fine_print

Challenges:

     A mixture of silicon, magnesia and alumina with traces of chromium, manganese, sodium and potassium oxides might involve many complex reactions.    Aluminum can reduce chromium oxide and manganese oxides. The small amount of alumina that forms due to these reactions will undergo carbothermic reduction. It doesn't seem as if there will be a problem. Sodium and potassium oxides will be reduced easily with carbon at high temperatures and boil out of the melt. K2O should be reduced by carbon at 850 C.; Na2O at 1000 C.; Cr2O3 at 1250 C. and MnO at 1400 C. Well below the temperatures needed to reduce alumina. Carbides of chrominum might form but these will decompose at the temperatures required to reduce alumina.

MgO could be boiled out of the alumina obtained by roasting Al2(SO4)3 at 1500 C. Traces of Na and K might boil off during reduction of alumina and be captured in a cool trap.

     High Temperature Materials Needed

     Finally, the solar furnace/retort must be made of a material that can handle intense heat and the cold of the lunar night. Pure silica like the Shuttle heat shield tiles has extremely low thermal expansion and is a candidate. There will be no water to cool the retort or outer furnace walls. There might be a helium gas cooling system and some really large area radiators. If the retorts are made out of ceramic bricks produced during magma electrolysis or simple cast basalt they won't be very costly, presuming we have cheap labor, human and robotic, to build retorts to replace the ones that crack. How will we glue these bricks together? Could they be welded together somehow, perhaps with microwave or lasers or electron beams?   Magma electrolysis of unbeneficiated moondust yeilds oxygen and iron and silicon. That leave a melt of magnesium-aluminum spinel mixed with CaO rich silicate. I suspect that sodium and potassium will boil off. What are the properties of ceramics made from this stuff? If temperature is not a problem, we can be sure that the walls of the furnace must be very thick, as they will be gradually reduced by the silicon, carbon and heat as well as the feedstock!   We may need something more exotic like a graphite crucible insulated by stabilized zirconia and alumina. Manufacturing all these things on the Moon will be quite a challenge. The carbon could come from volatile scavenging. Zirconia must be imported. Everything else must be made on the Moon from local resources.

It's easy to see why 95% of the materials used on the Moon will be cast basalt, glass, iron and steel! Unfortunately, there isn't much hydrogen on the Moon and we need a cheap way to make large amounts of aluminum and/or aluminum-silicon (both Al and Si burn at 13,000 BTU/pound) for rocket fuel. Given the constant Sun of the lunar day, it seems as if solar silicothermic and carbothermic reduction could be an efficient way to do this.

The Fine Print

    Rationale for H2SO4 Leaching and Solar Silico-carbothermic Reduction

    The Processes
     The set of processes I have described consist of 1) volatile harvesting 2) magnetic benefication 3) hydrogen reduction of ilmenite 4) the FFC process 5) sulfuric acid leaching 6) distillation of HF, HCl and H3PO4 7) calcination of sulfates 8) silicothermic and carbothermic reduction 9) aluminum boiling 10) zone refining 11) electrostatic separation 12) magma electrolysis 13) carbon recovery by methanation and pyrolysis 14) chlorination

     There are other processes that use electrolysis to get silicon, oxygen, calcium and aluminum, but I prefer this solar carbothermic reduction centered set of processes.

1.It does not require a flux like lithium fluoride.
2.It uses the direct application of solar energy rather than the generation of large quantities of electricity that would require massive solar panel farms and/or nuclear reactors. Only aluminum foil dish reflectors and polished metal secondary mirrors are needed.
3. High temperatures, but not nearly as high as plasma decomposition temperatures are involved. No free oxygen is released, thus there are no problems with oxygen attack upon materials of the smelter.
4.Temperatures in the furnace can be controlled precisely by using a reflector that consists of multiple units, like the petals of a flower, that can open and close individually as desired to control the amount of solar energy directed into the furnace. The reflectors will track the Sun slowly during the lunar day for roughly 4300 hours of operation annually.
5.Multiple metals are produced at the same time.
6.The carbochlorination and electrolysis of aluminum is uncessary.
7.The sulfuric acid leach yields products that have value as is like calcium sulfate that has agricultural uses and can be used for plaster (wall coatings, molds, medical uses, etc.) and sheet rock as well as a minor portland cement ingredient. Calcium sulfate can be decomposed with solar heat and perhaps carbon to get CaO for making cement and glass with controlled composition. Calcium metal, a conductor superior to copper, can be derived from CaO by reduction with aluminum. Also by reaction with fluorapatite and chlorapatite we get HF, HCl and H3PO4 after filtering and boiling off residual H2SO4 and formed water and distillation.

