| Basic Chemistry for Moon Miners
By Dave Dietzler Mining for Light Elements-H,C,N & S Hydrogen, helium, nitrogen and carbon blown out by the solar wind over the ages exist in lunar regolith. These could be extracted by heating the regolith. Often seen figures are 40-50 ppm hydrogen, 25 ppm helium 4, 100 ppm nitrogen, 200 ppm carbon and 500+ ppm sulfur. I have seen higher numbers for sulfur that include the sulfur in the form of troilite, FeS, of meteoric origin. These quantities in ppm seem to be averages taken from Apollo samples. In reality, by mining one million tons of regolith we could not expect to get 50 tons of hydrogen or 100 tons of nitrogen because recovery rates are never 100%. Additionally, hydrogen and carbon react with oxygen in the mineral grains of the regolith to form water, CO and CO2 when the lunar soil is heated. Methane also seems to form by reaction of carbon compounds with hydrogen. These gases could all be separated by cooling and liquefying some of them, membranes, pressure swing absorption and/or compression. At the University of Wisconsin, Dr. Kulcinski and his colleagues have designed a ten ton regolith mining machine called the Mark 3. They predict that one of their Mark 3 robotic miners could process six million tons of regolith per year and produce 201 tons of hydrogen, 109 tons of water, 0.033 tons of helium 3 (that’s 33 kg.), 102 tons of helium 4, 16.5 tons of nitrogen, 63 tons of carbon monoxide, 56 tons of CO2 and 53 tons of methane. The CO, CO2 and CH4 contain a total of 82 tons of carbon. These researchers have chosen to heat the regolith only up to 700 C. Beyond that temperature oxides of sulfur form that react with evolved water vapor aided by the catalytic properties of the regolith to form sulfuric acid that would be hard on equipment. The Mark 3 is not intended to extract sulfur but some future machine with a high silicon (11-14%) alloy iron furnace lined with cast basalt (CB resists up to 98% sulfuric acid) perhaps might mine the Moon for sulfur. The main use for sulfur would be the production of sulfuric acid for leaching minerals out of regolith. Given the higher concentrations of sulfur the miner would not have to cover areas as large as the Mark 3 will for helium 3 and other volatiles to get a respectable load of sulfur in the form of SO2, SO3, H2S and H2SO4. For more information about the Mark 3 miner, see: http://www.nasa-academy.org/soffen/travelgrant/gadja.pdf |
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| Problem: How is carbon obtained for steel production?
Carbon monoxide could be reacted with carbon monoxide at temperatures no higher than 700 C. to form CO2 and carbon. This happens all the time in sooty smoke stacks. 2CO ==> CO2 + C No more than half the carbon in the CO could be obtained this way. It might be possible to take the CO2 and subject it to electrolysis to get CO and oxygen then react the CO with CO again. Another way would be to heat the CO2 to 1,100 C. to partially dissociate it into CO and oxygen. 