This document provides an overview of dense medium separation. It begins by defining dense medium separation as a process that uses heavy liquids to separate minerals of different specific gravities, with those lighter than the liquid floating and those denser sinking. It then discusses the principle behind dense medium separation and different types of dense mediums that can be used, including suspensions of solids in water. The document also covers characteristics of dense medium separation such as its applications in coal preparation and mineral processing as well as factors that influence its separation efficiency.

1. PRESENTATION ON DENSE MEDIUM
SEPARATION
Presented By
Gulfam Hussain

2. WHAT IS DENSE MEDIUM SEPARATION?
Dense medium separation (or heavy medium separation (HMS),
or the sink-and-float process) is applied to the pre-concentration
of minerals, i.e. the rejection of gangue prior to grinding for final
liberation. It is also used in coal preparation to produce a
commercially graded end-product, clean coal being separated
from the heavier shale or high-ash coal.

3. 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 lighter than the liquid float, while those
denser than it sink.

4. GEOMETRICAL VIEW
Feed
Minerals of S.G -2.8 Fluid Medium S.G=2.8 Minerals of S.G +2.8 (Sinks)
(floats)

5. DIFFERENT DENSE MEDIUMS
Since most of the liquids used in the laboratory are expensive or
toxic, the dense medium used in industrial separations is a thick
suspension, or pulp, of some heavy solid in water, which behaves
as a heavy liquid. 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
densities below 2.96. Bromoform (sp. gr. 2.89) may be mixed with
carbon tetrachloride to give densities in the range 1.58-2.89. For
densities of up to 3.3, diiodomethane is useful, diluted as required
with triethyl orthophosphate. Aqueous solutions of sodium
polytungstate have certain advantages over organic liquids, such as
being virtually non-volatile, non-toxic and of lower viscosity, and
densities of up to 3.1 can easily be achieved.

6. Clerici Solution:
Clerici solution (thallium formate-thallium malonate solution) allows
separation at densities up to 4.2 at 20’C or 5.0 at 90’C.
Magnetohydrostatics:
Separations of up to 18 kg 1-1 can be achieved by the use of
magnetohydrostatics, i.e. the utilization of the supplementary weighting
force produced in a solution of a paramagnetic salt or ferrofluid when
situated in a magnetic field gradient. This type of separation is
applicable primarily to non-magnetic minerals with a lower limiting
particle size of about 50 microns.

7. SUSPENSION
A suspension is a homogeneous mixture containing solid particles
that are sufficiently large for sedimentation. Usually they must be
larger than 1 micrometer. An example of a suspension would be
sand in water.
Below a concentration of about 15% by volume, finely ground
suspensions in water behave essentially as simple Newtonian
fluids. Above this concentration, however, the suspension becomes
non-Newtonian. A fluid is said to be Newtonian if the viscous
stresses that arise from its flow at every point, are proportional to
the local strain rate — the rate of change of its deformation over
time.

8. In order to produce a stable suspension of sufficiently high density, with a
reasonably low viscosity, it is necessary to use fine, high specific gravity
solid particles, agitation being necessary to maintain the suspension and
to lower the apparent viscosity. The solids comprising the medium must be
hard, with no tendency to slime, as degradation increases the apparent
viscosity by increasing the surface area of the medium. The medium must
be easily removed from the mineral surfaces by washing, and must be
easily recoverable from the fine-ore particles washed from the surfaces. It
must not be affected by the constituents of the ore and must resist
chemical attack, such as corrosion. Galena was initially used as the
medium and, when pure, it can give a bath density of about 4.0. Above this
level, ore separation is slowed down by the viscous resistance. Froth
flotation, which is an expensive process, was used to clean the
contaminated medium, but the main disadvantage is that galena is fairly
soft and tends to slime easily, and it also has a tendency to oxidize, which
impairs the flotation efficiency.

9. ADVANTAGES
The process offers some advantages over other gravity processes. It has the
ability to make sharp separations at any required density, with a high degree of
efficiency even in the presence of high percentages of near-density material.
The density of separation can be closely controlled, within a relative density of
+0.005kg/l, -0.005kg/l 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
and the cost of the medium itself.

