Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD FOR ELECTROWINNING OF TITANIUM METAL OR ALLOY FROM
TITANIUM OXIDE CONTAINING COMPOUND IN THE LIQUID STATE
FIELD OF THE INVENTION
This invention relates to a method for the continuous electrowinning of
titanium
metal or titanium alloys from electrically conductive titanium oxide
containing
compounds in the liquid state such as molten titania slag, molten ilmenite,
molten
leucoxene, molten perowskite, molten titanite, and molten natural or synthetic
rutile.
BACKGROUND ART
Titanium metal has been produced and manufactured on a commercial scale since
the early 1950s for its unique set of properties: (i) high strength-to-weight
ratio, (ii)
elevated melting point, and (iii) excellent corrosion resistance in various
harsh
chemical environments. Actually, about 55% of titanium metal produced
worldwide is used as structural metal in civilian and military aircraft and
spacecraft
such as jet engines, airframes components, and space and missile
applications2.
Titanium metal is also employed in the chemical process industries (30%),
sporting
and consumer goods (14%), and in a lesser extend power generation, marine,
ordnance, architecture, and medical3. Titanium sponge, the primary metal form
during titanium production is still produced industrially worldwide by a
process
invented by Dr. Wilhelm Justin KROLL4 and patented in the 1940s5. The Kroll
Process consists to the metallothermic reduction of gaseous titanium
tetrachloride
with pure magnesium metal. However, today potential huge market such as
automotive parts are still looking forward to seeing the cost of the primary
metal to
decrease by 50-70%. Nevertheless, this cost is only maintained high due to the
expensive steps used to win the metal. Even if the Kroll's process has been
improved since its first industrial introduction it still exhibits several
drawbacks: (1)
it is performed under strictly batch conditions leading to expensive
downtimes, (2)
the inefficient contact between reactants leads to slow reaction kinetics, (3)
it
requires the preparation, purification, and use the volatile and corrosive
titanium
tetrachloride (TiCl4) as the dominant feed with its associated health and
safety
issues, (4) the process can only accept expensive natural rutile or rutile
substitutes
(e.g., upgraded titania slag, synthetic rutile) as raw materials, (5) the
magnesium
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and chlorine must be recovered from reaction products by electrolysis in
molten
salts accounting for 6% of the final cost of the sponge, (6) the specification
of low
residual oxygen and iron content of the final ingot requires expensive and
complex
refinning steps (e.g., vacuum distillation, and/or acid leaching) of the crude
titanium
sponge in order to remove entrapped inclusions accounting for about 30% of the
final cost of the ingot, finally (7) it only produces dendritic crystals or
powder
requiring extensive reprocessing before usable mill products can be obtained
(i.e.,
remelting, casting, forging) and wastage of 50% is common in fabricating
titanium
parts.
For all the above reasons, since the early 1970s there is a strong commitment
of
the titanium industry in synergy with several academic institutes to actively
pursue
new routes for producing titanium metal. Research and development focus has
been directed towards developing a continuous process to produce high-purity
and
low-cost titanium powder or ingots for metallurgical applications.
Although a plethora of alternative methods have been examined beyond a
laboratory stage or have been considered for preparing titanium crystals,
sponge,
powder, and alloys, none have reached industrial production.
Included in those known processes were: (i) gaseous and plasma reduction, (ii)
tetraiodide decomposition, (iii) calcio- and aluminothermic reduction, (iv)
disproportionation of TiCl3 and TiCl2, (v) carbothermic reduction, and (vi)
electrowinning in molten salts. Most were considered by the authoring National
Materials Advisory Board committee (NMAB)6 panel to be unlikely to progress to
production in the near future except electrowinning which seemed to be the
most
promising alternative route.
Actually, the extraction and preparation of pure metals from ores using an
electrolytic process is well known as electrowinning. This relatively
straightforward
process is based on the electrochemical reduction of metal cations present in
a
suitable electrolyte by electrons supplied by a negative electrode (i.e.,
cathode, -),
while at the positive electrode (i.e., anode, +) an oxidation reaction occurs
(e.g.,
anode dissolution, gas evolution, etc.). According to the first Faraday's law
of
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electrolysis the mass of electrodeposited metal is a direct function of
quantity of
electricity passed. Today among the current industrial electrolytic processes
several utilize an aqueous electrolyte to electrodeposit metal (e.g., Cu, Zn,
Ni, Pb,
Au).
Unfortunately, aqueous electrolytes exhibit a narrow electrochemical span and
are
unsuitable for preparing highly electropositive and reactive metals such as
titanium.
Actually, when cathodic (i.e., negative) potentials are applied to the
electrode, the
competitive process of the electrochemical reduction of protons occurs
together
with the evolution of hydrogen gas. This main parasitic reaction consumes the
major part of the reduction current thereby drastically decreasing the overall
current efficiency.
Despite the availability of cathode materials exhibiting a large hydrogen
evolution
overpotential (e.g., Cd, Hg, Pb), it has heretofore been quite impossible to
electrodeposit efficiently such metals despite numerous attemps reported in
the
literature' $ 9'° ~~.
Organic electrolytes were also tested'2 '3 '4 but despite their wide
decomposition
potential limits, organic solvents in which an appropriate supporting
electrolyte has
been dissolved have not yet been used industrially owing to their poor
electrical
conductivity which increases ohmic drop between electrode gap, the low
solubility
of inorganic salts, their elevated cost and toxicity.
By contrast, molten salt based electrolytes were already used industrially
since the
beginning of the 1900s in the electrolytic preparation of important structural
metals
(e.g., AI, Mg), and in a lesser extent for the preparation of alkali and
alkali-earth
metals (e.g., Na, Li, and Be).
Actually, fused inorganic salts exhibit numerous attractive features~5 '6 ~'
over
aqueous electrolytes, these advantages are as follows: (1) they produce ionic
liquids having a wide electrochemical span between decomposition limits (i.e.,
high
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decomposition potential) allowing the electrodeposition of highly
electropositive
metals such as titanium. (2) Based on the Arrhenius law, the high temperature
required to melt the inorganic salt promotes fast electrochemical reaction ~
kinetics
suitable to increase hourly yields. (3) The faradaic efficiencies are usually
close to
100%. (4) Due to their ionic state molten salts possess a high electrical
ionic
conductivity which minimizes the ohmic-drop and induces lower energy
consumption. (5) The elevated solubility of electroactive species in the bath
allows
to utilize high solute concentrations allowing to operate at high cathodic
current
densities.
Therefore, it has become clear that the most promising route for
electrowinning
titanium is to develop a high temperature electrolytic process conducted in
molten
salt electrolytes. However, despite the numerous attempts performed until
today
there are still no available electrolytic processes in molten salts for
producing
titanium metal industrially. In order to reach industrial success the new
electrochemical route must solve the major issues of the energy demanding and
labor intensive Kroll's process and also overcome the pitfalls that have lead
to
failures until today.
Actually, the electrolytic production of titanium metal has been extensively
investigated with the aim of developing a continuous process to replace
Kroll's
process. Several attempts were made in industry.
Early work was done since 1950 by National Lead Industries, Inc. and in 1956
at
the former U. S. Bureau of Mines (USBM) in Boulder City, Nevada. A small pilot
was built to investigate the electrowinning of titanium's. It consisted of a
12-inch
cylinder vessel lined with pure iron and containing a molten electrolyte made
of a
mixture of LiCI-KCI approximately at the eutectic composition with TiCl2
added.
Three equally spaced openings in the cell top accommodated: (i) the
replaceable
anode assembly, (ii) the titanium tetrachloride feed unit, and (iii) the
cathode.
Three slide valves combined with air-locks allowed the quick and easy
introduction
or removal of assemblies without contaminating the cell. The desired solute
(i.e.,
TiCl2) was produced in-situ either by the chemical reduction of stoichiometric
amount of TiCl4 with titanium metal scrap or by direct electrochemical
reduction of
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TiCl4 at the cathode. Actually, TiCl4, a covalent compound, does not ionize
and
must be converted to a ionic compound such as TiCl2. The concentration was
then
increased by operating only the feed cathode and anode and feeding one mole of
TiCl4 per two faradays of charge. In all cases gaseous TiCl4 was introduced
into
5 the bath close to the cathode with a feed nickel tube plated with molybdenum
and
dipped below the surface level of the melt. In order to avoid the oxidation of
the
newly formed Ti2+ and dragout of the dissolved TiCl4 with the chlorine evolved
at
the anode, a porous ceramic diaphragm made of alundum~ (i.e., 86 wt.% AI2O3-12
wt.%Si02)'9 surrounded the immersed graphite anode forming distinct anolyte
and
catholyte compartments. The reported optimum operating conditions identified
were: (1 ) an operating temperature above 500°C to prevent the
precipitation of
solute, and below 550°C to avoid severe corrosion of the alundum
diaphragm,
usually 520°C, (2) a solute content comprises between 2 and 4 wt.%
TiCl2, (3) a
cathodic current density of 1 to 5 kA.m 2, while the anodic current density
was
comprised between 5 and 10 kA.m-2, (4) a diaphragm current density of 1.5 kA.m-
2.
By conducting experiments with the above conditions USBM claimed that high-
purity titanium was electrowon with a Brinell hardness as low as 68 HB and a
current efficiency of 60%. However frequent failures of the diaphragm that
became
periodically plugged or loaded with titanium crystals proved troublesome. As
the
titanium content increased, the ceramic diaphragm became conductive and then
acted as a bipolar electrode and had to be removed rapidly from the bath. In
1972,
the same authors2° built a larger rectangular cell containing 226.8 kg
(i.e., 500 Ib.)
of bath in order to assess the actual performance of two kind of diaphragm
materials: (i) solid materials composite diaphragms, and (ii) loose fill
materials
composite diaphragms. For solid diaphragms, it was observed that alundum
coated nickel screen showed little deterioration but was subject to the same
current density limitations as the porous alundum diaphragm. On the other
hand,
cement coated nickel screens with loose fill material such as alumina was the
best
material in terms of strength, flexibility, resistance to corrosion, and low
replacement of titanium (0.2 to 1.0 wt.%).
In 1968, Priscu2' of the Titanium Metal Corporation (TIMET) disclosed that a
new
electrowinning cell was patented22, designed and operated in Henderson,
Nevada.
This electrolytic cell was a unique pilot based on a non diaphragm basket
cathode
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type. The cell used a suspended central metal basket cathode with sixteen
anodes
peripheral to the basket. The central basket cathode was a cubic box with the
four
sides made of perforated steel plates, while the bottom and top were blind
plates.
Four steel rods were used in the basket to act as cathode collectors while
TiCl4
was fed using a tube positioned at the center of the basket. TiCl4 was
initially fed at
a low rate into the center of the basket walls. This porous sidewall deposit
served
as a diaphragm to keep the reduced TiCl2 inside the basket while a mechanical
system was provided for withdrawing the large cathode deposits into an inert-
gas-
filled chamber, installing a new cathode, and reclaiming the inert gas for
reuse.
The average valence of dissolved titanium rations was maintained very low
generally no greater than 2.1 to obtain the electrodeposition of premium-grade
titanium metal. TIMET claimed that later models of pilot-plants have produced
up
to 363 to 408 kg (i.e., 800 to 900 Ib.) of titanium metal in one cathode
deposit. This
semi-works plant produced about 68 tonnes (i.e., 150,000 Ib.) of electrolytic
titanium sponge but discontinued the operation in 1968 owing of overcapacity
for
making sponge by Kroll's process.
Later in 1971, Hashimoto et al. have worked extensively on the electrowinning
of
titanium metal from its oxides or mixed oxides23 2a 25. Titanium solute was
introduced in a molten fluoride bath, as a solid compound such as Ti02,
FeTi03,
CaTi03, or MgTiO3. The melt chemistries tested were CaF2, MgF2, BaF2, NaF and
their mixtures. The first electrolysis study was conducted at temperatures
above
1600°C with graphite anode and cathode. Only in the cases of the CaF2-
Ti02 (1-
10% wt.) and CaF2-CaTi03 (10% wt.) melt systems molten titanium was obtained
but largely contaminated by' carbon and oxygen (2-4 wt.%). In other cases,
fine
titanium powder was only obtained. After the preliminary results, they
focussed on
the electrowinning of titanium from pure Ti02 carried out in molten salt baths
made
of CaF2, BaF2, MgF2, CaF2-MgF2, CaF2-NaF, CaF2-MgF2-NaF, CaF2-MgF2-NaF2,
and CaF2-MgF2-SrF2 at 1300-1420°. The titanium electrodeposited in CaF2
and
BaF2 baths was considerably contaminated by carbon owing to graphite
electrodes. In NaF-containing fused salts such as CaF2-NaF and CaF2-MgF2-NaF,
only fine powdery deposits were obtained due to simultaneous sodium reduction
that occurs. In the baths of MgF2, CaF2-MgF2, CaF2-MgF2-BaF2, and CaF2-MgF2-
SrF2, dendritic deposits were obtained. They pointed out that best result was
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obtained in the CaF2-MgF2 bath, but the purity of the deposit was not as high
as
that of the common grade titanium sponge required by the industry. In the
third
article, electrowinning of titanium was carried out in CaF2-MgF2 (50-50 wt.%)
molten salt bath at 1020-1030°C in an argon atmosphere by using a
completely
enclosed cell. In electrowinning from Ti02, the form of the electrodeposited
metal
changed from crystaline to spongelike with an increase in current density, or
cell
voltage, but when CaTi03 was used, deposits were spongelike. Despite the
material yield of titanium was superior to 95 wt.% it did not still meet the
requirements of commercial sponge.
Later in 1973, the Doriv Chemical Company in a close working relationship with
the
HOIlVMET group (i.e., subsidiary of the French Pechiney Ugine Kuhlmann (PUK)
Group) founded the D-H Titanium Company for producing continuously high-purity
electrolytic titanium at Howmet's plant in Whiteall, M126. Cell design,
operating
procedure, metal quality, proposed production, and economic projections have
been described by Cobel et a1.27. The technology was based on the cell
designed
in the previous work done at Dow Chemical by Juckniess et a128. Actually, a
major
cell improvement in the D-H Titanium design was the fabrication of a metal
screen
diaphragm that was electroless-plated with cobalt or nickel to give the
required
electrical and flow characteristics. The cell operated at 520°C under
argon
atmosphere with LiCI-KCI-TiCl2 (ca. 2 wt.% TiCl2) as molten salt electrolyte.
TiCl4
was fed continuously into a pre-reduction cathode compartment where reduction
to
dichloride TiCl2 takes place at a separate feed cathode within the cell. Final
reduction to metal was continuously done on separate deposition cathodes. The
cathodes were periodically removed hot and placed into a stripping machine
under
inert atmosphere. Metal-working cathodes were individually pulled, stripped,
and
replaced in the cell, in an argon atmosphere, by a self-positioning and
automatically operated mechanical device. A sealed, argon-shielded hopper
containing the titanium crystals and entrained electrode was cooled before
being
opened to discharge its contents. Crystalline metal and dragout salts were
crushed
to 3/8-inch size and leached in dilute 0.5 wt.% HCI solution. Then the spent
solution was neutralized with a mixture of Li2C03 and KOH in a ratio
equivalent to
that used in the electrolyte. Dragout of electrolyte varied with the titanium
crystal
sizes to about 1 kg per kg of fine titanium for coarse washed metal. Dragout
was
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dried and passed over a magnetic separator, and metal fines were removed by
screening to about 80 mesh (177 pm). They claimed that the sponge produced
exhibited both a low residual oxygen, nitrogen, iron and chlorine content, had
a
Brinell hardness of 60 to 90 HB and excellent melting characteristics.
According to
Cobel et a1.29, the direct current required for electrowinning (17.4 kWh/kg)
appears
to be only about half that required for the I<roll process. Although titanium
sponge
of apparently satisfactory purity was claimed to be produced in relatively
small
pilot-plant cells with a daily titanium capacity of up to 86 kilograms per
day, the
electrowinning of titanium was far from an industrial scale.
Unfortunately, in December 30th, 1982, according to American Metal Market, the
expenses for completing the joint program and the economic climate at that
time
have forced the dissolution of the D-H Titanium Company. With the breakup each
company (i.e., Dovv and Howmet) Dow has continued some research and
development work on the electrolytic process but without success while Hovvmet
apart having patented some work done in France3o 3~ has later focused in the
metals fabrication area.
