JP4880865B2 - Process for simultaneous electrochemical production of sodium dithionite and sodium peroxodisulfate - Google Patents

Description

  In particular, various bleaching processes that do not use chlorine, particularly in paper bleaching and pulp bleaching, are increasingly using a combination of oxidizing and reducing bleaching sequences. In this case, sodium dithionite is advantageously used as the reducing bleach and hydrogen peroxide as the oxidizing bleach. It has also been proposed to use peroxodisulfate or peroxomonosulfate, which can be produced electrochemically, as oxidative bleach (DE-PS 19803001). Peroxodisulfate is produced exclusively electrochemically (J. Balej, H. Vogt: Electrochemical Reactors. In: Fortschritte der Verfahrenstechnik. Vol. 22, page 361, VDI Verlag 1984).

  According to one electrochemical combination method, sodium peroxodisulfate can be produced from sodium sulfate together with caustic soda in a two-chamber cell equipped with a cation exchange membrane as a separator (EP 084194).

  It has also been proposed to use an alkaline solution of sodium peroxodisulfate and caustic soda stoichiometrically for the bleaching and oxidation processes (DE-PS 4430391).

  In contrast, sodium dithionite, which is used not only as a bleaching agent used in the textile and paper industries but also as a dyeing aid and printing aid, is advantageously produced by chemical methods ( W. Bruekner, R. Schliebs, G. Winter, K.-H. Bueschel: Industrielle anorganische Chemie. Weinheim: Verlag Chemie 1986). Dithionite is obtained by techniques of reducing sulfur dioxide using zinc, using sodium formate in a high pressure reaction, or using sodium tetrahydroborate. Dithionite can also be obtained by cathodic reduction of sulfur dioxide. However, until now, only an indirect electrolysis method using sodium amalgam as a reducing agent has been carried out on an industrial scale (Ullmanns Encyclopedia Industrial Chemistry, Vol. A 25, pages 483 to 484, Weinheim 1994). However, because of the ecotoxicological potential of mercury salts, this method is no longer epoch-making.

  Until now, direct cathodic reduction of sulfite ion or hydrogen sulfite ion has no industrial significance. This is mainly due to the significant yield loss that occurs as the electrolysis time increases because dithionite generates and decomposes thiosulfate and disulfite ions. This reaction occurs more rapidly at higher temperatures and proton concentrations. Therefore, it has been proposed to minimize the residence time of dithionite in the electrode gap by keeping the electrolyte temperature below 20 ° C. during electrolysis or reducing the cathode flow volume by internal and external cooling systems. (DE-PS2646825).

  In the patent specification (US-PS 3920551), the production of dithionite is combined with chlorine production, thereby utilizing the cathode and anode processes. Although the ion exchange membranes utilized today are highly selective, they cannot prevent chloride ions from reaching the cathode cycle during the electrolysis process. This is a problem because many uses require chloride-free dithionite.

  In order to meet the above requirements, it has been proposed to use a three-chamber cell (US-PS 3905879). The three-chamber cell has a drawback that an additional voltage loss is caused by the central chamber as compared to the two-chamber cell. Furthermore, an anion exchange membrane is required in addition to the cation exchange membrane, and this anion exchange membrane has a relatively low resistance to oxidation, and therefore the membrane must be frequently exchanged. In addition to the higher operating costs associated with this, the procurement cost of a three-chamber cell is also clearly higher than a two-chamber cell that is easily manufactured.

  The object of the present invention is to simultaneously and economically produce sodium dithionite and sodium peroxodisulfate by an electrochemical method.

This problem is solved by a combined electrolysis method as described in the features of claim 1. In this method, sodium peroxodisulfate at the anode, sodium dithionite at the cathode, an anode of niobium, tantalum, titanium or zirconium which is a valve metal made of smooth platinum or coated with platinum or diamond, and carbon, 1.5-6 kA / m ^{2 in} one or more electrolysis cells provided with a cathode made of a material coated with special steel, silver or platinum metal and divided in two by a cation exchange membrane At a current density of 20 to 60 ° C. In this case, sodium sulfate and water are supplied to the anolyte circulating through the anode chamber. Sodium ions released on the anode side reach the cathode chamber through the cation exchange membrane. The pH value is adjusted in the region of 4-6 by supplying sulfur dioxide, water and possibly sodium bisulfite to the catholyte circulating through the cathode chamber.

  In this case, important basic chemicals such as sodium peroxodisulfate and sulfite are obtained from sodium sulfate and sulfur dioxide, which are chemicals generated as unwanted or by-products in many industrial processes, or from sulfuric acid and bisulfite solutions. It becomes possible to produce sodium dithionate in the form of crystals.

