Under the background of energy shortage and environmental degradation, traditional graphite anodes and lead-based alloy anodes hinder their development in chemical, metallurgy, electroplating, protection, environmental protection and other industries due to high energy consumption, short life, frequent maintenance or lead pollution. People thus turn to research on new coated anodes with low energy consumption and long life. In 1965, Beer obtained the patent right of the RuO2-TiO2 coated electrode, and three years later, De Nora of Italy realized its industrial application in the chlor-alkali industry; in 1973, Bianchi and others obtained the patent right of the IrO2-Ta2O5 coated electrode. This kind of coating electrode has been applied in galvanizing of steel sheet [1]. Since then, the application of titanium-based noble metal oxide-coated electrodes has opened a new chapter. The author introduced the types and advantages of titanium-based noble metal oxide coated electrodes, reviewed the application progress of coated titanium electrodes, pointed out the existing problems in application, and prospected the development direction of coated titanium electrodes, in order to provide a more reasonable The application of noble metal oxide-coated titanium electrodes provides a reference.

1 Types and advantages of coated titanium electrodes

Coated titanium electrode is a kind of metal electrode material made of metal titanium as the base and coating the active coating with platinum group metal oxide as the main component on its surface. Usually the coating consists of active and inert ingredients. According to the different active components in the coating, the electrodes are generally divided into RuO2 and IrO2. Due to the difference in thermal expansion coefficient between the titanium substrate and the coating, and multiple thermal and cooling cycles during the preparation process, the coating has a porous and cracked morphology. Compared with lead-based alloy anodes and graphite anodes, coated titanium electrodes have the following advantages [3]: (1) The electrode size is stable, the electrode spacing is constant during electrolysis, which can avoid short circuits between electrodes; (2) Oxygen evolution (chlorine) potential (3) long working life; (4) high chemical stability, no pollution to cathode products; (5) light weight, high mechanical strength; (6) easy to make shape, high precision (7) The base titanium can be used repeatedly; (8) It can work under high temperature and high current density.

2. Application of Coated Titanium Electrodes

Since RuO2-based coated titanium electrodes and IrO2-based coated titanium electrodes have been applied in the chlor-alkali industry and steel galvanizing industry successively, the application fields of coated titanium electrodes have been continuously expanded, and energy-saving and environmental protection benefits have gradually emerged, and even promoted the chlor-alkali industry. The rapid development of some industries such as industry and metal foil materials.

2.1 Chemical industry

2.1.1 Chlorine to alkali [4]

In 1973, Tianyuan Chemical Plant in my country used the coated titanium electrode electrolyzer for the first time in the chlor-alkali industry. By the end of 1988, 2,800 coated titanium electrode electrolyzers had been put into use in my country alone. The coated titanium electrode used in Japan has an area of 100,000 m2, and the world is up to 1 million m2. When using the traditional graphite anode, the working current is 900A·m-2 and the cell voltage is 4.4V, while using the coated titanium electrode, the current density can be increased to 2000A·m-2, but the cell voltage is only 3.6V . It can be seen that the coated titanium electrode not only saves energy significantly, but also greatly improves the production capacity.

2.1.2 The production of sodium hypochlorite is different from the factory operation, and a small sodium hypochlorite generator can easily prepare a small amount of low-concentration sodium hypochlorite aqueous solution. Due to the advantages of simple structure, low price, convenient use and maintenance, etc., the sodium hypochlorite generator has been widely used. When using graphite anode, the life is short, only more than 1 month, while using RuIrTiSnMn oxide-coated titanium electrode, the life is about 5 years. In addition, the graphite particles exfoliated from the graphite anode often block the channel and affect the operation of the generator, and the coated titanium electrode can avoid these problems.

2.2 Industrial electroplating

2.2.1 High-speed electroplating High-speed electroplating is a process of high-speed deposition under extremely high cathode current density to obtain a high-quality coating. At present, IrO2-based coated titanium electrodes have been successfully applied to galvanized steel, tinned steel, printed circuit boards

(PCB) horizontal electroplating and other fields.

Typical galvanizing bath is 1mol·L-1H2SO4, 0.7mol·

L-1 Na2SO4. When electrolyzed at pH 0.5, 100A·dm-2, and 60℃, the working life of the coated titanium electrode Ti/IrO2-Ta2O5 is more than 417d, during which no maintenance is required, while the traditional lead-based alloy anode needs 10-20d. maintenance [5]. Representative companies include Nippon Steel Pipe Co., Ltd., Amco Steel Co., Ltd. of the United States, and Salmax Gm Co. of Germany.Tin-plated steel is commonly used in the food industry, and manufacturers do not use lead-based alloy anodes for safety reasons. Japan has used Ti/IrO2-Ta2O5/SnO2 electrodes to replace expensive Ti/Pt electrodes, which greatly saves investment costs. In the hole metallization process of the PCB manufacturing process, the Rohm and Haas Company of the United States applies Ti/IrO2-Ta2O5 electrodes; domestic scholars use Ti/IrO2-Ta2O5 electrodes.

