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A brief discussion on the harmfulness of the anode effect in aluminum electrolytic production. The anode effect is a phenomenon unique to molten salt electrolysis, and the performance of electrolytic aluminum production is obvious. When the anode effect occurs during production, the voltage of the electrolytic cell rises sharply, reaching 20 to 50V, and sometimes even higher. Its occurrence has a great impact on the entire electrolysis series, reducing the current efficiency, affecting various technical indicators of electrolysis, reducing the output and quality of aluminum, and destroying the stable power supply of the entire electrolysis series. In terms of processing methods, there are nothing more than two: Use an effect rod (wooden stick) to extinguish it, or lower the anode and increase the amount of aluminum oxide discharge. To achieve the purpose of extinguishing the anode effect. So far, no better method has been found. Today's society, especially in the West * * , the control of anode effect in aluminum electrolysis production is extremely strict. There has been a shift from fluoride for several years to greenhouse gases PFCs = CF4 + C2F6 in the anode effect (USEPA). The management idea of "aiming for zero effects" proposed by Haupin, a well-known international aluminum expert, is worthy of our consideration. Haupin believes that according to the current situation of the development of the aluminum industry, "zero effect" management is the most ideal. For this reason, I think: Today, when environmental protection is becoming increasingly important, the anode effect should be strictly controlled in aluminum electrolytic production, especially in large-scale prebaked tank production. As long as the electrolytic tank condition is normal, there is no need for anode effects. ""Zero effect" management is the future development direction of aluminum electrolytic production. 1. There are currently different opinions on the mechanism of the anode effect, but the one that better explains the mechanism of the anode effect is the "anode process change theory". This view holds that: The anode effect occurs because as the electrolysis process proceeds, the oxygen-containing ions in the electrolyte gradually decrease. When it reaches a certain level, fluorine is precipitated and interacts with the anode carbon to form carbon fluoride. The carbon fluoride precipitates into fine carbon particles when it decomposes. These carbon particles Attached to the anode surface, it prevents the contact between the electrolyte and the anode, so that the electrolyte cannot wet the anode well, just like water cannot wet the oiled surface, causing a poorly conductive gas film to form between the electrolyte and the anode, increasing the anode overvoltage, causing the anode effect. When new alumina is added, oxygen is precipitated on the anode. The oxygen reacts with the carbon powder, gradually making the anode surface quiet, reducing the resistance, and the electrolysis process tends to normal again. The mechanism of the anode effect is: Zc=RT/Fin{ic/ic－I} where Nc-concentration overvoltage that produces anode effect ； R-gas constant ； T-temperature, 0K ； F - Faraday's constant ； Ic--critical current density ； i--the maximum current density on any anode ； Nc--0。 00004308Tin{ ic/ic-I } Critical current density is a function of dissolved alumina concentration ； However, it is also affected by the electrolyte flow, electrolyte temperature, anode size (including the interface change of the anode after consumption) and the tank volume. The critical current density decreases with the decrease of alumina concentration (because Nc approaches 1 with ic). As the alumina concentration decreases, bubbles are generated on the anode, which increases the surface tension of the electrolyte and increases the overvoltage of the anode effect. lead to the occurrence of AEs. This view better explains why the anode effect occurs. Accepted by electrolysis technology workers. 2. The harm of anode effect in aluminum electrolysis production is not only reflected in the harm to production, but also extremely serious harm to the ecological environment. The author will elaborate on it from several aspects. 2. 1. The harm of anode effect to production. When the anode effect occurs during production, the temperature of the electrolyte rises sharply, from the normal value of 940°C to 955°C to 980°C to 990°C. The furnace side melts and becomes thinner, increasing the possibility of the side carbon blocks being eroded. The sharp increase in voltage causes the series current to fluctuate, affecting the output of the electrolyzer. Increased power consumption. The method to extinguish the anode effect in production is: Insert the effect rod (a branch about 2 to 3 meters in diameter and 2 to 4 cm in diameter) into the aluminum liquid to burn the wood rod to remove the gas film on the anode bottom and clean the anode bottom. In fact, the aluminum liquid is burning. The whole process lasts about 3 to 5 minutes. At this time, the electrochemical process of electrolysis is stopped. This is the reason why electrolysis workers often say that "aluminum is not produced during the effect time, and power is consumed." As a result, serious losses of aluminum liquid were caused. Take the 300KA intermediate blanking prebaking tank as an example: Effect coefficient 0. 3 times/tank day, effect time 5 minutes, current efficiency 93%, one anode effect produces less primary aluminum: 300×0. 3355×5÷60=8. 4kg, the power consumption per ton of aluminum increases by 158kwh. Most of this energy is converted into heat energy during production, causing the temperature between the poles of the electrolytic cell to rise sharply, and then conducts it to the surroundings of the anode, causing the temperature of the electrolytic cell to rise, causing a large amount of volatilization of aluminum fluoride in the electrolyte. Take our company’s electrolytic cell as an example: An effect time is 5 minutes, and the average molecular ratio increases by 0. 