JP2014224288A - Electrolytic tank drying method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolytic tank drying method for executing, on an occasion for forcibly heating and drying an Mg electrolytic tank based on bath salt retention, efficient heating and drying treatment while effectively preventing the leakage of the bath salt.SOLUTION: On an occasion for forcibly heating and drying a refractory material layer within an Mg electrolytic tank with a molten salt charged into the tank, the temperature of the intra-tank bath salt is elevated in the initial drying phase of 48 hours or less since the charging of the molten salt in such a way that the average temperature gain ratio of the intra-tank bath salt over the period of 48 hours or less will be 0.75 to 2.5°C/hr. In the electrolytic tank drying method, the average temperature gain ratio of the intra-tank bath salt since the commencement of the bath salt retention drying till the passage of 24 hours is greater than the average temperature gain ratio of the intra-tank bath salt since 24 hours after the commencement of the bath salt retention drying till the passage of 48 hours, whereas the average temperature gain ratio of the intra-tank bath salt since the commencement of the bath salt retention drying till the passage of 24 hours is 1.5 to 5°C/hr. The average temperature of the bath salt following the passage of 48 hours since the charging of the molten salt is assumed to be 620 to 750°C.

Description

  The present invention relates to a drying method for an Mg electrolytic cell used for the production of metallic Mg, and more particularly to a drying method before starting operation of a built-in Mg electrolytic cell, and more specifically, a bath salt holding for holding a molten salt in the bath. The present invention relates to an electrolytic cell drying method by drying.

  Conventionally, a reduction method called a crawl method has been used for producing sponge titanium. In the production of sponge titanium by the crawl method, titanium tetrachloride liquid is dropped into molten Mg in the reaction vessel, and the titanium tetrachloride is reduced with Mg to produce sponge titanium in the reaction vessel as a by-product. MgCl2 is formed. By-product MgCl2 is returned to metallic Mg by the molten salt electrolysis method and reused as a reducing agent in the crawl method.

  In the production of metal Mg by the molten salt electrolysis method, an electrolytic cell constructed of refractory bricks is used. This electrolytic cell is divided into an electrolytic chamber and an Mg recovery chamber. In operation, MgCl2 molten salt is charged into the tank, and Mg is generated by electrolysis in the electrolytic chamber. Mg generated in the electrolysis chamber is transported to the adjacent Mg recovery chamber by the convection of the molten salt, floats on the molten salt in the chamber, and is sequentially recovered. In the electrolytic chamber, chlorine gas is generated simultaneously with the generation of Mg.

  In the production of such metal Mg, increasing the current efficiency in the electrolytic cell is a very important requirement for reducing the production cost of Mg. There are various factors that decrease the current efficiency in the electrolytic cell, one of which is moisture in the electrolytic cell.

  That is, the current efficiency in the electrolytic cell changes over time depending on the number of days elapsed since operation. Usually, current efficiency decreases as the number of working days increases in the latter half of operation. The cause of this is considered to be wear of refractory bricks and electrodes constituting the electrolytic cell. However, this is not the only decrease in current efficiency, but there is also a change in current efficiency that is low immediately after operation and increases as the number of operating days increases. The cause of the decrease in current efficiency in the initial stage of operation is assumed to be moisture in the electrolytic cell. More specifically, it is considered that the current efficiency is adversely affected by the following mechanism.

  When water is mixed in the molten salt, MgO is formed by reaction with Mg or MgCl2. When the MgO concentration in the molten salt increases, the recovery efficiency of Mg decreases, and the reaction of Chemical Formula 1 causes a re-reaction in which the generated Mg or Cl 2 returns to MgCl 2, resulting in a large energy loss. This is a factor that reduces current efficiency. In addition to this, water may generate hydrochloric acid by reaction with chloride, which may cause corrosion of iron structures (cathodes, heat exchangers, etc.) in the electrolytic cell. Furthermore, the formed MgO reacts with the graphite (C), which is an electrode material, so that the electrode is worn and the current efficiency is lowered and the cell life is adversely affected in the late stage of electrolytic operation.

