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The effect of demagnetisation and ore contamination on the viscosity of the medium in a dense medium cyclone plant
Minerals Engineering. Vol. 3, No. 6. pp. 607-613, 1990
0892-6875/90 $3.00 + 00 Pergamon Press plc
Printed in Great Britain
THE EFFECT OF DEMAGNETISATION AND ORE CONTAMINATION ON THE VISCOSITY OF THE MEDIUM IN A DENSE MEDIUM CYCLONE PLANT T.J. NAPIER-MUNN§ and I.A. SCOTTt § Julius Kruttschnitt Mineral Research Centre, Isles Rd., Indooroopilly, Queensland 4068, Australia Renison Ltd, PO Box 20, Zeehan, Tasmania 7469, Australia (formerly of the JKMRC) (Received and accepted 13 February 1990)
ABSTRACT The influence o/medium viscosity on dense medium cyclone performance, and the influence of various factors on medium viscosity, are briefly reviewed. Experimental work was carried out in the DM cyclone preconcentration plant at Mount Isa Mines to investigate the effect of using demagnetising coils to reduce the viscosity of the circulating ferrosilicon medium. The effect on viscosity o/ natural fine contamination from the ore, including magnetic pyrrhotite, was also studied by replacing the medium charge with fresh medium and then allowing it to become re-contaminated. The viscosity of the circulating medium was monitored over a range of operating densities using an on-line viscometer. It was found, as expected, that operation of the demagnetising coil significantly reduced viscosity, and that the viscosity o/ the contaminated medium was much higher than that of the fresh medium. It is recommended that demagnetising coils be considered/or all DM cyclone plants operating with ferrosilicon media.
Keywords Dense medium separation; viscosity; demagnetisation; on-line viscometer; cyclone INTRODUCTION The viscosity of the medium in a dense medium (DM) separation plant has always been recognised as an important process variable. In the case of bath-type separators, the effect of an increase in viscosity is simply interpreted as reducing the terminal velocity of ore or coal particles, thereby increasing the probability of particles being misplaced to the wrong product. For DM cyclones, the influence is more complex because medium viscosity controls the separation both directly, through its influence over particle motions, and indirectly, through its effect on the medium behaviour in the cyclone itself, which in turn influences the ore/coal separation. In both cases, however, the effect of a higher viscosity is known to be detrimental, reducing the quality of separation and also affecting the density of separation. Determining quantitatively the influence of viscosity on separation has always been difficult because of the difficulty of measuring viscosity, and of decoupling the effects of viscosity from other variables, in particular medium density. Non-Newtonian rheological effects, such as the existence of a yield stress, also complicate the picture for both baths and cyclones [ 1,2].
607
608
T.J. NAPIER-MUNN and I. A. SCOTT
The consensus of the literature, however, is that an increase in medium viscosity decreases the separating density and efficiency of a DM cyclone. The effect increases with a decrease in the particle size of ore or coal, and above a certain particle size the effect is small or absent [3,4,5]. The factors controlling viscosity are well known, but will be repeated here for completeness: M e d i u m d e n s i t y - viscosity increases with solids concentration and thus with medium
density, in a non-linear manner, the increase being rapid above a certain critical concentration. S o l i d s d e n s i t y - this controls the solids concentration required to achieve a given
medium density; high density solids require a lower concentration (and thus a lower viscosity) to achieve a given medium density. Particle size distribution - finer medium particles produce higher viscosities. P a r t i c l e s h a p e - rounded or smooth particles produce lower viscosities than angular
or rough particles. F i n e c o n t a m i n a t i o n - contaminants, such as slimes from the ore or coal, usually
increase viscosity due both to their lower solids density and fine particle size. (There may also be additional physico-chemical effects). - commercial media are generally magnetic (ferrosilicon and magnetite) to allow simple recovery and regeneration processes. Passage through the magnetic separators in the medium recovery circuit induces a residual magnetisation which causes flocculation or agglomeration of the magnetised particles. This effect generally increases medium viscosity. The effect can be minimised by demagnetising or depolarising the medium after magnetic recovery [6]. Demagnetisation
