Aluminum electrolytic cell roasting start method and development trend

According to the bulk non-ferrous net reprint: I. Introduction

New aluminum electrolytic tanks and overhauling tanks must be subjected to roasting, start-up and initial production stages before they can be transferred to normal production.

The process of raising the temperature of the electrolyzer from room temperature to 600°C (referring to the surface temperature of the cathode carbon block, the same below) is called roasting; adding electrolyte and then energizing it to carry out electrolysis is called startup; after adjustment, the electrolyzer is adjusted. The temperature and electrolyte composition and the establishment of the tank ledge are called the initial production stage.

The purpose of roasting is to allow the electrolysis cell to be smoothly transferred from room temperature to high temperature production to avoid damage to the lining caused by thermal shock, to obtain good strength of the caulk paste, and to drive off moisture in the lining. A large number of studies have shown that most of the lining defects of the electrolytic cell are generated during the roasting start-up phase. Therefore, the roasting start-up has a great influence on the normal production of the aluminum electrolytic cell and the life of the electrolytic cell. In recent years, with the increasing capacity of aluminum electrolytic cells, the aluminum industry has paid more and more attention to the firing method. The roasting start method has been greatly improved. The average life span of aluminum electrolyzers in China is short, the production efficiency is low, and the method of roasting startup is not proper and the quality is low, which is one of the important reasons.

Since the 1970s, China's aluminum plants have used aluminum water roasting methods. In recent years, aluminum plants in China have paid more and more attention to roasting start-up methods, and roasting methods have been diversified. Many prebaked aluminum plants have changed from coke-bed roasting (also known as coke-grain method to coke-layer method), and Pingguo Aluminum has also developed its own gas roasting method (also known as fuel method, thermal method, and flame method). These are all welcome changes.

This paper introduces and analyzes the foreign countries' aluminum electrolysis cell roasting start-up methods and their development trends in recent years. According to the current status of China's aluminum electrolysis industry, five suggestions are proposed for the reference of aluminum electrolysis engineering technicians and researchers in China.

Second, roasting method

According to the source of heat, the roasting method can generally be divided into two major categories: resistance method and fuel method. The most commonly used resistance methods are the coke bed method and the aluminum water method. The most commonly used fuel in the fuel method is liquefied petroleum gas and natural gas, so it is often referred to as the gas method. The coke bed method is commonly used for prebaked cells and also for self-baked cells. When used in a self-baking cell, the working surface of the anode must be smoothed.

Aluminum water method is only used for the second starter tank. The gas method is applicable to all trough types, but it is more convenient for self-baking troughs than for pre-baking troughs. Directly filling the electrolyte to start the electrolytic cell and simultaneously starting the firing of the inner lining, firing the inner lining while baking the anode at the baking furnace, and using an electric heating element (resistance wire/rod) to roast the inner lining are extremely rarely used. This article does not discuss it.

Currently, in the Western countries, the most commonly used and most competitive roasting methods are actually only the char bed method and the gas method. Among them, most of the coke bed methods use a shunt and an anode soft tape. The former is to gradually increase the firing current, and the latter is to make the connection between the anode bus (beam) and the anode guide flexible. This article focuses on the coke bed method and gas method.

Third, the evaluation criteria

From a technical point of view, the roasting method can generally be evaluated from the following six aspects:

1. Controllability of heating rate. The average heating rate should not exceed 20°C/hour. The maximum heating rate should not exceed 50°C/hour. In the baking temperature range of 300-600 °C, the heating rate should be controlled at 10 °C / hour or less.

2. Temperature distribution of the cathode surface during roasting. Obviously, the more uniform and better the temperature distribution is. It is generally required that the relative standard deviation be less than 10%. At the end of the calcination, there is no "hot zone" at the surface of the cathode that exceeds 1000°C.

3. Vertical temperature gradient during calcination (from the surface of the cathode carbon block up to the insulating layer). There is currently a lack of quantitative data. It is generally required that the temperature gradient at the end of the calcination should be as close as possible to the temperature gradient during the normal production of the electrolytic cell.

4. The average temperature of the cathode surface at the end of the calcination. Ideally, this temperature should be as close as possible to the electrolyte temperature (950-970°C) during the normal production of the electrolytic cell to avoid thermal shock when the electrolyte is injected.

5. Anode current distribution. This is an important test indicator for roasting prebaked cells with a coke bed method. It is generally required that the relative standard deviation of the anode current distribution is less than 15%, preferably less than 10%.

6. Cathode current distribution. This is another important test index when roasting the electrolysis cell by the coke bed method. The requirement is the same as the anode current distribution, ie its relative standard deviation should be less than 15%, preferably less than 10%.

From an operational and economic point of view, it is of course simple and cost-effective. One of the most important factors that most aluminum plants currently consider when choosing a roasting method is simplicity.

The roasting heating curve should be mainly based on the characteristics of the caulking paste. This is because the caulking paste undergoes large chemical and physical changes during the roasting process (mainly caused by the carbonization of the binder coal pitch), whereas the cathode carbon block and the side blocks that have been subjected to high temperature treatment have only minor physical changes.

