The primary factor in the production of sintered bricks – raw materials
The primary factor in the production of sintered bricks – raw materials (basic knowledge)
Generally speaking, any raw material that can be fired into ordinary bricks can produce hollow bricks. However, hollow bricks have multiple holes, thin walls, and weak bodies, which require stricter requirements for the preparation of raw materials and the blending of internal fuels. Harmful impurities should be reduced, particle size distribution should be more reasonable, and mineral components should be fully dispersed, dispersed, and evenly distributed to ensure the plasticity and good bonding ability of the brick making raw materials. The basic requirements for raw materials mainly lie in their chemical composition, mineral composition, and physical properties.
1、 The influence of the main chemical composition of raw materials on brick making
Silicon dioxide (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; If it is too high, 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, often appears in the form of limestone (CaC03) in raw materials. It is a harmful substance with a content not exceeding 5%. Otherwise, not only will the sintering temperature range of the product be reduced, making it difficult to calcine, but when its particle size is greater than 2mm, it will also cause lime explosion, moisture absorption, loosening, and pulverization of the product.
Magnesium oxide (MgO): It is a harmful substance, the lower the content, the better, and should not exceed 3%. It, like calcium sulfate (CaS04) and magnesium sulfate (MgS04), will cause the product to experience frost, even delamination and weathering.
Sulfuric anhydride (SO3): It is best not to have it at all, and it cannot exceed 1% at most. Otherwise, the product will generate gas during calcination, causing the brick volume to expand, loosen and crush.
2、 Mineral analysis
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. For example, 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, the better. 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. Of course, these small particles must be clay or shale, coal gangue, or other materials with clay like properties. Otherwise, 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. Because it not only reduces the strength of the product, but also causes drying cracks due to uneven shrinkage.
2. Plasticity index
The plasticity index is an important parameter for evaluating the raw materials used in brick making. In the brick making industry, plasticity refers to the ability of a mixture of materials and water to be extruded into shape at its maximum viscosity and maintain its shape after being released from pressure. The magnitude of this ability is represented by the plasticity index.
Although a high plasticity index is beneficial for extrusion molding, it is prone to cracking during drying and roasting; The low plasticity index, although beneficial for drying and roasting, can also bring difficulties to molding. If the plasticity index is below 7, not only is extrusion molding difficult, 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, slag, fly ash, 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, rolling, aging, mixing, 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, so that a thin layer of bentonite is adhered to the surface of each fly ash particle. By mixing, homogenizing, and squeezing, the plasticity can be improved to meet the molding requirements.
It must be pointed out that when using multiple raw materials for production, the primary condition is thorough mixing. Otherwise, not only will it fail to achieve the expected effect, but it will also cause drying and shrinkage cracks due to the different shrinkage rates of various raw materials.
Due to the fact that the quality of extruded bricks not only depends on the plasticity of the raw materials, but also has a direct relationship with the friction coefficient between the raw material particles. 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%. Otherwise, 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
During the drying process, 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, that is, 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 “expand” 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.
Generally speaking, 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, the wider the sintering temperature range, the easier it is to control the roasting process. For hollow bricks, this temperature range should not be less than 50 ℃. This is because the cross-sectional temperature of the kiln chamber cannot be exactly the same, often with higher temperatures in the middle and lower temperatures at the edges, as well as certain temperature differences occurring near the top and bottom, inner and outer sides, kiln walls, and doors. If the firing temperature range is too narrow, it will inevitably occur on the same section where the middle brick is fired well and the side brick is underfired; Or if the bricks on the edge are burned, the middle bricks are burnt, or if this part is under fire and that part is over fire.
The sintering temperature of clay bricks is relatively low, about 900 ℃, while that of coal gangue bricks is relatively high, about 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, dispersed, 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, mixing, stirring, grinding, etc.
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.
For example, 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.
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.
In addition, powders with similar bulk densities are easier to mix well, while powders with significantly different bulk densities are more difficult to mix well. For example, 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. Therefore, 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.
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%. However, 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. Therefore, it is necessary to sprinkle water evenly at the beginning of mixing the powder to fully utilize its function.
(4) Grinding and grinding
The purpose of grinding is to further mix various mud materials thoroughly, allow water to penetrate further, make the overall performance of the mud materials uniform and consistent, facilitate molding, and prevent cracks caused by uneven shrinkage. Just like kneading dough, “the better the dough is kneaded, the better the Mantou tastes”.
At present, the most commonly used grinding equipment includes various fine crushing roller machines, wheel mills, kneading machines, and stirring extruders.
Fine crushing roller machine: The roller machine used in brick making not only crushes and squeezes the mud due to the significant difference in linear speed between the two roller surfaces, but also rubs and tears the squeezed “mud skin” for further mixing in the next process.
Wheel mill: 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. Moreover, 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. Therefore, 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 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