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The crystallization kinetics of potassium chloride in the presence of additives with common ions
Journal of Crystal Growth 383 (2013) 112–118
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
The crystallization kinetics of potassium chloride in the presence of additives with common ions Ayhan Abdullah Ceyhan a,n, A.Nusret Bulutcu b a b
Seljuk University, Aladdin Keykubat Campus, Faculty of Engineering and Architecture, Chemical Engineering Department, Konya Turkey Istanbul Technical University Department of Chemical Engineering, Istanbul, Turkey
art ic l e i nf o
a b s t r a c t
Article history: Received 19 March 2012 Received in revised form 26 July 2013 Accepted 20 August 2013 Communicated by: M. Uwaha Available online 7 September 2013
In this study, the growth and dissolution rates of closely sized KCl crystals that were classified with respect to their separation voltages in an electrostatic separator were investigated in the presence of K2SO4, CH3COOK, NH4Cl and BaCl2 additives, having common cations or anions. Separation voltages were taken as a measure of surface potential. The experiments were carried out in a typical stagnant medium type single crystal measurement system. From experimental results it is observed that thread-like growth behaviors resulting from surface nucleation were dominant and dead zones where there was no growth or dissolution did always exist. For additives having common cation, it was found that the growth in the thread-like structure increases with the increase of surface potential. It was also found that, additives having common anions, initially led to decrease and neutralize the surface potential, then oppositely charged the crystal surface, when the crystal growth/dissolution behavior changes were considered. This study obviously proved that, crystal growth and dissolution behaviors can be controlled by changing the surface potentials of crystals by using additives having common ions. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.
Keywords: A1. Additive A1. Dendrites A1. Surface structure A2. Growth from solutions A2. Single crystal growth B1. Potassium compounds
1. Introduction Crystallization is one of the oldest separation methods used in chemical industries. Besides purification and separation of matter, crystallization which also enables to produce crystals with certain preferred features, is affected by many factors like temperature, supersaturation, mixing rate etc. However; among these factors, additives dissolved in a crystallization medium have significant effects on crystallization with several mechanisms [1–3]. Even a small amount of additive may affect many features of crystallization; like nucleation rate, crystal growth rate, habit formation, physical and storage properties of the product. Additives also tend to incorporate into crystal structures and thereby impairing the purity of the crystals. However; for studied systems generalization of additive effects is not easy [1–3]. Additives affect crystallization with different mechanisms. Some additives link themselves into the crystal lattice during crystal growth and thereby affecting the growth centers while others act by changing the structural features in the solution or along the crystal-solution interface. There are many studies in the literature
n
Corresponding author. Tel.: þ 90 332 2232064; fax: þ90 332 2410635. E-mail addresses: [email protected], [email protected] (A.A. Ceyhan), [email protected] (A.Nusre. Bulutcu).
where the effects of additives and their modeling on crystallization were investigated in details [1–6]. It is a well known fact that crystals of the same matter having same sizes and subjected to same hydrodynamic conditions and at the same supersaturation grow at different rates. This phenomenon is known as growth rate dispersion (GRD) [7,8]. One of the reasons for GRD is the selective adsorptions of anions or cations on some sites on crystal surface, which leads to formation of electrical double layer around these sites and surface potential on the crystal surface and therefore leading to different growth or dissolution rates. It is not possible to measure surface potential for highly soluble salts at the present state of art. It is known that electrical double layer is depressed by increasing concentration of solution. This means that two layers become closer, but still they exist and therefore surface potential sh\ould remains. No physical evidence is known that surface potential disappear in concentrated solutions. In order to detect effects of surface potential on GRD experimentally, several indirect methods were used. These methods include changing the surface potentials by using polyelectrolytes [9–11], measuring the growth/dissolution rates of crystals classified by using electrostatic separator [12,13], investigation of growth/dissolution behaviors of crystals with different surface potentials under electrical field [14] and comparing growth /dissolution rates in the presence of additives [15,16]. Initially, it was thought that dispersion in crystal growth and dissolution rates existed only in growth region and it was surface
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A.A. Ceyhan, A.Nusret Bulutcu / Journal of Crystal Growth 383 (2013) 112–118
oriented rather than diffusional mass transfer to crystal surface. However, Fabian and Ulrich, in their study on K2SO4, showed that there is dispersion during dissolution (DRD), too [17]. Similar results were also obtained by Şahin and Bulutcu [12], and İlyaskarov and Bulutcu [18]. All these results indicate that surfaces of any individual crystal have its own features that directly affect the GRD and DRD. The most probable reason for selective ion adsorption on crystal surfaces is the different radii of ions at their hydrated state [9–16,19]. Physical adsorption which is a concentration dependent phenomenon develops in favor of one of the ions for pure salts. This assumption indicates that GRD and DRD is possible in pure systems. However; this equilibrium can be shifted by the effect of any additive having common ion. While an increase in concentration of selectively adsorbed ion in saturated solution leads to an increase in the adsorption rate, it can be delayed by adding additives that carry common ions with non adsorbed one. In literature, with the exception of the study conducted on K2Cr2O7 [15,16] no studies exist on systematic investigation of effects of additives with common anions or cations on the growth rate of crystals. In this study, the effects of additives with common cations or anions on the growth and dissolution rates of KCl crystals classified on an electrostatic separator based on their respective surface potentials were investigated.
