JPS63520B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for pickling hard components such as Mg(OH) ₂ and CaCO ₃ deposited on electrode parts during seawater or salt water electrolysis.

Facilities that come into contact with seawater, such as condensers and cooling systems for ships and land-based equipment, suffer great damage from the adhesion of marine organisms. For example, ships use large amounts of seawater for engine cooling and sanitary purposes, but these seawater systems often experience problems such as channel blockage due to marine organisms and abnormal corrosion of the metal materials used due to uneven seawater flow. This has a considerable impact on ship operation and safety.

Therefore, in order to prevent marine organisms from adhering to the seawater, a system is currently being adopted in which a portion of the cooling seawater is electrolyzed to produce chlorine, which has a sterilizing effect, and this is injected into the seawater intake, which is highly effective. ing.

However, in order to use seawater as an electrolyte, hard components in seawater precipitate on the cathode part by the following reaction, Mg ²⁺ +2OH ^- →Mg(OH) ₂ ↓ Ca ²⁺ +HCO ₃ ^- +OH ^- →CaCO ₃ ↓ +H ₂ O Blockage between the anode and cathode may occur, making electrolysis impossible. For this reason, magnesium hydroxide is
It is necessary to remove precipitates such as Mg(OH) ₂ and calcium carbonate CaCO ₃ , and an acid solution cleaning method is used.

In this acid solution cleaning method, electricity is stopped, the electrolyte is drained, and then an acid solution (usually HCl solution) is added, and the precipitates are dissolved and removed by the same acid solution.

Conventionally, platinum-plated electrodes have been used as current-carrying anodes for electrolysis, including seawater electrolysis, and titanium and titanium electrodes have been used as cathodes.
Stainless steel was mainly used. The electrode that generates chlorine through seawater electrolysis is the anode, and the corrosion resistance and lifespan of the anode material are important.

The platinum electrode has extremely excellent corrosion resistance, and there were no problems such as damage to the electrode during pickling, and the hardness components could be sufficiently removed by pickling.

However, platinum electrodes have the drawbacks of low chlorine generation efficiency of around 70% and high power consumption (KWH/Kgcl ₂ ) required to generate 1 kg of chlorine. Electrodes supported by sintering on materials have become popular.

This noble metal composite oxide electrode has a current efficiency of about 20% higher than that of a platinum electrode and consumes less power, so it is currently the mainstream electrode for seawater electrolysis in place of the platinum electrode.

Even if a high-performance noble metal composite oxide electrode is used as an anode, as long as seawater is used as the electrolyte, hardness components will precipitate, and it is necessary to remove the precipitates by pickling, as in the case of using platinum electrodes. However, in the case of a noble metal composite oxide electrode, the appearance during pickling is different from that of a platinum electrode. That is, oxide electrodes have extremely effective corrosion resistance when in the oxidized state, that is, when acting as an anode, but have the disadvantage that when they are in the reduced state, that is, acting as a cathode, they can be damaged by subsequent regular energization.

As mentioned above, in pickling, electricity is stopped, an acid solution is added in place of the electrolyte, and the acid solution is used to remove the electrolyte, so there is a potential difference between the anode (oxide electrode) and the cathode (titanium, stainless steel, etc.). occurs, and if the anode and cathode are short-circuited, a battery will be formed.

The direction of current flow during seawater electrolysis and pickling in the case of various energization methods will be explained below with reference to FIGS. 1 to 6.

Figure 1 shows the case during electrolysis of a single anode and cathode, where 1a is an electrolytic cell body, 2a is an electrolytic solution (seawater) taken into the electrolytic cell body 1a, a DC power source 5a, and a current-carrying line 6a.
Therefore, in the electrolyte 2a, a current i flows from the anode 3a (noble metal composite oxide) to the cathode 4a (titanium, stainless steel, etc.).

Figure 2 shows a case where several sets of single electrolytic cells are arranged in series and the electrolyte is passed through the series, 11a is the electrolytic cell body, 12a is the electrolyte (seawater), and the cell inlet 1
It enters the electrolytic cell body 11a from 2'a, exits from the cell outlet 12''a, and is discharged or circulated through each cell body.

