Note: Descriptions are shown in the official language in which they were submitted.
CA 02602801 2007-09-28
DESCRIPTION
PROCESS FOR PRODUCING Ti OR Ti ALLOY, AND PULL-UP ELECTROLYSIS
METHOD APPLICABLE TO SAID PROCESS
TECHNICAL FIELD
[0001]
The present invention relates to a method for efficiently
producing Ti or a Ti alloy at low costs and a pulling electrolysis method
in which Ca applicable to reduce Ti and other hard-to-reduce metals can
be obtained as a solid-state Ca and an electrolytic-bath salt.
BACKGROUND ART
[0002]
Metals such as Ti, Zr, Ta, Hf, and V each is a useful metal having
excellent properties. These metals are hard to be reduced and it is
necessary to separate coexisting homologous elements and impurities.
Therefore, usually oxides or halides thereof are formed after the metals
are refined through many processing steps, and the metals are produced
by reducing the oxides or halides with a metal such as Mg and Na having
strong reduction power.
[0003]
Among others, a metallic Ti and a Ti alloy are excellent in
corrosion resistance and design, and further light in weight and
excellent in mechanical properties. Therefore, the metallic Ti and Ti
alloy are widely used as aircraft materials, heat exchanger materials,
chemical plant materials, roof materials, golf-club heads, and the like.
Particularly, in recent years, the metallic Ti and Ti alloy are used in
instruments of medical fields because of a nontoxic metal for a human
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body, and the application of the metallic Ti and Ti alloy is being
increased.
[0004]
However, because the metallic Ti is difficult to smelt, the metallic
Ti is extremely expensive as a metal, development of a method of
producing the metallic Ti with high productivity at low costs is
demanded in an industrial scale. That is, in the conventional metallic
Ti smelting method, titanium tetrachloride (TiCl4) is reduced to obtain
the metallic Ti by the metal such as Mg and Na having the strong
reduction power (reducing metal). However, in the conventional
method, because the production is performed in a batch manner, there is
a problem in that the improvement in productivity is limited.
[0005]
For example, in a Kroll process which is of a typical method of
industrially producing the metallic Ti, molten Mg is loaded in a reactor
vessel, a TiCla liquid is fed from above a liquid surface thereof, and the
metallic Ti is produced by reducing TiCla with Mg near the liquid surface
of the molten Mg. However, in the Kroll method, a feed rate of TiC14 is
restricted because the reaction is generated near the liquid surface of
the molten Mg in the reactor vessel.
[0006]
Additionally, Ti granules produced are aggregated because of an
adhesion property between Ti and the molten Mg, and the Ti granules
are sintered to grow in sizes by the heat of the molten liquid.
Consequently, it is difficult to take out the produced Ti to the outside of
the reactor vessel, it is also difficult to continuously produce Ti, the
improvement in productivity is limited to thereby increase production
costs, and a product price becomes extremely high.
[0007]
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Therefore, various researches and developments have been
conducted on the Ti production method except for the Kroll process. For
example, U.S. Patent No. 2205854 discloses that Ca can be used as a
reducing agent for TiCla in addition to Mg. Ca has a stronger affinity
for Cl than that of Mg, and Ca is suitable in principle for the reducing
agent for TiCla. However, the metallic Ti production method in which
TiC14 is reduced with Ca is not put to practical use yet. This is because
that CaC12 is hardly electrolyzed.
[0008]
Another Ti production method includes an Olson process
disclosed in U.S. Patent No. 2845386. The Olson process is a kind of
oxide direct-reduction process in which TiO2 is directly reduced by Ca
without going the route of TiC14. Although the oxide direct-reduction
process has high efficiency, the oxide direct-reduction process is not
suitable for the high-purity Ti production at low costs. This is because
it is necessary to use expensive high-purity Ti02.
[0009]
That is, in the Ti production methods disclosed in U.S. Patent
Nos. 2205854 and 2845386, unfortunately it is not easy to refine Ca, and
it is difficult to handle Ca because it is easily oxidized. Additionally, in
the oxide direct-reduction process, there is the problem in that it is
necessary to use the expensive high-purity Ti02. Therefore, the Ti
production methods disclosed in U.S. Patent Nos. 2205854 and 2845386
are not put to practical use yet.
[0010]
However, Ca has the stronger affinity for Cl than that of Mg, and
Ca is suitable in principle for the reducing agent for TiCla. When Ca is
obtained at low costs, Ca can be used as the reducing agent for Ti and
hard-to-reduce metals such as Zr and Hf, and further Ta and V, any of
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which is caused to form chloride and then reduced by Mg, thus proving to
be industrially-useful..
[0011]
Currently, the metallic Ca is mainly produced by a vaporization
reducing method using a carbonated salt as a raw material. However, a
Germany technical document ("HANDBUCH DER TECHNISCHEN
ELEKTRO CHEMIE" DRITTER BAND (1934) p.128 to p.164 "Calcium,
Strontium, Barium." Von Dr. V. Makow) reports that, in early stage in
which the metallic Ca was industrially produced, the molten CaCl2 was
electrolyzed and Ca was produced by separating the attached salt by
re-melting.
[0012]
However, a voltage becomes extremely high during the
electrolysis in the Ca production by the electrolysis of the molten CaCl2,
described in the Germany technical reference. Therefore, it can be
predicted that the required electric power (currentxvoltage) becomes
large, huge electric energies are consumed, and the production costs are
increased.
DISCLOSURE OF THE INVENTION
[0013]
In view of the foregoing, a first object of the invention is to
provide a method of producing Ti or the Ti alloy through reduction by Ca,
particularly a method of efficiently producing Ti or the Ti alloy at low
costs. A second object of the invention is to provide a pulling
electrolysis method in which Ca applicable for reducing Ti can be
obtained at low electrolytic bath voltage and high current efficiency in
order to be applied to the method of producing the hard-to-reduce metal,
particularly Ti or the Ti alloy.
