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Magnetic separation using superconducting magnets
Physica C 357±360 (2001) 1272±1280
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Magnetic separation using superconducting magnets Takeshi Ohara a,b,*, Hiroaki Kumakura a, Hitoshi Wada a a
Tsukuba Magnet Laboratory, National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 16 October 2000; accepted 9 November 2000
Abstract Since the 1970s, magnetic separation has been increasingly used for puri®cation of liquids, such as heavy-metal ion removal from laboratory waste water, puri®cation of kaolin clay in the paper-coating industry, waste water recycling in the steel industry, and recycling of glass grinding sludge in cathode-ray tube polishing factories. In the 1980s, large superconducting magnets were adopted for the ®eld coils of high-gradient magnetic separation systems used for kaolin clay puri®cation. After the development of liquid-helium-free superconducting magnets in the 1990s, new national research projects were initiated in Japan to expand the application of superconducting magnetic separation to other areas. This paper discusses some of the fundamental characteristics of magnetic separation, the progress of superconductorization (use of superconducting magnets as ®eld coils in magnetic separation systems) and recent R&D activities at the National Research Institute for Metals in Japan. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 07.90.c; 47.55Kf.c; 83.50.Yt; 83.70Hq; 89.20.a Keywords: Magnetic separation; Superconducting magnet; High-gradient magnetic separation; Bi-2223 magnet
1. Introduction Recent progress in magnet technology has realized economically and operationally favorable cryocooler-cooled superconducting magnets with excellent operability, which can be used for commercial applications. Consequently, the dream of widespread use of superconducting magnets is being realized. In particular, magnetic separation is a promising application of large area, strong
* Corresponding author. Address: Tsukuba Magnet Laboratory, National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. Fax: +81-298-59-5602. E-mail address: [email protected] (T. Ohara).
magnetic ®elds, and in addition to industrial applications, can also be used to remove biogenic and anthropogenic pollutants from aquatic systems and industrial waste from streams. The fullscale application of magnetic separation processes will greatly contribute to preserving the global environment. In magnetic separation, magnetic forces act directly on ®ne particles to be separated. The force vanishes when the externally applied magnetic ®eld is turned o, so that the system can be continuously regenerated by cycling it on and o. Unlike conventional ®ltration methods that use blockingtype ®ltration, secondary waste is not produced in high-gradient magnetic separation (HGMS, also known as magnetic or electromagnetic ®ltration) systems. Furthermore, because HGMS systems use
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 5 3 0 - 5
T. Ohara et al. / Physica C 357±360 (2001) 1272±1280
much higher magnetic forces than conventional magnetic separation techniques, it can also be used to rapidly separate large quantities of dilute suspensions. In this paper, we discuss the principles of magnetic separation, and the historical development of commercial HGMS systems. 2. Principle of use 2.1. Three elements to enhance magnetic forces When ®ne particles are dispersed in air, water, sea water, oil, organic solvents, etc., their separation or ®ltration by using a magnetic force is called magnetic separation. To increase the separation eciency of these systems we must increase the magnetic force acting on particles by increasing the particle volume, relative magnetization between the particles (dispersoid) and the dispersion medium, and/or the magnitude of the magnetic ®eld gradients [1±3]. To understand the reason for this, consider the principle of magnetic forces. By calculating the gradient of the magneto-static energy dierence between magnetized particles of volume, Vp , and the dispersion medium of the same volume, DUp , the magnetic force on a particle, F m , is Fm
r DUp Vp l0 M p H r 2
Vp l0 M f H ; 2
n x; y; z;
2
where M H0
9 vp vf 3 vp 3 vf
Eq. (2) indicates that the magnetic force on particles depends on three key parameters: Vp , M , and rn H . For Fmn 6 0, Vp , M , and rn H are all non-zero. Therefore, Eqs. (2) and (3) indicate that to generate a magnetic force (Fmn 6 0), a dispersoid with susceptibility dierent from that of the dispersion medium (i.e., vp 6 vf ) must be placed in an inhomogeneous magnetic ®eld (i.e., rn H 6 0). When the dispersed particle is weakly magnetized, such as red blood cells, M vp vf H0 , and Fmn is proportional to the susceptibility difference between the dispersoid (red blood cell) and the dispersion medium (plasma); to H0 ; and to rn H . When the dispersoid is a ferromagnetic particle, Mp becomes saturated at a relatively low magnetic ®eld strength. The enhancement of Fmn through the use of strong magnetic ®elds is therefore limited. (Mp also becomes saturated at relatively small susceptibilities of vp 10 because Mp is proportional to vp = 1 N vp , where N is a demagnetizing factor.) Eective ways of enhancing the magnetic force include: (A) increasing Mp by ``magnetic seeding'' of the dispersoid, (B) increasing rn H , (C) using high intensity magnetic ®elds, and (D) selecting a dispersion medium with a large value of (vp vf ). In Section 2.2 we describe options (A), (B) and (C) for enhancing the magnetic force. Option (D) is applicable only when weakly magnetized particles are used, and we do not discuss this in this paper.
1
where Mp is the particle magnetization, Mf is the magnetization of the dispersion medium, and H is the magnetic ®eld. By assuming a spherical particle with volume magnetic susceptibility, vp , and uniform magnetization Mp , we obtain Fmn Vp l0 M rn H ;
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3
and where vf is the volume magnetic susceptibility of the dispersion medium, M is the relative magnetization between the dispersoid and the dispersion medium, and H0 is the applied magnetic ®eld.
