Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR PREPARING METALLIC TITANIUM BY ELECTROLYZING
MOLTEN SALT WITH TITANIUM CIRCULATION
FIELD OF THE INVENTION
The present invention relates to a technical filed of non-ferrous metal
metallurgy,
more particularly, to a method for preparing metallic titanium by
electrolyzing molten
salt.
DESCRIPTION OF RELATED ART
Titanium (Ti) is a metal with superior performances, and has many advantages
such
as low specific gravity, high specific strength, good corrosion resistance and
the like.
Titanium alloy also has various excellent properties, for example, good
high/low
temperature resistance, nonmagnetic, shape memory characteristics, hydrogen
absorption characteristics, superconductivity, and low damping characteristics
and the
like, thus it is an excellent constructional and/or functional material.
Because of
numerous advantages of titanium, titanium is called "Metal for outer space",
"Marine
Metal", "Metal of 21s' century", etc. Consequently, in modem society, metallic
titanium
and its alloy are increasingly applied to national defense, chemical industry,
metallurgy,
medical treatment, industrial and agriculture production and other fields,
especially to
high and new technology industries.
However, it is pretty difficult to extract titanium. Currently, Kroll process
is mainly
used to produce sponge titanium in the world, and sponge titanium obtained by
this
process is generally purified by using a method of vacuum arc remelting. In
particular,
the process of producing sponge titanium by using the Kroll method comprises:
firstly
preparing titanium tetrachloride (TiCl4) by chloridizing titanium dioxide
(Ti02) with
addition of carbon (C) (see the following reaction equation (1)), obtaining
sponge
titanium by thermally reducing metallic magnesium (see the following reaction
equation
(2)), and then vacuum distilling the obtained sponge titanium to obtain
commercial
sponge titanium.
Ti02 + 2C12 + 2C 4 TiC14 + 2CO (1)
2Mg + TiC14 4 2MgC12 +Ti (2)
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During producing sponge titanium through the Kroll method, metallic Mg is
obtained by electrolyzing MgCl2, and chlorine gas (C12) obtained through
electrolysis is
used to prepare titanium chloride in turn, thus the whole production process
comprises
four main steps, i.e. chloridization of TiO2, thermal reduction with
magnesium, vacuum
distillation and electrolysis of MgC12. The method is advantageous in high
productivity,
is capable of producing sponge titanium with good quality, and facilitates
commercialization, thus it seems that this method is irreplaceable so far. On
the other
hand, the method has disadvantages such as long process flow, excessive steps,
high
power consumption, and difficulty in achievement of continuous processes,
resulting in
high manufacturing cost of sponge titanium. For example, the sponge titanium
produced
by the Kroll method has a price much higher than steel, and has a price per
unit weight
three times higher than that of metal aluminum. Furthermore, Kroll process has
already
been a considerably mature process at present, and little improvement in it
can be made.
In this case, it is expected that a method for producing metallic titanium
with a
lower cost can be developed. At present, representative methods comprise: OS
method,
FFC process, QIT process, USTB method, PRP method, electrolysis of TiC14,
Hunter
method, Armstrong method (reduction with Na), SOM technique, producing Ti in
vacuum, etc., but the industrialization has not been realized so far due to
various
disadvantages thereof.
Since TiC14 is used as a raw material in Kroll method, people all conceive of
such a
method of obtaining metallic titanium by directly electrolyzing TiC14. As a
result, the
disadvantage existing in reduction and distillation of Kroll method can be
avoided.
However, studies have shown that TiCl4, the most common in titanium chlorides,
has a
pretty high vapor pressure at a temperature of molten salt electrolysis, and
has a pretty
small solubility (<1%) in melt, thus it is not feasible to use TiC14 as a raw
material for
directly electrolysis.
Studies on electrolysis of titanium chlorides mostly focus on a method of
electrolyzing TiC13. Cordner and Warner from Australia reported their research
results
at the earliest; in particular, they obtained titanium crystals having a
purity of 99% by
electrolyzing a mixed molten salt of LiCl-KC1 having TiC13 dissolved therein
at a
temperature of 550 C. However, it was difficult to realize industrialization
of this
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method due to the following problems: on one hand, there is no available
method of
preparing TiC13; on the other hand, there is no proper method to supply TiC13
to an
electrolytic bath. Additionally, TiC13 is decomposed due to easily absorbing
moisture
in air, causing pollution to the product.
