Causes And Countermeasures Of Hard Fracture And Soft Fracture Of Electrode Paste in Submerged Arc Furnace

When the working end of an electrode has already been well sintered and breaks during the smelting process for some reason, this is called electrode hard fracture.

① Frequent hot shutdowns cause temperature instability.

If the furnace is started and stopped repeatedly, the electrode temperature becomes unstable. This generates internal stress inside the electrode, which may cause it to break.

② Rapid load increase after a cold restart following a hot shutdown.

After a hot shutdown, the electrode temperature is low. If the load is raised too quickly during power restoration, the electrode is likely to break. This is especially true when the furnace condition was viscous before the shutdown. If the working end of the electrode was too long before shutdown, the electrode is also more likely to break after power restoration. If the shutdown lasts for a long time and the electrode sticks to the furnace charge, it may be pulled apart when power is restored.

③ Poor electrode paste quality during normal smelting.

During normal operation, electrode hard fracture is often caused by poor electrode paste quality, such as:

▪excessive ash content in the electrode paste;

▪unsuitable particle size distribution of the raw materials; or

▪improper storage of the electrode paste, resulting in contamination such as dirt on the surface.

These problems deteriorate the sintering quality of the electrode and lead to breakage.

① Deal with the fault as soon as possible after power off to reduce hot shutdown time.

② Remove the broken electrode stub from the furnace if possible. If it is difficult to remove, lower the electrode appropriately and add a proper amount of silica and lime to accelerate oxidation. All broken electrode stubs in the furnace must be removed, regardless of size; otherwise, the electrode cannot penetrate deeply into the charge after power restoration.

③ Remove any electrode stub longer than 100 mm. If the broken stub length exceeds 100 mm, it must be removed; otherwise, the load will be unstable after power restoration and deep electrode penetration will be affected. If the broken section is too long, the electrode should be re-baked.

④ Lower the electrode carefully according to sintering condition. After electrode breakage, if the working end is relatively short, the electrode may be lowered appropriately based on its sintering condition. Do not lower the electrode if it has not been properly sintered, to prevent the accident from expanding. After power restoration, reduce the load on that electrode phase and wait until the electrode is well sintered before lowering it.

⑤ Increase load slowly after power restoration. After clearing the electrode breakage fault, increase the load gradually during power restoration and monitor the electrode condition carefully to prevent further electrode accidents.

Electrode hard fracture is usually caused by: excessively long shutdown time, or too rapid load increase after power restoration.

Therefore, the fundamental preventive measures are:

① Reduce hot shutdown time and shutdown frequency.

② Avoid an excessively long working end before shutdown, and maintain the electrode properly after shutdown.

(1) It is frequently caused by insecure electrode fixing during electrode slipping. Especially when oil exists on the electrode shell surface, the electrode may slide down shortly after slipping and trigger soft fracture. Soft fracture can also happen if both steel strips are released simultaneously while slipping the electrode. Failing to reduce the load of this electrode phase during slipping, insufficient load reduction, or excessively rapid load increase will easily lead to electrode soft fracture.

(2) Electrode slipping beyond the specified length, frequent slipping with short intervals will feed unsintered electrode section downwards, inevitably resulting in soft fracture.

(3) For newly started furnaces, the temperature is low and electrode sintering proceeds slowly, so soft fracture tends to occur after electrode slipping. Therefore, before slipping electrodes on a new furnace, carefully inspect the electrode sintering condition. It is recommended to start electrode slipping only after operating under normal load for 8 hours.

(4) Excessively high volatile content in electrode paste prevents normal electrode sintering, which causes soft fracture after electrode slipping.

(5) Electrode soft fracture is prone to occur after electrode hard fracture or paste leakage accidents.

(1) Once electrode soft fracture or paste leakage is detected, cut off power immediately. Do not lift this phase electrode. Increase air volume and reduce cooling water flow of the copper shoes by 1/2 ~ 1/3 to prevent all liquid electrode paste from flowing into the furnace.

(2) If the solid electrode section is broken but not fallen down, clamp it tightly by copper shoes for a length of about 200~300 mm at once. If the broken solid electrode has fallen into the furnace, reposition it and clamp with copper shoes, then lower this electrode phase for sintering. Meanwhile, remove residual electrode paste inside the furnace as soon as possible. Prevent paste from flowing into the crucible. When moving the other two phases of electrodes, do not lift them too high to avoid paste entering the crucible.

After all inflowed electrode paste is completely cleaned, restore power supply at low load. Control the current of the other two electrodes to sinter this electrode phase. Lifting is forbidden before sintering is completed.

(3) If the solid electrode part is not fully broken after soft fracture, weld and repair the broken section with electrode shell steel plate, then clamp it by copper shoes. Lower this electrode phase and energize at low load for sintering.

(4) If all electrode paste flows out after soft fracture, cut off the broken section by gas welding, weld a new electrode bottom filled with small pieces of electrode paste. First bake it with firewood for around 4 hours. Then lift the other two electrodes and control the current for baking and sintering.

After this electrode phase is fully sintered, lift it and resume power supply under low load.