  For this reason, various measures have been taken to reduce moisture in the tank as much as possible before the operation of the electrolytic cell, and one of them is the use of a sufficiently dried refractory brick (Patent Document 1). In addition, although it is not aimed at active moisture reduction, as one of the general construction methods, local burner drying for firing and forming casters, which are one of the irregular refractory materials, in the tank Has also been implemented conventionally (Patent Document 2). However, in the case of conventional electrolyzers, even if aggressive measures against moisture such as using dry bricks are taken, a decrease in current efficiency cannot be avoided for a while from the start of electrolysis operation. As current efficiency declines, it has been considered an inevitable phenomenon. In particular, when an irregular refractory material is used as the refractory material, a drop in current efficiency in the initial stage of electrolytic operation was significant.

  To solve this problem, when an electrolytic cell was constructed and electrolytic operation was started in the electrolytic cell, the molten salt was put into the electrolytic cell and then melted without energization. It is bath salt holding drying which dries the refractory material in the tank while holding the salt in a molten state (Patent Document 3). That is, the molten salt containing MgCl2 is heated to several hundred degrees Celsius. When this is put into the constructed electrolytic cell, the entire interior of the cell is uniformly heated to a high temperature of several hundred degrees C without using any special energy, and the refractory material constituting the electrolytic cell is also heated to a high temperature of several hundred degrees C. By maintaining this state, moisture in the refractory material evaporates and drying proceeds. Part of the moisture discharged from the refractory material is absorbed by the molten salt in the tank. However, Mg and Cl2 are not present in the molten salt unless electrolysis is performed. For this reason, even if moisture enters the molten salt, no energy loss occurs due to re-reaction.

  More specifically, most of the water generated from the refractory material in the early stage of electrolysis is water from the mortar used as a joint material between the refractory bricks. This moisture reacts with Mg dispersed and mixed as droplets in the molten salt, and a large amount of problematic MgO is generated. If molten salt is put into an electrolytic cell, drying of mortar will proceed effectively and moisture will be absorbed into the molten salt, but Mg will not be generated in the molten salt unless electrolytic current is applied, which is problematic. The generation of MgO can be greatly suppressed, and only the drying of refractories including mortar proceeds. For this reason, it is an effective drying method to keep the molten salt in the electrolytic cell without conducting electrolysis.

  And naturally, the temperature of the molten salt is better from the viewpoint of improving the current efficiency immediately after the operation of the electrolytic cell by promoting drying and shortening the drying period. However, when the molten salt temperature during electrolytic operation (about 660 ° C.) + 200 ° C. becomes extremely high, thermal damage to the furnace material becomes a problem.

  However, when the present inventors actually carried out this bath salt holding and drying, the molten salt was electrolyzed even though the molten salt temperature was about 700 ° C., which was sufficiently lower than the upper limit of the furnace material heat resistance temperature. It has been found that there is a risk of leakage from an internal drying nozzle or the like that passes through the refractory material layer on the back side of the casing, which is called the iron shell of the tank. If the molten salt leaks out of the electrolytic cell, there is a risk in terms of safety, and it is necessary to carry out maintenance of the leaking portion, which requires extra costs. In the worst case, it is necessary to dismantle even though it is an electrolytic cell before operation, and it suffers a great deal of economic damage.

JP 58-157983 A JP 2002-80989 A JP 2006-328450 A

  An object of the present invention is to provide an electrolytic cell drying method for efficiently performing a heat drying process while effectively preventing leakage of a bath salt when a Mg electrolytic cell is forcibly heated and dried by holding a bath salt. .

