Preparation method of porous ceramic filter for purifying liquid magnesium alloy

Abstract

The invention discloses a porous ceramic filter for liquid magnesium alloy purification and a preparation method thereof, and the porous ceramic filter comprises the following raw materials in percentage by mass: 70-85wt% of ceramic particles, 8-15wt% of multi-element oxide system microcrystalline glass brazing filler metal and the balance of a binder, al2O3-B2O3-SiO2-MgO-CaO multi-element oxide system microcrystalline glass brazing filler metal is adopted, under the condition of air atmosphere high-temperature calcination, a compact reaction layer containing a low-melting-point glass phase, an aluminum oxide phase, a magnesium aluminate spinel phase, a calcium aluminosilicate phase, a borate phase and a silicate phase is generated on the surfaces of ceramic particles of a porous ceramic filter through a chemical reaction; the magnesium alloy filter has high strength and good high-temperature stability, and the problems that the filter loses efficacy and magnesium alloy melt is polluted due to the fact that the filter is eroded by magnesium alloy liquid are effectively solved.

Description

Technical field:

[0001] The invention relates to the technical fields of melt filtration and ceramic materials, and in particular to a method for preparing a porous ceramic filter for purifying liquid magnesium alloy. Background technology:

[0002] Magnesium alloys have the advantages of light weight, high specific strength, good thermal conductivity, good electromagnetic shielding performance, and strong damping, vibration reduction and noise reduction capabilities. They have broad application prospects in the automotive industry, aerospace, 3C electronics, biomedicine and other fields. In recent years, the development of structural lightweight technology and environmental protection needs have further stimulated the high-quality development of the magnesium alloy industry, and it is becoming one of the indispensable materials in modern industry.

[0003] A pure magnesium alloy melt is the fundamental prerequisite for obtaining high-quality magnesium alloy materials. However, in actual industrial production, liquid magnesium alloy easily reacts with O2, H2O, N2, etc. to form MgO, Mg3N2, etc., resulting in defects such as slag and impurities in the casting, reducing the purity of the magnesium alloy and affecting its casting performance and the mechanical properties of the final product. Therefore, how to effectively remove impurities and slag defects in the magnesium alloy melt is an issue that must be addressed in the production of high-quality magnesium alloy castings.

[0004] Currently, filtration is an effective way to solve this problem. There are two most commonly used filters: one is a woven glass mesh filter, which can effectively remove larger impurities in the molten metal. However, the glass fiber of this filter becomes very brittle during the magnesium alloy casting process, easily breaks, and broken objects are mixed into the casting; the other is a ceramic foam filter, which greatly improves the filtration effect by intercepting, adsorbing and depositing inclusions. However, the honeycomb pore blockage of this type of filter still reduces the filtration efficiency, and the compressive strength at the magnesium alloy casting temperature is low, making it impossible to filter under high pressure.

[0005] In view of this, this application is hereby filed. Summary of the invention:

[0006] The purpose of the present invention is to provide a porous ceramic filter for purifying liquid magnesium alloy and a preparation method thereof, which solves the problem of the existing technology that the filter is corroded by the magnesium alloy liquid, resulting in the filter breaking and failing, entering the magnesium alloy melt and being contaminated, or the problem of low filtration efficiency.

[0007] The present invention is achieved through the following technical solutions:

[0008] A porous ceramic filter for purifying liquid magnesium alloy, wherein the raw material composition of the porous ceramic filter is 100% by total mass percentage, including: 70-85wt% ceramic particles, 8-15wt% multi-element oxide system microcrystalline glass solder, and the rest is a binder; the multi-element oxide system microcrystalline glass solder has an average particle size of less than 5μm and is an Al2O3-B2O3-SiO2-MgO-CaO multi-element oxide system, and the raw material composition is 100% by total mass percentage, including: 15-45wt% Al2O3, 15-40wt% B2O3, 15-35wt% SiO2, 5-15wt% MgO, and the balance is CaO, preferably 15-35wt% Al2O3, 25-40wt% B2O3, 15-20wt% SiO2, 5-10wt% MgO, and the balance is CaO, and is prepared by the process of "rapid heating - short-term heat preservation - water quenching and crushing - drying treatment - ball milling and refining - screening and grading"; the binder is one or more of water glass, silica sol, phenolic varnish, methyl cellulose, polyacrylamide solution, and aluminum dihydrogen phosphate solution.

[0009] Preferably, based on the total mass percentage being 100%, the raw material composition of the porous ceramic filter includes: 70-78wt% ceramic particles, 12-15wt% multi-element oxide system microcrystalline glass solder, and the rest being a binder.

[0010] Preferably, the ceramic particles are made of one or more of Al2O3 ceramic, SiO2 ceramic, ZrO2 ceramic, or Al2O3-ZrO2 composite ceramic (with adjustable composition). The main reason for selecting ceramic particles of these materials as the structural support skeleton is that these oxide ceramic particles can react with the microcrystalline glass solder of the Al2O3-B2O3-SiO2-MgO-CaO multi-oxide system. The high-temperature resistant products generated by the reaction can increase the high-temperature strength of the porous ceramic filter. In addition, these ceramic particles have good high-temperature resistance and chemical stability, making them suitable for purification of magnesium alloy melts.

