JPS6249355B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrolytic treatment method with high current efficiency.

Electrolysis methods are put into practice in a wide range of sectors, from industrial-scale separation and purification of specific substances, decomposition and recovery treatment of wastewater, to measurement fields known as electrochemical measurement methods.

FIG. 1 shows an explanatory diagram of the principle of a conventional electrolytic method.

In Fig. 1, 1 is an electrolytic cell, and 2 is the same electrolytic cell 1.
3 and 4 are the anode and cathode disposed in the electrolyte 2, 5 and 6 are the anode 3
and lead wires having one side connected to the cathode 4, and 7 a power source to which the other ends of the lead wires 5 and 6 are connected. In such a configuration, electrolytic treatment is performed by supplying electricity. Although the power source 7 is a constant current or constant voltage DC power source, constant current is often used in ordinary practical scale electrolysis.

FIGS. 2 and 3 are examples of basic electrode configurations of electrolytic cells that are used in conventional electrolytic methods based on the principle shown in FIG. 1.

In FIG. 2, a flat anode 3 and a cathode 4 having a certain effective area face each other with an electrolyte flowing in and out in the direction of the arrow in the figure. There are many other inventions and ideas that are variations on the one shown in Figure 2, but all of them are based on the principle shown in Figure 1.

Figure 3 shows another example of the main anode 3A and the main cathode 4.
This is an electrolysis method consisting of a dispersed electrode group P in which bipolar fine particles, such as carbon particles, are interposed in an electrolytic solution portion flowing in and out in the direction of the arrows in the figure between A.

In this method, the surface area of the electrode in the electrolyte was changed from the two-dimensional concept of other conventional methods to a three-dimensional concept, and the three-dimensional method significantly increased the surface area of the electrode. The principle is the same as that shown in FIG. 1 or 2.

However, the above conventional method has the following drawbacks.

Generally, in order to maintain good current efficiency when performing an electrolytic reaction, measures should be taken so that the applied current is effectively used for the intended electrode reaction.

Now, in Figure 1, if a redox electrode reaction is occurring as shown in the following equation, xO + m , the concentration Cm of the reaction component in the solution body, the concentration Co at the electrode interface, the diffusion layer thickness δ, and the diffusion constant D of the reaction component, the following relationship is given.

i=nFD(Cm-Co)/δ...(2) where n is the number of reaction electrons and F is Faraday's constant.

In that case, the limiting current density il is Co=0, so il=nFD(Cm)/δ...(3), and the maximum limiting current density il depends on the concentration Cm of the reaction component in the solution body. .

However, since this Cm decreases as the electrolysis progresses, the change in the concentration of the components during the reaction progresses has the following relationship with the electrolysis time.

Cx=Cm・e ^- 〓 ^t ...(4) However, Cx is the concentration of the reaction component at time t.

Here, α is a factor determined by processing conditions such as electrode area S and processing liquid amount V, and can generally be expressed by the following equation.

However, A=0.43·Dγ/δ, and γ is a coefficient related to the shape of the electrode.

The above relational expression shows that since a certain amount of the liquid to be treated is always treated under a set condition (e.g. constant current condition) with a constant electrode area S, the target reaction component Cx in the treatment liquid is Although the electrolysis current density decreases with the electrolysis time t, it is shown that the electrolysis current density is processed under the condition of the initial concentration Cm of the target reaction component.

Therefore, the conventional electrolytic method essentially has the following drawbacks.

(1) As the target reaction component concentration Cx decreases, the critical current density il decreases, so in order to improve current efficiency it is necessary to decrease the current density accordingly, but in this case the electrolysis time t required for processing becomes significantly longer.

(2) If electrolysis is continued under certain conditions, current loss will occur, side reactions to the main reaction will increase, the yield at the end of the process will deteriorate, and the current efficiency will decrease.

Because of these drawbacks, the electrolytic method has been limited in its applicability to dilute components compared to other physical treatment methods.

The drawbacks of (1) and (2) above were confirmed using FIG. 4.

Figure 4 shows the results of decomposing and removing treated components by constant current electrolysis (line A) and the results of producing a certain target substance as a product (line B) as trend values. As shown by B, in both cases, the removal rate or yield decreases with increasing treatment time.

