The use of chlorine to extract, reclaim, and purify metals has attractive possibilities for extraterrestrial processing of local planetary resources. While a complete cyclic process has been proposed for the recovery of metallurgically significant metals and oxygen, herein the chlorination step of the cycle is examined. An experimental apparatus for reacting refractory materials, such as ilmenite, in a microwave induced plasma is being built. Complex equilibria calculations reveal that stable refractory materials can, under the influence of a plasma, undergo chlorination and yield oxygen as a by-product. These issues and the potential advantages for plasma processing in space are reviewed. Also presented is a discussion of the complex equilibria program used in the analysis

Lynch, D. C.

English

NASA Technical Reports Server

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Chlorination Processing of Local Planetary Ores for Oxygen
and Metallurgically Important Metals
D. C. Lynch
Department of Materials Science and Engineering
The University of Arizona /
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Abstract
The use of chlorine to extract, reclaim, and purify metals has attractive possibilities
for extraterrestrial processing of local planetary resources. While a complete cyclic
process has been proposed for the recovery of metallurgically significant metals and
oxygen, this progress report focuses on the chlorination step of the cycle. An
experimental apparatus for reacting refractory materials, such as ilmenite, in a
microwave-induced plasma is being built. Complex equilibria calculations reveal that
stable refractory materials can, under the influence of a plasma, undergo chlorination
and yield oxygen as a by-product. These issues and the potential advantages for
plasma processing in space are reviewed. Also presented in this report is a discussion
of the complex equilibria program used in the analysis.
Introduction
On Earth, the use of chlorine to extract, reclaim, and purify metals has long been
an attractive possibility. 1-s In space, plasma chlorination has even greater possibilities.
Those possibilities include the production of oxygen and chloride metal vapor. The
latter can be either reduced or decomposed to its metal and chlorine states for
recycling. In addition, a plasma chlorinator is versatile. It can be used to process a
number of different ores, and it can also be used to decompose metal chloride vapor
or waste products from other processes.
Our immediate goal is to complete construction of the plasma reactor and begin
testing. The unit will, for the next year, be the center of focus for our experimental
program. The reactor has been designed to meet both our immediate needs and to
serve as a service facility for other investigators participating in Center activities who
may wish to determine if a plasma can enhance their research.
In the same spirit of cooperation, we have assembled a software package for
complex equilibria calculations and an extensive data base. The software package is
based on Outokumpu's HSC program. The data base is oriented toward the extractive
metallurgical industry and has already found considerable use in this work. The HSC
program has been interfaced with LOTUS 123 for manipulation of data, Grapher and
Energraphics for data plotting, and Inset to cut and insert results from any program into
https://ntrs.nasa.gov/search.jsp?R=19910015893 2020-03-19T18:03:05+00:00Z
1-26
another. The entire package is up and running, and we plan to put together a brief
description regarding its operation.
Background
The general advantages to chlorination processing are the large number of
elements that form chlorides, the reactivity, volatility and solubility of metal chloride
products, and the reducibility of metallurgically important chlorides to metal. 1 Almost
all of the elements in the periodic table form chlorides, except for a few noble gases.
In addition, chlorides tend to volatilize or have comparatively low melting points. Both
physical phenomena provide a means for separation and subsequent refining.
Metal chlorides, while easily produced by reaction of chlorine with the metal, are
difficult to form from metal oxides. 2 This difficulty is usually overcome by employing
a CO-CI 2 gas mixture or by reacting the oxide with CI 2 in the presence of solid
carbon. In carbochlorination, carbon monoxide acts as a reducing agent, forming CO2,
while the chlorine oxidizes the metal, forming a volatile chloride. In this manner, a
new surface is continually exposed for reaction. This process is certainly viable, and
one worth considering for space application. However, there are technical complexities
with carbochiorination that may be ignored on Earth, but cannot be overlooked with
regard to space application.
A number of chlorinated oxy-carbon compounds are stable at elevated
temperatures. Landsberg et al.8 conducted an extensive study of the relationship
between various metal oxide carbochiorination reactions and C-CI-O compound by-
products. They found and quantified 136 identifiable C-CI-O by-product compounds
produced during carbochlorination of oxides of titanium, zirconium, and aluminum at
reaction temperatures between 600 and 1000°C. In an economic setting, such as that
which can be anticipated for a space base, the loss of scarce reactants and oxygen in
an unwanted by-product cannot be tolerated.
