WO2000050339A1 - Method for producing polysulfide by electrolytic oxidation - Google Patents

Abstract

A method for producing polysulfides which comprises introducing a sulfide ion-containing solution into an anode compartment of an electrolytic bath comprising an anode compartment having a porous anode, a cathode compartment having a cathode, and a diaphragm partitioning the anode compartment and the cathode compartment and effecting an electrolytic oxidation, to thereby produce polysulfide ions, characterized in that the cathode compartment has a pressure higher than that in the anode compartment. The method can be used for producing a digesting solution having a high content of polysufide-sulfur with high selectivity and with low electric power consumption, with markedly reduced by-production of a thiosulfate ion.

Description

 Description Method for producing polysulfide by electrolytic oxidation

 The present invention relates to a method for producing a polysulfide by electrolytic oxidation, and more particularly to a method for producing a polysulfide digestion liquid by electrolytically oxidizing a white liquor or a green liquor in a valve production process. Background art

 For efficient utilization of wood resources, increasing the yield of chemical pulp is an important issue. One of the technologies for increasing the yield of kraft pulp, which is the mainstream of this chemical harp, is the polysulfide digestion process.

The cooking liquor in the polysulfide cooking process is manufactured by oxidizing an alkaline aqueous solution containing sodium sulfide, a so-called white liquor, with molecular oxygen such as air in the presence of a catalyst such as activated carbon (for example, the following reaction formula 1). (Japanese Unexamined Patent Publication Nos. Sho 61-259754 and Sho 53-92981) ( ₃ ) By this method, the conversion of sulfide ion base is about 60%, and the selectivity is about 60%. 5 _s can and two-obtaining gZL about multi sulfides cooking liquor However, the Chio sulfate ion is not at all contribute to the digestion by side reactions when raising the conversion rate in this process (for example, the following reaction Scheme 2, 3) Due to the increase in by-products, it was difficult to produce a cooking liquor containing a high concentration of polysulfide at a high selectivity.

4Na ₂ S + 0 ₂ + 2H 0 → 2Na S + 4Na OH (1) 2 Na ₂ S + _{202 2} + H 0-→ 2 Na SO _{: ί} + 2 Na OH (2) 2Na ₂ S ₂ + 4NaOH → 2 Na ₂ S, O _{; j} (3) Here, polysulfide is also referred to as borosulfide sulfur (PS—S). For example, sodium polysulfide Na 2 SX has zero valence, ie, atom ( X-1 1) means the number of pieces. In this specification, the term “N”, which is equivalent to the number of oxidations in polysulfide ions—2 (SX ² -one atom equivalent to one atom) and sulfide ions (S 2−), is collectively referred to as N in this specification. a 2 S mode This specification In the book, the unit liter of capacity is represented by L.

 On the other hand, PCT International Publication WO95 / 0701 describes a method for electrolytically producing a polysulfide cooking liquor. In this method, an anode is used in which a carrier is coated with an oxide of ruthenium, iridium, platinum, or palladium. Specifically, a three-dimensional mesh electrode of a carrier in which a number of expanded metals are combined is disclosed. Also, ? Jing International Publication, No. 0977/41192, describes a process for the electrolytic production of polysulfide cooking liquors by the present applicants. In this method, a porous anode made of at least carbon is used as the anode, and in particular, an aggregate of carbon fibers having a diameter of 1 to 300 m is used.

 The present invention provides a process for producing a cooking liquor containing a high concentration of polysulfide by electrolysis from a solution containing sulfide ions, in particular, a white liquor or a green liquor in a pulp production process, by reducing the by-product of thiosulfate to a high level. It is intended to manufacture at a selectivity and low power. Disclosure of the invention

The present invention introduces a solution containing a sulfide ion into an anode chamber of an electrolytic cell having an anode chamber in which a porous anode is provided, a force sword chamber in which a force sword is provided, and a diaphragm separating the anode chamber and the cathode chamber. a method for producing a polysulfide to obtain polysulfide ions by electrolytic oxidation, carrying out the _third invention to provide a method for producing polysulfides which the pressure in the cathode chamber and feature is higher than the pressure in the anode chamber Best form to do

 Various shapes and materials can be used as the porous anode in the present invention. More specifically, for example, carbon fiber, carbon fiber, carbon paper, metal foam, reticulated metal, etc. Carbon. A metal electrode having a surface modified with platinum or the like can also be used.