[0011] The size of ceramic particles is determined according to the application scenario of the filter and the filtration accuracy requirements. The porous ceramic filter of the present invention is used in situations with high filtration accuracy. The size of ceramic particles is controlled between 800μm and 3mm to ensure a balance between filtration effect and porosity, preferably 800μm-1mm.

[0012] The multi-component oxide system microcrystalline glass solder is prepared by the "rapid heating-short-time heat preservation-water quenching and crushing-drying treatment-ball milling and refining-screening and grading" process. The specific method is as follows: the multi-component oxide system microcrystalline glass solder raw materials are placed in a graphite crucible of a high-frequency induction heating furnace according to the mass ratio, and the temperature is increased to 1350-1450℃ at a rate of 150-200℃ / min; then the temperature is kept for 2-5 minutes; the glass melt is quickly cooled to room temperature in water and then mechanically crushed into coarse particles, and then kept at 100-110℃ for 8-12 hours for drying; ball milling is carried out using a planetary ball mill with a rotation speed of 300-500r / min and a ball milling time of 4-6 hours, and then sieving and screening to obtain a multi-component oxide system microcrystalline glass solder with an average particle size of less than 5μm.

[0013] The glass-ceramic solder of the multi-oxide "Al2O3-B2O3-SiO2-MgO-CaO" system is a complex system, with its reaction products and phase behavior influenced by the component ratios, temperature, and reaction conditions. The metastable phases that can form in this system after the aforementioned "rapid heating-short holding-water quenching and fragmentation" cycle primarily include: ① a low-melting-point glass phase (an amorphous glass network structure rich in SiO2 and B2O3); ② a metastable boroaluminate phase (9Al2O3·2B2O3); ③ a metastable calcium aluminosilicate phase (such as CaAl2Si2O8); ④ a metastable borate phase (such as CaB2O4 or Mg3B2O6); and ⑤ a metastable silicate phase (such as MgSiO3 or 2CaO·MgO·2SiO2).

[0014] Binders are closely linked to the performance of porous ceramic filters, with different binders possessing distinct characteristics in terms of process compatibility, cost, and temperature resistance. Water glass is inexpensive and readily available, easily solidifying under natural drying conditions to provide strength to porous ceramic filter blanks. Its excellent high-temperature resistance (withstanding temperatures exceeding 800°C) makes it suitable for high-temperature sintering processes, resulting in ceramic filters with high rigidity and compressive strength. Silica sol exhibits excellent liquid fluidity, extremely low (nearly zero) volume shrinkage during sintering, and minimal high-temperature decomposition residue. Phenolic varnish carbonizes at high temperatures to form a high-temperature-resistant network structure, resistant to transient high-temperature shocks. The resulting bond layer is highly robust, enhancing the compressive resistance of porous ceramic filters. Methyl cellulose provides good bonding during the low-temperature molding stage and is easily removed after drying, reducing residual impurities. Polyacrylamide solution can enhance the bonding between slurry particles through charge neutralization and bridge adsorption, improving blank strength. A small amount of polyacrylamide reduces slurry viscosity and improves coating uniformity. Aluminum dihydrogen phosphate Aluminum dihydrogen phosphate can form a stable aluminum phosphate network at high temperature, with a temperature resistance of over 1200°C; it reacts with the surface of ceramic particles to form phosphate compounds, which strengthens the chemical bonding between particles and reduces interface defects.

[0015] The method for preparing the porous ceramic filter for purifying liquid magnesium alloy comprises the following steps:

[0016] Step S1, screening ceramic particles: selecting ceramic particles of appropriate size according to the target porosity and filtration accuracy requirements of the porous ceramic filter; the average particle size of the ceramic particles is 800 μm-3 mm, preferably 800 μm-1 mm;

[0017] Step S2, weighing and mixing raw materials: First, ceramic particles, multi-element oxide system glass-ceramics solder, and binder are weighed according to the mass ratio; then, the multi-element oxide system glass-ceramics solder and binder are mixed at room temperature and mechanically stirred to form a homogeneous slurry. Finally, the slurry is evenly coated on the surface of the clean ceramic particles using a three-dimensional mixing process;

[0018] Step S3, green blank molding and drying: The ceramic particles coated with the slurry are filled into a mold, and a pressure of 10-15 MPa, preferably 12-15 MPa, is applied and maintained to form the green blank. After demolding, the green blank is naturally dried, and then further heated to 100-110° C. for drying to obtain a porous ceramic filter green blank with a certain strength and controllable porosity.

[0019] Step S4, high-temperature sintering and cooling of the green body: placing the porous ceramic filter green body in a high-temperature furnace, calcining it at 1220-1420°C in an air atmosphere for 3-5 hours, and cooling it along the temperature gradient in the furnace to obtain a porous ceramic filter with high strength, high filtration efficiency and uniform pore distribution for magnesium alloy filtration.

[0020] Preferably, the rotation speed of the mechanical stirring in step S2 is 300-600 r / min, and the rotation speed adopted in the three-dimensional mixing process is 200-300 r / min.

[0021] In step S3, the holding time is 10-20 seconds, preferably 15-20 seconds, the natural drying time is 12-24 hours, and the heating drying time is 2-3 hours.

[0022] In step S4, the heating rate is 2-5°C / min, and the cooling rate is 10-15°C / min.

