U.S. patent application number 12/308367 was filed with the patent office on 2010-01-14 for method, apparatus and means for production of metals in a molten salt electrolyte.
Invention is credited to Kevin Dring, Eirik Hagen, Odd-Arne Lorentsen, Christian Rosenkilde.
Application Number | 20100006448 12/308367 |
Document ID | / |
Family ID | 38831961 |
Filed Date | 2010-01-14 |
United States Patent
Application |
20100006448 |
Kind Code |
A1 |
Dring; Kevin ; et
al. |
January 14, 2010 |
Method, apparatus and means for production of metals in a molten
salt electrolyte
Abstract
This invention describes a method of producing a metal, M.sub.1,
in an electrolytic cell consisting of a molten electrolyte,
M.sub.ZY-M.sub.ZO, at least one anode and at least one cathode,
characterised in that the passage of current between said anode(s)
and cathode(s) through said electrolyte, produces a metal, M.sub.1,
from a raw material, M.sub.1X, containing a non-metallic species,
X, under conditions such that the potential at the cathode causes
the reduction of the M.sub.Z cation and the formation of M.sub.Z at
activities less than one, and the potential at the cathode is
insufficient to cause formation of M.sub.Z metal as a discrete
solid or liquid phase, and the M.sub.Z so produced reduces the raw
material, M.sub.1X, at the cathode, to M.sub.1.
Inventors: |
Dring; Kevin; (Skien,
NO) ; Hagen; Eirik; (Porsgrunn, NO) ;
Lorentsen; Odd-Arne; (Porsgrunn, NO) ; Rosenkilde;
Christian; (Porsgrunn, NO) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
1030 15th Street, N.W.,, Suite 400 East
Washington
DC
20005-1503
US
|
Family ID: |
38831961 |
Appl. No.: |
12/308367 |
Filed: |
May 30, 2007 |
PCT Filed: |
May 30, 2007 |
PCT NO: |
PCT/NO2007/000183 |
371 Date: |
May 29, 2009 |
Current U.S.
Class: |
205/357 ;
204/243.1; 205/363; 205/367 |
Current CPC
Class: |
C25C 3/28 20130101; C22B
34/129 20130101; C22C 14/00 20130101; C25C 3/00 20130101 |
Class at
Publication: |
205/357 ;
205/367; 205/363; 204/243.1 |
International
Class: |
C25C 3/00 20060101
C25C003/00; C25C 3/36 20060101 C25C003/36; C25B 1/00 20060101
C25B001/00; C25C 7/06 20060101 C25C007/06; C25C 7/00 20060101
C25C007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 14, 2006 |
NO |
20062776 |
Claims
1. A method of producing a metal, M.sub.1, in an electrolytic cell
consisting of a molten electrolyte, M.sub.ZY-M.sub.ZO, at least one
anode and at least one cathode, characterised in that the passage
of current between said anode(s) and cathode(s) through said
electrolyte, produces a metal, M.sub.1, from a raw material,
M.sub.1X, containing a non-metallic species, X, under conditions
such that a. the potential at the cathode causes the reduction of
the Mz cation and the formation of Mz at activities less than one
b. the potential at the cathode is insufficient to cause formation
of Mz metal as a discrete solid or liquid phase c. and the M.sub.Z
so produced reduces the raw material, M.sub.1X, at the cathode, to
M.sub.Z.
2. A method of producing an alloy, M.sub.1-M.sub.2- . . . -M.sub.n,
in an electrolytic cell consisting of a molten electrolyte,
M.sub.ZY-M.sub.ZO, at least one anode and at least one cathode,
characterised in that the passage of current between said anode(s)
and cathode(s) through said electrolyte, produces the alloy,
M.sub.1-M.sub.2- . . . -M.sub.Z, from a mixture of raw materials,
M.sub.1X-M.sub.2X- . . . -M.sub.nX, containing non-metallic
species, X, under conditions such that a. the potential at the
cathode causes the reduction of the M.sub.Z cation and the
formation of M.sub.Z at activities less than one b. the potential
at the cathode is insufficient to cause formation of M.sub.Z metal
as a discrete solid or liquid phase c. and the M.sub.Z so produced
reduces the raw material at the cathode to an alloy
M.sub.1-M.sub.2- . . . -M.sub.n.
3. A method of producing a composite, M.sub.1-M.sub.2A, in an
electrolytic cell consisting of a molten electrolyte,
M.sub.ZY-M.sub.ZO, at least one anode and at least one cathode,
characterised in that the passage of current between said anode(s)
and cathode(s) through said electrolyte, produces a composite,
M.sub.1-M.sub.2A, from a raw material, M.sub.1X-M.sub.2A,
containing non-metallic species, X, and substance, A, under
conditions such that a. the potential at the cathode causes the
reduction of the M.sub.Z cation and the formation of M.sub.Z at
activities less than one b. the potential at the cathode is
insufficient to cause formation of M.sub.Z metal as a discrete
solid or liquid phase c. and the M.sub.Z so produced reduces the
raw material at the cathode to form a composite,
M.sub.1-M.sub.2A.
4. A method in accordance with claim 1, 2, or 3, characterised in
that the formation of Mz more preferably occurs at an activity
between 10.sup.-6 and 5.times.10.sup.-1.
5. A method in accordance with claim 1, 2, or 3, characterised in
that the formation of M.sub.Z and the reduction of M.sub.1X occur
at the same physical location.
6. A method in accordance with claim 1, 2, or 3, characterised in
that the said electrolyte (fused salt) more preferably comprises Ca
or Na, or a mixture thereof as the cation(s), M.sub.Z.
7. A method in accordance with claim 1, 2, or 3, characterised in
that the said electrolyte (fused salt) may also comprise at least
one of the following cations: Ba, Li, Sc, Sr or K
8. A method in accordance with claim 1, 2, or 3, characterised in
that the said electrolyte (fused salt) more preferably comprises Cl
as the anion, Y.
9. A method in accordance with claim 1, 2, or 3, characterised in
that the said electrolyte (fused salt) may also comprise F as the
anion, Y.
10. A method in claim 1, 2, or 3, where the electrolyte more
preferably contains 0.1 to 3 wt % CaO.
11. A method according to claim 1, 2, or 3, in which the raw
material contains M.sub.1 and X as constituents of a single phase
compound comprised of greater than two elements.
12. A method according to claim 1, 2, or 3, in which the non-metal
species, X, comprises at least one of the elements O, S, C or
N.
13. A method according to claim 1, 2, or 3, in which the non-metal
species, X, more preferably comprises the element O.
14. A method according to claim 3, in which the substance, A,
consists of at least one of the following: B, C, O, N, or Si.
15. A method in accordance with claim 1, 2, or 3, characterised in
that the metal species, M.sub.1, M.sub.2 . . . M.sub.n, being
produced comprises at least one of the following components: Ti,
Si, Ge, Zr, Hf, Sm, U, Al, Mg, Nd, Mo, Cr, Nb, V, Ta, Mb, W, Mn,
Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Be, Sr, Ga,
In, Tl, lanthanides or actinides.
16. An apparatus consisting of an electrolytic cell including an
anode, a cathode comprised of a current collector and raw material,
and a molten electrolyte, which operates under conditions such that
a metal, alloy or composite is formed from said raw material
whereby the potential at cathode is sufficient to reduce the
M.sub.Z cation and causes the formation of M.sub.Z at activities
less than one, and the potential at the cathode is insufficient to
cause formation of M.sub.Z metal in either the solid or liquid
phase, and the M.sub.Z so produced reduces the raw material, at the
cathode, to said metal, alloy or composite.
17. The anode defined in claim 15 further comprising either a. a
carbonaceous material such that CO and CO.sub.2 gas are evolved
during electrolysis, or b. a material such that the anodic
electrolysis product is oxygen gas.
Description
INTRODUCTION
[0001] This invention describes a method, apparatus and means for
production of a metal, metal alloy or metal composite in an
electrolysis cell.
PRIOR ART
[0002] Titanium and its alloys exhibit excellent mechanical
properties, unrivalled corrosion resistance and outstanding
biocompatibility; however, annual global titanium production is
dwarfed by commodity metals such as steel and aluminium. One needs
only to examine the complex and discontinuous production method,
the Kroll process, to correlate the high price of titanium with its
low consumption. Since the 1950's, alternate processing routes have
been sought, in vain; titanium oxides are extremely stable
compounds that are bound with increasing tenacity to oxygen as the
latter concentration decreases. A high solubility for oxygen in
metallic titanium necessitates carbo-chlorination of titanium
dioxide to produce an oxygen-free, chloride feedstock (TiCl.sub.4),
which is subsequently metallothermically reduced with liquid
magnesium. Historical electrowinning processes have failed to
successfully address the issue of dissolved titanium species, which
may be present in up to 3 different oxidation states (4.sup.+,
3.sup.+, and 2.sup.+). Furthermore, due to the liquid melt
processing of titanium during conventional arc or electron beam
melting, segregation or incomplete dissolution of alloying
additions may occur, limiting the range of compositions and types
of materials that may be economically produced using the present
process.
[0003] Several organisations including BHP Billiton, QinetiQ,
Cambridge University, Nippon Light Metal Company and British
Titanium have investigated the direct production of titanium from
its oxides using an electrolytic cell that includes a graphitic
anode and a CaCl.sub.2-based molten salt electrolyte. The operating
conditions for the processes are claimed to be greater than 1.3
volts and less than 3.5 volts over a temperature range of 600 to
1000.degree. C. Furthermore, CaO is specified as an electrolyte
constituent in all of these methods. The organisations may be
categorized according to the mechanism they believe to be
responsible for the actual conversion of metal oxide to metal.
Cambridge University Technical Services Ltd. (WO 99/64638-U.S. Pat.
No. 6,712,952) and British Titanium Plc (WO 2006/027612 A2) insist
that the operating potential at the cathode is insufficient to
cause deposition of the electrolyte cation or decomposition of the
electrolyte. Rather, it is their stated understanding that the
"reaction of the substance rather than deposition of the cation
occurs" and the "substance or substances dissolves into the
electrolyte" instead.
[0004] Claim 1 of U.S. Pat. No. 6,712,952 states a method that is
operated by " . . . applying a voltage between the electrode and
the anode such that the potential at the electrode is lower than a
deposition potential for the cation at a surface of the electrode
and such that the substance dissolves in the electrolyte." Claims 4
and 5 of patent application WO 2006/027612 A2 state, respectively "
. . . the voltage is applied such that the potential at the cathode
increases to a potential which is less than a potential for
continuous deposition of the cation from the electrolyte at the
cathode." and " . . . the voltage is applied such that the
effective potential between the cathode and the anode increases to
a potential which is less than a potential sufficient to cause
continuous decomposition of the electrolyte." It may be understood
from these prior arts that the (continuous) deposition of the
electrolyte cation is totally avoided according to the methods so
described. The electrolyte in the process described by these groups
acts merely as a solvent for the reaction product, rather than
participating in the electrochemical reduction reaction itself.
[0005] Conversely, BHP Billiton (U.S. Pat. No. 6,663,763) and
Nippon Light Metal Company. (US 2004/0237711 A1) forward an
alternate mechanism whereby the potential at the cathode is
sufficiently reducing to deposit the cation from the electrolyte,
since decomposition of the CaO constituent occurs at lower
potentials than those for decomposition of CaCl.sub.2. Claim 1 of
U.S. Pat. No. 6,663,763 describes " . . . operating the cell at a
potential that is above a potential at which Ca cations in the
electrolyte deposit as Ca metal on the cathode, whereby the Ca
metal chemically reduces the cathode titanium oxide." Claim 2 of
the same document states " . . . the Ca metal deposited on the
cathode is soluble in the electrolyte and can dissolve in the
electrolyte and thereby migrate to the vicinity of the cathode
titanium oxide." Within the description of the "Summary of the
Invention" (line 41), it was stated, " . . . the Ca metal that
deposited on electrically conductive sections of the cathode was
deposited predominantly as a separate phase . . . ". A separate
phase is equivalent to a condensed phase and calcium metal at unit
activity.
[0006] Patent application US 2004/0237711 A1 describes, in
paragraph [0049], "Moreover, the electrolysis of calcium oxide in
calcium chloride between the anode made of a consumable
carbonaceous anode material and the cathode made of non-consumable
cathode material forms calcium-saturated calcium chloride either
saturated with dissolved calcium or coexisting with pure calcium in
the vicinity of the cathode . . . " Thus the calcium formed by
electrolysis is maintained at the saturation concentration as may
be understood by the formation of "pure calcium" or Ca metal, which
may be interpreted as either a discrete solid or liquid, depending
on the operating temperature. Claim 1 of the same patent
application states " . . . the molten salt in the reaction region
is electrolysed thereby converting the molten salt into a strongly
reducing molten salt . . . ". Someone skilled in the art would
appreciate that the higher the extent of calcium saturation, the
greater the reducing strength of the electrolyte. Consequently, the
use of a "calcium-saturated" calcium chloride describes an
extremely reducing electrolyte that has reached the solubility
limit for dissolved calcium metal.
[0007] One of the inventors of the present application, has
previously observed the conversion of TiO.sub.2 to Ti--O solid
solutions in a sequential process whereby titanium metal is formed
via the lower oxides (Ti.sub.3O.sub.5, Ti.sub.2O.sub.3, TiO) Dring
et al. (J. Electrochem. Soc. v152, #3 (2005):E104). It was stated
that oxygen ionization was the dominant mechanism in a melt
containing a very low CaO content (approximately 100 ppm CaO).
Electrolysis was observed to occur at low current densities during
the initial stages of reduction, but proceeded more rapidly as the
decomposition potential of the electrolyte was approached. FIG. 1
depicts the current response to the potential waveform and there
are clearly electrochemical processes, labeled A, B, C and D,
occurring at potentials significantly positive of the reductive
limit (E) of the electrolyte, where calcium metal would form. These
electrochemical processes were attributed to the formation of
Ti.sub.3O.sub.5, Ti.sub.2O.sub.3, TiO and Ti--O, respectively.
Although metallic titanium was produced, significant quantities of
calcium were also generated during the reduction. Thus, it has been
established that a calcium-saturated melt or metallic calcium is
not necessary to effect reduction of TiO.sub.2 to the lower oxides
of titanium or metallic titanium containing oxygen.
[0008] The applicant has extensively investigated the
electrochemistry of the CaCl.sub.2--CaO--Ti--O system in an attempt
to rationalise the disagreement in mechanistic explanations. The
experimental work conducted by the applicant has proven that it is
impossible to operate under conditions where the electrolyte is not
decomposed and the cation of the electrolyte is not deposited,
owing to the thermodynamic and physical properties of the
electrolyte system. The applicant has established that it is
preferable to conduct the electrolysis under conditions where the
cation deposition process results in a dilute and controlled
concentration of dissolved calcium, which may effectively reduce
the metal oxide or deoxidize the metal.
Further Prior Art Literature:
TABLE-US-00001 [0009] US Patent U.S. Pat. No. 6,663,763 BHP
Billiton Documents U.S. Pat. No. 6,712,952 Cambridge University
Technical Services US 2004/0237711 A1 Nippon Light Metal Company
Ltd Foreign Patent WO 2006/027612 A2 British Titanium Plc Documents
Other R. Littlewood, J. Electrochem. Soc., v109, #6, Publications
(1962): 525. H. Fischbach, Steel Research, v56, #7, (1985): 365. K.
Dring, J. Electrochem. Soc., v152, #3, (2005): E113. K. Dring, J.
Electrochem. Soc., v152, #10 (2005): D184.
Present Invention:
[0010] The present invention describes a process in which, a metal,
M.sub.1, is produced in an electrolytic cell consisting of a molten
electrolyte, M.sub.ZY-M.sub.ZO, at least one anode and at least one
cathode, characterised in that the passage of current between said
anode(s) and cathode(s) through said electrolyte, produces a metal,
M.sub.1, from a raw material, M.sub.1X, containing a non-metallic
species, X, under conditions such that the potential at the cathode
causes the reduction of the M.sub.Z cation and the formation of
M.sub.Z at activities less than one. Additionally, the potential at
the cathode is insufficient to cause formation of M.sub.Z metal as
a discrete solid or liquid phase. The M.sub.Z produced in this
manner reduces the raw material, M.sub.1X, at the cathode, to
M.sub.1. The raw material feed may also contain both species
M.sub.1 and X, in a ternary or higher order oxide, of the form
M.sub.ZM.sub.1X, by way of example.
[0011] One suitable molten electrolyte (M.sub.ZY-M.sub.ZO) for
conducting such electrolysis is CaCl.sub.2--CaO electrolyte. This
molten salt used by both the applicant and other organisations is
comprised of Ca.sup.2+ cations with Cl.sup.- and O.sup.2- anions.