In 1985, the Italian company Elettrochimica Marco Ginatta S.p.A. (EMG) owned
by
the Italian scientist and businessman Marco Vincen~o Ginatta claimed a new
electrowinning process32 inspired from the previous attempts33. This new
upgraded
process for the electrolytic preparation of titanium uses always the
dissolution and
cathodic reduction of titanium tetrachloride in an electrolyte made of alkali
or
alkaline-earth metal halides and the electrodeposition of the dissolved
titanium
cations. The process was supported by RMI Titanium, and the company built a
pilot plant. Ginatta claimed that the current production capacity of this
plant
reached 70 tonnes per year in 198534. Unfortunately, in 1990 RMI closed the
plant
owing to inability to solve "engineering issues".
Later, in the period 1997-2000 Kawakami et a1.35 have proposed an electroslag
remelting process. The main idea was to avoid common dendritic electrodeposits
by producing the electrodeposited titanium metal in its liquid state. Direct
electrowinning of liquid titanium metal was the investigated techniques by
using a
direct current Electro-Slag Remelting (i.e., DC-ESR) apparatus. A small scale
DC-
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ESR unit of 110 mm inner diameter was operated in d.c. reverse polarity mode,
where a graphite rod was used as anode and a steel or a copper base-plate was
used as cathode. The used slag was Ca0-CaF2-Ti02 mixture. The current was
approximately 1.5 kA. Under certain experimental conditions, some amount of
titanium was electrodeposited in the metal pool. From the view point of heat
balance, the sufficient heat was supplied by Joule heating in a molten slag
phase.
It can be seen from the published results that unfortunately most of the
deposit
was obtained as TiC and the current efficiency for the reduction was only
1.5%.
In 1999, the process was improved36, the current efficiency for the reduction
was
up to 18% with a larger distance between the electrodes. Some amount of
titanium
was electrodeposited on the base-plate though its state changed with the
electrolytic condition. Pure titanium metal pieces were obtained in the
solidified salt
after the run with the bigger electrode distance. It was concluded that the
electrowinning of liquid titanium metal by the present process was possible if
sufficient heat to form a metal pool can be supplied at the bigger distance
between
the electrodes. The DC-ESR process was patented in 1988 and reconducted in
2000, and then recently presented at ECS meeting3~.
The idea to use a molten pool of titanium was also recenty claimed by Ginatta
Torino Technology (GTT) who patented a new process for electrowinning titanium
based on the recovery of the molten metal using a pool of liquid titanium as
cathode like for aluminium38.
The main idea of Ginatta is to avoid common dendritic electrodeposits by
producing the electrodeposited titanium metal in the liquid state such as for
aluminium. Nevertheless, the process which operates at 1750°C still
needs to
convert the expensive titanium dioxide to the titanium tetrachloride and the
dissolution of the feedstock into a molten salt electrolyte made of CaCl2-CaF2
and
containing calcium metal Ca.
Recently in 2000, based on early results obtained by Fray, Farthing, and
Chen39
float the Dept. of Materials Science of the Cambridge University, early trials
were
conducted and patented4~ 42at the Defence Evaluation and Research Agency
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(DERA) at Farnborough (Hampshire, U.K.). A new company British Titanium (BTi)
has been formed to commercialize the newly discovered process43 that the
scientific litterature has dubbed the Cambridge's or FFC's Process. The
process
claims the electrochemical deoxidation of solid titanium dioxide that was
originally
5 applied for refining titanium metal by Okabe et al. in 199344 4s 46. The
inventors
have demonstrated at the laboratory scale that the reduction reaction proceeds
at
950°C from a cathode made originally of solid Ti02 while oxidation of
oxygen
anions occurs at the graphite anode with evolution of carbon dioxide. Pure
calcium
chloride (CaCl2) was selected as molten salt electrolyte owing to its high
solubility
10 for oxygen and excellent migration transport properties for oxygen anions.
According to inventors, the process for the production of pure titanium metal
consists of the following sequences of operations. The pure titanium dioxide
powder is mixed with an appropriate binder to form a past or slip, and cast
into a
rectangular shape cathodes using one of the techniques common in the ceramic
industry, such as rolling or slip casting. The green cathode will be then
fired in an
air '.kiln to initiate sintering in order to produce a solid ceramic material.
After
sintering the shapes give solid cathodes. Reduction of titanium occurs in an
enclosed electrolytic cell with inert gas filling. The cell is designed for
continuous
operation with cathodes at different stages in their cycles being inserted and
removed through an automated air lock. By controlling the cathode potential,
oxygen can be removed from titanium dioxide allowing to leave behind a high
purity metal which is morphologically similar to the Kroll's sponge. The cell
voltage
is roughly 3 V, which is just below the decomposition voltage of CaCl2 (3.25 V
at
950°C), avoiding chlorine evolution at the anode but well above the
decomposition
voltage of Ti02 (1.85 V at 950°C). Sufficient overpotential is
necessary to reduce
the oxygen content of the titanium metal. The inventors claim that
stoichiometric
mixture of other metal oxides with Ti02 into the original cathode are also
concurrently reduced to metal leading to the possibility to produce also
titanium
alloys although the microstructure is different. The process has been
demonstrated
in a bench-scale reactor (i.e., 1 kilogram per day). The Cambridge's process
claimed that it overcomes several of the issues encountered by its
predecessors
but however there are several important pitfalls to be overcome in scaling-up
the
process for a future commercial development. Primarily, it has an extremely
low
space time yield, i.e., mass of titanium produced per unit time and cathode
surface
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area. This is related to the slow diffusion kinetics of oxygen across the
layer of
solid titanium metal at the cathode/electrolyte interface. Actually, several
hours are
required to completely reduce a porous pellet made of sintered Ti02 and huge
cathode surface areas are needed to compensate. Secondly, since the waste
CaCl2 can be only removed from the titanium by water leaching after the
completion of the reaction it is strictly a batch process. Finally, it
requires
expensive preparation of titanium dioxide pellets as feedstock itself produced
from
tetrachloride and a preliminary preparation to render the feedstock conductive
is
needed.
Also in 2000, Sharma4~ proposed the calciothermic reduction of pure titanium
dioxide with a zinc-calcium alloys performed in a molten salt mixture of CaCl2-
CaF2
at 800°C. Titanium powder was later recovered from the Zn-Ti alloys
formed by
vacuum distillation which is highly energy demanding.
In 2001, Fortin4$ proposed another process for obtaining titanium metal from
ilmenite using a so-called 'shuttle-alloys'. The process which comprises two
consecutive steps requires expensive materials and some having environmental
issues for an industrial process and is also energy demanding.
In 2001, Pal et al. from Boston University suggested a new way for
electrowinning
reactive metals including titanium using a solid oxide membrane (SOM)
process49.
The patented method consists to electrolyse a molten salt electrolyte
containing
the cations of the metal to electrodeposit at the cathode using a porous gas
diffusion anode separated from the high temperature melt by a solid ionic
membrane capable of transporting the anionic species of the electrolyte to the
anode5o 5'. Nevertheless, this process did not use the electrochemical
deoxidation
of a cathode and no mentions is made to use SOM as a unique electrolyte
immersed into a molten titania slag acting as liquid cathode material.
Heretofore, no processes described in the prior art have proven to be
satisfactory
or gained industrial acceptance. None of the prior art processes directly use
inexpensive titanium feedstocks such as crude titania slag for producing
electrochemically titanium metal and alloys. Actually, plenty of crude titania
slag is
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produced industrially by the carbothermic reduction of hemo-ilmenite or
ilmenite
ore concentrate with anthracite coal into an electric arc furnace (EAF) such
as
those produced industrially by Quebec Iron & Titanium Inc. (QIT) in Canada or
by
Richards Bay Minerals in South Africa. Indeed, titania slag exhibits a
semiconductive behavior and hence it can be used without any treatment and
additives as an electrode material. Its good electronic conductivity ranging
from 10
S.rri' for the bulk solid at room temperature until 1.21 x 104 S.m-' for the
melt
above its liquidus temperature is related to the sub-stoichiometric titanium
oxides it
contains. These oxides exhibit the typical Andersson-Magneli crystal
structure52
having the global chemical formula TI~02"_~, with n an integer at least equal
to 4
(e.g., Ti407, Ti509, TI6O~~). Actually, these oxides exhibit in their pure
state at room
temperature an electrical resistivity sometimes even lower than that of pure
graphite (e.g., as low as 630 NS~.cm for Ti407).