  In order to achieve the above-mentioned object, the present inventor in order to elucidate the cause of the leakage of bath salt when heat drying by holding the bath salt is performed on the constructed Mg electrolytic cell before operation. The dismantling investigation was conducted on the electrolytic cell in which this occurred. As a result, the penetration of the bath salt has progressed through the joints of the refractory brick, which is the refractory material of the electrolytic cell, and in the leaked part, the permeated bath salt reaches the casing of the electrolytic cell and spreads along its inner surface, It has been found that leakage has occurred from the nozzle for drying inside the casing.

  On the contrary, when the electrolytic cell where the leakage of bath salt did not occur was disassembled after the end of its useful life, the solidified layer of the bath salt formed by the molten salt solidifying in the refractory layer before the casing was effective. It was revealed that a protective layer was formed. And it became clear that the difference of this phenomenon originated in the difference of the viscosity by the temperature of bath salt, and the balance of the penetration rate to the refractory material layer by this, and the heat dissipation from the casing surface.

  That is, as described above, the temperature of the molten salt is set to a high value from the viewpoint of improving the current efficiency immediately after operation of the electrolytic cell by promoting drying and shortening the drying period. The bath salt penetrates into the layer in a molten state in a relatively short time, and the permeation rate is higher than the heat dissipation from the casing surface. As a result, the risk of leakage increases due to reaching the casing. On the contrary, if the bath salt is low temperature and its viscosity is high, the penetration rate into the refractory material layer becomes slow, the balance between the penetration rate into the refractory material layer and the heat radiation from the casing surface, and the solidified layer itself is near the casing. An effective protective layer is formed.

  In any case, the leakage of the bath salt is a problem on the balance between the heat dissipation from the casing surface and the penetration rate of the bath salt (bath salt temperature), and in particular, the formation of an effective protective layer is It has been found that it is governed by the balance in the initial stage of the drying process, which is 1 to 2 days after the bath salt is introduced into the electrolytic cell, that is, 1 to 2 days from the start of the bath salt retention drying.

From this, when the present inventors dried the refractory material layer in the electrolytic cell with the input bath salt, the input bath salt used was a low-temperature bath salt that was intentionally lowered from the optimum bath temperature for drying. Therefore, it is effective for preventing the bath salt from leaking, so that the penetration of the bath salt into the refractory material layer is suppressed, thereby forming an effective protective layer in the refractory material layer before the casing. After that, it is safe and efficient to quickly maintain the bath salt in the tank at a higher steady temperature optimum for drying, improve the current efficiency immediately after the operation of the electrolytic cell by promoting drying, and shorten the drying period. It was concluded that this is a reasonable and reasonable drying method.

  The electrolytic cell drying method of the present invention has been completed on the basis of such knowledge, and the bath salt retention drying is performed by forcibly heating and drying the refractory material layer in the Mg electrolytic cell with the molten salt charged into the electrolytic cell. In the initial stage of drying within 48 hours from the start of the bath salt retention drying, the bath salt in the bath is adjusted so that the average temperature rise rate of the bath salt in the bath is within a range of 0.75 to 2.5 ° C./hr within 48 hours. The temperature is raised.

As a typical example, when the steady-state temperature of the molten salt is 700 ° C, which is optimized from the viewpoint of improving the current efficiency immediately after the operation of the electrolytic cell by promoting drying and shortening the drying period, the low temperature of 600 ° C is lower by 100 ° C The hot bath is put into the electrolytic bath, and the bath salt is heated so that the bath temperature becomes 700 ° C. after 48 hours at the latest. At this time, the average temperature increase rate of the bath salt in the tank in the initial 48 hours is about 2 ° C./hr (100 ° C./48 hr) or more. As a result, an effective solidified layer is formed by the slow penetration of the bath salt into the refractory material layer at the initial stage of drying, and then the bath salt temperature is quickly returned to a higher steady temperature. Drying is performed. Even when a high-temperature bath salt is used, an effective solidified layer is formed at the early stage of drying before that, and leakage outside the bath due to penetration of the bath salt is suppressed as much as possible.

