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Status of magnetic separation
STATUS OF M A G N E T I C SEPARATION* F. J. F R I E D L A E N D E R , M. TAKAYASU**, T. NAKANO*** Purdue University, West Lafayette, IN 47907, USA
and D. R. K E L L A N D Francis Bitter National Magnet Laboratory +, MIT, Cambridge, MA 02139, USA
After a brief review of high gradient magnetic separation the capture process and filter design for strongly magnetic particles--magnetite--is discussed. A method for selective capture of diamagnetic particles is described and experimental results are given.
1. Introduction and review
The retention or separation of solids through the use of magnetic forces has been a well-established art for most of this century. However, until the 1960's most of the applications were confined to the removal of materials with permanent magnetic moments, i.e. ferro- or ferrimagnetic materials, and even for those materials only pieces larger than some minimum size could be removed efficiently. Over the years there had been steady progress in the design of devices for the removal of such magnetic materials. Though the equations for the forces on magnetic dipoles had been formulated during the previous century, there had not been any major theoretical consideration of the overall magnetic separation process. Perhaps the largest breakthrough in this field occurred when High Gradient Magnetic Separation (HGMS) was introduced just over ten years ago. There have been various reviews of more recent developments [1]. The theoretical basis for the early analysis of H G M S filters from a physical point of view can be found in a paper on electrostatic separation by Zebel [2]. There and in HGMS, high gradients (and hence, large forces) are obtained by applying a uniform electric or magnetic field to a small diameter fiber (i.e. dielectric in one case, ferromagnetic in the other case). In most practical H G M S filters, a *Work supported in part by National Science Foundation, Department of Energy and a NATO Grant. **Permanent address: Mie University, Department of Electronic Engineering, Tsu City, Japan. ***Permanent address: Oita University, Department of Electrical Engineering, Oita City, Japan. *Supported by the National Science Foundation.
matrix of ferromagnetic steel wo,,L (with a packing fraction of the order of only 5%) is placed in a magnetic field produced by an iron-clad electromagnet or by a superconducting coil and the substance(s) to be captured are passed through the matrix in a gaseous or, more commonly, a liquid carrier. It is possible to remove particles that are weakly paramagnetic (and only of the order of a few micrometers in size) from a water slurry in filters currently commercially available. Most physical theories of such filters are based on an analysis of particle capture on a single ferromagnetic fiber, essentially in an extension of the work of Zebel [2] with the results extended to an array of fibers. Capture on a single fiber can occur in various configurations of fiber orientation relative to the applied field and particle flow and is usually idealized. Because of the difficulties of modeling the fluid flow, work still remains to be done in this area, especially on the extension from single fiber capture to an analysis of filter behavior. Until a few years ago almost all experimental work had been carried out on entire filters, making correlation between theory and experiment quite indirect and often inconclusive, with no direct evidence of the adequacy of the physical models and assumptions used. More recently a number of groups have presented experimental data on the dynamic buildup of particles on single fibers [3] and on particle trajectories in liquid [4, 5] and gaseous [6] carriers. All of this work had been on the collection of paramagnetic particles. Here the extremes of magnetic properties for magnetic separation will be considered. At one end of the scale the collection of large amounts of ferromagnetic solids from dense slurries has been
Journal of Magnetism and Magnetic Materials 15-18 (1980) 1555-1558 ©North Holland
1555
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F.J. Friedlaender et al./ Status of magnetic separation
accomplished. The mechanism of capture and the design of a matrix for this application will be discussed. At the other end of the scale, the selective capture of diamagnetic particles will be described in a system in which capture forces on diamagnetic particles have been enhanced to a level comparable to that achieved in paramagnetic separations.
