JP3409920B2 - Lead dioxide electrode for electrolysis and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lead dioxide electrode for electrolysis and a method for producing the same, and more particularly to an electrolysis mainly used for water treatment and wastewater treatment, which is used at a relatively low current density of 20 A / dm ² or less. The present invention relates to a lead dioxide anode for treatment and a method for manufacturing the same.

[0002]

2. Description of the Related Art Wastewater treatment has become a serious problem due to the tightening of wastewater regulations. In particular, the metal components in the waste water have been conventionally precipitated mainly as an oxide-hydroxide sludge by the neutralization precipitation method and discarded. However, this method has a problem that the amount of waste increases and the metal component cannot be recovered even when the metal is a valuable resource. On the other hand, the organic matter in the wastewater remains as COD and TOC in the wastewater and requires expensive hydrogen peroxide, ozone, hypochlorous acid, etc. for its decomposition. It has also been pointed out that there is a possibility of secondary pollution, which is the generation of things.

In order to solve some of these problems, electrolytic wastewater treatment has been proposed. Treating wastewater with a cathode having a three-dimensional spread increases the chances of association between the metal in the wastewater and the cathode, and collects the metal ions in the wastewater by electrodeposition on the cathode surface at a low current density. Will be possible. Even if the wastewater cannot be completely purified by this treatment alone, a complete wastewater purification can be achieved by performing the final treatment with an ion exchange resin, etc., and metal chelates that cannot be treated by the neutralization precipitation method can also be treated by this electrolytic treatment. There is a feature called. On the other hand, COD and BO in wastewater
D and TOC can be decomposed and treated by electrolysis using a highly oxidizable anode. That is, by combining anodizing and cathodic treatment, metal removal or recovery and organic substance decomposition can be performed simultaneously, and a treatment device that can actually perform metal removal and organic substance decomposition simultaneously is commercially available.

The anode current density of a conventional wastewater treatment apparatus by electrolysis is 10 A / dm ² or less, and if it exceeds that, oxygen generation mainly occurs and the target decomposition of organic matter does not proceed as much as the current supply amount. Lead, lead alloys and lead dioxide are the most suitable anode materials for the decomposition of organic substances, but lead and lead alloys have a dissolution rate of several mg / AH, which is about 100 times that of lead dioxide. Therefore, lead dioxide is usually used. Platinum-plated titanium is also promising because it has a high potential as an anode, but its use is limited because it is expensive and its oxidizing power is weak relative to the potential.

A lead dioxide electrode most suitable for such wastewater treatment is usually provided with an intermediate layer for preventing oxidation and maintaining conductivity on a titanium or titanium alloy substrate, and the lead dioxide layer is provided on the intermediate layer. Formed and configured. The intermediate layer is formed because lead dioxide is a type of ceramics and has high conductivity, but is extremely brittle, and sufficient strength cannot be obtained only with the lead dioxide layer. As a result, expensive materials such as platinum and titanium oxide / tantalum must be used, and even if lead dioxide itself is inexpensive, there is a drawback that the finally manufactured electrode becomes expensive. Normally, a lead dioxide electrode is extremely stable even at a current density of 100 A / dm ² or more. For such a high current density, the specification having the above-mentioned intermediate layer may be used.
The current density is low, for example, it is an excessive specification for wastewater treatment, and it is expensive, so its use is limited.

[0006]

OBJECTS OF THE INVENTION The present invention solves the above-mentioned drawbacks of conventional lead dioxide electrodes and is a long-lived lead dioxide electrode which is inexpensive and easy to handle, is used mainly at low current density, and is particularly suitable for water treatment. The purpose is to provide a method.

According to the present invention, a porous carbon substrate impregnated with or plated with metallic lead, an α-lead dioxide layer formed on the substrate, and a surface of the α-lead dioxide layer are formed. β
A lead dioxide electrode for electrolysis comprising a lead dioxide layer and a method for producing the same.

