tincalconite (Na2B4O7), may be found in the vicinity of ancient lunar volcanic vents. In either case it should be possible to isolate the boron species using a combination of chemical and electrophoretic techniques. However, the details of this process cannot be specified until available boron resources on the Moon are more precisely characterized.
Terrestrial boron-containing minerals are either calcium or sodium berates. A calcium borate may be converted to a sodium borate by treatment with Na2CO3, yielding borax and CaCO3 which precipitates out of solution. (Calcium carbonate may be recycled by roasting to obtain CaO and CO2, from the latter of which elemental carbon can be recovered.) Sodium berates are reduced to boric oxide in two steps:
- Na2B4O7 + H2SO4 + 5H2O → 4H3BO3 + Na2SO4
- 2H3BO3 --(heat)--> B2O3 + 3H2O
The sodium and sulfur may be recycled by the following steps:
- Na2SO4 + CaCl2 → CaSO4 + 2NaCl
- CaSO4 + C → SO2 + CaO + CO
- 2NaCl --(electrolysis)--> 2Na + Cl2
(Sulfuric acid and calcium chloride are added to the list of process chemicals.)
Low-purity boron is prepared by reduction of B2O3 with Mg, followed by vigorous washing with sodium alkali and HF. The impurities are a mixture of oxides and borides. Almost pure boron (up to 99.9999% is available commercially by this method) for electronics applications may be prepared by vapor phase reduction of BCl3 (or BBr3) with hydrogen on electrically heated filaments. BCl3 is prepared by heating B and Cl2 directly at 800 to 1100 K. Possible filament materials have not been investigated, but the mass requirement is probably less than 1 kg. Chlorine is added to the process chemicals list, since F2 cannot be substituted for Cl2 for vapor phase purification.
Phosphorous and halogens. More than 200 minerals containing up to 5% phosphorus by weight are known on Earth, but the two main species available on the Moon are fluorapatite, Ca5(PO4)3F and chlorapatite, Ca5(PO4)3Cl. The other lunar phosphorus-bearing mineral, whitlockite, is generally given as Ca3(PO4)2 but often is found associated with Mg, Fe, F, and Cl. Fluorapatite is by far the most abundant and is also the major source of fluorine on the lunar surface. (Amphibole has a trace of fluorine, but this small amount is probably not worth the trouble to extract.) Chlorapatite, very rare by comparison, is the major source of chlorine on the Moon, except for lawrencite (a nickel/iron chloride believed derived from meteorites). Whitlockite is also very rare.
Apatite is separated from lunar soil by the electrophoretic process described above. The calcium phosphate is then reduced to P2O5 by heating with silica (available from the HF leach stage) yielding pure phosphorus when treated with carbon:
- 2Ca3(PO4)2 + 6SiO2 + 10C --(electric furnace)--> P4 + 6CaSiO3 + 10CO
Alternatively, calcium phosphate dissolved in sulfuric acid gives phosphoric acid plus insoluble calcium sulfate (which may be recycled, see below). The acid is then reduced with carbon to obtain elemental phosphorus.
The sulfuric acid technique appears best for halogen extraction. When acted upon by sulfuric acid, a natural mixture of fluorapatite and chlorapatite undergoes the following net reaction:
- 3Ca3(PO4)2·Ca(F,Cl)2 + H2SO4 → H3PO4 + HF + HCl + CaSO4
This results in a solution of the three acids. If heated to above 390 K (but below 486 K), the HF and HCl boil off leaving pure orthophosphoric acid behind. The evaporate is condensed, then separated into HF and HCl by either of two methods. First, the acid solution is desiccated in vapor form over anhydrous CaCl2, then cooled to 273 K. HF condenses and is removed in liquid form, leaving HCl gas to be electrolyzed to obtain H, and Cl. Or, second, after desiccation with CaCl2 the HF/HCl solution is electrolyzed with the release of H2 at one electrode and a mixture of F2 and Cl2 at the other. This mixture is cooled to 240 K which liquefies the Cl, (to be drained off) leaving F2 gas, which may be combined directly with the liberated H, to make HF. This entire problem may also be circumvented if fluorapatite and chlorapatite can be separated using electrophoretic beneficiation.
To recover sulfur, a valuable volatile, from the above process, the calcium sulfate is recycled by roasting according to:
- CaSO4 + C --(heat)--> SO2 + CO + CaO
Supporting reagents. Reagents necessary to ensure closure of the LMF chemical processing sector include sodium hydroxide, silane, sulfuric acid, nitric acid, freon, ammonia, calcium chloride and sodium carbonate. The derivation of each is briefly reviewed below.
