1 Control the composition ratio of electrolytic refining agent
When using electrolytic refining agent, it is especially important to ensure the content of Na3AIF6. Of course, NaCI cannot be used to replace KCI. The purpose of ensuring the content of Na3AIF6 is to reduce the wetting angle between the slag particles in the melt and the medium, that is, to reduce the interfacial tension between the slag and the medium and dissolve the slag, take out the slag particles, and improve the slag removal effect.
There is a lot of slag in the electrolytic aluminum liquid. When refining in the smelting furnace for the first time, it is advisable to use an electrolytic refining agent containing the above ingredients to reduce the slag content in the melt as much as possible.
2 Strictly prevent the flux from getting wet
Salts such as potassium and sodium are very easy to absorb moisture and agglomerate, forming attached water and crystal water. Water is the main source of gas content in the melt. If the flux contains 18g of attached water and crystal water, water reacts chemically with aluminum:
(1)
From formula 1-17, it can be seen that 1mol (18g) of water can produce 1mol of hydrogen. If hydrogen is regarded as an ideal gas, 1mol of hydrogen is 22.4L. Assuming that all of this hydrogen is absorbed by the aluminum melt, the gas content of 10t of aluminum melt can be increased by 0.224mL/100gAI. For pure aluminum melt, its gas content is required to be less than 0.12mL/100gAI. In fact, even dehydrated flux will deliquesce once it comes into contact with air, absorbing 3%~5% of water, and 1kg of flux may exceed 18g of water content. If 20kg of flux is used for 10t of aluminum liquid, its water content may exceed 300g. With such flux refining, it is very likely that the gas removed during refining is much less than the gas absorbed by the melt, so the more refined, the more gas content. Many manufacturers have encountered such a situation. Therefore, it is best to heat the electrolytic refining agent to 350℃ before refining to completely remove its attached water and crystal water.
The liquid electrolytic refining agent currently used is mainly carbon tetrachloride. Carbon tetrachloride reacts with aluminum liquid as follows:
The generated AICI3 has the same effect as above, but the generated free chlorine may react with hydrogen dissolved in aluminum liquid:
HCl is a gas and will escape from the melt. Therefore, a severe irritating odor can be smelled when refining with carbon tetrachloride. Carbon tetrachloride has a good degassing effect.
Carbon tetrachloride can be used as a carrier of foam lightweight bricks, that is, the foam lightweight bricks are immersed in carbon tetrachloride liquid. After the lightweight bricks are saturated with carbon tetrachloride, they will sink to the bottom of the container and can be taken out for use. However, lightweight bricks absorb water seriously, and one lightweight brick can absorb about 500g of water. Therefore, before use, the lightweight bricks must be kept warm at high temperature for more than 4 hours to completely remove moisture. The reaction rate cannot be controlled by this method of refining. At the beginning of refining, the reaction is violent, and as the refining proceeds, it gradually slows down and cannot fully exert the refining effect.
Another method is to use inert gas as a carrier, similar to siphoning, to bring carbon tetrachloride into the melt for refining. This method not only avoids the pollution caused by the water absorption of lightweight bricks, but also controls the reaction rate and ensures the refining effect. In the process of refining with carbon tetrachloride, this method is relatively feasible and effective.
The use of carbon tetrachloride is simple to operate, but the slag removal effect of carbon tetrachloride is poor, and the toxicity is relatively high, which seriously pollutes the environment. It is a restricted or banned product. At the same time, it will leave carbon elements in the aluminum liquid, which will have a certain negative impact on some products that require high quality. Generally, due to investment and scale limitations, this refining method basically does not require equipment investment for small and medium-sized enterprises, and is widely used; large enterprises have large production scales and are less used.
Gas electrolytic refining agent is an inert gas (nitrogen or argon) or a mixture of inert gas and active gas (nitrogen + chlorine or argon + chlorine). Pure inert gas refining is to add external dispersed nitrogen or argon bubble cores to the aluminum melt, and hydrogen diffuses into the bubbles and takes hydrogen out. It is a physical diffusion degassing property, and the degassing effect is limited. In order to increase the degassing rate, a certain amount of active gas-chlorine is added to the inert gas. The content of active gas added in furnace refining is generally 5%~10%, and in special cases, it can be increased to 15%; the active gas added during online refining is generally 1%~2%.
