2. Dense Medium Separation (DMS)
Dense medium separation (DMS) is also known as heavy medium separation (HMS) or the sink-
and-float process.
It has two principal applications:
1. the pre-concentration of minerals, that is, the rejection of gangue prior to grinding or final
liberation
2. in coal preparation to produce a commercially graded end-product, that is, clean coal being
separated from the heavier shale or high-ash coal.
In principle, it is the simplest of all gravity processes and has long been a standard laboratory
method for separating minerals of different specific gravity. Heavy liquids of suitable density are
used, so that those minerals less dense (“lighter”) than the liquid float, while those denser
(“heavier”) than it sink
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4. Since most of heavy liquids are expensive or toxic, the dense medium used in industrial
separations is a suspension of particles of some dense solid in water, which behaves as a heavy
liquid.
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Concentration Criterion:
5. THE DENSE MEDIUM
• It has two components
1. Liquids
Heavy liquids have wide use in the laboratory for the appraisal of gravity separation techniques on ores.
• Heavy liquid testing may be performed to determine the feasibility of DMS on a particular ore and to
determine the economic separating density, or it may be used to assess the efficiency of an existing dense
medium circuit by carrying out tests on the sink and float products.
• The aim is to separate the ore samples into a series of fractions according to density, establishing the
relationship between the high- and the low specific gravity minerals
• Examples:
I. Tetrabromoethane, having a specific gravity of 2.96, is commonly used and may be diluted with white spirit
or carbon tetrachloride (s.g. 1.58) to give a range of densities below 2.96.
II. Bromoform (s.g. 2.89) may be mixed with carbon tetrachloride to give densities in the range 1.582.89.
2. Suspensions:
• These are the fine particles to be suspended in the liquid medium to adjust the density of the medium.
These fine particles should not be more that 15% by volume of the total liquid.
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6. Jigs
• Jigging is one of the oldest methods of gravity concentration, yet the basic principles have only recently
been understood.
• In the jig, the separation of minerals of different specific gravity is accomplished in a particle bed, which is
fluidized by a pulsating current of water, producing stratification based upon density.
• The bed may be a specific mineral added to and retained in the jig, called ragging, composed of a certain
density and shape through which the dense particles penetrate and the light particles pass over the top.
• The aim is to dilate the bed and to control the dilation so that the heavier, smaller particles penetrate the
interstices of the bed and the larger high-specific gravity particles fall under a condition similar to hindered
settling.
• On the pulsion stroke the bed is normally lifted as a mass, then as the velocity decreases it tends to dilate,
the bottom particles falling first until the whole bed is loosened.
• On the suction stroke it then closes slowly again, and this is repeated at every stroke. Fine particles tend to
pass through the interstices after the large ones have become immobile.
• The motion can be obtained either by using a fixed sieve jig, and pulsating the water, or by employing a
moving sieve, as in the simple hand-jig shown in Figure.
7.
8.
9.
10. • The jig is normally used to concentrate relatively coarse material and,
if the feed is fairly close-sized (e.g.,310 mm), it is not difficult to
achieve good separation of a fairly narrow specific gravity range in
minerals in the feed (e.g., fluorite, s.g. 3.2, from quartz, s.g. 2.7).
• When the specific gravity difference is large, good concentration is
possible with a wider size range. Many large jig circuits are still
operated in coal, cassiterite, tungsten, gold, barytes, and iron ore
concentrators.
11. Jigging Action
• It was shown that the equation of motion of a particle settling in a viscous fluid is:
• where m is the mass of the mineral particle, dx/dt is the acceleration, g is the
acceleration due to gravity, m is the mass of displaced fluid, and D is the fluid resistance
due to the particle movement. At the beginning of the particle movement, since the
velocity x is very small, D can be ignored, as it is a function of velocity.
and since the particle and the displaced fluid are of equal volume,
where ρs and ρf are the respective densities of the solid and the fluid.
12. • The initial acceleration of the mineral grains is thus independent of
size and dependent only on the densities of the solid and the fluid.
Theoretically, if the duration of fall is short enough and the repetition
of fall frequent enough, the total distance travelled by the particles
will be affected more by the differential initial acceleration, and
therefore by density, than by their terminal velocities, and therefore
by size.
• In other words, to separate small heavy mineral particles from large
light particles, a short jigging cycle is necessary.
