JP4808551B2 - Method for producing persulfuric acid - Google Patents

Description

  The present invention relates to a method and an electrolytic cell for producing persulfuric acid that can be used as an oxidant for industrial applications in a concentrated state.

It has been reported that so-called electrolyzed water having oxidizing properties or reducing properties produced by water electrolysis can be used in various fields such as medicine and food. Also, in the electronic component cleaning process, washing with electrolytic water is attracting attention because it is an on-site type, and there is little risk associated with storage and transportation, and the wastewater treatment cost can be reduced.
The drugs used here are persulfuric acid and persulfates, which are known to be produced electrochemically, and have already been produced electrolytically on an industrial scale for over a decade, such as acidic sulfates such as Obtained by anodic oxidation of ammonium sulfate (NH ₄ ) ₂ SO ₄ . The production efficiency of persulfuric acid at this time depends on the sulfate ion concentration, and the higher the concentration of sulfuric acid, the higher the efficiency of persulfuric acid.

Platinum electrodes are usually used as electrodes for persulfuric acid production. However, the consumption amount is extremely large under the above-mentioned electrolysis conditions, so impurities cannot be ignored, the application is limited, and the electrodes are frequently replaced. There was a problem that had to be done.
In order to eliminate the drawbacks of electrolytic synthesis of persulfuric acid due to the use of a platinum electrode, the use of a conductive diamond anode has been proposed (Patent Document 1). Conductive diamond as an anode material has a high overvoltage for oxygen generation reaction. When electrolysis of aqueous solution containing sulfate ion using this conductive diamond as anode, oxygen generation reaction by water electrolysis and persulfate ion by oxidation of sulfate ion Although the formation reaction is a competitive reaction, the overvoltage for the oxygen generation reaction of the conductive diamond electrode is high, so the progress of the oxygen generation reaction is suppressed and the production of persulfate ions by oxidation of sulfate ions preferentially proceeds. There is.

The invention described in Patent Document 1 has the advantage that the generation of impurities is small and the anode life is long. On the other hand, since persulfuric acid generation is a competitive reaction with oxygen generation, the current efficiency does not increase and a high concentration of persulfuric acid is not obtained. There is a disadvantage that it cannot be obtained.
The inventions described in Patent Documents 2 and 3 describe a mode in which a cleaning liquid is circulated between a cleaning tank and an electrolytic reaction layer. In the cleaning tank, an object to be cleaned is washed with persulfuric acid, and a part is consumed by cleaning in the cleaning tank. Then, the cleaning liquid converted into sulfuric acid is circulated in the electrolytic reaction tank and electrolyzed to regenerate persulfuric acid, and then returned to the cleaning tank. A cleaning solution obtained by diluting 98% sulfuric acid with pure water is supplied to the cleaning tank at the beginning of operation, and even if this cleaning solution is electrolyzed, the concentration does not increase to a concentration exceeding the initial sulfuric acid concentration.
JP 2001-192874 A JP 2006-111943 A (Claim 1, paragraph 0047) JP 2006-114880 A

  An object of this invention is to provide the method and apparatus which can produce | generate a high concentration persulfuric acid efficiently, suppressing oxygen generating reaction.

In the present invention , a conductive diamond electrode is accommodated in an anode chamber, and the anode chamber is partitioned from the cathode chamber by a reinforced fluororesin cation exchange membrane or a porous fluororesin membrane subjected to hydrophilization treatment. A method for producing persulfuric acid is characterized in that electrolysis is performed by supplying 96% or more of concentrated sulfuric acid as an anolyte to the anode chamber of the electrolytic cell. even if the supply to the electrolytic sulfuric acid 70% or less as a catholyte have good. In the present invention, the high concentration persulfuric acid means 30% or more of persulfuric acid.

The present invention will be described in detail below.
In the present invention, a conductive diamond electrode is used as the anode, and 96% or more of concentrated sulfuric acid is electrolyzed with this conductive diamond electrode. Conductive diamond electrode has higher oxygen overvoltage than platinum electrode and lead dioxide electrode (platinum is several hundred mV, lead dioxide is about 0.5V, conductive diamond is about 1.4V) and reacts with water As shown in reaction formulas (1) and (2), oxygen and ozone are generated. Further, when sulfate ions or hydrogen sulfate ions are present in the anolyte, they react with them to generate oxygen and ozone as shown in the reaction formulas (3) and (4).

