Since March 1, 1989, attention was concentrated on the extraction of ilmenite from extraterrestrial materials and on the planning and development of laboratory facilities for carbonyl extraction of ferrous metal alloys. Work under three subcontracts was administered by this project: (1) electrolytic production of oxygen from molten lunar materials; (2) microwave processing of lunar materials; and (3) production of a resource-oriented space science data base

Lewis, John S.

English

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Extraction of Volatile and Metals From Extraterrestrial Materials .,_
Lunar and Planetary Laboratory
The University of Arizona
Abstract
Since 1 March 1989, we have concentrated our attention on the extraction of
ilmenite from extraterrestrial materials and on the planning and development of
laboratory facilities for carbonyl extraction of ferrous metal alloys. Work under three
subcontracts was administered by this project: electrolytic production of oxygen from
molten lunar materials (L. Haskin, Washington University), microwave processing of
lunar materials (T. Meek, University of Tennessee), and production of a resource-
oriented space science data base (D. Davis, Planetary Sciences Institute).
Introduction
As the first principal target of SERC's research, the production of propellants from
nonterrestrial materials, especially those found on the lunar surface, has necessarily
been the focal point of much of our research activity. Since the large majority of the
mass of an H/O propellant (89%) is liquid oxygen, and since programmatic
considerations seem to favor relatively early operations on the Moon, the focus of
efforts within this program has been the production of oxygen on the Moon. Our
research group has participated in this program in two very different ways. First, we
have studied the prospects for extracting reactive FeO-bearing minerals suitable for
oxygen production from the lunar regolith, concentrating on the properties and
extraction physics of ilmenite and the preparation of suitable stimulants for lunar-
derived ilmenite. Second, we have searched for simple ways to coproduce other
products along with oxygen, so as to maximize the total yield of useful products per
unit energy expended. This latter search has exposed ferrous metals and refractory
oxides as the most likely products to accompany lunar oxygen production. We have
chosen to begin this effort with an experimental investigation of the use of the gaseous
carbonyl process for recovery of ferrous metals in conjunction with the ilmenite
reduction and melt electrolysis schemes for lunar oxygen production.
Lunar Ilmenite and Simulants
As the result of a continuing literature search, several major concerns regarding
proposed schemes for oxygen production from lunar ilmenite have been identified.
These are the effects of other minerals on oxygen production, the chemical nature of
simulants of lunar ilmenite, procedures for beneficiation of ilmenite from lunar regolith
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feedstocks, and the merits of using lunar fines versus lithic fragments as the source of
ilmenite. •
Effects o/ Other Minerals. Hydrogen reduction experiments by Williams (1985),
using terrestrial ilmenite simulants containing trace amounts of iron sulfides, has
demonstrated that small amounts of sulfur have dramatic effects on the ilmenite
reduction process. During hydrogen reduction of the simulant, hydrogen sulfide formed
along with water vapor in such quantity as to force the termination of the experiment.
When such a mixture is electrolytically decomposed to regenerate hydrogen and
oxygen, sulfur is oxidized to sulfur trioxide and forms sulfuric acid, which is highly
corrosive. Since lunar soils typically contain approximately 0.5 wt% of the sulfide
mineral troilite (stoichiometric FeS) (Vaniman et al. 1989), some method of removing
sulfides from the feedstock must be devised. Because both troilite and lunar ilmenite
are nonmagnetic (stoichiometric FeS is antiferromagnetic whereas familiar terrestrial
pyrrhotite is ferrimagnetic), and since both minerals have nearly the same density,
devising an adequate separation scheme is nor trivial. Pre-oxidation of the sulfide by
roasting to release sulfur dioxide is a possibility, but this necessitates more
complexities: either (a) the SO2 must be discarded, which wastes oxygen, or (b) it must
be further reacted to make sulfuric acid for process use, which apparently requires the
demonstration of a whole new family of processes in which some practical and
profitable use can be made of the sulfuric acid, and in which the sulfuric acid can be
recycled. This introduces a wholly unwanted set of complications into what was
intended to be a simple, early processing scheme to make oxygen.