In addition, there were other research results reported that metallic
titanium was prepared by electrolyzing TiC12. However, it could not be
industrialized
yet, since there was not an available method for preparing TiC12, and it was
difficult to
add TiC12 to an electrolytic bath and to store it.
SUMMARY OF THE INVENTION
In one aspect, the present invention overcomes the disadvantages such
as long process flow; high power consumption or impossibility in realizing
industrialization in the conventional method for preparing metallic titanium.
Thus, one
aspect of the present invention provides a method for preparing metallic
titanium
having simple process, low power consumption and being capable of realizing
industrialization, by using Ti as a reducing agent to reduce titanium
tetrachloride
(TiCl4) to titanium trichloride (TiCl3) and/or titanium dichloride (TiCI2).
The present invention provides, in another aspect, a method for
preparing metallic titanium by electrolyzing molten salt with titanium
circulation
comprising: i) reducing titanium tetrachloride (TiCl4) to at least one of
titanium
trichloride (TiCl3) and titanium dichloride (TiCl2) in chloride molten salt by
metallic
titanium (Ti); and ii) electrolyzing the at least one of TiC13 and TiC12 in
the chloride
molten salt to form metallic titanium.
According to another aspect of the present invention, the metallic
titanium used in the step i) is a portion of metallic titanium obtained by
electrolyzing
the at least one of TiC13 and TiC12. A mole ratio of the metallic titanium for
reducing to
TiC14 is 1:1 to 1:3. The chloride molten salt is at a temperature higher than
the
eutectic temperature of the molten salt and lower than vaporization
temperatures and
decomposition temperatures of salts for forming the molten salt. According to
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preferable embodiments of the present invention, the chloride molten salt is
formed
by melting two or more of LiCI, NaCl, KCI, MgC12, CaC12, SrC12 and BaC12. The
chloride molten salt is at a temperature of 400 C to 850 C.
According to another aspect of the present invention, a small amount of
sponge titanium, titanium powder or titanium scrap is added as a titanium
source
before electrolyzing so as to react with TiCI4, thereby obtaining TiC12,
TiC13, or a
mixture of TiC12 and TiC13 used as electrolyte to induce electrolysis.
According to another aspect of the present invention, the method further
comprises enclosing an anode with a ceramic material article, and collecting
C12
liberated at the anode in the step ii).
According to another aspect of the present invention, the method further
comprises sequentially washing the generated titanium using hydrochloric acid
aqueous solution having a concentration of 0.5 wt%-5 wt% and water after
obtaining
titanium by electrolyzing the at least one of TiC12 and TiC13, and drying the
acid-
washed and water-washed titanium in a vacuum oven.
According to another aspect of the method for preparing metallic
titanium of the present invention, TiC12, TiC13 or a mixture thereof is
prepared and
electrolyzed continuously without changing the surrounding medium, thus the
problems in the preparation and feeding of TiC12 and/or TiC13 can be solved,
no
additional foreign substance is introduced in the electrolysis and the
operation can be
simplified. In addition, C12 generated at the anode can be recovered and
reused as a
byproduct.
According to still another aspect of the present invention, there is
provided a method for preparing metallic titanium by electrolyzing molten salt
with
titanium circulation, comprising: i) reducing titanium tetrachloride (TiCI4)
to at least
one of titanium trichloride (TiCl3) and titanium dichloride (TiCl2) in
chloride molten salt
by metallic titanium (Ti); and ii) electrolyzing the at least one of TiC13 and
TiC12 in the
chloride molten salt to form metallic titanium, wherein a small amount of
sponge
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titanium, titanium powder or titanium scrap is added as a titanium source
before
electrolyzing so as to react with TiCI4, thereby obtaining TiC12, TiCI3, or a
mixture of
TiC12 and TiCI3 as electrolyte to induce electrolysis, and wherein the
metallic titanium
used in the step i) is a portion of metallic titanium obtained by
electrolyzing at least
one of TiCI3 and TiC12.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a method for preparing metallic titanium
(Ti). In particular, the present invention provides a method for preparing
metallic
titanium by electrolyzing titanium trichloride (TiCI3) and/or titanium
dichloride (TiCl2) in
a molten salt with titanium circulation.
