Pergamon 0892-6875(98)00032-6

Minerals Engineering, Vol. 11, No. 6, pp. 501-509, 1998 @ 1998 Elsevier Science Ltd

All rights reserved. Printed in Great Britain 0892-6875/98 $19.004-0.00

EXPERIMENTAL INVESTIGATION INTO THE APPLICATION OF A MAGNETIC CYCLONE FOR DENSE MEDIUM SEPARATION

J. SVOBODA ~, C. COETZEE t and Q.P. CAMPBELL t

§ De Beers Diamond Research Laboratory, Mineral Processing Division, P.O. Box 1770, Southdale 2135, Johannesburg, South Africa. E-mail: jsvoboda@global.co.za

t Potchefstroom University, School of Chemical and Minerals Engineering, Potchefstroom 2520, South Africa

(Received 24 November 1997; accepted 20 March 1998)

ABSTRACT

The density differential between the overflow and underflow streams is one of the critical measurable parameters in the operation of dense medium cyclones. Our investigation of the application of a magnetic cyclone to dense medium separation showed that by a judicious positioning of a solenoid magnet along the axis of the cyclone, and by adjusting the strength of the magnetic field, it is possible to control the density differential in the cyclone. It is also possible to influence the cut-point density and the sharpness of the Tromp curve and the selectivity of separation. Typically, it was possible to vary the density differential in a wide range from -0.1 to 0.8 g/cm 3. The cut-point density was controlled between 2.8 and 3.2 g/cm 3 for 270D ferrosilicon, at a feed density of 2.35 g/cm 3, and between 3.1 and 3.4 g/cm3 for Cyclone 60, at a feed density of 2.45 g/cm 3. The Ep of separation was observed to decrease from 0.05 to 0.01 by the application of the magnetic field, accompanied by reduced cut-point density. © 1998 Elsevier Science Ltd. All rights reserved

Keywords Dense medium separation; hydrocyclones; magnetic separation

INTRODUCTION

The magnetic cyclone was developed in the late sixties as a natural extension of a conventional hydrocyclone with the aim of providing an additional external (magnetic) force to supplement the gravitational and centrifugal forces that cause classification and separation. By providing such an external force, which can vary in a wide range of values, it is, in principle, possible to manipulate the matter more efficiently under a wide spectrum of experimental conditions. More specifically, attempts were made to apply the magnetic cyclone to the beneficiation of strongly and feebly magnetic ores and the recovery of magnetisable heavy media.

Probably the first magnetic cyclone was developed and applied on a production scale in ex-Soviet Union [ 1,2]. The permanent-magnet based cyclone was applied to beneficiation of roasted magnetite ore and was reported to efficiently remove fine low-grade material into the tailings [3]. Somewhat more sophisticated systems were developed in the early eighties [4,5]. These magnetic cyclones employed electromagnets to separate a sand-magnetite mixture.

~r')l

502 J. Svoboda et aL

Boxmag-Rapid Ltd. have improved the above designs by employing a quadrupole electromagnet [6], in contrast to the dipole arrangement of the Watson and Fricker magnetic cyclones. The quadrupole arrangement generates a higher magnetic field gradient, which results in a higher magnetic force acting on particles. The Boxmag-Rapid magnetic cyclone was intended for fine iron-ore circuits and for dewatering a dilute heavy medium. (Recently, a magnetic cyclone system, similar to the original Russian design, this time equipped with rare-earth magnets, was tested [7].) A review of various designs of magnetic cyclones was published in [8].

It appears that the magnetic cyclone has not found desired acceptance by the mining industry. Apparently insufficient understanding of the theoretical principles of magnetic cycloning and a rather unsophisticated design of the magnetic circuits are the main reasons for the disappointing performance of such cyclones. Typically, the performance of magnetic cyclones was characterised by insufficient mineral recovery, undesirable flocculation of magnetic particles, poor concentrate grades and product accumulation in the cyclone.

