If there is a clear dependency of the compressibility on flowability, a tapped density measurement might help.

But:

Tapped density can be influenced by the stiffness of the powder. Thus a "stiff" powder is subjected to higher stresses than a "soft" material when consolidated at the tapped density measurement. This can yield to the soft material being less consolidated - and thus showing a smaller density, although its flowability can be worse! The problem is that the stress acting on the material at compression is not clearly defined. Not clearly defined conditions are a plobrem with many so-called "simple tests". When regarding a "simple test" one might think that this test is easy to understand, but in most cases nobody really knows all the quantitites having an influence. Thus, a valuable tester first must consolidate the powder in a defined way, and then the strength of the powder has to be measured. Only this way the flowability can be determined, and only this way the test duplicates the typical problem when a powder is handled: The powder is subjected to a certain stress (e.g. in a bin, a bigbag, a container...) and does not flow. Therefore, a tester should allow for the adjustment of the consolidation stress according to the stress acting in the application under consideration!

More information you will find on my website www.dietmar-schulze.com.

Best regards

Dietmar Schulze ■

There are great obstacles to attaining a simple method of characterizing powders for flow. First, because there are a number of interacting factors and second, one of these - the strength that is achieved under varied stress and stress history conditions, is both complicated and situation dependent.

However, the main features are: -

Bulk density, wall friction and shear strength.

Bulk density is the driving force for flow, so it should be measured, as far as practical, in the relevant condition; or at least in a consistent state for comparison purposes. Loose poured and tapped a few times in a consistant manner is a useful basic guide.

Wall friction is exceptionally important, particlularly in small installations, because this subtracts energy from the material, leaving less to promote flow. It is also a major factor in determining the inclination of a hopper wall for mass flow design. Incidentally, it is very useful to consider efficient flow geometry when considering difficult flow materials. see www.ajax.co.uk for details of technical guides and wall friction apparatus.

Shear strength is the technical nightmare. The Jenike procedure is vary awkward for the non-specialist. Proprietary instruments are available, as from the previous contributor, but these are not cheap or convenient for field use, but are invaluable for serious design work. There is a great industrial need for a more simple 'screening' method for initial characterization of powders for flow. The Working Party for the Mechanics of Particulate Solids of the EFCE have been examining this requirement for some time. A method of providing a quick, relative value is the vertical shear test. This is a 'one value' measurement of initial shear failure strength from a consolidated sample. It is not a measurement of flowing conditions, but is a measure of the difficulty of starting flow. Some care is needed to select the appropriate preapration conditions, but consistency is the main criteria for comparitive purposes. More details are available on the above web site.

Other fundamental consolidated powder properties, such as tensile strength and cohesive strength, can be useful as quality control devices or for some research or development and a load/compaction rig is also informative, if you know what to look for. Dynamic devices, such as the 'Avalanching' tester and the Freeman Rheometer, may produce interesting results, but the stress conditions encountered are complex and do not reflect confined flow conditions.

I have just completed the preparation of a 'Glossary of terms in powder and bulk technology' for the British Materials Handling board. This includes some useful background on bulk material properties and particularly concerning 'atributes' that may affect flow, storage and handling applications. Details will be supplied on request.

Lyn Bates ■

Dear Mr. Burry,

The problem of so called simple methods are well known. I have described it in the article Nr.2 in the literature list below. In this article the advantages and disadvantages of common shear testers are described. For understanding the performance of shear test I suggest you reed the article Nr. 11.

Please consult our website WWW.IPT-ONLINE.COM where you can find our products and services.

In October we will have our yearly course “Quality control and powder technology” which will be given at University of Delft.

At this course you will have the opportunity to measure by your self – if possible with your own powders.

Please send me your full address and we will send you the invitation.

Best regards

Dr. IvanPeschl ■