JP2007260492A - Electrolyzing method of water - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To quickly recover chlorine generation function of an electrolyzing apparatus for preventing slime trouble or the like. <P>SOLUTION: The electrolyzing apparatus 32 for chlorine generation is connected to a cooling tower 1 via a pipe 31. The electrolyzing apparatus 32 generates chlorine with a first electrode serving as an anode, and a second electrode as a cathode. When lowering of the chlorine generation function is detected by an ORP or the like, polarity inversion is performed for 5 sec. to 10 min. The first electrode is preferably formed by attaching a noble metal like platinum on the surface of a titanium plate by plating or the like. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method of electrolytically treating water in order to prevent slime generation in a water system in which slime generation is a problem, such as a cooling water system, and in particular, electrolysis of water in which an electrolysis process and a reversal process are performed alternately. It relates to the processing method.

  Waste heat generated in compressors and refrigerators in factories and buildings is cooled with cooling water (cooling medium) through a heat exchanger. In the heat exchanger, the cooling water whose temperature has risen due to heat exchange with waste heat evaporates by contacting with air in the open cooling tower, is dissipated and cooled, and is circulated again to the heat exchanger. Therefore, in such a circulation type cooling water system, operation is performed by supplying supplementary water corresponding to the amount of water reduced by evaporation or scattering in the cooling tower.

  In the circulation type cooling water system, there are cases where nutrient components such as organic components and phosphorus are included in the makeup water component, and depending on the external environment, nutrient components may be mixed into the cooling tower. Cooling water temperature is around 30 ° C and is in an environment where slime and mold are easy to propagate. If slime treatment is not performed, heat exchange porosity decreases due to slime adhesion to the heat exchanger, water resistance increases, or microbial corrosion Cause various driving problems.

  Therefore, to operate so as not to cause slime damage in the system, it is necessary to add a bactericidal agent and maintain it at an appropriate concentration. As the disinfectant, a chlorine disinfectant such as sodium hypochlorite and an organic disinfectant are widely used. In order to prevent slime in cooling water, it is important to use a metering pump to control the concentration of the disinfectant so that microorganisms do not always grow. However, these disinfectants may have skin irritation and toxicity at the concentration of the stock solution, and there is a problem of worker safety in handling the chemicals.

  In recent years, in view of the environmental impact on aquatic organisms after being discharged into the environment, a sterilization treatment that does not use chemicals is required.

  On the other hand, a method is known in which chloride ions contained in cooling water are converted into a chlorine-based oxidizing agent such as hypochlorous acid by electrolytic oxidation, and this chlorine-based oxidizing agent remains in the cooling water. (Patent Documents 1 and 2 below).

That is, the tap water or industrial water used as make-up water for the cooling water system, usually several mg-Cl - ^/ L~ number 10mg-Cl - / ^L about chlorides since the ions are contained, circulating cooling water system In this cooling water, chloride ions in this makeup water are concentrated by a 6-8 times high concentration operation. For this reason, by subjecting this cooling water to electrolytic treatment, residual chlorine (free chlorine) having a slime prevention effect can be generated from chloride ions in the cooling water. By returning the electrolytically treated water containing residual chlorine to the cooling water system, slime failure can be prevented.

  In this electrolytic treatment apparatus for generating a chlorinated oxidant, a DC voltage is applied between an anode and a cathode using an external power source, and cooling water is passed between both electrodes. As a result, chloride ions in the cooling water are oxidized on the surface of the anode, and residual chlorine having strong oxidizing power such as hypochlorous acid is generated. The generated residual chlorine sterilizes microorganisms that cause slime, or suppresses growth, so that generation of slime in the circulating cooling water system can be effectively prevented.

  When the above electrolytic treatment is performed, scales such as calcium carbonate gradually adhere to the cathode. In paragraph 0006 of Patent Document 1 described below, it is described that a polarity reversal process in which an applied voltage of an electrode is reversed periodically or arbitrarily is performed, and a scale attached during the polarity reversal process is removed.