The CaO dissolved in the electrolyte is present as Ca.sup.2+ and
O.sup.2- ions and there is no distinction between Ca.sup.2+ cations
that originate from CaO and those from CaCl.sub.2. The consequence
of this is that the cathode potential for reduction of Ca.sup.2+ is
unchanged by addition of CaO. Decomposition of the electrolyte to
form elemental calcium will occur once the cathode potential
exceeds a certain threshold value, which may be determined from
known thermodynamic data for a given temperature. At 900.degree.
C., the standard state potential for the reduction of the cation
Ca.sup.2+ to calcium is 3.211 V negative of the standard chlorine
electrode.
[0012] It has been established that many alkali and alkaline earth
metals exhibit high solubility in their chloride salts.
Unsurprisingly, calcium exhibits a high solubility in the
CaCl.sub.2--CaO electrolyte system (Fischbach). This has profound
consequences for the cathodic process, as the form in which the
cations of the electrolyte are present following reduction is no
longer restricted to a discrete solid or liquid (depending on the
electrolysis temperature). While cathode potentials equal or more
negative than the standard state reduction potential for Ca.sup.2+
give rise to saturated, unit activity calcium, there is a range of
potentials positive of this value that will result in the formation
of calcium at less than unit activity. The activity of calcium
metal is a frequently used term that corresponds to the
concentration of calcium metal in the electrolyte. Unit activity
denotes saturation and the presence of a condensed metallic calcium
phase and values less than one correspond to dilute solutions of
dissolved calcium metal. The exact correlation between electrode
potential, E, and calcium activity, a.sub.Ca, in equilibrium with
the electrode potential, is given by Equation 1. The potential
described by Equation 1 would be changed according to resistive and
polarisation effects.
E=-RT/2F ln a.sub.Ca (1)
Where:
[0013] E is measured in volts versus the standard state potential
for the reduction of Ca.sup.2+ to Ca.sup.0 R is the universal gas
constant (8,3144 Jmol.sup.-1K.sup.-1) T is the temperature in
Kelvin 2 is the number of electrons taking part in the reduction of
Ca.sup.2+ to Ca.sup.0 F is Faraday's constant (96484.6 Cmol.sup.-1)
a.sub.Ca is the activity of calcium
[0014] Table 1 lists the standard state reduction potential (100%
saturation of calcium in the electrolyte) of Ca.sup.2+ at
900.degree. C. and the potentials calculated from Equation 1,
corresponding to selected dissolved calcium activities less than
one (less than 100% saturation of calcium in the electrolyte).
[0015] The electrochemical spectrum that represents the continuum
of calcium activities from infinitely low values, (ostensibly, an
activity of zero) to saturation (unit activity) may be further
defined by a second variable of interest, such as melt calcium
oxide activity. Predominance diagrams for the Ti--O--Ca--Cl system
were constructed by Dring et al (J. Electrochem. Soc. v152, #10
(2005):D184), in the manner described by Littlewood (J.
Electrochem. Soc. 1965). These maps delineate the regions of
stability for the experimentally observed phases over a range of
electrode potentials and melt CaO contents. This representation of
the Ca--Cl--O system is shown graphically in FIG. 2 (J.
Electrochem. Soc. v152, #10 (2005):D184), and may be considered the
molten salt equivalent of Pourbaix diagrams. In the case of a
CaCl.sub.2--CaO molten salt, the x-axis is chosen as the negative,
base-ten logarithm of the CaO activity, (-log.sub.10[a.sub.CaO]) in
the melt, which is denoted by pO.sup.2-. For the sake of
completion, possible electrode reactions for oxide anions in the
presence of a graphite anode are shown.
[0016] As a rule, vertical lines on the diagram represent reactions
where an exchange of O.sup.2- occurs, such as in Reaction 2.
Horizontal lines depict reactions where no O.sup.2- is consumed or
liberated, but a change in oxidation state occurs (Reaction 3).
Sloping lines indicate a reaction that involves both electrons and
oxide anions, such as Reaction 4.
M.sup.2++O.sup.2-=MO (2)
M.sup.2++2e.sup.-=M (3)
MO.sub.2+2e.sup.-+Ca.sup.2+=MO+CaO (4)
[0017] As can be seen from the diagram, the addition of CaO to
CaCl.sub.2 has absolutely no effect on the potential at which
calcium forms. The only reactions that are modified by moving from
low to high pO.sup.2- (high to low CaO contents) are those
involving O.sup.2-. The evolution of CO, CO.sub.2 or O.sub.2 all
occur at increasingly positive potentials with decreasing melt CaO
content. This, of course, reflects the fact that when the melt has
lower amounts of CaO, a higher cell voltage is required to produce
CO, CO.sub.2 or O.sub.2 at an equivalent partial pressure.
[0018] The low cell voltage, relative to the standard state
decomposition potential of CaCl.sub.2, reported by the inventors of
competing processes, that may be applied in order to observe
electrolysis current is due to the fact that oxide anions are
oxidised at less positive potentials than chloride anions, and the
potential for CO and CO.sub.2 evolution on graphite anodes is
significantly less positive than for O.sub.2 evolution on inert
anodes at the same CaO content.
[0019] Dring et al (J. Electrochem. Soc. v152, #10 (2005):D184),
augmented FIG. 2 with the inclusion of the phase fields for various
titanium oxides within a CaCl.sub.2--CaO electrolyte. FIG. 3 shows
the regions of stability of various titanium-oxygen-calcium
compounds as a function of both melt calcium oxide activity and
electrode potential, which is, as shown earlier, synonymous with
calcium activity. FIG. 3 illustrates that the reduction of
TiO.sub.2 to all of its lower oxides and even low-oxygen metal
occurs without calcium metal at unit activity (either solid or
liquid) present, and that this may occur at calcium activities
below 10.sup.-3 in melts having low CaO contents. Under these
conditions, the melt contains neither calcium metal as a discrete
phase nor the high concentrations of Ca.sup.0 necessary to
constitute a "strongly reducing molten salt". Operation of the
electrolysis cell such that the cathode potential is less negative
than that corresponding to saturated calcium formation, which
results in the reduction of the electrolyte cation to produce
calcium in the solvated state, is equally effective for reduction
of the metal oxide. Thus it is not necessary to form the reductant
as a metal at unit activity and await its dissolution and
subsequent transport to the metal oxide.
[0020] Production of a calcium-saturated melt or calcium metal at
the cathode is believed by other organisations to be the most
effective method to reduce the titanium oxide. However, contrary to
conventional belief, operation at higher dissolved calcium contents
will result in lower production efficiency. This is because the
rate of production of the metal of interest is not equal to the
cathodic current. The latter is only an indication of the rate at
which calcium is formed. The rate at which calcium reduces the
titanium oxide determines the speed of the process. Although an
electrolyte saturated with calcium reductant may be desirable, from
a consideration of electrode kinetics and the thermodynamics of
Ca.sup.2+ reduction, there are numerous complications associated
with the formation of such high amounts of the calcium reductant
metal. First, the exothermic reaction with the oxide precursor may
result in the sintering of the feed material, with possible
entrapment of reaction products and/or slowing of subsequent
reduction rates. Any calcium reductant that does not reduce the
titanium oxide at the cathode is able to diffuse away from the raw
material, where it cannot do useful electrochemical work.
Consequently, the calcium may chemically react with the anode
material or anode off-gases resulting in excessive anode
consumption/erosion and, if graphite anodes are used, the
generation of free carbon or calcium carbides. Additionally,
increasing contents of calcium in the electrolyte impart a high
degree of electronic conductivity and create a short circuit path
for electrons within the electrolyte, which should ideally function
as an ionic conductor only. This final drawback is of the greatest
concern, since current efficiency is significantly worsened as a
consequence. The increased energy consumption leads not only to an
increase in financial costs, but added environmental burdens
arising from the prerequisite power generation.
[0021] The applicant has determined that since high amounts of
calcium are detrimental to the efficiency of the electrolysis, it
is preferable to ensure that throughout the reduction process the
concentration and activity of calcium and calcium oxide are
controlled. The preferred manner of control is via the use of a
reference electrode whose potential is not affected by changes in
the electrolyte composition, specifically the CaO concentration. By
controlling the potential at the cathode with respect to this
reference electrode potential, a fixed calcium
activity/concentration may be obtained. This serves to maximize the
reduction rate since the rate at which calcium is supplied to the
raw material can be controlled to match the rate at which the
reduction of metal oxide occurs. The melt calcium oxide content
must also be controlled, and this may be understood in the context
of an equilibrium between oxide ions In the titanium-containing raw
material, and in the electrolyte (Reaction 5).
