JPH0474880A - Electrolytic cell by ion exchange membrane method - Google Patents

Abstract

PURPOSE:To uniformize the current distribution in an ion exchange membrane by mounting a net-like body formed by knitting metallic wires on a support consisting of an expanded metal, etc., as a cathode, thereby decreasing the aperture of the cathode. CONSTITUTION:A unit electrolytic cell 1 is separated to an anode chamber and a cathode chamber by a partition plate 3 mounted to an electrolytic cell frame body 2. The anode chamber side of the partition wall is constituted of Ti or an alloy essentially consisting of Ti contg. Pd, Ru, etc. and the cathode chamber side of the partition wall is constituted of Ni, stainless steel. The partition wall 3 is constituted of the joined body of both. Cathode ribs 4 consisting of Ni, stainless steel, etc., are provided in the cathode chamber and the electrode support 5 made of the expanded metal formed from Ni, stainless steel, etc., is mounted to the cathode ribs 4. The wire-shaped body 6 formed by knitting the metallic wires is mounted by welding on the surface of the electrode support 5. Anode ribs 7 are provided on the anode chamber side and an anode 8 formed by coating a base body of the expanded metal consisting of Ti (alloy) with an anode catalyst material of a ruthenium oxide-titanium oxide system is mounted to the anode ribs 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an ion-exchange membrane electrolytic cell, and more particularly to an ion-exchange membrane electrolytic cell used for electrolysis of aqueous solutions of alkali metal chlorides.

[Prior Art] Conventionally, a method for producing alkali metal hydroxide and chlorine by electrolysis of an aqueous solution of alkali metal chloride involves decomposing an amalgam of alkali metals produced by electrolysis using mercury as a cathode. The mainstream was the mercury method, in which hydrogen and high-purity alkali metal hydroxides were obtained through a final process, but due to pollution problems caused by mercury, the production method was changed to the diaphragm method, which uses an asbestos diaphragm. However, in the diaphragm method, it is theoretically unavoidable that the aqueous alkali chloride solution supplied to the anode chamber is introduced into the cathode chamber through the diaphragm, and the resulting aqueous solution of alkali metal hydroxide of about 10% must be evaporated and concentrated. Although alkali metal chlorides are removed by evaporation and concentration, the contamination of alkali metal chlorides cannot be avoided, and although the energy required for electrolysis is less compared to the mercury method, the energy required for evaporation is large. , the overall energy consumption is large.

For this reason, the purity of the alkali metal hydroxide obtained is high, and since the concentration of the alkali metal hydroxide is over 30%, a cation exchange membrane is used, which requires less energy for concentration and lower overall energy consumption. Manufacturing methods are progressing toward an ion-exchange membrane method.

In the alkali chloride electrolysis method using the ion exchange membrane method, various measures have been taken to lower the electrolysis voltage. For example, various improvements have been made to the electrode catalysts formed in the electrodes used and the shape of the electrodes.Also, in electrolysis methods, the pressure in the cathode chamber is made higher than that in the anode chamber, and the cation exchange membrane is used as the anode. It has been adopted to lower the electrolytic voltage by bringing them into close contact.

[Problems to be Solved by the Invention] The method of bringing an ion exchange membrane into contact with the anode makes it possible to reduce the ohmic loss caused by the anolyte. The cause lies in the cation exchange membrane itself, and this ohmic loss appears as a voltage of about 300 to 400 mV under normal operating conditions. Therefore, it is of course important to improve the cation exchange membrane itself, but it is also important to equalize the current density within the cation exchange membrane and reduce the substantial ohmic loss in the cation exchange membrane. It is being done.

From this point of view, it is recommended to use a smoothed expanded metal for the anode (Japanese Patent Publication No. 1-46596) and to use a perforated plate instead of expanded metal for the electrode plate (Japanese Patent Publication No. 1-46596). 1-16915) has been proposed.

On the other hand, regarding the cathode, the cathode overvoltage is reduced by coating a cathode active catalyst on a substrate such as stainless steel or nickel that itself acts as a cathode, so-called an active cathode. Although the actual overvoltage has been obtained to be about 100 mV, which is about 300 mV lower than that of conventional nickel, little attention has been paid to the structure of the cathode.

In addition, electrolysis was performed by providing a gap of about 2 mm between the cathode and the cation exchange membrane, rather than bringing the ion exchange membrane into contact with the cathode. This is due to the high conductivity of the catholyte, such as sodium hydroxide and potassium hydroxide, and the fact that the bubbles of the water gas generated at the cathode are extremely finely dispersed in the catholyte, causing problems such as bubble removal. This is probably because it did not become a major problem.

However, in order to reduce the electrolytic voltage contributed on the cathode side, it is necessary to equalize the current density in the cation exchange membrane, lower the actual current density in the ion exchange membrane, and lower the ohmic loss. It's coming. Therefore, the so-called "zero gap" method, in which the cathode is essentially brought into contact with the ion exchange membrane, is used.
Or, ``An electrolysis method called membrane gap J, in which an ion exchange membrane is sandwiched between an anode and a cathode, is being used, but this involves treatments such as forming a porous hydrophilic layer on the surface of the ion exchange membrane. This method has been made possible by this, and is expected to be expanded in the future.

