Seed ciystals

Another method of fractional crystallization, in which advantage is taken of different ciystallization rates, is sometimes used. Thus, a solution saturated with borax and potassium chloride will, in the absence of borax seed ciystals, precipitate only potassium chloride on rapid coohng. The borax remains behind as a supersaturated solution, and the potassium chloride crystals can be removed before the slower borax crystalhzation starts. 

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. 

While most of the industrial designs in use today are built to reduce the problems due to excess nucleation, it is true that in some ciystal-lizing systems a deficiency of seed ciystals is produced and the prod-... 

In the case of lactose manufacture, crystals of uniform size are produced by first grinding part of a previous batch and taking a quantity with the required nunriiber of particles, then introducing these as seed crystals into a solution that is gradually cooled with gentle stirring. Variation in size of the seed ciystals does not affec t me size of the produc t ciystals. 

Fines and dust added with a batch of seed ciystals. 

Diaspore (P-Aluminum Oxide Hydroxide). Diaspore, found in bauxites of Greece, China, and the USSR, can also be obtained by hydrothemial transfomiation of gibbsite and boehmite. Higher (>200°) temperatures and pressure (>15 AlPa-150bar) are needed for synthesis and the presence of diaspore seed ciystals helps to avoid boehmite fomiation. 

Experiments in laboratoiy and industrial ciystallizers have shown that nuclei ate bom at supersaturations Ac Ac gj hom in presence of crystals (either product crystals or added seed ciystals). Such nuclei are called secondary nuclei. This secondary nucleation caused by the removal of preordered species on a crystal sm-face and attrition fragments can take place at very small supersaturations however, < c gj hetsecondary nuclei. In Fig. 8.4-1 the solubility c and the three metastable zone widths ACmet,hom, Ac gj het, a d Ac gj, gg Valid for homogeneous, heterogeneous, and secondary nucleation, respectively, are shown as a function of temperature T. 

In the forced-circulation-type crystallizer (Fig. 19-43) primaiy control over particle size is exercised by the designer in selecting the circulating system and volume of the body. From the operating standpoint there is little that can be done to an existing unit other than supply external seed, classify the discharge ciystals, or control the shiny... 

Figure 2. Change in average crystal size with time for ice ciystal seeds grown in 6% lactose under semi-batch conditions with removal of crystal slurry.

A single ciystal prepared in this way is already oriented for example, it is oriented parallel to the face of the cleaved mica ciystal base. Ciystals starting on this seed are pulled out of a melt. Another method of forming a single ciystal is to have it cut and polished. Cutting is done with a spark cutter on the ciystal cradled in the arms of a goniometer, which allows the angle of the spark cutter to be precisely oriented in the direction of the desired plane. 

When the attachment of the substrate to the precipitate to be formed is strong, the clusters tend to spread themselves out on the substrate and form thin surface islands. A special limiting case is the formation of a surface nucleus on a seed crystal of the same mineral (as in surface nucleation ciystal growth). As the cohesive bonding within the cluster becomes stronger relative to the bonding between the cluster and the substrate, the cluster will tend to grow three-dimensionally (Steefel and Van Cappellen, 1990). 

Fig. 23.4. sPPcO20 Relationship between ciystallization temperature and crystal thickness in the range around the point of intersection between the melting- and the crystallization line. Isothermal crystallizations were carried out with the aid of the self seeding technique [8,15]... 

A Figure 13.10 Sodium acetate readiiy forms supersaturated solutions in water, (a) When a seed crystal of NaC2H302 is added, excess NaC2H302 ciystallizes from the solution, as shown in (b) and (c). 

Growth of Large-Diameter CaAs Single Ciystals 245 Unloaded seed after dip... 

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