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Medium Stability

The stability of the medium is defined as the tendency of the solids in the medium to settle out. All conventional dense media are inherently unstable because the solids (e.g. ferrosilicon) have a higher density than the liquid in which it is suspended (water). The reciprocal of the rate at which the medium solids settle out under gravity is a measure of the medium stability.

So it is clear that stability and viscosity are directly related – a medium with high viscosity will have a high stability, and vice versa, as shown in the figure on the right. Any factor which affects the one, will also affect the other.

Stability is very important in determining the behavior of the medium in the separator. In general it is desirable to have a stable medium because if the solids settle out too readily, then separators, pipelines and pumps will sand up, and strong density gradients will be set up in the separator, which usually inhibits an efficient separation. Such density gradients are reflected in a difference in the density overflow and underflow medium streams from the separator.

The difference between the overflow and underflow medium densities (or sometimes the feed and underflow densities) is called the differential. Low or zero differentials are desirable in bath-type separators in order to reduce the incidents of artificial midlings – those particles of intermediate density which tend to accumulate in the bath. In cyclones, some positive differential (instability) is desirable to assist in the sorting process, but it should not normally exceed 400-500kg/m3. Media are much more stable in a cyclone because they are subjected to sedimentation forces many times those in a bath separator.

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