Separation of Products In chloride electrolysis, the chlorine gas can be separated under reduced gravity, particularly with the aid of suction if necessary. Separation of molten magnesium is more difficult, especially from electrolytes denser than magnesium. Separation by wetting of a porous cathode and use of lighter electrolytes have been discussed previously as possible methods of improving separation. Another alternative is the deposition of solid magnesium from electrolytes melting at temperatures below that of magnesium. Strelets (6) provides detailed information on the melting point, viscosity, conductivity, etc., of a variety of compositions, some of which have a sufficiently low melting point. However, solid deposits from fused salts tend to be irregular, which may cause difficulty in removal, interelectrode spacing, etc. Solid deposits may be considered if a feasible method of collecting molten magnesium cannot be developed. If a rotating module will be the norm for the overall lunar processing system, and approximately 1.0 g is attained, reduced gravitation will cease to be an imposed condition for cell operation. Anode Materials Graphite anodes are used in chloride electrolysis and carbon is used in the electric furnace reductions to produce the ferrosilicon for the Magnetherm process. The corrosion of graphite in the I. G. Farben anhydrous chloride process is small and should be tolerable. Improvements in feed purity, etc., may further minimize corrosion. Recovery of carbon through Fischer-Tropsch synthesis by using Co and H2 and burning to lampblack will probably not provide a suitable starting material for graphite anodes. Pyrolytic deposition from the Fischer-Tropsch products may directly provide a suitable anode. Other Electrolytic Methods Magnesium can be electrolyzed from a fused sulfide bath. Magnesium oxide can also be used as feed to a fused cryolite bath (7) similar to that used for aluminum production. In such a process the carbon anode is extensively oxidized. Insoluble anodes for use in oxide-fed cryolite baths have been considered. Kronenberg (8) tried depolarizing several anode materials with hydrogen, methane, and carbon monoxide. Water formed by hydrogen, however, caused hydrolysis and other undesirable side reactions. Silicon, as a reductant for MgO, may also be electrolyzed out of a fused fluoride bath, but the practicality of scaling up this method has not been determined (9). Investigations are now underway at Battelle to deposit silicon from nonaqueous electrolytes (10). Thin film deposits for photovoltaic devices are the current objective but electrowinning is a longer range possibility. CONCLUSIONS There are several options for direct or indirect electrolytic reduction to obtain magnesium metal from lunar materials. The most extensively developed process is the chloride process utilizing an anhydrous magnesium chloride feed. This process
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