O principal fator na produção de tijolos sinterizados – matérias-primas
O principal fator na produção de tijolos sinterizados – matérias-primas (conhecimento básico)
De um modo geral, qualquer matéria-prima que possa ser transformada em tijolos comuns pode produzir tijolos ocos. No entanto, tijolos ocos têm vários buracos, paredes finas, e corpos fracos, que exigem requisitos mais rigorosos para a preparação de matérias-primas e a mistura de combustíveis internos. As impurezas prejudiciais devem ser reduzidas, a distribuição do tamanho das partículas deve ser mais razoável, e componentes minerais devem ser totalmente dispersos, disperso, e distribuído uniformemente para garantir a plasticidade e boa capacidade de ligação das matérias-primas para fabricação de tijolos. Os requisitos básicos para matérias-primas residem principalmente na sua composição química, composição mineral, e propriedades físicas.
1、 A influência da principal composição química das matérias-primas na fabricação de tijolos
Dióxido de silício (SiO2): is the main component in the raw materials of sintered bricks, with a suitable content of 55-70%. When exceeded, the plasticity of the raw material is too low, making it difficult to form, and the volume slightly expands during firing, resulting in a decrease in the strength of the product.
Aluminum trioxide (AL2O3): The content in the raw materials for brick making should be 10-25%. When it is too low, it will reduce the strength of the product and not resist bending; Se for muito alto, it will inevitably increase the firing temperature, increase the coal consumption, and make the color of the product lighter.
Iron trioxide (Fe203): It is a coloring agent in brick raw materials, with a suitable content of 3-10%. When it is too high, it will reduce the fire resistance of the product and make its color more red.
Calcium oxide (CaO): also known as quicklime, muitas vezes aparece na forma de calcário (CaC03) em matérias-primas. É uma substância nociva cujo conteúdo não excede 5%. Por outro lado, não só a faixa de temperatura de sinterização do produto será reduzida, dificultando a calcinação, mas quando seu tamanho de partícula é maior que 2 mm, também causará explosão de cal, absorção de umidade, afrouxando, e pulverização do produto.
Óxido de magnésio (MgO): É uma substância nociva, quanto menor o conteúdo, o melhor, e não deve exceder 3%. Isto, como sulfato de cálcio (CaSO4) e sulfato de magnésio (MgSO4), fará com que o produto fique congelado, até mesmo delaminação e desgaste.
Anidrido sulfúrico (SO3): É melhor não ter nada, e não pode exceder 1% no máximo. Por outro lado, o produto irá gerar gás durante a calcinação, fazendo com que o volume do tijolo se expanda, soltar e esmagar.
2、 Análise mineral
Mineral analysis of raw materials helps to understand some of their physical properties, in order to take corresponding process measures and make changes to meet the requirements of brick making. Por exemplo, feldspar in the raw materials will reduce the frost resistance of the product, and when its content exceeds 15%, the product will not be frost resistant. Another example is montmorillonite (Pengrun soil), which has extremely high viscosity. After absorbing water, its volume expands dramatically, and after drying, it contracts strongly. Its linear shrinkage rate is higher than 13-23%, causing a large number of dry cracks in the body. Practice has shown that when the content of montmorillonite in the raw material reaches 20%, dry cracks cannot be avoided. High plasticity bentonite is often used as a binder for fly ash in production to produce various high-quality fly ash bricks.
3、 Physical performance
1. Particle composition: also known as particle size distribution.
Although the finer the particle size of the raw material, the larger its surface area, the better its water penetration, and the better its plasticity. But the finer the raw materials for making bricks, o melhor. Because all the fine raw materials are not conducive to the drying and roasting of the product, and the role of raw materials with different particle sizes in the product is different.
Powder particles with a particle size less than 0.05mm are called plastic particles, which are used to produce the required plasticity for molding. Claro, these small particles must be clay or shale, coal gangue, or other materials with clay like properties. Por outro lado, for river sand, no matter how finely ground it is, there is no plasticity.
The material particles with a particle size of 0.05-1.2mm become filling particles, which are used to control excessive shrinkage and cracking of the product and give a certain strength to the green body during plastic forming.
Coarse particles with a particle size of 1.2-2mm play a skeleton role in the green body, called skeleton particles, which are beneficial for draining water from the green body during drying.
