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

The present invention relates to a method for producing a titanium-based raw material for electro-refining for use in molten salt electro-refining to obtain pure metallic titanium or Ti—Al alloy, and a method for producing pure metallic titanium or Ti—Al alloy.

Commercially pure titanium and titanium alloys are generally produced using titanium sponge, which is pure metallic titanium obtained from titanium-bearing ore, by a method based on the Kroll process.

However, this method requires the production of titanium tetrachloride by chlorination of titanium-bearing ore, followed by purification of titanium tetrachloride and reduction of titanium tetrachloride with metallic magnesium, as well as it requires crushing of titanium sponge lumps, and electrolysis of magnesium chloride produced by the reduction, resulting in a large number of batch steps included. It is, therefore, difficult to say that titanium sponge, which is pure metallic titanium, can efficiently be produced at low cost. Further, coke (carbon) is used in the reaction for producing titanium tetrachloride, which will release carbon dioxide, a greenhouse gas that causes global warming.

The titanium production process described herein overcomes these issues by using a processing technique referred to as electro-refining. Specifically, the electro-refining method employed herein uses a molten salt bath (also referred to as “molten salt electro-refining”), to produce pure metallic titanium or Ti—Al alloys. These materials can, in turn, be used to easily produce commercially pure titanium or titanium alloys. This approach is substantially advantageous compared to the method based on the Kroll process. Examples of this type of technique include those described in Patent Literatures 1 and 2.

Patent Literature 1 describes: “a method of extracting a titanium product from a titanium-bearing ore, comprising: mixing a chemical blend comprising the titanium-bearing ore and a reducing agent, wherein a ratio of the titanium-bearing ore to the reducing agent corresponds to a weight ratio of titanium oxide component in the titanium-bearing ore: reducing metal in the reducing agent of 0.9 to 2.4; heating the chemical blend to initiate an extraction reaction, wherein the chemical blend is heated at a ramp up rate between 1° C. to 50° C./min; maintaining the chemical blend at a reaction temperature between 1500-1800° C. for a time period between 5 and 30 minutes; cooling the chemical blend to a temperature lower than 1670° C.; and separating a titanium product from a residual slag”, and “wherein the titanium-bearing ore comprises titanium oxide (TiO) and the reducing agent comprises aluminum (Al)”. Patent Document 2 also describes a similar method.

For the above “chemical blend,” Patent Literature 1 mentions:

In the electro-refining process using the molten salt bath, a conductive crude titanium-based material containing Ti, Al, and O is used on the anode side as a consumable anode raw material in the molten salt bath in the electrolytic cell, and an electric voltage is applied between the anode and the cathode. In some cases, the crude titanium-based material may contain impurities of elements other than Ti, Al, and O, and may be placed in a predetermined cage-like container such as a perforated container with many through holes, and may be used as a consumable component for the anode together with the cage-like container. By the above electro-refining, Ti is mainly removed from the crude titanium-based material of the anode, and a purified titanium-based material having a higher purity than that of the crude titanium-based material is deposited on the cathode surface. The pure metallic titanium or one or more Ti—Al alloys can be thus obtained as a titanium-based deposit on the cathode surface.

If the above electro-refining process is continued, the contents of Al, O, and impurities in the crude titanium-based material will relatively increase as Ti is removed from the crude titanium-based material of the anode, and, due to this, the electrical resistance gradually increases, and finally, substantially no electric current will flow and substantially no Ti will be removed from the crude titanium-based material. In particular, when a titanium-based raw material for electro-refining having a higher Al content and O content is used as the crude titanium-based material, a large amount of electric power is required from the beginning of electro-refining. Additionally, the increased resistance of the anode material shortens the effective electro-refining time period, thus preventing a sufficient amount of Ti from being removed to achieve an adequate yield.

In order to obtain the titanium-based raw material for electro-refining (the above crude titanium-based material), for example, as described in Patent Literatures 1 and 2, Al as a reducing agent can be allowed to react with titanium oxide contained in the titanium-bearing ore in a melt at elevated temperature to reduce the titanium oxide. However, for the methods described in Patent Literatures 1 and 2, there is still room for improvement in reducing the Al content and O content in the titanium-based raw material for electro-refining.

Further, when a relatively large amount of calcium fluoride (CaF) described in Patent Literatures 1 and 2 is used in the above reaction in the melt, F in the calcium fluoride forms a fluoride with Al in the melt, and some of the Al becomes consumed without contributing to the reaction, as well as there is a possibility that aluminum trifluoride (AlF), which may be affected on the human body, may be generated, although its amount is miner. Furthermore, if a chloride such as potassium perchlorate (KClO) is used in place of the above calcium fluoride, there is a possibility that a small amount of aluminum trichloride (AlCl), which may be affected on the human body, may be generated.