2.2. Generation of strong magnetic forces for practical uses Magnetic separation is an old technology, used to enrich iron ore and iron sands for over 150 years. Various devices have been used to generate strong magnetic forces, which are described in detail elsewhere [2±7]. In this paper we provide a brief summary of this technology. Magnetic ®eld gradients are produced from inhomogeneous magnetic ®elds, created either by arranging a series of permanent magnets with alternating orientation of the poles, or by applying a magnetic ®eld to a grid of ferromagnetic rods. Near the rods the magnetic ®eld is altered, creating magnetic ®eld gradients. Such technology is used
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in rotating-drum and rotating-disk magnetic separators. Rotating-drum magnetic separators are mainly used in mines. A good example of this technology is a magnetic separator operated in one of the largest anatase TiO2 mines in the world, located in Tapira, Minas Gerais, Brazil, used to remove ferromagnetic impurities in the ore. Other magnetic separators are used to enrich Ti after heat treatment. A rotating-disk magnetic separator [7] has a simple rotating steel disc that contains rod-shaped permanent magnets, so that the magnetic poles may be arranged with consistent intervals on the disc surface. This is used in so-called ferritic processes [7,8] to remove heavy metal ions, such as Fe, Mn, Zn, Ni, Co, Cu, and about 40 other elements, from waste water discharged from some university laboratories in Japan. This is an improved method of magnetic seeding, in which the substance to be separated is absorbed into ferromagnetic ferrite by using chemical reactions that form spinel ferrite, and the reacted ®ne ferrite particles are recovered by using a rotary disc magnetic separator. Other methods for increasing the magnetic ®eld gradient are to use superconducting coils in a drum-type separator [9], multi-pole superconducting coil separators [10], or HGMS systems [11±14]. In HGMS systems a magnetic ®eld is applied to a grid of thin magnetic wires. Other than through the use of HGMS, the induced magnetic force is not strong enough to separate particles smaller than several hundred microns, because under practical conditions, in these magnetic separators the drag and gravitational forces counteract the magnetic forces. Therefore, the volume of particles must be increased prior to magnetic separation, either by using ¯occulants, heat treatment to increase the quantity of the magnetic substance (used in ore sorting), or magnetic seeding of the slurry (used in the ferritic process described above). To enhance the separation process, chemical, physical, electrochemical, or biological techniques are used to increase the magnetism and/or the volume of the dispersoid. In HGMS, a ®eld gradient, j r H j, as high as 1:6 1010 A/m2 j r l0 H j 20; 000 T/m is
reached, and the magnetic force is enhanced by a factor of 1000±10; 000 [12]. Before the development of HGMS, magnetic separation was only performed on large-diameter ferromagnetic particles, but by using HGMS, weakly magnetized particles down to tens of microns can be magnetically separated in practical systems (see Section 3.4). High-gradient magnetic ®elds are generated near a ferromagnetic wire with several hundred microns in diameter placed under an applied uniform magnetic ®eld. If the magnetic ®eld is strong enough, in principle, all the particles with either positive or negative magnetic susceptibility dispersed in the medium are captured onto the wire. Weakly magnetized particles and typical dispersion media have magnetic susceptibilities much smaller than 1. Substituting this condition into Eq. (3) yields M vp vf H0 and Fm / H0 r H (Eq. (2)). Thus, use of high intensity, high-gradient magnetic ®elds is necessary to increase the magnetic force on magnetic particles. In conventional magnetic separators this force is less than 0.01% of the magnetic force acting on ferromagnetic particles, and is usually disregarded. However, because the magnetic ®eld gradient of HGMS systems is much higher than that used in conventional magnetic separators, separation of weakly magnetized particles is now possible. On the other hand, magnetization of ferromagnetic particles is saturated in comparatively weak magnetic ®elds, and therefore M in Eq. (2) remains small, nor does the magnetic force acting on ferromagnetic particles increase, even if the magnetic ®eld is increased beyond the point of saturation.
3. Progress of magnetic separation and superconductorization The history of the development of magnetic separation devices dates back to the 1800s, and although a detailed description of the historical development is beyond the scope of this work, we present developments from the 1970s to the present, when application of magnetic separation to water handling systems rapidly progressed.
T. Ohara et al. / Physica C 357±360 (2001) 1272±1280
3.1. Pioneers in the 1970s In the 1970s, a practical ferritic method [7,8] (see Section 2.2) was developed in Japan, which is still widely used. At nearly the same time, the HGMS method [11,12] was invented in the USA. This made high-speed processing of large quantities of slurry and colloidal suspensions possible, and was immediately used in the paper industry in Georgia for re®ning kaolin clay [14]. Since its development, it has also been used as a drainage recycling system for treating rolling-mill waste water in the steel industry [15,16]. Fig. 1 shows the magnetic ®lter part of the HGMS system for water treatment in the steel industry [16]. Models of various sizes were fabricated, with the largest canister bore of 3 m. About 100 systems are currently in use in Japan. Furthermore, in the 1970s progress in the theoretical understanding of magnetic separation included development of the socalled particle trajectory model by Watson [13],
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which was subsequently used for analyzing magnetic separation mechanisms and eciency. 3.2. Beginning of superconductorization in the 1980s In the 1980s, HGMS began to be used for removal of impurities in condensates at thermal power plants [17] and in boiling water reactors (BWR) [18]. About this time, conventional magnets were also replaced with superconducting magnets. We call this superconductorization. In 1984 in Germany, a prototype rotary drum magnetic separator [10] using a multi-pole superconducting magnet with a maximum ®eld strength of 3.0 T was manufactured. Although it had diculty simultaneously generating strong magnetic ®elds and high magnetic ®eld gradients at room temperature, from this development project a superconducting coil support method was developed. It is called a ``breathing ®xture'', which means a ®xture that shrinks and expands with the coils as they
Fig. 1. HGMS system for water puri®cation in the steel industry [16]. About 100 systems are currently in use in Japan. (Courtesy of Hitachi Plant Eng. and Const. Co. Ltd.)
