30+ YEARS OF EXPERIENCE IN CARBON INDUSTRY

As raw materials for binders, the coke residues remaining after self-baking and sintering of electrode paste can increase electrode strength and make its texture more uniform. The best binders are coal tar pitch and coal tar.

Coal tar is a group of fractions distilled at lower temperatures than coal tar pitch. It is a viscous liquid, ranging in color from dark brown to black, with a relative density of 1.05–1.25. The quality of tar depends mainly on the coking temperature conditions and the structure of the furnace. In furnaces of the same structure, under the same temperature conditions, tar with the same composition and physical properties can be obtained even when coking coal blends of different compositions. The coal tar discussed in this section is used after heat treatment to adjust the softening point of electrode paste for closed furnaces.

Pitch is a black, glossy solid substance with a relative density of 1.245–1.299. It is the residue remaining after other fractions in tar have been extracted at a distillation temperature above 360°C.

According to their softening points, pitches can be classified as soft pitch, 40–50°C; medium-temperature pitch, 65–75°C; and hard pitch, above 80°C. The following are the specifications for medium-temperature pitch upon receipt at the plant:

The moisture in pitch consists of two parts: mechanically suspended moisture and absorbed moisture. After heat treatment above 130°C for several days to remove the moisture and reduce expansion, it may be used in production.

Pitch contains free carbon, part of which is carried over from tar into the pitch, while another part is formed by the decomposition of high-molecular-weight compounds during tar distillation.

The most valuable part of pitch and tar is the coking portion, namely the binding coke. Binding coke can be obtained by removing free carbon from the coke residue.