JPH01127689A - Method for concentrating aqueous solution of alkali hydroxide - Google Patents

Abstract

PURPOSE:To produce aq. NaOH soln. having high concn. in a cathode chamber with a small amt. of electric energy, by supplying aq. NaOH soln. having low concn. to the anode and cathode chamber of the diaphragm cell having the anode made of hydrogen occlusion alloy and the cathode made of Ni, by applying an electric current and by circulating the hydrogen generated on the cathode to the anode chamber. CONSTITUTION:In the electrolytic cell separated into the anodic electrolyte chamber 4 and cathodic electrolyte chamber 5 with a cation-exchange member 3 consisting of sodium perfluorosulfonate, the anode 1 made of hydrogen occlusion alloy is arranged as an anode 1 on the one side of the anodic electrolytic chamber 4 and a hydrogen gas chamber 9 is formed at the rear. As the cathode, sintered alloy of carbonyl Ni powder is used and the upper part of the cathode chamber 5 is connected to the hydrogen gas chamber 9 on the rear of the anode with a pipe having a circulating pump 8. An aq. NaOH having about 30% concn. is supplied to both the chambers 4 and 5, and DC is applied to both the poles. The hydrogen generated on the cathode is sent to the hydrogen gas chamber 9 and adsorbed in the anode, and Na ion infiltrates to the cathode chamber 5 through the diaphragm 3. The aq. NaOH soln. having high concn. of about 50% is taken out from an exhaust port 11 and aq. NaOH soln. having low concn. of about 20% is taken out from an exhaust port 10.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for concentrating an aqueous alkali hydroxide solution, and more particularly to a method for concentrating a relatively low concentration aqueous alkali hydroxide solution produced by electrolysis of alkali chloride. .

Conventional technology An electrolytic cell for alkali chloride generally consists of a cathode, an anode, an ion exchange layer or asbestos diaphragm, a cathode electrolyte chamber, and an anolyte electrolyte chamber. When sodium chloride, for example, is used as the alkali chloride, the following Such an electrode reaction occurs.

Cathode: 2H20+2e →)b + 208-
(1) Positive [1: 2CI-→C12+2e
(2) Total reaction: 2H20+ 2NaC1-+
t12+ 2NaO1-1)Clz (3) The aqueous solution of sodium hydroxide produced at the cathode by this reaction needs to have a temperature of 11 degrees, which is about 50%, when considered as a product, but in reality it has a temperature of 30 to 35%. The reality is that it is suppressed. This is because, for example, in a sodium chloride electrolytic cell using the ion exchange membrane method, when the concentration of putrium hydroxide in the cathode electrolyte chamber is increased, hydroxide ions (OH-) leak to the anode electrolyte chamber side.
Not only do disadvantageous phenomena occur such as the dissolution of ruthenium oxide, which is commonly used as a catalyst for ion exchange, and the generation of oxygen other than the p-chlorine generation reaction at the anode, but also disadvantageous phenomena such as a decrease in N flow rate and a drop in ion exchange 11(7) IR. This is because it also happens.

Conventionally, the method of concentrating an aqueous solution of sodium hydroxide has been to evaporate water using heat, but when converting the thermal energy in this case into a unit of electric power, it is approximately 750 wh per 1 ton of sodium hydroxide. , becomes quite large.

On the other hand, recently, a method using a hydrogen-air fuel cell using a cation exchange membrane as a diaphragm has been proposed as a new method for concentrating sodium hydroxide. (E.

American Ele
ctrOCheliCalsociety, Spri
ng Meeting, May 1O-15 (19
This method uses a 30% aqueous sodium hydroxide solution produced by electrolysis of sodium chloride to the positive electrolyte chamber formed between the cation exchange membrane and the positive electrode (air electrode) of the fuel cell, and the cation exchange membrane and negative electrode ( When the fuel cell is operated by supplying hydrogen generated from the cathode with one solution of sodium chloride to the negative electrode electrolyte chamber formed between the negative electrode (hydrogen electrode) and the hydrogen electrode (hydrogen electrode), This is based on the principle that the m degree of the sodium hydroxide aqueous solution in the positive electrode electrolyte chamber increases (50%). Note that in this method, the concentration of the sodium hydroxide aqueous solution in the negative electrode electrolyte chamber decreases (about 21%), but this low 81 degree sodium hydroxide aqueous solution is returned to the cathode electrolysis chamber 1 of the sodium chloride electrolytic cell. . Moreover, the electric power obtained by operating this fuel cell is used for electrolyzing sodium chloride.