"Cold" plasma chlorination can eliminate this problem, enhance reaction rates,
minimize reactor size, and yield oxygen as a by-product. In addition, a plasma
chlorination reactor is versatile. A single reactor could be used to process more than
one ore, and it could also be used in the thermal reduction of chloride vapor.
Approach
"Cold" plasma processing of metallurgical materials is a natural progression of
technological development within the extractive industry. High-temperature or
"thermal" plasmas are used in many industrial applications in the extractive industry,
including such areas as process heating and melting, thermal decomposition, smelting,
1-27
and refining. 7.s Those applications rely primarily upon plasma heating to achieve
conventional results that could be achieved with other heating sources. A "cold"
plasma, unlike conventional heating sources, can significantly enhance reaction rates
and form compounds, ions, and reactive species that cannot be produced by normal
means.
In spite of their name, "cold" plasmas are not necessarily cold. Depending upon
the gas composition and pressure, a "cold" plasma can exist at molecular temperatures
up to 2000 K. The term "cold" is attached to those plasmas wherein there is a
significant difference between the molecular and electron temperatures, the latter often
being thousands of degrees greater than the former. The reactive nature of a "cold"
plasma is a direct result of the difference in the two temperatures.
Both the reactive nature and the thermodynamic character of a "cold" plasma are
represented by the sketch in Figure 1.16. In this sketch, the chemical potential of both
reactants and products are plotted against an arbitrary variable to represent potential
reactions. In a normal gas solid reaction, reactants combine to yield products and an
overall reduction in the chemical potential of the system. In a "cold" plasma, energetic
free electrons collide with gas molecules, leading to ionization and/or dissociation of
those molecules. Depending on the initial gas composition, the excitation process can
produce novel reactants, such as monatomic chlorine. 9,1° In addition, there occurs a
substantial increase in the overall chemical potential of the reactants, which, in turn,
allows a number of extra potential reactions, including the formation of metastable
phases not attainable under non-plasma conditions.
Reaction rates can be substantially increased in a "cold" plasma. The high
chemical potential of ionized and dissociated molecules reflects the reactive nature of
these molecules. The presence of broken bonds, partially filled orbitals, and
unbalanced charge makes these molecules highly reactive and can substantially
enhance kinetic processes. It is these characteristics of a "cold" plasma, and how they
can be used to enhance extractive processes, which are the focus of this investigation.
Results to Date
Experimental Apparatus. The primary effort in the past eight months has involved
design and construction of the experimental apparatus. A schematic diagram of the
apparatus is presented in Figure 1.17. The system includes a microwave source, an
applicator where the plasma is generated, a gas delivery system, a mass spectrometer
for monitoring the extent of the reaction, an optical pyrometer for monitoring the
temperature of the solid specimen in the plasma, a specimen holder that both rotates
and allows for vertical translation of the solid specimen, and a vacuum system.
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PLASMA
ACTIVATED
REACTANTS
PRODUCTS - METASTABLE
WITH RESPECT TO
ACTIVATED REACTANTS
_a
z
o
CONVENTIONAL
REACTANTS
PRODUCTS - METASTABLE
_ _'_ WITH RESPECT TO BOTH
\ CONVENTIONAL AND
TIVATED REACTANTS
STABLE PRODUCTS
Figure 1.16. Schematic representation of chemical potential for conventional and
plasma-assisted reactions.
1-29
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The microwave generator was moved from a southside industrial site to the Arizona
Material's Laboratory, where it has been set up and is now operational. The gas
delivery "system is in place; the specimen holder has been constructed, tested, and
deemed operational. The reaction chamber, which is housed in the applicator, has
been designed and built, while the applicator has been redesigned to accommodate the
mass spectrometer.
In addition to these activities, we are in the process of completing arrangements
for specimen preparation. This activity has involved construction of a vacuum system
for sealing specimens in silica ampules and setting up a sintering furnace. Specimens
prepared from powders will be isostatically pressed and then sealed in a silica ampule
under vacuum before sintering at approximately 1000°C. The solid specimens formed
in this manner will be characterized using a BET apparatus to determine surface area
and pore volume, with visual confirmation of those results provided through use of a
scanning electron microscope.