[O] in TiO.sub.x or Ti--O=[O] in electrolyte (5)
[0022] With a high electrolyte CaO content, there is a decreased
driving force for oxygen to exit the titanium oxide or
titanium-oxygen solid solution. Consequently, higher amounts of
calcium are required to compensate and push the reaction in the
direction of reducing the raw material. If calcium production is
allowed to proceed uncontrolled, such that a highly
calcium-saturated melt is formed, then the exterior regions of the
titanium oxide are rapidly reduced forming high quantities of CaO.
In such an event, the precipitation of solid CaO may occur when the
local solubility limit is exceeded. Consequently, the transport of
electrolyte and the CaO reaction product is greatly diminished, and
the transport of calcium that was formed at the cathode may slow
significantly or even fail to reach all areas of the cathode
containing the raw material. If calcium continues to be generated,
then this exterior surface may be reduced all the way to metal,
which may then act as an additional surface area for the cathodic
reduction of the Ca.sup.2+ cations in the melt. This would
exacerbate the situation further, since even more of the cathode
would be producing calcium at high activities.
[0023] The applicant has resolved that the optimum operating
conditions are such that the consumption of calcium reductant by
the raw material is matched by the rate of calcium generation at
the cathode. The cathode potential may be maintained at potentials
corresponding to the activities of calcium needed to effect each of
the reduction process in order to proceed from TiO.sub.2 to
metallic titanium. This is accomplished using a stable reference
electrode, which does not vary as the melt CaO composition changes.
The benefits of this are two-fold: a lower cell voltage may be
used, thus consuming less power; and significantly less calcium is
formed The latter effect avoids the numerous disadvantages
described above. The applicant believes that operation of the
electrolysis cell in the manner described above is the only way to
achieve a high quality metal, alloy or composite product at an
acceptable price.
TABLE OF FIGURES
[0024] The invention shall be further explained by examples and
figures where:
[0025] FIG. 1 is a current versus potential plot of TiO.sub.2 and
Mo in CaCl.sub.2 at 900.degree. C.
[0026] FIG. 2 is a predominance diagram showing the conditions of
electrode potential and melt oxide content corresponding to a given
electrochemical reaction for a system with a CaCl.sub.2--CaO
electrolyte and a graphite electrode(s).
[0027] FIG. 3 is a predominance diagram showing the conditions of
electrode potential and melt oxide content corresponding to a given
electrochemical reaction for a system with a CaCl.sub.2--CaO
electrolyte, a graphite anode(s), and a cathode consisting of
titanium oxide.
[0028] FIG. 4 is a schematic diagram of the electrochemical cell
used in conjunction with the present invention.
[0029] FIG. 5 is the x-ray diffraction pattern of cathode material
produced after 24 hours with a cell voltage of 500 mV.
[0030] FIG. 6 is the x-ray diffraction pattern of cathode material
produced after 24 hours with a cell voltage of 750 mV.
[0031] FIG. 7 is the x-ray diffraction pattern of cathode material
produced after 24 hours with a cell voltage of 1000 mV.
[0032] FIG. 8 is an optical image of partially reduced cathode
material exhibiting a metallic shell and a core consisting of
oxides.
[0033] FIG. 9 is a potential versus time plot for TiO.sub.2 reduced
under constant current
[0034] FIG. 10 Scanning electron micrograph of Ti-10V-2Fe-3Al alloy
produced via the present invention.
[0035] Table 1 lists the standard state reduction potential for
Ca.sup.2+ to Ca.sup.0, and the potentials calculated from Equation
1 corresponding to selected dissolved activities of Ca.sup.0 at
900.degree. C.
EXAMPLES
Example 1
Reduction of TiO.sub.2 at Very Low Activities of Calcium
[0036] A molten salt reactor, depicted in FIG. 4, was assembled
using vertical tube furnace with temperatures recorded using a
thermocouple (1) within the cell and a PC-based data acquisition
unit. A sealed inconel reaction (2) vessel housed alumina crucibles
(3), which contained the CaCl.sub.2--CaO electrolyte (4). This
electrolyte was obtained by mixing thermally dried
CaCl.sub.2.2H.sub.2O and 1 wt % CaO, and was subsequently heated in
the retort under flowing argon (5, 6) to 1173 K. Once the
electrolyte was molten, a graphite anode (7) was lowered Into the
melt along with the cathode (8), which consisted of a TiO.sub.2
pellet formed by pressing and sintering micron-scale powders. A
voltage of 500 mV was applied across the anode and cathode for a
time of 24 hours. A reference electrode (9) was used to monitor the
cathode potential. At the conclusion of the experiment, the preform
was lifted from the melt and allowed to cool in the upper chamber
of the argon-purged cell. The sample was subsequently pulverised
and analysed using x-ray diffraction. The experiment was repeated
under identical conditions except different operating voltages (750
and 1000 mV) were used. FIGS. 5-7 show that the reduction of
TiO.sub.2 to lower oxides is possible under conditions completely
devoid of calcium at unit activity.