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[0014]
The inventors focus on and study the method of reducing TiC14 by
Ca in order to solve the above problems. As a result of the study, the
inventors establish a method (hereinafter the method is referred to as
"the method of producing Ti or the Ti alloy through reduction by Ca" with
inclusion of various examples), wherein the CaC12-containing molten salt
in which Ca is dissolved is retained in the reactor vessel, the metallic
chloride containing TiC14 is caused to react with Ca in the molten salt to
generate Ti granules or Ti alloy granules in the molten salt, and the Ti
granules or Ti alloy granules generated in the molten salt are separated
and recovered from the molten salt, whereby continuously producing Ti
or the Ti alloy. Additionally, the inventors propose a pulling
electrolysis method enabling to obtain a solid-state Ca applicable to the
method. The development background and the obtained findings will be
described for each method while the one is "the method of producing Ti or
the Ti alloy through reduction by Ca" and the other is "the pulling
electrolysis method".
[0015]
(Method of Producing Ti or Ti Alloy through Reduction by Ca)
In the method of producing Ti or the Ti alloy through reduction by
Ca, CaC12 which is of a by-product in association with the Ti generation
is taken out to the outside of the reactor vessel and electrolyzed, and the
generated Ca can be used in the reaction for generating the metal such
as Ti in the reactor vessel. In this case, one of large advantages of the
method is in that rigorous separation of Ca and CaC12 is not required.
[0016]
According to the method of producing Ti or the Ti alloy through
reduction by Ca, the Ti generation reaction proceeds in the molten CaC12.
Accordingly, compared with the Kroll process in which the reaction field
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is limited to the proximity of the liquid surface because TiC14 is fed to a
liquid surface of the reducing agent (Mg) in the reactor vessel, the
reaction field is enlarged and the heat generation region is also spread to
easily perform the cooling, so that the method holds promise of largely
enhancing a feed rate of TiCl4 which is of the raw material for Ti to
largely improve productivity.
[00171
However, because the reaction of Ca and TiCla in the molten
CaCl2 is an exothermic reaction, in order to maintain the high
productivity, it is necessary to cool the heat generation region to
dissipate the heat. Therefore, the production efficiency is lowered and
the thermal efficiency is also lowered because of the large heat loss.
[00181
In the case where the heat rapidly generated due to the excessive
TiC14 feed rate exceeds the cooling capacity, the reaction field (region
where the reaction of Ca and TiC14 is generated) gets to an excessively
high temperature and the reactor vessel is severely worn. On the
contrary, when the temperature is excessively lowered, the reaction rate
is decreased. Therefore, in the TiC14 reduction process, it is necessary
to finely control the temperature to maintain the high productivity.
[00191
In order to solve the problem, the inventors further study
measures to overcome the difficulty in the reaction-field temperature
control while suppressing the decrease in production efficiency and
energy loss caused by repetition of the heat generation and heat
dissipation as much as possible. As a result, the inventors have an idea
in which the molten CaCl2 solution is electrolyzed while taken out from
the reactor vessel and the generated Ca is not returned into the reactor
vessel along with the molten CaC12 solution (i.e., as the CaCl2 solution
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whose Ca concentration is increased), but Ca and the molten CaC12
solution are recovered in the form of the solid substance and the solid
substance is returned to the reduction process.
[0020]
The realization of the method can absorb the heat generated by
the reaction of Ca and TiCl4 by utilizing latent heat of fusion possessed
by the solid substance containing Ca and CaC12. Therefore, the thermal.
efficiency is largely improved, the reaction temperature is easily
controlled, and the method of producing Ti or the Ti alloy through
reduction by Ca can be performed further efficiently.
[0021]
The Germany technical reference reports that a rod-shape Ca is
obtained by the electrolysis of the molten CaC12.
[0022]
Fig. 1 is a view showing a schematic configuration of a main part
of a calcium furnace for producing Ca by the electrolysis of the molten
CaC12, disclosed in the Germany technical document. In the calcium
furnace (electrolytic apparatus) having the schematic configuration
shown in Fig. 1, a calcium chloride (CaC12) 16 which is of an electrolyte is
loaded in a graphite crucible 15 (cooled steel vessel coated with a
graphite plate), and is melted and heated. A part of the electrolyte is
solidified to form a solidified electrolyte 20 by cooling in a bottom portion
and an inner wall portion of the crucible 15.
[0023]
Then, electricity is turned on between an anode (positive
electrode) 17 and a cathode (negative electrode) 18 to perform the
electrolysis. At this point, the cathode 18 is pulled such that variations
in current and voltage are decreased according to a degree of depositing
Ca on the cathode 18, and Ca 19 is grown in a rod shape. Because the
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solidified salt adheres to the surface of the calcium rod, the solidified
salt is re-melted and separated in the calcium chloride. The Germany
technical reference reports that, during the electrolysis, a cathode
current density is set to 125 A/cm2, the voltage ranges from 35 to 40V,
and purity of the re-melted metallic Ca ranges from 98 to 99%.
[0024]
Although Ca is generated by the electrolysis in the Germany
technical reference, the inventors perform experiments and studies on
the recovery of the solid substance containing Ca and the
electrolytic-bath salt (CaC12 is used) in the electrolysis process. As a
result, the inventors obtain the following findings: Ca generated by the
electrolysis is deposited on the cathode surface by gradually pulling the
cathode during the electrolysis, and a phenomenon in which CaClz
adheres as solidified substance near the deposited Ca is repeatedly
generated to obtain the solid substance in which the Ca and the
electrolytic-bath salt are mixed.
[00251
(Pulling Electrolysis Method)
In order to develop a pulling electrolysis method of being able to
obtain the hard-to-reduce metal, particularly Ca usable in the Ti
reduction, the inventors conducts comprehensive study for finding an
electrolysis condition that, using the electrolytic-bath salt containing
CaC12, the tank voltage (hereinafter simply referred to as "voltage") is
decreased while the high current efficiency (i.e., high Ca recovery
efficiency) is obtained during the electrolysis. As a result, the
inventors obtain the following findings for the bath temperature, the
cathode current density (hereinafter referred to as "cathode current
density" or simply "current density"), and the cathode pulling rate.
