Abstract Body

The Fukushima Daiichi nuclear power plant suffered an accident as a result of the 2011 earthquake. As a result, radioactive materials were released into the atmosphere. Afterwards, the radioactive materials were deposited in the soil due to rainfall. The government removed the topsoil and decontaminated the soil. Approximately 13 million cubic meters of contaminated soil were generated during the decontamination process, and processing it requires a huge amount of time and money. Therefore, there is a need for a method to efficiently treat contaminated soil. Therefore, we proposed a method to selectively remove clay minerals that have a high radiation dose due to their adsorption properties of caesium, which make up most of the contaminated soil. In conventional methods, magnetic separation of paramagnetic materials requires a combination of a strong magnetic field of about 5 T or more and a high gradient magnetic field with ferromagnetic filaments. Therefore, we have proposed a separation concept of the selection tube magnetic separation method in which the drag force acting on the particles to be separated is reduced as much as possible to an almost weightless state by fluid control, and a low magnetic field is used as the source of the magnetic field for the magnetic attraction force required for the separation. Under laminar flow, the particle size distribution of particles suspended in the tube depends on their streamlines (velocity distribution). The width of the particle size distribution can be precisely controlled by magnetic force. In our previous study, we have been shown from calculations that more precise separation is possible by using a solenoid-type superconducting magnet as the magnetic field source, placing a magnetic filter inside the selection tube, and controlling the magnetic force acting on the particles in a vertical downward direction. The results showed that smaller particles could be adsorbed at the same flow rate by using it in combination with magnetic separation. Thus, it was found that by using magnetic separation in combination, the water flow rate can be increased by 10 times when adsorbing particles of the same particle size, which may lead to an improvement in processing speed. And, for actual magnetic separation with selection tube, it is necessary to guide the targeted particles in a desired magnetic field area inside the bore of a superconducting magnet, and we had been succeeded in sending the particles in a desired magnetic field area by using a multi-stage selection tube [1], [2].

In this study, a superconducting solenoid magnet was applied as the magnetic field generator, sieved vermiculite particles (volume magnetic susceptibility of 7.0 × 10^-4) were measured with a particle size analyzer, and the flow velocity in the tube was determined from calculations with the targeted particle diameter of the particles to be separated as 4 µm, and a selection tube superconducting magnetic separation experiment was conducted. The experimental system is shown in Fig. 1-(a). A magnetic filter (made of SUS430 with a wire diameter of 0.6 mm) was placed in a selection tube and an external magnetic field of 2 T was applied. A two-stage selection tube was used, and the target particles were suspended in the separation area. As shown in Figure 1-(b), particles were captured on the magnetic filter, with a peak at about 4 µm particle size, confirming that vermiculite with a particle size appropriate for the calculated flow velocity was captured. High gradient magnetic separation experiments were conducted using a small-scale selection tube with a superconducting magnet of the solenoid type under a low magnetic field. Magnetic separation of paramagnetic particles of several microns in diameter was possible at a maximum applied magnetic field of 2 T, demonstrating that the magnetic separation method using selection tubes is effective for the separation of paramagnetic materials.

References

[1] Development of novel magnetic separation for paramagnetic particles using the selection tube., N.Nomura, F.Mishima, S.Nishijima, IEEE Trans. on Appl. Supercond., 32, (6), 2022.9

[2] Study on multi-stage magnetic separation device for paramagnetic materials operated in low magnetic fields., F. Mishima, A. Nagahama, N. Nomura, S. Nishijima, Progress in Superconductivity and Cryogenics, 25, (3), p.p.13-17, 2023.9

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number 24K15358.

Figure 1. (a) Concept illustration of magnetic selection tube applied Superconducting magnet. The selection tube is located above the center of the maximum magnetic field. Magnetic flux density (B-max) about 2T.

(b) Photo of vermiculite particles captured on a magnetic filter.

Keywords: Magnetic separation, paramagnetic particle, High Gradient Magnetic Separation System, low magnetic field, Selection tube