By-production gas suction method at molten salt electrolysis and molten salt electrolysis tank

Abstract

To provide a by-production gas suction method capable of resisting corrosion by a gas by-produced under a high temperature by molten salt electrolysis rather than a valve body of an austenitic stainless steel stock and having a valve body with high operation smoothness when the valve body operates, and a molten salt electrolysis tank.SOLUTION: A by-production suction method for sucking a gas by-produced by molten salt electrolysis when metal is manufactured, sucks the gas via a suction pressure control valve having a valve body composed of a carbon material. According to the by-production suction method, it is clarified that corrosion resistance of the valve body with respect to the gas by-produced at molten salt electrolysis can be remarkably improved. In addition, it is confirmed that operation during valve body operation becomes smooth. For this reason, according to the by-production suction method, a usable period of the valve body can be further extended.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for suctioning a gas by-produced during recovery of a metal from a metal compound by molten salt electrolysis, and also to a molten salt electrolytic cell used for molten salt electrolysis. [Background technology]

[0002] Molten salt electrolysis is often used industrially because it can produce metals that cannot be produced by aqueous solution electrolysis. For example, metallic titanium is industrially produced from sponge-like metallic titanium (hereinafter sometimes referred to as "sponge titanium") produced by the Kroll process. The manufacturing process of sponge titanium by the Kroll process is roughly divided into four steps: a chlorination distillation step, a reduction and separation step, a crushing step, and an electrolysis step. In the electrolysis step, which is one of these steps, metallic magnesium is recovered by performing molten salt electrolysis using magnesium chloride, which is a by-product in the reduction step in which titanium tetrachloride is reduced with metallic magnesium to produce sponge titanium. The molten salt electrolytic cell used in this electrolysis step is composed of an electrolysis chamber in which the molten salt is electrolyzed, and a metal recovery chamber in which the metallic magnesium refined by the electrolysis of the molten salt is recovered. Then, magnesium chloride is introduced into the molten salt electrolytic cell, and electrolysis is performed in the electrolysis chamber to produce metallic magnesium and chlorine gas. In this case, chlorine gas, which is a by-product when producing metallic magnesium, is recovered through a chlorine gas suction pipe by adjusting the inside of the electrolysis chamber to a slightly negative pressure environment. In order to keep the electrolysis chamber at a slight negative pressure, the suction pressure control valve controlled in the suction side pipe is very important. In the past, in order to control the suction pressure of chlorine gas generated from a plurality of electrolytic cells, a branch pipe of the chlorine gas suction pipe was provided in the electrolysis chamber, and a molten salt electrolysis method was considered in which the electrolysis chamber of each electrolytic cell was kept at a negative pressure by sucking each electrolytic cell collectively through the branch pipe with a suction machine provided in the main pipe. For example, JP-A-9-125281 discloses a molten salt device that includes a gas transport system in which a gate-type automatic valve having an air-air positioner is provided in each branch pipe, and a control system that detects the suction pressure of chlorine gas in a plurality of electrolytic cells and controls the opening degree of the automatic valve provided in each branch pipe so that each suction pressure is managed to a target value (see JP-A-9-125281). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 9-125281 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, austenitic stainless steel materials such as SUS316 are mainly used as the valve body material of the suction pressure control valve used in molten salt electrolysis. However, there is a problem that the suction pressure control valve using the above stainless steel material as the valve body material is corroded by the by-produced gas (corresponding to chlorine gas in the above example) at high temperatures. In general, the suction pressure control valve in a molten salt electrolytic cell is often installed above the electrolysis chamber where the by-produced gas is generated, so that there is a risk that cracks or breaks will occur in the valve body material weakened by the progression of corrosion, and a part of the valve body material will fall into the electrolysis chamber, and a part of the valve body will come into contact with the refined metal, resulting in quality contamination. In addition, when the corrosion of the valve body progresses, a catch occurs between the valve body surface worn by corrosion and other parts (e.g., packing part, etc.) in the gas suction hole during the operation of the valve body, impairing the smooth operation of the valve body. Due to this malfunction, the suction pressure control valve cannot perform appropriate pressure control, and the pressure in the electrolysis chamber cannot be maintained in an appropriate slightly negative pressure environment. Therefore, when the pressure in the electrolysis chamber becomes a negative pressure environment stronger than the target value, the air is sucked into the electrolysis chamber, and the oxygen and moisture contained in the sucked air react with the metal molten in the electrolysis chamber, resulting in a decrease in power efficiency and a decrease in the purity of the metal obtained by molten salt electrolysis. On the other hand, when the pressure in the electrolysis chamber becomes a positive pressure environment higher than the target value, there is a risk that the gas by-produced by the molten salt electrolysis will leak out of the molten salt electrolytic cell. In addition, due to the nature of molten salt electrolysis, metal compounds in the electrolytic cell may be sucked in as dust along with the sucked in gas by-product, and the sucked dust may accumulate on the corroded valve body surface to form a strong deposit. This leads to serious malfunction of the valve body, and the suction pressure control valve will not be able to perform appropriate pressure control.