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Fig. 2. Drum separator using a multi-pole superconducting magnet (Klochner Humbolt-Deutz Cologne, Germany).
change temperature, so that minimal prestressing at cryogenic temperatures is required. Separation tests on hematite ore, phosphorus ore, bauxite, etc., were made over a two-year period. A larger-scale device was developed afterwards. The outward appearance is similar to the conventional rotatingdrum magnetic separators described in Section 2.2, except that multi-pole superconducting magnets with the complicated shape shown in Fig. 2 were used, and the system used a helium lique®er. The superconductorization of the HGMS system for Georgia kaolin re®nement was done in 1986 [19]. In spite of being liquid-helium cooled, with its generated magnetic ®eld of 2.0 T and energy storage of 3.53 MJ, compared with previous systems, this superconductorized system resulted in a 58% reduction of the equipment's weight, a 66% reduction in volume, and a 95% reduction in power consumption. Moreover, the excitation time was shortened from several minutes to 1 min or less, and the operational eciency was also substantially improved. However, even this striking example does not fully show the merits of superconductivity. The reason is that in this application, the charging/ discharging cycling of the superconducting magnet occurred every 15 min, and this burdened the magnet and its cryogenic system with heavy ac power losses. Eventually only three of these systems with superconducting magnets were produced. During the 1980s, an HGMS system was developed in Japan for recycling glass grinding sludge [20], which used permanent magnets and electromagnets. This system has been used to recycle the waste sludge discharged from cathoderay tube (CRT) polishing factories for about 15
years. The volume of the discharged abrasion slurry in these factories is about 300±350 ton/h, and after iron separation and drying, the puri®ed sludge is supplied into the process as raw glass. The HGMS system is used to decrease the iron content in the sludge from about 3% to 0.3%. 3.3. Particle size limit of high-gradient magnetic separation In the early 1980s Takayasu et al. [21] demonstrated that HGMS could be used for separating submicron particles. Gerber et al. [22] suggested the particle size limit of HGMS is caused by Brownian motion (diusion), which dominates the kinematics of small, suspended particles. For particles in water, the theoretical particlesize separation limit vs vp of HGMS [22] is shown in Fig. 3 [3,23], and corresponds to ferromagnetic wires 100 lm in diameter, wire magnetization of 1 T, and an applied magnetic ®eld of either 2 (conventional magnets) or 7 T (superconducting magnets). Because of particle diusion, HGMS systems cannot trap particles below the area outlined by the circles (l0 H 2 T) or squares (l0 H 7 T) in Fig. 3. 3.4. Progress of superconductorization after the 1990s Research and development to use superconducting magnets more eectively in kaolin re®nement manufacturing processes was done in the United Kingdom [24] from the end of the 1980s and was completed in the early 1990s. Fig. 4 shows such a system.
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Fig. 3. Particle size limit of HGMS [3,22,23].
There were three main improvements made to superconducting magnets: (1) superconducting magnets were operated in a persistent current mode without an iron yoke, which reduced the burden on the cooling system; (2) a magnetic ®eld of either 5 or 6 T was used; and (3) two magnetic
®lters in the room temperature bore of superconducting magnets were reciprocated in and out of the magnetic ®eld. When one magnetic ®lter was separating particles, the other was being cleaned, thereby improving the operation eciency. The resulting separation capacity increased by 24 times
Fig. 4. Superconducting, reciprocating canister, HGMS system [24,25]. (Courtesy of Oxford Instruments and Outokumpu Technology, Inc. Carpco division.)
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T. Ohara et al. / Physica C 357±360 (2001) 1272±1280
compared to non-reciprocating devices [25]. As of February, 1999, there were about 30 such systems in the world [25]. In the 1990s, interest developed to use magnetic separation to reduce radioactive waste from nuclear power plants and for iron impurity removal from high-temperature boiler liquids in electrical power plants [26]. After the development of high-temperature oxide superconductivity technology, a commitment was made to replace liquid-helium cooled superconducting magnets with liquid-helium-free, high-temperature superconducting magnets. Some demonstration projects were started, such as HGMS systems for algae removal in lakes and marshes [27] where 1 T superconducting magnets are used, and for arsenic removal for safe use of geothermal water [28,29], in which 1.7 T superconducting magnets are proposed. The latter system is currently being evaluated using ®ne a-hematite sludge removal to simulate arsenic spinel ferrite removal. Open gradient magnetic separation (OGMS) devices [30] were also developed for removal of heavy metals from laboratory discharge euents. These devices use a Nb3 Sn superconducting magnet and have no magnetic wire ®lters in the center, but have a zone of high magnetic forces that acts on particles. Experiments on copper removal from arti®cial waste water showed that a removal eciency of more than 95% can be achieved for a superconducting magnet operating at temperatures of 8±9 K, and a generated magnetic ®eld of 5.5 T [30]. There are also R&D projects for developing magnetic chromatography systems (Section 4.3) [31,32], for application to kaolin clay re®nement [33], for purifying reclamation land water using electrolysis preprocessing, and for purifying inorganic slurry discharged from semiconductor manufacturing plants. 4. Research activities at National Research Institute for Metals and its related organizations 4.1. Development of Bi-2223 magnet and magnetic separation system We constructed a Bi-2223 magnet system with a 200 mm room temperature bore and demonstrated
