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Highly efficient magnetic separation using five-aligned superconducting bulk magnet

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2010 J. Phys.: Conf. Ser. 234 032015(http://iopscience.iop.org/1742-6596/234/3/032015)
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9th European Conference on Applied Superconductivity (EUCAS 09)
Journal of Physics: Conference Series 234 (2010) 032015

IOP Publishingdoi:10.1088/1742-6596/234/3/032015

Highly efficient magnetic separation using five-aligned
superconducting bulk magnet
Hiroyuki Fujishiro1, Takashi Miura1, Tomoyuki Naito1 and Hidemi Hayashi212Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
Kyushu Electric Power Co., Inc., Fukuoka 815-8520, Japan

E-mail: fujishiro@iwate-u.ac.jp
Abstract. We have constructed the highly efficient magnetic separation system using fivealigned superconducting bulk magnets, which has ten usable magnetic poles on both sides inopen space. We applied the bulk magnet system to the magnetic separation of ferromagnetic
particles (magnetite; Fe3O4) and paramagnetic ones (α-hematite; Fe2O3) dispersed in water forvarious average particle diameters d, flow speeds VF and initial concentrations C0 of theparticles. The multi-bulk magnet system has been confirmed to be effective for the magnetic
separation and the efficiency of the magnetic separation per one magnetic pole has been
estimated using the theoretical relation.

1. Introduction
The magnetic separation technique using permanent, electro and superconducting magnets has beenused to purify the dirty water for the environmental cleaning and to separate magnetic materials fromthe industrial waste. Recently, a superconducting bulk magnet, on which the trapped magnetic field BT
is as high as a few Tesla, has been developed because of the enhancement of superconducting
characteristics of the bulk materials and the improvement of magnetizing techniques. The maximum
trapped field BTFC of 17 T at 29 K has been achieved on the bulk by the field-cooled magnetization(FCM) using a superconducting magnet (SM) [1]. The investigations of the magnetic separation using
the superconducting bulk magnet as high as a few Tesla have been reported [2-4]. Under the magnetic
field B=µ0 H, the magnetic force FM for the spherical particles of b in radius with susceptibility χp in
the solvent with susceptibility χ f can be represented in the following [4],FM =9( " p # " f )
4 3
!b µ0
HgradH .
3(3 + " f )(3 + " p )(1)

In the equation, to increase the magnetic force FM using superconducting bulk magnets, it is necessary
to enhance not only the magnetic field H, but also magnetic field gradient (grad H) generated on thebulk. The increase of the number of bulk magnet is also an another solution to enhance the efficiencyof the magnetic separation. We have developed two- and five-aligned superconducting bulk magnet
system [5, 6]. The system has usable surface on both sides in open space, which can be realized onlyby the pulsed field magnetization (PFM) technique using a split-type copper coil. The PFM is asuitable technique to magnetize the superconducting bulk for practical applications because of anc 2010 IOP Publishing Ltd

1

9th European Conference on Applied Superconductivity (EUCAS 09)
Journal of Physics: Conference Series 234 (2010) 032015

