Practice of reducing the electrolysis temperature in 200KA pre-baked aluminum electrolyzers
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Practice of Reducing the Electrolysis Temperature in 200KA Prebaked Aluminum Electrolyzers. Aluminum electrolysis, temperature, practice. Abstract: This article focuses on improving the basic operations related to the control of technical parameters and the actual conditions of electrolytic production in 200KA prebaked aluminum electrolyzers. Through more than a year of practical efforts, optimizing the control of technical parameters has led to good results in reducing the temperature during electrolytic production and increasing current efficiency. Keywords: aluminum electrolysis; technical requirements; basic operations; introduction. Modern society has long been plagued by energy problems, and the aluminum electrolysis industry is a major consumer of energy; therefore, saving energy and reducing consumption has become a key focus for enterprises in this sector. How to reduce the electrolysis temperature and improve current efficiency in order to achieve energy savings and reduced consumption is naturally a technical challenge that needs to be addressed in electrolytic production. As the process parameters in the aluminum electrolysis industry are evolving from the \"four lows and one high\" principle (low alumina concentration, low temperature, low molecular ratio, low efficiency coefficient, high cell voltage) toward higher values, significant progress has been made in achieving low cell voltages and low aluminum levels while still adhering to these \"four lows\" criteria. During this process, it is crucial to optimize and control the low electrolysis temperature as well as other technical parameters. The 200KA pre-baked aluminum electrolysis cells at Chinalco’s Guizhou branch have continuously explored ways to reduce the electrolysis temperature through the optimized adjustment of technical parameters, and certain results were achieved by the end of 2006. 1 Optimization control of technical conditions 1.1 Control of electrolysis temperature The electrolysis temperature refers to the temperature of the electrolyte in aluminum electrolysis production. This temperature consists of two components: one is the initial crystallization temperature of the electrolyte, and the other is the superheat of the electrolyte. Currently, the electrolysis temperature in 160KA aluminum electrolyzers is maintained between 925 and 955 degrees. In industrial production, the electrolysis temperature is the most important factor affecting current efficiency. It primarily influences the secondary reactions of aluminum; too high an electrolysis temperature increases these secondary reactions, reduces current efficiency, can melt the sides of the furnace, and increase material consumption, thereby leading to faulty electrolyzers. There are mainly two methods to reduce the electrolysis temperature: one is to lower the initial crystallization temperature of the electrolyte, and the other is to reduce its superheat. Lowering the initial crystallization temperature of the electrolyte is mainly achieved by improving and optimizing its composition, by selecting electrolytes with a low melting point. In aluminum electrolysis production, the main method for adjusting the composition of the electrolyte is to add additives such as aluminum fluoride, calcium fluoride, magnesium fluoride, lithium fluoride, and alkalis to the electrolyte in order to lower its initial crystallization temperature. Magnesium fluoride and lithium fluoride have a relatively significant effect, while aluminum fluoride can achieve the same result only at high concentrations. Due to the high cost of magnesium fluoride and lithium fluoride, our factory does not use them extensively in production; (however, we are planning to conduct experiments using lithium salts to improve the electrolyte’s composition.) We adjust the ratio of components by adding aluminum fluoride in order to reduce the electrolyte’s initial crystallization temperature. However, reducing the electrolysis temperature merely by lowering the initial crystallization temperature of the electrolyte does not result in a significant improvement in efficiency; an excessively low initial crystallization temperature leads to difficulties in production operations, such as increased deposition at the bottom of the furnace, an elongated furnace bottom, increased anode