  Unlike single electrochemical production of sodium peroxodisulfate or sodium dithionite, twice the electrolysis current is used. This significantly reduces the specific equipment costs for the total product obtained, as well as the normal operating costs and, in this case, in particular the specific electrical energy consumption.

Also, dithionite is not contaminated by chloride as compared to the known electrochemical combinatorial process of producing dithionite at the cathode with simultaneous chlorine generation at the anode. In addition, the process is simpler than the process combined with chlorine separation. In this case, the coupling of the two electrode processes takes place by Na ⁺ ions moving from the anode chamber to the cathode chamber, which is evident by two simplified equations relating to the electrode reactions that take place mainly.
Anode reaction: 2Na ₂ SO ₄ -2e → Na ₂ S ₂ O ₈ + Na ⁺
Cathode reaction: 2SO ₂ ⁻ + 2Na ⁺ + 2e → Na ₂ S ₂ O ₄
However, release of Na ⁺ by the anode reaction, the consumption of Na ⁺ ions by moving and cathode reactions of Na ⁺ through the cation exchange membrane is dependent on completely different influence quantities, sodium balance, the material flow both electrodes solution solution It must be balanced by metering in.

  If the current efficiency of dithionite generation is greater than the movement of sodium ions, the sodium ions in the anolyte will decrease and the current efficiency of dithionite generation will decrease even if a preselected pH value is maintained. In this case, the total concentration of the required sodium ions can be adjusted by additionally metering sodium sulfite or sodium hydrogen sulfite or sodium hydroxide into the catholyte cycle.

Surprisingly, it has been found that when a pH value of 4 to 6 to be maintained in the catalyst is realized at a high SO ₂ concentration, the acid-catalyzed decomposition reaction of dithionite ions can be sufficiently suppressed even when the electrode solution temperature is high. It was.

  Therefore, preventing the reduction of catholyte sulfur dioxide by introducing sulfur dioxide during electrolysis, eg by using a gas-dispersed cathode, by an intensive gas radiator or by adding liquid sulfur dioxide Can do.

  If these conditions are observed, electrolysis can be performed even at temperatures up to 50 ° C. without causing significant decomposition of the generated dithionite ions and, consequently, a decrease in current efficiency.

  Advantageously, it is desirable that the average residence time of the produced sodium dithionite in the catholyte cycle is less than 30 minutes. This is made possible by minimizing the amount of catholyte circulating throughout the cathode cycle.

  To achieve optimal mass transport from and to the electrode surface, the relative velocity of the catholyte along the cathode is at least 0.1 m / s, preferably 0.3-0.5 m / It is desirable that s. The same flow rate and residence time are also advantageous for the formation of peroxodisulfate at the anode, which gives the advantage that both electrolyte circulation systems have a nearly symmetrical structure, in both electrode chambers. The pressure difference between the cation exchange membranes is almost the same, resulting in almost the same pressure formation.

Chemical reactors pre-installed by reacting sodium bisulfite or sodium sulfite with sulfuric acid in situ where sulfur dioxide is not obtained in gaseous form or where the use of liquid sulfur dioxide is not desired or impossible In both starting materials can be obtained in situ.
Na ₂ S ₂ O ₅ + H ₂ SO ₄ → Na ₂ SO ₄ + 2SO ₂ + H ₂ O
Na ₂ SO ₃ + H ₂ SO ₄ → Na ₂ SO ₄ + SO ₂ + H ₂ O
For this purpose, industrially available bisulfite solutions can also be used. It is advantageous to keep the residual amount of sulfur dioxide in the resulting sodium sulfate solution as low as possible by stripping with water vapor so that it can be fed directly into the anolyte.

  When using a sulfite solution, only about half of the amount of sodium sulfate required for the entire process is produced, while the other half can be added in solid form. This provides the advantage that the post-dissolution during electrolysis compensates for the consumption of sodium sulfate and maintains the high sulfate concentration required for high current efficiency of peroxodisulfate formation.

  For anode production of peroxodisulfate with an anode made of smooth platinum, optimum current efficiency is obtained with a high current density of 4-7 kA / m 2, while lower current density is advantageous for dithionite production. By adjusting the ratio of the electrochemically acting cathode surface to the platinum acting platinum surface at 1: 1 to 4, it is possible to meet the advantageous conditions for both reactions. For this purpose, a part of the platinum surface is covered with a mask made of, for example, tantalum, or a mesh electrode or strip electrode (Streifenelektroden) is used so as to obtain a current density distribution as uniform as possible even with a smaller anode surface area The platinum surface can be divided in form.

  This method has the advantage that, in particular, the current density in the cathode chamber and the cation exchange membrane is smaller than the current density in the anode and the anode chamber adjacent thereto, so that the required current density of the anode is reduced. Despite being high, the voltage drop and thus the specific electrical energy consumption is significantly reduced.

