KR800001113B1 - Process for concentration of caustic alkali solution produced by ion exchange membrane type electrolysis - Google Patents

Abstract

An alkali metal hydroxide soln. obtained from the anode compartment in the electrolysis of aq. alkali chloride soln. is conce. in a series of concn. units. Each of these units comprises a heat-exchange stage for the exchange between the aq. alkali hydroxide soln. of the cathode compartment and the anolyte of the anode compartment. Also, it compriese a vaporliq. sepn. step for evapn. of the hot hydroxide soln. which is passed successively with increasing concn. from the 1st to the next concn. unit to obtain a concd. alkali hydroxide soln. in the last unit. aq. alkali chloride soln. is recycled into the anode compartment.

Description

Concentration method of caustic alkali aqueous solution in ion exchange membrane method electrolysis

The accompanying drawings are schematic diagrams of devices used in one embodiment of the present invention.

The present invention relates to a method of concentrating a caustic alkali aqueous solution obtained by electrolyzing an aqueous alkali chloride solution into an ion exchange membrane by using electrolytic heat effectively without requiring a special heating means.

Conventionally, there are two kinds of methods that are practically used on a commercial scale as a method of producing caustic alkali. That is, it is a diaphragm method using the mercury, the electrolytic method, and the Asbestos diaphragm. By the way, the former has a possibility of mercury pollution, and the latter contains alkali chloride as an impurity, and the quality can be avoided, and each has its own difficulty. Therefore, as a third method, a so-called ion exchange membrane electrolysis method in which an anode cathode is placed in a state where the ion exchange membrane is sandwiched, a cathode chamber and a cathode chamber are formed, and an aqueous alkali chloride solution is used using an electrolytic cell is attempted to be put into practical use. The use of this method yields a relatively high concentration of caustic aqueous solution with high purity, but if a higher concentration, that is, an aqueous solution of caustic alkali having a concentration of 45 to 50% by weight NaOH, must be concentrated in any form. In the electric diaphragm method, the density | concentration of the caustic aqueous alkali solution obtained is low, about 10 to 13 weight%, and this is concentrated. In this concentration, steam was used as a heating source.

An object of the present invention requires no separate heating source by utilizing the sensible heat of a dilute aqueous alkali chloride solution (anode solution) obtained in the anode chamber as a heating source for concentrating the aqueous solution of caustic alkali obtained in the cathode chamber of the ion exchange method electrolytic cell. The present invention provides a method of concentrating a caustic alkali aqueous solution with little or no need thereof.

It is still another object of the present invention to provide a concentration method that can effectively utilize the sensible heat of the anolyte solution at such a high temperature of 80 to 120 ° C by heat-exchanging the caustic aqueous alkali solution and the anolyte solution in a specific process.

Another object of the present invention is to use an anolyte as a heating source to suppress the leakage current associated with the anolyte discharging to be extremely small, and to concentrate the aqueous solution of caustic alkali satisfactorily without adversely affecting the ion exchange membrane electrolysis. It is to provide a way to.

That is, since the anolyte solution has a relatively higher electrical resistance than the caustic alkaline aqueous solution, when the anolyte is taken out from the electrolytic cell, the amount of leakage of current accompanying such extraction is small, and the reduction in current efficiency due to current leakage is minimized. There is a number. Moreover, it is performed by the multi-effect concentrator which can prevent the electricity of the electrolytic cell material by such a current leakage to a minimum.

Therefore, the concentration of the caustic aqueous alkali solution obtained by the ion exchange membrane method can also be the same concentration method as the diaphragm method described above, but such a method is installed in a separate process because the steam generated by the boiler is used as a heating source, It is complicated and not economical.

Here, in the electrolysis of the ion exchange membrane method, the inventors of the present invention show that heat is generated by the electrical resistance of an electrolyte solution or an ion exchange membrane during electrolysis, and the electrolyte solution is heated to about 80 to 120 ° C., which is caustic obtained in such a cathode chamber. Note that the difference in the electrical resistance of the solution and the electrical resistance of the liquid held in the alkaline aqueous solution and the lean alkali chloride solution obtained in the anode chamber are higher than that of the caustic alkali aqueous solution, and the like as a heat source for concentrating the aqueous alkali solution. It was invented a method that enables the concentration of aqueous caustic aqueous solution without the need for a separate heating steam using heat retained in a dilute alkaline chloride aqueous solution.

