Powder Technology – Vital and Ignored

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Powder Technology – Vital and Ignored


Prof. Gisle G. Enstad, Tel-Tek, Department POSTEC, Telemark University College, Norway

Powder Technology emerged as a new field of science right after World War II. In spite of the fact that it is perhaps one of the oldest fields we know, and it is part of everyday life to an extent most people do not realise, it still seems to be grossly ignored, even by people who ought to be better informed. When we make food in the kitchen we apply a number of basic Powder Technology processes, like mixing of powders, adding and mixing liquids with powders, comminution of ingredients like spices, and heating and cooling of powders. As children we play with sand on the beach, utilizing the bind-ing forces between wet grains of sand, and when we make concrete we utilize chemical binding forces between parti-cles of different types of powders.

That Powder Technology is an old science is demonstrated by drawings made several thousands of years ago by the old Egyptians, showing separation of husks from grain by the use of wind screening, a fairly advanced Powder Technology process. As powders are parts of most industrial processes, from pharmaceutical and food industry, to polymer and cement production, powder metallurgy, ceramics and mining industry, bits and pieces of Powder Technology have been developed as parts of different sciences, often the same processes have been developed over and over again by different people who did not have any contact with each other, and did not have the slightest idea that the problem had been solved long time ago in a much better way by people working in completely different fields. The new idea is to collect all this fragmented knowledge into the new field named Powder Technology, to join forces for improving and developing it further by means of concerted scientific research, for the benefit of all those different branches depending on this knowledge. Everyone with some education in physics knows that a material can exist in three states. Depending on pressure and temperature it is a gas, a liquid or a solid. Except for compressibility gases and liquids are similar and are often denoted as fluids. There are a lot of text books on Fluid Mechanics and Solid State Mechan-ics, which are well established subjects that are taught as natural parts of any education on physics and technology. The gas equations, flow of fluids, and the bending of beams are all well covered. But real life is a lot more complicated. The fluid states are fairly well covered by common education, but the solid states are only covered as long as we are dealing with e.g. buildings or machines. If we are dealing with processes, chemical reactions, pharmaceutical or other industry, the solid state is not simply solid any more, as the solid has to be turned into small particles before it can be used in such industrial processes. Many production processes like crystallization, polymerization, spray drying, pelletization, granulation, and grinding give solids in the form of particles as their products. A solid that exists as small particles is not a traditional solid any more. It has become what we call a powder. Other terms like particulate solids, solids, and granular materials are also used, causing some confusion. Some people make distinctions where the term particulate solids is used for particles larger than say 100 µm, and powders for smaller particles. However, the term powder has been accepted by most workers as the term that is used in general for solids in particulate form. Solids divided into small particles become a new type of material, a powder. In principle it is still a solid, but the particles can move in relation to each other, at the same time as there will be attractive forces acting between particles in contact. There will be open spaces between the particles where there will be a fluid, usually air and water, and these fluids will interact with the particles, and influence the properties of the powder. This interaction can even be used for pneumatic or slurry transport. It is easy to understand that the powder will have properties completely different from the solid the particles consist of, and it is also completely different from the fluids filling the open spaces between the particles. A powder is neither a solid, nor a liquid or a gas. It is more like a fourth state of the material, a state that is a lot more complicated than the original three states. Looking at the mechanical or flow properties of a powder, the strength may vary a lot, from nearly no strength at all in an aerated state, to nearly the strength of a solid when compressed at high stresses like in tablet production or in powder metallurgy. Thus strength is not a powder property, but a function of its consolida-tion. It also depends on the particle size distribution, as well as the shape of the particles, the moisture content, the ambient air, and of time, to mention a few factors.

To describe all these relations a new field known as Powder Mechanics was developed by Andrew Jenike and his co-workers when the well known Jenike method of silo design was developed in the 1950-ies. New concepts like yield loci, time yield loci, flow functions, time flow functions, wall yield loci, angle of internal friction, effective angle of internal friction, and wall friction angles were introduced in order to describe the mechanical properties of powders by the new field named Powder Mechanics, constituting an important part of Powder Technology.

Characterization is perhaps the most complicated part of Powder Technology. The most basic characterization is probably determination of the particle size distribution. But what is the size of a particle? A number of different definitions are available, and a number of instruments for particle size measurements, based on different principles, are available, mostly giving different results for the same materials. The key issue is the particle shape, for which there is no agreed way of characterization.