In paragraph 0006 of Patent Document 1, as an electrode component, as an anode, for example, a corrosion-resistant material such as titanium is coated with a single element of platinum-based element such as platinum or iridium or / and its oxide. It is described that a material having good hypochlorous acid production efficiency can be suitably used, and stainless steel, aluminum, silver or the like can be used as the cathode.
JP 2001-62457 A JP 2003-269889

  When the water to be treated is passed through an electrolysis apparatus having an anode (anode) having at least a surface made of a noble metal, the chlorine generating ability gradually decreases with time. This is mainly due to the fact that an extremely thin oxide film is formed on the surface of the anode and the oxidation potential of chloride ions gradually increases. When used in a fresh water system having a low chloride ion concentration, the transport number of chloride ions (the ratio of chloride ions in the total electrical conductivity) becomes low, and the efficiency of hypochlorous acid generation becomes very poor. Because of this low generation efficiency, the reaction at the anode is mostly oxygen generation. An extremely thin oxide film is formed on the electrode surface widely used for the chlorine generating electrode, and the chlorine generating ability deteriorates with time. Oxide film can be reduced and removed by polarity conversion for the purpose of conventional scale removal, but repeated polarity conversion gradually deposits scales such as calcium carbonate on the electrode surface, reducing the effective electrode area, resulting in chlorine generation efficiency. Leading to a decline. An object of the present invention is to provide a water electrolytic treatment method capable of quickly recovering the chlorine generating ability during electrolysis.

  The method for electrolytic treatment of water according to claim 1 is characterized in that water to be treated containing chloride ions is passed through an electrolysis apparatus having at least a pair of a first electrode and a second electrode whose surfaces are made of a noble metal. Electrolysis is performed by applying a current so that the electrode becomes an anode and the second electrode becomes a cathode, and generating chlorine by electrolysis, and reversing the voltage application to the first electrode and the second electrode. The electrolytic treatment method includes a reversal step, and alternately repeats the chlorine generation step and the reversal step, and the time of the reversal step is 5 seconds to 10 minutes.

  The method for electrolytic treatment of water according to claim 2 is characterized in that, in claim 1, the electric potential in the inversion step is a negative electric potential more than 0.2 V (VS SHE, where SHE is a standard hydrogen electrode). It is what.

  The method for electrolytic treatment of water according to claim 3 is characterized in that, in claim 1 or 2, the chloride ion concentration of the water to be treated is 20 to 25000 mg / L.

  According to a fourth aspect of the present invention, there is provided a water electrolytic treatment method according to any one of the first to third aspects, wherein the Langeria index of water to be treated is 0 to 3.0.

  The method for electrolytic treatment of water according to claim 5 is that in any one of claims 1 to 4, when the chlorine generation rate is reduced to 90% or less of the initial value in the chlorine generation step, the process proceeds to the inversion step. It is characterized by.

  The method for electrolytic treatment of water according to claim 6 is the method according to claim 5, wherein the decrease in the chlorine generation rate is detected by measuring the oxidation-reduction potential (ORP) or residual chlorine concentration in the influent and effluent of the electrolyzer. It is characterized by.

  According to a seventh aspect of the present invention, there is provided a method for electrolytic treatment of water according to any one of the first to sixth aspects, wherein the second electrode is mirror-finished.

  The method for electrolytic treatment of water according to claim 8 is characterized in that, in any one of claims 1 to 7, the second electrode is made of a porous body having a porosity of 30 to 97%. It is.

  The method for electrolytic treatment of water according to claim 9 is characterized in that, in any one of claims 1 to 8, the method includes a cleaning step of periodically cleaning or removing the scale attached to the second electrode. is there.

  The method for electrolytic treatment of water according to claim 10 is characterized in that, in any one of claims 1 to 9, the water to be treated contains a scale inhibitor.

  When electrolytic treatment is performed by passing water to be treated through an electrolysis apparatus having a first electrode as an anode made of a noble metal such as platinum or iridium, a thin oxide film is formed on the anode surface as described above. Chlorine generation capacity gradually decreases. In the present invention, the oxide film is reduced and removed by performing inversion intermittently, and the chlorine generating ability of the first electrode is recovered. In addition, since the scale begins to adhere to the first electrode from which the oxide film has been removed if the time of the reversal process is excessively long, the time of the reversal process is a range in which the scale does not begin to adhere to the first electrode. That is, within 10 minutes.

