development and evaluation of the cavex dense medium cyclone

DEVELOPMENT AND EVALUATION OF THE

CAVEX DENSE MEDIUM CYCLONE

RICK HONAKER1, ROBERT HOLLIS2,DEBRA SWITZER3, AND TOM COKER4

1University of Kentucky, Lexington, Kentucky, USA2James River Coal, London, Kentucky, USA3Weir Minerals North America, Madison,Wisconsin, USA4Morris-Coker Inc., Beckley, West Virginia, USA

The CAVEX dense medium cyclone (DMC) was developed in the

later part of the 1990s as a result of the expertise developed by Weir

engineers in slurry pumping. The inlet area of the cyclone is

designed to minimize turbulence and to reduce wear at the feed entry

point, which provides more energy for particle separation at a given

feed pressure. A parametric study was performed on a 150mm diam-

eter unit to quantify separation efficiency as a function of feed press-

ure, apex diameter, medium density, and cone angle. The added

energy in the cyclone was confirmed by comparing the stability of

the medium in the CAVEX unit with that provided by a common

commercial unit having the same dimensions. A 500mm unit was

installed in parallel with an identically sized industrial unit in an

operating preparation plant treating 12� 1mm coal. The separation

efficiency values achieved by the CAVEX DMC were found to be

higher than those obtained by the standard industrial unit and the

This article was presented at the 2010 International Coal Preparation Congress

(ICPC), Lexington, KY. ICPC 2010 conference proceedings are published by the Society

of Mining, Metallurgy, and Exploration, Inc. (SME), Littleton, CO (www.smenet.org;

Tel.: 303-948-4200). SME’s permission to republish the article is gratefully acknowledged.

Address correspondence to Rick Honaker, University of Kentucky, Lexington,

Kentucky 40506, USA. E-mail: [email protected]

International Journal of Coal Preparation and Utilization, 30: 100–112, 2010

ISSN: 1939-2699 print=1939-2702 online

DOI: 10.1080/19392699.2010.497086

amount of improvement increased with a decrease in particle size.

The data from the pilot-scale and in-plant tests are presented and

discussed in this article.

Keywords: Dense medium cyclone; Parametric study; Pilot-scale

testing

INTRODUCTION

In the metallic ore industry, ultrafine particle size separations are often

required within a ball mill circuit using classifying cyclones with a diam-

eter of around 250mm. Due to the ball mill application, the feed can be

relatively coarse with a top particle size of 12mm and greater. Using con-

ventional classifying cyclones, the wear rates in the feed chamber are

high with replacements occurring every three to four weeks. The main

problems concern the scouring of the surfaces by the coarse particles

and the turbulence at the entry point into the cyclone, which is com-

monly referred to as the inlet shelf. As a result, a development project

was initiated in the late 1990s to redesign the inlet head of the classifying

cyclone in an effort to reduce wear while maintaining the performance

standards.

A critical issue regarding cyclone inlet design is the need to minimize

head loss as the feed slurry passes through the inlet and enters the cyc-

lone. Inlet head losses result in a reduction in the number of ‘‘g’’s (Ng)

experienced by the particles within a given cyclone as described by the

following expression [1, 2]:

Ng ¼ 2a2Vi

DcgDc

dc

� �2nþ1

; ð1Þ

in which a is a factor that accounts for inlet head losses. The value of acan be estimated by:

a ¼ 3:7Di

Dc

� �; ð2Þ

where Vi is the inlet velocity, Dc and Di the cyclone and inlet diameter,

respectively, and dc the radial position of particle. To quantify the impact

on particle movement within a cyclone, particle velocity (vt) toward the

wall of the cyclone can be determined by the following general hindered

CAVEX DENSE MEDIUM CYCLONE 101

settling equation:

vt ¼Nggd2ðqs � qmÞ

18mð1� /Þ3:65 ð3Þ

in which d is particle size, g the gravitational acceleration, qs and qm the

solid and medium densities, respectively, / the fractional volumetric

solid concentration, and m the medium viscosity.