In principle it would be desirable to control medium viscosity in DM plants, but in practice this is rarely if ever done because of the difficulty of continuous (on-line) measurement and uncertainties in the interpretation of results. Also it is difficult to control viscosity directly through any one fast-response control action. It is more usual to manage viscosity by selecting operating conditions which will limit it to an acceptable range based on previous experience. Factors to be considered will include: Selection of the correct medium grade (size distribution and shape). Control of medium density. Minimisation of contamination through efficient feed preparation (subject to the proviso that some contamination is sometimes tolerated or even encouraged to stabilise a coarse medium). Installation of demagnetisation coils in the plant. In the course of a larger study whose objective was to develop a model of DM cyclones [5,7], an opportunity occurred to examine specifically the effects of demagnetisation and non-magnetle contamination on medium viscosity in a commercial DM cyclone plant. This paper presents and discusses the results of this investigation. EXPERIMENTAL DETAILS The testwork was conducted in the dense medium cyclone preeoncentration plant at Mount Isa Mines. This plant rejects barren and low grade material ahead of the Lead-Zinc Concentrator. Details of the plant and its operation are given by Munro et al. in a number of papers [e.g. 8], and a flowsheet is shown in Figure 1.
Demagnetisation effect on viscosity in a DM cyclone plant
609
I1
i F
~p----~'Rejects
t
>
c)
S1knes
k
N/
9V
Fig.l Flowsheet of Mount Isa DMS plant (after Munro et al [8]) Key: 1. Mogenxn Sisere; 2. Feed prep. icreens; S. 400ram DM cyclones; 4. Sinks screens; 5. Floats Icreens; 6. Centrifugal densifiers; 7. Circulating medium sumps; 8. Densifying pump boxes; 9. Dilute medium sumps; 10. Demagnetkation coils; 11. Primary magnetic Rparators; 12. Thickening cyclones; 18. Secondary magnetic aeparatorl
The plant utilises 400ram cyclones at a m e d i u m - t o - o r e ratio of about 9:1 (approx. 60 t / h ore per cyclone), gravity-fed with an 8m head, with Samancor 150D ferrosilicon. Medium recovery and densification circuits are relatively conventional. At the time of the testwork, the job of the preconcentrator was to achieve a total reject rate of about 34% of the ore entering the plant, both through the removal of fines in the feed preparation section, and in the dense medium process itself. An interesting feature of the Mount Isa situation is the presence of pyrrhotite in the ore. This mineral is both magnetic and relatively soft, and therefore abrades and concentrates in the circulating medium; although the feed ore contains only about 3% pyrrhotite, the m e d i u m can contain over 20% of the mineral. This has a direct effect on viscosity, because the pyrrhotite is of lower density than the ferrosilicon medium (4650 k g / m 3 compared to 6800 k g / m 3) and a higher total solids concentration is therefore required to reach the normal operating density of about 2950 k g / m 3. The measurements were carried out in two groups in September 1986. In the first, the effect of demagnetisation was demonstrated by monitoring the medium viscosity in one circuit over an 8-hour period during which the demagnetising coil, mounted on the primary magnetic separator product line, was first switched on and then off again. (The coils are not normally run unless viscosity-related problems occur, usually apparent as a difficulty in maintaining density). The demagnetisation tests were conducted at normal operating density and levels of contamination. In the second group of measurements, the effects of contamination were studied by monitoring viscosity as a function of density, first at zero and then at normal contamination levels, by driving the circulating m e d i u m density up and down over the approximate range 2650-3150 k g / m 3. The entire contents of the circuit were first replaced by fresh (uncontaminated) 150D ferrosilicon, and the density-viscosity measurements carried out. The exercise was then repeated about a week later after the normal contamination levels had been reached again.