2. Experımental sectıon Analytical grade KCl crystals (Merck 1.04936) were closely sieved and one of the single sieve fraction of particles ( 212þ180 μm) were classified in an electrostatic separation unit by changing separation voltage. The electrostatic separation was carried out by a rotary type “Dings electrodynamic separator”. The principle of an electrostatic separation may be found elsewhere [12,18]. Then growth and dissolution rates of electrostaticaly classified single size KCl crystals were measured in the presence of additives containing 10, 100 and 1000 ppm CH3COOK, K2SO4, NH4Cl and BaCl2. Crystal growth and dissolution rates experiments were carried out in a stagnant medium type single crystal measurement system which consists of a crystal growth cell thermostated from jacket side, Pt-100 probe to measure actual temperature inside the cell with a precision of 0.01 1C, microscope and its associated camera together with a workstation. Details of experimental setup can be found in literature [12–14,19]. KCl, K2SO4, CH3COOK, NH4Cl and BaCl2 that were all analytical grade (Merck) were used in the experiments. Saturated KCl stock solution in de-ionized water with a conductivity of 0.067 mS/cm was prepared by using solubility data at 20 1C [20], filtered through a 0.45 μm membrane filter and used in all experiments. The filtered stock solution was kept in a shaker-bath at a temperature of about 1 to 2 1C above its saturation temperature. Growth rates of irregularly growing crystal were generally measured from the changing of equivalent diameter of projection area of seed crystal and in this work it was measured by using Image-pro plus software. But, substantial changing of particle shape during the crystal growth leads to difficulties to apply this method. For this reason, a new definition is necessary in the case of formation of new dendrites on crystal surface. It was observed that new dendrites were still a part of growing seed and linked to surface by very thin rod like bridges. Therefore dendrites formed outside of seed surface were assumed a part of crystal growth and surface area of seed and dendrites were measured separately and total projection area was used in calculation of equivalent diameter and growth rate (dL/dt) was based on the changing in diameter with respect to time. In fact, measured growth rates were not real growth rates, but still helpful to understand the impurity effects. Individual measurements on seed and dendrites showed
113
that equivalent diameter of each part change with time linearly (i.e. each part has constant growth rate), but they have different growth rates.
3. Results and discussion In an earlier work, it was proved that growth and dissolution rates of analytical grade KCl in the pure media showed two important aspects, one was the presence of a “dead zone” where growth or dissolution did not exist, even in the absence of impurities, and the second was that there were growth and dissolution rate dispersions which were shown to be as a functions of surface potential [13]. This result was contrary to the suppression of growth/dissolution rates by impurities or additives in the formation of dead zone. In other words, so called impurity effects proposed by Cabrera and Vermilyea [26] may be created also by specifically adsorbed ions in pure media. The results of Mohameed and Ulrich obtained in a fluidized bed crystal growth cell [21,22] also suggest the existence of a dead zone for KCl at different pH values. Existence of the dead zone was proved by earlier researchers for other potassium salts [16,23,24]. Also, it was shown that KCl in pure state showed growth anomalies, dendritical growth at very low supersaturation values and excessive surface nucleation at high supersaturation levels [13]. It is interesting that this is very similar to impurity effects which suppress the growth and dissolution and after such suppression the growth starts at a higher supersaturation which leads to very high growth rates and dentritical growth. Data for the pure system were added to figures throughout this article for comparison [13]. The growth behavior of KCl seed crystals observed in the presence of CH3COOK is very similar to that of pure solution. Dendritical growths at low supersaturations and needle-like growths as a result of excessive surface nucleation at high supersaturation are dominant. Also, similar to results of pure solution, as the separation voltage of the seed crystals increased, effects of surface nucleation diminish. Fig. 1 shows the growth behavior of seed crystals separated at 1 kV in the presence of 1000 ppm CH3COOK. As Fig. 1 b and c show, tiny fibers formed by means of surface nucleation stimulate further surface nucleation, thereby leading to the formation of new fibers. Such behavior was not observed in pure medium. If surface nucleation results from spots with surface potential, then it can be concluded that CH3COOK impurity increases that potential. This behavior is an expected one unless the accompanying CH3COO ion affects the crystallization in an unusual manner. Experimental results do not point out such an effect. When the surface potential forming ion is the K þ ion, addition of CH3COOK into the medium would increase the concentration of K þ ions when compared to Cl ions, which would lead to an increase in the adsorption of K þ ions and therefore the formerly mentioned effect could be observed. Consequently, the analysis of the effects of CH3COOK impurity on growth behavior shows that growth is controlled by the K þ ion, which causes surface potential. This outcome agrees with our previous studies [14,19]. Fig. 2 shows the variation in growth and dissolution rates of KCl crystals separated at 1 kV voltage, with respect to supersaturation at three different CH3COOK additive concentrations. Dead zones continued to form in the presence of CH3COOK additive. The rise in saturation (It is taken as the initiation of dissolution) temperatures when impurity concentrations increased can be linked to the drop in solubility as a result of common ion effect as well as Knapp's effect [25], which was already described as a decrease in solubility due to an increase in surface potential. It can be seen that existence of CH3COOK leads to a decrease in the dissolution rate of KCl crystals separated at 1 kV when compared to pure medium (Fig. 2). In fact, existence of dead zone makes it difficult to understand data. Growth rates are linear with respect to supersaturation and parallel
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Table 1 Dead zone width and dissolution characteristic of KCl crystals with particle size of ( 212 þ180) μm in the presence of NH4Cl and BaCl2. Additive concentration (ppm)
t=0
t=10
t=10
t=20
ΔC = 0,359 (g salt/100g sat.sol.)
t=0
t=10
t=20
ΔC = 0,213 (g salt/100g sat.sol.) Fig. 1. Growth behaviors of KCl crystals separated at 1 kV in the presence of 1000 ppm CH3COOK.
50 40
pure 10 ppm
G*10 8 (m/s)
30
100 ppm 1000 ppm
20 10 0 -10 -20 -0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
ΔC (g salt/100 g sat.sol.)