As described above, the present invention concentrates a caustic alkali aqueous solution obtained in an ion exchange membrane method alkaline chloride aqueous solution electrolytic cell, and comprises a heat exchange process between the caustic alkali aqueous solution and an aqueous alkali chloride solution (anode solution) that retains electrolytic heat obtained from the anode chamber of the electrolytic cell. This is carried out in a step of providing a unit concentration step including a gas-liquid separation step by evaporation of this caustic aqueous alkali solution which has undergone this heat exchange in a series of plural stages.

The aqueous solution of caustic alkali is first supplied to a first unit concentration step, and the caustic alkali concentration is increased, and then sent to the subsequent unit concentration step in sequence, and taken out as a product in the final unit concentration step. It is fed to the unit concentration process of the final stage and sent to the unit concentration coaxial of the sequential stage.

The anolyte liquid taken out in the first stage concentration process is added to the raw material salt to bring the alkali chloride saturated or close to saturation, and then recycled to the anode chamber.

Thus, the objective can be achieved by concentrating the caustic alkali aqueous solution through heat exchange by flowing countercurrent with the anolyte solution and the caustic aqueous alkali solution over these multiple-stage unit concentration processes. That is, the boiling point of the caustic aqueous alkali solution is lowered as the concentration decreases, so that the caustic alkali aqueous solution having low concentration is supplied to the unit concentration process of the first stage having a relatively low temperature, and the caustic alkali aqueous solution having an elevated concentration is sequentially evaporated. By supplying to the series in a high unit concentration step, the caustic alkali aqueous solution in each unit concentration step can be satisfactorily evaporated even though the temperature of the anolyte solution as the heat medium is not too high. In addition, in this invention, it is necessary to make each unit concentration process into a reduced pressure state in the meaning of reducing the boiling point of caustic aqueous alkali solution. It is preferable that the decompression state be kept in a decompression state as much as possible with respect to the temperature of the anolyte, which is a heat medium, and in particular, it is particularly preferable in practice that it is 60 to 90 Hgab. desirable.

In addition, in order to better concentrate the caustic aqueous alkali solution in the unit concentration step, it is preferable to circulate a portion of the concentrated caustic alkaline aqueous solution obtained by gas-liquid separation in the gas-liquid separation step of the unit concentration step to the heat exchange step of the same step, It is preferable that the unit concentration process include such a circulation process.

In the present invention, when the unit concentration step is provided in multiple stages in series, good results can be obtained. Particularly, it is more preferable to provide three to five stages in series from an economic point of view.

The apparatus which comprises the unit concentration process of this invention contains an evaporation pot and a heat exchanger.

The evaporating pot may employ a flush evaporating pot.

Examples of the material include alkali-resistant metals such as Fe and Fe alloys (such as Fe-Cr alloys and Fe-Cr-Ni alloys), Ni, and Ni alloys (Ni-Cu alloys, Ni-Fe alloys, and Ni-Cr alloys). Etc.) can be employed.

In particular, when the concentration of the caustic aqueous alkali solution in contact with the evaporator is 40 to 50% by weight in NaOH concentration, it is preferable to employ Ni, Ni alloy, especially Ni as the material. In addition, even when the concentration of the caustic alkali solution in contact with the evaporator is lower than the above range, Ni, Ni alloy can be employed as the material, but economically, stainless steel such as Fe-Cr alloy and Fe-Cr-Ni alloy is employed. It is desirable to. Therefore, although the material of each evaporator may be made of Ni and Ni alloys in the unit concentration process of a several stage, it is economically preferable to use the material of the evaporator of the comparatively low concentration range mentioned above as stainless steel. As for the heat exchanger, it is preferable to employ a heat exchanger having a composite partition wall of titanium and an alkali resistant metal as a heat exchange surface. By supplying the anolyte solution to the titanium surface side of such a heat exchanger, supplying a caustic alkali aqueous solution to the alkali-resistant metal surface side, and heat-exchanging through an electrical heat exchange surface, corrosion of the heat exchange surface can be prevented and a long life can be obtained. have. As alkali-resistant metals, iron, iron alloys, nickel and nickel alloys as mentioned above are employed. Especially, when a caustic alkali solution having a caustic alkali concentration of about 40% by weight or more is supplied, nickel and nickel are mainly used as alkali-resistant metals. It is preferable to employ an alloy.

