This document discusses practical operational aspects of dense medium cyclone separation. It addresses key factors that influence the efficiency of dense medium separation using cyclones, including cyclone design parameters like diameter and vortex finder to spigot ratio, operational parameters like head pressure and medium to ore ratio, and maintenance issues. The document provides equations, graphs, and tables to illustrate the effects of these parameters on separation efficiency and cut density. Maintaining proper operating conditions and medium properties is important to minimize classification, density differential, and particle hang-up issues that can reduce cyclone performance.

1. PRACTICAL OPERATIONAL ASPECTS OF DENSE MEDIUM CYCLONE SEPARATION
F Bornman
Multotec Process Equipment (Pty) Ltd
28 Forge Road, Spartan
Kempton Park, 1620, South Africa
(Author: faanb@multotec.com)
ABSTRACT
Reeves and Platt1
stated that dense medium separation is one of the most complex unit processes in mineral
processing today particularly if cyclones are used. It is also the most efficient gravity concentrating process available
to process engineers. The complexity of the process and large number of design and operating variables means that
the process has largely remained more an art than a science. Dense medium separation is well established in the
diamond, coal and iron ore industries but is gaining popularity as a pre-concentration step in other industries. The
maintenance issues are addressed that will affect the efficiency of the operation.
KEYWORDS
Dense medium, cyclone, ferrosilicon

2. INTRODUCTION
The purpose of the dense medium separation (DMS) process is to separate the valuable material from the
waste based on gravity differences. However, the specific gravity difference between the two fractions must be large
enough to achieve high efficiencies. Heavy particles sink through the dense liquid and light particles float.
DMS is also used for the pre-concentration of minerals before further downstream processes like
communition, in order to minimise the treatment costs. Examples are the pre-concentration of diamond ores or
metalliferous ores like nickel, copper and cobalt.
Dense medium separation is also used to upgrade the material to meet different market specifications, for
example in coal and iron ore applications.
A typical dense medium separation process for a diamond application is shown in Figure 1.
Figure 1 – Dense medium separation flow sheet (diamonds)
FEED PREPARATION
Vibrating screens
Slimes is defined as clay material and is normally referred to as the fraction smaller than 45 micron that is
not magnetic. Viscosity problems arise when slimes or clay contaminant is present. Viscosity influences the dense
medium separation of especially the finer particles.

3. Wash water
Effective de-sliming is therefore necessary before the dense medium separation of industrial minerals, ores
and coal. Wash water is used for de-sliming on the feed preparation vibrating screen. The amount of water necessary
for effective de-sliming must give a maximum solids concentration of 7% by volume in the underflow of the screen.
The formula to calculate the amount of water required is as follows:
A (m3
/hr) = 0.133 (a x S) (1)
ɣpr
Where a is the percentage of the feed solids to the screen smaller than the screen cut size, S is tonne per
hour of solids in the feed to the screen and ɣpr is the specific gravity of the solids. The +0.5mm material in the feed
has no effect on the total amount of de-sliming water required. It is recommended to use 45 m3
/hr water per meter
width of screen or 2-3 m3
/tonne for coal. Multistage washing is also recommended for a higher efficiency.
Bed depth
The allowable load C in t/h per m-screen width for the drain and rinse screen is:
C = 8 √d2
a x ɣ2
pr t/h/m (2)
Where da is the average particle size of the product in mm
ɣpr is the average specific gravity of the product
Depending on the top size of the material, the ideal bed depth should be less than 20 mm.
CYCLONE
The cyclone consists of various components as shown in Figure 2.
Figure 2 – The dense medium cyclone
Inlet Head
Cone
Vortex Extension
Barrel
Spigot
Vortex Finder

4. A cyclone is a constant volume device and it is critical that the cyclone receives a constant feed.
Factors influencing DMS operation are:
• Cyclone design parameters.
• Operational parameters.
Cyclone diameter
The centrifugal force as a function of cyclone diameter is shown in Figure 3 at an operating head of 9D.
The reduction in centrifugal force increases the particle size where the drag forces are larger than the centrifugal
forces and there is insufficient time to separate the particles effectively.
Figure 3 – The centrifugal force as a function of cyclone diameter
The minimum diameter of cyclone that must be used is determined by the top size of particle in the feed to
the cyclone. The size or number of cyclones installed is also a function of the feed or spigot capacity to be handled
as shown in Table 1.
Centrifugal Acceleration vs Cyclone Diameter
30.0
35.0
40.0
45.0
50.0
55.0
60.0
100 1000
Cyclone Diameter (mm)
CentrifugalAcceleration
FIGURE 10

5. Table 1 - Dimensions and capacities of Multotec cyclones
Multotec Standard Capacity Cyclones Multotec High Capacity Cyclones
Cyclone
Diameter (mm)
Max Particle
Size (mm)
Coal Feed
(t/h)
Cyclone
Diameter (mm)
Max Particle
Size (mm)
Coal Feed
(t/h)
510 34 54 510 51 99
610 41 81 610 61 145
660 44 97 660 66 175
710 47 114 710 71 207
800 53 149 800 80 270
900 60 196 900 94 355
1000 67 249 1000 100 454
1150 77 351 1150 115 638
1300 87 468 1300 130 854
1450 97 608 1450 145 1108
Vortex finder to spigot ratio
The spigot size determines the mass recovery to the underflow. The spigot diameter also affects the
differentials – the higher the spigot to vortex finder ratio, the lower the differentials. The spigot diameter must not
exceed 85% of the vortex finder diameter. The smaller the spigot, the higher the cut density and the lower the spigot
capacity. Larger spigots reduce the hang up of coarse particles and the excessive wear that takes place when hang up
occurs. Figure 4 shows the EPM and spigot to vortex finder ratio relationship.

