Method for producing sponge titanium

Abstract

To provide a manufacturing method of sponge titanium capable of manufacturing sponge titanium having a relatively small aluminum content.SOLUTION: A manufacturing method of titanium sponge according to the present invention includes reacting titanium tetrachloride and magnesium metal in a molten bath Bm in a reduction vessel 1 to form a titanium sponge mass TS. The metallic magnesium used in the reduction is unreacted metallic magnesium that remains without being used in the reaction in a reduction step different from the reduction step, and is separated from the sponge titanium mass TS in the reduction vessel 1 by heating under a reduced pressure atmosphere after the other reduction step.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This invention relates to a method for producing sponge titanium by generating a sponge titanium mass through the reaction of titanium tetrachloride and metallic magnesium.

Background Art

[0002] The Kroll process is widely used industrially as a method for producing sponge titanium. In the Kroll process, for example, a reduction step is performed in which molten metallic magnesium is previously stored in a reduction vessel to form a molten bath, and titanium tetrachloride is dropped and supplied onto the surface of the bath. During the reduction step, metallic magnesium acts as a reducing agent and titanium tetrachloride is reduced to metallic titanium, and the metallic titanium grows as a sponge titanium mass in the reduction vessel. At this time, magnesium chloride is generated as a by-product in the molten bath.

[0003] After the completion of the reduction step, a bath discharge step may be performed. Here, the molten bath containing metallic magnesium and by-product magnesium chloride that was not used in the reduction reaction is discharged from the reduction vessel while maintaining the molten state. Note that during the reduction step, the molten bath may be withdrawn from the reduction vessel. After the bath discharge step, as a vacuum separation step, the pressure inside the reduction vessel is reduced while heating the reduction vessel to a high temperature. Thereby, residues such as metallic magnesium inside the reduction vessel are separated from the sponge titanium mass.

[0004] After the completion of the vacuum separation step, the sponge titanium mass is taken out from the reduction vessel and subjected to crushing to obtain granular sponge titanium.

[0005] As a technology related to the reduction process and bath discharge process as described above, Patent Document 1 describes "When producing sponge titanium by the Kroll process, the mixed melt of unreacted Mg and MgCl2 is extracted from the reaction vessel and separated by specific gravity to produce MgCl2 with a reduced Mg contamination concentration. This MgCl2 is introduced into an electrolytic cell to produce metallic Mg, and this metallic Mg is used as a reducing agent in the production of the sponge titanium." This Patent Document 1 also describes "When producing high-purity sponge titanium and low-purity sponge titanium by the Kroll process, after each reduction reaction, the mixed melt of unreacted Mg and MgCl2 remaining in the reaction vessel is extracted from the reaction vessel and separated by specific gravity to reduce the Mg contamination concentration, and the resulting MgCl2 is introduced into an electrolytic cell to produce metallic Mg", and "The separated unreacted Mg is used as a reducing agent in the production of low-purity sponge titanium together with the metallic Mg produced in the electrolytic cell."

[0006] Regarding the crushing process, Patent Document 2 describes "In the production of sponge titanium by the Kroll process, in the vacuum separation process, the ratio (C / T) of the mass T (kg) of the sponge titanium mass to the total mass C (kg) of the metallic magnesium and magnesium chloride to be volatilized and separated is 0.40 to 0.58. In the crushing process, when the height of the sponge titanium mass is H (mm) and the diameter is D (mm), the lower limit position in the vertical direction for sampling is within the range of 0.05H to 0.10H from the lower end of the sponge titanium mass, and the outer limit position in the radial direction for sampling is within the range of 0.05D to 0.12D from the outer end of the sponge titanium mass. The portion below the lower limit position in the vertical direction and the portion outside the outer limit position in the radial direction are cut off and removed, and the remaining portion is sampled. This is a method for producing sponge titanium."