According to the present invention, the method for preparing metallic
titanium comprises the following steps:
1) A suitable molten salt system of chloride is selected as an electrolyte,
and in the system, TiCI3, TiC12 or a combination thereof with a certain
concentration is
prepared by using Ti and TiCI4. In particular, TiCI4 is reduced to TiCI3
and/or TiC12 by
metallic Ti in the molten salt system (see the following reaction equations
(3) and (4)).
Ti + TiCI4 - 2TiCI2 (3)
Ti + 3TiCI4 - 4TiCI3 (4)
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In this step, the chloride molten salt system is required to be dried to
sufficiently
remove various forms of moisture from the electrolyte.
In the present invention, titanium used at the early stage of electrolysis is
the added
metallic titanium such as sponge titanium, titanium powder, titanium scrap and
the like,
so that TiC14 is reduced to TiC12, TiCl3 or a combination thereof. Titanium
obtained by
electrolyzing is used after the electrolysis process is recycled.
2) TiC12, TiC13 or a combination thereof in the molten salt system is
electrolyzed to
generate metallic titanium and C12 (see the following reaction equations (5)
and (6)).
TiCl2 ->Ti + C12 (5)
2TiCI3 -32Ti + 3C12 (6)
According to the present invention, in the step of electrolyzing TiC12, TiC13
or a
combination thereof, an anode of the electrolytic bath is made of graphite,
and a cathode
thereof is made of low carbon steel which is spaced apart from the anode by
porous
material. Herein, a porous material commonly used in the art can be used to
enclose the
anode so as to collect chlorine gas liberated from the anode, and can further
prevent Tie},
Ti3+ in the molten salt from being oxidized to TiC14 by Cl2 at an anode
region. It is
preferable to enclose the anode using a ceramic material. The conditions of
electrolyzing TiCl2 and/or TiC13 to Ti include a voltage of 2-5V, an anode
current
density of 0.05-0.6A/cm2, and a cathode current density of 0.1-5 A/cm2.
3) The product obtained at the metal cathode is washed by hydrochloric acid
having a concentration of 0.5-5wt%, subsequently by distilled water so that
the filtrate
contains no chloride ion (Cl - ), and then acid-washed and water-washed
titanium is
dried in a vacuum oven, thereby obtaining metallic titanium having a
relatively high
purity.
In addition, it should be noted that TiC14 is continuously or discontinuously
introduced to the cathode during the electrolysis so that TiC14 contacts with
the titanium
at the cathode region. TiC14 may react with Ti to generate low valence-Ti
chloride at a
certain electrolysis temperature (i.e. the temperature of chloride molten
salt, 400-850 C),
and the amount and velocity of the introduced TiCl4 are controlled to ensure
the
electrolysis to proceed smoothly.
The chloride molten salt used in the present invention is a molten salt formed
by
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melting two or more of alkali metal chlorides and alkali-earth metal
chlorides, which
has a temperature (i.e. electrolysis temperature) higher than the eutectic
temperature of
the molten salt and lower than vaporization temperatures and decomposition
temperatures of the salts for forming the molten salt. In particular, alkali
metal chlorides
may be one or more of lithium chloride (LiCI), sodium chloride (NaCI) and
potassium
chloride (KC1), and alkali-earth metal chlorides may be one or more of
magnesium
chloride (MgC12), calcium chloride (CaC12), strontium chloride (SrC12) and
barium
chloride (BaC12). According to the present invention, the temperature of
chloride molten
salt is preferably in a range of 400 C to 850 C. For example, in the present
invention,
KCl-LiCl system may be selected with a mole ratio of 41%:59%, which has an
electrolysis temperature of 400 C to 600 C; NaCI-CaC12 system may also be
selected
with a mole ratio of 47.1 %:52.9%, which has an electrolysis temperature of
550 C to
750 C; or NaCI-KCI system may also be selected with a mole ratio of 50%:50%,
which
has an electrolysis temperature of 700 C to 850 C.
