JPS6367556B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for electrolyzing a dilute aqueous caustic solution using iron, nickel, or an alloy thereof as an electrode in an electrolytic cell separated by a cation exchange membrane.

Solutions containing caustic alkali are industrially discharged from various manufacturing, treatment or processing processes. Examples include reaction waste liquids from various chemical reaction processes, metal alkali treatment waste liquids, ion exchange resin regeneration waste liquids, alkali treatment waste liquids in petroleum refining processes, and the like. Recovering caustic alkali from these waste liquids is industrially important from the viewpoint of process economy or pollution control.

Therefore, conventionally, such waste liquid has been treated by
Various methods have been attempted to recover caustic alkali and render it harmless. Most of these caustic alkali-containing waste liquids are aqueous solutions with relatively low concentrations;
Because it contains many other inorganic or organic coexisting substances, it is often disposed of after undergoing detoxification treatment such as neutralization without being recovered for technical or economic reasons.

An electrolytic method using a cation exchange membrane is known as an effective means of efficiently recovering caustic alkali from such waste liquid. According to
Separates and recovers only alkali from alkaline wastewater,
A method for treating alkaline wastewater in which the wastewater is discharged as neutral is described.

However, in such electrolytic methods, the electrodes, especially the anode, are required to be highly durable to withstand the oxygen evolution reaction, and are not made of expensive precious metals or graphite, which is easily consumed and has various drawbacks in manufacturing and operation. The emergence of technically and economically superior electrolysis technology that can be used industrially has been desired. That is, iron, nickel, and their alloys such as stainless steel are inexpensive and have good workability, and have been conventionally used as electrodes for electrolyzing caustic aqueous solutions in water electrolysis and the like. However, these can be used in relatively high-temperature, highly concentrated aqueous solutions, and at low concentrations of caustic alkali of less than about 10%, especially less than 5%, the anode will be significantly oxidized due to the increase in electrolytic voltage, and oxides will form on the surface. Problems such as formation and inactivation or further elution of the anode surface occurred, and iron, nickel, etc. could not be used as electrodes in the electrolysis of low-concentration caustic alkaline aqueous solutions. Furthermore, when electrolyzing waste liquid containing various organic substances, heavy metals, etc., there is a problem in that these impurities adhere to and precipitate on ion exchange membranes, electrodes, piping, etc., making it difficult to proceed with electrolysis.

The present invention was made to solve the above-mentioned problems, and its purpose is to stably electrolyze a dilute aqueous caustic alkali solution over a long period of time using inexpensive electrodes made of iron, nickel, etc., and to efficiently remove caustic alkali. It is an object of the present invention to provide a new electrolysis method and an apparatus for the same that can be well recovered.

The present invention is an electrolysis method in which a dilute caustic aqueous solution is supplied to one electrode chamber of an electrolytic cell divided by a cation exchange membrane, and a concentrated caustic aqueous solution is recovered from the other electrode chamber. Using nickel or its alloy, each time electrolysis is performed by applying current in the forward direction for a predetermined period of time, the polarity is reversed and current is applied in the opposite direction for a certain period of time, and then the direction of supply and discharge of the electrolyte is reversed, This invention is characterized by a method for electrolyzing a dilute caustic aqueous solution, and an apparatus for electrolyzing a dilute caustic aqueous solution, which is characterized by performing electrolysis for a certain period of time by reversing the polarity and applying current in the opposite direction every time electrolysis is performed by applying current in the negative direction for a predetermined period of time. be.

As described above, the present invention performs similar electrolysis by periodically reversing the polarity of the electrode at a predetermined amount of current, and by operating the supply/discharge direction and the current direction of the electrolyte every electrolysis period for a certain period of time. As a result, we have achieved the above objectives and achieved the excellent effect of making it possible to stably electrolyze a dilute aqueous alkaline solution for a long period of time using inexpensive electrodes made of iron, nickel, etc. It is something to play.

The electrolytic cell used in the present invention has an electrode chamber divided by a cation exchange membrane, and can be of any type, such as a monopolar type or a bipolar type.

The electrolyzer shown in FIG.
are sandwiched to form electrode chambers 2 and 3, and electrodes 4 and 5 are sandwiched between them.
energize to perform electrolysis.

