Melt suspension crystallization

SaarM. O., Manga M., Cashman K. V., and Fremouw S. (2001) Numerical models of the onset of yield strength in crystal-melt suspensions. Earth Planet. Sci. Lett. 187, 367-379. 

Suspension crystallization crystals and melt same temperature design on degree of supersaturation separation of crystals from melt depends on density difference in countercurrent operation. Scraped wall crystallizer (Section 16.11.4.6). 

In both, layer and suspension crystallization solid material forms from the melt starting with a nucleus through which a solid/liquid interface is created. As crystallization proceeds the mass of solidified substance steadily increases which causes the interface to move. The impurity components remaining in the melt thereby enrich in front of the solid/liquid interface, forming a concentration boundary layer. The concentration profile in this boundary layer changes as the interface advances which is in literature referred to as moving boundary problem. ... 

In suspension crystallization the melt is cooled to below the stauration temperature, and crystals grow under adiabatic conditions. The degree of supersaturation is the driving force. Specialist knowledge is required to obtain crystals with a certain purity, structure, and particle size distribution. The residual mdt, which contains the impurities, must be mechanically separated from the crystals. 

In film crystallization the crystals grow on a cooled wall. Therefore, the crystals are colder than the melt (nonadiabatic process), and the driving force is the temperature gradient. Thus, rates of crystal growth that are 10-100 times higher than in suspension crystallization are attainable. 

The crystals have to be separated from the remaining melt (mother liquor) to achieve the intended purification. In case of layer crystallization, this is done by draining the remaining melt, collecting it separately, and melting down the crystal layer afterward (Fig. 8.2-11). In case of suspension crystallization the solid-liquid separation is done either by conventional filtration or by a sedimentation apparatus, with or without support of centrifugal forces. Another device repeatedly discussed in the context of solid-liquid separation is the wash column (Arkenbout 1995). 

Fig. 7-38. Jacket-cooled crystallizer with cooling disc mounted on a rotor to cool stepwise and to considerably avoid backmixing of melt or crystal suspension.

The melt crystallization can be carried out with the aid of a suspension crystallization or a layer crystallization, possibly in conjimction with a wash column or a centrifuge, or some other pvuification technique [9-11]. 

Concepts of plants can be divided into solid layer and suspension crystallization. Furthermore, these two techniques can be split into continuous and batchwise as well as into static and dynamic (stagnant or flowing melt) operating modes. A detailed overview of the different designs of existing and commercially available plants in solid layer as well as suspension crystallization is provided in Chapter 16. In the Sections... 

Melt crystallization is carried out either with a suspension of crystals or an advanciag front (layer) of soHds, although a more complete categorization of melt crystallization is available (71). FoUowiag is a brief review of processes ia which melt crystallization is used a more complete review, including a worked out case study for system design, is available (69). 

A suspension of crystals formed from the melt may be contacted by weU-mixed mother Hquor or the crystals may be moved countercurrently to hquor flow ia a vertical or horizontal column. In column crystallizers, crystals are moved ia a specific direction by gravity or rotating blades. The crystals are melted by the addition of heat when they reach a designated end of the crystallizer a portion of the melt is removed as product and the remainder is returned to the system to flow countercurrently to and to wash the product crystals. 

---