FIELD OF THE INVENTION
The present invention relates to dense medium cyclone used for separation of
low ash bearing carbon particles from high ash containing carbon particles
with the help of magnetite media particularly applicable for coarse coal (-
15+0.5 mm) beneficiation. In particular, the invention relates to dense
medium cyclone capable to minimize turbulence and reduce short circuiting of
feed to the overflow product stream during gravity separation process inside
the cyclone. More particularly, the invention relates to a cyclone for dense
medium separation of coarse coal to increase clean coal yield in a coal
benefication process.
BACKGROUND OF THE INVENTION
Dense medium cyclones are commonly used in the mineral processing
industry for beneficiation of coal. It separates low ash bearing carbon
particles from high ash containing carbon particles by utilising their specific
gravity difference. The separation density inside the cyclone is achieved
through designing of an unique feed inlet section as well as selecting a
geometrical profile of the cyclone so that the centrifugal forces creates a
density envelope for enhancing the separation.
The feed to the cyclones, constitute of raw coal and a superfine magnetite
suspension as the heavy medium, is introduced from the feed inlet near the
top of the cylindrical section of the cyclone. The medium density is adjusted
such that the feed remains in between low ash carbon particles and high ash
bearing coal rejects. Inside the cyclone, centrifugal force and drag force
affects the motion of the particles. The coal which is low in ash content is
lighter in weight, which moves towards the longitudinal axis and reports to
the vortex finder as an overflow material. The high ash content coal being
heavier moves along the walls of the cyclone and get discharged from the
spigot as an underflow material. Since the solid particles entering inside the

cyclones have their own size segregation, both being heavier than water, flow
patterns inside the cyclones is quite complex. This has led the designers to
use empirical models to predict the performance of the cyclones. Model
fitment of empirical equation depends on the available data sets of the
experiments. However, these empirical models have the inherent deficiency
for example, a model is valid only within the limits of the available data. Due
to this, more and more designers are using mathematical models based on
fluid mechanics to predict the performance of cyclones.
The known mathematical technique, “Computational Fluid Dynamics (CFD)” is
used for numerical treatment of the Navier-Stokes equations over a
structured/unstructured 3D grid within a cyclone body. Turbulence modelling
is achieved by both RSM (Reynold Stress Model) and LES (Large Eddy
Simulations) for incorporating the high swirling flow patterns observed in the
cyclones. CFD is used extensively to predict velocity profiles at wide range of
operating regimes. The numerical treatment of Navier-Stokes equations, the
basic of any CFD methodology, was started from the early 1980’s and has
undergone rapid improvement due to development and application of faster
processors and computers for better visualisation of turbulence inside the
cyclones.
At present, the dense medium cyclone (DMC) is one of the most efficient
processing unit used for beneficiation of coal having vast size difference from
75 mm lumps upto 0.5 mm size fines. The Indian coal having high near-
gravity-material (NGM) is particularly difficult to wash and needs a highly
efficient equipment for separating the valuables from the gangue particle. In
this context, DMC is an apt choice for separation of Indian coal. Over the
years DMC has gained popularity both in terms of its washing efficiency
including the separation phenomena which is taking place inside the cyclone.
A known Dense medium cyclone (DMC) is illustrated in Fig. 1. This
conventional dense medium cyclone includes a simple tangential feed inlet for