The particle velocity reductions predicted using Equations (1)–(3)

with 20% and 50% relative head losses are shown as a function of par-

ticle size in Figure 1. Given that particle residence time within a cyclone

is only a few seconds, a high particle velocity is needed for the particle to

report to the cyclone wall and into the underflow stream. The predictions

in Figure 1 show that particle velocity can be increased by over 50%

through reductions in inlet head loss. This observation indicates that

an advancement in cyclone inlet design that minimizes inlet head loss

increases particle velocity. In a classifying cyclone, the result would be

a decrease in the particle size cut-point (d50). Likewise, the impact in a

dense medium cyclone is a reduction in the density cut-point. Another

benefit of reduced head loss is that the desired cut-point within a given

cyclone can be achieved at a lower feed pressure, which reduces mainte-

nance and energy costs.

Figure 1. Impact of the cyclone inlet head loss on particle velocity toward the outer

cyclone wall.

102 R. HONAKER ET AL.

The inlet head of commercially available cyclones typically fall under

one of the designs shown in Figure 2a. The 75� involute has been a

design standard since the early 1950s. The major portion of the involute

is isolated from the cyclone body. Slurry enters the cyclone over a small

transfer arc of approximately 75� affording little opportunity for parallel

alignment with the already rotating mass. Examination of worn 75� invol-

ute liners in ball-mill-cyclone circuits showed the same localized wear

patterns as the often criticized tangential design.

An alternative design is the 180� volute. The major portion of the

volute is part of the cyclone body and more reasonably aligns new feed

parallel with the rotating mass. Intuitively, the much longer transfer

arc smooths the effect of new feed moving from the inlet shelf into the

cylinder thereby minimizing head loss. However, wear grooving immedi-

ately adjacent to each side of the involute are found with 180� volute

cyclones as commonly observed with the wear patterns of tangentially

fed cyclones [3].

The sharp 90� edge at the intersection of the inlet shelf and cyclone

cylinder was believed to be the cause of the turbulence and the significant

undercut immediately below the lip of the inlet shelf. To address these

issues, experience with pump design was incorporated to eliminate all

90� edges in the cyclone inlet. A new cyclone inlet design was developed

that includes several geometric relationships known to smooth slurry

Figure 2. Feed inlet chamber designs of (a) typical commercially available cyclones and (b)

CAVEX cyclone.


flow through the volute of a pump. The distinguishing feature of the new

cyclone feed chamber is a three-dimensional curvature along the inlet

path that forms a CAVEX (i.e., curved and spiraling) shape (Figure 2b).

In-plant tests of a 250mm CAVEX cyclone in ball-mill circuits used

for metallic ores found even wear characteristics rather than the channel

wear observed in conventional cyclones and wear life increased approxi-

mately 300%. In addition, classification efficiency improved and the sep-

aration size decreased at the same feed pressures, which follows the

aforementioned fundamental observations. This finding indicates lower

head loss through the feed inlet. As such, the potential exists to utilize

larger inlet sizes to achieve the same particle separation size, which

results in the ability to treat greater volumetric flow rates.

The CAVEX cyclone was recently evaluated in a dense medium

application for the treatment of 12� 1mm run-of-mine coal in a series

of laboratory tests and in an operating coal preparation plant. The

investigation was performed over a range of medium densities and

compared with those obtained from an industrial standard dense

medium cyclone. The results of the study are presented and discussed

in this article.

EXPERIMENTAL

Pilot-Scale Evaluation

A test program was performed to compare the coal-cleaning perfor-

mances achieved by an industrial standard cyclone and a CAVEX cyc-

lone when used as a dense medium separator. Both cyclones measured

150mm in diameter and were equipped with a 63.5mm diameter vortex

finder. The effective feed inlets were equal and the cone angle was 20�.

The cyclones were positioned 10� from the horizontal plane. As shown

in Figure 3, feed to the units was provided through the same line so that

feed coal characteristics and pressures were equivalent. The product

streams were recycled back to the feed sump to form a closed-circuit

arrangement.

The investigation involved a statistically designed test program that

evaluated the effects of three operating parameters at three value levels,

i.e., relative medium density: 1.4, 1.5, and 1.6; apex diameter: 38, 45, and

52mm; and feed inlet pressure: 21, 43, and 65 kPa. The medium-to-coal

ratio in the feed was maintained at 5:1.