1. The loss of aluminum fluoride is about 10~20kg. The traditional view is that: The anode effect can be used to separate carbon residue, clean the electrolyte, supplement the lack of heat in the electrolytic cell, and eliminate precipitation. However, with the improvement of anode quality and the application of intelligent fuzzy control computer systems and point blanking technology, the advantages of the anode effect have become increasingly small. Therefore, this traditional view can no longer adapt to today's modern electrolyzer production. 1. 2. The harm of anode effect to the environment. In aluminum electrolysis production, the anode effect is also accompanied by the production of PFCs (CF4·C2F6) gas that is destructive to the atmospheric ozone layer. Today's developed West * * The environmental protection requirements for aluminum electrolysis are extremely strict. Haupin, an internationally renowned aluminum expert, believes that the occurrence of PFCs is linearly related to the number of AE minutes per day and the level of voltage. However, analysis shows that the amount of PFCsd emitted during high voltage effects does not show the specific effect of longer effect time and greater emission. Individual experiments have shown that reducing the number of effects is more effective than reducing the time of effects in reducing the occurrence of PFCs. Because both CF4 and C2F6 are produced when the anode effect first occurs. After the electrolytic tank develops into the intermediate feeding prebaked tank, not only the number of anode effects is doubled, but also the effect time is shortened. * * shorten. At present, some foreign anode effect coefficients are lower than 0. 1 time/slot day. The root cause of PFCs=CF4+C2F6 is the anode effect (AE), but we * * For a long period of time, only control techniques were paid attention to. Still stuck in the traditional control of fluoride salts. Understanding the development of the aluminum industry in today's world, especially the "aiming at zero effect" of the famous aluminum expert Haupind, is of great benefit to improving the overall level of aluminum electrolysis in our country. us * * It is the signature of the international "Kyoto Protocol" * * , reducing the greenhouse effect and protecting the atmospheric environment are our unshirkable responsibilities. Therefore, the control of harmful gas emissions will definitely be strengthened in the future. In aluminum electrolysis production, strict control of the anode effect is the requirement of the times. 1. 3. The harm of anode effect to forests. There are three ways to extinguish the anode effect in aluminum electrolytic production.: (1) Use a spatula to extinguish the anode effect. (2) Use a large rake to extinguish the anode effect. (3) Use an effect rod (wooden rod) to extinguish the anode effect. The above three methods are commonly used in aluminum electrolysis production, especially in self-baking tanks. At present, almost all self-baking tanks in China have been transformed into intermediate feeding pre-baking tanks. However, the prebaked tank is produced with multiple sets of anodes, and the method of extinguishing the anode effect by large rakes and shovels loses its effect. The effect rod is a branch of approximately 2 to 3 meters with a diameter of 2 to 4 cm. Be the only way to extinguish the effect. At present, domestic aluminum electrolysis production is developing rapidly. In 2003, it exceeded 520 tons, making it the world's largest aluminum producer. The use of effect rods has increased sharply. If it is not controlled, it will inevitably cause serious damage to the forest. Taking our company as an example, the anode effect coefficient is controlled to be 0. 3 times/slot day, 1860 slot days per month, total 1860×0. 3 = 558 effects. It takes about 2 to 3 effect rods to extinguish an effect on a daily basis. Calculated as 3 rods, 558×3=1674 effect rods are needed per month. Calculated as 30 rods per bundle, approximately 1674÷30×12=672 bundles are needed per year. In addition to factors such as increasing the bus bar and compressing load, approximately 900 to 1000 bundles are needed per year. At present, the effect rods of various aluminum factories are basically supplied by the market. In order to seek personal interests, some people cut down or even steal trees to make effect rods required for aluminum electrolysis and sell them to electrolytic aluminum factories. Therefore, the sharp increase in the use of aluminum electrolytic anode effect rods will inevitably encourage some people to seek personal interests and cut down trees indiscriminately. This will give rise to * * The forest brings a disaster. Our country has extremely little forest coverage * * . From the 1950s to the 1990s, due to excessive deforestation and deforestation, fragile forest vegetation was severely damaged. The occurrence of land desertification and dust storms was nature's greatest revenge on human beings for deforestation and deforestation. With the implementation of my country’s policy of returning farmland to forests and planting trees and grass, * * A series of relevant policies have been formulated to strictly curb indiscriminate deforestation. * * Huge sums of money have been invested in restoring forest vegetation, closing mountains for afforestation in severely damaged areas, and planting trees and grass. The source of effect rods in aluminum electrolysis production must be strictly controlled. Aluminum electrolysis manufacturers in western my country should strictly control the anode effect in aluminum electrolysis production and minimize the use of effect rods. Analysis of the conditions for controlling the anode effect. Currently, self-baking electrolytic tanks have basically disappeared, and intermediate blanking prebaked tanks have become the main force in aluminum electrolytic production. The intermediate blanking prebaked tank is produced with low alumina concentration. An intelligent fuzzy computer control system is used to control the alumina concentration. The intermediate point blanking technology is used for timing shelling and blanking, which creates favorable conditions for reducing the anode effect coefficient. Haupin believes that controlling the anode effect and achieving zero effect mainly depends on: 1. Alumina quality: The main reason is that the electrostatic precipitator material in the alumina plant is less than 20 microns (μm) and has a slow dissolution rate. 2. The existing unloader is volumetric, not gravimetric, so the unloading is inaccurate. The key to "zero effect" is to develop a gravimetric unloader. 