separated from the raw lunar substrate by straightforward electromagnetic techniques directly as the raw input material leaves the input hopper. This Fe will be fairly pure, containing only about 5% nickel and 0.2% cobalt (Phinney et al., 1977).
Structural metals and metal oxides. Of all the chemical materials processing options studied to date, the hydrofluoric (HF) acid leach technique appears to have the best potential for minimum operating mass, ease of element separations to high purity, and favorable energy and heat rejection requirements (Amold et al., 1981;Waldron et al., 1979). HF acid leach (Waldon et al., 1979), shown in figure 5.41 in flowsheet form, uses low-temperature hydrochemical steps to separate the silica content of the lunar raw material from metallic oxides in minerals by converting them to fluorides and fluorosilicates. The silica is then vaporized as SiF4, leaving Ca, Al, Fe, Mg, and Ti fluoro salts to be separated by a variety of solution, precipitation, ion exchange, and electrolytic steps. These are then reduced to the pure metallic form with sodium metal, which is recycled. (HF is added as a major process chemical.)
Sodium for the reduction of metals and silicon may be obtained by a modified Castner cell process, which involves the electrolysis of molten NaOH to produce Na, O2, and H2. Iron electrodes can be used in this application. (NaOH must also be added to the process chemicals list.) Metal oxides and silicon dioxide can be obtained, where needed as ceramics, refractories, or for glasses, by hydrolysis of the fluoride or fluorosilicate with H2O steam (for the metal oxides), with NH3 (for silicon dioxide), or by ion exchange methods. (Water and ammonia are thus added to the list of process chemicals.) Electronics-grade silicon may be prepared through zone-refining and other techniques with up to nine-9s purity, although these processes have not been thoroughly investigated in the present study.
In a discussion of the HF acid leach technique, Criswell (1978) points out that the process with its various options is adaptable to several of the potential lunar minerals or concentrates including feldspars, pyroxenes, olivines, and even nonsilicates such as ilmenite and spinels. Beneficiation of these minerals (the major constituents of lunar soil) seems unnecessary since the appropriate separations are performed later on the fluorides and fluorosilicates. However, if necessary, this beneficiation can be accomplished using the electrophoretic method described above.
In addition to Fe, Al, Mg, Ti, Ca, Si, and O2, it is possible that the HF acid leach process may be used to prepare Cr and Mn. These two elements are present in pyroxene (up to 0.5% MnO, up to 1.25% Cr2O3), olivine and spinel (which contain Cr). CrF2 is slightly soluble in water; MnF2 is soluble, so the techniques described above should still be applicable although the details of this extension have not been extensively studied.
One final problem unique to the HF process is the question of containers. Process vessels and tubing normally employed in terrestrial industry are attacked by hydrofluoric acid. One solution is to use special carbon steel alloys for this purpose - these are customarily employed for storage of fluorine gas because a protective layer of iron fluoride forms which greatly impedes further chemical attack. A second alternative is to use hydrocarbon-based waxes, paraffins or plastics which are not attacked by HF, applied as a thin layer to the insides of pipes and containers. Yet a third option is to develop new structures perhaps based on sulfur and phosphorus (Allcock, 1974) and other inorganic polymers (Lee, 1979) which could be in reasonably plentiful supply in the lunar factory.
Extraction of volatiles. Lunar soil heated to 1300 K releases 0.1% by weight of the following trapped volatiles: CO, CO2, N2, H2, H2O, SO2, H2S, CH4, and inert gases (He, Ar, Ne, Kr, Xe). As much as 0.5-1.5% by weight may be released upon heating to 1700 K (Phinney et al., 1977). CO may be reduced to carbon by methanation followed by decomposition of the CH4 species over a refractory catalyst (such as MgO) to C and H2CO2 may be reduced to CO by making use of the reversible reaction:
| 800 K | ||
| 2CO | <---> | CO2 + C |
| 1300 K |
That is, CO2 passed over elemental C above 1300 K reduces to CO, which can then be methanated and further reduced to C over hot refractory. N2, H2, H2O, and SO2 are desirable process chemicals. H2S may be burned in O2 to yield SO2 and water. A sharply limited supply of O2 results in steam and sulfur vapor. If SO2 and H2S are mixed at room temperature, they react to form water and elemental sulfur. Finally, oxygen bubbled through an aqueous solution of H2S produces a precipitate of elemental sulfur.
Inert gases are useful in lasers and for providing a nonreactive atmosphere, and may be separated by fractional condensation using cold traps at various temperatures.
Boron production. Historically on Earth the most important source of boron has been borax or tincal (Na2B4O7·10H2O), though today the more common source is kernite or rasorite (Na2B4O7·4H2O) Other boron minerals include colemanite (Ca2B6O11·5H2O), ulexite (NaCaB5O9·8H2O), priceite (Ca4B10O19·7H2O), boracite (Mg3B7O13Cl) in salt beds, and sassolite (H3BO3).
Boron minerals on the Moon are likely associated with phosphorus-bearing apatite species (Dunning, personal communication, 1980), although it is possible that local concentrations of the most common anhydrous boron mineral,