610
T.J. NAPIER-MUNN and I. A. SCOTt
The viscosity was measured continuously using a Debex on-line viscometer at a bobbin speed of 400 rpm [9,10]. This device is calibrated with Newtonian oils, and provides a reading of equivalent apparent Newtonian viscosity, which is useful for comparative purposes. The medium density was measured using a radiation attenuation gauge and monitored using the plant TDC 2000 computer; these figures were checked at regular intervals by hand-sampling the circulating medium. Typical medium temperature during the testwork was 33°C. The demagnetising coils were Rapid HMD-6c, 415 V single phase AC, rated at 17.5 amps. Demagnetising coils are conventionally wound on a rectangular section to provide a uniformly increasing and then decreasing field, to achieve a final state of random orientation of magnetic domains in the ferromagnetic material. Such a randomly oriented material is said to be demagnetised or depolarised, and shows a decreased tendency to agglomerate. Von Wolff and Kessler note that the difficulty of demagnetisation is a function of the coercive force of the solid material, which increases with decrease in particle size [11]. Fine media and ferrosilicon therefore need higher demagnetising fields than coarse media and magnetite. RESULTS AND DISCUSSION The effect of contamination
Figure 2 shows the relationship between the medium viscosity and density for the contaminated (normal) and uncontaminated (fresh) medium. The effect of the contamination is clearly large. At a density of 2950 kg/m 3, the contaminated medium has a viscosity about 3 times that of the fresh material. As expected, this differential increases with the density. The familiar sharp rise in viscosity at the higher densities, usually modelled as an exponential function of solids concentration, means that processes operating at high densities are often vulnerable to small increases in viscosity. In the case of Mount Isa, however, computer simulation of the process had showed that the reject rate and associated metal loss were not sensitive to viscosity over the plant's normal operating range, as rises in viscosity due to (for example) increases in contamination could be compensated for by adjusting the density without loss of efficiency [12]. 200
ContaminatedM e d /
175 150 A
0.
._z, 8
125 100
c:
75 50 25
0 2600 2700 2800 2900 3000 3100 3200 Density (kg/m3) I
I
i
!
|
Fig.2 Viscosity vs density for clean and contaminated medium
Demagnetisation effect on viscosity in a DM cyclone plant
611
The effect of demagnetisatlon Figure 3 shows the viscosity monitored every 6 minutes over the 8-hour period during which the demagnetisation coil (normally off) was switched on and then off again. The viscosity was first monitored for 60 minutes to establish a viscosity norm. The coil was then switched on and allowed to run for 198 minutes before being switched off again. The density was also monitored every 6 minutes throughout the test period, the mean being 3036 k g / m s with a standard deviation of 27 k g / m 3. 130
120, Ira:• °1~_1~ =
II0
,1= ~la~ D I • [] • = ==== =
~>
I
,,%=
•==
100
==UP m
¢,
uP === • m ' E
[]
o
p= D
90, COILOF
COLON
80
COLOFF I
0
100
200
I
300
I
400
500
Time (minutes)
Fig.3 The effect of demagnetisation on medium viscosity Figure 3 shows that the coil had a rapid and substantial effect. The viscosity had started to drop by the 3rd monitoring increment (lS minutes) after the coil was activated, and continued to fall until about 3 increments after it was de-activated, when the viscosity began to rise. (The missing values towards the end of the monitoring period were due to plant interruptions and sampling blockages). The time at which the coil was de-activated was dictated by operational constraints, and it is not therefore known how much further the viscosity would have dropped had the coil remained on indefinitely. However, the rate of fall shows a distinct decline towards the end of the "on" period, and numerical extrapolation of the trend suggests a m i n i m u m viscosity value of about 90cP. This is a drop of 25% from the normal value of about 120cP, and about 1/3 of the difference between fresh and contaminated medium at that density - see Figure 2 (the contamination data for Figure 2 were obtained on a different day to the demagnetisation tests, and the viscosity at a density of 3036 k g / m 3 was somewhat higher in that case; the comparison is therefore not exact). The return to normal viscosity levels after the coil had been de-activated was rather erratic. It is not known why this should be, but clearly the magnetisation and demagnetisation are functions of the proportion of the circulating medium passing through the magnetic separators and demagnetising coil. In the present case, the initial rise in viscosity after the coil was switched off appeared to proceed at about the same rate as the initial decline. However after about 350 minutes, with some plant interruptions, the rise ceased. This might have been caused by a drop in ore feedrate and a corresponding decrease in the amount of ferrosilicon reporting to the dilute circuit. However, this is not known.