The feed coal was obtained from the same preparation plant in

which the in-plant tests were conducted. The run-of-mine medium vol-

atile, bituminous coal was extracted from the Amburgy coal seam. The

particle size fraction used for the pilot-scale tests was 6� 1mm.

In-Plant Evaluation

A 500mm diameter CAVEX dense medium cyclone (DMC) was installed

in parallel with an industrial standard DMC having the same diameter in

the LEECO 64 preparation plant located in eastern Kentucky and oper-

ated by James River Coal Company (Figure 4). The two DMC units

received equal feed splits containing nominally 12� 1mm run-of-mine

Figure 4. In-plant parallel cyclone configuration used in the in-plant test program.

Figure 3. Pilot-scale dense medium cyclone circuit.


coal from the Amburgy coal seam. The vortex finder and apex diameters

for both cyclones were 210mm and 140mm, respectively.

The particle size-by-size distribution and washability analysis data for

the feed coal are provided in Table 1. The washability data indicates that

the coal is very easy to clean with a cleanability index (1.3 Cum. Float

Weight=1.6 Cum. Float Weight) of nearly 0.75. The importance of this

fact is that differences in the separation efficiencies between the two

DMC units will not be easily detectable in the organic efficiency, product

quality, and yield values due to the significantly small amount of

near-gravity material present in the feed over the medium density range

studied. This statement assumes little to no bypass of low- or high-density

materials is realized by the two units. As such, the key performance para-

meter for comparison purposes is the probable error value.

The test program involved collecting samples around each unit while

operating under four different medium density values, i.e., 1.50 RD, 1.55

RD, 1.60 RD, and 1.65 RD. The underflow and overflow streams from the

two units were fed to separate drain-and-rinse screens that made the sam-

pling program easy and efficient. The feed was common to both units. For

each feed medium density setting, representative samples of each process

stream were taken and critical plant parameter values were recorded every

10 minutes for a period of about 80 minutes. Precision Testing Labora-

tories (Beckley, West Virginia) was contracted to perform all sample col-

lection and sample analyses as well as preliminary data analysis.

During the evaluation, the plant feed rate averaged 700 tph and ran-

ged from 672 tph to 718 tph. The medium-to-coal ratio in the feed

Table 1. Particle size-by-size washability data of feed coal

þ6mm

(37.0% Wt.)

6� 2mm

(42.1% Wt.)

2� 1mm

(18.5% Wt.)

1� 0.6mm

(1.9% Wt.)Specific

gravity Weight % Ash % Weight % Ash % Weight % Ash % Weight % Ash %

1.30 Float 38.47 2.86 38.05 2.39 33.30 1.87 32.63 2.04

1.40 Float 6.81 11.10 9.78 9.38 10.35 7.76 7.69 8.44

1.50 Float 3.31 21.02 2.27 21.41 2.60 19.92 3.09 17.50

1.60 Float 2.03 31.10 1.68 31.37 1.57 29.76 1.77 28.35

1.75 Float 1.43 40.75 1.18 40.43 1.31 38.97 1.32 36.94

1.90 Float 0.65 49.49 0.58 51.00 0.60 46.56 0.80 45.49

2.10 Float 0.84 63.01 0.69 65.86 0.77 57.44 1.25 62.28

2.10 Sink 46.46 91.64 45.77 91.16 49.50 91.34 51.45 91.64


stream was relatively low with an average of 3.10. There was no

measured difference in the feed pressure between the two DMC units

throughout the evaluation material. However, the pressure did increase

from a low of 80 kPa (11.8 psi) at a 1.50 RD medium density to 98 kPa

(14.4 psi) at 1.65 RD.

RESULTS AND DISCUSSION

Pilot-Scale Evaluation

Medium Stability. The first series of tests focused on the assessment of

medium stability in both cyclones under a range of relative medium

density values and feed inlet pressures. Medium stability was assessed

by measuring the difference in the medium density of the underflow

(qu) and overflow (qo). A large difference indicates an unstable

magnetite suspension and a differential value of 0.4 or less meets

industrial standards. The assessment was conducted in the absence of

coal in the medium.