IOP Publishingdoi:10.1088/1742-6596/234/3/032015

inexpensive and mobile experimental setup with no use of SM. However, the trapped field BTP by
PFM was pretty small, compared with BTFC because of the large temperature rise by the dynamical
motion of the magnetic flux. We have investigated the enhancement of BTP on the superconducting
bulk magnetized by PFM and have succeeded the highest field trapping of 5.20 T on the GdBaCuO
bulk of 45 mm in diameter at 30 K, and of 2.53 T on the vacuum sheath using a solenoid copper coil[7]. For the five-aligned superconducting bulk magnet system, the magnetic field higher than 2.3 T has
been attained on the vacuum sheath for each bulk, which has 10 usable magnetic poles on both sides
and has a potential for new practical applications [6].
In this paper, as a feasible experiment, we apply the five-aligned bulk magnet system to the highly
effective magnetic separation of ferromagnetic particles (magnetite; Fe3O4) and paramagnetic ones (αhematite; α-Fe2 O3) dispersed in water. The relation has been investigated between the magnetic
separation efficiency and the several parameters such as an average particle diameter d, a flow speed
VF and an initial particle concentration C0 dispersed in water.
2. Experimental procedure
Figure 1(a) shows the schematic view of experimental setup for the magnetic separation used in thisstudy. Figure 1(b) shows the structure of the five-aligned superconducting bulk magnet and theseparation container passing through the wastewater. The ferromagnetic Fe3 O4 particles (average
particle diameter d of 3.2 µm, 0.5 µm and 0.1 µm) were dispersed in water with initial concentrationsof C0 =400, 200, 100 ppm and were stirred by pumping and air bubbling in a tank of 20 litre. The
susceptibility χp of the Fe3 O4 particles was 2.5 x 10-4 (emu/mol/Oe) at 300 K, which was independentof the particle size. The flow speed VF was changed up to 5 L/min by controlling the valves of B1 andB2. Two separation containers made by acrylic resin were faced to the both sides of the five-aligned
bulk magnets and the magnetic separation was performed by an open gradient method. The purified
water passing the five magnetic poles was extracted from the valve B3 and the final purified water
after passing the ten magnetic poles was collected. In figure 1(b), the dimension of the separation
container is 80 mm in height and 30 mm in thickness. To make wastewater flow near the magnetic
poles effectively, the partition walls were inserted in the containers.
The magnetic separation experiment of paramagnetic hematite (α-Fe2O3) particles (d=1.0 µm) were&013&6

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Figure 1. (a) The schematic view of the magnetic separation system used in this study. (b) The
structure of the five-aligned superconducting bulk magnet system and the separation container passing
through the wastewater in which the partition walls were inserted.

2

9th European Conference on Applied Superconductivity (EUCAS 09)
Journal of Physics: Conference Series 234 (2010) 032015

IOP Publishingdoi:10.1088/1742-6596/234/3/032015

performed by the high gradient magnetic separation (HGMS) method using filters composed byferromagnetic fine wires, which can create high gradient magnetic field [8]. The concentration ofhematite powder was C0=500 ppm in water and the flow speed VF was changed up to 3 L/min. The
susceptibility χp of the α-Fe2O3 particles was 5.0 x 10-7 (emu/mol/Oe) at 300 K. In all the experiments,
the particle concentration C in purified water was measured by the visible spectrophotometer.
As shown in figure 1(b), c-axis oriented five rectangular-shaped GdBaCuO bulks (34 34 15
mm3, Nippon Steel Co., Ltd.) were tightly fastened with the copper bar from the side face (along theab-plane) and the bar was attached to the cold stage of a Gifford McMahon (GM) cycle helium
refrigerator [9]. An interval between the bulks was 60 mm. The temperature of the bulk was controlled
at 40 K. The pulse field of Bex=5.5 T ~ 6.0 T with a rise time of 12 ms was applied to the zero-fieldcooled bulks using a split-type copper coil dipped in liquid N2. After the completion of magnetizing abulk, the magnetizing coil was moved in parallel and another bulk was magnetized in turn in the samemanner. The successive pulse applications with identical strength (SPA) were performed to enhance
BTP [9]. The line scan profile of the magnetic field BT(x, z) was measured using an axial-type Hallsensor (F W Bell, BHA921) for the distance z=0, 3 and 6 mm from the vacuum sheath surface.
3. Theoretical estimation
When we assume the initial particle concentration in water as C0 and the magnetic separation
efficiency per one magnetic pole as k (0≤k≤1), the particle concentration C in purified water after
passing through the n-magnetic poles is defined as

C /C0 = (1" k) n .

