High temperature superconducting permanently magnetised discs and rings: Prospects for use in magnetic separation

Abstract

Magnetic separation is used widely in the mineral processing industry to concentrate and recover valuable minerals. High-Tc superconducting permanently magnetised discs and rings, operating in liquid nitrogen at between 65–77K, offer the opportunity to produce much stronger magnetic forces than conventional permanent magnets. In terms of the magnetic field B, the important factor determining the magnetic force is the gradient of B 2 and values as high as 60 (Tesla) 2.cm −1 can be obtained from superconducting discs. Values available from conventional permanent magnets are of the order 1–3 T 2.cm −1. Conventional permanent magnets have been used to provide fields and field gradients for drum separators in which magnetic forces are used to hold magnetisable mineral particles against the surface of a rotating drum against gravity in such a way that they are spatially separated from less magnetisable particles; in “open gradient” separators in which the particles are separated, by deflection with magnetic forces, from a falling stream of mixed particles, the so-called “falling curtain” technique. Separation has also been achieved by guiding particles on a belt moving over a permanently magnetised rotor. Superconducting permanent magnets may be applied to advantage in all the areas mentioned above. In previous work the examination of fully magnetised discs revealed problems in achieving high selectivity in the separation process. As will be discussed in this paper the difficulties of selectivity can be greatly reduced by the use of partially magnetised discs and rings. The industrial reliability required is not met by superconducting permanent magnets at present. The approach to the problem presented here is to use flux tubes in conjunction with flux pumps to provide cheap and reliable sources of superconducting permanent magnets.