The Process of Crystallization

TABLE 16.2. Maximum Allowable Supercooling AT (°C) and Corresponding Supersaturation AC (g/100g water) at 25°C  

The origin of this concept is that the fraction of the total area under a normal distribution curve between the 16 and 84% points is twice the standard deviation. The smaller CV, the more nearly uniform the crystal sizes. Products of DTB crystallizers, for instance, often have CVs of 30-50%. The number is useful as a measure of consis- 

The questions of interest are how to precipitate the crystals and how to make them grow to suitable sizes and size distributions. Required sizes and size distributions are established by the need for subsequent recovery in pure form and ease of handling, and by traditional commercial practices or consumer preferences. 

A precipitate may be formed as a result of chemical reaction between separately soluble gases or liquids. Commercial examples are productions of sodium sulfate, ammonium sulfate, and ammonium phosphate. 

Precipitation also can be induced by additives, a process generally called salting out because salts with ions common to those whose precipitation is desired are often used for this purpose. For instance, ammonium chloride is recovered from spent Solvay liquors by addition of sodium chloride and the solubility of BaCl2 can be reduced from 32% to 0.1% by addition of 32% of CaCl2. Other kinds of precipitants also are used, for instance, alcohol to precipitate aluminum sulfate from aqueous solutions. 

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The successive crops of crystals are systematically recrystallized as follows, using Norite if necessary. About two-thirds of the first crop is dissolved in about three parts of water and the hot solution allowed to deposit crystals by slow cooling (Note 3). The liquor is filtered or decanted, the remainder of the first crop is dissolved in it, and the process of crystallization is repeated. The remaining crops are then similarly recrystallized in succession from the same liquor, the solution being evaporated to the appropriate volume before each crystallization. The final mother liquor is evaporated in stages and the viscous residue is united with that from the original crystallization. The various crops are systematically recrystallized from fresh water until pure... 

Crystallization generally involves the evaporation and subsequent cooling of a solution to the point of supersaturation, whereupon the formation of crystals takes place. Modern technology often focus on the control of crystal size, since product demands frequently are rigorous in this regard. The process of crystallization is often conducted in evaporators. As in the evaporation of salt and in the recovery of salt and glycerin in soap manufacturing, salt separators are used to remove crystallized materials as rapidly as it settles. 

Crystalline 1 1 complex formation can be regarded as molecular recognition in the process of crystallization. Since the formation of a new type of the crystalline 1 1 complex depends on the shapes of R7 and R8 35), the influence of the spatial relationship between R7 and R8 on the complex formation was investigated. 

When a precipitate separates from a solution, it is not always perfectly pure it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances which are normally soluble in the mother liquor is termed co-precipitation. We must distinguish between two important types of co-precipitation. The first is concerned with adsorption at the surface of the particles exposed to the solution, and the second relates to the occlusion of foreign substances during the process of crystal growth from the primary particles. 

On the basis of the above data it has been hypothesized that the conductivity of PFCM is due not to the contact between the filler particles but the current passes across the thin (less than 1 -2 microns) polymer interlayers. The conductivity arises when a spontaneous pressure exceeding the threshold value develops in the material. The overstresses apparently arise as a result of PP crystallization in the very narrow gaps between the filler particles [312], Since crystallization must strongly affect the macromolecular conformation whereas the narrowness of the gap and fixed position of molecules on the filler prevent it, the heat released in the process of crystallization must, in part, be spent to overcome this hindrance, whereby a local high pressure may arise in the gap. This effect is possible only where there are gaps of the size comparable with that of macromolecules. The small gap thickness will also hamper pressure relaxation, since the rate of flow from such a narrow clearance should be negligibly small. 

A characteristic feature of the structure of samples obtained under the conditions of molecular orientation is the presence of folded-chain crystals in addition to ECC. Kawai22 has emphasized that the process of crystallization from the melt under the conditions of molecular orientation can be regarded as a bicomponent crystallization in which, just as in the case of fibrous structures in the crystallization from solutions, the formation of crystals of the packet type (ECC) occurs in the initial stage followed by the crystallization with folding . 