[00261
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1 T
(a) Because the voltage is increased when the cathode current
density is increased, in order to decrease the voltage, the cathode and
the anode are brought close to each other to shorten a distance between
the electrodes (inter-electrode distance), whereby decreasing the current
efficiency. On the contrary, when the inter-electrode distance is
lengthened, the current efficiency is improved while the voltage is
increased. That is, it is difficult to balance the decrease in voltage with
the improvement of the current efficiency.
[0027]
(b) When the electrodes are brought close to each other to
decrease the voltage while the current density is set within a
predetermined range (0.1 to 200 A/cm2), the inter-electrode distance
becomes not more than 7 cm and the voltage becomes not more than lOV.
[0028]
(c) When the cathode pulling rate is enhanced while the condition
(b) is satisfied, the current efficiency is increased. This effect is
observed when the pulling rate is not less than 0.05 cm/min. The
solid-state "Ca and electrolytic-bath salt" containing Ca generated
(deposited) by the electrolysis and the electrolytic-bath salt which
adheres to the surface of Ca and is solidified is recovered by pulling the
cathode.
[0029]
(d) When the bath temperature is raised under the condition (c), a
Ca concentration in the recovered Ca and electrolytic-bath salt is
increased. In the case of the high-temperature electrolytic-bath salt,
although the current efficiency is slightly decreased, the current
efficiency is increased by enhancing the cathode pulling rate. The
super current efficiency is obtained in the case where the pulling rate
satisfies the following equation (1):
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V_> 0.0035xt-2.4 (1)
where V: cathode pulling rate (cm/min), and
t: bath temperature ( C).
[0030]
The following unique advantage is generated by being able to
recover Ca in the form of the solid-state "Ca and electrolytic-bath salt".
[0031]
As described above, instead of the Kroll process in which the
continuous production is hardly performed, the inventors establish the
method, wherein the CaC12 containing molten salt in which Ca is
dissolved is retained in the reactor vessel, the metallic chloride
containing TiC14 is caused to react with Ca in the molten salt to generate
the Ti granules or the Ti alloy granules in the molten salt, and the Ti
granules or the Ti alloy granules are separated and recovered from the
molten salt to continuously produce Ti or the Ti alloy.
[0032]
As described above, in the production method, CaCl2 which is of
the by-product in association with the Ti generation is taken out to the
outside of the reactor vessel and electrolyzed, and the generated Ca can
be used in the reaction for generating the metal such as Ti in the reactor
vessel. In this case, one of the large advantages of the method is in that
rigorous separation of Ca and CaC12 is not required.
[0033]
According to the method of producing Ti or the Ti alloy through
reduction by Ca, the Ti generating reaction proceeds in the molten CaC12.
Accordingly, compared with the Kroll process (reaction field is limited to
the neighbor of the Mg liquid surface), the region where the reducing
reaction is generated (i.e., reaction field) is remarkably enlarged and the
heat generating region is also spread to easily perform the cooling, so
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that the TiC14 feed rate can largely be enhanced to remarkably improve
the productivity.
[00341
However, because the reaction of Ca and TiC14 in the molten
CaC12 is the exothermic reaction, in order to maintain the high
productivity, it is necessary to cool the heat generating region to
dissipate the heat. Therefore, the thermal efficiency is also lowered
because of the large heat loss.
[00351
In the case where the heat, rapidly generated due to the excessive
TiC14 feed rate, exceeds the cooling capacity, the reaction field becomes
an excessively high temperature, and the reactor vessel is severely worn.
On the contrary, when the temperature is excessively lowered, the
reaction rate is decreased. Therefore, in the TiCla reduction process, it
is necessary to finely control the temperature to maintain the high
productivity.
[00361
In order to solve the problem, the inventors further study the
measures to overcome the difficulty of the reaction-field temperature
control while suppressing the decrease in energy loss caused by
repetition of the heat generation and heat dissipation. As a result, the
inventors hit on an idea, in which the molten CaClz solution is
electrolyzed while taken out from the reactor vessel, the generated Ca
and the molten CaC12 solution are recovered in the form of the solid
substance, and the solid substance is returned to the reduction process.
[00371
The realization of the method can absorb the heat generated by
the reaction of Ca and TiC14 by utilizing latent heat of fusion possessed
by the solid substance containing Ca and CaC12. Therefore, the
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production efficiency and thermal efficiency are largely improved, the
reaction temperature is easily controlled, and the method of producing Ti
or the Ti alloy through reduction by Ca can be performed further
efficiently.
[0038)
Additionally, in producing Ti or the Ti alloy through reduction by
Ca, when the solid-state Ca and electrolytic-bath salt are fed as the Ca
source to the molten salt containing CaC12 in the reactor vessel, unlike
the case in which the solid-state metallic Ca is fed as the Ca source, the
solid-state Ca and electrolytic=bath salt are dissolved rapidly and evenly,
and the reaction of Ca and the metallic chloride containing TiC14 can be
caused to proceed evenly in a wide range inside the reactor vessel.
[00391
Thus, in the method of producing Ti or the Ti alloy through
reduction by Ca, the large effect is obtained by utilizing the solid-state
Ca and electrolytic-bath salt as the Ca source.
[00401
The invention is made based on study result on the method of
producing Ti or the Ti alloy through reduction by Ca, the involvement
with the technique of producing Ti or the Ti alloy through reduction by
Ca, and the findings (a) to (d). The summary of the invention includes
the following Ti or Ti alloy production methods (1) to (3) and pulling
electrolysis methods (4) and (5).
[0041]
(1) A method of producing Ti or a Ti alloy going through a
reduction process in which a metallic chloride containing TiC14 is caused
to react with Ca in a Ca-containing electrolytic-bath salt to generate Ti
or the Ti alloy in the electrolytic-bath salt, the method is characterized
in that the solid-state Ca and electrolytic-bath salt are fed to the
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reduction process.
[0042]
(2) A Ti or Ti alloy production method including a reduction
process in which a metallic chloride containing TiC14 is caused to react
with Ca in a Ca-containing electrolytic-bath salt to generate Ti or the Ti
alloy in the electrolytic-bath salt and an electrolysis process of
generating Ca by electrolyzing the electrolytic-bath salt taken out from
the reduction process, the method is characterized in that, in the
electrolysis process, a solid substance containing Ca and the
electrolytic-bath salt is recovered and the solid substance is delivered to
the reduction process.