[0005] For the above reasons, when corrosion of the valve body is discovered, it is necessary to replace the valve body promptly. Therefore, the corrosion of the valve body is a factor in reducing production efficiency and increasing production costs at manufacturing sites. Therefore, there is a demand for a by-product gas suction method and a molten salt electrolytic cell having a valve body that is more resistant to corrosion by by-product gases and has a longer usable period (hereinafter, sometimes referred to as the "valve body life") than conventional valve bodies.

[0006] An object of the present invention is to provide a by-product gas suction method and a molten salt electrolytic cell having a valve body which is more resistant to corrosion caused by gases by-produced at high temperatures during molten salt electrolysis than a valve body made of austenitic stainless steel and which also exhibits high operational smoothness when the valve body is in operation. [Means for solving the problem]

[0007] A by-product gas suction method according to a first aspect of the present invention is a method for suctioning gas generated as a by-product during the production of a metal by molten salt electrolysis, in which the gas is suctioned through a suction pressure control valve having a valve body made of a carbon material.

[0008] As a result of intensive research by the present inventors, it has become clear that this by-product gas suction method significantly improves the corrosion resistance of the valve body against the gas by-produced during molten salt electrolysis, compared to a by-product gas suction method using a suction pressure control valve whose valve body is made of austenitic stainless steel. It has also been confirmed that the valve body operates smoothly. Therefore, the by-product gas suction method according to the first aspect of the present invention makes it possible to further extend the usable life of the valve body.

[0009] Moreover, the by-product gas suction method according to the first aspect of the present invention functions particularly effectively when the by-product gas is chlorine gas.

[0010] As a result of intensive research by the present inventors, it has become clear that this by-product gas suction method significantly improves the corrosion resistance of the valve body against chlorine gas generated as a by-product during molten salt electrolysis. Therefore, it is possible to further extend the usable period of the valve body even during molten salt electrolysis involving chlorine gas.

[0011] A by-product gas suction method according to a second aspect of the present invention is the by-product gas suction method according to the first aspect, wherein the valve body has a plate shape.

[0012] According to the above configuration, the pressure in the electrolysis chamber can be maintained at an appropriate slightly negative pressure environment. Moreover, according to the above configuration, maintenance work such as replacement or adjustment of the valve body can be easily performed. Furthermore, the plate-shaped valve body is easier to manufacture than valve bodies of other shapes, and can be manufactured at a relatively low cost. Therefore, the material cost when purchasing a plate-shaped valve body can be reduced.

[0013] A by-product gas suction method according to a third aspect of the present invention is the by-product gas suction method according to the first or second aspect, wherein the valve body is made of a carbon fiber reinforced carbon composite material (hereinafter sometimes referred to as "C / C composite").

