Lyn,

I read your articles with keen interest. They are always quite insightful. It is apparent that you know your subject well. As a respected docent, I beg your theosophy and references on the matter of measurements influenced by:

1. Particle size distribition - filling of voidages

2. Particle shape - ideal consolidation of round verses slabby nested particle groups and others of interest

3. Plasticity and elasticity rheology and time dependency

4. Moisture by degree - light, critical (massive flow property change), and saturated

5. Multiple properties of particle groups as separtely measured and then integrated as a mass system.

Wishing You A Merry Christmas and Cheer Filled Holdiays,

Lawrence Nordell

Conveyor Dynamics, Inc.

email: nordell@conveyor-dynamics.com

web: www.conveyor-dynamics.com ■

Hi Lawrence,

Thanks for your interest, it is encouraging to learn that my contributions as useful.

Relating to your queries, the degree of voidage formed with different particle size distributions depends to some extent upon the behaviour of the finer fractions. There is an excellent publication by W.A.Gray in the powder technology series edited by J.C.Williams, entitled ‘The packing of solid particles’, published by Chapman and Hall. It is when inter-particle forces become strong, say at less than 50 micron sized particles, that geometric theory breaks down and the state of the powder and its condition of aeration becomes significant.

The behaviour of rounded and nodular grains are more easy to predict than flakes and highly irregular shapes because the condition of isotropy, or alignment structure, then has a strong influence on the nature of the bulk material by how the particles nest together. A bed of plate-shaped particles can suffer a huge change if vibration disturbs the structure to give closer alignment. The phenomena of ‘clay slips’ and ‘liquifraction’ has caused many landslides and unstable cargoes where the moisture content is able to fill the voids to excess if the particles align to a closer packing when vibrated. This has lead to the specification of a ‘safe transportable moisture content’ for ship cargo when handling damp minerals.

Plasticity and elasticity have a massive influence on flow behaviour. I have constructed a chart of a universal rheological model, with notes on the characteristics of the various elements, that I can send by post if you forward your address. This highlights the complexity of particulate solids behaviour under these various influences. Elastic granules, such as cork and rubber, form small flats on points of contact that are difficult to disturb. Plastic materials have a similar tendency under time consolidation.

Moisture has a variable effect. A small amount gives increasingly more difficult flow until the stage that some local regions become saturated, above which value the lubrication effect starts to outweigh surface tension and flow conditions ease. With mixed coal this poorest flow condition occurs in the region of 10 to 12 % moisture content.

So far as the considering the relationship between individual particle properties and those of the bulk assembly I think it is generally best to consider those most appropriate to the conditions of interest in any given situation. I also constructed a chart to outline the various aspects and relation of measurable attributes to application. I will send a copy of this also if of interest. Some features, such as segregation and degradation, need to take account of the operation form, scale and sequence to develop a clear understanding of effects.

I recently completed a ‘Glossary of terms in Powder and Bulk Technology’ that has been published by the British Materials Handling Board. This is more than a dictionary in that it includes much explanatory material and background information and standards. If of interest, I can send contact details. I hope that you find these notes useful.

Lyn Bates ■

Hello,

My name is Nickolas Seyve, I am a french Student in an Food Science and Nutrition school in Dijon and I have to do a research work on mixing different sort of powders.

I read through your article "Bulk powder density", which I found very interesting, and I was wondering if you could give me some piece of advice or give me some references about the density of powder such as wheat flour, corn flour, soybean flour, and animal protein flour... I would like to know if it is also hard to mix such componants and what are the best blenders to do this kind of mix.



The purpose of my work is to be able to find the theory to be able to calculate the dimension of a blender to produce a well mixed powder.

Thanks in advance, looking forward to hearing from you.

Cordialy,

Nickolas Seyve ■

A comprehensive file of typical bulk densities is available on the Ajax.co.uk web site if you care to submit the materials of interest. Regarding mixing of the fine dry powders you mention, these can be well mixed by batch or continuous type ribbon blenders. The choice of mxer in this case is usually determined by the scale and nature of the following process, except where there are a large number of ingredients to be mixed in variable proportions or btch tracability is required, in either of these cases a batch type machine is more usually needed. ■

Most bulk density problems arise with fine powders because of their tendency to entrain air in dilated flow or agitated conditions. One approach is to try and prevent the material dilating in the handling process. In general this requires an examination of the flow route and maximise the confinment of the flow. For example, transfer down a medium steep vee chute with a radius underside or fill a container through a full retracting chute that has a probe on the top to lift the bottom of the chute slowly, but stop lifting if the probe is uncovered. It is sometimes practical to pre-compact the feed by means of a suitably designed screw feeder. (This should only be used with powders that are compressible, not firm granular products).