IrO2-SnO2-Pd is used as the anode, and the hole metallization is carried out under the pulse conditions of 42ms, 12A·dm-2 in the forward direction and 2ms, 40A·dm-2 in the reverse direction. The results show that the service life is 8 months, which can meet the industrial requirements. .

2.2.2 In the early stage of precious metal electroplating, traditional Pt electrodes and Ti/Pt electrodes were used for gold plating, platinum plating, and rhodium plating. Ti/IrO2-Ta2O5 electrode has smaller overpotential, decomposition voltage and tip effect than Ti/Pt electrode, which can slow down the aging speed of gold plating solution. According to production practice, Ti/IrO2-Ta2O5 electrode can increase the utilization rate of gold salt from 70% when using Ti/Pt electrode to more than 85%.

2.3 Electrochemical metallurgy

From the perspective of anode consumption and application significance, electrochemical metallurgy is expected to become the second largest industry in the application of coated titanium electrodes after the chlor-alkali industry. Taking zinc electrolysis as an example, at common current densities, the overvoltage on PbO2 can reach

1V, resulting in useless power consumption of 1000kW·h/(t·Zn), which is about 30% of the total energy consumption of zinc electrowinning. Therefore, the research and application of energy-saving anode is of great significance.

2.3.1 Chloride solution system In the early stage, graphite anode was used to electrodeposit metal from chloride solution, but the graphite anode seriously polluted the cathode product [5]. For example, during normal electrowinning of cobalt,

The graphite anode tank pressure is 4.1V, and the service life is only a few months;

When the RuO2-based coated titanium electrode has a cell pressure of 3.7V, the life is 4 years, and the electricity saving is 11%. At present, the coated titanium electrode has been applied on a large scale in Jinchuan Company.

2.3.2 Ammonia complex solution system is often used for leaching zinc and copper in raw materials such as zinc-containing materials and waste circuit boards due to the high selectivity of ammonia-ammonium chloride solution to metal dissolution. For this chloride leachate, RuO2-based coated titanium electrode is the preferred anode; however, when the ammonia concentration is not controlled properly, a layer of deposits will appear on the anode surface, which will reduce the electrocatalytic active sites.

2.4 Material preparation

2.4.1 Copper foil manufacturing In recent years, the coated titanium electrode Ti/IrO2-Ta2O5 (Ir56%,

Ta44%) has been successfully applied to the green foil machine. The coated titanium electrode overcomes the disadvantage of unstable size of lead-based alloy anode, reduces energy consumption, improves the uniformity of copper foil, maintains the purity of electrolyte, and stabilizes the quality of copper foil products. Taking a single copper foil electrolytic cell with a current of 50kA as an example, the coated titanium electrode saves 400,000 electricity compared to the ordinary Pb-Ag anode.

kA h, the power saving effect is very obvious. In addition, the enhanced life of an improved coated titanium electrode Ti/IrO2-Ta2O5 has reached 72,000 kA h.

This will further improve the power saving efficiency [4].

2.4.2 Aluminum foil formation The current coated titanium electrode Ti/IrO2-Ta2O5 is suitable for ammonium adipate solution, but its working life is limited. Under the conditions of 0.8A·dm-2 and 40-70℃, the life of the electrode is 1 year; under the condition of 12A·dm-2, the life of the electrode is 4 months.

3 Problems in the application

3.1 Technical Standards

RuO2-based coated titanium electrodes have chemical industry standards (HG/T2471-2011), while IrO2-based coated titanium electrodes have no industry technical standards. Obviously, it is not conducive to the quality supervision and popularization of IrO2-based coated titanium electrodes.

3.2 Product suitability

There are often inorganic impurities such as F- and Mn2+ in industrial electrolytes, while organic additives such as thiourea are often found in electroplating solutions. These impurities and additives will affect the electrocatalytic performance and working life of the coated titanium electrode to a certain extent. At present, the domestic research on the influence of harmful impurities on the chemical properties of coated titanium electrodes is not sufficient, and the prepared coated titanium electrodes have poor adaptability.

4 Conclusion

Precious metal oxide-coated titanium electrode is an energy-saving and environmentally friendly metal anode with a wide range of applications. The practice of the chlor-alkali industry and the high-speed electroplating industry has shown that the coated titanium electrode is energy-saving and environmentally friendly. In view of the electrode price, investment factors and other reasons, although there are still difficulties in the application in some fields, and the applied research needs to be strengthened, it still has a relatively broad prospect.

The following aspects are worth discussing and researching: (1) Study on the failure mechanism of the coated titanium electrode under different environmental conditions. Through the in-depth study of the failure mechanism, a long-life coated titanium electrode can be developed in a targeted manner. (2) Development of coated titanium electrodes with less precious metals and multiple components. The aim is to improve the cost competitiveness of coated titanium electrodes and expand their application fields.