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Abstract: This paper analyzes the causes of bottom damage in intermediate-draw prebaked cell furnaces, and proposes specific repair methods and maintenance techniques for such damage, thereby effectively extending the service life of the damaged electrolyzers and achieving certain economic benefits. Keywords: intermediate discharging pre-baked cell ； Cracked furnace bottom ； Repair ； Maintenance ； Lifetime. Damage to electrolyzers during production is inevitable; after several years of operation, the bottom of the electrolyzer becomes damaged and operation must be halted due to the expansion of the cathode carbon blocks as a result of sodium absorption, as well as the erosion of the electrolyzer’s lining by molten aluminum and highly corrosive electrolytes. It affects yield and other economic benefits. For our company’s 75kA intermediate charging pre-baked cells, the time required to go from shutdown to restart after major maintenance is approximately 19 days. The cost of major maintenance for each cell is around 60,000 yuan; adding the costs associated with starting up the baking process and the daily losses incurred, the economic losses are significant. Therefore, strengthening the repair and maintenance of damaged electrolytic cells in order to maximize their service life is a matter of concern for the entire aluminum electrolysis industry. The author discusses the techniques for repairing and maintaining damaged electrolyzers over the years. 1. Methods for determining damage to pre-baked cells 1.1 Determination based on the increase in iron content in primary aluminum. An indication of cell damage is an increase in the iron content within the cell; the normal iron content in such cells is generally less than 0.14%. If, during normal operation of the cell, there is no melting of the anode steel claws, and no iron-containing tools or substandard aluminum are added, yet the iron content in the primary aluminum suddenly increases and continues to rise over time, it can be preliminarily concluded that the bottom of the cell has begun to damage. The high-temperature molten electrolyte and aluminum melt have seeped into the gaps in the damaged carbon blocks, starting to melt the cathode steel rods and thereby increasing the iron content in the primary aluminum. 1. 2 It can be determined based on the variation in the equal-spacing voltage drop across the cathode busbars. In a normally operating electrolyzer, the equal-spacing voltage drops across the various cathode steel rod busbars are generally the same, as the material, cross-section, and length of these busbars are identical. The equal-spacing voltage drop across the cathode busbars is proportional to the current flowing through them. In the case of damaged grooves, since a channel for molten aluminum has formed at the bottom of the furnace, the local resistance there decreases, the current flowing through it increases, which leads to an increase in the equal-spacing voltage drop across the cathode busbars. 1.2.1 Tools used for measurement The measurement method is very simple; no special tools are required, as an equal potential drop meter commonly used in anode operations can be utilized for this purpose. Measurement location: The aluminum soft strip between the cathode steel rod and the main busbar. During measurement, avoid the solder joints at both ends of the busbar; use a relatively flat piece of flexible aluminum tape and ensure good contact at the contact points. To reduce measurement errors. After completing the measurements and analyzing each measurement point, it was found that the equivalent pressure drop at the damaged areas on the bottom of the electrolyzer furnace increased significantly, indicating signs of damage around that cathode steel rod. 1.3 Determination using temperature differences of the cathode steel rods: Under normal circumstances, the structure at the bottom of the electrolytic cell is basically consistent. Therefore, the temperature difference between the cathode rods ranges from 5 to 25°C. When damage occurs around a particular cathode steel rod, creating a path for aluminum melt, this not only accelerates the heat transfer between the bottom of the cell and the cathode rod but also causes current to concentrate at the damaged area, resulting in an increased current density in that rod and a significant rise in the Joule heat generated at its tip. Raise the temperature of the cathode rod at the damaged area. This confirms signs of damage in that area. 1.3.1 Measurement tools and methods: Tool used for measurement: Infrared thermometer. Measurement site: Cathode rod tip. Remove dust from the surface before measurement. For some more noticeable cathode steel rods, multiple measurements should be taken to reduce errors. In practical work, the above three methods should be applied simultaneously and analyzed comprehensively to effectively reduce errors. Accurately determine the damaged area of the electrolyzer. For over a decade, we have used three methods to determine the damaged areas of electrolyzers, and there have been no cases of misjudgment. It effectively prevented trough leakage accidents caused by bottom damage of the furnace. 