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] By the way, for example, sponge titanium used in the semiconductor and other fields may be required to have a sufficiently low content of aluminum, which is an impurity, due to the requirement for high purity of the material.

[0009] Here, in the reduction process, generally, electrolytic magnesium obtained by molten salt electrolysis of magnesium chloride is used as metallic magnesium. The electrolytic cell used in this molten salt electrolysis is often constructed of bricks containing aluminum. Therefore, when mainly using electrolytic magnesium in the reduction process, sponge titanium may be contaminated with aluminum mixed in the electrolytic magnesium derived from the above bricks, and its aluminum content may increase.

[0010] To reduce the aluminum content of electrolytic magnesium, it is conceivable to purify electrolytic magnesium by distillation or the like, but the construction, maintenance, and management of the equipment for this purpose are required, increasing the manufacturing cost.

[0011] Patent Document 1 describes "extracting the mixed melt of unreacted Mg and MgCl2 remaining in the reaction vessel after the reduction reaction from the reaction vessel and performing specific gravity separation", and "using the separated unreacted Mg together with the metallic Mg generated in the electrolytic cell as a reducing agent in the production of low-purity sponge titanium". Even such metallic magnesium obtained by discharging from the reduction vessel in such a bath discharge process and performing specific gravity separation contains a certain amount of impurities such as aluminum. Therefore, even if such metallic magnesium is used in the reduction process, it is difficult to reduce the aluminum content of sponge titanium as expected.

[0012] Note that Patent Document 2 does not describe anything about improving the aluminum content of metallic magnesium for producing sponge titanium with a low aluminum content.

[0013] An object of the present invention is to provide a method for producing sponge titanium capable of producing sponge titanium with a relatively low aluminum content.

Means for Solving the Problems

[0014] As a result of intensive studies, the inventor has found that if it is unreacted metallic magnesium separated from a sponge titanium mass in a vacuum separation step, which is often performed after the reduction step and in many cases further after the bath discharge step, almost no aluminum is mixed therein and it can be effectively used in other reduction steps. In this regard, in the reduction step, a part of the aluminum contained in metallic magnesium exists in the molten bath, and the remainder reacts with the metallic titanium reduced from titanium tetrachloride to form titanium aluminum (TiAl), which is considered to be concentrated on the bottom side of the sponge titanium mass or the like. Most of the aluminum in the molten bath can be discharged together with the molten bath in the bath discharge step. Also, the aluminum in the titanium aluminum concentrated in the sponge titanium mass does not return to the metallic magnesium and remains integrated with the sponge titanium mass. Therefore, in the vacuum separation step, unreacted metallic magnesium containing almost no aluminum evaporates, and it can be recovered by cooling this. However, the present invention is not limited to such a mechanism.

[0015] The method for producing sponge titanium of the present invention includes a reduction step of reacting titanium tetrachloride and metallic magnesium in a molten bath in a reduction container to generate a sponge titanium mass, wherein the metallic magnesium used in the reduction step is unreacted metallic magnesium that remained without being used in a reduction reaction in a reduction step different from the reduction step, and the method includes the unreacted metallic magnesium separated and recovered by heating under a reduced pressure atmosphere from the sponge titanium mass in the reduction container after the different reduction step.

[0016] In the above manufacturing method, it is preferable that 35% by mass or more of the magnesium metal used in the reduction step is the unreacted magnesium metal.

[0017] When the recovered material recovered by heating under a reduced pressure atmosphere after the other reduction step contains magnesium chloride and the unreacted magnesium metal, the unreacted magnesium metal used in the reduction step is preferably obtained by separating from the recovered material based on the specific gravity difference from the magnesium chloride.

[0018] The above manufacturing method preferably includes a reduced pressure separation step of heating the inside of the reduction vessel under a reduced pressure atmosphere after the other reduction step is completed and discharging the molten bath in the reduction vessel, so as to separate the unreacted magnesium metal from the sponge titanium mass into a recovery vessel communicated with the reduction vessel.

