Production Method for Titanium Foil

A method for producing a titanium foil according to the present invention includes an electrodeposition step of performing electrolysis with electrodes including an anode and a cathode using a molten salt bath comprising titanium ions and having at least one molten chloride to deposit metal titanium onto an electrolytic surface of the cathode, wherein the electrodeposition step includes maintaining a ratio of a molar concentration of titanium ions to the total molar concentration of metal ions in the molten salt bath at 7% or more, and maintaining a temperature of the molten salt bath at 510° C. or less, and conducting a current to the electrodes under conditions where a continuous stop time of current conduction is less than 1.0 second, a current density is 0.10 A/cm2 or more and 1.0 A/cm2 or less, and a time for electrodepositing the metal titanium onto the electrolytic surface of the cathode is 120 minutes or less.

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Description
TECHNICAL FIELD

The present invention relates to a method for producing a titanium foil by performing electrolysis with an electrode including an anode and a cathode using a molten salt bath to deposit metal titanium.

BACKGROUND ART

Metal titanium is generally produced by a Kroll process suitable for mass production. In the production of metal titanium, titanium oxide contained in titanium ores is firstly allowed to react with chlorine in the presence of carbon sources such as cokes to produce titanium tetrachloride. The titanium tetrachloride is then reduced with metal magnesium to obtain sponge-shaped metal titanium, so-called sponge titanium.

Here, in order to produce metal titanium in the form of a foil having a relatively lower thickness using the above sponge titanium as a main raw material, the sponge titanium is generally molten and cast into a titanium ingot or titanium slab, and then subjected to forging, rolling or other processing. With such a process requiring melting and processing, it cannot be said that metal titanium having a foil shape can be produced efficiently and at low cost.

Under such circumstances, the use of molten salt electrolysis for depositing metal titanium with a molten salt bath in place of the above melting and processing is being studied in terms of reducing energy consumption and cost in the producing process.

Techniques relating to molten salt electrolysis are described in Patent Literatures 1 to 3, for example.

Patent Literature 1 describes “a method for producing high-purity titanium by a molten salt electrolysis method, wherein the electrolysis is performed in a chloride bath having a sodium ion content of 10 wt % or less as a bath composition”, and it also discloses that “when performing the electrolysis using an electrolytic bath having a low melting point of 400° C. or less, the electrolysis temperature should be in the range of 550 to 900° C.”.

The Patent Literature 1 teaches that:

    • “when a bath having a low melting point (400° C. or less) such as LiCl—KCl is used, the electrolysis is typically performed at 400 to 500° C. However, the electrodeposited Ti at this temperature results in a sponge shape or powdery shape. In this state, an amount of oxygen is larger and the loss is also larger, resulting in a poor yield”. It also teaches that:
    • “by setting the temperature of the bath to 550 to 900° C., preferably 600 to 750° C., as shown in Table 2, the shape of the electrodeposited Ti is changed to coarse crystals, specifically, a hexagonal plate shape or a dendrite shape, thereby reducing oxygen and improving the yield”.

Patent Literature 2 discloses “a method for producing titanium characterized by applying a voltage between a container filled with raw material titanium as an anode and an electrolytic container as a cathode when electrolytically refining the raw material titanium by molten salt electrolysis”. More particularly, it discloses:

    • “as the molten salt electrolysis step, first, from a state where a raw material titanium T and a titanium rod 3 are not charged into a container body 1a, a mixed chloride of NaCl—KCl mixed in advance at a molar ratio of 1:1 is introduced into the container body 1a. The mixed chloride is then heated to 650° C. under reduced pressure to dehydrate the mixed chloride well, an atmosphere in the furnace is purged with an argon atmosphere, and the temperature is then increased to 740° C. and maintained to melt the mixed chloride to form an electrolytic bath 4. Subsequently, the raw material titanium T and the titanium rod 3 are immersed in the electrolytic bath 4, and a lid 1b is closed. A liquid TiCl4 is blown from a feed pipe (not shown) into a bottom of the raw material titanium T at an appropriate flow rate to generate titanium ions in the electrolytic bath, and a DC current voltage is then applied to both of an electrolysis circuit 11 and an impurity elution prevention circuit 21, respectively. The voltage of the electrolysis circuit 11 is 100 to 1000 mV, and the voltage of the impurity elution prevention circuit 21 is 500 mV or less, and preferably 10 to 150 mV, and more preferably 30 to 100 mV.”

Patent Literature 3 proposes:

    • “a method for producing metal titanium by performing electrolysis using an anode and a cathode in a molten salt bath, the method comprising a titanium deposition step of depositing metal titanium onto the cathode using an anode containing the metal titanium as the anode, wherein, in the titanium deposition step, a temperature of the molten salt bath is at 250° C. or more and 600° C. or less, and an average current density of the cathode is maintained in a range of 0.01 A/cm2 to 0.09 A/cm2, from the start of the titanium deposition step to the laps of 30 minutes.” In Example section of Patent Literature 3, it discloses:
    • “a pulse current was applied to the anode and the cathode by repeating current conduction and stopping it at predetermined intervals, thereby performing electrolysis to dissolve the anode and deposit a metal titanium foil onto the cathode.” (See Table 1).

CITATION LIST Patent Literature

    • [PTL 1]
    • Japanese Patent Application Publication No. H03-291391 A
    • [PTL 2]
    • Japanese Patent Application Publication No. 2000-87280 A
    • [PTL3]
    • WO 2020/044841 A1

SUMMARY OF INVENTION Technical Problem

In the method described in Patent Literature 3, the metal titanium is thinly electrodeposited onto the cathode, and the metal titanium can be relatively easily peeled off from the cathode. It is, therefore, considered that a thin titanium foil can be obtained by this method.

