Patent Application
20070187255
The present invention relates to a method for producing Ti or Ti alloys through reduction by Ca, in which a metallic chloride containing titanium tetrachloride (TiCl4) is reduced by Ca to produce Ti metals or Ti alloys.
A Kroll method for reducing TiCl4 by Mg is generally used as a method for industrially producing the Ti metals. TiCl4 is obtained by chlorinating titanium oxide (TiO2). In the Kroll method, the Ti metals are produced through a reduction step and a vacuum distillation step. In the reduction step, TiCl4 is reduced by Mg in a reactor vessel. In the vacuum distillation step, unreacted Mg and MgCl2 formed as a by-product are removed from the sponge metallic Ti produced in the reactor vessel.
In the reduction step, the reactor vessel is filled with the molten Mg, and the TiCl4 liquid is supplied from above on a liquid surface of the molten Mg. This allows TiCl4 to be reduced by Mg near the liquid surface of the molten Mg to generate the particulate metallic Ti. The generated Ti metals move sequentially downward. At the same time, the molten MgCl2 which is of the by-product is generated near the liquid surface. A specific gravity of molten MgCl2 is larger than that of the molten Mg. The molten MgCl2 which is of the by-product moves downward due to the specific-gravity difference, and the molten Mg emerges in the liquid surface instead. The molten Mg is continuously supplied to the liquid surface by the specific-gravity difference substitution, and the reducing reaction of TiCl4 proceeds continuously.
In the production of Ti metals by the Kroll method, a high-purity product is produced. However, the production cost is increased and the price of the product becomes remarkably expensive. One of factors of increased production cost is the difficulty in enhancing a feed rate of TiCl4. The following items (a) to (c) are cited as the reason why the feed rate of TiCl4 is restricted.
(a) In order to improve productivity in the Kroll method, it is effective to enhance the feed rate of TiCl4, i.e., to enhance a supply amount of molten Mg to the liquid surface per unit area or unit time. However, when the feed rate of TiCl4 is excessively enhanced, the rate of the specific-gravity difference substitution cannot keep up with the reaction rate, so that while MgCl2 remains in the liquid surface, TiCl4 is supplied to the MgCl2. As a result, the supplied TiCl4 becomes lower grade chloride gases (referred to as “unreacted gas”) such as an unreacted TiCl4 gas and a TiCl3 gas, and the unreacted gas is discharged outside the reactor vessel, which reduces utilization efficiency of TiCl4. It is necessary to avoid the generation of the unreacted gas, because a rapid increase in inner pressure of the reactor vessel is associated with the generation of the unreacted gas. Thus, there is a limit of the feed rate of TiCl4 because of the above reasons.
(b) When the feed rate of TiCl4 is enhanced, Mg vapor generated from the liquid surface of the molten Mg reacts with TiCl4 vapor to increase a precipitation amount of Ti in the inner surface of the reactor vessel above the liquid surface of the molten Mg. On the other hand, the level of the liquid surface of the molten Mg rises as the reducing reaction proceeds. Therefore, the precipitated Ti in the inner surface of the upper portion of the reactor vessel is immersed in the molten Mg at a later stage of the reducing reaction, which causes the effective area of the Mg liquid surface to be decreased to reduce the reaction rate. In order to suppress the decrease of reaction rate, it is necessary that the feed rate of TiCl4 be restricted to prevent the Ti precipitation in the inner surface of the upper portion of the reactor vessel.
Japanese Patent Application Publication No. 8-295955 proposes a method in which the reaction efficiency is enhanced by supplying liquid TiCl4 in a dispersive manner to the liquid surface in which the molten Mg exists, and thereby the Ti precipitation is suppressed in the inner surface of the upper portion of the reactor vessel. However, the method proposed in Japanese Patent Application Publication No. 8-295955 is not enough to suppress the Ti precipitation.
(c) In the Kroll method, because the reaction is performed only near the liquid surface of the molten Mg in the reactor vessel, an exothermic area is narrowed and the temperature is locally elevated. Therefore, cooling becomes difficult, so that the feed rate of TiCl4 is restricted.