[0019] In this case, it is preferable that the other reduction step and the subsequent reduced pressure separation step are performed a plurality of times, the recovery vessel is used in the plurality of reduced pressure separation steps, and the unreacted magnesium metal is accumulated in the recovery vessel.

[0020] In the reduced pressure separation step, it is preferable to maintain the temperature of the internal space of the recovery vessel below the melting point of magnesium metal.

[0021] The above manufacturing method includes a crushing step of crushing the sponge titanium mass taken out from the reduction vessel after the reduction step to obtain sponge titanium. Among the sponge titanium obtained in the crushing step, the ratio of the mass of the sponge titanium having an aluminum content of 0.5 mass ppm or less to the mass of the sponge titanium mass is preferably 20% or more.

Advantages of the Invention

[0022] According to the method for producing sponge titanium of this invention, sponge titanium having a relatively low aluminum content can be produced.

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0024] Hereinafter, embodiments of the present invention will be described in detail. The method for producing sponge titanium according to one embodiment of the present invention produces sponge titanium with a relatively low aluminum content, and includes a reduction step of reacting titanium tetrachloride and metallic magnesium in a molten bath in a reduction container to produce a sponge titanium mass. In the reduction step, metallic magnesium containing unreacted metallic magnesium is used. This reduction step for producing sponge titanium with a relatively low aluminum content is referred to as "the said reduction step".

[0025] Here, the above unreacted metallic magnesium is metallic magnesium that remained without being used in the reduction reaction in a reduction step different from the said reduction step, and means metallic magnesium separated and recovered from the sponge titanium mass in the reduction container by heating under a reduced pressure atmosphere after another reduction step. More specifically, a sponge titanium mass is produced in the reduction container in another reduction step, and then, after passing through a bath discharge step, unreacted metallic magnesium is separated and recovered from the sponge titanium mass in the reduction container by heating under a reduced pressure atmosphere in the reduced pressure separation step. This unreacted metallic magnesium is used in the said reduction step.

[0026] The unreacted metallic magnesium recovered by vacuum separation contains a much smaller amount of aluminum than the electrolytic metallic magnesium obtained by molten salt electrolysis. By using such metallic magnesium containing unreacted metallic magnesium in the reduction step, aluminum contamination of the sponge titanium mass produced in the reduction step is sufficiently suppressed. As a result, for example, it becomes possible to collect a large amount of sponge titanium with a low aluminum content from each sponge titanium mass, and sponge titanium with high purity and almost no aluminum can be produced.

[0027] Note that, for the purpose of distinguishing from the reduction step, the reduction step from which the unreacted metallic magnesium used in the reduction step is obtained is referred to as "another reduction step". However, it is also possible to use the unreacted metallic magnesium recovered by vacuum separation after the reduction step in a reduction step other than the reduction step. That is, the reduction step may correspond to another reduction step. Also, it does not matter whether another reduction step itself has been performed or not. It is sufficient that the reduction step is performed and the unreacted metallic magnesium recovered by vacuum separation after another reduction step is used in the reduction step.

[0028] Preferably, the metallic magnesium used in the reduction step contains 35% by mass or more of the unreacted metallic magnesium recovered by vacuum separation. Thereby, the aluminum content of the sponge titanium can be further reduced. From this viewpoint, it is more preferable that the content of the unreacted metallic magnesium in the metallic magnesium is 40% by mass or more. It is also possible to use only the unreacted metallic magnesium as 100% by mass of the unreacted metallic magnesium recovered by vacuum separation in the reduction step. However, performing the reduction step using only the unreacted metallic magnesium recovered by vacuum separation may not be realistic from the viewpoint of production cost. The content of the unreacted metallic magnesium in the metallic magnesium may be, for example, 70% by mass or less, and further 50% by mass or less.

