Crystallization Introduction • Crystal is a solid body with plane faces in which the atoms are arranged in an ordinary repetitive array. Most crystals are anisotropic; their mechanical, electrical, magnetic and optical properties are often different in different directions. • The atomic arrangements and interatomic distances remain constant for the crystals of a particular solid irrespective of its size. Crystals can be classified according to seven general systems. The relative sizes of the faces of the crystals of a substance can vary considerably. Such a variation is called a modification of habit. • The relative growths of the faces of a crystal can be altered and often controlled by adjusting a number of factors such as the rate of cooling, presence of impurities, the type of solvent, etc. (Mullin, 2001). The study of geometry, properties and structure of crystals is known as crystallography. Types of crystal geometry Conti. Crystallization • Crystallization is a process where solid particles are formed from a homogeneous phase. When crystals are produced by cooling a saturated solution, process is called solution crystallization. • When crystals are produced by cooling a molten solid in absence of any solvent, it is called melt crystallization. • Some liquids (e.g., ammonium oleate) that exhibit anisotropy at temperatures just above their melting points are called liquid crystals because of their similarity with crystalline solids. Conti. • When the concentration of a liquid solution is increased (for example, by evaporation of the solvent, by cooling or by a combination of these), the solution gradually reaches the saturation level. On further evaporation or cooling, it attains a degree of 'supersaturation. If the concentration of a solution is more than the solubility of the solid at a particular temperature, it is called 'supersaturated. • Supersaturation can be created and maintained for almost all systems. However, the level of supersaturation attainable not only depends upon the particular system but also on how calm and free from disturbances the system is. • For example, a sugar solution can have a very high degree or level of supersaturation (the concentration may even be about 80% more than the saturation value), but in a solution of sodium chloride, the maximum attainable supersaturation is often too small to be measured. • Spontaneous formation and growth of tiny crystals, called nuclei, take place in a supersaturated solution. Also, if a nucleus or a seed crystal is added to a supersaturated solution, it gradually grows to a larger size. Conti. • The supersaturation in a solution provides the driving force for transport of the solute from bulk of solution to the crystal surface. • On reaching the surface, the solute molecules get oriented and integrated into the crystal lattice. The suspension or slurry containing the crystals and the solution is called the magma, and the solution remaining after removal of the crystals is called the mother liquor. • The process of production of crystals from a solution and 'downstream processing' consists of four major steps: (i) crystallization: (ii) separation of crystals from the mother liquor by filtration or centrifugation; (iii) washing the crystals with fresh solvent to remove the adhering mother liquor (the washings are recycled back to the crystallizer); and (iv) drying of the moist crystals. • Sometimes the crude crystals are washed, redissolved in the solvent and recrystallized to make a purer product. The sequence (Wibowo et al., 2001) is shown in Figure . Solid – Liquid phase equilibrium • Crystallization is opposite to dissolution of a solid in a liquid or solvent. Particles of a solid undergo dissolution so long as the solution concentration is less than the solubility of the solid at the given temperature. • Conversely, if we have a supersaturated solution, and a few seed crystals are dropped into it, a driving force for transportation of the solute from the bulk of the solution to the crystal surface will come into play. • Thus, the extent of supersaturation in a solution is the driving force for crystallization. It may be mentioned that the solubility of very small particles may be significantly larger than the normal solubility of a substance as given by Gibbs- Thompson equation (Mullin, 2001). Conti. Conti. • Since the smaller particles have more than normal solubility, they dissolve even in a saturated solution and raise the concentration creating supersaturation. The larger particles present in the suspension start growing. The overall process consists of dissolution of smaller particles making the larger particles grow. This phenomenon is called Ostwald ripening. • The solubility of a solid in a liquid at different temperatures is conveniently represented on a solid-liquid phase diagram. • To explain the characteristics of a phase diagram, we take the simple example of the naphthalene-benzene binary system shown in Figure. Conti. • A point on the line AEB represents the saturation concentration (solubility) of naphthalene in benzene at different temperatures. AEB is Solubility curve. Any point above the curve is unsaturated solution. • This type curve is called “liquidus”. P1→P2 (saturated): naphthalene starts crystallizing out (P2-E): State of solution change. P3 → P4 (saturated): The point E is called Eutectic point. • This is the lowest equilibrium solidification point possible for any solution. Complete solidification of solution occurs when the solution represented by point E is cooled. Mechanism of Crystallization Theory of Crystallization • When crystallization starts in a homogeneous mixture, a