2. Collection of strongly magnetic particles The collection of magnetite in a high gradient magnetic separator was first observed at M I T during research on the beneficiation of oxidized taconite iron ore [7]. A Mesabi (Minnesota) ore containing about 10% of magnetite by weight was observed to form dendrites along the magnetic field lines surrounding a 50 ~tm stainless steel wool strand such as are used as a matrix in a H G M S separator. This behavior viewed on a wire removed from the separator is shown in fig. 1 and ref. [8]. It is clear that particle collection occurs well beyond the region of magnetic field gradient which is caused by distortion of an otherwise uniform field by the presence of the wire. The concentration of the field by each newly captured magnetite particle provides a new capture site. Therefore the capacity of the matrix to collect material is much greater for ferromagnetic materials than for paramagnetic or diamagnetic particulates collected in the same matrix. In this photograph (fig. 1) hematite has been collected along with the magnetite and is concentrated closer to the wire although some appears collected along the dendrites. The magnetic field is in the plane of the photograph, transverse to the wire. There is apparently no collection in the near quadrant which is perpendicular to the field direction. It is in this quadrant that diamagnetic capture is observed. An application of H G M S - t y p e separators to the recovery of magnetite from heavy medium coalcleaning circuits was studied for the US Bureau of Mines Coal Preparation G r o u p (now Department of Energy) [9]. Magnetite is suspended in water to create an apparent specific gravity between that of coal and its impurities. Coal and magnetite and impurities with magnetite exit from the cleaning circuit and the magnetite is recovered from each
Fig. 1. An oxidized taconite iron ore containing magnetite which forms dendrites when collected on a 50/tin stainless steel wire. H = 0.8 T in the plane of the photograph and the magnification is 75 × . stream, very often by permanent magnet or electromagnet drum separators. The advantage of the H G M S separator when applied to the separation of highly magnetic material such as magnetite comes from the basic concept of the device. High field gradients are distributed throughout the working volume of the separator. The extent of these is small but, as shown before, their effect is extended by dendrite formation. Strong magnetic forces on magnetite particles hold them on the matrix even in high fluid flow rates. Therefore high material process rates are possible making H G M S an attractive alternative to conventional magnetic separation. N o n m a g netic coal particle entrapment is reduced because the material is collected a particle at a time instead of as a thick sludge layer. Some results of this H G M S application are listed in table 1. Very modest magnetic field values are required to recover magnetite even when the magnetite-coal stream is fed to the separator at high process rates. The separator was lab scale with a cross section of 31 cm 2. A throughput rate of 12 k g / s m 2 corresponds to a flow rate of about 20 c m / s with the slurry density 15.6% by weight.
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F. J. Friedlaender et al./ Status of magnetic separation
TABLE 1 Magnetite recoveryat high material throughput values in HGMS Magnetic field (kOe)
Materialr a t e (kg/sm 2)
Magnetitelost (%)
0.5 1.0 2.0 3.0 4.0 5.0 6.0
3.3 8.1 10.3 11.9 11.1 11.4 10.6
42.9 14.3 0.2 0.1 0.3 0.2 0.2
3. Matrix design for high loading The capture of magnetite in a H G M S device is accomplished at low field values. Of course, the capture occurs as a function of length along the matrix. In this case a short length is usually sufficient although for complete material recovery, matrix length is an important consideration. This is especially true if the average particle size is small. If the magnetite is mixed with a relatively large amount of coal the matrix must be an open structure to allow the highly dense slurry to pass through. Therefore, the capture probability, i.e., the probability of capture per unit length, is reduced. Compensation may be made by increasing the field strength or the matrix length. If the field strength is increased and the matrix design is limited to materials at hand, matrix plugging might occur because of high capture efficiency per unit length which results in the capture of most of the material at the beginning of the matrix. A simple way around this problem is to make a self-regulating matrix structure which will reach material capture saturation without completely impeding the slurry flow through the separator. The ideal structure would become completely filled with captured magnetite, thereby making optimum use of the magnetized volume on which is based the capital cost of the separator (and the power requirements). A structure of expanded metal screens was constructed with a space left for each screen to be bypassed by the slurry when it became filled with magnetite. Because the matrix was long, 60 cm, and the samples did not exceed 450 g of magnetite, we observed no "breakthrough." The best combi-
nation of high magnetite capture and low coal entrainment occurred at the highest flow rates and at the highest material throughput values.
4. Diamagnetic capture As has recently been demonstrated [10] the capture forces on diamagnetic particles can be greatly enhanced by dissolving a paramagnetic salt in a water slurry. If the carrier is made sufficiently paramagnetic, both diamagnetic particles, and paramagnetic particles with a lower per unit volume susceptibility than the liquid, will be captured in the "negative gradient" regions in which paramagnetic particles encounter repulsive forces.