FIG. 2 shows an example of a bipolar electrolytic cell according to the present invention, in which a cation exchange membrane 1 and a bipolar electrode 6 are sequentially provided between end electrodes 4' and 5'. . Reference numeral 7 indicates an intermediate electrode chamber, and it is of course possible to provide a plurality of 7 to form a multi-chamber bipolar electrolytic cell. In the present invention, since the same electrode material can be used for both the anode and the cathode, there is no need to combine different types of electrode materials, which is advantageous, especially when constructing a bipolar electrolytic cell.

As the cation exchange membrane 1, any known cation exchange membrane that can withstand an electrolytic environment can be used, but a fluororesin membrane such as a perfluoro ion exchange membrane with good alkali resistance is particularly suitable.

The electrodes 4, 5, or 6 are made of iron, nickel, or an alloy thereof. As the alloy material, for example, carbon steel, Fe-Ni alloy, stainless steel, alloy with Co, Cr or Mo, etc. can be used. The individual electrodes are
These electrode materials may be made of the same material, or may be made of different materials in combination. An electrolytic cell is usually attached with a supply/discharge mechanism for electrolyte and products, and in the present invention, including these, the electrolytic device is constructed in a symmetrical shape centering on the cation exchange membrane 1 or the electrode 6. However, the current supply direction and the liquid flow direction can be reversed at any time. In a multi-chamber bipolar electrolytic cell as shown in Figure 2, if an odd number of cation exchange membranes are used, the middle cation exchange membrane will be the center of symmetry, and if an even number is used, the middle bipolar electrode will be the center of symmetry. becomes.

For example, in FIG. 1, tanks 8, 8' piping 9, 9' pump 1 of the same shape can supply or discharge electrolyte to the left and right electrode chambers 2, 3.
0 and 10', and separate liquid supply pipe 1 as necessary.
1, 11', exhaust pipes 12, 12', etc., to construct an electrolyzer symmetrically with the cation exchange membrane 1 at the center.

Electrodes using iron, nickel, etc. as electrode materials are
It has good conductivity, can be easily formed into any shape such as a rod, plate, net, perforated plate, etc., and is inexpensive.
However, in the conventional electrolysis method, especially when used for electrolyzing dilute aqueous solutions or waste liquids containing caustic alkali,
The surface of the anode is oxidized to form oxides and become inactive, and impurities and the like are deposited on various parts of the electrolyzer, such as the cation exchange membrane, cathode, and piping, making it difficult to continue electrolysis.

Even if such an electrode is used, the present invention performs electrolysis for a certain period of time, in which the polarity is reversed every time the current is passed in the forward direction for a predetermined period of time, and the current is passed in the reverse direction, and then the direction of supply and discharge of the electrolyte is reversed. This work was made based on the new knowledge that if similar electrolysis was performed using a similar method, the above-mentioned conventional problems would be resolved, and a dilute caustic alkali-containing aqueous solution could be electrolyzed stably for a long period of time.

The energization method of the present invention will be specifically explained with reference to FIG.

First, one electrode chamber is used as an anode chamber, and a predetermined current value A is set.
Each time electricity is applied in the positive direction for a predetermined time T to perform electrolysis, the polarity is reversed, and electricity is applied in the reverse direction for a predetermined time t at a predetermined current value a, and this is repeated for a predetermined period L. After that, the electrode chamber is used as a cathode chamber, and the supply and discharge direction of the electrolyte is reversed, and the current is similarly applied in the negative direction for a predetermined time T', and each time the electrolysis is performed, the polarity is reversed and a predetermined current value is set.
At a', current is applied in the opposite direction for a predetermined time t', and this is continued for a predetermined period L'. Thereafter, electrolysis is continued in the same manner.

Although it is not necessarily clear why the above-mentioned effects of the present invention are achieved by doing so,
This is thought to be because periodic energization in the reverse direction prevents the progress of deactivation of the electrode and further restores its activity. In addition, energization in the opposite direction is effective in removing and cleaning impurity metals that are reduced and precipitated on the surface of the cathode and obstacles that are deposited and adhered to the cation exchange membrane.

