Process control and method for firing start of large-scale pre-baked aluminum electrolytic cell

The characteristics and control process of the two roasting start-up technology methods for large-scale pre-baked aluminum electrolytic cells. The advantages and disadvantages and specific operation steps of the two roasting methods are discussed. There are many factors that affect the life of the aluminum electrolytic cell during the start-up, and the measures taken for the early damage of the pre-baked aluminum electrolytic cell during the firing and preheating start-up process.

1 Overview

The roasting start of modern large-scale pre-baked aluminum electrolytic cells. In recent years, most of the newly-built electrolytic aluminum plants in China have adopted two methods: aluminum liquid roasting and coke roasting, especially coke roasting. At present, it is widely used in new electrolytic aluminum plants. The roasting and preheating process technology used has the advantages of shorter starting and preheating time and less temperature gradient than molten aluminum roasting, and can make up for the defects of tank lining and material quality problems. However, it also has its shortcomings, that is, it is relatively short. The start-up operation of molten aluminum roasting is complicated, the technical requirements are high, the cathode current distribution is uneven, the electrolyte carbon content is too high, and the energy consumption is increased. There are also two firing start-up methods that are graphite powder firing start-up technology and gas firing start-up technology. The price of the former is too high, resulting in increased costs and complicated operation (only the Dan** aluminum plant in France has used the 114.5kA aluminum electrolytic cell in the start-up), the latter is easy to oxidize carbon blocks, and the equipment used for start-up is complicated and difficult to operate. Therefore, these two methods are rarely used by aluminum electrolysis manufacturers.

The preheating and firing start of the aluminum electrolytic cell is one of the important factors affecting the life of the cell, and the cell life directly affects the stability of the production cost of the aluminum electrolysis, especially the firing start of the large pre-baked aluminum electrolytic cell. However, no matter which technical method is adopted, it is almost impossible to avoid cracks or pores in the cathode carbon block and the inner lining. However, it is possible to prevent the cracks and pores from being immersed in the aluminum liquid. The start-up method of coke pellet roasting has this advantage. , It was applied earlier in Baiyin Aluminum Plant, and it has been widely used in domestic new aluminum plants and pre-baked tank manufacturers that have rebuilt self-baked tanks in recent years.

2 Aluminum reduction cell firing start-up technology

How to extend the lining life of large-scale pre-baked aluminum electrolytic cells is an important research topic in the domestic aluminum industry. The current domestic pre-baked aluminum electrolytic cell lining life is shorter than that of foreign pre-baked aluminum electrolytic cells by 2-3 years. There are many factors that affect the life of the lining of the electrolytic cell, which can be divided into five aspects: design, furnace construction, materials, roasting start-up, and production management. Among them, the choice of roasting method can be said to be the key link that affects the life of the aluminum electrolytic cell. Especially the firing and preheating start, although this process is only a few days, it has a decisive influence on the service life of the aluminum electrolytic cell.

2. 1 Comparison of several kinds of roasting start electrolyzer technology

The roasting start-up method of large-scale pre-baked aluminum electrolytic cells at home and abroad is widely used to start the lining preheating roasting. The specific methods include liquid aluminum roasting start, coke grain roasting start, graphite roasting start, and gas roasting start four technical methods.

At the end of the 1990s, the domestic pre-baking tank roasting started and put into production more commonly used traditional aluminum liquid roasting technology. Its biggest advantage is simple operation, uniform temperature rise in the later stage, and convenient control. In recent years, with the construction of the third domestic aluminum electrolysis high. The trend is to increase the size of the electrolytic cell (200kA to 300kA). At present, almost all domestic manufacturers use coke pellet roasting to preheat the start. It is understood that the Guiyang Aluminum Magnesium Institute and the Guizhou Aluminum Factory have jointly researched and developed a new roasting start method. This method has been applied to the precious aluminum 230kA tank. The technical indicators are much better than the above four methods. The specific technical method is to use a mixture. The technology is currently applying for a patent.

2.2 Control of the three heating stages of preheating and roasting in the pre-baked aluminum electrolytic cell

2. 1. 1 Low temperature preheating stage

The average temperature of the tank lining is controlled below 200°C. The main purpose of controlling the temperature during this period is to remove the water in the tank lining material, and at the same time alleviate the cathode carbon block, paste, cathode steel rod, and tank shell at the initial stage of firing start-up. The thermal expansion deformation rate between the different materials reduces the damage of the thermal stress of the inner lining caused by the difference in the thermal expansion coefficient of various materials, especially when the temperature is below 200 ℃, the flexibility of the cathode steel rod is very small, and the thermal expansion coefficient It is about 3-4 times that of the cathode carbon block. If the temperature rise rate is well controlled, it will avoid the early cracks of the cathode carbon block. During this period of time, the heating rate should generally be controlled at about 5°C per hour.

