Petroleum coke calcination is one of the main processes for the production of anodes for aluminum. The purpose is to eliminate the volatiles of the raw materials and increase the density, mechanical strength, electrical conductivity and chemical stability of the raw materials. During calcination, petroleum coke changes from elemental composition to microstructure, and the physical and chemical properties of the raw materials are significantly improved after calcination.
The calcined petroleum coke is directly used for anode production, and the quality of the calcined coke will directly affect the quality of the anode carbon block. In the case where the raw coke quality is determined, the calcination quality mainly depends on the calcination temperature, so the influence of the calcination temperature on the quality of the calcined coke is significant.
1. Effect of calcination temperature on volatile matter discharge
The petroleum coke has a large amount of volatiles in the temperature range of 500 ° C ~ 700 ° C. When the temperature rises above 800 ° C, the removal rate of volatiles slows down, and the temperature rises to about 1300 ° C. The volatile content generally falls below 0.5%. The water in the coke is completely discharged, and most of the volatile matter is discharged. The relative content of the carbon is increased, the volume is contracted, and the porous internal crystal structure is formed, and the true specific gravity is large, and the electrical conductivity and mechanical strength are improved.
2. Effect of calcination temperature on structure and true density
The change of true density of petroleum coke during calcination is linear with the calcination temperature. Figure 2 shows the change of the true density of a certain petroleum coke with the calcination temperature. It can be seen that before 1000 °C, the true density increases linearly with the increase of the calcination temperature. It can be considered that in this temperature range, the true density of the coke is delayed. The increase is the result of pyrolysis and polycondensation reactions, and is also the result of a large amount of hydrogen, oxygen, nitrogen and other elements being continuously discharged, and the molecular structure of petroleum coke is rearranged.
The change in true density showed an inflection point around 1100 °C, and continued to rise after 1100 °C, mainly due to the coke structure. The true density and bulk density of petroleum coke increase with increasing temperature. As the heat treatment temperature increases, the lattice size of the petroleum coke grows and grows, and the Lc increases with temperature.
3. Effect of calcination temperature on resistivity
Determination of the electrical resistivity of the calcined coke is an important means of testing the degree of calcination. The change in powder resistivity of petroleum coke during calcination can be roughly divided into three stages:
(1) 500 ° C -800 ° C, the powder resistivity decreases linearly at this stage;
(2) 800 ° C -1000 ° C, with the increase of calcination temperature, the powder resistivity continues to decline, but the decline is small;
(3) At 100-1300 ° C, the powder resistivity still decreases slightly.
It can be seen that in the case of determining the raw material of raw petroleum coke, the calcination temperature is an important factor determining the quality of the calcined coke. After the anode production system of an aluminum plant in Northwest China was put into production, the product quality was poor. After investigation, it was considered that the calcination temperature was insufficient, which was one of the influencing factors. For this reason, the discussion and practice of high temperature calcination were carried out.