effects, and voltage fluctuations. Therefore, in aluminum electrolysis production, reducing the electrolysis temperature means not only lowering the initial crystallization temperature of the electrolyte, but more importantly, reducing its superheat. The superheat of the electrolyte is the difference between the electrolysis temperature and the initial crystallization temperature of the electrolyte; a lower superheat makes it easier to achieve better efficiency. Therefore, superheat is more important than the initial crystallization temperature. The relationship between superheat and current efficiency is such that lower superheat leads to higher current efficiency; an increase of 10°C in superheat results in a decrease of 1.2% to 1.5% in current efficiency. A lower superheat makes it easier to form side walls in the furnace, which reduces the mirror-like surface area of the cathode aluminum melt, thereby improving current efficiency. The main ways to reduce the superheat of the electrolyte in production are, first, improving the composition of the electrolyte, and second, enhancing the management of basic operations and improving the methods used in those operations. Through the efforts in 2006, including the optimized adjustment of various technical parameters and the management of on-site infrastructure work, as well as the improvement of process production methods, significant results were achieved only at the end of the year in terms of reducing the electrolysis temperature. The production indicators for 2006 are shown in Table 1 below: Table 1 Some Production Indicators of a Certain Work Area in 2006 Indicator Month April May June Cell Temperature (°C) 949 950.1 950.3 950 950.6 948.6 Mole Ratio 1.14 1.15 1.17 1.19 1.19 1.18 Voltage (V) 4.11 4.09 4.08 4.08 4.09 4.10 Two-Level Difference (cm) 21.1 22.2 20.7 21.9 20.8 21.4 21.2 21.2 20.8 20.7 21.1 20.8 Current Efficiency (%) 92.58 92.99 90.67 93.25 92.15 92.85 Indicator July August September October November December Average Cell Temperature (°C) 949.3 949.8 948.4 947.6 941.7 941.3 948.1 Mole Ratio 1.23 1.18 1.14 1.15 1.16 1.21 1.17 Voltage (V) 4.10 4.10 4.11 4.12 4.11 4.14 4.10 Two-Level Difference (cm) 20.5 21.2 19.8 19.9 19.5 20.8 19.8 20.7 19.2 20.9 19.5 20.0 20.3 21.0 Current Efficiency (%) 92.73 91.64 90.09 92.67 88.74 91.67 91.83 Selection of the 1.2 Ratio The mole ratio is one of the important parameters in aluminum electrolysis production; reducing the mole ratio can lower the initial crystallization temperature of the electrolyte, thereby reducing the electrolysis temperature. Although low-molecular-weight electrolytes have advantages such as a lower initial crystallization temperature, better fluidity, lower density, and significantly reduced secondary reaction losses of aluminum, they also have disadvantages including reduced solubility of alumina, increased resistivity, and increased viscosity of the electrolyte. Therefore, in the process of reducing the electrolysis temperature, it is not possible to rely solely on lowering the ratio; instead, it is necessary to select an appropriate ratio for control. Based on the relevant experimental data and formula calculations, the crystallization temperature of the electrolyte corresponding to a molecular ratio in the range of 1.19–2.5 is shown in Figure 1. Figure 1 Relationship between the electrolyte molecular ratio and the initial crystallization temperature. As can be seen from the figure, as the ratio decreases, the initial crystallization temperature of the electrolyte drops significantly; when the molecular ratio is small, the slope of the curve is large. This indicates that when the ratio is small, further reducing this ratio releases a large amount of heat; it becomes difficult to maintain the thermal equilibrium of the electrolyzer, and stable control of the electrolysis temperature is challenging. Therefore, in order to reduce the electrolysis temperature and maintain stable control while improving current efficiency, the ratio should be selected between 2.2 and 2.4; the corresponding initial crystallization temperature of the electrolyte is approximately 930°C. Based on the selection of the ratios mentioned above, it can be seen from Table 1 that the ratio control of our plant’s 200KA electrolyzers is generally in line with the requirements. However, the electrolysis temperature remains high, which indicates that reducing this temperature cannot be achieved solely by lowering the ratios.