(2)

The results of the calculation are indicated in figure 2. Even though the k value is as low as 0.3, thefinal C value can be decreased to 0.0282C0 for n=10. These results suggest that the weak!
ferromagnetic and/or nano-sized
ferromagnetic particles can be separated under weak FM using thesystem, and that the system can separate the ferromagnetic particles under large flow speed Vf.
4. Results and discussion
4.1. Characteristics of five-aligned bulk magnets
Figure 3(a) shows the line scan profiles of the trapped field BT(z) in open space, along the direction
through the centre of the bulk after the magnetization of the five bulks (#1~#5). The BT(z) profilechanges periodically and the maximum BT(z) value was 1.8 – 2.2 T at the vacuum sheath surface (z=0mm). The BT(z) value decreased with increasing z, and was 1.1 – 1.4 T at z=3 mm and 0.7 – 0.8 T at

Normalized Concentration C/C

0

100
10

C/C0=(1-k)n

-1

10-2
10

waste water-3

Separation rate
k=0.3
k=0.5
k=0.7
k=0.8
k=0.9

10-4
10-5

0

1

#1 #2 #3 #4 #5

Figure 2. The calculation of the normalized
concentration C/C0 as a function of the numberof the magnetic pole n for various magnetic
separation efficiencies, k, per one magneticpole.

#10 #9 #8 #7 #6 bulk

purified water

2
3
4
5
6
7
8
Number of magnetic pole n

9

10

3

9th European Conference on Applied Superconductivity (EUCAS 09)
Journal of Physics: Conference Series 234 (2010) 032015

z=0mm
z=3mm
z=6mm

(b)

200

dB/dx (T/m)

BT (T)

pole1~5

1.5
1.0

100
0
-100

0.5

#1

0
0

#2
50

#3

#4

100
150
200
Distance (mm)

#5
250

#1

-200
300

300

z=0mm
z=3mm
z=6mm

pole1~5

0

#2
50

#3

#4

100
150
200
Distance (mm)

(c)

250

300

pole1~5

100
0
-100
-200

#5

z=0mm
z=3mm
z=6mm

200
2

(a)

2.0

BdB/dx (T /m)

2.5

IOP Publishingdoi:10.1088/1742-6596/234/3/032015

-300

#1
0

#4

#5

100
150
200
Distance (mm)

250

#2
50

#3

300

Figure 3. The line scan profiles of (a) the trapped field BT(z), (b) the magnetic gradient dB(z)/dx
and (c) the B(z)dB(z)/dx values in open space, along the direction through the centre of the fivealigned bulk magnets from #1 to #5.
z=6 mm. Figures 3(b) and 3(c) show the dB/dx and BdB/dx profiles for the same bulk system. The
magnetic gradient dB/dx was as large as 150-200 T/m. The maximum BdB/dx was 250-300 T2/m and
50-70 T2/m at z=0 mm and z=6 mm, respectively, which contributes directly to FM as shown inequation (1). These specific values are about 10 times as large as those for the Nd-Fe-B permanent
magnet and are important parameters for the magnetic separation.
4.2. Magnetic separation of magnetite (Fe3 O4) particles
Figures 4(a) and 4(b) show the flow speed Vf dependence of the magnetite concentration C in purified
water passing through the 5 or 10 magnetic poles for various d of the Fe3O4 particles for the initialconcentration of C0=400 ppm and 100 ppm, respectively. In figure 4(a), the concentration C of the
Fe3 O4 particles with d=3.2 µm and d=0.5 µm decreases to 6 – 8 ppm in purified water passing throughthe 10 magnetic poles for Vf=1 L/min, and increases slightly with increasing Vf. The concentration C
for n=5 was higher than that for n=10. For the Fe3O4 particles with d=0.1 µm, however, the decrease in
the concentration C by the magnetic separation was not so large because of the smaller FM by smaller
d of the Fe3 O4 particles; the concentration C decreases to 170 ppm for n=5 and to 60 ppm for n=10 inpurified water for Vf=1 L/min. Since the susceptibility χp of magnetite powder was independent of d,
FM is proportional to d3 as shown in equation (1). As a result, the efficiency of the magnetic separation
should increase with increasing d. However, the efficiency was the highest for d=0.5 µm anddecreases for 3.2 µm and 0.1 µm in this order. The ratio of the efficiency for the d=0.1 µm, 0.5 µm
and 3.2 µm was (1/60):(1/6):(1/8) for n=10 and Vf=1 L/min. The discrepancy comes from theinsufficient mixing of the magnetite particles dispersed in water. In figure 4(b) for C0=100 ppm, the
400