[0014] As a result of extensive research by the present inventors, the by-product gas suction method using the valve body made of this carbon fiber reinforced carbon composite material has better toughness than valve bodies made of other carbon materials, such as graphite moldings, and therefore the usable life of the valve body can be further extended.

[0015] A by-product gas suction method according to a fourth aspect of the present invention is a by-product gas suction method according to any one of the first to third aspects, wherein the valve body is made of a carbon fiber reinforced carbon composite material (hereinafter sometimes referred to as a "surface-treated C / C composite").

[0016] As a result of intensive research by the inventors of the present application, it has been confirmed that the by-product gas suction method using a valve body made of a carbon fiber reinforced carbon composite material that has been subjected to this surface treatment makes the operation of the valve body even smoother. This makes it possible to further extend the usable period of the valve body. In addition, the smooth surface of the valve body makes it possible to suppress the accumulation of dust on the valve body surface.

[0017] A molten salt electrolytic cell according to a fifth aspect of the present invention includes an electrolytic chamber and a suction pressure control valve. The suction pressure control valve has a valve body that opens and closes a gas suction hole extending from the electrolytic chamber. The valve body is made of a carbon material.

[0018] As a result of intensive research by the present inventors, this molten salt electrolytic cell enables molten salt electrolysis of metal compounds accompanied by by-product gases having corrosive power that could not be tolerated by conventional molten salt electrolytic cells. This makes it possible to produce metals accompanied by by-product gases having high corrosive power. In addition, it is possible to improve the production efficiency and reduce the production cost when producing the metals. [Brief description of the drawings]

[0019] [Figure 1] 1 is a schematic cross-sectional view of a molten salt electrolysis apparatus for illustrating an embodiment of a by-product gas suction method and a molten salt electrolysis cell according to the present invention. FIG. [Diagram 2] FIG. 2 is a schematic front view of a suction pressure control valve for explaining an embodiment of a by-product gas suction method and a molten salt electrolytic cell according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, the embodiments of the present invention will be described in detail with reference to Figs. 1 and 2. However, the present invention is not limited to the embodiments described below. Figs. 1 and 2 are intended to aid in understanding the embodiments of the present invention, and the sizes and positional relationships of the illustrated members may not be accurate. Furthermore, in this specification, "upward" means the direction from the bottom wall 112 side of the molten electrolytic cell 110 to the top lid 120 side, as shown by the arrow in Fig. 1, and "downward" means the direction from the top lid 120 side to the bottom wall 112 side of the molten electrolytic cell 110.

[0021] The by-product gas suction method according to the embodiment of the present invention is a method for suctioning gas generated as a by-product when a metal is produced from a metal compound by molten salt electrolysis. In order to produce a metal from a metal compound by molten salt electrolysis, a molten salt electrolytic cell 110 as shown in Fig. 1 is used. The molten salt electrolysis apparatus 100 will be described in detail below.

[0022] <Molten salt electrolysis equipment> The molten salt electrolysis apparatus 100 shown in Fig. 1 includes a molten salt electrolysis cell 110, an anode 130, a cathode 140, an upper cover 120, an electrolysis chamber 150, and a metal recovery chamber 160. Note that a plurality of bipolar electrodes may be provided between the anode and the cathode.

[0023] 1.Molten salt electrolysis cell As shown in FIG. 1, the molten salt electrolytic cell 110 is composed of a bottom wall 112 and two pairs of side walls 114 connected to the bottom wall and extending upward. The molten salt electrolytic cell 110 stores a molten salt bath Bf made of molten salt containing metal chlorides supplied therein. The molten salt electrolytic cell 110 is partitioned into an electrolysis chamber 150 and a metal recovery chamber 160 by a first partition wall 116 and a second partition wall 117. The molten metal generated by electrolysis of the molten salt in the electrolysis chamber 150 is sent to the metal recovery chamber 160 due to the concentration gradient, and the molten salt is sent from the metal recovery chamber 160 to the electrolysis chamber 150. As a result, the molten metal and the molten salt are circulated in the molten salt bath. That is, the molten salt electrolysis apparatus 100 can ensure the flow of the molten salt bath indicated by arrow A (flow from the electrolysis chamber 150 to the metal recovery chamber 160) by forming the flow port 115 between the first partition wall 116 and the second partition wall 117. In addition, a passage through which the molten salt bath can flow is also formed on the lower surface side of the second partition wall 117, so that the flow indicated by arrow B (flow from the metal recovery chamber 160 to the electrolysis chamber 150) can be ensured.

