THE ARTEMIS PROJECT
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Ore Processing
Section 2.2.3.
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Silicon Production

Geoffrey A. Landis

Although silicon is abundant (second most abundant element, after oxygen, in the lunar crust), typically hydrogen, carbon, and halogens are required for existing technologies of refining and purification, although it is reasonable to expect that a process sequence adapted to lunar conditions may minimize usage of these materials. Calculations show that if recycling efficiency is good, transportation of these materials from Earth will not be a limiting factor.

The most common soil material is anorthite, CaAl2Si2O8, which can be considered to be a mixture of calcium, aluminum, and silicon oxides: CaO + Al2O3 + 2SiO2. In the literature several methods are proposed to process extraterrestrial ores, but none have been tested in situ. The primary criteria for the process selection are low power requirement and low requirement for non-lunar materials. Other important criteria include the end products provided, the process complexity, and suitability for manufacturing under 1/6 G.

Processes considered include fluoroacid leaching, vapor phase pyrolysis [1], carbothermal processing, and slag electrolysis. For the candidate process we considered aluminum reduction followed by electrolysis to recover the aluminum, as shown by the following reaction:

        3 SiO2 + 4 Al  -->  3 Si + 2 Al2O3,
        2 Al2O3 (electrolysis)  -->  4 Al + 3 O2

This process has been developed and demonstrated by Keller and co-workers at EMEC corporation [2]. The aluminum acts simultaneously as a reducing agent for the silicon dioxide and as a solvent for the silicon that has been formed, which can subsequently be crystallized out of the solution in a nearly pure form by cooling. Because the reduction is exothermic, the reaction vessel needs to be heated externally only during the initial phase of melting of the slag and the beginning of the reaction.

An alternative process is electroreduction from an alkaline-earth oxide melt, which has been demonstrated by DeMattei, Elwell, and Feigelson at Stanford [3], as shown in the reaction:

        3 BaSiO3 (electrolysis)  ->  Si + 3 BaO + 2 SiO2 + O2

This has the disadvantage that the flux metal, barium, is not available on the moon. It is possible that other metals can be used. This is not a problem as long as the material recycling efficiency is high.

The silicon obtained must be purified for use in semiconductor devices. A candidate process for purification is by float-zone recrystallization, where the silicon rod is melted in a zone that passes along the length of the rod. Impurities are segregated to the end of the material. This process is slow, but has the advantage of zero materials consumption.

References

(10) D.L. Anthony et al., "Dry Extraction of Silicon and Aluminum from Lunar Ores," Paper No. LBS-88-066, Lunar Bases and Space Activities in the 21st Century Symposium, 5-7 April 1988, Houston, TX.

(11) R.C. DeMattei, D. Elwell, and R,S. Feigelson, "Electrodeposition of Silicon at Temperatures above its Melting Point," J. Electrochem. Soc., 128, 8, 1712-1714 (1981).

(12) P.A. Taylor, "Silane: Manufacture and Applications," Solid State Technology, July 1987.

Content by Geoffrey A. Landis <geoffrey.landis@lerc.nasa.gov>

Ore Processing

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