The quality requirements of graphite electrodes for DC electric arc furnaces

Graphite electrodes used in DC arc furnaces have no skin effect when current passes, and the current is evenly distributed on the current cross section. Compared with AC arc furnaces, the current density through the electrodes can be appropriately increased. For the ultra-high power electric furnace with the same input power, the DC electric arc furnace uses only one electrode, and the diameter of the electrode is larger. For example, a 100t AC electric arc furnace uses an electrode with a diameter of 600mm, while a 100t DC electric arc furnace uses a 700mm diameter electrode. Electrodes, larger DC arc furnaces even require electrodes with a diameter of 750-800mm, and the current load is getting higher and higher. Therefore, the following requirements are put forward for the quality of graphite electrodes:

(1) The resistivity of the electrode body and the connector should be smaller. For example, the resistivity of the electrode body is reduced to about 5μΩ.m, and the resistivity of the connector is reduced to about 4μΩ.m. To reduce the resistivity of graphite electrodes, in addition to selecting high-quality needle coke raw materials, the graphitization temperature should also be increased accordingly.

(2) The linear expansion coefficient of the electrode body and the joint should be low, and the axial and radial linear expansion coefficient of the electrode body and the corresponding thermal expansion coefficient of the joint should maintain an appropriate proportional relationship according to the current density.

(3) The thermal conductivity of the electrode should be high. The high thermal conductivity can make the heat transfer in the graphite electrode body faster and reduce the radial temperature gradient, thereby reducing the thermal stress.

(4) It has sufficient mechanical strength. For example, the flexural strength of the electrode body reaches about 12Mpa, and the strength of the joint is much higher than that of the electrode body, generally higher than about 1 time. For the joint, the tensile strength should be measured, and the rated torque should be applied after the electrode is connected to maintain a certain tight pressure on both ends of the electrode.

(5) The porosity of the electrode should be low to reduce the oxidation consumption of the electrode surface.