The longitudinal magnetic field effect is caused by the characteristic flux motion driven by force-free torque. In this lecture, the peculiarity of this flux motion is discussed in association with the electric field induced. The reason why the variation principle cannot be used to derive the torque balance is also discussed. It is shown to be difficult to assume the flux cutting event as an explanation for various experimental results. Comparisons are performed relating to analogies in discovery between the theory of relativity and force-free torque from various aspects. Finally, the possibilities of superconducting power cables, the first application of the longitudinal magnetic-field effect, are introduced to show the enormous technological potential of this effect.

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This introductory article describes the progress of magnetic separation technology including its background, principles, and application to process a large quantity of dilute suspension (i.e., up to 50 kg/hr). Development of this technology dates back to the 1970s when researchers at the Massachusetts Institute of Technology (MIT) invented "high gradient magnetic separation" (HGMS). The HGMS could enable magnetic separation to be applied to a large class of weak paramagnetic materials, down to colloidal particle size. The strength of the magnetic force generated with HGMS has since been increased by a factor of 10³∼10⁴ compared with that of permanent magnets. Due to the availability of direct, selective, high-intensity magnetic forces on particles to be separated, HGMS devices have been used for beneficiation of kaolin clay in the paper industry, for wastewater purification in the steel industry, and for recovery and recycling of glass grinding sludge in the glass fabrication industry.
   In 1986, Machine Design magazine reported that the first superconducting magnetic separator designed for commercial use was operating at the Huber clay-processing plant in Wrens, Georgia. Since then, a new 230-ton HGMS separator has been built by Eriez Magnetics. It requires fewer chemical additives, less space (34%), and weighs less (42%) than a conventional separator. However, due to retrofitting, only three separators of this type have been installed in the plant without any consideration for the generation of AC loss in superconducting wires. With development of technical R&D on applied superconductivity, two large-scale applications became commercially successful in the mid 90s; namely, magnetic separation and magnetic resonance imaging (MRI). At the time of February 1999, the number of HGMS systems in operation using superconducting magnets for magnetic separation was as follows: seven in Brazil, five in the U.S.A., four in the U.K., three in Germany, two in India, two in Australia, one in Austria, and one in Egypt. These systems were all equipped with a reciprocating canister HGMS in order to utilize superconducting persistent current mode, and were operated mainly in the kaolin clay beneficiation industry.
   Since 1995, the so-called "new magneto-science" has been extensively researched in Japan to study the effects of strong magnetic fields on non-magnetic (extremely weak magnetic) substances in a wide variety of scientific areas including solid state physics, chemistry, metallurgy, biology, and medicine. The initial driving force behind this research was the invention of a liquid helium-free superconducting magnet system in 1993, which could be more easily operated due to its use of high critical temperature superconducting materials developed since the 1986 discovery of cuprate superconducting matter. Magnetic separation is a primary application field for superconducting magnets, and in 1999, many HGMS application projects were initiated in Japan aiming at arsenic removal from geothermal water, purification of water seeping from reclaimed land, and purification of wastewater from paper factories. Magnetic seeding methods to various non-magnetic particles dispersed in water have been developed, such as electro-coagulation, electro-chemical oxidation, magnetic flock, and magnetic precipitation. Magnetic separation is also a promising technology to remediate soil polluted with radioactive particles.

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Synopsis: In Korea superconducting HGMS has been applied for the treatment of various kinds of wastewater. The wastewater from paper factory was successfully purified by HTS superconducting magnet system after pre-treated with magnetite and coagulant. For the treatment of coolant of hot rolling process in iron manufactory, coagulation conditions were investigated. Three different types of magnetic separation systems were applied and the suspended solids in the coolant were effectively removed. The treatment of condenser water in thermal power plant which mainly consisted of iron oxides was studied by investigating the effect of magnetic field strength, magnetic filter wire size, and filter composition on the turbidity decrease. Recycling of abrasives from waste slurry by magnetic separation was also investigated. The studies demonstrated that superconducting HGMS is feasible to apply to various kinds of wastewater and suggested improvements for better performance of HGMS system.

