JP4199703B2 - Method for producing metal by molten salt electrolysis - Google Patents

Description

  The present invention relates to the recovery of metals from metal chlorides, and more particularly to a method for producing metals by molten salt electrolysis.

  Conventionally, single metal titanium has been produced by a crawl method in which titanium tetrachloride is reduced with molten magnesium to obtain sponge titanium, and production costs have been reduced by accumulating various improvements. However, since the crawl method is a batch process that repeats a series of operations discontinuously, there is a limit to the efficiency.

  For the above situation, a method of directly producing titanium metal by reducing titanium oxide with metallic calcium in molten salt (for example, see Patent Documents 1 and 2), or a reduction including a metal or alloy such as calcium An EMR method (see, for example, Patent Document 3) is disclosed in which a metallic metal is produced and a titanium compound is reduced by electrons emitted from the reducing metal to obtain metallic titanium. In these methods, calcium oxide by-produced by the electrolytic reaction is dissolved in calcium chloride, and then metal calcium is recovered and reused by performing molten salt electrolysis. However, the metal calcium produced by the electrolytic reaction has a problem that it has a high solubility in calcium chloride because it is in a liquid state, easily dissolves and dissipates, and the yield decreases.

  Furthermore, an attempt to deposit metallic calcium on a cathode in a solid state using a composite molten salt having a melting point lower than that of metallic calcium has been disclosed (for example, see Patent Document 4). However, this method requires special preparation of the composite molten salt, and the problem of increased cost has not been solved.

  As described above, the conventional method has a problem that it is difficult to efficiently recover a metal such as calcium metal, or the cost is high even if it is possible.

WO99 / 064638 JP 2003-129268 A JP 2003-306725 A US3226311

  The present invention has been made in view of the above circumstances, and for example, the metal used for reducing oxides or chlorides of titanium metal can be efficiently recovered, and is implemented by an inexpensive method. It aims at providing the manufacturing method of the metal by molten salt electrolysis which can be performed.

Method for producing a metal by molten salt electrolysis of the present invention includes an anode and a cathode, meet the molten salt containing metal chloride in an electrolytic bath arranged diaphragm composed of graphite between the anode and the cathode, the anode and cathode It is characterized in that molten salt electrolysis is performed by applying a voltage between the theoretical decomposition voltage of the molten salt and not more than twice the voltage between them.

  In metal chloride electrolysis, the reaction occurs in proportion to the voltage applied between the anode and the cathode, but an intermediate electrode (conductor) is installed between the anode and the cathode, and the voltage required for electrolysis is more than twice. When this voltage is applied, this intermediate pole is polarized and electrolysis occurs not only between the anode and the cathode, but also between the anode and the intermediate pole, and between the intermediate pole and the cathode, on one side of the anode and the intermediate pole. Chlorine is generated and metal is deposited on the other surface of the cathode and the intermediate electrode. According to the present invention, since the diaphragm made of graphite is disposed between the anode and the cathode, the chlorine generated at the anode and the metal deposited at the cathode are separated, and the reverse reaction can be suppressed. In addition, since a voltage less than twice the theoretical decomposition voltage of the molten salt is applied, the reaction can proceed efficiently without polarization of the diaphragm.

  Preferred embodiments of the present invention will be described below with reference to the drawings. The following is an example in which the metal is calcium metal, the metal chloride is calcium chloride, the refractory metal chloride is titanium tetrachloride, and the electrolytic bath is calcium chloride, but the metal is sodium metal and the metal chloride is sodium chloride. The same can be applied to the case.

  FIG. 1 shows a preferred apparatus configuration example for carrying out the present invention. In FIG. 1, reference numeral 1 denotes an electrolytic cell, which is filled with an electrolytic bath 2 made of calcium chloride (melting point 780 ° C.) and heated to a melting point of calcium chloride or higher by a heating means (not shown). It is kept in. Reference numeral 3 denotes an anode, and reference numeral 4 denotes a cathode, which is immersed in the electrolytic bath 2. A diaphragm 5 made of, for example, graphite is disposed between the anode 3 and the cathode 4.

  When the anode 3 and the cathode 4 are connected to a DC power source (not shown) and electrolysis of the electrolytic bath 2 is started, chloride ions in the electrolytic bath 2 are attracted to the anode 3 to release electrons, and become chlorine gas 6. Released outside. Calcium ions are attracted to the cathode 4 to receive electrons and become metal calcium 7 to be deposited on the surface of the cathode 4.

  Here, by setting the temperature of the electrolytic bath 2 to be equal to or lower than the melting point of metallic calcium (845 ° C.) and equal to or higher than the melting point of calcium chloride (780 ° C.), solid metallic calcium can be deposited on the cathode 4. On the other hand, even if the temperature of the electrolytic bath 2 is selected to be higher than the melting point of metallic calcium, the present invention can be carried out. In this case, the calcium metal deposited on the cathode 4 floats while being partially dissolved in the electrolytic bath 2 and stays on the surface of the electrolytic bath 2. The metal calcium and the electrolytic bath in which the metal calcium is concentrated can be used, for example, as a reducing agent by direct electrolysis of titanium oxide.