Demagnetising coils and their purpose In one of the author's experience (TJNM) there has long been a dispute as to whether demagnetising coils are necessary in a DM cyclone plant. The conventional wisdom has been
612
T.J. NAPIER-MUNNand I. A. SCOTT
that cyclones do not suffer the viscosity effects that DM baths suffer, and therefore that close control of medium viscosity is unnecessary. This is probably more true of coal preparation applications than mineral separations, which generally take place at higher densities and viscosities. Certainly, not all designers will install demagnetising coils in cyclone plants. It is now well established that cyclone performance in mineral applications can be a strong function of medium viscosity. However, the published literature on the influence of demagnetisation in this context is almost non-existent. The coils are generally seen by most operators of DM cyclone plants as non-critical to the operation and are often not utilised even if installed. This situation is compounded where there is a record of coll breakdown, usually due to short-circuiting by water following failure of the insulation, often caused by overheating [13]. Despite this, however, it is the authors' view that demagnetising coils should be installed and operated in DM cyclone plants because: .
It is now firmly established that elevated viscosities can have a deleterious effect on cyclone performance.
.
Sudden and unpredictable viscosity-related declines in cyclone performance can occur [14], and the use of coils would at least limit the probability of such events occurring.
.
The present work has shown that the coils do reduce the viscosity of circulating medium. CONCLUSIONS
Tests carried out in the Mount Isa Mines lead-zinc preconcentration dense medium cyclone plant have confirmed that normal contamination of the medium (principally by magnetic pyrrhotite) substantially increases the viscosity of the medium over that prevailing with fresh (uncontaminated) medium. T h i s increase can be reduced by about one third by running the demagnetising coils installed on the primary magnetic separator return lines. Computer simulation (not reported here) has shown that viscosity was not a critical parameter in the Mount Isa context (with current performance criteria). In general, however, viscosity is known to reduce the efficiency of DM cyclones, and it is therefore recommended that demagnetising coils be considered for all DM cyclone plants operating with ferrosilieon media. ACKNOWLEDGEMENTS This work was conducted with the cooperation and assistance of Mount Isa Mines Ltd., particularly the Manager and staff of the Lead-Zinc Concentrator. It was funded by a grant from the Australian Mineral Industries Research Association Ltd., for a collaborative research programme in mineral processing, of which Mount Isa Mines was sponsor. REFERENCES .
Whitmore R.L., Coal preparation: the separation efficiency of dense medium baths. J. Inst. Fuel, 31,583-591 (1958).
Demagnetisation effect on viscosity in a DM cyclone plant
613
.
Napier-Munn T.J., The mechanism of separation in dense medium cyclones. 2nd Int. Conf. on Hydrocyclones, Bath, 252-280 (BHRA) (1984).
.
Napier-Munn T.J., The mechanism of separation in dense medium cyclones. PhD. Thesis, University of London (1983).
.
Davis J.J. • Napier-Munn T.J., The influence of medium viscosity on the performance of dense medium cyclones in coal preparation. 3rd Int. Conf. on Hydrocyclones, Oxford, 155-165, Ed. P. Wood, BHRA (Sept. 1987).
.
Scott I.A., A dense medium cyclone model based on the pivot phenomenon. PhD. Thesis, University of Queensland (1988).
.
Williams M.F. & Hendrickson L.G., Depolarizing magnetite pulps. Trans. AIME, 201-209 (Feb. 1956).
.
Scott I.A., Davis J.J & Manlapig E., A methodology for the modelling of dense medium cyclones. Proc. 13th Cong. CMMI, (Aus. IMM), 67-76, Singapore (May 1986).
