JPS6353284A - Electrolysis method - Google Patents

Abstract

PURPOSE:To continuously and stably carry out electrolytic operation for a long time by passing a DC pulse current or a DC rectangular pulse current at the specified time in electrolysis to prevent the deposition of inorg. matter on both electrodes when seawater, etc., are electrolyzed. CONSTITUTION:Seawater 12 is supplied from a supply pipe 9 into an electrolytic cell 10 by a pump P, and an electric current is passed between an anode 13 of a Pt sheet and a cathode 14 made of stainless steel to produce NaOCl. In this case, inorg. matter such as Mg(OH)2 is deposited on the cathode surface due to the Mg ion in seawater, and inorg. matter such as FeOOH is disposed on the anode surface. The electrical resistance between both electrodes 13 and 14 is increased because of the deposition of inorg. matter, hence the electrolytic reaction of seawater is inactivated, and the product NaOCl cannot be stably produced. In this case, a DC pulse current or a DC rectangular pulse current is passed between both electrodes for a time shorter than the transition duration for the deposition of the inorg. matter on the electrode surfaces to remove the deposited inorg. matter, and the production of NaOCl by the electrolysis of seawater is stably carried out for a long period.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrolysis method that successfully prevents the deposition of inorganic substances on electrodes.

Inorganic substances here include, but are not particularly limited to,
To give a representative example and explain it, magnesium hydroxide scale E, which is simply Mg
(OH)2], and scales of iron oxyhydroxide and triiron tetroxide that precipitate on the anode as a result of the reaction between eluted Fa ions and dissolved oxygen in the electrolyte during electrolysis using Fe anodes [ Hereinafter, simply Fe0OH, Fe, 04
Therefore, the following explanation will focus on scale prevention technology in these cases, but the type of inorganic substance is not specified as mentioned above, and the word scale does not refer to the above. It should be understood that the illustrative examples have a wide range of meanings.

[Prior Art] Today, electrolysis is used in all fields and its importance is increasing.

As a technology that applies such electrolysis, there is a technology for producing sodium hypochlorite from seawater. As shown in FIG. 2 (principle diagram), this technology involves adding, for example, platinum to seawater 2 contained in an electrolytic cell 1 and anode 3. Direct current is passed through stainless steel as the cathode 4, and the following electrode reactions (1) and (2) occurring at the anode 3 and cathode 4 are utilized.

Positive 8i: 2 (1!--Cl3.+2e--...-
(1) Cathode: 2Na+2H20+2e--*)(2+
2NaOH (2) The following technique is also known in which Fe ions are eluted into the electrolyte and used for anticorrosion treatment. That is, the third
In the figure, reference numeral 6 denotes a condensate pipe, and the electrolyte is flowed on the inner circumferential surface of the condensate pipe 6 as shown by arrow a. By the way, an electrode (the anode 7 is made of Fe,
When a cathode 8 is made of stainless steel) and a current is passed through it, Fe ions are eluted from the positive Vi7 (Fe-F
e”+ 2 e-, and for cathode 8 2) 120+2e-
-*H2+20H- reaction occurs). The electrolyte solution contains various concentrations of dissolved oxygen depending on its properties and composition, so the dissolved oxygen and the Fe ions react to form Fe0OH, Fe3O4, etc. It is deposited on the inner surface of the condensate pipe 6 and anti-corrosion treatment is performed.

[Problems to be Solved by the Invention] The following problems have been pointed out in the above technology for producing sodium hypochlorite from seawater. That is, QH- ions are constantly generated at the cathode by electrode reaction (2), and these OH- ions react with Mg2'' ions in seawater to become Mg (OH) 2 and precipitate on the cathode. If Mg (OH) 2 is precipitated in this way, the current flow resistance between the electrode plates will increase, which will hinder the stabilization of sodium hypochlorite production.Also, the precipitation will become more intense and the space between the electrode plates will be blocked. This is something I often experience.

On the other hand, regarding the above technology for eluting Fe ions.

In this case, Mg(OH)2 is precipitated at the cathode in the same way as above, and Fe0OH etc. are precipitated little by little on the anode plate, causing the same problem as in the case of Mg(OH)2 precipitation.

The present invention has been made in consideration of these circumstances, and is designed to prevent the deposition of the inorganic substance on the electrode, and to
This is intended to provide an electrolysis method that can be stably operated continuously over a long period of time.

