The electrode paste is basically non-conductive before 450 ℃, and all the current passes through the electrode shell. At 500 ℃, the current through the electrode paste gradually increases. After 800 ℃, more than 50% of the current flows through the electrode paste. As the temperature continues to rise, with the oxidation and ablation of the electrode shell, the resistance also increases sharply, while the resistance of the electrode paste decreases correspondingly, and all the current is borne by the electrode paste. The effect of using electrode paste on the current distribution law at the same temperature is not obvious. For example, when using standard paste at 500 ℃, 0.82% of the current passes through the electrode paste, and when using special paste, it increases to 2.34%. At 600 ℃, the standard paste was 9.63%, and the special paste was 12%. Therefore, the low-temperature conductivity of the special paste was not significantly improved. It has good thermal conductivity and can improve the sintering temperature of the electrode, thus improving its conductivity. This is the main reason why the closed paste is suitable for use in the closed furnace.
3. Roasting temperature of electrodes in submerged arc furnace
The actual temperature of the electrode paste in different parts of the electrode must be known before the working characteristics of the electrode can be finally found out. In order to measure the actual baking temperature of the electrode paste under industrial production conditions, we installed two Ni Cr Ni Al thermocouples in the No. 3 electrode of the No. 2 electric furnace of a factory. One is fixed to the electrode and the other is installed at the position 50M / m from the edge of the electrode shell. As the electrode is pressed down, the thermocouple moves down gradually. According to the data obtained from this measurement, we have a clear concept of the heat source of electrode paste baking.
As for the existing electric furnace structure of a factory, the heat source of electrode baking can be analyzed in three stages:
The section is from the paste surface to the sealing ring, and the temperature rises from normal temperature to 270 ℃. The roasting heat source of this section is basically Joule heat generated when the current passes through. The temperature of the electrode paste increased by only 25 ℃ from about 235 ℃ at the time of copper tile to 270 ℃ at the time of sealing ring. The electrode paste will stay for three and a half days during this period, and the position will move down by more than one meter. However, the temperature only rises by 25 ℃, which shows that the effect of other heat sources (such as conduction and radiation) is very small. Joule heat of the electrode * and heat dissipation to the surrounding space have basically reached a balance. The temperature of the electrode paste tends to be stable and does not rise. Only by continuously increasing the load of the electric furnace and increasing the current intensity can this part of the electrode paste reach a higher temperature, but its potential will not be great (the rated power of the furnace is 16500kVA). This stage is the softening and preheating stage of the electrode paste.
The * section is from the upper plane of the sealing ring to 700 mm into the furnace. The temperature of the electrode paste in this section is increased from 270 ° C to 850 ° C, which is the main sintering stage of the electrode paste, with a large amount of volatiles volatilized. When the position of the electrode paste is at the same level with the lower surface of the furnace top, the temperature can reach 740 ℃, which is close to the burning state. It has certain electrical conductivity and mechanical strength, and the possibility of accidents is greatly reduced. The heat source of this part mainly depends on the heat conduction of furnace gas. During normal production, the furnace gas temperature is 450-550 ℃, and the electrode is in the high-temperature gas flow. In addition, the Joule heat generated by the self energization causes the temperature to rise rapidly. When the electrode is just put into the furnace, the heat conducted from the lower part along the fired electrode does not play a major role. It can be seen from the shape of the 800 ℃ isotherm that the temperature of the electrode * here is about 60 ℃ lower than the edge temperature, indicating that the heat transferred from the lower part is not enough to raise the temperature of the electrode * as high as the edge. At 850 ℃, i.e. after entering the furnace 700m / m, the distance from the slag surface is about 1200m / m, and the electrode * temperature and the edge temperature are equal, which proves that the heat transferred from the lower part along the electrode has accounted for a considerable proportion.
The third section is from the end of the electrode to 1200m / m above the slag surface, which is the working end of the fired electrode and is about 1.4-1.5m long. In this section, due to the good thermal conductivity of the fired electrode and its close proximity to the high temperature zone of 1500 ℃ ~ 1600 ℃, the heat of the electrode mainly comes from the heat conduction of the lower part, and the temperature of the electrode * is higher than the temperature of the electrode edge.