The cement can be densifiedprior to handling, but will dilate again is carried by a high speed screw, particlularly if the unit has hanger bearings that disturb the bulk. The options are either to feed the screw in a settled condition and move gently, as with a slow speed, short pitch screw, or to discharge the cement into a receiving hopper of a mass flow type that has means to accelerate de-aeration.

The best solution usually depends upon the layout and operation of the system and the purpose of densifying. Usually, this is to secure a better packing condition, either for sacks or big bags. Cement will tend to retain air for some time, particularly if in a warm condition, therefore high rates of flow can be awkward to de-aerate. ■

Lyn, Steve:

I too am interested in densifying cement powder by using better transfer chute designs. One idea suggested is to vibrate the flow field by placing mechanical vibrators attached to the impact station idlers under the receiving belt.

I have studied the gas discharge behavior. It occurs quite quickly. Chute flow, for a 2m/s belt, +3000 Blaine cement, will drop the apparent density by 50% in about 2 m. In the next 2m, it will drop another 10-15%. It then takes another 100-150m to see another 10% benefit. The question is: what benefit will a mechanical vibrator have on assisting the quick exhauting of entrained gas and by how much? What might the specifications be? Our interest is to minimize the belt size for the set belt velocity.

I am making a proposal to try the this approach unless you advise that it is folly. Hopefuly, we can increase the apparent density to +1150 kg/cm in about 4-6m of skirt length.

Further to the same discussion, we are modeling cement flow using DEM (Discrete Element Method) to control the variance in flow velocity during transfer either in height or angular postion.

I wonder if there is a body of published knowledge on the subject.

The idea is to do it without baghouse or other dust collection methods.

Awaiting your kind reply.

Lawrence Nordell

Conveyor Dynamics, Inc.

www.conveyor-dynamics.com ■

Lawrence,

The task of de-aerating cement is very temperature sensitive. It can be quite difficult if the powder is in a hot condition, whereas it will settle much more rapidly when cold. This is entirely due to the change in viscosity of air, which has to escape by percolating through the mass. The rate of escape obviously depends also upon the bed geometry, so a deep bed can retain excess air for a considerable period. Vibration will tend to accelerate the terminal stage of the process, when the particles are in closer alighnment and can transmit the force to cause the particles ot re-align to a closer order of packing. If the powder is in a loose condition, I would be wary of vibrating conveyor idlers as this may tend to spread the powder to spill or run back down down any inclined section.

I suggest the best way is to try to condition the cement before loading it to a belt conveyor. This can sometimes be done by natural settlement but may be accelerated by de-aeration frames. There will generally be some settlement of the material in transit along a belt conveyor, usually because the loading conditions have dilated the product.

Turning back to the original enquiry, even though this is no longer of interest to the originator, it is worth comment that compaction screws can densify powders, but only after they have attained a sufficient state of de-aeration that the entrained air will not elastically re-expand the bulk on removal of the compacting stresses. This feature equally applies to the formation of pills, tablets and rolled pressed items. Pre-conditioning of dilated materials is therefore a subject of some important in these processes, as well as for packing and storage interests.

With respect to discharge the cement should be handled as gently as possible down a focussed chute to avoid re-entrainment of air. The worst thing to do is allow free fall in a dispersed condition. ■

Lyn,

As always, you see to the heart of the matter.

We hope to vibrate only after transfering in the chute, in the skirt bed. During transfer, the cement will be guided and contained by the chute geometry with a more or less constant fall velocity, except for the last stage, so the cement does not expand and dialate. The last stage will use a compression curve in the flow stream. This, by itself, may be sufficient to consolidate the cement.

Do you have any knowledge of the use of circular chutes vs rectangular cross sections to assist consolidation? I believe the circular section will aid in consolidation. It produces a increased hydrostatic pressure over the rectangular section and less wetted area for turbulence.

The chute will be designed by DEM. The vibrating particle sizes, in the stream, are too fine to be modelled by DEM. They also has the Van Der Waal and Einstein forces for which we have no constitutive model.

Vibration is imposed to see if the bed can dearate faster than is observed by idler agitatation along the conveyor.

Any thougths on the throw oribit, frequency and amplitude? The present idler induced pulse rate is at 1.33 hz with an average 10 mm throw between idlers. I am thinking to increase it to 5 to 15 hz. over a 3 meter long bed with a =/-20 mm forward elliptical throw at the entry end and zero at the exit. The frequency and throw are to be variable. DEM can give some facts on the internal pressure gradients.

Your Appreciative Colleague,

Lawrence Nordell

Conveyor Dynamics, Inc.

website: www.conveyor-dynamics.com

email: nordell@conveyor-dynamics.com ■

Lyn,

By the way, what is a de-aeration frame?

Larry ■

Lawrence,

The crucial part of handling cement on a belt conveyor is getting the material onto the belt in a stable condition. If the powder is excessively dilated it will not only have a low density but will behave like a fluid. This will be to the detriment of containment at the fill point, the cross section that can be transferred and the inability to be carried up an inclined section following shortly after the point of loading. A marginal condition of dilation will tend to be disturbed on an inclined section of belt and 'run-back' can occur if the belt is highly loaded on the initial horizontal portion.

If the feed rate to the conveyor is controlled by prior equipment the clever part is to arrange a transfer chute that aids de-aeration by means of being at a relatively shallow inclination. One way is to transfer via a broad chute that is inclined about 10 degrees steeper than the angle of contact friction up to a shallow, vertical convergence onto the central portion of the belt. The ability to de-aerate is related to the depth of the bed and surface area available for percolation of the entrained air to an unconfined surface, so this allows optimum settlement during slip down the slope.

For a feed arrangement from a flood feed hopper outlet the hopper should be of mass flow design and have an extraction profile that is progressive over an extended length to give the lowest practical vertical flow velocity, as this detracts from the upwards gas flow during de-aeration. This is the situation that can be helped by a 'de-aeration frame'. The purpose of such a frame is to provide a multiplicity of preferential escape paths for the air to pass from the moving bed to the surface of the stored content. If the air has to pass through the mass of the bed it not only has to take a tortuous path through the interstices between the particles but is also inhibited by the solids counterflow down the hopper. Providing a number of vertical rods that are vibrated to whirl at a resonant speed form a group of 'holes' through the bed by which air can escape with little resistance.

As an alternative, for a situation where a flood feed is transferred via a chute, I suggest that this also should be inclined at a relatively shallow angle, rather than vertical. This will allow the escaping air to develop a path under the top surface of the chute unimpeded by the down-flow of solids and a shorter route through the bed for air escape. A round chute is more practical than a square one in offering a better cross section for the air path to develop. I would strongly reiterate my caution that hot cement takes much more time to de-aerate than cold cement.

If you care to send details of the feed geometry to lyn@ajax.co.uk, I will try to suggest an appropriate hopper or chute shape.

with best regards,

Lyn Bates ■