On the other hand, since the current direction and the flow direction of the liquid throughout the electrolyzer are periodically reversed every time electrolysis is performed for a certain period of time, the electrochemical effect described above becomes more effective, and at the same time, the physical effect due to the backflow of the liquid This is thought to be because the removal and cleaning of scale substances deposited on the membrane, pipes, etc. is performed effectively.

It is desirable that the energization times T and T' at the predetermined current values A and A' in the positive or negative direction are long for the purpose of performing electrolysis, but if they are too long, the electrodes will become inactive.
Furthermore, it is difficult to restore the activation even if current is applied in the opposite direction, so it is necessary to limit the time to a certain period. Generally, it is safe to set the time to about 15 minutes or less, and the electrode activity can be easily restored.

On the other hand, the current values a and a′ in the opposite direction and the current conduction time t,
Since energization at t' reduces the efficiency of the intended electrolysis, it is preferable to use a small amount of energization as long as it is possible to sufficiently restore the electrode activity. Normally, if the amount of energization a×t, a′×t′ in the reverse direction is about 3 to 30% of the amount of energization A×T, A′×T′ in the positive or negative direction, the object of the present invention can be effectively achieved. It was confirmed that this was achieved.
For example, if a=-A and T is 10 minutes, t will be about 18 seconds to 3 minutes according to the present invention.

Electrolysis is performed for a certain period of time while periodically applying current in the opposite direction, and then electrolysis is performed for a certain period of time by reversing the electrolyte supply/discharge direction and the current direction.
L' is performed, and thereafter, this is repeated to continue electrolysis for a long period of time. Period L for continuing electrolysis in the certain direction
Or L' can be determined as appropriate, but usually about 100 to 1000 hours is preferable in order to achieve the effects of the present invention.

In the energization pattern shown in Figure 3, the current value is the same in both the forward and reverse directions (A=A'=-
a=-a′), each energization time is constant (T=T′, t=t′,
When L=L'), the operation is the simplest since all you have to do is control the time for switching the polarity. However, as long as it does not depart from the purpose of the present invention, each current value A, A', a, a', energization time T, T', t, t', and electrolysis period L, L' may be changed as appropriate. .

Example 1 An electrolytic cell divided by a cation exchange membrane (trade name Nafion 315, manufactured by Dupont) was constructed as shown in Fig. 1, and both electrodes 4 and 5 had a size of 10 cm x 10 cm.
A stainless steel plate (SUS316) with a thickness of 2.5 mm was used as the electrode material. First, the left electrode chamber 2 is used as the anode chamber,
A NaOH aqueous solution was supplied as an electrolytic solution, and electrolysis was performed while periodically energizing in the opposite direction according to the energization pattern shown in FIG.

Next, the electrolytic solution was supplied to the right electrode chamber 3 by operating the valve, and electrolysis was continued in the same manner by using the electrode chamber 3 as the anode chamber and reversing the direction of flow of the solution and the direction of current supply. Those conditions are as follows.

Supply electrolyte: 2% NaOH aqueous solution Anode chamber effluent: 0.5% NaOH aqueous solution Cathode chamber effluent: 12% NaOH aqueous solution Electrolysis temperature: 55℃ Current density A=a: 30A/dm ² Current conduction time T=T': 60 seconds Reverse direction t=t': 6 seconds electrolysis period L=L': 300 hours As a result, the electrolytic treatment could be continued for 3000 hours without any trouble at a total current efficiency of about 71%.

On the other hand, in the case of electrolysis without periodic energization in the reverse direction, the current efficiency was about 86%, but the electrolysis voltage increased by more than 5V after about 100 hours of electrolysis, and electrolysis was continued beyond that point. I couldn't do it.

Example 2 An electrolytic cell separated by a cation exchange membrane (trade name Nafion 324, manufactured by Dupont) was constructed as shown in Fig. From alkaline wastewater from the Marox process during LPG refining using a nickel plate,
The NaOH aqueous solution was recovered.

The analytical values of the alkaline wastewater were as follows.