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This paper mainly described the work of Superconducting Magnet Engineering Center (SMEC), Institute of High Energy Physics (IHEP), Beijing, and its future development directions. During the last several years, the SMEC has successfully developed three big superconducting magnets, 1.0 T Beijing Spectrometer III (BESIII) Magnet, 3.0 T Superconducting Electromagnetic Iron Separator (SEIS) and 1.5 T Magnetic Resonance Image (MRI). And, there are some other Superconducting (SC) magnets under construction. With the help of SEIS magnet, arsenic magnetic separation experiments and turbid wastewater treatment experiments were carried out at IHEP.

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This study presents a newly developed superconducting magnetic separation system capable of continuous operation without interruption of feed-water owing to its unique structure, consisting of a continuously rotating disk-type magnetic filter and solenoid-type superconducting magnet. The continuously rotating disk-type magnetic filter, which is installed outwardly from the center axis of the magnet bore, is mounted on the cryostat and the superconducting magnet is energized using a permanent current switch, which consumes less electric power. This structure enables provision of a continuous operation system with high-speed and energy-saving separation treatment. We carried out a field test for two years including continuous operation for 1,000 hours. The test plant constructed had a capacity of 500 m³/day in order to study the possibility of applying the system for the purification of polluted lake and river water. The results show that the system has good performance, evidenced by a phosphorous removal rate of more than 85% and a suspended solid removal rate of less than 5 mg/L at a separation speed of 0.1 m/s. This system was found to be capable of being used practically for water pollution purification.

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This work examines the removal and recovery of phosphorus from treated wastewater using high-gradient magnetic separation (HGMS) applying a micron-sized ferromagnetic zirconium ferrite adsorbent. The characteristics of adsorption and desorption for zirconium ferrite were investigated. An experiment on magnetic separation with zirconium ferrite was performed. Excellent magnetic separation properties were obtained at a fluid velocity of 1 m/s and magnetic field of 2 T. Various parameters of a HGMS system for actual sewage plants were evaluated. These results show that this system could be applied to sewage plants.

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A magnetic separation experiment to recycle nickel from the waste fluid of electroless plating processes was conducted using the open-gradient magnetic separation technique and a HTS bulk magnet system. A magnetic pole containing Gd123-based bulk superconductors was activated to 3.44 T at 34.9 K using a 5 T superconducting solenoid and the field cooling method. The coarse precipitates of nickel sulfate are composed of phosphite ions, which are yielded during the plating reaction by controlling the temperature and pH. Next, the open-gradient magnetic separation technique was employed, with use of water channels to separate the nickel-sulfate crystals from the mixture of the nickel sulfate and phosphite compounds based on the difference in magnetic properties. From the concentrations of each precipitate attracted to the magnetic pole, we succeeded in collecting nickel-sulfate crystals preferentially to the phosphite ions soon after crystal growth began.

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To effectively separate the organic dyes in wastewater, we propose a method magnetic separation using superconducting bulk magnets. There are two key technologies in the magnetic separation process; magnetic seeding and magnetic separation. Two kinds of magnetic particles, magnetic activated carbon (MAC) and reactive nanoscale iron particles (RNIP), were used to adsorb organic dyes, orange II and crystal violet, as magnetic seeds. We set up a magnetic separator by placing an acrylic pipe between the magnetic poles of a face-to-face superconducting bulk magnet, and two types of magnetic separation were carried out: high gradient magnetic separation (HGMS) and open gradient magnetic separation (OGMS). The experimental results showed us that the adsorption ratio reached 93% and 97% for MAC and RNIP, respectively, and separation ratios of over 90% were achieved in HGMS both for the organic dyes.

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The separation characteristics of high-concentration magnetic sludge were studied using magnetic activated sludge, and the basic strategy for designing a magnetic separator using a permanent magnet was proposed. During the experiment, magnetic sludge was stuck on the magnet for only several seconds, until half the value of the saturation. The maximum magnetic compaction effect of the sludge was obtained when the concentration ratio of magnetite to activated sludge was from 1~3. The magnetic compaction was roughly twice that of gravitational settling in the experiment, and only several seconds was sufficient for the process. When the concentration ratio of separated sludge to suspended sludge is nearly 1, separated water can hardly be obtained from the magnetic separator. It is assumed that the concentration ratio of separated sludge to suspended sludge should be considerably smaller than 1. Under such condition, shortening of the scraping cycle seems to be effective for improving magnetic separator performance. It is believed that this basic strategy is useful to design a magnetic separator using a permanent magnet, electromagnet or superconductive magnet to separate high-concentration magnetic sludge.

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