  The method for heating the molten salt introduced into the electrolytic cell must be other than heating by electrolysis, and specifically heating by electricity such as electric resistance heating or heating by gas is preferable. When electrolytic current heating is performed, the moisture generated from the refractory material in the tank, in particular, the mortar between the refractory bricks reacts with Mg dispersed and mixed as droplets in the molten salt, causing the problematic MgO. Generate a large amount.

  The rate of temperature rise of the bath salt after the start of bath salt retention drying is basically better, and it is desirable from the viewpoint of drying efficiency to increase to the final temperature in a short time and shift to the steady temperature, If it is too fast, the balance between the heat dissipation from the casing surface of the electrolytic cell and the penetration rate (bath salt temperature) of the bath salt is lost, and the penetration of the bath salt proceeds rapidly, resulting in the risk of leakage. From this point of view, the average temperature rise rate of the bath salt in the tank in the initial 48 hours of drying is set to 0.75 to 2.5 ° C./hr, and preferably the average temperature rise rate of the bath salt in the tank in the initial 24 hours of drying. Set to 1.5 to 5 ° C./hr. The average temperature rise rate of the bath salt in the tank in the initial 24 hours of drying is 1.5-5 ° C./hr, and the temperature of the bath salt in the tank is raised to the steady temperature. Since the rate is 0.75 to 2.5 ° C./hr, the latter falls within the former range.

  It is possible to continue to raise the temperature after 48 hours have passed since the start of bath salt retention drying, but the temperature has been raised by 48 hours, preferably by 24 hours. After finishing the temperature, it is desirable from the viewpoint of drying efficiency that the bath salt temperature in the tank is maintained at a steady temperature optimum for drying. That is, the average temperature increase rate of the bath salt from the start of the bath salt holding and drying until the lapse of 24 hours is larger than the average temperature increase rate of the bath salt from 24 to 48 hours after the start of the bath salt holding and drying. Is desirable.

  The steady temperature herein is qualitatively the bath salt temperature for the purpose of drying the refractory material layer in the tank as described above, and specifically, the bath salt after 48 hours have elapsed since the start of bath salt retention drying. The average temperature until the end of holding and drying. It is desirable to maintain the temperature of the bath salt at a steady temperature after 48 hours from the start of the bath salt holding and drying. In this case, “maintenance at the steady temperature” does not necessarily need to be a constant temperature. Variation is allowed. Specifically, this variation rate is desirably ± 0.5 ° C./hr or less, and more desirably ± 0.3 ° C./hr or less.

  And the average value of the steady temperature is desirably 620 to 750 ° C. If this is low, drying of the refractory material layer in the electrolytic cell will not proceed sufficiently, so there will be a problem of moisture remaining at the start of energization or an extended drying period. On the other hand, if it is too high, a strong solidified layer has already been formed, so there will be no problem of leakage of molten salt, but more energy is required to raise the temperature of the molten salt, which reduces economic efficiency. Is a problem. Extremely high bath salt temperatures can adversely affect the refractory material.

  The temperature of the molten salt to be charged into the electrolytic bath, that is, the bath salt temperature at the start of bath salt holding and drying, is the average temperature rise rate of the bath salt in the initial stage of drying and after 48 hours have passed since the molten salt was added. If the bath salt temperature in the tank, that is, the steady temperature optimum for drying is determined, it is automatically calculated, and specifically, 530 to 670 ° C. is desirable. When this is low, the viscosity of the molten salt becomes high, and the charging operation is hindered. In addition, extra energy is required to raise the temperature after the addition, and the drying period becomes longer than necessary, which deteriorates the economy. On the other hand, if this temperature is too high, penetration of the molten salt into the refractory material layer is accelerated, and there is a risk of leakage.

  Note that the difference between the temperature of the molten salt immediately before being introduced into the electrolytic cell and the temperature of the molten salt immediately after being introduced into the electrolytic cell is negligible and can be ignored. That is, it is based on the temperature at the start of bath salt holding drying.

