Talking about the Harmful Effect of Anode Effect in Aluminum Electrolysis Production

The anode effect is a unique phenomenon of molten salt electrolysis, and the production performance of electrolytic aluminum is excellent. When the anode effect occurs in production, the cell voltage rises sharply, reaching 20 to 50 V, sometimes even higher. Its occurrence has a great influence on the whole electrolysis series, which reduces the current efficiency, affects various technical indicators of electrolysis, and reduces the output and quality of aluminum, destroying the smooth power supply of the entire electrolysis series. In the method of treatment, there are two kinds: the extinguishment with the effect rod (wood rod), or the reduction of the anode, and the increase of the amount of alumina to be fed. To achieve the purpose of extinguishing the anode effect. Until now, no better treatment has been found.

In today's society, especially Western countries, the control of anode effects in aluminum electrolysis production is extremely strict. There has been a shift from several years of fluoride to greenhouse gases PFCs = CF4 + C2F6 in the amount of anode effect (USEPA). [4] The management idea of ​​“Aiming for Zero Effect” proposed by the famous international aluminum expert Haupin is worth our thinking. Haupin believes that according to the current status of aluminum industry development, “zero effect” management is ideal. To this end, the author believes that: In today's increasingly important environment, the anode effect should be strictly controlled in the production of aluminum electrolysis, especially in the production of large pre-baking cells. As long as the cell conditions are normal, no effect is required. "Zero effect" management is the direction of future development of aluminum electrolysis production.

1. The mechanism of anode effect

At present, there are many opinions about the mechanism of the anode effect, but the explanation for the mechanism of the anode effect is "the theory of anodic process change." This view holds that [1]: The anode effect occurs because the electrolyte progresses along with the electrolysis process. The oxygen ion gradually decreases, and when it reaches a certain level, fluorine precipitates and reacts with the anode carbon to form a carbon fluoride, and when the fluoride of the carbon decomposes, fine carbon particles are precipitated. These carbon particles attach to the surface of the anode. The electrolyte is prevented from contacting the anode, so that the electrolyte does not wet the anode very well. Just like water cannot wet the oiled surface, an electrically conductive film is formed between the electrolyte and the anode, and the overvoltage of the anode increases. Anode effect. When new alumina is added, oxygen is further precipitated on the anode, and oxygen reacts with the carbon powder, so that the surface of the anode is gradually calmed, the resistance is reduced, and the electrolytic process becomes normal again.

The mechanism of anode effect is [4]:

Zc=RT/Fin{ic/ic-I}

In the formula, Nc- produces anode effect concentration overvoltage;

R-gas constant;

T-temperature, 0K;

F-Faraday constant.

Ic - critical current density;

I--larger current density at any anode;

Nc--0.00004308Tin{ ic/ic-I }

The critical current density is a function of the dissolved alumina concentration; however, it is also affected by electrolyte flow, electrolyte temperature, anode size (including interface changes at the anode after consumption), and groove volume. The critical current density decreases as the alumina concentration decreases (as Nc approaches 1 with ic). As the alumina concentration decreases, bubbles form on the anode, causing an increase in the surface tension of the electrolyte, which increases the anode voltage effect. high. Causes AE to occur.

This point of view explains better the cause of the anode effect. Accepted by electrolytic technology workers.

2. Anode effect hazard

The harmfulness of the anode effect in the production of aluminum electrolysis is not only manifested in the harm to production, but also extremely harmful to the ecological environment. The author will elaborate on several aspects.

2.1 Anode Effect Harmful to Production Hazards

When the anode effect occurs in production, the temperature of the electrolyte rises sharply, from 940°C to 955°C in the normal range to 980°C to 990°C, and the furnace melts and thins, increasing the possibility of erosion of the side carbon block. Sex. The sharp rise in voltage causes a series of current fluctuations that affect the cell's output. Increased power consumption. The method of extinguishing the anode effect in production is: Insert the effect rod (approximately 2 to 3 meters in diameter of 2 to 4 cm in diameter) into the aluminum liquid to burn the stick out of the gas film at the bottom of the anode, clean the bottom of the anode, and actually burn it. The aluminum liquid lasts about 3 to 5 minutes. At this time, the electrochemical process of electrolysis is stopped. This is why electrolysis workers often say that the “effect time is not aluminum production, but also the power consumption”. As a result, there is a serious loss of molten aluminum.