400

(a) C =400ppm(b) C =100ppm
0Concentration C (ppm)Concentration C (ppm)

0

100

10

Magnetite (Fe3O4)101

d=3.2µm (n=5)
d=3.2µm (n=10)
d=0.5µm (n=5)
d=0.5µm (n=10)
d=0.1µm (n=5)
d=0.1µm (n=10)

2
3
4Flow speed Vf (L/min)

5

100

10
Magnetite (Fe3O4)
1

6

0

1

d=3.2µm (n=5)
d=3.2µm (n=10)
d=0.5µm (n=5)
d=0.5µm (n=10)
d=0.1µm (n=5)
d=0.1µm (n=10)

2
3
4Flow speed Vf (L/min)5

6

Figure 4. The flow speed Vf dependence of the magnetite (Fe3 O4) concentration C in purified
water passing through the 5 or 10 magnetic poles for various d of the magnetite particles forthe initial concentration of (a) C0 =400 ppm and (b) C0=100 ppm, respectively.

49th European Conference on Applied Superconductivity (EUCAS 09)
Journal of Physics: Conference Series 234 (2010) 032015

IOP Publishingdoi:10.1088/1742-6596/234/3/032015

similar Vf dependence of the concentration C can be seen.
4.3. Estimation of magnetic separation efficiency k per one magnetic poleIn this subsection, we estimate the magnetic separation efficiency k per one magnetic pole for the
typical experimental results using equation (2). Figures 5(a) and 5(b) show the normalized
concentration C/C0 as a function of the number of the magnetic pole, n, for the results of d=0.5 µm and
d=0.1 µm, respectively. In these figures, calculated results using equation (2) are also shown forvarious k values. In figure 5(a), the C/C0 for the condition (C0=100 ppm, Vf=5 L/min) changes inaccordance with the equation (2) and the k value was estimated to be 0.2. The results for otherconditions suggest that the k value was estimated to be 0.3-0.4 for n=5, but decreased slightly for n=10.
These results come from the structural imperfection of the magnetic separation container andinsufficient dispersion of the particles. In figure 5(b) for d=0.1 µm, the k values are smaller than thatfor d=0.5 µm; the k values of C0 =400 ppm for Vf=1 L/min and 5 L/min (n=10) are 0.18 and 0.08,
respectively, and, decreases with decreasing C0.
4.4. Magnetic separation of hematite (α-Fe2 O3) particles
Figure 6(a) shows the flow speed Vf dependence of the concentration C of α-Fe2 O3 particles (d=1.0µm) in purified water passing through the 5 or 10 magnetic poles for the initial concentration of
C0=500 ppm. In the high gradient magnetic separation (HGMS), the particles are filtered by fine wires
without magnetic field. The results of the filtering effect are also shown in figure 6(a), in which theconcentration C for n=10 at Vf=1 L/min decreases to 180 ppm without magnetic field and 60 ppm withmagnetic field, respectively. The concentration slightly increases with increasing Vf and for n=5.
Figure 6(b) shows the normalized concentration C/C0 as a function of the number of the magnetic pole.
The k value was estimated to be 0.11 for Vf=3 L/min and 0.18 for Vf=1 L/min with magnetic field.

