Crystal spiral

Figure A3.14.il. Spiral waves imaged by photoelectron electron microscopy for the oxidation of CO by O2 on a Pt(l 10) single crystal under UHV conditions. (Reprinted with pennission from [35], The American Institute of Physics.)...

Lake Texcoco. Lake Texcoco, a few miles northeast of Mexico City, is in the lowest part of the Valley of Mexico. The lake is mostly dry and alkaH is recovered from brine weUs that have been drilled into the underlying stmcture. The brine is concentrated first in a spiral flow solar evaporation pond and further in conventional evaporators. This strong brine is carbonated and then cooled to crystallize sodium bicarbonate which is subsequently filtered and calcined to soda ash. Purity of this product is similar to Magadi material (9,29). 

FIG. 22-9 Center-fed column crystallizer with a spiral-type conveyor. 

Scale-up depends on the mechanical complexity of the crystal-transport system and techniques for removing heat. Vertical oscillating spiral columns are likely limited to about 0.2 m in diameter, whereas horizontal columns of several meters are possible. Scale-up is limited by design of melter and/or crystal-washing section. Vertical or horizontal columns of several meters in diameter are possible. 

Figure 3.23. A growth spiral on a silicon carbide crystal, originating from the point of emergence of a screw dislocation (courtesy Prof, S, Amelinckx).

Figure 5.5 Development of a crystal growth spiral staring from a screw dislocation...

FIG. 4 Experimental observation of growth spirals on a salt crystal. (Courtesy of K. Reichelt, Jiilich.)... 

Tellurium has only one crystalline form and this is composed of a network of spiral chains similar to those in hexagonal Se (Fig. 16.1c and d). Although the intra-chain Te-Te distance of 284 pm and the c dimension of the crystal (593 pm) are both substantially greater than for Scjt (as expected), nevertheless the closest interatomic distance between chains is almost identical for the 2 elements (Te Te 350 pm). Accordingly the elements form a continuous range of solid solutions in which there is a random... 

The BaBPOs compound was first prepared and structurally characterized by Bauer [12]. Figure 21.2 shows the crystal structure of BaBPOs. Its structure is similar to all stillwellite-like compounds with the space group P322. Its main structural elements are spiral tetrahedral chains [001] built of three-membered rings. The contact between the BO4 tetrahedra that form the central part of these chains are reinforced by PO4 tetrahedra and thus [BPO5] heterotetrahedral chain complexes are produced. 

This type of volume defect in the crystal is known as a "screw dislocation", so-called because of its topography. Note that the spiral dislocation of the growing lattice deposits around the Une defect at right angles to the line defect. 

Dislocations Dislocations are stoichiometric line defects. A dislocation marks the boundary between the slipped and unslipped parts of crystal. The simplest type of dislocation is an edge dislocation, involving an extra layer of atoms in a crystal (Fig. 25.2). The atoms in the layers above and below the half-plane distort beyond its edge and are no longer planar. The direction of the edge of the half-plane into the crystal is know as the line of dislocation. Another form of dislocation, known as a screw dislocation, occurs when an extra step is formed at the surface of a crystal, causing a mismatch that extends spirally through the crystal. 

Massive barite crystals (type C) are also composed of very fine grain-sized (several xm) microcrystals and have rough surfaces. Very fine barite particles are found on outer rims of the Hanaoka Kuroko chimney, while polyhedral well-formed barite is in the inner side of the chimney (type D). Type D barite is rarely observed in black ore. These scanning electron microscopic observations suggest that barite precipitation was controlled by a surface reaction mechanism (probably surface nucleation, but not spiral growth mechanism) rather than by a bulk diffusion mechanism. 

Figure 20-8 shows the features of a horizontal center-fed column [Brodie, Aust. Mech. Chem. Eng. Trans., 37 (May 1979)] which has been commercialized for continuous purification of naphthalene and p-dichlorobenzene. Liquid feed enters the column between the hot purifying section and the cold freezing or recovery zone. Crystals are formed internally by indirect coohng of the melt through the walls of the refining and recoveiy zones. Residue liquid that has been depleted of product exits from the coldest section of the column. A spiral conveyor controls the transport of solids through the unit. 

Another center-fed design that nas only been used on a preparative scale is the vertical spiral conveyor column reported by Schildknecht [Angew. Chem., 73, 612 (1961)]. In this device, aversion of which is shown on Fig. 20-9, the dispersed-crystal phase is formed in the freezing section and conveyed downward in a controlled manner by a rotating spiral with or without a vertical oscillation. 

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