Low porosity high density silicon nitride bonded silicon carbide brick made new type refractory brick has advantages such as high temperature strength, good thermal conductivity, good oxidization resistance, thermal shock resistance, corrosion resistance, high temperature creep resistance, etc. Currently, Si3N4 bonded SiC bricks are mainly applied in electrolytic bath side part of modern large scale aluminum factory, high-tech ultra-large scale iron making blast furnace bash, high class kiln furniture and so on.
The coarse, middle and fine grains of SiC and Si Powder are prepared with rational proportion, some quantity of additives and binder are also added into the raw refractory material. First, they are fully mixed in bell type blender mixer, and then casted into needed shapes, after drying, the green bodies are loaded into large scale nitriding furnace and sintered in pure nitrogen atmosphere. So the Si in raw refractory material transforms into Si3N4, so called Si3N4 bonded SiC composite material.
The application of silicon nitride bonded silicon carbide refractory can effectively reduce energy consumption, reduce product defect ratio, improve products quality, decrease unit kiln furniture production cost, is an ideal new type kiln furniture material.
Tempering
Before tempering, operators should check whether mixing roll can work normally or not, and impurities should be cleaned, for mixing roll that is not used for long term, it should be cleaned by grinding waste material.
Material Feeding Sequence
1. matrix material: 2.5~0.43mm, 1.43~0.5mm, 0.5~0mm, mixing for about 5 min.
2. matrix material addition quantity≤10%.
3. Adding dextrin liquid and mixing & grinding for 5 min.
4. Adding SiC fine powder and dextrin, then mixing and grinding for 5min.
5. Adding water to adjust flowability.
6. Mixing and grinding for at least 15 min.
Screening and Selecting
The fully tempered pug should be screened by 5mm screen before using, the rest refractory material can be collected and crushed for reusing, the impurities can be removed.
Molding
630T friction pressing machine can be used for the molding. Before molding, equipment operation condition, mould size, and hammer end flatness should be checked first, there should be no welder slag at hammer head, platform scale should be accurately adjusted to set unit weight. The quality of mould board should be checked and accepted by professional personnel. Mould size and flatness should be checked before using. The Si3N4 bonded SiC green body has a bulk density≥2.60g/cm3. During weight material, particles segregation should be avoided, when weighting and dumping pug refractory material, it should be scattered. During pressing molding, light –heavy-slowly lift hammer operation method should be adopted to ensure the integrity of side and corner, and to avoiding defective products.
Appropriately enhance semi-finished product standards, the dimension of molded brick green body should be checked one by one according to corresponding quality standards, the good quality refractory brick green body can be processed by next procedure. The green body should be lightly put on cotton cloth strips, 3~5cm gap should be left between green bodies.
Drying
The green bodies are naturally dried in drying ware, during this process, they should be turned over upside down according to refractory brick shapes and weather condition. The gaps between green bodies are 3~5cm, the dried bricks should contain no more than 0.1% of residual water. The natural drying temperature is between 20~30℃, the drying period should based on the strength and forming state of green body. After natural drying, the green bodies are delivered into tunnel kiln for further drying, the entrance temperature should not exceed 60℃.Thermal technical operation requirement
Covering furnace lid at 300℃, blowing water vapor at 500℃, pumping vacuum and charging nitrogen gas at 600℃, half an hour later, doing the second-time vacuum pumping and nitrogen charging, then nitrogen discharging dextrin to 750℃, when the pressure reaches to 0.03Mpa, begin to exchange the vent valve every 30min, to promise the even flowing of inner furnace current.Ensuring the furnace pressure is between 0.01~0.045Mpa.
When furnace is shut down, electricity current drops to zero, heating is stopped, and the kiln are cooled by blowing nitrogen, when temperature drops to 1000℃, the blowing can be stopped, all the vent valves are closed. Then open furnace lid, blowing air into furnace by blower to 600℃, and open furnace door.
Drawing Out of Kiln and Selecting
Selecting operation should be carful, every refractory brick should be cheeked one by one and stamped. Finished products and defective products should be packed at designated places. For defective products, defect responsibility analysis should be carried out according to refractory brick types and models and accurately recorded.
The main raw refractory materials of casted Si3N4-SiCproducts are industrial green silicon carbide, silicon powder. Coarse grains, middle grains and fine grains are proportionally prepared, and the matrix raw material high purity green silicon carbide are grinded to 1200 mesh by metal silicon current.
The grain shape of SiC material is the basic factor for producing high density and high strength silicon nitride bonded silicon carbide bricks. The particle shape of SiC grains are usually needle shape, flake shape, which may lead to bridging effect during molding process, and the molding density is not very high. So, the particle shapes of SiC should be modified. The grinding method are usually adopted for the modifying, so particle volumes are similar, then they are chemically treated, water washed and impurity are removed.
Industrial silica blocks are crushed by jaw crusher, roll crusher, and grinded into needed granularity. Then soaked by water for days, so metal silicide in silicon powder can fully react with water, after that bubbles are eliminated and silicon powder are dried.
Powder material mixing and pressing: Put coarse, middle size and fine SiC grains and Si powder on platform, according to batch formula, weight SiC sand and Si powder-load into high efficient blender mixer-mixing-quantitatively add water into mixer-evenly add dosed and water diluted additives and temporary binder-fully mix and stir for 15~20min-screen-aging in stock bin for 24h or more.
During nitriding process, green body almost has no shrinkage, so the density of Si3N4 bonded SiC refractory depends on the green body density, and the granularity composition of green body has large influence on its density. The density of Si3N4 bonded SiC refractory green body directly affects the sintering and final products physiochemical properties.
Since SiC is easy to sediment when it is made into slurry state, slurry absorption speed it very quick during casting, the strength of green body is very low, so proper additives should be added to make slurry have good suspension ability, flowablity, appropriate thixotropy and water content, which is very important for casting. Adding metal silicon and carbon containing material in reaction material, and adopting nanometer ludox as low temperature binder. When addition ratio is 0.5%~2%, material slurry has good suspension ability, no apparent thixotropy, low slurry absorption speed, high casting density, and high product quality.
Adopting multi-granularity coarse grains, middle size grains and fine grains preparation, the matrix raw material (high purity green silicon carbide) are grinded to 1200 mesh by metal silicon current.
Adding metal silicon and carbon containing material in raw material, high activity nanometer silicon carbide are synthesized by reaction, which can decrease sintering temperature and increase strength.
Green body adopts casting molding process, adopting nanometer ludox as low temperature binding agent to realize no cement binding, so there is low eutectic mixture content, high temperature strength is ensured.
The application of above mentioned craft measures can produce new type refractory material-high strength Si3N4 bonded SiC bricks, which have low apparent porosity, high strength, strong corrosion resistance, good high temperature strength, stable chemical property, high working temperature, strong wear resistance and so on. High strength Si3N4 bonded SiC bricks have longer service life than ordinary firebricks, are widely used in metallurgy, nonferrous metals industry, chemical industry, ceramics industry and so on, with very good economical benefits.
