Method for Producing Titanium-Based Electrolytic Raw Material and Method for Producing Metallic Titanium or Ti-Al Alloy

Abstract

Provided are a method for producing a titanium-based electrolytic raw material with relatively low Al and O contents while suppressing or eliminating the use of calcium fluoride and potassium perchlorate, and a method for producing pure metallic titanium or TiAl alloy. The method for producing a titanium-based raw material for electro-refining according to the present invention is a method for producing a titanium-based raw material for electro-refining used for molten salt electro-refining to obtain pure metallic titanium or TiAl alloy, the method comprising: a reaction step of bringing a titanium compound, at least a part of the titanium compound containing titanium oxide, into contact with, in melt, pure metal and/or alloy of aluminum as a reducing agent, and a melting accelerator, and causing reactions including deoxidation of a part of O in the titanium oxide to obtain a titanium alloy product comprising Al and O, wherein the melting accelerator comprises calcium oxide (CaO), and a content of calcium oxide in the melting accelerator is 80% by mass or higher.

Claims

1. A method for producing a titanium-based raw material for electro-refining used for molten salt electro-refining to obtain pure metallic titanium or TiAl alloy, the method comprising: a reaction step of bringing a titanium compound, at least a part of the titanium compound containing titanium oxide, into contact with, in melt, pure metal and/or alloy of aluminum as a reducing agent, and a melting accelerator, and causing reactions including deoxidation of a part of O in the titanium oxide to obtain a titanium alloy product comprising Al and O, wherein the melting accelerator comprises calcium oxide (CaO), and a content of the calcium oxide in the melting accelerator is 80% by mass or higher.

2. The method for producing a titanium-based raw material for electro-refining according to claim 1, wherein calcium fluoride (CaF.sub.2) and potassium perchlorate (KClO.sub.4) are not used in the reaction step.

3. The method for producing a titanium-based raw material for electro-refining according to claim 1, wherein in the reaction step, the temperature of the melt is 1450 C. or higher.

4. The method for producing a titanium-based raw material for electro-refining according to claim 1, wherein in the reaction step, the reducing agent consists of pure metal and/or the alloy of aluminum, and an Al content of pure metal and/or the alloy of aluminum is higher than 50% by mass.

5. The method for producing a titanium-based raw material for electro-refining according to claim 1, wherein in the reaction step, the titanium compound comprises TiO.sub.2-x (0x<1) and/or a titanate compound.

6. The method for producing a titanium-based raw material for electro-refining according to claim 1, wherein, when producing the melt, the reaction step comprises melting the titanium compound and the melting accelerator, and then adding pure metal and/or the alloy of aluminum.

7. A method for producing pure metallic titanium or TiAl alloy, the method comprising: electro-refining of dissolving a crude titanium-based material, which is a consumable anode raw material, in a molten salt bath, and depositing a purified titanium-based material on a cathode, wherein the titanium-based raw material for electro-refining produced by the method for a titanium-based raw material for electro-refining according to claim 1 is used for the consumable anode raw material as the crude titanium-based material.

8. The method for producing pure metallic titanium or TiAl alloy according to claim 7, wherein the molten salt bath is a chloride bath.

9. The method for producing pure metallic titanium or TiAl alloy according to claim 7, wherein the chloride bath comprises magnesium chloride and titanium dichloride.

Description

DESCRIPTION OF EMBODIMENTS

[0032] Embodiments of the present invention will be described below in detail.

[0033] The method for producing a titanium-based raw material for electro-refining according to an embodiment of the present invention is a method for producing a titanium-based raw material used for molten salt electro-refining to obtain pure metallic titanium or TiAl alloy. The method for producing a titanium-based raw material for electro-refining includes a reaction step.

[0034] In the reaction step, a titanium compound, at least part of which contains titanium oxide, is brought into contact with pure metal and/or alloy of aluminum, and a melting accelerator in a melt to cause reactions to obtain a titanium-based raw material for electro-refining. It should be noted that the melt preferably does not contain impurity oxides such as molybdenum oxide, vanadium oxide, and niobium oxide. In this case, the melting accelerator should contain calcium oxide (Cao), and a content of calcium oxide in the melting accelerator should be 80% by mass or higher. As a result, some of the O (oxygen) in the titanium oxide is deoxidized to produce various reaction products, and titanium alloy products containing Al and O (also simply referred to as titanium alloy product) in the melt as the reaction products, and a molten slag as its remaining substance or residue are obtained.

[0035] It is believed that by including the melting accelerator containing calcium oxide in the melt in the reaction step as described above, first, the reaction of calcium oxide with titanium oxide leads to the synthesis of calcium titanate, and causes eutectic melting between calcium titanate and CaO or TiO.sub.2, to produce a titanate compound (a compound containing titanium oxide as a component) in a molten state (i.e., the melt), and then Al deoxidizes the titanium oxide in the titanate compound to produce a titanium alloy product, which will be described below in detail. As a result, the titanium alloy product has a low Al content and a low O content.

[0036] This also allows the amount of calcium fluoride (CaF.sub.2) or potassium perchlorate (KClO.sub.4) typically used to be decreased, or the use of calcium fluoride or potassium perchlorate (KClO.sub.4) to be eliminated. It should be noted that when a large amount of calcium fluoride is used, for example, F in calcium fluoride combines with Al during the reaction to generate an aluminum monofluoride (AlF) gas, and when the gas is cooled, aluminum fluoride (AlF.sub.3) or similar compounds may be generated. Further, when potassium perchlorate (KClO.sub.4) is used, there is a possibility that aluminum trichloride (AlCl.sub.3) and similar compounds may be generated. Since such fluorides and chlorides are a cause for concern regarding their effects on the human body, they should be collected and treated to prevent them from leaking into the atmosphere, which requires equipment and work to perform these tasks. In addition, when such calcium fluoride or potassium perchlorate is used, Al is consumed in the production of aluminum fluoride or aluminum chloride, so that large amounts of aluminum and/or alloy containing the aluminum including the consumed amount are required. Also, when mainly calcium fluoride or potassium perchlorate is used, the Al content and the O content of the titanium alloy product obtained by the reaction may not be reduced significantly.