[0023] During the preparation of porous ceramic filters for purifying liquid magnesium alloys, a glass-ceramic brazing filler metal composed of the multi-oxide system "Al2O3-B2O3-SiO2-MgO-CaO" forms alumina, magnesia spinel, and borate whiskers on the ceramic particle surfaces. The detailed process is as follows: During sintering, the multi-oxide glass-ceramic brazing filler metal deviates from thermodynamic equilibrium, initially forming metastable phases. These metastable phases primarily consist of AlBO3, CaAl2O4, CaB4O7, Mg2B2O5, Ca2MgSi2O7, and an amorphous glass phase (containing SiO2 and B2O3). These metastable phases then decompose during the high-temperature continuous calcination of the ceramic blank. Through reactions within the glass-ceramic, the metastable phases react with the ceramic particle surfaces to form a low-melting-point glass phase, magnesia spinel, calcium aluminum silicate, borate, and silicate phases, with the magnesia spinel and borate phases forming whiskers. Excess unreacted alumina can grow into alumina whiskers. The main functions of whiskers in magnesium alloy melt filtration are: ① Physical interception and surface adsorption. The high aspect ratio and nanometer / micrometer size of whiskers provide a huge specific surface area, which captures tiny inclusions in the melt through physical interception. The porous network structure formed by whiskers and ceramic particles can enhance the efficiency of inclusion retention. ② Chemical bonding and interfacial reaction. The whisker surface is rich in active sites such as Al-O and BO, which can chemically react with metal oxides (such as FeO and MnO) in inclusions to form a stable composite phase to remove tiny inclusions. ③ B2O3 reduces the melt viscosity by adjusting the polymerization degree of the slag (such as converting the framework structure into a layered structure), making it easier for inclusions to migrate to the whisker surface and be captured. In summary, the alumina, magnesium aluminum spinel and borate whiskers generated by the reaction of the microcrystalline glass solder of the "Al2O3-B2O3-SiO2-MgO-CaO" multi-oxide system can achieve efficient adsorption of tiny inclusions through physical interception, chemical bonding and melt fluidity optimization. In addition, the microcrystalline glass solder of the multi-oxide system will also react with itself under the high-temperature calcination conditions in the air atmosphere in step S4 to generate a low-melting-point glass phase, an alumina phase, a magnesium aluminum spinel phase, a calcium aluminum silicate phase, a borate phase and a silicate phase. The phases generated by these reactions will coat the ceramic particles and react with and connect the oxide ceramic particles to generate porous ceramic components with high strength, high temperature resistance, high filtration efficiency, excellent compressive resistance and thermal shock resistance.

[0024] Therefore, the present invention also protects the porous ceramic filter for purifying liquid magnesium alloy obtained by the above preparation method and its application. When used to filter magnesium alloy melt, the porous ceramic filter does not pollute the melt and can efficiently intercept, adsorb and deposit slag and inclusions in the melt.

[0025] The beneficial effects of the present invention are as follows:

[0026] 1) The present invention uses an Al2O3-B2O3-SiO2-MgO-CaO multi-oxide system micro-ceramic glass solder. Under high-temperature calcination conditions in an air atmosphere, a chemical reaction forms on the surface of the ceramic particles of the porous ceramic filter. The resulting layer contains a low-melting-point glass phase, an alumina phase, a magnesium-aluminum spinel phase, a calcium-aluminum silicate phase, a borate phase, and a silicate phase. The layer has high strength and good high-temperature stability, effectively solving the problem of filter failure and contamination of the magnesium alloy melt caused by corrosion of the filter by the magnesium alloy liquid.

[0027] 2) The porous ceramic filter material prepared by the present invention has excellent compressive and thermal shock resistance: the compressive strength at room temperature is not less than 30 MPa, the compressive strength at 800°C is not less than 27 MPa, and the strength decay is minimal during high-temperature use. This improves the filter's ability to withstand higher pressures and effectively eliminates the problem of ceramic particles breaking and flaking into the magnesium alloy melt.

[0028] 3) The average porosity of the porous ceramic filter prepared by the present invention is adjustable between 32.3% and 37.4%. The porosity is adjusted by adjusting the average particle size and content of the raw ceramic particles, the green body molding pressure and holding time during the preparation process, and the green body high-temperature sintering temperature and heating rate. The filtration efficiency for inclusions of 5-50 μm is not less than 95%, and the filtration efficiency for inclusions ≤5 μm is not less than 90%. It has the advantages of high filtration precision, high filtration efficiency, and large filtration capacity.

[0029] 4) The preparation method provided in this application is simple in process, easy to operate, does not require vacuum equipment, and can realize batch preparation of porous ceramic filters in an air atmosphere, thereby reducing costs and being suitable for industrial production.

[0030] In summary, the preparation method of the present invention is simple, easy to operate, and low in cost, and obtains a new three-dimensional mesh structure ceramic filter with full through holes, adjustable pore size, uniform pore distribution, high strength, high filtration efficiency, excellent compressive resistance and thermal shock resistance, which significantly improves the filtration efficiency and filtration effect, thereby improving the purity and performance of the magnesium alloy material. Description of the drawings:

[0031] Figure 1 is a micro-CT image of the porous ceramic filter of Example 1 of the present invention;

[0032] Figure 2 The SEM images of the particle connections in the porous ceramic filter of Example 1 of the present invention are shown. The left image is a low-magnification SEM image of the connection between the ceramic particles and the whiskers (sintering neck), and the right image is a high-magnification SEM image of the connection between the ceramic particles and the whiskers (sintering neck).