raw coal and medium in aqueous slurry inside a cylindrical chamber, thus
forming a strong vertical flow. The high ash bearing particles usually have
relatively higher density move along the wall of the cyclone due to the
centrifugal force, where the velocity is the least and is discharged through the
spigot or nozzle located at the bottom of the cyclone. The low ash containing
carbon particles are comparatively lighter than the high ash bearing particles
and shifts towards the longitudinal axis of the cyclone due to the drag force
where a high velocity zone exists and passes through an overflow section,
namely vortex finder. It is widely observed during operation of such dense
medium cyclones, entrainment of fine or slower settling particles occurs in
the void spaces between the coarser, or faster settling, particles discharged
as the underflow. Turbulent fluctuation inside the cyclones is also expected to
be significant due to the collision of the inlet stream with the rotating stream.
Due to insufficient inlet geometry and body section design, the standard
dense medium cyclones are associated with large short-circuit flow and short
residence time of the internal upward flow. These cyclones through put
capacity and operational performance are based on the flow achieved
through the axial outlets. Hence, the actual flow input to these cyclones is
reduced in the plant to operate them at higher efficiencies as well as to
achieve the desired cut density for improved separation performance.
Accordingly, there is a need to design a new generation cyclone capable to
treat high NGM Indian coal with size range of -15+0.5 mm. This cyclone
should perform at higher efficiency (Ecart probability Ep) than the prior art
dense medium cyclones with an increased capacity to treat higher volume
slurries.
A new generation DMC for efficient coal separation has been developed using
a comprehensive CFD model. In particular, a CFD model of this DMC is
capable of predicting the performance of the developed cyclone, using Fluent,
by coupling component models of the air core, the magnetite medium

Lagrangian particle tracking for particle ranging in the size range from 0.5 to
15 mm. This has resulted in the invention described below.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a cyclone for
dense medium separation of coarse coal to increase clean coal yield in a coal
benefication process separation of coarse coal, which eliminates the
drawbacks of prior art.
Another object of the present invention is to propose a cyclone for dense
medium separation of coarse coal to increase clean coal yield in a coal
benefication process which minimizes short-circuiting of the feed and
increases residence time of coal particles inside the cyclone.
SUMMARY OF THE INVENTION
Accordingly, there is provided a cyclone for dense medium separation of
coarse coal to increase clean coal yield in a coal benefication process.
In one aspect of the invention, there is provided a cyclone for dense medium
separation of coarse coal, comprising :
a substantially rounded-wall shaped cyclone body having at least one nozzle
defining an interior space having an inner wall surface;
a vortex finder having a lower end that extends longitudinally into an upper
region of the interior space of said cyclone body;
an overflow section associated with an upper end of said vortex finder;
a feed inlet that is in fluid communication with the upper region of the interior
space of the cyclone body;
an outlet associated with a lower region of the interior space;

wherein the inner wall surface of the interior space is curved inwardly and
downwardly from the upper region of the interior space to the lower region of
the interior space.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 Schematic diagram of a known dense medium cyclone.
Fig. 2 Shows a Dense medium cyclone of the invention.
Fig. 3a and 3b show Predicted RMS radial velocity at axial position 750mm
and 1200mm at different feed velocities.
Fig. 4a and 4b show Predicted air volume fraction contours and mean axial
core diameter at different feed velocities.
Fig. 5 Simulated partition curve for -15+0.5mm coal fraction @11D condition.
Fig. 6 Simulated partition curve for different size fractions present in coal
@11D condition.
DETAIL DESCRIPTION OF THE INVENTION
As per CFD analysis, in a conventional dense medium cyclone (as shown in
Fig. 1) the high turbulent energy exists near the tip of the vortex finder. It is
evident that the sudden transition from a cylindrical section to a conical
section results in a clear source of turbulent fluctuations downwards through
the cyclone body. These fluctuations usually propagate a high turbulent
kinetic energy near the bottom of the vortex finder. According to the
invention, the cylindrical-conical shape of the conventional DMC has been re-
configured to a rounded wall shape as described immediately preceding
paragraph, which advantageously alleviates the turbulent fluctuations due to
separation at the intersection of cylindrical-conical sections. Further,