[0043]
(3) In the Ti or Ti alloy production method of (1) and (2), the
recovery of the solid substance containing Ca and the electrolytic-bath
salt may be performed by pulling a cathode while the solid substance is
caused to adhere to and solidified in a cathode surface. Additionally,
when a Ca-containing electrolytic-bath salt containing CaC12 is used as
the Ca-containing electrolytic-bath salt, desirably the method of
producing Ti or the Ti alloy through reduction by Ca proposed by the
inventors can further efficiently be performed.
[0044]
(4) A pulling electrolysis method of recovering a solid-state Ca
using a Ca-containing electrolytic-bath salt, the method is characterized
in that a electrolytic-bath salt is electrolyzed at a bath temperature of
680 to 900 C, a cathode current density of 0.1 to 200 A/cm2, and a
voltage of lOV or less, and Ca and the electrolytic-bath salt is recovered
in a solid state by pulling a cathode at a pulling rate of 0.05 cm/min or
more while a solid substance is caused to adhere to and solidified in a
cathode surface. The pulling electrolysis method can be applied as the
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electrolysis method of recovering Ca from the Ca-containing
electrolytic-bath salt in the electrolysis process described in the Ti or Ti
alloy production methods (1) and (2).
[0045]
(5) In the pulling electrolysis method (4), desirably the pulling
rate satisfies the following equation (1):
V? 0.0035xt-2.4 (1)
where V: cathode pulling rate (cm/min), and
t: bath temperature ( C).
[0046]
Additionally, when a Ca-containing electrolytic-bath salt
containing CaC12 is used as the Ca-containing electrolytic-bath salt,
desirably Ti or the Ti alloy production can further efficiently be
performed in the case of adopting the Ti or Ti alloy production methods
(1) and (2).
[0047]
As used herein, "solid-state" defined in the invention shall mean
that the solid substance in the cathode surface is apparently in the solid
state (including the state in which the surface is wetted during
solidification) at a time when the cathode is pulled. Specifically,
"solid-state" shall mean both the case in which the whole of the solid
substance is in the solid state (i.e., the solidification is completed) and
the case in which, although the solid substance is apparently in the solid
state, actually the solid substance is partially solidified and the
electrolytic-bath salt or the like in the molten state exists in the solid
substance.
[0048]
The Ti or Ti alloy production methods (1) and (2) each is the
method in which, in producing Ti or the Ti alloy through reduction of
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TiCl4 by Ca in the electrolytic-bath salt, the electrolytic-bath salt taken
out from the reduction process is electrolyzed to recover Ca and the
electrolytic-bath salt as the solid substance, and the recovered Ca and
electrolytic-bath salt are delivered to the reduction process. In the Ti
or Ti alloy production methods (1) and (2), heat generation is suppressed
in the reduction process to largely improve production efficiency and
thermal efficiency, a reaction temperature is easily controlled, and Ti
or the Ti alloy can efficiently be produced at low cost.
[00491
The pulling electrolysis methods (4) and (5) are the method of
recovering Ca by regulating the bath temperature, the cathode current
density, the voltage, and the pulling rate of the cathode in
predetermined ranges. According to these methods, Ca can be obtained
as the solid-state Ca and electrolytic-bath salt at low voltage and high
current efficiency, i.e., with relatively small electric power consumption.
BRIEF DESCRIPTION OF THE DRAWINGS
[00501
Fig. 1 is a view showing a schematic configuration of a main part
of a calcium furnace for producing Ca by electrolysis of a molten CaC12,
disclosed in a Germany technical document;
Fig. 2 is a view for explaining a pulling electrolysis method
according to the invention;
Fig. 3 is a view illustrating relationships between a cathode
pulling rate and current efficiency when the electrolysis method
according to the invention is implemented;
Fig. 4 is a view showing relationships between a bath
temperature and a cathode pulling rate in the pulling electrolysis; and
Fig. 5 is a view showing a configuration example of an apparatus
CA 02602801 2007-09-28
for producing a metallic Ti through reduction by Ca.
BEST MODE FOR CARRYING OUT THE INVENTION
[0051]
A Ti or Ti alloy production method according to the invention, a
pulling electrolysis method which can be applied thereto, and an
optimum production process of the invention in which these methods are
combined will be individually described below.
[0052]
1. Ti or Ti Alloy Production Method
The Ti or Ti alloy production method of the invention is a method
of producing Ti or a Ti alloy going through a reduction process in which a
metallic chloride containing TiCl4 is caused to react with Ca in a
Ca-containing electrolytic-bath salt to generate Ti or the Ti alloy in the
electrolytic-bath salt, the method being characterized in that the
solid-state Ca and electrolytic-bath salt are fed to the reduction process.
[0053]
A specific configuration as an example is "a Ti or Ti alloy
production method including a reduction process in which a metallic
chloride containing TiCl4 is caused to react with Ca in a Ca-containing
electrolytic-bath salt to generate Ti or the Ti alloy in the
electrolytic-bath salt and an electrolysis process of generating Ca by
electrolyzing the electrolytic-bath salt taken out from the reduction
process, the method being characterized in that, in the electrolysis
process, a solid substance containing Ca and the electrolytic-bath salt is
recovered and the solid substance is delivered to the reduction process.
[0054]
That is, the Ti or Ti alloy production method of the invention is
characterized in that the solid substance containing Ca and the
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electrolytic-bath salt is recovered in the electrolysis process and the
recovered solid substance is delivered to the reduction process in the
method of producing Ti or the Ti alloy by reducing TiCl4 by Ca in the
electrolytic-bath salt.
[0055]
A molten salt is used as the electrolytic-bath salt. Usually the
molten salt containing CaC12 is preferably used. However, the molten
salt is not limited to the molten salt containing CaC12. Any molten salt
can be used as the electrolytic-bath salt, because the reducing reaction
by Ca proceeds unless the molten salt having conductivity has extremely
small solubility for Ca. There is no limitation in a solid substance
recovering method. Ca generated in the electrolysis process is taken
out from the electrolytic bath as the solid substance containing the
electrolytic-bath salt, and Ca is delivered to the reduction process
(namely, Ca is loaded in a reactor vessel in which the reduction process
proceeds).