However, there has been little study on creating a special structure for the cathode, and only the same structure as the anode has been considered, but in order to further reduce the electrolytic voltage, [Means for solving the problem] Therefore, the present inventors developed a so-called zero gap method in which the cathode is brought into close contact with a cation exchange membrane for electrolysis. In the type electrolytic cell,
We have conducted intensive studies on the structure of the cathode that can equalize the current distribution within the cation exchange membrane, reduce the actual current density, and reduce the ohmic loss within the cation exchange membrane.
The present invention has now been completed.

That is, in the present invention, a cathode made of a net made of metal wires with a diameter of 2 mm or less and whose surface area is equal to or larger than the projected area is mounted on an expanded metal support, thereby generating cations. This is an ion exchange membrane electrolytic cell that makes it possible to reduce the electrolysis voltage in an electrolytic cell in which the exchange membrane and the cathode are in close contact.

In the case of an electrolysis method using a cation exchange membrane, water migrates together with alkali ions that move from the anode chamber side to the cathode chamber side as a result of electrolysis. Cation Exchange Membrane In the case of a so-called zero-gap electrolytic cell in which the king of the cathodes is used in close contact with each other, as a result of migration water moving from the anode side to the cathode side, halogen ions, which serve as the electrolyte, are formed on the anode surface. Since the anode is in direct contact with the ion-exchange membrane, water can pass through the ion-exchange membrane as long as the width of the area where water movement is not obstructed is approximately 2 mm or more. Therefore, a so-called dry zone where no liquid is supplied does not occur within the ion exchange membrane, and side reactions due to insufficient supply of ions hardly occur.

However, on the cathode side, it is necessary to ensure that this large amount of water containing alkali ions supplied quickly passes through the ion exchange membrane. If the passage is limited by the cathode, the excess water around the cathode will prevent the proper concentration of alkali hydroxide from being maintained, leading to an increase in the electrical resistance of the cation exchange membrane itself and the cathode. Non-uniformity in concentration also occurs in the liquid, and at the same time, hydrogen generated by electrolysis does not diffuse to the opposite side of the cation exchange membrane, leading to an increase in ohmic loss due to bubbles.

In order to prevent these, it is desirable to reduce the opening of the cathode and the area in contact with the cation exchange membrane. Expanded metal, which is commonly used in ion-exchange farming electrolyzers, cannot meet these conditions, but a mesh called woven mesh, which is a mesh made of thin metal wires, cannot be obtained. We have discovered that these things can be achieved by using the human body.

In the metal wire network electrode used as the cathode of the ion exchange farming electrolytic cell of the present invention, the metal wires constituting the network should be thinner, similar to ordinary expanded metal, and the thicker the metal wires, the closer the cations will be. As the area covering the exchange membrane becomes larger, the release of hydrogen gas bubbles to the opposite side of the cathode from the cation exchange membrane becomes worse, and the fan shape becomes thicker, the unevenness of the electrode surface becomes substantially larger. The portion away from the cation exchange membrane becomes larger.

Therefore, it is preferable that the wire diameter is 2 mm or less; if the wire diameter is larger than that, the problem arises that the sliding voltage increases. At the same time, it has been revealed that when the wire diameter is 2 mm or less, the entire surface of the wire in the mesh body effectively acts as an electrode. Also, it is better to line up the lines closely so that the apparent total surface area is large.

However, considering the backward escape of bubbles, it is preferable that the total surface area is 1 to 5 times, and more preferably 2 to 3 times, the projection plane, excluding the overlapping areas between lines. It's double.

In order to use such a network in substantially close contact with an ion exchange membrane, the physical strength of the network is not sufficient, so it is recommended to provide it on a porous material such as expanded metal with high strength. is necessary.

The porous material used to support the net-like cathode made of woven metal wires also serves as a power supply to the cathode, so it must be made with electrical conductivity in mind, and from the viewpoint of physical strength, it must be 1 mm or 1 mm thick.
．． 5mm nickel expanded metal is used. The opening of the expanded metal is not particularly limited, but preferably has a major axis of 10 mm to 20 mm, a minor axis of 5 mm to 10 mm, a strand of 1 mm to 2 mm, and an aperture ratio of 60% to 85%.

Expanded metal made of stainless steel can also be used, but in this case it is necessary to nickel plate the surface because it may corrode during electrolysis. In other words, the surface area of the woven metal wire network of the cathode is larger than the apparent projection surface, and since the activation treatment is performed by forming a coating with a cathode catalyst material, a low voltage of about 100 mV is required for hydrogen generation. Electrolysis can be expected. As a result, since the potential is lower than that of a commonly used cathode, corrosion can occur in stainless steel and iron even when they act as a cathode when energized. It is from.

The cathode, which is a net-like body made of braided metal wires, is used by being attached to the expanded metal of the base by a method such as welding, but it is preferable to form a coating of a catalytic material on the cathode before attachment.