[0033] Figure 3The whisker SEM morphology of the surface of the particles in the porous ceramic filter of the present invention: (a) Comparative Example 1; (b) Example 1; (c) Example 6; (d) Example 7. Specific implementation method:

[0034] The following is a further description of the present invention, but not a limitation of the present invention.

[0035] Example 1: Step S1, ceramic particle screening: selecting 60Al2O3-40ZrO2 (weight percentage) multiphase ceramic particles with an average size of 800 μm;

[0036] Step S2, weighing and mixing raw materials: first weighing 70wt% of 60Al2O3-40ZrO2 (weight percentage) composite ceramic particles and 15wt% of "35Al2O3-25B2O3-20SiO2-15MgO-5CaO" multi-oxide system microcrystalline glass solder (the average particle size is less than 5μm, and the raw material composition is calculated based on the total mass percentage of 100%, including: 35wt% Al2O3, 25wt% B2O3, 20wt% SiO2, 15wt% MgO, 5% CaO, and prepared by the process of "rapid heating-short-time heat preservation-water quenching and crushing-drying treatment-ball milling refinement-screening and grading"; at 200℃ / min and kept at this temperature for 2 minutes; the glass melt was quickly cooled to room temperature in water and then mechanically crushed into coarse particles, which were then kept at 110°C for 12 hours and dried; a planetary ball mill was used for refining at a speed of 500 r / min and a ball milling time of 6 hours, and glass solder with an average particle size of less than 5 μm was obtained by sieving), and 15 wt% binder (methyl cellulose: water glass: aluminum dihydrogen phosphate solution in a volume ratio of 1:1:3); the microcrystalline glass solder and binder were then mixed at room temperature and mechanically stirred (at a speed of 600 r / min) to form a homogeneous slurry, which was finally evenly coated on the surface of the clean ceramic particles using a three-dimensional mixing process (at a speed of 300 r / min);

[0037] Step S3, green body molding and drying: The ceramic particles coated with the slurry are filled into a mold, and a pressure of 15 MPa is applied and maintained for 20 seconds to form the green body. After demolding, the green body is naturally dried for 24 hours, and then further heated and dried at 110°C for 3 hours to obtain a porous ceramic filter green body with a certain strength and controllable porosity;

[0038] Step S4, high-temperature sintering and cooling of the blank: placing the porous ceramic filter blank in a high-temperature furnace, heating it to 1420°C at a rate of 2°C / min in an air atmosphere, and keeping it warm for 5 hours. Thereafter, cooling it to room temperature at a rate of 10°C / min in the furnace to obtain a porous ceramic filter with high strength, high filtration efficiency, excellent compressive resistance and thermal shock resistance for magnesium alloy filtration.

[0039] The porous ceramic filter prepared in this embodiment has a porosity of 32.6%, a compressive strength of 35.7 MPa at room temperature, and a compressive strength of 32.2 MPa at 800° C.; a filtration efficiency of 97.3% for inclusions of 5-50 μm and 91.8% for inclusions ≤5 μm.

[0040] Example 2

[0041] It is basically the same as Example 1, except that the ratio of raw materials used in the porous ceramic filter is different, the mass fraction of 60Al2O3-40ZrO2 composite ceramic particles is 78wt%, the mass fraction of "35Al2O3-25B2O3-20SiO2-15MgO-5CaO" multi-oxide system microcrystalline glass solder is 12wt%, and the mass fraction of the binder (methyl cellulose: water glass: aluminum dihydrogen phosphate solution volume ratio of 1:1:3) is 10wt%.

[0042] The porous ceramic filter prepared in this embodiment has a porosity of 33.1%, a compressive strength of 33.5 MPa at room temperature, and a compressive strength of 31.4 MPa at 800° C.; a filtration efficiency of 96.7% for inclusions of 5-50 μm and 91.2% for inclusions ≤5 μm.

[0043] Example 3

[0044] It is basically the same as Example 1, except that the ratio of raw materials used in the porous ceramic filter is different, the mass fraction of 60Al2O3-40ZrO2 composite ceramic particles is 85wt%, the mass fraction of "35Al2O3-25B2O3-20SiO2-15MgO-5CaO" multi-oxide system microcrystalline glass solder is 8wt%, and the mass fraction of the binder (methyl cellulose: water glass: aluminum dihydrogen phosphate solution volume ratio of 1:1:3) is 7wt%.

[0045] The porous ceramic filter prepared in this embodiment has a porosity of 33.5%, a compressive strength of 30.8 MPa at room temperature, and a compressive strength of 28.3 MPa at 800° C.; a filtration efficiency of 96.4% for inclusions of 5-50 μm and 90.7% for inclusions ≤5 μm.

[0046] Example 4

[0047] It is basically the same as Example 1, except that the average grain size of the 60Al2O3-40ZrO2 composite ceramic particles is 1.5 mm.

[0048] The porous ceramic filter prepared in this embodiment has a porosity of 34.7%, a compressive strength of 33.1 MPa at room temperature, and a compressive strength of 30.7 MPa at 800° C.; a filtration efficiency of 96.6% for inclusions of 5-50 μm and 90.8% for inclusions ≤5 μm.

[0049] Example 5

[0050] It is basically the same as Example 1, except that the average grain size of the 60Al2O3-40ZrO2 composite ceramic particles is 3 mm.

