JPS5943405B2 - Dechlorination method for returned salt water - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for dechlorinating return brine in a salt electrolysis process.

The effect of the present invention can be particularly exhibited by the ion exchange membrane method salt electrolysis. Furthermore, the greatest feature of the present invention is that by using a steam ejector to create a reduced pressure and using the steam discharged from the ejector as a heat source, the energy used for dechlorination is used extremely efficiently, thereby reducing energy consumption. The goal is to reduce

Conventionally, the fresh brine discharged from the electrolyzer in the aqueous electrolysis process of alkali metal chlorides (hereinafter also simply referred to as return brine) contains dissolved chlorine regardless of the electrolysis method, and this is directly used for resaturation and Circulating the brine into the refining process was considered undesirable due to corrosion problems and undesirable effects on the refining process.For this reason, in the case of mercury method brine electrolysis, returning brine is not recommended due to economic considerations. The salt water is usually dechlorinated by depressurization, air blowing, etc., and the chlorine content is usually reduced to about 10 to 50 PP [11].In addition, in the case of filterable diaphragm salt electrolysis, the salt water is usually left as is on the cathode chamber side. However, when the return salt water from the anode chamber is recycled, dechlorination is carried out by almost the same means for the same reasons as in the mercury method.The present invention provides a more efficient method for dechlorination. .

That is, the present invention introduces the returned brine to a vacuum distillation device, reduces the pressure using a steam ezekume, and uses the discharged steam of the ezekme as a heat source for heating the distillation device to dechlorinate the water, and then dechlorinates the water containing this chlorine. This is a method for dechlorinating returned salt water that makes extremely effective use of steam energy, which is characterized by introducing steam into a condenser, separating it into gas and liquid, and recovering chlorine gas. Although the present invention can of course be applied to any method such as the mercury method or the diaphragm method, it is particularly effective when applied to the ion exchange membrane method salt electrolysis process. In other words, in the ion-exchange membrane salt electrolysis process, unlike ordinary filtering diaphragms, the solution itself does not substantially pass through the membrane and migrate to the cathode chamber, but as sodium ions move, hydrated water As water escapes into the cathode chamber, the amount of returning salt water decreases accordingly, and a considerable amount of water must be replenished in terms of water balance. Therefore, in the ion-exchange membrane method salt electrolysis process, the returned brine is led to a vacuum distillation device, the pressure is reduced using a steam ejector, and the steam discharged from the ejector is used as a heat source for heating the distillation device to dechlorinate it, and then this chlorine-containing steam is removed. is introduced into the condenser, gas and liquid are separated to recover chlorine gas, and the condensate containing dissolved chlorine is led to the vacuum distillation equipment and returned to replenish the brine with water and eliminate the loss of dissolved chlorine. can be prevented. The present invention will be explained below with reference to the drawings.

FIG. 1 is an example of a flow sheet for implementing the present invention. Return brine discharged from the electrolytic cell is introduced into the vacuum distillation apparatus 1vc through line 20.

This vacuum distillation apparatus 1 may be of any type, for example, it may be a simple distillation apparatus or a rectification column type such as a packed column or plate column, but a column type is generally preferable. . Normally, the return salt water discharged from the electrolytic cell is 500 to 8
00P? It is introduced into a vacuum distillation apparatus with a sensible heat of about 50 to 95°C, where it is distilled under reduced pressure, and chlorine gas and a small amount of water vapor are distilled out from the upper part through line 21. In order to reduce the load on the pressure reduction device (steam sector), the distilled gas is usually extracted as suction gas (hereinafter also referred to as bleed gas) from the steam sector 2 after condensing and separating most of the entrained water vapor in the condenser 6. Ru. On the other hand, the steam industry sector 2 preferably uses high-pressure steam, such as 5K1dG or higher, or more preferably 10kg/DG, as the driving steam from the line 22.
Normally driven steam/intake gas =
Chlorine and water vapor are extracted from the vacuum distillation device at a ratio of about 20/1 to 2/1 (by weight), and the discharged steam (a mixture of 1 driving steam and distilled gas) is used as return salt water to be supplied to the vacuum distillation device. The return brine is heated by exchanging heat with the heat exchanger 3Itc, and chlorine is removed by distillation from the return brine.

