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1 Overview The distribution of heat losses in aluminum electrolysis cells is an important characteristic reflecting the operating conditions of these cells. By measuring, calculating, and analyzing the heat loss distribution in aluminum electrolysis cells, it is possible to conduct quantitative analysis and scientific evaluation of the energy distribution patterns in different areas, as well as the suitability of process parameters and operating procedures. This provides a scientific basis for implementing targeted technical improvements aimed at enhancing the key technical and economic indicators of aluminum electrolysis production. At present, there are many types of aluminum electrolysis cells in China, and this article focuses on the heat dissipation characteristics of large and medium-sized aluminum electrolysis cells in China with a capacity of 200KA to 300KA. 2 Methods for measuring heat loss in electrolyzers: The thermal balance system of an electrolyzer is defined as the closed physical interface formed between the aluminum electrolyzer and its environment, namely the top of the cell, the cell cover, the cell shell, and the bottom of the cell; this includes the ribs and cradle frames that are involved in heat transfer at these interfaces. Heat flow is calculated in units of kilowatts (Kw) of electrical power transferred per unit time. To accurately measure the heat loss in aluminum electrolyzer systems, it is necessary to divide virtual areas on the surface of the system using longitude and latitude lines, and to place measurement points at those locations. The side surface of the cathode tank shell is divided into a melt zone (electrolyte + molten aluminum), a cathode carbon block zone, and a refractory insulation zone along the latitude direction, with the cradle frame serving as the meridian to define the measurement point areas. The same method is used for zone division and point placement measurement for the groove cover, groove bottom, and groove top. In addition, the measurement points also include heat dissipation components such as the cradle frame that protrudes or emerges from the surface of the system, the anode rods, the cathode steel bars, and the trough edge plates. Based on the measured heat flux density and the area of the measurement zone, the surface heat loss of the groove system can be calculated ； Based on the measured flue gas flow rate and temperature, it is possible to calculate the heat carried away by the flue gas through the flue. 3 Basic characteristics of heat dissipation in large and medium-sized pre-baked aluminum electrolyzers in China 3.1 Basic characteristics Over the past two years, thermal balance measurements taken on the electrolyzers in more than 20 aluminum plants in China have revealed that the heat dissipation losses in these large and medium-sized pre-baked aluminum electrolyzers exhibit the following two notable characteristics: (1) The heat loss in China’s large and medium-sized pre-baked aluminum electrolyzers is around 2.0V; compared to 10 years ago, this value has decreased by approximately 200mV. (2) At the current stage, in China’s large and medium-sized aluminum electrolyzers, the proportions of heat losses in the upper and lower sections are 55.0% and 45.0%, respectively. As shown in the figure. It differs slightly from the previous ratios of 50.0% and 50.0% for heat loss in self-baking cells; the main reason for this difference is that the cathode carbon blocks in self-baking cells are relatively thin (about 400 cm), while the cathode steel rods are usually square-shaped and extend a considerable distance outside the cell shell (about 40 cm). As a result, the proportion of heat loss due to cooling of the cathode steel rods is relatively high (about 7%). Furthermore, the upper part of the self-baked cell consists of a single piece of anode, making it difficult for heat to dissipate; as a result, the heat loss from the upper part is slightly lower compared to pre-baked cells. 3.2 Detailed analysis of heat dissipation characteristics By testing and analyzing the heat dissipation characteristics of prebaked aluminum electrolyzers, it is possible to fully utilize the structural advantages of the prebaking mechanism and tap into energy-saving potential, thereby providing a basis for identifying key areas of focus and implementing targeted technical measures in the future. Here, the anode area (upper part) and the cathode area (lower part) of the electrolyzer are discussed separately. 