Materials Handling
Caking compromises the value of both raw materials and products in many processes. Tim Freeman shows how using powder rheometers can help to overcome the problem.
Die Klumpenbildung beeinträchtigt bei vielen Verfahren den Wert sowohl von Rohmaterialien wie von Produkten. Tim Freeman zeigt auf, wie der Gebrauch von Pulverrheometern dazu beitragen kann, das Problem zu lösen.
La concrétion compromet la valeur aussi bien des matières premières que des produits dans de nombreux procédés. Tim Freeman montre comment l’utilisation de rhéomètres à poudres peut contribuer à résoudre le problème.
Environmental factors impact on the caking behaviour of powders
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Fig. 1. Measuring BFE with a powder rheometer.
aking, the formation of agglomerated material from discrete particles or granules, is a widespread problem in powder processing. Many feed materials, as well as
the products used or manufactured by the food, chemical and pharmaceutical industries, are sold as free-flowing, easily processable powders. Caking compromises their value by adversely affecting either in-process or end-use performance. Agglomeration can occur over time via a
number of mechanical or chemical routes: one especially common mechanism is the migration or absorption of water. For achemical and process engineers, the key to its control lies in managing environmental conditions such that the feed or product remains in an optimal state. Described here, and illustrated by experimental studies, is the way in which powder rheometers can assist by measuring the changes in powder properties during the caking process - as a function of humidity and consolidation for example. Essential to solids processing is the ability to
control manufacturing variables in such a way as to produce powders with the required properties for specific applications. Packed into bags, kegs, bulk containers or tankers, these powders leave the manufacturing site in a relatively well-defined state. However, powder condition immediately before use may be markedly changed, by transportation and as
the result of storage. Caking is one of the primary mechanisms
by which powders deteriorate when stored, and humidity, temperature and consolidation are all influencing factors. Water can cause limited dissolution of the material and the subsequent formation of crystal bridges, which bind primary particles together into larger agglomerates. Prolonged consolidation on the other hand may promote mechanical aggregation by physically forcing the particles closer together. The sensitivity of different powders to these effects varies considerably, so storage conditions that suit one material may be unsuitable for another.
Powder rheometry Powder rheometers measure the dynamic properties of a sample, characterising flowability in a very direct way. Basic flow energy (BFE) is a key baseline measure and is defined as ‘the energy required
to rotate a helical blade down through a sample at a controlled rotational velocity. It is derived from precise measurements of both the axial and rotational forces acting on the blade as it passes through the powder (Fig. 1). Precision engineered instruments that use well-defined, automated test methodologies can measure BFE reproducibly, making it a highly differentiating parameter for powders across the entire cohesivity spectrum. Agglomerate formation during caking changes
the BFE of a sample, a number of mechanisms conspiring to increase BFE. Firstly, caking increases the strength of the inter-particulate bonds which have to be overcome before flow can occur. Secondly, the caked bulk is stiffer, the packing of the particles presenting greater resistance to movement. Fine powders tend to be relatively cohesive with the ability to trap air in inter-particulate pockets and as a result are often highly compressible, with an almost ‘spongy’ quality. With these materials, movement of the rheometer blade impacts only a small portion of the sample; the compressible powder easily absorbs the induced displacement and BFE is therefore low. In contrast, larger agglomerates pack more closely leaving little space for entrained air, so any displacement induced by the blade is transmitted very effectively through the bed, giving a much larger flow or transmission zone, and a corresponding increase in BFE. Finally, if caking is associated with a change in
moisture content it can increase BFE by changing the bulk density of the sample, as higher bulk densities often go hand in hand with a higher BFE. Together these changes make monitoring BFE a sensitive and productive way of detecting caking and assessing how it is influenced by environmental conditions, as the following study shows.
Consolidation on caking behaviour Comparing BFE profiles as a function of time for samples held under different conditions helps engineers to identify the optimum storage conditions for a given powder. The results of one such investigation are displayed in Fig. 2. Here two sets of data are shown: one for a sample held under no applied consolidating load, the other consolidated by an applied pressure of 9 kPa. For this powder blend, a slight increase in BFE
can be seen during the initial four day period for both the unconsolidated and consolidated powders. However, in the following days the BFE
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