MAGNETIC CYCLONE AS APPLIED TO HEAVY MEDIA SEPARATION

A feature common to all known magnetic cyclones is the use of a horizontally oriented magnetic field. It was proposed [9] to replace the horizontally oriented magnets by a simple solenoid wound around the cyclone, with its axis in the vertical direction. The arrangement is shown in Figure 1. The forces acting on a particle of, for example, heavy medium in such a magnetic cyclone are depicted in Figure 2. It can be seen that the resultant net force acting on a particle will be directed towards the central plane of the solenoid. The angle of this resultant force, with respect to the vertical, can be adjusted by varying the magnetic field strength.

OVERFLOW

SOLENOID

MAGNETIC FORCE

MAGNETIC FIELD

UNDERFLOW

Fig. 1 Schematic diagram of a magnetic cyclone for the control of density differential.

Application of magnetic cyclone for dense medium separation 503

Solenoid

/ Particle

F v l ~ Fc

Ft

F v 2 / F c

/ Fc - centrifugal force Fv - vertical force Ft - total force

Fg - force of gravity Fb - buoyancy force Fd - hydrodynamic drag Fm - magnetic force

Fvl - Fin + Fg - Fb - Fd Fv2 = Fm + Fb + F d - F g

VERTICAL FORCES

Fig.2 Forces in a magnetic cyclone with vertical magnetic field.

By a judicious positioning of the solenoid along the axis of the cyclone it is then possible to affect the distribution of the particles of the magnetic medium within the cyclone. Thus, by adjusting the magnetic field strength and by a suitable positioning of the magnet, it should be possible to control the density distribution of the heavy media and to set an optimum density differential, cut-point density and Ep of the Tromp curve of the cyclone.

One of the critical parameters that can be measured in the operation of dense medium cyclones is the density differential between the overflow and undertow streams produced by the cyclone. It is generally accepted that the density differential should have a value of between 0.2 and 0.5 g/cm 3. If the density differential is too high, there is a wide range of densities present in the cyclone and excessive middlings fraction with a high retention time is generated. If the density differential is too low, inadequate recovery of the valuable components is achieved.

In order to investigate the possibility of controlling the distribution of ferrosilicon in a dense medium cyclone, and thus to optimise operation of the cyclone by applying a magnetic field, an extensive experimental programme was undertaken. Results of this research project are described in the subsequent sections.

MI~ |I:6-B

504 J. Svoboda e t al.

EXPERIMENTAL

Figure 3 details the dimensions of the dense medium cyclone and the location of the test solenoid, referred to in the text as ' top' , 'middle' and 'bottom' positions. All the tests were conducted with a scaled-down dense medium cyclone unit developed by the De Beers Diamond Research Laboratory for the concentration of batch kimberlite samples.

tOOmm ,~ It,,

43 mm I i J i

' ' i

I ~ ~ "|DMS Cycloner'~' , ,~ ~ ~ ~ ( S . . . .... s l o p Position

/ / Solenoid =~ i=: n e e g i n = . . . . . . . . . . . . . . . ]

~ " ~ ~'" I~ '~=~--~ Mid die Position ,~ L , , ' - - ' ~ ' ~ ' " " " " ~ 20 mm / ' " ' " . . . . . . . . . ~ ~ ¢0;~ i

Bottom Position : . . . . . ~ . . . . . . . . . . . . . . ~ ' . ' ~ " " . " ~ . / . . . . . :

' ' 125mm , , ~ ~ ,

'Jl[ 320 mm ) ,

Fig.3 Schematic diagram of the experimental arrangement of the magnetic cyclone.

Th e cyclone is gravity-fed from a constant head mixing box to ensure a steady medium flow and pressure at the cyclone inlet. The cyclone axis is inclined toward the apex at an angle of 12 ° to horizontal to discharge the contained medium when the plant is stopped.