[0037] The titanium alloy product thus obtained can be used as a raw material for molten salt electro-refining, i.e., as the titanium-based raw material for electro-refining, and it can be used in the production of pure metallic titanium or TiAl alloy by molten salt electro-refining. In electro-refining, the titanium-based raw material described above is used as a consumable anode raw material. This crude titanium-based material is a raw material for molten salt electro-refining, in which an electric voltage is applied between the anode and the cathode, thereby depositing a purified titanium-based material on the cathode surface. At this time, the crude titanium-based material as the titanium-based raw material for electro-refining has a lower electrical resistance because it has a lower Al content and O content, so that power consumption during electro-refining can be maintained at a lower level.

[0038] Also, during the electro-refining process, the Al content and the O content of the crude titanium material gradually increase as Ti is removed, and prevents electric current from flowing. But when the Al content and the O content before electro-refining are lower, it will take a longer period of time until the current does not flow. Therefore, a larger amount of Ti is removed from the crude titanium-based material and deposited on the cathode than when using a crude titanium-based material with a higher Al content and a higher O content, so that the yield can be increased. In addition, when performing electro-refining to obtain pure metallic titanium having a higher purity, it is also possible to reduce the number of electro-refining steps performed, which is expected to significantly reduce the power consumption of the entire production.

[0039] It should be noted that this method for producing the titanium metal or the TiAl alloy by electro-refining reduces the amount of carbon used and the resulting carbon dioxide emissions compared to the method based on the Kroll process of chlorinating titanium-bearing ore, so that it can greatly contribute to the realization of carbon neutrality and, hence, a decarbonized society.

(Production of Titanium-Based Raw Material for Electro-Refining)

[0040] In the production of the titanium-based raw material for electro-refining, a reaction step is performed. If necessary, a separation step, a remelting step, and further, a casting step may be then performed in this order. However, at least one of the steps (separation step, the remelting step, and the casting step) may be omitted. Especially, in the embodiments described herein, the temperature of the melt during the reaction step may not be so high, and in this case, the titanium alloy product may form a solid phase, which may be separated from the slag, during the reaction step, without the separation step.

[0041] The reaction step is carried out by bringing a titanium compound, at least a part of which contains titanium oxide, into contact with pure metal and/or at least one alloy of aluminum, and a melting accelerator, in a melt. By melting a mixture containing the titanium compound, pure metal and/or the alloy of aluminum, and the melting accelerator, the melt containing them can be obtained. The reaction takes place in the melt.

[0042] When producing the melt, the order of melting the titanium compound, pure metal and/or the alloy of aluminum, and the melting accelerator is not particularly limited, but from the viewpoint of suppressing loss due to evaporation of Al, it is preferable to melt the titanium compound and the melting accelerator, and then add pure metal and/or the alloy of aluminum. It should be noted that the heating of the titanium compound and the melting accelerator may cause a solid phase reaction between the titanium oxide and calcium oxide contained therein even before melting them to generate calcium titanate, which will be described below.

[0043] In the melt containing the titanium compound (particularly titanium dioxide (TiO.sub.2)), pure metal and/or the alloy of aluminum, and the melting accelerator, it is presumed that reactions of the following formulae (1) or (1), (2) or (2) and (3) take place to provide a titanium alloy product containing Al and O(TiAlO):

##STR00001##

[0044] More particularly, it is believed that, first, as shown in the above formula (1) or (1) , titanium oxide of the titanium compound reacts with calcium oxide of the melting accelerator to generate calcium titanate (CaTio.sub.3, Ca.sub.3Ti.sub.2O.sub.7), and then, as shown in the formula (2) or (2) described above, eutectic melting occurs between the calcium titanate and unreacted titanium oxide or Cao to produce a melt containing the molten Ti, Ca, and O. The titanium oxide may have a melting point of about 1870 C., but due to the eutectic, it can be in the molten state at a lower temperature (for example, about 1450 to 1750 C.).

[0045] It is presumed that, then, pure metal and/or the alloy of aluminum acts as a reducing agent, and as shown in the above formula (3), deoxidation occurs for a part of the O in the titanium oxide in the melt to produce a titanium alloy product (TiAlO). In this case, alumina (Al.sub.2O.sub.3), a composite oxide composed of Al.sub.2O.sub.3 and CaO, and similar compounds are produced as slag. It is expected that the production of such alumina and composite oxide, in particular the composite oxide, significantly reduces the Al content and the O content in the titanium alloy product, and further lowers the melting points of the substances making up the melt.

[0046] The temperature of the melt in the reaction step is not particularly limited as long as the mixture containing the titanium compound, pure metal and/or the alloy of aluminum, and the melting accelerator can be maintained in the molten state. As described above, the eutectic between calcium titanate and titanium oxide or calcium oxide allows the titanium oxide to be melted at a temperature lower than its melting point. For this reason, the heating temperature may be, for example, 1450 C. or higher, although it depends on the composition of the melt.

[0047] From the viewpoint of promoting the reaction, a higher heating temperature is preferable, and it is desirable that it is 1800 C. or higher.

[0048] When the temperature of the melt is a high temperature of, for example, 1800 C. or higher, some calcium oxide would take O from titanium oxide to form calcium peroxide (CaO.sub.2), which would be evaporated. It allows the deoxidation of the titanium oxide to progress further to produce a titanium alloy product with a lower O content. When the above calcium peroxide is cooled after evaporation, it is decomposed into calcium oxide (CaO) and oxygen (O.sub.2), so that the gas containing calcium peroxide generated during the reaction can be cooled to recover calcium oxide therefrom. The calcium oxide, thus recovered, can be used again in the reaction step.

[0049] It is also believed that when the temperature of the melt is increased, a composite oxide composed of Al.sub.2O.sub.3 and Cao is generated in many regions in the melt. It leads to the deoxidation of titanium oxide in many regions, and the Al content and the O content of the titanium alloy product can be further reduced accordingly.

[0050] On the other hand, the heating of the melt to an excessively high temperature leads to the loss of thermal energy and an increase in heating costs. In view of this, the temperature of the melt may be 2500 C. or lower.

[0051] If the temperature of the melt is maintained below the melting point of the titanium alloy product, the titanium alloy product will be in a solid phase after it is formed. On the other hand, the slag having a melting point lower than that temperature is maintained in a liquid phase. Since the solid phase titanium alloy has a higher specific gravity than that of the slag, it settles on the lower side to be deposited. Therefore, the solid-phase titanium alloy can be separated from the slag, such as by pouring the slag into another container and removing it, or by removing the solid-phase titanium alloy product from the slag. In this case, the separation step as described below becomes unnecessary.

