Electrodes are the core of the smelting function of a submerged arc furnace, serving as a bridge for converting electrical energy into thermal energy. Domestic ferroalloy furnaces usually use low-cost self baking electrodes. In the smelting process of ferronickel furnaces, the normal consumption of electrode paste is about 8-12kg/t, with a very small cost proportion. However, the continuity of its consumption has an important impact on the stable operation and production indicators of submerged arc furnaces. With the gradual promotion of the “two carbon” measures by Chinese government, outdated production capacity is gradually phased out, and the construction of low-level smelting facilities is strictly prohibited. This will promote the development of submerged arc furnaces towards large-scale and efficient directions, and electrode hard fracture accidents will also have a more significant impact on the smelting of large submerged arc furnaces. Therefore, how to avoid hard breakage accidents of self baking electrodes is a long-term issue that needs to be taken seriously in the production of submerged arc furnaces.
A company operates two 36,000 kVA nickel-iron submerged arc furnaces, each equipped with three self-baking electrodes of 1,300 mm diameter. The electrode clamping system uses a modular clamp, and the electrode center-to-center diameter is 4,800 mm. During production, the company has experienced multiple incidents of electrode hard breaks, leading to significant losses.
In two consecutive months, the company’s nickel-iron furnaces encountered three electrode hard break incidents, resulting in a total downtime of 2,844 minutes and direct economic losses amounting to hundreds of thousands of yuan. According to production experience, a single electrode hard break incident typically requires at least five days to restore the furnace to normal conditions. If the break is too severe, it requires “dead phase” baking and gradual lowering of the electrode, which can cause even longer production delays.
① Internal causes of electrode hard breakage
The electrode paste undergoes a transformation within the electrode casing, progressing from a solid state to softening and melting, and finally sintering to form the electrode. The higher the degree of electrode graphitization, the greater its strength. If the electrode delaminates during the sintering process, it may experience hard breaks or top dropouts. The electrode paste is illustrated in Figure 1. During the smelting process, submerged arc operation is conducted on the nickel-iron furnace electrodes, and there are two current loops within the furnace: the primary loop, consisting of electrode-electrical arc-molten minerals-electrode, and the secondary loop, consisting of electrode-furnace lining-electrode. This creates a complex magnetic field within the furnace, where the electrode is not only involved in electrochemical reactions but also has to withstand high currents, high temperatures, electromagnetic forces, mechanical pressure, and material collapse impacts. As a result, the electrode must possess good thermal shock resistance, electrical conductivity, mechanical strength, oxidation resistance, and low impurity content. High-quality electrode paste is essential to meet the requirements of the ferroalloy furnace smelting process. The graphitized electrode has improved resistance to cracking and breakage. A schematic of the evolution of the electrode paste is shown in Figure 2.
Figure 1
Figure 2
②
External Causes of Electrode Hard Breaks
Excessive working end length: When the assessment of the working end length is inaccurate, frequent electrode pressing and lowering can result in an overly long working end. This increases current density, causing the electrode to lift, the crucible zone to shrink, and abnormal furnace conditions. Additionally, frequent operations on the electrode increase the risk of cracks at the electrode tip.
Low paste column height: A low paste column height can cause inadequate pressure at the lower section of the column, leading to loose filling, which is unable to sinter into a sufficiently strong electrode. This results in rapid electrode consumption and increased risk of breakage.
Unstable material supply: Multiple refractory failures in the rotary kiln can lead to frequent kiln shutdowns, causing significant fluctuations in the supply of calcined sand, requiring large-scale adjustments to the power of the submerged arc furnace in a short time, potentially resulting in electrode hard breaks.
Rapid power recovery after furnace shutdown: A quick recovery of current and power after a shutdown can raise the electrode temperature too rapidly, leading to electrode cracking and sintering delamination.
Water leakage from furnace cover: If cooling water from the furnace cover leaks onto the electrode surface, arcing can occur, causing uneven heating, with localized darkening. This can increase thermal stress in the electrode, leading to cracks and breaks. Examination of other electrodes that are functioning normally reveals no similar darkening, excessive cooling, or water/other liquid infiltration.
Electrode shell quality: Inspection of the electrode shells shows that the welds are fully penetrated, compact without slag inclusions or pores, and ground smoothly. There is no warping or bending, and both the interior and exterior are kept clean without foreign matter, meeting process requirements. This indicates that electrode shell quality is not a contributing factor to electrode hard breaks.
Improper operation: After investigation, it was found that only two individuals out of the three work teams lacked comprehensive knowledge of technical operations; however, their roles were not key contributors to electrode hard breaks.
Increase the secondary voltage; with higher voltage, the depth of electrode insertion becomes shallower. Raise the material level moderately and reduce the length of the feed pipe from 2.1 m to 2.0 m. When the feed pipe burns and shortens, the replacement frequency can be extended accordingly.
Rebuild the refractory lining of the rotary kiln to reduce downtime; if small segments of the refractory lining fall off, avoid frequent kiln shutdowns, opting for consolidated repair instead.
Disconnect the cooling water near the electrode on the furnace lid and switch to direct refractory casting. Control the electrode insertion depth and set the appropriate secondary voltage.
Adjust the electrode paste column height from 1.5 m to 2.8 m. Instruct personnel responsible for adding paste to regularly calibrate the rope to ensure the accuracy of the paste column height.
By implementing the relevant measures, the company’s ferro-nickel submerged arc furnaces have not experienced further incidents of electrode hard breakage. This has effectively improved equipment uptime and reduced production losses caused by electrode hard breaks, confirming that the electrode paste used by the company has no quality issues.
Conclusion: Based on production practices, statistical analysis revealed that the primary causes of hard breakage in ferro-nickel submerged arc furnace electrodes include excessive electrode tip length, unstable material supply, low electrode paste column height, water leakage from the furnace cover, and rapid recovery of current and power after furnace downtime. After implementing the corresponding corrective measures, incidents of electrode hard breakage in the submerged arc furnace have been successfully prevented.
