Physica C 471 (2011) 1506–1510

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Physica C
journal homepage: www.elsevier.com/locate/physc

Magnetic separation technique for groundwater by ﬁve HTS melt-processedbulk magnets arranged in a line
T. Oka a,⇑, H. Seki a, T. Kimura a, D. Mimura a, S. Fukui a, J. Ogawa a, T. Sato a, M. Ooizumi a, H. Fujishiro b,
H. Hayashi c, K. Yokoyama d, C. Stiehler eaNiigata University, 8050 Ikarashi-Nino-Cho, Nishi-ku, Niigata 950-2181, Japan
Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
Kyushu Electric Co., 2-1-47 Shiobaru, Minami-ku, Fukuoka 815-8520, Japand
Ashikaga Institute of Technology, 268-1 Ohmae-cho, Ashikaga, Tochigi 326-8558, JapaneIFW Dresden, Helmholzstr., 20-01069 Dresden, Germanybca r t i c l e

i n f oArticle history:
Available online 13 May 2011
Keywords:
Magnetic separation
Trapped ﬁeld magnet
Bulk superconductor
Magnetic ﬁeld generatora b s t r a c t
A magnetic separation study for groundwater puriﬁcation has been practically conducted by using themulti-pole magnet system. The magnetic pole was composed of 10 open magnetic spaces by arranging
ﬁve HTS melt-processed bulk magnets in a line in a vacuum sheath. The individual bulk magnets wereactivated by feeding intense pulsed magnetic ﬁelds up to 6 T. The magnetic ﬁeld distribution was estimated with respect to various pole arrangements. The actual groundwater samples of Sanjo City were
processed so as to form large precipitates by adding the coagulant and pH controlling. The maximum separation ratio of the iron-bearing precipitates has exceeded over 70% when slurry water was exposed to
10 magnetic poles of up to 2.5 T at a ﬂowing rate of less than 4.8 l/min. An obvious attraction of ﬂocks
to the magnetic poles was observed even when the water contains no magnetite powder at the ﬂow rateof 1.01 l/min. This implies the validity of the multi-pole magnet system with respect to the actual application to water puriﬁcation.
Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction
In Sanjo city in Japan, the groundwater has been used for incinerator furnace coolant in the waste material disposal works. Fig. 1
shows a schematic illustration of the conventional disposal furnace(a) and the water puriﬁcation process (b), respectively. Exhaust gasof 1200 K which is generated from burning waste materials iscooled to 600 K by gushing water before emitting it out of furnace.
The groundwater in this district contains Fe element with high concentration of around 14 ppm, as shown in the ﬁgure. As the furnace
would have been seriously damaged if the water nozzles would be
stopped up by accumulating rust, the water must be kept clearer
than usual to reduce the number of times of maintenance. As the
puriﬁcation processes requires the additives and coagulants, it has
been needed to propose more efﬁcient and cheaper ways than thoseat present. In the study, a magnetic separation technique using hightemperature superconductingbulk magnets (hereafter abbreviatedas HTS bulk magnets or bulk magnets) has been investigated toadapt them to the groundwater puriﬁcation process.
⇑ Corresponding author. Address: Faculty of Engineering, Niigata University, 8050
Ikarashi-Nino-Cho, Nishi-ku, Niigata 950-2181, Japan. Tel.: +81 25 262 7668; fax:
+81 25 262 7010.
E-mail address: okat@eng.niigata-u.ac.jp (T. Oka).
0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2011.05.226