According to the present invention, TiC13 and TiC12 in the molten salt system
are
electrolyzed to generate metallic titanium at the cathode, a portion of which
is used as a
reducing agent, and to liberate Cl2 at the anode, which is used as a byproduct
to be
recovered and reused.
In addition, according to the present invention, the concentration of low
valence-Ti
ion in the electrolyte will be reduced continuously as the electrolysis
proceeds, and the
concentration of low valence-Ti ion in the electrolyte is required to be
controlled within
a certain concentration range in order to carry out the electrolysis process
more
smoothly, thus a predetermined amount of TiCl4 may be introduced into the
chloride
molten salt periodically so that it reacts with a portion of titanium
generated by
electrolyzing to generate TiC13, TiC12 or a combination thereof Low valence-Ti
to be
consumed by electrolyzing in the electrolyte may be added in time, titanium
trichloride
and/or titanium dichloride generated in the molten salt continue to be
electrolyzed, and
the above procedures are repeated, thereby building up a complete process of
preparing
metallic titanium by electrolyzing molten salt with titanium circulation.
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Furthermore, it should be noted that there is only one molten salt system
formed in
the whole electrolysis, i.e. the molten salt system of chlorides of alkali
metal and/or
alkali earth-metal with TiC12/TiCl3, such as NaCI-KC1-TiC12/TiC13 or CaC12-KCI-
TiC12/TiC13, since titanium (Ti) is directly used as the reducing agent in the
present
invention. Therefore, no additional foreign substance is introduced in the
electrolysis
processes for preparing metallic titanium according to the present invention,
and the
operation parameters are kept the same in the whole electrolysis, thereby
reducing cost
and facilitating operation.
Hereinafter, the present invention will be further explained in connection
with
embodiments.
Embodiment 1
A chloride molten salt system was prepared. 117g of NaCl (analytical reagent)
and
149g of KCI (analytical reagent) were mixed uniformly and dried in an oven at
120 C
for 24 hours, followed by dried in a reactor made of stainless steel under
vacuum at
35011-4500 for 48 hours so that various forms of moisture contained in the
electrolyte
was sufficiently removed.
In this system, an electrolyte having a Ti2+ concentration of 5% was prepared
using
sponge titanium and TiC14. The temperature was then increased to 70011 in an
Ar
atmosphere to perform constant-current electrolysis. The anode was made of
graphite
and was spaced apart from the cathode by a porous material (preferably ceramic
material) so as to facilitate collecting C12 liberated from the anode, while
avoiding the
possibility of oxidizing Ti2+, Ti3+ in the molten salt to TiC14 by C12
liberated at the anode
region. The cathode was made of low carbon steel. The electrolysis system had
an
anode current density of 0.3A/cm2, a cathode current density of 1.2A/cm2, an
anode-
cathode distance of 6cm, a current intensity of 1OA, and 7g of metallic
titanium was
obtained based on 80% of current efficiency after lhr of electrolysis time. At
this time,
25g of TiC14 was introduced to the cathode region in the molten salt to
generate TiCl2,
and the generated TiC14 was electrolyzed for I hr under the conditions of a
current
intensity of 15A and other parameters (i.e. the anode current density, the
cathode
current density, and the anode-cathode distance) the same as above.
After electrolysis, the cathode product was firstly washed by 1 wt% of
hydrochloric
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acid to be colorless, and then washed by distilled water to have no chloride
ion,
followed by dried in a vacuum oven, thereby obtaining 12.08g of titanium
powder. The
results obtained by analyzing a sample were as follows: a Ti content of
99.83%, an
oxygen content of 0.085%, a carbon content less than 0.02%, and an iron
content less
than 0.045% by weight of the product. The current efficiency of TiCl2
electrolysis was
calculated as 89.98%.
Embodiment 2
After 40g of LiCI, 46g of NaCl and 22g of KC1 were weighed and mixed
uniformly, a chloride molten salt system was prepared through dehydrating
process in
the same manner as that in Embodiment 1.