However, in the method described in Patent Literature 3, an intermittent current such as a pulse current may be applied when conducting the current to the electrodes. In this case, it takes a certain period of time to electrodeposit the metal titanium onto the cathode due to the stop time of the current conduction by the pulse current. Therefore, there is still room for improving an efficiency of the production of the titanium foil.

An object of the present invention is to provide a method for producing a titanium foil, which can increase an amount of metal titanium electrodeposited per unit time without significantly reducing an easy peelability of the metal titanium electrodeposited onto a cathode from the cathode.

Solution to Problem

It is considered that the electrodeposition of metal titanium onto the cathode is promoted by increasing the concentration of titanium ions in the molten salt bath, increasing the temperature of the molten salt bath, and increasing the current density when the current is conducted to the electrodes. On the other hand, there is a concern that these factors may reduce the easy peelability of the metal titanium from the cathode.

As a result of intensive studies, the present inventors have newly found that an appropriate combination of the above conditions. This can reduce deterioration of the easy peelability of the metal titanium even if the stop time of the current conduction to the electrodes is sufficiently shortened or even if the current conduction is not stopped. Further, in this case, the stop time of the current conduction to the electrodes is shorter or the current conduction is not stopped, an increased amount of titanium metal deposited per unit time can be achieved.

A method for producing a titanium foil according to the present invention comprises an electrodeposition step of performing electrolysis with electrodes including an anode and a cathode using a molten salt bath comprising titanium ions and having at least one molten chloride to deposit metal titanium onto an electrolytic surface of the cathode, wherein the electrodeposition step comprises maintaining a ratio of a molar concentration of titanium ions to the total molar concentration of metal ions in the molten salt bath at 7% or more, and maintaining a temperature of the molten salt bath at 510° C. or less, and conducing a current to the electrodes under conditions where a continuous stop time of current conduction is less than 1.0 second, a current density is 0.10 A/cm2 or more and 1.0 A/cm2 or less, and a time for electrodepositing the metal titanium onto the electrolytic surface of the cathode is 120 minutes or less.

In the method for producing the titanium foil described above, it is preferable that the anode comprises Ti and the anode is consumed in the electrodeposition step.

Also, in the method for producing the titanium foil described above, it is preferable that the chloride comprises titanium dichloride and/or titanium trichloride.

In the electrodeposition step, it is preferable that the ratio of the molar concentration of titanium ions to the total molar concentration of metal ions in the molten salt bath is maintained at 10% or more.

Also, in the electrodeposition step, it is preferable that the temperature of the molten salt bath is maintained at 500° C. or less.

Also, in the electrodeposition step, it is preferable that the current density is 0.20 A/cm2 or more and 0.50 A/cm2 or less.

Advantageous Effects of Invention

According to the method for producing the titanium foil of the present invention, it is possible to increase an amount of metal titanium electrodeposited per unit time without significantly reducing an easy peelability of the metal titanium electrodeposited onto a cathode from the cathode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically showing an electrolytic device that can be used in a method for producing a titanium foil according to an embodiment of the present invention.

FIG. 2 is a partial cross-sectional perspective view schematically showing a cross-sectional view of portions other than electrodes according to another electrolytic device.

FIG. 3 is a cross-sectional view schematically showing still another electrolytic device.

FIG. 4 is a cross-sectional view schematically showing still another electrolytic device.

FIG. 5 is a photograph of metal titanium electrodeposited on a cathode of Example 1.

FIG. 6 is a photograph of metal titanium electrodeposited on a cathode of Comparative Example 2. FIG.

FIG. 7 is a cross-sectional view showing a peel strength test.

 DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below in detail. FIGS. 1 to 4 and 7 for describing the respective embodiments are schematic views showing configurations thereof. Therefore, the arrangement, size, etc. of each component shown in FIGS. 1 to 4 and 7 may not be accurate.

A method for producing a titanium foil according to an embodiment of the present invention includes an electrodeposition step of performing electrolysis with electrodes including an anode and a cathode using a molten salt bath containing titanium ions and having at least one molten chloride to deposit metal titanium onto an electrolytic surface of the cathode. The electrodeposition step includes maintaining a ratio of a molar concentration of titanium ions to the total molar concentration of metal ions in the molten salt bath at 7% or more, and maintaining a temperature of the molten salt bath at 510° C. or less. The electrodeposition step also includes conducing a current to the electrodes under conditions where a continuous stop time of current conduction is less than 1.0 second, a current density is 0.10 A/cm2 or more and 1.0 A/cm2 or less. A time for electrodepositing the metal titanium onto the electrolytic surface of the cathode is 120 minutes or less. In the descriptions of each embodiment, the ratio of the molar concentration of titanium ions to the total molar concentration of metal ions in the molten salt bath is also simply referred to as a “ratio of titanium ions”.

In such a type of molten salt electrolysis, an increase in the concentration of titanium ions in the molten salt bath, an increase in the temperature of the molten salt bath, and an increase in the current density during current conduction to the electrodes, tend to accelerate the electrodeposition and increase an amount of metal titanium deposited per unit time. However, if all of them are carried out, it will be difficult to peel off the metal titanium electrodeposited onto the cathode from the cathode. In particular, it will be difficult to physically peel off the electrodeposited metal titanium from the electrode.

In contrast, according to this embodiment, the concentration of titanium ions in the molten salt bath is increased to 7% or more, and the current density is 0.10 A/cm2 or more and 1.0 A/cm2 or less, which is higher to some extent, in the electrodeposition step. On the other hand, the temperature of the molten salt bath is a relatively low temperature of 510° C. or less. According to these conditions, it is possible to easily peel off the metal titanium from the cathode while increasing the amount of the metal titanium electrodeposited per unit time. Especially, physical releasing such as peeling of the electrodeposited metal titanium from the electrode can be easily carried out.

