Dense Medium Separation (also called Heavy Media Separation) is a well-established density separation process. Dense Medium Separation (DMS) uses the characteristic differences in density of the input material to enact a gravimetric-based separation. Due to the robustness of the process, DMS can be used in the separation of minerals, ore bodies and scrap metals. Dense Medium Separation is also capable of handling a wide range of feed densities, starting as low as 1.7 SG up to 4.5 SG for specific ore bodies.
Dense Medium Separation utilises a fine suspension of solids within an aqueous carrier fluid to create a medium of intermediate density, relative to the process feed. Particles more dense than the medium will report the process underflow as sinks, conversely less dense particles will report to the process overflow as floats.
For separation densities above 2.0, the usage of Ferrosilicon is recommended. DMS Powders produces specialized Ferrosilicon powders for Dense Medium Separation; these powders contain 14% - 16% Silicon (by weight) with a Relative Density of 6.8 – 7.1.
The production of Ferrosilicon for use in Dense Medium Separation processes requires sophisticated production and quality assurance techniques, which must incorporate tight control of chemical composition as well as physical properties. The use of Ferrosilicon produced in this fashion provides the following benefits:
DMS Powders produces a wide range of grades to suit any application. The different grades have different physical properties, which determine in-circuit behaviour. These properties should be correctly matched to the type of dense medium process being used and the nature of the required separation.
The more important properties of the dense suspension or ‘medium’, i.e. powder mixed with water to form heavy liquid, are density, viscosity and stability, which are interrelated. These properties are determined by the physical properties of the ferrosilicon powder, such as particle size distribution and shape, controlled in the production process to produce a wide range of products. The impact of ferrosilicon properties on that of the medium can be briefly described as follows:
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:
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.
|A high viscosity results from:
A low viscosity results from:
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.
A high stability results from:
A low stability results from:
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: