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Selection of the Medium

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.

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