2H ₂ O → O ₂ + 4H ⁺ + 4e ⁻ (1.23V) (1)
_{3H 2 O → O 3 + 6H} + + 6e - (1.51V) (2)
^{_{2SO 4 2- → S 2 O 8}} 2- + 2e - (2.01V) (3)
2HSO ₄ ⁻ → S ₂ O ₈ ²⁻ + 2H ⁺ + 2e ⁻ (2.12 V) (4)

As described above, in these reactions, oxygen generation reaction by water electrolysis and persulfate ion generation reaction by oxidation of sulfate ions are competitive reactions. However, when a conductive diamond electrode is used, generation of persulfate ions has priority.
This is because the diamond electrode has an extremely wide potential window, an overvoltage with respect to the oxygen generation reaction is high, and the target oxidation reaction can proceed in potential. Persulfate production occurs with current efficiency, and oxygen evolution occurs only slightly.
The height of the oxygen generation overvoltage of the conductive diamond can be explained as follows. On the normal electrode surface, water is first oxidized to form oxygen species, and oxygen and ozone are thought to be generated from this oxygen species, but diamond is charged with higher chemical stability than ordinary electrode materials. It is considered that the water that is not adsorbed on the surface is difficult to be adsorbed, and therefore, the oxidation of the water is unlikely to occur. On the other hand, sulfate ion is an anion, and it can be presumed that it is more likely to be adsorbed on the diamond surface functioning as the anode even at a low potential and more likely to occur than the oxygen generation reaction.

  Thus, when sulfuric acid electrolysis is performed using a conductive diamond electrode, persulfuric acid can be produced with higher efficiency than using a platinum electrode. However, since conventional sulfuric acid electrolysis uses dilute sulfuric acid, a large amount of water is present, generating a negligible amount of oxygen and ozone, which reduces current efficiency and hinders the increase in the concentration of persulfuric acid produced. .

  In the present invention, since 96% or more of concentrated sulfuric acid is used as an electrolytic raw material, the amount of water is 4% or less, and the persulfate ion generation reaction by oxidation of sulfate ions is significantly more advantageous than water electrolysis. Persulfate ions are selectively generated. Moreover, since the raw material itself is 96% or more concentrated sulfuric acid, the reaction of sulfate ions is promoted and persulfate ions are efficiently generated. The concentration of persulfuric acid obtained is influenced by the reaction time and temperature, but is about 30 to 60%.

In order to further increase the selectivity for the production of persulfuric acid, sodium fluoride or ammonium thiocyanate may be added to the electrolytic solution as in the past. Furthermore, an anion such as sodium, ammonium, potassium, cesium and rubidium may be added to increase the adsorption rate of sulfate ions on the electrode. The persulfuric acid generally includes peroxomonosulfuric acid (H ₂ SO ₅ ) and peroxodisulfuric acid (H ₂ S ₂ O ₈ ), and persulfate in the present invention mainly refers to the latter, but the former may be included. .

  The present invention electrolyzes more than 96% concentrated sulfuric acid using a highly selective conductive diamond electrode for the production of persulfuric acid. Since the raw material has a high concentration, the generated persulfuric acid also has a high concentration. Furthermore, since only 4% or less of water is present in the electrolytic solution, side reactions of oxygen and ozone generation due to water electrolysis are reduced, and persulfuric acid can be produced at a high current density.

Next, each member of the persulfuric acid production method and apparatus of the present invention will be described.
The conductive diamond anode used in the present invention is manufactured by supporting diamond, which is a reduced precipitate of an organic compound serving as a carbon source, on an electrode substrate.
The material and shape of the substrate are not particularly limited as long as the material is conductive, and is a plate, mesh, or porous material such as a vibrant fiber sintered body made of conductive silicon, silicon carbide, titanium, niobium, molybdenum, or the like. It is particularly preferable to use conductive silicon or silicon carbide having a thermal expansion coefficient that can be used. In order to improve the adhesion between the conductive diamond and the substrate, and to increase the surface area of the conductive diamond film and reduce the current density per unit area, it is desirable that the substrate surface has a certain degree of roughness.
When conductive diamond is used in the form of a film, it is desirable that the film thickness be 10 μm to 50 μm in order to reduce durability and occurrence of pinholes. Although a self-supporting film having a thickness of 100 μm or more can be used from the viewpoint of durability, it is not preferable because the cell voltage becomes high and the control of the electrolyte temperature becomes complicated.