[0052] Here, at least a part of the titanium compound used in the reaction step may contain titanium oxide. When the embodiment described herein is applied to the smelting of titanium-bearing ore, the titanium-bearing ore containing titanium oxide can be used as the titanium compound. Examples of the titanium-bearing ore include natural rutile, and upgraded ilmenite (UGI) or upgraded slag (UGS), which has been subjected to leaching or other upgrading treatments as necessary, and similar processes. The content of TiO.sub.2 in the titanium-bearing ore may be, for example, 50% by mass or higher, typically 80% by mass or higher, particularly 90% by mass or higher. In addition to or in place of such titanium-bearing ore, titanium oxide scrap, pure metallic titanium or titanium alloy scrap, or similar materials, may be included. Titanium oxide is composed of Ti and O, and is represented by Ti.sub.pO.sub.q, and includes TiO.sub.2, TiO.sub.2-x (0x<1), Tio, TiO.sub.1-x (0x<1), and the like.

[0053] The titanium compound may include a titanate compound containing an alkali metal or an alkaline earth metal in addition to Ti and O. The titanate compound as used herein is a titanium compound including titanium oxide as a component, which contains other elements (M) in addition to Ti and O, and is represented by M.sub.xTi.sub.yO.sub.z (x>0, y>0, z>0). Specific examples of the titanate compound include calcium titanate (CaTiO.sub.3, Ca.sub.3Ti.sub.2O.sub.7), which is a composite oxide of Ca and Ti, and magnesium titanate (MgTiO.sub.3, MgTi.sub.2O.sub.5), which is a composite oxide of Mg and Ti. When magnesium titanate is used in the reaction step, a reaction similar to that of the above formula (1) or (1) would occur with respect to titanium oxide in the magnesium titanate. It should be noted that in a narrow sense, only CaTio.sub.3 may be referred to as calcium titanate. However, as used herein, not only CaTio.sub.3 but also multiple types of composite oxides of Ca and Ti, including Ca.sub.3Ti.sub.2O.sub.7 and similar compounds, are collectively referred to as calcium titanate herein.

[0054] Further, pure metal and/or the alloy of aluminum are/is used as the reducing agent in the reaction step. Specifically, metallic aluminum, aluminum alloy scrap, or similar materials can be used as pure metal and/or the alloy of aluminum. The reducing agent refers to a material that directly reduces the titanium compound, such as titanium oxide, during the reaction in the reaction step. The Al content of the pure metal and/or the alloy of aluminum is preferably more than 50% by mass, and more preferably 70% by mass or higher, and particularly 80% by mass or higher.

[0055] Further, the melting accelerator forms a part of the titanate compound in the early stage of the reaction step to promote melting of the titanium oxide, and makes up a part of the melt. Here, the melting accelerator contains calcium oxide, and the content of calcium oxide in the melting accelerator is 80% by mass or higher. The content of calcium oxide in the melting accelerator can be preferably 85% by mass or higher, further 90% by mass or higher, and even 95% by mass or higher. The use of the melting accelerator containing mainly calcium oxide provides titanium alloy products with significantly lower Al and O contents.

[0056] In addition to calcium oxide, the melting accelerator may also contain calcium fluoride and/or potassium perchlorate, but calcium fluoride and/or potassium perchlorate may be undesirable, as described above. Therefore, it is preferable that the contents of calcium fluoride and potassium perchlorate in the melting accelerator are lower. Specifically, the total content of calcium fluoride and potassium perchlorate in the melting accelerator is preferably 15% by mass or lower, further preferably 10% by mass or lower, and even further preferably 5% by mass or lower. Furthermore, even if the melting accelerator does not contain calcium fluoride or potassium perchlorate, the reaction proceeds satisfactorily with calcium oxide. For this reason, it is particularly preferable to use a melting accelerator that does not contain calcium fluoride and potassium perchlorate, and not to use calcium fluoride and potassium perchlorate in the reaction step.

[0057] The ratio of the titanium compound, the reducing agent, and the melting accelerator used in the reaction step is not particularly limited, and it may be determined as needed in view of the amount of titanium-based raw material for electro-refining to be produced, its impurity content, the temperature of the reaction step, and the like.

[0058] In the reaction step, a water-cooled copper crucible, a water-cooled copper alloy crucible, a graphite crucible, a calcia (CaO) crucible, or the like can be used as a container for holding the melt. Preferably, the water-cooled copper crucible or the water-cooled copper alloy crucible is used, and the reaction takes place in the melt inside the crucible, with a thin-wall skull formed due to the solidification of a part of the melt being present just on the inner surface of the crucible. Further, in the reaction step, it is preferable to use an inert gas atmosphere such as an argon gas or a helium gas, rather than a vacuum atmosphere, in order to suppress evaporation of aluminum. This atmosphere may be in a pressurized state (1 atmosphere or more, or 0.1 MPa or more). More specifically, an arc melting method, a plasma arc melting method, or an induction skull melting method in an inert gas atmosphere can be used. It should be noted that substances that have reacted in the reaction step may contain many substances that are non-conductive at a lower temperature such as room temperature. Therefore, when using the above described melting methods, at least the pure metal and/or the alloy of aluminum may be heated, and a mixture containing pure titanium, a titanium alloy, or similar materials may be added and heated, from the viewpoint of ensuring electric conductivity when the melt is obtained. Pure titanium or one or more titanium alloys that may be used herein include those obtained using pure metallic titanium or the TiAl alloy produced by the production method according to the present invention, and those produced by another method (for example, titanium scrap, more specifically titanium scrap contaminated with O, Fe, etc.).

[0059] At the end of the reaction step, the separation step may be carried out to separate a titanium alloy product, which is one of the reaction products in the melt, from the slag, if necessary. Here, to separate the titanium alloy product sufficiently, a difference between the melting points of the titanium alloy product and the slag is utilized. Specifically, when the temperature of the melt is 1800 C. or higher, the temperature of the melt is decreased to a temperature lower than 1800 C. This results in a solid phase of the titanium alloy product containing mainly Ti, Al and O, whereas the slag containing Cao, Al.sub.2O.sub.3 and CaO Al.sub.2O.sub.3 composite oxide is maintained in a liquid phase. The solid phase titanium alloy product has a higher specific gravity than that of the liquid phase slag, so that the former settles and accumulates on the lower side. Alternatively, if the temperature at the bottom of the container is lower, the solid-phase titanium alloy product is preferentially formed at the bottom. Then, the slag of the melt is removed by pouring it into another container or by taking out the solid-phase titanium alloy product in the melt, to obtain a solid titanium alloy product that has been separated from the liquid slag. Alternatively, both the titanium alloy product and the slag, which are separated in the container, may be taken out from the container, and then the titanium alloy product and the slag may be separated with a sieve using the difference in specific gravity or in size.