In this system, an electrolyte having a Ti3+ concentration of 6% was prepared
using
sponge titanium and TiC14. The temperature was then increased to 6000 in an Ar
atmosphere to perform constant-current electrolysis. By using the same anode
and
cathode devices as those in Embodiment 1, the electrolysis system had an anode
current
density of 0.5A/cm2, a cathode current density of 3A/cm2, an anode-cathode
distance of
5cm, and a current intensity of 6A, and 4.6g of metallic titanium was obtained
based on
85% of current efficiency after lhr of electrolysis time. At this time, 55g of
TiC14 was
introduced to the cathode region in the molten salt to generate TiC13, and the
generated
TiC13 was electrolyzed for 2hr under the conditions of a current intensity of
8A and
other parameters the same as above.
After electrolysis, the cathode product was firstly washed by 5wt%
hydrochloric
acid to be colorless, and then washed by distilled water to have no chloride
ion,
followed by dried in a vacuum oven, thereby obtaining 12.65g of titanium
powder. The
results obtained by analyzing a sample were as follows: a Ti content of
99.72%, an
oxygen content of 0.072%, a carbon content less than 0.02%, and an iron
content less
than 0.060% by weight of the product. The current efficiency of TiCl3
electrolysis was
calculated as 88.37%.
Embodiment 3
After 70g of CaCl2 and 80g of KCl were weighed and mixed uniformly, a chloride
molten salt system was prepared through dehydrating process in the same manner
as
that in Embodiment 1.
In this system, an electrolyte having a combined concentration of Ti2+ and
Ti3+ of
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10% was prepared using sponge titanium and TiC14. The temperature was then
increased to 8000 in an Ar atmosphere to perform constant-current
electrolysis. By
using the same anode and cathode devices as those in Embodiment 1, the
electrolysis
system had an anode current density of 0.1 A/cm2, a cathode current density of
0.5A/em2,
an anode-cathode distance of 5cm, and a current intensity of 12A, and 8g of
metallic
titanium was liberated based on 85% of current efficiency after lhr of
electrolysis time.
At this time, 65g of TiC14 was introduced to the cathode region in the molten
salt to
generate a mixture of TiC12 and TiC13, and the generated TiC12 and TiC13 were
electrolyzed for 2hr under the conditions of a current intensity of 8A and
other
parameters the same as above.
After electrolysis, the cathode product was firstly washed by 0.5wt%
hydrochloric
acid to be colorless, and then washed by distilled water to have no chloride
ion,
followed by dried in a vacuum oven, thereby obtaining 10.20g of titanium
powder. The
results obtained by analyzing a sample were as follows: a Ti content of
99.67%, an
oxygen content of 0.09%, a carbon content less than 0.03%, and an iron content
less
than 0.05% by weight of the product. The current efficiency of TiC12 and TiC13
electrolysis in combination was calculated as 78.07%.
It can be known from the experiment results of Embodiments 1-3 that the
contents
of Ti in the products were all above 99.6%, which showed that the generated
titanium
powders have high purity and can be directly cast, and that the current
efficiencies in
electrolyzing were all above 87%, which showed that the method for preparing
metallic
titanium according to the present invention can produce metallic titanium with
high
yield and low power consumption.
It can be known from the above embodiments that TiC12, TiC13 or a mixture
thereof
is prepared and electrolyzed continuously in the method for preparing metallic
titanium
according to the present invention, without changing the surrounding medium,
thereby
solving the problems in the preparation and feeding of TiC12 and/or TiC13. In
addition,
C12 generated at the anode can be recovered and reused as a byproduct.
Therefore,
compared with the conventional method for preparing metallic titanium by
electrolyzing,
the method for preparing metallic titanium by electrolyzing molten salt with
titanium
circulation according to the present invention can simplify preparation
process, reduce
power consumption, and realize industrialization.
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The above description is only to illustrate some principles of the present
invention,
and does not limit the present invention to the scope as described above.
Therefore, all
available corresponding modifications and equivalents fall into the scope
claimed in the
present invention, without departing from the spirit and scope of the present
invention
as defined in the claims and their equivalents.
