FR2585039A1 - Electrolyser for a process with an ion exchange membrane - Google Patents

Abstract

Vertical electrolyser of the single-pole type for a process with an ion exchange membrane 2 which has gas-liquid separation interfaces in its electrolyte compartments 16, characterised in that the overflow orifice 14 defining the catholyte level is situated at a position above the end of the cathode 5.

Description

ELECTROLYSER FOR ION-EXCHANGING MEMBRANE PROCESS
The present invention relates to an electrolyser for the ion exchange membrane process, in particular for the electrolysis of sodium chloride, having improved corrosion resistance.

 The mercury process for the electrolysis of sodium chloride has been replaced by the diaphragm process using asbestos, due to the environmental pollution linked to mercury. The latter method, however, has the drawback of producing less pure caustic soda. As a result, the transformation is currently in progress to pass from the diaphragm process to process 9 the ion exchange membrane, using an ion exchange membrane and capable of producing a caustic soda of high purity with more energy consumption. low.

The concentration of caustic soda produced in the electrolyser for the ion exchange membrane process is approximately 30-35% (value approximately three times higher than in the diaphragm process), and the risk of corrosion from the iron cathode, which is out of the question in the diaphragm process, has been discussed in the past as a possible problem in this new process. But it is now clear that when an iron cathode is charged with a negative voltage and is at a potential capable of releasing hydrogen, the iron remains in the so-called "immunity" region, and corrosion does not occur. not even when immersed in a high concentration caustic soda solution. This phenomenon is reported for example in Kurose and Ohta, "Corrosion Behavior of Iron in Concentrated Caustic Soda
Solutions "(Proceedings of the 6th soda Industry Technical
Conference, p. 73-76, 1983).

 The case is different, however, for an activated cathode coated with a substance that lowers cell voltage, such as nickel. A cathode of this type has a lower hydrogen overvoltage, which makes it more noble in potential than an ordinary iron cathode. As a result, corrosion of the iron caused, for example, by pitting in the coating, is likely to progress in the activated cathode. The problem of iron corrosion is far from negligible, in particular when using an activated cathode in an electrolyser having a gas-liquid separation interface in its cathode compartment. In addition, the iron dissolved in the catholyte is deposited on the surface of the activated cathode, raising the cell voltage in a relatively short time.

 The present invention is explained below with reference to the accompanying drawings.

Figure 1 is a partially sectional view of an ion exchange membrane electrolyser having a gas-liquid separation region in its cathode compartment
Figure 2 is a plan view thereof;
Figure 3 is a sectional view along line AA of Figure 2 (conventional electrolyser);
FIG. 4 is a sectional view along the line AA in FIG. 2 (the electrolyser of the present invention) and
FIG. 5 represents graphs in which the cell voltage is plotted as a function of the elapsed time (in days) for the electrolysers.

 Figure 1 is a partially sectional view of a sodium chloride electrolyser for the ion exchange membrane process transformed i from a vertical electrolyser of the mono-pole type for the asbestos diaphragm process (such 1 DS type electrolyser from Diamond Shamrock and H type electrolyser from Hooker).

The remodeling of the electrolyser (MBC; registered trademark of
Chlorine Engineers Corp., Ltd.) is described in French patents including Nos. 8,116,084 and 8,219,683. A series of anodes 4, each surrounded by a bag of ion exchange membrane 2, are on the base plate 1 of the electrolyser. Metallic fabric cathodes (called solid tubes) 5 are mounted in the cathode sheath 3 in opposition to the anode i through the ion exchange membrane. A metal wire cathode (called end edge wire) 15 is fixed to a plate parallel to the solid tubes 5 on the internal wall of the cathode sheath. A vertical lateral metallic fabric 6 is placed on the plane perpendicular to the solid tubes and to the half-tubes.

Above the cathode sheath 3 is a partition plate 8 having an opening corresponding to the opening on the upper part of the bag of ion exchange membrane 2. An upper cover 7 of the anode compartment is above each compartment to cover the openings of the bags of ion exchange membrane. A distributor 9 has a small pipe leading to the various anode compartments.

Brine is supplied through the brine supply orifice 12. The chlorine gas separated from the liquid inside the distributor is discharged through the chlorine gas outlet orifice 10, and the catholyte is withdrawn through the anolyte discharge port 13.