2. 2. 2 Middle temperature roasting stage

During this period, the temperature is controlled between 200°C and 600°C. The purpose is to discharge the volatile matter and crystal water in the lining material. At this time, the cathode steel rod has been transformed from thermal expansion deformation to creep deformation (the yield point of steel). 200℃), the flexibility increases. During this time, the main task is to bake the solid paste between the cathode gaps and the groove peripheral slits, so as to improve the bonding performance of the solid paste and the cathode carbon block. In a period of time, the heating rate can reach 10-20°C per hour.

2. 2.3 High temperature coking stage

The temperature is in the range of about 600°C. During this period of time, the side lining is baked at a high temperature. The purpose of temperature control is to make the cathode carbon block and the solid paste fully bond and coke into a whole rest to reach the conditions of normal production. During this period of time, it should be noted that after the temperature reaches 500℃, the solidified paste will expand and deform freely due to coking and transform into shrinkage deformation. At this time, the cathode carbon block is still in expansion and deformation. Therefore, the periphery of the groove is solidified. The paste itself will produce certain shrinkage cracks, so the heating rate must be controlled in time.

3 The technical method of starting the aluminum electrolytic cell by roasting molten aluminum

The use of molten aluminum as a resistor is called molten aluminum baking start (also called molten aluminum preheating method). A certain amount of liquid aluminum is injected into the electrolytic cell to be put into production to cover the surface of the cathode and contact the anode to make it It forms a current loop and generates heat to preheat the electrolyzer. Since the aluminum liquid itself has low resistance, most of the heat is generated by the cathode and anode. For large-scale pre-baked anode electrolyzers, the resistance value of the anode is very small after high temperature baking, such as the cathode. The semi-graphitized carbon block has a small resistance value, so the total calorific value is not too large. In this way, the aluminum liquid roasting starts the electrolytic cell to pass the full current at one time, and the anode is tightly fixed on the anode bus bar at a time, without adding the middle conductor. Compared with the start of the roasting of coke particles, the operation procedure is simplified, the temperature distribution is uniform, and the electrolytic cell does not appear to be overheated locally. It can reduce the fracture of the cathode carbon block due to the expansion deformation stress caused by the uneven distribution of heat.

Since the total amount of heat generated by the starting method of molten aluminum baking is not large, the anode must be insulated. Generally, cryolite is used to cover the anode and fill the gap between the anodes. In order to increase the heat to achieve the preheating effect, the anode can be slowly raised within a certain period of time after startup, increasing the pole distance, so as to achieve the purpose of increasing the temperature.

3.1 Technical characteristics of the start-up of molten aluminum incubation

does not use soft connections, shunts. The technical method is simple, easy to operate, no need to add any temporary facilities, uniform temperature distribution in the tank, no local overheating phenomenon, can better reduce the cracks of the cathode carbon block, can completely avoid the oxidation of the anode, the electrolyte after commissioning is pure, No impurities, save labor and materials.

3.2 Technical shortcomings of molten aluminum roasting start

A. When pouring high-temperature aluminum liquid (about 800-900℃), the cathode carbon block can be subjected to a strong thermal shock, which affects the life of the cathode carbon block and the lining.

B. The poor quality of carbon paste can cause the aluminum liquid to penetrate and cause early damage to the aluminum electrolytic cell.

C. Due to the low resistance of molten aluminum, the preheating temperature rises slowly.

4 The technical method of firing coke particles to start an aluminum electrolytic cell

The firing of coke particles is started (also called the preheating method of coke particles). The coke particles used are calcined coke particles with strong oxidation resistance and small volume density changes. A layer of forged coke is laid between the cathode and the anode. For granules, the particle size is about 2—3mm, the use of coke powder below 1mm is strictly controlled, and the laying thickness is about 10—20mm. The coke grain layer acts as a resistive conductor to generate heat between the cathode and anode, preheating the electrolytic cell, and at the same time, the resistance of the cathode and anode itself also generates heat, which is preheated inside, and temporary conduction is used between the anode guide rod and the anode bus bar. The flexible connection bus bar is connected so that the full weight of the anode is pressed on the coke particles to ensure good contact between the anode and the coke particles. The anode guide rod is tightly fixed on the anode bus bar before production is started, and the temporary flexible connection bus bar is removed.

According to the investigation and information, at present, the current of the large-scale pre-baked aluminum electrolytic cell manufacturers in China is not uniform, and most manufacturers decide according to the actual situation on the spot.

4.1 Preheat the electrolyzer

Build electrolyte and cryolite around the tank, and separate the cryolite from the coke particles with a separator, so that the carbon blocks at the edges and the surrounding paste are protected from oxidation during the preheating process. Part of the current is introduced during the preheating process. Increase the current to reach full current in about 24 hours under normal circumstances, but the increase speed of the current can be determined according to the preheating speed of the cell. After the current is fully loaded, the electrolytic cell should continue to be preheated for a certain period of time (depending on the cell temperature). So that the surface temperature of the cathode reaches 900-950°C before starting to start.

