1. Use.
Coal pitch is a by-product in the process of coal tar processing, and its yield accounts for 30%-40% of the total coal tar. At present, coal asphalt is mainly used in the production of anode paste, impregnated asphalt, cable asphalt, tire special asphalt, graphite electrode and other products. However, most of them are used in the production of anode paste, which is an indispensable raw material in the production of aluminum electrolysis. With the development of the aluminum industry, the production of aluminum has increased significantly, and the demand for anode paste is also increasing. Therefore, it is of great significance to continuously research and develop the application methods of coal asphalt and improve the processing depth of coal asphalt to reduce the amount of carbon residue in the anode paste, reduce the energy consumption of electrolytic aluminum, improve the conductivity of carbon products, and improve the production capacity of the electrolytic cell.
Second, the main role.
1.As a binder for carbon paste. In the anode paste used in aluminum electrolytic cells, coal pitch exists as a binder. In the production of anode paste, it is first necessary to use tar asphalt as the base asphalt, and then the polymer formed by oxidative polymerization reaction as the binder to ensure that the anode paste can maintain a certain strength when used. The network structure of the binder is mainly formed by the formation of hydrogen bonds between the macromolecular chains of polymers and the association with each other. Under certain temperature and time conditions, the base asphalt undergoes oxidative polymerization reaction to form a polymer binder of a certain molecular weight, and the residual active groups form hydrogen bonds between the asphalt molecules to form a network structure, so that the tar asphalt has good cohesiveness.
2.Reduce the expansion rate of anode paste. Coal pitch is a complex mixture of polycyclic aromatic hydrocarbons, most of which are volatile components with low viscosity and low melting point, and the volatilization of these components causes the volume of coal pitch to shrink. At the same time, because coal pitch contains a large number of components with high aromatity, these substances are prone to heat shrinkage when heated. Therefore, coal asphalt has the characteristics of obvious reciprocating change of thermal expansion and contraction. After the coal pitch is added to the anode paste and then kneaded, formed and roasted, the volume shrinkage rate is generally 20%-30%, and the high volume shrinkage rate will seriously affect the roasting quality of carbon products and the quality of primary aluminum. Adding an appropriate amount of coal pitch can significantly reduce the volume shrinkage of the anode paste. All other things being equal, the volume shrinkage of the anode paste prepared with only base asphalt was 32%, while the volume shrinkage of the anode paste formulated with both base asphalt and modified asphalt decreased to 14%. At the same time, the addition of modified asphalt can improve the oiliness of the mixture, increase the amount of liquid phase in the mixture, and also play a role in improving the expansion rate.
3.Improves the conductivity of anode carbon blocks. After adding coal pitch, the conductivity of carbon products can be improved. The main reason is that coal pitch will soften and deform at a certain temperature, and the hydrogen atoms in the naphthenic hydrocarbon group and the hydrogen atoms on the benzene ring in its molecular structure can achieve orientation movement and free diffusion at a lower temperature, and with the increase of temperature, this orientation movement and free diffusion become more and more intense. At the same time, because coal pitch contains a large number of long-chain alkanes, naphthenes and aromatic hydrocarbons, these long-chain macromolecular substances can play the role of insulators under high temperature conditions, and their resistance value gradually decreases with the increase of temperature. In addition, coal pitch contains many residual polar groups such as phenolic hydroxyl groups, hydroxyl groups, etc., which all have certain conductivity. Moreover, these polar groups can dissociate trace amounts of H+ and OH- ions under high temperature conditions and have certain acidity and alkalinity, so they can adsorb oxygen and nitrogen ions in the gas and increase the number of conductive ions.