Structure formation solid—liquid separation

Crystallization is an important separation process that purifies fluids by forming solids. Crystallization is also a particle formation process by which molecules in solution or vapour are transformed into a solid phase of regular lattice structure, which is reflected on the external faces. Crystallization may be further described as a self-assembly molecular building process. Crystallographic and molecular factors are thus very important in affecting the shape (habit), purity and structure of crystals, as considered in detail by, for example, Mullin (2001) and Myerson (1999). In this chapter the internal crystal structure and external particle characteristics of size and shape are considered, which are important indicators of product quality and can affect downstream proeessing, such as solid-liquid separation markedly. Larger particles separate out from fluids more quickly than fines and are less prone to dust formation whilst smaller particles dissolve more rapidly. 

In summary, for solid-type materials, molecules can form regular arrays leading to crystals, and this also may occur with colloidal-scale particles. For liquid-type materials, molecules may be dispersed on a molecular scale (i.e., dissolved) in a liquid, or they may cluster together into a separate and homogeneous phase (i.e., show phase separation). Colloidal-scale particles may also exist as separate particles in a liquid, but these particles may also cluster into a dense phase. Whether the size of the building blocks is molecular or colloidal, the phenomena of phase separation, clustering and structure formation show many similarities. 

A unique feature of H20 is the formation and sharing of hydrogen bonds with other molecules. Such bonds play a major role in determining the structure of both liquid and solid phases of H20. It is believed that for intermolecular spacings of less than 2 A, the two water molecules exert strong repulsive forces on each other. As such, there exists a hard sphere radius of little interpenetration of the molecules. Usually, the repulsive part of Lennard-Jones 6—12 potential can be considered appropriate to describe these repulsive characteristics. At distances of separation greater than 5 A, dipole-dipole interaction plays a dominant role. This is reasonable, because each H20 molecule has a large dipole moment, p = 1.84 D. 

PORE. I A minute cavity in epidermal tissue as in skin, leaves, or leather, having a capillary channel to the surface that permits transport of water vapor from within outward but not the reverse. 2. A void of interstice between particles of a solid such as sand minerals or powdered metals, that permits passage of liquids or gases through the material in either direction. I11 some structures, such as gaseous diffusion barriers and molecular sieves, the pores ate of molecular dimensions, i.e 4-10 A units. Such microporous structures are useful for filtration and molecular separation purposes in various industrial operations. 3. A cell in a spongy structure made by gas formation (foamed plastic) that absorbs water on immersion but releases it when stressed. 

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