Highly pure form of these titanium oxides were first suggested as electrode
material by Hayfield53 from IMI and are now produced and commercialized under
the trade name Ebonex~54 by the British company Atraverda Ltd.55.
First experimental trials performed at RTIT to deoxidize electrochemically
solid
titania slag with calcium chloride as electrolyte at 950°C indicated
that the process
works but only produces a thin and brittle layer of titanium-iron alloy at the
slagielectrolyte interface. The overall electrochemical reaction corresponds
to the
carbothermic reduction of titanium dioxide with the following reaction scheme:
Ti02(sol.) + C(sol.) = Ti(sol.)~~ + C02(gas)T
The experimental results demonstrated that the electrochemical reaction
exhibits
both an extraordinarily high specific energy consumption and extremely low
space
time yield. These poor performances were attributed mainly to the newly formed
titanium metal layer at the slag/electrolyte interface that impedes proper
mass
transfer by diffusion of oxygen anions. In other words, as soon as a thin
layer of
solid titanium is produced, the process is "choked" and proceeds little
further.
Deoxidizing at higher temperatures up to 1350°C was also achieved but
despite
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improved performance the process remained unsatisfactory for a profitable
industrial process.
Thus, there remains an important need for an improved deoxidizing process for
titanium oxide containing compounds.
SUMMARY OF THE INVENTION
In general terms, the present invention provides an improved deoxidizing
process
for titanium oxide containing compounds. Thus, the present invention, provides
a
method for electrowinning of titanium metal or titanium alloys from conductive
titanium oxide containing compounds selected from titanium oxides, ferro-
titanium
oxides, titanium compounds and mixtures thereof. The method comprising the
steps of
(a) providing the conductive titanium oxide containing compound at
temperatures corresponding to the liquid state so as to provide a molten
material;
(b) pouring the molten material into an electrochemical reactor to form a
pool of electrically conductive liquid acting as molten cathode material;
(c) covering the molten cathode material with a layer of electrolyte,
preferably molten salts or a solid state ionic conductor hence providing an
interface between the molten cathode material and the electrolyte;
(d) providing at least one anode in said electrolyte, said anodes) being
operatively connected to an electrical current source;
(e) deoxidizing electrochemically the molten cathode at the interface with
the electrolyte by electrolysis induced by said current source and circulating
between the anode and cathode;
(f) recovering the resulting deoxidized titanium metal or titanium alloy .
In another related embodiment, the method comprises the steps of:
(a) providing the conductive titanium oxide containing compound at
temperatures corresponding to the liquid state so as to provide a molten
material to be used as a molten cathode material;
(b) providing a molten electrolyte, preferably molten salts or a solid state
ionic conductor in an electrochemical reactor;
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14
(c) pouring the molten cathode material into said electrolyte and allowing
separation based on relative densities with settling of the molten cathode
material as a layer under the molten electrolyte, hence providing a clean
interface between the molten cathode material and the electrolyte;
(d) providing at least one anode in said electrolyte, said anodes) being
operatively connected to an electrical current source;
(e) deoxidizing electrochemically the molten cathode at the interface with
the electrolyte by electrolysis induced by said current source and circulating
between the anode and cathode;
(f) recovering the resulting deoxidized titanium metal or titanium ally .
In another related embodiment, the electrolyte is not molten and is simply
part of a
gas diffusion anodes) which is dipped in the molten cathode of titanium oxide
containing compounds
In a preferred embodiment, the method is conducted as part of a continuous
process.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic illustration of the electrochemical reactor with a
molten salt
electrolyte and a consumable carbon anode.
Figure 2 is a schematic illustration of the electrochemical reactor with a
molten salt
electrolyte and an inert dimensionally stable anode.
Figure 3 is a schematic illustration of the electrochemical reactor with a
solid
oxygen anion conductor electrolyte and a gas diffusion anode.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FOR CARRYING
OUT THE INVENTION
Generally speaking, this invention relates to a method for the electrowinning
of
titanium metal or its alloys from electrically conductive titanium mixed oxide
compounds in the liquid state such as molten titania slag, molten ilmenite,
molten
perowskite, molten leucoxene, molten titanite, and molten natural or synthetic
rutile.
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Referring to Figures 1-3 there is .shown an apparatus (10) for conducting the
method of the present invention. The apparatus shown in Figures 1-3 only
differ in
the choice of anodes. The method preferably involves tapping by gravity or by
5 siphoning the crude and molten titanium slag (12) directly from an operating
electric arc furnace currently used for the smelting of hemo-ilmenite or
ilmenite ore
with anthracite coal. Pouring the molten titanic slag at the bottom of an
electrolytic
cell (14) to form a pool acting as liquid cathode material (-) (12). The
liquid cathode
(12) is covered with a layer of molten salt electrolyte (16) such as molten
calcium
10 fluoride (i.e., fluorspar) or a solid-state oxygen ion conductor (e.g.,
yttria stabilized
zirconia, beta-alumina). Reducing cathodically by direct current electrolysis
at high
temperatures the molten titanic slag with either at least one of a consumable
carbon anode (18), an inert dimensionally stable anode shown as numeral (20)
on
Figure 2 or a gas diffusion anode fed with a combustible gas (+) shown as
numeral
15 (22) on Figure 3. The electrochemical deoxidation initially produces
droplets of
metallic impurities such as metallic iron and other transition metals more
noble
than titanium (e.g., Mn, Cr, V, etc.). Hence iron metal and other metals
droplets
sink by gravity settling to the bottom of the electrolytic cell forming a,
pool of liquid
metal while oxygen anions diffuse and migrate through the molten salt
electrolyte
to the anode(s). In the case of a consumable carbon anode carbon dioxide gas
is
evolved at the anode. Once all the iron and other metals are removed
electrolitically the pool is siphoned or tapped at the taphole (24). The
apparatus
(10) is provided with water cooled flanges (26) and slide gate valves (28) to
permit
removal and insertion of materials without electrolytic cell contamination.
Once the first deoxidized metals or alloys are removed, then the temperature
of the
melt is increased by Joule's heating to compensate the concentration in
titanic
content. Then droplets of liquid titanium metal , are electrodeposited at the
slag/electrolyte interface while oxygen anions diffuse and migrate through the
electrolyte to the anode(s). Owing to the higher density of the liquid
titanium
compared to that of the molten titanic slag, the liquid titanium droplets sink
by
gravity settling to the bottom of the electrolytic cell forming after
coalescence a
pool of pure liquid titanium metal (30). The pure liquid titanium metal is
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16
continuously tapped by gravity or siphoning under an inert atmosphere and cast
into a dense, coherent, and large ingots.
The first and optional step consists in tapping or siphoning crude molten
titanium
slag directly from an operating electric arc furnace (EAF) currently used for
the
smelting of hemo-ilmenite or ilmenite ore concentrate with anthracite coal.
Transferring the hot molten liquid to an electrochemical reactor using
techniques
well known in the metallurgical industry (e.g., tapholes, slidegates). The
transfer is
intented to keep the sensible and latent heat of the molten titanic slag
unchanged
in order to maintain energy consumption lower as possible without the need of
melting it again. The temperature of molten titanic slag usually ranges
between
1570°C to 1860°C depending on its titanic content which is
usually comprised
between 77 to 85 wt.% Ti02 for crude titanic stags and until 92-96 wt.% for
melts
made of upgraded titanic slag, natural or syntetic rutile.
Preferably, the molten titanic slag is flowed into a furnace that already
contains an
electrolyte made of molten inorganic salts or their mixtures such as alkali-
earth
metals halides, but more preferably alkali-earth metals chlorides or fluorides
with a
final preference for metallurgical grade fluorspar (i.e., fluorite or calcium
fluoride
CaF2).