  In addition, the second electrode serves as an anode in the inversion step, and may corrode due to an anodization phenomenon in the inversion step. However, in the present invention, the inversion time is set to an extremely short time of 10 minutes or less. Therefore, the second electrode also has an effect that it does not corrode.

  In the present invention, in the process of reducing and removing the oxide film on the surface of the noble metal such as platinum, the oxide film is reduced by adsorption of hydrogen on the surface of the noble metal. Therefore, in the present invention, the potential of the first electrode in the inversion step is set to a negative potential of more than 0.2 V with respect to SHE (electrode potential of the standard hydrogen electrode), and preferably the potential of the first electrode with respect to SHE is set to be lower. Set the hydrogen generation potential or higher.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a system diagram of an open circulating cooling water system showing an embodiment of the method of the present invention.

In this open circulation cooling water system, the cooling water is supplied from the cooling tower 1 to the heat exchanger 3 through the circulation pipe 2 having the pump P ₁ , and the return water is returned to the cooling tower 1 through the pipe 4. 5 is a supply water introduction pipe, and 6 is a blow pipe.

  The cooling tower 1 is provided with an electric conductivity meter 20 for detecting the electric conductivity of the cooling water, and the detected value of the electric conductivity meter 20 is input to the concentration management device 21, The valve 5a of the makeup water pipe 5 is controlled so that the electric conductivity falls within the specified range. As shown in Japanese Patent Application No. 2005-338995, the blow piping is overflow type.

In addition, the water in the cooling tower 1 is circulated through the pipe 31 having the pump P ₂ and the electrolytic treatment apparatus 32. The electrolytic treatment apparatus 32 has a first electrode and a second electrode facing each other in an electrolytic cell, and a voltage is applied between the electrodes to generate a chlorine-based oxidant from chloride ions in water.

  The cooling tower 1 is provided with an ORP meter 30 for measuring the ORP (oxidation-reduction potential) of the cooling water, and the electrolytic treatment apparatus 32 is configured so that the detected ORP of the ORP meter 30 falls within a predetermined range. The applied voltage between the electrodes is controlled.

  The electrolytic treatment apparatus 32 is provided with a voltage measurement system for measuring the electrolytic voltage. When scale or slime adheres to the electrodes of the electrolytic treatment apparatus 32, the voltage applied between the electrodes increases in order to pass a predetermined current. Therefore, the state of the electrolytic treatment apparatus 32 can be monitored by measuring the applied voltage. it can.

  The electrolytic processing apparatus 32 is a casing in which a first electrode and a second electrode are arranged facing each other in parallel at an interval of about 2 to 20 mm. The number of each first electrode and second electrode is arbitrary.

  The first electrode that becomes the anode in the chlorine generation step is preferably a plate in which a catalyst mainly composed of platinum and iridium is plated or baked on a titanium plate.

  In the present invention, it is preferable to mirror-finish the second electrode because scale adhesion to the surface of the second electrode can be suppressed. Since the scale adheres to an active point called a kink, the scale adherence is suppressed by making the electrode surface a mirror surface and substantially eliminating the kink.

  The material of the second electrode that becomes the cathode in the chlorine generation step is arbitrary, but since the second electrode becomes the anode in the inversion step, it is preferable to have corrosion resistance, and in particular, austenitic stainless steel is preferable. .

  Note that the second electrode may be a porous body having a porosity of 30% or more. The scale adheres to the second electrode, which becomes the cathode in the chlorine generation process, in the chlorine generation process. However, if the second electrode is a porous body having a porosity of 30% or more, the scale adheres because the surface area is large. However, the increase in current density due to the decrease in effective electrode area is slow. For this reason, the occurrence of a voltage rise trouble due to scale adhesion to the cathode is suppressed.