.
Munro P.D., Schache I.S., Park W.G. & Watsford R.M.S., The design, construction and commissioning of a dense medium plant for silver-lead-zinc ore treatment Mount Isa Mines Ltd., XIV Int. Min. Proc. Cong., Toronto, Reprints Vol. 6 Paper 6 (CIMM) (1982).
.
Reeves T.J., On-line viscometer for mineral slurries. Trans. Inst. Min. Met., C201-C208 (1985).
94,
10.
Napier-Munn T,J., Reeves T.J. & Hansen J.O., The monitoring of medium rheology in dense medium cyclone plants. Proc. Aus.IMM, 294, no.3, 85-93 (May 1989).
11.
Von Wolff W . T . E . & K e s s l e r I . I . M . , Laboratory tests on magnetite powders used for coal washing. Coal, Gold and Base Minerals of S.A., 42-51 (Nov. 1966).
12.
Scott I.A., A simulation study of the benefits of decreasing the cut-size at the Mount Isa DMS preconcentrator. Julius Kruttschnitt Mineral Research Centre, confidential report to Mount Isa Mines Ltd., (August 1987).
13.
Munro P.D., Personal Communication, Mount Isa Mines Ltd., (1989).
14.
Napier-Munn T.J., Dense medium cyclones in diamond recovery. MSc. Thesis, University of the Witwatersrand (1977).
0892-6875/90 $3.00 + 00 Pergamon Press plc
Printed in Great Britain
THE EFFECT OF DEMAGNETISATION AND ORE CONTAMINATION ON THE VISCOSITY OF THE MEDIUM IN A DENSE MEDIUM CYCLONE PLANT T.J. NAPIER-MUNN§ and I.A. SCOTTt § Julius Kruttschnitt Mineral Research Centre, Isles Rd., Indooroopilly, Queensland 4068, Australia Renison Ltd, PO Box 20, Zeehan, Tasmania 7469, Australia (formerly of the JKMRC) (Received and accepted 13 February 1990)
ABSTRACT The influence o/medium viscosity on dense medium cyclone performance, and the influence of various factors on medium viscosity, are briefly reviewed. Experimental work was carried out in the DM cyclone preconcentration plant at Mount Isa Mines to investigate the effect of using demagnetising coils to reduce the viscosity of the circulating ferrosilicon medium. The effect on viscosity o/ natural fine contamination from the ore, including magnetic pyrrhotite, was also studied by replacing the medium charge with fresh medium and then allowing it to become re-contaminated. The viscosity of the circulating medium was monitored over a range of operating densities using an on-line viscometer. It was found, as expected, that operation of the demagnetising coil significantly reduced viscosity, and that the viscosity o/ the contaminated medium was much higher than that of the fresh medium. It is recommended that demagnetising coils be considered/or all DM cyclone plants operating with ferrosilicon media.
Keywords Dense medium separation; viscosity; demagnetisation; on-line viscometer; cyclone INTRODUCTION The viscosity of the medium in a dense medium (DM) separation plant has always been recognised as an important process variable. In the case of bath-type separators, the effect of an increase in viscosity is simply interpreted as reducing the terminal velocity of ore or coal particles, thereby increasing the probability of particles being misplaced to the wrong product. For DM cyclones, the influence is more complex because medium viscosity controls the separation both directly, through its influence over particle motions, and indirectly, through its effect on the medium behaviour in the cyclone itself, which in turn influences the ore/coal separation. In both cases, however, the effect of a higher viscosity is known to be detrimental, reducing the quality of separation and also affecting the density of separation. Determining quantitatively the influence of viscosity on separation has always been difficult because of the difficulty of measuring viscosity, and of decoupling the effects of viscosity from other variables, in particular medium density. Non-Newtonian rheological effects, such as the existence of a yield stress, also complicate the picture for both baths and cyclones [ 1,2].
