Technical Support

Medium density is controlled by the proportion of ferrosilicon solids present in the medium; it is increased by adding more ferrosilicon, and reduced by adding water.

A high medium density results from:

• A high concentration of solids in the medium
• A high absolute density of the solids in the medium

Thus ferrosilicon with a relative density of 6.8 will always produce a higher medium density at the same solids concentration than magnetite which has a relative density of 4.5.

The medium density determines to a large extent the density at which the separation will take place, and is the main variable available to the operator for controlling separation. In static bath separators, such as drums and cones, the separating density (or cut-point) is generally close to the medium density. In dynamic separators, such as cyclones and tri-flows, the separating density is usually somewhat higher than the medium density.

Medium viscosity is the measure of the medium’s resistance to flow – its consistency. It is determined by the concentration, shape and size distribution of the solids making up the medium.

Once the ferrosilicon grades have been selected, the most important variable affecting the viscosity is the medium density, determined by the solids concentration. The relationship between the viscosity and solids concentration is non-linear, a shown in the figure to the right.

At low densities, there is little change of viscosity with density (as determined by the solids concentration), but there comes a point at which viscosity rapidly increases with density. In practice it is unwise to operate even close to this point because the medium is then too viscous to use, and small changes in density will result in large changes in viscosity.

Apart from ferrosilicon of a particular grade (size and shape), the solids present in the medium often include fine contaminants from the feed being treated. Because these contaminants will nearly always be of a lower density than the ferrosilicon, a contaminated medium will need to contain a higher amount of solids concentration to achieve the same medium density, and will therefore have a higher medium viscosity than a clean medium.

The viscosity is an important property of the medium, but because it is difficult to measure, its influence on the separation is not always well understood. In general, however, high viscosity media are undesirable because it reduces the velocity of the particles being separated increases, increasing the chance of particle misplacement and thus reducing the efficiency of separation.

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.

The differential in a dynamic separator is diagnostic of the condition of the separation, and operating conditions, including the medium grade, can often be tuned to optimize the differential and to maximize separation efficiency.

The most important factor which must be taken into account when selecting a medium grade for a particular application, is the operating density. This will be determined by the separation, which is desired, the characteristics of the feed, and the nature of the separating vessel being used.

Once the operating density has been selected, it is the stability and viscosity, which will then determine the appropriate ferrosilicon grade. The ideal medium is one with high stability and low viscosity. Any selection involves a compromise between the two mutually exclusive properties.

The following guidelines should be followed:

• For high operating densities (say above 3.2 RD), only atomized grades can be used, as the milled grades are too viscous at the high solids concentrations required.
• Atomised grades may also be appropriate where corrosive conditions or highly porous feed (leading to high medium losses and thus operating costs), are suspected.
• Otherwise milled grades are to be preferred because of their lower cost.
• Dynamic separators require finer grades than bath separators, due to the greater tendency to instability (large differentials) in dynamic separators. High-pressure dynamic vessels (large static heads of high pump delivery pressures) require finer grades than low-pressure systems, to preserve stability.
• Deep bath separators in which relative quiescent conditions prevail, (e.g. cones) require finer grades than shallow baths (e.g. drums), in which greater turbulence helps to maintain stability. Other things being equal, coarse grades are preferred to finer grades because under most conditions medium losses occur preferentially in the finer sizes. The coarser grades are also often cheaper.
• The separation of fine feed particles in dynamic separators favours the use of finer grades than those appropriate for coarse separations, because high differentials are particularly deleterious to fine particle separation. However, this should not be at the expense of excessive viscosities, which are also damaging to the fine particle separations.
• The appropriate grade will often change with the life of a plant. At commissioning, before any of the separators, pipelines, pumps, transfer points or indeed the medium itself have been ‘run in’, a finer grade is often more appropriate than at a later stage.
• Mixtures of grades may be necessary to achieve the right combination of properties. Some research suggests that the correct bi-modal size distribution gives the desired properties of high stability and low viscosity.
• Other factors to take into account include the following:
  • Most plants operate with varying degrees of fine solids contamination in the medium. This is due to incomplete washing or in-circuit breakdown of the feed, which is a function of operating efficiency, plant design, age of the plat, feed type, etc. Contamination will act to increase the viscosity and stability of a given grade of ferrosilicon. Some operators deliberately permit a certain degree of contamination to stabilize particularly coarse grades of ferrosilicon.
  • Residual magnetization of the medium, caused by passage through the magnetic separators, increases viscosity and stability. This effect can be reduced by using demagnetisation coils. Atomised ferrosilicon requires higher demagnetising fields than milled ferrosilicon.
  • A useful guide to medium selection is of coarse the current practice in other, similar plants. However, the golden rule is to select the medium to optimise the process, and then design the plant to handle the medium.