Materials Handling


Fig. 2. Investigating the impact of consolidation on caking by tracking changes in BFE as a function of time.


Fig. 3. Comparing powder transit through a hopper under a) mass flow and b) funnel flow conditions.


rises rapidly, in a more pronounced way for the consolidated powder. At five and a half, the consolidated powder is twice as resistant to flow as it was when loaded into the storage vessel, whereas the unconsolidated powder takes until day eight to achieve a similar state. In both cases, BFE continues to rise rapidly with no sign of a plateau. In a bin and hopper the powder sits under the


consolidating pressure of its own weight. The properties of this powder suggest that minimising this pressure will be beneficial in terms of caking, so how can this be achieved? One option is to run the bin at a relatively low fill level,


topping up at a regular frequency with lower volumes. This potentially confers advantage in two ways. Firstly it reduces the head of powder acting on the material in the hopper, where the material is under greatest consolidating pressure. Secondly it reduces the residence time of powder in the bin, thereby limiting the extent of caking. However this second point deserves closer scrutiny because in fact, residence time will only be uniformly reduced if the bin is running in a mass flow, rather than funnel flow regime. With mass flow, all the powder in the bin is in motion:


as material is withdrawn from the hopper, powder transits through the unit on a first in, first out basis. A hopper with walls steeper than a limiting value defined on the basis of the


How to make sure a new product goes through the customer’s plant


handleability and flow of the material through downstream process plants. This is what happened when Imerys wanted to change the format of the clay used in papermaking around Europe from a spraydried clay with 5 per cent moisture to a granulated clay with 18 – 20 per cent moisture. The natural question was:


W 32 www.engineerlive.com


hen a manufacturing process for a powder or granular material changes, it can affect the


Will this new material flow reliably through our customers’ plants?


Imerys worked together with The Wolfson Centre for Bulk Solids Handling Technology to carry out a comprehensive series of tests, using the existing product and prototypes of the new product. In response to identified differences in behaviour between the old and new product form, some adjustments were made to the new manufacturing process, and the improved material re-tested.


Imerys worked with their customers to advise them how to readjust their plant and limited ‘preview’ shipment trials were undertaken at a number of plants, to show that the predictions made about the behaviour of the new material


shear properties of the powder will deliver this performance. Where these criteria are not met, funnel flow can develop. The hopper angle is insufficiently steep to ensure that powder flows smoothly down the walls so material builds up (Fig. 3) and the residence time of powder within the hopper becomes non-uniform. Some powder remains in the vessel only briefly, entering the centre of the ‘funnel’ and exiting almost immediately. More importantly from the point of view of caking, other portions of the powder population remain towards the base of the vessel for considerable amounts of time, under the consolidating pressure of the material above. These conditions are ideal for caking and are to be avoided. While the above example illustrates the importance of


considering caking within the dynamic environment that applies in a feed bin, many associate the phenomenon more closely with the ‘static’ condition of powder stored for a certain period of time in, for example, a closed keg. Here, studies such as the one detailed earlier quantify the likely extent of a caking problem and inform decisions about how often the material needs to be tumbled or agitated to keep it in a fit state for subsequent processing. n


Tim Freeman is Director of Operations with Freeman Technology, Welland, UK. www.freemantech.co.uk


were accurate and give customers confidence that the new material would behave well in their plants.


By managing the change-over in this way, the risks were isolated and dealt with before they became real and when the new material started to go to customers it flowed without trouble in their handling systems. n


Enter 32 or ✔ at www.engineerlive.com/epe


The Wolfson Centre for Bulk Solids Handling is based at University of Greenwich, Chatham Maritime, Kent, UK. www.bulksolids.com Imerys is based in Par, Cornwall, UK. www.imerys.com


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