Although the feed rate of TiCl4 is not directly affected, in the Kroll method, Ti is generated in the particulate form near the liquid surface of the molten Mg, and aggregated because of wetting properties (adhesion properties) of the molten Mg, and the Ti particles is made move downward while aggregated, and then the Ti particles are sintered to grow the Ti particles by the heat generated from the molten liquid during the downward travel. Therefore, it makes difficult to recover the generated Ti by taking out Ti as fine particles to the outside of the reactor vessel, whereby the continuous production is difficult to perform and the improvement of the productivity is fettered. By reason of this, the Ti is produced in the batch process in the form of the sponge titanium.
With reference to the Ti production methods except for the Kroll method, for example, U.S. Pat. No. 2,205,854 describes that, in addition to Mg, for example, Ca can be used as the reducing agent of TiCl4. U.S. Pat. No. 4,820,339 describes a method for producing Ti through the reducing reaction by Ca, in which the molten salt of CaCl2 is held in the reactor vessel, the metallic Ca powder is supplied into the molten salt from above, Ca is dissolved in the molten salt, and TiCl4 gas is supplied from below to react the dissolved Ca with TiCl4 in the molten salt of CaCl2.
In the reduction by Ca, the Ti metals are generated from TiCl4 by the reaction of the following chemical formula (1), and CaCl2 as the by-product is also generated at the same time:
TiCl4+2Ca→Ti+2CaCl2 (1)
Ca has an affinity for Cl stronger than that of Mg, and Ca is suitable to the reducing agent of TiCl4 in principle. Particularly, in the method described in U.S. Pat. No. 4,820,339, Ca is used while dissolved in the molten CaCl2. When the reducing reaction by Ca is utilized in the molten CaCl2, an area (reaction field) where the reaction is created is enlarged compared with the Kroll method in which TiCl4 is supplied to the liquid surface of the reducing agent in the reactor vessel. Therefore, because the exothermic area is also enlarged to facilitate the cooling, the feed rate of TiCl4 can be largely enhanced, and the remarkable improvement of the productivity can be also expected.
However, the method described in U.S. Pat. No. 4,820,339 is hardly adopted as the industrial Ti production method. In the method, because the highly expensive metallic Ca powder is used as the reducing agent, the production cost is higher than that of the Kroll method.
U.S. Pat. No. 2,845,386 describes another Ti production method (Olsen method) in which TiO2 is directly reduced by Ca not through TiCl4. The method described in U.S. Pat. No. 2,845,386 is a kind of oxide direct-reduction method and is highly efficient. However, the oxide direct-reduction method is not suitable to the production of the high-purity Ti because it is necessary to use high-purity TiO2.
It is an object of the present invention to provide a method for economically producing a high-purity Ti metals or high-purity Ti alloys with high efficiency, without using an expensive reducing agent.
In order to achieve the above object, the present inventors consider it indispensable that TiCl4 be reduced by Ca, and the present inventors look into the method for utilizing Ca dissolved in the molten salt of CaCl2 described in U.S. Pat. No. 4,820,339.
In the method described in U.S. Pat. No. 4,820,339, Ca in the molten salt is consumed in the reducing reaction reactor vessel as the reaction expressed by the chemical formula (1) proceeds, and it is necessary to continuously supply the metallic Ca powder to the reduction reactor vessel. However, in order to industrially establish the method for producing Ti through reduction by Ca, the present inventors propose a method for controlling a dissolved Ca concentration in the molten salt by electrolysis, in consideration of the fact that it is necessary that the consumed Ca of the molten salt in the reducing reaction is economically replenished.