The particle size of raw materials for producing hollow bricks should not exceed 2mm.
The reasonable particle composition should be plastic particles accounting for 35-50%; Filling particles account for 20-65%; Skeleton particles<30%; No particles larger than 3mm are allowed. Porque não só reduz a resistência do produto, mas também causa rachaduras devido ao encolhimento irregular.
2. Índice de plasticidade
O índice de plasticidade é um parâmetro importante para avaliação das matérias-primas utilizadas na fabricação de tijolos. Na indústria de tijolos, A plasticidade refere-se à capacidade de uma mistura de materiais e água ser extrudada em forma com sua viscosidade máxima e manter sua forma após ser liberada da pressão.. A magnitude dessa habilidade é representada pelo índice de plasticidade.
Embora um alto índice de plasticidade seja benéfico para moldagem por extrusão, tem tendência a rachar durante a secagem e torrefação; O baixo índice de plasticidade, embora benéfico para secar e assar, também pode trazer dificuldades à moldagem. Se o índice de plasticidade estiver abaixo 7, a moldagem por extrusão não é apenas difícil, but the strength of the product is also lower.
It is generally believed that when the plasticity index is greater than 15, it is called high plasticity clay, and when it is less than 7, it is called low plasticity clay. Only when the plasticity index is between 7-15, medium plasticity clay is most suitable for extrusion molding. The higher the porosity of the product, the more complex the pore shape, the thinner the wall, and the higher the required plasticity index during molding.
The plasticity index of various raw materials varies greatly. The plasticity index of clay is relatively high, some can reach 25 or above, coal gangue is relatively low, sometimes less than 7, and muddy shale is in the middle, often 7-18.
Through certain technical means, the plasticity index of raw materials can be adjusted within a certain range or the technical performance during extrusion molding can be improved. For raw materials with high plasticity index, appropriate addition of coal ash, escória, cinzas volantes, or waste brick powder can be used for thinning; For raw materials with poor plasticity index, early extraction, natural weathering, appropriate increase in fineness, water leaching and loosening, as well as mechanical methods such as stirring, rolando, envelhecimento, misturando, and extrusion should be used to improve. Clay and shale powder with higher plasticity index can also be added in an appropriate amount to adjust the overall plasticity index of the mixture. When producing fly ash bricks, an appropriate amount of bentonite can be added as a binder, de modo que uma fina camada de bentonita adira à superfície de cada partícula de cinza volante. Ao misturar, homogeneizar, e apertando, a plasticidade pode ser melhorada para atender aos requisitos de moldagem.
Deve-se ressaltar que ao utilizar múltiplas matérias-primas para produção, a condição principal é uma mistura completa. Por outro lado, não só não conseguirá alcançar o efeito esperado, mas também causará rachaduras de secagem e encolhimento devido às diferentes taxas de encolhimento de várias matérias-primas.
Devido ao fato de que a qualidade dos tijolos extrudados não depende apenas da plasticidade das matérias-primas, mas também tem relação direta com o coeficiente de atrito entre as partículas da matéria-prima. Particles with a smooth and spherical surface are more likely to flow and compact with each other when squeezed compared to particles with a rough surface instead of a spherical one. By adding certain additives to the raw materials and black liquid from paper mills, as well as waste acid from chemical and electroplating factories, the special ion adsorption and lubrication effect it produces on the surface of the mud particles greatly improves the flowability of the mud when squeezed, making it easier to compact and produce smooth and dense brick billets.
3. Shrinkage rate
During the drying process of the body, due to the evaporation of water, the particles naturally approach and shrink in volume, which is called drying shrinkage. The percentage of its shrinkage length to the original length of the billet is called the drying line shrinkage rate. For raw materials with a high shrinkage rate in the drying line, their products should be dried slowly, otherwise serious drying cracks will appear in the body and waste will be formed. In production, it is required that the linear shrinkage rate of the raw materials be less than 6%. Por outro lado, the raw materials should be thinned.
During calcination, due to a series of physical and chemical changes and the loss of certain substances in the raw materials, the finished product is not only lighter than the brick, but also slightly shrinks in volume, which is called calcination shrinkage. The percentage of its shrinkage length to the length of the dried body is called the firing shrinkage rate.
