Experiments during the first year of the project were directed towards generating elemental fluorine via the electrolysis of anhydrous molten fluorides. Na2SiF6 was dissolved in either molten NaBF4 or a eutectic (minimum-melting) mixture of KF-LiF-NaF and electrolyzed between 450 and 600 C to Si metal at the cathode and F2 gas at the anode. Ar gas was continuously passed through the system and F2 was trapped in a KBr furnace. Various anode and cathode materials were investigated. Despite many experimental difficulties, the capability of the process to produce elemental fluorine was demonstrated

Balasubramanian, Rajan

Burt, Donald M.

Roberts, Jeffery J.

Tyburczy, James A.

English

NASA Technical Reports Server

N93-26680
Lunar Mining of Oxygen Using Fluorine
Donald M. Burt, James A. Tyburczy, Jeffery J. Roberts,
and Rajan Balasubrsmanlan
Arizona State University
Department of Geology
Abstract
Experiments dudng the first year of the project were directed towards generating elemental fluorine
via the electrolysis of anhydrous molten fluorides. Na2SiF6was dissolved in either molten NaBF4 or
a eutectic (minimum-melting) mixture of KF-LiF-NaF and electrolyzed between 450° and 600 ° C to
Si metal at the cathode and F2 gas at the anode. Ar gas was continuously passed through the
system and F2 was trapped In a KBr furnace. Various anode and cathode materials were
investigated. Despite many experimental difficulties, the capability of the process to produce
elemental fluorine was demonstrated.
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https://ntrs.nasa.gov/search.jsp?R=19930017491 2020-03-17T05:39:10+00:00Z

introduction
We are engaged in experiments to produce fluodne by the electrolysis of anhydrous molten
fluorides, with application to the mining of oxygen =on the Moon. Fluorine, as the most reactive
element, readily releases oxygen from all oxygen-bearing rocks and minerals. Stable Isotope
geochemists have carded out this reaction successfully for more than 30 years in Ni crucibles at
a temperature of about 500° C.
Ruodne can only be produced by electrolysis.Conventionally, an HF-KF mixture is electrolyzed at
moderate temperatures to yield H2 at the cathode and F2 at the carbon anode (e.g., Ellis and May,
1986). This route does not readily lend itself to recycling of the fluorine, all of which would have to
be brought from Earth (as a stable inert salt, in the present process). The problem is that the
hydrogen produced at the cathode is a relatively poor reducing agent.
An alternative fluorine-generating process, devised by D.M. Burt in 1989 (unpublished), produces
by-product silicon (for solar cells) as well as oxygen, and results in all of the F2 being recycled (in
principle, at least). A lunar silicate rock is partially fluorinated by F2 gas, yielding pure 02 gas, then
the residue is heated in the lunar vacuum to release gaseous SiF4. The SiF4 is trapped in
NaF-bearing anhydrous molten salts as Na2SiF6,which is electrolyzed to Si metal at the cathode
and F2 gas at the anode, thus completing the cycle. Inasmuch as SiF4 is the most volatile common
fluoride, this process should work with any unbeneflciated lunar silicate rock.
Personnel
The apparatus was largely designed and set up this spring by Dr. Jeff Roberts, a former student of
Prof. Jim Tyburczy. He left in June to accept a job at Lawrence Livermore Labs and was replaced
by Mr. Rajan Balasubramanlan, an ASU Chemical Engineering graduate student. Mr.
Balasubramanian finished designing and assembling the apparatus in early July and has carried out
the actual experiments since then. Prof. Tyburczy left for a sabbatical year at SUNY Stony Brook
in August.
Proqres_
Since our progress report of May 28, 1992 we have finished setting up the electrolysis line and have
carried out 16 experiments, the first 15 of which are summarized on the appended table. We have
successfully prepared fluorine by electrolysis, beginning with our third try on July 23. Not
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unexpectedly, we also have encountered many experimental difficulties.
We had, somewhat surprisingly, no problems with corrosion of the Inconel 601 electrolysis vessel,
its water-coooled stainless steel top, or the Inner alumina crucible in which we melted the
electrolytes, at run temperatures up to 600 C (a technique used by Mamentov and Laher, 1989). The
KBr furnace at about 200 C seemingly trapped nearly all of the F2 formed as KF by releasing Br2;
we had no detectable releases of F2 gas from the system.
Our initialfew runs Involved sheet graphite cathodes and anodes and low-melting NaBF4 (MP. 408°
C) as the electrolyte (Mamentov and Laher, 1989). After run 4, we abandoned NaBF4 in favor of a
eutectic (minimum-melting) mixture of KF-LiF-NaF (M.P. 459° C: e.g., Minh and Redey, 1987) owing
to the high volatility of the former. NaBF 4 releases gaseous BF3 when heated, which gas was not
trapped by our KBr furnace at 200° C, even with NaF placed at the downstream end. We detected
this problem by the characteristic fuming of BF_when it contacted the water we had at the end of
the line to measure gas flow via bubbting rate ('ropchiev et al., 1959, p. 25). White borate deposits
partly blocked the glass tube, which locally was etched by the HF formed by the reaction of BF3
with the water.
We abandoned the graphite electrodes after 9 runs because of their relatively poor electrical
conductivity (reducing the electrode spacing did not help much, revealing that salt conductivity was
not the major problem). (We did produce some Si on the cathode by using long run times and small
electrode spacing.) The metals that we tried later (inconel, copper, titanium, and nickel, to date)
were much better conductors of electricity, although they suffered more severe anode corrosion.
Problems and Solutions
Our other experimental difficultiesare summarized below, together with the measures we undertook
to alleviate the problems. Many of these difficultiesundoubtedly resulted from our Inexperience in
molten salt electrolysis.
1. The KF-LiF-NaF mixture (especially KF) is highly hygroscopic. To minimize this problem,
samples were weighed out rapidly just before a run, then placed in a stream of dry argon, and left
to dry out at T > 300o C for an hour before proceeding with the run. Cell resistivity was then
monitored as the salt was heated to melting.
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iI
i
2. The maximum interqal temperatures were much cooler (50-100° C) than those measured
by the external thermocou_e, as revealed by our initial failure to melt the charge. We placed an
_ ;._ _ i-'___
alumina-sheathed internal thermocouple in later runs.
3. The closely- and evenly-spaced furnace windings, short length, and water-cooled top of our
electrolysis vessel resulted in a undesirably highvertical temperature gradient in the electrolysis cell
(100° C over a few cm, as revealed by raising and lowering the Internal thermocouple). The
alumina-sheathed thermocouple and especially the two cold metal electrode leads immersed in the
salt made the problem worse, as did the cold stream of argon being directed downwards through
the top. The only remedy was to carry out the experiments at much higher temperatures (more than
100° C greater) than initiallyplanned. After run 9, we reshaped the furnace (removed the top) so as
to place the bottom of the reaction vessel at the hot spot of the furnace, rather than above it. This
slightly alleviated, but did not cure, the problem.
4. The high vertical temperature gradient made the salts remain solid (crystalline) on top. They
tended to form a impermeable crust on top of the molten electrolyte, clinging to and even climbing
the metal electrode supports. In one run, #3, this crust trapped the first fluorine gas we produced
as tiny fluid inclusions, which decrepitated "explosively" (in tiny gas puffs) after the run was opened
and the adsorbed water began to dissolve the salts. The remedy was, again, to carry out the
electrolysis at a high enough temperature (about 600° C) to make sure all of the salt was melted.
The problem worsened when we began using metal electrodes; at this point we started preheating
the furnace to over 700° C, then cooling it down for the actual electrolysis.
5. Even with these measures, KF-LiF-NaF melts that were relatively rich in Na2SiF6 (up to 4%)
tended to climb up the walls of the container and also up the electrodes. Placing a small ceramic