  The drying period by holding the bath salt from the start of bath salt holding drying to the start of electrolysis is preferably 4 to 15 days, and particularly preferably 6 to 10 days. If the drying period is short, there is a problem of a decrease in current efficiency due to the generation of MgO immediately after operation of the electrolytic cell due to residual moisture. If the drying period is too long, not only energy for maintaining the bath salt temperature is required, but also the rest period of the electrolytic cell that has been constructed is lengthened, and the economic efficiency is lowered from both sides.

  For electrolytic tanks that have been constructed, gas heating drying is performed before drying by bath salt retention, and this reduces the moisture in the refractory material in the tank prior to bath salt retention drying, It is desirable from the viewpoint of shortening the entire drying period and improving the water removal efficiency. If high-temperature molten salt is introduced into a low-temperature electrolytic cell, there is a concern that the joint of the refractory material will be damaged by thermal shock. However, if gas heating drying is performed before bath holding drying, the refractory material is preheated by gas heating drying. Therefore, the concern is removed, and the temperature drop due to the addition of molten salt is also prevented.

  The gas used in the gas heating drying may be air. Other gases such as nitrogen can be used, or other gases can be mixed with the atmosphere, but there is no particular problem with heating and using the atmosphere. That is, the atmosphere has some moisture, but the refractory material can be dried by heating to 100 ° C. or higher. Air heating drying is easier to handle gas than using dry gas, and the cost is greatly reduced. As the heating means, a gas burner, an electric heater or the like can be used. The most rational and efficient method is heated air circulation drying in which the atmosphere is heated with a burner and blown into the tank and exhausted from other places.

  The period of gas heat drying is preferably 2 days in order to increase the drying effect, and more preferably 4 days or more. The upper limit of the drying time is preferably 10 days or less, and more preferably 7 days or less because excessive drying deteriorates the economy. A desirable atmospheric gas temperature is 100 to 400 ° C. If the atmospheric gas temperature is low, drying does not proceed sufficiently. When too high, the oxidation of the graphite member used for an electrolytic cell becomes remarkable. Within the range of 100 to 400 ° C., more desirably within the range of 300 to 400 ° C., the refractory can be efficiently dried while suppressing oxidation of the graphite member. When a member to be used cannot withstand a rapid temperature change and causes cracks or the like, a method of raising the temperature gradually or stepwise between 100 to 400 ° C. is desired. If the graphite member is removed from the tank, a heating temperature exceeding 400 ° C. is possible.

  The electrolytic cell drying method of the present invention is performed by heating the bath salt in the bath at the initial stage of bath salt holding drying in which the refractory material in the bath is forcibly heated and dried by the molten salt charged into the electrolytic cell. The temperature of the bath salt can be lowered from the steady temperature optimum for drying. Thereby, the viscosity of the bath salt can be increased, a solidified layer can be formed in the refractory material layer of the electrolytic cell, and the leakage of the bath salt to the outside of the cell can be effectively prevented. By quickly raising the temperature to the initial steady temperature after charging, efficient drying can be performed, thereby reducing the risk of leakage of bath salt and current efficiency immediately after operation of the electrolytic cell. In addition to the improvement, it is possible to shorten the drying period, and it is possible to significantly reduce the electrolysis cost including the avoidance of repair costs due to the leakage of bath salt.

It is the graph which illustrated the temperature rising pattern of the molten salt thrown into the electrolytic cell. It is the graph which illustrated another temperature rising pattern of the molten salt thrown into the electrolytic cell. It is the graph which illustrated another temperature rising pattern of the molten salt thrown into the electrolytic cell.