0100
Normalized Concentration C/C

Normalized Concentration C/C

0

5. Summary
We have constructed the highly efficient magnetic separation system using five-aligned
superconducting bulk magnets, which have ten usable magnetic poles on both sides in open space. We
applied the system to the magnetic separation of ferromagnetic particles (magnetite; Fe3O4) andparamagnetic particles (α-hematite; Fe2O3) dispersed in water for various average particle diameters d,
flow speeds VF and initial concentrations C0 of the particles. Important experimental results andconclusions are summarized as follows.
1) The magnetic separation of magnetite with d=0.5 µm was the most effective and the separation
efficiency decreases with increasing Vf and with decreasing the number of the magnetic pole n.

k=0.1

(a)

k=0.2

10-1

magnetite 0.5µm
nC/C0=(1-k)

400ppm(1L/min)
100ppm(1L/min)
400ppm(5L/min)
100ppm(5L/min)

10-2

0

1

k=0.3
k=0.4

2
3
4
5
6
7
8
Number of magnetic pole n

9

10

100
(b)

k=0.05magnetite 0.1µm

k=0.1

n

C/C0=(1-k)

400ppm(1L/min)
100ppm(1L/min)
400ppm(5L/min)
100ppm(5L/min)

10-1

0

1

k=0.2

2
3
4
5
6
7
8
Number of magnetic pole n

910

Figure 5. The normalized concentration C/C0 as a function of the number of the magnetic polen for the results of (a) d=0.5 µm and (b) d=0.1 µm, respectively.

59th European Conference on Applied Superconductivity (EUCAS 09)
Journal of Physics: Conference Series 234 (2010) 032015
100

400
Normalized Concentration C/C

Concentration C (ppm)

0100Magnetite (Fe3O4)
101

k=0.030(b) C =100ppm

10

IOP Publishingdoi:10.1088/1742-6596/234/3/032015

d=3.2µm (n=5)
d=3.2µm (n=10)
d=0.5µm (n=5)
d=0.5µm (n=10)
d=0.1µm (n=5)
d=0.1µm (n=10)

2
3
4Flow speed Vf (L/min)

5

6

(b)
k=0.1

k=0.05

Hematite 1.0µm

C/C0=(1-k)nwithout mag. (1L/min)with mag. (1L/min)without mag. (3L/min)with mag. (3L/min)

10-1

0

1

k=0.2

2
3
4
5
6
7
8
Number of magnetic pole n

910

Figure 6. (a) The flow speed Vf dependence of the concentration C of hematite (α-Fe2 O3)particles (d=1.0 µm) in purified water passing through the 5 or 10 magnetic poles for the initialconcentration of C0 =500 ppm. The results without magnetic field are also shown. (b) The
normalized concentration C/C0 as a function of the number of the magnetic pole for Vf=1 and 3
L/min.
2) The experimental results roughly follow the equation (2). The magnetic separation efficiency k per
one magnetic pole can be estimated for each experimental condition using the relation.
3) The magnetic separation effect by the high gradient magnetic separation (HGMS) was confirmed
using paramagnetic α-Fe2 O3 particles.
4) The multi-bulk magnet system has been confirmed to be effective for the magnetic separation. The
higher efficient separation, the separation for higher Vf by the improved separation container andthe larger FM are in progress.
Acknowledgments
This work was partially supported by Japan Science and Technology Agency under a Research for
Promoting Technological Seeds 2009 (02-054).
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Supercond. 13 1592
[3] Hayashi H, Nagabuchi N, Saho N, Mizumori T and Asano K 2004 Physica C 412-414 766[4] Takeda S and Nishijima S 2007 IEEE Trans. Appl. Supercond. 17 2178
[5] Fujishiro H, Fujiwara A, Tateiwa T, Oka T and Hayashi H 2006 IEEE Trans. Appl. Supercond.
16 1080[6] Fujiwara A, Tateiwa T, Fujishiro H, Hayashi H, Nagafuchi T and Oka T 2006 Physica C 445448 399[7] Fujishiro H, Tateiwa T, Fujiwara A, Oka T and Hayashi H 2006 Physica C 445-448 334[8] Oberteufer J A 1973 IEEE Trans. Mag. MAG-9 436[9] Hiyama T, Fujishiro H, Tateiwa T, Naito T, Hayashi H and Tone K 2008 Physica C 468 1469

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