Although the reason is not clear, it can be presumed as follows. By increasing the ratio of titanium ions in the molten salt bath as described above, the titanium ions will be difficult to be depleted in the vicinity of the cathode, even if the titanium ions are deposited as metal titanium onto the cathode by current conduction to the electrodes. It is believed that this suppresses a current bias in the vicinity of the cathode, suppresses the deposition of dendrite-like metal titanium due to the current bias, and deposits the metal titanium in the form of a foil onto the cathode. In addition, the electric power is not concentrated due to the formation of dendrites, so that an increase in the temperature on the cathode surface can be suppressed. Furthermore, since the temperature of the molten salt bath is maintained at the appropriately low level, the increase in the temperature on the cathode surface is appropriately suppressed. As a result, interdiffusion of the metal between the cathode and metal titanium deposited thereon is suppressed. Therefore, it is believed that even if the stop time of the current conduction to the electrodes is sufficiently short, such as the continuous stop time of current conduction of less than 1.0 second, or even if the current is constant without stopping the current conduction, the electrodeposited metal titanium will be able to be easily peeled off from the cathode. However, the present invention is not limited to the above theory.

Also, in this embodiment, the continuous stop time of current conduction is less than 1.0 second, so that the amount of metal titanium electrodeposited per unit time can be increased, thereby improving a production efficiency of the titanium foil. In addition, the titanium foil thus produced has improved smoothness due to the suppression of dendrite formation.

(Molten Salt Bath)

Molten salts for forming the molten salt bath in the electrolytic bath are molten chlorides. Preferably, the molten salt bath is made by melting only chlorides as compounds. Specific chlorides include, for example, MgCl2, NaCl, KCl, CaCl2, LiCl, BaCl2, and CsCl.

The molten salt bath preferably contains one or more chlorides, or two or more chlorides, selected from the group consisting of MgCl2, NaCl, KCl, CaCl2, LiCl, BaCl2 and CsCl. The molten salt bath preferably contains one or more, two or more, or three or more chlorides selected from the group consisting of MgCl2, NaCl, KCl, CaCl2 and LiCl. The molten salt bath preferably contains one or more, two or more, or three or more chlorides selected from the group consisting of MgCl2, NaCl, KCl and CaCl2. Specific examples of such preferred chlorides include NaCl—KCl—MgCl2, LiCl—KCl—MgCl2, NaCl—KCl—CaCl2), LiCl—KCl—CaCl2), NaCl—LiCl—KCl—MgCl2, NaCl—KCl—LiCl—CaCl2) and the like. The containing of the above chlorides can allow a molten state of the molten salt bath to be satisfactorily maintained even at a low temperature to some extent, so that the above-mentioned low temperature range of the molten salt bath in the electrodeposition step can be easily achieved.

A ratio of the total molar concentration of magnesium ions, sodium ions, potassium ions, calcium ions, lithium ions, barium ions, and cesium ions to the total molar concentration of metal ions in the molten salt bath is preferably 80% or more, and more preferably 90% or more. However, in view of the operating temperature and the like, the specific types and contents of the chlorides can be determined as needed. The molar concentration of each metal ion is calculated by ICP emission spectrometry and atomic absorption spectrometry.

It is desirable that the molten salt bath does not contain fluoride ions. Components of the molten salt bath often remain on the surface of the titanium foil obtained by peeling off the metal titanium deposited on the cathode in the electrodeposition step, and washing such as water washing may be performed in order to remove those components. In this case, if fluorides are contained in the components of the molten salt bath remaining on the surface of the titanium foil, harmful hydrogen fluoride or hydrofluoric acid is generated upon contact with water. Further, when the molten salt bath is formed by dissolving lithium fluoride, the lithium fluoride exhibits poor solubility in water, so that a large amount of water is required to remove it from the titanium foil by washing with water. In order to reduce the burden on workers and the environment, it is preferable to use a molten salt bath that does not contain fluoride ions.

The molten salt bath also contains titanium ions. In order to contain the titanium ions in the molten salt bath, a titanium raw material may be previously dissolved in the molten salt bath prior to the electrodeposition step, and/or, as described later, an anode containing Ti may be dissolved prior to the electrodeposition step or during the electrodeposition step.

When the titanium raw material is previously dissolved in the molten salt bath, more specific examples of the titanium raw material include titanium chloride and/or low-purity titanium containing impurities such as titanium scrap and titanium sponge. Among these, the low-purity titanium containing impurities may contain relatively large amounts of Fe and O as impurities, for example. When the titanium scrap or titanium sponge is used as the titanium raw material, they can be brought into contact with TiCl4 to generate lower titanium chloride such as titanium dichloride (TiCl2) and/or titanium trichloride (TiCl3), which is then dissolved to form the molten salt bath containing the titanium ions. Here, since the titanium raw material is dissolved in the molten salt bath and the metal titanium is then deposited onto the cathode, any contamination of impurities into the metal titanium can be suppressed, even if the titanium raw material contains relatively large amounts of impurities.

(Electrolytic Device)

Various electrolytic devices can be used for the present invention. As an example, an electrolytic device 1 shown in FIG. 1 includes: an electrolytic bath 2 in a form of a closed container containing a molten salt bath Bf therein; electrodes 3 including an anode 3a and a cathode 3b arranged so as to be immersed in the molten salt bath Bf in the electrolytic device 1; and a power supply 4 connected to the anode 3a and the cathode 3b to conduct a current to the anode 3a and the cathode 3b. Although not shown, in general, the electrolytic bath 2 is partially openable, and the electrodes 3 can be arranged in the electrolytic bath 2 using the opening. On the other hand, it is also possible to seal the opening, so that it is possible to prevent air from entering the electrolytic bath 2 from the external environment while conducting the current to the electrodes 3. In the anode dissolution step and/or the electrodeposition step, the interior of the electrolytic bath 2 may be maintained in a reduced pressure atmosphere or an inert gas atmosphere such as argon gas.