[Means for Solving the Problems] The electrolysis method according to the present invention is such that when performing electrolysis by inserting an anode and a cathode into an electrolyte, the transition time for the deposition of an inorganic substance on the electrode surface is shorter than that of the inorganic substance on the electrode surface. The gist is that electrolysis is carried out by passing a DC pulse current or a DC rectangular wave current that includes a current flow time.

[Function] The present inventors have developed a means for preventing the deposition of inorganic substances on the electrode plate, including electrode materials and current patterns.

We have been conducting investigations from various viewpoints such as current density and concentration of the electrolyte, and in the process we came up with the idea that the above precipitation phenomenon could be prevented by devising the current pattern. Therefore, we conducted a study to make the above current pattern more concrete, and found that it is extremely effective to use DC pulse current or DC square wave current (hereinafter referred to collectively as DC pulse current, etc.). I learned that. Since such current has a non-current period, the formation of OH- and Fe''' is suppressed, and the ions formed during the current period are washed away with the running water, leaving room for them to precipitate as inorganic substances. It is expected that it will disappear.

However, after repeated electrolysis experiments using various DC pulse currents, etc., it was found that the above effect could not be stably obtained no matter what DC pulse current was used, and it was found that the method of applying the DC pulse current was not stable. It was found that there were large differences in the degree of effectiveness depending on the factors. Therefore, the present inventors felt the need to study the precipitation of inorganic substances in more detail in relation to the form of DC pulse current and the like.

First, the present inventors considered the process in which the above-mentioned precipitation occurs by dividing it into the following stages.

■Process in which ions that trigger precipitation are formed by electrode reaction; ■Process in which these ions react with ions in the electrolyte and deposit on the electrode; ■By repeating the above steps ■ and ■, the amount of precipitation gradually increases. If the increasing process is analyzed in this way, if the above-mentioned items (1) to (3) are to proceed sufficiently, the current must be passed continuously for a certain period of time or more. That is, the current must be continuously applied for the time required for the formed ions to react with dissolved oxygen and other ions in the water to be treated and to precipitate (hereinafter referred to as precipitation transition time). If the continuous current application time is less than the above-mentioned precipitation transition time, the progress of the above-mentioned reaction (2) will be inhibited and ions will be flowed out, so that no inorganic substance should be deposited.

Therefore, the present inventors came up with the present invention, which uses a DC pulse current or a DC rectangular wave current having a current conduction time shorter than this precipitation transition time as its wave form.

The present invention is configured as described above, and is not subject to any limitations as long as the configuration is satisfied in which a DC pulse current or a DC rectangular current is passed that includes a current application time shorter than the transition time for depositing an inorganic substance on the electrode surface. For example, the following configuration may also be adopted.

(1) Regarding inorganic substances and precipitation transition time: As inorganic substances that can be considered to be deposited on the electrode, the above-mentioned Mg (OH
)2. In addition to Feoo) (, Fe3O4, CaCO3 is mentioned.

On the other hand, the precipitation transient time is an amount that is influenced by many factors, such as the type of the inorganic substance, the electrolytically generated ions constituting the inorganic substance, the concentration of the relevant ions in the electrolyte, water temperature, and stirring conditions. An example of this is as follows.

Mg(OH)z: on the order of seconds or less CaCO3 and iron scale: on the order of seconds to minutes (however, these vary depending on the conditions) (2) When other substances are added to the electrolyte Regarding addition: If you add an acid to make pt (less than 7) or add a scale inhibitor, inorganic substances [Mg(OH), Fe0
The precipitation of OH, Fe, 04, etc. can be more reliably prevented.

Here, the above acids include H2S04 and HCfl.

As the sodium sulfamic acid monobisulfite and the scale inhibitor, condensed phosphoric acid-based or amine-based compounds such as sodium hexametaphosphate and Flocon 100 (trade name) are used.

The acid and scale inhibitor described above are thought to have the effect of neutralizing OH- formed by the electrolytic reaction and preventing the growth of scale nuclei formed by the electrolytic reaction. These effects can be made more reliable by providing an energization suspension period.

(3) Number of occurrences of DC pulse current or DC square wave current,
Although there is no particular restriction on the ratio between the energization time and the rest time, it is recommended to select a desired value with consideration to the following points (a) to (d).