607
608
T.J. NAPIER-MUNN and I. A. SCOTT
The consensus of the literature, however, is that an increase in medium viscosity decreases the separating density and efficiency of a DM cyclone. The effect increases with a decrease in the particle size of ore or coal, and above a certain particle size the effect is small or absent [3,4,5]. The factors controlling viscosity are well known, but will be repeated here for completeness: M e d i u m d e n s i t y - viscosity increases with solids concentration and thus with medium
density, in a non-linear manner, the increase being rapid above a certain critical concentration. S o l i d s d e n s i t y - this controls the solids concentration required to achieve a given
medium density; high density solids require a lower concentration (and thus a lower viscosity) to achieve a given medium density. Particle size distribution - finer medium particles produce higher viscosities. P a r t i c l e s h a p e - rounded or smooth particles produce lower viscosities than angular
or rough particles. F i n e c o n t a m i n a t i o n - contaminants, such as slimes from the ore or coal, usually
increase viscosity due both to their lower solids density and fine particle size. (There may also be additional physico-chemical effects). - commercial media are generally magnetic (ferrosilicon and magnetite) to allow simple recovery and regeneration processes. Passage through the magnetic separators in the medium recovery circuit induces a residual magnetisation which causes flocculation or agglomeration of the magnetised particles. This effect generally increases medium viscosity. The effect can be minimised by demagnetising or depolarising the medium after magnetic recovery [6]. Demagnetisation
In principle it would be desirable to control medium viscosity in DM plants, but in practice this is rarely if ever done because of the difficulty of continuous (on-line) measurement and uncertainties in the interpretation of results. Also it is difficult to control viscosity directly through any one fast-response control action. It is more usual to manage viscosity by selecting operating conditions which will limit it to an acceptable range based on previous experience. Factors to be considered will include: Selection of the correct medium grade (size distribution and shape). Control of medium density. Minimisation of contamination through efficient feed preparation (subject to the proviso that some contamination is sometimes tolerated or even encouraged to stabilise a coarse medium). Installation of demagnetisation coils in the plant. In the course of a larger study whose objective was to develop a model of DM cyclones [5,7], an opportunity occurred to examine specifically the effects of demagnetisation and non-magnetle contamination on medium viscosity in a commercial DM cyclone plant. This paper presents and discusses the results of this investigation. EXPERIMENTAL DETAILS The testwork was conducted in the dense medium cyclone preeoncentration plant at Mount Isa Mines. This plant rejects barren and low grade material ahead of the Lead-Zinc Concentrator. Details of the plant and its operation are given by Munro et al. in a number of papers [e.g. 8], and a flowsheet is shown in Figure 1.
Demagnetisation effect on viscosity in a DM cyclone plant
609
I1
i F
~p----~'Rejects
t
>
c)
S1knes
k
N/
9V
Fig.l Flowsheet of Mount Isa DMS plant (after Munro et al [8]) Key: 1. Mogenxn Sisere; 2. Feed prep. icreens; S. 400ram DM cyclones; 4. Sinks screens; 5. Floats Icreens; 6. Centrifugal densifiers; 7. Circulating medium sumps; 8. Densifying pump boxes; 9. Dilute medium sumps; 10. Demagnetkation coils; 11. Primary magnetic Rparators; 12. Thickening cyclones; 18. Secondary magnetic aeparatorl
The plant utilises 400ram cyclones at a m e d i u m - t o - o r e ratio of about 9:1 (approx. 60 t / h ore per cyclone), gravity-fed with an 8m head, with Samancor 150D ferrosilicon. Medium recovery and densification circuits are relatively conventional. At the time of the testwork, the job of the preconcentrator was to achieve a total reject rate of about 34% of the ore entering the plant, both through the removal of fines in the feed preparation section, and in the dense medium process itself. An interesting feature of the Mount Isa situation is the presence of pyrrhotite in the ore. This mineral is both magnetic and relatively soft, and therefore abrades and concentrates in the circulating medium; although the feed ore contains only about 3% pyrrhotite, the m e d i u m can contain over 20% of the mineral. This has a direct effect on viscosity, because the pyrrhotite is of lower density than the ferrosilicon medium (4650 k g / m 3 compared to 6800 k g / m 3) and a higher total solids concentration is therefore required to reach the normal operating density of about 2950 k g / m 3. The measurements were carried out in two groups in September 1986. In the first, the effect of demagnetisation was demonstrated by monitoring the medium viscosity in one circuit over an 8-hour period during which the demagnetising coil, mounted on the primary magnetic separator product line, was first switched on and then off again. (The coils are not normally run unless viscosity-related problems occur, usually apparent as a difficulty in maintaining density). The demagnetisation tests were conducted at normal operating density and levels of contamination. In the second group of measurements, the effects of contamination were studied by monitoring viscosity as a function of density, first at zero and then at normal contamination levels, by driving the circulating m e d i u m density up and down over the approximate range 2650-3150 k g / m 3. The entire contents of the circuit were first replaced by fresh (uncontaminated) 150D ferrosilicon, and the density-viscosity measurements carried out. The exercise was then repeated about a week later after the normal contamination levels had been reached again.