Fig. 2. Growth and dissolution rates versus supersaturation changing of KCl crystals separated at 1 kV, in pure medium and in the presence of 10, 100 and 1000 ppm CH3COOK.
to each other. If the saturation concentration is taken as the initiation of growth rate, data on growth zone should shift to the same initiation of growth rate point and in this case no difference in growth rate behavior is observed. Growth and dissolution rates behavior with respect to supersaturation obtained for crystals separated at 16 kV were similar to Fig. 2. In both cases, nonlinear change of dissolution rates with respect to supersaturation shows that not only diffusion step but also surface reactions have an impact on dissolution. As a result, CH3COOK additive at 3 different concentrations have no noticeable influence on the growth and dissolution rates of KCl crystals. This conclusion agrees with the suggestion that it is the K þ ion that creates the surface potential [14].
Saturation temperature (1C)
Dead zone width (1C)
NH4Cl
NH4Cl
BaCl2
BaCl2
10
1 3 9 16
19.4 19.3 19.1 19.05
19.6 20.0 19.5 19.7
1.00 1.05 1.05 1.00
1.00 1.40 1.15 0.85
100
1 3 9 16
19.3 19.2 19.2 19.2
19.8 19.8 19.7 19.8
1.25 1.15 1.15 0.85
1.15 1.15 1.10 1.00
1000
1 3 9 16
19.5 19.6 19.6 19.75
19.75 19.75 19.80 19.80
0.85 0.90 1.00 1.05
0.95 1.15 0.85 0.85
t=20
ΔC = 0,409 (g salt/100g sat.sol.)
t=0
Separation voltages of seed crystals (kV)
Growth behaviors of KCl seed crystals separated at 1 kV in the presence of 100 ppm K2SO4 were very similar to the one given in Fig. 1, obtained in the presence of CH3COOK. No significant variations in growth behaviors with respect to separation voltages were also detected comparing to pure media and in the presence of CH3COOK. Growth and dissolution rates versus supersaturation changing of KCl crystals separated at 1 kV, in pure medium and in the presence of 10, 100 and 1000 ppm K2SO4 were very similar to Fig. 2. All these results imply the importance of surface potential controlled by K þ ions. This is a consistent finding with an earlier study [13]. Experimental results showed that either CH3COO or SO4 ions did not have any significant effect on growth behaviors and growth/dissolution rates. Effects of increasing concentration of additives with common anions were estimated to decrease the surface potential of KCl as additive concentrations increase, then further increase in concentration leads to neutralize and then to increase it again by donating opposite charges to the surface. NH4Cl and BaCl2 were used as substances having common anions. Dead zone formation in the presence of NH4Cl and BaCl2 continued to exist. Table 1 shows experimental values for the saturation temperature (taken as the initiation temperature of dissolution) and dead zone width. This result shows that additions of NH4Cl and BaCl2 lead to narrow the dead zone since it was given as 1.3 1C in average value in pure medium [14]. Saturation temperature given in Table 1 is controlled by three factors. The first one is the initial surface potentials of seed crystals and higher the surface potential is lower the saturation concentration and higher the saturation temperature [25]. The second one is the concentration of an excess common ion which leads to decrease in solubility and therefore an increase in the saturation temperature and the third one is an expected drop in surface potential as a result of the increasing concentration of Cl ion, which competes with surface potential dominating K þ ion concentration. This last effect is more complex one and its effect depends also on the initial surface potential and additive concentrations. Under these complex effects, it is still possible to distinguish the influence of NH4Cl on the saturation temperature given in Table 1. First and third factor may be detected in the presence of 10 ppm and 1000 ppm NH4Cl, respectively. But in the case of BaCl2 variations in dissolution temperature are low. This is attributed to additional effect of Ba ion. Fig. 3 shows the growth behavior of the KCl crystals that were separated at 1 and 9 kV, as a function of time, in the presence of 10, 100 and 1000 ppm NH4Cl. It is observed that an increase in the separation voltage (i.e. decreasing of surface potential) of the KCl seed crystals leaded to lessen the surface nucleation type growth behavior. This tendency was also observed for crystals separated at 3 and 16 kV.
A.A. Ceyhan, A.Nusret Bulutcu / Journal of Crystal Growth 383 (2013) 112–118
115
10 ppm
1 kV
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
100 ppm
1000 ppm
10 ppm
9 kV
100 ppm
1000 ppm
Fig. 3. Growth behaviors of KCl crystals separated at 1 and 9 kV in the presence of 10, 100 and 1000 ppm NH4Cl and at 16.8 1C.
Fig. 3 shows that KCl crystals that were separated at 1 kV displayed a strong trend of surface nucleation in the presence of 10 ppm NH4Cl; while at 100 ppm NH4Cl concentration, surface nucleation was weakened and at 1000 ppm NH4Cl concentration, surface nucleation began again. On the other hand, no surface nucleation was observed on the crystals that were separated at 9 kV in the presence of 10 ppm NH4Cl; however surface nucleation was initiated at 100 ppm NH4Cl concentration and got stronger when the NH4Cl concentration was raised to 1000 ppm. These results agree with our assumption that surface potential which dominates growth mechanism can be controlled by additives having common ions. Fig. 4 shows the growth and dissolution rates versus supersaturation behavior of the KCl crystals that were separated at 3 kV, in the presence of 10, 100 and 1000 ppm NH4Cl. As it is seen, growth rates were depressed comparing to pure state and dissolution rates were less affected and turn from curvature shape seen in Fig. 2 to diffusion controlled linear form. This indicates that existence of excess amount of common ion comparing to surface potential dominating ion leads to decrease the surface potential and partly or totally eliminates the surface effect in dissolution.
