paper  AIAA-­‐2011-­‐701,  49th  AIAA  Aerospace  Sciences  Conference,  Orlando  FL,  4-­‐7  January  2011.  

Calcium Reduction as a Process for Oxygen Production from Lunar Regolith Geoffrey A. Landis1 NASA John Glenn Research Center, Cleveland OH 44135

A calcium process is proposed as a method to produce of oxygen by reduction of lunar soil. In this sequence, metallic calcium is reacted with regolith to produce calcium oxide plus reduced metals, and then the calcium oxide is electrolyzed in a CaO/CaCl2 molten-salt eutectic to produce oxygen and to regenerate the reactant calcium. The baseline calcium reaction is done at a temperature of 900 - 1000°C, but lower temperatures can be achieved by use of a flux material, such as calcium chloride. The electrolysis reaction is done at a temperature of 825-900°C. These temperatures are considerably lower than the temperatures proposed for direct electrolysis of lunar soil, and are comparable to temperatures proposed for hydrogen reduction.

A

I. Introduction

lunar base, and eventual long-term lunar settlement, will require the ability to process available lunar resources to produce useful product to reduce the requirements for resupply from Earth. It is well accepted that the most useful product that can be produced from lunar regolith will be oxygen. Oxygen is the major, by mass, component of rocket fuel; it is also required for life support, and finally it is the main (again by mass) component of water, which is also required for life support. Oxygen is an abundant component of the lunar surface; lunar rocks and regolith consist of about 45% oxygen by mass1. However, the oxygen is tightly bound in the form of silicates. Because of the network-forming properties of the silicate, silicate rocks have high melting temperature, and reduction of the rock to produce oxygen (and byproduct metals and silicon) is difficult. While the first material, and most valuable material to be produced from lunar regolith will certainly be oxygen, in the long term, in the interests of future lunar industry, it will become desirable to produce other materials from lunar materials as well. Some volatiles materials, the gaseous elements adsorbed onto the surface and implanted into grains of regolith (presumably originating from the solar wind) will be easily produced by heating the regolith, although in small quantities. These will be of considerable use for replacing life-support consumables. In addition, hydrogen is known to be available in the form of water ice (or permafrost) in cold-traps near the lunar poles, as well as available in the form of hydrated minerals in the soil in high-latitude regions. In the longer term, other desirable elements to be produced from regolith are aluminum, iron, and titanium for use as structural metals; silicon for use as a semiconductors; aluminum for use as an electrical conductors, and oxides to be used in glassmaking. It may also be valuable to produce metals or silane for use as a rocket fuel. Thus, it is desirable that a regolith reduction process be one which produces highly-reduced byproduct that can be further refined to useful materials, rather than a slag material of mixed tightly-bound oxides. Many processes for reduction of lunar regolith to produce oxygen have been proposed, too many to review in depth. Numerous reviews are available elsewhere,2,3,4 and a summary of NASA's recent work can be found in Sanders et al..5 1

Scientist, Power and In-Space Propulsion Division, NASA Glenn mailstop 302-1, 21000 Brookpark Road, Cleveland OH 44135. AIAA Associate Fellow. 1 American Institute of Aeronautics and Astronautics

paper  AIAA-­‐2011-­‐701,  49th  AIAA  Aerospace  Sciences  Conference,  Orlando  FL,  4-­‐7  January  2011.  