610
T.J. NAPIER-MUNN and I. A. SCOTt
The viscosity was measured continuously using a Debex on-line viscometer at a bobbin speed of 400 rpm [9,10]. This device is calibrated with Newtonian oils, and provides a reading of equivalent apparent Newtonian viscosity, which is useful for comparative purposes. The medium density was measured using a radiation attenuation gauge and monitored using the plant TDC 2000 computer; these figures were checked at regular intervals by hand-sampling the circulating medium. Typical medium temperature during the testwork was 33°C. The demagnetising coils were Rapid HMD-6c, 415 V single phase AC, rated at 17.5 amps. Demagnetising coils are conventionally wound on a rectangular section to provide a uniformly increasing and then decreasing field, to achieve a final state of random orientation of magnetic domains in the ferromagnetic material. Such a randomly oriented material is said to be demagnetised or depolarised, and shows a decreased tendency to agglomerate. Von Wolff and Kessler note that the difficulty of demagnetisation is a function of the coercive force of the solid material, which increases with decrease in particle size [11]. Fine media and ferrosilicon therefore need higher demagnetising fields than coarse media and magnetite. RESULTS AND DISCUSSION The effect of contamination
Figure 2 shows the relationship between the medium viscosity and density for the contaminated (normal) and uncontaminated (fresh) medium. The effect of the contamination is clearly large. At a density of 2950 kg/m 3, the contaminated medium has a viscosity about 3 times that of the fresh material. As expected, this differential increases with the density. The familiar sharp rise in viscosity at the higher densities, usually modelled as an exponential function of solids concentration, means that processes operating at high densities are often vulnerable to small increases in viscosity. In the case of Mount Isa, however, computer simulation of the process had showed that the reject rate and associated metal loss were not sensitive to viscosity over the plant's normal operating range, as rises in viscosity due to (for example) increases in contamination could be compensated for by adjusting the density without loss of efficiency [12]. 200
ContaminatedM e d /
175 150 A
0.
._z, 8
125 100
c:
75 50 25
0 2600 2700 2800 2900 3000 3100 3200 Density (kg/m3) I
I
i
!