Fig. 5 shows the growth behavior of the KCl crystals separated at 1 kV, as a function of time, in the presence of 10, 100 and 1000 ppm BaCl2 and at two different supersaturation levels. It was observed that surface-nucleation-type growth behavior depressed as the BaCl2 concentration increased and this tendency is similar to the effect of NH4Cl. At lower supersaturation level, growth anomaly of KCl was highly depressed at all BaCl2 concentrations but at high supersaturation this effect can be obtained at high additive concentration. The growth behaviors of the crystals separated at 3, 9 and 16 kV were similar to the growth behavior of the crystals separated at 1 kV. Fig. 6 shows the changing of growth and dissolution rates with respect to supersaturation for crystals separated at 1 and 3 kV and in the presence of 10 ppm BaCl2. It is seen that growth rates of crystals separated at 3 kV are higher than the ones of crystals separated at 1 kV and dissolution rates have reverse behavior. It is interesting that in the absence of additive, growth rate of crystals separated at 1 kV exhibited higher value than the one of crystals separated at 3 kV [13]. The reason for this contradiction results from the ratio of surface potential neutralization by BaCl2 additive.
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It is obvious that neutralization of surface potential of crystals separated at 3 kV is more effective than the one of crystals separated at 1 kV. The remaining surface potential controls growth and dissolution rates. Fig. 7 shows the changing of growth and dissolution rates versus supersaturation, for KCl crystals separated at 1 kV, measured in the
presence of 10, 100 and 1000 ppm BaCl2. According to Fig. 7, the presence of BaCl2 impurity causes the dissolution rate of KCl to decrease and turn from curvature shape seen in the case of pure media to diffusion controlled linear form. This is the similar result obtained in the case of NH4Cl. It is a fact that obtained dissolution 60
70
50
60 50
G*10 8 (m/s)
40 30
G*10 8 (m/s)
pure 10 ppm 100 ppm 1000 ppm
20
40
1 kV
30
3 kV
20 10
10 0
0
-10
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
-10 -20
-20
-0.4
ΔC (g salt/100 g sat.sol.) Fig. 4. Growth and dissolution rates versus supersaturation changing of KCl crystals separated at 3 kV, in pure medium and in the presence of 10, 100 and 1000 ppm NH4Cl.
-0.3
-0.2
-0.1
0
0.1
10 ppm
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
100 ppm
1000 ppm
ΔC = 0,3845 (g salt/100g sat.sol.)
10 ppm
100 ppm
0.3
0.4
0.5
0.6
Fig. 6. Growth and dissolution rates versus supersaturation changing of KCl crystals separated at 1 and 3 kV, in the presence of 10 ppm BaCl2.
ΔC = 0,4335 (g salt/100g sat.sol.)
1 kV
0.2
ΔC (g salt/100 g sat.sol.)
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
1000 ppm Fig. 5. Growth behaviors of KCl crystals separated at 1 kV in the presence of 10, 100 and 1000 ppm BaCl2.
A.A. Ceyhan, A.Nusret Bulutcu / Journal of Crystal Growth 383 (2013) 112–118 60
40
G*10 8 (m/s)
crystallization were weak. They lead to slightly decrease in dissolution rates of KCl. In spite of this, increase of concentration of ions having common anion (NH4Cl and BaCl2) lead to decrease surface potential and they have deeper effects on growth behavior and growth/dissolution rates. While curvature change of dissolution rates in pure solution and in the presence of K2SO4 and CH3COOK shows other effects different then diffusional phenomena which gives linear tendency. This, in fact, shows the importance of surface property on crystallization. Additives having common anion (NH4Cl and BaCl2) lead to turn it from curvature shape to diffusion controlled linear form.
pure 10 ppm 100 ppm 1000 ppm
50
30
117
20 10 0 -10 -20 -0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
ΔC (g salt/100 g sat.sol.) Fig. 7. Growth and dissolution rates versus supersaturation changing of KCl crystals separated at 1 kV, in pure medium and in the presence of 10, 100 and 1000 ppm BaCl2.
Notation t
ΔC
G rates at all additive concentrations are close and not separated as in the case of growth rates. It can be seen that growth rates increase in the presence of 10 ppm BaCl2 and then decrease again. This fact is interpreted as decreasing the value of surface potential and it is in a good agreement with the result obtained from Fig. 5. All these results indicate that the effect of BaCl2 impurity is a function of the surface potential of the seed crystals and BaCl2 additive at different concentration can be used to modify it. When these results are compared to those obtained in the presence of NH4Cl additive, a similar influences but at different additive concentrations can be noticed. These combined results also show that the growth behavior of the KCl crystals is modified by additives having common ion which is not surface potential dominating ion in the case of pure saturated solution.
4. Conclusions The effects of additives having common cation (CH3COOK and K2SO4) and common anion (NH4Cl and BaCl2) on growth and dissolution rates of KCl crystals that were closely sized and classified in an electrostatic separator with respect to their surface potentials were investigated. Main aim of this work was to detect the importance of surface potential of individual crystals on the growth behavior and growth/dissolution rates and to control it by additives having common ion. The experiments were conducted with three different concentrations of each additive (10 ppm, 100 ppm and 1000 ppm) in a stagnant type single crystal cell and results were compared with values obtained in pure solution [13]. It was found that, formations of dead zone continued in the presence of all kinds of additives and at all their concentrations. But all additives lead to narrowing the width of the dead zone. But, additives having common anion with KCl, i.e. NH4Cl and BaCl2, have deeper effect on this zone width. Since surface potential dominating ion of KCl crystals were given as potassium ion [13], counter ion (Cl ) would lead to a decrease in the adsorption rate of potassium ions and therefore surface potential. It was already suggested that dead zone formation results from the impurity effect [26]. Our results imply that dead zone formation in pure solutions is a result of surface potential dominating ion which acts as an impurity. No normal growth behavior, either dendritically growth or excessive surface nucleation was encountered in the studied system. Both growth types are functions of the crystals surface potentials. While increasing of concentration of additives having common cation (K2SO4 and CH3COOK) leads to slightly increase in the surface nucleation. Therefore effects of additives having common cation on
Time (min) Supersaturation (g salt/100 g saturated solution) Growth rate (m/s)
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The crystallization kinetics of potassium chloride in the presence of additives with common ions Ayhan Abdullah Ceyhan a,n, A.Nusret Bulutcu b a b
Seljuk University, Aladdin Keykubat Campus, Faculty of Engineering and Architecture, Chemical Engineering Department, Konya Turkey Istanbul Technical University Department of Chemical Engineering, Istanbul, Turkey
art ic l e i nf o
a b s t r a c t
Article history: Received 19 March 2012 Received in revised form 26 July 2013 Accepted 20 August 2013 Communicated by: M. Uwaha Available online 7 September 2013
In this study, the growth and dissolution rates of closely sized KCl crystals that were classified with respect to their separation voltages in an electrostatic separator were investigated in the presence of K2SO4, CH3COOK, NH4Cl and BaCl2 additives, having common cations or anions. Separation voltages were taken as a measure of surface potential. The experiments were carried out in a typical stagnant medium type single crystal measurement system. From experimental results it is observed that thread-like growth behaviors resulting from surface nucleation were dominant and dead zones where there was no growth or dissolution did always exist. For additives having common cation, it was found that the growth in the thread-like structure increases with the increase of surface potential. It was also found that, additives having common anions, initially led to decrease and neutralize the surface potential, then oppositely charged the crystal surface, when the crystal growth/dissolution behavior changes were considered. This study obviously proved that, crystal growth and dissolution behaviors can be controlled by changing the surface potentials of crystals by using additives having common ions. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.