1.3 Maintaining reasonable levels of molten aluminum and electrolyte: Keeping a high level of molten aluminum and an appropriate level of electrolyte has many advantages: (1) It facilitates the transfer of excess heat at the bottom of the electrolytic cell to the area around the anode, thereby increasing thermal stability； (2) Keep the bottom of the electrolyzer flat to ensure uniform current flow across the bottom of the furnace ； (3) It can better protect the cathode carbon blocks in the electrolyzer, reduce the formation of aluminum carbide, and extend the lifespan of the electrolyzer ； (4) Maintaining a high level of molten aluminum facilitates the separation of carbon slag from anode gases ； (5) Reduce or mitigate the influence of the magnetic field, thereby minimizing fluctuations in the aluminum melt within the groove and maintaining stable voltage. Maintaining an appropriate electrolyte level can: (1) better dissolve Al2O3, keep the Al2O3 concentration relatively stable, and reduce the anode effect ； (2) Reduce bottom sedimentation and lower the bottom pressure drop ； (3) Increasing the wettability of the anode helps to maintain voltage stability. The electrolyte level is determined based on the principle of not submerging or eroding the anode steel claws. Since 2005, the level of molten aluminum in our plant has generally been maintained between 19 and 22 cm, while the level of the electrolyte has generally been kept between 20 and 21 cm. Production experience shows that for 160KA pre-baked electrolytic cells, maintaining appropriate levels of molten aluminum and electrolyte is beneficial for stable production and improving various economic and technical indicators. 1.4 Control of alumina concentration: In current industrial aluminum electrolysis processes, the difficulty in controlling the alumina concentration lies in the fact that it cannot be measured in real time; instead, such control is achieved by analyzing data such as the resistance of the electrolyte. As the alumina concentration increases, the voltage shown on the voltage curve decreases over a certain period of time; conversely, as the alumina concentration decreases, the electrolyte resistance drops, causing the voltage shown on the voltage curve to increase over a certain period of time. If the alumina concentration in the electrolyzer remains stable within a narrow range during production, it reduces the cell voltage and controls the occurrence of anodic effects, which is highly beneficial for energy savings, reduced consumption, and stable production. Determining the relationship between the cell resistance of aluminum electrolyzers and the alumina concentration is a prerequisite for achieving such control and identifying the optimal alumina concentration. In the aluminum electrolysis process, the relationship between cell resistance and alumina concentration, as well as the impact of this relationship on aluminum electrolysis production, are extremely important. In the production at our factory, practice shows that the analysis of the alumina and calcium fluoride concentrations as well as their ratio in electrolyte samples is carried out every four days. Practice has shown that when the alumina concentration is kept below 3.5%, the current efficiency can reach over 90%. When the alumina concentration is within the range of 1.5% to 3.0%, the current efficiency can reach 92% to 93% or higher; in other words, as the alumina concentration decreases, the current efficiency increases. To meet the production requirements of electrolysis at low temperatures, the electrolysis plant selected the area with a lower alumina concentration as the range for feed control, keeping the alumina concentration within the range of 1.8% to 3.5%. It not only reduces the formation of deposits at the bottom of the electrolyzer furnace, but is also easy to monitor and identify using computers, thereby achieving stable operating conditions and higher current efficiency. It also creates conditions for predicting anode effects, which helps to reduce the anode effect coefficient. In the production process at electrolysis plants, the normal interval between discharges from 200KA aluminum electrolyzers is 4.3 minutes; the amount discharged each time is 4×1.8 kg, with discharges occurring simultaneously at four points. The issue of controlling the alumina concentration is essentially a matter of feeding interval, a matter of \"degree\". The intelligent fuzzy control system determines the alumina concentration based on the resistance of the cell, and automatically adjusts the feeding interval for cells with concentrations that are too high or too low. At the same time, by manually reviewing computer-generated reports and adjusting the feeding interval after performing edge processing on the electrolyzers to control the alumina concentration, so that underfeeding and overfeeding occur alternately as quickly as possible, both the alumina concentration in the electrolyte and the cell resistance tend to stabilize. A production process with a low alumina concentration is employed to achieve higher current efficiency. 