[0024] 2.Anode The anode 130 is a carbon electrode having a porous structure. The carbon electrode is produced by forming a carbon raw material (such as graphite, coke, or pitch) and then firing it.

[0025] 3.Cathode The cathode 140 is, for example, a steel cathode.

[0026] 4.Top lid The top lid 120 serves to insulate the inside of the molten salt electrolytic bath 110 from the outside because the molten salt bath Bf is at a high temperature. By placing the top lid 120, the inside of the molten salt electrolytic bath 110 becomes a closed space. The top lid 120 is also provided with a first gas intake hole 124. The top lid 120 is further provided with a second gas intake hole 126 and a supply and exhaust hole 128.

[0027] (1) First gas intake hole The first gas intake hole 124 is used to collect chlorine gas generated by electrolysis performed in the electrolysis chamber 150. Therefore, the first gas intake hole 124 is provided in the region where the electrolysis chamber 150 is located. As shown in FIG. 1, a first gas suction pipe 171, which serves as a passage for collecting gas generated from the anode 130 by electrolysis of the molten salt, extends upward from the first gas intake hole 124 relative to the first gas intake hole 124. A first suction pressure control valve 170 is provided on the first gas suction pipe 171 so as to be capable of opening and closing the internal passage of the first gas suction pipe 171.

[0028] (2) Second gas intake hole The second gas intake hole 126 is used to collect the remaining gas generated by electrolysis that has not been collected by the first gas intake hole 124 and has flowed to the metal recovery chamber 160. For this reason, the second gas intake hole 126 is provided in the area where the metal recovery chamber 160 is located. As shown in FIG. 1, a second gas suction pipe 181 extends upward from the second gas intake hole 126 relative to the second gas intake hole 126. The second gas suction pipe 181 serves as a passage for collecting the remaining gas that has not been collected by the first gas intake hole 124 and has flowed to the metal recovery chamber 160. A second suction pressure control valve 180 is provided on the second gas suction pipe 181 so as to be capable of opening and closing the internal passage of the second gas suction pipe 171.

[0029] (3) Suction pressure control valve The first suction pressure control valve 170 and the second suction pressure control valve 180 play a role in adjusting the inside of the molten salt electrolytic cell 110 to a slightly negative pressure environment with respect to the external environment in order to prevent leakage of chlorine gas generated from the anode 130 during molten salt electrolysis. The configuration of the first suction pressure control valve 170 will be described below with reference to Fig. 2. Note that the configuration of the second suction pressure control valve 180 in the molten salt electrolytic cell according to the embodiment of the present invention is similar to that of the suction pressure control valve 170 shown in Fig. 2, and therefore description thereof will be omitted here.

[0030] The suction pressure control valve 170 mainly comprises a valve element 210, a valve body 220, and a handle portion 230. The valve element 210 of the molten salt electrolytic cell according to the embodiment of the present invention is made of a carbon material having better corrosion resistance than austenitic stainless steel material. This provides better corrosion resistance than a valve element made of austenitic stainless steel material against gases by-produced during metal production, such as chlorine gas (Cl2) by-produced when regenerating metallic magnesium (Mg) from magnesium chloride (MgCl2) produced in the electrolysis step in the Kroll process.