The titanium-alkali-resistant metal composite partition wall is preferably joined via an alloy layer of titanium and an alkali-resistant metal, and such bonding hardly causes peeling of the titanium and the alkali-resistant metal, resulting in poor thermal conductivity or strength. There is no fear of deterioration.

In the composite partition wall joined through such an alloy layer of titanium and an alkyl-resistant metal, for example, when nickel is used as the alkali-resistant metal, the thickness of the nickel layer is 1 µm or more, particularly 10 µm or more in terms of corrosion resistance. desirable. On the other hand, it is preferable that the thickness of a titanium layer is 10 micrometers or more, especially 100 micrometers or more.

Moreover, the thickness of the titanium-nickel alloy layer is preferably 1 to 10 mu, in particular 3 to 8 mu, in terms of mechanical strength.

In addition, when iron is used as an alkali-resistant metal, it is preferable that the thickness of an iron layer is 2 micrometers or more, especially 15 micrometers or more from a corrosion resistance point. On the other hand, it is preferable that the thickness of titanium is 10 micrometers or more, especially 100 micrometers or more. In addition, the thickness of the iron-titanium alloy layer is preferably 2 to 12 in terms of mechanical strength.

In order to form the titanium-alkali-resistant metal complex bulkhead as described above, the explosive press welding for joining two different metals using the explosive force of gunpowder, or the hot diffusion for high-temperature treatment of two different metals under reduced pressure Although it can also carry out by crimp welding, it is preferable to join and heat-treat titanium and an alkali-resistant metal from the point which is easy to form an alloy layer of a desired thickness, and the workability mentioned later. In such a case, the titanium and the alkali metal may be joined by mechanical contact, but it is preferable to coat the alkali metal on the titanium substrate or the titanium metal on the alkali metal substrate.

Next, the present invention will be clarified by describing preferred embodiments of the case where the titanium-nickel composite partition wall is formed.

First, the nickel substrate is coated with a thickness of 4 µm or more, preferably 5 µm or more, by means of electroplating, chemical plating, or vapor deposition on a titanium substrate, and preferably 450 ° C or more, preferably 600 ° C or more, and more preferably in a non-oxidizing atmosphere. When it heats more than 750 degreeC, a favorable result can be obtained.

Conversely, titanium may be coated on the nickel substrate in the same manner and heat-treated in a non-oxidizing atmosphere. In this case, since a uniform coating layer of titanium is difficult to obtain, the method of coating nickel on the titanium substrate as described above is desirable. The heat treatment is performed in a non-oxidizing atmosphere in order to prevent titanium and nickel from oxidizing and deterioration of the junction between titanium and nickel by this oxide. The non-oxidizing atmosphere can be achieved by setting it as a reduced pressure, an inert gas atmosphere, or a reducing atmosphere.

In addition, when forming a titanium-iron composite partition, it is preferable to heat-process at 600 degreeC or more.

The formation method described above is advantageous in terms of workability when the heat-transfer surface having a complicated mold is used as a titanium-nickel composite.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Hereinafter, the present invention will be described based on the accompanying drawings, in which the accompanying drawings are apparatus flow diagrams using an embodiment and an embodiment of the present invention, wherein the unit concentration step includes a heat exchange step, a gas-liquid separation step, and a circulation step. It is a case where a process is provided in three steps in series.

In the drawing, the caustic soda aqueous solution produced at the cathode chamber side 7 of the electrolytic cell is a caustic soda aqueous solution supply pump 9 and the suction pipe 17 of the No. 3 circulation pump 10 of the third stage unit concentration process. Supplied to. The caustic soda solution supplied to the third stage unit concentration process is circulated by a No. 3 circulation pump 10 together with the partially concentrated caustic soda solution in the No. 3 evaporator 6.