The porous anode used in the present invention preferably has a physically continuous three-dimensional network structure. By forming a three-dimensional network structure, the surface area of the anode can be increased, and a desired electrolytic reaction occurs on the entire surface of the electrode, thereby suppressing generation of by-products. Also, if the anode is made into a physical continuum rather than an aggregate of fibers, the anode As a result, the cell voltage can be further reduced because it exhibits sufficient electrical conductivity and the IR drop at the anode can be reduced.

 The network structure is a physically continuous structure, and may be continuously connected by welding or the like. Specifically, a physically continuous three-dimensional fine structure made of nickel or a nickel alloy containing at least 50% by weight of nickel is preferred. For example, porous nickel obtained by plating nickel on the skeleton of a foamed polymer material and then baking off the polymer material inside can be given.

 The anode having a three-dimensional network structure preferably has a diameter of 0.01 to 2 mm at a portion corresponding to a yarn of a network constituting the network. If the diameter is less than 0.01 mm, manufacturing is extremely difficult, costly, and handling is not easy. When the diameter exceeds 2 mm, a large surface area of the anode cannot be obtained, the current density on the anode surface increases, and not only is it easy to generate by-products such as thiosulfate ions, but also the anode is made of metal. In this case, anodic dissolution is likely to occur, which is not preferable. It is particularly preferred if the diameter is between 0.02 and 1 mm.

The average pore size of the anode network is preferably 0.001 to 5 mm. If the average pore size of the mesh is larger than 5 mm, the anode surface area cannot be increased, the current density on the anode surface increases, and not only is it easy to generate by-products such as thiosulfate, but also as an anode If a metal is used, anodic dissolution is likely to occur. If the average pore size of the mesh is smaller than 0.001 mm, clogging occurs when solid components are mixed in the electrolytic cell, and problems in the electrolytic operation when the pressure loss of the solution increases become large. It is not preferable because it may occur. The average pore size of the anode mesh is 0. If it is 2 to 2 mm is still more preferably les, the _c present invention, in the electrolytic operation, the pressure force Sword chamber is carried out in large conditions also Ri by the pressure in the anode chamber . An electrolytic cell generally has a structure in which a diaphragm is sandwiched between an anode and a cathode. From the viewpoint of assembly accuracy and protection of the diaphragm, the anode and the cathode are arranged at a relatively large distance. Specifically, a distance of about several mm is often provided. The diaphragm placed between them will approach the anode side or approach the force side depending on the electrolysis conditions. In the present invention, the diaphragm is Improve current efficiency, etc. by forcibly contacting the node at all times, eliminating the space between the anode and the diaphragm, and introducing all the anode liquid into the porous anode. Things. In the present invention, as the means, the electrolysis operation is performed under the condition that the pressure in the force source chamber is higher than the pressure in the anode chamber. By doing so, the diaphragm is pressed against the anode, so that the anolyte can sufficiently flow inside the porous anode, and a high selectivity is realized.

 In the present invention, as a means for increasing the pressure in the cathode chamber higher than the pressure in the anode chamber, a solution (hereinafter, referred to as anolyte) for introducing a flow rate of a solution (hereinafter, referred to as “force sword solution”) to the sword chamber into the anode chamber is used. And increasing the cathode liquid outlet resistance by reducing the diameter of the outlet pipe on the cathode side.

 In the present invention, it is preferable that at least the surface of the porous anode is made of nickel or a nickel alloy containing nickel in an amount of 50% by weight or more. Since at least the surface portion of the anode is made of nickel, the anode has practically sufficient durability in the production of polysulfide. Nickel is a suitable material in the present invention because nickel is inexpensive and its elution potential including its oxide is higher than the potential for generating polydisulfide sulfate.

Further, the porous anode in the present invention, the surface area per effective current area of the diaphragm which separates the anode chamber and the cathode-de chamber ^{2~ 1 0 O m 2 Zm 2} 2 m 2 is preferred anode surface area in the range of Zm 2 If the anode is a metal, the current density on the anode surface increases, and not only is it easy to generate by-products such as thiosulfate ions, but also the anode is liable to dissolve when the anode is a metal. If the anode surface area is larger than 10 O m ² / m 2, the pressure loss of the porous anode itself becomes high, and it is difficult to make the pressure in the force source chamber higher than the pressure in the anode chamber. It is not preferable because it may be caused. More preferably, the anode surface area is 5 to 50 m ² m 2 per effective conducting area of the membrane.

