JPS644594B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method 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, in practice it is often disposed of after being detoxified, such as neutralized, without being recovered, mainly for 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. In other words, alloys such as iron, nickel, and stainless steel are inexpensive and have good workability, and have traditionally been used as electrodes for caustic alkaline aqueous solutions in water electrolysis. However, at low concentrations of caustic alkali of less than about 10%, particularly less than 5%, the anode will be significantly oxidized due to the increase in electrolytic potential, and oxides will be formed on the surface, making it inactive, or the surface will become inactive. Problems such as elution occurred, and iron, nickel, etc. could not be used as electrodes for electrolysis of low-concentration caustic alkaline aqueous solutions. The present invention was made to solve the above problems, and its purpose is to stably electrolyze a dilute aqueous caustic alkali solution for a long period of time using inexpensive electrodes made of iron, nickel, etc., and efficiently remove caustic alkali. The object of the present invention is to provide a new electrolytic method that can be recovered. The present invention provides an electrolysis method in which a dilute caustic aqueous solution is supplied to an anode chamber of an electrolytic cell in which an anode chamber and a cathode chamber are separated by a cation exchange membrane, and a concentrated caustic aqueous solution is recovered from the cathode chamber. Iron, nickel, or an alloy thereof is used as the electrode material for the and cathode, and each time electricity is applied in the positive direction for a predetermined period of 15 minutes or less, the polarity is reversed and 3 to 30% of the amount of electricity is applied. This is characterized by energization in the opposite direction. As described above, the present invention achieves the above object by periodically reversing the polarity of the electrode at a predetermined amount of current, and as described in detail below, uses inexpensive electrodes such as iron and nickel. It has the excellent effect of making it possible to perform electrolysis of a dilute aqueous alkaline solution stably for a long period of time. The electrolytic cell used in the present invention has an anode chamber and a cathode chamber separated by a cation exchange membrane, and is of a known monopolar type or bipolar type as described in JP-A-52-16859, for example. Applicable to any model. 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, any known cation exchange membrane that can withstand an electrolytic environment can be used, but fluororesin membranes such as perfluoro ion exchange membranes with good alkali resistance are particularly suitable. In the present invention, iron, nickel, or an alloy thereof is used as the electrode material. As the alloy material, for example, carbon steel, Fe--Ni alloy, stainless steel, alloy with Co, Cr, or Mo, etc. can be used. These electrode materials may be the same for both the anode and the cathode, or may not be the same. Electrodes using these electrode materials have good conductivity, can be easily formed into any shape such as a rod, plate, net, or perforated plate, and are inexpensive. However, especially when used for electrolysis of a dilute aqueous caustic solution, in the conventional electrolysis method, the surface of the anode is gradually oxidized and becomes inactive due to the formation of oxides, making it unusable. The present invention solves the above-mentioned conventional problems by reversing the polarity and energizing in the opposite direction every time electrolysis is performed by applying current in the forward direction for a predetermined period of time. This was done based on new knowledge that it can be done. The energization method in the present invention will be specifically explained with reference to the drawings. FIG. 1 shows a conventional electrolytic energization method, in which the anode is energized in the positive direction at a predetermined current value A for a required time T. On the other hand, in the present invention, the current values A ₁ , A ₂ ,
Every time T _{1 ,} T ₂ , T ₃ are energized for a predetermined time at A 3 and electrolyzed, the polarity is reversed and the predetermined constant current value a ₁ ,
Electricity is applied at a ₂ and a ₃ for predetermined times t ₁ , t ₂ , and t ₃ . Although it is not necessarily clear why the effects of the present invention are achieved as described above, by periodically energizing in the opposite direction, progress of deactivation of the electrode is prevented and further activation is achieved. This is thought to be due to the revival of In particular, it was confirmed that at the anode, the oxides formed as electrolysis progressed were reduced and disappeared, and the active surface was restored. Further, although impurity metal ions and the like are usually reduced and deposited on the surface of the cathode, the surface is cleaned by applying current in the opposite direction. Such a cleaning action is also effective in removing obstacles that deposit and adhere to the cation exchange membrane being used. It is desirable that the current energization time in the forward direction be as long as possible for the purpose of performing electrolysis, but if it is too long, the electrode will become inactive, and furthermore, it will be difficult to restore activity even by energizing in the reverse direction. Must be limited to a certain amount of time. Generally, it is safe to set the time to about 15 minutes or less, and the electrode activity can be easily restored, allowing stable electrolysis to be performed for a long period of time. On the other hand, since energization in the opposite direction reduces the efficiency of the intended electrolysis, it is desirable to use as little energization as possible, but it is necessary to set the amount of energization to be sufficient to restore electrode activity. Normally, the amount of current in the reverse direction is about 3 to the amount of current in the forward direction.
It was confirmed that the purpose of the present invention can be sufficiently achieved and effective if the ratio is set to 30%. FIG. 2 shows a typical energization pattern in the electrolytic method of the present invention, in which a constant current A ₁ is applied for a predetermined time T ₁
Electrolyze by passing current in the positive direction, then apply the same current (a ₁ = -
At A ₁ ), electricity is applied in the opposite direction for a certain period of time t ₁ , and this is repeated to continue the desired electrolysis. In this case, the amount of energization is represented by A ₁ ×T ₁ and −A ₁ ×t ₁ (the area of the shaded part in the figure), respectively, and the ratio thereof is determined only by the ratio of each energization time. Therefore, the operation is the simplest since it is only necessary to control the time for switching the polarity. For example, if T ₁ is 10 minutes, the current application time t ₁ in the reverse direction is about 18 seconds to 3 minutes according to the present invention. Then, by setting t ₁ to an appropriate time within that range, for example 1 minute, it is easy to perform electrolysis by automatically controlling the power supply of the electrolytic cell using an automatic timer or the like so that the electrode polarity is changed at that period. . The energization pattern shown in Figure 3 is a positive direction current.
This is an example in which electrolysis is performed by changing both the current _{a 2 in} the opposite direction and the energization time t ₂ with respect to the A ₂ energization time T 2 . In addition, in the case shown in Fig. 4, the energizing time T ₃ in the forward and reverse directions is the same (T ₃ = T ₃ ), and the energizing current is set to A ₃ , which is smaller in the opposite direction than A ₃ in the forward direction, to conduct electrolysis. This is an example of doing so. As described above, in the present invention, any energization method may be used as long as the amount of energization in the reverse direction is periodically about 3 to 30% of that in the forward direction. Example 1 An electrolytic cell separated by a cation exchange membrane (trade name Nafion 315, manufactured by Dupont) was constructed, and both the anode and cathode were made of stainless steel plates (SUS316) with a thickness of 1 mm.
was used as the electrode material. A 0.5% NaOH aqueous solution was supplied to the anode chamber, and electrolysis was carried out at a temperature of 60° C. and a current density of 30 A/dm ² by changing the energization time in the opposite direction according to the energization pattern shown in FIG. 2. The first 10% of the head is in the cathode chamber.
Fill with NaOH aqueous solution, then 0.2% from the anode chamber
Drain the NaOH aqueous solution and 12% NaOH from the cathode chamber.
The aqueous solution was extracted. The results are shown in Table 1. In addition, the electrode life is determined when the electrolytic voltage is lower than the initial value.
This was done depending on the point at which the voltage rose by 2.0V.