Other particle properties are particle density, particle poros-ity, surface texture, and elasticity, to mention a few. Then there are the bulk properties, like flow properties covered by Powder Mechanics, bulk density, which is depending on degree of consolidation, fluidization properties, dustiness, segregation properties, permeability, and behaviour in pneumatic conveying, again only to mention a few. These examples demonstrate what is meant when we claim that Powder Technology is complicated. It may not be difficult, but it is complicated and complex. Compare it with fluid mechanics. Broadly speaking a fluid is characterized by its density and its viscosity. These two well defined parameters, which are easy to measure, are sufficient to characterize a fluid. As mentioned before, fluid mechanics is well established in most universities or other institutions teaching physics at a certain level, and there are a number of text books available in this field. In contrast Powder Technology is taught only in a few places, few text books are available, and most engineers who start a career in industry have not been taught anything about Powder Technology. In real life most of them will discover that the problems they have to struggle with, in most cases are related to Powder Technology, which they then have to start to learn about. As you know, learning in real life often means trial and error, which can be quite an expensive way of learning. It seems that the simple and straight forward subjects are taught, whereas the most complicated and complex subject is left out, and ignored. The idea of POSTEC was to help industry in this situation. By becoming members of POSTEC, industry would enable us to do basic research, which would provide us with the knowledge we needed to assist them with the problems they might have with their Powder Technology. It was also the idea that we would educate people in industry to enable them to solve their problems by themselves, and over the years a number of courses have been given not only in Norway, but also in countries as far away as in India. Many commercial projects have also contributed to increase the competence both of our clients and ourselves. Since POSTEC was established more than 20 years ago, its total turnover for the whole period may now approach 200 mill. NOK. In this period the industry we have served has become very competitive, and is making more profit than ever. I do not claim that this is a direct result of our activity, but I am convinced that we have given a solid contribution to this development. I am convinced that at least 200 mill. NOK per year in increased profit can be traced back to our activity, may be the increase in profit could be counted in billions if it were checked. Knowing how we have struggled to survive economically all this time, it is a great satisfaction to see how well our current and former members are doing. In 1988 also formal education in Powder Technology for our master students at the University College of Telemark here in Porsgrunn was started.

This was made possible by POSTEC employees. In 1990 the first masters of engineering left us with knowledge in Powder Technology for the first time in Norway. However, instead of growing, the two subjects in Powder Technology have been cut to one, and the remaining one is no longer compulsory, but can be chosen by those students who are interested. Similar developments are seen worldwide.

Institutions involved in Powder Technology are closed down, and the subject is not taught any longer in as many places as before. In the early days there was a lot of optimism, a lot of research activity in the field was initiated, and education was expanding. But this trend did not last, and still the majority of new masters of engineering have not heard about Powder Technology. The simple Fluid Mechanics they know well, but the complicated Powder Mechanics is completely unknown, and they have to learn about it when their real life in industry begins. Therefore, the need for research groups like POSTEC is still there, and those in industry who are aware of the importance of Powder Technology must continue to support us by active memberships. We need young, dedicated people who will continue our work to increase our knowledge in the old and new field of Powder Technology, until it becomes the natural part of our basic knowledge it ought to be.

What is Tel-Tek?

The Telemark Technological Research and Development Centre was founded in 1986 by Telemark County and local business interests to promote the transfer of competence, technology and research results from the then Telemark Institute of Technology to local industries, as well as to promote the formation of new enterprises. Some of the departments are headed by professors at what is now the Faculty of Technology of Telemark University College, and its personnel are allowed full access to the Faculty’s facilities. In addition to Powder Technology, groups have been established in Simulation of Industrial Flow Processes, Chemometrics, Polymeric Membranes, Process Equipment, Combustion and Environment and Water and Waste Water Treatment.

...and POSTEC?

POSTEC is the Department of Powder Science and Technology at Tel-Tek. The name originates from the time that a multi-client programme was set up to support the research activities in Powder Technology at Chr. Michelsen Institute, Bergen, in 1983. The programme continues to this day, and members receive concessional hourly rates, discounts on services, and access to the results of all general research projects. It is one of the longest surviving multi-client programmes of its kind in the


Reprinted from.

POSTEC Newsletter No. 24, 2006.

This Newsletter is a report on the research activities in Powder Technology carried out at the Telemark Technological Research and Development Centre and Telemark University College. POSTEC Newsletters are published annually and distributed free of charge to anyone who indicates an inter-est in receiving them. Please fill in the appropriate form on the last page of this issue if you want your own copy. We will publish articles from our partners from other parts of the world, if we consider such articles to be topical and of interest to our readers. Such articles are to be submitted to the editor by November. The editor, however, neither guarantees publication nor the return of any material so sent.In order to support the costs of publi-cation and distribution, we welcome advertising. Advertising copy must also be sent to the editor by May.


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