  In addition, since the intensity | strength of a 2nd electrode will become low when the porosity of a 2nd electrode becomes higher than 97%, it is preferable that the porosity of a 2nd electrode is 30 to 97%. More preferably, it is 60-95%, More preferably, it is 80-90%.

In the chlorine generation step, water is passed through the first electrode as an anode (anode) and the second electrode as a cathode (cathode). As a result, chloride ions are oxidized on the surface of the anode and residual chlorine (free chlorine) is generated. The flow velocity (linear velocity) in the space between the first electrode and the second electrode in this chlorine generation step is preferably about 0.01 to 0.1 m / sec. Current density at the chlorine generating step is suitably 0.5~2.0A / dm ^2.

  When this chlorine generating step is performed, an oxide film is gradually formed on the anode surface, the applied voltage for generating chlorine increases, and the chlorine generating ability decreases. Therefore, the electrodes are intermittently reversed and energized so that the first electrode is a cathode and the second electrode is an anode.

  Since the first electrode having the noble metal provided on the surface of the titanium plate as described above undergoes slight degradation (for example, peeling of the surface noble metal such as platinum) every time the polarity is reversed, the polarity inversion is as low as possible. And That is, it is preferable to take a long time for one chlorine generation step. Specifically, one chlorine generation step is preferably 30 to 120 minutes, particularly preferably about 60 to 90 minutes. If the inversion step is performed for 5 seconds or more, particularly 15 seconds or more, the chlorine generating ability is almost fully recovered.

  The reversal process is less than 1/10 of the chlorine generation process. If the time for the inversion step is excessively long, the second electrode serving as the anode in the inversion step will corrode, and therefore the one inversion step should be 10 minutes or less, particularly 5 minutes or less. preferable. In addition, since the scale may adhere to the surface of the first electrode serving as the cathode during this reversal process, the shorter reversal process is better in this sense. In the present invention, it is preferable to end the inversion step before the scale starts to adhere to the first electrode during the inversion step.

  In general, hydrogen is generated and becomes alkaline in the vicinity of the cathode in the water passing space between the anode and the cathode. For this reason, bicarbonate ions dissociate into carbonate ions in the vicinity of the cathode, and calcium carbonate and magnesium carbonate are generated from Ca ions and Mg ions, and these adhere to the electrode surface as a scale.

  The Langerian index (hereinafter referred to as LSI), which is an index for measuring the corrosion / scale tendency of a solution, shows a tendency of scale precipitation as the value becomes positive, and a tendency toward corrosion as the value becomes negative. Also not shown. By removing both the hardness component and bicarbonate ion and lowering the pH, the LSI is managed so as to have a value of about 0 to 3.0, particularly about 0.5 to 1.0, and a concentration close to supersaturation at which scale does not precipitate. By leaving the scale component at, it is possible to reduce the corrosion rate of the piping in the system and the heat exchanger.

The calcium hardness in the circulating water is preferably operated at 80 to 120 mg CaCO ₃ / L which does not cause the scale to deposit on the heat exchange section of the cooling water system, the piping, the filler of the cooling tower, and the M alkalinity is 80 to 120 mg CaCO ₃ / L. L is preferable.

  It is preferable that the chloride ion concentration of to-be-processed water is about 20-25000 mg / L.

  The applied voltage in the inversion step is preferably more negative than 0.2 V with respect to SHE (electrode potential of the standard hydrogen electrode). In addition, since there exists a possibility that a 2nd electrode may corrode when this applied voltage is too high, it is preferable that the reduction voltage of a reversal process shall be 0-0.2V (VS SHE). In addition, when this applied voltage is about 0 V (VS SHE), it is preferable that the inversion step is 5 minutes or less.

  If the scale adheres to the electrode during the above chlorine generation or reversal process, remove the scale by spraying a jet water stream on the electrode, scraping it off with a scraper, or washing with an acid. .

  In addition, when the scale inhibitor is contained in the inflow water of the electrolyzer, scale adhesion to the electrode is suppressed.

  If the scale adheres to such an extent that it cannot be removed, the electrode may be replaced.

