CHALLENGING THE RECOMMENDED COMPARATIVE DESIGN
RATIOS FOR DMS CYCLONES
IGT Smith and D Lincoln
Multotec Process Equipment (Pty) Ltd, Spartan, South Africa, 1619
ABSTRACT
Cyclone efficiency was monitored while different vortex finder lengths were tested. The main
purpose was to address the issue regarding vortex finder length and the insertion of a barrel.
Preliminary tests with tracers show that the vortex finder length may be increased, whether a
barrel is included or not. Compared to the exclusion of a barrel, the separation efficiency
doesn’t decrease when a barrel is inserted at the standard vortex finder length for the C360.
INTRODUCTION
Are you getting the most out of your dense medium cyclone? An on-going test program is
pushing the boundaries of the Dutch State Mine (DSM) recommendations to maximize the
efficiency of dense medium cyclones. The program is investigating the hypothesis that, by
simply changing some critical dimensions at the correct ratios, it is possible to improve the
overall efficiency of the cyclone and ultimately reduce the cost per ton treated.
The vortex finder length is the first variable to be tested. The general recommendation by
industry is that the length of the vortex finder should change when a barrel is added to the
cyclone, while standard practice is to keep the length of the vortex finder the same. The test
work is aimed to explore this principal further.
The motivation behind this endeavour is to see whether separation efficiency was impaired
by increasing cyclone capacity in a milieu where the quality of feedstock is ever decreasing
as demand increases. The barrel is included to increase capacity and residence time to
achieve better separation efficiency in more difficult to separate material.
The test program is a lengthy process due to the many variables. However, the learning
process is expected to have a significant impact on the understanding of dense medium
cyclone design.
BACKGOUND
In September 1945, Driessen proposed a way to segregate fine coal (minus 3.2 mm at that
time) by using heavy liquids and suspensions in cyclone washers, which introduce a
centrifugal force rather than using gravity (Van der Walt, 1950). The Dutch State Mines then
developed the heavy medium washer where the employed centrifugal force sped up the
separation process and also improved on the overall efficiency (Van der Walt, 1950). These
washers are now known as dense medium separating cyclones.
When the Dutch State Mines developed the dense medium cyclone washers, they gave a few
recommendations on the cyclone geometry. Table 1 shows the recommended comparative
design ratios by Dutch State Mines and those for Multotec’s C360 cyclone (cyclone diameter
(DC) = 350 mm). Note that the spigot on the C360 cyclone during the test work had a
diameter of 100 mm, to be in line with the recommended ratio.
DSM Multotec’s C360
Inlet Diameter [DI] 0.20 DC 0.20 DC
Vortex Finder Diameter [DV] 2.15 DI ≡ 0.43 DC 0.42 DC
Spigot Diameter [DS] Max 0.85 DV ≡ 0.37 Dc Max 0.85 DV ≡ 0.37 Dc
Cone Angle Standard 20o Standard 20
o
Cylindrical Part Length 0.66 DC 0.57 DC
Barrel Length 1.84 DC 0.66 DC
Combined Length 2.50 DC 1.23 DC
Table 1
Comparative design ratios: Dutch State Mines vs. Multotec
The barrel
The geometries regarding the inlet, vortex finder and spigot diameters are on par with the
DSM recommendation, and the only discrepancy is the length of the cylindrical part prior to
the conical section. The C360 cyclone is a bit shorter in this regard whether a barrel is present
or not.
The Dutch State Mines doesn’t differentiate between the inclusion and exclusion of a barrel,
but increases the cylindrical part to a total of 2.50 of the cyclone diameter when the near
density material in the feed is 30% or more. The Dutch State Mines defines near density
material as the +0.10 and -0.10 increment from the cut density. Multotec on the other hand
includes a barrel to increase cyclone capacity and also allow for larger residence time in the
cyclone to achieve a better separation efficiency and less misplaced material in more difficult
to separate material.
