BULK MATERIALS HANDLING | MACHINERY
Keeping materials on the move
Reducing downtime, enhancing flexibility and providing a safe working environment are all key elements of a cost-effective and efficient bulk materials handling system in the compounding plant. Mark Holmes finds out more
An efficient compounding operation requires resins, fillers and additives, as well as finished compounded materials, to be in the right location in the plant at the right time. Pneumatic conveying and handling of materials on the shop floor is an essential tool in achieving this. However, materials such as calcium carbonate, barium sulphate, magnesium hydroxide and alumina trihydrate - all commonly used in the compounding industry to provide opacity, improve surface finish, increase impact strength and stiffness, and improve flame retardant properties – can provide particular handling problems. “Most of these fillers are in a particle size range
of 200 microns to less than 1 micron,” says Dr John Lawrence, Research Director at the Bulk Solids Innovation Center of Kansas State University in the US. “At this particle size range, these materials exhibit high cohesive strength and are difficult to
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flow into the hoppers used to feed material in a pneumatic conveying system.” Lawrence says the characteristics of flow behaviour difficulty can be expressed using Andrew Jenike’s flow function, which was pub- lished in bulletin 123 back in 1964. This detailed flow function and how it relates to the cohesive strength of the material (basically, the higher the cohesive strength of the material, the lower the flow function value). Flow function is defined as the ratio between the applied normal force and unconfined yield strength of the material. “Plastic compounding fillers exhibit higher unconfined yield strength and flow function values are always small, meaning difficult-to-flow materi- als,” Lawrence says. “For a flow function value of less than 2, the hopper angle for mass flow must always be greater than 80° – a very steep slope. In practice, most hopper angles are between 60 and
Main image: Moving raw materials quickly and reliably is a key requirement in efficient
compounding. This image shows Coperi- on’s Test Centre for Bulk Material Handing at Weingarten in Germany
February 2018 | COMPOUNDING WORLD 31
PHOTO: COPERION
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