Simulants. Lunar ilmenite is almost stoichiometric FeTiO 3 and, hence, contains no
ferric iron. Therefore, terrestrial ilmenite, which is a solid solution of Fe203 in FeTiO 3,
is not a good thermodynamic or kinetic analogue of lunar ilmenite. Synthetic ilmenites
are useful for determining the effects of minor substitution of other (lunar) constitutents
of the kinetics of ilmenite reduction. The most abundant solute in lunar ilmenite is
MgTiO 3, which is never the dominant impurity in terrestrial ilmenites. Synthetic
iimenites, although useful for kinetic studies, do not suffice either for beneficiation
experiments or for determining the effects of other native lunar minerals on the
reduction process.
In the search for a better, more relevant analogue of lunar ilmenite, we have
considered in some detail the use of many different types of meteorites. The closest
analogue is found in the eucritic (basaltic) meteorites, ilmenite-bearing rocks with
overall chemistry and oxidation state similar to lunar basalts. Indeed, with respect to
oxidation state, the eucrites are known to be formed under conditions strikingly similar
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(but not identical) to those under which lunar basalts formed, at far lower oxygen
fugacities than terrestrial basalts. Table 1.4 compares the composition of lunar and
eucritic ilmenite.
Table 1.4 Composition of natural lunar and eucritic ilmenites, a
Lunar Eucritic
Moiety Mean Range (wt%) Mean Range (wt%)
W
FeO 43.3 35.20- 47.70 44.20 42.80- 45.00
TiO 2 52.7 49.90- 55.50 52.80 52.50- 53.00
AI203 -- 0.10- 0.30 0.06 0.03- 0.11
Cr203 0.64 0.50- 2.12 0.09 0.03- 0.27
MgO 2.23 0.00- 9.50 1.05 0.46- 2.35
MnO 0.42 0.00- 0.95 0.88 0.82- 0.93
CaO -- 0.00- 0.30 0.10 0.03- 0.16
aLunar data from a compilation of 133 ilmenites from high-Ti basalts (Vaniman et
al. 1989), except for AI203 and CaO, which are Apollo 11 ilmenites (Masan and Nelson
1970). Ilmenite data for eight eucrites are from Bunch and Keil (1971).
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Bene/iciation. Due to its lack of ferric iron, lunar ilmenite has a low magnetic
susceptibility, similar to that of the coexisting silicate phases. Attempts to beneficiate
eucritic ilmenites (which also lack ferric iron) magnetically were unsuccessful. It
appears that electrostatic separation is the most viable presently conceived technique
for use with lunar feedstocks. The amount of recoverable ilmenite in the lunar soils
(about 1-2% ilmenite) is far lower than in high-titanium basaltic rocks (8-16%) (Vaniman,
personal communication). Moreover, experiments by William Agosto have shown that
electrostatic separation experiments on terrestrial soil simulants lacking agglutinates
(shock-fused glass and rock fragments) produced fairly satisfactory enrichments of
iimenite. This suggests that lithic fragments from high-titanium basalts may be a much
better ilmenite source than soils.
Unfortunately, the use of rocks as a feedstock requires a crushing step before
beneficiation, and crushing may not, by itself, efficiently liberate single ilmenite grains.
During a recent attempt to separate ilmenite from a crushed eucrite, the rock did not
disaggregate cleanly along grain boundaries and most of the ilmenite remained trapped
in composite grains. In addition, a serious "static cling" problem was observed, in
which individual mineral and composite grain-sized lithic fragments were coated with a
fine adherent dust produced during crushing. This coating introduced a large amount
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of contamination, including FeS, into even the °'ilmenite grain" fraction. This problem
is traditionally combated by passing moist air over the material, a solution most
inappropriate for use on the Moon. Further, static (and van der Waals) adhesion of
particles is likely to be much more severe in the lunar environment than in our
laboratory, since lunar grains lack adsorbed multilayer coatings of "sticky" (high-
dipole-moment, easily condensed, or highly polarizable) gases that, on Earth, inhibit the
"welding" of these grains to each other at their points of contact. At the very least,
study of a variety of methods of disaggregating strong rocks is needed.