it for separation of particles from slurries containing ®ne a-hematite paramagnetic particles (Fig. 5) [28,29]. The magnet uses a Giord±McMahon (GM) cryocooler, generates 1.7 T at the center and more than 1 T in its 11 l room temperature bore. It is excited up to its maximum magnetic ®eld in 1 min, and the magnet temperature increases during excitation from an initial temperature of 12 K. After being cyclically excited up to the maximum magnetic ®eld about twenty times with the cycle length of nine minutes, the magnet temperature stabilizes to between 35±38 K. To obtain high operational eciency in magnetic separation systems, rapid energizing and de-energizing are important, because separation should be done at maximum ®eld strength, and separated particles should be removed at H0 0. Our tests showed that nearly 100% of the hematite particles were separated from the slurry, and that the Bi-2223 magnet was thermally stable against repetitive excitation, and therefore suitable for magnetic separation applications. 4.2. Development of a computational ¯uid dynamics-based numerical model We also developed a numerical method for designing and evaluating HGMS processes [34]. We use a computational ¯uid dynamics (CFD) computer program to simulate the eect of ¯uid ¯ow, particle diusion, and magnetic forces on the timedependent spatial and temporal concentration of particles. By calculating the fraction of particles that penetrate through the system, we can calculate the capture eciency of the system, which is an important parameter for magnetic ®lter systems. 4.3. Development of a magnetic chromatography system We proposed magnetic chromatography (MC) as a new technique for ultra-®ne particle separation, which separates chemically similar but magnetically dissimilar materials, such as lanthanide and actinide elements, and extends the particle size limits of HGMS to smaller particles [31,32]. We developed an MC simulator to model the transient
T. Ohara et al. / Physica C 357±360 (2001) 1272±1280
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Fig. 5. Bi-2223 superconduting magnet and magnetic separation system developed by the National Research Institute for Metals [28].
particles in an HGMS behavior of 300 and 500 A system by accounting for the eect of ¯uid ¯ow, particle diusion, and magnetic ®eld. Simulation results suggest that ¯ow velocities from 10 4 to 10 3 m/s, and magnetic ®eld strengths from 0.5 to 5 T are suitable for practical particle separation systems. 5. Concluding remarks HGMS has made the high-speed separation of ®ne particles in liquid suspensions economically feasible. HGMS is expected to contribute greatly to the reduction of secondary waste produced by the separation and concentration of useful or harmful materials that are discharged from either frontier or atomic power industries. HGMS is useful for both separation and recovery processes, as well as for promoting the reuse of waste particles created in the following industries: semiconductor, atomic power, wire manufacturing, electronic, chemical, petrochemical, iron and steel, plating, ®ber and dyeing, pulp and paper, marine products, and other basic materials production industries. Magnetic separation is also useful for
promoting the reuse of ®ne particles from ``urban'' and abandoned mines. To promote new applications for superconducting magnetic separation, the fusion of science and technology from diverse areas is required. An important role in this will be played through the interchange and cooperation of researchers working in dierent ®elds: including superconductivity, electrical engineering, and mechanical engineering for equipment improvement; chemical engineering and applied chemistry for separation systems; and environmental, sanitation, and resource engineering for practical utilization of these systems. Organizations that promote the exchange of research information from dierent technical ®elds and collaboration are very valuable.
Acknowledgements We thank Dr. Evan R. Whitby of Chimera Technologies, Inc. for his helpful and patient discussions of an earlier version of this paper. This work was supported by STA multi-core project for Superconducting Materials Research.
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References [1] T. Ohara, IEEE Trans. Magn. MAG-20 (1984) 436. [2] T. Ohara, Japanese Science and Technology, vol. 23, no. 216, 1982, p. 57 (in Japanese). [3] T. Ohara, H. Kumakura, H. Wada, Proc. Symp. New Magn. Sci.'99, The Third Meeting, 1999, p. 164. [4] J.A. Oberteuer, IEEE Trans. Magn. MAG-10 (1974) 223. [5] P.G. Marston, World Electrotech. Congress, Moscow, USSR, 1977. [6] S. Uchiyama, Oyo buturi 45 (1976) 152 (in Japanese). [7] T. Nagashima, J. Mag. Soc. Jpn. 2 (1978) 46 (in Japanese). [8] T. Takada, M. Kiyama, Proc. Int. Conf. Ferrites (1970) 69. [9] T. Janowski, S. Kozak, IEEE Trans. Magn. 29 (1993) 3261. [10] K.P. Juengst, G. Ries, S. Foerster, F. Graf, G. Obermaier, W. Lehmann, Cryogenics 24 (1984) 648. [11] H.H. Kolm, US patent 3,567,026 (1971)/3,676,337, 1972. [12] J.A. Oberteuer, IEEE Trans. Magn. MAG-9 (1973) 303. [13] J.H.P. Watson, J. Appl. Phys. 44 (1973) 4209. [14] J. Iannicelli, IEEE Trans. Magn. Mag-12 (1976) 489. [15] J. Yano, I. Eguchi, Proc. Int. Conf. Ind. Appl. Magn. Sep. (1978) 134. [16] K. Takino, S. Matsuno, H. Fujita, Y. Kato, Kogyoyosui 227 (1977) 54 (in Japanese). [17] M. Uchida, Y. Yuasa, K. Mizojiri, T. Nagano, S. Sato, S. Yoshida, Karyoku-Gensiryoku-Hatsuden 35 (1984) 141 (in Japanese). [18] F. Soda, T. Yukawa, K. Ito, J. Mag. Soc. Jpn. 8 (1984) 209 (in Japanese). [19] A.J. Winters, J.A. Selvaggi, Chem. Engng. Prog. 86 (1990) 36.