The present invention will be described in detail below. When a valve metal such as titanium is used as the base material like the conventional lead dioxide electrode, the surface of the base material becomes passivated due to the oxidation of lead dioxide and also due to the high potential, and it cannot be used as it is.Platinum or titanium-tantalum It was necessary to form a thin corrosion resistant layer of oxide + platinum, etc., as the surface layer. Therefore, in addition to the valve metal itself being expensive, the cost required for forming these surface layers also becomes enormous, and the electrode could not be manufactured at low cost. In view of this problem, an inexpensive carbon material is used as the base material in the present invention. The shape of the carbon material can be an appropriate shape such as a plate shape, a rod shape, a net shape, or a porous shape, depending on the shape of the final electrode. The surface state may be smooth, but it is desirable that the surface state is porous to improve the affinity with lead, which will be described later.

Since the specific gravity of lead dioxide is about 9 for both α-lead dioxide and β-lead dioxide, they are extremely heavy, and therefore have a large physical strength as a carbon base material for holding these lead dioxide layers on the base material. It is essential to use carbon materials,
For example, what is commercially available as so-called carbon paper (carbon paper or carbon paper) obtained by sintering fibrous carbon is preferable, and it is particularly desirable that the carbon paper is graphitized in order to improve conductivity and corrosion resistance.

When a lead dioxide layer is formed directly on this carbon substrate, a part of the lead dioxide layer is destroyed, and the exposed carbon substrate reacts selectively to volatilize as carbon dioxide, which is consumed by oxidation of the substrate. The life of the entire electrode is shortened. In order to avoid this, in the present invention, the carbon base material is impregnated with metallic lead or the surface thereof is plated with a metallic lead layer. Impregnation of the carbon substrate with lead is preferably performed by immersing the substrate in molten lead, and in the case of plating, the substrate is electroplated with lead using a lead borofluoride bath or the like. If the carbon substrate is not porous, impregnation is not appropriate and lead is plated on the substrate surface by electroplating.

The conditions for impregnating the molten lead are not particularly limited, but if the wettability between the base material and the molten lead is kept good, the lead enters the pores of the base material and the lead is held in the carbon base material. It In order to improve the wettability, it is desirable to wash the surface of the base material with an organic solvent such as alcohol such as methanol or ethanol before impregnation to reduce the surface tension and increase the affinity between the base material and lead. The temperature at the time of impregnation is not particularly limited as long as lead exists in a liquid state, but it is desirable to raise it slightly above 300 ° C. to reduce the viscosity of lead and improve the wettability. When the molten lead and the carbon base material are brought into contact with each other in this state, the carbon base material sufficiently absorbs lead, and in combination with the anchor effect, an impregnated lead layer having sufficient strength and sufficient conductivity is formed. On the other hand, in the case of using electroplating, a commercially available borofluoride bath lead plating solution can be used, and as a typical bath, 150 g / liter of lead, 150 g / liter of borofluoric acid, and 2 to 4 g / peptone of peptone are used. There are baths containing liters.
A plating layer having a thickness of about 40 to 50 μm can be formed by using these baths and using lead as an anode at a temperature of 20 to 25 ° C. and a current density of about 3 A / dm ² for about 1 hour.

The thickness of the plating layer formed is not particularly limited, but if it is less than 10 μm, the formation of through holes cannot be completely prevented. Even if a through hole is formed, it does not hinder normal use, but the carbon base material may be corroded by the electrolytic solution over a long period of use.
It is preferably 10 μm or more. Also, the thickness of the plating layer
If the thickness is more than 100 μm, the weight increases and the lead dioxide layer formed on the surface of the plating layer is easily peeled off. Therefore, the thickness of the plating layer is preferably 100 μm or less. A plating layer having a desired thickness can be obtained by adjusting the electrolysis time of the plating operation.

