Regolith refining

Refining Moondust

BELOW) Illmenite is separated from other iron bearing compounds electrostatically and is subjected to heat and hydrogen to produce water that is electrolyzed to recover hydrogen and gain oxygen.
FeTiO3+ H2=>Fe+TiO2+H2O
The spent solids consist of iron and titanium dioxide. The iron is separated from the non-magnetic TiO2 with magnets after centrifugal grinding to break up fused TiO2 and iron particles then combined with carbon and smelted in solar furnaces to make steel. The titanium dioxide is melted to make hi-temp ceramics and electrolyzed or 'de-oxidized' with the FFC (Fray-Farthing-Chen) process to get titanium metal and more oxygen.

BELOW) The non-magnetic 'de-ironed' moondust is leached with sulfuric acid after it has been melted to a glass, cooled and ground fine in rod and ball mills. A 12.5 m radius concave reflector could melt 1 ton of anorthositic material to a glass in about one hour if the material reflects 50% of the light falling on it.

Silica is used to make glass. It can be dexodized in a FFC cell to get silicon and oxygen. This avoids high temperatures and does not demand the use of highly corrosive hydrofluoric acid or fluorine gas. CaSO4 is used for plaster. Some CaSO4 is calcined or reduced with carbon to get lime-CaO, which is heated in solar furnaces with the right amounts of silica and aluminous moondust to form "clinkers" which are ground and mixed with a small percentage of CaSO4 to make Portland cement.

From ten tons of anorthite reacted with 14.1 tons of H2SO4 we get 4.9 tons of CaSO4, 12.3 tons of aluminum sulfate that could be calcined to yeild 3.7 tons of alumina and 2 tons of aluminum, 4.3 tons of silica and 2.6 tons of water that will be use to reconstitute acid stocks. The 4.9 tons of CaSO4 could yield 2 tons of lime, CaO. If the anhydrite is not roasted to lime in order to recover SO3 for reconstituting acid stocks, about 1.33 tons of sulfur must be burned with oxygen to generate SO2 for making fresh H2SO4. The hydrogen stays in the recycling loop in the form of water.

The sulfates are heated and dried. Water, unreacted sulfuric acid, HCl, HFl and H3PO4 formed by reaction of acid with chlorapatite and fluorapatite and phosphates are boiled off, condensed and electrolyzed to obtain chlorine, fluorine and phosphorus in small amounts. The sulfates are then roasted to oxides, mostly Al2O3, and reacted with silicon and carbon in a solar furnace as depicted below to obtain metals.

There might be significant qtys of MgO in the Al2O3 as well as some Na2O. Heating to 1500 C.+ in vacuum could boil these off.

l2(SO4)3+heat=Al2O3+3SO3 or 3SO2 + 1.5O2 Carbon is added to the mixture in the retort and alumina is reduced. According to Murray of the CSM, 3SiO2 +9C=3SiC+6CO at 1500-1600 C.; 2Al2O3+3C=Al4O4C+2CO at 1600-1900 C.; Al4O4C+3SiC=4Al+3Si+4CO at 1950-2200 C.;

See:

An alloy of aluminum+silicon containing small amounts of Cr and Mn remains. The CO and SO2 are recycled to maintain acid and carbon supplies. CO is shifted to methane and water in a Sabatier reactor. The methane and water are then decomposed to recover carbon & hydrogen and get oxygen. The SO2 is reformed into sulfuric acid.

BELOW) An alloy that is 4 parts Al and 3 parts Si is not useful for much, although 10-15% silicon in Al makes a good alloy. I have taken the brute force approach and suggest simply boiling the Al away from the Si with solar energy. Temperature control will be possible by adjusting the angle of reflectors so we don't boil it all away. Direct application of solar energy will be very efficient compared to the production of electricity.