The melt-processed HTS bulk magnet systems in cooperation
with small-sized refrigerators are characterized as intense magnetic ﬁeld generators [1–3]. An intense magnetic ﬁeld generator
which contains ﬁve bulk magnets in a line was prepared for thisstudy. As Fujiwara et al. [4] reported in 2006, it emitted an intense
ﬁeld of 1 T in the open space outside the vacuum chamber.
Since it is difﬁcult to activate the multi-pole bulk magnets bymeans of the ﬁeld cooling method, the pulsed ﬁeld magnetization
technique (PFM) are conducted in this experiment. PFM techniqueis well known to be easy and compact way to activate bulk magnets in comparison with the ﬁeld cooling method which requires
large scale superconducting solenoid magnets [5]. In the past, various PFM techniques have been investigated to let the bulk magnets trap intense magnetic ﬁelds. Fujiyama et al. [6] discussed on
the ﬁeld trapping property of inhomogeneous bulk materials.
Fujishiro et al. [7] reported that the performance of trapped magnetic ﬁeld reached 5.2 T by applying multi-pulse ﬁelds in the process of stepwise temperature falls. Ida et al. [8] attempted toactivate the bulk magnets by the method in which the bulk magnets were sandwiched by vortex-type pulse coils settled face-toface, and showed that the vortex type coil was effective to obtainthe conical trapped ﬁeld distribution. Hiyama et al. [9] reported
that ﬁve-aligned Gd-based bulk magnets were successfully activated by pulsed ﬁelds with use of sprit-type pulse coils. Fujishiro

T. Oka et al. / Physica C 471 (2011) 1506–1510

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Fig. 1. Schematic illustration of waste material disposal furnace (a) and water puriﬁcation process of groundwater (b).

et al. [10] attempted to apply this magnet system to the magnetic
separation technique. In this experiment, we have adopted thisequipment to purify the groundwater and to show the effectiveness in the practical environmental issue.
Fig. 1b shows a conventional groundwater puriﬁcation process.
The precipitates containing Fe element are condensed and ﬁlteredin the process. The clear water with Fe concentration of 1 ppm is
directly showered in the furnace through the ﬂush nozzle ‘‘waterjet 2’’ in Fig. 1b. Beside this main process, the disposed slurry
which still includes water with high concentration is ﬁltered again
and led in the furnace through ‘‘water jet 1’’. The Fe concentrationof the ﬁltered water through this sub process is estimated to 20–
240 ppm. The experiment was operated to purify the water indicated as ‘‘water jet 1’’.
We know a couple of magnetic separation systems. One is thehigh gradient magnetic separation (HGMS, Okada et al. [11]) andthe other is open gradient magnetic separation (OGMS, Fukui
et al. [12]). It has been reported by Oka et al. that the Fe in the
groundwater was effectively removed by HGMS technique withuse of face-to-face HTS bulk magnets, in which the iron balls of
3 mm in diameter were installed in a water channel between themagnetic poles emitting magnetic ﬁeld over 2 T [13].
As mentioned above, Fujishiro et al. [10] reported that the Feoxide powder mixed beforehand in the water has been effectively
removed by passing it through the multi-pole magnet which contains ﬁve activated bulk magnets. On the other, Oka et al. [13] clariﬁed that HGMS technique with use of the bulk magnets is effective
to purify the groundwater. As we know there are a lot of application candidates of magnetic separation techniques and we must
conﬁrm the validity of the technique on the view point of eachpractical application, we attempted to adopt the multi-pole magnet system to the groundwater puriﬁcation to verify if the technique is practical or not.
In this discussion, we deal with the performances on the separation ratio of Fe against the ﬂowing rate of groundwater whichcontains the magnetized ﬂocks which are formed by adding theﬁne Fe oxide powder after the former magnetic separation experiment which was performed with use of ﬁve-aligned bulk magnets
system [10].

condition from outside. They were cooled by Gifford McMahon
refrigerator (AISIN SEIKI Co. Ltd., type GD-251) and then activatedby the PFM technique using a split-type pulse coil. The pulse coil
was settled on the both sides of the sheath to sandwich it, asshown on the right hand side in Fig. 2.
As shown in Fig. 3, the c-axis oriented Gd–Ba–Cu–O bulk magnets of rectangular-shape (34  34  15 mm3, Nippon Steel Co.,
Ltd.) were embedded in the thermal conduction bar of copper,
which tightly ﬁxed the bulk samples from their side faces whichwere perpendicular to the a–b plane of the crystal. A distance between each bulk magnet was designed to be 25 mm. The conduction bar was connected to the cold head of GM refrigerator andcooled to 30 K.
The sheath-type magnetic pole is activated by PFM. The activation was performed by applying pulsed ﬁelds of 5.5–5.8 T with arise time of 12 ms from the 60 mF condenser bank by using a
split-type copper coil, which is dipped in liquid nitrogen. The
pulsed magnetic ﬁeld was applied just once for every bulk magnet
in the following order, applied magnetic ﬂux densities and discharging voltages; #1(5.8 T, 700 V), #5(5.2 T, 600 V), #4(5.5 T,
650 V), #2(5.8 T, 700 V), and #3(5.8 T, 700 V) [9]. After each magnetizing for a bulk magnet, the magnetizing coil was shifted sideward along the sheath, and next bulk was activated in turn in thesame manner. In addition, the direction of ﬁeld application waschosen in two cases. Five bulk magnets were magnetized in the