[0029] (Reduction Step) In the reduction process, a reduction container 1 made of clad steel, stainless steel, or the like as exemplified in FIG. 1 can be used. This reduction container 1 includes a cylindrical or other tubular side wall 2, a bottom 3 that seals one end side in the axial direction of the side wall 2 (the lower end side in FIG. 1), and a lid 4 attached to the opening at the other end side in the axial direction of the side wall 2 (the upper end side in FIG. 1). A discharge pipe 5 connected to a portion near the bottom 3 of the side wall 2 or the like is provided on the side wall 2, and a supply pipe 6 is provided on the lid 4. In the illustrated example, the discharge pipe 5 is connected to the side wall 2, but the discharge pipe 5 may be connected to the bottom 3. Regardless of whether the discharge pipe 5 is connected to either, it is possible to discharge the molten bath or remove the molten bath. Generally, on the bottom 3 side inside the reduction container 1, as in the illustrated example, a punch 7 that operates to push up the sponge titanium mass TS when taking out the sponge titanium mass TS from inside the reduction container 1 is arranged. Note that the punch 7 may be an integrally molded product, or it may have a lost tool on which the portion where the sponge titanium mass TS is placed is detachable. When the punch 7 has a lost tool, the separation between the sponge titanium mass TS and the punch 7 becomes easy. The reduction container 1 is arranged in a reduction furnace 8, and the reduction process is performed.

[0030] To perform the reduction process using the reduction container 1, for example, molten magnesium (Mg) as a reducing agent is stored in the reduction container 1 in a molten state, and the inside of the reduction container 1 is made into a molten bath Bm. Then, while heating the reduction container 1 in the reduction furnace 8, local cooling is also used in combination, and titanium tetrachloride (TiCl4), which is a raw material, is dropped and supplied onto the bath surface Sb of the molten bath Bm from above through the supply pipe 6 provided on the lid 4. The titanium tetrachloride dropped in this way comes into contact with the magnesium in the molten bath Bm and is reduced by the magnesium based on the reaction of the formula: TiCl4 + 2Mg → Ti + 2MgCl2. The metallic titanium (Ti) generated by the reduction of titanium tetrachloride becomes a sponge titanium mass TS on the punch 7.

[0031] Magnesium chloride (MgCl2) produced as a by-product in the molten bath Bm by this reaction sinks downward from the bath surface Sb because its specific gravity is greater than that of metallic magnesium. On the other hand, metallic magnesium in the molten bath floats upward toward the bath surface Sb because of its relatively small specific gravity. Due to such a specific gravity difference between magnesium chloride and metallic magnesium, a bath flow is generated and metallic magnesium is located on the bath surface Sb. For this reason, a reaction continuously occurs between metallic magnesium and the dropped titanium tetrachloride on the bath surface Sb, and mainly sponge titanium lumps TS grow in the molten bath Bm. Magnesium chloride that has sunk to the lower side can be withdrawn outside the reduction vessel 1 through the discharge pipe 5 on the bottom 3 side of the reduction vessel 1, for example, intermittently at regular intervals.

[0032] Note that the titanium tetrachloride used in the reduction step can be, for example, liquid purified titanium tetrachloride after being purified in a rectification column. This purified titanium tetrachloride is obtained, for example, by purifying crude titanium tetrachloride produced by reacting a raw ore such as titanium ore with a carbon source such as coke and chlorine gas in a rectification column. However, as long as it can be used in the reduction step, the titanium tetrachloride is not limited to the above-mentioned purified titanium tetrachloride.

[0033] In addition, magnesium chloride produced in the reduction step can be decomposed into metallic magnesium and chlorine gas by being subjected to molten salt electrolysis in an electrolytic cell. The metallic magnesium thus obtained (also referred to as "electrolytic metallic magnesium") may be used again in the reduction step.

