JPH021917B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for producing chlorine dioxide in substantially pure form, ie, substantially free of chlorine.

Chlorine dioxide is widely used as a bleaching chemical and is known to be produced by the reduction of sodium chlorate in an acidic aqueous reaction medium. The reaction in which chlorine dioxide is formed is represented by the following equation: ClO ₃ ⁻ +Cl ⁻ +2H ⁺ →ClO ₂ +1/2Cl ₂ +H ₂ O Therefore, chlorine is usually produced together with chlorine dioxide.

reducing the co-produced chlorine chemically, for example by using sulfur dioxide or methanol;
Operations are known which consist in producing chlorine ions for the above-mentioned chlorine dioxide generating operations in situ. Because the operation uses sulfuric acid as the acid source, it produces a sodium sulfate byproduct.

Additionally, the use of electrolytic operations to produce chlorine dioxide has been proposed in the industry. In this regard, US Pat. No. 3,904,495, US Pat.
It can be known from the specifications of the same No. 3920801, the same No. 4308117, the same No. 4324635, and the same No. 4456510.

With the exception of the last patent specification, in each case chlorine dioxide is produced together with chlorine from a chloric acid solution in the anode compartment of a multicompartment electrolyzer. US Patent No.
In the specification of No. 4456510, chlorine dioxide is produced by electrolyzing a sodium chlorite aqueous solution.

The process of the invention is therefore an electrolytic process for producing chlorine dioxide carried out in the cathode compartment of an electrolytic cell divided in a cation exchange membrane, in which the co-produced chlorine is transferred to a three-dimensional conductive surface. The object of the present invention is to provide a method for electrical reduction in a cathode compartment equipped with a high surface area cathode. Typically, chlorate ions in the form of an aqueous sodium chlorate solution are charged to the cathode compartment which is supplied with hydrogen and chloride ions. Chlorate ions are reduced by hydrogen ions and chloride ions in the cathode compartment to produce chlorine dioxide, while an electric current is applied to cycloconvert the co-produced chlorine to chloride ions, which are then evacuated from the cathode compartment. Hydrogen ions are electrolytically formed in the anode compartment and migrate from the anode compartment to the cathode compartment. High purity chlorine dioxide is thus produced electrolytically from a chlorate charge.

Chlorine dioxide is chemically generated in the cathode compartment by the following formula: NaClO ₃ +2H ⁺ +2Cl ^- →ClO ₂ +1/2Cl ₂ +NaCl+H ₂ O The chlorine produced along with chlorine dioxide in the cathode compartment by the above formula is electrolytically produced. The product is reduced to chloride ions and removed from the cathode compartment as a substantially chlorine-free product, chlorine dioxide.

After the initial charge of oxyacid, water is charged to the anolyte compartment of the electrolyzer. The electrolytic reaction that takes place in the electrolytic cell produces hydrogen ions and oxygen gas which is exhausted from the anode compartment, and the hydrogen ions are transferred through the cation exchange membrane to the cathode compartment where the chemical reaction produces chlorine dioxide. Produces hydrogen ions for the reaction. For every gram atom of chlorine that is electrochemically reduced to Cl ^- in the cathodic compartment, 1 mole of H ⁺ is transferred to the cathodic compartment, thereby reducing the hydrogen and chloride ions required for continuous operation of the chlorine dioxide production reaction. Supply 1 mole of 2 moles. Therefore, the cathode compartment requires a charge of 1 mole of sodium chlorate, 1 mole of hydrogen ions, and 1 mole of chloride ions to maintain chlorine dioxide production as a continuous operation.

Alternatively, 1/2 mole of chlorine can be charged to the cathode compartment along with 1 mole of sodium chlorate.
In this case, 2 moles of H ⁺ are transferred from the anode compartment to the cathode compartment to meet the hydrogen ion requirements of the operation, while 1/2 moles of chlorine are charged to the cathode compartment.
2 moles and 1/2 mole of chlorine co-produced in the cathode compartment are electrochemically cyclized to provide 2 moles of chloride ions.

First, FIG. 1 will be described. FIG. 1 shows an electrolytic cell 1 for producing chlorine dioxide according to one embodiment of the present invention.
0 is written. The aqueous sodium chlorate solution is introduced via conduit 12 into the cathode compartment 14 of the electrolytic cell 10, which is provided with three-dimensional electrodes. An acid, preferably hydrochloric acid, is also charged to the cathode compartment 14 via conduit 16.