2. Experimental
Fig. 2 shows a photo of experimental setup of the bulk magnet
system used in this study [4,9,10]. Five HTS bulk magnets were installed in the vacuum sheath with keeping thermally insulated

Fig. 2. Strong ﬁeld generator containing ﬁve-aligned bulk magnets in the sheathand magnetizing coil to activate them.

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Fig. 3. Schematic illustrations of bulk magnet system and magnetizing apparatus to apply pulsed ﬁelds.

same magnetic poles and alternating poles by changing current
directions on their magnetizing processes. The strength of thetrapped magnetic ﬁelds and their distributions were precisely
measured by scanning a Hall sensor (F.W. Bell, BHT921) at the surface of the vacuum sheath for the direction normal to the sheathsurface, and analyzed by three dimensional and line scan proﬁles.
The magnetic ﬂocks were formed by adding poly-aluminum
chloride (PAC) and coagulants to the groundwater with the concentrations of 700 ppm and 0.5–1%, respectively. The magnetite
ﬁne powder of 50 ppm was added to the water and ﬂocks to givethem ferromagnetic property. The performance of magnetic separation ratio was compared to the cases with and without magnetite
addition.
Fig. 4 shows the structure of magnetic separation channels inwhich the groundwater ﬂows. A couple of separation channels
made of acrylic resin were settled face-to-face on the both sidesof the vacuum sheath which emits magnetic ﬁelds on its surface.
The magnetic separation was performed by an open gradientmethod, as shown in Fig. 4. The dimensions of the water channels
were 80 mm in height and 30 mm in thickness. In order to let thewater ﬂow near the magnetic poles, the partitions were settled in
the channels. The water and ﬂocks were thoroughly stirred and led
to the water channels. The ﬂow rates were controlled up to 4.8
l/min by the valves of B1 and B2. The puriﬁed water passing thefront water channel was derived from the valve B3 after undergoing ﬁve magnetic poles and the ﬁnally puriﬁed water was drained
from the back channel after passing 10 magnetic poles.
The puriﬁed water was precisely examined in terms of the concentration of Fe ions by the inductively coupled plasma (ICP) analysis. The separation ratios S were estimated against the ﬂow rates
of water, where S was deﬁned as S = (c0 c)/ c0. c0 was iron concentration in the sample water with ﬂocks just before the experimentand c was that at each ﬂow rate.

Fig. 4. Schematic structure of magnetic separation experiment with use of a coupleof water channels which are attached beside the multi-pole magnetic sheath.

3. Results and discussion
Fig. 5a shows the three-dimensional magnetic ﬁeld distribution
when ﬁve bulk magnets were activated to the same magnetic polesin the sheath. The proﬁle of the ﬁeld distribution shows cone-shapes
and the maximum magnetic ﬁelds were located at each center of thebulk surfaces. The maximum value of them was 2.2 T at the surface
out of #4 bulk magnet, shown in the ﬁgure. The shape of trapped
ﬁeld strongly reﬂects the rectangular shape of the bulk magnets. A
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Fig. 5. Three dimensional distribution of trapped magnetic ﬁeld on the sheath surface (a), and cross sectional distribution with same and alternating magnetic polearrangement (b).