II. Requirements The optimum regolith reduction process would have the following desirable characteristics: 1. it should work on average lunar regolith, without extensive requirements for beneficiation 2. it should produce large amounts of oxygen; that is, it should reduce the majority mineral constituents of lunar regolith 3. it should recycle all reactants or use only reactants that must be produced from lunar material (and preferably both. Since real-world recycling of reactants will be less than 100% efficient, it is desirable that the all reactants can be replenished from local materials.) 4. it should occur at a reasonable temperature, minimizing the need for exotic materials for reaction crucibles, and reducing the requirements on solar concentrators. In addition, for a process that can be adapted to future lunar industrialization, it is desirable that the process should further: 5. Produce a reduced byproduct either in easily-separable elemental form, or in a state that may be easily refined for use in other manufacturing processes. In earlier work,6,7 it was proposed that this reduction might be done by a process of anion substitution, specifically, by the process of fluoridation. In the initial stage, regolith is reacted with fluorine. The fluorine substitutes for oxygen in the rock, converting oxides (primarily silicates) into fluoride salts, and producing oxygen as a product. The fluorine reactant must then be recovered from the process, which is done by molten-salt electrolysis of the product salts. Because of the network-forming properties of the silicate, silicate rocks have high melting temperature. The basic approach is that substituting fluorine for the oxygen breaks up the silicates, which the difficult to deal with, by converting the minerals into a chemical form amenable to reduction by electrolysis at a lower temperature. This process has several advantages over other proposed reduction methods, but also has some disadvantages. Fluorine is not present in great abundance on the moon, so it is necessary that the recycling of the reactants be extremely efficient, or else frequent resupply from Earth is needed. And, although the temperature of electrolysis of fluorine salts is considerably lower than that required for melting and electrolysis of the soil directly ("magma electrolysis"), it still does require a high-temperature step. And, finally, the process requires working with fluorine, a corrosive and highly reactive gas, at high temperatures. The process proposed here is cation substitution, which is the complement of that process. In the proposed process, the silicates of the soil are broken up by substitution of an alternate cation. To do this requires a cation that will displace silicon. This comprises the following requirements 1: a metallic cation that is more reactive than silicon, i.e., more electropositive on the electromotive scale. The alkali (lithium, sodium) and alkali earth (calcium, beryllium) metals are suitable in this regard. 2: the product of this substitution reaction must themselves be easier to reduce than the original silicates, and should be reduced by a reaction that returns the reactant to the original state. In addition, a third requirement is that the cation used itself is desirably an element that can be produced from lunar regolith. Any reaction sequence will not be one hundred percent effective in use of materials, and some losses of reactant in the processing chain will be inevitable. Use of a reactant that is not itself available on the moon would produce stringent requirements for nearly lossless processing, which will increase the cost and complexity of the process. Use of a material that is itself produced in the processing, on the other hand, means that the process can tolerate loss of material, for example due to side reactions or incomplete reaction. Several of the well-studied reactions proposed for production of oxygen from lunar regolith can be considered versions of cation substitution. Hydrogen reduction, carbothermal reduction, and aluminothermic reduction all work by reducing one of the cations of the silicate by having the oxygen bind to hydrogen, carbon, or aluminum, respectively. These reactions all are unsatisfactory in one respect or another. Hydrogen and carbon are only capable of reducing a very limited subset of the minerals found on the moon. Hydrogen reduction, for example, done at a temperature of 900-1100 °C, is useful only for reducing ilmenite, FeTiO3, a very small component of lunar regolith.8,9,10 (Even so, hydrogen reduces ilmenite by only a single oxygen atom per molecule, with a byproduct of FeTiO2).4,8 Carbothermal reduction typically occurs at even higher temperature, e.g., 1625 °C for methane reduction of olivine and pyroxene,11 up to 1800°C or above for anorthite4, and still only partially reduces the oxides. Lunar rock consists mainly (>97%) of three silicate minerals: anorthite (CaAl2Si2O8), pyroxenes ([MgFeCa]Si2O6), and olivine [MgFe]2SiO4, with a fourth mineral, ilmenite (FeTiO3) composing a variable (but small) amount of the remainder12. Thus, the five metallic elements Ca, Al, Mg, Fe and Ti, in addition to silicon, are the primary ones to be reduced. 2 American Institute of Aeronautics and Astronautics