That is, when the molten CaCl2 is electrolyzed in a reactor cell, electrode reactions expressed by the following chemical formulas (2) and (3) proceed to generate a Cl2 gas near the surface of a anode electrode while generating Ca near the surface of a cathode electrode, which allows the Ca concentration to be increased in the molten salt. Therefore, when TiCl4 is supplied to CaCl2 so as to react with Ca generated on the cathode electrode side, because Ca consumed in the Ti generation is replenished as needed, the replenishment of metallic Ca from the outside or extraction of metallic Ca becomes unnecessary, which allows the Ti metals to be economically produced.
Anode electrode: 2Cl−→2e−+Cl2 (2)
Cathode electrode: Ca2++2e−→Ca (3)
The method for replenishing Ca, consumed in the reduction of TiCl4, with Ca generated by the electrolysis can also be achieved by respectively performing the reduction and the electrolysis in a reduction cell and an electrolytic cell to circulate the molten CaCl2 between the cells. However, when TiCl4 is supplied to the molten CaCl2 in the reactor cell so as to react with Ca generated on the cathode electrode side by the electrolysis, the reactor cell can commonly be used as the reduction cell and the electrolytic cell. Therefore, because it is not necessary to separately provide the reduction cell and the electrolytic cell, there is also a great advantage from a viewpoint of installation cost compared with the case in which the molten CaCl2 is circulated between the reduction cell and the electrolytic cell.
The present invention is made based on the above conception, and the gist of the present invention pertains to a method for producing Ti or Ti alloys.
That is, a method for producing Ti or Ti alloys through reducing reaction by Ca includes: a reduction electrolysis step comprising holding a molten salt in a reactor cell to perform electrolysis in the molten salt in the reactor cell, the molten salt containing CaCl2 and having Ca dissolved in the molten salt and generating Ti or Ti alloys in the molten salt by supplying a metallic chloride containing TiCl4 to the molten salt so as to react with Ca generated on a cathode electrode side by the electrolysis; and a Ti separation step of separating Ti or the Ti alloy from the molten salt in the reactor cell or outside the reactor cell.
The method of the present invention for producing Ti or Ti alloys through reduction by Ca is a method of reducing TiCl4 in which a high-purity material is easily obtained, so that the method of the present invention can produce high-purity Ti metals or high-purity Ti alloys.
Ca is used as the reducing agent to cause the metallic chloride containing TiCl4 to react with Ca in the molten salt containing CaCl2, so that the feed rate of TiCl4 can be increased. Because the Ti particles or Ti alloy particles are generated in CaCl2, the aggregation of the particles and the particle growth caused by the sintering are significantly lessened, whereby it becomes possible to discharge these particles outside reactor cell, thus enabling the continuous operation to be performed. The reducing reaction and the electrolytic reaction are simultaneously caused to proceed, and Ca is replenished by the electrolytic reaction while consumed in the reducing reaction, which allows Ca to be utilized in the state in which Ca is always dissolved in the molten salt.
Accordingly, the production method of the present invention can efficiently and economically produce high-purity Ti metals or high-purity Ti alloys.
In the method according to the present invention for producing Ti or Ti alloys through reduction by Ca, for example, when the molten CaCl2 is held as the molten salt in a reactor cell to supply TiCl4 to the molten salt in the reactor cell, TiCl4 is reduced by Ca dissolved in the molten salt to generate Ti metals in the form of the particulate or powder (hereinafter referred to as “Ti particles”). Although the Ca dissolved in the molten salt is consumed in association with the generation of the Ti particles, Ca is generated on the cathode electrode side to replenish the consumed Ca dissolved in the molten salt because the electrolysis of the molten CaCl2 proceeds simultaneously with the reducing reaction in the reactor cell.
One of the reasons why the Ca is not conventionally used in the industrial production of the Ti metals is the difficulty in separating Ca and CaCl2. Mg is produced by electrolyzing MgCl2, and the generated Mg can efficiently be recovered because Mg is hardly dissolved in MgCl2. Similarly to Mg, Na can efficiently be produced by electrolyzing NaCl. On the other hand, Ca is produced by electrolyzing CaCl2, and it is difficult to efficiently separate only Ca because the generated Ca is dissolved in CaCl2 by about 1.5%. There is also a phenomenon in which the dissolved Ca generates CaCl2 by a back reaction (reaction in which Ca generated on the cathode electrode side is combined with Cl2 generated on the anode electrode side to return to CaCl2). Therefore, the production efficiency of Ca becomes worse. In this regard, although a recovery factor of Ca is improved by applying the contrivance such as cooling the electrode, the production cost of Ca inevitably remains to be still high.
