Flow of Powders

FLOW OF POWDERS

The gravity flow of powders in chutes and hoppers and the movement of powders through a constriction occur in tabletting, encapsulation, and many processes in which a powder is subdivided for packing into final containers. In many cases, the accuracy of weight and dose depends on the regularity of flow. The flow of powders is extremely complex and is influenced by many factors. A profile, in two dimensions, of the flow of granular solids through an aperture is shown in Figure 5.3. Particles slide over A while A itself slides over B. B moves slowly over the stationary region E. Material is fed into zone C and moves downward and inward to a tongue D. Here, packing is less dense, particles move more quickly, and bridges and arches formed in the powder collapse. Unless the structure is completely emptied, powder in region E never

FIGURE 5.3 Profile of the flow of granules through an orifice.

flows through the aperture. If, in use, a container is partially emptied and partially filled, this material may spoil. If the container is narrow, region E is absent and the whole mass moves downward, the central part of region C occupying the entire tube (Brown and Richards).

For granular solids, the relation between mass flow rate, G, and the diameter of a circular orifice, D_o, is expressed by the equation

G = Constant D^a_oH^b

where H is the height of the bed and a and b are constants. For a wide variety of powders, the constant a lies between 2.5 and 3.0. If the height of the bed is several times that of the orifice, H lies between 0 and 0.05. The absence of a pressure-depth relation, already observed in a static bed, therefore, seems to persist in dynamic conditions.

The relation between mass flow rate and particle size is more complex. With an orifice of given size and shape, the flow increases as the particle size decreases until a maximum rate is reached. With further decrease in size and increase in cohesiveness, flow decreases and becomes irregular. Arches and bridges form above the aperture, and flow stops. The determination of the minimum aperture through which a powder will flow without assistance is a useful laboratory exercise. The distribution of particle sizes also affects the flow in a given system. Often, the removal of the finest fraction will greatly improve flow. On the other hand, the addition of very small quantities of fine powder can, in some circumstances, improve flow. This is probably due to adsorption of these particles onto the original material, preventing close approach and the development of strong cohesional bonds. Magnesia and talc, for example, promote the flow of many cohesive powders. These materials, which can be called glidants, are useful additives when good flow properties are required of a powder.

Vibration and tapping may maintain or improve the flow of cohesive powders by preventing or destroying the bridges and arches responsible for irregular movement or blockage. Vibration and tapping to initiate flow are less satisfactory because the associated increase in bulk density due to closer packing renders the powder more cohesive.