[0051] The porous ceramic filter prepared in this embodiment has a porosity of 37.4%, a compressive strength of 31.8 MPa at room temperature, and a compressive strength of 29.4 MPa at 800° C.; a filtration efficiency of 95.7% for inclusions of 5-50 μm and 90.2% for inclusions ≤5 μm.

[0052] Example 6

[0053] This embodiment is basically the same as Example 1, with the only difference being that the raw material ratio of the multi-oxide system microcrystalline glass solder is different. The raw material composition, calculated based on the total mass percentage of 100%, includes: 35wt% Al2O3, 30wt% B2O3, 15wt% SiO2, 10wt% MgO, and 10% CaO, which is recorded as "35Al2O3-30B2O3-15SiO2-10MgO-10CaO".

[0054] The porous ceramic filter prepared in this embodiment has a porosity of 32.3%, a compressive strength of 37.2 MPa at room temperature, and a compressive strength of 33.8 MPa at 800° C.; a filtration efficiency of 96.5% for inclusions of 5-50 μm and 90.7% for inclusions ≤5 μm.

[0055] Example 7

[0056] This embodiment is basically the same as Example 1, with the only difference being that the raw material ratio of the multi-oxide system microcrystalline glass solder is different. The raw material composition, calculated based on the total mass percentage as 100%, includes: 45wt% Al2O3, 20wt% B2O3, 15wt% SiO2, 15wt% MgO, and 5% CaO, recorded as "45Al2O3-20B2O3-15SiO2-15MgO-5CaO".

[0057] The porous ceramic filter prepared in this embodiment has a porosity of 32.7%, a compressive strength of 30.6 MPa at room temperature, and a compressive strength of 27.3 MPa at 800° C.; a filtration efficiency of 96.1% for inclusions of 5-50 μm and 89.6% for inclusions ≤5 μm.

[0058] Example 8

[0059] This embodiment is basically the same as Example 1, with the only difference being that the preparation process of the multi-oxide system microcrystalline glass solder "35Al2O3-25B2O3--20SiO2-15MgO-5CaO" is different: heating to 1350°C at 200°C / min and keeping warm for 2 minutes; the glass melt is quickly cooled to room temperature in water and then mechanically crushed into coarse particles, and then kept warm and dried at 110°C for 10 hours; the planetary ball mill has a refining speed of 500 r / min and a ball milling time of 4 hours, and the glass solder with an average particle size of less than 5 μm is obtained by sieving.

[0060] The porous ceramic filter prepared in this embodiment has a porosity of 32.6%, a compressive strength of 32.1 MPa at room temperature, and a compressive strength of 30.3 MPa at 800° C.; a filtration efficiency of 96.1% for inclusions of 5-50 μm and 91.3% for inclusions ≤5 μm.

[0061] Example 9

[0062] This embodiment is basically the same as Example 1, with the only difference being that the preparation process of the multi-oxide system microcrystalline glass solder "35Al2O3-25B2O3--20SiO2-15MgO-5CaO" is different: heating to 1350°C at 150°C / min and keeping warm for 2 minutes; the glass melt is quickly cooled to room temperature in water and then mechanically crushed into coarse particles, and then kept warm and dried at 110°C for 10 hours; the planetary ball mill has a refining speed of 500 r / min and a ball milling time of 4 hours, and the glass solder with an average particle size of less than 5 μm is obtained by sieving.

[0063] The porous ceramic filter prepared in this embodiment has a porosity of 32.7%, a compressive strength of 30.7 MPa at room temperature, and a compressive strength of 28.4 MPa at 800° C.; a filtration efficiency of 95.4% for inclusions of 5-50 μm and 90.3% for inclusions ≤5 μm.

[0064] Example 10

[0065] This embodiment is basically the same as Example 1, with the only difference being that the blank forming process is different: the ceramic particles coated with the slurry are filled into a mold, a pressure of 12 MPa is applied and maintained for 15 seconds to form the blank, and after demolding, the blank is naturally dried for 12 hours, and then further heated and dried at 110°C for 3 hours to obtain a porous ceramic filter blank with a certain strength and controllable porosity.

[0066] The porous ceramic filter prepared in this embodiment has a porosity of 33.2%, a compressive strength of 34.6 MPa at room temperature, and a compressive strength of 31.7 MPa at 800° C.; a filtration efficiency of 97.1% for inclusions of 5-50 μm and 91.5% for inclusions ≤5 μm.

[0067] Example 11

[0068] This embodiment is basically the same as Example 1, except that the blank forming process is different: the ceramic particles coated with the slurry are filled in the mold, a pressure of 10 MPa is applied and maintained for 10 seconds to form it, and after demolding, it is naturally dried for 12 hours and further heated and dried at 100°C for 3 hours to obtain a porous ceramic filter blank with certain strength and controllable porosity.

[0069] The porous ceramic filter prepared in this embodiment has a porosity of 32.4%, a compressive strength of 33.8 MPa at room temperature, and a compressive strength of 31.1 MPa at 800° C.; a filtration efficiency of 97.2% for inclusions of 5-50 μm, and a filtration efficiency of 91.4% for inclusions ≤5 μm.

[0070] Example 12

[0071] This embodiment is basically the same as embodiment 1, except that the sintering process is different: the heating rate is 5°C / min

[0072] The porous ceramic filter prepared in this embodiment has a porosity of 32.3%, a compressive strength of 33.1 MPa at room temperature, and a compressive strength of 31.3 MPa at 800° C.; a filtration efficiency of 96.3% for inclusions of 5-50 μm and 90.8% for inclusions ≤5 μm.