As shown in Figure 5, the medium was highly unstable in the

CAVEX cyclone relative to the stability in the industrial standard cyc-

lone. The only acceptable condition for the CAVEX cyclone was

Figure 5. Medium stability comparison on the basis of the medium density differential

between the underflow and overflow stream in the absence of coal.


achieved under the lowest feed inlet pressure and the highest medium

density. Given that the only difference in the two cyclones was the inlet

design, the data is further confirmation that the CAVEX inlet design pro-

vides lower head loss that results in higher centrifugal forces under the

same feed pressure. The enhanced gravity field accelerates particle

movement that causes the finest particles in the magnetite that forms

the dense medium to move independently of the water toward the outer

cyclone wall. For the tests conducted to evaluate coal-cleaning perform-

ance, the lowest feed inlet pressure was used, which minimized the den-

sity differential between the cyclones.

Separation Performance. Repeatability of the separation performance

for both cyclones was assessed by conducting five experiments under the

same operating conditions using a relative medium density of 1.50. The

results showed that the CAVEX cyclone consistently provided lower

product ash values as indicated by the data in Table 2. The mass yield

was slightly lower for the CAVEX unit. However, by comparing to feed

washability data, the average organic efficiency achieved by the CAVEX

was higher than that achieved by the standard industrial cyclone. The

higher organic efficiency counters the possible explanation for the lower

product ash values, which could be that lower effective density

cut-points resulted from a less stable medium.

Tests were performed over a range of medium density values,

apex diameters, and feed pressures. A comparison of the separation

Table 2. Separation performance achieved from five tests under identical condition to

evaluate repeatability; relative medium density¼ 1.50

CAVEX Standard

Test

number

Product

ash (%)

Tailings

ash (%)

Recovery

(%)

Product

ash (%)

Tailings

ash (%)

Recovery

(%)

1 5.92 86.10 86.8 6.39 86.87 87.7

2 5.75 86.25 87.0 6.18 86.86 87.7

3 5.63 86.41 87.1 6.30 85.89 86.7

4 5.76 86.00 86.7 6.17 87.67 88.6

5 6.11 86.18 87.0 6.22 86.22 87.0

Average 5.83 86.18 86.9 6.23 86.70 87.6

Theoretical Recovery (%) 89.5 91.1

Organic Efficiency (%) 97.1 96.2


performance revealed similar differences between the CAVEX and stan-

dard cyclone. The differences were relatively small in part due to the easy

cleaning characteristics of the feed coal.

In-Plant Evaluation

Separation Efficiency. Partition curves were developed from each test to

quantify the probable error and separation density achieved from the

two DMC units. The partition curves shown in Figure 6a show that

the separation performance was fairly constant over a particle size range

of 12� 1mm with the ‘‘breakaway’’ performance starting to occur for

particle sizes smaller than 1mm. Also, the separation efficiency as

defined by the slope and bypass amounts remained relatively unchanged

over the range of medium density values tested.

Probable error values approaching a value of zero reflect improving

separation efficiency. As shown in Figure 7, the CAVEX DM cyclone

tended to provide slightly higher separation efficiencies, especially for

the finer particle size fractions. This trend agrees well with the findings

previously reported when using the CAVEX cyclone as a classifier. The

performance improvement is believed to be due to lower turbulence as

the feed enters the cyclone and lower head loss. The lower head loss

would result in a prolonged time of high centrifugal forces within the cyc-

lone that would assist fine high-density particles to report to the outer

wall of the cyclone and out the underflow stream. The probable error

values obtained from tests conducted with a medium density of 1.65

Figure 6. Partition curves generated from the performances of the CAVEX cyclone over (a)

a range of particle sizes and (b) a range of medium density values.


RD did not follow the same trend. Under this condition, the probable

error values associated with the CAVEX cyclone were slightly inferior

(Table 3).

The overall separation efficiency performances achieved by both

DMC units are within the industrial standard range as shown by the

probable error values in Table 3. Also, the organic efficiency values

achieved in all tests were statistically equal, which is reflective of the rela-

tively close efficiency performances and the low amount of near-gravity

material in the feed coal.