The sodium chlorate aqueous solution charged through the conduit 12 has a predetermined flow rate of the sodium chlorate aqueous solution,
A relatively high concentration of sodium chlorate in the cathode compartment 14, typically about 5 molar or more, preferably about 5 to 6.5 molar.
Have sufficient concentration to establish a molar concentration.

Electrolytic cell 10 includes a cation exchange membrane 18 for separating anode compartment 20 and cathode compartment 14 . After the initial charge of oxyacid, usually flowing acid, water is charged to the anolyte compartment 20 via conduit 22 and the hydrogen ions produced by electrolysis of the anolyte migrate through the cation exchange membrane 18 to the cathode compartment 14. The anolyte sulfuric acid solution is recycled by conduit 23.

The hydrogen ions traveling through the cation exchange membrane 18 and the charge of hydrochloric acid via conduit 16 establish a total acid normality in the cathode compartment 14 of at least about 0.01 normal, preferably at least about 0.05 normal.

Oxygen co-produced in the electrolytic process in the anode compartment is exhausted from the anode compartment 20 by conduit 24.

In the cathode compartment 14, the sodium chlorate charged by conduit 12 is mixed with hydrogen and chloride ions charged by conduit 16, hydrogen ions produced by electrolysis traveling through a cation exchange membrane, and as described below. It reacts chemically with the chloride ions produced by electrolysis in the cathode compartment 14 to form chlorine dioxide and chlorine as follows: NaClO ₃ +2H ⁺ +2Cl ^- →ClO ₂ +1/2Cl ₂ +NaCl+H ₂ O Hydrogen Half of the ion requirement is provided by the acid charged by conduit 16, and the remainder of the hydrogen ion requirement is provided by hydrogen ions traveling from the anode compartment 20.

The co-produced chlorine is reduced under electrochemical conditions present in the cathode compartment 14 that are selective for the chlorine dioxide present in the cathode compartment 14. The chloride ions produced as described above provide half of the chloride ions for the chemical reduction of chlorate, with the remainder of the chloride ions being supplied by hydrochloric acid in conduit 16 or by chloride such as sodium chloride. The ions may be supplied from some other convenient external source.

Depending on the electrolytic conditions in the cathode compartment, chloride ions can be produced directly from chlorine co-produced by an electrochemical ring element according to the formula below.

1/2Cl ₂ +e→Cl ^- Alternatively, it may be produced indirectly by chemically ring-forming chlorite ion produced from chlorine dioxide by electrolysis according to the following formula.

In this latter operation, controlling the formation of chlorite ions avoids further electrolytic reduction of chlorite which would result in inefficient production of chloride ions.

The chlorine concentration in the product exhaust gas stream in conduit 26 is monitored and controlled by the electrical current applied to the electrolyzer.

Charge of sodium chlorate via conduit 12 and charge of chloride ions via conduit 16 and electrochemically produced chloride ions are typically at least about 1/2
1, preferably establishing a chlorate/chloride ion ratio of about 2/1 to 4/1.

the electrode potential applied to the cathode is more noble than -1 volt relative to a saturated calomel electrode (SCE) as measured by a current supply device to the cathode, and less noble than the open circuit potential under typical conditions; Preferably -0.2
It's meaner than Bolt.

The electrode potential of the cathode is related to the solution potential measured with a current supply device in a similar manner as for the plate electrode. Three-dimensional electrodes, such as those used in the present invention, have an inherent potential distribution within the electrode structure, the exact potential depending on the measurement location, and may be less than -1 volt relative to SCE.

Cathode compartment 14 is maintained at an elevated temperature to increase the rate of chlorine dioxide formation. Usually about 50℃ or higher,
Preferably a temperature of about 60-70°C is used.

Alternatively, chlorine may be supplied into the cathode compartment 14 in place of the hydrogen ions and chloride ions in conduit 16 to selectively reduce the chlorine to chloride ions with selective reduction of the co-produced chlorine. good. In this case, the anolyte charge is increased to double the amount of hydrogen ions transferred through the cation exchange membrane 18 to provide the entire hydrogen ion requirement of the cathode compartment 14.

Chlorine dioxide produced during the chemical reaction in the substantial absence of chlorine is exhausted from cathode compartment 14 by conduit 26 as a product gas stream. This chlorine dioxide stream can be further utilized, for example, as described below with respect to the embodiment of FIG.