10. CHARACTERISTICS
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 particle size, as separation efficiency decreases with size due
to the slower rate of settling of the particles. Particles should preferably
be larger than about 4 mm in diameter, in which case separation can
be effective on a difference in specific gravity of 0.1 or less. Separation
down to 5001xm, and less, in size can, however, be made by the use
of centrifugal separators. Providing a density difference exists, there is
no upper size limit except that determined by the ability of the plant to
handle the material.

11. USES
The most important use of DMS is in coal preparation where a relatively simple
separation removes the low-ash coal from the heavier high-ash discard and
associated shales and sand-stones. DMS is also used to pre concentrate tin and
tungsten ores, and non-metallic ores such as fluorite, barite, etc. It has a very
important use in the pre concentration of diamond ores, prior to recovery of the
diamonds by electronic sorting. Diamonds are the lowest grade of all ores mined,
and concentration ratios of several million to one must be achieved. DMS
produces an initial enrichment of the ore in the order of 100-1000 to 1 by making
use of the fact that diamonds have a fairly high specific gravity (3.5), and are
relatively easily liberated from the ore, since they are loosely held in the parent
rock. Gravity and centrifugal separators are utilized, with ferrosilicon as the
medium, and separating densities of between 2.6 and 3.0 are used. Clays in the
ore sometimes present a problem by increasing the medium viscosity, thus
reducing separating efficiency and the recovery of diamonds to the sinks.

12. EFFICIENCY OF DENSE MEDIUM
SEPARATION
Laboratory testing assumes perfect separation and, in such batch tests,
conditions are indeed close to the ideal, as sufficient time can be taken
to allow complete separation to take place. In a continuous production
process, however, conditions are usually far from ideal and particles can
be misplaced to the wrong product for a variety of reasons. The
dominant effect is that of the density distribution of the feed. Very dense
or very light particles will settle through the medium and report to the
appropriate product quickly, but particles of density close to that of the
medium will move more slowly and may not reach the fight product in the
time available for the separation. In the limit, particles of density the
same as, or very close to, that of the medium will follow the medium and
divide in much the same proportion.

13. Other factors also play a role in determining the efficiency of separation.
Fine particles generally separate less efficiently than coarse, again
because of their slower settling rates. The properties of the medium, the
design and condition of the separating vessel, and the feed
conditions, particularly feed rate, will all influence the separation. The
efficiency of separation can be represented by the slope of a Partition or
Tromp curve, first introduced by K.F. Tromp (1937). It describes the
separating efficiency for the separator whatever the quality of the feed
and can be used for estimation of performance and comparison between
separators. The partition curve relates the partition coefficient or partition
number, i.e. the percentage of the feed material of a particular specific
gravity which reports to either the sinks product (generally used for
minerals) or the floats product (generally used for coal), to specific gravity
(Figure). It is exactly analogous to the classification efficiency curve, in
which the partition coefficient is plotted against size rather than specific
gravity.

14. PARTITION OR TROMP CURVE

15. The ideal partition curve reflects a perfect separation in which all particles having a
density higher than the separating density report to sinks, and those lighter report to
floats. There is no misplaced material. The partition curve for a real separation
shows that efficiency is highest for particles of density far from the operating density
and decreases for particles approaching the operating density. The area between
the two curves is called the "error area" and is a measure of the degree of
misplacement of particles to the wrong product. Many partition curves give a
reasonable straight-line relationship between the distribution of 25 and 75%, and the
slope of the line between these distributions is used to show the efficiency of the
process.
The probable error of separation or the Ecart probable (Ep) is defined as half the
difference between the density where 75% is recovered to sinks and that at which
25% is recovered to sinks, i.e. from Figure,
Ep=(A-B)/2
The density at which 50% of the particles report to sinks is shown as the effective
density of separation, which may not be exactly the same as the medium
density, particularly for centrifugal separators, in which the separating density is
generally higher than the medium density.

16. The lower the Ep, the nearer to vertical is the line between 25 and 75%
and the more efficient is the separation. An ideal separation has a
vertical line with an Ep = 0 whereas in practice the Ep usually lies in the
range 0.01-0.10. The Ep is not commonly used as a method of
assessing the efficiency of separation in units such as
tables, spirals, cones, etc., due to the many operating variables (wash
water, table slope, speed, etc.) which can affect the separation efficiency.
It is, however, ideally suited to the relatively simple and reproducible
DMS process. However care should be taken in its application, as it
does not reflect performance at the tails of the curve, which can be
important.