The present invention improves the drawbacks of the conventional electrolytic method described above, and achieves higher current efficiency and shorter processing time (i.e., results as shown in dotted lines A' and B' in FIG. 4). The present invention provides an electrolytic treatment method that can be used.

That is, the present invention provides an electrolytic treatment characterized in that electrolytic treatment is always carried out at a constant current by changing the electrolytic current density and electrode area in response to changes in the concentration of the target reaction component in the liquid to be treated due to the progress of electrolysis. This relates to a processing method.

By the way, the reaction rate (i.e. current density) i _L
From equations (3) and (4) above, i _L = nFD/δ・Cm・e ^- 〓 ^t ...(6), and when the electrode area is S, the total electrolytic current I is =i _L S=K ₁・Cm・e ^- 〓 ^t・S ...(7) (However, K ₁ =nFD/δ). As is clear from equations (6) and (7), the electrolytic current I decreases exponentially with time, and the reaction rate i _L decreases.

Therefore, the electrode area S is changed over time as follows: S=K ₂ e〓 ^t ...(8) However, K ₂ is a constant, and ideally K ₂ = 1, but the current efficiency is not necessarily constant depending on the current density. Therefore, K ₂ is also determined depending on the actual situation. ), the total electrolytic current I becomes I=K ₁・Cm・e ^- 〓 ^t・K ₂・e〓 ^t ...(9), and the current I corresponding to the initial concentration Cm , the reaction time T is expressed as: T=nFCmV/Wg ⁻ ·I (10) (where Wg is the atomic weight of the reaction component).

For example, in the case of concentrated liquids, it is rare to carry out electrolysis operation with the current density increased to the critical current density il, and an appropriate current density is selected based on current efficiency, reaction time, and electrolyzer cost. As electrolysis progresses and the target component concentration becomes dilute, mass transfer becomes rate-limiting, so it is necessary to lower the current density. However, since this reduces the reaction rate, it is necessary to increase the electrode area S accordingly. Ideally, as mentioned above, S=
Although e= is an exponential function of ^t , the purpose can be sufficiently achieved even if it is changed stepwise as S = αt ₁ → αt ₂ → αt ₃ ...αt _o .

Note that the present invention is not limited to concentrated liquids, and can process from concentrated liquids to dilute liquids with high current efficiency and high reaction rate.

Hereinafter, specific operational aspects of the method of the present invention will be explained with reference to the accompanying drawings.

FIGS. 5A and 5B show an example of an electrolytic cell used in carrying out the method of the present invention, FIG. FIG.

The ones in Figure 5 A and B have metal electrode plates 11a and 1.
Four plates 1b, 11c, and 11d are housed in an electrolytic cell, and 12 ₁ , 12 ₂ , and 12 ₃ indicate plastic side plates. Electrode plates 11a, 11b, 11c, 11
d has areas Sa, Sb, Sc, and Sd that increase successively from the electrolyte inlet 13 toward the outlet 14, and is supported in the electrolytic cell by a plastic support plate 15 that also serves as an insulating plate. The electrolytic solution is designed to flow through a small hole 16 formed in the middle of the support plate 15.

The apparatus shown in FIGS. 5A and 5B has three electrolytic chambers, but the number of electrolytic chambers can be increased or decreased as required as shown in FIGS. 5A and 5B.

If current is applied from the outside of the electrode plates 11a to 11d in such a device, the anode and cathode current densities are respectively anode current density ia ia ₁ = I/Sa, ia ₂ = I/Sb, ia ₃ = I/Sc cathode The current density ic ic ₁ = I/Sb, ic ₂ = I/Sc, ic ₃ = I/Sd gradually decreases, but the total current is I, which corresponds to the concentration reduced between the electrode plates 11a and 11b. This means that the optimum current density can be maintained.

The changes in the areas Sa to Sd of the electrode plates 11a to 11d or the current densities ia ₁ to ia ₃ and ic ₁ to ic ₃ can be changed in any way by changing the geometrical structure of the electrolytic cell. can be selected arbitrarily depending on the characteristics of the electrolytic system.

6 and 7 show other examples of electrolytic cells, FIG. 6 being a conical electrolytic cell and FIG. 7 being a spherical electrolytic cell.