Simple magnetic beneficiation is not enough. Lunar regolith consists of shocked and impact melted particles. Most iron and ilmenite bearing grains are embedded in silicate grains. The magnetic iron bearing material must be ground up and sieved, then separated magnetically again. It will not be necessary to grind all regolith-an energy and time consuming task for the machines, but just the magnetic fraction.

Drum magnet image based on design by Mark Prado. After all the magnetic material from several passes through the hopper/magnet system is extracted and ground fine then passed through the first drum magnet, ground again and passed through the second drum magnet, a concentrate of ilmenite and other iron bearing minerals is obtained. Centrifugal grinders will be used as these do not have abrasive wheels or grit that will wear down with heavy use.

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OBJECTIONS TO CARBOTHERMAL REDUCTION[1]

The extent of reduction of Ca, Mg, Ti, Mn, and Cr and their distribution in the slag and vapor phase is not well established.
There will be no calcium in the mixture. Mn and Cr are present in small amounts. There's a lot of Mg which is to be reduced with silicon and boiled off. There should be little or no titanium in the mix.
The formation of carbides, oxycarbides, and suboxides complicates the reaction mechanism which requires detailed study of the proper temperature and proportion of carbon to-silicate ratio in the feedstocks.
This is true. Detailed study is necessary. The author suggests reduction of a mixture of sulfates to oxides to metals rather than reduction of silicates in the form of anorthite. Carbides and suboxides will form but they can be decomposed at hi temp.
Pilot plant operation with alumina and silica showed that the formation of carbides and oxycarbides made it impossible to obtain a melt of Al and Si below 2000oC. Even at 2300oC, the extent of metal losses due to promotion of products into vapor phase is not well established. The metal losses are promoted when the process operates under high vacuum conditions existing in space.
Vaporized metal can be condensed and refluxed back into the solar furnace. Pressure can be maintained in the furnace by controlling the release of gases in the retort with a valve. SO2 will be released first during thermal decomposition of sulfates and removed. CO will form but this should float above metallic vapors and be vented off with clever retort design to prevent reversion.
Anorthite contains significant amounts of CaO, which further complicates the process. At 2000oC, CaO is also reduced, yielding a Ca-Al-Si alloy that must be further purified.
CaO will not be contained in the mixture.
Separation of CaO before the reduction process increases the amount of capital investment and requires large amounts of materials not readily available on the lunar surface.
The capital investment may be increased by the sulfuric acid leach, but this author contends that this is worthwhile at advanced Moon Bases. Sulfur and hydrogen are common enough on the Moon to make sulfuric acid which will be recycled. Glass, plaster and feedstock for cement and silicon will be obtained by the sulfuric acid leaching.
Despite several years of research, no plant for producing aluminum through carbothermic reduction exists.
This is also true; however, this author believes that a solar carbothermal process might succeed where blast furnace and electric arc furnace based processes failed and that this might be of use on the Moon because it require no fluoride flux or electrical power sources.

Limited quantities of aluminum sulfate and trace sulfates will dissolve in aqueous sulfuric acid solution, so the saturated acid solution is boiled off leaving sulfate precipitate, it is condensed and dripped through the 'de-ironed' moondust repeatedly until all acid has reacted to form sulfates and water. Silica and calcium sulfate remains in top tank to be dumped through after the sulfates in the bottom tank are dumped. It will be necessary to dry the material with heat while in the leaching tanks to prevent the loss of water.

Magnets are used to draw illmenite (FeTiO3), other iron bearing mineral particles, free iron containing nickel and cobalt, and iron sulfide out of anorthositic highland regolith that has been roasted to obtain volatiles like H,C,N, S, He4, Ne, and valuable fusion fuel He3. This is done to obtain fairly pure anorthite: CaAl2Si2O8.

Mare soil will yield more iron bearing minerals and titanium bearing ilmenite and will be processed to get those metals, but highland soil will have some mixed in due to meteoric "splashing" of regolith, and we want to get fairly pure anorthite for aluminum.