[0034] Both the present reduction step and another reduction step can be carried out in the same manner as the above-described reduction step. However, in the present reduction step, as described above, at least a part of the metallic magnesium used therein is the unreacted metallic magnesium recovered in the subsequent vacuum separation step. In another reduction step, it does not matter whether or not the unreacted metallic magnesium recovered in the vacuum separation step is used.

[0035] In addition, in the reduction step, while the sponge titanium mass TS is growing due to the reaction between titanium tetrachloride and metallic magnesium, it is desirable not to add metallic magnesium into the reduction vessel 1. That is, it is preferable to store all the metallic magnesium used in the reduction step in the reduction vessel 1 before the start of the reduction step. If metallic magnesium is added into the reduction vessel 1 while the sponge titanium mass TS is growing, aluminum that may be contained in the metallic magnesium, even in a very small amount, may migrate into the sponge titanium mass TS. In particular, electrolytic metallic magnesium contains a relatively large amount of aluminum, so it is preferably not added while the sponge titanium mass TS is growing. Thereby, the aluminum content of the sponge titanium can be further reduced.

[0036] (Bath discharge step) After the completion of the reduction step, the inside of the reduction vessel 1 stores a molten bath Bm containing metallic magnesium that was not used in the reduction reaction in the reduction step (also referred to as "unreacted metallic magnesium") and by-products such as magnesium chloride generated in the reduction reaction, and the sponge titanium mass TS is immersed in the molten bath Bm. In the bath discharge step, the molten bath Bm is discharged from the inside of the reduction vessel 1 using, for example, the discharge pipe 5 while leaving the sponge titanium mass TS in the reduction vessel 1.

[0037] In another bath discharge step after the reduction step, unreacted metallic magnesium may be separated from the molten bath Bm discharged from the inside of the reduction vessel 1 based on the specific gravity difference from magnesium chloride or the like. The unreacted metallic magnesium recovered in this way in the bath discharge step can also be used as part of the metallic magnesium in the reduction step. However, even in this case, the metallic magnesium used in the reduction step shall include the unreacted metallic magnesium recovered in the vacuum separation step described later. This is because the aluminum content of the sponge titanium cannot be reduced so much with the unreacted metallic magnesium recovered in the bath discharge step.

[0038] (Vacuum separation step) In the pressure reduction separation process performed after the bath discharge process, for example, a recovery container 11 as shown in FIG. 2 can be used. The recovery container 11 is connected and communicated with the reduction container 1 by a connecting pipe 21 connected to each lid body 4, 14, etc. There is no particular limitation on the timing of communicating the recovery container 11 with the reduction container 1 as long as they are communicated before the start of the pressure reduction separation process. The communication between the recovery container 11 and the reduction container 1 can be performed, for example, after the bath discharge process, or it can also be performed before the reduction process. Note that the recovery container 11 with the vertical direction reversed from the illustrated direction may be directly connected to the reduction container 1 without using the connecting pipe 21.

[0039] Note that the recovery container 11 includes a cylindrical side wall 12 such as a cylinder, a bottom 13 that seals one end side in the axial direction of the side wall 12 (the lower end side in FIG. 2), and a lid body 14 attached to the opening at the other end side in the axial direction of the side wall 12 (the upper end side in FIG. 2), and may be substantially the same as the reduction container 1. The reduction container 1 is disposed in a heating furnace (not shown), and the recovery container 11 is cooled by a coolant such as cooling water, a gas such as air, or other refrigerants from around it.

[0040] In the pressure reduction separation process, while heating the inside of the reduction container 1, suction is performed from the recovery container 11 side through the discharge pipe 15 to make the inside of the recovery container, for example, a vacuum or reduced pressure atmosphere of 30 Pa or less. As a result, the unreacted metallic magnesium in the reduction container 1 is separated from the sponge titanium mass TS and is sucked into the recovery container 11 through the connecting pipe 21 from the reduction container 1. At this time, since the recovery container 11 is cooled, the unreacted metallic magnesium solidifies in the recovery container 11 and adheres to the inner surface of the side wall 12, etc.