Here, of the anode 3a and the cathode 3b to be immersed in the molten salt bath Bf in the electrolytic bath 2, the anode 3a preferably contains Ti. The anode 3a can have various shapes such as a sheet shape, cylindrical shape, pillar shape, plate shape, massive shape, powdery shape, granular shape, fibrous shape, or briquette shape. Specifically, sponge titanium, titanium scrap, titanium rod material and/or titanium plate material can be used as the anode 3a. Further, as the anode 3a, the sponge titanium and/or titanium scrap can be specifically used. When the sponge titanium is used as the anode 3a, massive sponge titanium may be placed in a cage made of Ni or the like, and a current may be conducted through the cage. Since Ni has a lower ionization tendency than that of Ti, only sponge titanium can be eluted as the anode 3a without eluting Ni. In this case, the above cage is also included in a part of the anode 3a, and the anode 3a contains Ti and Ni. In the anode 3a including the cage and its contents (sponge titanium, etc.), only the contents containing Ti are consumed during the anode dissolution step or the electrodeposition step, and the cage is not consumed in many cases. Further, as described above, the briquette-shaped material can be used as the anode 3a. When the briquette shape is used, the anode can be constructed without using the basket made of Ni or the like.

Here, the material of the cathode 3b is not particularly limited as long as Ti is electrodeposited. In some cases, the cathode 3b contains Mo, W, Ta, Nb, or any of their alloys on the electrolytic surface on which metal titanium is to be electrodeposited. Among them, as the cathode 3b, at least the electrolytic surface preferably contains 90% by mass or more of Mo, and more preferably 99.9% by mass or more of Mo. Since Mo is difficult to be dissolved into Ti at 600° C. or less, the electrolytic surface of the cathode 3b containing 90% by mass or more of Mo does not adhere to the metal titanium deposited thereon, so that the metal titanium can be easily peeled off, and any contamination of impurities such as Mo into metal titanium can be suppressed.

When the cathode 3b has a plurality of layers made of different materials, it is possible to form an electrolytic surface containing 90% by mass or more of Mo on at least a surface layer of the layers by coating the surface of the cathode. At least the electrolytic surface of the cathode 3b may contain less than 10% by mass of impurities other than Mo, and the impurities include Ti and the like. When the cathode 3b is repeatedly used, the cathode 3b may contain Ti to some extent. In addition, not only the electrolytic surface of the cathode 3b but also the entire of the cathode 3b may be composed of Mo of 90% by mass or more.

Each of the anode (the contents of the above cage if it is included) and the cathode can be, for example, generally rod shaped, strip shaped, plate shaped, or cylindrical or other pillar shaped, or massive shaped. In particular, as illustrated in FIG. 2, it is preferable that at least a part of the electrolytic surface of the cathode 33b, on which metal titanium is electrodeposited, has a curved surface shape. However, each of the anode and the cathode may be plate shaped. As for the cathode, a plate-shaped one may be preferably used in some cases. An electrolytic device 31 shown in FIG. 2 has substantially the same structure as that of the electrolytic device 1 shown in FIG. 1, with the exception that the shapes of the anode 33a and the cathode 33b are changed. The electrolytic device 31 includes a pillar shaped cathode 33b having a cylindrical surface and a cylindrical anode 33a disposed surrounding the cathode 33b. When both the surface of the anode 33a and the surface of the cathode 33b are thus curved, it is easy to provide a constant distance between the electrodes even if the cathode 33b is configured to be movable, so that metal titanium can be more uniformly deposited over a wider area on the surface (electrolytic surface) of the cathode 33b. From this point of view, the surface of the anode 33a and the opposing surface of the cathode 33b preferably have similar shapes to each other.

Another electrolytic device 11 is shown in FIG. 3. The electrolytic device 11 of FIG. 3 has a cylindrical or pillar shaped cathode 13b as a so-called cathode drum placed in a closed electrolytic bath 12 so that a part of the cylindrical surface is immersed in the molten salt bath Bf. Further, in the electrolytic device 11, a plate-shaped anode 13a curving along the surface of the cylindrical cathode 13b is arranged in the molten salt bath Bf so as to face the surface of the cathode 13b.

In the electrolytic device 11 of FIG. 3, as an electric current is conducted through the electrodes 13 from a power source (not shown) while rotating the cylindrical or pillar shaped cathode 13b around the central axis, mainly a portion opposing to the anode 13a of the circumference portion on the surface of the cathode 13b immersed in the molten salt bath Bf will serve as an electrolytic surface for depositing metal titanium, and a foil-shaped metallic titanium Ts is deposited on that electrolytic surface. The portion of the surface of the cathode 13b that is immersed in the molten salt bath Bf changes as the cathode 13b rotates, and the electrolytic surface moves along the circumferential direction of the cathode 13b accordingly. Here, by winding up the metal titanium Ts with a winding roll 15 further provided in the electrolytic device 11, a titanium foil as a long metal titanium Ts can be continuously produced while being peeled off from the surface of the cathode 13b.

Still another electrolytic device 21 shown in FIG. 4 has a strip-shaped cathode 23b as a cathode strip, which is arranged so as to be annularly wound over a pair of rotating rolls 26a, 26b. Further, here, a plate-shaped anode 23a such as a flat plate is arranged in the molten salt bath Bf so as to face a portion of the cathode 23b in the molten salt bath Bf. The cathode 23b is positioned in the closed electrolytic bath 22 such that a portion of the surface facing outward, which has annularly wound, is immersed in the molten salt bath Bf. In this electrolytic device 21, mainly a portion opposing to the anode 23a of the surface of the cathode 23b immersed in the molten salt bath Bf will serve as the electrolytic surface.

