Lunar pyroxene contains up to 0.2% and lunar plagioclase up to 1.5% Na2O (Williams and Jadwick, 1980). Specific pyroxene minerals containing Na are acmite or aegirite, Na2O·Fe2O3·4SiO2 and jadeite, Na2O·Al2O3·4SiO2. Among plagioclase feldspars are anorthoclase, albite, and andesine, Na2O·Al2O3·6SiO2. After these minerals are obtained by electrophoresis, roasting causes the Na2O component to sublime above 1200 K. By 1800 K as much as 70% of the available Na2O may have evaporated, leaving behind a still solid residue of iron, silicon, and aluminum oxides (Williams and Jadwick, 1980). The liberated sodium oxide is dissolved in water to give NaOH. The small amount of Na produced during boron reduction may be added directly to the HF leach system as metal, or hydrated to form NaOH, as required.
Silane for microelectronic wafer fabrication may be prepared in either of two ways. First, elemental silicon may be heated in the absence of air with magnesium to form the silicide, which is then hydrolyzed with sulfuric acid to silanes and MgSO4 (which can be recycled for sulfur much like calcium sulfate). This hydrolysis gives about 25% yield of silicon hydrides, comprised of 40% SiH4, 30% Si2H6, 15% Si3H8, 10% Si4H10, and 5% of Si5H12 and Si6H14. These may be separated by fractional distillation; or, if cooled to below 258 K, all species liquefy except SiH4, which remains a gas and can be removed. A second process suggested by Criswell (1980a) involves hydrolysis of the Mg2Si with HCl, with the magnesium chloride hydrolyzed by steam to recover the HCl.
Sulfuric acid is relatively simple to prepare, provided a suitable catalyst is available. In the two-step contact process, SO2 is burned in oxygen and in the presence of catalyst to the trioxide, which is then dissolved in water to yield the acid. The usual catalyst was, traditionally, finely powdered platinum, and more recently vanadium pentoxide. If possible, the use of these substances should be avoided as Pt and V are rare in the lunar regolith. Fortunately, practically all refractory substances have some degree of catalytic activity in the contact process, provided they are immune to impurities. Alternative and plentiful viable catalyst agents include pumice (SiO2·Al2O3), porcelain or powdered ceramic, and ferric oxide (Fe2O3), all of which are active and readily available in the LMF.
Nitric acid is more difficult to prepare, primarily because of the difficulty of "fixing" nitrogen chemically. The two most common commercial processes for acid production involve the use either of existing nitrate stocks or of platinum (for the catalytic oxidation of ammonia), neither of which is feasible at the LMF. A third method, not feasible commercially because of its low energetic efficiency, is the electric arc technique first discovered by Priestley in 1772. Elemental nitrogen and oxygen are passed through a spark discharge, producing nitric oxide with a yield of 2.5% under ideal conditions. After rapid quenching of the reaction mixture, the NO reacts rapidly below 873 K in an excess of O2 to form NO2, which makes nitric acid upon contact with water. Biological nitrogen fixation using Rhizobium and Azotobacter microorganisms is an interesting alternative and should be investigated further.
Freon (CF4) is prepared by fluorination of methane with elemental fluorine. The resulting mixture of CF4 and HF is separated by dissolution in water. There are two potentially feasible methods for producing ammonia. First is the standard Haber process, in which elemental nitrogen and hydrogen are combined directly at 800 K in the presence of iron and aluminum oxide catalysts. In the second process, magnesium is ignited at 600 K in a nitrogen atmosphere to form the nitride, which is then hydrolyzed to yield ammonia and magnesium hydroxide. Water and MgO are recycled by roasting the hydroxide.
Only very limited amounts of CaCl2 are needed, so direct combination of the elements (both of which are already available) is the preferred production pathway. Sodium carbonate for boron production is obtained by bubbling CO2 gas through an aqueous solution of NaOH, then gently heating to recover the solute.
5E.3 Quantitative LMF Materials Closure
The arguments presented in section 5E.2 demonstrate that a surprisingly simple system involving 18 elements and perhaps two dozen mineral species and process chemicals can probably achieve virtually 100% materials processing closure. Reagents necessary for electronics parts fabrication were included so that the lunar SRS has the materials needed to replicate its own computer and robot equipment While the above is P,,bably not the minimum size chemical processing plant that can retain closure, it is certainly one example of such a system. Other possibilities should be pursued in future research. Of course, once a growing seed reaches full adult size, it can install a whole new series of production equipment (say, for the recovery of platinum group metals) making possible a new range of capabilities that were unnecessary during the early growth/replication phases.
Quantitatively, in order to rigorously demonstrate complete materials closure it would be necessary to work through every chemical process described above, calculate the exact materials mass for every structure, robot, and other LMF device on an element-by-element basis, then verify that enough of each could be produced by the system. Such a detailed computation clearly lies beyond the scope of the present study. However, the team has attempted to estimate some of the most critical throughputs and analyze their anticipated effects upon total system closure. In this context, "closure" is a relationship between given machine design and a particular substrate from which the machine's chemical elemental constituents are to be drawn. Hence, the numerical calculation of closure requires a knowledge of the precise composition both of the