[0041] The unreacted metallic magnesium recovered in the recovery container 11 in this way is used as at least a part of the metallic magnesium used in the reduction process.

[0042] The recovered material recovered by the recovery container 11 may contain magnesium chloride in addition to the unreacted metallic magnesium described above. In this case, the recovered material can be heated and melted, and the unreacted metallic magnesium contained therein can be separated from magnesium chloride by a specific gravity difference. For example, magnesium chloride that settles downward during heating of the recovered material can be separated from the unreacted metallic magnesium that accumulates on the upper side by discharging it from the lower side. The unreacted metallic magnesium obtained from the recovered material can be used in the reduction process.

[0043] When separating the recovered material from the reduction container 1 into the recovery container 11 in the vacuum separation process, it is preferable to maintain the temperature of the internal space of the recovery container 11 below the melting point of metallic magnesium (650 °C). More specifically, it is preferable that the space temperature at least at the center position of the cross section orthogonal to the depth direction of the recovery container 11 is below the melting point of metallic magnesium. In the recovery container 11, the recovered material adheres to the inner surface of the side wall 12, and the adhesion layer gradually grows from the inner surface side toward the center side of the cross section. For this reason, it becomes difficult to cool from the periphery of the recovery container 11 at the center of the cross section. On the other hand, the recovered material containing unreacted metallic magnesium sucked from the reduction container 1 passes through the internal space inside the adhesion layer that grows from the inner surface of the side wall 12 toward the center side. Therefore, if the recovery container 11 is cooled so that the temperature of the internal space is below the melting point of metallic magnesium, the unreacted metallic magnesium flowing into the recovery container 11 is likely to solidify and be captured there. In this case, the unreacted metallic magnesium often solidifies before reaching the vicinity of the bottom 13 in the recovery container 11, and it is also possible to suppress clogging of the discharge pipe 15 due to the accumulation of unreacted metallic magnesium near the bottom 13. Thereby, it becomes easy to maintain the suction vacuum from the discharge pipe 15.

[0044] The recovery container 11 can accumulate unreacted metallic magnesium therein by being repeatedly used in a plurality of vacuum separation steps. For example, when another reduction step, and subsequent bath discharge step and vacuum separation step are performed multiple times, the same recovery container 11 can be repeatedly used in each vacuum separation step after each different reduction step. In other words, the recovery container 11 used in the vacuum separation step after one reduction step can be repeatedly used in the vacuum separation step after another different reduction step. As a result, the unreacted metallic magnesium separated from the sponge titanium mass TS in each reduction container 1 accumulates in the same recovery container 11. By accumulating the unreacted metallic magnesium in the recovery container 11, it becomes easier to use a relatively large amount of unreacted metallic magnesium in the reduction step.

[0045] In addition, when another reduction step and subsequent vacuum separation step are performed multiple times, different recovery containers 11 may be used in each vacuum separation step. At this time, the unreacted metallic magnesium accumulated in each recovery container 11 can be transferred and collected into one of the recovery containers 11 or other containers. In that case, the unreacted metallic magnesium can be directly transferred between containers such as the recovery container 11, or a separate transfer container may be used.

[0046] (Crushing step) The sponge titanium mass TS taken out from the reduction container 1 can be crushed, for example, in a crushing step, into sponge titanium in the form of granules of a predetermined size or the like. Here, for example, first, chipping is performed manually or the like, and further, using a guillotine-type shear or the like, the upper part, lower part, outer peripheral part, etc. of the sponge titanium mass TS, which often contains a large amount of impurities, are removed. Thereafter, the desired part of the sponge titanium mass TS can be crushed to a certain size.

[0047] Of the sponge titanium thus obtained, the ratio of the mass of the sponge titanium having an aluminum content of 0.5 mass ppm or less to the mass of the sponge titanium mass is preferably 20% or more, more preferably 40% or more. In this embodiment, since the reduction step is performed as described above, a large amount of sponge titanium with a low aluminum content can be produced.