[0060] The temperature of the melt to be decreased in the separation step may be at least a temperature at which the molten state of the slag is maintained and at most 1800 C. For example, the temperature after the melt is decreased may be 1400 C. or higher. The time period during which such a temperature is maintained may be, for example, 3 minutes or longer, further 5 minutes or longer, or preferably 60 minutes or shorter in view of productivity. However, if the solid-phase titanium alloy product settles to be separated from the liquid-phase slag during cooling by slowing down the cooling rate or the like, it may not be necessary to maintain it at a temperature higher than the temperature at which the molten state of the slag is maintained. For example, if the solid-phase titanium alloy product is separated from the liquid-phase slag while the slag is at a temperature higher than or equal to that at which the slag remains in the molten state during slow cooling, it may be decreased to a lower temperature (e.g., room temperature).

[0061] When the temperature of the melt is 1800 C. or lower, the titanium alloy product is produced as a solid phase during the reaction and is separated by sedimentation. Therefore, in this case, there is no need to carry out the separation step.

[0062] In the separation step, it is preferable to use an induction skull melting method that easily maintains the above temperature. Alternatively, if the temperature is not maintained in the separation step and the temperature is decreased at a slow rate, it may be preferable to use a plasma arc melting method. The separation step is also preferably performed in an inert gas atmosphere.

[0063] Although the plasma arc melting method can easily perform high-temperature heating, it may be difficult to control the temperature range of 1800 C. or lower. Further, although the induction skull melting method has difficulty in heating and melting the substance to be reacted in the reaction step, it is easy to stably control the temperature at 1800 C. or lower. From the above point of view, the reaction step may be performed using the plasma arc melting method, and the separation step may be performed using the induction skull melting method.

[0064] The solid titanium alloy product separated from the slag in the separation step may be melted again by heating in a reduced-pressure atmosphere in a remelting step, if necessary. When the remelting step is performed, some of the Al and O that may be contained in the titanium alloy product are evaporated and removed in the form of atomic aluminum and Al.sub.2O (aluminum suboxide), so that the Al content and the O content of the titanium-based raw material for electro-refining can be further reduced.

[0065] In the remelting step, the degree of vacuum is preferably 110.sup.3 Pa to 1 Pa in order to evaporate aluminum and Al.sub.2O well. Further, the remelting step is not particularly limited as long as the temperature is such that the titanium alloy product is melted, but the temperature may be 1800 C. to 2500 C. Furthermore, the time period during which the molten metal is maintained may be, for example, 5 minutes to 1 hour. In the remelting step, it is preferable to use a high vacuum electron beam melting method or the induction skull melting method in order to effectively remove many of the impurities as described above.

[0066] Furthermore, if necessary, a casting step may be performed after the remelting step. In the molten salt electro-refining, it is advantageous that the titanium-based raw material for electro-refining to be dissolved has a large surface area, and from this point of view, granular titanium-based raw materials for electro-refining may be used. On the other hand, the titanium-based raw material for electro-refining having low Al and O contents has higher toughness and may be difficult to crush in order to be used in the molten salt electro-refining. In such a case, the casting step is performed to form it into a predetermined shape, and the product can be used as it is as a consumable anode raw material in the molten salt electro-refining. When performing the casting step, the titanium alloy product melted in the above remelting step can be poured into a casting die or mold in the form of the anode while maintaining the molten state without cooling it to the melting point or lower.

[0067] The titanium-based raw material for electro-refining produced as described above contains Ti, Al, O, and inevitable impurities, and has electrical conductivity. The titanium-based raw material for electro-refining preferably has an Al content of 31% by mass or lower, 8% by mass or lower, further 5% by mass or lower, or further 3% by mass or lower, and an O content of 8% by mass or lower, further 5% by mass or lower, or even 3% by mass or lower. The Al content of the titanium-based raw material for electro-refining may be 0.5% by mass or higher, further 1.0% by mass or higher, particularly 2% by mass or higher, and the O content may be 0.3% by mass or higher, 0.5% by mass or higher, 1.0% by mass or higher, further 2.0% by mass or higher, particularly 2.5% by mass or higher. The specific resistance of the titanium-based raw material for electro-refining as measured at room temperature is, for example, 10 .Math.m to 150 .Math.m. The specific resistance values are measured at room temperature, and based on their magnitude relationship of the specific resistance values at room temperature, the magnitude relationship of the specific resistance values at a high temperature during molten salt electro-refining can be inferred.

(Production of Pure Metallic Titanium or TiAl Alloy)

[0068] In order to produce pure metallic titanium or the TiAl alloy, the molten salt electro-refining can be performed using the titanium-based raw material for electro-refining described above. In the molten salt electro-refining, an electrical voltage is applied between electrodes composed of an anode and a cathode while immersing the electrodes in a molten salt bath in an electrolytic cell. At this time, the titanium-based raw material for electro-refining is set on the anode side, and the titanium-based raw material for electro-refining is used as a crude titanium-based material for a consumable anode raw material. By applying the electrical voltage between the electrodes, Ti is removed from the crude titanium-based material of the consumable anode raw material, and is electrodeposited onto the cathode surface to deposit a purified titanium-based material.

[0069] The consumable anode raw material is not particularly limited as long as it contains the titanium-based raw material for electro-refining, which is a crude titanium-based material in electro-refining. For example, the plate-shaped titanium-based raw material for electro-refining obtained after casting in the above casting step can be used as the consumable anode raw material. Alternatively, a granular or powdered titanium-based raw material for electro-refining may be placed in a perforated cage-like container having many through holes and capable of conducting electricity. The perforated cage-like container may have an outer shape such as a plate-shape and cylindrical shape, and may be made of nickel, a nickel-based alloy, Hastelloy, or steel coated with nickel or a nickel-based alloy, and may have many through holes. The cathode that can be used herein may be made of titanium for at least its surface, and can be, for example, a titanium plate or a titanium rod made entirely of titanium. Although it is considered that a bipolar electrode is disposed between the anode and the cathode, the bipolar electrode may not be used.