1.5 Adjustment of operating voltage The cell voltage of an industrial aluminum electrolyzer refers to the voltage drop between the anode bus and the cathode bus of the electrolyzer, and this voltage can be measured directly using a voltmeter. The cell pitch of an electrolyzer refers to the vertical distance between the anode and the cathode in an aluminum electrolyzer, that is, the distance from the bottom of the anode to the horizontal surface of the aluminum melt. With factors such as current intensity, anode current density, and electrolyte conductivity remaining constant, the distance between the electrodes affects the magnitude of the cell voltage. Increasing the pole pitch helps improve current efficiency, while also increasing power consumption; however, improving current efficiency can reduce power consumption. Practice has shown that using a high pole pitch to achieve high current efficiency also helps to reduce power consumption. The following figure shows the relationship between slot voltage, current efficiency, and power consumption. As can be seen from Figure 2, as the slot voltage increases from V1 to V2, the current efficiency rises from A to B, while the power consumption decreases from w1 to w2. Figure 2 shows the relationship curves between cell voltage, current efficiency, and power consumption. To ensure the normal and stable operation of the electrolyzer, its pole distance cannot be adjusted arbitrarily; rather, a reasonable pole distance must be determined based on factors such as the cell design, current intensity, electrolysis temperature, and molecular ratio. Only by setting a reasonable pole distance can the balance of energy, materials, and voltage in the electrolyzer be maintained at normal temperatures and optimal production efficiency. With the advancement of technology in the aluminum electrolysis industry, the electrolysis temperature, molecular ratio, and anode effect coefficient are gradually decreasing. To maintain a proper thermal balance in the electrolyzer, it becomes very important to keep a large pole distance. However, if the pole pitch is kept too high, it leads to a disruption of thermal balance, resulting in the formation of heat pockets; if the pole pitch is kept too low, it reduces the heat input to the electrolyzer, increases the likelihood of secondary reactions involving aluminum, and lowers current efficiency. Therefore, maintaining a reasonable pole pitch is essential to ensure the efficient and low-energy operation of the electrolyzer. Considering the actual conditions of the 160KA aluminum electrolysis cells, the electrolysis plant sets the cell voltage between 4.10 and 4.20 volts, with a pole distance of around 5 cm. 1.6 Control of effect coefficients In aluminum electrolysis production, anode effects have both advantages and disadvantages. On the one hand, it enables monitoring of the normal operation of the electrolyzer, accelerates the separation and removal of carbon slag from the electrolyte, and makes the electrolyte cleaner ； On the other hand, it will lead to increased energy and material consumption, affecting the economic and technical indicators. But overall, the anode effect has more disadvantages than advantages, especially when intelligent fuzzy control is used; its impact is even greater, as it can raise the electrolysis temperature, cause the furnace walls to melt and thin out, reduce current efficiency, and increase power consumption. In electrolysis plants, maintaining the alumina concentration between 1.5–3.5% in 160KA aluminum electrolyzers allows for smooth exhaust of the anode gas, reduces gas film resistance, and minimizes the occurrence of anode effects. Low temperatures and a low molecular ratio also cause the anode effect coefficient to decrease. Therefore, the anode effect coefficient of the 160KA aluminum electrolyzer has been reduced from over 0.2 per cell-day to below 0.1 per cell-day. Practice has shown that proper control of the anode effect not only ensures the normal and stable operation of the electrolyzer but also reduces DC power consumption. After optimizing and improving various technical conditions, the production indicators for the period from January to June 2007 are shown in Table 2 below: Table 2: Some production indicators for a certain production area from January to June 2007. Indicator: January, February, March, April, May, June, Average; Tank temperature (°C): 941.6, 940.6, 940.4, 941.3, 942.2, 941, 941.2; Mole ratio: 1.19, 1.20, 1.18, 1.15, 1.20, 1.21, 1.19; Voltage (V): 4.13, 4.13, 4.12, 4.13, 4.13, 4.12, 4.126; Two-level value (cm): 19.8, 20.3, 20.0, 20.6, 20.1, 20.6, 20.2, 20.4, 19.6, 20.9, 19.5, 20.6, 19.9, 20.6; Current efficiency (%): 92.75, 92.8, 93.02, 93.17, 92.76, 92.86, 92.89. 