  Embodiments of the present invention will be described below. The electrolytic cell drying method of the present embodiment is directed to an Mg electrolytic cell for regenerating metal Mg from MgCl2 by-produced by the crawl method, and more specifically, in the electrolytic cell used for the construction of the Mg electrolytic cell. Used for drying refractory materials. This Mg electrolytic cell is constructed by stacking dry refractory bricks with mortar and forming a refractory layer on the inside of an iron skin as a casing. The electrolytic cell drying of the present embodiment is to perform gas heating drying as pre-treatment and bath salt retention drying as main treatment to the Mg electrolytic cell completed by installing auxiliary equipment such as electrodes in the constructed tank. Is the method.

  In the electrolytic cell drying method of the present embodiment, regarding the completed electrolytic cell, from the viewpoint of drying the refractory material in the electrolytic cell, an optimal gas heating temperature is set in advance from the range of 300 to 400 ° C. and an optimal bath salt The steady average temperature in the holding drying is set in advance from the range of 620 to 750 ° C. In addition, from the viewpoint of preventing leakage of the bath salt in the initial stage of bath salt holding and drying, it is set in advance within a range that satisfies the average temperature increase rate of 0.75 to 2.5 ° C./hr in the initial 48 hours of bath salt holding and drying. deep. From these, the bath salt temperature at the start of bath salt holding drying is set in advance within a range of 530 to 670 ° C.

  When the electrolytic cell is completed, a molten salt (low temperature bath) having a temperature of 530 to 670 ° C. is put into the electrolytic cell after gas heating and drying. When charging of the molten salt (low temperature bath) is completed, the molten salt in the tank is heated to a high temperature bath having a steady average temperature (620 to 750 ° C.) at least 48 hours after the charging. At this time, the average temperature rise rate in 48 hours needs to satisfy 0.75-2.5 degrees C / hr.

  When the introduced low temperature bath becomes a high temperature bath having a predetermined steady temperature, the temperature of the high temperature bath is maintained for a predetermined drying period, and the drying process is terminated. FIG. 1 to FIG. 3 show the temperature change pattern of bath salt in the bath in bath salt holding drying carried out after 5 days of gas heating drying (300 ° C.). The temperature of the low temperature bath introduced into the electrolytic cell, that is, the bath salt temperature at the start of bath salt holding and drying is 600 ° C., and the temperature of the high temperature bath after the temperature rise is 700 ° C. As long as the low temperature bath at 600 ° C. becomes a high temperature bath at 700 ° C. after the start of the low temperature bath, that is, 48 hours after the start of the bath salt holding drying, the average temperature of the bath salt in the initial 48 hours of drying The rate of increase is about 2 ° C./hr.

  The temperature rising pattern of the bath salt in the tank shown in FIG. 1 will be specifically described. A in FIG. 1 shows a temperature increase pattern in which the temperature increase in the initial stage of drying was performed at a constant rate over 48 hours. B is a temperature rising pattern in which the temperature of the bath salt is raised to a steady temperature in the first 24 hours and the steady temperature is maintained for the next 24 hours. The temperature rising rate in the first 24 hours is about 4 ° C./hr (100 ° C./24 hr). C is a two-stage temperature increase pattern in which the temperature increase rate in the first 24 hours is larger than the temperature increase rate in the subsequent 24 hours. Specifically, the temperature is increased by 75 ° C. in the first 24 hours. The temperature was raised at 25 ° C. over the next 24 hours. The rate of temperature rise in the first 24 hours is about 3 ° C./hr (75 ° C./24 hr), and the rate of temperature rise in the subsequent 24 hours is about 1 ° C./hr (25 ° C./24 hr).

In any of the patterns, the average temperature increase rate from the start of drying to the lapse of 48 hours is all about 2 ° C./hr (100 ° C./48 hr). This solves the problem of bath salt leakage which becomes a problem in the early stage of drying. After the temperature is raised to the optimum steady temperature for drying, efficient drying is performed by maintaining the steady temperature. As a result, it is possible to perform efficient drying in a short period of time while suppressing the occurrence of bath salt leakage problems, and the efficient drying improves current efficiency immediately after operation of the electrolytic cell and shortens the drying period. be able to. By the way, the bath salt retention drying period is 8 days here. In particular, in the B pattern and the C pattern, since the heating rate in the first half was increased and the heating rate in the second half was decreased, the drying efficiency by holding the bath salt increased, and the drying period was more than 8 days.

