properties orders of magnitude smaller than ferromagnetic iron) micron size particles from a fluid slurry in HGMS devices which are available commercially. A wide range of magnetic properties are encountered in particulate systems. At one end of the scale, the collection of large amounts of ferromagnetic solids from dense slurries has been accomplished (2). At the other end of the scale, the selective capture of diamagnetic (again, magnetic properties small but negative, i.e., repelled by a field that would attract paramagnets) particles has been observed in a system in which capture forces on these particles have been enhanced to a level comparable to that achieved in paramagnetic separations (3). The collection of magnetite in a high gradient magnetic separator was first observed at MIT during research on the beneficiation of oxidized taconite iron ore (4). A Mesabi (Minnesota) ore containing about 10% of magnetite by weight was observed to form dendrites along the magnetic field lines surrounding a 50 /xm stainless steel wool strand such as those used as a matrix in HGMS. Figure 1 illustrates the general principles of magnetic separation. A magnetic field gradient (d/7/dr) is required for a force to be exerted on magnetizable material. That force is given in one dimension as
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