4. Dry sensitivity coefficient
Durante o processo de secagem, as the moisture in the green body gradually evaporates, the volume also gradually decreases. Due to the fact that the drying rate and shrinkage rate inside and outside the billet are always fast on the outside and slow on the inside, isso é, the surface has already dried and started to shrink, while the inside is still “unchanged”, once the amount of shrinkage exceeds the elastic coefficient of the mud (1-2%), it will “expandir” the surface of the billet, producing network cracks. This is called the drying sensitivity of the mud, and is represented by the drying sensitivity coefficient. The larger the drying sensitivity coefficient, The threat of cracking during the drying process of the billet is also more severe. When the drying sensitivity coefficient is less than 1, the problems during the drying process are smaller. Once the drying sensitivity coefficient is greater than 2, the risk of cracking during the drying process is also very serious. It is necessary to thin the raw materials to reduce the drying sensitivity coefficient.
De um modo geral, the higher the plasticity index of clay, the higher its drying linear shrinkage rate and drying sensitivity coefficient.
5. Burning temperature range
If the temperature continues to rise beyond the firing temperature, the billet will gradually soften and deform, even melt, and the billet stack will collapse. Obviously, raw or underfired bricks will appear below the firing temperature; When the firing temperature is higher than the firing temperature, there will be overburning, burning flow, and even collapse of the billet stack. Obviously, quanto mais ampla for a faixa de temperatura de sinterização, mais fácil é controlar o processo de torra. Para tijolos ocos, esta faixa de temperatura não deve ser inferior a 50 ℃. Isto ocorre porque a temperatura da seção transversal da câmara do forno não pode ser exatamente a mesma, muitas vezes com temperaturas mais altas no meio e temperaturas mais baixas nas bordas, bem como certas diferenças de temperatura que ocorrem perto da parte superior e inferior, lados internos e externos, paredes do forno, e portas. Se a faixa de temperatura de queima for muito estreita, inevitavelmente ocorrerá na mesma seção onde o tijolo do meio é bem queimado e o tijolo lateral é mal queimado; Ou se os tijolos da borda estiverem queimados, os tijolos do meio estão queimados, ou se esta parte estiver sob fogo e aquela parte estiver sobre fogo.
The sintering temperature of clay bricks is relatively low, sobre 900 ℃, while that of coal gangue bricks is relatively high, sobre 1000-1100 ℃, with shale in the middle.
3、 Preparation and processing of raw materials
In order to make the mineral composition and particle size distribution of the raw materials more reasonable, ensure the uniformity and consistency of the entire raw material, have suitable plasticity index and drying sensitivity coefficient, and facilitate the extrusion molding of raw materials and the production of qualified products, it is necessary to systematically process and treat the raw materials.
1. Remove impurities
The tree roots, grass, large pebbles, sandstone, limestone, etc. in the raw materials can be manually removed, but mainly by mechanical means, and can be removed by stone removal rollers and various sieve stone removal machines. The file type stone removal machine effectively removes the clay wrapped around the pebbles, and the extrusion purification machine can also effectively remove tree roots, grass, and gravel while stirring and extruding the mud. Due to the use of a pre installed impurity removal device, impurities can be removed without stopping the machine, which is very convenient.
2. Natural weathering
The extracted raw materials are stored in the open air for a period of time, allowing them to be exposed to sunlight, wind, rain, or frozen. With the help of the power of nature, they can be dispersed, disperso, and water can penetrate evenly. This is a simple and effective method to homogenize, increase their plasticity, and improve their drying performance. For shale, the weathering process can also decompose large blocks into smaller ones, reducing the burden of the crushing process.
3. Stuck material retention (also a form of aging):
After being crushed, mixed, and appropriately mixed with water, the mud material is piled up and stored in the material warehouse for more than 72 hours to fully penetrate the water, loosen and homogenize the mud material. This can not only improve plasticity and facilitate molding, but also reduce stress during drying and roasting, and reduce cracks. We have this experience: the mud that has not been used up after stirring on the first day is particularly easy to extrude and shape the next day, which is the reason.
4. Mechanical processing
The purpose of mechanical treatment is to improve certain technical properties of mud. Its methods include crushing, misturando, mexendo, grinding, etc.
(1) Crushing
The purpose of crushing is to reduce particle size, increase specific surface area, enable the mud to come into contact with water more fully, shorten the path for water to penetrate the mud, and make the mud evenly and fully wet. Suitable equipment should be selected based on the physical properties, block size, and degree of crushing required for the material.