The method for supporting the conductive diamond on the substrate is not particularly limited, and any conventional method can be used. Typical conductive diamond production methods include a hot filament CVD (chemical vapor deposition) method, a microwave plasma CVD method, a plasma arc jet method, and a physical vapor deposition (PVD) method. The use of a microwave plasma CVD method is desirable because it is easy to obtain a uniform film.
In addition to this, a diamond electrode in which a synthetic diamond powder produced at an ultra-high pressure is supported on a substrate by using a binder such as a resin can also be used, and in particular when a hydrophobic component such as a fluororesin is present on the electrode surface This makes it easier to capture sulfate ions and improves the reaction efficiency.

  The microwave plasma CVD method uses a mixed gas obtained by diluting a carbon source such as methane and a dopant source such as borane with hydrogen as a conductive material such as conductive silicon, alumina, or silicon carbide connected to a microwave transmitter through a waveguide. This is a method in which a conductive diamond film is introduced into a reaction chamber where a substrate is installed, plasma is generated in the reaction chamber, and conductive diamond is grown on the substrate. In the plasma by microwaves, ions hardly vibrate, and a pseudo high temperature is achieved in a state where only electrons are vibrated, and the chemical reaction is promoted. The plasma output is 1 to 5 kW, and the larger the output, the more active species can be generated, and the growth rate of diamond increases. The advantage of using plasma is that diamond can be deposited at high speed using a substrate with a large surface area.

  In order to impart conductivity to the diamond, trace amounts of elements having different valences are added. The content of boron or phosphorus is preferably 1 to 100,000 ppm, more preferably 100 to 10,000 ppm. As a raw material for this additive element, boron oxide, diphosphorus pentoxide, or the like having a low toxicity can be used. The conductive diamond supported on the substrate thus manufactured is a flat plate, stamped plate, wire mesh, powder sintered body made of a conductive material such as titanium, niobium, tantalum, silicon, carbon, nickel, tungsten carbide. In addition, it can be connected to a power feeding body having a form such as a metal fiber body or a metal fiber sintered body.

The electrolytic cell to be used is a two-chamber electrolytic cell that is divided into an anode chamber and a cathode chamber by a diaphragm such as a reinforced fluororesin cation exchange membrane or a hydrophilized porous fluororesin membrane. The persulfate ions once generated in step 1 are prevented from coming into contact with the cathode and being reduced to sulfate ions.
The material of the electrolysis chamber is preferably PTFE or NewPFE having high temperature resistance and high chemical resistance in terms of durability. As the sealing material, porous PTFE such as GORE-TEX or POREFLON, rubber sheet or O-ring wrapped with PTFE or NewPFE are desirable.
The cathode used in the present invention may be a hydrogen generating electrode or an oxygen gas electrode as long as the concentrated sulfuric acid has durability, and these materials plated with conductive silicon, glassy carbon, and noble metal can be used. In the case of an oxygen gas electrode, the amount of oxygen supplied is about 1.2 to 10 times the theoretical amount.