[0031] The carbon material referred to here is not particularly limited, but may include carbon fiber, carbon fiber reinforced plastic (CFRP), glassy carbon (glassy carbon), graphite molding material, carbon fiber reinforced carbon composite material (C / C composite), and these materials subjected to surface treatment. Among them, in the molten salt electrolytic cell according to the embodiment of the present invention, it is preferable to use a graphite molding material for the valve body of the suction pressure control valve, it is more preferable to use a carbon fiber reinforced carbon composite material (C / C composite), and it is particularly preferable to use a carbon fiber reinforced carbon composite material subjected to surface treatment (C / C composite subjected to surface treatment).

[0032] The above-mentioned surface treatment is not particularly limited, but it is preferable to form a coating of pyrolytic carbon on the surface of the carbon fiber reinforced carbon composite material by chemical vapor deposition (CVD) or chemical vapor impregnation (CVI). The thickness of the coating formed on the surface of the carbon fiber reinforced carbon composite material by the above-mentioned surface treatment is preferably 1 μm or more and 10 μm or less, and more preferably 2 μm or more and 5 μm or less.

[0033] The by-product gas suction method according to the embodiment of the present invention is a method for suctioning a by-product gas using a molten electrolytic cell according to the embodiment of the present invention, and therefore exhibits superior corrosion resistance to the conventional method using a valve body made of austenitic stainless steel material.

[0034] (4) Supply / discharge hole The supply and discharge hole 128 is used for recovering the molten metal produced by electrolysis in the electrolysis chamber 150 and for supplying the molten salt to the molten salt electrolytic cell 110. For this reason, the supply and discharge hole 128 is provided in the region where the metal recovery chamber 160 is located.

[0035] 5.Electrolysis chamber In the electrolysis chamber 150, a molten metal is generated by electrolyzing the molten salt by applying a predetermined voltage between the anode 130 and the cathode 140. For example, when the molten salt is magnesium chloride, the electrolysis of magnesium chloride generates metallic magnesium, which is the molten metal, and chlorine gas, which is the by-product gas. In this case, in addition to the above-mentioned magnesium chloride as the molten salt, calcium chloride (CaCl2), sodium chloride (NaCl), calcium fluoride (CaF2), etc. may be mixed as a supporting salt.

[0036] 6. Metal Recovery Room The metal recovery chamber 160 is in communication with the electrolysis chamber 150. Therefore, in the metal recovery chamber 160, the molten metal produced by the electrolysis carried out in the electrolysis chamber 150 can be recovered.

[0037] Next, the electrolysis process for producing metals from metal compounds using the molten salt electrolysis apparatus 100 according to the embodiment of the present invention will be described in detail.

[0038] <Electrolysis process> In the electrolysis step, electrolysis of metal compounds contained in the molten salt bath Bf is carried out. The molten salt bath Bf flows from the electrolysis chamber 150 through the flow port 115 to the metal recovery chamber 160 as shown by the arrow A in FIG. 1, and flows from the metal recovery chamber 160 through the lower side of the second partition wall 117 to the electrolysis chamber 150 as shown by the arrow B in FIG. 1. In the electrolysis chamber 150, the metal compounds in the molten salt bath Bf are electrolyzed to generate molten metal and by-product gas. This molten metal flows into the metal recovery chamber 160 by the flow of the molten salt bath Bf. Thereafter, the molten metal, which has a smaller specific gravity than the molten salt, floats above the liquid surface in the metal recovery chamber 160 and accumulates. The molten metal that floats above the liquid surface in the metal recovery chamber 160 is recovered by inserting a recovery pipe or the like into the supply and discharge port 128.

[0039] The above-mentioned by-product gas is generated from the anode 130 during molten salt electrolysis and mainly remains in the electrolysis chamber 150. At this time, the first suction pressure control valve 170 provided on the first gas suction pipe 171 keeps the pressure inside the molten salt electrolytic cell 110 at a slightly negative pressure environment with respect to the external environment, thereby preventing the by-product gas from leaking outside the molten salt electrolytic cell 110. When the first suction pressure control valve 170 is opened, the by-product gas remaining in the electrolysis chamber 150 is collected via the first gas intake hole 124.