Reference numerals 13a and 14a are an anode and a cathode respectively disposed in each tank body 11a, and are short-circuited by a current-carrying wire 16a. In this case, when the DC power supply 15a is activated, the current i enters the cathode 14a from the anode 13a via the electrolyte 12a, and the current that entered the cathode 14a of the first tank passes through the current carrying wire 16a and enters the cathode 14a.
The current flows to the anode of the tank and returns to the DC current 15a in the same way.

Figure 3 shows an energization system called bipolar (plural) type, where 101a is an electrolytic cell body, 102a
The electrolytic solution (seawater) enters the tank from the tank inlet 102'a and flows out from the tank outlet 102''a.

Current enters the current-carrying anode 103a from the DC power source 105a via the current-carrying wire 106a, passes through the electrolyte, and enters the bipolar electrode 107a arranged between the current-carrying cathode 104a.The incoming side is the cathode 104'a, and the exit side is the anode 103'. a is charged, and the electrolyte solution 102 is sequentially charged.
a and the electrode 107a to enter the current-carrying cathode 104a. In this case, a noble metal composite oxide is supported on the anode 103'a side of the electrode 107a,
The side of the cathode 104'a is a base material, such as a titanium surface, supporting a composite oxide.

In this way, during electrolysis, in either method, the current i flows out from the noble metal oxide that is the anode and flows into the cathode made of titanium, stainless steel, etc. through the electrolyte.

However, during pickling, a current flows in the opposite direction. Figures 4 to 6 show the direction of current flow during pickling.

Figure 4 shows the case of a single electrolytic cell; 1b is the electrolytic cell body;
2b is an acid solution, and hydrochloric acid HCl is usually used. 3b is a noble metal composite oxide electrode, which serves as an anode during electrolysis. 4b is an electrode made of titanium or stainless steel, and serves as a cathode during electrolysis. To stop electrolysis during pickling, the noble metal composite oxide electrode is made of titanium,
The natural potential is nobler than that of stainless steel, creating a potential difference. As a result, when the composite oxide electrode and titanium are in a short-circuited state, a battery is formed, and the current 9b flows from the titanium surface through the acid solution to the composite oxide in a direction different from that during electrolysis. Since the DC power supply includes a rectifying element resistor 5b, it becomes difficult to form a battery, but it does not become completely short-circuited, so the current 9
b will flow.

Figure 5 shows the current flow during pickling when several sets of single electrolytic cells are arranged in series.
12b is an electrolytic cell body, and 12b is an acid solution, usually hydrochloric acid, which enters from a tank inlet 12'b, exits from a tank outlet 12''b, and is circulated or discharged through each tank.

13b is a noble metal composite oxide electrode; 14b
is a cathode material such as titanium or stainless steel. 16b
is a current-carrying wire, and 15b is a rectifying element resistance in the DC power supply. When pickling is carried out in such a system, the cathode material 14b in the front tank and the noble metal composite oxide 1 in the rear tank are
3b, the cathode material 14b is connected to the noble metal composite oxide electrode 13b via the acid solution 12b.
A current 18b flows from the point as shown by a dotted line.
Further, a current 19b flows from the titanium material 14b to the noble metal composite oxide electrode 13b via the rectifying element resistor 15b of the DC power supply and the current carrying wire 16b.

FIG. 6 shows the current flow during pickling in the case of a bipolar type. 101b is an electrolytic cell body;
102b is an acid solution such as hydrochloric acid, and the tank inlet 102'b
The precious metal composite oxide is circulated or discharged through the tank outlet 102″b.
b is a cathode material such as titanium or stainless steel, and a bipolar electrode 107b is disposed between them. The titanium surface 104'b is supported on the energized anode (noble metal composite oxide) 103b side of this electrode 107b, and the noble metal composite oxide is supported on the energized cathode 104b side.
During electrolysis, they act as a cathode and an anode, respectively. When pickling is carried out in such a system, the titanium surface 10
Current 10 from 4'b to noble metal composite oxide 103'b
8b flows.

Further, the electrode 1 is connected to the cathode material 104b via the rectifying element resistor 105b of the DC power supply and the current-carrying wire 106b.
A current 109b flows through 07b to the noble metal composite oxide electrode 103b.