Figure 7. Particle size-by-size separation efficiency comparisons.

Table 3. Comparison of probable error values on a particle size-by-size basis

Probable error values

1.50 Medium

density

1.55 Medium

density

1.60 Medium

density

1.65 Medium

densityParticle size

fraction

(mm) CAVEX Standard CAVEX Standard CAVEX Standard CAVEX Standard

þ6 0.026 0.035 0.031 0.036 0.033 0.035 0.039 0.033

6� 2 0.035 0.039 0.035 0.042 0.039 0.042 0.042 0.040

2� 1 0.049 0.049 0.046 0.052 0.047 0.053 0.051 0.049

1� 0.6 0.070 0.078 0.073 0.085 0.071 0.079 0.076 0.074

þ0.6 0.035 0.038 0.038 0.043 0.043 0.044 0.045 0.038


Separation Density Offset

The CAVEX DM cyclone generally produced a lower separation density

and thus had lower density offsets for all test conditions as shown in

Figure 8. The differences tended to be greater for the finer particle size

fractions. This observation could also be reflective of both the lower tur-

bulence and higher energy within the cyclone due to lower head losses. It

is noted that the medium split to the underflow stream of the CAVEX

cyclone was consistently higher than that of the standard DMC, which

could also explain the lower separation density values.

Negative density offsets are noted by the data in Figure 8, which

sometimes reflect medium instability issues. However, the medium den-

sity difference between the underflow and overflow streams remained

below 0.4 units through the test program, which generally indicates

acceptable medium stability characteristics.

SUMMARY AND CONCLUSIONS

Performance data obtained from ball-mill-classification circuits employ-

ing the CAVEX cyclone revealed the novel inlet design resulted in

improved efficiencies and low particle size cut-points relative to indus-

trial standard cyclones under the same operating pressure. The new inlet

Figure 8. Separation density comparison on a particle size-by-size basis.


design provides complete curvature of the slurry entry into the cyclone,

which reduces turbulence and thus head loss.

The data reported in this article is based on a study that focused on

assessing the benefits of employing the new cyclone design in densemedium

cyclone applications. Initial medium stability data obtained from pilot-scale

150mm diameter units of the CAVEX and an industrial standard provides

further evidence supporting the claim of lower head loss through the new

cyclone inlet. The suspended magnetite medium was found to be relatively

unstable compared to the standard unit under nearly all feed pressures and

medium density values tested. The geometries and operating conditions for

both units were identical. This finding indicates that the CAVEX unit can

achieve performance approximately equivalent to the same size industrial

units at lower feed pressures, which reduces maintenance and energy con-

sumption. The separation performances achieved by the 150mm CAVEX

cyclone using dense medium to clean 6� 1mm coal provide lower product

ash values and higher efficiencies than the industrial standard unit.

An in-plant test program was conducted to evaluate and compare

the separation performance provided by the CAVEX dense medium cyc-

lone (DMC) with those obtained by a dense medium cyclone technology

that is considered the industrial standard. A 500mm diameter CAVEX

unit was installed in parallel with the industrial standard in an operating

preparation plant located in eastern Kentucky. The vortex finder and

apex diameters were equal between the two units.

The CAVEX unit achieved process efficiencies that meet typical

industrial DMC standards over a medium density range of 1.50 RD to

1.65 RD. Under most conditions, the CAVEX cyclone tended to provide

equal or better separation efficiency over a particle size range of around

12� 0.6mm and was especially effective on the finer particle size frac-

tions. Probable error values between 0.035 and 0.045 were achieved with

no bypass of high- or low-density particles.

REFERENCES

1. Bradley, D. 1965. The Hydrocyclone. Oxford: Pergammon Press.

2. Zanker, A. 1977. Hydrocyclones: Dimensions and performance. Chemical

Engineering 84: 122–125.

3. David, D. 1966. HMS cyclone development at argyle. In Proceedings of the

Australian Institute of Mining and Metallurgy Annual Conference. Carlton, Victoria,

Australia: Australian Institute of Mining and Metallurgy.


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development and evaluation of the cavex dense medium cyclone

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