Much useful understanding of the processes of crystal-field theory, however, can be had from a study of just the free-ion ground terms. Application of the simple process in Section 3.7 identifies the ground terms for d free ions as D, D,... 

The structures of the solids isolated from strongly alkaline solutions cast a shadow of doubt over the simple model in which the only species formed are Be(OH)3 and BelOH) . Of course what crystallizes from a solution may well be a species with low equilibrium concentration as the process of crystallization is driven by the insolubility of the product. Nevertheless it is clear that relatively little effort has... 

Atoms or ions of solute leave solution during the process of crystallization to form a regular repeat lattice. 

To answer this question, we must consider all energetic considerations occurring during the process of crystallization, possibly including phenomena not directly related to the actual processes inside the beaker. 

The process of crystallization is governed by kinetic and thermodynamic effects. The consideration of both of these is important in controlling the polymorphic form of a product. The available states (polymorphs) that a system may reach are defined by thermodynamics, but it is kinetics that guides the path taken between these states, and control when a given state will be reached in a process. In the time constrained environment of a production plant it is often... 

Although disordered structure is fairly common in the known protein structures, this is undoubtedly one of the cases in which the process of crystallization induces a bias on the results observed. Since extensive disorder makes crystals much harder to obtain, it seems probable that disordered regions are even more prevalent on the proteins that do not crystallize. 

The solid product is seldom pure when obtained from a chemical reaction, being contaminated with various impurities, reagents and byproducts. For purification, the process of crystallization, sometimes called recrystallization, is generally employed When dealing with large quantity formulas, the utmost care should be taken to obtain the maximum yield of a pure crystallized compound. 

The. folded-chain lamella theory arose in the last 1950s when polymer single crystals in the form of thin platelets termed lamella, measuring about 10,000 A x 100 A, were grown from polymer solutions. Contrary to previous expectations, X-ray diffraction patterns showed the polymer chain axes to be parallel to the smaller dimension of the platelet. Since polymer molecules are much longer than 100 A, the polymer molecules are presumed to fold back and forth on themselves in an accordionlike manner in the process of crystallization. Chain... 

On crystal faces bounding a polyhedral crystal, step patterns resembling the contour lines on a topographic map or striations in one direction are observable depending on the nature of the face. These show the process of crystal growth or dissolution at an atomic level, and are referred to as the surface morphology or surface microtopography. 

The processes of crystal growth are divided into the following three stages. 

These intergrowth relations are formed through the processes of crystal growth, phase transformation or decomposition associated with a decrease in temperature and pressure, or metasomatism due to the supply of new components from outside. 

What happens in the electrociystallization process How do metals electrogrow on other metals I here are, strictly speaking, two aspects to this question. The first (Fig. 7.120) involves the process of deposition, i.e., the path taken by an ion in solution to move up to and be incorporated in the lattices of the crystals that make up the electrode. The second aspect (Fig. 7.121) concerns the process of crystallization, or... 

Schlesinger and Marton (15) studied the nucleation and growth of electrolessly deposited thin nickel (Ni-P) films. These studies were later extended and complemented by the studies performed by Cortijo and Schlesinger (19, 20) on radial distribution functions (RDFs). RDF curves were derived from electron diffraction data obtained from similar types of films as well as electrolessly deposited copper ones. Those studies, taken together, have elucidated the process of crystallization in the electroless deposition of thin metal films. 

Further hydrolysis proceeds much slower with very small heat evolution (for R = Et and Bun its value is zero within the accuracy of the experiment, while for R = Pr1 it does not exceed 20% of the overall reaction heat). Composition of the hydrolysis products for all h values approximately corresponds to Ti01s(0R) yR0H, where y = 0.15-1 depending on the nature of alcohol and concentration of alkoxide. Solvating alcohol in the hydrolysis products was confirmed by chemical analysis and IR spectroscopy of the products of their thermal decomposition. Residual carbon on thermal treatment in air is eliminated in two steps — at 300°C with formation of amorphous black powder and then in the process of crystallization at 400 to 500. A mixture of anatase and rutile is usually thus formed, calcination at higher temperature gives pure rutile. 

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