As the notes on structural arching seemed to be well received I thought the following brief notes on segregation may be useful.

As segregation problems are prone to arise in circumstances that involve relatively large masses of bulk material, the behaviour of materials in storage hoppers and silos are of much related interest. The various mechanisms that lead to the different fractions diverging to occupy separate locations are prominent during unconfined flow, therefore are mostly prone to occur during the filling of storage containers, but only become manifest in the discharge process. It is therefore important to understand the various flow regimes that prevail in these two circumstances. A more detailed description of forces, mechanisms and processes that result in the segregation of bulk solids are given in the book – ‘User Guide to Segregation’ published by The British Materials Handling Board and a description of the differing flow regimes that develop during the various geometrical forms of hopper and silos presented in the Educational Resources in Powder Technology web site, www.erpt.org/992q/batesbates.

A good starting point when considering a problem of this kind is to say that segregation is a pernicious phenomenon that is virtually impossible to prevent with free flowing products of variable composition. It also will take place at every location and process where the bulk material moves up to the point of use, so the only thing that can be done is to restrain this to an acceptable condition which, whether it affects the process, product quality or its value, is ultimately determined by the user. Under these circumstances it is usually not practical for a manufacturer to guarantee a solution to a problem that will eliminate segregation, or reduce it to a condition that is acceptable to others whose criteria is unknown. What can usually be done is to put forward a design modification based on experience and best practice that offers to significantly reduce the lack of homogeneity within a reasonable cost.

For example, one process that is well known for separating fractions is that of unconfined flow down the surface of a ‘poured repose’ cone, such as is formed when a batch of material is loaded into storage from a single point fill. This normally results in the peripheral regions accumulating a different composition from the material that is deposited in the centre of the pile. If the contents then discharge completely from a hopper that is not of mass flow design, there will be four stages of zone discharge.

1.Material will initially empty a small diameter 'core' of product immediately above the outlet, with a cross section roughly equal in diameter to the outlet size, until material that was initially on the surface of the contents reaches the outlet.

2.The space vacated by this extraction will be followed by a progressive reduction of the 'poured repose' pile by the material running into a 'drained repose ' cone to flow down the core flow channel. This phase will continue until the growing diameter of the draining product reaches the wall of the container.

3.Material will slough off the surface of the contents over the full container diameter into the central flow channel in even layers, until the boundary reaches the start of the converging cone of the hopper construction.

4. The residual material resting between the drained repose cone and the cone of the hopper section will steadily runs out, with a small, final stage as an empty region develops in the centre of the outlet

The consequences of this flow regime with a segregating product is that the discharge in phase one will result in a small quantity initially emptying will have a shortage of the component that migrates to the boundary of a poured repose cone, which is usually, but not always, the larger fractions.

Material in phase two will show a progressive increase in restoration of the balance of ingredients as the content draining into the central flow channel erodes the initial surface pile and draws down an increasingly uniform compostion.

The third phase of discharge will be evacuating layers that are even cross sections of the deposited contents, and hence should reflect the composition as originally filled into the hopper.

It is during the terminal period of discharge that the most extreme conditions of segregation are manifested as the boundary contents of the deposited material has no centrally deposited contents to dilute the concentration of fractions that roll to the outside of the cone of repose formed during filling.

The effects of segregation are therefore more distict in large diameter, squat containers as a large proportion of the material contained in the body section of a tall silos may reconstitute the infeed proportion of filling by collecting from the complete cross section of the stored material which has a similar particle size distribution to that entered, even though these were not deposited at the same time. The respective volumes of these phases can be calculated from knowledge of the hopper geometry and respective angles of poured and drained repose. It should be noted that if the container is refilled before emptying there will be a more complicated segregation that gives rise to an intermediate 'flash' of segregation as the discharge passes through the refill level.

There are various approaches that can be taken to address this pattern of segregation. Contrary to many published statements, mass flow will not fully rectify the situation because the flow velocity profile in a converging channel will result in the material resting on the wall of the converging section of the hopper being progressively slower to empty when the level of material reduces to the bottom of the parallel body section of the hopper. The degree of segregation may not be as strong as with a non-mass flow construction, but the teminal portion of discharged product will tend to contain a higher proportions of the peripherally deosited fractions.

The two main avenue of retrofit approach are either to reduce the radial imbalance of deposits by devices fitted at the hopper or silo inlet, or to modify the draw pattern of discharge by installing flow modification devices affecting the flow pattern with which the contents discharge. Examples of these fittings and their effects can be seen on www.ajax..co.uk/flowvideosflowvideos. Various degrees of sophistication of either of these methods is possible, according to many factors, but reasonable results can usually be obtained by relatively crude fittings designed by experienced persons who need to take account of access facilities and overall economics. ■