  The chlorine gas and metallic calcium generated as described above tend to diffuse and cause a reverse reaction with each other. However, in the present invention, a diaphragm is provided between the anode and the cathode, so that the reverse reaction can be suppressed. it can.

  When the voltage applied to the anode and the cathode is increased, the amount of current supplied to the electrolytic cell 1 increases, and the metal deposition rate can be increased. However, as the applied voltage increases, both surfaces of the diaphragm 5 are polarized, and when the applied voltage reaches twice the theoretical decomposition voltage, metal is deposited on the anode side of the diaphragm 5 and chlorine gas is deposited on the cathode side of the diaphragm 5. Begins to occur. Chlorine gas generated on the cathode side of the diaphragm 5 causes a reverse reaction with the metal deposited on the cathode 4 and reduces the yield of calcium metal. Therefore, the voltage applied to the anode 3 and the cathode 4 is preferably an electrolysis voltage that does not cause polarization of the diaphragm 5. Such a voltage range is greater than or equal to the theoretical decomposition voltage of calcium chloride and less than or equal to twice, specifically, a range of 3.2V to 6.4V.

  The anode used in the present invention is required to be a material that can withstand high-temperature chlorine gas, and as such a material, graphite is preferable. Graphite not only withstands high-temperature chlorine gas, but also has durability in high-temperature electrolytic baths, and also has good conductivity. In many cases, the anode penetrates the upper lid of the electrolytic cell 1 (not shown) and is immersed in the electrolytic bath 2, and the surface of the anode 3 made of graphite penetrating the upper lid may be coated with ceramic. With such a configuration, it is possible to minimize the wear of graphite.

Since no chlorine gas is generated from the cathode, any material that can withstand a high-temperature molten salt may be used, and it can be made of general carbon steel or stainless steel. The cathode is preferably made of carbon steel because there is a risk of producing refined metal and carbide. Carbon steel is preferred because it withstands hot molten salts or metallic calcium. Moreover, it is inexpensive, durable and practical.

  Like the anode, the diaphragm used in the present invention is required to be composed of a material having durability against high-temperature calcium chloride or chlorine gas, and specifically, graphite is preferable. Although the whole diaphragm may be made of graphite, the strength at a high temperature can be maintained for a long time by making the central part of ceramics and the outside of graphite.

  Although the diaphragm is required to be as dense as possible, the porosity of the diaphragm is a hindrance to the practice of the present invention even if there is a gap that does not allow metal calcium produced at the cathode 4 to permeate and move to the anode side. There is no. Further, the lower end of the diaphragm does not need to reach the bottom of the electrolytic cell, and it is sufficient that the metal calcium produced at the cathode 4 or the calcium chloride layer enriched with metal calcium cannot move to the anode. .

  The generated chlorine gas is extracted out of the system and can be used, for example, for chlorination reaction of titanium ore. Metal calcium can be used for the reduction reaction of titanium oxide or titanium chloride using a molten salt.

  As described above, the diaphragm is made of graphite, and the voltage applied to the anode and the cathode is not less than the theoretical decomposition voltage of calcium chloride and not more than twice the theoretical decomposition voltage. Metallic calcium can be efficiently generated without generating gas.

[Example 1]
The apparatus shown in FIG. 1 was used, a graphite diaphragm was used, an electrolytic bath composed of calcium chloride was maintained at 850 ° C., and 5.0 V was applied between the carbon anode and the cathode composed of carbon steel. A voltage was applied to carry out molten salt electrolysis of calcium chloride. A part of the metallic calcium produced at the cathode was dissolved in the calcium chloride bath, and the rest floated on the calcium chloride bath surface as molten metallic calcium to form a concentrated metallic calcium layer. After being extracted out of the system from the metallic calcium concentrated layer, it was used for direct reduction of titanium oxide. Metal calcium corresponding to 75% of the theoretical value calculated from the amount of current applied to the cathode could be recovered.

[Comparative Example 1]
In Example 1, when the molten salt electrolysis of calcium chloride was performed with the voltage applied to the anode and the cathode set to 7.0 V, metal calcium corresponding to 20% of the theoretical value could not be recovered.

  The metal used for reducing metal titanium oxide or chloride can be recovered with high efficiency.

It is a schematic cross section which shows the electrolytic cell in the molten salt electrolysis of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrolysis tank 2 Electrolytic bath 3 Anode 4 Cathode 5 Diaphragm 6 Chlorine gas 7 Metal calcium

Claims (4)

An electrolytic cell provided with an anode and a cathode, and having a diaphragm made of graphite disposed between the anode and the cathode, is filled with a molten salt containing a metal chloride, and has a voltage not less than the theoretical decomposition voltage of the molten salt and not more than twice the voltage A method for producing a metal by molten salt electrolysis, characterized by subjecting to molten salt electrolysis. The method for producing a metal by molten salt electrolysis according to claim 1 , wherein the metal is recovered as a mixture with a molten salt or a molten material. The method for producing a metal by molten salt electrolysis according to claim 1 or 2, wherein the metal is magnesium, sodium, or calcium. The method for producing a metal by molten salt electrolysis according to any one of claims 1 to 3 , wherein the molten salt is sodium chloride, magnesium chloride, or calcium chloride.

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