[0040] Meanwhile, as described above, there is also by-product gas that has moved to the metal recovery chamber 160 side without being recovered through the first gas intake hole 124. In this case, the second suction pressure control valve 180 provided on the second gas suction pipe 181 keeps the pressure inside the molten salt electrolytic cell 110 at a slightly negative pressure environment with respect to the external environment, thereby preventing the by-product gas from leaking outside the molten salt electrolytic cell 110. When the second suction pressure control valve 180 is opened, the by-product gas remaining in the metal recovery chamber 160 is recovered through the second gas intake hole 126.

[0041] <Features of the by-product gas suction method according to the embodiment of the present invention> According to the by-product gas suction method of the embodiment of the present invention, as described above, the by-product gas generated from the electrolysis chamber 150 in the molten salt electrolytic cell 110 can be collected through the first gas suction hole 124 or the second gas suction hole 126 by maintaining the pressure in the molten salt electrolytic cell 110 at an appropriate slightly negative pressure environment using the first suction pressure control valve 170 or the second suction pressure control valve 180. Furthermore, the molten salt electrolytic cell according to the embodiment of the present invention is provided with the suction pressure control valve 200 having the valve body 210 made of a carbon material that is more corrosion resistant than austenitic stainless steel material, thereby enabling electrolysis of metal compounds accompanied by highly corrosive by-product gas.

[0042] <Modification>

[0043] (A) In the above embodiment, the present invention is applied to a molten salt electrolysis apparatus for producing metallic magnesium, which is a molten metal, from magnesium chloride, which is a molten salt, but the present invention may also be applied to a molten salt electrolysis apparatus that regenerates pure metals from other metal compounds.

[0044] (B) In the previous embodiment, the top lid 120 is provided with the first gas intake hole 124, the second gas intake hole 126, and the supply and exhaust hole 128, but the present invention may also be applied to cases where the top lid 120 is provided with only the first gas intake hole 124, where the top lid 120 is provided with the first gas intake hole 124 and the second gas intake hole 126, where the top lid 120 is provided with the first gas intake hole 124 and the supply and exhaust hole 128, etc.

[0045] (C) In the previous embodiment, the first gas intake hole 124 is provided in the area where the electrolysis chamber 150 is located, but the present invention may also be applied to cases where the first gas intake hole 124 is provided in the area where the metal recovery chamber 160 is located.

[0046] (D) In the previous embodiment, the second gas intake hole 126 is provided in the area where the metal recovery chamber 160 is located, but the present invention may also be applied to cases where the first gas intake hole 126 is provided in the area where the electrolysis chamber 150 is located.

[0047] (E) In the previous embodiment, the first suction pressure control valve 170 and the second suction pressure control valve 180 are provided on the first gas suction pipe 171 and the second gas suction pipe 181, respectively, but the present invention may also be applied to a case where the first gas suction pipe 171 and the second gas suction pipe 181 are not provided. In this case, for example, the first suction pressure control valve 170 and the second suction pressure control valve 180 are provided at positions where they can open and close the first gas suction hole 124 and the second gas suction hole 126.

[0048] (F) In the previous embodiment, the configuration of the second suction pressure control valve 180 was similar to the configuration of the first suction pressure control valve 170, but the present invention may be applied in cases where the configurations of the first suction pressure control valve 170 and the second suction pressure control valve 180 are different.

