Because of the different genetic types of kaolin, its composition, structure and use conditions are different, it is difficult to use the same standard to measure. The kaolin used in ceramic production requires high refractoriness, certain plasticity, bonding and drying strength, as well as the whiteness and mechanical strength after calcination, so the corresponding quality requirements should be put forward.
1. Chemical composition
Coloring compounds in the raw material, such as Fe2O3 and TiO2, should be minimized. For usually aluminum-rich kaolin Fe2O3 and TiO2 content should generally be required to be less than 1.2%. For “silicon-rich” kaolin with low aluminum content and high silicon content of clay, the highest can not exceed 0.90%. However, its mineral composition is still kaolinite or polywater kaolinite. Due to the small amount of aluminum required by this kaolin, it generally contains less colored oxides. For the content of SO3, it is true that the less the better, the SO3 formed by high temperature escape, may cause billet bubble or enamel pinhole defects.
2, fineness requirements
Usually in the production of kaolin fineness, with 250 mesh screen (aperture should be 0.061mm) to test. Because the fineness reflects the merits of its plasticity, bonding and drying strength, as well as the amount of non-kaolin components (such as quartz, mica, etc.), it is required that the screening residue rate generally not exceed 1%.
3, refractoriness and sinter
The refractoriness and sinter of kaolin mainly depend on its chemical composition, of which the ratio of Al2O3/SiO2 is very important, the larger the ratio, the higher the refractoriness, the wider the sintering range. For aluminum-rich kaolin, its refractoriness should not be lower than 1650℃(SK26 triangular cone flask). Sintering property is when it is heated to a certain temperature, the porosity drops to the lowest value, the density rises to the maximum sintering state, at this time the corresponding temperature is called the sintering temperature, although there is generally no clear requirements, but always hope that the sintering range is wider, which will bring benefits to the selection of glaze formula, determine the firing temperature and formulate the firing system.
4. Other requirements
Other requirements for the plasticity, binding, drying and firing shrinkage and thixotropy of kaolin may be increased as appropriate. However, for other types of clay except kaolin, the corresponding quality requirements should also be made. The main mineral composition of porcelain stone is sericite (or hydromica), quartz and a small amount of kaolinite, etc., also known as China clay after powder fine, with clay properties, however, the Al2O3 content is generally 13% to 18%, the SiO2 content of 70% to 78%,Fe2O3 – generally 0.6% to 0.8%. In order to facilitate production, some corresponding enterprise quality standards have been formulated for porcelain stone, such as chemical composition provisions: SiO2>65%, Al2O3>15%, Fe2O3<0.7%, CaO<1%, fineness is based on different processing forms for the corresponding requirements, the quality index of porcelain stone to increase the plasticity index (or plasticity index) requirements, it is necessary, but also can promote its fine work in the process of processing and grinding, improve its plasticity, Make full use of raw materials.
Kaolinite minerals
The difference in crystallinity of kaolinite minerals affects some of its physical and chemical properties, such as plasticity, shrinkage, cation exchange capacity, iron and titanium content, high-temperature mullitization, etc., which is shown in the following aspects.
Kaolinite with intact crystal shape and small specific surface area; Kaolinite with poor crystallization has a large specific surface area because of its irregular crystal shape and fine particles. The size of specific surface area caused by particle fineness can directly affect its plasticity. Therefore, kaolin composed of well-crystallized kaolinite has the worst plasticity. The shrinkage rate of well-crystallized kaolinite is small because it does not contain interlayer water. The shrinkage rate of dry kaolinite with poor crystallization is larger due to interlayer water.
The degree of replacement of well-crystallized kaolinite is also very poor (the general cation exchange capacity is only 2 to 5mmol/100g). The poor crystallization of kaolinite (B-axis disordered kaolinite and polyaqueous kaolinite), because of the small grain size and large surface area, the cation exchange capacity generated by the “broken bond” is very large (up to 30 ~ 40mmol/100g). When calcined at high temperature, well-crystallized kaolinite only develops into mullite spicules, while poorly crystallized kaolinite can obtain larger mullite crystals.
The differential heat curves and weight loss curves of kaolinite minerals are different because of their different crystalline forms. Due to impurities such as brown carbon and subcarbon rich in organic matter, the appearance is black; It is also due to the large amount of organic matter, so the plasticity and adhesion are quite strong. The technological physical properties of the trademark kaolin are shown in Table 1. The comprehensive analysis and judgment of mineral composition, chemical composition and technological physical properties of kaolin show that the characteristics of various kaolin are as follows.
• White block kaolin (high fire mud), low Fe2O3 content (<0.3%), high firing whiteness and refractoriness, fine particles, firing shrinkage, so it is a high-quality raw material for manufacturing electric porcelain, high-grade daily fine porcelain, but the plasticity is slightly less than the pink mud, the amount of up to 60% in the formula. The whiteness of the finished product is more than 80%, and it is also a good raw material for glazing.
• Pink flake kaolin (pink mud), with high firing whiteness, low Fe2O3 content (<0.5%), high plasticity and excellent properties, but the whiteness is slightly lower than high fire mud. The amount of daily porcelain and electric porcelain can reach 50%, which is a high-quality raw material for manufacturing advanced daily porcelain and electric porcelain, and can also be used as raw material for making glaze.
• Orange block kaolin (yellow sand mud), high iron content (>1%), strong color ability, suitable for making stoneware or rubber fillers.
• Purple red flake kaolin (pig liver mud), high Fe2O3 content (1% ~ 2%), medium plasticity, refractoriness less than 1380℃, currently only for the production of inverting kiln saggar raw materials, the amount of up to 50% ~ 60%.
• Grayish white block kaolin (low fire mud), Fe2O3, Na2O content is high, can not be used in the manufacture of advanced daily fine porcelain, but can be used in domestic porcelain instead of feldspar, is also a good flux raw material for the trial production of stoneware or hotel porcelain.
• White block containing albite kaolin (sand and white mud), high Na2O content (>7%), no plasticity and viscosity, currently rarely used in daily porcelain, but low Fe2O3 content, high whiteness, suitable for firing low-temperature ceramics or as ceramic glaze.
Original Source: https://www.pairshorseschem.com/news/what-are-the-requirements-of-kaolin-for-ceramic-production/
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