paper  AIAA-­‐2011-­‐701,  49th  AIAA  Aerospace  Sciences  Conference,  Orlando  FL,  4-­‐7  January  2011.   The choices considered for the substitution cation are the three lightest (and most common) alkali metals, lithium, sodium, and potassium, and the alkali earth metal calcium. All of these are capable of reducing the primary components of interest, Al, Mg, Fe, Ti, and Si. Sodium is not capable of reducing calcium, and potassium is only marginally more electropositive than calcium, so if Na or K were selected, the calcium component of the regolith (about 3-7% by mass) would remain as an unreduced component (i.e., slag). Of these choices, lithium and calcium were chosen for further study. Calcium was selected as the final choice for a number of reasons, partly for the known technology to process the product, calcium oxide, to liberate oxygen and return the calcium to metallic form, but also because calcium itself is a significant component of the regolith, and hence can be generated in situ, while lithium is a trace component of lunar soil at the level of a few parts per million in average soil.

III. Process The process is straightforward, consisting of two parts, first reducing the regolith, and then regenerating the metallic reactant: (1) Metallothermic reduction. This is done by heating of the regolith in the presence of (liquid) metallic calcium, to convert the silicates into metals plus calcium oxide. (2) Molten Salt Electrolysis: this stage electrolyses the calcium oxide in a molten salt at 825-900 °C, to produce metallic calcium and oxygen. These processes require considerably lower temperatures than direct electrolysis ("Magma electrolysis") reactions, and produce oxygen with considerably higher efficiency than hydrogen or carbothermal reduction methods. The byproduct of the reaction is a metallic alloy, comprising the reduced metals and silicon. Further separation and purification steps can be taken from this point to produce the reduced elements into refined product for use in other processing. A. Metallothermic Reduction The metallothermic reduction has been used for production of metals on Earth.13 This is the common production process used for reduction of rare earth elements, as well as a reaction that has been used for production of Manganese, chromium, vanadium, zirconium, and niobium.14 In particular, the process proposes here is calciothermic reduction. This is done by heating of the regolith in the presence of (liquid) metallic calcium at a temperature greater than the melting point of calcium, 845°C. At this temperature, the calcium reacts with the rock to convert the silicates into metals plus calcium oxide. Calcium reduction was described by Alexander15 at a temperature of 900 to 1000°C. As described by Alexander, the process is done directly, and the reaction rate is enhanced by use of finely-ground reactants as well as an excess of calcium. An example metallothermic reaction for olivine, with an example composition MgFeSiO4, is:

MgFeSiO4 + 4Ca → 4 CaO + MgFeSi

The reaction product, calcium oxide, is a refractory material, and production of a layer of oxide tends to limit the reaction. This can be dealt with by use of finely ground reactant material,15 or by addition of a flux to fluidize the slag.13 Since both calcium and calcium oxide are soluble in calcium chloride, either CaCl2 or a CaO/CaCl2 eutectic mix can be used as a flux to accomplish the calciothermic reaction in a liquid solution. Mishra and Olson,16 for example, describe using CaCl2 flux for the reduction of titanium oxide to titanium using calcium; while Gupta and Krishnamurthy17 describe the use of a CaCl2/NaCl melt for calciothermic production of rare earth elements at 710790°C. Calcium iodide and calcium sulfide can also be used as a flux.16 Since the calciothermic reaction is exothermic, once it is initiated the reaction is self-heating. The byproduct is a metallic alloy, with the main components of silicon, magnesium, and iron; a smaller amount of aluminum, and traces (depending on the feedstock) of titanium. If the reaction temperature remains low, the byproduct will be in the form of the reduced alloy forming a shell on the outside of individual grains of unreacted regolith. Hence, maximizing the reacted fraction requires increasing the surface area for reaction by reducing the particle size in the feedstock. This can be done by an added crushing or grinding step, or (more reasonably) by sieving regolith to use the fine particle fraction. Alternately, it may be acceptable to leave some fraction of the regolith unreacted, and either discard the byproduct, or send it to an additional step to recover the alloy by either physical, thermal (melting or distilling) or chemical processing for other use.