[0049] (G) In the previous embodiment, the first suction pressure control valve 170 and the second suction pressure control valve 180 are gate-type automatic valves, but the present invention may also be applied when other types of suction pressure control valves are used. <Examples and Comparative Examples> The present invention will be described in more detail below with reference to examples and comparative examples, although the present invention is not limited to these examples. EXAMPLES

[0050] Using the molten salt electrolysis apparatus 100 shown in FIG. 1, magnesium chloride was electrolyzed in the molten salt electrolyte 110 by the above-mentioned method to generate metallic magnesium and chlorine gas. The electrolysis was continued for 360 days, and then the electrolysis was terminated. Thereafter, a graphite molding (bulk density: 1.77 Mg / m 3The condition of the valve body 210 made of a commercially available valve with a bending strength of 43 MPa and a degree of corrosion were checked. The results are shown in Table 1. EXAMPLES

[0051] The valve body 210 is made of C / C composite (bulk density: 1.58 Mg / m 3 The molten salt electrolysis apparatus 100 was operated in the same manner as in Example 1, except that a valve body 210 made of a C / C composite was used (a commercially available product with a bending strength of 185 MPa). Thereafter, the state of the valve body 210 and the degree of corrosion were confirmed. The results are shown in Table 1. EXAMPLES

[0052] The material constituting the valve body 210 was a C / C composite with a 3 μm thick pyrolytic carbon coating formed on the surface by the chemical vapor impregnation (CVI) method (surface-treated C / C composite (bulk density: 1.58 Mg / m 3 The molten salt electrolysis apparatus 100 was operated in the same manner as in Example 1, except that a C / C composite was used in which a 3 μm thick pyrolytic carbon coating was formed on a commercially available C / C composite with a bending strength of 185 MPa. Thereafter, the condition of the valve body 210 made of the surface-treated C / C composite and the progress of corrosion were confirmed. The results are shown in Table 1.

[0053] Comparative Example 1 The molten salt electrolysis apparatus 100 was operated in the same manner as in Example 1, except that an austenitic stainless steel material (hereinafter, sometimes referred to as a "stainless steel valve body") was used as the constituent material of the valve body 210. Thereafter, the state of the stainless steel valve body 210 and the progress of corrosion were confirmed. The results are shown in Table 1.

[0054] The evaluation results of the valve bodies 210 in Examples 1 to 3 and Comparative Example 1 are shown below.

[0055] (Evaluation items) -Corrosion resistance of valve body: The valve body after electrolysis was checked for rust or other corrosion by comparing images of the valve body before and after use. The results are shown as an index, with the corrosion resistance of stainless steel valve body set at 100. -Toughness of valve body: The valve body after electrolysis was checked for cracks or breaks by visual and microscopic observation. The results are shown as an index, with the toughness of a stainless steel valve body set at 100. - Smoothness of valve operation: The smoothness of the valve operation during use was periodically evaluated to check whether the valve opened and closed smoothly during the electrolysis. The evaluation was confirmed by measuring the operating speed of the valve in response to air control or electric control. The results are shown as an index, with the smoothness of the stainless steel valve being set at 100. Valve life: Indicated as an index with the life of a stainless steel valve being 100.

[0056] [Table 1]

[0057] (Evaluation Criteria) The evaluation results for each item of the stainless steel valve body are shown as relative evaluation values, with the evaluation result for each item being set at 100. Complete resistance: This means that no rust or loss of any part due to corrosion by the by-produced gas was observed during the evaluation period. From the above investigation, it was confirmed that the stainless steel valve body (in the case of Comparative Example 1) had rust on the entire valve body 5 months after the start of the electrolysis, and that a part of the valve body had been lost due to corrosion. It was also confirmed that the stainless steel valve body had a problem in smooth operation due to a jamming that occurred when opening and closing the valve body 3 months after the start of the electrolysis.

[0058] On the other hand, in the case of the valve body using graphite molding material as the constituent material of the valve body (in the case of Example 1), even 9 months after the start of the above-mentioned electrolysis, no rust or partial loss due to corrosion was found in the valve body, and it was confirmed that there were no problems with the smooth operation of the valve body.

