I thnk that it would be difficult to construct a reliable general model for this task.

It is important to note that there are two aspects to increasing the density fo a particulate solid. Restructuring the bed to secure a closer order packing is the ultimate objective, but this behaviour is impeded by the need to reduce the volme of the void fluid, usually air, particularly in the case of fine particles. Excess air in the voids is the most common reason why the there is a need to increase the bulk density of a bulk material, so the formation conditions, structure, composition and geometry of the bed all have to be taken into account when considering the best way to accelerate the de-aeration. I suspect that, although the material is said to be granular, it is relatively fine and takes a long time to settle. A loose condition of the bulk can give rise to many problems in packing and handling, so 'state' control' of the material in a flow system is more than just securing flow reliability, but making sure that at each point on the route it is in a suitable condition for the operation in hand.

Addessing the user requirements therefore may involve a review of the preceeding equipment, to see how the material attained the undesirable condition, rather than just concentrating on redressing the situation.n ■

You have raised a few issues. Depending on the ratios of the fractions and the flow circumstances, there may be a segregation problem if the bulk material is retained in a loose condition. However, in general, vibration is a densifying mechanism with granular materials, not fines. The optimum frequency and amplitude then depend on the scale and nature of the structure. The objective of this vibration is to de-stabilise the points of contact, to allow the particles to re-orient by gravity to the most stable matrix of load paths through the bed. Too violent, and the bulk is disrupted and cannot settle well.

For fine materials and products that have a sufficient fines content to fill the space between the larger particles, densification, apart from mechanical compaction, is almost invariably a matter of reducing the air content in the voids. Even with high mechanical compaction, as with pressing pills, it is necessary to remove excess air from the voids to avoid the compact exploding when unconfined due to trapped gas. This is a more tricky subject, to long to discuss here. If you care to send your email address to lyn@ajax.co.uk, I will send an article on the de-aeration of fine powders. Specific operating problems may be dealt with on a consiulting basis. ■