Powders cohesive

Spouted beds Wiirster coaters Moderate (layered) 50 ton/hr continuous cohesive powders, good for coating applications detergents Batch pharmaceuticals, agricultural chemicals, nuclear wastes... 

An absorbent may also be necessary when the formulation contains a hygroscopic ingredient, especially when absorption of moisture produces a cohesive powder that will not feed properly to the tablet press. In such instances, silicon dioxide has been found to be of particular value. 

In some cases, especially for very fine powders, researchers have looked at other properties to explain or classify product behavior. For example, Gel dart et al. (1984) have found that the ratio of tapped to aerated bulk density provides a good indication of the likely fluidization characteristics of fine and cohesive powders. 

Group C 20 Fine cohesive powders, difficult to fluidise and readily form channels. Flour Fine silica... 

In simple terms, a cohesive powder can be defined as a material where the adhesive forces between particles exceed the particle weight by at least an order of magnitude. In such systems, particles no longer flow independently rather, they move in chunks whose characteristic size depends on the intensity of the cohesive stresses. 

In summary, scale-up and scale-down of blenders for cohesive powders is a risky enterprise. Caution is strongly advised. 

For cohesive powders, mixing depends on shear rate, and rotation rates are very important. 

The dry-blending operation is a critical process in case of low-dosage forms. In order to obtain a high degree of mixing, cohesive powder components have to be disagglomerated. For this purpose, it is often advantageous to proceed as follows ... 

Group C powders Very fine, cohesive powders are classified into this category, which are incapable of fluidization in the strict sense and tend to rise as a slug of solids. Their sizes are usually less than 30 pin, and they easily give rise to channeling. Examples of group C materials are talc, flour, and starch. 

Physical form powder powder rubbery plastic plastic cohesive powder... 

Microspheres based on phenol formaldehyde oligomers (PhFO s) are brown non-cohesive powders with particle sizes of 10—300 pm, apparent densities of 100 to 150 kg/m3, and sinkages of less than 5—10 mass% u,24 26). 

Chirone, R., Massimilla, L. and Russo, S. (1992). Bubbling Fluidization of a Cohesive Powder in an Acoustic Field. In Fluidization VII. Ed. Potter and Nicklin. New York Engineering Foundation. 

Finally, fluidized-bed mixers are rapid mixers, but cannot, of course, deal with cohesive powders or a very wide distribution of particulate sizes neither are they suitable for powder mixtures with pronounced size, density, and shape variations because of segregation problems. They generate small static electric charges. 

A Carr index less than 21 for a powder mixture indicates good flow, while one between 21 and 25 suggests a marginal flow property. A more commonly used term is the Hausner ratio, which is simply p,/pb, or the tapped density divided by bulk density. This ratio was introduced by Hausner in 1967 to characterize metal powders,58 but is commonly used today for pharmaceutical powders. The higher the Hausner ratio, the poorer the flow. The Hausner ratio varies from about 1.2 for a free-flowing powder to 1.6 for cohesive powder. 

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