In contrast, in the method of the present invention for producing Ti or Ti alloys through reduction by Ca, Ca dissolved in the molten CaCl2 is used and the separation of Ca is not necessary, so that the electrolysis production cost of Ca can be decreased.
When the reduction by Ca is utilized in the molten CaCl2, the reducing reaction field is expanded and the heat generation/exothermic area is also enlarged. Mg has vapor pressure of 6.7 kPa (50 mmHg) at 850° C. while Ca has extremely small vapor pressure of 0.3 kPa (2 mmHg). Therefore, in the case where Ca is used for the reduction, the Ti precipitation amount becomes dramatically lessened in the inner surface of the upper portion of the reactor cell compared with Mg. Accordingly, in the method of the present invention for producing Ti or Ti alloys through reduction by Ca, the feed rate of TiCl4 can largely be increased.
In addition, Ca is inferior in wetting properties (adhesion properties) to Mg, and Ca adhering to the precipitated Ti particles is dissolved in CaCl2, so that the aggregation in the generated titanium particles and the particle growth caused by the sintering are significantly lessened. Therefore, the generated Ti can be taken out from the reactor cell in the form of particles, and the Ti production can continuously be operated.
For a supply mode of TiCl4 to the molten CaCl2 liquid, it is particularly desirable that TiCl4 be directly supplied in the gaseous state inside the molten CaCl2 liquid, because the contact efficiency of TiCl4 to Ca in the molten CaCl2 liquid is enhanced. Alternatively, it is also possible that TiCl4 is supplied in the gaseous or liquid state to the liquid surface of the molten CaCl2 liquid, or it is also possible that the liquid or gaseous TiCl4 is supplied to the liquid surface or inside the molten Ca liquid held on the molten CaCl2 liquid.
In the case where the reducing reaction is performed by supplying the TiCl4 liquid to the liquid surface of the molten Ca held on the surface of the molten CaCl2 liquid, it is desirable that the molten Ca liquid be held in a thin state to an extent in which Ca in the molten CaCl2 liquid can be utilized. When the Ca layer is thin, because Ca in the molten CaCl2 liquid is also involved in the reaction, the reaction is rendered to take place at the molten Ca layer as well as at the molten CaCl2 layer, and the Ti can continuously be generated even if the specific-gravity difference substitution cannot keep up with the reaction rate due to the increase in feed rate of TiCl4.
With reference to the supply of the TiCl4 gas, an advantage of the method of the present invention for producing Ti or Ti alloys through reduction by Ca over the Kroll method will be described as below.
In the Kroll method, the TiCl4 liquid is supplied to the liquid surface of the molten Mg liquid. It is tried that the TiCl4 gas is supplied into the molten Mg liquid in order to enlarge the reaction field. However, as described above, because the Mg has the high vapor pressure, the Mg vapor intrudes in a supply nozzle of the TiCl4 gas to react with TiCl4, which causes a supply nozzle to be choked.
On the other hand, it is also tried that the TiCl4 gas is supplied inside the molten MgCl2 liquid. Although a choking frequency of the supply nozzle is decreased, the supply nozzle choking problem still remains. This is attributed to the fact that the melt is agitated by bubbling of the TiCl4 gas and sometimes the molten Mg reaches the supply nozzle. As much as anything, even if TiCl4 is supplied inside the molten MgCl2 liquid, the reducing reaction is difficult to occur because Mg is hardly dissolved in the molten salt.
On the contrary, in the method of utilizing the reduction by Ca, the nozzle choking is hardly generated and the TiCl4 gas can be supplied inside the molten CaCl2 liquid. One of the reasons why the nozzle choking is hardly generated is the small vapor pressure of the molten Ca.