The critical temperature range affecting the sintering paste paste quality is 200-500°C, and the heating rate must be slow in this temperature range. If the heating rate is too high, the decomposition and volatilization rate of the asphalt will also increase, the decomposition and volatilization will increase, the cracks in the caulking paste will increase, and the mechanical strength will decrease. This is one of the main reasons leading to a short lifetime of the electrolytic cell and operational difficulties.

Since the heating curve of the firing process is based on the surface temperature of the cathode carbon block, and the temperature of the caulking paste lags behind the temperature of the cathode carbon block, the actual firing process generally starts from 300-350° C. to reduce the heating rate. It should be noted that it is generally advantageous that the temperature of the caulking paste lags behind the temperature of the carbon block. Because this means that the caulking paste can retain plasticity for a longer period of time, which helps to eliminate the expansion stress generated during the firing process.

Fourth, the advantages and disadvantages

In general, the coke bed method has the advantages of being simple, requiring no complicated equipment, requiring no fuel, substantially no problem of burning of the cathode carbon block, short firing time, and a plurality of electrolytic cells that can be fired at a time. The gas method has the advantages of uniform temperature distribution, good temperature controllability, small vertical temperature gradient, no need to remove coke after startup, no current distribution problems, and no influence on the operation of the same series of production tanks. The biggest advantages of the aluminum water method are simplicity and low smoke volume.

However, due to the defects such as large thermal shock during aluminum filling, low viscosity, low viscosity, preferential penetration of aluminum water into the lining cracks, and the inability of the caulking defects of the caulking paste to be detected after the end of the firing and timely remediation, most foreign countries The aluminum plant has long since not used this method for roasting new tanks or overhauling tank linings, but is limited to roasting of the second starter tank.

The coke bed method is a widely used old method. It has a large number of skilled workers and technicians and has many years of experience. This is another great advantage of the coke bed method over the fuel method. In addition, when a fine-grained coke bed is used, the coke particles can be burned in a shorter period of time after the start of the electrolytic cell without manual removal. The biggest weakness of the coke bed method is that the control of the heating rate is not as good as the gas method and the temperature distribution is not uniform enough. With the use of shunts and anode soft tapes, these weaknesses have been greatly improved, but they are still inferior to the gas method. In addition, the shunt consumes about 20% of the electrical energy, increasing the cost of firing.

The first level of the gas law in the promotion process is subject to review and approval by the security department. This will take quite some time in some areas. Although gas fired so far, no safety accident has occurred. However, many aluminum electrolysis plants still have concerns about the safety of gas roasting when choosing the roasting method.

The use of liquefied petroleum gas as a fuel requires large-scale high-pressure vessels, which increases the safety risk and is inconvenient for some aluminum electrolysis plants. The use of oil as a fuel may eliminate the problem of high-pressure vessels, but it is unfortunate that oil-fired roasting equipment is not as easy to operate and control as gas-fired roasting equipment. Tests for improved fuel roasting are still in progress. With aluminum gas plants that supply natural gas, it is more convenient to use the gas method.

For the pre-baking cell, how to cover the cell for heat preservation and oxidation prevention is another problem that the gas method encounters. The coverage of the self-baking tank is much easier, which is why the gas method is more suitable for self-baking tanks. In addition, it takes time and investment to purchase gas roasting equipment and personnel training.

In spite of these difficulties, the use of gas-fired aluminum electrolysis plants has continued to increase in recent years. Currently, Alcoa/Reynolds, Elkem, Alusaf, Hydro, Alusuisse, Sor-Norge, and Kubal have adopted Alcoa/Reynolds in all or part of foreign aluminum companies/aluminum plants.

It should be noted that the firing start method is associated with the electrolyzer's carbon lining material. In the gas-fired method, the strength of the caulking paste after baking is larger than that of other calcination methods and may exceed the strength of the cathode carbon block. When a large thermal shock is encountered, the carbon block may break and cause damage to the electrolytic cell. Therefore, when the gas method and the negative impact resistance of the cathode carbon block are adopted, the formulation of the gap filling paste should be properly adjusted to reduce the strength after firing.

V. Development Trends

In recent years, the development trend of the aluminum electrolytic cell roasting start method is to strengthen the control of the heating rate, to improve the uniformity of the roasting temperature distribution and to achieve no-effect start-up. The purpose is to reduce the thermal shock to the lining as much as possible, so that the electrolytic cell can be transferred to normal production as soon as possible.

When using the coke bed method, in order to strengthen the control over the heating rate, each aluminum company has developed a shunting technology, and uses an anode soft tape to connect the anode busbars and the anode guide rods.

For example, all types of aluminum electrolytic cells have developed corresponding standard flow dividers. The firing current is generally divided into more than four steps and gradually increases to full series current in not less than 24 hours. Softband connection is also a very important technology. Practice has proved that it has the following effects: to improve the current distribution of the anode and cathode, reduce the possibility of "hot spots" on the cathode surface, keep the anode and the coke layer in good contact and eliminate the need to repeatedly tighten the anode clamp operation, etc. .