[20] Y. Kenmoku, S. Sakata, IEEE Trans. Magn. MAG-20 (1984) 1198. [21] M. Takayasu, R. Gerber, F.J. Friedlaender, IEEE Trans. Magn. MAG-19 (1983) 2112. [22] R. Gerber, M. Takayasu, F.J. Friedlaender, IEEE Trans. Magn. MAG-19 (1983) 2115. [23] T. Ohara, S. Mori, Y. Oda, Y. Wada, O. Tsukamoto, Trans. IEE Jpn. 116-B (1996) 979. [24] J.H.P. Watson, Supercond. Sci. Technol. 5 (1992) 694. [25] J.H.P. Watson, Int. Symp. Novel Uses Magn. Force, AIST, Tsukuba, March 8, 1999. [26] H. Okada, K. Fukushima, J. Tamura, Trans. IEE Jpn. 119B (1999) 1181 (in Japanese). [27] N. Saho, H. Isogami, T. Takagi, M. Morita, IEEE Trans. Supercond. 9 (1999) 398. [28] H. Kumakura, T. Ohara, H. Kitaguchi, K. Togano, H. Wada, H. Mukai, K. Ohmatsu, H. Okada, Appl. Supercond. Conf. (ASC2000), Virginia Beach, VA, USA, 17±22 September, 2000. [29] T. Tada, A. Chiba, H. Okada, T. Ohara, H. Wada, Extended Abstracts of the 61st Autumn Meeting, Jpn. Soc. Appl. Phys. 1 (2000) 395 (in Japanese). [30] M. Franzreb, K.P. Juengst, W.H. Hoell, J.A. Good, R. Hall, Proc. MT15, Beijing, China, 1997, p. 744. [31] X. Wang, T. Ohara, E.R. Whitby, K.C. Karki, C.H. Winstead, Trans. IEE Jpn. 117-B (1997) 1466 (in Japanese). [32] T. Ohara, X. Wang, H. Wada, E.R. Whitby, Trans. IEE Jpn. 120-A (2000) 62 (in Japanese). [33] J. Iannicelli, J. Pechin, M. Ueyama, K. Okura, K. Hayashi, K. Sato, A. Lauder, C. Rey, IEEE Trans. Appl. Supercond. 7 (1997) 106. [34] H. Okada, T. Tada, A. Chiba, T. Ohara, H. Wada, Appl. Supercond. Conf. ASC2000, Virginia Beach, VA, USA, 17±22 September, 2000.
www.elsevier.com/locate/physc
Magnetic separation using superconducting magnets Takeshi Ohara a,b,*, Hiroaki Kumakura a, Hitoshi Wada a a
Tsukuba Magnet Laboratory, National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 16 October 2000; accepted 9 November 2000
Abstract Since the 1970s, magnetic separation has been increasingly used for puri®cation of liquids, such as heavy-metal ion removal from laboratory waste water, puri®cation of kaolin clay in the paper-coating industry, waste water recycling in the steel industry, and recycling of glass grinding sludge in cathode-ray tube polishing factories. In the 1980s, large superconducting magnets were adopted for the ®eld coils of high-gradient magnetic separation systems used for kaolin clay puri®cation. After the development of liquid-helium-free superconducting magnets in the 1990s, new national research projects were initiated in Japan to expand the application of superconducting magnetic separation to other areas. This paper discusses some of the fundamental characteristics of magnetic separation, the progress of superconductorization (use of superconducting magnets as ®eld coils in magnetic separation systems) and recent R&D activities at the National Research Institute for Metals in Japan. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 07.90.c; 47.55Kf.c; 83.50.Yt; 83.70Hq; 89.20.a Keywords: Magnetic separation; Superconducting magnet; High-gradient magnetic separation; Bi-2223 magnet
1. Introduction Recent progress in magnet technology has realized economically and operationally favorable cryocooler-cooled superconducting magnets with excellent operability, which can be used for commercial applications. Consequently, the dream of widespread use of superconducting magnets is being realized. In particular, magnetic separation is a promising application of large area, strong
* Corresponding author. Address: Tsukuba Magnet Laboratory, National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. Fax: +81-298-59-5602. E-mail address: [email protected] (T. Ohara).
