Low-Temperature Oxidation of Coal Tar Pitch in a Thin Layer

Abstract

Coal tar pitch is derived from coal tar. It is used as a binder for electrodes and anode mass. It is mainly produced as the <360°C fraction in the distillation of coal tar. However, the coal tar pitch obtained by this means does not always meet the requirements for binder pitch. It may be characterized by low softening point and insufficient content of binder fractions (β and α₂). Thermal oxidation is used to improve pitch quality. The present article describes experiments on the oxidation of low-temperature pitch (59°C) produced by the distillation of coal tar (from EVRAZ ZSMK) and medium-temperature grade B electrode pitch (71°C) in a thin layer. Such oxidation is not often used and requires further investigation. Thermal oxidation in a thin layer is studied on laboratory apparatus in a 4-L reactor. Air is supplied by a pipe of 9-mm diameter. The mass of the pitch samples is selected so that they sit in a thin layer on the reactor floor. The range of oxidation temperature is 260–300°C. The rate of air supply is 4800 L/kg h for low-temperature pitch and 2400 L/kg h for medium-temperature grade B electrode pitch. The oxidation product (pitch) is collected. The pitch yield, its softening temperature, and its content of components insoluble in toluene and quinoline are determined. The fractional composition of the oxidation products is analyzed. The products are compared with the initial pitch samples. For medium-temperature grade B electrode pitch, the dependence of the softening point and fractional composition of the oxidation products on the duration of oxidation is considered. The conditions in which the content of the α₁ fraction may increase on low-temperature oxidation are identified. The increase in the content of α₂ and α₁ fractions in the oxidation products is estimated. It is found that oxidation in a thin layer increases the yield of carbonized pitch. For the given pitch samples, the yield of carbonized pitch–coke mixture is higher than the yield of carbonized pitch–coke mixture based on grade C electrode pitch.