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paper  AIAA-­‐2011-­‐701,  49th  AIAA  Aerospace  Sciences  Conference,  Orlando  FL,  4-­‐7  January  2011.   If the temperature is sufficiently high, the molten metal/silicon byproduct, which is denser than the oxide or the reactant calcium, will settle to the bottom of the crucible, where it can be decanted as a liquid, or separated mechanically.17 B. Molten Salt Electrolysis Following the reaction, the oxygen is in the form of the reaction products, calcium oxide. To generate oxygen (and recover the reactant, elemental calcium), this oxide is electrolyzed:

Ca+2 + 2 e- → Ca (metal) O-2 → ½ O2 + 2 enet: CaO → Ca (metal) + ½ O2 (gas)

With a melt temperature of 2580 °C, calcium oxide is too refractory a material to be electrolyzed from the melt in its pure form. This problem is overcome by electrolysis of the calcium oxide from a molten salt eutectic.18 This can be done using a molten calcium chloride/calcium oxide mixture, with a eutectic point of 750 °C. The electrolysis in CaO/CaCl2 has been demonstrated at an operating temperature of 774 °C,18 but is more commonly done at temperature from 850 to 900 °C16,18,19 Mishra and Olson16 also point out that the two steps of calciothermic reduction and electrolysis need not be separated; a single reaction crucible can be used to accomplish both steps simultaneously, using "in-situ generated metallic calcium" regenerated by electrolysis as the reaction proceeds. This combined processing describes the FFC ("Fray, Farthing, Chen") electrochemical reduction process.20, which has been used to produce metallic titanium, molybdenum, and tungsten from oxides.21 The FFC process has been demonstrated at industrial levels for production of titanium.22 A difficulty in the FCC process is that, since calcium metal is soluble in the CaO/CaCl2 melt, the oxygen produced at the anode is subject to reaction with Ca in solution, in effect short-circuiting the reaction. While the chemical energy of reaction is not lost (it is returned to the system to heat the molten salt) it reduces the effective coulomb efficiency and hence decreases the reaction rate. A number of solutions to this problem exist, including using a solid-oxide ionic conductor that is permeable to oxygen ions but not calcium ions at the anode, or designing a process to withdraw the oxygen generated at the anode before it reacts with the dissolved calcium. The advantage of the FFC process over other electrical reduction processes proposed for lunar regolith are several: 1. It does not require the regolith itself to be melted, and thus can be accomplished at temperatures as low at the CaO/CaCl2 eutectic point of 750 °C. 2. It does not require electrical conductivity of the regolith 3. The process will reduce all the components of lunar regolith, up to and including magnesium and titanium. Process Calcium reduction Hydrogen reduction of Ilmenite Magma electrolysis Carbothermal reduction Vacuum pyrolysis of ilmenite Vacuum pyrolysis of regolith

temperature 825-900 °C 800°C5 - 1100 °C9,10 1300°C10 - 1450 °C23 1625 °C11 - 1800°C4,5 >1200°C24 1400°C24 - 2500 °C25

yield fraction 100% 1.5%5 30%10 205,11 - 45%11 1.5% 4%26 - 35%25

notes (1) (2) (3)(4) (2)

notes: (1) temperature may be as low as 750°C (2) yield calculated for average regolith; up to 90% for soil that is beneficiated to pure ilmenite (3) 30% conversion yield is for a one-pass system. (4) temperature listed is minimum; higher temperatures result in better performance.

Table 1: Comparison of oxygen production technologies. Yield fraction is defined here as the fraction of the oxygen content in lunar soil which is recovered; the mass fraction of oxygen recovered equals the yield times the oxygen mass fraction of lunar soil, about 60%.