The plant includes a wet drum magnetic separator to remove non-magnetic fine solids from the circulating medium and return clean ferrosilicon to the correct medium storage tank to minimise medium viscosity. The density of the circulating medium is maintained at the pre-set operating density by an automatic density controller and clean water dilution valve.

Two different grades of ferrosilicon were used: a finely milled 270D and a coarser atomised Cyclone 60. Twenty 2-mm density tracers, from 2.0 g/cm 3 to 3.7 g/cm 3, at 0.1 g/cm 3 steps, were added into the system in each run. The tracers were retrieved by a 1-mm laboratory sieve from the overflow and underflow streams of the cyclone.

The response of the cyclone with 270D ferrosilicon was tested at three different feed densities, namely 2.35, 2.45 and 2.55 g/cm 3. For Cyclone 60 the feed densities tested were 2.45, 2.55 and 2.65 g/cm 3 . Tests were done at each solenoid position and at different settings of the coil current representing different values of the magnetic field. These settings were varied from 0 to 120 Gauss. Each test was repeated three times to ensure reproducibility.

Parameters recorded during each run were: the cyclone feed, undertow and overflow densities, the undertow and overflow flow rates and the distribution of density tracers. The partition curves were constructed from the density tracer data, from which the cut-point densities and the mean probable errors were calculated.

Application of magnetic cyclone for dense medium separation

RESULTS AND DISCUSSION

505

Figures 4 and 5 summarise the effect of the magnetic field on the density differential in the cyclone, for 270D and Cyclone 60 ferrosilicon, respectively. It can be seen that by applying the magnetic field, the density differential between the overflow and underflow decreases until it reaches a minimum. With a further increase of the magnetic field strength the density differential begins to rise. This pattern is common to both grades of ferrosilicon, to all three feed densities and to all positions of the solenoid magnet investigated in this study. It can, however, be seen that the greatest reduction in the density differential is achieved with the magnet close to the overflow. It can be easily understood by realising that the closer the magnet is to the overflow, the more uniform the distribution of ferrosilicon within the cyclone. It can be seen in Figure 5 that it is even possible to induce inversion of density resulting in a negative density differential.

0.5

0.4

0.2

¢3

• ~ 0.o c o ~3

0.0

• Feed 2.35 g/cm a, bottom magnet • Feed 2.35 g/cm 3, magnet middle • Feed 2.35 g/cm 3, magnet top v Feed 2.55 g/cm 3, magnet bottom • Feed 2.55 g/cm 3, magnet middle

I eonem raagaet ~ a • Feed 2.55 g/cm 3, magnet top

0 20 40 60 80 100 120

Magnetic Field Strength [G]

Fig.4 Density differential as a function of magnetic field strength for various positions of the magnet and for two feed densities. Ferrosilicon: 270D.

The results also show that the pattern of the density differential is practically independent of the feed density, in the range investigated (i.e. between 2.35 and 2.65 g/cm3). It can also be seen from Figures 4 and 5 that the minimum density differential was achieved at approximately 80 G for 270D ferrosilicon, while for Cyclone 60 the minimum was observed at 40 G. After reaching the minimum, the density differential begins to rise with increasing magnetic field. This is the result of the onset of magnetic flocculation [10] of the ferrosilicon particles, increased settling of the magnetic floes and distortion of the flow pattern within the cyclone.

506 J. Svoboda e t al.

,,V"

E

D

._m

=_-

ore {/p c

c~

1.0

0.8

0.6

0.4

0.2

0.0

Feed 2.45 g/cm 3, bottom magnet Feed 2.45 g/cm 3, magnet middle Feed 2.45 g/cm 3, magnet top Feed 2.65 g/cm 3, magnet bottom ;eed 2.65 g/cm 3, magnet middle Feed 2.65 g/cm 3, magnet top

-0.2

0 20 40 60 80 100 120

Magnetic Field Strength [G]

Fig.5 Density differential as a function of magnetic field strength for various positions of the magnet and for two feed densities. Ferrosilicon: Cyclone60.