The by-product sodium chloride from the chemical production of chlorine dioxide is removed from the cathode compartment as an aqueous solution by conduit 28. The aqueous sodium chloride solution may be sent to a chlorate electrolyzer for electrolytic conversion to an aqueous sodium chlorate solution and then recycled to cathode compartment 14 to provide at least a portion of the sodium chlorate in conduit 12. can.

The cathode used in cathode compartment 14 is a high surface area electrode with a three-dimensional electrolyte contacting surface that allows for prolonged contact between the reactants.

The term "high surface area" in reference to a cathode refers to a type of electrode in which a large surface area of the electrode is exposed to the electrolyte compared to the physical dimensions of the electrode. The electrode is formed with a gap through which the electrolyte flows, thereby having a three-dimensional surface in contact with the electrolyte.

High surface area cathodes can be of the so-called "flow-through" type, in which the electrolyte is made of a layer of conductive porous material, e.g. flows through the porous structure and is thereby exposed to the high surface area mesh of the electrode.

In addition, the high surface area cathode has a so-called "flow-by"
This type of electrode consists of a packed bed of individual conductive particles, and the electrolyte flows through the packed bed normally perpendicular to the current while undergoing electrolysis, thereby increasing the conductivity of the high surface area packed bed. exposure to sexual particles.

The electrode may consist of a material with a low overpotential, preferably a high overpotential, for the Cl ₂ →Cl ⁻ reaction, preferably graphite. As is well known to those skilled in the electrochemical industry, the electrochemical reaction Cl ₂ /Cl ^-
The overpotential of an electrode relative to the electrochemical reaction refers to the relationship of the potential applied to the electrode compared to the equilibrium potential required to maintain the electrochemical reaction at a reasonable rate. If the electrode potential is close to the equilibrium potential, the electrode is considered to have a low overpotential, but if a very base potential is required to obtain a practical reduction rate, the electrode can be considered to have a high overpotential. It is considered to have an overpotential.

Materials constituting low overpotential electrodes such as those described above are known and are used in the form of so-called dimensionally stable electrodes. The electrodes are usually made of a substrate which is titanium, zirconium, tantalum or hafnium and a noble metal such as platinum, a noble metal alloy such as a platinum-iridium alloy, a metal oxide such as ruthenium oxide or titanium dioxide, a platinic acid such as lithium or calcium platinate, or It consists of a conductive coating of a mixture of two or more of the above-mentioned substances. Any of the materials described above can be used to provide the material of construction for the low overpotential cathode.

The electrolytic cell 10 in which the electrolytic production of chlorine dioxide is carried out according to the method of the invention can have any convenient construction. Usually, the electrolytic cell has an anode compartment (anolyte compartment) using an ion exchange membrane (ordinary cation exchange membrane 18).
20 and a catholyte compartment (catholyte compartment) 14, thereby promoting the movement of hydrogen ions and preventing the interaction of the gases usually oxygen and chlorine dioxide occurring at the anode and electrical reduction at the cathode. The anode of the electrolytic cell can be constructed of any desired electrically conductive material, such as graphite or metal.

Next, FIG. 2 will be described. FIG. 2 shows the integration of an electrolytic cell (chlorine dioxide generator) 10, a chlorate electrolytic cell 30, and a chlorite generating electrolytic cell 32, as described in detail below. In this embodiment, the sodium chloride byproduct in conduit 28 is routed to a chlorate electrolyzer 30 where the sodium chloride is electrolyzed to form sodium chlorate, which is then sent via conduit to an electrolyzer (chlorine dioxide generator) 10. Recycle. Byproduct hydrogen from electrolysis in chlorate electrolyzer 30 is exhausted via conduit 34.

The chlorine dioxide formed in the chlorine dioxide generator 10 is conveyed via conduit 26 to the cathode compartment 36 of a chlorite electrolyzer 32 . Sodium chloride is delivered by conduit 38 to the anode compartment 40 of the chlorite cell 32. While the anodic electrolysis produces chlorine, the sodium ions migrate through the cation exchange membrane 42 that separates the cathode compartment 36 and the anode compartment 40. In the cathode compartment 36, the chlorine dioxide delivered by conduit 26 forms chlorite ions such that sodium chlorite is exited from the cathode compartment 36 by conduit 44. The sodium chloride depleted solution discharged from the anode compartment 40 is recycled by conduit 46. The chlorine formed in the anode compartment 40 is conveyed by conduit 48 to the cathode compartment 14 of the chlorine dioxide generator 10. Unlike in Figure 1, all hydrogen and chloride ions for chlorine dioxide cathode production are
is produced in the cathode compartment 14 from the movement of chlorine and hydrogen ions charged by. This result is achieved by increasing the current applied to the electrolytic cell (chlorine dioxide generator) 10 from 1 to 2 Fradays per mole of chlorine dioxide produced.