[0056]
At this point, the whole amount of Ca generated in the
electrolysis process may be delivered in the form of the solid substance to
the reduction process, or Ca may partially be delivered in the form of the
solid substance to the reduction process while the residue may be
returned in the form of the CaC12 solution whose Ca concentration is
increased. Even in this case, the above effects (such as the
improvement of the production efficiency and thermal efficiency and the
improvement of the reaction temperature control) are obtained
depending on an extent of the delivered solid substance.
[0057]
Adopting the "solid substance recovery-delivery" method can
decrease heat loss to largely improve the thermal efficiency while absorb
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the heat generation to enhance the production efficiency by utilizing
latent heat of fusion possessed by the solid substance containing Ca and
CaC12. Because cooling capability in the reaction system is increased as
a whole, the reaction temperature is easily controlled, and the
raw-material loading rate can be enhanced to further efficiently produce
Ti or the Ti alloy through the reduction by Ca at low costs. The compact
apparatus can also be realized.
[00581
In the method of producing Ti or the Ti alloy through reduction by
Ca, the reaction rates in the reduction process and electrolysis process
are possibly changed during operation (or the reaction rates are
inevitably changed). In such cases, an added effect that the solid
substance containing Ca and CaC12 is retained (temporarily stored) as
the Ca source and used according to need can be sufficiently expected.
[0059)
That is, the solid substance containing Ca and CaC12 can act as a
buffer in adjusting the reaction rates in the reduction process and
electrolysis process. When the CaC12 solution whose Ca concentration
is increased is kept in a high-temperature state, or when CaC12 solution
is regained to be in a dissolution state when in use after cooled once, the
energy loss becomes extremely large. In addition, because the solid
substance containing the high-concentration Ca can be recovered, an
amount of Ca transported to the reduction process can be decreased
compared with the case in which Ca is returned to the reduction process
along with CaC12 solution.
[0060]
In the Ti or Ti alloy production method of the invention, the
recovered Ca and the Ca concentration in the solid substance containing
the electrolytic-bath salt can be adjusted by controlling the temperature
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of the Ca-containing electrolytic-bath salt. For example, the Ca
concentration in the solid substance can be adjusted to 20% by weight by
setting the electrolytic temperature to 720 C, and the Ca concentration
in the solid substance can be adjusted to 30% by weight by setting the
electrolytic temperature to 800 C.
[0061]
That is, the Ca concentration in the solid substance containing
Ca and the electrolytic-bath salt can be controlled by managing the
electrolytic temperature. For example, in the case where a temperature
in a reaction field where the reducing reaction is generated is lowered,
the electrolytic temperature is lowered to decrease the Ca concentration
in the solid substance. On the other hand, in the case where a reaction
rate is enhanced in the reduction process, the electrolytic temperature is
raised to increase the Ca concentration in the solid substance. Thus,
the solid substance containing Ca and the electrolytic-bath salt can be
suitably selectively used as the operation situation demands.
[00621
2. Pulling Electrolysis Method
As described above, the pulling electrolysis method of the
invention is a method characterized in that an electrolytic-bath salt is
electrolyzed at a bath temperature of 680 to 900 C, a cathode current
density of 0.1 to 200 A/cm2, and a voltage of lOV or less, and the
electrolytic-bath salt and Ca is recovered in a solid state by pulling a
cathode at a pulling rate of 0.05 cm/min or more while a solid substance
is caused to adhere to and solidified in a cathode surface.
[0063]
Fig. 2 is a view for explaining the pulling electrolysis method of
the invention. As shown in Fig. 2, an electrolytic-bath salt 2 is retained
in an electrolytic tank 1, and a positive electrode 3 and a cathode 4 are
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CA 02602801 2007-09-28
provided. When electricity is passed between the electrodes to start the
electrolysis, chlorine (Cl2) is generated at the anode 3 and Ca is
deposited at the cathode 4.
[00641
At this point, when the cathode 4 is gradually pulled upward as
shown by an arrow in Fig. 2, because the temperature is rapidly lowered
in an exposed portion above the liquid surface of the electrolytic-bath
salt 2, the electrolytic-bath salts adhering to Ca generated (deposited) by
the electrolysis start the solidification one after another. On the other
hand, because the conductivity is maintained, Ca is continuously
deposited while the electrolytic-bath salt is solidified.
[00651
The deposition of Ca and the adhesion and solidification of the
electrolytic-bath salt near the deposited Ca repeatedly undergo as the
cathode 4 is pulled, and the solid-state Ca and electrolytic-bath salt 5 in
which Ca and the electrolytic-bath salt are contained in a mixed state
are formed downward from the portion which is dipped in the
electrolytic-bath salt 2 in starting the electrolysis of the cathode 4.
[00661
In the solid-state Ca and electrolytic-bath salt, Ca is dispersed in
fine granules, and Ca has an extremely large surface area. Accordingly,
in the method of producing Ti or the Ti alloy through reduction by Ca,
the solid-state Ca and electrolytic-bath salt have a property of being
easily dissolved in the molten salt containing CaC12 in the reactor vessel
when the solid-state Ca and electrolytic-bath salt are used as the Ca
source. For the "state in which Ca and the electrolytic-bath salt are
mixed", a mixing ratio and unbalance of the mixing are not particularly
defined. Any solid-state Ca and electrolytic-bath salt may be used as
long as they are rapidly and evenly dissolved in the molten salt
CA 02602801 2007-09-28
containing CaC12 when fed into the molten salt as the Ca source.
[0067]
In the electrolysis method of the invention, the electrolytic-bath
salt having any composition can be used as long as the temperature of
the electrolytic-bath salt can be adjusted within the above-described
temperature range and Ca is generated by the electrolysis. Usually a
mixture of a halogenated salt of Ca is used. Examples of the mixture of
a halogenated salt of Ca includes a binary-system mixed salt such as a
calcium fluoride and a calcium chloride and the calcium chloride and a
potassium chloride and a ternary-system mixed salt such as the calcium
chloride, the calcium fluoride, and the potassium chloride. Because a
melting point of the electrolytic-bath salt can be changed by the use of
the mixed salt, the electrolytic-bath salt can be selected according to the
setting bath temperature.
