Bir maddenin aynı boyuta sahip kristallerinin aynı ortam ve şartlarda birbirlerinden farklı hızlarda büyüdükleri uzun zamandır bilinmektedir. Bu olay, büyüme hızındaki saçılma (Growth Rate Dispersion) olarak tanımlanır. Çözeltide bulunan anyon ya da katyonların kristal yüzeyine seçimli adsorpsiyonları nedeniyle kristaller farklı yüzey potansiyelleri kazanırlar. Bu olay büyüme hız saçılımının nedenlerinden bir tanesidir. Bu çalışmada büyümede saçılım gösterdiği bilinen KCl kristallerinin büyüme ve çözünme hızları üzerine yüzey potansiyelinin etkisi durgun ortam tek kristal ölçüm sisteminde incelenmiştir. Deneylerde, elektrostatik ayırıcıda sırasıyla 1, 3, 5, 9 ve 16 kV ayırma voltajları uygulanarak yüzey potansiyellerine göre sınıflandırılmış olan (-425+300) μm boyut bölgesindeki KCl aşı kristalleri kullanılmıştır. Saf ortamda gerçekleştirilen deneylerde, büyüme ve çözünmenin olmadığı bir ölü bölgenin varlığı tespit edilmiştir. Deneylerde elde edilen bir diğer sonuç, büyümenin başladığı duruma göre nispeten düşük sayılabilecek aşırı doygunluklarda dentritik büyüme görülürken, yüksek aşırı doygunluklarda çok farklı bir büyüme mekanizmasının ortaya çıkmasıdır. Yüksek aşırı doygunluklarda dentritik büyüme yanında yüzeye dik olarak fışkıran uzun ve çok ince iğnecikler zamanla büyüyerek çok farklı büyümüş kristal görünümünü ortaya çıkartmaktadır. Bu davranışla farklı voltajlarda ayrılan tüm kristallerde karşılaşılmıştır. Bu tip büyüme davranışının, artan ayırma voltajının artışına bağlı olarak azalma gösterdiği görülmüştür. Yapılan bu deneysel çalışma ile, KCl kristallerinin sahip oldukları yüzey potansiyellerinin büyüme ve çözünme hızları üzerinde önemli rol oynadıkları gösterilmiştir. Anahtar Kelimeler: Kristalizasyon, yüzey potansiyeli, büyüme hız saçılımı, elektrostatik ayırma, KCl.Crystallization is a very complex operation. The main reason of this complexity is the number of mass transfer steps involved in the process. In a supersaturated solution, the first step is the creation of a new surface by nucleation; then solvated growth units diffuse to the surface and are adsorbed onto it. Surface migration then occurs followed by desorption back into solution or incorporate into the crystal lattice (Davey, 1976). These steps are governed by different physical laws which are not explained completely. Growth rate dispersion (GRD) is one of the phenomena which is difficult to explain. Growth rate dispersion can be described as the difference in the growth rate of crystals which are of the same size at the time when the growth process was started (Ulrich, 1989). Explanation of this difference is based on a well-known BCF theory developed by Burton et al., (1951). They suggested that the different growth rates were resulted from the distribution of dislocation sites on the crystal surfaces. Dislocation sites which have low attachment energy lead to the formation of the specific ion adsorption on the surface. The probable reason of specific ion adsorption is the different hydrated diameters of the ions. Details of this double layer formation can be found elsewhere (Kuşkay and Bulutcu, 2003). The surface potential resulting from electrical double layer formation dominates the growth and dissolution behavior through two different mechanisms. The first mechanism was explained by Knapp (1922). According to this theory, the higher the surface potential, the lower the equilibrium concentration. For this reason, crystals having higher surface potential will grow relatively in higher supersaturated conditions. The second mechanism was related to electrical double layer. The surface potential acts as an additional resistance to mass transfer, so that it will reduce the growth rate of the crystals. Surface charge can be measured easily in the case of sparingly soluble substances. But in the case of highly soluble salts it is difficult or sometimes impossible to measure it by the present technologies. But its effect can be still detected by indirect methods; such as changing the crystal growth rates in the presence of polyelectrolytes in different charge density (Sayan, 1995; Titiz, 1995). Measuring the crystal growth rates of crystals separated with respect to their surface potential is another indirect method. Şahin and Bulutcu (2001) and İlyaskarov (2002) made experiments systematically using different types of crystals which were separated in an electrostatic separator and the crystal growth /dissolution rates were determined in each crystal group having different surface potential. KCl is one of the crystalline material showing surface potential distributions originating from specific ion adsorption (or desorption) to defected sites on the crystal surface, in its own solution. The aim of this work is to investigate the effect of surface potential on the growth rate and growth behavior of KCl in pure KCl solution. Closely sized KCl crystals were separated to sub fractions in an electrostatic separator at different voltages in the range of 0-1, 1-3, 3-5, 5-9 and 16 kV and separation voltages were taken as a measure of surface potential of each sub fraction. Crystal growth and dissolution rates of these crystal fractions were measured at around 20 oC in a stagnant type single crystal cell using an image analyzer system. Experiment showed that growth and dissolution rates versus supersaturation plots had a dead zone in which no detectable growth was observed. Widths of dead zone were slightly different for the crystals having different surface potential. During the growth of KCl crystals surface nucleation was always dominant, even at very low supersaturations. At moderate and high supersaturation levels very unusual growth behavior were observed. In this region growth of crystals were dominated by the very thin but very long rod-like crystal branches growing perpendicular to the surface and their separately growing products on the brances. For this reason, seed crystals lost their original shape during the growth process. Frequency of the formation of branches from growing surface was the function of supersaturation and surface potential. From the experimental data the role of surface potential was shown to explain the reason of growth and dissolution rates dispersions of KCl. Keywords: Crystallization, surface charge, growth rate dispertion, electrostatic separation, KCl