[0073] Example 13

[0074] This embodiment is basically the same as embodiment 1, except that the ceramic particles are Al2O3 ceramics.

[0075] The porous ceramic filter prepared in this embodiment has a porosity of 32.9%, a compressive strength of 32.8 MPa at room temperature, and a compressive strength of 30.4 MPa at 800° C.; a filtration efficiency of 97.1% for inclusions of 5-50 μm and 91.3% for inclusions ≤5 μm.

[0076] Example 14

[0077] This embodiment is basically the same as the embodiment 1, except that the ceramic particles are "65 wt% Al2O3 ceramic + 35 wt% SiO2 ceramic".

[0078] The porous ceramic filter prepared in this embodiment has a porosity of 32.3%, a compressive strength of 33.7 MPa at room temperature, and a compressive strength of 31.1 MPa at 800° C.; a filtration efficiency of 96.5% for inclusions of 5-50 μm and 90.4% for inclusions ≤5 μm.

[0079] Example 15

[0080] This embodiment is basically the same as the embodiment 1, except that the ceramic particles are "40 wt % ZrO 2 ceramic + 60 wt % SiO 2 ceramic".

[0081] The porous ceramic filter prepared in this embodiment has a porosity of 32.5%, a compressive strength of 34.2 MPa at room temperature, and a compressive strength of 31.4 MPa at 800° C.; a filtration efficiency of 96.8% for inclusions of 5-50 μm and 90.7% for inclusions ≤5 μm.

[0082] Comparative Example 1

[0083] This embodiment is basically the same as embodiment 1, and the only difference is the sintering process: the sintering temperature is 1480°C.

[0084] The porous ceramic filter prepared in this embodiment has a porosity of 33.8%, a compressive strength of 17.3 MPa at room temperature, and a compressive strength of 14.2 MPa at 800° C.; a filtration efficiency of 94.1% for inclusions of 5-50 μm, and a filtration efficiency of 85.6% for inclusions ≤5 μm.

[0085] Comparative Example 2

[0086] This embodiment is basically the same as embodiment 1, and the only difference is the sintering process: the heating rate is 15° C. / min.

[0087] The porous ceramic filter prepared in this embodiment has a porosity of 34.6%, a compressive strength of 26.1 MPa at room temperature, and a compressive strength of 19.8 MPa at 800° C.; a filtration efficiency of 95.4% for inclusions of 5-50 μm and 87.3% for inclusions ≤5 μm.

[0088] Comparative Example 3

[0089] This embodiment is basically the same as embodiment 1, and the only difference is the furnace cooling process: the furnace cooling rate is 30° C. / min.

[0090] The porous ceramic filter prepared in this embodiment has a porosity of 34.9%, a compressive strength of 22.4 MPa at room temperature, and a compressive strength of 17.3 MPa at 800° C.; a filtration efficiency of 96.3% for inclusions of 5-50 μm, and a filtration efficiency of 88.6% for inclusions ≤5 μm.

[0091] By testing the porosity, room temperature compressive strength, compressive strength at 800°C, and filtration accuracy for inclusions ≤ 5 μm of the porous ceramic filters for liquid magnesium alloy purification prepared in different embodiments, the following conclusions can be drawn:

[0092] From the comparison of Example 1, Example 2 and Example 3, it can be seen that when the proportion of ceramic particles in the raw materials used to prepare the porous ceramic filter increases and the proportion of multi-oxide microcrystalline glass solder and binder decreases, the porosity of the porous ceramic filter increases slightly, the compressive strength at room temperature and the compressive strength at 800°C decrease, and the filtration efficiency of 5-50μm / less than 5μm inclusions decreases slightly. Therefore, based on the total mass percentage of 100%, the raw material composition of the porous ceramic filter preferably includes: 70-78wt% ceramic particles, 12-15wt% multi-oxide system microcrystalline glass solder, and the rest is binder. In general, within the scope of the requirements of this patent, the adjustment of the proportion of raw material components has no significant effect on the various indicators of the porous ceramic filter. The main reason is that the high proportion of ceramic particles ensures the stability of the skeleton and offsets some fluctuations in porosity and strength. The microcrystalline glass solder provides additional bonding force through high-temperature reaction. The optimization of process parameters buffers the negative impact of the ratio adjustment.

[0093] Comparison of Examples 1, 4, and 5 shows that when the average grain size of the ceramic particles in the raw materials used in the porous ceramic filter increases (within the range of 0.8-3 mm), the porosity of the porous ceramic filter increases significantly. This is because the increase in particle size reduces the contact points between particles, resulting in larger gaps and smaller sintering necks between particles. The increase in porosity means that the solid part of the porous ceramic decreases, and the area that effectively bears pressure decreases, which in turn slightly reduces the compressive strength at room temperature and at 800°C. The increase in pore size and the decrease in the specific surface area of ​​the whiskers on the particle surface weaken the interception and adsorption of inclusions, thereby slightly reducing the filtration efficiency for inclusions of 5-50 μm / less than 5 μm. Therefore, the average particle size of the ceramic particles is preferably 800 μm-1 mm.