No. 2 heat exchanger of the second stage unit concentration process (anode liquid formed by the discharge tube 36 from 2 and heat exchange in No. 3 heat exchanger 3, wherein the heated caustic soda solution is No. 3 The gas is separated by the flush in the evaporator 6, the gas (vapor) is introduced into the vacuum condenser 38 from the outlet pipe 32 of the evaporator 6 and discharged as condensed water, and the liquid is partially concentrated. Caustic soda is concentrated to form a caustic soda concentrate, which is then fluorinated to flow into the suction pipe 21 of the No. 2 circulation pump 11 of the second stage unit concentration process through the caustic soda conveying pipe 20. The anolyte solution after heat exchange in the heat exchanger 3 is circulated to the anode chamber side 8 of the electrolytic cell via the discharge tube 37. Since the anolyte liquid taken out of the discharge tube 37 has a low NaCl concentration, After adding the raw material salt to saturation of NaCl or near saturation, it is made to the anode chamber side 8 of the electrolytic cell. Class is. Further, in this hanhu a state close to a saturated or saturated, electrolytic, such as Ca ^2+, Mg ²⁺ is fed to the anode chamber side (8) of the electrolytic cell. In addition, near saturation or saturated saline solution in this same If it contains components that adversely affect it, it is necessary to purify the anolyte to remove these components as much as possible.

In the second stage unit concentration process, the concentrated liquid from the evaporator 6 introduced through the caustic soda conveying pipe 20 is No. 2 circulating pump 11 together with the predetermined amount of the circulating concentrated liquid from the evaporator 5. Heat exchange with the anolyte from the first heat exchanger 1 of the first stage unit concentration process via the discharge amount 22 of the heat exchanger 2, and the No. 2 evaporator 5 through the discharge tube 23 ), The gaseous liquid is separated in the same manner, and the separated gas (vapor) is introduced into the vacuum condenser 38 in the entrance pipe 31 and discharged as condensed water, and the caustic soda aqueous solution is concentrated to caustic soda concentrate. And flows overflow into the suction pipe 25 of the No. 1 circulation pump 12 of the 1st stage unit concentration process from the caustic soda delivery piping 24. On the other hand, the anolyte passing through the heat exchanger 2 is supplied to the heat exchanger 3 of the third stage unit concentration process as described above.

In the first unit concentration process, the anolyte that recovers and retains the heat of electrolysis from the anode chamber side 8 of the electrolytic cell is sequentially passed through the anolyte transfer pump suction tube 33, the transfer pump 14, and the discharge tube 34. .1 enter the heat exchanger (1); On the other hand, the concentrate liquid that has been overflowed from the evaporator pot 5 of the second stage unit concentration process flows into the suction pipe 25 of the circulation pump 12 through the transfer pipe 24 and from the evaporator pot 4 with the circulating concentrate liquid. After entering the heat exchanger (1) through the circulation pump 12 and the heat exchange with the anolyte solution, the gas is separated into the same by entering the evaporator (4), the separated gas is passed through the outlet pipe (30) It is introduced into the vacuum condenser 38 and is discharged as condensed water, and the aqueous caustic soda solution is concentrated to become a caustic soda concentrate, a part of which is circulated to the circulation pump 12 and the other is overflowed. It is taken out through the extraction pump 13 and the discharge pipe 29 via the concentrated liquid extraction pump suction pipe 28. The caustic soda concentration of the caustic soda concentrate extracted here depends on the electrolytic solution concentration and the amount of heat generated during electrolysis, and may or may not reach a practical temperature of 45 to 50%. If it does not reach the practical concentration, it is further sent to the concentration process using steam as a heating source, and it is concentrated to 45 to 50% of the practical concentration. Even in such a case, the amount of steam used as a heating source can be greatly reduced as compared with the case of heating and concentrating only with steam, thereby reducing the cost.

In addition, although the material of the evaporation pots 4, 5, and 6 of No. 1, No. 2, and No. 3 is sufficient as NI and Ni alloy, even if the material of No. 3 evaporation pot is made of stainless steel economically, do.

In addition, No.1, No.2, and No.3 heat exchanger (1) (2) (3) has a titanium-alkali-resistant metal composite partition as a heat exchange surface, and the caustic alkali aqueous solution has an anolyte solution on the alkali-resistant metal surface side. Is supplied to the titanium surface side.

The present invention can be employed for the concentration of an aqueous sodium chloride solution using an ion exchange membrane and an aqueous sodium hydroxide solution obtained by electrolysis, or a concentration of an aqueous potassium hydroxide solution obtained by electrolysis of an aqueous potassium chloride solution using an ion exchange membrane.

Example 1

The concentration of the NaOH aqueous solution obtained by electrolytic ion exchange method NaCl aqueous solution electrolysis was performed according to the system diagram as shown in the figure using the concentration apparatus provided with the unit concentration process shown in an accompanying drawing in three steps in series.

