Back in the day we used to have plenty of people at the mill to do all kinds of nifty stuff. For instance, if a big-bag wouldn’t empty you had your choice of burly men to kick it for you. This is not the case anymore. In today’s manufacturing industry a great deal of both raw materials and products may be delivered in the form of dry bulk powders. When dealing with these materials you want them to obey your process and flow in a controlled way during logistics, transport as well as in application. All this needs to be happen without blunt force. The bulk material supplier should have a tight quality grip on the flow parameters in their bulk product.
As a customer you don’t necessarily see, or may even be unaware of, everything going on at the producer’s end what comes to tailoring bulk flows. You or your colleague may have come across issues or difficulties in handling some bulky powder material? Maybe a jammed hopper has required some serious physical attention or a bulk screw transporter has suddenly overflowed leaving bulk powder all over the place? These are examples of typical extreme cases in bulk flow. Nevertheless, these can be avoided by the correct bulk rheology. Studying the complete picture from individual particles, their interactions all the way to their behavior as bulk can allow for adjustments of the bulk flow ensuring smooth handling. In the bulky world of grains varying problems may arise from the tiny particles themselves: their shape and physics but most of all from their level of interaction.
Let’s have a closer look at what’s making a pile of powder stay or bulk material to flow. Generally, powders are challenging and can be classified more complex than both liquids and gases. In a bulk material we may have solid particles, liquid and gas making a three-phase system. Such recipes render bulk materials more multifaceted than any of the single basic phases alone. Considering the particles themselves they can behave as a solid when consolidated enough. Recall e.g.the making of tablets in the pharmaceutical industry where a bulk product is compressed at high pressure to become a solid pill. In other cases, particles may display elasticity and may withstand compressive forces, as in case with metal spheres or quartz sand grains.
Considering bulk powder flow characteristics some key parameters can be utilized. A particulate flow can be described by its tendency to resist shear deformation. Here the micron level inter-particle adhesion forces are responsible for the bulk material’s internal cohesion. We have all experienced an overflowing flour bag, dusting in our kitchen or seen a powder that is supposed to go down a funnel but just doesn’t. This parameter is decisive for the flow characteristics and can be related to a series of primary characteristics in the particles and their interactions. Amongst these we may distinguish e.g.:
- particle size and size distribution,
- particle shape, density and porosity
- humidity around and moisture in particles
- particle roughness and chemical composition
- particle surface chemistry (adhesion and cohesion) and
- bulk physics (elasticity and compressibility)
These parameters are to some level involved in the interparticle friction and will be decisive for the particles how they react as bulk material on external forces (Schultze, 2008). As mentioned above, the bulk material air contents will greatly influence its behavior, especially when in transit or during logistic phases when subjected to external and/or internal stresses. At high enough air contents, the bulk material will resemble a fluid, this is utilized in fluid-bed applications as e.g.in combustion chambers. Nevertheless, any comprehensive measurement of all the above relevant properties is quite an impossible task, despite today’s materials technology.
From basic surface and interfacial chemistry, we know that adhesional and cohesive forces in any materials are central to its interactive and bulk level behavior, respectively. Adhesion describes the natural uniting tendency between two components. Cohesion describes the similar principle in a single material. These interaction mechanisms arise from various interparticle forces present in chemistry (Allen, 1993). In a bulk material, cohesion would largely describe interparticle dynamics (i.e.internal friction) while adhesion to a degree more the bulk e.g.flow at pipe and hopper walls. Generally, there’s a link between adhesion and friction as stronger adhesion generally results in increased friction (Carpick, Sridharan, & Flater, 2004). The interparticle cohesive forces will affect the bulk material fluidity as particles stick and cling together forming larger aggregates. You may have experienced spreading a bit moist icing sugar onto your freshly baked danishes? Or ditto table salt? Nope, no success there… Both these particle types need to be seemingly dry to flow as bulk. Hence, at higher levels of particle cohesion – for example due to increased moisture – the bulk material’s threshold level to become fluid upon shear is elevated. In practice we may experience this as an impaired bulk flow. Generally, the range of adhesive forces can be described by a few physical and chemical mechanisms:
Mechanical forces (particle level)
- Physical interlockingsbetween particles, these may arise due to rough surfaces, uneven particle geometries, variant porosity or unfavorable particle size distributions. These interactions relate to particle size and may exist in the range 10-3–10-5
- Liquid bridgesin moist powders, when free water is present between or on particles the surface tension and capillary pressure of the liquid may be a source of strong attractive forces between particles. Apparent at distances below 10-3
- Secondary forces (intermolecular level)
- Van der Waals’ forcesthat are based on interactions between (constant and/or induced) electric dipoles in the molecules making up the individual particles. Intermolecular forces are primarily Coulomb (electrostatic) in nature. Only discernible at distances below 10-6
- Electrostatic forces that arise due to differences in electric potential between particle surfaces. Discernible at distances below 10-4–10-7
- Primary valence forces (molecular level)
- Ionic, covalent and metallic bonds, responsible for chemical bonding in molecular structures. Effective at atomic and molecular scale levels; below 10-7
Several theories behind the primary and secondary forces exist, have evolved and been debated but are here considered beyond scope (Woodward, 2000). In our case, the particle-level interactions described above are mainly involved in the flow of granular materials – the bulk rheology. The concept of bulk flowability, or bulk rheology, was originally developed by (Jenike, 1961)for characterization and classification of bulk solids in material handling processes. By shear testing of a bulk sample information on the unconfined yield strength describing the bulk material cohesive strength as a function of compaction pressurecan be gained (Bell, 2001).