 In the cathode sheath 3, the cathode compartment is separated by the bags 2 of ion exchange membrane. A dilute solution of caustic soda or water is sent through the catholyte supply port (not shown). The concentrated caustic soda solution formed is drawn off in the form of an overflow by the overflow opening 14 of the catholyte provided in the cathode sheath 3. Hydrogen gas is evacuated through the outlet outlet of the hydrogen 11.

 In this type of electrolyser, the gas-liquid separation region is established in the orifice of the cathode compartment; consequently, the gas-liquid interface is the site of a formation of foam under the effect of the hydrogen gas released by the cathode, and the gas phase contains numerous splashes. As a result, the gas-liquid interface formed on the surface of the cathode is always fluctuating, and the part of the cathode exposed to the gas phase is constantly wetted with splashes by the solution of concentrated caustic soda.

 There is no corrosion of the cathode when it is immersed in a concentrated caustic soda solution as long as it remains in the region of immunity. However, splashes fixed on the surface of the iron exposed to the gas phase allow natural corrosion to progress, and the concentration cell formed by the difference in concentration at the gas-liquid interface on the cathode can cause corrosion in this region. .

 Such iron corrosion can be avoided if the gas phase is excluded from the cathode compartment, but the establishment of the gas-liquid separation region in the cathode compartment makes the cell structure very complex and is not a good way to solve the problems.

 The present invention provides a relatively simple means for avoiding cathodic corrosion, for lowering the iron concentration in the caustic soda formed and for maintaining optimal activated cathode performance over long periods.

 In a conventional electrolyser (shown in FIG. 3, a sectional view along line AA in FIG. 2), the upper part of the wire mesh of the terminal edge 15 and the solid tube 5 (wire mesh cathode) are gas phase of the gas-liquid separation region in the cathode compartment 16.

On the contrary, in the electrolyser of the present invention (shown in FIG. 4, a sectional view of the same part as above), the upper part of the metallic fabric of the terminal edge 15 and the solid tube 5 are completely submerged below the catholyte level. When an electrolyser for the existing asbestos diaphragm process is redesigned into an electrolyzer for the ion exchange membrane & process, it is advantageous to cut the upper part of the cathode due difficulties in installing the opening of the catholyte overflow above the cathode. +
In the electrolyser of the present invention, in which the cathode is completely immersed in the catholyte, the iron of the cathode, if it is brought into contact with the catholyte through a puncture, is always at a hydrogen release potential and corrosion is minimal. As a result, the electroplating of iron on the activated cathode surface is greatly reduced, thereby maintaining mechanical strength and electrochemical performance over extended periods of time.

 The present invention not only applies to the new electrolyser manufactured for the ion exchange membrane process, but it is also used to transform a vertical electrolyser of the single-pole type for the asbestos diaphragm process, such as Diamond Shamrock type DS electrolyser and Hooker's type H electrolyzer, into an electrolyser for the ion exchange membrane process.

 The effects produced by the present invention will emerge more clearly on reading the following example.

(Example)
Electrolysis of an aqueous sodium chloride solution is carried out in commercial electrolysers using a laminate of a perfluorocarboxylic acid membrane and a perfluorosulfonic acid membrane as the cation exchange membrane and the coated iron. of a nickel-type coating as a cathode, approximately 90 ° C., at a current density of 2
KA / m2, and producing a caustic soda solution i 30 & 35%. The results are summarized in FIG. 5, in which the cell voltages are plotted as a function of the elapsed time (in turns). In the cell in which the upper orifice of the cathode is above the gas-liquid interface and exposed to the gas phase (curve B), the cell voltage continues to increase from the start of the implementation, gradually approaching the level at which an ordinary iron cathode was used (curve A). In the other cell, in which the anode was completely immersed in the catholyte, on the other hand, the increase in cell voltage was very slight, as shown by curve C.

Claims (3)

1. Vertical electrolyser, of the single-pole type for an ion-exchange membrane process (2) having gas-liquid separation interfaces in its electrolytic compartments (16), characterized in that the overflow orifice (14) defining the catholyte level is located at a position above the end of the cathode (5). 2. Electrolyser for an ion exchange membrane process according to claim 1, characterized in that the cathode (5) is an activated cathode coated with a substance which reduces the hydrogen overvoltage. 3. Electrolyser for an ion exchange membrane process according to claim 1 or 2, characterized in that said electrolyser is obtained by modification from an electrolyser for asbestos diaphragm process.