Porous ceramic filters and their applications in the metallurgical industry

1 Introduction

Porous ceramic filters, also known as microporous ceramics, ceramic foam filters, alumina ceramic foam filters, etc., have uniformly distributed micropores (porosity can be as high as 50% to 90%), low volume density, and?an interconnected three-dimensional network skeleton structure.?Ceramic products.

It has a developed specific surface area and unique physical surface characteristics. It has selective permeability, energy absorption, or pressure resistance characteristics for liquid and gas media. In addition, the ceramic material itself has unique high-temperature resistance, corrosion resistance, and other excellent characteristics, making it Porous ceramic filters are widely used in many aspects such as gas and liquid filtration, purification and separation, chemical catalytic carriers, advanced insulation materials, biological implant materials, sound absorption and shock absorption, and sensor materials.

Porous ceramic filters were first successfully developed by the United States in 1978. They used alumina, kaolin, and other ceramic slurries to make Porous ceramic filters for aluminum alloy casting filtration, which can significantly improve the quality of castings and reduce the scrap rate.?And?published their research results at the American Foundry Annual Meeting in April 1980. Since then, Britain, Russia, Germany, Japan, Switzerland, and other countries have?researched Porous ceramic filters and developed Porous ceramic filter materials of various materials suitable for different uses.?The technical equipment and production processes have become increasingly advanced, and the products have been serialized and standardized. , forming an emerging industry.

my?country began to develop porous ceramics in the early 1980s.?It has?been widely used?in non-ferrous metal alloys, black alloys, and gas purification catalyst carriers.?For example, my country's porous ceramic filter plates used for the filtration of non-ferrous metal melts are produced stably and have reached a?certain?scale, and their?product performance is comparable to similar foreign products.

2 Properties of Porous Ceramic Filters

2.1 Porosity

The porosity of foam ceramics is 70% to 90%, which is the highest for Porous ceramic filters. The porosity of honeycomb ceramics is about 60%, and the porosity of ceramic particle sintered bodies is about 30% to 50%.

2.2 Bending strength

The strength of foam ceramics mainly depends on the ceramic material and the thickness of the network skeleton.?The thickness of the skeleton can be expressed by the volume density of foam ceramics.?

Table 1 lists the flexural strengths of different ceramic materials and?different?bulk densities.

If the skeleton is made thicker, the bulk density can?be increased and?the mechanical strength of the product can?be?increased.?But if it?is increased?too much, the air pores will be blocked by the slurry?and?the pressure loss will increase.

For honeycomb ceramics, in the parallel direction of the lattice, the strength is very different in the vertical direction and the slope direction, while?foam ceramics is a three-dimensional structure with consistent directions, and its strength has no directional change.

2.3 Thermal shock stability

As a filter material for molten metal, foam ceramics must have good thermal shock resistance when used in situations where temperature changes rapidly. The properties of foam ceramic products developed by Shandong Industrial Ceramics Research and Design Institute?are shown?in Table 2.

Due to?the?different viscosity, density, and fluidity?of metal melts, filter plate mesh apertures of?different?sizes should?be selected. The mesh size of foam ceramics can generally be controlled?within the range of 0.2~3mm and?is usually divided?into

There are?3?levels of coarse, medium, and?fine?pores.

3. Manufacturing process of porous ceramics

3.1 Extrusion molding process

The extrusion molding process is one of the most commonly used methods for manufacturing porous ceramics. Its process flow is raw material synthesis - mixing - extrusion - drying - firing - finished product. The porous ceramic body made by this process has uniform pore size, shape, and?porosity,?and is suitable for mass production. However, the difficulty in manufacturing small-pore products is a shortcoming of this process.

In the production process, the production of extrusion molds is the core technology.?At present,?the honeycomb ceramic extrusion mold used in my country's production has reached the specification of 400 holes/i8.?The United States and Japan have developed high-density, ultra-thin honeycomb ceramic molds with 600 to 900 holes An2.

3.2 Organic foam impregnation process

The ceramic slurry?is impregnated?with organic foam, and the organic foam?is burned off?after drying to produce a Porous ceramic filter body. Since the organic foam has an open-cell three-dimensional network skeleton structure, the prepared slurry?is coated?on the foam, and the pores formed after burning off the organic foam are mesh-shaped.

This process can produce products with high strength and high porosity, but it cannot?produce?products with?small pores?and closed?pores,?the shape is limited?and?the density is difficult to control.

To ensure the?quality of the product, the gasification temperature of the foam?should be lower than the firing temperature of the ceramic body, and it will not pollute the ceramic body.?The foam must have a certain degree of hydrophilicity and sufficient resilience?and can?be tightly combined with the ceramic slurry to ensure that it can return to its original shape after the excess slurry?is squeezed out.