The electricity consumption rate during the usual electrolysis of sodium chloride is 2200 kWh per ton of sodium hydroxide.

750 kw for concentration by evaporation, total 2950 kw
h, but in the case of a system that uses this fuel cell in combination, it is significantly reduced to 1950 kwh, and can be said to be a layered proposal.

Problems to be Solved by the Invention The method of condensing an aqueous alkali hydroxide solution using thermal energy has problems because the thermal energy is large.

Furthermore, the above-mentioned fuel cell combination method has a problem in that hydrogen produced by electrolysis of alkali chloride is consumed; in other words, hydrogen is merely converted into electrical energy. In addition, air contains 0.03% pi of carbon dioxide, but when this carbon dioxide mixes into the water WJ liquid of sodium hydroxide, which is the product, alkali carbonate is produced, which reduces the purity of the product. There is a problem that the amount of energy decreases. Regarding this point, it would be possible to supply air from which carbon dioxide has been removed beforehand to the fuel cell, but there is a drawback that the cost for this is quite large.

Further, a complicated control system is required to operate the fuel cell, and furthermore, a new power supply control 15AW1 is required to use the electric power generated by the fuel cell for electrolysis of alkali chloride. Therefore, fuel cells and power control 2Ilv
The large initial investment in equipment is also an obstacle from an industrial perspective.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides an anode mainly made of a hydrogen storage alloy, a cathode as a hydrogen generating electrode, a cation exchange membrane, an anode electrolyte chamber, and a cathode electrolyte. An electrochemical device equipped with a chamber is prepared, and an alkali hydroxide aqueous solution at a relatively low temperature of 11 degrees to be concentrated is supplied to both the anode electrolyte chamber and the cathode electrolyte chamber. By passing a DC current through R, an electrolytic oxidation reaction of hydrogen is caused at the anode, a hydrogen generation reaction is caused at the cathode, and the hydrogen generated from the cathode is recycled to the anode and stored. II of the aqueous alkali hydroxide solution in the electrolyte chamber? It is characterized by the fact that it is made to be made into a three-layered rice cake.

A working anode mainly made of a hydrogen storage alloy that causes electrolytic oxidation of hydrogen is arranged, a hydrogen generating electrode is arranged as a cathode, and a cation exchange membrane is arranged between the picture electrodes. 30 to 35 in the anode electrolyte chamber formed between the anode and the catholyte electrolyte chamber formed between the cation exchange membrane and the cathode.
When an aqueous solution of sodium hydroxide or potassium hydroxide is supplied and a direct current is passed between the anode and cathode from an external power source, the following electrode reaction occurs.

Anode: H2+ 20H--+ 2820+28
(4) Cathode: 2H20+28-) 82 + 2
Day 0 - (5) That is, hydrogen in the hydrogen storage alloy is electrolytically oxidized at the anode, and gaseous hydrogen is generated at the cathode. Therefore,
As for the entire reaction, only hydrogen migrates from the anode side to the cathode side, and apparently nothing happens, and the theoretical electrolysis voltage is OV. However, in reality, there is an IR drop due to the overvoltage of each of the 74 electrodes and the resistance of the cation exchange membrane and solution part, so 0.
A voltage of 3-0.4V is required.

On the other hand, cation exchange membranes allow alkali metal ions to pass through (
(from the anode side to the cathode side), but hydroxide ions do not pass through, so
Due to the reaction of formula (5), alkali hydroxide is rapidly produced in the catholyte chamber, and its concentration increases. In other words, alii of alkali hydroxide occurs. On the other hand, in the anode electrolyte chamber, water is generated by the reaction of equation (71), so
The concentration of alkali hydroxide decreases. This aqueous alkali hydroxide solution that has become low in concentration may be returned to the cathode electrolyte chamber of the alkaline chloride electrolytic cell.