throughput in the DMCof the invention is high compared to the conventional
cyclones described above.
Furthermore, the rounded wall shape of the cyclone body significantly
increases the residence time of particles within the cyclone which would be
advantageous for better separation of the feed.
At the roof of the cyclone, for example, the outlet part of the vortex finder is
fixed and then tapers in the direction of the nozzle of the cyclone body along
the longitudinal axis of the cyclone. The degree of taper varies in the range of
5-10 degree from the longitudinal axis towards the cyclone body and extends
upto just below the opening feed inlet involute inside the cyclone body.
The curvature of the inner wall surface preferably extends substantially along
the longitudinal length of the cyclone body. The radius of curvature of the
cyclone starts immediately below the opening of feed inlet involute inside the
cyclone body and extends up to the nozzle of the cyclone. The radius of
curvature of the inner wall varies in the range of 5100-5800mm depending on
the desired throughput of the dense medium cyclone.
The vortex finder inside the cyclone body is provided with an outer surface
that tapers outwardly and extends in the direction of the nozzle of the
cyclone along the longitudinal axis of the cyclone. The preferred degree of
the taper of the outer surface of the vortex finder is 9 degree from the
longitudinal axis of the cyclone body. The overflow outlet may be positioned
as desired relative to the lower end that extends longitudinally into the upper
region of the cyclone body with the same centre of axis as that of the cyclone
itself.
The feed inlet is connected in tangential position of the cyclone body near the
roof of the cyclone and takes any suitable configuration. It is preferred that
the feed inlet diameter kept in the range of 0.26-0.3 Dc (where Dc is the
diameter of the cyclone barrel below the vortex finder inside the cyclone

body) includes a scrolled involute conduit that extends around a portion of
the circumference of an upper end of the cyclone body. More particularly, the
involute conduit preferably extends horizontally along the circumference of
the upper end of the cyclone and includes a rear wall that tapers inwardly
and that reduces the turbulence of feed material while entering inside the
main cyclone body.
The upper region of the interior space may be in the form of substantially
cylindrical barrel that extends upto 1.25 Dc upto roof of the cyclone. The
substantially cylindrical barrel merges with and into the more rounded inner
wall surface of the cyclone for handling higher throughput compared with
conventional cyclones. The overall length of the cyclone is generally 3.25 -3.6
Dc which is effectively 2.15 m for a 610 mm cyclone (taking ratio of 3.5).
The superior inlet design described above, including the preferred form for
the vortex finder and the preferred form for the feed inlet, advantageously
helps in reducing the short-circuiting of high gravity fraction into the
overflow. Further, this design enhances centrifugal forces in the upper region
of the cyclone body.
Simulation Results
In these comparative tests, a cyclone made by renowned cyclone supplier
was tested against the CFD simulated dense medium cyclone of the
invention.
The simulation attempted in ANSYS’s Fluent with 3D body fitted grids and
accurate geometric model of the selected 610 mm cyclone geometry. The
schematic diagram of the new generation dense medium cyclone is given
below in Fig. 2 with improved feed inlet profile, curvi-linear geometry of the
cyclone body for reducing turbulence at the cylindrical-conical intersection,
better vortex-finder design for reducing short-circuiting.

A number of simulations have been performed to simulate the volumetric
capacity of this new cyclone operating at different operating pressures. Table
1 shows various feed conditions of the cyclone used in the two-phase
simulation and the predicted volumetric flow rates. It is observed that as inlet
velocity increases, the pressure drop across the cyclone increases.

Fig. 3 shows the predicted RMS radial velocities at different axial positions for
different feed inlet velocities which corresponds to the inlet pressure of the
slurry. From Fig. 3, it is observed that the RMS radial velocity component is
increasing with feed flowrate. This change is significantly high beyond 4.4
m/s of inlet velocity which corresponds to 11D condition. Hence 11D pressure
is identified as optimum pressure in this cyclone for obtaining best
performance with minimum misplacement.
Fig. 4a and 4b shows the predicted air phase volume fraction contours and its
axial variation across the cyclone at different feed velocities. At the optimum
feed pressure, say 9D to 11D, the occupied air-core effective diameter is
observed to be fairly constant which indicates stable flow regime.