We are currently awaiting arrival of the mass spectrometer and mechanical pumps.
The mass spectrometer, an EXTRELL C-50 Quadropole, was ordered last Fall. The
instrument is currently being tested by the manufacturer, and the expected shipping
date is March 17, 1989. Three corrosion-resistant pumps are on order and are
expected to be shipped before the end of March 1989. Upon receipt of this equipment,
we will complete construction of the apparatus and begin experimentation.
Thermodynamic Calculations. In many systems, chlorination is limited by the
dissociation of diatomic chlorine. In Figure 1.18, the equilibrium partial pressure of
monatomio chlorine in equilibrium with the diatomic species is plotted assuming the
sum of the partial pressures of both the monatomic and diatomic species equals 1 atm.
Also in Figure 1.18, the partial pressure is compared to the partial pressure of
monatomic chlorine that can be achieved in a "cold" plasma. The latter value, based
on a total plasma pressure of 0.1 atm and total dissociation of diatomic chlorine,
intersects the equilibrium-calculated curve at 1560 K. Depending upon the
concentration of monatomic chlorine in the plasma, it is possible to enhance the
chlorination kinetics greatly.
The chemistry which takes place in a plasma is usually quite complex and involves
a large number of elementary reactions which are initially generated by collisions
between energetic free electrons and gas molecules. 10,11 While we cannot model the
thermochemistry of the plasma at this early stage, we are in a position to estimate its
potential impact on chlorination processing. The impact is estimated through complex
equilibria calculations, using the computer program described earlier.
1-31
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i,560K2780K
PCI & PH
I I I
0 0.5 1.0 1.5
lit . 103
Figure 1.18. Comparison of the partial pressures of monatomic chlorine and hydrogen
produced by thermal heating (total pressure 1 atm) and plasma (total
pressure 0.1 atm).
The chlorination of ilmenite has been modeled using the system presented in Table
1.5. It has been assumed that, in plasma processing, diatomic chlorine is completely
decomposed. Accordingly, the results presented below regarding the plasma represent
the best-case scenario and do not take into consideration the effect of the plasma on
other reactants and products. Moreover, it is important to remember that the results
presented in this section are based on thermodynamic calculations and thus represent
only what is energetically possible. Kinetic processes may also limit what can be
achieved.
1-32
Table 1.5. System used for thermodynamic calculations.
Vapor Phase
Cl 2 and/or CI, 02, CIO, CIO 2, FeCl, FeCl 2, FeCI 3, Fe2Cl 8, TiCI, TiCl 2, Ti2CI 8, TiCI 4,
TiOCI, TiOCI2, TiO
Solids
FeTiO 3 TiO 2 TiCI 2
TiCI 3 TiCI 4 FeO
Fe2 03 Fe3 04 FeCI 2
FeCI 3 FeOCI
Ilmenite reacts with chlorine at 1100°C. The extent of this reaction is represented
in Figure 1.19 as a function of the moles of chlorine available per mole of ilmenite.
The results in the figure and the raw data reveal that the iron is volatilized as chlorides
while elemental oxygen is released. The process continues until iron is eliminated,
leaving only rutile ('1"iO2) remaining in the solid. At this point, the reaction stops for all
practical purposes.
The extent of the chlorination of ilmenite in a plasma can be enhanced, as the
results shown in Figure 1.20 reveal. Even after all the iron has been volatilized, the
chlorination process continues. Titanium chlorides are volatilized and elemental
oxygen is released. A comparison of Figures 1.19 and 1.20 reveals that the chlorine
may be better utilized in a plasma reactor.
The potential for plasma chlorination is dramatically illustrated in Figure 1.21.
Rutile, as noted earlier, is not readily chlorinated under standard conditions. The
results in Figure 1.21 reveal that rutile, a highly refractory material, may readily be
reacted in a plasma. It is this reactive nature that holds exceptional promise for the
use of "cold" plasmas in the processing of local planetary resources.
Summary and Future Work
The investigation is in its initial stage. Considerable effort has been expanded in
the design and construction of the plasma reactor. This initial work is expected to
continue for three to four months. Upon completion of the apparatus, a number of
preliminary experiments will be undertaken.