The difference in the behaviour of 270D and Cyclone 60 ferrosilicon can be explained by realising that Cyclone 60 is coarser and more magnetic than 270D ferrosilicon, as can be seen in Table 1. Coarser and more magnetic Cyclone 60 requires a lower magnetic field to manipulate the distribution of the ferrosilicon particles within the cyclone. The onset of magnetic flocculation of the Cyclone 60 particles also occurs at a lower magnetic field than that for 270D.

TABLE 1 Physical characteristics of 270D and Cyclone 60 ferrosilicon [11]

Ferrosilicon Particle size Saturation Remanent Coercive -45 l a r n magnetisation magnetisation force

Bs(kG ) B r (G) H c (Oe)

270D 85-93% 7.6 117 36

Cyclone 60 65-78% 9.4 187 40

Figure 6 summarises the effect of the magnetic field on the mean probable error Ep. It can be seen that the application of the magnetic field reduces the Ep; for instance with 270D ferrosilicon the Ep decreased from 0.06 at zero magnetic field, to less than 0.02 at about 35 G. A further rise in the magnetic field strength increases the mean probable error, the increase being more dramatic for Cyclone 60 ferrosilicon, for reasons discussed above. Figure 7 depicts the dependence of the mean probable error and of the density differential on the magnetic field strength. Such a chart offers, for the first time in DMS practice, a tool for determination of the optimum density differential, at which the selectivity of separation is maximum. It is of interest to note that the minimum value of Ep, for 270D, occurred at a density differential of 0.25 g/cm 3, in agreement with plant experience [12].

Application of magnetic cyclone for dense medium separation

0.20 --r

tl,I t _

o

_e ,.0 m

.o 0 . c Q =E

0.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

• FeSi 270D t • FeSi Cyclone 60 . . . . . . . . . . . . . . I i

1

. . . . . . . . . . . . . . . . . . /--i ................ ~ -m-- i . . . . . . . . - - ~

.2.... . . . . . -:- . . . . . ~ . . . . . .

Magnetic Field Strength [G]

0 10 20 30 40 50 60 70 80

Fig.6 Mean probable error Ep as a function of the magnetic field.

507

0.09 0.5

0.08

0.07

0.06 IM @

0.05 .o

la. 0.04

0.03

0.02

0.01

. . . . . ~ . . . . l . . . . . . . ~ . . . . . . . . . . . . . . . . l . . . . . . ~ . . . . . . . . 0 . 4

ii ~ " ii I e D e n s i t y d i f f e r e n t i a l ~ . . N _ ~ • ED ,

. . . . . - ~ . . . . . . . . . . . . . . . i 0 . 3 . = .

', \ i F-\ o ' , / °

, i 0 20 40 60 80 100 120

Magnetic Field Strength [G]

Fig.7 Mean probable error Ep and density differential, as a function of the magnetic field, for 270D ferrosilicon.

Figures 8 and 9 depict the dependence of the cut-point density on the magnetic field strength. It can be seen that the cut-point density decreases with an increasing magnetic field, the maximum reduction amounting to about 5 per cent at the optimum magnetic field (where Ep is the lowest). The decrease in the cut-point density is the result of the fact that the application of the magnetic field reduces the density differential and

508 J. Svoboda et ai.

thus the density of the underflow. It also transpires from Figures 8 and 9 that the decrease in the cut-point density seems to be more pronounced for lower feed densities, although the reduction in the density differential is independent of the feed density.

3.3

3.2

i 3.1

3.0

"~ 2.9

d 2.8

2.7

• Feed 2.35 g/cm s, magnet bottom • Feed 2.35 g/cm s, magnet middle & Feed 2.35 g/cm 3, magnet top v Feed 2.55 g/cm a, magnet bottom • Feed 2.55 g/cm s, magnet middle

~ . Feed 2.55 g/cm3, magnet top

. . . . . . . . . i . . . . . . .

i i 0 10 20 30 40 50 60 70 80

Magnetic Field Strength [G]

Fig.8 The cut-point density as a function of the magnetic field for 270D ferrosilicon.