It is preferable that the anode surface area per anode chamber volume is 500 to 2000 m2 / _m3 . If the surface area of the anode is smaller than 50 Om3, the current density at the anode surface increases, and by-products such as thiosulfate are formed. In addition to this, it is not preferable that the anode is made of a metal because the anode is likely to dissolve. If the surface area of the anode is more than 2000 m ² / m 3, it is not preferable because a problem in electrolysis operation may occur when the pressure loss of the liquid increases. The anode surface area of the anode chamber per volume, 1 0 0 0~ 2 0 0 0 0 m and even more preferably in the range of ² / m ^3.

Current density at the diaphragm surface is 0. 5~2 0 k A / m Les Shi preferred is to operate at ^2,. If the current density at the diaphragm surface is less than 0.5 kA / m ² , the electrolysis equipment will be unnecessarily large, which is not preferable. If the current density at the diaphragm surface exceeds ² 0 k A / m 2 is Chio sulfate, not only increases sulfate, by-products such as oxygen, since if the anode is a metal which may cause anodic dissolution Not preferred. If the current density at the diaphragm surface is ² ~1 5 k A / m 2 is still more preferably les, the _c present invention, the area of the diaphragm, in Anodo surface because of the use of large anodes surface area It can be operated in a range where the current density is small.

Current density at the surface of the anode each portion is assumed to be uniform that, if the anode of the surface area was determined current density at the anode surface, the value is 5~ 3 0 0 0 A / m 2 Is preferred. A more preferred range is from 10 to 150 A / m 2. If the current density on the anode surface is less than 5 A / m 2, unnecessarily large electrolytic equipment will be required, which is not desirable. When the current density at the anode surface exceeds 300 A / m2, not only the by-products such as thiosulfuric acid, sulfuric acid, and oxygen are increased, but also when the anode is a metal, anodic dissolution may occur. Is not preferred.

 The average superficial velocity of the anolyte is preferably 1 to 30 cmZ seconds. If the time is longer than 30 c seconds, an unnecessarily large electrolytic facility is required, which is not preferable. If the average superficial velocity of the anolyte is too low, not only is by-products such as thiosulfuric acid, sulfuric acid, and oxygen increased, but if the anode is a metal, the anode may be dissolved, which is not preferable. It is more preferable that the average superficial velocity of the anolyte is 1 to 15 cs, particularly 2 to 10 cMz. Although the flow rate of the force solution is not limited, it is determined by the magnitude of the buoyancy of the generated gas, the allowable pressure of the electrolytic cell, the concentration of alkali generated on the cathode side, and the like.

In order for the electrolytic reaction to occur efficiently in the anode, the liquid to be treated must flow through the anode. Therefore, the anode itself preferably has sufficient voids, and the porosity of the porous anode is preferably 30 to 99%. If the porosity is less than 30%, the pressure loss may increase, which is not preferable. If the porosity is more than 99%, it is difficult to increase the anode surface area. It is particularly preferable that the porosity is 50 to 98%.

 A current is supplied to the anode through the anode current collector. As a material of the current collector, a material having excellent alkali resistance is preferable. For example, nickel, titanium, carbon, gold, platinum, stainless steel, and the like can be used. The current collector is attached to the back surface or the periphery of the anode. If the current collector is mounted on the back of the anode, its surface may be planar. The electric current may be supplied simply by mechanical contact with the anode, but it is preferable that the electric current is physically bonded by welding or the like.

 As the force sword material, an alkali-resistant material is preferable, and nickel, Raney nickel, nickel sulfide, steel, stainless steel and the like can be used. The force sword is used in the form of a flat plate or mesh, one or more of which are in a multilayer structure. A three-dimensional electrode combining linear electrodes can also be used.

 As the electrolyzer, a two-chamber electrolyzer comprising one anode chamber and one power sword chamber is used. Electrolyzers combining three or more rooms are also used. Multiple cells can be arranged in a monopolar or bipolar configuration.

 It is preferable to use a cation exchange membrane as a diaphragm, that is, a membrane that separates and separates the anode chamber and the power source chamber. The cation exchange membrane directs cations from the anode compartment to the force sword compartment, preventing the transfer of sulfide and polysulfide ions. As the cation exchange membrane, a polymer membrane in which a cation exchange group such as a sulfonic acid group or a carboxylic acid group is introduced into a hydrocarbon-based or fluororesin-based polymer is preferable. If there is no problem in terms of alkali resistance, a bipolar membrane or an anion exchange membrane can be used.