3.3 Foaming process

This process?is to add?organic or inorganic chemical substances to the ceramic components,?generate?volatile gases through chemical reactions, etc., and?then bake?them into porous ceramic bodies after drying.?The main chemicals used as foaming agents are:

Calcium carbide, calcium hydroxide, aluminum powder, aluminum sulfate, hydrogen peroxide: sulfide, and sulfate mixed foaming agent: hydrophilic polyurethane?plastic,?and ceramic slurry?are simultaneously foamed?to produce porous ceramics, etc.

The advantage of using the foaming process to produce porous ceramics is that it is easy to control the shape, composition, and density?of the?product,?and is particularly suitable for?the production of?closed-pore ceramic materials.?However, its disadvantages are high requirements for raw materials and difficult-to-control process conditions.

3.4 Adding pore-forming agent process

This process involves introducing an artificial pore-forming agent into the ceramic ingredients. Since the pore-forming agent occupies a?certain?space in the green body, the pore-forming agent volatilizes through firing, thus making porous ceramics. The shape and size of the pore-forming agent particles determine the shape and size of the pores of the porous ceramic material. There are two types of pore-forming agents: inorganic and organic. Inorganic pore-forming agents include ammonium carbonate, ammonium bicarbonate, ammonium chloride, coal powder, carbon powder, etc.; organic pore-forming agents mainly?include:?natural fibers, high molecular polymers, organic Acid, etc., the molding method of porous ceramics is the same as that of ordinary ceramics. This process can produce ceramic products with complex shapes and various pore structures, but the disadvantage is poor pore distribution and low porosity.

3.5 Sol-gel process

This method is a color process for manufacturing porous ceramics. It uses the accumulation of colloidal particles during the gelation process and the formation of a controllable porous structure during gel treatment and heat treatment to prepare nanoscale porous ceramic bodies. For?example:?using this process to prepare alumina porous ceramics, compared with other processes, can further improve the control of pore size distribution, phase change, purity, and microstructure of alumina porous ceramics. However, the disadvantage of this process is that the?shape of the product?is subject to certain restrictions.

4, Applications of porous ceramics

The performance of porous ceramic materials?is determined?by the surface chemical properties of the micropores and the size characteristics of the micropores.?The factors that determine the chemical properties of microporous surfaces include the?composition and state of the ceramic and the treatment of the microporous surface.?The adsorption performance?is determined?by the chemical composition of the microporous surface characteristics, crystal structure, non-quality, and the presence or absence of OH.?Among the size?characteristics?of micropores, micropore diameter, distribution, form, specific surface area,?etc. have a?great?impact on?its filtration and separation performance.?The properties of porous ceramics prepared by different processes are also different. According to the different properties of porous ceramics, there are different applications. For example, the uniformity of porous ceramics can?be used?to manufacture various filters, separation devices, fluid distribution elements, mixing elements, infiltration elements, throttling elements, etc.; the absorption properties of porous ceramics can?be used?as various sound-absorbing materials, reducers, etc. Vibration materials, etc.;?the developed specific surface area of porous ceramics can be used?to make?various?porous electrodes, catalyst carriers, heat exchangers, gas sensors, etc.;?the characteristics of low density and low thermal conductivity of porous ceramics can be used?to make various thermal insulation materials.?materials, lightweight structural materials, etc.

1. Catalyst carrier

Porous ceramics have the advantages of high specific surface area, heat resistance, wear resistance, no pollution, low density, and low processing cost.?As catalyst carriers, they can be used?in organic and inorganic chemical production fields.?For example, it?is used for the treatment of?toxic, odorous, and other harmful gases.?In terms of?automobile exhaust gas purification, Porous ceramic filters exert unparalleled excellent properties. Installing the porous ceramic exhaust catalytic converter in the automobile exhaust pipe can convert harmful gases such as CO, EC, and HOx emitted by gasoline vehicles into CO2, H20, and N2, with a conversion rate of over 90%. When the porous ceramic filter element is full of carbon particles, it can be regenerated using the catalytic oxidation method or the electronically controlled combustion method to eliminate the deposited carbon particles and achieve different gas compositions on the high-pressure side during long-term use. When two immiscible liquids pass through the porous ceramic body, such as oil distributed in water as tiny droplets, the porous ceramic body can separate the oil and water because the diameter of the oil droplets is large and the specific gravity is different from that of water.?When the gas contains particles, let?the gas?pass through the ceramic body, and?the particles?contained?in it will be filtered.

3. Energy-saving thermal insulation materials.?Currently, the best thermal insulation materials in the world?are porous ceramic materials.?Traditional kilns and high-temperature electric furnaces?are mostly lined?with porous ceramics. It?is also used?to insulate the outer shell of the space shuttle. Evacuating the internal gas of the porous ceramic body can also enhance its thermal insulation properties.