Fig. 5b. In the ﬁgure, plotted are the data obtained in both cases ofsame and alternating magnetic poles, respectively. One can see thatexcellent trapped ﬁelds in absolute values were obtained by alternating magnetic pole arrangement. The maximum value was 2.5 T
at #1 bulk magnet. It is inferred that the distance between the bulk
magnets, which was designed as 25 mm, was not enough to eliminate the returning path of the ﬂux lines. Therefore the trapped ﬁeld
for alternating pole arrangement was slightly enhanced in comparison with same magnetic pole arrangement. For alternating poles,
the trapped ﬂux along the c-axis increases with decreasing the distance between the magnets located side by side. In contrast, thesame magnetic poles arranged in a line must be placed apartto avoid their mutual interferences. Furthermore, the distributionof parallel direction to the sheath surface must be estimated to discuss on the performance of the total separation system.
A photograph of the experiment is shown in Fig. 6. The groundwater and ﬂocks were poured to the water channel from the righthand side of the picture, and ﬂew out after passing through thespaces in front of ﬁve magnetic poles. When another water channel
was attached on the back side of the sheath, the water undergoes
the magnetic ﬁeld spaces for 10 times. One can easily see thatthe ﬂocks were attracted to the magnetic poles, reﬂecting theirrectangular shape. It is obvious that the dark-colored water shownat the beginning changed clearer and clearer as the water undergoes the magnetic ﬁeld spaces in the channel.

Fig. 7 shows the separation ratios of Fe element against the ﬂow
rates of the water and ﬂocks as functions of the magnetite concentration and the number of the times of magnetic poles the water
passes. A couple of cases were examined with respect to the sameand alternating pole arrangements, respectively, as shown in
Fig. 7a and b. The data measured after passing ﬁve-poles at thefront and back sides of the sheath were also plotted in Fig. 7b,

Fig. 6. Photo of the experiment by open gradient magnetic separation technique.

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Fig. 7. Flow rate dependence of separation ratio for same (a) and alternating (b) magnetic pole arrangement.

Fig. 8. View of the magnetic separation experiment for the ﬂocks with nomagnetite powder addition with a ﬂowing rate of 1.01 l/min.

respectively. Although it is obvious that the separation ratios decrease with increasing ﬂowing rates in every case, the performances are quite different between the cases whether themagnetite powder was added to the ﬂocks or not. We must notethat there are no substantial differences between the cases inwhich the bulk magnets are activated in the same and alternating
pole arrangements. This may be attributed to the fact that thetrapped ﬁelds did not show large differences enough to be reﬂectedto the separation performance. However, when the distance between the magnets is designed to be closer and the magnets arealternately activated, the magnetic ﬁeld would be highly improved.
Fig. 8 shows a view of separation experiment operated withoutmagnetite addition. One can see apparent adhesion of the ﬂocks oneach magnetic pole when the ﬂow rate is 1.01 l/min with use of 10
magnetic poles. This phenomenon corresponds to the separation
ratio of 56.3%, as plotted in Fig. 7b. To observe sharply, we would
see a precipitation of ﬂocks on the bottom of the channel. Thismeans that some improvements on the design of water channels
would easily enhance the separation performances in near future.
We believe that 10-time applications of such strong magnetic
ﬁelds to the water enable us to develop a novel water puriﬁcation
technique with high efﬁciency.
4. Conclusions
The magnetic separation experiment was carried out with respect to the practical environmental issue with use of newly constructed multi-pole magnetic ﬁeld generator containing ﬁvealigned Gd-based bulk magnets. The PFM was operated by the

split-type coils, and strong magnetic ﬁeld of 2.5 T was generated
at the sheath surface. Aligned bulk magnets arranged side by sidemutually affected to each trapped ﬁeld distribution even in thecase that the distance between the bulk magnets was designed
as 25 mm. This means that the trapped ﬁeld must be improved
when the distance is shortened and the magnets are alternately
activated.
The separation ratios of the magnetic ﬂocks which contain magnetite powder of 50 ppm have remained high performances of over
70% for under the ﬂow rates less than 5 l/min. Although an additionof magnetite was dominant on the separation ratios rather thanother parameters like ﬂow rates or number of poles, we have observed an obvious attraction of ﬂocks to the multi-pole magnets
even when the water contains no magnetite powder at all. This implies that the further improvement on the water channel design
must bring us the practical performance of water puriﬁcation in actual environmental issues.
Acknowledgements
This work has been conducted in cooperation with Sanjo Cleaning Centre. The authors appreciate their help. In this experiment,
Mr. C. Stiehler joined our activity from Dresden IFW. The authors
thank Prof. B. Holzapfel and Dr. K. Iida in IFW for their advice
and cooperation.
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