In a preferred embodiment, the electrolytic cell (14) which is designed for
continuous operation consists of a high temperature furnace with consumable
carbon anodes (18) or inert dimensionally anodes (20) or gas diffusion anodes
(22)
that can be inserted and removed from the electrochemical reactor at different
stages in their cycles without any entries of air and moisture through tight
air locks
which are closed by means of large gate valves (28). The refractory walls are
water-cooled externally (32) in order to maintain a thick and protective
frozen layer
(banks) of both titanium metal, titanic slag and electrolyte. This is done to
self-
contain this ternary system at high temperature and avoid any corrosion
issues.
During electrolysis, heat is only provided to the electrochemical reactor by
Joule's
heating. The electrolysis is pertormed under galvanostatic conditions (i.e.,
at
constant current) by imposing a direct current between the molten titanic slag
cathode (-) and the anode (+) by mean of an d.c. electric power supply or a
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17
rectifier. Usually high cathodic current densities of 5 kA.m 2 are imposed
with a cell
voltage of less than 3 volts. Owing to the high operating temperature which is
above the melting point of titanium metal (1660°C) and the higher
density of pure
liquid titanium (4082 kg.m-3) compared to that of the molten titanic slag
(3510 kg.m-
3), the electrodeposited titanium at the slag/electrolyte interface forms
droplets of
liquid metal that sink by gravity settling at the bottom of the electrolytic
cell forming
a pool of pure liquid titanium metal. The pool also acts as an efficient
current
collector and never impedes the oxygen diffusion at the slag electrolyte
interface.
While oxygen anions removed from the titanic diffuse and migrate to the carbon
anode where carbon dioxide is evolved. The overall electrochemical reaction
corresponds to the carbothermic reduction of titanium dioxide with an overall
reaction scheme which is given by:
TiO2 (liq.) + C (sol.) = Ti (liq).~ + C02 (gas) T
The level of molten titanium slag in the electrolytic cell is permanently
adjusted in
order to insure continuous operating electrolysis. The liquid titanium metal
is
continuously tapped under an inert argon atmosphere and cast into large dense,
and coherent titanium ingots. The titanium metal ingots produced exhibited a
high
purity and other characteristics that satisfies at least the grade EL-110 in
accordance with the standard B299-99 from the American Society for Testing
Materials (ASTM)56 such as a low residual oxygen, nitrogen, iron and chlorine
content, a Brinell hardness of 60 HB. The electrowinning process always
exhibits a
specific energy consumption lower than 7 kWh per kg of titanium metal
produced.
Therefore, the present invention resolves many if not all of the previous
issues
related to the electrolytic production of the titanium metal by: (1)
Deoxidizing
electrochemically, continuously and in one step a raw and electrically
conductive
titanium mixed oxide compound such as crude titanic slag far less expensive
than
previous feedstocks such as titanium tetrachloride or pure titanium dioxide.
(2)
Using the molten titanic slag as cathode material, preferably as is, without
any
prior treatment or introduction of additives. (3) Taking advantage of the
elevated
sensible and latent heat of the molten titanic slag because it is can be
siphoned
directly from an electric arc furnace used industrially for the smelting of
ilmenite. (4)
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Operating the electrolysis at a temperature greater than the liquidus
temperature of
titanic slag and melting point of titanium metal allowing to collect quickly
by gravity
settling the droplets of electrodeposited titanium as a pool of liquid metal
at the
bottom of the electrolytic cell below molten titanic slag owing the difference
of
densities. (5) Utilizing high temperature electrolytes with elevated boiling
points
which are excellent oxygen anions carrier such as molten halide salts (e.g.,
calcium fluoride, strontium chloride) or solid-state oxygen anion conductors
(e.g.,
yttria-stabilized zirconia, beta alumina). (6) Cooling externally the walls of
the
electrochemical reactor in order to maintain a protective frozen layer of both
titanium metal, titanic slag and electrolyte. This is done to self-contain the
ternary
system at high temperature and prevent potential corrosion issues. During
electrolysis, the heat necessary to maintain the melt liquid is preferably
only
provided by Joule's heating. (7) Using either a consumable carbon electrode or
an
inert dimensionally stable anode or a gas diffusion electrode fed with a
combustible
gas such as hydrogen, hydrocarbons, natural gas, ammonia, carbon monoxide or
process smelter gas (i.e., carbon monoxide and hydrogen mixtures). (8)
Continuously siphoning or tapping of the pure liquid titanium metal and
casting it
under inert atmosphere into large titanium ingots.
EXAMPLES
Example 1: (Reference Example) This example is only intended to provide the
performances of the electrochemical deoxidation of solid titanic slag. This in
order
to serve as reference experiment to allow later comparison with the
performances
of the present invention. For instance, a mass of 0.100 kg of crude titanium
slag
from Richards Bay Minerals (see Table 1) with at least 85 wt.% Ti02 is crushed
and ground to a final particle size comprised between 0.075 mm and 0.420 mm
(i.e., 40 and 200 mesh Tyler). This step is required at the laboratory scale
only in
order to facilitate the removal of inert minerals present in the crude titanic
slag
(e.g., silicates, sulfides) and facilitate the removal of associated chemical
impurities
(e.g., Fe, Si, Ca, Mg). Secondly, the finely ground titanic slag undergoes a
magnetic separation step. The strong ferromagnetic phases such as for instance
free metallic iron entrapped in the titanic slag during the smelting process
and its
intimately bound silicate minerals are efficiently removed using a low
magnetic
induction of 0.3 tesla and separated with the magnetic fraction which is
discarded.
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Then the remaining material undergoes a second magnetic separation conducted
with a stronger magnetic induction of 1 tesla. The non magnetic fraction
containing
all the diamagnetic mineral phases (e.g., free silica and silicates) is also
discarded.
The remaining material consists of a finely purified ground titania slag.
Thirdly, the
ground material is poured into a pure molybdenum crucible of 5.08 cm inside
diameter and 10.16 cm tall and introduced in a high temperature furnace with a
graphite heating element. The furnace chamber is closed by means of water
cooled flanges, the proper tightness is insured by o-ring gaskets made of
fluoroelastomers (e.g., Viton~) or annealed ductile metals (e.g., Cu, Au). The
components of the apparatus were selected to achieve a vacuum tight cell at
elevated temperatures. Before reaching the temperature of 1200°C, the
furnace is
purged from background contaminants by medium vacuum pumping (i.e., 0.01
mbar). When the temperature is reached the vacuum circuit was switched to a
pure argon stream. The argon stream is purified by passing it through both a
water
and oxygen traps (i.e., getter made of zirconium turnings heated at
900°C). Then
the temperature is increased to 1700°C and maintained steady during
about 1
hour. Once totally molten the titania slag is cooled down inside the crucible.
After
complete solidification the typical electrical resistivity of the material at
room
temperature currently ranges between 600 and 5000 pS2.cm. An inorganic salt
consisting of 0.200 kg of pure calcium chloride (CaCl2) is then added and
serves
as electrolytic bath. Once again, the furnace is tighly closed and heated
under
medium vacuum until the temperature of fusion of the pure calcium chloride is
reached (i.e., 775°C). At that point the vacuum circuit was switched to
a pure argon
stream and the temperature is increased until the final operating temperature
of
950°C. Then a 1.905 cm diameter rod of consumable carbon anode (e.g.,
semi-
graphite from SGL Carbon) is immersed into the electrolyte with an inter-
electrode
spacing of 1.5 cm from the titania slag. Once thermal equilibrium is reached,
the
electrolysis is performed under galvanostatic conditions (i.e., at constant
current)
by imposing a direct current between the consumable carbon anode (+) and the
solid titania slag cathode (-) by mean of a DC electric power supply. A
progressive
cathodic current ramp of 0.5 kA.m ~.min-~ is applied up to a final steady
cathodic
current density of 5 kA.m-2. During this electrolysis the average cell voltage
is less
than 4.0 volts. At the slag/electrolyte interface the electrochemical
deoxidation
produce a solid layer of titanium alloy. While the oxygen anions removed from
the
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titanic diffuse extremely slowly through this layer and migrate across
electrolyte to
the carbon anode where carbon dioxide is finally evolved. The overall
electrochemical reaction corresponds to the carbothermic reduction of titanium
dioxide and overall reaction scheme is given by:
5
Ti02(sol.) + C(sol.) = Ti(sol.) + C02(gas)T
After completion of the reaction, that is, when an anode effect occurs owing
to
depletion of oxygen anions in the bath, the crucible is cooled down and the
calcium
10 chloride is removed easily by washing it with hot water. The surface of the
titanic
slag exposed to the melt revealed a thin metallic layer of few millimeters
thickness
mainly composed of a titanium alloy with the average chemical composition:
69 wt.% Ti,
wt.% Fe,
15 2.5 wt.% Mn,
2.0 wt.% Cr,
1.5 wt.% Si.