  D in FIG. 2 is a pattern in which the first 24 hours are kept in the low temperature bath and the temperature is raised to the steady temperature in the next 24 hours. E is a pattern in which the bath salt is charged, that is, the temperature starts to rise after 12 hours have elapsed from the start of bath salt holding and drying, the temperature is raised to the steady temperature in 24 hours, and the remaining 12 hours are maintained at the steady temperature. The temperature increase rate at the time of temperature increase is about 4 ° C./hr (100 ° C./24 hr), which is the same as the B pattern in FIG. 1. However, the higher the temperature increase, the better. D pattern order.

  In the temperature rise pattern indicated by F in FIG. 3, the temperature rise from the introduction of the 600 ° C. low temperature bath charged into the electrolytic cell is continued for 3 days (72 hours), and the low temperature bath is changed to a 700 ° C. high temperature bath. did. Since the bath temperature after the lapse of 48 hours from the introduction is about 670 ° C., the average temperature increase rate in the initial 48 hours of drying is about 1.5 ° C./h. Since the temperature rise to 700 ° C., which is a steady temperature, was 3 days (72 hours), the bath salt retention drying period was extended from 8 days to 9 days. The average bath temperature from the end of 48 hours to the end of drying is slightly lower than the steady temperature of 700 ° C., but is still close to 700 ° C. and within the range of 620 to 750 ° C.

  In the G pattern in FIG. 3, as with the F pattern, the temperature increase from the introduction of the low temperature bath was continued for 3 days (72 hours). The heating rate was slowed down to the hour. Although the temperature rise is continued for 72 hours, the temperature is raised to about 685 ° C. by the 48th hour, and the substantial temperature rise is finished. [The temperature rise rate during this period is about 1.8 ° C./hr (85 ° C./48 hr) ] Since the temperature rising pattern approaches the A pattern in FIG. 1 as much as possible, the bath salt retention drying period was the same 8 days as the A pattern in FIG.

  The above temperature rising patterns include those that are relatively unrealistic in order to clarify the range, but the realistic temperature rising pattern is the range where the leakage of bath salt does not occur immediately after the bath salt is added. FIG. 1 shows the A pattern, B pattern, and C pattern in FIG. 1 that rapidly rise in temperature and shift to a steady temperature by 48 hours. In particular, in the case of the B pattern in FIG. 1, the steady temperature time in the initial stage of drying is longer than in the other patterns. Therefore, the maintenance time of the steady temperature after the initial stage of drying can be shortened accordingly, and the overall drying period can be reduced. It is also possible to shorten the time from the illustrated 8 days to about 7 days.

  As an example, the actual electrolytic cell was actually heated and dried for 7 days (7 days), then molten salt was added, and the pattern was raised according to the patterns shown in FIGS. Warm up. The period of bath salt retention drying was 8 days. As Comparative Example 1, a molten salt of 670 ° C. was charged into an electrolytic cell, and this was maintained for 8 days at a constant temperature of 670 ° C. without initial temperature rise. Similarly, as Comparative Example 2, 700 ° C. constant temperature bath holding and drying (8 days) without initial temperature increase was performed. We investigated the effect of bath salt steady temperature, initial temperature rise pattern (average temperature rise rate), and the type of initial temperature rise pattern on bath leakage rate, electrolytic cell casing repair area, and current efficiency in the initial electrolysis operation. . The survey results are shown in Table 1.