Since all the hydrogen ions and/or chlorine ions required by the chlorine dioxide generator 10 are supplied within the device and there is no need to process the amount of chlorine generated, the chlorine dioxide generator 10 and the electrolytic cell for chlorite generation are used. The entire operation during 32 hours theoretically requires no additional input of hydrogen and/or chloride ions. Furthermore, a chlorate electrolyzer will be integrated to create a device that inputs only sodium chloride and electricity to produce only sodium chlorite, hydrogen, and oxygen.

The modification of the operation shown in FIG.
0 to the chlorate electrolyzer 30. Additionally, hypochlorous acid in the hydrogen exhaust gas stream in conduit 34 can be condensed and recycled to cathode compartment 14 of chlorine dioxide generator 10.

FIG. 3 illustrates the integration of the caustic soda-chlorine electrolyzer 50 and the chlorine dioxide generator 10. In this case, the by-product sodium chloride as well as unreacted sodium chlorate is sent by conduit 28 to the anode compartment 52 of the caustic soda-chlorine electrolyzer 50. The electrolyte is conveyed by conduit 54 to a cathode compartment 56 separated from anode compartment 52 by a cation exchange membrane 58 in a caustic soda-chlorine electrolyzer 50 .

The chlorine produced in the anode compartment 52 is conveyed by conduit 60 to the cathode compartment 14 of the chlorine dioxide generator 10 as an aqueous solution of unreacted sodium chlorate.
Sodium hydroxide is recovered from cathode compartment 56 as product in conduit 62 and byproduct hydrogen gas is exhausted by conduit 64.

The overall operation in the embodiment of FIG. 3 is described by the following equation: H ₂ O + NaClO ₃ +2e →ClO ₂ +NaOH + 1/2O ₂ +1/2H _2Power to produce chlorine dioxide, sodium hydroxide, oxygen and hydrogen. It is necessary to add sodium chlorate. Sodium hydroxide is useful elsewhere in the plant and by-product gases can be vented.

The experimental electrolyzer was assembled as described in FIG. The electrolytic cell was filled with graphite felt [Union Carbide Corporation] in the cathode compartment.
It was a conventional MP electrolyzer from Electrocell AB that was modified to accommodate a three-dimensional electrode formed by inserting a 3D electrode (manufactured by Carbide Corporation). The electrolytic cell is divided into an anode compartment and a cathode compartment by a cation exchange membrane (NAFION 120). The area of the cation exchange membrane is 100 cm ² (1 dm ² ), and the area of the cathode is estimated to be about 100 to 1000 times the area of the cation exchange membrane. An oxygen-evolving dimensionally stable electrode was used as the anode.

The raw material charged to the cathode compartment is sodium chlorate.
8.626 mol, sodium chloride 2.356 mol and
HCl is 1.536 mol. 6N H ₂ SO ₄ was used as the anolyte. An electrode potential of approximately -0.7 volts vs. SCE was applied to the cathode at 70° C. for 4 hours at a current density of 1.97 KA/m ² . The effluent from the cathode compartment contained 7.659 moles NaClO ₃ and 3.548 moles NaCl. Analysis of exhaust gases revealed that
It contained 0.626 mole _ClO2 and 0.068 mole _Cl2 .

Chlorine dioxide has a purity of 90.2% and was produced with a chemical efficiency of 82.2%.