That is, in the method of the present invention for producing Ti or Ti alloys through reduction by Ca, it is particularly desirable that TiCl4 be directly supplied in the gaseous state inside the molten CaCl2 liquid, and this supply mode can be applied without any problem in the actual operation. It is also possible that the liquid or gaseous TiCl4 is supplied to the liquid surface of the molten CaCl2 liquid, or it is also possible that the liquid or gaseous TiCl4 is supplied to the liquid surface or inside the molten Ca liquid held on the molten CaCl2 liquid.
In separating the Ti particles generated in the molten CaCl2 liquid, it is possible to separate the Ti particles from the molten CaCl2 liquid either in the reactor cell or outside the reactor cell. However, the separation becomes the batch process when the separation is performed in the reactor cell. In order to improve the productivity, the Ti particles and the molten CaCl2 liquid may be separated from each other outside the reactor cell by utilizing the Ti generated in the particulate form to discharge the Ti particles outside the reactor cell along with the molten CaCl2 liquid. The Ti particles can simply be separated from the molten CaCl2 liquid by a squeezing operation and the like by means of mechanical compression.
In the case where Ti is produced by the method of the present invention, usually TiCl4 is used as a raw material. The Ti alloy can also be produced by using a mixture of TiCl4 and other metallic chloride. Because TiCl4 and other metallic chloride are simultaneously reduced by Ca, the Ti alloy can be produced by this method. Said other metallic chloride may be used either in the gaseous or liquid state.
In the method of the present invention for producing Ti or Ti alloys through reduction by Ca, the back reaction and the wearing of the reactor material become of issues. In the back reaction, Ca (Ca generated on the cathode electrode side or unreacted Ca) in the molten CaCl2 is combined with Cl2 generated on the anode electrode side to return to CaCl2. The wearing of the reactor material is caused by high reactivity of Ca.
When the back reaction is generated, the electrolytic current is consumed for the back reaction, which decreases current efficiency. Particularly, for the back reaction in which Ca generated on the cathode electrode side is combined with Cl2 generated on the anode electrode side, it is desirable to separate the inside of the cell into the anode electrode side and the cathode electrode side by providing a partition wall (see
For the problem of the wearing of the reactor material, the molten salt is formed not by single CaCl2 but by the mixed salt, and a melting point of the molten salt is decreased to effectively decrease the temperature of the molten salt (namely, bath temperature).
That is, in the method of the present invention for producing Ti or Ti alloys through reduction by Ca, basically CaCl2 having the melting point of 780° C. is used as the molten salt. However, a binary system molten salt such as CaCl2—NaCl and CaCl2—KCl and a ternary system molten salt such as CaCl2—NaCl—KCl can also be used such that at least one kind of other salts (for example, NaCl, KCl, LiCl, and CaF2) is mixed to CaCl2 to form a multiple system molten salt. Therefore, because the melting point of the salt is decreased, the temperature of the molten salt (bath temperature) can be decreased. For example, when CaCl2 and NaCl (having the melting point of about 800° C.) are mixed together, the melting point can be decreased to about 500° C. at the lowest.
As a result, the extension of reactor material life and the reactor material cost reduction can be achieved, and further the vaporization of Ca or the salt can be suppressed from the liquid surface.
An embodiment mode of the present invention will be described below with reference to the drawing.
In the reactor cell 1, the molten CaCl2 which is of the molten salt is electrolyzed by passing the electric current between a anode electrode 2 and a cathode electrode 3, the Cl2 gas is generated on the side of anode electrode 2, and Ca is generated on the side of cathode electrode 3. In the example, the inside of the reactor cell 1 is divided into the anode electrode side and the cathode electrode side by a partition wall 4. However, in the partition wall 4, the lower portion is opened in order that the transfer of the molten salt is not prevented.