[0070] The molten salt bath may be a chloride bath mainly containing a metal chloride, for example, it may contain an alkali metal chloride and/or an alkaline earth metal chloride in an amount of, for example, at least 70 mol %, further at least 90 mol %, even at least 95 mol %. Such a chloride bath is preferred because it is less corrosive, has less environmental impact, and is less expensive than a fluoride bath, a bromide bath and an iodide bath. In particular, when the chloride bath containing magnesium chloride (MgCl.sub.2) is used, it is possible to obtain a purified titanium-based material in which not only the O content but also the Al content are sufficiently reduced. The magnesium chloride content in the chloride bath is preferably 30 mol % or higher, further 50 mol % or higher, further 80 mol % or higher, further 85 mol % or higher, particularly 95 mol % or higher. The chloride bath may contain one or more metal chlorides selected from lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), rubidium chloride (RbCl), cesium chloride (CsCl), beryllium chloride (BeCl.sub.2), calcium chloride (CaCl.sub.2)), and strontium chloride (SrCl.sub.2) and barium chloride (BaCl.sub.2) in an amount of, for example, 70 mol % or lower, further 50 mol % or lower, further 20 mol % or lower, further 10 mol % or lower, further 5 mol % or lower.

[0071] Further, the molten salt bath may optionally contain titanium subchloride having a lower Ti valence than titanium tetrachloride, specifically titanium dichloride (TiCl.sub.2), titanium trichloride (TiCl.sub.3), or similar compounds. The content of Ti ions in the molten salt bath may preferably be 3 mol % or higher, more preferably 5 mol % or higher, still more preferably 6 mol % or higher, and even more preferably 10 mol % or higher, and preferably 20 mol % or lower.

[0072] In particular, when the molten salt bath is a chloride bath, the chloride bath preferably contains magnesium chloride and titanium dichloride. In this case, a purified titanium-based material with a further reduced Al content can be obtained, which is suitable for producing pure metallic titanium. In addition, in the chloride bath, a part of the titanium dichloride, as described above, may be converted to titanium trichloride or titanium tetrachloride and atomic Ti due to a disproportionation reaction.

[0073] The contents of the metal chlorides, the metal ions and the metal in the molten salt bath can be measured by ICP emission spectrometry or atomic absorption spectrometry. The content of Ti ions is determined as a percentage of the total content of the metal ions in the molten salt bath.

[0074] As conditions for electro-refining, for example, the temperature of the molten salt bath may be 450 C. to 900 C., and the initial current density at the cathode may be 0.01 A/cm.sup.2 to 3 A/cm.sup.2. The current density can be calculated by the equation: current density (A/cm.sup.2)=current (A)/macroscopic electrodeposition area (cm.sup.2). In addition to being capable of continuously passing the current through the electrodes, a pulse current may be passed through the electrodes. In the pulse current, a current-carrying stop period in which the current value is zero is provided, and the current-carrying period and the current-carrying stop period are alternately repeated. The maximum voltage between the electrodes may be, for example, 0.2 V to 3.5 V. During the electro-refining, the interior of the electrolytic cell is preferably maintained in an inert gas atmosphere such as argon.

[0075] The electro-refining can be repeated multiple times in order to further refine the resulting purified titanium-based material. When the electro-refining is performed multiple times, in each successive electro-refining, the purified titanium material deposited on the cathode surface in the previous electro-refining is used as a crude titanium-based material, and the crude titanium-based material is used as the consumable anode raw material. As a result, in the next electro-refining, a refined titanium-based material that has further removed impurities from the crude titanium-based material is deposited on the cathode surface. By performing electro-refining multiple times, it is also possible to produce high-purity pure metallic titanium containing substantially no impurities.

[0076] However, from the viewpoint of suppressing increases in energy consumption and costs, it is desirable to reduce the number of electro-refining operations. Since the titanium-based raw material for electro-refining produced by the method as described above has a reduced Al content and O content, it is possible to produce good pure metallic titanium or TiAl alloy even with a small number of electro-refining processes. It is preferable to perform electro-refining only once rather than twice or more times. However, even if the electro-refining is performed twice or more, it is possible to achieve the suppression of power consumption and the improvement of the yield due to the lower Al content and the lower O content of the titanium-based raw material for electro-refining.

[0077] When producing pure metallic titanium as described above, the pure metallic titanium finally obtained has an Al content of, for example, 0.1% by mass or lower, preferably 0.01% by mass or lower, and an O content of, for example, 0.2% by mass or lower, preferably 0.10% by mass or lower, and more preferably 0.05% by mass or lower. Alternatively, when producing the TiAl alloy, the TiAl alloy finally obtained has an Al content of, for example, 3% by mass or lower, preferably 2% by mass or lower, and an O content of, for example, 0.3% by mass or lower, preferably 0.15% by mass or lower.

Examples

[0078] Next, the method for producing a titanium-based raw material for electro-refining and the method for producing pure metallic titanium or TiAl alloy according to the present invention were 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.

(Test 1)

[0079] The reaction step and the separation step as described above were performed, and the titanium alloy product was separated from the slag in the separation step, and this was used as a titanium-based raw material for electro-refining.

[0080] Each titanium compound, reducing agent and melting accelerator shown in Table 1 were used in the reaction step. In Table 1, in each item of Titanium Compound, Reducing Agent and Melting Accelerator, each numerical value in the parentheses means the mass percentage of each substance in the titanium compound, the reducing agent, or the melting accelerator. The unit of Purity in Table 1 is % by mass. The items Heating Furnace and Crucible and Cooling Holding Furnace and Crucible in Table 2 indicate the types of furnaces and crucibles used in the reaction step, and A represents an induction heating furnace equipped with a calcia crucible (flowing Ar atmosphere), B represents an induction skull melting furnace equipped with a water-cooled copper crucible (flowing Ar atmosphere), and C represents a plasma arc melting furnace equipped with a water-cooled copper crucible (flowing Ar atmosphere or He atmosphere). The same is true for Table 3 of Test 2, which will be described below.