According to the electrolytic device 21 of FIG. 4, for example, by rotating the rotating roll 26a on the drive side, the strip-shaped cathode 23b and its electrolytic surface move around the rotating rolls 26a, 26b as shown by the arrow in FIG. 4, and the rotating roll 26b on the following side rotates accordingly. At this time, by conducting the current through the electrodes 23 from the power source (not shown), a foil-shaped metal titanium Ts is deposited onto the electrolytic surface on an outer side of the cathode 23b. The metal titanium Ts is peeled off from the surface of the cathode 23b and wound up by the winding roll 25, whereby a long titanium foil of the metal titanium Ts can be continuously produced.

Although the distance between the electrodes of the anode and the cathode is not particularly limited, but it is preferably 0.5 cm or more and 10.0 cm or less on any of their opposing surfaces. The distance between the electrodes of the anode and the cathode is preferably 1.0 cm or more and 8.0 cm or less, and more preferably 1.0 cm or more and 5.0 cm or less. The distance between the electrodes of 0.5 cm or more can lead to suppression of a short circuit generated between the electrodes. Further, the distance between the electrodes of 10.0 cm or less can lead to suppression of any unintended increase in voltage, and to saved power consumption. The distance between the electrodes means the shortest distance between the surface of the anode and the surface of the cathode. When the anode has the cage made of Ni or the like as described above and the sponge titanium or the like disposed therein, it is to understand that the distance between the electrodes is the shortest distance from the end of the cage to the surface of the cathode.

In the following descriptions, the electrolytic device 1 shown in FIG. 1 will be described as an example, but the electrolytic devices 11, 21 and 31 shown in FIGS. 11, 21, 23 may also be used.

(Anode Dissolution Step)

Prior to an electrodeposition step, an anode dissolution step can optionally be performed by consuming the Ti-containing anode 3a and feeding titanium ions to the molten salt bath Bf. However, the anode dissolution step may be omitted.

In the anode dissolution step, a current of an appropriate magnitude is allowed to flow between the anode 3a and the cathode 3b immersed in the molten salt bath Bf while maintaining the molten salt bath Bf at a predetermined temperature, in substantially the same manner as in general molten salt electrolysis.

As a result, the Ti-containing anode 3a is dissolved into the molten salt bath Bf, and titanium ions are present in the molten salt bath Bf. That is, here, the anode 3a functions to feed the titanium ions to the molten salt bath Bf, as in a so-called consumable electrode.

The temperature of the molten salt bath Bf in the anode dissolution step can be 250° C. to 800° C. on the premise that it is in a molten state, and the current density of the cathode 3b is 0.01 A/cm2 to 2.00 A/cm2. This allows the dissolution of the anode 3a to carried out satisfactorily.

Here, the current density of the cathode 3b can be calculated by the equation: current density (A/cm2)=current value (A)/electrolysis area (cm2). Here, for the cathode 3b having the cylindrical surface, for example, the electrolysis area is calculated based on the equation: electrolysis area (cm2)=cathode immersion surface area=cathode diameter (cm)×π×cathode height (cm). Also, the current value is an average value of the current allowed to flow during a predetermined period of time for obtaining the current density. For example, if a constant current is allowed to flow, the value of that current will be the above current value. If the value of the current changes over time, for example, the measured values of the current can be obtained at equal time intervals during the current conduction, and the above current value can be determined by “the total of the measured values of the current/the number of measurements”. The current density of the cathode 3b can be calculated in the same manner in the electrodeposition step, which will be described below.

In the anode dissolution step, the cathode 3b can be replaced after the feeding of titanium ions to the molten salt bath Bf is completed and prior to the electrodeposition step. In the anode dissolution step, a metal other than Ti may be deposited onto the cathode 3b. Therefore, if the electrodeposition step is performed using the cathode 3b in this state, the purity of the metal titanium obtained in the electrodeposition step may decrease. Also, there is a risk that the metal titanium electrodeposited onto the cathode 3b in the electrodeposition step may be alloyed, resulting in a decrease in peelability. Therefore, it is preferable to replace the cathode 3b after feeding the titanium ions to the molten salt bath Bf in the anode dissolution step.

(Electrodeposition Step)

In the electrodeposition step, the electrolysis is performed at the electrodes 3 by conducting the current from the power supply 4 to the electrodes 3 including the anode 3a and the cathode 3b, and the titanium ions in the molten salt bath Bf are deposited as metal titanium onto the cathode 3b.

Here, the electrolysis is performed so that the ratio of the molar concentration (Mt) of titanium ions to the total molar concentration (Mm) of metal ions in the molten salt bath Bf (percentage of Mt/Mm) is maintained at 7% or more. If the ratio of the titanium ions in the molten salt bath Bf is less than 7%, the titanium ions around the cathode 3b become deficient, which may result in a biased current distribution around the cathode 3b and formation of dendrites in metal titanium on the cathode 3b. The formation of the dendrites themselves is undesirable because they impair the smoothness of the titanium foil obtained from the metal titanium on the cathode 3b, and also makes it difficult to peel off the metal titanium from the cathode 3b. From this point of view, the ratio of the titanium ions in the molten salt bath Bf is preferably maintained at 10% or more. The upper limit of the ratio Mt/Mm is not particularly limited, and the ratio of the titanium ions can be changed as appropriate in a range within which the molten salt bath can be maintained.

The molar concentration of each metal ion, including titanium ions, in the molten salt bath Bf, is calculated by solidifying a molten salt sample taken from the molten salt bath and then analyzing components of the sample by ICP emission spectrometry and atomic absorption spectrometry. If the molten salt bath contains MgCl2, NaCl, KCl, CaCl2, LiCl, TiCl2 and TiCl3, the total of the molar concentrations of metal ions (Mm) is determined by adding the molar concentration of magnesium ions, the molar concentration of sodium ions, the molar concentration of potassium ions, the molar concentration of calcium ions, the molar concentration of lithium ions, and the molar concentration of titanium ions (Mt). The ratio of the titanium ions can be calculated by dividing the molar concentration (Mt) of the titanium ions by the total molar concentration (Mm) of the metal ions and expressing it as a percentage.