new solid phase is created. Basic steps of nucleus formation or nucleation and crystal growth are overall crystallization process. • Nucleation means formation of tiny new crystals in a supersaturated solution. A new crystal thus formed is called a nucleus. • Nucleation is of two types: primary and secondary. Primary nucleation is also divided into two categories; homogeneous and heterogeneous. Contact, shear, fracture and attrition are secondary nucleation Primary Nucleation • It is the phenomenon of formation of new crystals independent of the presence of other crystals in the medium. • It is of two types- Homogenous and Heterogeneous. • In a homogenous solution. as a result of rapid random fluctuation, molecules may come together and associate into a cluster. • A nucleus is formed if the cluster size exceeds a critical size. A theoretical analysis of the energetics of cluster formation shows that a high degree of supersaturation is necessary in order that a cluster of size larger than the critical size is formed. • The theoretical rate of homogeneous nucleation can be derived from energy considerations (see Randolph and Larson, 1988: Myerson, 2002). Conti. • Here A’ a constant; vM = volume of a molecule; k = Boltzmann constant; T= absolute temperature; s = degree (or level) of supersaturation. • At normal levels of supersaturation, homogeneous nucleation proceeds very slowly. It increases rapidly if the supersaturation level is high. • The formation of clusters of small molecules is slowed down if the solution viscosity is high. • So, an increase in temp. has an additional +ve effect on the nucleation rate because of the reduced viscosity. Conti. • The formation of crystals of a solute on tiny suspended foreign solid particles or on the surfaces of the crystallizer is called heterogeneous nucleation. • The rate of heterogeneous nucleation depends not only on supersaturation but also on the availability of the 'active sites' for nucleation (Mullin, 2001). • It has been found that if a solution is very pure and clean (i.e. foreign particles are absent), a very high degree of supersaturation is required for heterogeneous nucleation to occur. Secondary nucleation • Now whenever we will be talking about the secondary nucleation the formation of the new crystals actually from the existing microscopic crystals in the magma actually is called secondary nucleation. • So for that we have 4 different types of nucleation. The first one is this contact nucleation. It is the most common type of nucleation in the industrial crystallizer. • If a suspension of crystals is vigorously agitated, fracture of crystals occurs by impact with the impeller (Bennett et al., 1973). Attrition between two crystals in suspension or between a crystal and the crystallizer surface also leads to the formation of small fragments. • In either case, small crystals or nuclei are formed which grow to larger sizes in the supersaturated solution. However, it has been found that more nuclei are formed by contact nucleation than by fracture or attrition. • In the process of crystallization, ion-pairs or molecules of a solute reach the surface of a crystal by diffusive or convective transport from the bulk of the solution. The solute molecules are continuously adsorbed on a crystal and are simultaneously accommodated into the lattice, resulting in crystal growth. Conti. • But accommodation of the ion-pairs or molecules in the crystal lattice does not occur instantaneously. Contact of a crystal with the agitator wall causes displacement of the adsorbed layer of solute on the crystal surface. • Some of the 'cluster of molecules or "loosely ordered phase in an adsorbed layer get displaced by the action of the agitator or due to shear at the wall and give rise to a new crystal nucleus. At a high level of supersaturation, the adsorbed layer is thicker and more particles are formed by contact nucleation. • When there is a high relative motion between a fluid and a crystal, the "loosely ordered phase near the crystal surface is carried into the bulk of the liquid (this is shear nucleation). Such a cluster of molecules acts as a secondary, nucleus. Secondary nucleation is sometimes called breeding. • The formation of a secondary nucleus by contact nucleation involves three major steps (i)the generation of a cluster of molecules near or at the crystal surface, (ii) the removal of the cluster, and (iii) the growth of the cluster to form a nucleus. Conti. • The degree or level of supersaturation is a very important factor. Numerous evidences exist in this regard. For example. Garade et al. (1979) reported their experimental results on size distribution of potassium alum nuclei generated by contact nucleation.They conducted experiments at constant energy input but at varying levels of supersaturation. Conti. • The intensity of agitation, is an important factor in contact nucleation. An intense agitation or a high agitator speed helps in quickly dislodging an 'adsorbed cluster of molecules from the crystal surface. • The 'hardness' of the impeller is also important. Below figure shows the effect of impeller material (its hardness) on the nucleation rate. A steel impeller is more effective to cause nucleation than a plastic impeller. Besides dislodging the cluster of molecules, a hard impeller causes breakage of crystals, particularly at the corners, to form secondary nuclei. Conti. • In connection with the design and analysis of a crystallizer, a simple powerlaw nucleation correlation of the following type is generally used to calculate the overall rate of nucleation by all of the above mechanisms (Garside, 1985; Myerson, 2002).