2. Improvement of basic operations: The electrolysis temperature consists of the initial crystallization temperature of the electrolyte and its superheat; reducing the electrolysis temperature involves lowering both of these values. The optimized matching of the technical conditions mentioned earlier, along with the control of electrolyte composition and ratios, are primarily aimed at regulating the crystallization temperature of the electrolyte and the energy input to the electrolyzer. When it comes to reducing the overheating of the electrolyte, fundamental efforts to lower the electrolyte temperature are also of crucial importance. In electrolyzer production, daily operations mainly include anode replacement, quenching effect, edge processing, and removal of carbon slag, all of which also have a significant impact on electrolytic production. Anode replacement represents an interruption in the production process of the electrolyzer, but it also provides an opportunity to observe and adjust its condition. During this process, it is possible to assess the status of the remaining anode, the level of the electrolyte, the amount of carbon sludge, and the performance of the adjacent electrodes. It is feasible to deal with excessive carbon sludge, add materials to replenish the electrolyte, and repair the furnace walls at the site of anode replacement. After the anode is replaced and the furnace walls are repaired, it is necessary to shape the new anode and add insulating material; by adjusting the width of the heat dissipation zone and thinning the insulating material, it is possible to naturally form furnace walls that facilitate better heat dissipation, thereby helping to reduce the superheat of the electrolyte. The anode effect has certain advantages in electrolytic production, but its impact is greater; therefore, in daily production it is necessary to suppress the anode effect promptly and reduce its duration, which is generally kept within 3 minutes. Edge processing is carried out to repair the side walls over large areas; it serves to feed material, reduce temperature, cause contraction, and push the molten aluminum and electrolyte out of the furnace chamber. After edge processing, it is essential to tidy up the edges in accordance with the operating procedures. Removing carbon sludge is caused by poor anode quality and anodic oxidation; the excessive accumulation of carbon sludge in the cell poses many drawbacks to electrolyzer operation, so carbon sludge must be removed at least three times per shift. 3. Conclusion Practice has shown that by making appropriate optimizations to the process technical parameters and improving the basic operations of 160KA pre-baked aluminum electrolyzers, it is possible to achieve the following: (1) By optimizing the technical parameters, the electrolysis temperature of the electrolyzers can be reduced, allowing for stable operation of the production process with those parameters remaining constant. (2) Improvements in the basic operations of electrolysis production help to reduce the superheat of the electrolyte and keep it within a narrow range, thereby achieving the goal of lowering the electrolysis temperature. (3) Low-temperature electrolysis production helps improve current efficiency and reduces DC power consumption, thereby achieving the goal of energy savings and reduced waste. References: Yin Ensheng. Production Process and Management of 200KA Center-feeding Pre-baked Aluminum Electrolysis Cells. Central South University of Technology Press, 1997. Qiu Zhuxian. Principles and Applications of Aluminum Electrolysis. China University of Mining and Technology Press, 1998. He Junrong. Selection of Ratio Values for 200KA Aluminum Electrolysis Cells and Corresponding Production Measures. Qinghai Science and Technology, 2004(3):47. Liu Jianxin. Application of Low Ratio Values in Aluminum Electrolysis Production. Southern Metals, 2005(2):55. Dong Shiyi, Liu Yongqiang. A Brief Discussion on How to Achieve High Current Efficiency in Aluminum Electrolysis Cells. World Non-ferrous Metals, 2006(1):30. Li Zhenzhong, Yuan Weijin. Optimizing Technical Conditions in Aluminum Electrolysis Processes to Improve Current Efficiency. Aluminum and Magnesium Communications, 2005(2):33. Feng Naixiang. Aluminum Electrolysis. Chemical Industry Press, 2006. Qiu Zhuxian. Aluminum Smelting Using Pre-baked Cells. Metallurgical Industry Press, 2005.
If the temperature is too low, how can the solubility of alumina be addressed? It seems there is no good solution yet.
If the alumina concentration in the electrolyzer remains stable within a narrow range during production, it reduces the cell voltage and controls the occurrence of anode effects, which is highly beneficial for saving energy, reducing consumption, and maintaining stable production.