2CO2 ==> 2CO + O2 Then react the CO formed with CO again after separation of the oxygen from the CO. The problem here is that CO2 is fairly stable and only some of the molecules will dissociate at high temperatures. Another way would be to react CO and CO2 with hydrogen in the presence of a catalyst to form methane that would be decomposed (pyrolyzed) to carbon and hydrogen by passing it over hot refractories at 900 C. The carbon would be removed from the refractories by scraping, wire brushing or ultrasonics and the hydrogen reused to convert more CO and CO2 to CH4. Some CH4 comes out of the regolith when it is roasted at up to 700 C. as in the Mark 3 miner and that will go straight to pyrolysis. Methane breaks down with less energy than CO2 does and that’s why it is worthwhile to convert or “shift,” as a chemist might say, the CO and CO2 to CH4 then pyrolyze it. CO2 + 4H2 ==> nickel catalyst ==> CH4 + 2H2O CO + 3H2 ==> Ni catalyst ==> CH4 + H2O CH4 + heat ==> C + 2H2 2H2O + electrolysis ==> 2H2 + O2 The carbon obtained can be used to make steel or filter water and other liquids. The hydrogen obtained can be reused to process more CO and CO2 to CH4. If CO is desired for reducing metallic oxides for instance, the RWGS (Reverse Water Gas Shift) reaction can be used. CO2 + H2 ==> iron-chrome catalyst ==> CO + H2O Carbon monoxide and hydrogen in combination are called “synthesis gas” because by adjusting their proportions to each other and using the right catalysts many organic chemicals can be made. CO + 2H2 ==> ZnO/Cr2O3 catalyst ==> CH3OH methanol CO + CH3OH ==> at 200 C. and 50 atm. pressure ==> CH3COOH acetic acid 8CO + 17H2 ==> Fe-Co catalyst ==> C8H18 octane These chemicals would be used as solvents and as starting points for synthesizing other chemicals. 2CO + 4H2 ==> Fe cat. ==> C2H4 (ethylene) + 2H2O At high pressures and in the presence of a titanium chloride catalyst ethylene polymerizes and forms polyethylene, the most common plastic in use today. This plastic or a better one will be essential for wire insulation on the Moon. Other substances of lunar interest are carbides. These are made by reacting oxides at high temperatures in electric furnaces with an excess of carbon. CaO + 3C ==> CO + CaC2 calcium carbide CaC2 + 2H2O ==> Ca(OH)2 + C2H2 acetylene Acetylene is of course used for oxyacetylene welding that would be done inside pressurized habitat on the Moon to recover the CO2 and H2O that forms when C2H2 burns. SiO2 + 3C ==> SiC + 2CO Silicon carbide (SiC) is hard and abrasive. TiO2 + 3C ==> TiC + 2CO Titanium carbide (TiC) might be used for molten silicate electrodes or as turbine parts. Sodium silicate (Water Glass) is another interesting chemical. It is not sodium orthosilicate- Na4SiO4. It is composed of sodium or potassium salts of silicic acids (H6Si2O7, H4Si3O8, H2SiO3…) made by boiling silica (SiO2) in a water solution of NaOH (sodium hydroxide, lye) or KOH (potassium hydroxide). Water glass is used as an adhesive; a cloth, wood and paper fireproofing agent; a cement for wood, glass, pottery and stoneware; and to preserve eggs by sealing the pores in the shells. It can also be used to paint with. See: http://www.lunar-reclamation.org/art/painting_exp.htm |
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| The chart above shows us that substantial quantities of H, He, C and N can be obtained when mining lunar regolith for helium 3 and other solar wind implanted volatile elements and compounds. At roughly $10,000/lb. to the lunar surface the value of these substances will be on the order of billions of dollars. |
| Sulfur, etc.