IV. Conclusion A calcium process is proposed as a means for production of oxygen from lunar soil. In this process sequence, metallic calcium is reacted with the regolith to produce calcium oxide, and then the calcium oxide is electrolyzed in a CaO/CaCl2 molten-salt eutectic to produce oxygen and calcium. The baseline calcium reaction is done at a 4 American Institute of Aeronautics and Astronautics

paper  AIAA-­‐2011-­‐701,  49th  AIAA  Aerospace  Sciences  Conference,  Orlando  FL,  4-­‐7  January  2011.   temperature of 900 - 1000°C, but lower temperatures can be achieved by use of a flux material, such as calcium chloride. The electrolysis reaction is done at a temperature of 825-900°C. Table 1 compares the calcium reduction technique with several other proposed regolith reduction methods. The temperature required is considerably lower than the temperatures proposed for direct electrolysis of lunar soil, and is slightly lower than temperatures proposed for hydrogen reduction. The individual reactions of the proposed process have all been demonstrated, and some of these are currently in use, e.g., calciothermal reduction is used for production of rare-earth elements. The steps may also be combined, using the FFC process, in which the electrolysis and the thermal reduction steps occur simultaneously. This calcium sequence has the advantage over many other proposed reduction processes that it does not require selection of a particular mineral for feedstock, or beneficiation of the soil to enhance one component (although it would be desirable to beneficiate the soil to the extent of using the fine component, rather than bulk rock for feedstock, since the reaction rate will be proportional to surface area). The process should be able to remove the oxygen from all the mineral components found in average soil. This means that it has very high oxygen yield, as seen in Table 1. Another advantage of the process is that the primary reactant, metallic calcium, is itself a component of lunar soil, and thus reactant that is lost in processing can be replaced by material generated from local resources. The byproduct of this production process, comprising a metal-silicon alloy, can in principle be further refined to produce other materials that may be useful for lunar industrialization, although these further refining steps have not yet been analyzed.

References 1. McKay, D., et al., “The Lunar Regolith,” Chapter 7, Lunar Sourcebook, Heiken, G., Vaniman, D., and French, B., eds., Cambridge University Press, Cambridge, 1991, pp. 285–356. 2. Lewis, J., Matthews, M.S., and Guerrieri, M. L., eds., "Part II: the Moon," in Resources of Near-Earth Space, University of Arizona Press, Tucson AZ, 1993, pp. 17-448. 3. Hepp, A., Linne, D., Landis, G., Wadel, M., and Colvin, J., "Production and Use of Metals and Oxygen for Lunar Propulsion," Journal of Propulsion and Power, Vol. 10, No. 6, 1994, pp. 834-840. 4. Schrunk, D. G., Sharpe, B., Cooper, B. L., and Thangavelu, M., "Appendix E: Proposed Processes for Lunar Oxygen Extraction," in The Moon: Resources, Future Development and Settlement, Wiley-Praxis, John Wiley & Sons, 1999, pp. 305-352. 5. Sanders, G. B., Larson, W. E., Sacksteder, K. R., and Mclemore, C. A., "NASA In-Situ Resource Utilization (ISRU) Project –Development & Implementation," paper AIAA-2008-7853, AIAA Space-2008 Conference & Exposition, San Diego CA, 9-11 September 2008. 6. Landis, G. A., "Materials Refining for Solar Array Production on the Moon," NASA Technical Memorandum TM-214014, December 2005. 7. Landis, G. A., "Materials Refining on the Moon," Acta Astronautica, Vol. 60, No. 10-11, May-June 2007, pp. 906– 915. 8. Yoshida, H., Watanabe, T., Kanamori, H., Yoshida, T., Ogiwara, S., and Eguchi, K., "Experimental Study on Water Production by Hydrogen Reduction of Lunar Soil Simulant in a Fixed Bed Reactor," Second Space Resources Roundtable, Golden, USA, Nov. 2000. Available http://www.chemenv.titech.ac.jp/watanabe/Media/PDF-conf/SRR00.pdf 9. Zhou, G., and Mardon, A. A. , "Lunar Oxygen Production through Hydrogen Reduction at High Temperatures," abstract 3012, Annual Meeting of the Lunar Exploration Analysis Group, Washington DC, September 14–16 2010. 10. McCullough, E. D., and Cutler, A. H., "ISRU Lunar Processing Research at Boeing," paper AIAA-2001-0938, 39th AIAA Aerospace Sciences Meeting and Exhibit, Jan. 8-11 2001, Reno NV. 11. Gustafson, R. J., White, B. C., Fidler, M. J., and Muscatello, A. C., "Demonstrating the Solar Carbothermal Reduction of Lunar Regolith to Produce Oxygen," paper AIAA 2010-1163, 48th AIAA Aerospace Sciences Meeting, January 4 - 7 2010, Orlando FL. 12. Korotev, R. L., Chemie der Erde 65, 2005, p 309. 13. Habashi, F., "Chapter 9: Metallothermic Reactions," Principles of Extractive Metallurgy: General principles, Volume 3: Pyrometallurgy, Gordon and Breach, 1986, p. 131. 14. Gupta, C. K., "4.6.4: Pyrometallurgy: Calciothermy," in Chemical Metallurgy: Principles and Practice, Wiley-VCH, Weinheim, 2003, p. 379-386. 15. Alexander, P. P., "Reduction of Ores by Metallic Calcium," United States Patent 2,043,363 June 9, 1936. 16. Mishra, B., and Olson, D. L., "Molten Salt Applications in Materials Processing," Journal of Physics and Chemistry of Solids, 66, 2005, pp. 396-401. 17. Gupta, C. K., and Krishnamurthy, N., "4.9.3, Metallothermic Reduction in Molten Salt," in Extractive Metallurgy of Rare Earths, CRC Press 2004, p. 247. 18. Threadgill, W. D., "Preparation of Metallic Calcium by Electrolysis of Calcium Oxide Dissolved in Molten Calcium Chloride," J. Electrochemical Soc., Vol. 111, No. 12, December 1964, pp. 1408-1411.