3. Since the superheat of the electrolyte is very small (8℃~10℃), the anode effect will be caused when the series current and voltage change. 4. Poor lining of the electrolytic cell, such as poor quality of the cathode carbon block and poor contact between the cathode rod and the carbon block, leads to uneven cathode current distribution and is also an important reason for the anode effect. 5. The quality of the anode is poor, and the replacement of the anode is not standardized and accurate. If individual anodes are consumed too quickly and the cross-section is sharply reduced, AE will occur. According to the viewpoint of the author, combined with the actual situation of domestic aluminum electrolysis, the author believes that when the basic conditions of aluminum electrolysis production are relatively stable, the control of the anode effect coefficient mainly depends on the quality of the anode carbon block and the characteristics of alumina. 2. 1 Anode carbon block with high anode quality has the following characteristics: 1. Good electrical conductivity. In order to ensure that the anode current density is increased, the production capacity of the aluminum electrolytic cell is increased and the power consumption is reduced. 2. It has good thermal shock resistance and oxidation resistance. 3. The anode quality is uniform and stable to ensure stable electrolysis production, high efficiency and low consumption. 4. It has certain tensile strength, flexural strength and large thermal expansion rate. At the same time, the anode is also required to have less ash, low specific resistance, low porosity, less harmful elements, and dense structure. In foreign advanced prebaked tanks, due to the excellent anode quality and less carbon slag in the electrolyte, it has little impact on production. There is almost no carbon slag removal during production, and there is no process of removing carbon slag. The anode effect is controlled at a low level, generally at 0. 05～0. 1 time/slot day. It is currently moving towards "zero effect" control. The quality of domestic prebaked anodes lags behind that of foreign countries due to raw materials, technology and standards. The anodes have poor oxidation resistance and serious falling off and slag. Removing carbon residue is an important process in production. Traditional management technology believes that the use of the anode effect can promote the separation of carbon residue in the electrolyte, and can also supplement heat and control precipitation in the tank. Therefore, improving the quality of domestic anodes is a key factor in reducing the anode effect. 2. 2. The quality of alumina. Aluminum electrolysis production requires alumina to have small water absorption, be able to dissolve quickly in molten cryolite, and at the same time require good activity, sufficient specific surface area, and uniform particle size, so that it can effectively absorb HF gas. Sand-like alumina can meet these conditions. Sand alumina has the characteristics of strong melting performance, good fluidity, uniform particle size, small wear coefficient and strong ability to absorb hydrogen fluoride. In China, because the bauxite used to produce alumina is diaspore type, the melting temperature of alumina production is as high as 240°C and above, and the decomposed seeds have poor activity, making it difficult to produce sandy alumina. Although Chinalco has successfully tested it, some technical indicators are still far behind those of foreign countries, especially The wear index is larger than that of foreign countries. The wear index of foreign countries is generally less than 15%, while the wear index of alumina tested by Chinalco Shanxi Branch is around 25%. Moreover, it will take a certain amount of time to achieve industrial production, so it will take some time for domestic aluminum electrolytic production to be able to use sandy alumina. Due to the quality of the anode and the alumina, there are certain restrictions on reducing the anode effect coefficient during production, but the author believes that the anode effect coefficient should be controlled at 0. It can still be done with less than 2 times/slot day. 3. The way to control the anode effect is to comprehensively analyze the actual situation of the prebaked tank in my country and absorb the foreign experience in controlling the anode effect in the prebaked tank. The author believes that controlling the anode effect and minimizing the number of anode effects should be improved in the following aspects. 1). Use sandy alumina if possible, improve the processing and cutting system, ensure sufficient raw materials, ensure the unblocked discharge port of the electrolytic cell, and prevent uneven cutting. 2). Make sure there is a sufficient amount of electrolyte in the electrolyzer to prevent electrolyte shrinkage. Ensure stable production. Maintain appropriately high electrolyte levels. Like our company's 75KA intermediate feed pre-baked tank, the author believes that the electrolyte level should be ≧18cm. aluminum level