[0081] In Test 1, in the reaction step, all the blended raw materials containing the titanium compound, the reducing agent, and the melting accelerator were charged into the furnace with a total weight of 10 kg, and then heated to the maximum melt temperature shown in Table 2 to cause the reaction in the melt produced by melting them. In each of Comparative Examples 1-1 to 1-3 and Examples 1-1 to 1-21, the ratio of the raw materials blended was titanium compound:reducingagent:melting accelerator=9:4:7 in a mass ratio, and in each of Examples 1-22 to 1-24, the ratio was titanium compound:reducingagent:melting accelerator=6:3:2 in a mass ratio, and in each of Comparative Examples 14 to 1-5 and Examples 1-25 to 1-26, the ratio was titanium compound:reducingagent:melting accelerator=4:5:2 in a mass ratio. In the following tables, the terms Ex. mean Examples and the terms Comp. mean Comparative Examples.

[0082] Subsequently, it was cooled to each cooling temperature shown in Table 2 to convert the titanium alloy product to a solid phase, which was separated from the liquid-phase slag and removed, and this titanium alloy product was used as a titanium-based raw material for electro-refining. In addition, since each of Comparative Example 1-1, Comparative Example 1-4, Example 1-3, Example 1-10, and Examples 1-22 to 1-25 was naturally cooled after the above heating, the item Cooling Holding Furnace and Crucible was represented as None, and the item Cooling Temperature was represented as -. Example 1-4 is an example in which the melt after the reaction was poured into another furnace and the temperature of the melt was then decreased, that is, different furnaces and crucibles were used for reaction and cooling. Table 2 also shows the Al concentration, the O concentration, and the Fe concentration of the titanium-based raw material for electro-refining thus obtained. The Al concentration and the Fe concentration were measured by ICP emission spectrometry using PS3520UVDDII manufactured by Hitachi High-Tech Corporation, and the O concentration was measured by inert gas fusion infrared absorption method using a model ON736 manufactured by LECO. In addition, the titanium-based raw material for electro-refining contained Al, O, and Fe, the balance other than Al, O and Fe being titanium and unavoidable impurities contaminated from the blended materials and the like.

[0083] As shown in Table 2, all the titanium-based raw materials for electro-refining according to Examples 1-1 to 1-24 had at least lower O concentrations than those of the titanium-based raw materials for electro-refining according to Comparative Examples 1-1 to 1-3, with keeping the Al concentrations lower than or equal to those in Comparative Examples 1-1 to 1-3, and had the sufficiently reduced Al and O concentrations. Examples 1-25 to 1-26 were cases where the amount of Al as the reducing agent was extremely high, and provided titanium-based raw materials for electro-refining containing the higher Al concentration, although the O concentration was lower. The titanium-based raw materials for electro-refining according to Examples 1-25 to 1-26 had lower Al and O concentrations than those of the titanium-based raw materials for electro-refining according to Comparative Examples 14 to 1-5. Since Examples 1-1 to 1-26 either did not use calcium fluoride or potassium perchlorate in the blended raw materials, or used a miner amount of calcium fluoride or potassium perchlorate even if it was used, substantially no aluminum chloride (AlF.sub.3) or aluminum trichloride (AlCl.sub.3) was generated.

TABLE-US-00001 TABLE 1 10 kg of Blended Raw Materials Melting Titanium Compound Reducing Agent Accelerator Comp. 1-1 Purity 95% Natural Rutile (100%) Al (100%) CaF (100%) Comp. 1-2 Purity 95% UGI (100%) Al (100%) KClO (100%) Comp. 1-3 Purity 95% UGI (100%) Al (100%) CaO (78%), KClO (10%), CaF (12%) Comp. 1-4 Industrial Waste Purity 97% TiO (100%) Al (100%) CaF (100%) Comp. 1-5 Industrial Waste Purity 97% TiO (100%) Al (100%) CaF (100%) Ex. 1-1 Purity 95% UGI (100%) Al (100%) CaO (100%) Ex. 1-2 Purity 95% UGI (100%) Al (100%) CaO (100%) Ex. 1-3 Purity 95% UGI (100%) Al (100%) CaO (100%) Ex. 1-4 Purity 95% UGI (100%) Al (100%) CaO (100%) Ex. 1-5 Purity 5% UGI (100%) Al (100%) CaO (100%) Ex. 1-6 Purity 50% UGI (100%) Al (100%) CaO (100%) Ex. 1-7 Purity 95% Natural Rutile (50%), Al (100%) CaO (100%) Purity 95% UGS(50%) Ex. 1-8 Industrial Waste Purity 97% TiO (100%) Al (100%) CaO (100%) Ex. 1-9 Industrial Waste Purity 93% Ti.sub.2O.sub.3 Al (100%) CaO (100%) (Ti-33mass% O)(50%), Industrial Waste Purity 95% TiO (Ti-25mass% O)(50%) Ex. 1-10 Purity 95% UGI (70%), Al (100%) CaO (100%) Industrial Waste Purity 97% Ti O (Ti-10% O)(30%) Ex. 1-11 Purity 95% UGI (70%), Al (100%) CaO (100%) Purity 97% CaTiO (30%) Ex. 1-12 Purity 95% UGI (70%), Al (100%) CaO (100%) Purity 97% CaTiO.sub.3 (20%), Purity 97% MgTiO.sub.3 (5%), Purity 94% MgTi O (5%) Ex. 1-13 Purity 95% UGI (100%) Al- % Mg (100%) CaO (100%) Ex. 1-14 Purity 95% UGI (100%) Al-5% Ca (100%) CaO (100%) Ex. 1-15 Purity 95% UGI (100%) Al-35% Si-10% Fe (100%) CaO (100%) Ex. 1-16 Purity 95% UGI (100%) Al-35% Si-10% Fe-10% Mn (100%) CaO (100%) Ex. 1-17 Purity 95% UGI (100%) Al (100%) CaO (96%), CaF (4%) Ex. 1-18 Purity 95% UGI (100%) Al (100%) CaO (92%), CaF (8%) Ex. 1-19 Purity 95% UGI (100%) Al (100%) CaO (82%), KClO (18%) Ex. 1-20 Purity 95% UGI (100%) Al (100%) CaO (100%) Ex. 1-21 Purity 95% UGI (100%) Al (100%) CaO (100%) Ex. 1-22 Purity 95% UGI (100%) Al (100%) CaO (100%) Ex. 1-23 Purity 95% UGI (100%) Al (100%) CaO (100%) Ex. 1-24 Purity 95% UGI (100%) Al (100%) CaO (100%) Ex. 1-25 Purity 95% UGI (100%) Al (100%) CaO (100%) Ex. 1-26 Purity 95% UGI (100%) Al (100%) CaO (100%) indicates data missing or illegible when filed