magnetic ®elds, and in addition to industrial applications, can also be used to remove biogenic and anthropogenic pollutants from aquatic systems and industrial waste from streams. The fullscale application of magnetic separation processes will greatly contribute to preserving the global environment. In magnetic separation, magnetic forces act directly on ®ne particles to be separated. The force vanishes when the externally applied magnetic ®eld is turned o, so that the system can be continuously regenerated by cycling it on and o. Unlike conventional ®ltration methods that use blockingtype ®ltration, secondary waste is not produced in high-gradient magnetic separation (HGMS, also known as magnetic or electromagnetic ®ltration) systems. Furthermore, because HGMS systems use
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 5 3 0 - 5
T. Ohara et al. / Physica C 357±360 (2001) 1272±1280
much higher magnetic forces than conventional magnetic separation techniques, it can also be used to rapidly separate large quantities of dilute suspensions. In this paper, we discuss the principles of magnetic separation, and the historical development of commercial HGMS systems. 2. Principle of use 2.1. Three elements to enhance magnetic forces When ®ne particles are dispersed in air, water, sea water, oil, organic solvents, etc., their separation or ®ltration by using a magnetic force is called magnetic separation. To increase the separation eciency of these systems we must increase the magnetic force acting on particles by increasing the particle volume, relative magnetization between the particles (dispersoid) and the dispersion medium, and/or the magnitude of the magnetic ®eld gradients [1±3]. To understand the reason for this, consider the principle of magnetic forces. By calculating the gradient of the magneto-static energy dierence between magnetized particles of volume, Vp , and the dispersion medium of the same volume, DUp , the magnetic force on a particle, F m , is Fm
r DUp Vp l0 M p H r 2
Vp l0 M f H ; 2
n x; y; z;
2
where M H0
9 vp vf 3 vp 3 vf
Eq. (2) indicates that the magnetic force on particles depends on three key parameters: Vp , M , and rn H . For Fmn 6 0, Vp , M , and rn H are all non-zero. Therefore, Eqs. (2) and (3) indicate that to generate a magnetic force (Fmn 6 0), a dispersoid with susceptibility dierent from that of the dispersion medium (i.e., vp 6 vf ) must be placed in an inhomogeneous magnetic ®eld (i.e., rn H 6 0). When the dispersed particle is weakly magnetized, such as red blood cells, M vp vf H0 , and Fmn is proportional to the susceptibility difference between the dispersoid (red blood cell) and the dispersion medium (plasma); to H0 ; and to rn H . When the dispersoid is a ferromagnetic particle, Mp becomes saturated at a relatively low magnetic ®eld strength. The enhancement of Fmn through the use of strong magnetic ®elds is therefore limited. (Mp also becomes saturated at relatively small susceptibilities of vp 10 because Mp is proportional to vp = 1 N vp , where N is a demagnetizing factor.) Eective ways of enhancing the magnetic force include: (A) increasing Mp by ``magnetic seeding'' of the dispersoid, (B) increasing rn H , (C) using high intensity magnetic ®elds, and (D) selecting a dispersion medium with a large value of (vp vf ). In Section 2.2 we describe options (A), (B) and (C) for enhancing the magnetic force. Option (D) is applicable only when weakly magnetized particles are used, and we do not discuss this in this paper.
1
where Mp is the particle magnetization, Mf is the magnetization of the dispersion medium, and H is the magnetic ®eld. By assuming a spherical particle with volume magnetic susceptibility, vp , and uniform magnetization Mp , we obtain Fmn Vp l0 M rn H ;
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3
and where vf is the volume magnetic susceptibility of the dispersion medium, M is the relative magnetization between the dispersoid and the dispersion medium, and H0 is the applied magnetic ®eld.
2.2. Generation of strong magnetic forces for practical uses Magnetic separation is an old technology, used to enrich iron ore and iron sands for over 150 years. Various devices have been used to generate strong magnetic forces, which are described in detail elsewhere [2±7]. In this paper we provide a brief summary of this technology. Magnetic ®eld gradients are produced from inhomogeneous magnetic ®elds, created either by arranging a series of permanent magnets with alternating orientation of the poles, or by applying a magnetic ®eld to a grid of ferromagnetic rods. Near the rods the magnetic ®eld is altered, creating magnetic ®eld gradients. Such technology is used
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in rotating-drum and rotating-disk magnetic separators. Rotating-drum magnetic separators are mainly used in mines. A good example of this technology is a magnetic separator operated in one of the largest anatase TiO2 mines in the world, located in Tapira, Minas Gerais, Brazil, used to remove ferromagnetic impurities in the ore. Other magnetic separators are used to enrich Ti after heat treatment. A rotating-disk magnetic separator [7] has a simple rotating steel disc that contains rod-shaped permanent magnets, so that the magnetic poles may be arranged with consistent intervals on the disc surface. This is used in so-called ferritic processes [7,8] to remove heavy metal ions, such as Fe, Mn, Zn, Ni, Co, Cu, and about 40 other elements, from waste water discharged from some university laboratories in Japan. This is an improved method of magnetic seeding, in which the substance to be separated is absorbed into ferromagnetic ferrite by using chemical reactions that form spinel ferrite, and the reacted ®ne ferrite particles are recovered by using a rotary disc magnetic separator. Other methods for increasing the magnetic ®eld gradient are to use superconducting coils in a drum-type separator [9], multi-pole superconducting coil separators [10], or HGMS systems [11±14]. In HGMS systems a magnetic ®eld is applied to a grid of thin magnetic wires. Other than through the use of HGMS, the induced magnetic force is not strong enough to separate particles smaller than several hundred microns, because under practical conditions, in these magnetic separators the drag and gravitational forces counteract the magnetic forces. Therefore, the volume of particles must be increased prior to magnetic separation, either by using ¯occulants, heat treatment to increase the quantity of the magnetic substance (used in ore sorting), or magnetic seeding of the slurry (used in the ferritic process described above). To enhance the separation process, chemical, physical, electrochemical, or biological techniques are used to increase the magnetism and/or the volume of the dispersoid. In HGMS, a ®eld gradient, j r H j, as high as 1:6 1010 A/m2 j r l0 H j 20; 000 T/m is
reached, and the magnetic force is enhanced by a factor of 1000±10; 000 [12]. Before the development of HGMS, magnetic separation was only performed on large-diameter ferromagnetic particles, but by using HGMS, weakly magnetized particles down to tens of microns can be magnetically separated in practical systems (see Section 3.4). High-gradient magnetic ®elds are generated near a ferromagnetic wire with several hundred microns in diameter placed under an applied uniform magnetic ®eld. If the magnetic ®eld is strong enough, in principle, all the particles with either positive or negative magnetic susceptibility dispersed in the medium are captured onto the wire. Weakly magnetized particles and typical dispersion media have magnetic susceptibilities much smaller than 1. Substituting this condition into Eq. (3) yields M vp vf H0 and Fm / H0 r H (Eq. (2)). Thus, use of high intensity, high-gradient magnetic ®elds is necessary to increase the magnetic force on magnetic particles. In conventional magnetic separators this force is less than 0.01% of the magnetic force acting on ferromagnetic particles, and is usually disregarded. However, because the magnetic ®eld gradient of HGMS systems is much higher than that used in conventional magnetic separators, separation of weakly magnetized particles is now possible. On the other hand, magnetization of ferromagnetic particles is saturated in comparatively weak magnetic ®elds, and therefore M in Eq. (2) remains small, nor does the magnetic force acting on ferromagnetic particles increase, even if the magnetic ®eld is increased beyond the point of saturation.