Therefore, there is a need for a method of reducing titanium oxide without using calcium fluoride or potassium perchlorate as much as possible to obtain a titanium-based raw material for electro-refining (crude titanium-based material) with lower Al and O contents.

An objective of the present invention is to provide a method for producing a titanium-based raw material for electro-refining 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 Ti—Al alloy.

As results of extensive studies, the inventors have found that by using a melting accelerator, i.e., a substance that provides melting benefits and takes part in the chemical reaction, containing calcium oxide (CaO) in place of most or all of the conventionally used calcium fluoride and potassium perchlorate, the calcium oxide takes part in the reaction, thereby enabling the Al content and O content of a titanium alloy product (titanium-based raw material for electro-refining) to be reduced.

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 Ti—Al 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 the content of calcium oxide in the melting accelerator is 80% by mass or higher.

If the above melt contains an oxide other than titanium oxide, for example, at least one oxide selected from oxides of Mo, V and Nb, the reducing agent may reduce the oxides of Mo, V, and Nb in addition to titanium oxide. In this case, a large amount of reducing agent is required for sufficiently reducing titanium oxide with Al, and impurities such as Mo, V, and Nb will be mixed into the titanium-based raw material for electro-refining produced. From the viewpoint of addressing this problem, in the production of the titanium-based raw material for electro-refining, it is preferable that the melt does not substantially contain impurity oxides other than titanium oxides.

Preferably, calcium fluoride (CaF) and potassium perchlorate (KClO) are not used in the reaction step.

In the reaction step, the temperature of the melt is preferably 1450° C. or higher.

In the reaction step, it is preferable that the reducing agent consists of pure metal and/or the alloy of aluminum, and the Al content of pure metal and/or the alloy of aluminum is higher than 50% by mass.

In the reaction step, the titanium compound may comprise TiO(0≤x<1) and/or a titanate compound.

When producing the melt, the reaction step preferably comprises melting the titanium compound and the melting accelerator, and then adding pure metal and/or the alloy of aluminum.

The method for producing pure metallic titanium or Ti—Al alloy according to the present invention comprises 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 any one of the methods for producing a titanium-based raw material for electro-refining is used for the consumable anode raw material as the crude titanium-based material. It should be noted that the consumable anode raw material means a raw material that is provided on a side of the anode used in electro-refining, and is consumed as Ti is removed during electro-refining. If a cage-like container is used, the cage-like container also forms a portion of the anode, but the cage-like container is essentially not consumed during electro-refining.

Preferably, the molten salt bath is a chloride bath.

In this case, the chloride bath preferably comprises magnesium chloride and titanium dichloride.

According to the method for producing a titanium-based raw material for electro-refining of the present invention, it is possible to produce a titanium-based raw material for electro-refining with a relatively low Al content and O content while suppressing or eliminating the use of calcium fluoride or potassium perchlorate.

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

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 Ti—Al alloy. The method for producing a titanium-based raw material for electro-refining includes a reaction step.

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.

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, 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.

This also allows the amount of calcium fluoride (CaF) or potassium perchlorate (KClO) typically used to be decreased, or the use of calcium fluoride or potassium perchlorate (KClO) 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) or similar compounds may be generated. Further, when potassium perchlorate (KClO) is used, there is a possibility that aluminum trichloride (AlCl) 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.

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 Ti—Al 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.

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.

It should be noted that this method for producing the titanium metal or the Ti—Al 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.

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.

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.

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.

In the melt containing the titanium compound (particularly titanium dioxide (TiO)), 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):

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, CaTiO), 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.).

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 (AlO), a composite oxide composed of AlOand 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.

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.

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

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), 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), 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.

It is also believed that when the temperature of the melt is increased, a composite oxide composed of AlOand 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.

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.

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.

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 TiOin 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 TiO, and includes TiO, TiO(0≤x<1), Tio, TiO(0≤x<1), and the like.

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 MTiO(x>0, y>0, z>0). Specific examples of the titanate compound include calcium titanate (CaTiO, CaTiO), which is a composite oxide of Ca and Ti, and magnesium titanate (MgTiO, MgTiO), 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 CaTiomay be referred to as calcium titanate. However, as used herein, not only CaTiobut also multiple types of composite oxides of Ca and Ti, including CaTiOand similar compounds, are collectively referred to as calcium titanate herein.

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.

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.