The 300KA intermediate blanking prebaked cell is taken as an example: the effect coefficient is 0.3 times/slot day, the effect time is 5 min, the current efficiency is 93%, and one anode effect produces less primary aluminum: 300×0.335×5÷60=8. 4kg, tons of aluminum power consumption increased 158kwh,

This energy is mostly converted into heat energy in production, so that the temperature between the poles of the electrolyzer rises sharply, and then conducts around the anode, so that the temperature of the electrolyzer increases, causing a large amount of volatilization of the aluminum fluoride in the electrolyte. Take the electrolyzer of our company as an example: For an effect time of 5 minutes, the ratio of molecules increases by an average of 0.1. About 10 to 20 kg of aluminum fluoride is lost.

The traditional view is that the anode effect can be used to separate the carbon residue, clean the electrolyte, supplement the heat of the electrolytic cell, and precipitate. However, with the improvement of the anode quality and the use of intelligent fuzzy control computer systems and point-loading technologies, the advantages of the anode effect have become smaller and smaller, so the traditional view can no longer meet the needs of today's modern electrolytic cell production.

1.2 Anode Effect Harm to the Environment

In the production of aluminum electrolysis, the anode effect is also accompanied by the production of PFCs (CF4·C2F6) which are destructive to the atmospheric ozone layer. Today's western developed countries have extremely strict environmental protection requirements for aluminum electrolysis.

The internationally renowned aluminum expert Haupin [4] believes that the incidence of PFCs is directly related to the daily AE minutes and the voltage level. However, the analysis shows that the emission of PFCsd does not show the long-distributed effect at high voltage effects. Individual trials showed that reducing the number of effects more effective than reducing the effect of time to reduce the incidence of PFCs. Because both CF4 and C2F6 are generated at the time of the anode effect, after the electrolytic cell is developed into the intermediate blanking prebaking cell, not only the number of anode effects is doubled, but also the effect time is greatly shortened. At present, foreign anode effect coefficients have been lower than 0.1 times/slot day.

The origin of PFCs=CF4+C2F6 is the anode effect (AE), but our country only pay attention to control technology for a long time. Also stays in the traditional control of fluoride salts. Understanding the development of the aluminum industry in today's world, especially the well-known aluminum expert Haupind's "Aiming Zero Effect" [4] is of great benefit to improving the overall level of aluminum electrolysis in China.

Our country is a signatory country for the international "Kyoto Protocol". Reducing the greenhouse effect and protecting the atmosphere are unshirkable responsibilities. Therefore, the control of harmful gas emissions will certainly be strengthened in the future. In the production of aluminum electrolysis, strict control of the anode effect is the requirement of the times.

1. 3 Anode Effect Harm to Forests

There are three ways to extinguish the anode effect in aluminum electrolysis:

(1) Use a shovel to extinguish the anode effect. (2) Extinguish the anode effect with a large file. (3) Extinguish the anode effect with the effect rod (wood rod).

The above three methods are commonly used in aluminum electrolysis production, especially self-baking tanks. At present, almost all self-baking tanks have been converted into intermediate blanking pre-baking tanks. The pre-baking tanks use a plurality of anodes to produce, and the method of extinguishing the anode effect by the large spade and the skip shovel loses its effect. The effect rod is a branch of about 2 to 3 meters in diameter of 2 to 4 cm. Become a way to extinguish the effect.

At present, the domestic aluminum electrolysis production has developed rapidly. In 2003, it had exceeded 520 tons, and it has become the world's leading aluminum producer. The use of effect rods has increased dramatically. If it is not controlled, it will inevitably cause serious damage to the forest.

