Suspension solid-liquid separation technique

Figure 7.12 Impurity content as a result of suspension (melt) crystallization of diphenyl from 80% diphenyl/20% naphthalene mixtures and application of different solid-liquid separation techniques (reproduced with permission from Ref (6j).

All of the drained casting techniques, e.g. slip casting, pressure casting and centrifugal casting, involve a solid-liquid separation process to form a dense green body (Figure 9.8). A mould is filled with a suspension and the liquid is separated from the solid particles. 

Flotation. The use of ak or gas bubbles to separate mineral particles by preferential attachment and subsequent flotation has been a feature of the mineral processing industry for many decades, but the application of the principle to solid-liquid separations is of more recent origin. The techniques have similarities but differences. In mineral flotation it is more common to use chemicals to enhance the collection of the particles and the stability of the ffoth and to produce the bubbles by beating air into the suspension, but in solid liquid separation it is more usual to dissolve air into the liquid under pressure, allowing the gas to expand out of solution under the lower pressure of the separation chamber, producing bubbles for the preflocculated particles to collect on without the need for further chemical addition. 

The separation of solids in suspension in liquids can be achieved by either of the following techniques... 

Leoni [366] observed that in the extraction preconcentration of organochlo-rine insecticides and PCB s from surface and coastal waters in the presence of other pollutants such as oil, surface active substances, etc., the results obtained with an absorption column of Tenax-Celite are equivalent to those obtained with the continuous liquid-liquid extraction technique. For non-saline waters that contain solids in suspension that absorb pesticides, it may be necessary to filter the water before extraction with Tenax and then to extract the suspended solids separately. Analyses of river and estuarine sea waters, filtered before extraction, showed the effectiveness of Tenax, and the extracts obtained for pesticide analysis prove to be much less contaminated by interfering substances than corresponding extracts obtained by the liquid-liquid technique. Leoni et al. [365] showed that for the extraction of organic micro pollutants such as pesticides and aromatic polycyclic hydrocarbons from waters, the recoveries of these substances from unpolluted waters (mineral and potable waters) when added at the level of 1 xg/l averaged 90%. 

Centrifugation alone is not sufficient to separate Np solid material from the solution (j) ). Thus, the Np solution concentrations and oxidation state analyses were determined from aliquots filtered through approximately 1.8 nm pore-size Centriflo membrane cones (Amicon Corp., Lexington, MA). Each filter cone was pretreated with an initial aliquot of the suspension to be filtered. Tests using successive filtration confirmed that no significant amounts of Np were being sorbed by the filter cones. The solutions were alpha counted by liquid scintillation techniques. Major cations in the solutions were determined by inductively coupled plasma spectrometry (ICP) analyses. 

Recently an improvement in this separation technique was reported, which seemed to indicate that enantioselective inclusion in the lattices of chiral hosts could be employed on a large scale. [11] When crystalline hosts such as R,R)-(-)-S (m.p. 196 °C), [12] (/ ,/ )-(-)-9 (m.p. 165 °C), [12] and (5,5)-(-)-10 (m.p. 128 °C) are suspended in hexane or water, chiral guest molecules form the same inclusion compounds as from solution. This is by no means self-evident, since inclusion compounds have different crystal lattices than the pure host crystals. Thus crystal/liquid reactions occur, and phase rebuildings analogous to those observed in gas/solid reactions [13] must take place. Yet this suspension technique is more selective and more effective than the initially developed solution technique. Numerous racemic alcohols like 11, -hydroxy esters like 12, epoxy esters like 13, and epoxy ketones like 14 were stirred a few hours with appropriate hosts (suspensions of 8, 9, and 10) and formed 1 1 complexes that could be filtered off in yields of > 85 % and with ee values of > 97 % (the complex of 12 and 9 formed in hexane only 80% ee in one step). Recrystallization of the inclusion... 

Filtration is a separation technique used to eliminate a solid in suspension from a liquid by passing it through a filter medium consisting of a porous layer that traps the solid particles. Filtering generally refers to the clarification of a liquid, while filtration is more often used to describe the technical... 

Oirect Filtration (Contact Flocculation). This is a mixture of filtration and flocculation in which the suspension entering the bed has been flocculated, but the floes have not fiiUy formed prior to bed entry. The technique is applied to suspensions of low concentration of colloidal matter. The shear induced inside the deep bed provides excellent conditions for orthokinetic flocculation. The main advantage of direct filtration is a reduction in the capital cost over the provision of a flocculator and separate vessel for solid-liquid... 

There are a large number of processes in the chemical industries that handle a variety of suspensions of solid particles in liquids. The application of filtration techniques for the separation of these heterogeneous systems is sometimes very costly. If, however, the discrete phase of the suspension largely contains settleable particles, the separation can be effected by the operation of sedimentation. The process of sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling. This unit operation is divided into thickening,... 

Solids of different densities can be separated by immersing them in a fluid of intermediate density. The heavier solids sink to the bottom and the lighter float to the surface. Water suspensions of fine particles are often used as the dense liquid (heavy-medium). The technique is used extensively for the benefication (concentration) of mineral ores. 

Mixed solid fertilizers can be made by either direct granulation methods (40%) or bulk blending (40%). Bulk blending is made by mechanical mixing of the separate granular intermediate materials. It is usually done in small plants near the point of use. This technique is employed because the fertilizer can be tailor-made to fit the exact requirements of the user. Fluid or liquid fertilizers (clear, suspension, and slurry) account for 20% of all NPK mixed fertilizers. 

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