Keywords: A1. Additive A1. Dendrites A1. Surface structure A2. Growth from solutions A2. Single crystal growth B1. Potassium compounds
1. Introduction Crystallization is one of the oldest separation methods used in chemical industries. Besides purification and separation of matter, crystallization which also enables to produce crystals with certain preferred features, is affected by many factors like temperature, supersaturation, mixing rate etc. However; among these factors, additives dissolved in a crystallization medium have significant effects on crystallization with several mechanisms [1–3]. Even a small amount of additive may affect many features of crystallization; like nucleation rate, crystal growth rate, habit formation, physical and storage properties of the product. Additives also tend to incorporate into crystal structures and thereby impairing the purity of the crystals. However; for studied systems generalization of additive effects is not easy [1–3]. Additives affect crystallization with different mechanisms. Some additives link themselves into the crystal lattice during crystal growth and thereby affecting the growth centers while others act by changing the structural features in the solution or along the crystal-solution interface. There are many studies in the literature
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Corresponding author. Tel.: þ 90 332 2232064; fax: þ90 332 2410635. E-mail addresses: [email protected], [email protected] (A.A. Ceyhan), [email protected] (A.Nusre. Bulutcu).
where the effects of additives and their modeling on crystallization were investigated in details [1–6]. It is a well known fact that crystals of the same matter having same sizes and subjected to same hydrodynamic conditions and at the same supersaturation grow at different rates. This phenomenon is known as growth rate dispersion (GRD) [7,8]. One of the reasons for GRD is the selective adsorptions of anions or cations on some sites on crystal surface, which leads to formation of electrical double layer around these sites and surface potential on the crystal surface and therefore leading to different growth or dissolution rates. It is not possible to measure surface potential for highly soluble salts at the present state of art. It is known that electrical double layer is depressed by increasing concentration of solution. This means that two layers become closer, but still they exist and therefore surface potential sh\ould remains. No physical evidence is known that surface potential disappear in concentrated solutions. In order to detect effects of surface potential on GRD experimentally, several indirect methods were used. These methods include changing the surface potentials by using polyelectrolytes [9–11], measuring the growth/dissolution rates of crystals classified by using electrostatic separator [12,13], investigation of growth/dissolution behaviors of crystals with different surface potentials under electrical field [14] and comparing growth /dissolution rates in the presence of additives [15,16]. Initially, it was thought that dispersion in crystal growth and dissolution rates existed only in growth region and it was surface
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A.A. Ceyhan, A.Nusret Bulutcu / Journal of Crystal Growth 383 (2013) 112–118
oriented rather than diffusional mass transfer to crystal surface. However, Fabian and Ulrich, in their study on K2SO4, showed that there is dispersion during dissolution (DRD), too [17]. Similar results were also obtained by Şahin and Bulutcu [12], and İlyaskarov and Bulutcu [18]. All these results indicate that surfaces of any individual crystal have its own features that directly affect the GRD and DRD. The most probable reason for selective ion adsorption on crystal surfaces is the different radii of ions at their hydrated state [9–16,19]. Physical adsorption which is a concentration dependent phenomenon develops in favor of one of the ions for pure salts. This assumption indicates that GRD and DRD is possible in pure systems. However; this equilibrium can be shifted by the effect of any additive having common ion. While an increase in concentration of selectively adsorbed ion in saturated solution leads to an increase in the adsorption rate, it can be delayed by adding additives that carry common ions with non adsorbed one. In literature, with the exception of the study conducted on K2Cr2O7 [15,16] no studies exist on systematic investigation of effects of additives with common anions or cations on the growth rate of crystals. In this study, the effects of additives with common cations or anions on the growth and dissolution rates of KCl crystals classified on an electrostatic separator based on their respective surface potentials were investigated.