Ferrous Metal Extraction
Extraction and use of nonterrestrial ferrous metals is very attractive for several
reasons that are quite unrelated to the terrestrial mining experience. First, the iron-
group elements are extremely abundant in the solar system. The terrestrial planets,
their moons, and the asteroids are generally about 30% iron and nickel by mass.
Further, these elements are often found as native metals, instead of being tied up in
thermodynamically stable oxides and sulfides as they are in Earth-surface ore deposits.
Indeed, some belt asteroids, and even one near-Earth asteroid, have reflection spectra
characteristic of pure native metal. The amounts of ferrous native metals available in
nearby space are not trivial; one of the metallic (M-type) asteroids in the belt, 16
Psyche, has a diameter of 250 km. If the surface composition is representative of its
interior, Psyche contains about 6 x 1018 tonnes of ferrous native metals, equivalent to
200,000,000 years of global iron and steel production at present rates.
On a more modest scale, several opportunities for acquiring ferrous native metals
for early use in space present themselves. First, there are native metals found in the
lunar regolith, both tiny fragments of asteroidal metal from the explosion of impactors
and nearly pure elemental iron made by reduction of lunar FeO-bearing minerals by
solar wind-implanted hydrogen. These forms of native metals make up about 1000 ppm
of the regolith, most of which is readily extractable by a magnetic rake. Second, the
ilmenite reduction process leaves an intimate, partly sintered mixture of metallic iron
and the titanium oxide rutile (TiO2). Each ton of oxygen extracted leaves behind 4.5
tons of metallic iron, 5.7 tons of rutile, and an unknown amount of unreacted ilmenite
and dross. Third, electrolysis of molten silicates produces a cathode deposit of metals,
dominantly iron during the early stages of electrochemical reduction. The mixed
metals in the cathode deposit also include nickel, cobalt, chromium, and manganese, all
of which form gaseous carbonyls. Further electrolysis produces abundant silicon,
leaving a refractory oxide melt rich in calcium, magnesium, titanium, and aluminum.
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Beyond the Moon, asteroids are abundant, rich sources of ferrous native metals.
Concentrations of 10 to 30% by weight of native metals are typically present in
undifferentiated (chondritic) meteorites, and many asteroids in the inner part of the belt
have metallic (M-type) reflection spectra. There is even a near-Earth asteroid that has
an M-type spectrum. These bodies may be up to 99% native ferrous metal alloy. The
concentration of free metals in the average meteorite is about 20%, some 200 times as
high as in the lunar regolith.
In order to extract and purify these metals with minimal energy consumption, it is
highly desirable to avoid forming intermediate compounds that are strongly bonded.
Attractive schemes include physical separation by magnetic beneficiation and
volatilization by the gaseous carbonyl process. Efficient magnetic separation, even in
the case of the lunar regolith,= requires crushing to a fine enough size to liberate
particles of metal. Both native metal minerals, kamacite (alpha iron; 4 to 7% Ni
content) and taenite (gamma iron; 20 to 60% Ni), are ferromagnetic, as are the
widespread trace minerals schreibersite, (Fe, Ni)3P, and cohenite, (Fe, Ni)3C. The
sulfide mineral troi!ite, FeS, is antiferromagnetic and, hence, follows free metals during
magnetic extraction only because of the common textural association of FeS with metal
in intergrowths. In carbonaceous chondrites, the metals are progressively oxidized,
culminating in zero free metal and abundant magnetite in the highly oxidized, carbon-
and volatile-rich Ci chondrites. Magnetite is ferrimagnetic and, hence, it is almost as
easily extracted as free metal. Strong heating of CI. chondrites leads to extensive
autoreduction, with release of carbon oxides and water and the formation of metallic
iron.