Ros o_ 2.51
_
.-
--
a
i
20 E E
15
•~ .
Ho × f (cgsemu/cc/Oe)
io
~
15 2o 8.5
°i[_ j go
Fig. 2. Normalized saturation buildup radius Ras of CuO on 125 #m nickel wire as a function of susceptibility Xf of fluid, varied by changing concentration of MnCl2 dissolved in water used as carrier fluid. (a) and (b) show saturation collection as viewed by means of a TV system [3] for each case indicated. Applied field, H0 = 1 T; Average fluid velocity, vo = 0.72 cm/s.
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F. J. Friedlaender et al./ Status of magnetic separation
S i m u l t a n e o u s c a p t u r e of two different particle types, one of w h i c h is m o r e p a r a m a g n e t i c a n d the other less p a r a m a g n e t i c (e.g. d i a m a g n e t i c ) than the liquid, is also possible with each particle type b e i n g collected in the a p p r o p r i a t e region on the collecting fiber. Since the c a p t u r e force is p r o p o r t i o n a l to I X p - Xr[ = X where Xp a n d Xf are the p e r unit v o l u m e susceptibilities of the p a r t i c l e a n d the fluid, respectively, selective filtering is a c h i e v e d b y setting X equal to zero for those particles that are to be a l l o w e d to pass t h r o u g h the filter. T h e range of values of Xp over which X c a n be m a d e equal to zero is, of course, limited b y the solubility of the p a r a m a g n e t i c salt u s e d to c h a n g e Xf, a n d the value of the susceptibility of the p a r a m a g n e t i c ion in the salt. In fig. 2 the n o r m a l i z e d s a t u r a t i o n b u i l d u p , Ras of C u O particles is given as a f u n c t i o n of Xf, c a l c u l a t e d a c c o r d i n g to eq. (6) of ref. [10]. It s h o u l d be n o t e d that this m e t h o d is not o n l y suitable for selective filtering, as shown here, b u t c o u l d also be used as a m e a n s of m e a s u r i n g Xp Xf f r o m the zero crossing of Ra~, as has b e e n suggested for a s o m e w h a t different a r r a n g e m e n t
Ill]. T h e d a t a for fig. 2 were o b t a i n e d for the axial configuration, with the fiber (125 /~m d i a m e t e r nickel wire) axis p a r a l l e l to the flow a n d the field p e r p e n d i c u l a r to the wire axis. P h o t o g r a p h s of p a r t
of the TV m o n i t o r screen s h o w i n g p a r a m a g n e t i c a n d d i a m a g n e t i c collection, respectively, have been inserted a d j a c e n t to the d a t a points, see lower p a r t of fig. 2.
References [1] A good cross section of papers on HGMS can be found in the IEEE Trans. Magn. MAG-12, No. 5 (1976). [2] G. Zebel, J. Colloid Sci. 20 (1965) 522. [3] F. J. Friedlaender, M. Takayasu, J. B. Rettig and C. P. Kentzer, IEEE Trans. Magn. MAG-14 (1978) 1158. [4] F. Paul, D. Melville and S. Roath, IEEE Trans. Magn. MAG-15 (1979) 989. [5] H. Schewe, M. Takayasu and F. J. Friedlaender, IEEE Trans. Magn. MAG-16 (1980) in press. [6] R. R. Treat, W. F. Lawson and J. L. Johnson, J. Appl. Phys. 50 (1979) 3596. [7] E. Maxwell and D. R. Kelland, Dig. lntermag. Conf., London (1975). [8] F. J. Friedlaender and M. Takayasu, in: Industrial Applications of Magnetic Separation, ed. Y. A. Liu (IEEE No. 78CHI447-2MAG) p. 154. [9] D. R. Kelland and E. Maxwell, IEEE Trans. Magn. MAG-14 (1978) 401. [10] F. J. Friedlaender, M. Takayasu, T. Nakano and W. H. McNeese, IEEE Trans. Magn. MAG-15 (1979) in press. [11] Y. Zimmels and V. Yaniv, IEEE Trans. Magn. MAG-12 (1976) 359.
and D. R. K E L L A N D Francis Bitter National Magnet Laboratory +, MIT, Cambridge, MA 02139, USA
After a brief review of high gradient magnetic separation the capture process and filter design for strongly magnetic particles--magnetite--is discussed. A method for selective capture of diamagnetic particles is described and experimental results are given.