As explained in FIGS. 4 to 6, when the noble metal composite oxide electrode and the cathode material are short-circuited, a current opposite to that during electrolysis flows during pickling.

The reason why a reverse current flows during pickling is that the natural potentials of the noble metal composite oxide and the cathode material are different, and a battery is formed in a short-circuited state. That is, since the noble metal composite oxide has a noble potential, it serves as a cathode (positive electrode), and the cathode material such as titanium or stainless steel is base, so it serves as an anode (negative electrode).

Figure 7 shows the time course of the reverse current when seawater is electrolyzed using the electrolytic cell shown in Figure 4 using a noble metal composite oxide as the anode and titanium as the cathode, and then pickled with 10% hydrochloric acid in a completely short-circuited state. It shows change.
In this case, the distance between the electrodes is 5 mm, and the liquid temperature is 20 to 25 mm.
It is ℃.

As shown in the figure, during pickling, a reverse current of approximately several tens of μA/cm ² (although it varies depending on the pickling conditions) is generated.

As a result, the noble metal composite oxide electrode is placed in a reducing atmosphere, and the oxide MO, which is the main component of the electrode, is reduced to the metal M as shown in equation (1) below.

MO+H ₂ O+2e ^- →M+2OH ^- (1) When seawater is passed through and a regular current is applied, an oxide is generated again by the reverse reaction of equation (1), but the composition of the oxide (e.g. palladium, lead , tin-based, etc.), perfect MO is not produced, but MO with defects is produced. Therefore, corrosion of the electrode material progresses from the defective portion, causing early wear of the electrode.

As a means of preventing this electrode wear and tear, there is a method of bringing the noble metal composite oxide and the cathode material into a completely non-short-circuited state during pickling so that a battery circuit is not formed. In the case of the electrolysis method shown in Figures 4 and 5,
It becomes possible to cut off the current-carrying wires 6b and 16b by incorporating a knife switch or the like. However, in the case of seawater electrolysis, a current of about 1000A is passed, which requires large-capacity switches, and in the case of the energization method shown in Figure 5, the number of electrolyzers is required, which increases the initial cost. However, there is a disadvantage that maintenance of the switch part becomes complicated. on the other hand,
In the case of FIG. 6, since a battery is formed in the electrode 107b itself, it is impossible to prevent battery formation even if the current-carrying wire 106b is turned off using a switch, etc., and the performance due to the wear and tear of the noble metal composite oxide electrode. , the durability will deteriorate at an early stage.

The present invention does not require switches on the energized wire,
Furthermore, the present invention provides a method for pickling electrodes for electrolysis which can prevent the formation of batteries even during pickling in the case of an electrolytic method such as the bipolar type shown in FIG.

That is, the present invention provides an acid cleaning method for an electrode for electrolysis in seawater or salt water using a noble metal composite oxide electrode as an anode, in which a DC voltage of the same polarity as during electrolysis,
Provided is a method for pickling an electrode for electrolysis, characterized in that cleaning is performed with an acid solution while a current is applied.

The reverse current that occurs during pickling is due to the formation of a battery, and if this could be prevented, electrode wear would be eliminated. If the noble metal composite oxide electrode and the cathode material are completely free from short-circuiting, battery formation can be prevented, but there are problems as described above.

In the method of the present invention, the pickling is carried out while supplying current with the same polarity as during the electrolysis shown in FIGS. 1 to 3. For the DC power supply, use the power supply for electrolysis as is. An embodiment in which the present invention is applied to a single electrolytic cell is shown in FIG.

In Figure 8, 1c is an electrolytic cell body, 2c is an acid solution (e.g.
Hcl), 3c is a noble metal composite oxide electrode, 4c is a counter electrode (titanium, stainless steel, etc.), 5c is a DC power supply,
6c is a current-carrying wire, and i is a current.

In this way, there is no battery formation and the noble metal composite oxide electrode is maintained in an oxidized state, so the electrode part does not wear out and high performance durability can be maintained, and the hardness components deposited on the electrode part can be effectively removed. Can be removed. The current density can be appropriately set to an optimum value depending on the electrode type, liquid properties, etc.