[0068]
The bath temperature is set to a range of 680 to 900 C. When
the bath temperature is lower than 680 C, the electrolytic generation of
Ca is hardly performed due to the excessively low reaction temperature.
On the other hand, when the bath temperature exceeds 900 C, it is
difficult to obtain high current efficiency (i.e., Ca recovery efficiency)
because a dissolution amount of generated Ca in the electrolytic-bath
salt is increased.
[0069]
The cathode current density is set in the range of 0.1 to 200 A/cm2.
A lower limit of the current density depends on a rate at which Ca
generated by the electrolysis is re-dissolved. When the current density
is less than 0.1 A/cm2, because the rate at which Ca is dissolved in the
electrolytic-bath salt is faster than the Ca generation rate, Ca cannot be
recovered. On the other hand, the reason why an upper limit of the
21
CA 02602801 2007-09-28
current density is set to 200 A/cm2 is that, when the electrolysis is
performed at the current density exceeding 200 A/cm2, the voltage
cannot be decreased even if the inter-electrode distance is adjusted, and
the electric power consumption is increased.
[00701
When the cathode current density is set in the range of 0.1 to 70
A/cm2, the voltage can be set to 5V or less to largely reduce the electric
power consumption, which is desirable. Further, when the cathode
current density is set to the range of 10 to 50 A/cm2, at least 90% current
efficiency can be obtained in addition to the large reduction of the
electric power consumption.
[00711
That is, the demands of the decrease in voltage and the current
efficiency improvement, which conflict with each other, can be satisfied.
Accordingly, the above range is the compatible range for the cathode
current density, and desirably the commercial operation is performed
while the cathode current density is set in the range of 10 to 50 A/cmz.
[00721
The reason why the voltage is set to lOV or less during the
electrolysis is that the increase in electric power consumption is
suppressed as much as possible. In the description of the Germany
technical document, it is considered that the electrolysis is performed at
high voltage (35 to 40V) in order to deposit the metallic Ca. However,
the high voltage is not required in the electrolysis method of the
invention, because Ca is recovered as the solid substance in which the Ca
and the electrolytic=bath salt are mixed. Although the lower limit of
the voltage is not particularly determined, it is necessary that the
voltage be higher than at least a decomposition voltage (about 3.2V) of
the molten CaC12 in order that the electrolysis proceeds to deposit Ca.
22
CA 02602801 2007-09-28
[0073]
The cathode pulling rate is set to 0.05 cm/min or more. When
the pulling rate is slower than 0.05 cm/min, it is difficult to cause the
generated Ca to adhere to the cathode surface. This is because the
generated Ca is dissolved and widely spread into the bath.
[0074]
The upper limit of the pulling rate is not particularly defined.
As regulated in the electrolysis method of the invention, when a
manipulation for pulling the cathode while the solid substance is caused
to adhere to and solidified in the cathode surface is performed, the upper
limit of the pulling rate is determined by itself. That is, when the
pulling rate is excessively fast, the pulled solid substance has an
excessively small sectional area (i.e., becomes excessively thin) and the
pulled solid substance is cut in the middle, so that the pulling cannot
continuously be performed. In consideration of the restriction on the
pulling manipulation, desirably the pulling rate is set to 10 cm/min or
less.
[0075]
In the electrolysis method of the invention, when the pulling rate
further satisfies the equation (1), Ca and the electrolytic-bath salt can be
recovered at good current efficiency.
[00761
Fig. 3 is a view illustrating relationships between the cathode
pulling rate and the current efficiency when the pulling electrolysis
method of the invention is implemented. Fig. 3 shows electrolysis
examples at the voltage of lOV or less and the interelectrode distance of
7 cm or less. In Fig. 3, a mark "+" and a solid line indicate the case in
which bath temperature is set to 720 C during the electrolysis
(electrolysis at 720 C), the mark "* " and a broken line indicate the case
23
CA 02602801 2007-09-28
in which the bath temperature is set to 800 C (electrolysis at 800 C), a
mark "O" indicates the case in which a columnar cathode whose diameter
is 8 mm is used, a mark "=" indicates the case in which a columnar
cathode whose diameter is 5 mm is used, and a mark "0" indicates the
case in which a columnar cathode whose diameter is 15 mm is used.
(0077]
At this point, the current efficiency is expressed by a ratio
(percentage) of the Ca amount in the solid substance (i.e., solid-state Ca
and electrolytic-bath salt) of the cathode surface to the Ca deposition
amount (theoretical deposition amount) determined from the electricity
amount based on a Faraday's law. Because Ca in the solid substance
does not include Ca which is dissolved or peeled off after once deposited
on the cathode surface, the current efficiency used herein is synonymous
with Ca recovery efficiency.
[0078)
As is clear from Fig. 3, the cathode pulling rate is closely related
with the current efficiency, the current efficiency is improved when the
pulling rate is increased irrespective of the bath temperature. This is
attributed to the fact that, although the generated Ca is partially
dissolved and spread in the bath from the neighbor of the cathode, the
generated Ca is exposed from the surface of the electrolytic-bath salt
before dissolved in the bath by increasing the pulling rate, which allows
the dissolution to be suppressed to enhance the Ca recovery efficiency
(i.e., current efficiency). In the electrolysis at 800 C, an influence of
the shape (diameter in section) of the cathode is not observed as far as
the investigation is performed.
[0079]
In the case of the high bath temperature, the current efficiency is
slightly lowered. In the example shown in Fig. 3, the electrolysis at
24
CA 02602801 2007-09-28
800 C is lower than the electrolysis at 720 C in the current efficiency
over the whole range of the pulling rate. This is attributed to the fact
that the dissolution amount of generated Ca into CaC12 is increased to
decrease the Ca recovery amount when the bath temperature becomes
higher. Accordingly, in the case where the electrolytic-bath salt is used
at a particularly high temperature, desirably the pulling rate is
increased to perform the electrolysis on the condition that the current
efficiency is enhanced.