  In order to obtain the maximum current efficiency of peroxodisulfate formation, it is necessary to add an additive that increases the potential, in particular sodium thiocyanate, to the anolyte. However, other known electrolysis additives such as, for example, sodium cyanamide, thiourea, fluoride, chloride, etc. can also be used advantageously in this combination method.

  In the catholyte, suitable additives such as phosphoric acid and / or phosphate can be used for the stabilization of dithionite or to maintain the desired pH value.

  Processing the resulting aqueous solution of sodium dithionite and sodium peroxodisulfate, further containing sulfite or sodium sulfate and sulfuric acid, in a known manner, into a crystalline solid end product. In this case, the mother liquor obtained from the crystallization process can also be returned to the electrolyte cycle.

  However, in many cases it is advantageous to use the resulting solution as a reducing and oxidative bleach, either directly or after crystallization of bisulfite and / or sodium sulfate.

  In particular, the combined use of both electrolysis products is advantageous, for example for oxidative and reductive bleaching sequences in pulp bleaching. At this time, the regenerated sodium sulfate can be separated and returned to the combined electrolysis process.

Example 1:
FIG. 1 shows a flow chart illustrating an electrolyzer equipped with a pre-reactor for producing sulfur dioxide and sodium sulfate in situ from a bisulfite solution. Prereactor 1 produces sulfuric acid at 2 and bisulfite solution at 3 and, on the one hand, the amount of sulfur dioxide consumed during the process, on the other hand so that the sodium present reacts to become almost completely sodium sulfate. Dosage at a proper ratio. The nearly concentrated sodium sulfate solution obtained at the bottom of the reactor is fed to the anolyte cycle at 4 and the sulfur dioxide exiting at the top of the reactor is fed to the catholyte cycle at 5.

  The catholyte is guided to the cycle by the circulation pump 6 through the cathode chamber 7 of the electrolysis cell 8 and the gas separator 9. At 10, meter the amount of water required to obtain the desired final concentration into the catholyte cycle. In 11, the separated anode gas flows out, and in 12, the amount of catholyte corresponding to the amount of liquid supplied overflows with the concentration of sodium dithionite increased. The cathode chamber is separated from the anode chamber 14 by a cation exchange membrane 13. The anolyte is circulated by the circulation pump 15 through the anode chamber and the gas separator 16 and the dissolution vessel 17. In this dissolution vessel, crystalline sodium sulfate is added at 18 to saturate the anolyte. In 19, an electrolysis additive that improves the potential is metered in, and in 20, the separated anode gas flows out. The produced sodium peroxodisulfate solution flows out from upstream 21 of the dissolution vessel.

Example 2
In a small-scale industrial experimental apparatus in which Example 1 is changed and no pre-reactor is provided, the anolyte and catholyte are pumped through the cycle through the electrode chamber and gas separator of the electrolysis cell by a circulation pump. To do. In addition, the anolyte cycle incorporates a dissolution vessel for sodium sulfate as shown in FIG. The anolyte cycle is metered with deionized water containing sodium thiocyanate additive (as an additive to increase potential) by a metering pump. Further, water-free solid sodium sulfate is metered into the dissolution vessel. In the catholyte cycle, gaseous sulfur dioxide is supplied from a gas cylinder and a sodium sulfite solution is supplied by a metering pump. Sodium sulfite is used to compensate for the disadvantages of sodium compounds caused by less migration of sodium ions from the anode chamber to the cathode chamber. The pH value is adjusted to approximately 5.8 by the addition of sulfur dioxide. Thereby, the supplied amount of SO ₂ can be optimally adapted to the amount of movement of sodium ions.

As electrolysis cell, a bipolar filter press electrolysis cell is used as described in DE 4419683 for the production of peroxodisulfate. This electrolysis cell is composed of three electrode plates provided on a tightening frame, two edge plates provided with a current supply unit, and an intermediate bipolar electrode plate. This forms two electrolysis cells that are electrically connected in series and connected in parallel with respect to the electrolyte flow. The electrode plate is made of impregnated graphite provided with a built-in cooling passage and an inlet / outlet attached for electrolyte solution and cooling water. In the anode, an insulating plate made of PVC and a gasket made of EPDM and having a thickness of about 3 mm are provided. On this insulating plate, a laterally provided platinum film strip is arranged as an anode, which is in lateral contact with the graphite substrate on the underside of the gasket. Both electrode chambers are separated by a cation exchange membrane of model number Nafion450 (DuPont). The cathode chamber is attached to the substrate in the form of a flow-through (4 mm depth) flowed in parallel. Since the cell is 2000 mm high, the cross-sectional cross section of the anode and cathode chambers can be kept very small, about 1.5 cm ² , thereby obtaining a high flow velocity along both electrodes. . The liquid volume during the cathode cycle is minimized, so that the shortest possible residence time can be achieved.