The vortex finder
In the beginning of centrifugal washers short circuiting of coarse particles to the overflow
occurred in hydro cyclones. This is due to the close proximity of the inlet and overflow
opening as described in Figure 1. The vortex finder was implemented to prevent this short-
circuiting from happening. On the other hand, coarse particles may also be entrained to the
overflow when the vortex finder tip extends into the conical section. This is due to pressure
turbulence and swirls forming near the vortex finder inlet when it is extended too much into
the cyclone (Martinez et al., 2008).
The sole purpose of the vortex finder was to counter the short-circuiting effect, and no clear
recommendation was given on the length of the vortex finder. This was also evident in test
work done by Van der Walt (1950), where by-pass of “heavy” material decreased by
including the vortex finder in a dense medium cyclone treating coal. Van der Walt (1950)
also tried to find the optimum vortex finder length, but found that the efficiency remained
constant with the increase of the vortex finder up until a drastic decrease in cyclone
efficiency when the vortex finder was increased too much (vortex finder length to cyclone
diameter ratio of 1.74, where the next preceding setting was at a ratio of 1.00). The current
ratio of vortex finder length to cyclone diameter used for the C360 cyclone is 0.42 DC.
Figure 1
Illustrating how short circuiting was prevented by adding the vortex finder (Adapted
from Martinez et al., 2008)
Later Lotzien et al. (1978) investigated a number of cyclone geometries which included
extensive vortex finder length and diameter test work as well. This research was primarily
done to improve the separation performance of dense medium cyclones for the ultra-fine
particles. Lotzien et al. (1978) found that the e.p.m. optimum was between the ratios of 0.7
and 0.8.
Cyclone operation
Normally when a cyclone isn’t performing as it should, processes that influence the medium
and material characteristics and behaviour are to blame when the cyclone geometries are
correct and isn’t worn or damaged. The operating pressure is also an important variable, but
if the medium doesn’t behave favourably, the cyclone will not perform adequately.
The rheology of the medium plays an integral role, and when the viscosity is too high smaller
particles cannot move through the medium (He and Laskowski, 1994). Multotec test for
instability or viscosity problems by taking the feed density minus the overflow density,
divided by the feed density, and the obtained figure should be between 3% and 12%. When it
is above 12% the differential is too high and stability issues is encountered, and if it is below
3% viscosity problems are prominent.
Another way to see whether the medium behaves as it should is to determine the extent of
medium segregation by using the densities of the overflow and underflow. Here the
differential is the underflow density minus the overflow density. If the differential (medium
segregation) is too large, particle hang-up can occur. This differential should be between 200
and 500 kg/m3 (He and Laskowski, 1994).
For the purpose of this paper the two types of differentials are distinguished between the
differential described by the difference in overflow and underflow density (medium
segregation extent – Type 2) and the differential described by relating medium segregation to
the feed (Type 1).
EXPERIMENTAL
The cast iron C360 cyclone was used during the test work. It has an inner diameter of
350 mm and was operated at 9D pressure (43 kPa). The un-scrolled inlet head was used
during the test work, with the purpose of comparing the inclusion and exclusion of a barrel.
The standard cone angle of 20o and vortex finder diameter of 150 mm (0.43 DC) was used
with a 100 mm spigot diameter (0.29 DC ≡ 0.67 DV). Subsequent test work will vary these
geometries as well, and a large test work matrix will be completed.
For the purposes of this paper the vortex finder was varied in increments of 50 mm, and the
separation efficiency noted. Tests were also done at the standard vortex finder length for the
C360 cyclone (which is 147 mm or 0.42 DC), and vortex finder lengths that will correspond
to the same distance from the conical section between the scenarios where a barrel is present
and not. This means, that when a barrel is present, the vortex finder length was 415 mm and
35 mm from the cone, for example, and 177 mm in length to obtain the same distance from
the conical section (35 mm) when no barrel is inserted. Refer to Table 2 for the entire vortex
finder length range tested.
