Magnetic separation using a switchable system of permanent magnets (abstract)

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Abstract
Permanent magnets have been used in magnetic filtration, particularly in drum separators, thus, obviating the need for solenoids and electrical power. In the area of high gradient magnetic separation, which uses a matrix magnetized by an external field, the use of permanent magnets has been limited, because the field needs to be switched off periodically, due to the loss of filter efficiency owing to the accumulation of captured material, necessitating periodic cleaning. Cleaning requires the field in the matrix to be reduced, which is not possible with a nonswitchable field source. The solution presented here is to employ a novel arrangement of permanent magnets as the field source that permits the flux density in the matrix to be switched between distinct maximum and minimum levels. As a result, optimum capture is carried out with the field at maximum, then cleaning is accomplished by backwashing the filter with the field at minimum. The aim of this work is to develop a viable filtration system utilizing this principle. Early work has centered on a simple configuration of this system with two-dimensional symmetry and has demonstrated that the basic principles are sound. This apparatus has been incorporated into a separator using a ferromagnetic stainless steel wool matrix and its effectiveness demonstrated using paramagnetic wolframite particles. More sophisticated configurations are being developed with axial symmetry and with more powerful magnetmaterials, the basic outlines being optimized by means of computer-aided-design. It is anticipated that these subsequent models will provide the basis for a large operating volume, self-contained and fully reusable magnetic separation system.

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