SIZE ENLARGEMENT OF PARTICLES

The size of particles may be increased from molecular dimensions by growing them by crystallisation from both solutions and melts as discussed in Chapter 15. Here, dissolving and recrystallising may provide a mechanism for controlling both particle size and shape. It may be noted, as also discussed in Chapter 15, that fine particles may also be condensed out from both vapours and gases. 

There are essentially two types of processes that can cause agglomeration of particles when they are suspended in a fluid  

According to Sohnel and Mullin(29), the change in agglomerate size as a function of time may be represented by equations of the form  

The stability of the aggregates may be increased by the effects of mechanical interlocking that may occur, especially between particles in the form of long fibres. 

As discussed in Chapter 15, the size distribution of particles in an agglomeration process is essentially determined by a population balance that depends on the kinetics of the various processes taking place simultaneously, some of which result in particle growth and some in particle degradation. In a batch process, an equilibrium condition will eventually be established with the net rates of formation and destruction of particles of each size reaching an equilibrium condition. In a continuous process, there is the additional complication that the residence time distribution of particles of each size has an important influence. 

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For many purposes, lumps of materials of intermediate sizes are the most desirable forms, neither too small nor too large. For instance, beds of overly small granules of catalysts exhibit too great resistance to flow of reacting fluids, and too small particles in suspensions settle out or filter too slowly. Other situations that benefit from size enlargement of particles are listed in Table 12.11. 

This monograph on size enlargement of particles is the first of a series which will together form a Handbook of Powder Technology, primarily intended for engineers and scientists working in industry. 

See Size enlargement Size measurement of particles Size reduction. 

Humidity high humidity may cause enlargement of particle size, thus reducing effectiveness. 

Oil - [COALCONVERSIONPROCESSES - CARBONIZATION] (Vol6) - [COALCONVERSIONPROCESSES - GASIFICATION] (Vol6) - [SIZE ENLARGEMENT] (Vol 22) -fuel for limestone kilns [LIME AND LIMESTONE] (Vol 15) -hydrogen from [HYDROGEN] (Vol 13) -measurement of particles m [SIZE MEASUREMENT OF PARTICLES] (V ol 22) -m nuts [NUTS] (Vol 17) -as petroleum lubricant [LUBRICATION AND LUBRICANTS] (Vol 15) -pipeline transport [PIPELINES] (Vol 19) -sample analysis by ms [MASS SPECTROMETRY] (Vol 15) -from shale [OIL SHALE] (Vol 17) -steel quenching m [STEEL] (Vol 22) -sulfur removal from [SULFUR] (Vol 23) -tanks for [TANKS AND PRESSURE VESSELS] (Vol 23) -use of fluidization [FLUIDIZATION] (Vol 11)... 

Size enlargement of fine particles in liquid suspension can be accomplished in a number of ways. Electrolytes can be added to a suspension to cause a reduction in zeta potential and allow colliding particles to cohere. Examples include the use of trivalent aluminum and iron ions to flocculate the particles responsible for the turbidity of many water supplies and the flocculation of metallurgical slimes by pH adjustment to the isoelectric point. Alternatively, polymeric flocculants can be added to suspensions to bridge between the particles. A wide range of such polymeric agents [1] is available today to aid the removal of fine particles from water. 

Therefore, there is a quickly increasing emphasis on size enlargement of these finely divided particulate solids. Tasks are to increase the size of powders to particles that are large and heavy enough to avoid scattering by wind or water and to produce permanent bonds that are waterproof, survive freeze-thaw cycles, and, preferably, immobilize leachable compounds. 

Calculations and measurements have shown that condensation is the dominant effect. As condensation only occurs until the temperature of the solids reaches the dew point temperature that corresponds to the relative humidity of the gas atmosphere, particle inlet temperature has an important influence on the amount of water that can condense. Steam jet agglomeration is normally applied for the size enlargement of water soluble materials in the food industry to obtain products with instant characteristics (see also Section 5.4). Since during the condensation of steam not only a thin, uniform coating of water is formed on the solids but also thermal energy is transmitted, a maximum of the water soluble material is dissolved which produces recrystallization bonds during drying (see Section 5.1.1)... 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

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