On the other hand, gaseous hydrogen generated from the cathode is once taken out of the electrochemical device, recirculated to the anode, and stored in the hydrogen storage alloy of the anode, making the hydrogen system a kind of closed system. Furthermore, in order to recirculate hydrogen to the hydrogen storage alloy in this way, it is necessary to use the anode as a kind of gas diffusion electrode and to form a gas chamber on the back side of the electrode. To this end, the method for manufacturing the hydrogen plate J1'f 4 poles is to use a hydrogen-absorbing alloy powder alone or a mixed powder with a metal powder that does not have hydrogen-absorbing properties such as nickel powder, and a binder with old water properties such as fluororesin. It is best to add and pressure mold. Rare earth elements and nickel are hydrogen storage alloys.

Multi-element alloys with cobalt, aluminum, and other metals are preferred, but are not necessarily limited to these.

As the cation exchange membrane, one having a perfluorocarbon skeleton and having a sulfonic acid group, a carbon M group, or both of these ion exchange groups is suitable.

Embodiment The figure shows a cross-sectional structure of an electrochemical sodium hydroxide concentrator according to an embodiment of the present invention.

Electrochemical sodium hydroxide aqueous solution concentrator is LaN
l□, 5cOyuΔ1゜1. 100 parts of hydrogen-absorbing alloy powder having a composition of , carbonyl nickel powder in a hydrogen stream at 850 °C
It is composed of a cathode 2 sintered at 19 °C, a cation exchange membrane 3 made of sodium perfluorocarbon sulfonate, and an anode electrolyte chamber 4 and a cathode electrolyte chamber 5 separated by the cation exchange 11A3. has been done.

The 30% sodium hydroxide aqueous solution to be concentrated is supplied from the sodium hydroxide aqueous solution supply port 6 to the anode electrolyte chamber 4.
and the catholyte chamber 5. Next, when a direct current is passed between the anode 1 and the cathode 2 at a current density of 20 A/dnt, the voltage becomes 0.4V. Further, during this energization, hydrogen generated from the cathode 2 is taken out from the hydrogen derivation lower and supplied to a hydrogen gas chamber 9 formed on the back surface of the anode 1 by a circulation pump 8.

Thus, the concentration of the sodium hydroxide aqueous solution drawn out from the anolyte electrolyte outlet 10 becomes 20%, and the concentration of the sodium hydroxide aqueous solution drawn out from the catholyte electrolyte outlet 11 becomes 20%.
la has become 50%.

Effects of the Invention When concentrating a 30% sodium hydroxide aqueous solution to 50% by the method of the present invention, the electric power consumption per ton of sodium hydroxide is 107 kwh, and the electric power consumption when concentrating with thermal energy is 750 kwh. Actually 14%
becomes.

As described above, the present invention provides a method for concentrating an aqueous alkali hydroxide solution with extremely little energy, and is an industrial method for concentrating an aqueous alkali hydroxide solution. li is big.

[Brief explanation of the drawing]

The figure is a schematic cross-sectional view of an electrochemical sodium hydroxide concentration device according to an embodiment of the present invention. 1...Anode 2...Pin scratch 3
...Cation exchange membrane

Claims (1)

[Claims] In an electrochemical device comprising an anode mainly made of a hydrogen storage alloy, a cathode as a hydrogen generating electrode, a cation exchange membrane, an anode electrolyte chamber, and a cathode electrolyte chamber, By supplying a low-concentration alkaline hydroxide aqueous solution to the anode and passing a direct current between the anode and cathode, an electrolytic oxidation reaction of hydrogen occurs at the anode, a hydrogen generation reaction occurs at the cathode, and hydrogen is further removed from the cathode. A method for concentrating an aqueous alkali hydroxide solution, which comprises increasing the concentration of the aqueous alkali hydroxide solution in a cathode electrolyte chamber to a relatively high concentration while recirculating and occluding generated hydrogen to an anode.