1. Field of the Invention
The present invention relates to improvements in a process for producing alkali hydroxides using a conventional double electrode diaphragm-type electrolysis tank with asbestos diaphragms for producing alkali hydroxides, the improvements comprising that the asbestos diaphragm is replaced by a cationic ion exchange membrane and a lower electrolysis cell voltage is applied to obtain alkali hydroxides of high purity and high concentration.
2. Description of the Prior Art
The mercury method for producing alkali hydroxides through the electrolysis of alkali chlorides has been replaced by the diaphragm method using an asbestos diaphragm from the view-point for preventing pollution. Today the diaphragm method is most popular and preferred for the production of alkali hydroxides.
One of the typical diaphragm types of electrolysis tanks using an asbestos diaphragm for the production of alkali hydroxides is a vertical double electrode diaphragm-type electrolysis tank described below.
A number of electrolysis tanks are set in a series with separating walls therebetween and finger-anodes having an approximately U-shaped cross-section extend from one side of the separating wall. On the other side of the separating wall are placed cathode backscreens having some distance from the separating wall. Finger-cathodes having almost the same shape as the finger-anodes but in the reversed direction extend from the cathode backscreen. Electricity supply bars penetrating the separating walls hold the fingers of the anode and cathode and supply electricity. In an electrolysis tank separated by the separating walls, a finger-anode and a finger-cathode, each extending from different separating walls are so arranged that the finger-anode projects into a small gap of the finger-cathodes, and vice versa, their arrangement having a close distance between. Further an asbestos diaphragm is composed beforehand by forming layers of asbestos slurry on a net of a finger-cathode.
For supplying electricity to the electrolysis tank, the anode at one end of a series of electrolysis tanks is connected to the plus terminal of the electricity supplier and the cathode at the other end is connected to the minus terminal.
In addition, each electrolysis tank is divided into anode and cathode compartments by an asbestos diaphragm. More particularly, the peripheral part of the cathode backscreen is connected to the flange on the side of the cathode compartment so that the inside of the finger-cathode and the space between the separating wall and the cathode backscreen form the cathode compartment. Using the asbestos diaphragm which is formed by depositing asbestos slurry on a bag-like finger-cathode, the electrolysis of alkali chloride is conducted with a level of the alkali chloride solution supplied to the anode compartment kept at a higher level, to obtain alkali hydroxide at the cathode compartment.
In a diaphragm-type electrolysis tank using an asbestos diaphragm, however, the solution of alkali hydroxide obtained at the cathode compartment is very dilute and contains a large amount of alkali chloride due to the permeability of the asbestos diaphragm toward the liquid. For instance, in an electrolysis of sodium chloride, a sodium hydroxide solution obtained is 10-13% by weight and contains 15-18% by weight of sodium chloride. For industrial applications, however, the solution chould be further concentrated and sodium chloride deposited during the process should be separated. In this case, the produce sodium hydroxide solution about 50% by weight stil contains about 1% by weight of sodium chloride, which brings about a difficulty in the direct application to such fields as the rayon industry.
In the meantime, an electrolysis method using an ion exchange membrane has recently been developed as a diaphragm-type electrolysis method which permits alkali hydroxide to be obtained in a high concentration without contamination of the sodium chloride.
Both methods belong to the diaphragm method in a general sense, therefore it may be possible to apply a cation exchange membrane as a diaphragm in place of an asbestos diaphragm as in the previous diaphragm-type electrolysis tanks. Thus, it is expected to obtain a high purity and a high concentration of alkali hydroxides without a large amount of expenditure on the apparatus, giving large merit to industrial manufacturing.
The present inventors have investigated the effects of a cation exchange membrane installed in place of asbestos in the double electrode diaphragm-type electrolysis tank mentioned in detail above in the production of alkali chloride by the electrolysis of alkali chloride.
In the investigation, however, several problems were noticed in simply exchanging the diaphragms, as discussed below.
At first each finger-cathode was covered by a bag-like membrane of a cation exchange material and sealed to assemble the electrolysis tank. Electrolysis was carried out maintaining the level of the sodium chloride solution supplied to the anode compartment at a higher level than the level of the sodium hydroxide solution in the cathode compartment and pumping out the hydrogen gas evolved at the upper portion of the cathode compartment. The result was that a very high voltage was needed across the electrolysis tank and the product of sodium hydroxide contained more iron than that in the case of the asbestos diaphragm and even more unfavorably, the sodium chloride content of the sodium hydroxide was increased as time went on with accompanying degrading current efficiency in producing sodium hydroxide in the cathode compartment.
When the present inventors used a higher pressure for the hydrogen gas in the cathode compartment than that of the chlorine gas in the anode compartment, they encountered serious difficulties of a high voltage to be applied across the electrolysis tank and a low current efficiency to obtain sodium hydroxide in the cathode compartment.