3.50

3.45

3.40

i 3.35

..L~ 3.30

3.25

"O0. 3.20

U 3.15

3.10

3.05

I • Feed 2.45 g/cm s, magnet bottom l • Feed 2.45 g/cm 3, magnet middle

• Feed 2.45 g/cm 3, magnet top . ! v Feed 2.65 g/cm 3, magnet bottom

. . . . ~ . . . . . . . . . . . . . t • Feed 2.65 g/cm 3, magnet middle • :; • Feed 2.65 g/cm 3, magnet top

0 10 20 30 40 50 60

Magnetic Field Strength [G]

Fig.9 The cut-point density as a function of the magnetic field for Cyclone60 ferrosilicon.

Application of magnetic cyclone for dense medium separation 509

CONCLUSIONS

It has been confirmed experimentally that the distribution of ferrosilicon in a cyclone can be manipulated by a weak, externally applied, vertically oriented magnetic field. This manipulation enables control of the density differential and the mean probable error. It was also determined that the field strength required to achieve optimum results is dependent on the type of ferrosilicon used.

It was also possible, for the first time, to establish an experimental procedure that allows to relate the density differential to the mean probable error. By determining the minimum Ep it is possible to identify the density differential at which the cyclone should operate to provide the highest selectivity of separation.

Although more work is needed to evaluate the full potential of the magnetic cyclone technique for dense medium separation, it is apparent that the approach described in this paper offers a tool for determination of the optimum operating conditions of a cyclone and for maintaining such conditions. A magnetic field can also efficiently assist in rectification of operational problems often experienced at DMS plants. In view of the very low magnetic field strength required to optimise the operation of the cyclone, the capital cost is low and the running costs are negligible. In the case of our 100 mm cyclone, the input power did not exceed 15 W, under optimum conditions.

ACKNOWLEDGMENTS

This paper is published by permission of De Beers Diamond Research Laboratory. Numerous fruitful discussions with E. Hyland are gratefully acknowledged.

1.

2.

3.

4.

5.

6. 7.

8. 9.

10. 11. 12.

REFERENCES

Yurov, P.P. et al., Beneficiation of roasted ores in a reconstructed section of the CGOK Beneficiation Plant. Chernaya metallurgiya, No.9 (1970), 3 (in Russian). Yurov, P.P. et al., Magnetic cyclone of a new design for desliming oxidised and magnetite ores. Chernaya metallurgiya, No. 7 (1981), 31 (in Russian). Yurov, P.P. et al., The application of magnetic cyclones to the beneficiation of iron ores. Gorny Zh., No. 4 (1986), 34 (in Russian). Watson, J.L. & Amoako-Gyamphi, K., Cycloning in magnetic fields. SME-AIME Preprint No. 83-335, (1983). Fricker, A.G., Magnetic hydrocyclone separator. Trans. Instn. Min. Metall. Sect. C: Mineral Process. Extr. Metall., 94 (1985), C158. Anon: Mining J., (January 7, 1983), p. 6. Freeman, R.J. et al., The development of a magnetic cyclone for processing finely ground magnetite. IEEE Trans. Magn., 30, 4665 (1994). Freeman, R.J. et al., The progress of the magnetic hydrocyclone. Magn. Electr. Sep., 4, 139 (1993). Svoboda, J. & Campbell, Q.P., Magnetic cyclone and method of operating it. South African Patent No. 96/4132, (1996). Svoboda, J., Magnetic Methods for the Treatment of Minerals. Elsevier, Amsterdam (1987). Svoboda, J., unpublished results (1997) Ferrara, G. & Schena, G.D., Design criteria and control strategies for dynamic dense media separation processes treating fine ores. Proc. XVI Int. Min. Proc. Congress, 885 (1988).

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