[0059] Furthermore, in the case of the valve body using the C / C composite as the constituent material of the valve body (in the case of Example 2), no rust or partial loss due to corrosion was found in the valve body even 12 months after the start of the electrolysis. In addition, it was confirmed that there were no problems with the smooth operation of the valve body and that the toughness of the valve body was improved compared to the valve body using the graphite molding material.

[0060] Furthermore, in the case of the valve body (in the case of Example 3) in which a C / C composite with a surface treatment was used as the constituent material of the valve body, no rust or partial loss due to corrosion was found in the valve body even 15 months after the start of the electrolysis. In addition, it was confirmed that there were no problems with the smooth operation of the valve body, and that the toughness of the valve body was improved compared to the valve body using a graphite molding material.

[0061] <Summary> In the by-product gas suction method according to the embodiment of the present invention, a valve body made of a carbon material is used. As a result of the inventor's intensive research, it has become clear that the valve body made of the above-mentioned carbon material can withstand a corrosive environment caused by gas generated as a by-product during molten salt electrolysis, compared to a valve body made of a conventional austenitic stainless steel material. It has also been confirmed that the valve body made of a carbon material operates more smoothly when the valve body is activated than a conventional valve body. From this, it has become clear that the by-product gas suction method according to the embodiment of the present invention significantly reduces the occurrence of corrosion wear of the valve body compared to the conventional method. As a result, it has become clear that the frequency of replacement work for the valve body is significantly reduced compared to the conventional method.

[0062] Furthermore, the inventors of the present application have found through their intensive research that when a carbon material is used for the valve body material, the valve body toughness is further improved when a carbon fiber reinforced carbon composite material (C / C composite) is used for the valve body material compared to when a graphite molding material is used for the valve body material, and therefore it has become clear that the usable life of a valve body made of a carbon fiber reinforced carbon composite material is further extended compared to when a valve body made of a graphite molding material is used.

[0063] Furthermore, the inventors of the present application have made it clear that, compared to when a carbon fiber reinforced carbon composite material is used as the valve body material, when a surface-treated carbon fiber reinforced carbon composite material (a surface-treated C / C composite) is used, the valve body surface becomes smoother, and dust accumulation on the valve body surface can be suppressed. Therefore, it has become clear that the usable period of a valve body made of a surface-treated carbon fiber reinforced carbon composite material can be further extended compared to when a valve body made of a carbon fiber reinforced carbon composite material is used. [Explanation of symbols]

[0064] 100 Molten salt electrolysis device 110 Molten salt electrolysis cell 112 Bottom wall 114 Side wall 115 Distribution port 116 First Bulkhead 117 Second Bulkhead 120 Top lid 121 Back of the lid 124 First gas suction hole 126 Second gas suction hole 128 Supply / discharge hole 130 Anode 140 Cathode 150 Electrolysis Room 160 Metal Recovery Room 170 First suction pressure control valve 180 Second suction pressure control valve 171 First gas suction pipe 181 Second gas suction pipe A, B arrows Bf molten salt bath 210 Valve body 220 Valve Body 230 Handle

Claims

1. A method for suctioning gas produced as a by-product during the production of metals by molten salt electrolysis. The gas is sucked through a suction pressure control valve having a valve body made of a carbon material. A method for suctioning by-product gases during molten salt electrolysis.

2. The valve body is plate-shaped. The method for sucking out by-product gas during molten salt electrolysis according to claim 1.

3. The valve body is made of a carbon fiber reinforced carbon composite material.

3. A method for sucking out by-product gas during molten salt electrolysis according to claim 1 or 2.

4. The valve body is made of a carbon fiber reinforced carbon composite material that has been subjected to a surface treatment.

3. A method for sucking out by-product gas during molten salt electrolysis according to claim 1 or 2.

5. 2. The by-product gas suction method according to claim 1, wherein the gas is chlorine gas.

6. An electrolysis chamber; a suction pressure control valve having a valve body made of a carbon material for opening and closing a gas suction hole extending from the electrolysis chamber; A molten salt electrolysis cell comprising:

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