If the powder is held in a highly dilated form it is necessary to accelerate the escape of air from the voids. The normal path through a deep bed is tortuous and the rising air replaces air that is moving from the upper levels. By-pass channels can be created by the use of long vertical rods that are vibrated to whirl, so forming open paths through which air can rise without resistance or re-aerating the upper regions.

Another technique for a robust vessel is to seal the container amd connect to a vacuum cylinder sized to reduce the void pressure to an acceptable level. This works best with virtually full containers. To avoid a dust problem, the vacuum vessel should be reduced in pressure before connecting to the storage unit and have a mass flow discharge shape, so that any dust can settle and be drawn off before the next vacuum cycle. This connection will generate an increased pressure differential to draw out air from the bed and, when exposed to ambient pressure the bed will compact. Note that powders prone to adopt a fluidised condtion are also prone to have very poor flow properties when compacted.Excess air content in a powder bed is more difficult to disperse in warm or hot conditions because the air is more viscous at elevated temperatures.

Each application has to be considered on its individual merits for cost and effectiveness. Ajax is always prepared to review problem areas associated with product flow and bulk state control via lyn@ajax.co.uk ■

Lynn,

I was hoping you might be able to shed some light on a particular matter.

I am interested in maximising the packing density of some crystalline powders (typical particle sizes range from about 20-300 micron). One option is to optimise the packing through choice of particle size distributions. I have done a bit of reading of some related papers (Sohn/Moreland, Yu/Standish etc) which propose a number of packing models, but none appear to provide what I'm after.

This is not my area of expertise and as a result I'm a little lost when it comes to determining the state of play of various packing models. For example, which is the most applicable to small scale (say 1dm3) packing of multiple sized crystallline particles?

Also I was thinking of using an ultrasonic bath to "close-pack" the material but have seen relatively little on this subject.

If you have any ideas - references to models, techniques or software that could help me in this situaton it would be appreciated. I am now chasing down a copy of Gray (thanks to your article) but wont have it for a while.

Cheers,

Phil ■

Phil,

A basic difficulty associated with calculating a theoretical packing array is that it does not address the mechanics of bed formation, which is influenced by a host of factors. Looking at the overall situation it is considered important to distinguish between freely orienting particle structures and beds where particle cohesion restrains the relative motion of the constituents. There are also different considerations to take into account when the bulk is slow to de-aerate. If all the particles are inert and larger than 100 microns the mass is usually free flowing, i.e. the particles will re-orient easily, and the bulk is relatively quick to settle to a stable state. A bed with a significant proportion of 20 micron particles may be expected to inhibit the diffusion of air in the voids and be cohesive and non-free flowing when settled, but potentially offer a more dense condition. Intermediate compositions may be expected to show roughly proportional behaviour.

Homogenisation of a mixture containing fines may be achieved by an agitated, tumbling device, such as a rotating drum fitted with blades that disturb the bulk. The container should be ‘dump-loaded’ if practical, rather than have the contents loosely poured. The application of vibration will initially tend to disassociate the load path in the particle structure and allow the mass to reform with a closer order of packing. However, sustained vibration will tend to cause migration of the finer particles under the larger members to ultimately ‘float’ larger particles to the surface of the bed. An initial compaction will therefore be followed by a reduction in bulk density as voids are created between the elevated coarse fractions.

A tapping motion mobilises gravity and inertia to accelerate close packing. An effective mechanism suggested for the volume indicated is to impart vertical shocks equivalent to dropping the container about 25mm about 50 times onto a hard surface. The effect will be enhanced if there is a weight uniformly distributed over the bed surface during this process. The operation will be even more effective if it can be carried out in an atmosphere of reduced pressure that is suddenly released to atmospheric pressure when the tapping ceases. The pressure differential between normal atmospheric pressure and that prevailing in the voids will apply an additional consolidating load to give further compaction.

You may find that experimenting with different compositions and techniques is more productive, and interesting, than conducting a deep theoretical analysis that suggests an optimum composition that is hard to attain in practice. Good luck, and please let me know how you go on.

