to detect the presence of the solid phase in the solution. These differences cannot be a problem since the MZW of potassium alum is wide. This differences can be reduced by changing the properties of sensor such as its frequency or amplitude. The accuracy and applicable of this measuring technique has been tested also, e.g., for potassium chloride previously.7 In many instances, small amounts of impurities have dramatic effects on crystal growth, morphology, and nucleation. In this contribution, the influence of Cr3+, Cu2+, and Mg2+ ions on the MZW of potassium alum solution saturated at 30 °C at a cooling rate of 5 K/h and the influence of Mg2+ ions on the MZW of potassium chloride solution saturated at 30 °C at a cooling rate of 10 K/h were investigated using ultrasonic measuring technique. Figures 3, 4, 5, and 6 show these effects, respectively. Figure 3 shows that the MZW of potassium alum increases until approximately an ion concentration of 25 ppm Cr3+ and then decreases with the increasing concentration of Cr3+ ions. The similar tendency can
Influence of Additives on the Width of the Metastable Zone
Figure 3. The variation of the width of metastable zone of potassium alum in the presence of Cr3+ ions.
Figure 4. The variation of the width of metastable zone of potassium alum in the presence of Cu2+ ions.
Figure 5. The variation of the width of metastable zone of potassium alum in the presence of Mg2+ ions.
also be seen in Figures 4 and 5 in the presence of Cu2+ and Mg2+ ions. The MZW of potassium alum increases up to an ion concentration of 100 ppm Cu2+. By addition of more than 100 ppm Cu2+ ions, the MZW of potassium alum decreases. However, it can be seen that an ion concentration of 2.5 ppm Mg2+ is enough to enhance the MZW of potassium alum from 6.89 to 26.26 K. As can be seen from Figure 6, the MZW of potassium chloride increases up to an ion concentration of 25 ppm Mg2+. Afterward, the MZW of potassium chloride decreases with the increase of the Mg2+ ion concentration. The MZW decreases linearly in the range of 100-500 ppm Mg2+ ion concentration. This type of tendency is not common, but the similar effect has been reported by Mullin and Jancic in the presence of Cr3+ ions for
Crystal Growth & Design, Vol. 2, No. 5, 2002 373
Figure 6. The variation of the width of metastable zone of potassium chloride in the presence of Mg2+ ions.
potassium alum solution.8 They have investigated the influence of Cr3+ ions on the MZW of seeded potassium alum solution at 3 K/h cooling rate. However, they reported no mechanism as explanation. Sayan and Ulrich have investigated the influence of Cu2+ and Mg2+ ions on the MZW of ammonium sulfate in the range of 0-20 ppm and they have reported that both impurities have an enlargement effect on the MZW of ammonium sulfate.9 They have also investigated the influence of Mg2+ ions on the MZW of boric acid which has a very narrow MZW as in the case of potassium chloride.10 They have found that the MZW of boric acid decreases linearly with the increase of concentration of Mg2+ ions in the range of 200-1000 ppm. Unfortunately, it is not possible to compare the effect of Mg2+ ions on the MZW of potassium alum and potassium chloride and literature data here on are not available. It has been determined that the difference between nucleation temperatures determined by means of the ultrasonic measuring technique and the optically observation in the presence of impurities is narrow compared to pure solutions. The influence of impurities on the MZW can take place by different mechanisms. In general, the enlargement effect of impurities on MZW can be explained based on the adsorption of impurity molecules on the surface of subcritical embroyes in the solution. These impurities suppress the growth of embroys to larger than critical size and induce the enlargement of MZW. On the other hand, the presence of impurities can increase the formation of clustering in the solution. This induces promotion of nucleation and tends to narrow MZW. In small amounts, the suppressing effect on nucleation is dominant in larger amounts the structuring of the solution. Besides the decreasing and enhancement effect of impurities on MZW, the presence of impurities may change the equilibrium solubility or the solution structure. In the literature, there exist suggestions that even traces of Cr3+ ions can significantly affect the solubility of potassium sulfate (e.g., Ulrich and Stepanski).11 They showed that the solubility of potassium sulfate in the presence of Cr3+ ions is always lower than the equilibrium solubility. In this study, it has also been seen that the impurities investigated change the saturation temperature of potassium alum solutions depending on impurity concentrations. Figure 7 shows the change of saturation temperature of potassium alum solution corresponding to the concentration of Cr3+. As
374
Crystal Growth & Design, Vol. 2, No. 5, 2002
Titiz-Sargut and Ulrich
alum or of potassium chloride. All impurities investigated have both enlargement or suppression effects on the width of metastable zone of both substances potassium alum and potassium chloride depending on impurity concentration content. The impurities also affect the saturation temperature, e.g., at potassium alum solutions depending on their concentrations. It means the amount of impurities should be controlled in the crystallization process of potassium alum and potassium chloride to obtain crystal products with desired properties such as crystal shape and purity as a result of the nucleation and growth processes.