The preliminary experiments will focus on the power absorbed by the plasma,
which will be evaluated as a function of the gas content (chlorine versus inert gas
content, as well as the choice of inert gas), the total pressure, reactor size, and power
1-33
1_ _r_-.____ O x e n
i /k_/x.."
70 o\
_o_ Fe in Sol ids
5400 _°_o
02 i n gos //c
OOO______O_ _43 , , ,
0 I 2 3 4
Mo I es Chl or ine/ Mole I Imeni te
Figure 1.19. Extent of reaction of ilmenite with chlorine under conventional heating at
1100oC and total pressure of 0.1 atm.
_I\ "\
80L\ \_
_°I /Z '
'°r /, °-
0 I 2 3 4 5
Mo I as Ch Iorine/ Mole I Imeni te
Figure 1.20. Monatomic chlorine in a plasma can readily react with ilmenite at 1100°C
and a total pressure of 0.1 atm.
1-34
80
70
40
I0
O(
0
Figure 1.21. A plasma can effectively chlorinate refractory oxides.
provided. The apparatus is equipped with meters to measure both the forward and
reflected power, the difference being the absorbed power. The level of absorbed
power reflects the extent of radical formation. The results of this work will be used to
identify optimum conditions for the chlorination reaction. This work is expected to take
no longer than two months.
Chlorination studies will begin upon completion of the initial power studies. The
kinetics of chlorination will be examined as a function of temperature, chlorine content
in the gas, total pressure, power level, and plasma density. These experiments are
expected to be in progress throughout the remainder of the year.
Student Participants
Mr. Daniel Bullard and Mr. Robert Ortega, both U.S. citizens, are working on this
project. Daniel, who joined our research group in January, completed his
undergraduate program at the University of Wisconsin at Milwaukee in December,
where he received a double degree in Materials Engineering and Chemistry. Robert is
in his last year of undergraduate study at the University of Arizona. Upon completion
of his Bachelor's degree in May, he plans to commence graduate work. Robert began
working on the project in December.
1-35
References
1. A. Landsberg and F. E. Block, A Study o! the Chlorination Kinetics o! Germanium,
Silicon, Iron, Tungsten, Molybdenum, Columbium, and Titanium, USBM RI 6649, 1965.
2. I. Barin and W. Schuler, "On the Kinetics of the Chlorination of Titanium Dioxide in
the Presence of Solid Carbon," Metall. Trans. B 11B, 199-207, June 1980.
3. A. Landsberg, C. L. Hoatson, and F. E. Block, "The Chlorination Kinetics of
Tungsten, Molybdenum, and Their Alloys," J. Electro. Chem. Soc. 118, 1331-36,
August 1971.
4. A. Landsberg, C. L. Hoatson, and F. E. Block, "The Chlorination Kinetics of
Zirconium Dioxide in the Presence of Carbon and Carbon Monoxide," Metall.
Trans. 3, 517-23, February 1972.
5. A. Landsberg, "The Chemical Reactivity and Equilibria of Au, Pt, Pd, Ir, Rh, Re,
Mo, and W With Chlorine," private communication, May 1988.
6. A. Landsberg, R. D. Wilson, and W. Burns, "Conditions Affecting the Formation of
Chlorinated Carbon Compounds During Carbochlorination," Metall. Trans. B 19B,
477-82, June 1988.
7. W. H. (3auvin, M. G. Drouet, and R. J. Munz, "Developments in Plasma Processes for
Extractive Metallurgy," J. Metals 39, 14-17, December 1987.
8. C. W. Chang and J. Szekely, "Plasma Applications in Metals Processing," J. Metals
34, 57-64, February 1982.
9. F. H. Dorman and F. K. McTaggart, "Absorption of Microwave Power by Plasmas,"
J. Microwave Power 5, 4-16, January 1970.
10. A. T. Bell, "An Introduction to Plasma Processing," Solid State Tech., pp. 89-94,
April 1978.
11. A. T. Zander and (3. M. Hieftje, "Microwave-Supported Discharges," Applied
Spectroscopy 35, 357-71, April 1981.


Chlorination processing of local planetary ores for oxygen and metallurgically important metals

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