Crystal encrustations

Both vacuum and evaporating crystallizers generally have considerable height requirements to provide sufficient free space above the boiling liquor surface to allow entrained liquid droplets to disengage from the upflowing vapours. To minimize liquid droplet entrainment into the vapour space above the boiling liquid surface, a common cause of crystal encrustation on the walls, the evaporation rate must be controlled to limit the upward vapour velocity, max, an estimate of which may be made from the relationship... 

Experimental techniques for studying the early stages of crystal encrustation have been reported by Chandler (1964), Veverka and Nyvlt (1979), Duncan and Phillips (1979), Goldman and Spott (1981) and Chianese, Di Cave and Mazzarotta (1990). Examples of precautions that should be taken in the design and operation of crystallizers to minimize encrustation are given by Nyvlt (1978), Wohlk and Hofmann (1980), Shock (1983), and Sohnel and Garside (1992). 

The most frequent site for crystal encrustation is on a compatible solid surface within a zone of high supersaturation and low agitation. Selection of a less compatible material having a smooth surface can avoid the major excesses of encrustation. Duncan and Phillips (1979) and Shock (1983), respectively, reveal a connection between the metastable zone width of crystallizing solutions and their propensity to encrust. It is well known that judicious crystal seeding can also help minimize encrustation. Simple laboratory tests are recommended to determine all these issues before the plant is built. 

Crystallization process control is desirable from a number of standpoints. The primary objective is often to meet customer requirements by achieving consistent product quality to a desired specification of crystal size, size distribution and purity. Secondly, process requirements often dictate maintenance of stable crystallizer operation, the avoidance of fines and encrustation, and the minimization of subsequent downstream processing. 

Shock, R.A.W., 1983. Encrustation of crystallizers. Journal of separation process technology, 4, 1-13. 

There is almost no degree of freedom in the choice of the crystallizer type Crystallization in fine chemicals manufacture is usually carried out in jacketed stirred tanks. Coils can also be used to enhance cooling, but crystals are often formed on their surface. This encrustation results in a large increase of the thermal resistance, and, consequently, a decrease of the cooling capability of the tank. Any roughness of inner surfaces of the tank can be a germ for crystals, particularly the cooled surfaces. Therefore, crystallizers are often made with polished cooled surfaces. 

The horizontal continuous Brodie melt crystallizer is basically an indirectly cooled crystallizer with an internal ribbon conveyor to transport crystals countercurrent to the liquid and a vertical purifier for final refining. Figure 20-8 describes the operation of a single tube unit and Fig. 20-13 depicts a multitube unit. The multitube design has been successfully commercialized for a number of organic chemicals. The Brodie purifier configuration requires careful control of process and equipment temperature differences to eliminate internal encrustations and is limited by the inherent equipment geometry to capacities of less than 15,000 tons per year per module. 

Cu and Zn enter sedimentary material in substantial proportions, both in the structure of minerals (carbonates, clays) and adsorbed on surfaces. Boyle (1981) showed that foraminiferal tests may contain Zn in excess of a few ppm. Partitioning of Cu and Zn between water and carbonates has been investigated by Rimstidt et al. (1998). The crystal chemistry of Cu and Zn in goethite has been investigated by EXAFS by Manceau et al. (2000). Typical Zn and Cu concentrations in FeMn nodules and encrustations are 500-1000 ppm and 800-6000 ppm, respectively (e.g., Albarede et al. 1997b). 

Power ultrasound also has an additional property which is particularly beneficial in crystallisation operations namely that the cleaning action of the cavitation effectively stops the encrustation of crystals on cooling elements in the crystallisation vat and thereby ensures continuous efficient heat transfer. 

If iron wire is used exposed in a bundle to the hydrogen sulphide, it readily becomes encrusted with tiny crystals, silver-white in appearance when first prepared. The crystals 1 are regarded as belonging to the hexagonal system. 

Decay of stone can take place through efflorences which are encrustations formed at the surface they produce discoloration at the facades of buildings and upon repeated crystallization and hydration, they mechanically disintegrate the stone. 

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