Lyn ■

The 'flowability' of a bulk material cannot be expressed as a single figure because it depends on many factors. The main resistance to bulk flow is given by the 'strength' of the material. This varies according to the degree to which it is compacted, so shear strength measurments are necessary in conditions that reflect the circumstances of application. It should also be noted that the permeability of the bulk material is relevant to how quickly the bulk material expands and compacts as this affects the rate at which a fine powder will sellte from a dilate condition and expand from a compacted state. The frictional resistance to slip is also an important feature, as flow requires product to move relevant to a contact surface.

An Ajax publication, 'Characterisation, Industrial Practice', sets out the main aspects relating to bulk material behaviour and is available from www.ajax.co.uk ■

Lyn,

Thank you so much for your article, I enjoyed reading it. I just have a small question. You discuss things that help compacting a powder, but how do you prepare a bin of powder that needs to be unpacked. So how do you go from hard packed to loose packed without dropping the container on the ground?

I have powder material coming in and the shipping process has condensed the powder into a very hard pack, and I am currently trying to figure out how to loosen it so that the powder can be sucked out. The hard packed powder will not flow the way i need it to however. Any suggestions?

Thank you,

Brian ■

Dear Brian,

What kind of powder are you referring to?

Can the powder being sucked out from the top?

What vacuum installation do you have in mind?

How much powder in tons/year are you going to handle?

bast regards

Teus ■

Teus,

Well we are working with a PCTFE monomer resin and yes the resin is currently being sucked out from the top but unfortunately we experience constant feed losses due to the powder resin not sucking up and flowing to the vacuum lance correctly. I was thinking that maybe aerating the bin might help but am not quite sure how to go about doing that. We currently run through approximately 1000 tons a year with this. Any input would be wonderful.

Thanks,

Brian ■

Much depends on the type of container. Two methods used to assist the discharge of big bags are vibrating bases and squashing devices, but these are not suitable for boxes or drums. If it is not practical to invert the container, the material can only be loosened by agitation from the top or injecting a controlled amount of air at pressure to the bottom of the mass by means of a thin lance. However, putting air in is a delicate matter and may cause dust or spillage problems in some cases. The effectiveness will depend on the permeability of the bulk and will not give instant results without undue agitation. You could try giving a very short blast to deep into the bed and leaving it to some time to allow the air to percolate though the bulk.

An alternative would be to fit some thin rods projecting from the end of a vacuum pick up wand to scrape the suface to loosen it as it draws the disturbed material away.

Aaax has made some drum dischargers for dealing with packed or set contenrs that advances a large screw into the product. It works well, but is not cheap. There may be other mechanical solutions, but each application has to be examined on its merits and I do not know of any universal method. ■

Dear Brian,

As Lyn said, air injection into the cargo is probably creating more problems than solutions.

Nevertheless, with an ordinary 1” pipe connected with a valve to your main air supply system (or a small mobile compressor) you can try this method.

Be sure that there are no health hazards.

A mechanical solution is an articulated arm with a rotating cutting suction nozzle.

This method is applied on pneumatic cement unloaders.

The arm is hydraulically manipulated and takes away physical efforts of the operators.

Success

Teus ■

Dear all,

I am trying to mix two powders of vastly different bulk densities, one of which is a chlorine source. I have two ranges of granule sizes: 300 - 700 microns and 700 - 1200 microns.

These powders are mixed in an octagonal blender.

However, there seems to be some seggregation happening because the measured chlorine is not constant. This seggregation for some reason is more with the smaller granules.

Is it possibe that the seggregation is because of the difference in BD? What can I do to rectify the problem?

The second powder is incompressible - the granulation of this powder is done in a rapid mixer granulator. ■

Free flowing powders of different particle size will tend to segregate in any situation that allows them to move. The tumbling action of a rotating blender provides the dilates surface conditions of repose flow that permits easy separation of fractions, so is not the proper type of mixer for the duty. One way to stop segregating mechanisms, such as percolation, is to introduce a small amount of moisture or inert liquor, to increase the resistance of movement between particles. If this is not practical you may consider reducing the particle size difference or making one or the other into a cohesive condition by reducing one or both of the components to less than 50 microns.

Alternatively, a simple static mixer coud be employed, using calibrated feeders to deliver the required ration of the respective materials. Care should be taken with any subsequent handling as this could undo the balance of the blend so prefereably the mixer output should deliver directly to the point of use or packing. ■

hey lyn

i would to ask how one can measure the density of a powder during high shear mixing?

is there something called dynamic density of a powder?

does the density of a powder stay the same during high shear mixing?

im interested of the density during the roping regime of the mixing.

i hope you will be able to help

tawanda ■

The density of a bulk material is likely to vary considerably during high shear mixing, according to the location relative to the surface promoting motion and geometry of the container. Zones of excessive dilatation will probably be close to regions where the material is being strongly compacted and any material vigorously agitated at unconfined boundaries may well have particles that are not in contact, so density at this condition is meaninless.