3.2.1 Analysis of heat dissipation characteristics in the anode area: The heat dissipation loss in the anode area is the sum of the heat dissipated from the top of the cell, the cell cover plates, the anode rods, and the cell side plates, plus the heat carried away by the flue gas. The heat loss in the anode area of large and medium-sized electrolyzers in our country is generally around 1.0V–1.2V, with an average value of around 1.1V. There are significant differences in the heat loss from the anode area among various aluminum plants. This is mainly due to the influence of flue gas flow rates and the condition of the material on the upper part of the anode area. However, we cannot assume that cells with high heat loss from the anode area are in poor condition, nor that those with low heat loss are in excellent condition. A comprehensive analysis taking into account both the heat loss from the anode area and the overall heat loss of the cell is necessary to draw correct conclusions. Since the heat loss due to the trough side plates and anode guide rods in the anode cooling area is relatively small, the heat dissipation in the anode area can be analyzed primarily by examining the heat dissipation characteristics of the flue gas, the trough top, and the trough cover plates. Under normal circumstances, a higher flue gas flow rate results in more heat being carried away, while the opposite is true ； When the condition of the electrolyzer cover material remains unchanged, the heat dissipation of the tank cover plate is significantly affected by the flue gas flow rate ； The heat dissipation characteristics of the slot top are basically similar to those of the slot cover plate; furthermore, the alumina deposition on the slot top also has an impact on them. At present, the actual flue gas flow rate in large and medium-sized pre-baked aluminum electrolyzers in China generally differs significantly from the design value (usually less than 3/4 of the design value). This indicates that increasing the flue gas flow rate can allow for the removal of about 5% more heat (in Kw), thereby providing manufacturers with more room for heat dissipation in order to increase the current applied. 3.2.2 Analysis of heat dissipation characteristics in the cathode area: The heat dissipation loss in the cathode area is the sum of the heat dissipation amounts from the melt zone, the cathode area, the refractory insulation zone, the cradle frame, and the bottom of the tank (including both the bottom of the tank and the cradle frame). In domestic large and medium-sized aluminum electrolysis plants, the heat dissipation loss in the cathode area is generally around 0.85V–1.05V, with an average value of around 0.9V. The heat dissipation characteristics of the cathode area, the refractory insulation area, and the side cradle frame in the cathode heat dissipation zone differ not significantly; therefore, the focus here is on analyzing the heat dissipation losses in the melt area and the furnace bottom. The amount of heat dissipation in the melt zone is related to the material and thickness of the lining on the sides of the electrolyzer, the degree of regularity of the furnace walls, and the electrolysis temperature ； The amount of heat dissipation from the furnace bottom is primarily related to the design and material of the lining layer at the bottom of the electrolyzer, the degree of damage to the furnace bottom, and the lifespan of the electrolyzer. The materials and thicknesses of the cathode side blocks in electrolyzers used in various factories in our country vary. There are basically two types of structures: for 200KA-class electrolyzers, ordinary carbon blocks with a thickness of 120 cm are generally used, while for 300KA-class electrolyzers, SiC materials with a thickness of 90 cm are typically employed. In 200KA-class electrolyzers, the temperature of the cell shell in the melt zone is generally between 240–290°C, with the heat dissipation rate typically ranging from 15–25%. In 300KA-class electrolyzers, the temperature of the cell shell in the melt zone is generally between 260–330°C, with the heat dissipation rate typically ranging from 18–25%. The proportion of heat dissipation in the melt zone directly reflects the thickness and regularity of the side walls of the electrolyzer. In electrolyzers with thick side walls, the temperature of the tank shell in the melt zone is lower (around 260°C for ordinary carbon blocks, and below 300°C for SiC materials), and the heat density is lower as well (below 4200 W/m2 for ordinary carbon blocks, and below 6400 W/m2 for SiC materials). In contrast, in electrolyzers with poor side wall quality, the temperature of the tank shell in the melt zone is higher (around 290°C for ordinary carbon blocks, and around 370°C for SiC materials), and the heat density is higher (around 6500 W/m2 for ordinary carbon blocks, and around 9500 W/m2 for SiC materials). The thickness and regularity of the side walls depend mainly on the initial crystallization