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1 Ceramic foam filter
Foam ceramic is a type of porous material with a three-dimensional network structure and high porosity. Due to the special structure of foam ceramics, it has the advantages of low density, high porosity, high specific strength, good thermal shock resistance, and high-temperature resistance. Therefore, foam ceramics are widely used in gas and liquid filtration, purification and separation, chemical catalysis, sound absorption and shock absorption, advanced insulation materials, biological implant materials, and special strong materials and sensors [I’ship. Casting is one of the most widely used industries for ceramci foam filters. Its function is to make the chaotic and churning molten metal smooth, uniform, and clean after passing through the foam ceramic honeycomb holes, thereby greatly reducing the casting rejection rate caused by casting defects such as non-metallic inclusions and saving production costs.
2 Production process of foam ceramics
Ceramic foam filter (CFF, CeraIIlic Fo filter) uses polyurethane foam as a carrier, which is immersed in a slurry made of ceramic powder, binder, sintering aid, suspending agent, etc., and then squeezed out The excess slurry is used to evenly coat the ceramic slurry on the carrier skeleton to form a green body, which is then dried and sintered at high temperature. This process, also known as the organic foam impregnation method, is a relatively mature production process in China.
2.1 Process flow
2.1.1 Sponge processing process
The organic foam used mostly refers to polyurethane porous sponges, which are classified according to different pore sizes, including 10PPi, 15PPi, 20PPi, 30PPi, etc. The mesh classification of sponges is different from that of product meshes. Generally speaking, the larger the PPi (PorePer Inch, number of holes per inch) value, the smaller the corresponding pore size, and the smaller the filtered inclusions. As the first step, the sponge processing process is also very critical. The first is the choice of sponge. Because each sponge body may have different meshes. Even if the same batch of sponges is used, the mesh standards must be carefully checked before processing: secondly, the size of the sponge cutting must be accurate; finally, the processed sponge products must be guaranteed to have no tilt.
2.1.2 Selection of pulp mixing process formula
It is necessary to ensure the optimal consistency and fluidity of the slurry to ensure that the product can be evenly sized during the sizing process. Achieve the specified sizing weight. Because this is the prerequisite to ensure that the product's strength and porosity are qualified. The performance of the prepared slurry is judged by its specific gravity and consistency.
2.1.3 Sponge modification process
Sponge modification is to prepare for the sizing process. Improve the slurry hanging performance of the sponge. Make the sizing even.
2.1.4 Impregnation and Sizing Process
The modified dry sponge product is evenly coated on the roller press with the adjusted slurry to form a green body. Sponge products with different meshes require slurries of different consistencies for sizing, otherwise the sizing effect will not be achieved.
2.1.5 Drying process Drying process
The main purpose is to let the semi-finished products with good pulp evaporate the water, which is generally controlled below 1.0%. For products with larger specifications, it is necessary to control the dry environment, such as temperature, humidity, etc., to prevent drying deformation and crack defects.
2.1.6 Firing process Firing process
It is the last process in production, due to the improvement of the formula. Take into account the cost of production. At present, most SiC foam ceramics fired by foam ceramic companies do not require atmosphere protection, and the firing temperature is generally between 1350 and 1450°C. 2.1.7 Quality inspection process Due to its porous structure, foam ceramics. The burned products all have slag shedding to a greater or lesser extent. For foundries, in addition to considering the strength and mesh of the filter, the phenomenon of slag shedding is also a hot issue that they are most concerned about. Because the filter itself is used to filter out inclusions. However, its slagging will have the opposite effect, leading to the scrapping of the casting. Therefore, in the quality inspection process, in addition to testing the appearance and internal quality, it is also necessary to clean up the residue.
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3 Filtration mechanism and usage suggestions of foam ceramics for casting
3.1.1 Filter Cake Effect The complex foam ceramic structure can efficiently carry out mechanical slag blocking. When the molten metal passes through the complex structure foam ceramic filter, the filter medium removes pores larger than the filter surface through mechanical separation. Larger diameter inclusions are filtered out and precipitated at the liquid metal inlet end of the filter. As the number of inclusions accumulated on the filter surface increases, a layer of "filter cake" will gradually form, further narrowing the molten metal flow channel. Therefore, the surface of the newly added filter medium can filter out smaller inclusions. At the same time, the inside of the medium also has a filtering effect. Among the many small holes that penetrate the ceramic body, some have tiny slits and some have dead ends. These areas with different changes are possible locations for intercepting inclusions, and there is also a "filter cake" effect inside the filter.