2. Methods for repairing damaged electrolyzers 2.1 Identification of the repair area When repairing an electrolyzer with damage to its bottom, it is first necessary to determine the location, extent, and severity of the damage in order to take appropriate actions. The inspection method is as follows: a long iron rod is made according to the width of the electrolyzer furnace chamber; for example, if the width of our company’s electrolyzers is 2.93 meters, a rod with a length of 2–3 meters will suffice. This long iron rod is bent into a right-angled hook measuring 10–20 cm, with the tip of the hook pointing downward toward the cathode base. By following the vertical and horizontal arrangement of the base plates and seams, the hook is used to probe gradually in the areas where damage is suspected, in order to locate the damaged spots. Based on years of experience, we have found that at the sites of damage, the resistance is low so more electricity flows through those areas; as a result, the bottom surface there is clean and free of deposits. This characteristic makes it easy to identify the damaged areas. During the inspection, since everyone’s perception varies to some extent, it is necessary to have multiple people conduct the inspection and carry out a comprehensive analysis of the perceptions in order to avoid individual bias. One must also be careful and thorough, applying even force while avoiding excessive pressure that could worsen the damaged area. 2. Preparation of materials and tools for repair: Materials: magnesia, calcium fluoride, crushed furnace bottom slag (particle size ≤ 4 cm), and furnace scraping material. Tools: Ordinary operating tools used regularly are sufficient; there is no need to create specialized tools. Calcium fluoride and magnesia are substances that remain in a precipitated state and are insoluble in aluminum melt; they are suitable materials for repairing bottom damage in furnaces. For ease of use when repairing damaged grooves, a patching material in the form of a cake about 10 cm thick can be prepared by mixing aluminum melt with dry magnesia and calcium fluoride. Its size depends on the size of the area to be repaired. 2.3 Specific procedures for repairing damaged electrolyzers: Before making repairs, it is necessary to preheat the repair tools, especially in winter. Lift out the anode, remove any sediment from the bottom of the furnace, place the prepared material for repairing the furnace on a preheated ladle, hold it in place with a soldering iron or a large hook, and deliver it precisely to the damaged area; this process requires coordination between two people. After placing the materials for repairing the furnace, take a few aluminum ingots and gently press them down on top of those materials. Use a soldering iron to apply pressure for a few minutes, so that the area around the repair materials cools down and a crust can form; this helps the repair materials to cover the damaged area. Carefully monitor the changes in the iron content in the raw aluminum; if the iron content starts to decrease after one or two days, it indicates that the repair was successful; otherwise, the repair must be repeated. The repair material forms a crust at the damaged area, preventing the penetration of molten aluminum. Therefore, a decrease in iron content is a sign of successful repair. However, its effectiveness will not last long; it needs to be checked at regular intervals and repaired again. 2.4 Minimize electrical conduction at the damaged area. By accurately locating the cathode steel rods surrounding the damaged area, given the high temperatures and voltage drops, the flexible straps connected to the main busbars should be cut off (no more than 2 sets should be cut off in our company’s 75kA pre-baked cells), thereby breaking the circuit and preventing the cathode steel rods from melting faster due to excessive electrical conduction. Care must be taken during operation to accurately identify the steel rod that is being melted; otherwise, the opposite result will occur. 3. Remedial methods for electrolyzers with bulging and damaged bottoms. Such damaged electrolyzers suffer damage primarily due to the expansion of carbon materials caused by the absorption of sodium; the damage usually occurs in the middle of the electrolyzer, and its exact location is difficult to determine. It tends to occur in electrolyzers that have been in use for more than 3 years, and the success rate of repairing such electrolyzers is extremely low, at only 5% (based on Long Aluminum’s experience). The main material used for repair is calcium fluoride. The repair method is to add 500–800 kg of calcium fluoride in several batches to the material tank, and then mix it with alumina before adding it to the tank. Due to its high specific gravity, calcium fluoride tends to settle at the bottom of the furnace, covering the damaged areas there and preventing further penetration by the aluminum melt and electrolyte. This process needs to be repeated in order to successfully repair a small number of damaged electrolytic cells. Our company’s electrolyzer No. 39 has been in use for over 4 years, and it suffers from severe bulging at the bottom of the furnace. At the beginning of May this year, the iron