Change of electrode paste during firing
1. Roasting heat source
Electrode baking heat source mainly comes from three aspects
(1) The conduction heat of the molten pool: its heat accounts for about 60% of the total heat of the baking electrode, so the molten pool must be protected during operation
(2) Resistance heat: including the resistance heat generated by the electrode current above the material surface and the resistance heat generated by the contact between the copper and the electrode
(3) Furnace surface radiant heat: its heat conduction and radiation to the electrode have a lot to do with the complete sintering of the electrode.
2. The softening of the binder (pitch, coal tar) and the discharge of volatile matter in the electrode paste.
The solid electrode paste added into the electrode barrel will change from solid to plastic state as the temperature rises with the conduction and radiation of heat. At this time, volatiles are discharged.
3. Coking of binder
As the temperature rises, when the temperature is 4000-5000°C, a large amount of volatile matter of the electrode paste gas escapes, and the binder starts to be semi-coked due to decomposition and condensation reaction at this temperature. At this time, the main reason is that the degree of coking continues to increase, and a small amount of volatile matter is discharged. When the temperature reaches 6000-7500°C, the coking of the binder ends (becomes a coke network). The stacking fault coke network formed after the binder is coked firmly binds various solid particles together, the electrode reaches the final strength, and the electrode baking process is completed.
It should be noted that between 4000-5000℃, the physical properties of the binder change the most. At this time, if the firing speed is too fast, the carbon loss in the binder will be large, the anxiety will be reduced, and the internal stress of the electrode will increase. The so-called delamination phenomenon greatly reduces the strength of the electrode and is prone to hard fracture. If the firing speed is too slow, the electrode consumption and firing balance will be destroyed, and the binder in the liquid electrode paste will easily reverse ooze out of the unfired electrode, segregation will occur, and the electrode will break easily. In this temperature zone, the electrodes must be strictly managed and controlled.