Neutral membranes such as the product name Poreflon and cation exchange membranes such as the product names Nafion, Aciplex, and Flemion can be used as fluororesin-based cation exchange membranes, but from the standpoint that the products in the bipolar chamber can be separated and manufactured. The use of the latter cation exchange membrane is desirable. Furthermore, the fluororesin cation exchange membrane can rapidly proceed with electrolysis even when the conductivity of the electrolytic solution is low. Fluororesin-based cation exchange membrane with packing (reinforcing cloth) that has a stable dimension even at low moisture content, with a thickness of 50 μm, to make it less susceptible to the effects of water concentration gradients and to lower the cell voltage The following fluororesin cation exchange membranes and non-laminated fluororesin cation exchange membranes are desirable. Under the coexistence with a substance having a low equilibrium water vapor pressure, such as 96% sulfuric acid, there is a problem that the ion exchange membrane has a low water content in the environment and the specific resistance value increases and the electrolytic cell voltage increases. When supplying high concentration sulfuric acid such as 96% sulfuric acid in order to obtain persulfuric acid with high efficiency in the anode chamber, it is preferable to supply 70% or less sulfuric acid to the cathode chamber in order to supply water to the ion exchange membrane. .
In the present invention, in addition to the fluororesin cation exchange membrane, a porous fluororesin membrane subjected to hydrophilic treatment such as IPA (isopropyl alcohol) treatment can also be used as the diaphragm. Other than the ion exchange membrane, a porous fluororesin membrane having a trade name such as Gore-Tex or Poaflon does not undergo electrolysis unless a hydrophilic treatment such as IPA treatment is performed. The porous fluororesin membrane is hydrophobic, cannot pass sulfuric acid, and does not proceed with electrolysis. When the porous fluororesin membrane is hydrophilized, the resin membrane can contain water and concentrated sulfuric acid, and can also conduct electricity by sulfuric acid, thus functioning as an electrolytic cell diaphragm. . Since the porous fluororesin film not subjected to this treatment remains air-containing in the pores and cannot conduct electricity, electrolysis does not proceed. When a hydrophilic porous fluororesin membrane is used for the diaphragm, there is a problem that the product of the bipolar chamber is slightly mixed through the diaphragm, compared to the case where the fluororesin cation exchange membrane is used for the diaphragm. However, the diaphragm itself does not generate resistance and can be operated at a low electrolytic cell voltage.

In the IPA treatment, for example, only the portion of the porous fluororesin film that comes into contact with the liquid during electrolysis is dropped and applied with 98% IPA, and then the entire porous fluororesin film is immersed in pure water and allowed to permeate. The IPA and pure water are replaced and stored while immersed in pure water until installed in the electrolytic cell. When installing in the electrolytic cell, assemble quickly and let the sulfuric acid flow through so that the wetted part does not dry. Further, after replacing IPA and pure water, it is more preferable to substitute only 98% sulfuric acid for the portion in contact with the electrolyte during electrolysis because it does not dry and does not bring extra water into the electrolytic cell during assembly.
IPA has a hydrophilic part of —OH group and a hydrophobic part of —CH ₃ group, and the hydrophobic part is adsorbed on the surface of the porous fluororesin film by the above treatment, and the hydrophilic part is more porous. It becomes wet by adsorbing water or the like to this hydrophilic portion and can be used for electrolysis.
The porous fluororesin membrane is made hydrophilic by the IPA treatment, thereby obtaining conductivity and not only the electrosynthesis reaction of persulfuric acid that does not progress in the porous fluororesin membrane not subjected to the IPA treatment. Persulfuric acid can be synthesized at a lower voltage than when an ion exchange membrane is used.
The hydrophilic treatment is preferably IPA treatment, but may be hydrophilized by other hydrophilic treatment than IPA treatment. In addition to this, a commercially available porous fluororesin membrane that has already been made hydrophilic may be used.

This diaphragm may be sandwiched between two protective plates, and this protective plate is a plate made of PTFE or NewPFE with holes formed by punching or the like, or an expanded mesh.
The conductive diamond electrode has a high oxidizing power, and organic substances that come into contact with the positively polarized conductive diamond surface are decomposed and most of them are converted to carbon dioxide. The diaphragm in the electrolytic cell vibrates between the anode and the cathode under the influence of the discharge pressure of the liquid supply pump used to supply the liquid to the electrolytic cell, and without the protective plate, it contacts the conductive diamond anode. May wear out. Further, when the vibration is caused without the protective plate, the distance between the electrode and the diaphragm changes, and the cell voltage may also change.

  In order to protect the conductive diamond electrode and its substrate from the press pressure for sealing between the members of the electrolytic cell, it is desirable to sandwich a cushioning material such as rubber. Moreover, a power feeding body can also be used by using a conductive material such as a metal fiber sintered body or a metal web.

  Next, examples of persulfuric acid production will be described in the present invention, but the examples do not limit the present invention.