|
Fig.2 Viscosity vs density for clean and contaminated medium
Demagnetisation effect on viscosity in a DM cyclone plant
611
The effect of demagnetisatlon Figure 3 shows the viscosity monitored every 6 minutes over the 8-hour period during which the demagnetisation coil (normally off) was switched on and then off again. The viscosity was first monitored for 60 minutes to establish a viscosity norm. The coil was then switched on and allowed to run for 198 minutes before being switched off again. The density was also monitored every 6 minutes throughout the test period, the mean being 3036 k g / m s with a standard deviation of 27 k g / m 3. 130
120, Ira:• °1~_1~ =
II0
,1= ~la~ D I • [] • = ==== =
~>
I
,,%=
•==
100
==UP m
¢,
uP === • m ' E
[]
o
p= D
90, COILOF
COLON
80
COLOFF I
0
100
200
I
300
I
400
500
Time (minutes)
Fig.3 The effect of demagnetisation on medium viscosity Figure 3 shows that the coil had a rapid and substantial effect. The viscosity had started to drop by the 3rd monitoring increment (lS minutes) after the coil was activated, and continued to fall until about 3 increments after it was de-activated, when the viscosity began to rise. (The missing values towards the end of the monitoring period were due to plant interruptions and sampling blockages). The time at which the coil was de-activated was dictated by operational constraints, and it is not therefore known how much further the viscosity would have dropped had the coil remained on indefinitely. However, the rate of fall shows a distinct decline towards the end of the "on" period, and numerical extrapolation of the trend suggests a m i n i m u m viscosity value of about 90cP. This is a drop of 25% from the normal value of about 120cP, and about 1/3 of the difference between fresh and contaminated medium at that density - see Figure 2 (the contamination data for Figure 2 were obtained on a different day to the demagnetisation tests, and the viscosity at a density of 3036 k g / m 3 was somewhat higher in that case; the comparison is therefore not exact). The return to normal viscosity levels after the coil had been de-activated was rather erratic. It is not known why this should be, but clearly the magnetisation and demagnetisation are functions of the proportion of the circulating medium passing through the magnetic separators and demagnetising coil. In the present case, the initial rise in viscosity after the coil was switched off appeared to proceed at about the same rate as the initial decline. However after about 350 minutes, with some plant interruptions, the rise ceased. This might have been caused by a drop in ore feedrate and a corresponding decrease in the amount of ferrosilicon reporting to the dilute circuit. However, this is not known.
Demagnetising coils and their purpose In one of the author's experience (TJNM) there has long been a dispute as to whether demagnetising coils are necessary in a DM cyclone plant. The conventional wisdom has been
612
T.J. NAPIER-MUNNand I. A. SCOTT
that cyclones do not suffer the viscosity effects that DM baths suffer, and therefore that close control of medium viscosity is unnecessary. This is probably more true of coal preparation applications than mineral separations, which generally take place at higher densities and viscosities. Certainly, not all designers will install demagnetising coils in cyclone plants. It is now well established that cyclone performance in mineral applications can be a strong function of medium viscosity. However, the published literature on the influence of demagnetisation in this context is almost non-existent. The coils are generally seen by most operators of DM cyclone plants as non-critical to the operation and are often not utilised even if installed. This situation is compounded where there is a record of coll breakdown, usually due to short-circuiting by water following failure of the insulation, often caused by overheating [13]. Despite this, however, it is the authors' view that demagnetising coils should be installed and operated in DM cyclone plants because: .
It is now firmly established that elevated viscosities can have a deleterious effect on cyclone performance.
.
Sudden and unpredictable viscosity-related declines in cyclone performance can occur [14], and the use of coils would at least limit the probability of such events occurring.
.
The present work has shown that the coils do reduce the viscosity of circulating medium. CONCLUSIONS
Tests carried out in the Mount Isa Mines lead-zinc preconcentration dense medium cyclone plant have confirmed that normal contamination of the medium (principally by magnetic pyrrhotite) substantially increases the viscosity of the medium over that prevailing with fresh (uncontaminated) medium. T h i s increase can be reduced by about one third by running the demagnetising coils installed on the primary magnetic separator return lines. Computer simulation (not reported here) has shown that viscosity was not a critical parameter in the Mount Isa context (with current performance criteria). In general, however, viscosity is known to reduce the efficiency of DM cyclones, and it is therefore recommended that demagnetising coils be considered for all DM cyclone plants operating with ferrosilieon media. ACKNOWLEDGEMENTS This work was conducted with the cooperation and assistance of Mount Isa Mines Ltd., particularly the Manager and staff of the Lead-Zinc Concentrator. It was funded by a grant from the Australian Mineral Industries Research Association Ltd., for a collaborative research programme in mineral processing, of which Mount Isa Mines was sponsor. REFERENCES .
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