[0080)
As the bath temperature is increased, the Ca concentration is
increased in the recovered solid-state Ca and electrolytic-bath salt. For
example, the Ca concentration is 30% by weight at 800 C while the Ca
concentration is 20% by weight at 720 C. Although the detailed
phenomenon is unknown, this is attributed to the fact that, in the case of
the high bath temperature, the electrolytic-bath salt adhering to the
cathode surface (deposited Ca surface) during the pulling runs off to be
easily separated from Ca before the electrolytic-bath salt is solidified
and Ca in the recovered solid substance is condensed.
[00811
Accordingly, the Ca concentration can be controlled in the
recovered solid-state Ca and electrolytic-bath salt by adjusting the bath
temperature, and the Ca concentration can be learned and arbitrarily
determined when the solid-state Ca and electrolytic-bath salt are used as
the Ca source.
[00821
In the equation (1), the relationship between the bath
temperature and the pulling rate in which the good current efficiency is
obtained is determined based on the relationship among the bath
temperature, the cathode pulling rate, and the current density during
CA 02602801 2007-09-28
the electrolysis shown in Fig. 3. In the electrolysis method of the
invention, when the equation (1) is satisfied, Ca can efficiently be
recovered as the solid-state Ca and electrolytic-bath salt.
(0083]
Fig. 4 is a view showing a relationship between the bath
temperature and the cathode pulling rate in the pulling electrolysis. A
portion hatched in Fig. 4 is a region, where the pulling rate is not lower
than 0.05 cm/min and the good current efficiency (i.e., good Ca recovery
efficiency) expressed by the equation (1). The lower limit of the region
is expressed by an equation (V=0.0035xt-2.4, where 700<=t<=900) in the
case where both sides of the equation (1) are equal to each other.
[0084]
In the electrolysis method of the invention, Ca and the
electrolytic-bath salt are recovered in the solid state. As described
above, "solid-state" means that the Ca and the electrolytic-bath salt are
apparently in the solid state. For example, in the case where a large
difference is made between the bath temperature and the melting point
of the electrolytic-bath salt due to the high bath temperature, sometimes
the molten electrolytic-bath salt exists inside the solid substance on the
cathode surface. The electrolytic-bath salt adhering to the pulled
cathode surface is hardly solidified, and Ca generally has the melting
point higher than that of the electrolytic-bath salt. Therefore, Ca is
deposited in the form of the solid substance from the beginning, or Ca
becomes immediately the solid substance even if Ca is initially in the
molten state, so that the unsolidified electrolytic-bath salt is taken into
the solid substance.
[00851
In the case of the small difference between the bath temperature
and the melting point of the electrolytic-bath salt, because the
26
CA 02602801 2007-09-28
electrolytic-bath salt is easily solidified, the whole of the solid substance
on the cathode surface is recovered as the solid state.
[0086]
Thus, according to the electrolysis method of the invention, Ca
can be obtained as the solid-state Ca and electrolytic-bath salt at low
voltage and high current efficiency (i.e., with the relatively small
electric power consumption). The solid-state Ca and electrolytic-bath
salt is extremely effectively used as the Ca source when the method of
producing Ti or the Ti alloy through reduction by Ca is implemented.
[0087]
3. Production Process
A production process into which the pulling electrolysis method of
the invention is incorporated in producing Ti or the Ti alloy through
reduction by Ca will be described below.
[0088]
The production process is a method in which solid-state Ca and
electrolytic-bath salt recovered by the pulling electrolysis method of the
invention is used as Ca caused to react with a metallic chloride
containing TiC14 in implementing the method of producing Ti or the Ti
alloy through reduction by Ca, i.e., the Ti or Ti alloy production method
including a reduction process of causing a metallic chloride containing
TiC14 to react with Ca in a Ca-containing electrolytic-bath salt to
generate Ti or the Ti alloy in the electrolytic-bath salt and an
electrolysis process of generating Ca by electrolyzing the
electrolytic-bath salt taken out from the reduction process.
[0089]
Fig. 5 is a view showing a configuration example of an apparatus
for producing the metallic Ti through reduction by Ca. In the example,
TiC14 is used as the raw material, and the Ca-containing
27
CA 02602801 2007-09-28
electrolytic-bath salt containing CaC12 is used as the Ca-containing
electrolytic-bath salt. In the example, a separation process and a
chlorination process are included in addition to the reduction process
(below-mentioned process proceeding in the reactor vessel 6) and the
electrolysis process. In the separation process, the generated metallic
Ti is separated and recovered. In the chlorination process, TiCla is
produced by utilizing the chlorine (Cl2) generated through the
electrolysis.
[00901
Referring to Fig. 5, a reducing agent feed pipe 7 for feeding Ca (in
this case, feeding the solid-state Ca and electrolytic=bath salt) which is
of a reducing agent is provided in a ceiling portion of a reactor vessel 6.
A bottom of the reactor vessel 6 is tapered downward while a diameter of
the reactor vessel 6 is gradually reduced in order to promote discharge of
produced Ti granules. A Ti discharge pipe 8 is provided in a central
portion at lower end of the reactor vessel 6 to discharge the produced Ti
granules.
[0091]
On the other hand, a cylindrical separation wall 9 is disposed
inside the reactor vessel 6 while a predetermined gap is provided
between the separation wall 9 and an inner surface of a straight body
portion of the reactor vessel 6. A molten salt discharge pipe 10 is
provided in an upper portion of the reactor vessel 6 to discharge CaCl2 in
the vessel to the side. A raw material feed pipe 11 for feeding TiC14
which is of a raw material to Ti is provided in a lower portion of the
reactor vessel 6 while piercing through the separation wall 9 to reach a
central portion of the vessel.