[Example 1]
A 20 μm diamond layer was formed by microwave plasma CVD using methane and diborane (10000 ppm with respect to methane) on a 6-inch diameter silicon substrate (base) having a thickness of 3 mm, to obtain a conductive diamond electrode. A width of 1.8 cm of the outer peripheral portion of this electrode was used as a flange portion, and an anode having an electrolytic area of about 1 dm ² was obtained. A cathode was supported on glassy carbon having the same dimensions as the anode to form a cathode. Gore Select (manufactured by Japan Gore-Tex Co., Ltd.), a fluororesin cation exchange membrane woven with a reinforcing cloth as a diaphragm, is used, and the anode and cathode are arranged on both sides of the fluororesin cation exchange membrane. The system cation exchange membrane and each electrode were made into an anode chamber through which an electrolyte solution passed with a space of 3 mm, and the liquid circulation type ion exchange membrane type electrolytic cell shown in FIG. 1 was constructed.

  The electrolytic cell 1 has an anode chamber 4 in which the conductive diamond anode 3 is accommodated by a fluororesin cation exchange membrane 2 and filled with concentrated sulfuric acid, and a cathode in which the platinum cathode 5 is accommodated and filled with dilute sulfuric acid. It is partitioned into a chamber 6. An anolyte circulation pipe 7 is connected to the anode chamber 4, and concentrated sulfuric acid as an anolyte is circulated between the anode chamber 4 and the anolyte tank 8 by an anolyte circulation pump 9 through the circulation pipe 7. . Further, a catholyte circulation pipe 10 is connected to the cathode chamber 6, and the catholyte is circulated between the cathode chamber 6 and the catholyte tank 11 by the catholyte circulation pump 12 through the circulation pipe 10. The lower portions of the anolyte tank 8 and the catholyte tank 11 are immersed in a thermostatic chamber 13 to maintain the liquid temperature substantially constant.

Using this electrolytic cell, persulfuric acid was produced under the following conditions. The electrolytic solution was circulated by an aired pump.
Current value: 20 A / dm ²
Anode circulating fluid: 96% EL sulfuric acid (manufactured by Kanto Chemical Co., Inc.)
Cathode circulating fluid: 70% sulfuric acid (prepared by diluting anode circulating fluid with pure water)
Anolyte flow rate: 1 L / min Catholyte flow rate: 1 L / min Initial anolyte temperature: 25 ° C.
Initial catholyte temperature: 25 ° C
Electrolysis time: 60 minutes

By electrolysis for 60 minutes, about 40% persulfuric acid was obtained with a current efficiency of 95%, and the cell voltage was 14V.
After electrolysis, the electrolytic cell was disassembled and the fluororesin-based cation exchange membrane was visually observed. A white portion having the same shape as the electrolysis area was formed on the anode side surface of the fluororesin-based cation exchange membrane. . When this white portion was observed by SEM, scaly marks were generated. This is because, during electrolysis, the fluororesin cation exchange membrane is intermittently contacted by the ion exchange membrane and the anode due to the vibration of the electrolyte pressure generated from the circulation pump, so that the fluororesin cation exchange membrane is oxidized and altered. It was speculated that there was a mark. No scaly mark was formed on the cathode side.

[Example 2]
Persulfuric acid was produced under the same conditions as in Example 1 except that a fluororesin cation exchange membrane was sandwiched between two protective plates. The protective plate was made of PTFE having a thickness of 2 mm, and a plurality of circular holes having a diameter of 0.6 cm were formed so that holes were formed in about 29% of the whole.
By electrolysis for 60 minutes, about 40% persulfuric acid was obtained with a current efficiency of 95%. The cell voltage was 14V.
After electrolysis, the electrolytic cell was disassembled and the fluororesin cation exchange membrane was visually observed. No white portion was formed on any surface of the fluororesin cation exchange membrane.

[Example 3]
When persulfuric acid was produced under the same conditions as in Example 2 except that Nafion 117 (trade name of DuPont), which is a fluorinated sulfonate ion exchange membrane, was used instead of Gore Select as a diaphragm, about 20 % Persulfuric acid was obtained with a current efficiency of 48%. The cell voltage was 40V.
After the electrolysis, the electrolytic cell was disassembled and the fluororesin cation exchange membrane was visually observed. As a result, a plurality of holes were formed in the ion exchange membrane portion in contact with the flange. This can be presumed to be due to contraction of the ion exchange membrane due to heat generation and dehydration. The reason why the current efficiency is lower than that in Example 2 is considered to be that persulfuric acid generated due to the generation of pores in the ion exchange membrane leaked to the cathode chamber.