Some work on tomography indicates variable density conditions in a dynamic bed. The question is - what is the specific interest in obtaining a value?, could it be to assess the maximum state of compaction because of a threshold capacity of the material, or what? ■

Hey Lyn,

Thank you for your reply, you mentioned some work on tomography which indicate variable density conditions in a dynamic bed.

May i please have the reference of this work.

thank you in advance,

Tawanda ■

Lynn,

I wrote to you a while ago, asking for references to the work on tomography which indicates variable density conditions in a dynamic bed.

its been close to a month now, should continue waiting or what?

if you dont have them its okay.

tawanda ■

Tomography is a technique that I have not undertaken, but some work of this type was conducted at Brimingham University on examining mixing behaviour. There are many references on the internet under'Tomography in powder beds' and I am sure many other search titles. I regret that I cannot help more directly. ■

Fine powders tend to be slow to settle from a dilate condition when hot because the excess air in the voids has a higher viscosity at elevated temperatures, so cannot escape as freely from the voids, particualrly in a deep bed. The corollary is that when they are settled to a long-term equilibrium condition, it can be quite difficult to get them to expand to a flowable state because air connot easily penetrate the surface layers to reduce the pressure differential of the void demand of bulk expansion. A dilated bulk solid develops a void gas pressure when settling, which supports part of the weight of the bulk and hence reduces the particle contact pressure, making it easier for the particles to enjoy relative movement. It has the effect of reducing the bulk strength, strictly speaking, not by reducing the internal friction coeficient, but by reducing the internal friction value due to the lower normal load between the particles.

To sum up, there is no unique value for density of aparticulate solid. It varies from the point at which the particles are all totally separate to the state of ultimate compaction at a mass solid with no void content, a condition never reached in practice. A meaningful statement about bulk density should therefore include a description of the manner of its formation. In practice, coarse, rounded particles, that settle easily to a stable condition are not likely to vary widely in density but a bed of fine partiles is much more sensitive to how it is prepared. ■

Dear Lyn,

Do you know of any studies that quantify the rate of consolidation and the means to increase consolidation rate.

Specifically, are there any scientific publications on quickly (3-5 seconds) consolidating fine cement by 50% or greater from its loose bulk density state (1000 kg/cm)? Our process capacity is around 1000 t/h.

We are looking at vibration and compaction via screw conveyors entering at 1000 kg/cm and discharge at >1500 kg/cm. Is this possible? ■

Dear Nordell,

Cement density = 3100 kg/m3

Cement with a bulk density=1000 kg/m3 is in a fluidized state.

The bulk volume is then 1 ton/bulk density = (1000kg)/(1000 kg/m3) = 1 m3

The cement volume is (1000 kg)/(3100 kg/m3) = 0.3225 m3

The entrapped air volume at a bulk density of 1000 kg/m3 = 1 – 0.3225 = 0.6775 m3

Cement with a bulk density=1500 kg/m3 is in a settled state.

The bulk volume is then 1 ton/bulk density = (1000kg)/(1500 kg/m3) = 0.6667 m3

The cement volume is (1000 kg)/(3100 kg/m3) = 0.3225 m3

The entrapped air volume at a bulk density of 1500 kg/m3 = 0.6667 – 0.3225 = 0.3442 m3

To reach the settled state, it is necessary to remove 0.6775-0.3442 = 0.3225 m3/ton

For 1000 ton/hr. this means that 322.5 m3/hr of air has to be removed.

In a silo or in a just loaded ship, this air escapes slowly as the cement particles force the air out by gravity.

This is the reason that ships, when ready with loading cement, have to stay in port for 5 to 10 hrs, before departing to sea.

Compacting screws, do exist for feeding cement into a pressure pneumatic conveying system.

The compaction is created by a decreasing screw pitch in a tight barrel (Fuller Kinyon pump).

However, with this screw, also the entrapped air is compressed and when the cement is released again into atmospheric air, the fluidized state is restored by the expansion of the compressed air.

It is also a known fact that as soon as compacted cement is handled and moved again, it fluidizes right away into the fluidized state. (Which makes cement suitable for pneumatic conveying).

A consolidating screw seems not to be a viable solution.

The maximum consolidated density I have ever measured was 1500 kg/m3 in a silo after a storage time of 6 to 8 weeks.

