The overall objective of this project was to develop a novel carbothermal reduction process for production of oxygen from lunar ilmenite. The specific objective was to use a reaction sequence in which a wide variety of carbonaceous compounds (including carbonaceous wastes) can be used as reducing agents. During the first phase, two reactor systems were designed, constructed, and operated to study the reaction fundamental important in this process. One system is a small fluidized bed, and the other is a thermo-gravimetric reactor system. Preliminary experiments on synthetic ilmenite are conducted to study the effect of carbon type, carbon loading, temperature, and gas flow rate. Results indicate that a reaction path based on carbon gasification can be used to promote the overall kinetics. A unique temperature and concentration-programmed reaction procedure was being developed for rapid parametric study of the process

Shadman, F.

Zhao, Y.

English

NASA Technical Reports Server

N91-25 O y
Production of Oxycjen From Lunar"lmenite /
F. Shadman and Y. Zhao iO' /b
Department of Chemical Engineering
The University of Arizona
s
Abstract
The overall objective of this project is to develop a novel carbothermal reduction
process for production of oxygen from lunar ilmenite. The specific objective is to use
a reaction sequence in which a wide variety of carbonaceous compounds (including
carbonaceous wastes) can be used as reducing agents. During the first phase of this
work, two reactor systems were designed, constructed, and operated to investigate the
fundamentals of the reactions important in this process. One system is a small fluidized
bed, and the other is a thermo-gravimetric reactor system. Preliminary experiments on
synthetic ilmenite are conducted to study the effect of carbon type, carbon loading,
temperature, and gas flow rate. Results indicate that a reaction path based on carbon
gasification can be employed to promote the overall kinetics. A unique temperature
and concentration-programmed reaction procedure is being developed for rapid
parametric study of the process.
Introduction
Oxygen is a consumable material which needs to be produced continuously in most
space missions. Its use for propulsion, as well as life support, makes oxygen one of
the largest volume chemicals to be produced in space. Production of oxygen from
lunar materials is of particular interest and is a very attractive possibility.
There is a significant amount of oxygen on the moon, although very little is readily
available as water or gaseous oxygen. The only practical source of oxygen on the
lunar surface is igneous materials, which contain typically 40-50% oxygen as oxides.
Minerals present in these rocks include ilmenite (the most abundant opaque material in
lunar rocks), anorthite, and olivine. The igneous rocks have been pre-crushed to form
regolith, the lunar equivalent of soil, by meteoritic bombardments. This reduces the
need for crushing and simplifies the mining and separation problem greatly. 1,2
Extraction of oxygen from iron oxide present in lunar ilmenite is of particular
interest because it is energetically more favorable than extraction from silicon,
aluminum, titanium, calcium, or magnesium oxides. Iron oxide reduction is also
attractive because of its potential for producting iron as a co-product. A number of
processes have been suggested for oxygen production from ilmenite in lunar regolith.
Table 1.1 shows a list of these processes. Most of these processes require imported
https://ntrs.nasa.gov/search.jsp?R=19910015892 2020-03-19T18:03:58+00:00Z
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reagents to be recycled in the process. Therefore, low loss per mass is a critical
factor and requirement for process feasibility.
Table 1.1. Selected proposed processes.
Hydrogen reduction
Low-temperature carbothermal reduction
Slagging (high-temperature) carbothermal reduction
Slag electrolysis
Leaching: Fluoroacid
HCI
Caustic
Carbo-chlorination
Vaporization/rapid quenching, arc electrolysis, plasma reduction, alkali
cycling, vapor-phase pyrolysis, etc.
metal thermal
Among the proposed processes, the hydrogen and carbothermal reductions of
ilmenite appear very promising. Hydrogen reduction has a relatively simple process
configuration; the individual steps are relatively well studied. However, the major
problem is the large heating and cooling loads required to condense the water and
then heat the hydrogen to its reaction temperature. Handling and storage of large
amounts of hydrogen is also a problem. Hydrogen reduction of lunar ilmenite has been
studied by several investigators. 3 Carbothermal reduction of terrestrial ilmenite has also
been extensively studied, but the application to lunar ilmenite is still an open area for
investigation. 4
Even though a considerable body of literature has been published on various
aspects of the reduction of terrestrial ilmenite, the mechanism of transformation and the
rate-controlling step in the overall reduction have not been clearly established. Most