TABLE-US-00002 TABLE 2 Furnace and Heating Holding Conditions Cooling Product (Titanium-Based Raw Heating Material for Electro-Refining) Furnace Maximum Furnace Al O Fe and Temperature and Cooling Concentration Concentration Concentration of Temperature (mass %) (mass %) (mass %) Comp. 1-1 A 1700 C. None 2.2 Comp. 1-2 C 2000 C. C 8.5 2.5 Comp. 1-3 C 2000 C. C Comp. 1-4 A 1700 C. None 33.5 0.9 0.3 Comp. 1-5 C 2000 C. C 1500 C. 0.9 0.3 Ex. 1-1 C 2000 C. C 1500 C. 6.8 6.8 Ex. 1-2 A 2000 C. A 1500 C. 2.1 Ex. 1-3 B 1700 C. None 7.2 7.2 2.2 Ex. 1-4 C 2000 C. A 6.4 6.4 Ex. 1-5 C 2000 C. C 6.7 Ex. 1-6 C 2000 C. C 6.7 7.1 Ex. 1-7 C 2000 C. C 1500 C. 2 Ex. 1-8 C 2000 C. C 1500 C. 6.8 0.4 Ex. 1-9 C 2000 C. C 1500 C. 6.7 6.4 Ex. 1-10 B 1700 C. None Ex. 1-11 C 2000 C. C 1500 C. 0.8 Ex. 1-12 C 2000 C. C 1500 C. 0.7 Ex. 1-13 C 2000 C. C 1500 C. 7 1.9 Ex. 1-14 C 2000 C. C 2 Ex. 1-15 C 2000 C. C Ex. 1-16 C 2000 C. C 7.9 4.8 Ex. 1-17 C 2000 C. C 1500 C. 7.2 1.9 Ex. 1-18 C 2000 C. C 1500 C. 7.4 1.9 Ex. 1-19 C 2000 C. C 1500 C. 7.5 2 Ex. 1-20 C 2000 C. C 1700 C. 1.9 Ex. 1-21 C 2000 C. C 1700 C. 6.2 6.4 1.8 Ex. 1-22 C 2000 C. None Ex. 1-23 C 1500 C. None Ex. 1-24 C 1400 C. None 7.9 Ex. 1-25 A 1700 C. None 2.1 Ex. 1-26 C 2000 C. C 1500 C. indicates data missing or illegible when filed

(Test 2)

[0084] Example 2-1 shown in Table 3 conducted the test under substantially the same condition as those of Example 1-1 of Test 1, with the exception that in the reaction step, the titanium compound and melting accelerator were firstly melted, and then the reducing agent was added. Examples 2-2 to 2-7, as shown in Table 3, changed the blended raw materials from Example 2-1. For each of Examples 2-1 to 2-7, the ratio of the blended raw materials was titanium compound:reducingagent:melting accelerator=9:4:7 in a mass ratio.

[0085] For each titanium-based raw material for electro-refining according to Examples 2-1 to 2-7, the Al concentration, the O concentration, and the Fe concentration were measured by the same method as that of Test 1. The results are shown in Table 3. When Examples 2-1 to 2-7 shown in Table 3 were compared with Example 1-1 shown in Table 2, the Al concentration and the O concentration of the titanium-based raw materials for electro-refining were further reduced. This would be because in Examples 2-1 to 2-7, the reducing agent containing Al was added later, thereby suppressing the loss due to evaporation of Al, and using most of it for the reaction.

TABLE-US-00003 TABLE 3 Furnace and Heating Holding Conditions Product(Titanium-Based Raw Cooling Material for Electro-Refining) 10 of Heating Maximum Holding Al O Fe Blended Raw Materials Furnace Temper- Furnace Cooling Concen- Concen- Concen- Titanium Reducing Melting and ature and Temper- tration tration tration Compound Agent Accelerator of Melt ature (mass %) (mass %) (mass %) Ex. 2-1 95% Al (100%) CaO (100%) C 2000 C. C 1600 C. 5.4 5.5 1.9 (100%) Ex. 2-2 Industrial Waste Al (100%) CaO (100%) C 2000 C. C 1600 C. 5.4 5.2 0.4 Purity 97% (100%) Ex. 2-3 Industrial Waste Al (100%) CaO (100%) C 2000 C. C 1600 C. 5.3 5.1 0.5 Purity 93% Ti.sub.2O.sub.3 (Ti- mass % O) (50%) Industrial Waste Purity 96% TiO (Ti-25mass%O)(50%) Ex. 2-4 Purity 95% Al (100%) CaO (100%) C 2000 C. C 1600 C. 5.3 5.5 0.7 UGI (70%) Purity 97% CaTiO.sub.3 (30%) Ex. 2-5 Purity 95% Al (100%) CaO (100%) 2000 C. C 1600 C. 5.2 5.5 0.8 UGI (70%) Purity 97% CaTiO.sub.3 (20%) Purity:97% MgTiO.sub.3 (5%) Purity 94% MgTi.sub.2O.sub.3 (5%) Ex. 2-6 Purity 95% Al-3% CaO (100%) C 2000 C. C 1600 C. 5.6 5.5 1.9 UGI (100%) Mg (100%) Ex. 2-7 Purity 95% Al-5% CaO (100%) C 2000 C. C 1600 C. 5.3 5 2.1 UGI (100%) Ca (100%) indicates data missing or illegible when filed

(Test 3)

[0086] The titanium alloy product obtained in the reaction step and the separation step according to Example 1-1 was subjected to the remelting step and the casting step in a vacuum of 0.01 Pa to 0.1 Pa to obtain a titanium-based raw material for electro-refining. More details are as follows.

[0087] In Example 3-1, a titanium-based raw material for electro-refining obtained by Example 1 was remelted in an electron beam melting furnace using the so-called drip melt method and remelted drips fell down into a water-cooled copper crucible. It should be noted that the temperature during drip melting could not be measured.

