Small ciystals

Trimaonesic Dicabbonate— (MgCO,),MgH,0, -f 2 Aq-22C 4 36-is formed in small ciystals when a solution of MgSO is precipitated with excess of Na,CO, and the mixture boiled. 

Crystallizers with Fines Removal In Example 3, the product was from a forced-circulation crystallizer of the MSMPR type. In many cases, the product produced by such machines is too small for commercial use therefore, a separation baffle is added within the crystallizer to permit the removal of unwanted fine crystalline material from the magma, thereby controlling the population density in the machine so as to produce a coarser ciystal product. When this is done, the product sample plots on a graph of In n versus L as shown in hne P, Fig. 18-62. The line of steepest ope, line F, represents the particle-size distribution of the fine material, and samples which show this distribution can be taken from the liquid leaving the fines-separation baffle. The product crystals have a slope of lower value, and typically there should be little or no material present smaller than Lj, the size which the baffle is designed to separate. The effective nucleation rate for the product material is the intersection of the extension of line P to zero size. 

The transfer of supersaturated liquor from the vaporizer (point B, Fig. 18-69) often causes salt buildup in the piping and reduction of the operating cycle in equipment of this type. The rate of buildup can be reduced by circulating a thin suspension of solids through the vaporizing chamber however, the presence of such small seed ciystals tends to rob the supersaturation developed in the vaporizer, thereby lowering the efficiency of the recirculation system. 

Scraped-Surface Crystallizer For relatively small-scale apph-cations a number of ciystallizer designs employing direct neat exchange between the shiny and a jacket or double wall containing a cooling medium have been developed. The heat-transfer surface is scraped or agitated in such a way that the deposits cannot build up. 

The nitric acid is warmed gently on the water-bath in a larj O flask (I litre) with the addition of the vanadium pentoxidc. It then placed in the fume cupboard and the cane sugar at onc f added.. As soon as torrents of brown fumes begin to be evolved, the flask is placed in cold water. After the reaction has ceased tin liquid is left for twenty-four hours when colourless crystals of the acid separate. A further small quantity may be obtained fiom the mother liquor on standing. The ciystals are drained oia a small porcelain funnel without filter paper, and recrystallised from a very small quantity of water. Yield, 15—20 grams. 

The particle size of precipitated potassium heptafluorotantalate is one of the more important parameters. In order to achieve a certain particle size, potassium salts are added to the hot tantalum strip solution as a hot solution. The mixture is cooled down at a specific rate in order to enable the precipitation and ciystallization of K-salt in the form of small, individual crystals. 

When a ciystal breaks into smaller pieces, flagmentation occurs along crystal edges. Small crystals have the same characteristic angles as larger crystals. 

A ciystal lattice can be broken down to a small repeating unit called a unit cell. There are only seven distinct unit cells possible, and are shown in Figure 2.1. 

The small octaazacryptand, 2, is selective for fluoride [44], with extremely high affinity, log Ka = 10-11 in aqueous solution [44-46], As anticipated, the ciystal structure of 2 with fluoride showed the halide to be encapsulated Figure 5) [44], Later theoretical and modeling studies indicated that the small size of the cavity could preclude encapsulation of larger anions [45],... 

The precipitated precursor can be dissolved and re-ciystallized from fluorine-free solutions. This provides excellent conditions for deep purification of the material and reduction of problematic impurities such as titanium, fluorine, etc. Peroxometalates decompose at relatively low temperatures forming tantalum or niobium oxides containing small amount of absorbed water. The absorbed water separation is achieved by further thermal treatment - drying and calcination - of the product ... 

Potassium heptafluorotantalate, K2TaF7, precipitates in the form of transparent needles. The precipitated particles must not be too fine, since fine powder usually promotes co-precipitation and adsorption of some impurities from the solution. Even niobium can be adsorbed by the surface of K2TaF7 developed during precipitation, as shown by Herak et al. [535]. On the other hand, the precipitation of large K-salt crystals should not be strived for either. Laboratory and industrial experience indicates that excessively large ciystals usually contain small drops of solution trapped within the ciystals. This occluded solution can remain inside of the ciystal until diying and will certainly lead the hydrolysis of the material. 

The exceptional large value of forbidden gap width seems to be any doubtful. Perhaps it means that not all of the calculations results obtained for small clusters can be used for prediction of crystal structures properties. Must be noted that calculated bonds lengths in squares and hexagons and period of ciystal lattice (1.607 A, 1.452 A, 5.894 A) differ from the values (1.503 A, 1.380 A, 5.545 A), obtained in [8]. However one can not expect the better accordance from the results of clusters calculations performed moreover in the frames of semi-empirical PM3-basis. 

The term crystal habit is often used to describe the relative sizes of the faces of a crystal. Crystal habit is readily modified by conditions of nucleation and growth, and it is rather difficult to prepare ciystals with all faces of the same form equally developed (M2). Small amounts of soluble impurities, especially dyes, which may be adsorbed selectively on the different faces of a crystal, cause these faces to be suppressed in favor of others. This can alter the external geometry of a crystal completely, except for its interfacial angles. Many examples of crystal habit modification are reported in the literature (B8), and in some commercial... 

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