Next, an α-lead dioxide layer is formed on the surface of the substrate having the impregnated lead or lead plating layer. This α-lead dioxide layer may be obtained by oxidizing the surface of the lead plating layer or by electrolytic plating. That is, a base material having a lead-plated layer can be converted into an α-lead dioxide layer by anodic polarization in a water solution to oxidize metallic lead. The aqueous solution may be acidic or basic, but is preferably basic. That is, when the carbon substrate is used as an anode in a 10 to 20% caustic soda bath at a current density of about 1 A / dm ^{2 for} electrolysis for 30 minutes to 1 hour, oxygen is generated and the surface is oxidized to form an α-lead dioxide layer.

Instead of this anodic polarization, the
In order to form the lead dioxide layer, electrolysis is carried out using a carbon substrate plated with lead as an anode in a solution in which lead oxide (PbO) is saturated and dissolved in ca. 20% caustic soda. A typical condition for this electrolysis is a temperature of 25 to 50 ° C, preferably 40 ° C.
The current density is 0.5 to ^{2 A} / dm ^{2. In} this case, oxygen generation does not occur, and at the current density of 1 A / dm ² , the α-lead dioxide layer is formed with a current efficiency of almost 100% in about 30 minutes. This α-lead dioxide layer is for maintaining good bonding between the impregnated lead or lead-plated layer of the carbon substrate and the surface β-lead dioxide layer described later, and the thinner the more uniform, the better.

Next, a β-lead dioxide layer is formed on the surface of this α-lead dioxide layer. The thickness of the β-lead dioxide layer is not particularly limited, but considering stability, it is preferably 50 to 200 μm, and the conditions for obtaining the β-lead dioxide layer having this thickness are, for example, 600 to 900 g. / Liter of lead nitrate aqueous solution at 60 ° C. and a current density of ^{2 to} 4 A / dm ² for 1 to 2 hours using the carbon substrate as an anode. β-lead dioxide tends to have a large electrodeposition strain, and in order to prevent this, ceramic powder or a fluororesin can be added to the β-lead dioxide layer. These substances can be dispersed in the β-lead dioxide layer by suspending them in an electrolytic solution and performing electrodeposition with sufficient stirring, and it is not necessary to change other electrolysis conditions. The material of the ceramic powder and the fluororesin is not particularly limited and is preferably stable and has no conductivity with respect to the wastewater to be treated, and silica is a typical ceramic powder,
Alumina, titania, tantalum oxide, and the like, and typical Teflon dispersion (trade name), which is a fluororesin having hydrophilicity, is a typical fluororesin.

The particle size of these particles is preferably about 1 to 20 .mu.m, and if it exceeds this value, particles larger than the thickness of the lead dioxide layer are present, sometimes causing through holes. Further, by adding a water repellent substance to the lead dioxide layer to make the electrode surface water repellent, an electrode suitable for selective decomposition of organic substances can be manufactured. In order to make the surface of the electrode water repellent, a water repellent substance is added to the electrolytic solution and a surfactant is added to suspend it in the electrolytic solution. Although the kind of the surfactant is not particularly limited, a nonionic surfactant is suitable, and the surfactant is preferably a graphite fluoride having a particle diameter of 1 to 20 μm, a pitch fluoride, a polytetrafluoroethylene resin (PTFE), a fluorine. Ethylene propylene resin (FE
P) and other water-repellent substances are suspended and the above α-
A β-lead dioxide layer in which a water repellent substance is dispersed is formed on the surface of the lead dioxide layer. The electrolysis conditions differ depending on the type of surfactant, but generally the temperature is 40 to 50 ° C, the current density is 1 to 4 A / d.
m ^{2 It is} preferably 1 to ^{2 A} / dm ² .

[0016]

EXAMPLES Next, examples for producing the lead dioxide electrode of the present invention will be described, but the examples do not limit the present invention.