Gaseous carbonyl extraction is appealing for several reasons (Lewis et al. 1988):
The only reagent necessary is carbon monoxide, which is released upon heating every
known class of meteorite. The compounds formed, principally iron pentacarbonyl and
nickel tetracarbonyl, are weakly bonded, can be readily separated from each other by
distillation, and can also be readily separated into their component elements by gentle
thermal decomposition (or by isothermal pressure reduction). Separation of ferrous
metals is not practical in any operation involving melting of a metal mixture. It is
indeed remarkable that the low-energy route to ironmaking also gives a natural and
highly effective route to making pure metal separates. This is very desirable, because
it permits the formulation of a wide variety of alloy compositions.
We have surveyed the literature on the thermodynamics and kinetics of gaseous
carbonyl formation and developed several designs for extraction systems. We have
• acquired several of the crucial components and will be assembling them in the near
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future, so we can resume the experimentation we originally began in 1982. The ideal
materials for the system construction are brass and aluminum, which are completely
invulnerable to carbonyl attack. The initial experiments will be aimed at validating the
safety and functional design of the system. Safety is paramount in handling carbonyls,
because they are highly toxic. The next step will be the volatilization of pure samples
of iron and nickel (and possibly cobalt, if we are confident in the reliability of the
system at the high pressures necessary for cobalt carbonyl formation). We may then
move directly to volatilization of meteoritic iron. Iron meteorite samples are available
in large quantities and at low prices, and nothing is to be gained by lengthly and
expensive excursions into simulant production and validation. Indeed, magnetic
separates from chondritic meteorite powders are themselves very good first-order
stimulants of lunar regolith native free metals and will permit us to prepare for
requesting lunar samples for similar study.
We will then use native iron produced in ilmenite reduction experiments and
mixed-metal cathode deposits from Haskin's electrolysis project as feedstock for the
carbonyl extraction. Analytic services will be provided by Boynton (neutron activation)
and by Ruiz (inductively coupled plasma mass spectrometer). Platinum-group metal
extraction from the carbonyl volatilization residues will be carried out by Freiser's
group (Strategic Metals Recovery Research Facility).
We are not presently planning extensive experimentation on fabrication of metal
products in our laboratory. However, we have already begun discussions with
Vaporform Products regarding collaborative research on codeposition of Fe/Ni alloys.
They have the greatest body of expertise in the world on fabrication of nickel products
by chemical vapor deposition (CVD) from gaseous nickel carbonyl, but have no
experience with the effects Of admixtures of iron carbonyl as an "extender." It is,
however, well established that pure nickel tetracarbonyl produces nickel deposits
("castings") of excellent quality, whereas pure iron pentacarbonyl produces only a fine
black powder of ultra-high-purity iron. The intermediate compositional regime is ripe
for exploration. The facilities required for large-scale experimentation with mixed-
carbonyl CVD are available only at their facility in Pennsylvania.
References
Bunch, T. E. and Keil, K., 1971, "Chromite and ilmenite in nonchondritic meteorites,"
Amer. Mineral. 56, 146-157.
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Lewis, d. S., Jones, T. D., and Farrand, W., 1988, "Carbonyl Extraction of Lunar and
Asteroidal Metals," pages 111-122 in Space 88, AIAA, New York.*
Mason, B. and Nelson, W. G., 1970, The Lunar Rocks, J. Wiley and Sons, N. Y.
Vaniman, D., Heiken, G., and Muhich, T., 1989, "Lunar [Imenite," preprint.
Williams, R. J., 1985, "Oxygen Extraction From Lunar Materials: An Experimental Test of
an Ilmenite Reduction Process," in Lunar Bases and Space Activities o/ the 21st
Century, LPI, Houston.
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"See Appendix C.


Extraction of volatile and metals from extraterrestrial materials

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