In FIG. 6, the radius of the first electrode ₁₁₁ is
r ₁ , the nth electrode 11n is the radius rn, and the electrode 1
If the distance between 1 ₁ and electrode 11n is ln, and the distance between the apex of this cone and electrode 11 ₁ is l ₁ , then rn: (l ₁ + ln) = r ₁ : l ₁ rn = r ₁ (l ₁ + ln) /l ₁ ...(11), and the area Sn of the electrode 11n can be expressed as Sn=[r ₁ (l ₁ +ln)/l ₁ ] ² π...(12), so the current density is becomes.

In addition, the one in FIG. 7 has a central hemispherical electrode 11 ₁
are constructed as concentric spheres, the surface area S ₁ of the central hemispherical electrode ₁₁₁ is S ₁ =2πr〓, and the surface area Sn of the n-th hemispherical electrode 11n is
is Sn=2πr〓, and the current density decreases in inverse proportion to the square of the distance rn from the center as in=1...n=I/2πr〓...(14).

In addition to the above, it is extremely easy to incorporate changes in the area of the electrode plate into various function systems, and these can be appropriately selected depending on the electrolytic system.

Examples of the present invention will be described below.

Examples In addition to mainly organic wastewater, some inorganic wastewater was also used, and experiments were conducted on four to five types of wastewater under the following experimental conditions.

(1) Experimental example 1 1 Electrode material; PbO ₂ / anode, SuS / cathode 2 Current density: changed from 10A/dm ² to 1A/dm ² 3 Wastewater treatment amount: 0.5 and 1 (2) Experimental example 2 1 Electrode Material: Ti-Pt/anode and C material/anode, SuS/cathode 2 Current density: 10A/dm ² →1A/dm ² and 5A/
dm ² → 1A/dm ² Wastewater treatment amount: 0.5 _The experimental results are shown in Figure 8 ^. As shown by the trend value of the solid line D, it is closer to the theoretical curve shown by the dotted line A' than the conventional method shown by the dotted line A, indicating that the method of lowering the current density at the end of the process is effective in obtaining a high yield. I understood.

Experimental example The following electrode reaction 2Cl ^- →Cl ₂ +2e is carried out using chlorine ions in salt water as the target reaction component, and a hypochlorous acid solution is extracted by the following reaction.

Cl ₂ +2NaOH→NaClO+NaCl+H ₂ O In that case, in order to concentrate and electrolyze the product concentration to 7000×10000ppm, in the conventional electrolytic method, as the concentration of hypochlorous acid produced increases, 6ClO ^- +3H ₂ O→2ClO ₃ ^- +6H ⁺ +4Cl ^- +3/2O ₂ +
6e; Discharge of ClO ^- ClO ^- +2H ⁺ +2e→Cl ^- +H ₂ O; Main side reactions such as reduction of ClO ^- occurred, resulting in a significant decrease in performance.

In order to improve these drawbacks, experiments were carried out according to the present invention under the following experimental conditions.

1 Electrode material; Ti-Pt, oxide electrode/anode, SuS
and Ti/cathode 2 Current density: 3 → 30 A/dm ² , 10 → 30 A/dm ² 3 Coulometric concentration: 2 to 20 AH Since the invented method approached the theoretical curve indicated by the dotted line B' and achieved high performance, it was confirmed that it is effective to increase the current density at the end of the treatment to increase the reaction rate of the electrode.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the principle of the conventional electrolytic method, and Figures 2 and 3 are examples of the basic electrode configuration of an electrolytic cell used in the conventional electrolytic method based on the principle of Figure 1. Figure 4 is a chart showing the results of decomposition and removal of treated components and the production of target substances by the conventional electrolytic method, Figures 5A and B, Figures 6 and 7 are diagrams showing the results of the method of the present invention. FIG. 8 is a diagram showing an example of an electrolytic cell used in this case, and a chart showing results obtained in an experimental example using the method of the present invention.

Claims (1)

[Claims] 1. An electrolytic treatment method characterized by changing electrolytic current density and electrode area in response to changes in the concentration of a target reaction component in a liquid to be treated due to progress of electrolysis.