A virtual test cyclone of 610 mm diameter, with an angle 20 degree slanted
to the horizontal plane and a dense medium made of superfine quality of
magnetite powder was used for the simulations. The feed pressure to the
cyclone used in the simulations was in the range of 0.75 to 1.7 bar which
corresponds to 7 to 15 times the cyclone diameter. The predicted overflow
and underflow of the cyclone were noted and used for the calculation of
product densities.
At the end of each completed simulation, the turbulence analysis has been
done before coal particle tracking process, to understand the flow structure
and swirling pattern inside the cyclone. After completion of each simulation,
the partition characteristics of the DMC were modeled using Lagrangian
particle tracking for particles ranging in size from 0.5 to 15 mm. Partition
numbers or co-efficients were calculated according to techniques well known
in the art. It is basically an empirical measure of the average probability of
the particles in the respective density fraction reporting to one or other of the
products, for example to the cyclone overflow. The efficiency of separation
for a dense medium cyclone is usually represented by the Ep value, which is
calculated as given below:
Ep = (D75- - D25)/2
Where D75 is the density at which the probability of reporting to the overflow
is 75% and D25 is the density at which the probability of reporting to the
overflow is 25%.
Fig. 5 presents the simulated coal partitioning curve for 1.46 density @ 11D
condition, operating at inlet velocity of 3.987 m/s using superfine magnetite
and -15+0.5mm coal fraction. The predicted overall Ep is 0.01 and cut
density of 1.52. The steeper the partition curve, or the smaller the Ep value,
the better the separation. The partition curves for the experiments conducted
are shown in Fig. 6 @11D condition for the entire range of -15+0.5 mm

fraction. It is observed that the coarser particles demonstrate lower Ep value
which actually obtains in the plant cyclones.
It is observed from the CFD simulation that this new generation cyclone with
different design features than the conventional dense medium cyclones show
lower Ep values which signifies better separation of high ash bearing carbon
particles from low ash containing carbon particles. For the same diameter
dense medium cyclone from internationally repute cyclone manufacturer
demonstrates Ep value of 0.02 in the coal washery whose performance is
superseded by the predicted performance of this new generation cyclone.
From the simulation results, it is evident that new generation cyclone will
perform better in the coal washery for entire -15+0.5mm coal fraction
producing more clean coal at the same product ash level. Due to its unique
design, the diameter at the roof of the cyclone is higher than that of the
conventional cyclones which signifies higher capacity and longer residence
time of particles in this cyclone. Since the simulation results of this cyclone is
superior than the existing cyclones it will obviously show higher clean coal
yield figures compared to the other known cyclones.

WE CLAIM :
1. A cyclone for dense medium separation of coarse coal to increase clean coal yield in
a coal beneficiation process, comprising a tangential feed inlet involute, a vortex
finder fixed with the roof of the cyclone, extending longitudinally towards the spigot
of the cyclone along the cyclone longitudinal,
an overflow outlet connecting the vortex finder for taking out the lighter density
fractions,
and a continuous curved outer shell of the cyclone and spigot at the bottom of the
cyclone along the cyclone longitudinal axis for taking out the denser particles from
the cyclone,
wherein the vortex finder is tapered in the range of 5-10 degree made with the
cyclone longitudinal axis and the feed involute enters the cyclone in swirled involute
manner and its diameter gradually decreases and finally merged into the outer shell
of cyclone, the continuous curved outer shell of radius of curvature 5100-5800mm
extends longitudinally along its centre line towards the spigot of the cyclone and
merged with the starting section of the spigot.
2. The cyclone as claimed in claim 1, wherein the diameter of the feed inlet constricts
while entering the cyclone body and wherein at the point of entry, the feed diameter
gradually constricts to 0.8 to the lower end of the vortex finder.
3. The cyclone as claimed in claim 1, wherein the cyclone body is of and fitted with a
spigot or a nozzle having about 0.9 times the diameter of the feed inlet diameter.