The NaOH aqueous solution (NaOH concentration 30 weight%, temperature 95 degreeC) obtained in the cathode chamber of an electrolyzer was made to flow by the unit concentration process of No.3 and No.2M No.1 sequentially, and the evaporation pot of the unit concentration process of No.1 was carried out. An aqueous NaOH solution with a NaOH concentration of 48% by weight was obtained.

On the other hand, the lean NaCl aqueous solution (NaCl concentration 20 weight%, temperature 100 degreeC) obtained in the anode chamber of an electrolytic cell is made to flow through the unit concentration process of No.1, No.2, and No.3 sequentially, The lean NaCl aqueous solution taken out from the heat exchanger was refine | purified by making the raw material salt into saturated NaCl saturation, and it circulated to the anode chamber of the electrolytic cell.

The NaOH aqueous solution, the temperature of the lean NaCl aqueous solution, the concentration of the NaOH aqueous solution, and the pressures of the No. 1, No. 2, and No. 3 evaporator in each unit concentration step of No. 1, No. 2, and No. 3 were measured. It is presented in Table 1.

The Nos. 1 to 3 evaporators were made of Ni, and the Nos. 1 to 3 heat exchangers used titanium-nickel composite partition walls as heat exchange surfaces.

TABLE 1

Example 2

A 50 wt% NaOH aqueous solution was obtained by concentrating the same NaOH aqueous solution as in Example 1 using an apparatus provided with a unit process in 5-stage series as shown in the accompanying drawings, using the same lean NaCl aqueous solution as in Example 1. lost.

The concentration of the aqueous NaOH solution in each unit concentration step, the temperature of the NaOH aqueous ag aqueous NaOH aqueous solution, and the pressure of each evaporator were investigated, and the results are shown in Table 2.

TABLE 2

Example 3

The inner tube made of titanium (length 1,000mm × inner diameter 15mm × 3mm) is inserted into the outer shell made of titanium (length 800mm × inner diameter 40mm × thickness 2mm) to make the structure of the heat exchanger, electroplating nickel on the inner tube inner surface, and thickness of 30μ nickel A coating layer was formed.

Next, this was put into a vacuum electric furnace (1 × 10 ³ torr) and heated at 900 ° C. for 30 minutes. The inner tube treated as described above was cut out, and the cut surface was observed under an electric microscope. A 5μ nickel-titanium alloy layer (20% nickel and 80% titanium) was recognized between titanium and nickel, and the thickness of the nickel layer after treatment was observed. Was 25μ.

Then the same 50% by weight NaOH concentration within the tube of a heat exchanger manufactured by the, NaOH aqueous solution of temperature 80 ℃ supplying 1m ³ / hr, 15% by weight of NaCl concentration on the annular portion of the inner tube and the exterior temperature 95 ℃ The anolyte solution of the bulkhead method salt electrolyzer was circulated and supplied at 5 m ³ / hr, and the heat exchange operation was performed for 8,600 hours.

After that, the inner tube was removed and the inner and outer surfaces of the inner tube were observed, but corrosion was not recognized.

TABLE 3

Claims (1)

In preparing a caustic alkali by electrolyzing an aqueous alkali chloride solution in an electrolytic cell in which an anode and a cathode are disposed while the ion exchange membrane is sandwiched, an alkali chloride solution having an aqueous solution of caustic alkali obtained from the cathode side of the cell and an electrolytic heat obtained from the anode side. A unit concentration process including a heat exchange step with an aqueous solution and a gas-liquid separation step by evaporation of the caustic alkaline solution subjected to the heat exchange step is provided in series, and the caustic alkaline aqueous solution from the cathode side of the electrolytic bath is first-stage unit. It is supplied to the concentration step, while increasing the caustic alkali concentration is transported to the unit concentration step of the subsequent stage and taken out as a product in the unit concentration step of the final stage, while the aqueous alkali chloride solution from the anode side of the electrolytic cell first of the final stage Fed to the unit concentration process and this is the unit concentration process of sequential shearing Transport, and finally an aqueous solution of caustic alkali concentration of the method according to the ion exchange membrane electrolysis process, characterized in that the take-out unit in the concentration step of the first stage is recycled to the anode of the electrolytic cell to.

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