Fine, warm powders are slow to de-aerate as the viscosity of air increases with temperature and the escape route for excess air through a deep bed is long, tortuous and progressively diminishing, whilst replenishing air escaping from the upper regions.

The situation is usually aggrevated by the development of a short-circuit flow regime, as a powder in a state of low internal friction will form a self-reinforcing narrow flow channel through the stoarage bed to discharge fresh product from the surface in the loosest condition.

Preferential air escape routes can be provided by a de-aeration frame that, combined with flow inerts to spread the flow channel, can maximise the residence time and avoid the slow and inefficient percolative route of normal settlement.

Dilated powder cannot be readily compacted as the entrained air is compressed and with tend to expode the compact when the forces are released. However, when sufficient air has escaped for the material to develop internal strength, it can be compacted to a stable, dense condition. Hence a suitable hopper and discharge screw can offer progressive extraction from a greater storage volume, hence more time for de-aeration, and compact the discharging material by a suitable screw and casing form.

The ultimate condition has to be handled by subsequent equipment and a compacted fine powder will not be free flowing so the total installed system has to be considered as a whole. ■

Many of our clients worldwide have the same problem with many different materials ranging from aluminium oxide to cereal husk. When these clients installed our acoustic cleaners to prevent material build up on the side walls and/or prevent material bridging over the discharge - they found that as the acoustic sound waves exciterd the material so it densified it, of course at the same time facilitating maxium flowrate at maximum silo capacity.

It maybe is worth a try. Have a look at our web site and download the silo questionnaire. The web site is at - www.primasonics.com

Regards

Donald Cameron ■

We manufacture a vacuum densifier for just such an application. It's essentially a screw feeder that conveys the material thru a vacuum chamber that removes much of the air from between the particles. We have successfully densified kaolin clay at companies here in the U.S. like Burgess Pigment, Dry Branch Kaolin, ECC International, Engelhard Corporation, J.M. Huber, KT Clay, and Nord Kaolin.

If you're interested, please email me at mpayton@carmanindustries.com with the final bulk density required in your bags and the rate at which you're going to densify. If there's downtime due to changing bags (as in super-sacks), be sure to specify the instantaneous rate of densification.

Mark Payton

Application Manager ■

OK

Some ideas

you could try a de aeration screw up front - this is a large screw with a discharge weir at the end of the screw - the material has to jam up na dpack up to discharge - some Germany co makes these

Else, look for de aearators equipment.

Suggest some testing before spending $$

Silo vibration is another way to pack it in....

Do not know of your discharge system and flow sheet to your packers.

For the packers.....look into bags with holes for de aearation plus a good packer with impellors...as used in the cement industry. In the bag filling cycle..have a pause in the fill to relieve the bag.

Use transfer chutes with slides and not drops to avoid re-aerations

Cheers

James ■

Deaeration and densification of fine powder in Silos

Many of our clients worldwide have the same problem with many different materials ranging from aluminium oxide to cereal husk. When these clients installed our acoustic cleaners to prevent material build up on the side walls and/or prevent material bridging over the discharge - they found that as the acoustic sound waves exciterd the material so it densified it, of course at the same time facilitating maxium flowrate at maximum silo capacity.

It maybe is worth a try. Have a look at our web site and download the silo questionnaire. The web site is at - www.primasonics.com

Regards

Donald Cameron ■