Particle drying forces

Adhesion and cohesion forces in the immobile liquid films between individual primary powder particles. These forces may not contribute significantly to the final granule strength once the granules have been dried. These forces arise from the thin liquid layers around particles that may be generated due to moisture uptake by the powders. This immobile liquid film tends to decrease the inter-particle distances leading to an increase in the van der Waals forces between particles. 

One of the main problems is that the wafer cleaning is much more difficult after the polishing process. As the wafer becomes smoother, particles adhere more securely. There are several possible reasons for this. As the surface is made cleaner, the work of adhesion W increases so that the force F for removal, F = WD, increases. The presence of water or humidity keeps the adhesion down, but if the particles dry, then the adhesion can increase significantly. Figure 13.12b shows a particle on a wafer with a water meniscus condensed from the humid atmosphere. Silicic acid corrosion products form in the meniscus over a period of weeks, gluing the particle to the surface and giving strong adhesion, up to 50 UN. ... 

Inertial mechanisms used for particle removal depend on the fact that the radius of the path of a particle in a rapidly moving, curving airstream is larger than the path of the stream as a whole. Therefore, when a gas stream is spun by vanes, a fan, or a tangential gas inlet, the particulate matter may be collected on a separator wall because the particles are forced outward by centrifugal force. Devices utilizing this mode of operation are called dry centrifugal collectors. 

Dry mixing technology brought to center stage surface properties and powder particle interaction forces. The consequences were zeolite surface charges and sorption properties became as important as, for example, lEC. 

Other Centrifugal Collectors. Cyclones and modified centrifugal collectors are often used to remove entrained Hquids from a gas stream. Cyclones for this purpose have been described (167—169). The rotary stream dust separator (170,171), a newer dry centrifugal collector with improved collection efficiency on particles down to 1—2 pm, is considered more expensive and hence has been found less attractive than cyclones unless improved collection in the 2—10-pm particle range is a necessity. A number of inertial centrifugal force devices as well as some others termed dynamic collectors have been described in the Hterature (170). 

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