Barrel
Present
No Barrel
Present Correspondence
0 0 Same distance from
inlet 50 50
100 100
147 -
200 -
250 -
300 -
350 -
385 147 65 mm From Cone
415 177 35 mm From Cone
- 207 5 mm Into Cone
Table 2
Vortex finder lengths tested and explaining which correspond to the immediate cyclone
geometry
The conical section was never entered (except for one length tested where no barrel was
included). If the vortex finder is too long, it only reduces the capacity without any gain in
efficiency. Each of the vortex finder adjustment runs were repeated five times, and previous
work showed that the repeatability is extremely high. The standard deviation was also
determined.
Since the fields of interest are the commodities of coal and diamonds, the test work
commenced with magnetite (FeSi is naturally the next step). The magnetite slurry was made
up to a relative density of 1400 kg/m3 and 85% material passed 45 µm. The particle size
degradation of the magnetite was also monitored. 8 mm tracers from Tenova Mining &
Minerals were used and 50 tracers of each density class were charged directly into the pipe
system. There were 9 SG classes that ranged from 1.2 to 1.9 in increments of 0.10. SG
increments of 0.05 were requested, but weren’t available.
RESULTS AND DISCUSSION
Separation efficiency
The separation efficiency was measured by means of the Écart Probable Moyen (Ep) and
obtained by the use of tromp curves. The Ep represents the error or misplaced particles and
the lower the Ep the higher the separation performance. The individual Tromp curves were
modelled by equation 1, and solved for the correct Ep and ρ50 values, where is the partition
number and is the density of the relative density interval. Typical partition curves are
shown in Figure 2 and Figure 3 (examples of the actual tests), where the latter shows a
sharper separation curve and hence, a lower Ep.
[ ( ) ( )
]
(1)
Equation 1
Partition curve (Tromp curve) model (Wills and Napier-Munn, 2006)
By using the model and mathematical regression a fairly accurate estimate is made regarding
the correct values for Ep and ρ50 opposed to reading off the Ep and density cut-point. The Ep
values and cut-point for the respective vortex finder lengths in Figure 2 and Figure 3 are
0.040 and 1.455, and 0.019 and 1.498 respectively. It is interesting to note that the increase in
separation efficiency is due to less misplacement of light material to the underflow (which
was the case throughout) with extension of the vortex finder towards the conical section.
The Ep values (actual) obtained for different vortex finder lengths are given in Figure 4 and
Figure 6 for the exclusion and inclusion of a barrel respectively. A schematic of the vortex
finder position within the cyclone is also given, and comprise of Figure 5 and Figure 7 (also
for the exclusion and inclusion of a barrel respectively). The vortex finder lengths portrayed
in Figure 5 and Figure 7 are the actual lengths used as given in Table 2, and the vortex finder
length to cyclone diameter ratio is also included in Figure 5 and Figure 7 in brackets below
the vortex finder length given in millimetres.
Figure 2
Tromp curve: un-scrolled inlet head with barrel and VFL = 0
Figure 3
Tromp curve: un-scrolled inlet head with barrel and VFL = 350 mm (Ratio to DC = 1.0)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
Pa
rtit
ion
Nu
mb
er
Relative Density
8mm U/F Model 8mm U/F
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
Part
itio
n N
um
ber
Relative Density
8mm U/F Model 8mm U/F
Figure 4
Un-scrolled inlet head without a barrel: Ep values
Figure 5
Schematic of vortex finder position within cyclone: Without barrel
0.00
0.01
0.02
0.03
0.04
0.05
0.00 0.10 0.20 0.30 0.40 0.50 0.60
Ep
Va
lue
Vortex Finder Length to Cyclone Diameter Ratio (DC = 350 mm)
8mm Tracers Standard: 0.42 on C360
Figure 6
Un-scrolled inlet head with barrel included: Ep values
Figure 7
Schematic of vortex finder position within cyclone: With barrel included
0.00
0.01
0.02
0.03
0.04
0.05
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
Ep
Va
lue
Vortex Finder Length to Cyclone Diameter Ratio (DC = 350 mm)
8mm Tracers Standard: 0.42 on C360
It is evident that separation efficiency is increased by extending the vortex finder towards the
conical section. It is also noted that the cyclone performs poorer when the vortex finder is
shorter than the standard length, but performs roughly the same as the scenario where no
barrel is included at the standard vortex finder length. However, the cyclone performs better
when a barrel is present with a longer vortex finder, especially when the vortex finder is
situated close to the conical section. This is when the vortex finder is 65 mm from the cone
with a barrel (corresponding to a 147 mm vortex finder with no barrel) and 35 mm from the
cone with a barrel (corresponding to a 177 mm vortex finder with no barrel). The bottom-line
is: according to the tracer tests, the separation efficiency for 8 mm tracers is increased with
increase of vortex finder length. Material should also be tested to make this a definitive
statement.
