Dense
medium separation or Heavy medium separation (HMS), or the sink and float
process
is
applied to the pre-concentration of minerals (including Coal), i.e, the
rejection of gangue prior
to grinding for final liberation. It is also
used in coal preparation to produce a commercially gradedend-product,
clean coal being separated from the heavier shale or high ash Coal.
Dense medium separation is applicable to any ore in which, after a Suitable
degree of liberation by crushing, there is enough difference in specific
gravity between the
particles to separate those which will repay the cost of further treatment from
those which will not.
The process is most widely applied when the density difference occurs at a
coarse paticle size, as
separation efficiency decreases with size due to the slower rate of settling of
the particles.
Particles should
preferably be larger than about 3mm in diameter, in which case separation can
be effective ona
difference in specific gravity of 0.1 or less, explains Mr. Padmesh Gupta.
The
process offers some advantages over other gravity processes. It has the ability
to make sharp Separation
at any required density, with a high degree of efficiency even in the presence
of high percentage of near-density material (NGM).
The
density of separation can be closely controlled, within a relative density of +-0.005 Kg/ltr
and can be maintained, under normal conditions, for indefinite periods. The separating density
can, however, be changed at will and fairly quickly, to meet
varying requirements. The process is, however, rather expensive, mainly due to
the ancillary equipment needed to clean the medium, elaborates
Mr. Padmesh Gupta.
THE
DENSE MEDIUM:
Heavy
liquids have wide use in the laboratory for the appraisal of gravity-separation techniques
on ores. Heavy liquids testing may be performed to determine the feasi-bility of dense medium separation 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 sample
into a
series of fractions according to density, establishing the relationship between
the high and
low specific gravity materials.
Tetrabromoethane
(TBE),having a specific gravity of 2.96, is commonly used and may be Diluted
with white spirit or carbon tetrachloride (sp. Gr.1.58) to give a range of
density Below 2.96 .Bromoform (sp.gr 2.89) may be
mixed with carbon tetrachloride to give densities In
the range 1.58- 2.89 The most widely used medium for metalliferous ores is now ferrosilicon,
while magnetite is used in coal preparation. Recovery of medium in both cases is
by magnetic separation. Magnetite (sp. Gr 5.1) is relatively cheap, and is used
to maintain bath
densities of up to 2.5 Kg/Ltr.
BY:
SRI PADMESH GUPTA
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