Below this metallic layer. it is possible to identify from top to bottom
discoloured
20 underlying layers from bluish gray to golden brown and finally dark brown
made of
various oxygen depleted titanic slag regions confirming the progressive
deoxidation process. Because the iron and other impurities remain entrapped in
the titanium layer, the final purity of the metal is effectively poor and
obviously
never satisfies the commercial specifications of titanium sponge. Moreover in
these
25 conditions the electrowinning process exhibits extremely poor performances
(see
Table 3) such a huge specific energy consumption of 700 kWh per kg of titanium
metal and faradaic efficiency of 0.5% both related to the poor kinetic for
diffusion of
oxygen anions accross the metallic layer and increased distance from oxygen
rich
slag.
Example 2: The experimental conditions depicted in the following example just
differs from that of the example 1 in that the temperature of electrolysis is
now
increased to 1100°C. Even in that case, despite electrochemical
performances are
improved (see Table 3) compared to the previous example with a specific energy
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21
consumption of 346 kWh per kilogram of titanium produced and a faradaic
efficiency close to 2.4 % the final purity of the titanium alloy is quite
identical
because the feedstock material remained the same.
Example 3: The experimental conditions depicted in the following example just
differs from that of the example 1 in that the temperature of electrolysis is
now
increased to 1350°C. Even in that case, despite electrochemical
performances
being greatly improved (see Table 3) compared to the previous example with a
lower specific energy consumption of 31 kWh per kilogram of titanium produced
and a faradaic efficiency close to 13 % the final purity of the titanium alloy
is quite
identical because the feedstock material remained the same.
Example 4: The experimental conditions depicted in the following example just
differs from that of the example 3 in that the titanic slag is sintered prior
to be
electrochemically deoxidized. Actually after crushing and sizing the fraction
having
a particle size of 20/35 mesh (i.e., 425 to 850 pm) is sintered under an argon
atmosphere at 1450°C. The solid sintered mass was then used as cathode
material in the same set-up devised in the examples 1 and 2. Because the
active
cathode surface area was enhanced by the sintering process the electrochemical
performances are improved with a lower specific energy consumption of 18 kWh
per kilogram of titanium produced and a faradaic efficiency close to 36 % but
the
final purity of the titanium alloy is quite still the same because the
feedstock
material remained the same.
Example 5: The experimental conditions depicted in the following example just
differs from that of the example 2 in that (i) the cathode is now molten crude
titanic
slag from Richards Bay Minerals without any prior treatment. (ii) The molten
electrolyte is pure molten calcium fluoride (CaF2) and (iii) the electrolysis
temperature is 1700°C. During electrolysis the average cell voltage is
about 2.0
volts. At the slaglelectrolyte interface the electrochemical deoxidation
produces in
a first step dense droplets of liquid iron metal which is first to be
electrodeposited
along with other metals more noble than titanium (e.g., Mn, Cr, V, etc.) while
oxygen anions diffuse and migrate through the molten salt electrolyte to the
carbon
anode where carbon dioxide is evolved. The first electrochemical reaction
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22
corresponds to the carbothermic reduction of metallic oxides with a reaction
scheme given by:
MxOy(liq.) + (yl2)C(sol.) = xM(liq.)~ + (yi2)C02(gas)T
Owing to the higher density of the liquid iron (i.e., 6886 kg.m-3 at
1700°C) and
other metals compared to that of the molten titania slag (3510 kg.m-3 at
1700°C),
the liquid metal droplets sink quickly by gravity settling at the bottom of
the
electrolytic cell forming after coalescence a pool of liquid metal which is
continuously tapped. Once all the iron and other metallic impurities are
removed by
this selective electrodeposition, the temperature is increased to
1800°C to
compensate the enhanced content of TiO~ of the purer titania slag. Now
electrochemical deoxidation carries on with the electrodeposition of droplets
of
liquid titanium metal at the slag electrolyte interface. Meanwhile oxygen
anions
diffuse and migrate through the molten salt electrolyte to the carbon anode
where
carbon dioxide gas is evolved. Because the molten titania slag has a low
dynamic
viscosity and exhibits a much lower density (e.g., 3510 kg.m-3 for 80 wt.%
Ti02 at
1700°C) than that of pure liquid titanium (e.g., 4082 kg.m 3 at
1700°C), the pure
liquid titanium droplets fall by gravity settling at the bottom of the
electrolytic cell
forming after coalescence a pool of pure liquid titanium metal that accumulate
at
the bottom of the crucible which is continuously tapped under an inert argon
or
helium atmosphere. The overall electrochemical reaction corresponds to the
carbothermic reduction of titanium dioxide with a reaction scheme given by:
TiO~(liq.) + C(sol.) = Ti(liq.)~~ + C02(gas)T
Completion of the reaction occurs when an anode effect takes place owing to
depletion of oxygen anions in the bath. The titanium metal small ingot
produces
exhibits at least 99.9 wt.% Ti and the final purity of the metal always meets
the
sponge grade EL-110 of standard 8299-99 from the American Society for Testing
Materials (ASTM)5'. Moreover electrochemical performances are also greatly
improved with a lower specific energy consumption of 6.8 kWh per kilogram of
titanium produced and a faradaic efficiency close to 90 %.
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10
20
Example 6 - The experimental conditions depicted in the following example just
differs from that of the example 5 in that the cathode is now molten crude
titanic
slag with at least 78 wt.% Ti02 such as those produced by Quebec Iron &
Titanium
Inc (e.g., Sorelslag~).
Example 7 - The experimental conditions depicted in the following example just
differs from that of the example 5 in that the cathode is now molten upgraded
titanic slag with at least 94 wt.% Ti02 such as those produced by Quebec Iron
&
Titanium Inc (e.g., UGS~).
Example 8 - The experimental conditions depicted in the following example just
differs from that of the example 5 in that the cathode is now molten synthetic
ruble
with at least 94 wt.% TiO~ such as those produced artificially in Australia or
India
from weathered ilmenite and leucoxene, the temperature of electrolysis is
1850°C.
Example 9 - The experimental conditions depicted in the following example just
differs from that of the example 5 in that the cathode is now molten ACS
reagent
grade titanium dioxide from Fischer Scientific with at least 99 wt.% Ti02 and
the
electrolysis temperature is 1860°C.
Example 10 - The experimental conditions depicted in the following example
just
differs from that of the example 4 in that the molten salt electrolyte is
replaced by
a thick solid-state oxygen anion conductor such as yttria-stabilized zirconia
and the
anode is a gas diffusion anode feeded with a combustible gas such as either
hot
natural gas or smelter gas having the volumic composition of 85 vol.% CO and
15
vol.%H2.
Example 11 - The experimental conditions depicted in the following example
just
differs from that of the example 4 in that the molten salt electrolyte is
replaced by a
thick solid oxygen anion conductor such as beta-alumina and the anode is a gas
diffusion anode feeded with a combustible gas such as either hot natural gas
or
smelter gas having the volumic composition of 85 vol.% CO and 15 vol.%H2.
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24
Description of electrochemical quantities used in the examples:
Electrochemical equivalent (Eq):
E _ »xF
v° x M
Faradic (current) efficiency (~):
~I(%)=100x m =100x m xEq
>nth i x dt
Electrochemical conversion rate (dmldt):
c7nZ v M i i
at = ~F ~, _ ~1. Eq
Overall cell voltage (U~en):
U~~n = (E~ + ~ ~~1 a,k - ~7~,x ~ + i~ Rx
tlermodynamic cello oltage
overpotentials olunic drops
Specific energy consumption (em):
~U~eu > x ~ idt Z j~eu x i x 0t Uceu x Eq
gm = _ -
m m ~1
Space-Time Yield (Yt)
Y=~r x.7
Eq
Energy efficiency (~E):
~~ (%) =100 x UT'' x m ~ E~ - ~~ X ~I
Ucell l~t
With the following physical quantities in SI units (in practical units):
Eg electrochemical equivalent in C.kg-' (Ah/kg),
n dimensionless number of electrons involved,
F Faraday's constant 96485.309 C.mol-~ (26.8 Ah/mol),
vo stoichiometric coefficient,
M atomic or molar mass of electroactive species in kg.mol-~,
dmldt electrochemical conversion rate kg/s (kg/h),
U~eii average overall cell voltage, in V,
CA 02450978 2003-12-15
WO 03/046258 PCT/CA02/01802
Ea,~ Nernst anodic and cathodic electrode potentials in V,
r~a,~ anodic and cathodic overpotentials (e.g., activation, diffusion,
passivation), in V,
R resistances (e.g., electrodes, electrolyte, busbars, contacts) in S2,
5 i current intensity, A,
m mass of product, in kg,
n dimensionless number of~electrons involved,
dimensionless faradic or current efficiency,
sE dimensionless energy efficiency.