Density cut-point
Figure 8 and Figure 9 show the density cut-point as the vortex finder was extended through
the cyclone. The 8 mm tracers show an initial steady increase at a vortex finder length to
cyclone diameter ratio of 0.3 to 0.4. This is as expected, and happens due to the density shift
within the cyclone. Density shift can be described as smaller particles separating lower down
in the cyclone at a higher density band than larger particles which separate quickly and higher
up in the cyclone at a lower density band. This is due to the drag force present in the cyclone
that causes smaller particles to travel at a slower rate through the cyclone.
Figure 8
Un-scrolled inlet head without barrel: Cut-density
1.42
1.44
1.46
1.48
1.50
1.52
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Cu
t-D
ensi
ty
Vortex Finder Length to Cyclone Diameter Ratio (DC = 350 mm)
8mm Tracers Standard: 0.42 on C360
Figure 9
Un-scrolled inlet head with barrel included: Cut-density
Medium behaviour
The medium behaviour of a cyclone is extremely important, and the differentials of a cyclone
should be between 3% and 12% (Type 1). The differentials (the block figures read off on the
left vertical axis as indicated by the arrow) varied from 5% to 7% and the medium behaviour
was good at an average of 5.9% for the entire vortex finder length test work, refer to
Figure 10.
Figure 10
Summarising the medium behaviour – differentials by Type 1 and 2 (With barrel)
1.42
1.44
1.46
1.48
1.50
1.52
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
Cu
t-D
ensi
ty
Vortex Finder Length to Cyclone Diameter Ratio (DC = 350 mm)
8mm Tracers Standard: 0.42 on C360
0
100
200
300
400
500
600
700
800
900
3%
4%
5%
6%
7%
8%
9%
10%
11%
12%
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
Dif
fere
nti
al [k
g/m
3]
Dif
fere
nti
al [%
rel
ate
d t
o f
eed
]
Vortex Finder Length to Cyclone Diameter Ratio (DC = 350 mm)
Type 1 Type 2 Standard: 0.42 on C360
Another key method to determine medium behaviour is by addressing Type 2 (the circle
figures read off on the right vertical axis in indicated by the arrow). If the medium is
segregated to a too large extent, material hang-up occurs. Hang-up was noticed and found to
be imminent when the difference in overflow and underflow density reaches 450 kg/m3. This
was determined by observing no hang up for vortex finder lengths 0 mm and 50 mm, while at
100 mm particle hang-up occurred (also see Figure 10). This is the case for vortex finder
length to cyclone diameter ratios of 0.29 and higher.
Figure 11
Summarising the medium behaviour – differentials by Type 1 and 2 (Without barrel)
It is also noted that the difference in overflow and underflow density remained fairly constant
up to a vortex finder length to cyclone diameter of 1.00 (Figure 10), and it may be assumed
that normal cyclone operation can be expected when the vortex finder is extended up to this
point. Further down the cyclone the medium segregation will be at a too great extent as well
as the particle hang up. Particle hang-up of near density particles has the potential to rapidly
increase cone wear. The same behaviour is seen when no barrel is present (Figure 11), but the
differentials don’t increase to the same extent as when the barrel is included – stays around
500 kg/m3.