Take this company as an example, the anode effect coefficient is controlled to 0. 3 times/slot day

1860 troughs a month, a total of 1860 × 0. 3 = 558 effects

The daily extinguishment of an effect requires about 2 to 3 effect bars. Calculating 3 pieces each month requires 558×3=1674 effect bars. Calculating each bundle of 30 pieces requires about 1674×30×12=672 bundles per year. It takes about 900 to 1,000 bales a year to lift large buses and compress loads.

At present, various aluminum effect rods are basically supplied by the market. Some people use efficiencies in order to obtain personal interests, and hack or even cut down trees to make aluminium electrolysis requirements rods for sale to electrolytic aluminum plants. Therefore, aluminum electrolysis anode effect rods are used. The rapid increase will inevitably encourage some people to seek personal interests and indiscriminately cut trees. This will bring disaster to the national forest.

Our country is a country with very little forest coverage. In the 1950s and 1990s, due to excessive hacking and deforestation, the fragile forest vegetation was severely damaged. The desertification of the land and the occurrence of dust storms were the revenge of nature for wild deforestation. With the implementation of China's policy of returning farmland to forests and planting trees and grass, the state has formulated a series of related policies to severely curb the phenomenon of indiscriminate cutting. The state has invested heavily in restoration of forest vegetation, and has closed mountains and afforested areas and planted tree species in areas with severe damage. grass. The source of effect bars in aluminum electrolysis production is bound to be strictly controlled. As the aluminum electrolysis manufacturers in the western part of China, they should strictly control the anode effect in the production of aluminum electrolysis, and greatly reduce the effect of the rod.

2. Analysis of conditions for controlling anode effects

At present, the self-baked electrolytic cell has basically disappeared, and the intermediate blanking pre-baked cell has become the main force of aluminum electrolytic production. The middle blanking prebaking cell is produced with low alumina concentration. The intelligent fuzzy computer control system is used to control the alumina concentration, and the intermediate point type blanking technology is used to periodically shell and blank, which creates favorable conditions for reducing the anode effect coefficient.

Haupin believes that controlling the anode effect [4], the zero effect mainly depends on:

1. Alumina quality: The main reason is that alumina plant's electric dust collection material is less than 20 microns (μm) and the dissolution rate is slow.

2. The existing feeder is a volumetric type, not a weighted type, and the following materials are not accurate. The development of a weighted feeder is the key to "zero effect".

3. Since the superheat of the electrolyte is very small (8°C to 10°C), the anode effect is caused by a series of current and voltage changes.

4. Poor electrolyte lining, such as poor quality of cathode carbon block, poor contact between cathode rod and carbon block, resulting in uneven distribution of cathode current, is also an important cause of anode effect.

5. The quality of the anode is poor, and the replacement of the anode is not yet standardized and inaccurate. The rapid consumption of individual anodes and the sharp reduction in cross section can cause AEs to occur.

According to the viewpoint of the domestic aluminum electrolysis, the author believes that under the condition that 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 the alumina.

2.1. Anode quality

High quality anode carbon block has the following features [5]:

1. Good electrical conductivity. To ensure that the anode current density is increased, the capacity of the aluminum electrolytic cell is increased and the power consumption is reduced.

2. Good thermal shock and oxidation resistance.

3. The anode quality is uniform and stable to ensure stable electrolysis production, high efficiency and low consumption.

4. Has a certain degree of tensile strength, bending strength and greater thermal expansion rate. At the same time, the anode is required to have less ash content, lower specific resistance, lower porosity, less harmful elements, and dense structure.

The foreign advanced pre-baked cells, due to the excellent quality of the anode, the carbon residue in the electrolyte is less, not enough impact on the production, production almost no carbon residue, no carbon residue cinder this process. Anode effect control is relatively low, generally between 0. 05 ~ 0. 1 times / tank day. At present, it is tending to "zero effect" control.