4.2 Technical characteristics of coke grain roasting method

The cathode carbon block and anode carbon block can be gradually raised from room temperature, avoiding strong thermal shock. The coke particle layer protects the surface of the cathode. In the case of using a shunt, the heating rate of the electrolytic cell can be controlled, and the cathode itself The heat generated can make the cathode carbon block start to be dried from the inside, thereby avoiding the crack aluminizing phenomenon in the sticking gap.

4.3 Disadvantages of coke roasting method technology

The surface of the cathode will have a local high temperature phenomenon, which is complicated to operate and increases the difficulty of the operation. It is also easy to cause poor preheating of the solid paste around the groove, and it is easy to cause cracks or gaps. After the start of production, the carbon content in the electrolyte was too high, which increased the workload of manually picking up carbon residue.

5 Started production of large-scale aluminum reduction cell

After the preheating of the aluminum electrolytic cell reaches 900-950℃, the production can be started. Its main task is to melt a sufficient amount of liquid electrolyte in the electrolytic cell. In the method and technology of starting the production, at present, the dry method and the wet method are usually used in the country to start the production of the aluminum electrolytic cell.

5.1 Dry start

Dry start-up is usually the technical method used in new aluminum electrolysis plants. Because there is no ready-made liquid electrolyte when starting production, it greatly increases the power consumption. Generally, it is used in the first one or two electrolysis cells.

Dry start is to use the high temperature arc generated between the cathode and the anode to melt the solid cryolite into liquid, and gradually add cryolite to the electrolytic cell, slowly lift the anode to generate a strong arc and form a high temperature. After the proper height of the liquid is in it, the anode effect can be initiated, thereby speeding up the melting of cryolite. When the liquid electrolyte in the electrolytic cell reaches 15-17cm, the Al2O3 extinguishing effect can be added to inject the liquid aluminum water to electrolyze the aluminum. The trough enters normal production.

At the beginning of dry start-up, due to the strong arc generated between the two poles, it will damage the surface of the cathode carbon block and the anode carbon block. In severe cases, it will affect the life of the electrolytic cell. When lifting the anode, you must be cautious and slowly lift it to prevent bounce. Blasting and destroying the tank lining is usually monitored by the level of the tank voltage. Generally, the tank voltage should be controlled at about 10-15V.

5.2 Wet effect start

The start of the wet effect is to inject a certain amount of liquid electrolyte into the electrolytic cell to be produced, and gradually raise the anode to cause artificial effects. The cell effect voltage should be controlled at about 20V. After the liquid electrolyte reaches 15-17cm, Al2O3 is added to extinguish the effect. At this time, the cell voltage should be kept high, and the cell voltage should be 6-8V, and a certain amount of liquid aluminum water should be injected into the cell as the in-producing aluminum, and the anode protective material should be added to make the aluminum electrolytic cell Enter normal production.

Wet start is more power-saving than dry start, simple operation, low labor intensity, safe and reliable, and will not damage the cathode carbon block and lining, but it is prone to chemical furnace and chemical anode steel claws.

5.3 Wet start without effect

In recent years, some aluminum electrolysis manufacturers have also adopted non-effect wet start. This method mainly injects liquid electrolyte into the electrolytic cell to be started, then raises the voltage to about 10V, and slowly melts the solid materials, but the start-up time is longer. , Its advantage is that the material volatilization loss is small during the start-up period, and the working site environmental conditions are better. This method should be used to start the electrolytic cell preheating should be appropriately increased by about 50 ℃ compared with other start-up methods to prevent the injected electrolyte from solidifying.

6 Conclusion

In the roasting start-up work of the large-scale pre-baked aluminum electrolytic cell, no matter what technical method is adopted, the input solid materials must be fully melted, and the electrolyte temperature should be slightly higher than the electrolyte temperature in normal production. This is because If the initial input solid material is not fully melted, it will be deposited at the bottom of the electrolytic cell. When the liquid aluminum water is injected, the temperature of the furnace bottom will decrease, making it difficult to melt and solidify, and it will form a hard block at the bottom of the electrolytic cell for a long time. The object affects the normal production of the electrolytic cell, increases the power consumption, and affects the life of the cathode and the lining of the cell. In addition, the heat dissipation loss on the newly-started electrolyzer is too large, and the tank lining will absorb a lot of heat energy for a long time after startup. If the electrolyte temperature is too low during startup, the electrolyte will easily drop rapidly, and it will be at the bottom of the electrolyzer. Precipitation is produced, causing the bottom of the electrolytic cell to be deformed.

Throughout the entire roasting and preheating start-up process, as long as the electrolytic cell is built, the solidification quality is qualified, and the quality of the lining material meets the requirements, and the roasting start-up is strictly operated in accordance with the regulations, the life of the electrolytic cell will reach or exceed the design value. The start-up and production will be smoothly transferred to normal production, and the best technical and economic indicators will be obtained.