Effect of Physical Properties of Raw Materials on Sintering Bricks

11. Particle composition: or particle gradation.

Although the finer the particle size of the raw material, the larger the surface area, the better the moisture penetration, and the better the plasticity of the raw material. But the brick raw material is by no means the finer the better. Because the raw materials that are all too fine are not conducive to the drying and roasting of the products, the role of raw materials with different particle sizes in the products is different.

Powder particles with a particle size of less than 0.05mm are called plastic particles and are used to produce the plasticity required for molding. Of course, these fine particles must be clay or shale, coal gangue or other material with clay-like properties. Otherwise, as for river sand, no matter how fine it is ground, it is not plastic.

The material particles with a particle size of 0.05~1.2mm become filled particles, and their function is to control the excessive shrinkage and cracks of the product and give the blank a certain strength during plastic forming.

The coarse particles with a particle size of 1.2~2mm play a skeleton role in the green body, called skeleton particles, which are beneficial to discharge the water in the green body during drying. Hollow brick production raw material particles should not be greater than 2mm.

Reasonable particle composition should be plastic particles accounted for 35 ~ 50%; Filling particles accounted for 20 ~ 65%; Skeleton particles <30%; Never allow particles larger than 3mm. Because it will not only reduce the strength of the product, but also cause drying cracks due to uneven shrinkage.

2Plasticity index

Plasticity index is an important parameter for evaluating brick-making raw materials. In the brick-making industry, plasticity refers to the ability of a mixture of material and water to be extruded at its maximum consistency and to maintain the shape after the pressure is released. The magnitude of this ability is expressed in terms of the plasticity index.

Although the high plasticity index is conducive to extrusion molding, it is easy to produce cracks during drying and roasting; the low plasticity index is conducive to drying and roasting, but it will bring difficulties to molding. If the plasticity index is less than 7, not only extrusion molding is difficult, but also the strength of the product is low.

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 7~15, the medium plasticity clay is most suitable for extrusion molding. The higher the hole rate of the product, the more complex the hole type, the thinner the wall, and the higher the plasticity index required for molding.

The plasticity index of various raw materials is very different. The plasticity index of clay is high, some can reach more than 25, coal gangue is low, sometimes less than 7, argillaceous 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 of extrusion molding can be improved. For raw materials with high plasticity index, coal ash, slag, fly ash or waste brick powder can be appropriately mixed for thinning. For raw materials with poor plasticity index, it should be improved by mining in advance, natural weatherization, appropriate improvement of fineness, water drenching and mechanical methods such as stirring, rolling, aging, mixing and extrusion. The clay and shale powder with higher plasticity index can also be mixed appropriately to adjust the overall plasticity index of the mixture. In the production of fly ash bricks, bentonite can also be added as a binder, so that the surface of each fly ash is stuck with a thin layer of bentonite, through mixing, homogenization, extrusion to improve plasticity, to meet the molding.

It must be pointed out that when using a variety of raw materials for production, the primary condition is to fully mix. Otherwise, it will not only fail to achieve the expected effect, but will cause drying and shrinkage cracks due to the different shrinkage rates of various raw materials.

Because the quality of the extruded brick not only depends on the plasticity of the raw material, but also has a direct relationship with the friction coefficient between the raw material particles. Particles with smooth and spherical surfaces are more likely to flow closer to each other when squeezed than particles with rough rather than spherical surfaces. By adding some additives to the raw materials and the black liquor of the paper mill, the waste acid liquid of the chemical plant and the electroplating plant, due to its special ion adsorption and lubrication with the surface of the mud particles, the flow performance of the mud is greatly improved when it is extruded, and it is easier to be squeezed and compacted to produce a smooth and dense brick.

3Shrinkage

During the drying process of the green body, due to the evaporation of water, the particles naturally converge and the volume shrinks, which is the so-called drying shrinkage. The percentage of the length of the shrinkage and the original length of the blank is called the shrinkage rate of the drying line. For raw materials with large drying line shrinkage, their products should be dried slowly, otherwise, the blank will have serious drying cracks and become waste blanks. In production, the linear shrinkage rate of raw materials is required to be less than 6%. Otherwise, when the raw materials are subjected to thinning treatment and roasting, due to a series of physical and chemical changes and the burning loss of some substances in the raw materials, the finished product is not only lighter than the brick blank, but also slightly shrunk in volume, which is called firing shrinkage. The length of its shrinkage is called the percentage of the length of the green body after drying.

During the drying process, the volume of the body is gradually reduced while the moisture in the body is gradually evaporated. Since the drying speed and shrinkage speed inside and outside the blank are always fast outside and slow inside, that is, the surface layer has dried and started to shrink, while the inside is still "intact", once the shrinkage amount exceeds the elastic coefficient (1 ~ 2%) of the clay, the surface of the blank will be "burst" and mesh cracks will occur, which is called the drying sensitivity of the clay, and is expressed by the drying sensitivity coefficient, the greater the drying sensitivity coefficient, when the drying sensitivity coefficient is less than 1, the problem in the drying process is small. Once the drying sensitivity coefficient is greater than 2, the risk of cracks in the drying process is very serious. It is necessary to add thin raw materials to reduce the drying sensitivity coefficient.

Generally speaking, the higher the plasticity index of the clay, the higher the drying line shrinkage and drying sensitivity coefficient.

4Drying sensitivity coefficient

During the drying process, the volume of the body is gradually reduced while the moisture in the body is gradually evaporated. Because the drying speed and shrinkage speed inside and outside the body are always fast outside and slow inside, that is, the surface layer has dried and started to shrink, while the inside is still "intact". Once the shrinkage quantity exceeds the elastic coefficient of clay (1 ~ 2%), the surface of the body will be "burst" and mesh cracks will occur. This is called the drying sensitivity of mud and is expressed by the drying sensitivity coefficient. The greater the drying sensitivity coefficient, the more serious the threat of cracks in the drying process. When the drying sensitivity coefficient is less than 1, the problem in the drying process is small. Once the drying sensitivity coefficient is greater than 2, the risk of cracks in the drying process is very serious. Generally speaking, the higher the plasticity index of the clay, the higher the drying line shrinkage and the drying sensitivity coefficient.

5Sintering temperature range

More than the firing temperature continues to heat up, the body will slowly soften deformation, or even melt, the stack collapse. Obviously, when the firing temperature is lower than the firing temperature, there will be raw bricks or under-fired bricks; when the firing temperature is higher than the firing temperature, there will be over-burning, burning flow and even collapse of the billet stack. Obviously, the wider this sintering temperature range, the easier it is to grasp the firing. For hollow bricks, this temperature range should not be less than 50°C. This is because: the cross-sectional temperature of the kiln chamber can not be exactly the same, often the middle temperature is higher, the edge temperature is lower, and the top and bottom, inside and outside, kiln wall and kiln door near, there will be a certain temperature difference. If the firing temperature range is too narrow, it will definitely appear on the same section that the middle brick is well burned and the side brick is under fire. Or the side brick is burnt, the middle brick is burnt, or this part is under fire and the other part is over fire. The sintering temperature of clay brick is low, about 900 ℃, while that of coal gangue brick is high, about 1000~1100 ℃, and the shale is in the middle.