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paper  AIAA-­‐2011-­‐701,  49th  AIAA  Aerospace  Sciences  Conference,  Orlando  FL,  4-­‐7  January  2011.   19. Ferro, P. D., Mishra, B., Olson, D.L., and Averill, W. A., "Application of Ceramic Membrane in Molten Salt Electrolysis of CaO-CaCl2", Waste Management, Vol. 17, No. 7, 1997, pp. 451-461. 20. Chen, G. Z., Fray, D. J., and Farthing, T. W., "Direct Electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride," Nature 407, 21 September 2000, pp. 361-364. doi:10.1038/35030069. 21. Bhagat, R., Jackson, M., Inman, D., and Dashwood, R., "Production of Ti-W Alloys from Mixed Oxide Precursors via the FFC Cambridge Process," Journal of the Electrochemical Society 156: E1-7, (2009). doi:10.1149/1.2999340. 22. Suzuki, R. O., "Direct Reduction Processes for Titanium Oxide in Molten Salt," JOM Journal of the Minerals, Metals and Materials Society, Vol. 59, Number 1, January, 2007, doi 10.1007/s11837-007-0014-7, pp. 68-71. 23. Colson, R. A. and Haskin, L. A., "Producing Oxygen by Silicate Melt Electrolysis," in Resources of Near-Earth Space, University of Arizona Press, Tucson AZ, 1993, pp. 109-127. 24. Cardiff, E. H., Pomeroy, B. R., Banks, I. S. and Benz, A., "Vacuum Pyrolysis and Related ISRU Techniques, NASA Goddard Space Flight Center document 20070014929, Space Technology and Applications International Forum, Albuquerque NM, 15-17 Feb. 2007; AIP Conference Proceedings, Volume 880, pp. 846-853. 25. Cardiff, E. H., Pomeroy, B. R., and Matchett, J. P., "A Demonstration of Vacuum Pyrolysis," abstract 2015, Space Resources Roundtable VII, Golden CO, 2005. 26. Senior, C. L., "Lunar Oxygen Production by Pyrolysis," in Resources of Near-Earth Space, University of Arizona Press, Tucson AZ, 1993, pp. 179-197.

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