Example

[0048] Next, the method for producing sponge titanium of the present invention was experimentally implemented and its effects were confirmed, and will be described below. However, the description here is for the purpose of mere illustration and is not intended to be limited thereto.

[0049] A reduction step was performed in which titanium tetrachloride was dropped onto metallic magnesium in a reduction vessel and they were reacted. As a result, 8 tons of sponge titanium mass was generated in the reduction vessel. In the following Comparative Examples and Examples 1 to 4, the same amount of metallic magnesium was used. Here, the total amount of metallic magnesium used in the reduction step was previously stored in the reduction vessel before the start of the reduction step, and no metallic magnesium was added while the sponge titanium mass was growing.

[0050] After the reduction step, a bath discharge step and a vacuum separation step were performed to separate magnesium chloride and unreacted metallic magnesium from the sponge titanium mass. Thereafter, a crushing step was performed on the sponge titanium mass taken out from the reduction vessel to produce sponge titanium. In the vacuum separation step, the reduction vessel and the recovery vessel were communicated with each other and the inside of the recovery vessel was maintained in a vacuum state of 30 Pa or less. Also, the temperature inside the recovery vessel was maintained below 650°C.

[0051] (Comparative Example) All of the metallic magnesium used in the reduction process was electrolytic metallic magnesium (electrolytic Mg) obtained by molten salt electrolysis of magnesium chloride, and sponge titanium ingots were produced. Among the sponge titanium obtained from the sponge titanium ingots, the ratio of the mass of sponge titanium having an aluminum content of 0.5 mass ppm or less to the mass of the sponge titanium ingots (low Al sponge ratio) was 5%.

[0052] (Example 1) Twenty mass% of the metallic magnesium used in the reduction process was unreacted metallic magnesium (unreacted Mg), and the remaining 80 mass% was electrolytic metallic magnesium (electrolytic Mg). Here, as the unreacted metallic magnesium, that which remained without being used in another reduction reaction and was separated and recovered by heating under a reduced pressure atmosphere from the sponge titanium ingots in the reduction vessel after that other reduction process was used. Since the recovered material also contained magnesium chloride, specific gravity separation was carried out to obtain unreacted metallic magnesium. In the separation by heating under a reduced pressure atmosphere, the inside of the reduction vessel and the inside of the recovery vessel were made to communicate, and the inside of the recovery vessel was brought to a reduced pressure state of 30 Pa or less. Also, the temperature inside the recovery vessel was maintained below 650°C. The same applies to the unreacted metallic magnesium recovered by reduced pressure separation in Examples 2 to 4 below.

[0053] Among the sponge titanium obtained from the sponge titanium ingots produced in the reduction process, the ratio of the mass of sponge titanium having an aluminum content of 0.5 mass ppm or less to the mass of the sponge titanium ingots (low Al sponge ratio) was 20%.

[0054] (Example 2) All of the metallic magnesium used in the reduction process was unreacted metallic magnesium (unreacted Mg) recovered by reduced pressure separation after another reduction process. Among the sponge titanium obtained from the sponge titanium ingots produced thereby, the ratio of the mass of sponge titanium having an aluminum content of 0.5 mass ppm or less to the mass of the sponge titanium ingots (low Al sponge ratio) was 75%.

[0055] (Example 3) Thirty-five mass% of the metallic magnesium used in the reduction process was unreacted metallic magnesium (unreacted Mg) recovered by vacuum separation after another reduction process, and the remaining 65 mass% was electrolytic metallic magnesium (electrolytic Mg). Among the sponge titanium obtained from the sponge titanium mass produced in this way, the ratio of the mass of sponge titanium with an aluminum content of 0.5 mass ppm or less to the mass of the sponge titanium mass (low Al sponge ratio) was 45%.

