Segregation 1 - Hopper Filling & Transfer Points

Segregation is the process by which different fractions within a bulk solid separate, leading to a poorly mixed material which behaves differently, potentially affecting product quality. The risk of segregation occurring is greatest when transferring the material. In the first Top Tips on segregation we are focussing on filling a storage hopper and transfer points.

1. Avoid long repose slopes when filling hoppers

Dispersing plates or 'overflow' pots should be used with a small drain to avoid single point fill. However, care should be taken that any distributing device fitted has no feed or extraction bias, otherwise the situation may be exacerbated.

2. A Mass Flow pattern can re-mix products segregated during filling a storage container. However, a Mass Flow pattern is not a 'cure-all'

When the level of material in the storage system falls to the point where the walls begin to converge, material flows faster in the centre than at the walls. Consequently, the last material of a batch to discharge has flowed along the wall. This also leads to regions with mainly coarse fractions. A central insert can help by increasing the area of draw-down, or diluting local aggregations by drawing product from multiple locations.

3. Transfer batches of bulk as quickly as practical

Swift transfers restrict the time over which segregation processes can act, and reduces the change in particulate structure that facilitates the movement of fractions.

4. Avoid biased transfer to process lines, conveyors or storage

The flow stream of a belt conveyor delivering material to a hopper will eject the coarse further than the fines, and material running from the side of an inclined chute will tend to allow fines to leave first and coarser product to carry further on the sloping surface. By diverting multiple parts of the flow stream to different receiving regions it is possible to remix the separated material.

5. Be careful where samples are taken from

Select as close as practical to the points of major significance and be aware of the subsequent handling sequence. Take running samples as a 'slice' from across the full flow stream as effects can flow from inaccessible regions. For detailed guidance see the 'User Guide to Segregation', published by the British Materials Handling Board.

6. Watch carefully the initial and terminal effects of both batch and normally continuous operations, particularly if the product is stored at this stage, and refill takes place before total discharge. Modulating or attenuating techniques may be needed to avoid 'End Effects' at both the start and the completion of production feed streams.

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