NaOH 6.0% TOC 20g/ Ca ⁺⁺ 20mg/ Mg ⁺⁺ 5mg/ Mn ⁺⁺ 5mg/ SO ₄ ^-- 20mg/ This alkaline wastewater is used as an electrolyte and first supplied to the left electrode chamber 2, and the electrode chamber is The chamber was used as an anode chamber, and electrolysis was carried out while periodically energizing in the opposite direction according to the energization pattern shown in FIG. Next, the electrolytic solution was supplied to the right electrode chamber 3 by operating the valve, and electrolysis was continued in the same manner by using the electrode chamber 3 as the anode chamber and reversing the direction of flow of the solution and the direction of current supply. Shadow of electrode room/
After reversing the positive polarity and starting electrolysis, the concentrated NaOH aqueous solution obtained in the cathode chamber was returned to the original waste solution for 15 minutes, so that only the pure NaOH aqueous solution was recovered.

The conditions for electrolysis are as follows.

Supply electrolyte NaOH concentration: 6.0% Anode chamber effluent NaOH concentration: 0.6% Cathode chamber effluent: 12% NaOH aqueous solution Electrolysis temperature: 55℃ Current density A = a: 30A/dm ² Current application time T = T': 60 seconds Reverse direction t=t': 6 seconds electrolysis period L=L': 168 hours (1 week) As a result, the electrolytic treatment could be continued for 4500 hours without any trouble at a total current efficiency of about 73%. In addition,
Almost no deposits were observed to adhere to the cation exchange membrane.

On the other hand, in the case of electrolysis without periodic energization in the reverse direction, the current efficiency was about 88%, but the electrolysis voltage increased by more than 5V after about 100 hours of electrolysis, and electrolysis was continued beyond that point. I couldn't do it.

In addition, when electricity is periodically reversed but the electrode chamber is not periodically reversed between negative and positive, the total current efficiency is approximately 73%, and operation was possible for approximately 1,500 hours without any problems. The electrolysis voltage gradually increased.
When the tank was dismantled, a small amount of non-conductive oxide was observed on the surface of the anode plate, and precipitates thought to be impurities in the alkaline waste solution supplied were attached to the surface of the cation exchange membrane. The electrical resistance of the exchange membrane is 2
It was observed that the number had increased by about twice as much.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an example of an electrolyzer according to the present invention, FIG. 2 is an explanatory diagram showing another example of an electrolyzer according to the present invention, and FIG. 3 is an example of an energization pattern for electrolysis according to the present invention. FIG. 1: Cation exchange membrane, 2, 3, 7: Electrode chamber,
4, 5: electrode, 6: bipolar electrode, 8, 8': tank,
9,9': Piping, 10,10': Pump, 11,1
1': Liquid supply pipe, 12, 12': Exhaust pipe.

Claims (1)

[Claims] 1. An electrolysis method in which a dilute caustic aqueous solution is supplied to one electrode chamber of an electrolytic cell divided by a cation exchange membrane for electrolysis, and a concentrated caustic aqueous solution is recovered from the other electrode chamber, Iron, nickel, or an alloy thereof is used as the electrode material, and each time electrolysis is performed by applying current in the forward direction for a predetermined period of time, the polarity is reversed and current is applied in the opposite direction for a certain period of time, and then electrolysis is performed in the supply and discharge direction of the electrolyte. A method for electrolyzing a dilute caustic aqueous solution, characterized in that electrolysis is performed for a certain period of time by reversing the polarity and applying current in the negative direction for a predetermined period of time. 2. The method according to claim 1, wherein the aqueous solution supplied has a caustic alkali concentration of 10% or less. 3. The method according to claim 1, which supplies caustic alkali treatment waste liquid in a petroleum refining process. 4. The method according to claim 1, wherein the energization time in the positive or negative direction is 15 minutes or less. 5. The method according to claim 1, wherein the amount of energization in the reverse direction is 3 to 30% of the amount of energization in the positive or negative direction. 6. The method according to claim 1, wherein the electrolysis period is 100 to 1000 hours. 7 In an electrolysis device using an electrolytic cell separated by a cation exchange membrane, both electrodes are made of iron, nickel, or an alloy thereof, and both electrode chambers and their electrolyte supply and discharge mechanisms have the same shape. Alternatively, a dilute alkaline aqueous electrolysis device comprising a symmetrical electrolytic cell centered on an electrode, and in which the polarity of the electrodes and the direction of supply and discharge of the electrolytic solution can be reversed at any time.