Example 1 A 1.5 mm thick sintered carbon paper manufactured by Oji Paper Co., Ltd. was used as a base material, which was immersed in ethyl ether for 15 minutes and then naturally dried in order to improve the wettability of the surface. . This base material was dipped in molten lead at 350 ° C., held for 3 minutes, taken out, and hot air was blown onto the surface with a hair dryer to remove excess lead. The entire surface of the substrate was covered with a thin lead layer. The base material on which this lead layer was formed was immersed in a 10% caustic soda aqueous solution at 40 ° C to form an anode,
This was connected to a nickel plate cathode and the current density was 1 A / dm ²
Electrolysis was performed for 1 hour. After the electrolysis was stopped, the surface condition of the substrate was observed by X-ray diffraction, and it was found that the dense α-lead dioxide was covered with a thickness of about 25 μm.

This substrate was further electrolyzed at a temperature of 60 ° C. and a current density of 4 A / dm ² by using an aqueous solution of 800 g / liter lead nitrate in which silica powder having a particle size of 1 to 10 μm was suspended. As a result, β-lead dioxide having a thickness of 50 μm could be formed, and no bending due to electrodeposition strain was observed.
The base material on which this β-lead dioxide layer is formed is used as an anode at 60 ° C.
Electrolysis in 150 g / l of sulfuric acid
Even at a current density of A / dm ² , there was no increase in ohmic loss due to the base material, and electrolysis for 2000 hours at this current density operated without any problem.

[0018]

Example 2 The same carbon paper as in Example 1 was pretreated in the same manner, connected as a cathode in a commercially available lead borofluoride-based lead plating bath, and subjected to lead plating at an apparent current density of 3 A / dm ² for 1 hour. went. As a result, the surface of the carbon paper substrate was plated with about 1 kg of lead per m ² . A 20% caustic soda aqueous solution saturated with lead oxide (PbO) was used as an electrolytic solution, and the base material was used as an anode at 40 ° C. and 1 A /
Electrolysis was carried out at dm ² for 1 hour to form an α-lead dioxide layer on the surface of the base material. The thickness of α-lead dioxide was about 35 μm.

A nonionic surfactant is added to an aqueous solution of 800 g / liter of lead nitrate so as to be 10 g / liter, and 50 g / liter of graphite fluoride powder having a particle size of 1 to 10 μm is added.
It was suspended so that the volume became 1 liter. The substrate was electrolyzed in this suspension at 45 ° C. and 2 A / dm ² for 2 hours to form a β-lead dioxide layer containing graphite fluoride and having a thickness of about 50 μm. This β-lead dioxide layer did not show any wettability with water. When this electrode was used for electrolysis at a current density of 3 A / dm ² , electrolysis could be continued without any problem even after 3000 hours had passed.

[0020]

Example 3 Electrolysis was carried out in an aqueous solution of 150 g / liter of sulfuric acid containing 5 g / liter of humic acid using the electrodes of Examples 1 and 2 as anodes. It was found to decompose humic acid at a rate about 3 times that of the electrode.

[0021]

The present invention provides a porous carbon substrate impregnated or plated with metallic lead, an α-lead dioxide layer formed on the substrate, and a β-lead dioxide layer formed on the surface of the α-lead dioxide layer. Is a lead dioxide electrode for electrolysis. The lead dioxide electrode of the present invention uses a porous carbon substrate as a substrate, and does not have the titanium-based substrate and the intermediate layer used in the conventional lead dioxide electrode.
The lead dioxide electrode with this titanium-based substrate and the intermediate layer is 10
It is extremely stable even at a current density of 0 A / dm ² or more,
Since the material used is expensive, even if inexpensive lead dioxide is used as the electrode material, the cost of the electrode as a whole becomes considerably high. Although the lead dioxide electrode used for electrolysis at a current density of 100 A / dm ² or more is unavoidably expensive, electrolytic operation at a low current density such as water treatment uses an expensive corrosion-resistant material. Therefore, it is not necessary to configure the electrodes.

