The magnetic separation technique in combination with high temperature superconducting bulk magnets has been investigated to purify the ground water which has been used in the coolant system for the incinerator furnace to cool the burning gas. The experiment has been operated by means of the newly-built alternating channel type magnetic separating device. The separation ratios of ferromagnetic flocks including fine magnetite powder have been estimated by means of the high gradient magnetic separation method with small iron balls filled in the water channels. As the magnetic force acting on the magnetic particle is given by the product of a magnetization of the material and a gradient of magnetic field, and as the ferromagnetic stainless steel balls yield the steep gradient of magnetic field around them in a strong magnetic field, the system has exhibited a quite excellent performance with respect to the separation ratios. The separation ratios of the flocks which contain the magnetite powder with the values more than 50 ppm have remained over 80% for under the flow rates less than 5 L/min.
Keywords: Bulk superconductor; Trapped field magnet; Magnetic field generator; Magnetic separation
1. Introduction
The melt processed HTS bulk magnet systems in cooperation with small-sized refrigerators are characterized as intense magnetic field generators with steep magnetic gradients in narrow spaces [1], [2], [3] and [4]. In the study, a magnetic separation technique using HTS bulk magnets has been investigated to adapt them to the purification process of the ground water which has been led into the coolant system of the incinerator to cool the burning gas. Fig. 1 shows a schematic illustration of the coolant system in the incinerator furnaces for garbage disposal processes in Sanjo city in Niigata prefecture in Japan. The hot exhausted gas with the temperature 1200 K from the burning garbage is cooled to 600 K by showering water before emitting it outside the factory. In Sanjo city, the ground water is used for such gushing coolant water, however, as it contains Fe ions of around 14 ppm in this area, the water pipes and flush nozzles must have been regularly replaces to prevent stopping up caused by the accumulated rusts in them. As the purification processes requires adding a lot of additives and coagulants, as shown in Fig. 2, it has been needed to propose more efficient and cheaper ways than usual. The precipitates containing Fe element are condensed and filtered in the process, resultant clear water with typical content of 1 ppm is directly splashed in the furnace through the 2nd flush nozzle to cool the burning gas. Beside this main process, the disposed slurry which still includes water with high concentration is also filtered to drain water. The Fe concentration of the filtered water through this sub process is estimated to about 20-240 ppm. The experiment was conducted to purify the water at this 1st flush nozzle process, as indicated by the dashed circle in Fig. 2. As the furnace would have taken crucial damages if the water nozzles would be stopped up by the rust which has accumulates in the water pipes, the water must be purified to the clearer level than usual to reduce the number of the periodical replacement of pipes.
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Fig. 1. Coolant system in garbage disposal furnace.
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Fig. 2. Present water purification process of incinerator furnace coolant.
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As is expressed in Eq. (1), the magnetic force Fm is given by the product of three elements of volume, susceptibility and magnetic field [5]. We note that the presence of steep gradient and enlargement of precipitates are necessary to obtain strong magnetic force in addition to strong magnetic fields.
(1)We know a couple of magnetic separation systems. One is the high gradient magnetic separation (HGMS, by Okada et al. [6]) and the other is open gradient magnetic separation (OGMS, Fukui et al. [7]). As shown in Fig. 3, when some ferromagnetic membranes lie in magnetic fields, the flux lines are attracted to them and change their distributions, resultantly yielding steep magnetic gradient around the membranes. In the paper, we follow HGMS with use of a couple of small iron balls with diameters of 3 mm and 5 mm as the ferromagnetic membranes. The magnetic force acting on the precipitates called as the flocks is given by the product of a magnetization of particles and a gradient of magnetic field [8].
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Fig. 3. An illustration of high gradient magnetic separation technique.
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In the present investigation, we deal with the performances on the separation ratio against the flowing rate of ground water which contains the magnetized flocks which are formed by adding the fine magnetite powder.
2. Experimental
The magnetite-bearing precipitates which are formed by adding coagulants and poli-aluminium chloride (PAC) to the ground water, as shown in Fig. 4. It is obvious that the ground water in the beaker turns its color dark by the oxidation of Fe contents, as shown in Fig. 4a. And the flocks are deposited, keeping the clear water above it when the magnetite powder with PAC and coagulants are added, as shown in Fig. 4b. The averaged size of magnetite powder is 1 μm.
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Fig. 4. Sample waters: (a) ground water and (b) cohered sample water containing magnetite powder.
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A bulk magnet system as shown in Fig. 5 was prepared for the experiment. The face-to-face type magnetic poles were activated by the pulsed field magnetization (PFM). The magnetic poles contained a pair of Sm-Ba-Cu-O bulk samples which were mounted on the respective cold stages of the GM refrigerators in each vacuum vessel [8] and [9]. The bulk samples with a size 60 mm in diameter were cooled to 38 K and then magnetized by feeding the pulsed currents from the 60 mF condenser bank to the pulse coils. The rise time of the magnetic pulses was 10 ms.
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Fig. 5. Face to face type magnetic field generator (a) and its field distribution map on the left side pole (b).