The main technical data used are as follows:
Anode area (platinum) electrode plates one per 300 cm ^2, a total of 600 cm ²
The cathode area (Graphite): electrode plates one per 1200 cm ^2, a total of 2400 cm ²
Current intensity: 2 × 150 A = 300 A Current capacity Current density: Anode 0.5 A / cm ² , Cathode 0.12 A / cm ²
Catholyte cycle volume: 2.5 l
Anolyte cycle volume (including dissolution vessel): 6.5 l
Circulation amount Anolyte = Catholyte: 400 l / h
Speed along the electrode surface: about 0.4 m / s
Number of circulations per hour: catholyte 160, anolyte 61.5
The quantities dispensed are as follows:
Catholyte: solution of 95 g / l Na ₂ SO ₃ 4.6 l / h
Gas inflow: SO ₂ approx. 680 g / h (temporal average)
Anolyte: 3.6 l / h water containing 0.15 g / l NaSCN
Dissolution container: Na ₂ SO ₄ 2000 g / h
The cooling of the cathode was adjusted so that the temperature of the circulating catholyte was about 35 ° C. and the temperature of the anolyte was adjusted to about 48 ° C. The cell voltage was 5.5V (total voltage 11V).

  After a break-in time of about 6 hours, a more stable operating state is obtained, and thereafter, the amount of electrode solution shown below having a composition relating to the above-mentioned overflow is circulated in the electrode solution cycle.

Anolyte: 4.1 l / h (Na ₂ S ₂ O ₈ 229 g / l + Na ₂ SO ₄ 215 g / l + H ₂ SO ₄ 8 g / l)
In the anolyte cycle, the average residence time was about 95 minutes. The total yield of sodium peroxodisulfate is 939 g / h, which corresponds to a current efficiency of 70.5%.

Catholyte:
5.4 l / h, including the following composition:
142 g / l Na ₂ S ₂ O ₈ , Na ₂ HSO ₃ approximately 70 g / l, Na ₂ SO ₃ approximately 20 g / l, Na ₂ S ₂ O ₃ approximately 10 g / l
In the catholyte cycle, the average residence time was about 28 minutes. The total yield of sodium dithionite is 767 g / h, which corresponds to a current efficiency of 78.7%.

Flow chart illustrating an electrolyzer with a pre-reactor for producing sulfur dioxide and sodium sulfate in situ from caustic bisulfite.

Claims (12)

In a method for simultaneously producing sodium peroxodisulfate and sodium dithionite,
In an electrolytic cell divided in two by a cation exchange membrane, in the aqueous solution at a current density of 1.5-6 kA / m ² and a temperature of 20-60 ° C., at the anode, sodium sulfate is oxidized to peroxodisulfuric acid. Peroxo, characterized in that sodium dithionite is produced at a pH value of 4-6 from sodium ions released at the anode and transferred through the cation exchange membrane and supplied SO ₂ at the cathode. A method for simultaneously producing sodium disulfate and sodium dithionite. The method according to claim 1, wherein Na ₂ SO ₄ and water are supplied to the circulating anolyte, and SO ₂ and water are supplied to the circulating catholyte.   The process according to claim 1, wherein sulfur dioxide and sodium sulfate are produced from sodium sulfite or sodium hydrogen sulfite and sulfuric acid in a pre-reactor.   Use a valve metal niobium, tantalum, titanium or zirconium anode made of smooth platinum or coated with platinum or diamond and a cathode made of carbon, special steel, silver or a material coated with platinum metal The method of claim 1.   The method according to any one of claims 1 to 4, wherein an anode made of smooth platinum is used, and the ratio of the cathode area to the anode area is adjusted to 1 to 4.   6. A method according to any one of claims 1 to 5, wherein the flow velocity along the electrode is at least 0.1 m / s.   7. A process as claimed in any one of claims 1 to 6, wherein the produced sodium dithionite has an average residence time in the catholyte cycle of up to 30 minutes. 8. One or more potential enhancing additives selected from sodium thiocyanate, sodium cyanamide, thiourea, fluoride and chloride are present in the anolyte according to any one of claims 1-7. Method. The presence of a stabilizer of phosphoric acid and / or phosphate in the catholyte, any one process of claim 1 to 8. Peroxodisulfate anolyte was circulated containing sodium sulphate is worked up in order to obtain crystalline sodium peroxodisulfate, any one process of claim 1 to 9. 11. A method according to any one of claims 1 to 10 , wherein the circulated catholyte containing sodium dithionite is post-treated to obtain crystalline sodium dithionite. Circulating the anolyte solution and catholyte solution was directly used as an oxidation or reduction bleaching solution, any one process of claim 1 to 9.

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