JPS6218630B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for purifying brine used in the electrolysis of alkali metal chloride salts. More specifically, the present invention relates to a method for purifying brine, which is characterized by reducing the total amount of calcium and magnesium ions in the brine used for ion-exchange membrane electrolysis of alkali metal chlorides. It is known that in the usual electrolysis industry that generally uses alkali metal chloride salts as raw materials, impurities contained therein, such as calcium, magnesium, etc., are removed and purified before being used as an electrolytic solution. As a conventionally known industrial method for purifying aqueous alkali metal chloride solutions, calcium is precipitated and removed as calcium carbonate and magnesium as magnesium hydroxide by treatment with addition of caustic alkali or slaked lime and alkali carbonate. This method is widely used. In this method, impurities in alkali metal chloride salt water are precipitated and separated as solid precipitates, but there are limits to the separation by sedimentation, and the contamination of solids into the clarified liquid cannot be completely avoided. The clarified liquid after sedimentation separation is further filtered and then supplied to the electrolytic cell as an electrolytic solution. However, in addition to the sedimentation separation described above, the purified alkali metal chloride brine obtained by filtration still contains a total amount of calcium and magnesium of 2.
Contained at a concentration of ~20ppm. Furthermore, in the conventional electrolysis of alkali metal chloride salt water using the mercury method and the diaphragm method, the concentrations of calcium, magnesium and solids contained within these concentration ranges were not particularly problematic. The reason for this is that in the mercury method, calcium ions are discharged at the mercury electrode and become amalgam, and this amalgam is decomposed and released. It did not cause any problems. On the other hand, in the asbestos diaphragm method, the diaphragm is porous, and the anolyte (alkali metal chloride salt water) is pumped through the micropores to the cathode side, so calcium hydroxide and magnesium are not deposited inside the diaphragm. It was difficult, so it wasn't really a problem. On the other hand, in the electrolysis of alkali metal chloride salt water using an ion exchange membrane, when using salt water purified by conventional purification methods, the electrolytic voltage increases, the current efficiency decreases significantly, and the ion exchange membrane may be damaged. A phenomenon is observed. The electrolysis method of alkali metal chloride salt water using an ion exchange membrane mentioned here means that the space between the anode and the cathode is divided into an anode chamber and a cathode chamber, and the aqueous solution of alkali metal chloride is placed in the anode chamber and the cathode is This is an electrolytic method in which the chamber is filled with an aqueous caustic solution and a voltage is applied between the two electrodes to obtain chlorine gas from the anode chamber and hydrogen gas and the generated caustic alkaline solution from the cathode chamber. In this electrolysis method of alkali metal chloride brine using an ion exchange membrane, alkali metal ions in the alkali metal chloride brine electrically permeate the cation exchange membrane and are transported to the cathode chamber; Cations other than alkali metal ions contained in salt water also permeate through the cation exchange membrane. an anode chamber filled with alkali metal chloride brine;
The inside of the cation exchange membrane that divides the cathode chamber filled with aqueous caustic alkaline solution is in equilibrium with both external solutions and contains a fairly high concentration of hydroxide ions, so alkali metal ions, etc. can easily pass through. However, ion species that form salts with low solubility with hydroxide ions, such as calcium ions and/or magnesium ions, form sparingly soluble salts with hydroxide ions inside the ion exchange membrane during permeation, causing fine particles to form within the membrane. This causes a rise in electrolytic voltage and a decrease in current efficiency over time. In particular, the phenomenon of decrease in current efficiency is remarkable, and if operation continues as it is, the internal structure of the ion exchange membrane may be destroyed and the membrane may be damaged. As a result of various studies and examinations to solve these problems, the present inventors have determined that the total amount of calcium and/or magnesium in the alkali metal chloride brine supplied to the cation exchange membrane method electrolytic cell should be 0.08 ppm or less. It has been discovered that by maintaining the cation exchange membrane at a temperature of 100 mL, no precipitation of calcium hydroxide and/or magnesium hydroxide is observed inside the cation exchange membrane. Based on this knowledge, the total amount of calcium and/or magnesium in the alkali metal chloride brine