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The trapped field distribution was measured by scanning a Hall sensor (F. W. Bell BHA921) on each pole surface with a gap 0.3 mm. The trapped field distribution which was measured at the centre of the open space between the poles is shown in Fig. 5b. The maximum magnetic field has reached 2.35 T. The profile of the field distribution shows a cone-shape and the maximum magnetic field is located at the centre of the surface. This is an important characteristic of the trapped field of bulk superconductors. The HTS bulk magnets are characterized as compact and strong magnets with steep gradient, whereas the superconducting solenoid magnets are featured as field generators with vast spaces and uniform fields [10].
A photograph and a schematic illustration of the experimental setup are shown in [Fig. 6] and [Fig. 7]. A couple of water channels with a cross section of 30 mm - 60 mm are alternately inserted in the space between the magnetic poles by the motor-driven moving slider so that the slurry water channels periodically changes their positions. The stainless steel balls with sizes of 3 mm and 5 mm in diameter are filled in the channels, respectively. The amount of the balls is 1 kg in each channel. When they are exposed to the magnetic field, these media attract ferromagnetic particles flowing in the channels. The magnetic flux lines are attracted to the media, changing the distribution, resultantly yielding steep gradient of magnetic field around each media. When the channel moves to the space out of the field, the compressed air is gushed into the channel to push the precipitates out. The channels alternatively change their positions every 1 min. 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 original ground water and c was that at each flow rate. The values of dc susceptibility of sample flocks were estimated by using the superconducting quantum interference device (SQUID) magnetometer (Quantum Design) under the magnetic field less than 10 kOe. The scanning electron microscope (SEM) morphology of the cohered flocks with magnetite powder was estimated by means of the electron probe micro-analyzer (EPMA, JOEL, JXA-8621).
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Fig. 6. Alternating magnetic separation apparatus.
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Fig. 7. Operation of the water channels.
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3. Results and discussions
Fig. 8 shows SEM images of the flocks which contain magnetite (Fe3O4) powder with the concentration of (a) 0 and (b) 50 ppm. According to the SEM images, the size of the precipitates are estimated less than 0.1 mm, and one can see no major differences in shape between (a) and (b). It was confirmed from the observation on EPMA mapping on Fe element that fine magnetite powder uniformly invades into all the flock particles and gives them sufficient magnetic property. The laser particle size analyzer revealed that the mean size of the flock was 0.15 mm, which roughly corresponds to the SEM images in Fig. 8.
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Fig. 8. SEM observation of cohered precipitates with (a) 0 ppm and (b) 50 ppm magnetite powder.
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Fig. 9 shows the results of magnetization curve measured by SQUID magnetometer. The initial dc susceptibility for the precipitates without adding any magnetite powder was estimated to be 0.2. Since the corresponding value for the magnetite is reported as about 6000, we suspect that these flocks would be possible to be magnetically separated when we would apply sufficient magnetic fields to the large particles and examine in slow flowing rates.
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Fig. 9. Initial magnetization curve of flocks without magnetite powder.
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Fig. 10 shows the separation ratios of Fe precipitates against the flow rates of slurry water as a function of magnetite content. The data were obtained by using the HTS bulk magnet and HGMS system using different sizes of stainless steel balls. As shown in Fig. 10a, although it is easy to understand that the separation ratios decrease with increasing flowing rates, the performances get better with increasing magnetite contents, and it is exhibited that the data which is obtained when the addition of magnetite powder is 50 ppm remain more than 80% until the flow rates reach to 5 L/min. Whereas, an inferior performance is shown in Fig. 10b when the iron balls of 5 mm in diameter were utilized. It is implied that the opening space among the media balls and the accumulated ball surface area strongly affect the property of separation ratios in this experiment.
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Fig. 10. The separation ratios of Fe precipitates against the flow rates of slurry water as a function of magnetite content, with (a) 3 mm and (b) 5 mm balls.
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It is worthwhile noting that the data obtained without adding magnetite powder exhibit the separation ratio up to 60% at low flow rate region. As the iron ions which are originally contained in the ground water are coagulated and the data of dc susceptibility expresses sufficient value to be attracted to strong fields, this behaviour is not strange. But, since it has been impossible to see this phenomenon without adding magnetite powder, we can conclude that adopting such a strong field by HTS bulk magnet that is practically optimized in near future would results in the excellent performances.
The authors are attempting to adapt this system to the practical application to the incinerator facility. The equipments will be needed to be constructed compactly when the systems will be adapted to the practical applications. Since the HTS bulk magnets are characterized as compact and strong magnets with steep gradient, this system would be possible to take it to the practical disposal lines. On the contrary, the superconducting solenoid magnets are featured as their wide closed spaces and uniform fields. And we must choose the most appropriate device to adapt to the practical processes depending on the conditions such as size, cost and so on.
4. Conclusions
The research has been conducted by the alternating channel type magnetic separating device, which has been developed to be adapted to this experiment. The separation ratios of the flocks which contain magnetite powder more than 50 ppm have remained over 80% for under the flow rates less than 5 L/min. The separation ratios reached up to 60% even when the flocks contain no magnetite powder. This implies that a novel and compact separation system can be constructed without adding any magnetic additives by focusing on the magnetic property which Fe ions in the ground water originally possesses by means of HTS bulk magnets.