Bulk rheology characterization
In a typical shear test cell, a sequence of shear tests is performed at various levels of normal stress. By recording the shear stress (response) at various normal stresses (input) we can define the bulk materials yield loci – the point where flow is induced – from a normal-shear stress plot. The phenomena could be compared with fresh snow below zero: the less you have compressed (normal stress) a snow ball in your palm the easier it will break (shear stress). The same principal behavior is present in other particulate matter (Freeman Technology, 2014). When we plot these yield loci in a stress plot and determine their average slope we get a so-called flow function. The smaller the slope the easier the bulk material flows – this is indicated as smaller number in the flow-function.
Considering other bulk rheology phenomena, we may experience formation of so-called rat-holes in the hopper. This implies hourglass-type vertical bulk flow where only the middle part of the hopper contents is moving downwards. From the above shear test data, it is possible to estimate a critical rat-hole diameter for a given bulk product. The number relates to the bulk material compressibility and the hopper wall friction, also derived from the stress test data, providing a numerical estimate of a rathole diameter. The larger the diameter the better the situation for a given hopper.
Principally all hoppers and bulk material handling systems are individuals to some degree (Laurén, 2017). At Chemigate we have the tools available for tailoring the bulk rheology in accordance with the customer bulk hopper. Since customer bulk handling systems are unique to some degree this also requires knowledge of both the bulk process and end-application. It is not uncommon that initial problems tend to arise in older bulk handling systems originally designed for other bulk materials. This happens frequently nowadays when customers are changing starch raw materials e.g. from potato to cereal grades. But no reason for worries, we have the recipes for such situations too.
Bulk rheology deviation leading to handling disturbances are not uncommon in moisture-sensitive bulk materials. As discussed above the bulk starch particles mainly interact via mechanical forces and interlockings. By introducing efficient and inert flow agents, we have full control of the bulk rheology. Nevertheless, moisture interactive materials are more challenging. At some point of increased moisture, the interaction forces will intensify as they step down in size-scale, especially at the level where condensed or free water starts to occur. This is definitely an undesired state for any process dealing with hydrosensitive bulk materials. Latest at this point we are facing bulk rheology problems.
At Chemigate we have the experiences and technology to avoid these during the bulk material processing. Our starch products are safe frommoisture. We have the know-how and skills to produce great powders with tailored, well-controlled rheology in accordance with the customer needs. The customer buys not just great powder but the skills to use it as well.
Tom Lundin D.Sc. (Tech.)
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Bell, T. (2001). Handbook of Conveying and Handling of Particulate Solids(Vol. 10). (A. Levy, & H. Kalman, Eds.)
Carpick, R. W., Sridharan, K., & Flater, E. E. (2004). The effect of surface chemistry and structure on nano-scale adhesion and friction. Polymeric Materials(90), 197-198.
Chen, W., & Roberts, A. W. (2017). A modified flowability classification model for moist and cohesive bulk solids.Powder Technology, 325, 639-650. doi:https://doi.org/10.1016/j.powtec.2017.11.054
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Freeman Technology. (2014). Shear testing. Retrieved 07 27, 2017 from About the FT4 Powder Rheometer: http://www.freemantech.co.uk/_powders/powder-testing-shear-cells
Jenike, A. (1961). Storage and flow of solids. Bulletin of the University of Utah, 52(123).
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Schultze, D. (2008). Powders and Bulk Solids. Behavior, Characterization, Storage and Flow.Berlin, Heidelberg: Springer-Verlag. doi:https://doi.org/10.1007/978-3-540-73768-1_3
Woodward, R. P. (2000). Prediction of Adhesion and Wetting from Lewis Acid Base Measurements. TPOs in Automotive 2000.Novi, Michigan: Executive Conference Management.