As mentioned above, after adding active gas, in addition to the inert bubble core participating in degassing, AICI3 bubble core participating in degassing is generated in the aluminum melt. At the same time, chlorine also directly combines with hydrogen in the melt to form HCI, so the refining agent composed of inert gas and active gas has a better degassing effect. Large aluminum processing enterprises generally use this refining degassing.
To produce high-quality double zero aluminum foil and PS aluminum plate base by direct casting and rolling of electrolytic aluminum liquid, in addition to using solid flux refining in the smelting furnace to remove most of the inclusion particles in the melt, it is more appropriate to use active gas in the static furnace and online refining: it can remove most of the gas in the melt, and can also remove sodium and other substances remaining in the melt from electrolysis. The application of electrolytic refining agent can further improve the refining effect.
3 Ensure the purity of refining gas and the role of electrolytic refining agent
Try to reduce the water and oxygen content in the gas as much as possible, and it is best to reduce the oxygen and water content to less than 0.0005%. Water is the main source of gas content in aluminum, needless to say. When oxygen is contained in the refining gas, although it does not increase the gas content, it hinders the degassing effect.
Oxygen reacts with aluminum to form aluminum oxide. When the refining gas is passed into the aluminum melt, it becomes small bubbles one by one, dispersed in the aluminum liquid. Indeed, at this time, the hydrogen partial pressure in the bubble is equal to 0, and the hydrogen dissolved in the melt diffuses into the bubble. The migration speed of hydrogen from the aluminum liquid to the bubble is relatively fast, that is, the partial pressure in the bubble can easily establish a dynamic balance with the hydrogen partial pressure in the aluminum liquid, bringing it out of the melt surface to obtain a satisfactory degassing effect. However, when the bubble contains oxygen, the oxygen reacts with the aluminum melt around the bubble, that is, a dense aluminum oxide film is generated, which slows down the migration speed and hinders the smooth passage of hydrogen. Generally speaking, the migration speed of hydrogen in aluminum oxide is only about 1/10 of that in aluminum melt, which greatly reduces the degassing effect. Therefore, it is necessary to take deep dehydration and deoxidation measures for the refined gas to reduce the water and oxygen content to a minimum.
The structure principle of the deep deoxidation and dehydration device, that is, the high-purity purification device of the mixture of inert + active gas, deoxidation and dehydration is shown in Figure 1.
Figure 1 Schematic diagram of the high-purity purification device of refined gas
Practice has proved that the refined gas after high purification has better refining effect. After a series of degassing treatments, the gas content of the electrolytic aluminum liquid can be reduced from 0.4~0.5mL/100gAI to 0.10~0.13mL/100gAI. The pinholes of 0.006~0.007mm aluminum produced by a certain factory are less than 28/m2. However, when the deep deoxidation and dehydration system fails and the high purity of the refined gas fails, the degassing efficiency is significantly reduced, and the pinholes of the 0.007mm aluminum foil produced suddenly increase to more than 300/m2.
4 All refining should be reasonably designed refiners
Under certain conditions, the bubble diameter should be reduced as much as possible so that the bubbles can be distributed in the aluminum melt in a small, uniform and diffuse manner. The smaller the bubble, the larger its specific surface area, which increases the total surface area of the bubble in contact with the aluminum melt, and the shorter the path for hydrogen to diffuse into the bubble; the slower the bubble rises in the melt, the longer it stays in the aluminum melt, the greater the probability of the bubble absorbing hydrogen, and the more hydrogen is brought out under the condition of using the same refining gas. In the process of transferring the aluminum melt from the melting furnace to the static furnace mentioned above, the gas ejected from the injection pipe is atomized into small bubbles one by one when the siphon closed transfer is used and the nitrogen chlorine or argon chlorine gas is sprayed for refining; SNIF or Alpur is used for refining, and its rotating head is distributed with fine holes (see Figure 2 and Figure 3), which crush the gas into fine bubbles.
Figure 2 Schematic diagram of degassing equipment operation
Figure 3 Silicon nitride rotating head
Figure 2 is a double rotating nozzle, and Figure 4 is a single rotating nozzle. The gas passes through the rotor of the nozzle to form dispersed fine bubbles, and as the rotor stirs the melt, the bubbles are evenly dispersed in the entire melt, thereby producing the effect of degassing and deslagging. The gas blown in is nitrogen or argon (argon is generally used. At high temperatures, nitrogen may react with aluminum to form aluminum nitride, blocking the nozzle). Sometimes 1% to 2% chlorine is added to improve the refining effect. This method is suitable for large-scale continuous production.
