Crystallization
Crystallization is a process that leads to solids with a uniform patern of atoms or molecules, i.e. a crystal. The uniform nature of a crystalline solid can be contrasted with amorphous solids in which atoms or molecules lack regular organization. Crystallization can occur by various routes including precipitation from solution, freezing of a liquid, or deposition from a gas. Attributes of the resulting crystal can depend largely on factors such as temperature, air pressure, cooling rate, or solute concentration.
Crystallization occurs in two main phases. The first is nucleation, the appearance of a crystalline phase from either a supercooled liquid or a supersaturated solvent. The second step is known as crystal growth, which is the increase in the size of particles and leads to a crystal state. An important feature of this step is that loose particles form layers at the crystal's surface and lodge themselves into open inconsistencies such as pores, cracks, etc.
Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs. In chemical engineering, crystallization occurs in a crystallizer. Crystallization is therefore related to precipitation, although the result is not amorphous or disordered, but a crystal.
Process
The crystallization process consists of two major events, nucleation and crystal growth which are driven by thermodynamic properties as well as chemical properties.Nucleation is the step where the solute molecules or atoms dispersed in the solvent start to gather into clusters, on the microscopic scale, that become stable under the current operating conditions. These stable clusters constitute the nuclei. Therefore, the clusters need to reach a critical size in order to become stable nuclei. Such critical size is dictated by many different factors. It is at the stage of nucleation that the atoms or molecules arrange in a defined and periodic manner that defines the crystal structure – note that "crystal structure" is a special term that refers to the relative arrangement of the atoms or molecules, not the macroscopic properties of the crystal, although those are a result of the internal crystal structure.
The crystal growth is the subsequent size increase of the nuclei that succeed in achieving the critical cluster size. Crystal growth is a dynamic process occurring in equilibrium where solute molecules or atoms precipitate out of solution, and dissolve back into solution. Supersaturation is one of the driving forces of crystallization, as the solubility of a species is an equilibrium process quantified by Ksp. Depending upon the conditions, either nucleation or growth may be predominant over the other, dictating crystal size.
Many compounds have the ability to crystallize with some having different crystal structures, a phenomenon called polymorphism. Certain polymorphs may be metastable, meaning that although it is not in thermodynamic equilibrium, it is kinetically stable and requires some input of energy to initiate a transformation to the equilibrium phase. Each polymorph is in fact a different thermodynamic solid state and crystal polymorphs of the same compound exhibit different physical properties, such as dissolution rate, shape, melting point, etc. For this reason, polymorphism is of major importance in industrial manufacture of crystalline products. Additionally, crystal phases can sometimes be interconverted by varying factors such as temperature, such as in the transformation of anatase to rutile phases of titanium dioxide.
In nature
There are many examples of natural process that involve crystallization.Geological time scale process examples include:
- Natural crystal formation ;
- Stalactite/stalagmite, rings formation;
Crystals can be formed by various methods, such as: cooling, evaporation, addition of a second solvent to reduce the solubility of the solute, solvent layering, sublimation, changing the cation or anion, as well as other methods.
The formation of a supersaturated solution does not guarantee crystal formation, and often a seed crystal or scratching the glass is required to form nucleation sites.
A typical laboratory technique for crystal formation is to dissolve the solid in a solution in which it is partially soluble, usually at high temperatures to obtain supersaturation. The hot mixture is then filtered to remove any insoluble impurities. The filtrate is allowed to slowly cool. Crystals that form are then filtered and washed with a solvent in which they are not soluble, but is miscible with the mother liquor. The process is then repeated to increase the purity in a technique known as recrystallization.
For biological molecules in which the solvent channels continue to be present to retain the three dimensional structure intact, microbatch crystallization under oil and vapor diffusion have been the common methods.