To summarize the disclosure herein, the present invention provides a novel electrolytic process for producing substantially pure portable chlorine dioxide. Modifications may be made within the scope of the invention.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an electrolytic cell for producing chlorine dioxide according to one embodiment of the invention, and FIG. 2 is a schematic diagram of an electrolytic cell for producing chlorine dioxide and chlorine dioxide according to another embodiment of the invention. FIG. 3 is a schematic diagram showing the integration of an electrolytic cell for producing sodium acid
The figure is a schematic diagram showing the integration of an electrolytic cell for producing chlorine dioxide and an electrolytic cell for producing sodium hydroxide and chlorine. In the figure, 10... electrolytic cell (chlorine dioxide generator), 12... conduit, 14...
Cathode compartment, 16... Conduit, 18... Cation exchange membrane, 20... Anode compartment, 22... Conduit, 23...
Conduit, 24... Conduit, 26... Conduit, 28... Conduit, 30... Chlorate electrolytic cell, 32... Electrolytic cell for chlorite generation, 34... Conduit, 36... Cathode compartment, 38 ... Conduit, 40 ... Anode section, 42 ...
Cation exchange membrane, 44... conduit, 46... conduit,
48... Conduit, 50... Caustic soda-chlorine electrolyzer, 52... Anode compartment, 56... Cathode compartment, 58
... Cation exchange membrane, 60 ... Conduit, 62 ... Conduit, 64 ... Conduit.

Claims (1)

[Claims] 1. An electrolytic method for producing chlorine dioxide, comprising: (a) an electrolytic cell comprising a cathode compartment comprising a three-dimensional high surface area cathode and an anode compartment separated from the cathode compartment by a cation exchange membrane; (b) charging chlorate ions into the cathode compartment and supplying hydrogen ions and chloride ions into the cathode compartment; (c) discharging chlorate ions with hydrogen ions and chloride ions in the cathode compartment; reducing to form chlorine dioxide and simultaneously applying an electric current to the cathode compartment to reduce the chlorine produced with chlorine dioxide to chloride ions; (d) exhausting the produced chlorine dioxide from the cathode compartment; and (e) An electrolytic process for producing chlorine dioxide, characterized in that hydrogen ions are electrolytically formed in an anode compartment and the hydrogen ions are transferred from the anode compartment through an ion exchange membrane to the cathode compartment. 2. The hydrogen ions and chloride ions in the cathode compartment are partially supplied by the chlorine ions produced by electrolytic reduction of the hydrogen ions and co-produced chlorine transferred from the anode compartment to the cathode compartment and are charged into the cathode compartment. Claim 1 partially supplied by hydrochloric acid
Method described. 3 Hydrogen ions and chlorine ions in the cathode compartment are supplied by hydrogen ions moving from the anode compartment to the cathode compartment, and chlorine ions are supplied by chlorine ions produced by electrolytic reduction of co-produced chlorine. 2. The method of claim 1, wherein the method is partially supplied by chlorine ions produced by electrolytic reduction of chlorine which is partially supplied and charged to the cathode compartment from an external source. 4. An electrolytic operation for producing sodium chlorite comprising electrolytically reducing exhausted chlorine dioxide to chlorite ions and electrolytically forming chlorine and sending it to the cathode compartment to provide the chlorine charge for the cathode compartment. 3. An electrolytic operation for the production of chlorate comprising, optionally, electrolyzing by-product sodium chloride from the cathode compartment to provide a sodium chlorate charge to the cathode compartment. Method described. 5. The method of claim 1, comprising an integrated caustic soda chlorine electrolyzer comprising electrolyzing by-product sodium chloride from the cathode compartment to provide a chlorine charge to the cathode compartment. 6. the catholyte of the cathode compartment has a chlorate ion concentration of at least 5 molar, preferably about 5 to 6.5 molar;
Preferably the chlorate ion/chloride ion ratio is about 2/1.
~4/1 and the total acid normality is at least about
6. A method according to any one of claims 1 to 5, characterized in that it is 0.01 normal, preferably at least about 0.05 normal. 7. Claims 1 to 6 wherein the catholyte in the cathode compartment has a temperature of at least about 50°C, preferably about 60-70°C.
The method according to any one of the above. 8. The electrode potential applied to the cathode is more noble than -1 volt relative to a saturated calomel electrode when measured with a current supply device to the cathode, and less noble than the open circuit potential under typical conditions, and is preferably 8. A method according to any one of claims 1 to 7, wherein: is less base than -0.2 volts. 9 consisting of a stack of conductive mesh-like material passing through the cathode percolating the chlorate solution normally parallel to the current, or a solid conductor percolating the chlorate solution normally perpendicular to the current. comprising a packed bed of reactive particles, and preferably a bed of reactive particles.
9. The method as claimed in claim 1, wherein the method is composed of a material having a high overpotential for _Cl2 - ^Cl- , in particular graphite or other carbonaceous material. 10 The chlorate ions charged to the cathode compartment are approximately 3
10. A method according to claim 1, wherein the method is an aqueous sodium chlorate solution having a concentration of .about.7 molar.