cap or "protector" over the place where the electode joined the metal carrier, especially for the
cathode, helped somewhat. In later runs we tried to retract the electrodes and thermocouple before
cooling off the run, in order to facilitate their examlnation and cleaning (otherwise they were trapped
in rock-hard frozen electrolyte and had to be laboriously chipped or dissolved free); vertical creep
of the electrolyte made this Impossible insome Si-rich runs (crystallized electrolyte crust "froze them
in place" during the run).
6. In runs with less Si or those electrolyzed for longer periods or at higher currents, all of the
Si dissolved in the electrolyte was apparently used up (by deposition at the cathode) and at that
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point the metallic anode became highly corroded. Ruorine produced at the anode reacted with it
and the resulting fluoride dissolved In the melt; the metal reprecipitated at the cathode dendritically
(and the fibrous dendrites eventually shorted out the cell). At this point the cell was producing no
F2gas but was merely electroreflning the metal of the anode. This was especially visible in the runs
with Cu electrodes (#12 and #13), but it also happened to inconel (#14) and Ti (#15). TiF4 is a gas
at high temperatures, but much of it appears to have been re.absorbed by reaction with the melt.
7. Our major problem was the failure of the cold trap (a simple U-tube) to trap the Br2released
into the Ar gas stream by reaction of F2 with KBr !n our horizontal tube fluorine-trapping furnace.
We tried switching from an ice-water bath to a dry Ice bath, but this didn't help. Using a liquid
nitrogen bath resulted in condensation of liquid argon from the argon gas stream; this rapidly
blocked the U-tube and was abandoned. Our resident fluorine expert, Prof. Paul Knauth, tells us that
a simple U-tube works for trapping diffuse Br2 in a vacuum; apparently the thermal vibrations of all
of the accompanying Ar atoms in our experiments prevent the Br molecules from condensing (they
are "hustled on through" the cold U-tube, instead of sticking to its walls). We could try using dry N2
In place of Ar as the gas stream, with liquid nitrogen bathing the U-tube, or could try using a
vacuum (our present set-up is not vacuum tight). In order to measure fluodne production
quanititatively, we shall have to come up with a different method of measurement, such as an
ion-sensitive electrode.
We can, nevertheless, demonstate that we have been making considerable fluorine, if not exactly
how much. Not only did tiny fluid inclusions of F2gas in salt crust react explosively with water after
run #3, as mentioned above (the reaction F2 + H20 = 2HF + 02), but also optical examination of
the material filling the KBr fumace after 13 runs revealed that much of it had been converted to KF.
This was accomplished by examining a crushed sample on a glass microscope slide under a cover
slip using an oil of refractive index 1.555, which nearly matches that of KBr (1.559), making KBr
grains appear nearly invisible. Any conversion to KF is detected by the much lower refractive index
of the latter (1.363).
.
2.
Ruorine was produced.
KF-LiF-NaF eutectic better than NaBf4 owing to volatility of latter.
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3. Graphite anode seemingly nonreactive with F2, but such a poor conductor that only tiny
currents could be maintained; some Si was produced.
4. Metal anodes (Cu, Ti, inconel, Ni) corroded rapidly and metal reprecipated at the cathode
(electroreflning) after Si in the melt was used up.
Major Discoveries
1. Ruorine can be produced by anhydrous molten salt electrolysis; this can be carded out in
alumina crucibles Inside an inconel or nickel reaction vessel at temperatures up to 600 C.
2. The eutectic (minimum melting) mixture of KF-LiF-NaF worked much better as the
electrolyte than NaBF4, owing to the high volatility of the latter.
3. Sheet graphite anodes were less subject to corrosion than the metals we tried (inconel, Ni,
Cu, and Ti) and conveniently broke off when it was time to free them from the electrolyte, but were
such poor conductors of electricity that only long run times and close electrode spacing allowed
visible production of Si at the cathode.
4. The metal electrodes corroded rapidly at the anode and the metal reprecipitated
dendritically at the cathode (electroreflning); this problem was probably caused by the low initial Si
contents of the melts; all of the Si was precipitated at the cathode quite rapidly. Two
differently-colored layers of precipitate were commonly observed on the cathode after these
experiments, the inner one presumably being Si and the outer one the cathode metal.
5. Many experimental problems remain to be solved.
Future Research
Continued funding is requested to carry out the following experiments:
1. Continue electrolysis experiments with other metals and forms of carbon other than graphite
as the anode material.
2. Determine the composition of the metals deposited at the cathode by using electron
microprobe microanalysis.
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3. Devise a different method of measuring fluodne production quantitatively, either by replacing
Ar by N2 in the gas stream, or by redesigning the apparatus to work in a vacuum, or by using an
Ion sensitive electrode to measure F in aqueous solution.
4. Redesign the fumace and reaction vessel to minimize temperature gradients, thereby
minimizing several experimental problems Involving electrolyte crusts and vertical creep, and also
minimizing anode corrosion by allowing electrolysis at lower temperatures. Two modifications that
could be tried with our present set-up involve air instead of water cooling of the top of the reaction
vessel and pre-heatlng of the argon streambefore_ it enters the reaction vessel.
5. Initiate experiments on partial fluorination of silicate rocks and on SiF4release by pyrolysis
of partly fluorinated rocks.
References
Ellis, J.F. and G.F. May. Modern fluorine generation. J. F/uorine Chem. 33 (1986): 133-47.
Mamantov, G. and T.M. Laher. Production of fluodne by the electrolysis
calcium-fluoride-containing tetrafluoroborate melts, J. Electrochem. Soc. 136 (1989): 673-76.
of
Minh, N.Q. and I_ Redey. Reference electrodes for molten electrolytes, in Mo/ten Sa/t Techniques
(ed. D.G. Lovering and R.J. Gale) (New York: Plenum, 1987): 105-287.
Topchiev, A.V., S.V. Zavgorodnii, and Y.M. Paushkln. Boron F/uor/de and its Compounds as
Catalysts in Organic Chemistry (transl. by J.T. Greaves). (New York: Pergamon Press, 1959).
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Lunar mining of oxygen using fluorine

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