[0056] (Example 4) Fifty mass% of the metallic magnesium used in the reduction process was unreacted metallic magnesium (unreacted Mg), and the remaining 50 mass% was electrolytic metallic magnesium (electrolytic Mg). Of the 50 mass% of unreacted metallic magnesium, 25 mass% was unreacted metallic magnesium recovered by vacuum separation after another reduction process, and 25 mass% was unreacted metallic magnesium recovered by discharging the molten bath from the reduction vessel after another reduction process.

[0057] Among the sponge titanium obtained from the sponge titanium mass produced in this way, the ratio of the mass of sponge titanium with an aluminum content of 0.5 mass ppm or less to the mass of the sponge titanium mass (low Al sponge ratio) was 50%.

[0058] [Table 1]

[0059] From the above, it was found that according to this invention, sponge titanium with a relatively low aluminum content can be produced. [Explanation of Signs]

[0060] 1 Reduction vessel 11 Recovery vessel 2, 12 Side walls 3, 13 Bottom 4, 14 Lid 5, 15 Discharge pipe 6 Supply pipe 7 Punches 8 Reduction Furnace 21 Connecting Pipe TS Sponge Titanium Mass Bm Molten Bath Sb Bath Surface

Claims

1. A method for producing sponge titanium, comprising: a reduction step of reacting titanium tetrachloride and metallic magnesium in a molten bath in a reduction vessel to produce a sponge titanium mass; wherein the metallic magnesium used in the reduction step is unreacted metallic magnesium that remained without being used in a reduction reaction in a reduction step different from the reduction step, and is the unreacted metallic magnesium recovered by separating it from the sponge titanium mass in the reduction vessel by heating under a reduced pressure atmosphere after the different reduction step; a method for producing sponge titanium, wherein 20% by mass or more of the metallic magnesium used in the reduction step is the unreacted metallic magnesium recovered by heating under the reduced pressure atmosphere.

2. The method for producing sponge titanium according to claim 1, wherein 35% by mass or more of the metallic magnesium used in the reduction step is the unreacted metallic magnesium recovered by heating under the reduced pressure atmosphere.

3. The recovered product recovered by heating under a reduced pressure atmosphere after the different reduction step contains magnesium chloride and the unreacted metallic magnesium, The method for producing sponge titanium according to claim 1 or 2, wherein the unreacted metallic magnesium recovered by heating under the reduced pressure atmosphere and used in the reduction step is obtained by separating it from the recovered product based on the difference in specific gravity from the magnesium chloride.

4. The method for producing sponge titanium according to any one of claims 1 to 3, comprising a reduced pressure separation step of heating the inside of the reduction vessel under a reduced pressure atmosphere after the different reduction step is completed and the molten bath in the reduction vessel is discharged, and separating unreacted metallic magnesium from the sponge titanium mass into a recovery vessel communicated with the reduction vessel.

5. The different reduction step and the subsequent reduced pressure separation step are performed a plurality of times, The method for producing sponge titanium according to claim 4, wherein the recovery vessel is used in the plurality of reduced pressure separation steps, and the unreacted metallic magnesium is accumulated in the recovery vessel.

6. The method for producing sponge titanium according to claim 4 or 5, wherein in the reduced pressure separation step, the temperature of the internal space of the recovery vessel is maintained below the melting point of metallic magnesium.

7. After the reduction step, the method includes a crushing step of crushing the sponge titanium mass taken out from the reduction vessel to obtain sponge titanium. Of the sponge titanium obtained in the crushing step, the ratio of the mass of the sponge titanium having an aluminum content of 0.5 mass ppm or less to the mass of the sponge titanium mass is 20% or more. The method for producing sponge titanium according to any one of claims 1 to 6.

8. In the reduction step, while the sponge titanium mass is growing in the reduction container, no metallic magnesium is added. The method for producing sponge titanium according to any one of claims 1 to 7.

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