Open-gradient Magnetic Separator With Racetrack Coils Suitable for Cleaning Aqueous Solutions

Superconductivity enables magnetic fields in a magnitude of 5-10 times greater than conventional electromagnets. Applied to magnetic separation, use of superconducting magnets opens the possibility to separate sub-micron sized paramagnetic particles. Open-gradient magnetic separation (OGMS) enables continuous operation without the need of matrix-elements. Such a separator consisting of liquid helium cryostat and an Nb₃Sn coils were designed and constructed for testing the separation of various slurries. The separation parameters, such as size and magnetic susceptibility of particles to be separated, applied magnetic force density, and flow velocity have to be experimentally optimized. In isodynamic separation, high gradient and nearly constant and homogenous magnetic force density distribution in separation zone are created by magnet design. Here the superconducting OGMS was used in purification of synthetically made solutions emulating industrial wastewaters. Performance of the used Nb₃Sn magnet is discussed and proposals for developing the performance of racetrack magnets are given. Finally, a simple permanent magnet separator was constructed and tested with wastewater

I. Introduction

Magnetic separation can be described as a method of separating particles in relation to their magnetic properties. This long-standing engineering practice is mainly used by the mining industry for concentration of minerals and for purification of different slurries. Efficient usage of magnetic separation in minerals processing has encouraged its use also in wastewater treatment. During the 1970's, development of high-field superconducting magnets along with high-gradient separation process made it possible to separate fine, weakly magnetic particles [1], [2]. Later for example viruses, algae, phosphate, and even dissolved pollutants have been removed by magnetic seeding technique, which enhances the magnetic properties of materials to be removed [3]–[6]. Magnetic purification accelerates considerably the separation of sludge from liquid, and thus leads to increased processing rates compared to conventional decantation and mechanical filtration.