6. Figure 4 – EPM values for the spigot to vortex finder ratio
Feed size distribution
The smaller the particle, the less centrifugal force exerted, resulting in an increase in the fine “sinks”
reporting to the “floats”.
The separation efficiency (Ep value) is normally measured per size range – at a specific bottom size the
separation efficiency will decrease. Smaller diameter cyclones are employed for finer feed materials in order to
reduce the breakaway size. The breakaway size for a specific cyclone diameter is calculated by:
db = (6 x Ø1.64
) x 10-5
(3)
Figure 5 shows the breakaway size for different cyclone sizes.
Figure 5 – The breakaway size versus the cyclone diameter
Breakaway Size vs Cyclone Diameter (mm)
4.99
6.28
7.67
9.18
0.22
0.36 0.51
0.93 1.07
1.20 1.65
1.45
1.99
2.22
2.28
2.52
2.85
3.46
4.20
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Cyclone Size (mm)
BreakawaySize(mm)
FIGURE 12
S p ig o t S iz e ( R a tio o f V o r te x F in d e r S i ze )
0 .0 1
0 .0 1 1
0 .0 1 2
0 .0 1 3
0 .0 1 4
0 .0 1 5
0 .6 0.65 0 .7 0 .7 5 0 .8 0 .8 5 0.9
S p ig o t t o V o r te x F in d e r R a ti o
EPMValue

7. Density shift
The density separation of ore particles is time dependent – larger particles separate faster than small
particles. The separation of large particles will take place at a higher level within the cyclone and hence lower
density. As the solids get smaller in size, so the d50 cut point for that particular size of solids increases. This is
referred to as the density shift.
OPERATING PARAMETERS
Medium to ore ratio
The medium to ore ratio is important to prevent “over-crowding” of the ore. Typical ratios are:
• easy to wash coals 3 : 1.
• diamond washing 5 – 7 : 1.
• other minerals 5 : 1.
Figure 6 shows the effect of the M:O ratio on the EPM.
Figure 6 – The EPM and M:O relationship
Higher ratios appear to have very little effect in terms of improved efficiency and result in increased
equipment size, higher operating cost and increased medium losses.
Head/pressure
At a high feed inlet pressure, the slurry flow rate to the cyclone increases. The tangential velocity of the
particles increases which increases the centrifugal force which in turn improve the separation process especially of
fine particles despite the shorter residence time. Unfortunately a high feed inlet pressure increases the classification
of the media, the density differential of the media increases which is measurable by determining the underflow and
overflow densities respectively and calculating the density differential. A fluctuating head affects the medium to ore
ratio, especially when the solids feed rate is left unchanged.

8. Recommended feed heads are:
• coal 9 x D.
• diamonds 12 – 15 x D.
• other minerals 12 – 20 x D.
It is generally accepted that below 7D the cyclone air core becomes unstable and the cyclone performance is
likely to deteriorate.
Table 2 shows the relationship between pressure fluctuation and inlet velocity/spigot density changes.
Table 2 – Relative differences in parameters
Total Head ∆ Pressure ∆ Inlet Velocity ∆ Spigot Density
182 kPa -10% 15.5% 1.1%
182 kPa -5% 8.0% 0.6%
182 kPa -0% 0.0% 0.0%
182 kPa +5% -8.7% -0.9%
182 kPa +10% -18.4% -2.1%
Figure 7 shows the relationship between relative pressure and the relative wear rate.
Figure 7 – Effect of pressure on wear rate
Differentials
A dense medium cyclone performs a separation within a separation. Classification of the medium solids
(separation via size) occurs; therefore the medium solids will separate to a certain degree. The feed, overflow and
underflow will have different medium densities. Factors affecting differentials are:
• medium viscosity/stability.
• feed head (pressure).
• cyclone diameter.