I have seen figures indicating that sulfur is present in lunar regolith at from 540ppm to 1700ppm. Some might be from the solar wind and some is in the form of meteoric troilite, FeS. A robotic miner with a high silicon (11-14%) alloy iron furnace, piping and storage tanks might be needed since this alloy resists corrosion by H2SO4. It’s used widely in the chemical industry partly because it is cheaper than stainless steel. We don’t have that much chromium and nickel for stainless steel on the Moon but we have plenty of iron and silicon. The equipment might also be lined with cast basalt since CB resists up to 96% H2SO4. This miner would heat the regolith up over 700 C. to get the sulfur out in the form of sulfur vapors and SO2, SO3, H2SO4 due to catalytic action of the regolith, and H2S. The main use for sulfur will be production of sulfuric acid. Some of this will be gotten during regolith roasting. The SO3 gas can be bubbled into water to make H2SO4. The SO2 can be reacted with oxygen over a platinum or vanadium pentoxide (V2O5) catalyst to form SO3 that is then reacted with H2O. On the Moon we might just use regolith for a cheap catalyst. H2S could be burned to SO2 and H2O. Sulfuric acid is produced in larger quantities than any other industrial chemical. It is sometimes called “the bread of chemistry.” It can be used to remove rust from steel. This is called “pickling.” Rusting steel won’t be a problem out in the vacuum, but steel will rust on inner walls of welded steel plate modules unless we paint them or coat them with a vapor deposited thin film of aluminum; and other items might rust due to humidity inside habitat. Sulfuric acid can convert phosphate rock to superphosphate fertilizer. On the Moon we might leach KREEP bearing rocks and regolith with sulfuric acid. The phosphorus in KREEP probably exists in the form of apatite. From: http://www.islandone.org/MMSG/aasm/AASM5E.html#5e 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. HF can be used to extract silicon from regolith by formining SiF4 gas. Silicon and silane can be obtained by treating silicon from magma electrolysis with hot HCL gas. See: Lunar…Rockets. Phosphoric acid would be used to make phosphate fertilizer and detergents. Calcium sulfate, CaSO4, is plaster. It has many uses including wall board by laying a layer of wet plaster between two sheets of woven glass fiber cloth, medical and dental casts, making cement ( 5% CaSO4 in cement retards setting time), making molds for casting aluminum and magnesium, conditioning garden soil, etc. Calcium sulfate can also be obtained by sulfuric acid leaching regolith that has had the iron and titanium bearing minerals removed magnetically and electrostatically. Insoluble silica (actually silicic acids but when dried they become silica) will form and also barely soluble CaSO4 that can be filtered out with a glass fiber filter that allows the water solution of aluminum, magnesium and sodium sulfates pass thru. The SiO2 and CaSO4 could be separated electrostatically. See: Electrostatic Separation. Silica has many uses including glass making. These sulfates could be heated until they break down into oxides like Al2O3 for emery, synthetic gems, refractories and filters for corrosive liquids. Magnesium sulfate (MgSO4) is epsom salts. If thermally decomposed the MgO formed could be used for firebrick and also as an antacid. See: Added Value Products The Al2(SO4)3 can be reacted with lime (CaO) to form aluminum hydroxide. Al2(SO4)3 + 3Ca(OH)2 ==> 2Al(OH)3 + 3CaSO4 The lime can be obtained by thermally decomposing CaSO4 or reacting it with carbon. CaSO4 + C + heat ==> CaO + SO2 + CO Aluminum hydroxide is used for paper making, water purification and sewage treatment, water proofing fabrics and as a textile mordant. If Al(OH)3 is precipitated from a dye solution, the precipitate becomes colored. The Al(OH)3 absorbs the dye and holds it too the cloth. The aluminum hydroxide is called a mordant and the colored product a lake. Aluminum hydroxide in water forms a gelatinous material that sinks and carries down muddy material suspended in the water and even bacteria. Another way to get silica, CaSO4 and Al2(SO4)3 would be to take anorthositic regolith that has been heated to 1800 – 2000 K to make cement powder and leach that with H2SO4. This would get some of the other salts out of the way. We could also take calcium aluminate, CaAl2O4, obtained by roasting highland regolith at over 1500 C. to drive out all the FeO, SiO2, MgO, Na2O and K2O, and leach that to get only CaSO4 and Al2(SO4)3 that are separated by filtration of the water solution that forms. CaAl2O4 + 4H2SO4 ==> CaSO4 + Al2(SO4)3 + 4H2O |
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| Recommended Reading for Moon miners interested in chemistry: The Case For Mars by Robert Zubrin Mining the Sky by John Lewis General Chemisty by Linus Pauling Hawley's Condensed Chemical Dictionary Any chemistry text books you can aquire that are collecting dust in basements or book shelves, at yard sales, or by going to public or college libraries. Some of my data came from College Chemistry by Nebergall & Schmidt (1957) and General Chemistry 4th edition by Ebbing (1993) |