During the electrodeposition step, the titanium ions in the molten salt bath Bf are consumed as metal titanium is electrodeposited onto the cathode 3b. On the other hand, in order to maintain the high concentration of titanium ions in the molten salt bath Bf as described above, it is preferable to use the anode 3a containing Ti in the electrodeposition step. In this case, as the electrolysis progresses, the anode 3a is consumed, and the Ti contained therein is converted to the titanium ions, which are fed into the molten salt bath Bf. This makes it easier to maintain the titanium ions in the molten salt bath Bf at the predetermined ratio.

The temperature of the molten salt bath Bf in the electrodeposition step is maintained at 510° C. or less, and preferably 500° C. or less, and more preferably 480° C. or less. If the temperature of the molten salt bath Bf is too high, crystal grains of the metal titanium electrodeposited onto the cathode 3b are likely to coarsen, and dendrite growth may proceed. If the molten salts forming the molten salt bath Bf can be maintained in a molten state and electrolysis using the molten salt bath Bf is possible, the temperature of the molten salt bath Bf can be sufficiently lowered.

Further, when conducting the current to the electrodes 3, the current density is preferably 0.10 A/cm2 or more and 1.0 A/cm2 or less, and more preferably 0.10 A/cm2 or more and 0.50 A/cm2 or less, and even more preferably 0.20 A/cm2 or more and 0.50 A/cm2 or less. By setting the current density to such a relatively high value, the metal titanium is electrodeposited onto the cathode 3b in a short period of time. Moreover, even with the high current density as described above, in this embodiment, the metal titanium can be easily peeled off from the cathode 3b. If the current density is less than 0.10 A/cm2, an amount of metal titanium electrodeposited onto the cathode 3b per unit time decreases, resulting in a decrease in an efficiency of titanium foil production. If the current density is higher than 1.0 A/cm2, there is a risk that the metal titanium cannot be easily peeled off from the cathode 3b. It should be noted that when the current varies during electrolysis in the electrodeposition step, the above current density means an average value from the start to the end of electrolysis.

In the electrodeposition step according to this embodiment, the continuous stop time of current conduction to the electrodes 3 (that is, the time during which the current does not flow continuously) is set to less than 1.0 second to provide a sufficient short continuous stop time of current conduction to the electrodes 3, or the current continuously flows without stopping the current conduction. Depending on the embodiments, the current may continue to flow without stopping the current conduction to the electrodes 3. When the continuous stop time of the current conduction to the electrodes 3 is set to less than 1.0 second, the amount of the metal titanium electrodeposited onto the cathode 3b per unit time can be satisfactorily increased. Even if the continuous stop time of the current conduction is provided, the continuous stop time of the current conduction is preferably very shorter than the current conduction time, for example, the ratio of the total continuous stop time to the electrodeposition time for electrolytically depositing the metal titanium onto the electrolytic surface of the cathode is 20% or less. In addition, it should be noted that the stop of the current conduction as used herein means that the forward current for electrolysis for electrodepositing the metal titanium onto the cathode is stopped. Therefore, even if the reverse current flows during at least a part of the stop time of the current conduction, the forward current does not flow during that time, which thus corresponds to the stop of the current conduction.

Particularly preferably, the current conduction to the electrodes 3 is not stopped, and a constant current that does not significantly change the current value or current density is used. Also in this case, the current density is preferably in the range described above.

In addition to the conditions described above, the time for electrodepositing the metal titanium onto the electrolytic surface of the cathode is 120 minutes or less. This allows the production efficiency of the titanium foil to be improved, and the formation of dendrites in the metal titanium on the cathode 3b to be suppressed, so that the smoothness of the titanium foil can be improved. In the electrolytic device 11 shown in FIG. 3 and the electrolytic device 21 shown in FIG. 4, the electrolytic surfaces of the cathodes 13b, 23b move. In this case, the time for electrodepositing the titanium metal onto the predetermined surface positions serving as the electrolytic surfaces of the cathodes 13b, 23b may be 120 minutes or less, and the time after the electrolytic surfaces move and electrodeposition of the titanium metal onto another surface positions begins is not included in the electrodeposition time on the predetermined surface portions. The time for electrodepositing the metal titanium onto the electrolytic surface of the cathode is preferably 80 minutes or less, and more preferably 60 minutes or less.

By adjusting the various conditions as described above, the metal titanium electrodeposited onto the cathode 3b in the electrodeposition step can be easily peeled off from the cathode 3b. The peeling as used herein means that the metal titanium is physically peeled off from the cathode 3b without using leaching or the like.

For example, even if the electrolytic surface on the surface of the cathode 3b is set to 78 cm2 or more and a foil-shaped titanium metal having a relatively large size is deposited, the metal titanium can be satisfactorily peeled off from the surface of the cathode 3b. Furthermore, even if the electrolytic surface on the surface of the cathode 3b is set to 500 cm2 or more, good peelability may be ensured.

Each of surface areas of the front and back surfaces of the titanium foil obtained by peeling-off from the cathode 3b may be 78 cm2 or more, and even 500 cm2 or more. The titanium foil preferably has an average thickness of 10 μm to 1000 μm, and more preferably 50 μm to 500 μm. To calculate the average thickness of the titanium foil, a cross section in the thickness direction along one side of the foil is observed with an optical microscope at magnifications of 100, the thicknesses are determined at 10 points, and an average value thereof is determined to be the average thickness of the titanium foil. It should be noted that the titanium metal on the cathode 3b tends to become thicker as the electrodeposition time is longer.

Further, here, since the titanium foil is produced by depositing the metal titanium onto the cathode 3b by electrolysis as described above, the contents of oxygen and iron that can be contained in the titanium foil can be lower than those contained in the titanium raw material such as the anode 3a. For example, in the titanium foil produced according to this embodiment, the oxygen content can be reduced to 400 ppm by mass or less. The oxygen content can be measured by an inert gas fusion method.

