Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
2001-03-08
2002-12-03
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S516000
Reexamination Certificate
active
06488833
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electrolytic process of an alkali chloride by the ion exchange membrane method using a gas diffusion cathode, particularly to a process for feeding an oxygen-containing gas and an aqueous alkali hydroxide solution or water in the electrolytic process of an alkali chloride by the ion exchange membrane method.
BACKGROUND ART
It is known to obtain a caustic alkali by electrolyzing an aqueous alkali chloride solution by the ion exchange membrane method using a gas diffusion cathode. In this process, roughly speaking, an anode chamber having an anode and containing an aqueous alkali chloride solution and a cathode chamber having a cathode and containing water or an aqueous caustic alkali solution are partitioned by an ion exchange membrane, usually a cationic exchange membrane. When electrolysis is effected by sending an electric current between both electrodes, a gas diffusion cathode which is made of, at a portion of the cathode, a porous substance and has, at its back, a gas chamber to be fed with an oxygen-containing gas is used, whereby the caustic alkali is formed in the cathode chamber. No hydrogen gas is generated at the cathode so that this process is accompanied with such an advantage as a marked reduction of an electrolytic voltage.
Examples of the patent literature which discloses such an electrolytic process include JP-A-54-97600 (the term “JP-A” as used herein means an “unexamined published Japanese patent application), JP-A-56-44784, JP-A-56-130482, JP-A-57-152479, JP-A-59-133386, JP-A-61-266591, JP-B-58-441156 (the term “JP-B” as used herein means an “examined published Japanese patent publication”), JP-B-58-49639, JP-B-60-9595 and JP-B-61-20634.
In addition, a number of proposals have been made on a production process of a gas diffusion cathode and improvement in the performance of it, but they hardly include a process for properly feeding an oxygen-containing gas.
In the conventionally known electrolysis of an alkali chloride by using the ion exchange membrane method without a gas diffusion cathode, an anode chamber having an anode is partitioned from a cathode chamber having a cathode by an ion exchange membrane. An aqueous alkali chloride solution is fed to the anode chamber for generating a chlorine gas at the anode, while a caustic alkali or water is fed to the cathode chamber for generating the caustic alkali and a hydrogen gas at the cathode.
Usually in the electrolysis of an alkali chloride in accordance with the ion exchange membrane method by using a gas diffusion cathode, on the other hand, an anode chamber having an anode is separated from a cathode chamber having a gas diffusion cathode by an ion exchange membrane. An aqueous alkali chloride solution is fed to the anode chamber for generating a chlorine gas at the anode. A caustic alkali or water is fed to the cathode chamber and an oxygen-containing gas is fed to the gas chamber of the gas diffusion cathode, whereby the caustic alkali is generated at the cathode.
The comparison of these two electrolytic processes indicates that they are utterly same in the reaction at an anode, but differ largely in the reaction at a cathode. The electrolysis by the ion exchange membrane method using a gas diffusion cathode is characterized in that no hydrogen gas is generated.
Various types of a gas diffusion cathode to be used for the latter process have been proposed. Examples include a microporous gas-permeable sheet obtained by hot pressing a mixture of carbon powders and polytetrafluoroethylene powders. It may hold thereon a catalyst, for example, a noble metal such as platinum or silver, or an alloy thereof or may be reinforced with a metal mesh for raising its strength or conductivity. This gas diffusion cathode is usually equipped with a gas chamber on the back side of the electrode surface. An oxygen-containing gas is fed to this gas chamber to cause the reaction which will be described later, whereby generation of a hydrogen gas on the electrode surface can be prevented.
In the electrolysis of an alkali chloride by using a gas diffusion cathode, appropriate supply of an oxygen-containing gas is very important. A gas containing oxygen in an amount exceeding the stoichiometric amount required for the reaction must be supplied. When an oxygen amount is insufficient, hydrogen is inevitably produced at the gas diffusion cathode. There is a danger of this hydrogen reacting with oxygen and causing explosion. In addition, the performance of the gas diffusion cathode drastically lowers at this time. So an oxygen-containing gas is usually fed in excess. Too much supply however leads to waste of the raw material. Although an appropriate excess ratio of oxygen to be fed varies depending on the properties of the gas diffusion cathode, a preferred excess ratio of oxygen relative to the stoichiometric amount is considered to be larger than a predetermined value. The extent of the excess ratio varies, depending on various conditions so it cannot be determined in a wholesale manner. The higher the oxygen concentration of the oxygen-containing gas, the better the performance of the gas diffusion cathode. In this case, the excess ratio is said to be set at not so high.
Air, which is most easily available and exists abundantly, as the oxygen-containing gas is inexpensive as a raw material gas, but use thereof deteriorates the oxygen reducing performance of the gas diffusion electrode owing to its low oxygen concentration. Use of pure oxygen on the other hand leads to a cost increase, though a gas diffusion electrode exhibits a sufficient performance. A PSA apparatus serves to separate air by the adsorption method. It does not permit the formation of pure oxygen, but makes it possible to prepare a gas containing oxygen in an amount of at least 90% at a low cost. This apparatus can be used effectively in this process. Even if the oxygen-containing gas from this PSA apparatus is used, the running cost of the gas diffusion cathode depends largely on the excess ratio of the oxygen-containing gas to be fed newly.
As an ordinarily employed electrolytic cell of an alkali chloride having a gas diffusion cathode, that having a filter press structure is usually used. It has a structure wherein a plurality of units each of which is formed of an anode chamber having an anode, an ion exchange membrane, a cathode chamber and a gas diffusion cathode (equipped with a gas chamber) in the order of mention have been stacked one after another. It is the common practice, upon supply of each of gas chambers with an oxygen-containing gas, to adopt one flow-rate control system for one electrolytic cell or a plurality of electrolytic cells and feed each gas chamber with a uniformly dispersed gas through a convenient system such as orifice, because adjustment of the flow rate of the oxygen-containing gas to be fed to each of the gas chambers leads to a cost increase. Flow rates differ to some extent with gas chambers. In order to prevent shortage in an amount of the oxygen-containing gas in each gas chamber, an excess ratio must be set greater, causing a waste of the raw material.
The electrolytic cell of an alkali chloride equipped with a gas diffusion cathode is usually a triple chamber type. Since the electrolytic cell of a triple chamber type has three chambers, that is, a cathode chamber, a catholyte chamber and a gas chamber partitioned by an ion exchange membrane and a liquid-impermeable gas diffusion cathode, it is called “triple chamber type”.
A double chamber type using a liquid-permeable gas diffusion electrode is also under investigation. In this double chamber type, one unit is formed of an anode chamber having an anode, an ion exchange membrane, a gas diffusion cathode and a gas chamber also serving as a cathode chamber in the order of mention. In this double chamber type, the electrolytic cell has two chambers divided by the ion exchange membrane, that is, anode chamber and the gas chamber also serving as a cathode chamber.
Since the gas diffusion cathode of this electrolytic cell is li
Saiki Koji
Sakata Akihiro
Watanabe Takeshi
Phasge Arun S,.
Toagosei Co. Ltd.
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