In order to demonstrate the usefulness of the method of the present invention, FIG.
Electrolytic cell shown in the figure (anode...precious metal composite oxide, cathode...
...Titanium), pickling under a completely short-circuit condition (curve A), pickling under a completely non-short-circuit condition (curve B)
And, when pickling according to the method of the present invention (curve C) is carried out, the change in chlorine generation current efficiency after pickling of the noble metal composite oxide electrode with respect to the number of pickling times is shown. In this case, the acid solution used was 10% HCl, and the pickling time was
30 minutes, liquid temperature 20-25℃, current density
It was set to 0.5A/ ^dm2 .

In Figure 9, when the counter electrode is made of titanium and pickled in a completely short-circuited state (curve A), a decrease in efficiency is observed from around the 4th time, and it drops to 85% by the 6th time, causing damage to the electrode surface. was recognized. On the other hand, in pickling in a completely non-short-circuited state (curve B) without battery formation, the efficiency was maintained at 95% or more even after ten or more picklings, and the electrode surface was also good.

In the case of pickling according to the method of the present invention (curve C), there is no decrease in efficiency as in the case of completely no short circuit, and it can be seen that the method of the present invention is extremely useful.

As described above, according to the method of the present invention, there is no need to cut off the current-carrying wire using switches, it is possible to prevent wear of the electrode during pickling, and at the same time, it is possible to effectively remove hard components adhering to the electrode surface. The value is extremely large.

In addition, the method of the present invention is extremely effective not only for seawater electrolysis, but also when hardness components are precipitated, and when electrode parts are contaminated with electrolyte components, they are cleaned and removed using an acid solution. It is something.

That is, salt water electrolysis is based on the same principle as sea water electrolysis. This system electrolyzes a solution of salt dissolved in city water to produce chlorine, which has a disinfecting effect, and uses this to disinfect water supplies. In this case, 100% pure common salt (NaCl) is not used, and hardness components (Ma, Ca) are present because the salt is dissolved and prepared in city water. Compared to seawater, the amount of hardness components is small, but after long-term electrolysis, precipitation of Mg(OH) ₂ , CaCO ₃ , etc. occurs, requiring acid cleaning.

Saltwater electrolysis also requires an anode with excellent chlorine generation ability, and noble metal composite oxide electrodes are often used.
The effects of the present invention can be obtained.

Furthermore, there is soda electrolysis, whose main products are chlorine produced at the anode and alkali produced at the cathode by electrolyzing salt water.

This soda electrolysis also provides the effects of the present invention for the same reason as the salt water electrolysis described above.

[Brief explanation of the drawing]

Figures 1 to 3 are cutaway diagrams showing the state of electrolysis using a conventional single electrolyzer, multiple electrolyzer, and bipolar electrode type electrolyzer, respectively, and Figures 4 to 6 are explanatory diagrams showing the state of electrolysis using the above-mentioned devices, respectively. Figure 7 is an explanatory diagram showing the state during pickling in a cut-away manner. Figure 8 is an explanatory diagram showing a cutaway example of the method of the present invention carried out using the apparatus shown in Figure 4, and Figure 9 is a diagram showing the current efficiency of chlorine generation after pickling in the method of the present invention and a comparative example. It is a graph showing changes with respect to the number of times of pickling. 1a, 1b, 1c, 11a, 11b, 101
a, 101b...electrolytic cell body, 2a, 12a, 10
2a... Electrolyte (seawater), 2b, 2c, 12b,
102b...Acid liquid, 3a, 3b, 3c, 13a,
13b... Anode, 4a, 4b, 4c, 14a, 1
4b... cathode, 107a, 107b... bipolar electrode, 5a, 5b, 5c, 15a, 15b,
105a, 105b...DC power supply, 6a, 6b,
6c, 16a, 16b, 106a, 106b...
Current-carrying wire, i...current.

Claims (1)

[Claims] 1. A method for acid cleaning an electrode for electrolysis in seawater or salt water using a noble metal composite oxide electrode as an anode, which is characterized by cleaning with an acid solution while applying a DC voltage and current with the same polarity as during electrolysis. Pickling method for electrodes.