1. Introduction and review
The retention or separation of solids through the use of magnetic forces has been a well-established art for most of this century. However, until the 1960's most of the applications were confined to the removal of materials with permanent magnetic moments, i.e. ferro- or ferrimagnetic materials, and even for those materials only pieces larger than some minimum size could be removed efficiently. Over the years there had been steady progress in the design of devices for the removal of such magnetic materials. Though the equations for the forces on magnetic dipoles had been formulated during the previous century, there had not been any major theoretical consideration of the overall magnetic separation process. Perhaps the largest breakthrough in this field occurred when High Gradient Magnetic Separation (HGMS) was introduced just over ten years ago. There have been various reviews of more recent developments [1]. The theoretical basis for the early analysis of H G M S filters from a physical point of view can be found in a paper on electrostatic separation by Zebel [2]. There and in HGMS, high gradients (and hence, large forces) are obtained by applying a uniform electric or magnetic field to a small diameter fiber (i.e. dielectric in one case, ferromagnetic in the other case). In most practical H G M S filters, a *Work supported in part by National Science Foundation, Department of Energy and a NATO Grant. **Permanent address: Mie University, Department of Electronic Engineering, Tsu City, Japan. ***Permanent address: Oita University, Department of Electrical Engineering, Oita City, Japan. *Supported by the National Science Foundation.
matrix of ferromagnetic steel wo,,L (with a packing fraction of the order of only 5%) is placed in a magnetic field produced by an iron-clad electromagnet or by a superconducting coil and the substance(s) to be captured are passed through the matrix in a gaseous or, more commonly, a liquid carrier. It is possible to remove particles that are weakly paramagnetic (and only of the order of a few micrometers in size) from a water slurry in filters currently commercially available. Most physical theories of such filters are based on an analysis of particle capture on a single ferromagnetic fiber, essentially in an extension of the work of Zebel [2] with the results extended to an array of fibers. Capture on a single fiber can occur in various configurations of fiber orientation relative to the applied field and particle flow and is usually idealized. Because of the difficulties of modeling the fluid flow, work still remains to be done in this area, especially on the extension from single fiber capture to an analysis of filter behavior. Until a few years ago almost all experimental work had been carried out on entire filters, making correlation between theory and experiment quite indirect and often inconclusive, with no direct evidence of the adequacy of the physical models and assumptions used. More recently a number of groups have presented experimental data on the dynamic buildup of particles on single fibers [3] and on particle trajectories in liquid [4, 5] and gaseous [6] carriers. All of this work had been on the collection of paramagnetic particles. Here the extremes of magnetic properties for magnetic separation will be considered. At one end of the scale the collection of large amounts of ferromagnetic solids from dense slurries has been
Journal of Magnetism and Magnetic Materials 15-18 (1980) 1555-1558 ©North Holland
1555
1556
F.J. Friedlaender et al./ Status of magnetic separation
accomplished. The mechanism of capture and the design of a matrix for this application will be discussed. At the other end of the scale, the selective capture of diamagnetic particles will be described in a system in which capture forces on diamagnetic particles have been enhanced to a level comparable to that achieved in paramagnetic separations.