The lead dioxide electrode of the present invention is an electrode used in a low current density region, and as described above, an inexpensive porous carbon material is used as a base material to avoid the use of an expensive titanium base material or an intermediate layer. is doing. However, if the α-lead dioxide layer and the β-lead dioxide layer are formed directly on the surface of the porous carbon substrate, the consumption of the porous carbon substrate may be promoted through the through holes that may be formed in both the lead dioxide layers. Therefore, in the present invention, a lead plating layer is formed between the carbon base material and the α-lead dioxide layer, or the carbon base material is impregnated with lead. Since the lead plating layer and the impregnated lead can be formed inexpensively, the cost of the electrode is slightly increased and an inexpensive lead dioxide electrode for a low current density region can be provided.

Further, it is desirable that the porous carbon base material is a base material having a high physical strength such as a sintered body of fibrous carbon. When these base materials are used, lead dioxide having a high specific gravity can be surely held and stabilized. The obtained electrode can be obtained. It is also possible to disperse ceramics and / or fluororesin powder in the β-lead dioxide layer, and these substances effectively prevent electrodeposition strain that tends to occur in β-lead dioxide. Further, a water-repellent fluororesin and / or graphite fluoride can be dispersed in the β-lead dioxide layer to make the electrode surface water-repellent. By making the electrode surface water repellent, the electrode can be made suitable for selective decomposition of organic substances.

According to the method of the present invention, a porous carbon substrate is immersed in molten lead to impregnate the substrate with lead, and then the substrate is electrolyzed in an electrolytic solution containing lead ions to form an α A method for producing a lead dioxide electrode for electrolysis, characterized in that a lead dioxide layer is electrodeposited, and then a β-lead dioxide layer is electrodeposited by electrolyzing the above-mentioned substrate in a lead nitrate-containing electrolytic solution as an anode. As described above, the method can provide a lead dioxide electrode that is inexpensive and can be used in a low current density region. Further, instead of impregnating lead in the substrate of the method, a metallic lead layer can be formed on the surface of a porous carbon substrate having a hydrophilic surface by electrodeposition. A lead dioxide electrode can be provided.

Continuation of the front page (56) References JP-A-52-19230 (JP, A) JP-A-6-57474 (JP, A) JP-B-45-20164 (JP, B1) (58) Fields investigated (Int .Cl. ⁷ , DB name) C25B 1/00-15/08

Claims (7)

(57) [Claims] 1. An electrolysis comprising a porous carbon substrate impregnated or plated with metallic lead, an α-lead dioxide layer formed on the substrate, and a β-lead dioxide layer formed on the surface of the α-lead dioxide layer. Lead dioxide electrode. 2. The lead dioxide electrode according to claim 1, wherein the porous carbon substrate is a sintered body of fibrous carbon. 3. The lead dioxide electrode according to claim 1, wherein ceramics and / or fluororesin powder is dispersed in the β-lead dioxide layer. 4. The lead dioxide electrode according to claim 1, wherein a water-repellent fluororesin and / or graphite fluoride is dispersed in the β-lead dioxide layer to make the surface water-repellent. 5. A porous carbon base material is immersed in molten lead to impregnate the base material with lead, and then the base material is electrolyzed in an electrolyte solution containing lead ions to form α-lead dioxide. A process for producing a lead dioxide electrode for electrolysis, which comprises electrodepositing a layer, and then electrolyzing the β-lead dioxide layer by electrolyzing the substrate in a lead nitrate-containing electrolytic solution using the substrate as an anode. 6. An α-lead dioxide layer, which is obtained by forming a metallic lead layer on the surface of a porous carbon substrate having a hydrophilic surface by electrodeposition and electrolyzing the substrate in an electrolytic solution containing lead ions using the substrate as an anode. And electrodepositing the β-lead dioxide layer by electrolytically using the above substrate as an anode in a lead nitrate-containing electrolytic solution, to produce a lead dioxide electrode for electrolysis. 7. A lead metal layer is formed by electrodeposition on the surface of a porous carbon substrate having a hydrophilic surface, and the lead metal layer is positively polarized to form an α-lead dioxide layer on the surface, A method for producing a lead dioxide electrode for electrolysis, which comprises electrolyzing a β-lead dioxide layer by electrolyzing the substrate as an anode in a lead nitrate-containing electrolytic solution.