[0094] Comparison of Example 1, Example 6 and Example 7 shows that when the ratio of the multi-oxide microcrystalline glass solder in the raw materials used for the porous ceramic filter changes, there is no significant effect on the porosity of the porous ceramic filter, showing that with the increase of the proportion of Al2O3 and the decrease of the proportion of B2O3, the porosity increases slightly; however, it has a significant effect on the compressive strength at room temperature and the compressive strength at 800°C, and generally shows an increase with the increase of the proportion of Al2O3. This is because the increase in the proportion of magnesium aluminum spinel phase generated by the reaction at the interface connection leads to an increase in strength; the filtration efficiency of 5-50μm / less than 5μm inclusions is reduced, which is because the aspect ratio of the whiskers on the particle surface of the porous ceramic filter becomes smaller (see Figure 3 ) leads to the weakening of the adsorption effect on small-sized inclusions.

[0095] From the comparison of Example 1, Example 8 and Example 9, it can be seen that the preparation process of the multi-oxide microcrystalline glass solder in the raw materials used for the porous ceramic filter has no significant effect on the porosity of the porous ceramic filter, but the increase in the heating rate and the holding temperature during the preparation of the multi-oxide microcrystalline glass solder raw materials helps it retain more metastable phases, so that it can subsequently react with the surface of the ceramic particles to form a stronger connection, thereby improving the room temperature compressive strength and the compressive strength at 800°C of the porous ceramic filter; the filtration efficiency for 5-50μm / less than 5μm inclusions is slightly reduced, mainly because the retained metastable phase changes the microstructure of the porous ceramic after reacting with the surface of the ceramic particles, resulting in irregular changes in the pores.

[0096] Comparison of Examples 1, 10, and 11 reveals that during the preparation of porous ceramic filter blanks, greater applied pressure and dwell time decrease the porosity and increase the room temperature compressive strength and compressive strength at 800°C. This is because: within a reasonable range, increasing pressure significantly improves the density and mechanical interlocking effect of the porous ceramic filter blank; exceeding the pressure easily leads to ceramic particle breakage and stress concentration, which in turn reduces blank performance; a reasonable dwell time allows for sufficient particle rearrangement; adding a natural drying step avoids cracking caused by rapid dehydration; a reasonable low-temperature drying temperature removes bound water and prevents decomposition of organic additives; and excessive drying time increases energy consumption without significantly improving strength. The lack of significant effect on the filtration efficiency of 5-50μm / less than 5μm inclusions is due to the lack of significant differences in porosity and surface whiskers in the porous ceramic filter particles.

[0097] By comparing Example 1 and Example 12, it can be seen that during the high-temperature sintering process of the porous ceramic filter blank, within the scope required by this patent, the synergistic effect of low heating rate and high sintering temperature can more easily obtain a porous ceramic filter with lower porosity, higher room temperature compressive strength and compressive strength at 800°C, and slow heating (such as 2°C / min) can promote the full decomposition of organic matter and binder, reduce internal defects caused by residual gas, and allow the pores to slowly shrink and connect, eventually forming a relatively uniform pore structure with a slightly reduced porosity; slow heating / high temperature sintering promotes densification, and the neck connection between grains is more complete, making the porous ceramic filter stronger; when the low heating rate and high sintering temperature work together, the pore distribution is uniform, the average pore size deviation is within a small range, the mechanical interception effect is enhanced, and the adsorption effect is simultaneously improved.

[0098] By comparing Example 1, Example 13, Example 14 and Example 15, it can be seen that the influence of the matrix ceramic particles on the various properties of the porous ceramic filter is not significant. The reason is that the pore formation mechanism prepared by the particle stacking method in this patent mainly relies on the retention of the stacking gaps between the particles, and its porosity is determined by the particle size, morphology and sintering conditions; no matter what kind of ceramic particles are selected, the mechanism of forming sintering necks between particles through liquid phase sintering or solid phase diffusion during high-temperature sintering is universal, and the final strength difference mainly depends on the type of sintering aid and process parameters (such as sintering temperature, holding time), rather than the particle type itself; the filtration efficiency of inclusions is mainly determined by the pore connectivity and pore size distribution, and these two are less affected by the particle type.

[0099] By comparing Example 1 with Comparative Examples 1-2 alone, it can be seen that the sintering temperature that is too high / the heating rate that is too fast has no significant effect on the porosity, but will significantly reduce the room temperature compressive strength of the porous ceramic filter, the compressive strength at 800°C, and the filtration efficiency of 5-50μm / less than 5μm inclusions. The main reasons are: the sintering temperature is too high / the heating rate is too fast, the whiskers on the surface of the porous ceramic filter particles are coarsened, microcracks are generated at the joints, and the pore size distribution is deteriorated, which will significantly reduce the mechanical properties and filtration efficiency of the porous ceramic, while the porosity remains stable due to the balance between densification and volatilization / decomposition.

[0100] A comparison of Example 1 and Comparative Example 3 shows that during the high-temperature sintering of the porous ceramic filter blank, excessively rapid cooling will slightly increase the porosity of the porous ceramic filter, but significantly reduce the room-temperature compressive strength and compressive strength at 800°C of the porous ceramic filter. Slow cooling allows for the release of residual stress, making the pore structure of the porous ceramic filter more stable and reducing the porosity. Rapid cooling causes dramatic shrinkage at the particle junctions, resulting in microcracks caused by residual stress within the junction necks, which in turn reduces the room-temperature and high-temperature strength of the porous ceramic filter. At the same time, excessively rapid cooling causes the pores formed by the porous ceramic filter particles to shrink irregularly, making the originally evenly distributed pores unevenly sized, allowing tiny inclusions to pass more easily, significantly reducing the filtration efficiency for inclusions smaller than 5μm.