[0092]
The molten CaCl2 solution in which Ca is melted is retained as
28
CA 02602801 2007-09-28
the molten salt in the reactor vessel 6. A level of the molten CaC12
solution is set higher than the molten salt discharge pipe 10 and lower
than an upper end of the separation wall 9. In this state of things, a
TiCl4 gas is supplied to the molten CaC12 solution located inside the
separation wall 9 through the raw material feed pipe 11. This enables
TiC14 to be reduced by Ca in the molten CaC12 solution inside the
separation wall 9 to produce the granular metallic Ti in the molten CaC12
solution.
[00931
The Ti granules produced in the molten CaC12 solution located
inside the separation wall 9 in the reactor vessel 6 moves downward
through the molten CaC12 solution and deposited on the bottom of the
reactor vessel 6. The deposited Ti granules are appropriately taken out
downward from the Ti discharge pipe 8 along with the molten CaCla
solution and delivered to a separation process 12.
(0094)
The molten CaC12 solution whose Ca is consumed by the reducing
reaction inside the separation wall 9 rises along the outside of the
separation wall 9 though the bottom of the separation wall 9, and the
molten CaC12 solution is discharged from the molten salt discharge
pipe 10. The discharged molten CaC12 solution is delivered to an
electrolysis process 13.
L00951
Inside the separation wall 9, Ca is replenished by feeding the
solid-state Ca and electrolytic-bath salt to the molten CaC12 solution
from the reducing agent feed pipe 7.
I0096]
On the other hand, in the separation process 12, the Ti granules
taken out of the reactor vessel 6 along with the molten CaC12 solution are
29
CA 02602801 2007-09-28
separated from the molten CaC12 solution. Specifically, the Ti granules
are compressed by squeezing the molten CaCl2 solution. The molten
CaC12 solution obtained in the separation process 12 is delivered to the
electrolysis process 13.
L00971
In the electrolysis process 13, a molten CaC12 solution 13b
introduced into an electrolytic tank 13a from the reactor vessel 6 and the
separation process 12 are separated into Ca and the C12 gas through the
electrolysis.
[0098]
Ca generated on the side of a cathode 13d is recovered by the
manipulation for pulling the cathode 13d in the form of solid-state Ca
and electrolytic-bath salt 13e in which Ca and the electrolytic-bath salt
are mixed, and the solid-state Ca and electrolytic-bath salt 13e are
returned into the reactor vessel 6 to replenish Ca. The total of Ca may
be replenished (fed) with the solid-state Ca and electrolytic-bath salt, or
part of Ca may be replenished with the solid-state Ca and
electrolytic-bath salt while the residue is replenished with the CaC12
solution whose Ca concentration is increased.
(0099]
The solid-state Ca and electrolytic-bath salt 13e delivered in the
reactor vessel 6 are easily dissolved, so that they are rapidly and evenly
dissolved in the reactor vessel. In the case where the molten
electrolytic-bath salt exists in the solid-state Ca and electrolytic-bath
salt, because the dissolution proceeds further rapidly, the even reaction
between TiC14 and Ca proceeds effectively in the wide range of the
vessel.
[01001
Additionally, the solid-state Ca and electrolytic-bath salt 13e are
CA 02602801 2007-09-28
melted by absorbing the heat generated in association with the reaction
between TiC14 and Ca, which allows the production efficiency and the
thermal efficiency to be largely improved. Because the cooling
capability in the reaction system is enhanced as a whole, the reaction
temperature is easily controlled, and raw-material loading rate can be
enhanced to produce further efficiently Ti or the Ti alloy through
reduction by Ca. The latent heat of fusion can maximally be utilized
when the whole of the solid-state Ca and electrolytic-bath salt are in the
solid state.
[0101]
In the method of producing Ti or the Ti alloy through reduction by
Ca, the reaction rates in the reduction process and electrolysis process
are possibly changed during operation (or the reaction rates are
inevitably changed). In such cases, an added effect that the solid-state
Ca and electrolytic-bath salt are retained (temporarily stored) as the
buffer of the Ca source and used according to need can sufficiently be
expected.
[01021
The C12 gas generated on the side of a positive electrode 13c in
the electrolysis process 13 is delivered to a chlorinaton process 14. In
the chlorinaton process 14, TiOz is chlorinated to produce TiC14 in the
presence of carbon powders (C). Oxygen which is of a by-product is
discharged in the form of C02 by simultaneously using the carbon
powders (C). The produced TiC14 is introduced into the reactor vessel 6
through the raw material feed pipe 11. Thus, Ca and Clz gas which are
of the reducing agent are circulated by the circulation of CaC12.
(0103]
As described above, according to the production process, the
metallic Ti can continuously be produced only by actually replenishing
31
CA 02602801 2007-09-28
Ti02 and C. At this point, the solid-state Ca and electrolytic-bath salt
which are recovered by the electrolysis method of the invention can
suitably used as the Ca source.
INDUSTRIAL APPLICABILITY
[0104]
According to the production method of the invention, in producing
Ti or the Ti alloy through reduction by Ca, the electrolytic=bath salt
taken out from the reduction process is electrolyzed to recover the Ca
and electrolytic-bath salt in the form of the solid substance, and the
recovered Ca and electrolytic-bath salt is delivered to the reduction
process. Therefore, the heat generation in the reduction process is
suppressed by utilizing the latent heat of fusion possessed by the solid
substance, the production efficiency and the thermal efficiency are
largely improved, the reaction temperature is easily controlled, and the
raw-material loading rate can be enhanced to efficiently produce Ti or
the Ti alloy at low cost. In the pulling electrolysis method of the
invention, Ca is recovered while the bath temperature, the cathode
current density, the voltage, and the cathode pulling rate are regulated
within the predetermined ranges. Therefore, the solid-state Ca and
electrolytic-bath salt can be obtained at low voltage and high current
efficiency, i.e., with the relatively small power consumption. When the
solid-state Ca and electrolytic-bath salt are used as the Ca source in
producing Ti or the Ti alloy through reduction by Ca, the solid-state Ca
and electrolytic-bath salt are dissolved rapidly and evenly in the reactor
vessel, and the excessive heat generated by the reaction of the metallic
chloride containing Ca and TiC14 is suppressed by melting the Ca and
electrolytic-bath salt during the reducing reaction, so that Ti or the Ti
alloy can efficiently be produced.
32