3.1.2 Surface effect When molten metal flows through a ceramic body with a complex structure. It is divided into many small streams, which increases the contact area and probability of inclusions in the molten metal and the filter medium. Since the surface of the filter is an extremely small uneven surface, the size of the concave blocks is about 1 to 10 μm, which is very important for inclusions. It has electrostatic adsorption and adhesion interception effects.
3.1.3 Rectification effect When the molten metal flows through the porous ceramci filter, it is divided into many small unit streams with smaller diameters, which reduces the Reynolds number (Re_vd/r) and makes the liquid flow tend to be laminar. sports. When the molten metal is in a laminar flow state. Since the density of the molten metal is much greater than the density of the inclusions, the inclusions have sufficient time to float up and be removed. That is, the foam ceramic filter can assist the runner in preventing slag. After a filter is placed in the gating system, the resistance to the flow of molten metal increases. The molten metal flowing in the runner is prone to full motion and reduces the flow rate, which is conducive to the floating of inclusions and retention on the top surface of the runner.
3.2 Recommendations for the use of alumina ceramic foam filters
(1) Select filters of appropriate materials according to the melting point of the alloy to avoid excessive temperature. The function of the filter is damaged and the filtering effect cannot be achieved.
(2) Select the corresponding mesh, and the purification effect must match the requirements of the casting.
(3) The casting temperature should be as high as possible to increase metal fluidity.
(4) When the filter is placed horizontally under the intersection cup or on the parting surface. The casting height should not exceed 20cm. The molten metal should rush onto the wall of the intersection cup. Do not rush directly to the filter.
(5) The filter must be handled with care. When not in use, keep it in a dry and ventilated place. To prevent moisture absorption from affecting the strength of the filter.
4 Conclusion
The organic foam impregnation method is currently the most widely used production process for foam ceramic filters. However, with the intensification of market competition, rising raw material prices, and other factors, how to further improve the process, improve production efficiency, improve product quality, and reduce production costs has become an important topic in current research. Compared with foreign foam ceramic filters, there is a big gap in appearance and product quality between domestic ones. Moreover, the existing varieties are single and can only occupy part of the middle and low-end markets. Although there are also domestic foam ceramic products exported, the prices are not ideal. Supply is also unstable. On the other hand, national standards for foam ceramic filters should be formulated as soon as possible. It is conducive to the development of the foam ceramic industry.
When it comes to filtration, ceramic materials offer a massive amount of options and solutions for users. Our raw materials are used in many applications such as ceramic membranes for pharma/chemicals, wastewater and oil/water separation, ceramic foam filters used in molten steel, aluminum and iron casting, and catalyst supports and diesel particle filters for the industrial and automotive industry. We design our raw materials to meet the flexural strength, porosity, and thermal properties as desired and requested by our clients, fully compatible with shaping processes such as extrusion and impregnation methods.
Ceramic membranes for liquid-solid or liquid-liquid separations are used in water & oil separation, pharma, chemical and industrial & public wastewater treatment. We optimize our materials to meet the separation requirements, from the size and shape, working pressures, temperatures, cycling rates to the pH and corrosion resistance.
Aluminas are widely used due to their workability at low and high temperatures, at pH of 0-14 and at relatively low cost. Coarser white fused aluminas will make up the support layer enabling the desired porosity. Fine and very fine aluminas or zirconia powders will find their place in the intermediate and filtration layer.
Ceramic foam filters represent another area where we supply a variety of Zirconias and Aluminas. Molten metals, more specifically aluminium and steel (stainless, alloys), require a purification from solid and liquid impurities prior to casting to avoid part defects. Ceramic foam filters are the best solution to remove these impurities. Thermal shock resistance, operational temperature and corrosion resistance, as well as processability, are important factors to consider when selecting ceramic raw materials.
Our main raw materials for membranes & foam filters can be supplied from our plants in Germany, China, Switzerland and the USA. ZIONICTM Zirconia and ALODUR® Alumina are the most widely used. Our TIMREX® graphite is primarily used as a pore former. We also offer certain natural minerals that may be used as binders.
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