2. Experımental sectıon Analytical grade KCl crystals (Merck 1.04936) were closely sieved and one of the single sieve fraction of particles ( 212þ180 μm) were classified in an electrostatic separation unit by changing separation voltage. The electrostatic separation was carried out by a rotary type “Dings electrodynamic separator”. The principle of an electrostatic separation may be found elsewhere [12,18]. Then growth and dissolution rates of electrostaticaly classified single size KCl crystals were measured in the presence of additives containing 10, 100 and 1000 ppm CH3COOK, K2SO4, NH4Cl and BaCl2. Crystal growth and dissolution rates experiments were carried out in a stagnant medium type single crystal measurement system which consists of a crystal growth cell thermostated from jacket side, Pt-100 probe to measure actual temperature inside the cell with a precision of 0.01 1C, microscope and its associated camera together with a workstation. Details of experimental setup can be found in literature [12–14,19]. KCl, K2SO4, CH3COOK, NH4Cl and BaCl2 that were all analytical grade (Merck) were used in the experiments. Saturated KCl stock solution in de-ionized water with a conductivity of 0.067 mS/cm was prepared by using solubility data at 20 1C [20], filtered through a 0.45 μm membrane filter and used in all experiments. The filtered stock solution was kept in a shaker-bath at a temperature of about 1 to 2 1C above its saturation temperature. Growth rates of irregularly growing crystal were generally measured from the changing of equivalent diameter of projection area of seed crystal and in this work it was measured by using Image-pro plus software. But, substantial changing of particle shape during the crystal growth leads to difficulties to apply this method. For this reason, a new definition is necessary in the case of formation of new dendrites on crystal surface. It was observed that new dendrites were still a part of growing seed and linked to surface by very thin rod like bridges. Therefore dendrites formed outside of seed surface were assumed a part of crystal growth and surface area of seed and dendrites were measured separately and total projection area was used in calculation of equivalent diameter and growth rate (dL/dt) was based on the changing in diameter with respect to time. In fact, measured growth rates were not real growth rates, but still helpful to understand the impurity effects. Individual measurements on seed and dendrites showed
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that equivalent diameter of each part change with time linearly (i.e. each part has constant growth rate), but they have different growth rates.
3. Results and discussion In an earlier work, it was proved that growth and dissolution rates of analytical grade KCl in the pure media showed two important aspects, one was the presence of a “dead zone” where growth or dissolution did not exist, even in the absence of impurities, and the second was that there were growth and dissolution rate dispersions which were shown to be as a functions of surface potential [13]. This result was contrary to the suppression of growth/dissolution rates by impurities or additives in the formation of dead zone. In other words, so called impurity effects proposed by Cabrera and Vermilyea [26] may be created also by specifically adsorbed ions in pure media. The results of Mohameed and Ulrich obtained in a fluidized bed crystal growth cell [21,22] also suggest the existence of a dead zone for KCl at different pH values. Existence of the dead zone was proved by earlier researchers for other potassium salts [16,23,24]. Also, it was shown that KCl in pure state showed growth anomalies, dendritical growth at very low supersaturation values and excessive surface nucleation at high supersaturation levels [13]. It is interesting that this is very similar to impurity effects which suppress the growth and dissolution and after such suppression the growth starts at a higher supersaturation which leads to very high growth rates and dentritical growth. Data for the pure system were added to figures throughout this article for comparison [13]. The growth behavior of KCl seed crystals observed in the presence of CH3COOK is very similar to that of pure solution. Dendritical growths at low supersaturations and needle-like growths as a result of excessive surface nucleation at high supersaturation are dominant. Also, similar to results of pure solution, as the separation voltage of the seed crystals increased, effects of surface nucleation diminish. Fig. 1 shows the growth behavior of seed crystals separated at 1 kV in the presence of 1000 ppm CH3COOK. As Fig. 1 b and c show, tiny fibers formed by means of surface nucleation stimulate further surface nucleation, thereby leading to the formation of new fibers. Such behavior was not observed in pure medium. If surface nucleation results from spots with surface potential, then it can be concluded that CH3COOK impurity increases that potential. This behavior is an expected one unless the accompanying CH3COO ion affects the crystallization in an unusual manner. Experimental results do not point out such an effect. When the surface potential forming ion is the K þ ion, addition of CH3COOK into the medium would increase the concentration of K þ ions when compared to Cl ions, which would lead to an increase in the adsorption of K þ ions and therefore the formerly mentioned effect could be observed. Consequently, the analysis of the effects of CH3COOK impurity on growth behavior shows that growth is controlled by the K þ ion, which causes surface potential. This outcome agrees with our previous studies [14,19]. Fig. 2 shows the variation in growth and dissolution rates of KCl crystals separated at 1 kV voltage, with respect to supersaturation at three different CH3COOK additive concentrations. Dead zones continued to form in the presence of CH3COOK additive. The rise in saturation (It is taken as the initiation of dissolution) temperatures when impurity concentrations increased can be linked to the drop in solubility as a result of common ion effect as well as Knapp's effect [25], which was already described as a decrease in solubility due to an increase in surface potential. It can be seen that existence of CH3COOK leads to a decrease in the dissolution rate of KCl crystals separated at 1 kV when compared to pure medium (Fig. 2). In fact, existence of dead zone makes it difficult to understand data. Growth rates are linear with respect to supersaturation and parallel
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Table 1 Dead zone width and dissolution characteristic of KCl crystals with particle size of ( 212 þ180) μm in the presence of NH4Cl and BaCl2. Additive concentration (ppm)
t=0
t=10
t=10
t=20
ΔC = 0,359 (g salt/100g sat.sol.)
t=0
t=10
t=20
ΔC = 0,213 (g salt/100g sat.sol.) Fig. 1. Growth behaviors of KCl crystals separated at 1 kV in the presence of 1000 ppm CH3COOK.
50 40
pure 10 ppm
G*10 8 (m/s)
30
100 ppm 1000 ppm
20 10 0 -10 -20 -0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
ΔC (g salt/100 g sat.sol.)