content there exceeded 0.55%. After comprehensive analysis, it was determined that this electrolyzer is damaged, though it is difficult to identify the exact location of the damage. This is the only way to remedy it. 700 kg of calcium fluoride was added to the tank in two batches, and certain maintenance measures were taken; after one week, the iron content dropped to 0.19%, and the electrolyzer returned to normal after half a month. The efficiency reached 90%. However, due to the upward movement of the furnace bottom, daily maintenance is difficult. Production continued until November 2004, when shutting down the tank was planned. During tank shutdown, the iron content in primary aluminum remains stable at 0.15%. There are no signs of damage. During furnace trimming, traces of melting were found on the 5th and 6th cathode rods in the middle, indicating that the repairs carried out in May were successful. 4. Maintenance techniques for damaged electrolyzers: Once an electrolyzer is damaged, every effort should be made to extend its service life, depending on production requirements and efficiency considerations. In addition to repairing damaged electrolyzers, necessary measures must also be put in place to enhance maintenance. Through long-term practice, we have developed a set of maintenance methods for repairing damaged grooves. 4.1 Adjustment of technical conditions: After repairing the electrolyzer, in order to allow the repaired material to cool and harden, it is necessary to cool the bottom of the furnace. This is achieved by raising the aluminum level by 2–3 cm, thereby improving heat dissipation from the furnace bottom; this helps the repaired material to harden and reduces the interval between material additions, while also ensuring that there is a certain amount of sediment at the furnace bottom. The pressure drop at the repaired furnace bottom increases by about 30–60 cm, which allows the cell voltage to be increased by 50–100 mV. During maintenance, it is essential to strictly control the anode effect and avoid prolonged burning effects, keeping the anode effect below 0.3 times per cell per day. The technical conditions for maintenance are shown in Table 1. Table 1 shows the technical parameters for damaged electrolyzers. Parameter categories include aluminum level in cm, electrolyte level, molecular ratio, cell temperature in °C, efficiency coefficient, and cell voltage. For normal cells: 20–22 cm for aluminum level, 20–22 cm for electrolyte level, a molecular ratio of 2.4–2.65, a cell temperature of 935–945 °C, an efficiency coefficient of ≤0.4, and a cell voltage of 4.25–4.3 V. For damaged cells: 24–27 cm for aluminum level, 17–20 cm for electrolyte level, a molecular ratio of 2.5–2.75, a cell temperature of 935–950 °C, an efficiency coefficient of ≤0.3, and a cell voltage of 4.3–4.35 V. Maintaining an appropriate balance of these technical parameters is essential for ensuring that the electrolyzer can return to normal operation after repair; otherwise, the effectiveness of the repair efforts will be significantly reduced. These technical parameters have played an important role in our company’s efforts to repair damaged electrolyzers. It has been promoted in our company. 4.2 Daily maintenance management of damaged electrolyzers To enable damaged electrolyzers to gradually return to normal operation, in addition to adjusting the technical parameters, appropriate operating procedures must also be established; maintaining stability is of utmost importance to prevent fluctuations in the electrolyzer’s performance caused by improper operation. In the maintenance of cells with damage, the author believes the following points should be emphasized: 1) For electrolytic cells that have been repaired successfully, it is necessary to regularly check the condition of the repaired areas, and repairs should be carried out multiple times – it’s not enough to fix them just once. Repatch the damaged areas every 20 to 30 days. 2) Adjust the feeding interval appropriately to ensure the normal operation of the shell-breaking and feeding system, and strictly control the anode effect by keeping its duration under 4 minutes. 3) Strictly control the aluminum output rate to prevent large fluctuations. Maintain stable groove conditions. 4) Closely monitor changes in iron content, measure the temperature of the tank daily, and prepare the necessary tools to prevent furnace leakage accidents. 5) Improve the operating quality of the anode, prevent the occurrence of abnormal voltages, and regularly remove carbon slag. Keep the electrolytes clean. 5. Effect analysis: Over the years, we have done a great deal of work in repairing and maintaining damaged electrolyzers, accumulating extensive experience in doing so. This has enabled us to effectively extend the service life of these damaged units, thereby generating certain economic benefits for the company. 6. Conclusion In aluminum electrolysis production, the damage of electrolyzers is inevitable; however, by improving the management of damaged electrolyzers, carrying out effective and scientific repairs, and providing proper maintenance, it is possible to get some of these damaged electrolyzers back to normal operation and extend their service life.