(a) If the number of generation times is too large, the energization pause time will be shortened, making it impossible to secure enough time for the ions generated at the electrode to flow out, and achieving the desired generation efficiency of electrolytically generated ions. decreases.

(b) If the number of occurrences is too small, the continuous current application time becomes long and the concentration of electrolytically generated ions on the electrode becomes high.

(c) If the ratio is too large, the energization stop time becomes short, resulting in the same problem as in (a) above.

(d) If the ratio is too small, the current application time per unit time will be shortened, and the current value will have to be increased to obtain the same electrolytic effect. The current efficiency of electrolytically generated ions becomes low.

From this point of view, the present inventors have defined the respective values, and examples thereof are as follows. The number of occurrences is preferably 60 times/second to 10 times/minute, more preferably 10 times/second to 10 times/minute. The above ratio is preferably 0.5:1 to 10:1, and 1:1 to 10:1.
3:1 is more preferred.

(4) Stirring the electrode: If the electrode is sufficiently stirred, the ions formed by the reaction in ① above can be diffused from around the electrode to the entire area of the electrolyte. Precipitation of inorganic substances can be prevented.

However, the above-mentioned stirring actually has a certain limit, and ions generated near the electrode cannot necessarily be sufficiently diffused (boundary film part). Although the significance of the present invention lies in these points, in view of the above-mentioned stirring, the effects of the present invention can be made even more remarkable by adding the above-mentioned stirring operation to the structure of the present invention.

Regarding the production of electrode reaction products (e.g., sodium hypochlorite), there is no problem because it is possible to obtain approximately the same amount of product per cycle of pulsed current (or square wave current) as with continuous energization. There isn't.

The present invention will be specifically explained below by giving examples together with comparative examples, but the present invention is not limited to these examples and may be modified as appropriate in keeping with the spirit of the preceding and following descriptions. Can be done.

[Example] Comparative Example 1 As shown in FIG. 4, seawater as the liquid to be treated is supplied to the electrolyzer 4Q10 via the electrolytic cell water supply pipe 9 by the electrolytic cell water supply pump P, and on the other hand, the seawater to be treated is supplied through the electrolytic treatment water pipe 11. 1
Derive 2. Therefore, the electrolytic cell 10 is constantly filled with fresh seawater 1.
This means that it has been replaced by 2. By the way, 13 is an anode made of platinized titanium, and 14 is stainless steel (
The cathode was made of SUS304), and sodium hypochlorite was generated by continuously supplying current from a DC power source 15 between these two electrodes. The distance between the two poles is 10m
m, current density is 40 mA/cm2, seawater flow rate is 1.6
At 117 minutes, the generated effective chlorine concentration was 10 tag/Jl.

As a result of continuing this energization for two months, the electrolytic voltage was initially 5.
．． It was found that the voltage rose from 0■ to 12.5V. Yin f again! 14 is a white Mg(OH) with a thickness of about 3 mm.
) was coated with 2 scales.

Example 1 FIG. 1(a) is a schematic explanatory diagram of an apparatus for carrying out the method of the present invention, and FIG. 1(b) is a diagram showing the mode of current flowing through the apparatus.

In Comparative Example 1 above, the current was passed continuously, but instead, a DC rectangular wave current (the ratio of the energization time to the energization stop time was 3:1) as shown in Figure 1(b) was passed. . The DC rectangular wave current value (o-b) was set using the DC rectangular wave current generator 16 shown in FIG. controlled.

After continuing in this way for about two months, the effective chlorine concentration in the electrolyzed water was the same value as in Comparative Example 1, that is, 10 mg.
/fl, and the electrolytic voltage (normally medium) is 5.0μ at the initial stage.
Even after 2 months, the amount of Mg(oH)z scale deposited on the cathode had decreased and its thickness was less than 0.5 mm. Ta.

FIG. 5 is a flow diagram showing another embodiment of the method of the present invention. In this embodiment, H230 is supplied via the electrolytic tank water supply pipe 9.
4 is added, and the pH of the seawater in the electrolytic cell 10 is adjusted to 6.5. Other than that, the same method as in Example 1 was adopted.

Effective chlorine concentration is approximately 10mg/J! The electrolytic voltage (during energization) hardly changes from the initial 5.0 μ even after two months, and the Mg (OH)2 at the cathode
Almost no precipitation was observed.

