Application of Ceramic Foam Filter in Aluminum Alloy Production

1 Structure and Filtering Function of Ceramic Foam Filter

1.1 Structure

The usual method of making foam ceramic filter plate is to infiltrate polyurethane sponge into a ceramic slurry, squeeze out the excess part, dry it, and finally sinter it at high temperature to form a three-dimensional porous ceramic material related to the original foam structure, which is a winding, interconnected and unevenly thick pipe. It is precisely this uneven pipe wall that plays a key role in capturing fine inclusions in the aluminum melt. Such a structure also increases the path of passage, which increases the chance of aluminum melt contacting the ceramic part of the filter plate, thereby increasing the probability of inclusion particles being adsorbed.

1.2 Function

The ceramic foam filter can not only effectively remove large pieces of heterogeneous impurities in the aluminum liquid, but also filter out several micrometers of fine inclusions that traditional processes cannot handle. Since hydrogen atoms and other harmful ions in aluminum liquid are often adsorbed on inclusions, and inclusions can become the core of bubble formation, filtering out inclusions can also reduce the content of these harmful elements and gases in aluminum liquid. Some studies also believe that the ceramic foam filter filters out many fine inclusions, thereby reducing the effective crystal nuclei required for aluminum liquid solidification. This can promote the nucleation and growth of aluminum liquid under large supercooling conditions, refine the organization, and improve product performance.

2 Filtration mechanism of foam ceramic filter plate

The main purpose of using foam ceramic filters is to remove inclusions in aluminum melt.

Inclusions usually exist in three forms in the melt: metal oxides; foreign particles such as refractory fragments; and particles introduced during melt processing. Generally speaking, there are roughly three ways to remove particles suspended in the melt: sedimentation, particles accumulate at the bottom of the melt; floating, inclusions float and gather on the surface of the melt; filtration, particles are separated from the melt through porous media.

Natural sedimentation generally occurs when the particle density and diameter are large. Particles smaller than 90 μm are not very effective in static sedimentation. Bubbles or molten flux generated during traditional gas dehydrogenation or flux treatment can bring inclusion particles to the surface of the melt. This mechanism is effective for removing inclusions larger than 30 μm in most cases. Filtration refers to the process of separating inclusion particles from the melt by passing the melt through densely packed spheres, glass fiber fabrics, metal meshes, foam ceramic filter plates, or other porous media. The advantage of foam ceramic filtration is that it can remove inclusion particles with a diameter much smaller than its through holes. Foam ceramic filter plates separate inclusions through three mechanisms: screening, cake layer, and deep bed filtration.

2.1 Screening

When the size of the particles or their clusters is larger than the pore size of the filter plate surface, the inclusion particles accumulate on the filter plate surface, just like a sieve to prevent large particles from passing through. Traditional glass fiber fabric filtration is this mechanism.

2.2 Cake layer

When larger particles accumulate more and more, a loose cake layer is formed on the surface of the filter plate, and the melt can still pass through; as the thickness of the cake layer increases, the size of the particles that can pass through becomes smaller and smaller, and more and more inclusions will be captured on the surface of the cake layer and in the gaps inside the cake layer. At the same time, the resistance to the passage of the melt increases, and the surface liquid level rises. When it reaches a certain level, the aluminum liquid is difficult to pass through, and it is necessary to consider replacing the filter plate. In the actual filtration process of aluminum alloy production, the obvious sign of entering the cake layer mechanism is to observe that the pressure head of the aluminum liquid gradually increases.

In fact, screening and cake layer are two stages of a mechanism. The length of this process depends on the amount of slag in the melt and the size of the filter plate aperture. Generally speaking, the more inclusions in the aluminum liquid, the smaller the filter plate aperture, and the faster the liquid level rises. Vice versa. To solve this problem, you can first filter with glass fiber fabric or coarse-pore filter plate and then filter through a fine filter plate.

2.3 Deep bed filtration

When the melt flows through the filter plate, inclusions that are much smaller than the pore size of the filter plate enter the filter plate with the aluminum liquid and come into contact with the surface of the ceramic part of the filter plate, resulting in particles adhering to the surface in contact with the melt.

Inside the ceramic filter plate, the path of the melt is tortuous, and there is even a phenomenon of cross-flow reversal in a small local area. This mainly plays two roles: (1) it can greatly increase the chance of particles contacting the filter medium; (2) it can make the flow rate and flow direction of the aluminum liquid change faster. This makes it easy for the tiny inclusions in the aluminum liquid to be left behind in a corner after the collision, which is conducive to the capture of particles. For inclusions with small diameters and not large numbers, the deep filtration mechanism is the most effective. At present, it is more common to use PHi (pore per inch) to indicate the size of the filter plate pore size. A 40ppi filter plate can remove 50% of 5jm-sized inclusions in one filtration.

In addition, whether the particles can be removed depends not only on the collision probability but also on the adsorption capacity of the surface of the filter plate in contact with the melt. The stronger the affinity between the material constituting the filter plate and the inclusion particles, the more microscopic defects (or roughness) on the surface of the pores, the more conducive to the adsorption and retention of inclusion particles.

In actual production, when the foam ceramic filter plate is filtered, the above three mechanisms work in combination, but the role of various mechanisms is different at different stages. In the initial stage, the surface screening intercepts large particles, while small particles are removed by the deep filtration mechanism: with the formation of the surface cake layer, the cake layer can intercept both large and small particles, and the small inclusion particles that escape the net can be captured by the deep bed filtration mechanism. At this time, the filter plate filtering effect will be better. When the adsorbed particles reach saturation, that is, when the number of particles adsorbed from the melt is equal to the number of particles carried away by the flowing melt, the slag removal capacity decreases.