Fig. 2. Growth and dissolution rates versus supersaturation changing of KCl crystals separated at 1 kV, in pure medium and in the presence of 10, 100 and 1000 ppm CH3COOK.
to each other. If the saturation concentration is taken as the initiation of growth rate, data on growth zone should shift to the same initiation of growth rate point and in this case no difference in growth rate behavior is observed. Growth and dissolution rates behavior with respect to supersaturation obtained for crystals separated at 16 kV were similar to Fig. 2. In both cases, nonlinear change of dissolution rates with respect to supersaturation shows that not only diffusion step but also surface reactions have an impact on dissolution. As a result, CH3COOK additive at 3 different concentrations have no noticeable influence on the growth and dissolution rates of KCl crystals. This conclusion agrees with the suggestion that it is the K þ ion that creates the surface potential [14].
Saturation temperature (1C)
Dead zone width (1C)
NH4Cl
NH4Cl
BaCl2
BaCl2
10
1 3 9 16
19.4 19.3 19.1 19.05
19.6 20.0 19.5 19.7
1.00 1.05 1.05 1.00
1.00 1.40 1.15 0.85
100
1 3 9 16
19.3 19.2 19.2 19.2
19.8 19.8 19.7 19.8
1.25 1.15 1.15 0.85
1.15 1.15 1.10 1.00
1000
1 3 9 16
19.5 19.6 19.6 19.75
19.75 19.75 19.80 19.80
0.85 0.90 1.00 1.05
0.95 1.15 0.85 0.85
t=20
ΔC = 0,409 (g salt/100g sat.sol.)
t=0
Separation voltages of seed crystals (kV)
Growth behaviors of KCl seed crystals separated at 1 kV in the presence of 100 ppm K2SO4 were very similar to the one given in Fig. 1, obtained in the presence of CH3COOK. No significant variations in growth behaviors with respect to separation voltages were also detected comparing to pure media and in the presence of CH3COOK. Growth and dissolution rates versus supersaturation changing of KCl crystals separated at 1 kV, in pure medium and in the presence of 10, 100 and 1000 ppm K2SO4 were very similar to Fig. 2. All these results imply the importance of surface potential controlled by K þ ions. This is a consistent finding with an earlier study [13]. Experimental results showed that either CH3COO or SO4 ions did not have any significant effect on growth behaviors and growth/dissolution rates. Effects of increasing concentration of additives with common anions were estimated to decrease the surface potential of KCl as additive concentrations increase, then further increase in concentration leads to neutralize and then to increase it again by donating opposite charges to the surface. NH4Cl and BaCl2 were used as substances having common anions. Dead zone formation in the presence of NH4Cl and BaCl2 continued to exist. Table 1 shows experimental values for the saturation temperature (taken as the initiation temperature of dissolution) and dead zone width. This result shows that additions of NH4Cl and BaCl2 lead to narrow the dead zone since it was given as 1.3 1C in average value in pure medium [14]. Saturation temperature given in Table 1 is controlled by three factors. The first one is the initial surface potentials of seed crystals and higher the surface potential is lower the saturation concentration and higher the saturation temperature [25]. The second one is the concentration of an excess common ion which leads to decrease in solubility and therefore an increase in the saturation temperature and the third one is an expected drop in surface potential as a result of the increasing concentration of Cl ion, which competes with surface potential dominating K þ ion concentration. This last effect is more complex one and its effect depends also on the initial surface potential and additive concentrations. Under these complex effects, it is still possible to distinguish the influence of NH4Cl on the saturation temperature given in Table 1. First and third factor may be detected in the presence of 10 ppm and 1000 ppm NH4Cl, respectively. But in the case of BaCl2 variations in dissolution temperature are low. This is attributed to additional effect of Ba ion. Fig. 3 shows the growth behavior of the KCl crystals that were separated at 1 and 9 kV, as a function of time, in the presence of 10, 100 and 1000 ppm NH4Cl. It is observed that an increase in the separation voltage (i.e. decreasing of surface potential) of the KCl seed crystals leaded to lessen the surface nucleation type growth behavior. This tendency was also observed for crystals separated at 3 and 16 kV.
A.A. Ceyhan, A.Nusret Bulutcu / Journal of Crystal Growth 383 (2013) 112–118
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10 ppm
1 kV
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
100 ppm
1000 ppm
10 ppm
9 kV
100 ppm
1000 ppm
Fig. 3. Growth behaviors of KCl crystals separated at 1 and 9 kV in the presence of 10, 100 and 1000 ppm NH4Cl and at 16.8 1C.
Fig. 3 shows that KCl crystals that were separated at 1 kV displayed a strong trend of surface nucleation in the presence of 10 ppm NH4Cl; while at 100 ppm NH4Cl concentration, surface nucleation was weakened and at 1000 ppm NH4Cl concentration, surface nucleation began again. On the other hand, no surface nucleation was observed on the crystals that were separated at 9 kV in the presence of 10 ppm NH4Cl; however surface nucleation was initiated at 100 ppm NH4Cl concentration and got stronger when the NH4Cl concentration was raised to 1000 ppm. These results agree with our assumption that surface potential which dominates growth mechanism can be controlled by additives having common ions. Fig. 4 shows the growth and dissolution rates versus supersaturation behavior of the KCl crystals that were separated at 3 kV, in the presence of 10, 100 and 1000 ppm NH4Cl. As it is seen, growth rates were depressed comparing to pure state and dissolution rates were less affected and turn from curvature shape seen in Fig. 2 to diffusion controlled linear form. This indicates that existence of excess amount of common ion comparing to surface potential dominating ion leads to decrease the surface potential and partly or totally eliminates the surface effect in dissolution.