As a result of various studies and examinations on methods to maintain the concentration below 0.08 ppm, we found that even if the alkali metal chloride brine filtered with a conventional sand filter is processed by passing it through a tower filled with chelate resin, calcium and /or total amount of magnesium
It was found that it was difficult to maintain the concentration below 0.08 ppm. That is, when the calcium and/or magnesium contained in the alkali metal chloride brine is in a dissolved state, it is adsorbed on the chelate resin, but when it is present in the form of suspended solids, it is not adsorbed on the chelate resin and remains as it is. It turned out that this was due to passing through the chelate resin tower. It was also found that most of the components of the solid suspension were calcium and/or magnesium. That is, in order to maintain the total amount of calcium and/or magnesium in the alkali metal chloride brine to 0.08 ppm or less, a filter that is more precise than the conventional sand filter is used to remove solid suspended matter in the alkali metal chloride brine. By supplying the chelate resin tower with a concentration of 3 ppm or less, calcium and/or
It has also been found that the total amount of magnesium can be maintained at 0.08 ppm or less. The reason for this is not yet clear, but it is thought to be as follows. Calcium and/or magnesium present in the alkali metal chloride salt solution is dissolved as each ion to an amount close to saturation concentration, and the remaining calcium and/or magnesium is in the form of carbonate, hydroxide, etc. It exists as a fine solid substance in
When such an alkali metal chloride salt solution is supplied to a chelate resin tower, the calcium ions and magnesium ions in the solution are adsorbed by the chelate resin, and the concentration of calcium ions and magnesium ions in the solution decreases. Calcium and magnesium are eluted from the solid substance containing calcium and magnesium until the concentration approaches the saturation concentration of the solution and become ions, which are also adsorbed by the chelate resin. It is thought that by continuing such a process up to the outlet of the chelate resin tower, calcium and magnesium in the solution are adsorbed and removed. However, if there is a large amount of solid matter in the solution, not all of it will be eluted before the exit of the chelate resin tower, and the solid matter will remain, resulting in a high concentration of calcium and magnesium ions in the solution. This means that if the operating conditions of the chelate resin tower (e.g., temperature, space velocity (hr ^-1 ) of the feed solution, tower height, etc.) are controlled, the outlet side will be The total amount of calcium ions and magnesium ions in
Although it would be possible to reduce the solid content to 0.08 ppm or less, the present inventors believe that under normal operating conditions of the chelate resin tower, the solids concentration at the inlet of the chelate resin tower should be reduced to 3 ppm or less. This is what we discovered. On the other hand, a leaf filter or a cartridge filter using a filter aid such as activated carbon or α-cellulose is suitable as a precision filter to reduce the concentration of solids in the brine supplied to the chelate resin tower to 3 ppm or less. . Furthermore, even a sand filter can be used as long as the filtration rate is reduced to prevent solid matter from leaking. Example In the cation exchange membrane electrolysis of salt, the salt water at the outlet of the clarification tank is filtered using a conventional sand filter or a leaf filter pre-coated with activated carbon.
The liquid was purified by passing it through a chelating resin at SV=40 hr ^-1 . When this purified salt water was supplied to a cation exchange membrane electrolytic cell and electrolyzed for 90 days, the results shown in the following table regarding intramembrane precipitation were obtained. The operating conditions of the cation exchange membrane electrolytic cell were: current density: 20 A/dm ² , electrolytic cell temperature: 90° C., caustic soda concentration: 40%, and salt water decomposition rate: 50%. 【table】

Claims (1)

[Claims] 1. In a method of purifying salt water for electrolysis by passing it through a chelate ion exchange resin layer, the total amount of calcium ions and magnesium ions in the salt water at the inlet of the chelate ion exchange resin layer is 0.4 to 0.4.
7ppm and the suspended solids concentration is 3ppm or less, and the total amount of calcium ions and magnesium ions in the brine at the outlet of the chelate ion exchange resin layer is 0.08ppm or less. Ion-exchange membrane method A method for purifying salt water for electrolysis.