The temperature of the anode compartment is preferably between 70 and 110 ° C. If the temperature of the anode chamber is lower than 70 ° C, not only is the cell voltage increased, but also sulfur deposition and by-products are easily generated.If the anode is a metal, the anode may be dissolved, which is preferable. Nare, The upper temperature limit is practically limited by the material of the cell or diaphragm. The anode potential, S ₂ 2-a oxidation product of sulfide ^{_{ions, S 3 2-, S 4 2}} -, S 5 2 - polysulfide ions such as (SX ² -) is generated, the Chio sulfate ions It is preferable to maintain it so as not to produce by-products. The operation is preferably performed so that the anode potential is in the range of 0.75 to 0.25 V. If the anode potential is lower than 0.75 V, the formation of polysulfide ions does not substantially occur, which is not preferable. If the anode potential is higher than +0.25 V, not only is by-products such as thiosulfate ions formed, but also if the anode is a metal, it may cause anode dissolution, which is not preferable. In this specification, the electrode potential represents a potential measured with respect to a Hg / Hg 2 C 12 reference electrode in a 25 ° C. saturated KC 1 solution.

 If the anode is a three-dimensional electrode, it is not easy to measure the anode potential accurately. Therefore, industrially, it is preferable to control the manufacturing conditions by controlling the cell voltage and the current density on the diaphragm surface, rather than controlling the manufacturing conditions by controlling the potential. The electrolysis method is preferably a constant current electrolysis, but the current density can be changed.

 The solution containing sulfide ions supplied to the anode compartment can be at least partially circulated to the same anode compartment after being electrolytically oxidized in the anode compartment. In addition, a process for supplying to the next step without performing such a circulation, that is, a so-called one-pass process can also be adopted. If the solution containing sulfide ions is white liquor or green liquor in the manufacturing process of the valve, the electrolytically oxidized white liquor or green liquor flowing out of the anode compartment is not circulated to the same anode compartment. It is preferable to supply to the next step. As the counter cation of sulfide ion in the anolyte, an alkali metal ion is preferable. As an alkali metal, sodium or power beam is preferred.

The method of the present invention is particularly suitable for a method of treating a white liquor or a green liquor in a pulp 'production process to obtain a polysulfide cooking liquor. In this specification, the term “white liquor” or “green liquor” includes liquids that have been subjected to concentration, dilution, or separation of solids, respectively, for the white liquor or green liquor. When incorporating the polysulfide production process according to the present invention into the pulp production process, at least a part of the white liquor or green liquor is extracted, treated in the polysulfide production process of the present invention, and then supplied to the digestion process . The composition of white liquor is, for example, that of white liquor used in current kraft pulp digestion, usually contains 2 to 6 mol 1 ZL as alkali metal ions, of which 90% or more is sodium ion. Yes, the rest is almost force-ion. Anions are mainly composed of hydroxide ion, sulfide ion and carbonate ion, and also include sulfate ion, thiosulfate ion, chloride ion and sulfite ion. It also contains trace components such as potassium, silicon, aluminum, phosphorus, magnesium, copper, manganese, and iron. On the other hand, the composition of the green liquor is that sodium sulfide and sodium hydroxide are the main components of the white liquor, whereas sodium sulfide and sodium carbonate are the main components. Other anions and trace components in the green liquor are the same as in the white liquor. When such white liquor or green liquor is supplied to the anode compartment according to the present invention to perform electrolytic oxidation, sulfide ions are oxidized to generate polysulfide ions. As a result, alkali metal ions move to the force source chamber through the diaphragm.

