Flow properties from powder bulk densities

Flow properties from powder bulk densities... 

The behaviour of powders is often quite different from the behaviour of liquids and gases. Engineers and scientists are used to dealing with liquids and gases whose properties can be readily measured, tabulated and even calculated. With particle systems the picture is quite different. The flow properties of certain powders may depend not only on the particle size, size distribution and shape, but also on surface properties, on the humidity of the atmosphere and the state of compaction of the powder. These variables are not easy to characterise and so their influence on the flow properties is difficult to predict. In the case of particle systems it is almost always necessary to perform appropriate measurements on the actual powder in question rather than to rely on tabulated data. The measurements made are generally measurements of bulk properties, such as bulk density and shear stress. 

Crystallization from solution is an important industrial separation and purification process for a variety of materials ranging from bulk coimnodity chemical to pharmaceuticals. Besides purity, spedfications on the produd crystals often includes such properties as crystal size distribution (or average size), bulk density, filterability, slurry viscosity and flow properties of the dry powder. All of these properties depend entirely on the crystal size distribution and the crystal shape. 

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. 

It is well known that particle shape affects many secondary properties relevant to powder handling such as the bulk density, failure properties or particle-gas interaction. For non-spherical particles, the results obtained with different methods of particle size measurement are, in general, not comparable. From the point of view of powder handling, flaky or stringy particles like wood shavings, mica or asbestos fibres are known to be difficult because they interlock and form obstructions to flow. 

Since a powder assembly can generally be regarded as a biphasic system the permeability of a powder, or the ability of a gas to pass through a collection of particles or bed of powder, has now been recognised as a bulk powder property. The magnitude of permeability as a function of bulk density may be used in the determination of the powder flow rate and discharge of powders from hoppers. 

MagChem 50 is a high purity technical grade of angnesium oxide processed from magnesium-rich brine. This fine white powder has a very high reactivity index, low bulk density, and excellent flow properties. 

Since most rigid PVC extrusion is from powder blends, and the extrusion process—especially twin-screw—is sensitive to powder blend properties of flow, compact density, and bulk density, the need to be aware of bulk density variations is most important. This awareness is especially necessary with the supplier s requirement to reduce the residual VCM content of PVC resin, which in turn can mean more severe drying cycles after a steam-stripping operation in the process. Consequently, the bulk density of PVC resin, and its particle size distribution, may be somewhat different now than previously—and the resin particle surface may be a little harder, or less absorptive to liquid additives. Many resins are much drier than before. In short, the dry-blend, powder flow, and bulk density differences of current PVC resins may require subtle changes in mixing procedures and extrusion conditions to maintain product quality (more on this later). Knowing this before the resin is blended and extruded can minimize a host of production problems. 

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