3. Progress of magnetic separation and superconductorization The history of the development of magnetic separation devices dates back to the 1800s, and although a detailed description of the historical development is beyond the scope of this work, we present developments from the 1970s to the present, when application of magnetic separation to water handling systems rapidly progressed.
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3.1. Pioneers in the 1970s In the 1970s, a practical ferritic method [7,8] (see Section 2.2) was developed in Japan, which is still widely used. At nearly the same time, the HGMS method [11,12] was invented in the USA. This made high-speed processing of large quantities of slurry and colloidal suspensions possible, and was immediately used in the paper industry in Georgia for re®ning kaolin clay [14]. Since its development, it has also been used as a drainage recycling system for treating rolling-mill waste water in the steel industry [15,16]. Fig. 1 shows the magnetic ®lter part of the HGMS system for water treatment in the steel industry [16]. Models of various sizes were fabricated, with the largest canister bore of 3 m. About 100 systems are currently in use in Japan. Furthermore, in the 1970s progress in the theoretical understanding of magnetic separation included development of the socalled particle trajectory model by Watson [13],
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which was subsequently used for analyzing magnetic separation mechanisms and eciency. 3.2. Beginning of superconductorization in the 1980s In the 1980s, HGMS began to be used for removal of impurities in condensates at thermal power plants [17] and in boiling water reactors (BWR) [18]. About this time, conventional magnets were also replaced with superconducting magnets. We call this superconductorization. In 1984 in Germany, a prototype rotary drum magnetic separator [10] using a multi-pole superconducting magnet with a maximum ®eld strength of 3.0 T was manufactured. Although it had diculty simultaneously generating strong magnetic ®elds and high magnetic ®eld gradients at room temperature, from this development project a superconducting coil support method was developed. It is called a ``breathing ®xture'', which means a ®xture that shrinks and expands with the coils as they
Fig. 1. HGMS system for water puri®cation in the steel industry [16]. About 100 systems are currently in use in Japan. (Courtesy of Hitachi Plant Eng. and Const. Co. Ltd.)
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Fig. 2. Drum separator using a multi-pole superconducting magnet (Klochner Humbolt-Deutz Cologne, Germany).
change temperature, so that minimal prestressing at cryogenic temperatures is required. Separation tests on hematite ore, phosphorus ore, bauxite, etc., were made over a two-year period. A larger-scale device was developed afterwards. The outward appearance is similar to the conventional rotatingdrum magnetic separators described in Section 2.2, except that multi-pole superconducting magnets with the complicated shape shown in Fig. 2 were used, and the system used a helium lique®er. The superconductorization of the HGMS system for Georgia kaolin re®nement was done in 1986 [19]. In spite of being liquid-helium cooled, with its generated magnetic ®eld of 2.0 T and energy storage of 3.53 MJ, compared with previous systems, this superconductorized system resulted in a 58% reduction of the equipment's weight, a 66% reduction in volume, and a 95% reduction in power consumption. Moreover, the excitation time was shortened from several minutes to 1 min or less, and the operational eciency was also substantially improved. However, even this striking example does not fully show the merits of superconductivity. The reason is that in this application, the charging/ discharging cycling of the superconducting magnet occurred every 15 min, and this burdened the magnet and its cryogenic system with heavy ac power losses. Eventually only three of these systems with superconducting magnets were produced. During the 1980s, an HGMS system was developed in Japan for recycling glass grinding sludge [20], which used permanent magnets and electromagnets. This system has been used to recycle the waste sludge discharged from cathoderay tube (CRT) polishing factories for about 15
years. The volume of the discharged abrasion slurry in these factories is about 300±350 ton/h, and after iron separation and drying, the puri®ed sludge is supplied into the process as raw glass. The HGMS system is used to decrease the iron content in the sludge from about 3% to 0.3%. 3.3. Particle size limit of high-gradient magnetic separation In the early 1980s Takayasu et al. [21] demonstrated that HGMS could be used for separating submicron particles. Gerber et al. [22] suggested the particle size limit of HGMS is caused by Brownian motion (diusion), which dominates the kinematics of small, suspended particles. For particles in water, the theoretical particlesize separation limit vs vp of HGMS [22] is shown in Fig. 3 [3,23], and corresponds to ferromagnetic wires 100 lm in diameter, wire magnetization of 1 T, and an applied magnetic ®eld of either 2 (conventional magnets) or 7 T (superconducting magnets). Because of particle diusion, HGMS systems cannot trap particles below the area outlined by the circles (l0 H 2 T) or squares (l0 H 7 T) in Fig. 3. 3.4. Progress of superconductorization after the 1990s Research and development to use superconducting magnets more eectively in kaolin re®nement manufacturing processes was done in the United Kingdom [24] from the end of the 1980s and was completed in the early 1990s. Fig. 4 shows such a system.
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Fig. 3. Particle size limit of HGMS [3,22,23].