studies on terrestrial ilmenite show that both hydrogen reduction reactions take place in
two main stages. In the first stage, the ferric iron component of the ilmenite was
rapidly reduced to ferrous iron. In the second stage, which was much slower than the
first stage, the ferrous iron was reduced to the metallic state. During the carbothermal
process, terrestrial ilmenite starts to react at about 860°C at the contact points between
the reactants. Up to 1020°C, solid-state reduction appears to be the main reaction
mechanism, while above this temperature, a rate increase has been observed and has
been contributed to a change in the mechanism to a gaseous reduction of ilmenite by
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regenerated CO. 5,6 It has been reported that the reaction rate between synthetic
ilmenite and graphite is increased significantly by the addition of ferric chloride, which
promotes the nucleation of iron, and is decreased by the addition of rutile.
The above mechanisms and the available kinetic information are not directly
applicable to lunar ilmenite, which is very different from terrestrial ilmenite in both
physical and chemical properties. Moreover, the emphasis in reduction of terrestrial
ilmenite has been on metal oxide recovery and not on oxygen production. However,
the existing technical background on carbothermal processing of terrestrial ilmenite
indicates that its application to lunar ilmenite should be pursued. In comparison with
the hydrogen reduction, the carbothermal process has a more complicated flow
configuration, but is energetically more favorable, and can potentially use a wide
variety of carbonaceous compounds as reducing agents. A comparison of the primary
features of the carbothermal and hydrogen reduction processes can be made using
Tables 1.2 and 1.3.
Table 1.2. Hydrogen reduction.
Advantages
Simple flow sheet
Relatively well-known chemistry
Continuous operation
Disadvantages
Low hydrogen utilization
Problems in hydrogen transport, handling, and storage
High heating/cooling demand
No useful co-products (low integration value)
Key Research Areas
High-temperature water dissociation
High-temperature ceramic membranes
Sensitivity to the physical and chemical properties of the feed
Process design
1-13
Table 1.3. Carbothermal reduction.
Advantages
Known technology for terrestrial ilmenite
Available carbonaceous makeup
Almost 100% utilization of carbon
Useful co-products (high integration value)
Ease of carbon transport, storage, and handling
Disadvantages
More complex flow sheet
More solid handling in continuous operation
Key Research Areas
Reaction kinetics and mechanism for lunar ilmenite
Ceramic membranes application
CO 2 reverse fuel cell
Sensitivity to the physical and chemical properties of the feed
Process Design
Obiectives
The overall objectives of this study can be described as follows:
• Study of the mechanism and kinetics of carbothermal reduction of simulated
lunar ilmenite; determination of the rate-limiting steps; investigation of the
effect of impurities, particularly magnesium; search for catalysts suitable for
enhancement of the rate-limiting steps.
• Development of new reduction methods which are based on the use of a waste
carbonaceous compound as a carbon source for the process; development of a
novel carbothermal reaction path which utilizes gasification of a carbonaceous
solid for reducing gaseous species (hydrocarbons and carbon monoxide) to
facilitate the reduction reaction kinetics and make the process more flexible in
using various forms of carbonaceous feeds.
• Development of advanced gas separation techniques, including the use of high-
temperature ceramic membranes.
• Development of an optimized process flow sheet for carbothermal reduction
and comparison of this process with the hydrogen reduction scheme, as well
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as a general comparison with other leading oxygen production schemes; use of
new and advanced material processing and separation techniques.
Method of Approach
In the first phase of the work, the emphasis has been on the development of
experimental methods for studying the kinetics and mechanism of reduction of
simulated lunar ilmenite. The experimental facilities and procedures are summarized
below.
Experimental Apparatus. Two reactor systems have been designed, fabricated, and
put into operation during the past 9 months: A mini-fluidized bed reactor for quick
response times and a thermo-gravimetric reactor system with an electronic
microbalance for the continuous measurement of rate and sample conversion.
Schematic diagrams of these two reactor systems are given in Figures 1.5, 1.6, and 1.7.
The components of these two reactor systems are described below.
V
m
I I
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B7
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I
COl ''_ '
I
U-tube
reactor
I 1
= l
I 1
= I
CO IR }
I
CO z IP,
F-iowmeier
t