Further, the electrolysis method of the present invention can be connected to a system including a reverse osmosis seawater desalination device 17 as shown in FIG. That is, a thread path 18 that supplies seawater to the reverse osmosis seawater desalination device 17
H2so4 is introduced into the cell, thereby setting the pH to 6.5, and then branching to the electrolytic cell 1o. In the electrolytic cell vi10, electrolysis similar to that shown in FIG.
2So4 before introduction). The safety filter 120 is a concentrated water discharge pipe, 21 is a pressure reducing valve, and 22 is a desalinated water discharge pipe.

The same effect as shown in FIG. 5 was also obtained in the electrolytic cell 10. By connecting to the reverse osmosis seawater desalination device 17 in this way, sterilization treatment before introduction into the device 17 can be performed continuously and reliably.

Embodiment 3 FIG. 7 is a flow diagram showing another embodiment of the method of the present invention. In this embodiment as well, the electrolytic cell 10 is connected to a system including a reverse osmosis seawater desalination device 17 as in FIG. 6 above. However, the liquid to be treated that is led to the electrolytic cell 10 is
This is not salt water before concentration as in the case of the figure, but salt water after concentration supplied from the reverse osmosis seawater desalination device 17h). The electrolysis conditions were the same as those shown in FIG. 6 above, but the concentration of the feed water (?IA shrinkage brine) was 1.6 times higher than in the case shown in FIG. . Therefore, the effective chlorine concentration in the electrolytically treated water was 11 mg/u (current efficiency increased from 78% to 84%). However, despite this, almost no Mg(OH)2 precipitation was observed on the cathode after two months had passed.

Comparative Example 2 Dough water containing about 3,000 mg/jZ of salt was used as the treatment liquid supplied to the electrolytic cell 10 shown in FIG.
Electrolysis was carried out under the same electrolysis conditions as in Comparative Example 1, using iron for the electrode and stainless steel (SUS 304) for the cathode. The total iron concentration at this time was 6 mg/It.

Dissolved oxygen was 8 mg/λ.

After continuous energization for one week in this way, a mixed scale 23 of Fe0OH and Fe=04 and a mixed scale 24 of Mg(OH)2 and CaCO3 were formed on the positive g113 as shown in FIG. The space between the boards was almost completely occluded.

Ability d Tsuji A The same procedure as Comparative Example 2 was used except that the device shown in FIG. 1(a) was used and the rectangular wave current as shown in FIG. 1(b) was passed from the DC rectangular wave current generator 16 of the device. Electrolysis was performed under electrolytic conditions.

The amount of iron scale deposited on the anode after one week is:
The adhesion thickness is 0.5, which is drastically reduced compared to the case of Comparative Example 2 above.
It was below am. Even after about two weeks, when the anode became thinner and it became necessary to replace the electrode, the above-mentioned electrode blockage was not observed, and the voltage remained at the initial level of 3.2mm.
Only a slight increase was observed from 3.3■ to 3.3■.

[Effects of the Invention] Since the present invention is configured as described above, it is possible to prevent the precipitation of inorganic substances on the electrode plates, and thereby to stably perform long-term continuous operation. It was possible to provide an electrolysis method that can increase the production efficiency for electrolysis purposes.

[Brief explanation of drawings]

Figure 1(a) is a schematic explanatory diagram of an apparatus for carrying out the method of the present invention, Figure 1(b) is a diagram showing the current pattern flowing through the apparatus, Figure 2 is a diagram of the principle of electrolysis, and Figure 3. is an explanatory diagram showing a state in which the inner peripheral surface of the condensate pipe is coated with iron oxide, Fig. 4 is an explanatory diagram showing a state in which electrolysis is performed while flowing the liquid to be treated, and Figs. 5 to 7 8 is a diagram showing an example of the method of the present invention, and FIG. 8 is an explanatory diagram showing a state of electrode closure in the case of iron ion elution. 1 and 10...electrolytic cell

Claims (1)

[Claims] When performing electrolysis by inserting an anode and a cathode into the electrolyte, conduct the electrolysis by passing a DC pulse current or a DC square wave current with a current application time shorter than the transition time for the deposition of inorganic substances on the electrode surface. An electrolysis method characterized by