2. Collection of strongly magnetic particles The collection of magnetite in a high gradient magnetic separator was first observed at M I T during research on the beneficiation of oxidized taconite iron ore [7]. A Mesabi (Minnesota) ore containing about 10% of magnetite by weight was observed to form dendrites along the magnetic field lines surrounding a 50 ~tm stainless steel wool strand such as are used as a matrix in a H G M S separator. This behavior viewed on a wire removed from the separator is shown in fig. 1 and ref. [8]. It is clear that particle collection occurs well beyond the region of magnetic field gradient which is caused by distortion of an otherwise uniform field by the presence of the wire. The concentration of the field by each newly captured magnetite particle provides a new capture site. Therefore the capacity of the matrix to collect material is much greater for ferromagnetic materials than for paramagnetic or diamagnetic particulates collected in the same matrix. In this photograph (fig. 1) hematite has been collected along with the magnetite and is concentrated closer to the wire although some appears collected along the dendrites. The magnetic field is in the plane of the photograph, transverse to the wire. There is apparently no collection in the near quadrant which is perpendicular to the field direction. It is in this quadrant that diamagnetic capture is observed. An application of H G M S - t y p e separators to the recovery of magnetite from heavy medium coalcleaning circuits was studied for the US Bureau of Mines Coal Preparation G r o u p (now Department of Energy) [9]. Magnetite is suspended in water to create an apparent specific gravity between that of coal and its impurities. Coal and magnetite and impurities with magnetite exit from the cleaning circuit and the magnetite is recovered from each
Fig. 1. An oxidized taconite iron ore containing magnetite which forms dendrites when collected on a 50/tin stainless steel wire. H = 0.8 T in the plane of the photograph and the magnification is 75 × . stream, very often by permanent magnet or electromagnet drum separators. The advantage of the H G M S separator when applied to the separation of highly magnetic material such as magnetite comes from the basic concept of the device. High field gradients are distributed throughout the working volume of the separator. The extent of these is small but, as shown before, their effect is extended by dendrite formation. Strong magnetic forces on magnetite particles hold them on the matrix even in high fluid flow rates. Therefore high material process rates are possible making H G M S an attractive alternative to conventional magnetic separation. N o n m a g netic coal particle entrapment is reduced because the material is collected a particle at a time instead of as a thick sludge layer. Some results of this H G M S application are listed in table 1. Very modest magnetic field values are required to recover magnetite even when the magnetite-coal stream is fed to the separator at high process rates. The separator was lab scale with a cross section of 31 cm 2. A throughput rate of 12 k g / s m 2 corresponds to a flow rate of about 20 c m / s with the slurry density 15.6% by weight.
1557
F. J. Friedlaender et al./ Status of magnetic separation
TABLE 1 Magnetite recoveryat high material throughput values in HGMS Magnetic field (kOe)
Materialr a t e (kg/sm 2)
Magnetitelost (%)
0.5 1.0 2.0 3.0 4.0 5.0 6.0
3.3 8.1 10.3 11.9 11.1 11.4 10.6
42.9 14.3 0.2 0.1 0.3 0.2 0.2
3. Matrix design for high loading The capture of magnetite in a H G M S device is accomplished at low field values. Of course, the capture occurs as a function of length along the matrix. In this case a short length is usually sufficient although for complete material recovery, matrix length is an important consideration. This is especially true if the average particle size is small. If the magnetite is mixed with a relatively large amount of coal the matrix must be an open structure to allow the highly dense slurry to pass through. Therefore, the capture probability, i.e., the probability of capture per unit length, is reduced. Compensation may be made by increasing the field strength or the matrix length. If the field strength is increased and the matrix design is limited to materials at hand, matrix plugging might occur because of high capture efficiency per unit length which results in the capture of most of the material at the beginning of the matrix. A simple way around this problem is to make a self-regulating matrix structure which will reach material capture saturation without completely impeding the slurry flow through the separator. The ideal structure would become completely filled with captured magnetite, thereby making optimum use of the magnetized volume on which is based the capital cost of the separator (and the power requirements). A structure of expanded metal screens was constructed with a space left for each screen to be bypassed by the slurry when it became filled with magnetite. Because the matrix was long, 60 cm, and the samples did not exceed 450 g of magnetite, we observed no "breakthrough." The best combi-
nation of high magnetite capture and low coal entrainment occurred at the highest flow rates and at the highest material throughput values.
4. Diamagnetic capture As has recently been demonstrated [10] the capture forces on diamagnetic particles can be greatly enhanced by dissolving a paramagnetic salt in a water slurry. If the carrier is made sufficiently paramagnetic, both diamagnetic particles, and paramagnetic particles with a lower per unit volume susceptibility than the liquid, will be captured in the "negative gradient" regions in which paramagnetic particles encounter repulsive forces.