In addition, various catalyst materials can be used to form the cathode, but in the electrolytic cell of the present invention, the cathode and the cation exchange membrane are used in close contact with each other, so the surface of the cathode is It is desirable to form a coating with a smooth surface using a platinum group metal or its oxide as an electrocatalyst material that will not damage the cation exchange membrane even if it comes into contact with the membrane.

[Function] The present invention provides an electrode support such as an expanded metal with an apparent surface area of 1 to 5 times the projected area and a diameter of 2 mm.
An ion exchange membrane method electrolytic cell equipped with a cathode made of a mesh made of metal wires as shown below, the cathode and cation exchange membrane
It is possible to operate at a low electrolytic voltage with the three anodes in close contact.

[Example code] EXAMPLES Hereinafter, the present invention will be further explained by showing examples of the present invention.

To explain the present invention with reference to the drawings, Fig. 1 shows a partially cutaway perspective view of a unit electrolytic cell viewed from the cathode side when the ion exchange membrane method electrolytic cell of the present invention is a bipolar electrolytic cell. However, the unit electrolytic cell 1 is separated into an anode chamber and a cathode chamber by a partition wall 3 attached to an electrolytic cell frame 2. The anode chamber side of the partition wall is made of titanium or an alloy containing titanium such as palladium or ruthenium, and the cathode chamber side of the partition wall is made of nickel or stainless steel.The partition wall is made of a combination of both.

The cathode chamber is provided with a cathode rib 4 made of nickel, stainless steel, etc., and an electrode support 5 made of expanded metal made of nickel, stainless steel, etc. is attached to the cathode rib. A net-like body 6 made of woven metal wires is attached to the surface by welding.

In addition, an anode rib 7 is provided on the anode chamber side, and the anode rib is an anode formed by forming a coating of a ruthenium oxide-titanium oxide anode catalyst material on an expanded metal base made of titanium or its alloy. 8 is installed.

A gasket is provided on the flange surface 9 of the unit electrolytic cell, and cation exchange membranes are stacked alternately to assemble an ion exchange membrane method electrolytic cell.

EXAMPLE The surface of a net made of nickel wires with a diameter of 1 mm arranged vertically and horizontally at 4 mm intervals was plated using a nickel plating bath in which ruthenium oxide was suspended.

The cathode overvoltage of this electrode at 30 A/dm2 in a 30% sodium hydroxide aqueous solution at a temperature of 60°C is 110
It was mV.

The surface area of this network was approximately 1.9 relative to the projected area. This was spot welded to an electrode support made of nickel expanded metal with a thickness of 1.0 mm, openings of 15 mm in major axis, 7 mm in minor axis, and strands of 1.2 mm, and installed in a filter press type electrolytic cell. .

As a cation exchange membrane, Nafion 902 manufactured by DuPont
The surface of 09 was made hydrophilic by plasma treatment, and the anode had a thickness of 0.5 mm and an opening with a major diameter of 6 mm.
, a titanium expanded metal base with a short diameter of 4 mm and a strand of 0.8 mm was coated with a composite oxide based electrode catalyst of ruthenium oxide and titanium oxide, and the cation exchange membrane was coated with an anode and an anode. The cathode was completely adhered, the anolyte was a saturated saline solution of 200 g/l, and the catholyte was a 32% sodium hydroxide aqueous solution at a current density of 40 A/dm2 and a temperature of 90°C. When electrolysis was performed, the sliding voltage was 3.15V.

Comparative example A nickel plate with a thickness of 1.2 mm was cut into a width of 2 mm as a cathode.
The cathode was made of a smooth expanded metal with a major diameter of 14 mm and a minor diameter of 7 mm, and was subjected to the same nickel plating treatment in which ruthenium oxide was suspended as in the example to form an electrode catalyst coating. When the same anode as in Example was brought into close contact with a cation exchange membrane and electrolysis was carried out under the same conditions, the sliding voltage was 3.30V.

[Effects of the Invention] The ion exchange membrane method electrolytic cell of the present invention has a net-like body made of woven metal wires attached to a support such as an expanded metal as a cathode, and thereby the opening of the cathode can be made small. At the same time, since the area in which the cathode contacts the cation exchange membrane can be reduced, the current distribution in the ion exchange membrane can be made uniform, and operation at a low electrolysis voltage is possible.

[Brief explanation of the drawing]

FIG. 1 shows a partially cutaway perspective view of a unit electrolytic cell viewed from the cathode side when the ion-exchange membrane electrolytic cell of the present invention is applied to a bipolar electrolytic cell. 1... Unit electrolytic cell, 2... Electrolytic cell frame 3...
・Partition wall, 4... Cathode rib, 5... Electrode support base 6
... Netting of metal wires 7 ... Anode ribs, 8
...Anode, 9...Flange surface

Claims (2)

[Claims] (1) An ion exchange membrane method characterized in that a cathode made of a mesh of metal wires with an apparent surface area of 1 to 5 times the projected area and a diameter of 2 mm or less is mounted on a porous electrode support. electrolytic cell. (2) The ion exchange membrane method electrolytic cell according to claim 1, characterized in that a catalytically active coating is formed on the surface of the cathode.