CA 02450978 2003-12-15
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26
N
O O N ~ N O
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CA 02450978 2003-12-15
WO 03/046258 PCT/CA02/01802
27
M
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CA 02450978 2003-12-15
WO 03/046258 PCT/CA02/01802
28
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CA 02450978 2003-12-15
WO 03/046258 PCT/CA02/01802
29
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CA 02450978 2003-12-15
WO 03/046258 PCT/CA02/01802
Thus, the preferred method of the present invention confers numerous benefits
heretofore unfound in the prior art. These benefits are most apparent when
inexpensive titanic slag is used as a feedstock. Indeed, the benefits are: (1)
the
excellent electronic conductivity of the molten titanic slag reduces the ohmic
drop
5 and hence the overall cell voltage resulting in a much lower specific energy
consumption; (2) taking advantage of the elevated sensible and latent heat of
the
molten titanic slag because it can be transferred directly from an electric
arc
furnace allows to achieve electrolysis at high temperatures; (3) the elevated
operating temperature preferably ranging between 1570°C and
1860°C depending
10 on the Fe0 content and other impurities of the titanic slag allows an
excellent
electrochemical reaction kinetics. (4) above liquidus temperature titanic slag
exhibits a low dynamic viscosity and a much lower density (e.g., 3510 kg.m-3
for 80
wt.% Ti02 at 1700°C) lower than that of pure and liquid titanium (e.g.,
4082 kg.m-3
at 1700°C). Hence firstly iron metal and other metals more noble than
titanium
15 (Mn, Cr, V, etc.) are first to be deoxidized electrochemically. This allows
separation
of these metals for the later produced deoxidized titanium. Owing to the
higher
density of the pure liquid iron (e.g., 6886 kg.m-3 at 1700°C) and other
metals
compared to that of the molten titanic slag (3510 kg.m-3 at 1700°C),
the liquid
metal droplets sink quickly by gravity settling to the bottom of the
electrolyser
20 forming a pool of metallic alloy while oxygen anions diffuse and migrate
through
the molten salt electrolyte to the consumable carbon anode where carbon
dioxide
gas is evolved. This first electrochemical reaction corresponds to the
carbothermic
reduction of metallic oxides with a reaction scheme given by:
25 MXOy(liq.) + (y/2)C(sol.) = xM(liq.).~ + (y/2)C02(gas)T
Once all the iron and other metallic impurities are removed by this selective
electrodeposition, the temperature is preferably increased to 1800°C to
compensate the enhanced content of Ti02 of the purer titanic slag. Meanwhile,
30 electrochemical deoxidation carries on with the electrodeposition of
droplets of
liquid titanium metal at the slag electrolyte interface while oxygen anions
diffuse
and migrate through the molten salt electrolyte to the anodes) where carbon
dioxide gas is evolved. Because the molten titanic slag has a low dynamic
viscosity and exhibits a much lower density (e.g., 3510 kg.m-3 for 80 wt.%
Ti02 at
CA 02450978 2003-12-15
WO 03/046258 PCT/CA02/01802
31
1700°C) than that of pure and liquid titanium (e.g., 4082 kg.m-3 at
1700°C), the
liquid titanium droplets fall by gravity settling at the bottom of the
electrolytic cell
forming after coalescence a pool of pure liquid titanium metal that accumulate
at
the bottom of the electrolyser. This pool of pure liquid titanium metal never
impedes the oxygen diffusion at the slag electrolyte interface and allows the
straightforward continuous tapping of the titanium metal under inert
atmosphere for
casting large titanium ingots without requiring labor intensive and energy
demanding steps to transform a.sponge into ingots. This is of great benefit
when
comparing the cost-efficiency of the present inventive method to known
processes
for making titanium sponge. The overall electrochemical reaction corresponds
to
the carbothermic reduction of titanium dioxide with a reaction scheme given
by:
Ti02(liq.) + C(sol.) = Ti(liq.).~ + C02(gas)T
In addition conducting the electrolysis into appropriate electrolytes having a
wide
decomposition potentials, elevated ionic conductivity, low vapor pressure, and
excellent capability to dissolve large amount of oxygen anion permit to
operate at
elevated current densities of several kA.m-2 impossible in the prior art.
CA 02450978 2003-12-15
WO 03/046258 PCT/CA02/01802
32
Notes:
CARDARELLI, F (2001 ) - Materials Handbook: A Concise Desktop Reference. -
Springer-Verlag, London, New York, pages 115-135.
2 GAMBOGI, J. - Titanium and Titanium Dioxide - from Mineral Commodity
Summaries.- U.S. Bureau of Mines (1995) p.180.
3 GAMBOGI, J. - Annual Report: Titanium-1992 - U.S. Bureau of Mines (1993) p.
1.
4 KROLL, W.J. - Trans. Electrochem. Soc. 112(1940)35-47.
KROLL, W.J. - Method for the manufacturing titanium and alloys thereof. - U.
S.
Pat. 2,205,854, June 25 1940.
6 NATIONAL MATERIALS ADVISORY BOARD Committee on Direct Reduction Processes
for the Production of Titanium Metal. 1974. ' Report # NMAB-304, National
Academy of Sciences, Washington, DC.
KOLTHOFF, M.; and THOMAS, J. - Polarography in acetonitrile of titanium
tetrachloride and tetraiodide in various supporting electrolytes - J.
Electrochem.
Soc. 111 (9) ( 1964) 1065-1074.
$ SINHA, N.H.; and SwARUF, D. - Indian Mining J. Spec. 1(1975)134.
9 KUDRYATSEV, V.N.; LYAKHOV, B.F.; ANUFRIEV, W.G.; and PEDAN, K.S. - Hydrog.
Met. Proc. Int. Cong. 2nd. Pergamon Press, Oxford 1977, page 5.
'° SOFRONKOV, A.N.; PRVII, E.N.; PRESNOV, V.N.; and SEMIZOROV, N.F. -
Zh. Prikl.
I~Chim. 51 (1978)607
11 BRIBIESCA, S.L.; CONTRERAS, E.S.; and TAVERA, F.J. -Electrowinning of
titanium
from sulfuric acid titanium solutions. - Proc. Titanium'92, The Minerals,
Metals,
and Materials Society 1993, pages 2443-2444.
'2 BIALLOZOR, S.; and LISOWSKA, A. - Electrochim. Acta 25(1980)1209.
13 LlsowsKA, A.; and BIALLOZOR, S. - Electrochim. Acta 27(1982)105.
14 ABBOTT, A.P.; BETTLEY, A.; and SCHIFFRIN, D.J. - J. Electroanal. Chem.
347( 1993) 153-164.
~5 CARDARELLI, F., TAXIL, P., and SAVALL, A. - Tantalum Protective Thin
Coating
Techniques for the Cherriical Process Industry: Molten Salts Electrocoating as
New
Alternative - Int. J. Refractory Metals & Hard Materials 14(1996)365-381.
16 DELIMARSKII, Iu.K.; and MARKOV, B.F. (1961) - Electrochemistry of Fused
Salts. -
Sigma Press Publishing, New York.
~~ LANTELME, F.; INMAN, D.; and LOVERING, D.G. (1984) - Electrochemistry-I, In
Molten Salt Techniques, LOVERING, D.G.; GALE, R.J. (Eds.) Vol. 2., Plenum
Press,
New York, pp. 138-220.