EXAMPLES

Next, the method for producing the titanium foil according to the present invention was experimentally carried out, and effects thereof were confirmed as described below. However, descriptions herein are merely for illustration, and are not intended to be limited thereto.

Using the electrolytic device shown in FIG. 2, electrolysis was performed in a molten salt bath by applying an electric current to the anode and the cathode. The dimensions and shape of the bath portion of the electrolytic device were 500 mmϕ×800 mm in depth. The molten salt bath used lower titanium chlorides (titanium dichloride and titanium trichloride) as the titanium raw material, and the ratio of the molar concentration of the titanium ions to the total molar concentration of the metal ions in the molten salt bath was maintained at 6 to 10%, the values shown in Table 1, and the balance was NaCl, KCl and MgCl2. A JIS class 2 titanium plate (having a thickness of 6 mm) was used as the anode. The cathode had a molybdenum plate having a thickness of the outermost layer of 0.2 mm, which was formed into a cylindrical shape having an inner diameter (diameter) of 96 mm. In Example 5, the material of the outermost surface of the cathode was changed from molybdenum to tantalum. The cathode was placed on an inner side of the cylindrical anode in the electrolytic bath of the electrolytic device. Here, the height directions of the anode and the cathode were substantially parallel to the depth direction of the molten salt bath, and the central axes of the anode and the cathode were at the same position. As a result, the distance between the electrodes was constant over the entire circumference of the anode and cathode.

The conditions were changed as shown in Table 1, and the electrodeposition step was carried out for Examples 1 to 6 and Comparative Examples 1 to 4 to deposit relatively large foil-shaped metal titanium onto the surface of the cathode. During the electrodeposition step, a constant current was passed through the electrode without stopping the current conduction for Examples 1 to 6 and Comparative Examples 1, 2 and 4. On the other hand, for Comparative Example 3, a pulse current was applied, the current density when the pulse current was ON was 0.18 A/cm2, the ON time was 1.5 seconds, and the current density was zero when the current was OFF (no current flowed), the OFF time was 1.5 seconds, and the average current density was 0.09 A/cm2. The “Electrodeposition time” in Comparative Example 3 is the total from the start to the end of the electrodeposition, so it includes the OFF time. Further, the temperature of the molten salt bath was maintained at each value shown in Table 1 during the electrodeposition step.

After the electrodeposition step, the cathode on which metal titanium was electrodeposited was pulled up from the molten salt bath. The metal titanium electrodeposited onto the cathode had the appearance shown in FIG. 5 for Example 1 and the appearance shown in FIG. 6 for Comparative Example 2.

Subsequently, the metal titanium on the cathode was washed with water to remove the molten salt adhering to its surface. Then, a peel strength test, which will be described later, was conducted. Table 1 shows the easy peelability at that time.

Compar- Compar- Compar- Compar- ative ative ative ative Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 1 ple 2 ple 3 ple 4 Bath Components NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl KCl KCl KCl KCl KCl KCl KCl KCl KCl KCl MgCl2 MgCl2 MgCl2 MgCl2 MgCl2 MgCl2 MgCl2 MgCl2 MgCl2 MgCl2 Ratio (%) of molar 10 10 8 10 10 10 6 10 6 10 concentration of titanium ions to total molar concentration of metal ions in molten salt bath Temperature (° C.) 500 480 500 500 500 500 500 500 500 700 of molten salt bath Current density 0.10 0.10 0.10 0.50 0.10 0.10 0.10 0.10 0.09 pulse 0.10 (A/cm2) Electrodeposition 60 60 60 12 60 60 60 180 133 60 time (min) Anode structure Ti pipe Ti pipe Ti pipe Ti pipe Ti pipe Ti pipe Ti pipe Ti pipe Ti pipe Ti pipe Cathode surface Mo Mo Mo Mo Ta Mo Mo Mo Mo Mo Electrodeposition 700 area (cm2) Distance between 3 3 3 3 3 1.7 3 3 1.7 1.7 electrodes (cm) Easy peelability ∘ ∘ Δ Δ ∘ ∘ x x Δ x Dendrite number density ∘ ∘ Δ Δ ∘ ∘ Δ x ∘ x (Number of dendrites/cm2) Thickness (μm) of 90 90 70 20 90 100 60 150 90 20 metal titanium Electrodeposition evaluation ∘(90 μm) ∘ Δ ∘(100 μm) ∘ ∘ Δ x x x of metal titanium

The easy peelability was determined by the peel strength test to determine whether it was “circle”, “triangle” or “X”. The “circle” means that the peel strength was 0.2 N/mm or less, the “triangle” means that the peel strength was more than 0.2 N/mm and 1.0 N/mm or less, the “X” means that the peel strength was more than 1.0 N/mm. The evaluations as “circle” and “triangle” are acceptable, and the evaluation as “circle” means that the evaluation is better. The evaluation as “X” is unacceptable.

The peel strength test was conducted as shown in FIG. 7. First, a sample 103 having 70 mm×10 mm was cut out from the metal titanium electrodeposited onto the cathode and the cathode with a cutter or the like. The sample 103 was then placed on a stage 111 of a 90° peeling tester, 10 mm of the titanium metal 101 was peeled off from the cathode 102 at one end of the sample 103, and the peeled portion of the titanium metal 101 was clamped with a chuck. Each of the cathode 102 at one end of the sample 103 on the stage 111 and the other end of the sample 103 located on the side opposite to the one end was then fixed with a fixing jig 112. Subsequently, as indicated by the arrow in FIG. 7, the chuck was lifted vertically upward at 20 mm/sec, and the stage 111 was moved horizontally at 20 mm/sec. From the load measured at this time, the peel strength was determined using the equation: peel strength=average load (N)/width of metal titanium foil (mm). Here, the average load means an average value of the loads acting on the chuck in the vertical direction while the stage 111 is horizontally displaced by 20 mm from 5 mm to 25 mm. The width of the titanium metal means the width of the metal titanium 101 on the stage 111 along a direction orthogonal to the moving direction of the stage 111 (the depth direction of the paper surface of FIG. 7). An angle formed by the peeling direction of the metal titanium 101 and the surface of the cathode 102 was 90° as measured from the surface of the cathode. As the peeling tester, a digital force gauge ZTS-200N (measurable load: 200 N) and a slide table P90-200N for 90 degree peel test manufactured by Imada Co., Ltd. were used.