        2Ca(PO4)2*Ca(F,Cl) + H2SO4 ==> H3PO4 + HF + HCl + CaSO4

     Reaction of phosphoric acid with minerals can yield phosphate fertilizer and HCl plus sodium makes common table salt, NaCl which has obivious uses and can make sodium hydroxide by electrolysis of aqueous NaCl solutions. HF can etch glass and be used to make high purity semiconducter grade silicon in small batches. Fluorine and chlorine can be used to make CFC refrigerants when combined with carbon and dry cleaning fluid-carbon tetrachloride. Chlorine is also used to make silicones essential for airlock seals. Sodium and chlorine are also used in polycarbonate synthesis. Polycarbonate makes unbreakable vehicle windows and space helments. From two million tons of regolith obtained by mining a square kilometer (250 acres) to a depth of about one meter and roasting the soil we can get about 80-100 tons of hydrogen, 200-400 tons of carbon, 200 tons of nitrogen, and 1000-2000 tons of sulfur to make and reconstitute H2SO4.   There will be no sulfuric acid shortage if some of the sulfate salts are not decomposed but used as is. By acid leaching two million tons of Moon dust we will get 50 tons of chlorine and 340 tons of fluorine, at 100% recovery. In reality we will get less. And it might take years to do this. These figures are based on the average concentrations in regolith. Susequently, I feel that the sulfuric acid leaching step is worthwhile. Melting, cooling, and grinding with rod and ball mills of anorthositic highland regolith must be done before acid leaching to make it more easily attacked by H2SO4.

   .Glass Formulations

      Pure silica fibers bound by a matrix of lower melting point glass like alumino-silicate glass made by controlled additions of CaO, MgO, Al2O3, Na2O and K2O can make glass-glass composites that are lightweight and very strong. Magnesium and aluminum can be reacted with H2SO4 to form MgSO4 and Al2(SO4)3, decomposed to oxides at 850 C and 1040 C. respectively, and added to silica in desired amounts. Sodium and potassium can be reacted with water to form hydroxides and added to glass in desired amounts. CaO will be added accordingly. Potassium hydroxide can also be used as alkaline fuel cell electrolyte.

     Metal Base Rocket Fuels

     The aluminum/silicon alloy might also be ground as used as rocket fuel. It could be melted in a solar furnace and sprayed through a nozzle to make microspheres that cool by radiation. These would then be shattered and flattened in impact grinders that require no grit or abrasive wheels that can wear down.

    Magma electrolysis can be used to get oxygen, ferrosilicon and ceramic bricks. With the proper electrodes there will be no consumption of them. Ferrosilicon can be mixed with iron from hydrogen reduction of ilmenite to get high silicon alloy iron for sulfuric acid handling equipment. It might even be possible to make a low performance monopropellant from ferrosilicon mixed with LUNOX for short range sub-orbital "Moon Hoppers." Iron burns in pure oxygen and silicon has about 13,000 BTU per pound. Bricks can be used for construction. If the voltage is high enough, magnesium will boil out of the melt and if divided from oxygen it will not burn, but this might be unecessary as plenty of magnesium will be gained by solar silicothermic reduction of oxides derived by solar heat calcining of sulfates.

     Materials Aplenty

     If huge tonnages of regolith are roasted for helium 3 mining there will be no shortage of H, He, C, N or S. We will be able to produce silicones and plastics for their most vital purposes like electrical wire insulation. Processing of several hundred thousand tons of regolith annually for silicon, aluminum, iron and steel, titanium and other elements will make surface vehicle (wheeled and rail), habitat (steel modules to lava tubes sealed with sulfur cement), solar power satellite, space station and spaceship industry (Ti frames, Al skins) possible. We will also have enough sulfur for large masses of sulfur concrete, simple cast basalt bricks, tiles, pottery etc., portland cement and plaster for interior constructions, glass, glass-glass composites, ceramics, metals and oxygen for large dwellings within lava tubes. Early bases could be composed of steel frames and plates welded together to form pressurized volume with life support, computers and communications from amphibious wheeled landing vehicles envisioned by Peter Kokh of the Moon Miner's Manifesto dubbed "Frogs."

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