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Figure 1.5. Mini-fluidized bed reactor system.
1-15
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ZZZ
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m _
I
I I I
I s_ ]
I I I J,p'_)"'_
I z I
I I
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Figure 1.6. Micro-gravimetric reactor system.
Carrier Gas '
Hangdown Wire
Thermocouple
Sample Pan
I
I
I
I '
I
Figure 1.7. Schematic of the quartz reactor.
1-16
a. Gas PreparationSection: This includesa UHPnitrogendelivery systemwith
critical flow elements for mass flow control.
b. The Mini-Fluidized Bed Reactor: This is a quartz reactor with a U
configuration. The reactor is mounted on the stand and can be moved
vertically in and out of an electric furnace. A chromel/alumel type K
thermocouple is placed adjacent to the fluidized bed to monitor the
temperatures independently during the course of an experiment. The ilmenite
particles are supported by a stainless steel screen fitted inside the reactor.
This reactor has low dead volume and a very rapid response.
c. Thermo-(3ravimetric Reactor: In this reactor system, the sample is suspended
on one arm of an electronic recording balance. A specially designed quartz
reactor, which surrounds the sample with ports for inlet and dilution gas
entry, is mounted vertically on wheels. This configuration allowed rapid
start-up and quenching of the experiments for accurate measurement of initial
and instantaneous rates. The furnace is equipped with a PID controller for
both steady and temperature-programmed reactions.
d. Gas Analysis Section: This includes nondispersive infrared analyzers for
measurement of carbon monoxide and carbon dioxide, as well as a gas
chromatograph for a more complete gas analysis.
e. Other facilities used in our experimental study include an X-ray diffraction
spectrometer, Mossbauer spectrometer, and scanning electron microscope
with EDX.
Experimental Procedure. The work so far has focused on the study of the
mechanism and kinetics of reactions important for carbothermal reduction of lunar
ilmenite. The samples used have been synthetic ilmenite (iron titanate) obtained from
CERAC, Inc., with a purity of 99.9% and particle size between 63 and 74 microns. To
simulate lunar ilmenite, it was important to make sure that the iron in the sample was
only Fe**. To confirm this, Mossbauer spectroscopy was performed on the synthetic
ilmenite. A typical Mossbauer spectra of the sample is shown in Figure 1.8. The
results indicate that the amount of impurities, including Fe***, was less than 0.8% on a
metal basis.
In each experimental run, a well-characterized mixture of ilmenite and carbon with
known particle size and composition was placed in the reactor while the furnace was
heated to a desired temperature. The gas flow inside the reactor was then stabilized
and maintained for purging the reactor and the sample while the furnace temperature
approached the pre-set value. To start the run, the reactor was lowered into the furnace
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(in the fluidized bed system) or the furnace was raised around the reactor (in the
thermo-gravimetric system). The course of the reaction was studied by monitoring the
gas composition and the sample weight in the gravimetric reactor.
The other methods used for characterization of the starting ilmenite were scanning
electron microscopy and X-ray diffraction spectroscopy. Application of these
techniques to samples at different levels of conversion can help us to understand the
morphological and phase changes that occur in the samples during the course of
reaction.
Discussion of Results
Due to the heterogeneous nature of the reduction reactions, the kinetics of the
process depend on the physical structure of the solid. Figures 1.9 and 1.10 are the
micrographs of the original synthetic ilmenite. The small particles on the surface are
tiny chiPs generated during the grinding process. Figures 1.11 and 1.12 show partially
reacted samples and, in particular, the porosity developed during the reaction. The
SEM micrographs show a significant change in the structure and porosity of the
particles as the reduction reaction proceeds. It appears that the new products formed
on the surface of the particles have a lower melting point compared with that of
ilmenite (melting point of iron titanate is 1327°C). Further characterization of this phase
change is in progress.
So far, two types of experiments have been performed in the reactor systems.
Most of the results are tests of the accuracy, reproducibility, and calibration of the
newly developed reactor systems. More recently, however, experiments for determining
the kinetics and mechanisms have been started. Among the first type of experiments
are tests of the differential nature of the reactors. This is important to make sure that
the kinetics are characteristics of the reaction and not the reactor geometry; this is an
important point often neglected in similar studies. As an example, tests with different