Fig. 5 shows the growth behavior of the KCl crystals separated at 1 kV, as a function of time, in the presence of 10, 100 and 1000 ppm BaCl2 and at two different supersaturation levels. It was observed that surface-nucleation-type growth behavior depressed as the BaCl2 concentration increased and this tendency is similar to the effect of NH4Cl. At lower supersaturation level, growth anomaly of KCl was highly depressed at all BaCl2 concentrations but at high supersaturation this effect can be obtained at high additive concentration. The growth behaviors of the crystals separated at 3, 9 and 16 kV were similar to the growth behavior of the crystals separated at 1 kV. Fig. 6 shows the changing of growth and dissolution rates with respect to supersaturation for crystals separated at 1 and 3 kV and in the presence of 10 ppm BaCl2. It is seen that growth rates of crystals separated at 3 kV are higher than the ones of crystals separated at 1 kV and dissolution rates have reverse behavior. It is interesting that in the absence of additive, growth rate of crystals separated at 1 kV exhibited higher value than the one of crystals separated at 3 kV [13]. The reason for this contradiction results from the ratio of surface potential neutralization by BaCl2 additive.
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It is obvious that neutralization of surface potential of crystals separated at 3 kV is more effective than the one of crystals separated at 1 kV. The remaining surface potential controls growth and dissolution rates. Fig. 7 shows the changing of growth and dissolution rates versus supersaturation, for KCl crystals separated at 1 kV, measured in the
presence of 10, 100 and 1000 ppm BaCl2. According to Fig. 7, the presence of BaCl2 impurity causes the dissolution rate of KCl to decrease and turn from curvature shape seen in the case of pure media to diffusion controlled linear form. This is the similar result obtained in the case of NH4Cl. It is a fact that obtained dissolution 60
70
50
60 50
G*10 8 (m/s)
40 30
G*10 8 (m/s)
pure 10 ppm 100 ppm 1000 ppm
20
40
1 kV
30
3 kV
20 10
10 0
0
-10
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
-10 -20
-20
-0.4
ΔC (g salt/100 g sat.sol.) Fig. 4. Growth and dissolution rates versus supersaturation changing of KCl crystals separated at 3 kV, in pure medium and in the presence of 10, 100 and 1000 ppm NH4Cl.
-0.3
-0.2
-0.1
0
0.1
10 ppm
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
100 ppm
1000 ppm
ΔC = 0,3845 (g salt/100g sat.sol.)
10 ppm
100 ppm
0.3
0.4
0.5
0.6
Fig. 6. Growth and dissolution rates versus supersaturation changing of KCl crystals separated at 1 and 3 kV, in the presence of 10 ppm BaCl2.
ΔC = 0,4335 (g salt/100g sat.sol.)
1 kV
0.2
ΔC (g salt/100 g sat.sol.)
t=0
t=10
t=20
t=0
t=10
t=20
t=0
t=10
t=20
1000 ppm Fig. 5. Growth behaviors of KCl crystals separated at 1 kV in the presence of 10, 100 and 1000 ppm BaCl2.
A.A. Ceyhan, A.Nusret Bulutcu / Journal of Crystal Growth 383 (2013) 112–118 60
40
G*10 8 (m/s)
crystallization were weak. They lead to slightly decrease in dissolution rates of KCl. In spite of this, increase of concentration of ions having common anion (NH4Cl and BaCl2) lead to decrease surface potential and they have deeper effects on growth behavior and growth/dissolution rates. While curvature change of dissolution rates in pure solution and in the presence of K2SO4 and CH3COOK shows other effects different then diffusional phenomena which gives linear tendency. This, in fact, shows the importance of surface property on crystallization. Additives having common anion (NH4Cl and BaCl2) lead to turn it from curvature shape to diffusion controlled linear form.
pure 10 ppm 100 ppm 1000 ppm
50
30
117
20 10 0 -10 -20 -0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
ΔC (g salt/100 g sat.sol.) Fig. 7. Growth and dissolution rates versus supersaturation changing of KCl crystals separated at 1 kV, in pure medium and in the presence of 10, 100 and 1000 ppm BaCl2.
Notation t
ΔC
G rates at all additive concentrations are close and not separated as in the case of growth rates. It can be seen that growth rates increase in the presence of 10 ppm BaCl2 and then decrease again. This fact is interpreted as decreasing the value of surface potential and it is in a good agreement with the result obtained from Fig. 5. All these results indicate that the effect of BaCl2 impurity is a function of the surface potential of the seed crystals and BaCl2 additive at different concentration can be used to modify it. When these results are compared to those obtained in the presence of NH4Cl additive, a similar influences but at different additive concentrations can be noticed. These combined results also show that the growth behavior of the KCl crystals is modified by additives having common ion which is not surface potential dominating ion in the case of pure saturated solution.
4. Conclusions The effects of additives having common cation (CH3COOK and K2SO4) and common anion (NH4Cl and BaCl2) on growth and dissolution rates of KCl crystals that were closely sized and classified in an electrostatic separator with respect to their surface potentials were investigated. Main aim of this work was to detect the importance of surface potential of individual crystals on the growth behavior and growth/dissolution rates and to control it by additives having common ion. The experiments were conducted with three different concentrations of each additive (10 ppm, 100 ppm and 1000 ppm) in a stagnant type single crystal cell and results were compared with values obtained in pure solution [13]. It was found that, formations of dead zone continued in the presence of all kinds of additives and at all their concentrations. But all additives lead to narrowing the width of the dead zone. But, additives having common anion with KCl, i.e. NH4Cl and BaCl2, have deeper effect on this zone width. Since surface potential dominating ion of KCl crystals were given as potassium ion [13], counter ion (Cl ) would lead to a decrease in the adsorption rate of potassium ions and therefore surface potential. It was already suggested that dead zone formation results from the impurity effect [26]. Our results imply that dead zone formation in pure solutions is a result of surface potential dominating ion which acts as an impurity. No normal growth behavior, either dendritically growth or excessive surface nucleation was encountered in the studied system. Both growth types are functions of the crystals surface potentials. While increasing of concentration of additives having common cation (K2SO4 and CH3COOK) leads to slightly increase in the surface nucleation. Therefore effects of additives having common cation on
Time (min) Supersaturation (g salt/100 g saturated solution) Growth rate (m/s)
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