Figure 7. The variation of saturation temperature of potassium alum in the presence of Cr3+ ions.
can be seen from Figure 7, there is a tendency which is contrary to the change of MZW of potassium alum with increasing concentration of Cr3+. Since, MZW is a function of concentration, the change of equilibrium saturation temperature tends to change of the MZW of potassium alum depending on an ion concentration of Cr3+. As can be seen in Figures 3 and 7, the MZW increases with decreasing the equilibrium saturation temperature and decreases with increasing the equilibrium saturation temperature. The similar effect on the equilibrium saturation temperature and the MZW of potassium alum also could be seen in the presence of Cu2+ and Mg2+ ions. It has been determined experimentally that the added amounts of Mg2+ ions have a negligible effect on the saturation temperature of potassium chloride solution. Although it is difficult to give general explanations of the phenomenon of nucleation suppression or enhancement by the use of impurities, based on all the results obtained in here, the concentration of impurities investigated is very effective on the enlargement or suppression of the MZW besides the type of impurities. Therefore, the relationship between the MZW and impurities must be known and the concentration of impurities in solution should be controlled to obtain crystal products with desired physical properties because all crystallization processes take place always in the metastable zone. Conclusion The ultrasonic measuring technique can be used to determine the width of metastable zone of potassium
Acknowledgment. The authors gratefully acknowledge the support for parts of the works which led to the results by the EU and by the Volkswagen Foundation, respectively. References (1) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heineman, Oxford, UK, 1993. (2) Nyvlt, J. J. Cryst. Growth 1968, 3-4, 377. (3) Nyvlt, J.; Karel, M.; Pı´saøı´k, S. Cryst. Res. Technol. 1994, 29-3, 409-415. (4) Dunuwila, D. D.; Berglund, A. K. J. Cryst. Growth 1997, 179, 185-193. (5) Omar, W.; Ulrich, J. Application of ultrasonics in the control of crystallization processes, 4th International Workshop on Crystal Growth of Organic Materials; CGOM 4 Ulrich, J., Ed.; Shaker Verlag: Aachen, Germany, 1997; pp 294-301. (6) Seidel, A.; Linke, W. F. Solubilities of Inorganic and MetalOrganic Compounds, 4th ed.; McGregor and Werner Inc., Washington, DC, 1958; p 261. (7) Strege, C.; Omar, W.; Ulrich, J. Measurements of metastable zone width by means of ultrasonic devices, 7th international Workshop on Industrial Crystallization, BIWIC, Ulrich, J., Ed.; Martin-Luther-Universita¨t Halle-Wittenberg, 1999; pp 219-230. (8) Mullin, J. W.; Jancı´c, S. J. Trans IChemE, 1979, 57, 188193. (9) Sayan, P.; Ulrich, J. Determination of the metastable zone width of ammonium sulfate in the presence of impurities by an ultrasound technique, 8th International Workshop on Industrial Crystallization, BIWIC, Jansens, P., Kramer, H. Roelands, M., Eds.; Doc Vision Delft, Delft, Netherland, 2001; pp 269-276. (10) Sayan, P.; Ulrich, J. Cryst. Res. Technol. 2001, 36, 4-5, 411-417. (11) Ulrich, J.; Stepanski, M. The effect of additives on the growth behaviour of reaction diffusion controlled growing crystals, In Industrial Crystallization 78, Nyvlt, J., Zacek, S., Eds.; Academia Praha, Praha, 1989; p 253.
CG020011Z