Moreover, in Table 1, the dendrite number density is obtained by measuring the number of dendrites per unit area. Specifically, using a scanning electron microscope (SEM), the number of dendrites present on the surface of the titanium metal on the cathode was measured for each of five fields of view at magnifications of 50, and an average value of the numbers of dendrites in these five fields of view was converted to the number per 1 cm2 (rounded off to the first decimal place). The “circle” means that the dendrite number density was less than 1/cm2, the “triangle” means that the dendrite number density was 1/cm2 or more and less than 2/cm2, and the “X” means that the dendrite number density was 2/cm2 or more. The evaluation as the “circle” and “triangle” are acceptable, and the evaluation as “circle” means that the evaluation is better, and the evaluation as “X” is unacceptable.

In Table 1, the amount of metal titanium electrodeposited was evaluated from the results obtained by converting the thickness of the metal titanium electrodeposited onto the cathode per 60 minutes of electrodeposition time. The “circle” means that the thickness of the metal titanium per 60 minutes of electrodeposition time was 80 μm or more, and the “triangle” means that the thickness of the metal titanium per 60 minutes of electrodeposition time was 60 μm or more and less than 80 μm, and the “X” means that the thickness of metal titanium per 60 minutes of electrodeposition time was less than 60 μm. The evaluation as the “circle” and “triangle” are acceptable, and the evaluation as “circle” means that the evaluation is better. The evaluation as “X” is unacceptable.

It is found from Table 1 that in Examples 1 to 6, the metal titanium was easily peeled off from the cathode, and the metal titanium was electrodeposited in a sufficiently high thickness per unit time of electrodeposition. On the other hand, in Comparative Examples 1 to 4, there were cases where the peeling of the metal titanium from the cathode was not easy, and cases where the thickness of the metal titanium electrodeposited per unit time of electrodeposition was thinner. Also, it can be said that Examples 1 to 6 generally suppressed the formation of dendrites on the cathode.

Therefore, according to the present invention, it was found that the amount of metal titanium electrodeposited per unit time can be increased without greatly deteriorating the easy peelability of the metal titanium electrodeposited on the cathode.

DESCRIPTION OF REFERENCE NUMERALS

    • 1, 11, 21, 31 electrolytic device
    • 2, 12, 22, 32 electrolytic bath
    • 3, 13, 23, 33 electrode
    • 3a, 13a, 23a, 33a anode
    • 3b, 13b, 23b, 33b cathode
    • 4, 34 power supply
    • 15, 25 winding roll
    • 26a, 26b rotating roll
    • 101 metal titanium
    • 102 cathode
    • 103 sample
    • 111 stage
    • 112 fixing jig
    • Bf molten salt bath
    • TS metal titanium

Claims

1. A method for producing a titanium foil, the method comprising:

an electrodeposition step of performing electrolysis with electrodes including an anode and a cathode using a molten salt bath comprising titanium ions and having at least one molten chloride to deposit metal titanium onto an electrolytic surface of the cathode,
wherein the electrodeposition step comprises maintaining a ratio of a molar concentration of titanium ions to a total molar concentration of metal ions in the molten salt bath at 7% or more, and maintaining a temperature of the molten salt bath at 510° C. or less, and conducting a current to the electrodes under conditions where a continuous stop time of current conduction is less than 1.0 second, a current density is 0.10 A/cm2 or more and 1.0 A/cm2 or less, and a time for electrodepositing the metal titanium onto the electrolytic surface of the cathode is 120 minutes or less.

2. The method for producing a titanium foil according to claim 1, wherein the anode comprises Ti and the anode is consumed in the electrodeposition step.

3. The method for producing a titanium foil according to claim 1, wherein the chloride comprises titanium dichloride and/or titanium trichloride.

4. The method for producing a titanium foil according to claim 1, wherein, in the electrodeposition step, the ratio of the molar concentration of titanium ions to the total molar concentration of metal ions in the molten salt bath is maintained at 10% or more.

5. The method for producing a titanium foil according to claim 1, wherein, in the electrodeposition step, the temperature of the molten salt bath is maintained at 500° C. or less.

6. The method for producing a titanium foil according to claim 1, wherein, in the electrodeposition step, the current density is 0.20 A/cm2 or more and 0.50 A/cm2 or less.

7. The method for producing a titanium foil according to claim 1, wherein the molten salt bath does not comprise fluoride ions.

8. The method for producing a titanium foil according to claim 1, wherein a surface of the anode and an opposing surface of the cathode have similar shapes to each other.

9. The method for producing a titanium foil according to claim 1, wherein a distance between the electrodes of the anode and the cathode is 0.5 cm or more and 10.0 cm or less.

Patent History
Publication number: 20240084469
Type: Application
Filed: Feb 22, 2022
Publication Date: Mar 14, 2024
Inventors: Takumi Kaneko (Kanagawa), Yuta Nakajo (Kanagawa), Daisuke Suzuki (Kanagawa), Matsuhide Horikawa (Kanagawa), Hideki Fujii (Kanagawa)
Application Number: 18/275,509
Classifications
International Classification: C25D 1/04 (20060101); C25D 3/66 (20060101);