Electromagnetic separation techniques in metal casting. I. Conventional methods

Conventional methods for electromagnetic separation of small inclusions in metal casting are analyzed. The electromagnetic separation implies flotation of nonconducting inclusions in a conducting fluid (molten metal) by Archimedes´ electromagnetic force, and their subsequent removal. The magnitudes of the separation force are calculated for the following methods: (1) separation by an electromagnetic induction coil; (2) separation by a traveling magnetic field; (3) pinch-effect separation; and (4) separation with superimposed current and superimposed magnetic field. Theoretically estimated force magnitudes appear on the order of the gravitation buoyancy force. They are not as high as observed in some laboratory-scale experiments. Power efficiency of the electromagnetic separation is studied with respect to the accompanying Joule heating of molten metal. A power efficiency coefficient is introduced to compare various separation techniques. Typically, this coefficient is very low, on the order of 10^-4 and less for inclusions of 10-μm size in molten aluminum. The highest values of the power coefficient are obtained for the traveling magnetic field. The pinch-effect separation mechanism is found to be most energy-consuming. We found that the use of dc superconducting coils can drastically improve the power efficiency of an electromagnetic separation process if the injection current is used as the origin of the Lorentz force. This mechanism, as well as that of magnetohydrodynamic separation without superimposed current in very strong magnetic fields, are subjects of an upcoming publication