Since Porous ceramic filters have huge porosity and have a considerable heat exchange area under high-temperature conditions, they can be used as heat exchange materials and achieve sufficient heat exchange.?If the porous ceramic body?is placed?at the flue opening of the heating furnace, the high-temperature gas in the?furnace?can enter the flue through the porous ceramic body, and at the same time, the ceramic body?is heated?to a temperature close to the?furnace.?Then, the porous ceramic body will radiate heat energy into the?furnace?and reduce heat energy loss. According to relevant information, it can save 30% of energy. In short, porous ceramics have significant energy-saving effects as thermal insulation materials and heat exchange materials.

5. Technical level of porous ceramics at home and abroad

5.1 Foreign technical level

Since the invention of the United States in 1978, foam ceramics have?been successfully developed?using ceramic slurries such as alumina and kaolin for aluminum alloy casting filtration. After that, Britain, Guizhou, Russia, Germany, Switzerland, and other countries competed to?carry out?research. The production process is becoming increasingly advanced, and technical equipment is?increasingly?developing towards mechanization and automation. Foam ceramic filters of various materials suitable for different purposes have been developed, such as A1203, Zr02, SiC, SiN, and other high-temperature foam ceramics, and the products have?been serialized. , standardization, forming an emerging industry.?At present, foam plastic precursors are produced?in the world?using dipping roller-forming machines.?The green body is microwave-dried, and the high-temperature roller?is continuously fired?under computer monitoring. The detection and packaging are also mechanized. The entire production process has reached a very high level of technology. The application fields of foam ceramics have developed from metal melt filtration to the chemical industry, environmental protection, energy saving, etc.?Metal melt filtration has developed from aluminum alloy to?refining filtration of high-temperature steel melt. At present,?the metal melt filtration process has been widely used in the manufacturing industries of more industrially developed countries in the world and has achieved good results. According to the Moscow Institute of Scientific and Technological Information,?the use of foam ceramic filters in the production of?pig iron castings has made the castings more suitable.

The frame rate?is increased?to 80%. When gray iron and burnable cast iron are purified using foam ceramic filters to produce crankshafts for automobiles, the amount of waste in the carbon workshop alone?is reduced?from 35% to 0.3%. In continuous cast steel, when foam ceramic filtration?is used, the content of non-metallic impurities in stainless steel is reduced by approximately 20%.

5.2 Domestic technical level

my country started research on foam ceramics in the early 1980s.?In the past 20 years, more than a dozen scientific research institutions and manufacturers have?conducted exploration and?research?on?foam ceramic products.?Recently, the production of foam ceramic filter plates used for filtering non-ferrous metal melts, that is, aluminum and copper alloy melts, has been stable in my country and has formed a?certain?production scale. Its product performance is comparable to the foam ceramics produced by Selee Company in the United States.

Foam ceramic products used for high-temperature black alloys and other purposes?are made?of silicon carbide, alumina, zirconia, and other materials.?They have not yet formed a production scale, and some are in?the?development?stage. Shandong Industrial Ceramics Research and Design Institute is an early unit in?the research and development of?foam ceramics in China.?The product variety, quality, and production capacity currently developed rank among the top in the country, and it has formulated industry standards for foam ceramic filter plate building materials.

However,?the?overall technical level of foam ceramics?in my country?is far behind that of foreign countries.?① There are few varieties, small specifications, low output, and unstable quality. The forming process is manual or semi-mechanized, and the pass rate of large products is low. ② The quality of the foam plastic precursor is unstable, and its elasticity and porosity are relatively poor,?which directly affects?the quality of foam ceramic products. ③?For the firing of?foam ceramics, inverted flame pots are?mostly?used in China. The production efficiency is low, the density of the kiln is low, and the production cost is high.

With the increasing requirements for simplifying processes, improving efficiency, and reducing costs in production, new preparation technologies are constantly emerging, and these methods also show that simplicity, cheapness, and economy are necessary conditions for practical use. In addition, due to the need for performance and structure, composite Porous ceramics came into being, and therefore?composite?preparation methods that comprehensively utilize various preparation technologies have?been developed. Problems to?be solved?in these preparation technologies in the future include: Preparation technology control of the accuracy of porous ceramic structures, including pore size, distribution, shape,?etc.:?Reasonable coordination of the relationship between porosity and strength, etc.

Due to the many excellent properties of porous ceramics, the further development of modern science and technology has attracted people's attention to new porous ceramic materials. There has been further development in its application as a catalyst carrier in?automobile exhaust gas purification treatment. The core part of the automobile exhaust gas purifier is the porous ceramic catalytic system, which will be a focus in future development. The research and development of porous ceramics in energy saving and filtration have given porous ceramics broad application prospects as environmentally friendly green materials.