temperature of the electrolyte and the degree of superheating. In the electrolyzer, the interface between the furnace wall and the electrolyte is a dynamic process of solidification and melting of the furnace wall; when the temperature at this interface is higher than the initial crystallization temperature of the electrolyte, the furnace wall melts ； When the temperature at the interface is below the initial crystallization temperature of the electrolyte, the solidification of the electrolyte causes the furnace walls to thicken. Therefore, a relatively stable initial crystallization temperature of the electrolyte and control of a low superheat are crucial for forming well-structured furnace walls. A stable molecular ratio, fluoride content, and thus electrolyte composition, along with maintaining the alumina concentration within a narrow range, contribute to the stability of the electrolyte’s initial crystallization temperature ； Reducing the effect coefficient, shortening the effect time, and minimizing voltage fluctuations are all beneficial for controlling overheating and for the formation of the furnace lining. The heat dissipation from the bottom of the furnace is primarily determined by the thickness of the refractory insulation layer on that area and the materials used. Currently, the thermal conditions at the bottom of large and medium-sized aluminum electrolysis cells in China are generally good; the temperature of the cell floor is usually between 80°C and 110°C, with heat dissipation accounting for around 6.7% to 9.2% of the total heat loss. However, if the mechanism base is damaged, the local temperature can exceed 160°C, with heat dissipation accounting for over 11%. 3.3 Characteristics of the heat flow distribution in a good electrolyzer: During stable operational conditions, if the voltage, current, and current efficiency remain constant, as do the electrolysis temperature, the amount of alumina consumed per ton of aluminum produced, as well as the gross and net losses at the anode, then the total heat loss of the electrolyzer also remains essentially unchanged. Under normal circumstances, the heat loss from the bottom of the furnace changes little; if the furnace walls are well-structured, then the heat loss from the sides remains relatively constant, which in turn results in a constant heat loss from the upper part as well. On the other hand, if the furnace walls are irregular or the furnace chamber is relatively empty, it is possible to increase heat dissipation at the upper part by thinning the lining material on the shell surface and increasing the flue gas flow, thereby promoting the formation of side furnace walls ； Conversely, if the operations are not carried out properly, the condition of the covering material cannot be maintained constant, the flue gas flow is not steady, and the heat dissipation from the upper part is not consistent, this is highly detrimental to the formation and maintenance of the furnace lining. With the continuous advancement of aluminum electrolysis technology, the cell voltage continues to decrease (to around 3.9 V). Better insulation is provided at the upper part of the electrolyzer, and the side walls are made thicker and more uniform, which will further reduce the total heat loss in the electrolyzer to below 1.8 V. 4 Conclusion By considering the characteristics of heat loss in large and medium-sized aluminum electrolysis cells in China, it can be seen that in future efforts to save energy and reduce consumption, it is necessary first to create a well-structured furnace interior within the electrolysis cells, to minimize heat loss from the sides, and at the same time, to adjust the heat loss from the upper part to an appropriate level, taking into account the cell voltage. On the other hand, when increasing the current, companies must pay close attention to maintaining stable thermal balance on the sides. By appropriately reducing the thickness of the material affected by smut and increasing the flow rate of smoke, it is possible to ensure that any excess energy (heat) is dissipated from the upper part of the cell, thus preserving an ideal internal structure for the electrolyzer and enabling its long-term, stable, and efficient operation. References: Shen Xianchun, Zhang Ailing. Physical field characteristic parameters of aluminum electrolysis cells and their measurement techniques. Light Metals. 1997(12): 25-29. Shen Xianchun, Zhang Ailing, et al. Testing and analysis of the physical fields in 160KA aluminum electrolysis cells – Energy balance testing and analysis. Light Metals. 1999(2): 27-33. Qiu Zhuxian. Aluminum smelting by pre-baking method. Beijing: Metallurgical Industry Press, 2005. Feng Naixiang. Aluminum electrolysis. Chemical Industry Press, 2006