Crystallization from a melt

Continuous fractionation is carried out in countercurrent flow. For the continuous fractionation of a solution containing two dissolved substances, a split column is used (Fig. 7-30). In the upper column section, the crystallization column, the less soluble component crystallizes during the counter-current contact of the mother liquor and the crystal. In the lower section, the concentrating or enrichment column, the more soluble component is favorably extracted from 

CC Crystallization column ES Enrichment section EC Evaporator crystallizer FI Filter UR Upper reflux LR Lower reflux 

The required number of stages for fractional crystallization are determined by methods given in [7.1]. The Ponchon-Savarit and the McCabe-Thiele diagram are both commonly used. 

Conversely, in recrystallisation processes, only the crystalline phase is purified by further treatment, not the mother liquor. 

Crystallization from a pure melt consisting of only one component is a simple formulation process for the preparation of the sales product, and may be carried out on water-cooled steel belts or on internally cooled drums which dip into the melt (Fig. 7-31). The operating conditions for the crystallization unit are given by the path of the melting point pressure curve of the component to be treated (see Chapter 1.4.5.2). 

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Powder Preparation. There are several routes to preparing SiC powders having variable purity levels, crystal stmcture, particle size, shape, and distribution. Methods that have been examined include growth by sublimation from the vapor phase, carbothermic reduction, and crystallization from a melt. 

Chromite crystallization from a melt, or its re-equilibration with a melt, creates an even more reduced boundary layer at the crystal-melt interface as a consequence of the incorporation of Cr " and Fe " relative to divalent ions. The reduced conditions trigger the precipitation of IPGE minerals on the surface of crystallizing chromite that can be later on entrapped during chromite growth (Finnigan etal. 2008). 

Backscattered electron image of a Type CAI in the Allende CV3 chondrite. This object crystallized from a melt and has an outer mantle of melilite (Mel) surrounding an inner zone of pyroxene (Px) and anorthite (An). Tiny grains of spinel (Sp) are abundant in the inner portion of the inclusion. Image courtesy of A. Krot. 

Dry fractionation is defined as crystallization from a melt without dilution with solvent (O Brien, 1998 Anderson, 1996 Bailey, 1950). This process is the simplest and least expensive process for separating high and low-MP fatty derivatives (Illingworth, 2002 Kellens and Hendrix, 2000 O Brien, 1998 Anderson, 1996 Krishnamurthy and Kellens, 1996). It is also the most commonly practiced form of fat fractionation technologies currently in use (Illingworth, 2002 O Brien, 1998). 

Accumulated minerals These are crystals in lavas that are in excess of the amounts that could crystallize from a melt in a closed system. Like cumulates, some cases of accumulated crystals may be impossible to detect, but others stand out because they result in rock compositions different from all or most terrestrial melts. 

The Avrami equation (1), originally developed for the crystallization of metals from melt, has been applied by many researchers to the crystallization of oils and fats in order to elucidate information on their crystallization mechanism. The Avrami equation is based on the model of a growing sphere crystallizing from a melt of uniform density without impingement. The usual Avrami exponent, used to draw conclusions with respect to the crystallization mechanism of the system, is observed to be about three or four for oils and fats after rounding off to whole integers. 

Derivation Crystallized from a melt of Na20 and Si02 at approximately 1089C. 

An intimate intergrowth of two different compounds or crystal structures as a consequence of their simultaneous crystallization from a melt. 

Closely allied with the concepts of metastable equilibrium and suspended transformations is Ostwald s rule (Ostwald s step rule or law of successive reactions). Essentially Ostwald s rule states that in all processes it is not the most stable state with the least amount of free energy that is initially obtained but the least stable state lying nearest to the original state in free energy (Ostwald 1897). It is easy to see how this rule and the concept of suspended transformations can explain the production of a metastable polymorph through crystallization from a melt or solution. 

Polyphosphate is defined in this work as any linear, condensed phosphate, in which the phosphate, but not necessarily the system containing the phosphate exhibits the conditions 1 < M2O/P2O5 < 2, where M is any single or mixed metals with a total equivalency of unity. The definition is required to differentiate the total composition of a system from the polyphosphates crystallizing in a melt. An example is a polyphosphate crystallizing from a melt of ultraphosphate composition where several metal oxides may be involved. 

Three possibilities exist when a salt with a polyphosphate x-ray pattern crystallizes from a melt containing an excess of phosphorus pentoxide. 1. The phosphorus pentoxide is incorporated into the polyphosphate chains converting the chains to crystalline ultraphosphates. 2. The excess phosphorus pentoxide does not enter the polyphosphate crystal structure, but forms an amorphous phase between the crystals of polyphosphate. The amorphous phase is not detected by x-ray. 3. The excess phosphorus pentoxide does not enter the crystal structure of the polyphosphate, but forms as an ultraphosphate between the crystalline polyphosphate crystals as a eutectic phase. (This latter case is precisely what happens in the calcium sodium ultraphosphate system from which calcium phosphate fibers are grown (21) and the phase diagram of Hill et. al. is obeyed as it should be.)... 

PRIMARY NUCLEATION. In scientific usage, nucleation refers to the birth of very small bodies of a new phase within a supersaturated homogenous existing phase. Basically, the phenomenon of nucleation is the same for crystallization from solution, crystallization from a melt, condensation of fog drops in a supercooled vapor, and generation of bubbles in a superheated liquid. In all instances, nucleation is a consequence of rapid local fluctuations on a molecular scale in a homogenous phase that is in a state of metastable equilibrium. The basic phenomenon is called homogeneous nucleation, which is further restricted to the formation of new particles within a phase iminfluenced in any way by solids of... 

As an example, we will grow a crystal from a melt of oxides whose melting points exceed 1800 C. Automatically, we are limited to use of an Ir crucible and we will use an R.F.- generator for the power source. 

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