In the reactor cell 1, the gaseous TiCl4 is injected in the dispersive manner inside the molten salt on the cathode electrode side in parallel with the electrolysis of the molten salt. Therefore, the injected TiCl4 is reduced to generate the particulate metallic Ti by the Ca dissolved in the molten salt. The generated Ti particles moves downward by the specific gravity difference and accumulated at the bottom on the cathode electrode side in the reactor cell 1.
The Ti particles accumulated at the bottom of the reactor cell 1 are discharged from the reactor cell 1 along with the molten salt existing at the bottom of the reactor cell 1, and the Ti particles and the molten salt are sent to the Ti separation step (not shown). In the Ti separation step, the Ti particles discharged along with the molten salt from the reactor cell 1 are separated from the molten salt. Specifically, the Ti particles are compressed to squeeze the molten salt. The Ti particles obtained in the Ti separation step is melted to yield Ti ingots.
On the other hand, the molten salt separated from the Ti particles in the Ti separation step is the molten salt after use, in which Ca is consumed to decrease the Ca concentration. It is desirable to reuse the molten salt after use by returning it to the reactor cell. Usually, both the above separated molten salt and the molten salt after use separately discharged from the reactor cell 1 are introduced to the anode electrode side in the reactor cell 1.
Ca in the molten salt is consumed on the cathode electrode side in the reactor cell 1 as the Ti particles are generated by the reducing reaction. However, Ca is generated near the surface of the cathode electrode 3 in the cell by the electrolysis which proceeds simultaneously in the cell, and a consumed amount of Ca is replenished by the Ca generated by the electrolysis. That is, TiCl4 supplied into the molten salt is sequentially reduced in a direct manner by Ca generated near the surface of the cathode electrode 3.
On the other hand, in the desirable mode, the molten salt after use is sent from the Ti separation step onto the anode electrode side in the reactor cell 1. Therefore, a unidirectional flow of the molten salt is formed from the anode electrode side toward the cathode electrode side in the reactor cell 1 to avoid the flow of Ca generated on the cathode electrode side into the anode electrode side. When the partition wall 4 shown in
It is desirable that the Cl2 gas generated on the anode electrode side in the reactor cell 1 be reused in a chlorination step (not shown). In the chlorination step, TiCl4 which is of the raw material of Ti is generated by the chlorination of TiO2. The generated TiCl4 is introduced to the reactor cell 1, and TiCl4 is circularly used to generate the Ti particles by the Ca reduction.
As described above, in this embodiment mode, the generation of the Ti particles by the Ca reduction, i.e., the Ca consumption and the Ca replenishment by the electrolysis are concurrently performed in the reactor cell 1. Therefore, it is not necessary to replenish or take out Ca in the solid state, and the high-quality Ti particles are continuously and economically produced by the Ca reduction. The reactor cell 1 is commonly used as the reduction cell and the electrolytic cell, which contributes largely to an economical merit from the viewpoint of installation. The flow of Ca generated on the cathode electrode side into the anode electrode side is avoided in the reactor cell 1, so that the back reaction in which Ca reacts with the Cl2 gas generated on the anode electrode side can be prevented.
During the operation, the molten salt is managed at a temperature higher than the melting point (about 780° C.) of CaCl2 in the reactor cell 1.
According to the method of the present invention for producing Ti or Ti alloys through reduction by Ca, the feed rate of TiCl4 which is of the raw material can be enhanced, and the continuous production can be realized. Further, the reducing reaction and the electrolytic reaction are simultaneously caused to proceed in the reactor cell, and Ca consumed in the reducing reaction can be replenished by the electrolytic reaction, so that it is not necessary to independently handle Ca by itself.
Accordingly, the production method of the present invention can effectively be used as means for efficiently and economically producing high-purity Ti metals or high-purity Ti alloys, so that the production method of the present invention can widely be applied as the industrial method for producing Ti or Ti alloys.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-044827 | Feb 2004 | JP | national |
| 2004-281341 | Sep 2004 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/02291 | 2/16/2005 | WO | 8/18/2006 |