[Table] As is clear from the results in Table 1, it can be seen that the life of the electrode is dramatically improved by periodically applying current in the opposite direction. In addition, if the amount of current flowing in the reverse direction increases, the electrode life will be extended, but the overall current efficiency will decrease, so in order to maintain the current efficiency above 50%, the amount of current flowing in the reverse direction should be approximately equal to that of the forward direction. 3
It should be about ~30%. Example 2 An electrolytic cell was constructed in the same manner as in Example 1 except that Ni plates were used for both the anode and cathode, and the anode chamber contained 4%
A NaOH aqueous solution was supplied, a 2% NaOH aqueous solution was discharged, a 12% NaOH aqueous solution was extracted from the cathode chamber, and electrolysis was performed in the same manner. The results are shown in Table 2.

【table】 [Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing a normal energization pattern according to a conventional method, and FIGS. 2, 3, and 4 are explanatory diagrams illustrating an energization pattern according to the present invention. A, A ₁ , A ₂ , A ₃ : Positive direction current value, a ₁ , a ₂ ,
a ₃ : Reverse direction current value, T, T ₁ , T ₂ , T ₃ : Forward direction energization time, t ₁ , t ₂ , t ₃ : Reverse direction energization time.

Claims (1)

[Scope of Claims] 1. An electrolysis method in which a dilute aqueous caustic alkaline solution is supplied to the anode chamber of an electrolytic cell in which an anode chamber and a cathode chamber are separated by a cation exchange membrane for electrolysis, and a concentrated aqueous caustic alkaline solution is recovered from the cathode chamber. In this method, iron, nickel, or an alloy thereof is used as the electrode material for the anode and cathode, and each time electrolysis is performed by applying current in the positive direction for a predetermined period of 15 minutes or less, the polarity is reversed to
A method for electrolyzing a dilute caustic alkaline aqueous solution, which is characterized by energizing 30% in the opposite direction. 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, wherein the anode chamber is supplied with a caustic alkali treatment waste liquid in a petroleum refining process.