3 Selection and use of filter plates

3.1 Selection of filter plates

When the quality of the filter plates themselves is not a problem, the first consideration when selecting filter plates should be the quality requirements of the product, combined with the user's practical experience and respective usage conditions, and finally, whether the cleanliness of the product meets the quality requirements should be used as the criterion.

Filter plates with different ppi and different thicknesses have different filtering effects on particles. The larger the ppi and the thicker the thickness, the better the filtering effect. However, if the thickness is too thick, the speed of the melt passing through will slow down. In actual production, if a certain flow rate is to be achieved, the size of the filter plate must be increased. The general standard filter plate thickness is 50mm. When the thickness is constant, the particle capture ability of filter plates with different ppi is also different. The larger the Ppi, the larger its specific surface area and the stronger its ability to capture small particles. Figure 2 shows the filtering effect measured by SELBE for filter plates with different PPI.

In addition, in actual production, the selection of filter plates should also consider the aluminum flow rate of the chute during the casting of the alloy liquid, the total amount of filtration, the cleanliness of the aluminum liquid itself, and the production cost. Selecting a filter plate with too high specifications will increase production costs and reduce production efficiency. The specific selection should be based on experience and actual conditions. The recommended liquid flow rate is generally 10-19mm/s. Table 1 is some empirical reference data for selecting filter plates.

3.2 How to use the filter plate

3.2.1 Preparation of the filter bag.

The filter bag should be thoroughly cleaned before use to prevent the refractory residue from the previous use from falling into the aluminum liquid; the size specifications of the filter plate and the filter bag should be matched, and the matching gap and deviation angle between the two should be as small as possible.

3.2.2 After the filter bag is cleaned, apply a layer of refractory paint on the inner surface of the box, and then put the filter plate in.

The gap between the filter plate and the filter bag should be filled with refractory materials such as asbestos rope.

3.2.3 Preheating.

Correct preheating is an important part of the use of filter plates. Incorrect preheating methods often lead to the following problems: a. Cracks appear during preheating; b. The aluminum liquid does not pass through the overheating plate or partially does not pass through; the pressure head is too high; d. The filter plate floats.

The equipment usually used for preheating is a natural gas flame spray gun. The size ratio of natural gas and air can be adjusted according to the actual production situation.

After igniting the torch, preheat the filter bag first. After the filter bag is preheated to the specified requirements, place the torch nozzle on the filter plate and heat it evenly and slowly until the overheating plate gradually turns fiery red, indicating that the preheating is in place.

During the use of the filter plate, the liquid level on the filter plate is usually called the "pressure head". Under normal filtration, the pressure head increases slowly over time, which is due to the gradual thickening of the cake layer on the surface of the filter plate. If the pressure head increases too quickly, it may be that there is too much slag in the aluminum liquid or the temperature is low (the viscosity of the aluminum melt increases); if the pressure head drops suddenly, it may be that the filter plate is broken or there are seams around it, as shown in Figure 3.

table 2

In actual production, after pouring begins, pay attention to the changes in the aluminum hydraulic head. Under normal circumstances, the starting pressure head is 75-150mm. When the aluminum liquid passes through, the pressure head will drop below 25mm, and then the pressure head will slowly rise again.

3.3 Technical requirements for making filter plates

The above-mentioned usage method is based on the reasonable selection of the filter plate size and ppi, and the quality of the filter plate is not a problem.

Generally, more formal foam ceramic filter plate manufacturers will use a more complete set of measures to ensure the stability of the product. For example, according to the definition of PPI, polyurethane foam with uniform pore size and size as close to the specified range as possible is selected as the precursor of the foam ceramic filter plate. There is a set of strict inspection and control measures for the viscosity, slurry amount and uniformity, drying, and sintering process of the slurry. Even if there is a fluctuation, it is within a specified range to ensure that the final product strength, through-porosity, geometric size, appearance quality, physical and chemical properties, etc. meet the standards. Table 2 is the recommended standard.

Figure 4

3.4 Requirements for filtering aluminum waves

In actual production, to achieve better-filtering effects, the alloy aluminum melt upstream of the filter plate must have a certain degree of cleanliness, that is, the upstream aluminum liquid can be treated with traditional gas dehydrogenation or flux to remove inclusion particles above 30um. After that, it is recommended to use multi-stage filtration technology and filter with different PPI filter plates to achieve satisfactory results. Without the above measures as a prerequisite, filtering and purifying the aluminum alloy melt with high hydrogen and oxygen content, many non-metallic inclusions and large size alone will not only have low filtration efficiency, but also easy clogging of the filter, resulting in increased costs, and thus the purification effect will not be very good. After taking the above measures, the hydrogen content in the alloy melt has dropped to a very low level. The size of the oxide inclusions in the melt is also small. Therefore, these fine and dispersed oxide inclusions can be effectively removed during the filtration process of the ceramic filter plate, thereby improving the filtration efficiency, reducing the cost of filtration purification, and obtaining a significant filtration effect. In addition, the filtered high-quality aluminum liquid must be protected from secondary contamination, that is, there should be more stringent requirements for the refractory materials downstream of the filter box, and the oxide scale on the surface of the downstream aluminum liquid should be protected.

4 Summary

Users can reasonably select and use filter plates based on the cleanliness of the aluminum melt and the requirements for the final product, combined with production practice experience. Different manufacturers have their considerations for using various specifications and types of filter materials and methods, which largely depend on the type and requirements of the products produced, as well as the manufacturer's process equipment conditions. Of course, in actual production, due to various reasons, ceramic filter plates have their weaknesses during use, such as unstable filtration efficiency in large-scale production.