Figure 4 The fine bubbles generated in the rotating head of the refiner are evenly dispersed into the fluid
The Alpur method is basically the same as the SNIF method. Both use a rotating nozzle to disperse the bubbles in the melt. The difference is that in the SNIF method, the melt contacts the bubbles at the gas nozzle, and then the bubbles are dispersed into the melt with the help of the centrifugal force of the rotating rotor; while the Alpur method allows the stirred melt to enter the nozzle and contact the bubbles, making the bubbles smaller, more dispersed and more uniformly distributed, and the purification effect better. The nozzle structure is shown in Figure 5.
Figure 5 Schematic diagram of the Alpur method device a-rotating nozzle: 1-gas discharge hole; 2-melt through hole; 3-gas; 4-rotating shaft; 5-rotating impeller; 6-device overall: 1-gas inlet; 2-melt population; 3-melt outlet
The refining effects of the SNIF method and the Alpur method vary with the rotation speed of the rotating head and the size of the ventilation volume. The faster the rotation speed and the greater the ventilation pressure, the more and smaller the bubbles blown into the aluminum liquid, the more evenly distributed, the greater the bubble interface rate, and the longer the time it acts in the aluminum. As long as it does not cause the surface of the aluminum melt to stir or tumble, the degassing effect is better. In this process, the use of electrolytic refining agent can also enhance the degassing effect, and further accelerate the removal of gas by reacting with the gas in the aluminum liquid.
In the past, the rotating heads of online refiners were mostly made of graphite. In this rotor, carbon reacts with oxygen near the interface where the melt and air intersect to generate carbon dioxide, which reduces the life of the rotor. In order to extend the service life, some people reduce the rotation speed, which reduces the degassing effect.
In order to solve the problem of rotor service life, Sanming Metal Mining Company has successfully developed a silicon nitride rotating head (see Figure 3). It is not corroded by aluminum water, does not react chemically with oxygen, has a long service life, and a rotor can generally be used for more than three years; it has a good degassing effect. The best degassing effect of the silicon nitride head is shown in Figures 6 and 7.
Figure 6 Degassing effect of silicon nitride rotor
Figure 7 Changes in hydrogen content in the melt during the residence time in the degassing chamber
As mentioned earlier, part of the slag is removed while degassing. Its deslagging effect is shown in Figure 8. In this process, the electrolytic refining agent can further optimize the removal of slag and ensure the purity of the aluminum liquid during the refining process.
5 The gas pressure should be reasonably adjusted during refining in the furnace
When the refining gas is passed into the aluminum liquid, the wave height of the aluminum liquid is between 50 and 100 mm. If the pressure is too high and the wave height is greater than 100 mm, the wave will fall in a mushroom shape, and the air will be wrapped in the aluminum melt again. The air contains oxygen and water. Water reacts with aluminum, reabsorbs hydrogen, and regenerates alumina slag, causing secondary pollution of the aluminum melt. The pressure is too low, and the wave height is less than 50mm. There are few bubbles generated by refining, which are difficult to be fine and evenly dispersed, and the refining effect is not sufficient.
Figure 8 Slag removal effect during online refining and degassing
6 Correctly grasp the refining time in the furnace
The refining time depends on the weight of the melt and the gas content. The larger the melt capacity, the longer the refining time; the higher the gas content of the melt, the refining time should also be appropriately extended. However, in production practice, in order to achieve the purpose of thorough refining and degassing, the refining time is extended arbitrarily, inappropriately, and unlimitedly. Some even repeat the refining for two, three, or four times continuously, thinking that it is difficult to remove all the gas in the melt without doing so. In fact, this is a misunderstanding. Each refining method has its limit degassing efficiency. When the limit efficiency of the method is reached, even if the time is extended or the number of refining times is continued to increase, the gas content of the melt will not continue to decrease. On the contrary, it may sometimes cause some negative effects.
7.Make good use of supplementary refining and electrolytic refining agent
After the aluminum melt in the static furnace is refined in the smelting furnace and the static furnace, if the casting and rolling cannot be completed in a short time, supplementary refining must be carried out; or gas analysis should be carried out on it. If the gas content exceeds the allowable range, supplementary refining should be carried out immediately to ensure that the gas content of the aluminum melt is controlled within the allowable range to ensure the high quality of the final product. In the process of supplementary refining, the use of electrolytic refining agents helps to improve the degassing efficiency, make supplementary refining more thorough, and ensure the quality of aluminum liquid.