sample mass were conducted and the initial rates were compared. The results are
shown in Table 1.4. The fact that the initial rate did not change significantly with
sample size confirms that the reactor is differential and that the rates are intrinsic
values. Similar results were obtained when flow rate was varied.
One of the important process parameters is the optimum carbon/ilmenite ratio in
the reactor. The preliminary results are shown in Figure 1.13. I't appears that the initial
rate increases slightly with an increase in the carbon content. The significant
difference is in the rate profile. As carbon content decreases, the variations in rate
with time and conversion become more significant. The variations in rate are very
important in the design of a large-scale reactor.
1-19
OR!CINAL PAGE IS
OF POOR (_JALITY
Figure 1.9. The surface microstructure of synthetic ilmenite (particle size z 70 #m;
sample coated with Au/pd); specimen tilt, 30°; magnification, 600X; left bar
length, 10 p.m.
Figure 1.10. The surface microstructure of synthetic ilmenite (particle size z 70 #m;
sample coated with Au/pd); specimen tilt, 30°; magnification, 1030X; left
bar length, 10 p.m.
1-20
Figure 1.11. The surface microstructure of the reduced synthetic ilmenite (particle size
70 #m; sample coated with Au/pd); specimen tilt, 30°; magnification,
490X; left bar length, 10 p.m.
Figure 1.12. The surface microstructure of the reduced synthetic ilmenite (particle size
-= 70 p.m; sample coated with Au/pd); specimen tilt, 30°; magnification,
1050X; left bar length, 10 p.m.
1-21
Table 1.4. Effect of sample mass on reaction kinetics (temperature ,-
1120°C; flow rate - 75 cc/min).
Run Number Sample Size Carbon Content Initial Rate
(g) (wt%) (g CO/g ilmenite-s)
1 0.986 20 3.3 x 10-4
2 0.498 20 3.1 x 10-4
2.0
carbon content
o_ _._ \_ _=_7 o_
g 1.0 \\\
0.0/ I I I _'--" I I I I I
0,0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0,9
Conversion
1.0
Figure 1.13. Effect of carbon content on ilmenite reduction kinetics.
1-22
Another important finding is the influence of carbon type on the kinetics of the
process. As shown in Figure 1.14, the reaction rate and even product distribution for
two types of carbon are quite different. It seems that carbon forms with higher surface
activity give a higher overall reduction rate. These preliminary results indicate that the
gasification of carbon is an important (and possibly rate-determining) step in the
overall process. Therefore, the use of a catalyst to promote this reaction should be
investigated. One of the objectives of this study is to find a suitable catalyst to
promote the gasification reaction.
The chemical reactions taking place during the carbothermal reduction are as
follows:
C + FeTiO 3 - CO + Fe + TiO 2
CO + FeTiO 3 - CO 2 + Fe + TiO 2
CO 2 + C - 2CO
(1)
(2)
(3)
Reaction 1 is expected to start at the points of contact between carbon and ilmenite.
However, the desirable path which will be emphasized in this project is a combination
of Reactions 2 and 3. This path does not require solid contact and can use a wide
variety of carbonaceous compounds for ilmenite reduction.
Finally, a new temperature and concentration-programmed reaction is being
developed to investigate the mechanism of the reduction reaction. This technique is
suitable for studying the effect of various reaction conditions on rate using relatively
few experiments. One example showing the effect of temperature on rate is shown in
Figure 1.15.
References
1. W. N. Agosto, "Electrostatic Concentration of Lunar Soil Ilmenite in Vacuum
Ambient," in Lunar Base and Space Activities in the 21st Centry, W. Mendell, Ed.,
Lunar and Planetary Institute, Houston, 1986.
2. R. J. Williams, D. S. McKay, D. Giles, and T. E. Bunch, "Mining and Beneficiation of
Lunar Ores," in Space Resources and Space Settlements, NASA SP-428, J. Billingham,
W. Gilbreath, and B. O'Leary, Eds., U.S. Government Printing Office, Washington,
1979.
3. M. A. Gibson and C. W. Knudsen, "Lunar Oxygen Production Vis Fluidized Bed Iron
Oxide Reduction," 1988 Annual Meeting of American Institute of Chemical Engineers,
Washington, 1988.
1-23
]raphite
T = 1080 °C
carbon content = 20 wf_
activated carbon
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Conversion
1.0
Figure 1.14. Effect of carbon type on ilmenite reduction kinetics.
E
E
P_
E
0
E
_j
0
L)
1100
Temp
(c)
1'60
.... CO:
-- CO
Ti=e (m._)
Figure 1.15. Temperature-programmed reaction results.
1-24
4. A. H. Cutler and P. A. Krag, "Carbothermal Scheme for Lunar Oxygen Production," in
Lunar Base and Space Activities in the 21st Centry, W. Mendel I, Ed., Lunary and
Planetary Institute, Houston, 1985.
5. D. (3. Jones, J. Appl. Chem. Biotechnol. 25, 561-582, 1975.
6. M. I. EI-Guindy and W. G. Davenport, Met, Trans. Am. lnst. Min. Eng. 1, 1929-1970,
1970.
7. K. G. Suresh, V. Rajakumar, and P. Griereson, Met. Trans. B 18B, 713-718, 1987.


Production of oxygen from lunar ilmenite

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