would be needed again for charging. Magnetic separators employing such magnets can be cleaned without turning off the magnet. In fact, truly continuous separators operate in this mode (6,7). Using niobium-tin coils would allow helium or pumped hydrogen to be used as a refrigerant and both of these may be available on the Moon. Except for the leads during charging and the refrigerator, there is no heat loss from such magnets. For the purposes of magnetic separation it makes no difference in principle how the magnetic field is provided. In terrestrial applications, economics dominate the choice of magnet systems. For magnetic fields less than 2 tesla (T) about the magnetization value in magnetically saturated iron, conventional copper coils are used with iron return paths contributing much of the field.* Beyond 2 T, the field can be increased only by increasing the current and the power costs become prohibitive (8). For many years, continuous fields up to about 22.5 T have been generated for research purposes at the National Magnet Laboratory at MIT by copper coil magnets alone. Now, hybrids consisting of coils in conjunction with superconducting magnets have pushed the d.c. fields available up to values above 30 T; 50 T hybrid magnets are under construction. For the first practical use in space, coils with iron, perhaps sintered from lunar soil iron, or permanent magnets probably would be chosen. Ceramic or oxide magnets offer two advantages for lunar use; they are lighter, having a specific gravity about one-half of that of metallic magnets, and they are relatively free of eddy current effects. While the magnetic energy density is lower in ceramic than in metal or alloy magnets, they still offer a better energy density to weight ratio. Freedom from eddy current effects in ceramics reduce the demagnetization problems which might occur in a device where the field was to be shunted on and off. Heat losses would also be reduced (9). LUNAR PROCESSING Any process proposed for separation on the lunar surface should have desirable characteristics concerning material needs from Earth, reliability, and so on. Waldron, Erstfeld, and Criswell present a comprehensive list of process features important to consider in choosing processes to be used on the Moon (10). Magnetic separation appears as an excellent candidate for a lunar based process when examined against this list. 1. Avoid process steps that require long completion times. HGMS is a fast process with nearly immediate results. 2. Avoid steps with low concentration input material. HGMS can treat at least 40% slurries. Concentrations in space or lunar atmosphere need to be studied. 3. Avoid volatiles. None would be used in dry magnetic separation. 4. There will be no phase separations from viscous suspensions. 5. Avoid reactions (processes) with low conversion per pass. Magnetic separation is usually highly effective in a single pass. 6. There is no need for handling and storage of large gas volumes. Superconducting magnets use cryogenic liquids which would need to be reliquified however. 7. /Avoid large heat transfers through fluid-vapor heat exchangers. Refrigeration systems for superconducting magnets supply only losses, small at lunar temperatures. *The magnetic field of the Earth is less than one oersted or about 0.5 x 10~4 T.
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