  For the examples in which the initial temperature was raised, the average temperature rise rate of the bath salt at the initial stage of drying was set to a value from the start of drying to 24 hours, a value from 24 hours to 48 hours, and 48 times from the start of drying. The values up to the passage of time are displayed. The bath leak rate is the ratio of the electrolyzer that caused even a slight bath leak among a plurality of electrolyzers, and the repair area of the electrolyzer casing is the average value of the repair areas in the electrolyzer that caused the bath leak. is there. These were expressed as relative values with the current efficiency as well as the value in Comparative Example 2 in which constant temperature bath salt holding drying (8 days) at 700 ° C. without initial temperature increase was performed.

  In addition, the evaluation of the hot water leak rate, casing repair area, and initial current efficiency is displayed in parentheses. For the hot water leak rate and casing repair area, “less than 50” is indicated as “× (impossible)” and “50 or higher”. “Yes (Yes)”. Regarding the initial current efficiency, “less than 90” is “5”, “91 to 93” is “4”, “94 to 96” is “3”, “97 to 98” is “2”, and “99 or more” is “ 1 ”. That is, the smaller the evaluation number, the higher the evaluation.

  These comprehensive evaluations are shown in Table 1. The overall evaluation was a five-step evaluation from “5” to “1”. Like the initial current efficiency, the one with a smaller evaluation number was rated high. When any evaluation of the hot water leak rate and the casing repair area is “× (impossible)”, the lowest evaluation “5” is set as the overall evaluation regardless of the evaluation of the initial current efficiency. When the evaluation of both the hot water leak rate and the casing repair area is “◯ (possible)”, the five-stage evaluation of the initial current efficiency was taken as the overall evaluation.

  Comparative Examples 1 and 2 are conventional examples in which the temperature was not raised at the initial stage of drying, and the bath salt temperature at the start of bath salt holding drying and the bath salt temperature at the end of bath salt holding drying were the same. In Comparative Example 2 in which the bath salt temperature, that is, the steady-state temperature is 700 ° C., the bath leakage and the accompanying casing repair became a big problem. However, since the drying of the refractory material layer is progressing, the initial current efficiency at the start of the electrolytic operation is high. Similarly, in Comparative Example 1 in which the temperature was not raised at the initial stage of drying, the bath salt temperature was 670 ° C., which was slightly lower than that in Comparative Example 2. The decrease of was remarkable.

On the other hand, in the patterns A to G in which the initial temperature increase was performed, there are some differences depending on the patterns, but as a whole, a decrease in current efficiency at the initial stage of the electrolysis energization operation is achieved while effectively suppressing bath leakage. I was able to stop it effectively. The specific tendency in each pattern was consistent with the above explanation.

Claims (6)

  In the bath salt holding drying in which the refractory material layer in the Mg electrolytic bath is forcibly heated and dried by the molten salt charged into the bath, within 48 hours from the start of the bath salt holding drying within 48 hours. The electrolytic bath drying method of heating the bath salt in the bath so that the average temperature rise rate of the bath salt in the bath is 0.75 to 2.5 ° C./hr.   2. The electrolytic cell drying method according to claim 1, wherein the temperature of the bath salt is maintained at a steady temperature after 48 hours from the start of the bath salt retention drying.   3. The electrolytic cell drying method according to claim 1, wherein the average temperature rise rate of the bath salt in the bath from the start of the bath salt holding drying to the elapse of 24 hours is from 24 hours to 48 hours after the start of the bath salt holding drying. An electrolytic cell drying method that is greater than the average temperature rise rate of the bath salt in the cell.   The electrolytic cell drying method according to any one of claims 1 to 3, wherein the average temperature rise rate of the bath salt in the bath from the start of bath salt retention drying to the lapse of 24 hours is 1.5 to 5 ° C / hr. Tank drying method.   The electrolytic cell drying method according to any one of claims 1 to 4, wherein an average temperature of the bath salt after lapse of 48 hours from the start of the bath salt retention drying is 620 to 750 ° C.   The electrolytic cell drying method according to claim 1, wherein gas drying is performed on the refractory material layer in the cell before the start of bath salt retention drying.

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