When used in the valve digestion process, the strength depends on the sulfide ion concentration in the white liquor or green liquor. The PS-S concentration in the solution obtained by electrolysis (polysulfide digestion solution) is 5 to 15 g / L is preferred. If it is less than 5 gZL, the effect of increasing the pulp yield during cooking may not be sufficiently obtained. When the concentration of PS—S is higher than 15 g / L, the amount of Na 2 S decreases, so that the pulp yield does not increase and thiosulfate ions are easily produced during electrolysis. When the average value of X of the existing polysulfide ions (SX ^2- ) exceeds 4, thiosulfate ions are similarly produced as a by-product during electrolysis. It is preferable to perform the electrolysis operation so that the average value of X of the polysulfide ions in the cooking liquor is 4 or less, particularly 3.5 or less, because dissolution is likely to occur. The conversion (reaction rate) of sulfide ions to PS—S is preferably 15% or more and 75% or less, more preferably 72% or less. Although various reactions can be selected for the reaction in the power source chamber, it is preferable to use a reaction in which hydrogen gas is generated from water. Alkali hydroxide is generated from the resulting hydroxide ions and alkali metal ions that have migrated from the anode compartment. The solution introduced into the cathode chamber is preferably a solution substantially consisting of water and an alkali metal hydroxide, particularly preferably a solution consisting of water and a hydroxide of sodium or magnesium. Although the concentration of the alkali metal hydroxide is not limited, for example, l to 15 mol Z L, preferably 2-5mo 1ZL. Depending on the case, if a solution having an ionic strength lower than the ionic strength of the white liquor flowing through the anode chamber is used as the catholyte, it is possible to prevent insoluble components from depositing on the diaphragm.

 Hereinafter, the present invention will be described in more detail with reference to Examples, but it goes without saying that the present invention is not limited to these Examples.

 [Example 1 ]

A two-chamber electrolytic cell was assembled as follows. A nickel foam (available from Sumitomo Electric Industries, trade name Celmet, height 10 OmmX width 2 OmmX thickness 4 mm) was electro-welded to the nickel current collector plate. A fluororesin-based cation exchange membrane (Flemion, manufactured by Asahi Glass Co., Ltd.) was prepared as a force sword using mesh Raney nickel as a diaphragm. The anode chamber is fitted with a 5 mm thick anode chamber frame, and a diaphragm, a force sword, a 4 mm thick cathode chamber frame, and a cathode chamber plate are stacked on top of each other and pressed down and fixed.The anode chamber has a height of 100 mm and a width of 100 mm. 2 Omm, and a thickness of 5 mm, the shape of the cathode-de chamber height 10 Omm, a width 20 mm, thickness 5 mm, the effective area of the diaphragm is 2 O cm ^2. During the electrolysis operation, both the anolyte and the power source fluid flowed from the bottom to the top in the height direction of each chamber so that the pressure in the power source chamber was higher than that in the anode chamber.

 At this time, the physical properties and electrolysis conditions of the anode are as follows.

 Anode chamber thickness: 4 mm Anode thickness: 4 mm

 Apparent anode volume ratio to anode chamber volume: 100%

 Porosity in the anode chamber: 95% Average liquid superficial velocity in the node chamber: 4 cmZ seconds Node surface area per anode chamber volume: 7000 m 2 / m 3

Average pore size of the mesh: 0.5 1 mm Surface area per diaphragm area: 28 m ² / m ² Electrolysis temperature: 85 ° C Current density at the diaphragm: 6 kA / m ²

As anolyte, model white liquor (N a ₂ S: Iou terms of atom in 1 6 g / L, N a OH: 90 gZL, N a 2 CO 3: 34 g / L) was 1 L preparation, § Roh one It was circulated at a flow rate of 192 mLZ (average superficial velocity in the anode chamber: 4 cs) while introducing from the lower side of the anode chamber and extracting from the upper side. Using 2 L of 3 N: Na〇H aqueous solution as the power source liquid, while introducing from the lower side of the power source chamber and extracting from the upper side, Circulation was performed at a flow rate of 80 mL / min (superficial velocity: 1.3 cmZ seconds). Heat exchangers were provided on both the anode and cathode sides, and the anolyte and catholyte were heated and introduced into the cell.

A constant current electrolysis is performed at a current of 12 A (current density at the diaphragm: 6 kA / m ² ) to synthesize a polysulfide digest, measure the cell voltage at a predetermined time, and sample the circulating fluid. Then, PS-S, sulfide ion and thiosulfate ion in the solution were analyzed and quantified. The analysis was performed based on the method described in Japanese Patent Application Laid-Open No. 7-92148.

The time courses of the quantitative values of the concentrations of various sulfur compounds and the measured values of the cell voltage were as follows. Set configuration of polysulfide cooking liquor of 1 hour 3 0 minutes after the start of electrolysis is, PS- S is 1 0. 6 GZL, the N a ₂ S and 5. l gZ in Iou atoms terms, increased Chio sulfate ions Was 0.25 gZL in terms of zeta atom, and the average value of X of the polysulfide ion (S _x ^2- ) was 3.0. During this period, the PS-S current efficiency was 95% and the selectivity was 97%.

