where x is the magnetic susceptibility relative to the fluid carrier, defined as the ratio of the magnetization to the magnetizing field, V is the material volume and dH/dx is the gradient of the field over the volume of the material. In the example of Fig. 1, unsymmetrical poles produce a magnetic field which varies in space. Magnetic particles, if free to move, do so toward the pointed pole under the influence of the magnetic force. (A number of separators have been used over the years that have shaped poles.) Figure 1 illustrates a slurry of particles passing through such a separator. The magnetic particles migrate transversely in the slurry stream; the stream is then split and the materials are separated. Unfortunately, the gradient, which normally needs to be large in order to produce a large magnetic force, is not large between widely separated pole pieces. A high gradient magnetic separator, in contrast, uses materials such as steel wool or expanded metal to fill the space occupied by the field. Ferromagnetic steel wool strands 50 to 100 /xm in diameter cause large, local distortions in the field. In the case of HGMS, the gradients extend over micrometers. Therefore, the local gradients are much larger than those attainable with conventional methods. Very large field gradients can be obtained when high field strengths are applied. Hence the force, which depends directly on the gradient as well as susceptiblity and particle size, is very large, and very small, weakly magnetic particles can be trapped. The creation of high magnetic fields is the specialty of the Francis Bitter National Magnet Laboratory.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==