Metals can be purified with zone refining. Silicon rods can be refined to 99% purity. On Earth, only small amounts of material are zone refined in inert gas filled glass cylinders which prevent oxidation or combustion of the hot silicon. The surface tension of the molten zone keeps the rod from falling apart in Earth's gravity. In the low gravity of the Moon or weightlessness of space, and free vacuum, it will be possible to zone refine large masses of metals since we won't have to worry about the rod separating at the molten zone or oxidation. This will be especially useful for mass producing pure silicon for solar cells which will be needed in huge quantities for SPSs and power generation on the Moon.

Some Cr and Mn will boil away with the Al if it is possible to separate Al from Si by boiling. Perhaps these can be concentrated by ZR. Al is not zone refined on Earth, but in the low G or weightlessness and free vacuum, it might be possible. This is worth experimenting with in space at the ISS perhaps. If we grind the Al rod ends, treat with Cl gas and form chlorides, boil off the SiCl4 if any at 57.6 C and Al2Cl6 at >178 C we can extract MnCl2 with alcohol. The alcohol can be recycled. CrCl2 and CrCl3 remain. The chloride salts can be electrolyzed to get pure Mn and Cr. Vanadium at 114 ppm and Zr at 311 ppm in lunar regolith will remain in Si because they have high boiling points. These could also be extracted from impuritiy concentrations in Si rod ends. Since boiling points are lower at reduced pressure actual experimentation is necessary. In any case, zone refining concentrates trace elements as well as purifies elements like silicon. It will be easier to extract the concentrated trace elements from rod ends which are melted off or cut off with lasers perhaps than it would be to extract trace elements directly from lunar regolith. Lasers won't wear out like carbide cutting wheels. Grinding of rod ends could could be done with SiC wheels or they could be melted and sprayed into the vacuum to form microdroplets that cool by radiation to powders.

1) Rao, Choundry, Erstfeld, Williams and Yang. "V5. Extraction Processes for the Production of Aluminum, Titanium, Iron, Magnesium and Oxygen for Non-terrestrial Resources." http://www.nas.nasa.gov/About/Education/SpaceSettlement/spaceres/V-5.html

ABOVE) An FFC cell

Sieve can be removed for cleaning/unclogging.

CaAl2Si2O8 + 4H2SO4 ==> CaSO4 + Al2(SO4)3 + 2SiO2 + 4H2O

NEXT) The silica (SiO2) and CaSO4 (calcium sulfate, anhydrite, gypsum, plaster of Paris) are separated electrostatically.

Many added value substances can be obtained from the acid leaching. See: Added-Value-Leaching

+7800 V

This page was inspired by T.A. Heppenheimer's Colonies in Space Stackpole Books; 1976.
Now on-line at http://www.nss.org/settlement/ColoniesInSpace/index.html
Copyright 1977, 2007
by T. A. Heppenheimer
see chapter 7 to read about the H2SO4 process that I have suggested modifications for on this page.
http://www.nss.org/settlement/ColoniesInSpace/colonies_chap07.html

Also of interest, is another article about H2SO4 leaching by K. Eric Drexler, "Systems for the Production of Aluminum, Glass and Oxygen from Lunar Materials." Appendix IX Space Manufacturing Facilites-Space Colonies, ed. Jerry Grey; AIAA, New York, NY: 1977

1) Jean P. Murray
Engineering Division, Colorado School of Mines
SOLAR PRODUCTION OF ALUMINUM BY DIRECT REDUCTION OF ORE TO AL-SI ALLOY
<http://www.kenes.com/Ises.Abstracts/Htm/0450.htm>

see catalyst note bottom
of page

CATALYST NOTE: We might not have to import vanadium pentoxide.

[http://www.islandone.org/MMSG/aasm/AASM5E.html
"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"

If we can make the acid catalyst from moondust we are ahead of the game. The only vanadium we import will be for making tool steels.