[Example 4]
Poreflon WP-045-40 (manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) was cut to the same dimensions as the electrode, and 98% IPA was dropped and applied only to the wetted part. This was immersed in pure water for 1 hour to replace it, and then taken out and subjected to a hydrophilic treatment. Immediately before the assembly of the electrolytic cell, 96% sulfuric acid was dropped and applied only to the wetted part of the hydrofluorinated poreflon.
When persulfuric acid production was performed under the same conditions as in Example 2 except that the pore filter used in the above IPA treatment was used in place of Gore Select as the diaphragm, about 45% persulfuric acid had a current efficiency of 95%. Obtained. The cell voltage was 10V.
After electrolysis, the electrolytic cell was disassembled and the ion exchange membrane was visually observed. As a result, no change was observed.

[Comparative Example 1]
The persulfuric acid production was tried under the same conditions as in Example 2 except that untreated pore flon (manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) was used instead of Gore Select as the diaphragm, but electrolysis did not proceed.

[Example 5]
When perforated sulfuric acid was produced under the same conditions as in Example 2 except that hydrophilic pore-flon WPW-045-40 (manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) was used as a diaphragm, the same electrolysis results as in Example 2 were obtained. Hydrophilic pore flon has hydrophilicity and can be electrolyzed without IPA treatment. However, after the electrolysis test, the hydrophilic pore fluorocarbon used in the test was washed with pure water, dried, and immersed again in pure water. As a result, the hydrophilicity was lost and water did not penetrate.

  The results of Examples 2 to 5 and Comparative Example 1 are summarized in Table 1.

[Example 6]
Electrosynthesis of persulfuric acid was carried out under the same conditions as in Example 2 except that the electrolysis time was 1 minute.
After 1 minute of electrolysis, 5 ml of the liquid in the electrolyte tank is collected and diluted with water to make 5% sulfuric acid, and the total amount is subjected to redox titration by iodine titration to produce persulfuric acid (H ₂ S ₂ O ₈ ). As a result, the current efficiency was calculated to be 95% as shown in Table 2.

[Comparative Examples 2 and 3]
When the current efficiency was measured under the same conditions as in Example 6 except that 50% sulfuric acid (Comparative Example 2) and 10% (Comparative Example 3) were used instead of 96% sulfuric acid, 83% as shown in Table 2. (Comparative Example 2) and 77% (Comparative Example 3).

Schematic of the liquid circulation type ion exchange membrane type electrolytic cell used by the Example and the comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrolyzer 2 Diaphragm (Fluoro resin cation exchange membrane, porous fluorine resin membrane)
DESCRIPTION OF SYMBOLS 3 Conductive diamond anode 4 Anode chamber 5 Cathode 6 Cathode chamber 7 Anolyte circulation pipe 8 Anolyte tank 9 Anolyte circulation pump 10 Catholyte circulation pipe 11 Catholyte tank 12 Catholyte circulation pump 13 Constant temperature bath

Claims (2)

Electrolysis in which a conductive diamond electrode is accommodated in an anode chamber, and the anode chamber is partitioned from the cathode chamber by a diaphragm which is a fluorinated resin-based cation exchange membrane or a porous fluorine-based resin membrane subjected to hydrophilic treatment. There the row electrolysis is supplied to the anode chamber 96% or more concentrated sulfuric acid as the anolyte tank, a manufacturing method of persulfate, characterized in that the electrolytic synthesis persulfate. Electrolysis in which a conductive diamond electrode is accommodated in an anode chamber, and the anode chamber is partitioned from the cathode chamber by a diaphragm which is a fluorinated resin-based cation exchange membrane or a porous fluorine-based resin membrane subjected to hydrophilic treatment. 96% or more of concentrated sulfuric acid in the anode compartment of the tank and the anolyte, have rows electrolysis by supplying a catholyte sulfuric acid 70% or less in the cathode chamber, excessive characterized by electrolytic synthesis persulfate A method for producing sulfuric acid.

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