The quality of domestic prebaked anodes has a certain gap with foreign countries due to raw materials, technology, and standards. The anode has poor oxidation resistance and severe detachment. Fishing carbon residue as an important process in production. Traditional management techniques believe that the anode effect can promote the separation of carbon residue in the electrolyte. It can also add heat and control the precipitation in the tank. Therefore, improving the domestic anode quality is a key factor in reducing the anode effect.

2. 2 alumina quality

Aluminum electrolytic production requires that alumina has low water absorption, can dissolve in molten cryolite quickly, and requires good activity and enough specific surface area, as well as uniform particle size, so as to effectively absorb HF gas. It is sand-like alumina that satisfies these conditions.

Sandy alumina [2] has the characteristics of strong melting performance, good fluidity, uniform particle size, small wear coefficient, and strong ability to absorb hydrogen fluoride. In China, the production of alumina bauxite is a diaspore type, the melting temperature of alumina production is as high as 240° C., and the seed activity of seed decomposition is poor, so it is difficult to produce sand-like alumina. Although the company has successfully tested, there is still a certain gap between individual technical indicators and foreign countries, especially in the wear index and abroad, the foreign wear index is generally lower than 15%, while the alumina branch trialed by Chinalco Shanxi Branch The loss index is about 25%, and it takes a certain amount of time to achieve industrial production. Therefore, it takes a certain amount of time for domestic aluminum electrolytic production to use sand-like alumina.

Based on the reasons of anode quality and alumina, the reduction of the anode effect coefficient in production is limited, but I believe that controlling the anode effect coefficient below 0.2 times/slot day can still be achieved.

3. Ways to control anode effects

Comprehensive analysis of the actual situation of China's pre-baking tank, absorbing foreign experience in controlling the anode effect on the pre-baking tank, the author believes that controlling the anode effect and minimizing the number of anode effects should be improved in the following aspects.

1) . Conditional use of sand alumina, improve the processing and feeding system to ensure adequate raw materials to ensure that the electrolytic tank feed port open, prevent uneven feeding.

2) . Make sure there is enough electrolyte in the cell to prevent the electrolyte from shrinking. Ensure stable production. Maintain proper high electrolyte levels. Like our company's 75KA Intermediate Blanking Prebaked Cell, I believe the electrolyte level should be 18cm. Aluminum level <22cm.

3). Improve the insulation effect of the electrolytic cell, reduce heat loss, and appropriately increase the thickness of the insulation material on the anode. Keep the thickness above 12cm.

4). Smooth power supply, reduce current fluctuations, and select better current intensity.

5). The computer intelligent fuzzy control technology is used to control the electrolysis cell, so as to increase the hanging rate and reduce the number of manual operations. Increase the effect interval. Set the effect interval to more than 100 hours.

6). Change the idea of ​​anode effect management, align the relationship between electrolyzers and effects, and establish a management philosophy that “as long as the condition of the tank is normal, it does not have to be effective”.

7). Optimize the lining structure of the electrolyzer, use the semi-graphite cathode carbon block, use the new dry impervious material, improve the insulation effect of the cathode bottom, and use the silicon carbide composite material in the electrolysis cell measurement part.

8) . Do a good job of the electrolytic cell roasting start-up work to ensure that the electrolysis cell can build polymer furnaces within a specified time.

9) . Improve the quality of anode work, standardize operating procedures, increase anode exchange rate, and reduce interference with electrolyzers. Save the electrolytic cell to produce stable.

4. in conclusion

The hazard of the anode effect deserves our attention, especially its destructiveness to the environment and forests that we have not considered in the past. The times are developing and society is improving. An understanding of the anode effect in aluminum electrolysis production. And management methods are extremely partial. Already not meet the requirements of the modern era. "Zero effect" management is the direction of future development of aluminum electrolysis production.

Therefore, in the production of aluminum electrolysis, as long as the electrolysis cell is in normal production, the less anode effect, the better. Taking into account the current domestic anode quality, alumina physical properties, and other factors, the anode effect is controlled at 0. 2 times / tank day is achievable.

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