Por exemplo, for raw materials that are brittle, hard, and have low natural moisture content, it is advisable to use impact type crushing equipment. If various hammer crushers and impact crushers are used to finely crush various medium hard coal gangue and shale; Use a cage crusher to finely crush various harder and naturally high water content shale; Using a roller crusher to crush soft shale with high natural moisture content; Use a dry ball mill to grind hard coal gangue or shale with a natural moisture content below 3%; Using jaw crushers or larger impact crushers to crush coal gangue or shale with medium hardness or higher; Using a toothed roller machine to crush soft shale and clay.
(2) Mixing
The purpose of mixing is to fully mix powders with different properties, “penetrate” each other, complement each other’s strengths and weaknesses, and improve the overall performance of the powders. Due to the dispersion of dry powder particles, there are more opportunities for mutual contact, making it easy to mix, while wet powder has formed larger clusters, with fewer opportunities for mutual contact between powders, making it difficult to mix thoroughly.
Além disso, powders with similar bulk densities are easier to mix well, while powders with significantly different bulk densities are more difficult to mix well. Por exemplo, the bulk density of fly ash is only about half of that of shale or coal gangue, and if it is light, it will float upwards, making it difficult to mix well. Portanto, in the production of fly ash shale bricks or fly ash coal gangue bricks, several types of dry powder are often sent in proportion to the cage crusher or the hammer crusher with the sieve bar (sieve plate) removed for dry mixing, which has the best effect.
(3) Stirring
The plasticity of powder is achieved by the thorough mixing and penetration of water. The main function of adding water and stirring is to ensure that the water and powder are fully mixed, and to make the water fully penetrate into the interior of each powder as much as possible to form the required plasticity for molding. Experiments have shown that for the same raw material, when stirred for only two minutes, the dry cracking of extruded bricks can reach up to 4%. No entanto, when stirred for more than three minutes, under the same conditions, the dry cracking is only 1%. A strong mixer (mixing extruder) is used to mix the mud and then force it out through a double screw mud tank, allowing water to penetrate better into the mud particles and achieve better results.
As mentioned above, the purpose of stirring is to thoroughly mix the water and mud. Portanto, it is necessary to sprinkle water evenly at the beginning of mixing the powder to fully utilize its function.
(4) Grinding and grinding
O objetivo da moagem é misturar ainda mais vários materiais de lama, permitir que a água penetre mais, tornar o desempenho geral dos materiais de lama uniforme e consistente, facilitar a moldagem, e evitar rachaduras causadas por encolhimento irregular. Assim como amassar massa, “melhor a massa é amassada, melhor é o sabor do Mantou”.
Atualmente, o equipamento de moagem mais comumente usado inclui várias máquinas de rolos de britagem fina, moinhos de rodas, máquinas de amassar, e agitando extrusoras.
Máquina de rolo de britagem fina: A máquina de rolos usada na fabricação de tijolos não apenas esmaga e comprime a lama devido à diferença significativa na velocidade linear entre as duas superfícies do rolo, mas também esfrega e rasga o espremido “pele de lama” para mistura adicional no próximo processo.
Moinho de rodas: The linear speed of the roller surface of the double roller machine is about 10 meters per second, and the high-speed fine crushing double roller is above 12 meters per second. The time for the mud to be forcefully squeezed at the intersection of the two rollers is extremely short. The linear speed of the wheel surface of the wheel mill is less than 2m/second, so its action time on the mud is much longer. Além disso, the linear velocity of each point in the axial direction of the wheel surface varies with its distance from the center of the grinding disc. Portanto, it not only crushes and crushes the mud, but also has various functions such as kneading, tearing, and mixing, thereby playing a comprehensive effect on the roller and mixer combined.
Plasticizing materials
Plasticizing materials are additive materials that can improve the plasticity, bonding ability, and flowability of raw materials. High plasticity clay and shale are used as plasticizers. Other plasticizers mainly include soda, sodium silicate, hydroxymethyl cellulose (CMC), caustic soda, and methyl cellulose (MC).
Inorganic plasticizers are mostly plasticized by changing the pH value and the adsorbed cations on the surface of the raw material particles, thereby affecting the thickness of the hydration film. The molecules of organic plasticizers in water