Vibrating the cement must be done in such a way that the entrapped air is released and removed, otherwise, when extra air is entrapped the fluidization only increases.

As soon as the cement is handled again, the fluidization is there again.

A cement company should have more experience in this case.

Success

Teus ■

Dear Teus,

I appreciate your time in responding.

Our goal is develop an gas separation from the fine cement to the degree stated. I do not know if it is possible. The rate is quite high and the time is short. Is there a hope in finding a positive separation in one of more mechanical system? ■

Dear Nordell,

A place where cement is really packed (densified) is in cement unloaders.

In pneumatic vacuum cement unloaders (my field of experience), cement is drawn into a kettle under vacuum (less air than under atmospheric conditions).

When the kettle is filled, the vacuum is shut off and pressure is put on the kettle and the cement.

When the vacuum is shut off and the pressure reaches even atmospheric pressure, the level tester in the kettle indicates almost instantly “No cement”.

This indicates a sudden level drop in the kettle, which also means a rise in bulk density.

Before the kettle is ready to empty under pressure, the internal fluidization is not 100%, because the cement is like a hard pack of vacuumed coffee and only air channels are created.

The full fluidization occurs when the cement starts flowing.

However, vacuuming 1000 tons/hr of cement is already a big challenge, getting the cement out of the vacuum tank without re-fluidizing the cement seems almost impossible.

I am afraid that mechanical systems all have the disadvantage of re-fluidizing the cement.

What is the purpose of increasing the density of cement?

Have a nice day

Teus ■

Dear Teus,

The direction of my thread may be a little of the mark of the posted initial question and I seek a more comprehensive understanding than may be admissible here. Can you offer your email to discuss in more detail?

My email is listed below. ■

As teus points out, a rapid change in volume would takes place almost entirely at the expense of the void air content as the solid is virtually incompressible. Assuming the entrained air to be initially at ambient, reducing the void space by 50% creates a pressure of around one bar, which is probably sufficient to explode the compact formed, in a like manner to the flaking of tables or briquettes when pressed from a contdition of high dilatation. The rate of exedous of air from cement is highly dependent on the cement temperature due to the elevated viscosity of gasses when hot. It follows that the more settled the powder, the greater prospect there is for deveoping the bulk strength sufficient to contain the reduced internal pressure and allow the air to diffuse out over time. There is a catch 22 in that settlement reduces the flow potential of the material, so that a feed system would require a residence time to settle out some air and an integral, reliable feed screw that provided the compacting force. A plug forming method has been developed by Ajax equipment that eliminates the disruption of compact that is caused in the Fuller Kinyon type of machine, but has not been empoyed for such large feed rates. A screw pump of this form would enable a degree of compaction to be secured, but the duty specified is quite challenging.

Success is claimed for continuously compacting calcined kaolin clay, which is light and fluffy and has a bulk density of 10 to 15 lbs/cub.ft, by means of converging belt conveyors under compacting loads, US Patent 20090271953. This method of direct loading would seem to offer the most efficient mechanical means to secure high rate densification, if the product can be contained. ■

Originally Posted by Donna A.

Dear Lyn,

Im with the R&D Group of one noodle seasoning manufacturing company. I would like to know if there are standard methods on how to measure/quantify the flowability of a certain seasoning powder mix. The mix generally contains granulated sugar and salt with particle size of 180 microns which is about 50%, 45% are fine powders and the remaining 5% are dehydrated vegetables with particle size of about 2-4mm.

What are other elements of powder morphology that we may measure so we can establish standards for consistent packing?

thanks very much!

There are Standard methods of measuring bulk density in British Standards and ASTM Standards that essentially pour the powder into a cylindrical container of known volume, trim off leveland weigh the nett cotents. In practice a bulk material will vary in bulk density according to the way in which it is handled, but a mixture is also sensitive to segregation that causes a bias distribution of the mass. Segregation control requires a thorough design review through the total flow route, with special attention to any storage situations and unconfined flow channels. Consistent density in dymanic conditions requires uniform stress conditions, for example mass flow discharge, rather than the erratic collapse of arches from a funnel flow hopper. This also demands a systematic examination of the flow route to ensure that any intermittent behaviour is served by a closely reproducible action. Fine cohesiive products are prone to vary in bulk density and have a corrosponding difference in flow behaviour. As the bulk strength corrolates with density, a simple shear test will give a measure of consistency. As a rule, it is better to employ mass flow hoppersandsecure flow down inclined chutes, rather than allow free fall. ■