Ros o_ 2.51
_
.-
--
a
i
20 E E
15
•~ .
Ho × f (cgsemu/cc/Oe)
io
~
15 2o 8.5
°i[_ j go
Fig. 2. Normalized saturation buildup radius Ras of CuO on 125 #m nickel wire as a function of susceptibility Xf of fluid, varied by changing concentration of MnCl2 dissolved in water used as carrier fluid. (a) and (b) show saturation collection as viewed by means of a TV system [3] for each case indicated. Applied field, H0 = 1 T; Average fluid velocity, vo = 0.72 cm/s.
1558
F. J. Friedlaender et al./ Status of magnetic separation
S i m u l t a n e o u s c a p t u r e of two different particle types, one of w h i c h is m o r e p a r a m a g n e t i c a n d the other less p a r a m a g n e t i c (e.g. d i a m a g n e t i c ) than the liquid, is also possible with each particle type b e i n g collected in the a p p r o p r i a t e region on the collecting fiber. Since the c a p t u r e force is p r o p o r t i o n a l to I X p - Xr[ = X where Xp a n d Xf are the p e r unit v o l u m e susceptibilities of the p a r t i c l e a n d the fluid, respectively, selective filtering is a c h i e v e d b y setting X equal to zero for those particles that are to be a l l o w e d to pass t h r o u g h the filter. T h e range of values of Xp over which X c a n be m a d e equal to zero is, of course, limited b y the solubility of the p a r a m a g n e t i c salt u s e d to c h a n g e Xf, a n d the value of the susceptibility of the p a r a m a g n e t i c ion in the salt. In fig. 2 the n o r m a l i z e d s a t u r a t i o n b u i l d u p , Ras of C u O particles is given as a f u n c t i o n of Xf, c a l c u l a t e d a c c o r d i n g to eq. (6) of ref. [10]. It s h o u l d be n o t e d that this m e t h o d is not o n l y suitable for selective filtering, as shown here, b u t c o u l d also be used as a m e a n s of m e a s u r i n g Xp Xf f r o m the zero crossing of Ra~, as has b e e n suggested for a s o m e w h a t different a r r a n g e m e n t
Ill]. T h e d a t a for fig. 2 were o b t a i n e d for the axial configuration, with the fiber (125 /~m d i a m e t e r nickel wire) axis p a r a l l e l to the flow a n d the field p e r p e n d i c u l a r to the wire axis. P h o t o g r a p h s of p a r t
of the TV m o n i t o r screen s h o w i n g p a r a m a g n e t i c a n d d i a m a g n e t i c collection, respectively, have been inserted a d j a c e n t to the d a t a points, see lower p a r t of fig. 2.
References [1] A good cross section of papers on HGMS can be found in the IEEE Trans. Magn. MAG-12, No. 5 (1976). [2] G. Zebel, J. Colloid Sci. 20 (1965) 522. [3] F. J. Friedlaender, M. Takayasu, J. B. Rettig and C. P. Kentzer, IEEE Trans. Magn. MAG-14 (1978) 1158. [4] F. Paul, D. Melville and S. Roath, IEEE Trans. Magn. MAG-15 (1979) 989. [5] H. Schewe, M. Takayasu and F. J. Friedlaender, IEEE Trans. Magn. MAG-16 (1980) in press. [6] R. R. Treat, W. F. Lawson and J. L. Johnson, J. Appl. Phys. 50 (1979) 3596. [7] E. Maxwell and D. R. Kelland, Dig. lntermag. Conf., London (1975). [8] F. J. Friedlaender and M. Takayasu, in: Industrial Applications of Magnetic Separation, ed. Y. A. Liu (IEEE No. 78CHI447-2MAG) p. 154. [9] D. R. Kelland and E. Maxwell, IEEE Trans. Magn. MAG-14 (1978) 401. [10] F. J. Friedlaender, M. Takayasu, T. Nakano and W. H. McNeese, IEEE Trans. Magn. MAG-15 (1979) in press. [11] Y. Zimmels and V. Yaniv, IEEE Trans. Magn. MAG-12 (1976) 359.