[0088] In Example 3-2, a titanium-based raw material for electro-refining obtained by Example 1 was remelted in an electron beam melting furnace with a water-cooled copper hearth. Then it was poured into a water-cooled copper mold and the temperature of the melt was 2100 C.

[0089] In Example 3-3, a titanium-based raw material for electro-refining obtained by Example 1 was remelted in a water-cooled copper crucible of an induction skull melting furnace while keeping temperature at 1850 C. Then it was poured into a graphite crucible and a plate-shaped titanium-based raw material for electro-refining was produced.

[0090] For each titanium-based raw material for electro-refining obtained in Examples 3-1 to 3-3, the Al concentration, the O concentration, and the Fe concentration were measured by the same method as that of Test 1. The results are shown in Table 4. Table 4 also lists the Al concentration, the O concentration, and the Fe concentration of the titanium-based raw material for electro-refining before the tests according to Example 1-1 shown in Table 2. It is found from Table 4 that each of the titanium-based raw materials for electro-refining according to Examples 3-1 to 3-3, also has a low O concentration and a low Fe concentration, which are comparable to those of the titanium-based raw material for electro-refining according to Example 1-1. The Al concentration was further reduced as compared to Example 1-1. In each of Example 3-2 and Example 3-3, the plate-shaped titanium-based raw material for electro-refining was obtained, which could be used as it was as the anode for the molten salt electro-refining.

TABLE-US-00004 TABLE 4 Product (Titanium-Based Raw Material for Electro-Refining) Al Concentration O Concentration Fe Concentration (mass %) (mass %) (mass %) Ex. 1-1 6.8 6.8 1.8 Ex. 3-1 4.4 6.4 2 Ex. 3-2 4 6.6 2.2 Ex. 3-3 4.1 6.5 2.3

(Test 4)

[0091] Using each titanium-based raw material for electro-refining obtained in Comparative Example 1-2, Example 1-1, Example 2-6, or Example 3-2 as a crude titanium-based material, the molten salt electro-refining was performed to produce pure metallic titanium or TiAl alloy. The specific electric resistance value of each crude titanium-based material was measured by a two-terminal measurement method using a low resistance meter 3566-RY manufactured by Tsuruga Electric Corporation. This measurement of the specific electric resistance value was carried out at room temperature, and based on its magnitude relationship of the specific electric resistance values at room temperature, the magnitude relationship of the specific electric resistance value at a high temperature during molten salt electro-refining can be inferred.

[0092] In the molten salt electro-refining, an electric voltage was applied between the anode and the cathode in a molten salt bath having the composition shown in Table 5 to remove Ti from the crude titanium-based material of the anode, and to deposit a purified titanium-based material onto the cathode. The molten salt bath contained TiCl.sub.2 in a range of 3% to 8% by mass, and further contained other components as shown in Table 5. For the anode, among those described above, each of the titanium-based raw materials for electro-refining according to Comparative Example 1-2, Example 1-1 and Example 2-6 was crushed to obtain a crude titanium-based material, which was placed in a perforated cage-like container made of nickel having many through holes and used as the anode. For the plate-shaped titanium-based raw material for electro-refining according to Example 3-2, the perforated cage-like container was not used, and the plate-shaped material was used as the anode as it was. The cathode was a titanium plate.

[0093] The electro-refining was carried out by applying a constant voltage, and ended when the current value sharply decreased, it was determined that further electrolysis could not take place, and the weight of the remaining crude titanium-based material in the perforated cage-like container of the anode or a plate-shaped titanium-based material was measured and the yield was calculated using the following equation. The results are shown in Table 5. Yield (%)=100(weight of crude titanium-based material before electro-refiningweight of remaining residue at the end)/weight of crude titanium-based material before electro-refining.

[0094] At the end of the electro-refining, the cathode was taken out, and pure metallic titanium or the TiAl alloy deposited on the cathode was collected, washed with dilute hydrochloric acid, and then washed with water, and further air-dried at a temperature of about 50 C. to 60 C. The Al concentration and the O concentration of pure metallic titanium or the TiAl alloy were measured in the same manner as the measurement for the titanium-based raw material for electro-refining in Test 1. The results are shown in Table 5.

[0095] In each of Examples 4-1 to 4-9, the yield was improved as compared to each of Comparative Examples 4-1 to 4-3. Further, pure metallic titanium or the TiAl alloy obtained in each of Examples 4-1 to 4-9 had the sufficiently low O concentration. Furthermore, in each of Example 4-2, Example 4-3, Example 4-5, Example 4-6, Example 4-8, and Example 4-9, the Al concentration was also sufficiently reduced, and pure metallic titanium was produced. As described above, pure metallic titanium or the TiAl alloy could be produced by performing one molten salt electro-refining process.

TABLE-US-00005 TABLE 5 Crude Titanium-Based Material Molten Salt Pure Metallic Titanium Specific Electro-refining or TiAl Alloy Al O Fe Resistance Bath Temp. Al O Fe Pre- (mass %) (mass %) (mass %) ( .Math. m) Composition ( C.) Yield (%) (mass %) (mass %) (mass %) Comp. 4-1 Comp. 1-2 8.5 2.5. 160 750 52 3.6 1.3 <0.01 Comp. 4-2 MgCl.sub.2, 800 55 1.6 2.2 <0.01 Comp. 4-3 NaCl, KCl, 600 51 2 1.7 <0.01 MgCl.sub.2, Ex. 4-1 Ex. 1-1 6.8 1.8 93 NaCl, KCl, 750 79 2.1 0.3 <0.01 Ex. 4-2 MgCl.sub.2, 800 80 0.6 0.2 <0.01 Ex. 4-3 NaCl, KCl, 600 78 0.3 0.2 <0.01 MgCl.sub.2, EX. 4-A Ex. 2-6 5.6 5.5 1.9 55 NaCl, KCl, 750 82 2.3 0.17 <0.01 Ex. 4-5 MgCl.sub.2, 800 84 0.1 0.12 <0.01 Ex. 4-6 NaCl, KCl, 600 82 0.09 0.13 <0.01 MgCl.sub.2, Ex. 4-7 Ex. 3-2 4 2.2 34 NaCl, KCl, 750 88 1.6 0.1 <0.01 Ex. 4-8 MgCl.sub.2, 800 90 0.05 <0.01 EX. 4-9 NaCl, KCl, 600 89 0.007 0.04 <0.01 MgCl.sub.2, indicates data missing or illegible when filed