Example 2
Reduction of TiO.sub.2 Under Conditions of Constantly High Ca
Activity
[0037] The experimental apparatus used in Example 1 was reproduced
identically, except for the electrolysis voltage, which was fixed
at 3V, and the duration of electrolysis, which was 12 hours. The
sample was removed from the electrolyte, allowed to cool, and
washed in water. A cross section of the sample (FIG. 8) revealed a
metallic .alpha.-titanium case that enclosed a darker powder, which
was identified by x-ray diffraction as a titanium sub-oxide. The
thickness of the metallic layer was approximately 100-200 microns,
which effectively acted as a diffusion barrier preventing full
reduction of the titanium dioxide pellet.
Example 3
Reduction of TiO.sub.2 Under Constant Reduction Current
[0038] An identical reactor to that used in Example 1 was employed
to reduce 10-micron thick TiO.sub.2 layers thermally formed on a
titanium substrate. A potentiostat was used in conjunction with a
graphite counter electrode, nickel/nickel chloride reference
electrode and TiO.sub.2 working electrodes. A constant reduction
current was applied to the working electrode and the potential,
with respect to the reference electrode, was recorded over time
(FIG. 9). The reduction current was terminated when the working
electrode potential reached a steady state value that did not
continue to decrease over a long period of time. Since the
TiO.sub.2 layer was of finite thickness, the reduction current at
the conclusion of the experiment must have been comprised primarily
of calcium formation. Reduction was observed to occur in a
sequential manner (events A, B, C, D, and E), with distinct
transition times corresponding to the formation of the lower oxides
of titanium and then titanium metal. The cathode potentials at A,
B, C and D corresponded to dissolved calcium activities
significantly less than unity. Specifically, at the onset of
process A, the dissolved calcium activity in equilibrium with this
potential is approximately 10.sup.-12, the dissolved calcium
activity at B is approximately 10.sup.-11. Events C, D and E begin
at dissolved calcium activities of 10.sup.-6, 10.sup.-5 and
10.sup.-1, respectively. The end potential for this experiment was
observed to be -1950 mV (versus the Ni/NiCl.sub.2 reference), which
resulted in the formation of .alpha.-Ti containing less than 1 wt %
oxygen, as determined by x-ray diffraction analysis.
Example 4
Production of Conventional Titanium Alloy (Ti-10V-2Fe-3Al)
[0039] Reagent grade oxide powders from Alfa Aesar (TiO.sub.2
99.5%, FeTiO.sub.3 99.8%, Al.sub.2O.sub.3 99.9% and V.sub.2O.sub.5
99%, 1-2 .mu.m particle size) were mixed, as-received, with a small
amount of distilled water, which acted as a binding agent, to
achieve a final composition of 10 wt % V, 2 wt % Fe, 3 wt % Al with
the balance of titanium. The powder was then ground with a mortar
and pestle for 5 minutes to break down large agglomerates prior to
uniaxial compaction on a 15 mm diameter die at 100 MPa to obtain
the desired preform shape. These preforms were placed in an alumina
firing trough and sintered in air at 1373 K for 2 hours using a 3
Kmin.sup.-1 heating rate and a 6 Kmin.sup.-1 cooling rate. The
preforms were placed inside the same reactor described in Example
1, then lowered into the electrolyte whilst suspended on a CP Gr 2
titanium wire and reduced under applied voltages of 1500 mV against
a graphite anode rod for an initial period of 1 to 2 hours. The
voltage was subsequently increased at a constant linear rate to
3100 mV for the remaining duration of the 24 hour reduction time.
Analysis of the pellet indicated that a low-oxygen, .alpha.-.beta.
titanium alloy with a nominal composition of 8.6 V, 3.1 Al and 1.5
Fe was formed. FIG. 10 depicts the fine-grained microstructure and
fully reduced alloy obtained from the present invention.
* * * * *