The return salt water dechlorinated in this way is taken out from line 24, and, depending on the case, is further subjected to a second dechlorination treatment 5, such as contacting with an adsorbent such as activated carbon, or adding a reducing agent. This is carried out as necessary, and the salt is supplied to the electrolytic cell through the salt dissolution and purification steps.

In addition, to further explain the distillate gas in detail, the distillate gas discharged from the line 21 is normally passed through the condenser 6 as described above.
This condenses the entrained water vapor, increasing the degree of pressure reduction and reducing the burden on the engineering sector.

Of course, if Ezek Shioichi has sufficient capacity, this condenser 6 is not necessary. The bleed gas that has passed through the Ezeku Shioichi is then mixed with driving steam, and serves as a heat source for heating the return brine in the vacuum distillation apparatus through an indirect heat exchanger as discharged steam from the industrial sector. The waste fluid from the industrial sector after heat exchange generates drainage depending on the amount of heat exchanged, but in some cases, it may be further drained in order to improve separation from non-condensable chlorine gas. The separator 4 may have a function to help condense water vapor, or a separate condenser may be provided. Of course, if the steam is sufficiently condensed by heat exchange with the returned salt water, or if the recovered chlorine can contain a large amount of water, such steam condensation equipment is not required. Most of the condensed water after the heat exchange is derived from the driving steam and contains a saturated amount of chlorine at that temperature. Therefore, if this is disposed of as is, it will not only result in loss of chlorine, but also cause environmental pollution in some cases. Therefore, it must be used for sterilization and other effective purposes as much as possible. However, in processes that require water replenishment due to the water balance of the salt water system, such as the ion exchange membrane method or the filterable diaphragm method that produces return salt water, the condensed water is introduced into the vacuum distillation apparatus through the 25 VC line. can do. Therefore, in the case of a vacuum distillation column system, the condensate may be refluxed to the top of the column. In this case, a portion of the bottom liquid is withdrawn to the next step, and the remainder is returned to the distillation column as a bottom circulation stream via line 23 through a reboiler heated by the steam discharged from the industrial sector. In addition, if the heat amount of the steam discharged from the industrial sector is insufficient to heat the returning salt water, an additional line 2 is added.
It is also possible to compensate for the shortage by introducing steam at a relatively low pressure.

As described above, the present invention makes it possible to reduce the chlorine concentration in returned brine to the order of several ppm much more easily than conventional methods by dechlorinating returned brine using a vacuum distillation method. This is extremely meaningful for the salt water purification process and other electrolytic processes as a whole.

Example According to the flow sheet shown in Figure 1, the temperature was about 80°C and 70% chlorine was added.
13 dAr of return brine from the ion-exchange membrane salt electrolysis process containing 0 ppm is fed to a vacuum distillation column filled with a Raschich ring with approximately 3 equivalent stages for dechlorination.

At the top of the tower, the driving steam of 10Kg/CTlG is 200k9/H.
A water-cooled condenser is installed in the middle of the industrial sector to be used.
After heat exchange, approximately 9 kg/hr of chlorine is recovered from the industrial sector discharge gas along with uncondensed water vapor. All of the condensate was refluxed to the top of the column. The bottom liquid is approximately 13.2 m3 Ar and contains only 1 f:. of chlorine. It was about 2 ppm.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing one embodiment of the present invention. 1 is a vacuum distillation column, 2 is a pressure reducer (engineering sector), 3 is an indirect heat exchanger, 4 is a gas-liquid separator, and 6 is a condenser.

Claims (1)

[Scope of Claims] 1. Dechlorination is carried out by introducing the returned brine into a vacuum distillation device using a steam ejector, and using the steam discharged from the ejector as a heat source for heating the distillation device. A method for dechlorinating returned salt water. 2. The method according to claim 1, which uses return brine from the ion-exchange membrane salt electrolysis process, wherein the condensate after separating chlorine gas is circulated to a vacuum distillation device. The method described in paragraph 1. 3. Claim 1, wherein the vacuum distillation apparatus is a distillation column, and the dechlorinated brine is taken out from the bottom of the column.
The method described in section.