  About the timing which performs said inversion, it is preferable to monitor the chlorine generation amount by an electrolyzer, and to switch from a chlorine generation process to a inversion process when this chlorine generation amount becomes below setting. In order to monitor the amount of chlorine generated by the electrolyzer, it is preferable to measure the redox potential (ORP) or residual chlorine concentration, particularly ORP, of the inflow water flowing into the electrolyzer and the effluent water flowing out of the electrolyzer. When the chlorine generation capacity detected from the ORP or the like falls below a specific value of 90% or less of the initial value (for example, selected from the range of 25 to 80%), the chlorine generation process is switched to the reversal. Switch to the process.

Hereinafter, examples and comparative examples will be described. The conditions of this example and the comparative example are as follows.
Constant current: current density 1.6 A / dm ²
Electrode area 0.5 dm ²
LV: 0.03m / sec
Water temperature 30 ° C
10mm distance between electrodes
Electrical conductivity 120mS / m
Chloride ion concentration: 180mg / L

  In Example 1, the anode was a Pt—Ir-coated titanium electrode, the cathode was a porous SUS electrode, and an operation of alternately repeating an electrolysis process of 60 minutes and a reversal process of 30 seconds was performed.

  In Comparative Example 1, both the anode and the cathode were made of Pt—Ir-coated titanium electrodes, and an operation of alternately repeating the electrolysis process for 60 minutes and the inversion process for 60 minutes was performed.

The results are shown in FIG.
As shown in FIG. 2, in Example 1, the performance is maintained even after 270 hours.

  On the other hand, in Comparative Example 1, the recovery of chlorine generation ability at the initial stage of electrolysis exceeds that of Example 1, but the reversal time is long (60 minutes). The performance drops. In the present invention, since the inversion time (30 seconds) is short, the above problem does not occur.

It is a systematic diagram of a cooling water system to which the electrolytic treatment method according to the embodiment is applied. It is a graph which shows the result of an Example and a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cooling tower 3 Heat exchanger 7 Metal ion generator 20 Electrical conductivity meter 21 Concentration management device 30 ORP meter 32 Electrolytic processing device

Claims (10)

Water to be treated containing chloride ions is passed through an electrolysis apparatus having at least a pair of a first electrode and a second electrode whose surfaces are made of a noble metal, and the first electrode serves as an anode and the second electrode serves as a cathode. A chlorine generating step for generating chlorine by energizing and electrolyzing,
A reversing step of performing electrolysis by reversing the voltage application to the first electrode and the second electrode,
An electrolytic treatment method that alternately repeats a chlorine generation step and a reversal step,
The method for electrolytic treatment of water, wherein the time of the reversal step is 5 seconds to 10 minutes.   2. The method for electrolytic treatment of water according to claim 1, wherein the potential at the inversion step is more negative than 0.2 V (VS SHE; SHE is a standard hydrogen electrode).   3. The method for electrolytic treatment of water according to claim 1, wherein a chloride ion concentration of the water to be treated is 20 to 25000 mg / L.   4. The method for electrolytic treatment of water according to any one of claims 1 to 3, wherein the Langerian index of water to be treated is 0 to 3.0.   5. The method for electrolytic treatment of water according to claim 1, wherein when the chlorine generation rate is reduced to 90% or less of the initial value in the chlorine generation step, the process proceeds to a reversal step.   6. The method for electrolytic treatment of water according to claim 5, wherein a decrease in the chlorine generation rate is detected by measuring an oxidation-reduction potential (ORP) or residual chlorine concentration in the influent and effluent of the electrolyzer.   7. The water electrolysis method according to claim 1, wherein the second electrode is mirror-finished.   8. The method for electrolytic treatment of water according to claim 1, wherein the second electrode is made of a porous body having a porosity of 30 to 97%.   9. The method for electrolytic treatment of water according to claim 1, further comprising a cleaning step of periodically cleaning or removing the scale attached to the second electrode.   The method for electrolytic treatment of water according to any one of claims 1 to 9, wherein the water to be treated contains a scale inhibitor.

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