 The cell voltage was constant at about 1.1 V for about one hour from the start of electrolysis, but then gradually increased. The thiosulfate ion concentration was 1.3 V at 1 hour and 40 minutes after the concentration began to increase, and after another 20 minutes, the voltage increased to about 2 V, and the nickel leaching reaction began to proceed.

 "Current efficiency" and "selectivity" are defined as follows when the generated PS-S concentration is A (gZL) and the generated thiosulfate ion concentration is B (gZL) in terms of zeolite atoms. . Until the nickel elution reaction takes place during the electrolysis operation, only PS-S and thiosulfate ions are generated, so they may be defined as follows.

 Current efficiency = (A / (A + 2 B)) X 100%

 Selectivity-(A / (A + B)) X I 00%

 [Example 2 (Comparative example)]

The same electrolytic cell as in Example 1 was used, but in contrast to Example 1, constant-current electrolysis was performed under conditions where the pressure in the anode chamber was higher than the pressure in the force-side chamber. This maintained a 1 mm gap between the anode and the diaphragm, increasing the thickness of the anode compartment to 5 mm and reducing the thickness of the force sword compartment to 4 mm. Average empty space of liquid in cathode chamber and cathode chamber In order to set the speed to the same value as in Example 1, the flow rate of the anolyte was changed to 24 OmLZ and the flow rate of the catholyte was changed to 64 mLZ. The time courses of the quantitative values of the concentrations of various sulfur compounds and the measured values of the cell voltage were as follows. One and a half hours after the start of electrolysis, the composition of the polysulfide cooking liquor was as follows: PS-S was 10. O g / L, Na ₂ S was 5.4 gZL in terms of zeo atoms, and the increased thiosulfate ion was zeolite. It was 0.64 g / L in terms of atoms, and the average value of X of the polysulfide ion (S _x 2-) was 2.9. During this period, the current efficiency of PS-S was 89% and the selectivity was 94%. The cell voltage from the start of electrolysis to about one hour was constant at about 1.3 V. The thiosulfate ion concentration began to rise, and the voltage was 1.4 V per hour and forty minutes. After one hour, the voltage rose to about 2 V, and the nickel elution reaction began to progress. Compared with Example 1, the pressure loss was lower and the elution time of nickel was found to be slower, but the current efficiency of PS-S was poor. Industrial applicability

 ADVANTAGE OF THE INVENTION According to this invention, the by-product of a thiosulfate ion is extremely small, a high concentration polysulfide is contained, and the cooking liquor which has a lot of residual Na2S state can be maintained at a low power while maintaining a high selectivity. Can be manufactured. In particular, the use of the polysulfide cooking liquor thus obtained from the white liquor or green liquor in the halve production process for the cooking can effectively increase the yield of rubbish and rub.

Claims

The scope of the claims 1. A solution containing sulfide ion is introduced into the anode chamber of an electrolytic cell having an anode chamber in which a porous anode is provided, a force sword chamber in which a force sword is provided, and an anode chamber and a cathode chamber. A method for producing polysulfide, wherein polysulfide ions are obtained by electrolytic oxidation, wherein the pressure in the cathode chamber is higher than the pressure in the anode chamber.  2. The method for producing a polysulfide according to claim 1, wherein the porous anode has a physically continuous three-dimensional network structure.  3. The method for producing a polysulfide according to claim 2, wherein the porous anode has at least a surface made of nickel or a nickel alloy containing 50% by weight or more of nickel. 4. Method for producing polysulfides according to any one of the above porous § Roh claim surface area once it is effective per energization area 2~ 1 0 0 m ^2/2 of the diaphragm 1-3.  5. The method for producing a polysulfide according to any one of claims 1 to 4, wherein a current density in the electrolytic oxidation is 0.5 to 20 kA / m2 per effective conducting area of the diaphragm.  6. The method for producing a polysulfide according to any one of claims 1 to 5, wherein the solution containing the sulfide ion is passed through the anode chamber at an average superficial velocity of 1 to 30 cmZ seconds. 7. The method for producing a polysulfide according to any one of claims 1 to 6, wherein the solution containing the sulfide ion is a white liquor or a green liquor in a pulp production process.  8. The method for producing a polysulfide according to claim 7, wherein the electrolytically oxidized white liquor or green liquor flowing out of the anode chamber is supplied to the next step without being circulated to the anode chamber.