There were three main improvements made to superconducting magnets: (1) superconducting magnets were operated in a persistent current mode without an iron yoke, which reduced the burden on the cooling system; (2) a magnetic ®eld of either 5 or 6 T was used; and (3) two magnetic
®lters in the room temperature bore of superconducting magnets were reciprocated in and out of the magnetic ®eld. When one magnetic ®lter was separating particles, the other was being cleaned, thereby improving the operation eciency. The resulting separation capacity increased by 24 times
Fig. 4. Superconducting, reciprocating canister, HGMS system [24,25]. (Courtesy of Oxford Instruments and Outokumpu Technology, Inc. Carpco division.)
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compared to non-reciprocating devices [25]. As of February, 1999, there were about 30 such systems in the world [25]. In the 1990s, interest developed to use magnetic separation to reduce radioactive waste from nuclear power plants and for iron impurity removal from high-temperature boiler liquids in electrical power plants [26]. After the development of high-temperature oxide superconductivity technology, a commitment was made to replace liquid-helium cooled superconducting magnets with liquid-helium-free, high-temperature superconducting magnets. Some demonstration projects were started, such as HGMS systems for algae removal in lakes and marshes [27] where 1 T superconducting magnets are used, and for arsenic removal for safe use of geothermal water [28,29], in which 1.7 T superconducting magnets are proposed. The latter system is currently being evaluated using ®ne a-hematite sludge removal to simulate arsenic spinel ferrite removal. Open gradient magnetic separation (OGMS) devices [30] were also developed for removal of heavy metals from laboratory discharge euents. These devices use a Nb3 Sn superconducting magnet and have no magnetic wire ®lters in the center, but have a zone of high magnetic forces that acts on particles. Experiments on copper removal from arti®cial waste water showed that a removal eciency of more than 95% can be achieved for a superconducting magnet operating at temperatures of 8±9 K, and a generated magnetic ®eld of 5.5 T [30]. There are also R&D projects for developing magnetic chromatography systems (Section 4.3) [31,32], for application to kaolin clay re®nement [33], for purifying reclamation land water using electrolysis preprocessing, and for purifying inorganic slurry discharged from semiconductor manufacturing plants. 4. Research activities at National Research Institute for Metals and its related organizations 4.1. Development of Bi-2223 magnet and magnetic separation system We constructed a Bi-2223 magnet system with a 200 mm room temperature bore and demonstrated
it for separation of particles from slurries containing ®ne a-hematite paramagnetic particles (Fig. 5) [28,29]. The magnet uses a Giord±McMahon (GM) cryocooler, generates 1.7 T at the center and more than 1 T in its 11 l room temperature bore. It is excited up to its maximum magnetic ®eld in 1 min, and the magnet temperature increases during excitation from an initial temperature of 12 K. After being cyclically excited up to the maximum magnetic ®eld about twenty times with the cycle length of nine minutes, the magnet temperature stabilizes to between 35±38 K. To obtain high operational eciency in magnetic separation systems, rapid energizing and de-energizing are important, because separation should be done at maximum ®eld strength, and separated particles should be removed at H0 0. Our tests showed that nearly 100% of the hematite particles were separated from the slurry, and that the Bi-2223 magnet was thermally stable against repetitive excitation, and therefore suitable for magnetic separation applications. 4.2. Development of a computational ¯uid dynamics-based numerical model We also developed a numerical method for designing and evaluating HGMS processes [34]. We use a computational ¯uid dynamics (CFD) computer program to simulate the eect of ¯uid ¯ow, particle diusion, and magnetic forces on the timedependent spatial and temporal concentration of particles. By calculating the fraction of particles that penetrate through the system, we can calculate the capture eciency of the system, which is an important parameter for magnetic ®lter systems. 4.3. Development of a magnetic chromatography system We proposed magnetic chromatography (MC) as a new technique for ultra-®ne particle separation, which separates chemically similar but magnetically dissimilar materials, such as lanthanide and actinide elements, and extends the particle size limits of HGMS to smaller particles [31,32]. We developed an MC simulator to model the transient
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Fig. 5. Bi-2223 superconduting magnet and magnetic separation system developed by the National Research Institute for Metals [28].
particles in an HGMS behavior of 300 and 500 A system by accounting for the eect of ¯uid ¯ow, particle diusion, and magnetic ®eld. Simulation results suggest that ¯ow velocities from 10 4 to 10 3 m/s, and magnetic ®eld strengths from 0.5 to 5 T are suitable for practical particle separation systems. 5. Concluding remarks HGMS has made the high-speed separation of ®ne particles in liquid suspensions economically feasible. HGMS is expected to contribute greatly to the reduction of secondary waste produced by the separation and concentration of useful or harmful materials that are discharged from either frontier or atomic power industries. HGMS is useful for both separation and recovery processes, as well as for promoting the reuse of waste particles created in the following industries: semiconductor, atomic power, wire manufacturing, electronic, chemical, petrochemical, iron and steel, plating, ®ber and dyeing, pulp and paper, marine products, and other basic materials production industries. Magnetic separation is also useful for
promoting the reuse of ®ne particles from ``urban'' and abandoned mines. To promote new applications for superconducting magnetic separation, the fusion of science and technology from diverse areas is required. An important role in this will be played through the interchange and cooperation of researchers working in dierent ®elds: including superconductivity, electrical engineering, and mechanical engineering for equipment improvement; chemical engineering and applied chemistry for separation systems; and environmental, sanitation, and resource engineering for practical utilization of these systems. Organizations that promote the exchange of research information from dierent technical ®elds and collaboration are very valuable.
Acknowledgements We thank Dr. Evan R. Whitby of Chimera Technologies, Inc. for his helpful and patient discussions of an earlier version of this paper. This work was supported by STA multi-core project for Superconducting Materials Research.
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