When the gas method is used, computer control technology has been introduced in recent years to better control the heating rate and monitor the roasting condition. By adjusting the fuel flow rate, the temperature increase rate can be accurately controlled according to a preset temperature increase curve. The actual temperature rise of each point can be seen in the monitoring room. By adjusting the flow rate of each spray gun and the direction of the nozzle, the temperature increase rate of each point can be adjusted.

The set temperature rise curve during roasting, the actual temperature rise curve at each point, and the anomalous conditions are displayed on the computer screen and recorded to facilitate monitoring, analysis, and improvement. And these are hard to do with the coke bed method. This is because when the coke bed method is used, people have limited control over the roasting power, that is, the roasting power cannot be easily adjusted at any time according to the wishes of people. Assuming that the roasting power remains constant for a period of time, as the temperature rises, the heat loss from the electrolysis cell and the specific heat of the carbon material increase, and thus the heating rate slowly decreases.

The resistance of the coke bed during roasting will be greatly reduced, and the shunt resistance will increase, so the firing current will increase. The increase depends mainly on the relative changes in the cell resistance (including the coke resistance) and the shunt resistance. The firing voltage and firing power also change with changes in resistance and current. The specific changes are related to the selected shunt material, size, coke bed material, coke thickness, coke particle size, cathode carbon block type, and temperature. All these changes cannot be controlled during the roasting process. Regarding the shunt material, steel, aluminum, or nichrome is generally used. With respect to coke bed materials, some aluminum electrolysis companies/plants have abandoned petroleum coke in recent years and switched to graphite.

In order to improve the uniformity of the roasting temperature distribution (ie, to decrease the horizontal temperature gradient and the vertical temperature gradient), in addition to strengthening the control of the heating rate, it is also necessary to strengthen the monitoring of temperature changes in various parts of the lining. For this purpose, no less than four thermocouples shall be embedded in the surface layer of the cathode carbon block (about 2 cm from the surface) and the mat between the cathode carbon block and the firebrick, respectively, and buried under the refractory brick layer and the thermal insulation layer. Not less than two thermocouples.

After the roasting begins, the temperature changes and life of each thermocouple should be recorded and analyzed in detail to provide first-hand information for the timely adjustment of the roasting process and the subsequent improvement of the roasting method and analysis of the lining damage mechanism. With the coke bed method, monitoring of the anode current distribution and cathode current distribution must also be enhanced.

Due to the large amount of strong greenhouse gases CF4 and C2F6 and other toxic gases emitted by the electrolytic cell during the anode effect, western aluminum companies have been trying to reduce the number and duration of anode effects in recent years. The realization of the electrolysis cell without effect start to reduce the emission of toxic gases has become one of the goals pursued by the aluminum industry. To achieve no-effect start-up, the following points should be made:

1. Increase the baking temperature. The final calcination temperature (carbon block surface) should reach 920°C or higher.

2. Decrease the vertical temperature gradient. It is generally required that the temperature of the bottom surface of the cathode carbon block reach 800°C at the end of the baking. In order to achieve the above two requirements, it is necessary to extend the roasting time appropriately. When using the coke bed method, the baking time should not be less than 48 hours. When using the gas method, the baking time is about 72 hours.

3. Avoid lining cooling when starting. For this reason, the insulation material used for firing is generally not removed, and the pole pitch is reduced to 1-2 cm.

4. Irrigation electrolyte and electricity should be as fast as possible. There should be a sufficient amount of hot electrolyte and there should be no aluminum in the electrolyte.

5. The pole pitch should be raised very slowly.

After the roasting temperature of the electrolytic cell is increased, the cell voltage at the initial stage of the operation of the electrolytic cell is reduced, the fluctuation of the cell voltage is also reduced, and the electrolytic cell can be quickly transferred to the normal production.

Six, some suggestions

1. In view of the poor thermal shock resistance of carbon lining materials in aluminum reduction cells in China, it is particularly important to reduce the thermal shock at the start of firing. Reducing the heating rate during baking, in particular, lowering the heating rate in the baking temperature range of 300-600°C (corresponding caulking paste temperature of about 200-500°C), should be the focus of our improved roasting start-up method.

2. The design institute (institution) should cooperate with the aluminum electrolysis plant when designing the aluminum electrolytic cell, and at the same time design the corresponding shunt and anode soft tape. The selection of diverter materials, the arrangement of the diverter, and the behavior of the diverter during use should be carefully documented and analyzed for improvement and standardization.

3. The aluminum electrolysis plant should strengthen the monitoring and recording of parameters such as temperature and current distribution during the roasting process, establish the electrolysis cell roasting start profile, and provide first-hand information for improving the roasting method and analyzing the lining damage mechanism.

4. The gas-fired method should be actively applied to the aluminum electrolysis plants with natural gas supply and near the refinery. Consider the introduction of complete sets of equipment and technologies combined with self-developed approaches to develop gas roasting methods that are suitable for China's national conditions.

5. Strengthen the research on the damage mechanism of the electrolytic cell. Damaged troughs should always be dry dissected, and the broken parts should be carefully recorded and analyzed for their causes of damage.