The mean density of hang-up material is roughly the same as the medium density, where
mostly the 1.4 SG tracers hang-up. This is in accordance with the theory regarding how a
dense medium cyclone operates. A balance between the centrifugal force radially outward
and the drag force radially inward exists. Heavy particles tend to move outward to the
cyclone wall and down to the apex, while lighter particles move toward the air column and up
the vortex finder. The heavy particles either exit through the apex (spigot), or are re-
circulated to the vortex, penetrating the dense medium profile again and move towards the
cyclone wall. This creates a density profile within the cyclone that may be higher in density
0
100
200
300
400
500
600
700
800
900
3%
4%
5%
6%
7%
8%
9%
10%
11%
12%
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Dif
fere
nti
al [k
g/m
3]
Dif
fere
nti
al [%
rel
ate
d t
o f
eed
]
Vortex Finder Length to Cyclone Diameter Ratio (DC = 350 mm)
Type 1 Type 2 Standard: 0.42 on C360
than the dense medium fed to the cyclone, Van der Walt (1950). Thus, it can be deduced that
the material that gets hanged-up within the cyclone should be near in density to the medium
density, and the cut-density should be higher (which is the case and common knowledge).
CONLUSIONS
When the vortex finder is at standard length (147 mm or 0.42 DC) the same separation
efficiency was obtained with and without a barrel. In both cases the separation efficiency is
increased with increasing the vortex finder length, and the highest efficiency is obtained
when the vortex finder was 35 mm from the conical section in the case without a barrel and
65 mm from the conical section when the barrel is present. This corresponds to a vortex
finder length to cyclone diameter ratio of 0.5 (177 mm) and 1.1 (385 mm) with and without a
barrel respectively. The down side, however, is that medium segregation becomes very high
at these vortex finder settings, and material hang-up may be high. This in turn may cause
wear in the cone.
Medium segregation remains constant through vortex finder length to cyclone diameter ratios
of 0.42 to 1.00, where 0.42 is the standard vortex finder length to cyclone diameter for the
C360 cyclone. It can then be concluded that particle hang-up and cone wear will be the same
within this ratio range. The best separation efficiency that was obtained within this range was
at a vortex finder length to cyclone diameter ratio of 1.00, which relates to a vortex finder
length of 350 mm – this was for the case when a barrel is present. It must be stressed,
however, that the differentials are influenced by a number of factors, which include:
• shape of inlet head (square or rectangular)
• inlet design (tangential or evolute)
• spigot size
• medium density
• cyclone diameter
• feed pressure
• medium viscosity and stability.
With no barrel present, the vortex finder was increased to just before the conical section starts
(0.5 vortex finder length to cyclone diameter ratio). It also seems that the differential at this
vortex finder length is also acceptable at 500 kg/m3.
It is recommended to undertake another study that involve the explanation of the stepwise
increase in separation efficiency as the vortex finder is extended through the cyclone by
means of computational fluid dynamics (CFD).
REFERENCES
He Y.B. and Laskowski J.S., 1994, Effect of dense medium properties on the separation
performance of a dense medium cyclone, Minerals engineering, 7 209-221.
Lotzien R., Hober H. & Schneider F.U., 1978, Cleaning of ultra-fines in heavy liquid
cyclones, Aufbereitungs-Technik, 10 563-570.
Martinez L.F., Lavin A.G., Mahamud M.M. & Bueno J.L., 2008, Vortex finder optimum
length in hydrocyclone separation, Chemical engineering and processing, 47 192-199.
Van der Walt P.J., 1950, A study of the operation of the cyclone washer and its application to
Witbank fine coal, Journal of the chemical, metallurgical and mining society of South Africa,
none 18-101.
Wills B.A. and Napier-Munn T.J., 2006, Wills’ mineral processing technology, 7th
ed.,
Elsevier Science & Technology Books, Burlington MA, p. 264.
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