'$ LEONE, O.Q., KNUDSEN, H., and CoucH, D.E. - High-purity titanium electrowon
from titanium tetrachloride. - J. O. M. 19(3)(1967)18-23
'9 CoucH, D.E., LEONE, O.Q., LANG, R.S., and BLUE, D.D. - Evaluation of
diaphragm materials for electrowinning high-purity titanium. Proc. Extractive
Met.
Div. Symp., Met. Soc. RIME Chicago IL December 11-13 1967, pp. 309-323.
CA 02450978 2003-12-15
WO 03/046258 PCT/CA02/01802
33
20 LEONE, O.Q, and CoucH, D.E. (1972) - Use of composite diaphragms in the
electrowinning of titanium. - Report Investigation #7648, U.S. Dept. of
Interior,
Bureau of Mines, Washington D.C.
2' PRiscu, J.C. - Symp. on Electrometallurgy, Proc. AIMS Extractive Metallurgy
Div, Cleveland Ohio, December 1968, page 83.
22 PRiscu, J.C. - Electrolytic cell for the production of titanium. - U. S.
Pat.
3,282,822, Nov. 1 1966.
23 HASHIMOTO, Y.; URIYA, K.; and KONO, R. - Electrowinning of titanium from
its
oxides. Part I. Fused salt electrolysis at temperatures above the melting
point of
the metal. - Denki Kagaku 39(6) (1971)516-522.
24 HASHIMOTO, Y. - Electrowinning of titanium from its oxides. Part II.
Influences of
fluoride salt baths on fused-salt electrodeposition of titanium metal from
titanium
dioxide. - Denki Kagaku 39(12) (1971 )938-943.
25 HASHIMOTO, Y. - Electrowinning of titanium from its oxides. Part III.
Electrowinning of titanium from titanium dioxide or calcium titanate in
calcium
fluoride-magnesium fluoride molten salt baths. - Denki Kagaku 40(1) (1972) 39-
44.
26 ANONYMOUS - Costwise they call it less of a sponge. - Amer. Metal Market
May
3, page 1-3 1973.
2' CoBE~, G.; FISHER, J.; and SNYDER, L.E - Electrowinning of titanium from
titanium.
tetrachloride: pilot plant experience and production plant projections 1969-
1976. -
Conference: Titanium '80, Science and Technology, Vol. 3, Kyoto, Japan, 19-22
May 1980 TMS/AIME, Warrendale, Pa.
~$ Juc~eNiESS, P.R., and JOHNSON, D.R. - Method for electrowinning titanium. -
U. S.
Pat. 4,118,291; October 3 (1978).
29 Ma~° COBEL G., FISHER J., and SNYDER L. (1980) - Electrowinning of
titanium from
titanium tetrachloride. - 4th International Conference on Titanium, May 1980,
Kyoto, Japan.
so ARMAND, M. - Process for.the preparation of titanium by electrolysis. -
IJ.S. Pat.
4,381,976, May 3 (1983).
31 ARMAND, M. - Novel Apparatus and process for the TiCl4 feed to electrolysis
cells for the preparation of titanium. - U. S. Pat. 4,396,472, August 2
(1983).
32 GINATTA, M.V.; ORSELLO, G.; and BERRUTI, R. - A method for the electrolytic
production of a polyvalent metal and equipment for carrying out the method: -
PCT
Int. Appl., 33 pp. WO 8910437 (1990).
33 GiNa-rTa, M.V.; and ORSELL.O, G. - Plant for the electrolytic production of
reactive
metals in molten salt baths. - Eur. Pat. Appl. EP 210961 Apr. 02, 1987.
34 DIMARIA, E. - RMI gets license to make new type of titanium. - Metalworking
News, February 15t., 1988.
35 ~wA~MI, M.; OoisHi, M.; TAKENAKA, T.; and SuzuK~, T. - The possibility of
electrowinning of liquid titanium using ESR apparatus. - Proc. Int. Conf.
Molten
Slags, Fluxes Salts '97, 5th (1997) 477-482 Iron and Steel Society,
Warrendale,
PA.
CA 02450978 2003-12-15
WO 03/046258 PCT/CA02/01802
34
3s TAKENAKA, T.; SUZUKI, T.; ISHIKAWA, M.; FUKASAWA, E.; and KAWAKAMI, M. -
The
new concept for electrowinning process of liquid titanium metal in molten
salt. -
Electrochemistry 67(6) (1999) 661-668.
3' TAKENAKA, T.; ISHIKAWA, M.; and KAWAKAMI, M. - Direct electrowinning of
liquid
titanium metal by using direct current electro slag remelting apparatus. -
Proc.
Electrochem. Soc. (Molten Salts XII) (2000) 99-41 578-584.
38 GINATTA, M.V. - Process for the Electrolytic Production of Metal. - U.S.
Pat.
6,074,045, June 13 2000.
39 CHEN, G.Z; FRAY, D.J.; and FARTHING, T.W. - Direct electrochemical
reduction of
titanium dioxide to titanium in molten calcium chloride. - Nature 407(2000)361-
364.
ao FRAY, D.J.; FARTHING, T. W.; and ChiEN, G.Z. - Removal of oxygen from metal
oxides and solids solutions by electrolysis in a fused salt. - Int. Pat. Appl.
WO
99/64638 Dec. 16, 1999.
41 WARD-CLOSE, C. M.; and GoDFREY, A.B. - Electrolytic reduction of metal
oxides
such as titanium dioxide and process applications. - Int. Pat. Appl. WO
01162996
Feb. 20, 2001.
42 WARD-CLOSE, C. M.; and GoDFREY, A.B. - Method of manufacture for ferro-
titanium and other metal alloys by electrolytic production. - Int. Pat. Appl.
WO
01 /62994 Feb. 19, 2001
43 Financial Times December 21 st, 2000, page 12.
44 NAKAMURA, M.; OKABE, T.H.; OISHI, T.; and ONO. K. - Electrochemical
deoxidation
of titanium. - Proc. Electrochem. Soc. Molten Salt Chemistry and Technology
(1993)93-99.
45 OKABE, T.H.; NAKAMURA, M.; OISFiI, T.; and ONO. K. - Metall. Trans. 8
24B(1993)449-455.
46 OKABE, T.H.; DEURA, T.N.; OISHI, T., ONO. K.; and SADOWAY, D.R. - J. Alloys
Compounds 282(1996)155-164.
4' SHARMA, R.A. - Molten salt process for producing titanium or zirconium
powder. -
U. S. Pat. 6,117,208 Sept. 12, 2000.
4$ FORTIN, C. - Process for obtaining titanium or other metals using shuttle
alloys. -
U.S. Pat. 6,245,211, June 12, 2001.
49 PAL, U.B.; WOOLLEY, D.E.; and KENNEY, G.B. - Emerging SOM technology for
the
green synthesis of metals from oxides. - J.O.M. October (2001) 32-35.
so PAL, U.; BRITTEN, S.C. - Method and apparatus for metal extraction and
sensor
device related thereto. - U. S. Pat. 5,976,345, November 2, 1999.
5~ PAL, U.; BRITTEN, S.C. - Apparatus for metal extraction. - U. S. Pat.
6,299,742,
October 9, 2001.
52 ANDERSSON, S.; COLLEN, B.; KUYLENSTIERNA, U.; and MAGNELI, A. Acta Chem.
Scand. 11 ( 1957) 1641.
CA 02450978 2003-12-15
WO 03/046258 PCT/CA02/01802
53 ~YFIELD, P.C.S. - Electrode material, electrode and electrochemical cell. -
U. S.
Pat. 4,422,917 December 27, 1983.
54 CARDARELLI, F (2001 ) - Materials Handbook: A Concise Desktop Reference. -
Springer-Verlag London Ltd., London, pages 329-330.
55 CL~~ R.; and PARDOE, R. - Applications of Ebonex~ conductive ceramic
electrodes in effluent treatment. - in Genders, D; and Weinberg; N.L. -
Electrochemistry for a Cleaner Environment. - Electrosynthesis Company Inc.,
East Amherst, N.Y. (1992) Chap. 18, pages 349-363).
56 ASTM B299-99 - Standard Specification for Titanium Sponge. - American
Society for Testing and Materials (ASTM)
5' ASTM B299-99 - Standard Specification for Titanium Sponge. - American
Society for Testing and Materials (ASTM)