[0101] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A porous ceramic filter for purifying liquid magnesium alloy, characterized in that: The raw material composition of the porous ceramic filter, calculated based on the total mass percentage content as 100%, includes: 70-85wt% ceramic particles, 8-15wt% multi-element oxide system microcrystalline glass solder, and the rest is a binder; the multi-element oxide system microcrystalline glass solder has an average particle size of less than 5μm, and is an Al2O3-B2O3-SiO2-MgO-CaO multi-element oxide system, and its raw material composition, calculated based on the total mass percentage content as 100%, includes: 15-45wt% Al2O3, 15-40wt% B2O3, 15-35wt% SiO2, 5-15wt% MgO, and the rest is CaO, and is prepared by the "rapid heating-short-time heat preservation-water quenching and crushing-drying treatment-ball milling and refining-screening and grading" process; the binder is one or more of water glass, silica sol, phenolic varnish, methyl cellulose, polyacrylamide solution, and aluminum dihydrogen phosphate solution.

2. The porous ceramic filter for purifying liquid magnesium alloy according to claim 1, characterized in that: The Al2O3-B2O3-SiO2-MgO-CaO multi-oxide system has a raw material composition based on the total mass percentage of 100%, including: 15-35wt% Al2O3, 25-40wt% B2O3, 15-20wt% SiO2, 5-10wt% MgO, and the rest is CaO.

3. The porous ceramic filter for purifying liquid magnesium alloy according to claim 1, characterized in that: Based on the total mass percentage being 100%, the raw material composition of the porous ceramic filter includes: 70-78wt% ceramic particles, 12-15wt% multi-element oxide system microcrystalline glass solder, and the rest being a binder.

4. The porous ceramic filter for purifying liquid magnesium alloy according to claim 1, characterized in that: The material of the ceramic particles is one or more of Al2O3 ceramics, SiO2 ceramics, ZrO2 ceramics or Al2O3-ZrO2 composite ceramic materials.

5. The porous ceramic filter for purifying liquid magnesium alloy according to claim 1, characterized in that: The size of the ceramic particles is controlled between 800 μm and 3 mm.

6. The porous ceramic filter for purifying liquid magnesium alloy according to claim 5, characterized in that: The size of ceramic particles is controlled at 800μm-1mm.

7. The porous ceramic filter for purifying liquid magnesium alloy according to claim 1, characterized in that: The multi-component oxide system microcrystalline glass solder is prepared by adopting the process of "rapid heating-short-time heat preservation-water quenching and crushing-drying treatment-ball milling and refining-screening and grading". The specific method is as follows: the multi-component oxide system microcrystalline glass solder raw materials are placed in a graphite crucible of a high-frequency induction heating furnace according to the mass ratio, and the temperature is increased to 1350-1450℃ at a rate of 150-200℃ / min; then the temperature is kept for 2-5 minutes; the glass melt is quickly cooled to room temperature in water and then mechanically crushed into coarse particles, and then kept at 100-110℃ for 8-12 hours for drying; ball milling is carried out in a planetary ball mill with a rotation speed of 300-500r / min and a ball milling time of 4-6 hours, and then the multi-component oxide system microcrystalline glass solder with an average particle size of less than 5μm is obtained by sieving.

8. The method for preparing the porous ceramic filter for purifying liquid magnesium alloy according to claim 1, characterized in that: The method comprises the following steps: Step S1, Ceramic particle screening: Select ceramic particles of appropriate size according to the target porosity and filtration accuracy requirements of the porous ceramic filter; the average particle size of the ceramic particles is 800 μm-3 mm; Step S2, weighing and mixing raw materials: First, ceramic particles, multi-element oxide system glass-ceramics solder, and binder are weighed according to the mass ratio; then, the multi-element oxide system glass-ceramics solder and binder are mixed at room temperature and mechanically stirred to form a homogeneous slurry. Finally, the slurry is evenly coated on the surface of the clean ceramic particles using a three-dimensional mixing process; Step S3, green body forming and drying: filling the ceramic particles coated with the slurry into a mold, applying a pressure of 10-15 MPa and maintaining the pressure to form the green body, demolding the mold, and then drying the green body naturally, and then further heating to 100-110°C to dry the green body to obtain a porous ceramic filter green body with a certain strength and controllable porosity; Step S4, high-temperature sintering and cooling of the green body: placing the porous ceramic filter green body in a high-temperature furnace, calcining it at 1220-1420°C in an air atmosphere for 3-5 hours, and cooling it along the temperature gradient in the furnace to obtain a porous ceramic filter with high strength, high filtration efficiency and uniform pore distribution for magnesium alloy filtration.

9. The preparation method according to claim 8, characterized in that In step S2, the speed of mechanical stirring is 300-600 r / min, and the speed used in the three-dimensional mixing process is 200-300 r / min; in step S3, a pressure of 12-15 MPa is applied, the holding time is 10-20 s, the natural drying time is 12-24 h, and the heating drying time is 2-3 h; in step S4, the heating rate is 2-5 ° C / min, and the cooling rate is 10-15 ° C / min.

10. Use of the porous ceramic filter for purifying liquid magnesium alloy according to claim 1, characterized in that: Used for filtering magnesium alloy melts.

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