9. • inlet design.
• barrel section.
• spigot size.
• medium density.
The differential is calculated as:
RDfeed
If the differentials are too big, hang
the cyclone is causing the hang-up. Coarse sinks particle hang
shows the acceptable medium properties.
DENSE MEDIUM
Table 3 lists a number of observations and identifies
recommended action.
The differential is calculated as:
feed - RDoverflow = 3 -12%
RDfeed
If the differentials are too big, hang-up of particles in the cyclone can occur. The “near dense” material in
up. Coarse sinks particle hang-up is a concern in the diamond industry. Figure 8
shows the acceptable medium properties.
Figure 8 – Medium properties
DENSE MEDIUM CYCLONE CHECK AND PROBLEM SOLVING
Table 3 lists a number of observations and identifies a possible problem for the observations as well as the
12% (4)
les in the cyclone can occur. The “near dense” material in
up is a concern in the diamond industry. Figure 8
CYCLONE CHECK AND PROBLEM SOLVING
a possible problem for the observations as well as the

10. Table 3 – Dense medium cyclone separation fault finding
Observation Possible problem Action
Feed to the preparation screen is
fluctuating.
Inconsistent feed rate from ore
preparation section.
Investigate problem and rectify.
Not enough water on screens. Water pressure too low, water not
open, spray nozzles blocked.
Verify water pressure, ensure water
valves are open, unblock spray
nozzles, add sufficient water to
remove clay. Minimize clay balls
and cyclone blockages by ensuring
the spray nozzles on the spray bars
are open, rather have too much water
than too little.
Screen blockage, no proper draining. Type of material treated,
contamination with vegetation,
require different type of screen
panel, screen motion.
Prevent contamination of medium.
De-blind screen where required,
maintain aperture size, verify correct
panels for the application, ensure
screen stroke is sufficient.
Have a weir bar between the drain
and rinse to prevent medium
carryover to the rinse section, use
enough water on the rinse section to
minimize adhering medium losses.
Coarse material in effluent and
medium.
Holes in screen panels. Check condition of screen panels
and rectify.
Fluctuating cyclone pressure.
Cyclone pressure increase.
Pressure decreases.
Mixing box level not constant,
insufficient medium in circuit.
Medium to mixing box is
fluctuating.
Poor cyclone efficiency.
Accelerated wear.
Blocked cyclone inlet (product
disappear from screens).
Increase in medium density.
Mixing box level too high.
Cyclone feed pump speed too high.
Decrease in medium density.
Ensure header tank level.
Check correct medium sump level,
add medium if required.
Maintain level in mixing box, open
medium flow to mixing box. Check
for obstructions in medium flow to
mixing box.
Low pumping efficiency, check
condition of correct medium pump,
make sure ore feed rate is constant.
Operate as constant as possible.
Cyclone pressure to be in the
specified operating range.
Stop plant and unblock inlet / check
for source of oversize.
Check density and rectify – add
water.
Adjust level by restricting medium
flow to box.
Check speed indication / VSD
malfunctioning.
Check density and rectify, may
require FeSi.

11. Excessive concentrate.
Reduction in concentrate, possible
diamond loss.
Partial blockage in cyclone feed
hose.
Mixing box level too low.
Cyclone feed pump impeller
blocked.
Cyclone feed pump speed too low.
Drop in medium density.
Excessive medium contamination.
Excessive medium magnetisation.
Worn spigot.
Worn vortex finder.
Change in ore characteristics.
High medium density.
Excessive cyclone pressure.
Change in medium grading.
Change in ore feed characteristics.
Clear out cyclone feed hose.
Open medium flow to box, check
sump level.
Open impeller and clear obstruction.
Check speed indication / VSD
malfunctioning.
Check density on pulp density scale.
Check viscosity, clear medium by
running without feed.
Excessive quantity passing through
the magnetic separator - check
demagnetising coil.
Check monthly and replace when
worn out by 10%.
Check monthly and replace when
thin or when a “hap” shows.
Investigate increasing separation
density.
Check mixing box density.
Check pressure indication.
Check medium – coarse/fine.
Investigate and change medium
density if required.
Inadequate densification (cannot
maintain circulating medium
density).
Densifying cyclone defective.
Insufficient level in dilute media
sump. Insufficient bleed from floats
drain panel.
Check cyclone for wear and
blockages. Check sprays and rectify.
Increase bleed to dilute media side.
CONCLUSION
Dense medium separation is used to separate the valuable material from the waste.
Slimes, defined as the -45 micron clay particles, cause viscosity problems in the medium which influence
the separation of near density particles. It is recommended to add 2-3 m3
water/tonne coal to the preparation screen
for de-sliming. The bed depth should be less than 20 mm depending on the top size of the feed material.

12. A fluctuation of 10% in the cyclone pressure will result in a fluctuation of 15% in the inlet velocity to the
cyclone. The aim should always be to keep the cyclone pressure as constant as possible. Cyclone operation should
not be at a too high pressure (>20D), as the pressure to wear rate ratio is an exponential function. The differential
over the cyclone can be calculated to determine whether the operation is in the acceptable range.
REFERENCES
1) Reeves, TJ & Platt, BI 1988, ‘Maximizing efficiency in dense medium plants’, The Third Samancor Symposium
on Dense Medium Separation 29 February – 1 March 1988, Riviera International Hotel and Country Club
Vereeniging, South Africa.
2) Bekker, E 2012, ‘DMS Cyclone Presentation – DMS Basics’.
3) Communication with J Engelbrecht, Director International Business Division, January 2013.