Typical equipment
Equipment for the [|main industrial processes for crystallization].- Tank crystallizers: Tank crystallization is an old method still used in some specialized cases. Saturated solutions are allowed to cool or evaporate in tanks. After a period of time the mother liquor is drained and the crystals removed. Nucleation and size of crystals are difficult to control. Typically, labor costs are very high. Tank crystallizers can come in multiple forms:
- * Cooling crystallizers can be used in the case where solubility varies significantly with temperature. Temperature decrease is achieved through the use of a coolant liquid circulating within an intermediate jacket. A mixer is often included inside the tank to provide internal circulation and temperature equilibration.
- * Evaporative crystallizers employ open tanks to remove solvent over time, thus precipitating crystals by increasing the solute concentration above the solubility threshold. This process is insensitive to change in temperature. these tanks can be further subdivided into tubular evaporators and plate evaporators.
- Mixed-Suspension, Mixed-Product-Removal : MSMPR is used for much larger scale inorganic crystallization. MSMPR can crystalize solutions in a continuous manner.
- Draft Tube and Baffle ''crystallizers'': Conceptualized in the late 1950s, DTB crystallizers contain an internal circulator that moves the solution upwards at a very low velocity, and a settling area in an annulus around the tank. Large crystals rise and settle against the annulus continuously, while smaller crystals are filtered through a side-tube in the settling area, re-dissolved, and returned to the mother liquor to sustain supersaturation and promote large crystal growth.
Thermodynamic view
The molecules within a pure, perfect crystal, when heated by an external source, will become liquid. This occurs at a sharply defined temperature. As it liquifies, the complicated architecture of the crystal collapses. Melting occurs because the entropy gain in the system by spatial randomization of the molecules has overcome the enthalpy loss due to breaking the crystal packing forces:
Regarding crystals, there are no exceptions to this rule. Similarly, when the molten crystal is cooled, the molecules will return to their crystalline form once the temperature falls beyond the turning point. This is because the thermal randomization of the surroundings compensates for the loss of entropy that results from the reordering of molecules within the system. Such liquids that crystallize on cooling are the exception rather than the rule.
The nature of the crystallization process is governed by both thermodynamic and kinetic factors, which can make it highly variable and difficult to control. Factors such as impurity level, mixing regime, vessel design, and cooling profile can have a major impact on the size, number, and shape of crystals produced.
Dynamics
As mentioned above, a crystal is formed following a well-defined pattern, or structure, dictated by forces acting at the molecular level. As a consequence, during its formation process the crystal is in an environment where the solute concentration reaches a certain critical value, before changing status. Solid formation, impossible below the solubility threshold at the given temperature and pressure conditions, may then take place at a concentration higher than the theoretical solubility level. The difference between the actual value of the solute concentration at the crystallization limit and the theoretical solubility threshold is called supersaturation and is a fundamental factor in crystallization.Nucleation
Nucleation is the initiation of a phase change in a small region, such as the formation of a solid crystal from a liquid solution. It is a consequence of rapid local fluctuations on a molecular scale in a homogeneous phase that is in a state of metastable equilibrium. Total nucleation is the sum effect of two categories of nucleation – primary and secondary.Primary nucleation
Primary nucleation is the initial formation of a crystal where there are no other crystals present or where, if there are crystals present in the system, they do not have any influence on the process. This can occur in two conditions. The first is homogeneous nucleation, which is nucleation that is not influenced in any way by solids. These solids include the walls of the crystallizer vessel and particles of any foreign substance. The second category, then, is heterogeneous nucleation. This occurs when solid particles of foreign substances cause an increase in the rate of nucleation that would otherwise not be seen without the existence of these foreign particles. Homogeneous nucleation rarely occurs in practice due to the high energy necessary to begin nucleation without a solid surface to catalyze the nucleation.File:Crystallization diffraction TEM.png|thumb|433x433px|Time-resolved TEM diffraction patterns of ZrO2 showing the transition from an amorphous to a crystalline structure under electron beam exposure, evidenced by the emergence of Bragg peaks.
Primary nucleation has been modeled as follows:
where