Technological characteristics of kaolin

White brightness

Whiteness is one of the main parameters for the technological performance of kaolin, and high purity kaolin is white. The whiteness of kaolin is divided into natural whiteness and calcined whiteness. The whiteness after calcination is more important for ceramic raw materials, and the higher the calcined whiteness, the better the quality. The ceramic process stipulates that drying at 105 ℃ and calcining at 13000 ℃ are the grading standards for natural whiteness. ℃ is the grading standard for calcined whiteness. Whiteness can be measured using a whiteness meter. A device for measuring the reflectance of light at a wavelength of 3800-7000 (i.e. 1 angstrom=0.1 nanometer) using a whiteness meter. In a whiteness meter, the contrast reflectance between the sample to be tested and the standard sample (such as BaSO4, MgO, etc.) is the whiteness value (such as whiteness 90, which represents 90% of the reflectance of the standard sample).

The brightness and whiteness are similar, equivalent to the whiteness under 4570 wavelength light irradiation.

The color of kaolin is mainly related to metal oxides or organic matter. Generally containing Fe2O, three are rose red and brown yellow; Fe2 appears light blue and light green; Light brown color with MnO2; The organic matter is light yellow, gray, green, black, and other colors. The presence of these impurities reduces the natural whiteness of kaolin, and iron and titanium minerals can also affect the calcined whiteness, leading to spots or melt scars on porcelain.

Particle size distribution

Particle size distribution refers to the proportion (in percentage) of particles in natural kaolin in a given continuous range of different particle sizes (represented by mm or micron sieve). The particle size distribution of kaolin is of great significance to the ore washability and process application. Its particle size has a significant impact on its plasticity, mud viscosity, ion exchange capacity, formability, drying performance, and combustion performance. Kaolin ore requires technical processing. Whether it is easy to process to the required fineness has become one of the standards for evaluating ore quality. Each industrial department has specific particle size and fineness requirements for different uses of kaolin. For example, the requirement for kaolin in coatings in the United States is less than 2 μ The content of m accounts for 90-95%, and the paper filling material is less than 2 μ The proportion of m is 78-80%.

Plasticity

The mud formed by the combination of kaolin and water can deform under external forces. After the external force is removed, this deformation property can still be maintained, i.e. plasticity. Plasticity is the foundation of the forming process of kaolin in ceramic bodies, and it is also the main technical indicator of the process. The plasticity index and plasticity index are usually used to represent the magnitude of plasticity. The plasticity index refers to the liquid limit moisture content of kaolin slurry minus the plastic limit moisture content, expressed as a percentage, i.e. W plasticity index=100 (W liquid limit W plasticity limit). The plasticity index represents the formability of kaolin slurry. The load and deformation of the clay ball during crushing can be directly measured using a plasticity instrument. The higher the plasticity index, the better the molding performance. The plasticity of kaolin can be divided into four levels.

Associativity

Binding property refers to the formation of a plastic slurry by combining kaolin with non plastic raw materials, which has a certain drying strength. The determination of binding capacity is to add standard quartz sand to kaolin (its mass composition is 0.25-0.15 particle size accounting for 70%, and 0.15-0.09mm particle size accounting for 30%). According to the maximum sand content and flexural strength after drying, the more sand is added, the stronger the bonding capacity of kaolin is. Generally, kaolin with strong plasticity has strong binding capacity.

viscosity

Viscosity refers to the characteristic that hinders the relative flow of a fluid due to internal friction. Its magnitude (acting on internal friction per unit area) is represented by viscosity, in Pa · s. The rotational viscometer is usually used to measure the velocity of kaolin mud containing 70% solids. In the production process, viscosity is of great significance. It is not only an important parameter in the ceramic industry, but also has a significant impact on the paper industry. Data shows that the viscosity of foreign kaolin coatings in low-speed coatings is about 0.5Pa · s, and the requirement for high-speed coating is less than 1.5Pa · s.

Thixotropy refers to the characteristic that mud becomes fluid after being stressed, gradually becomes original after being static, and then gradually becomes viscous and original. Its size is represented by the thickness coefficient of the outflow viscometer and the capillary viscometer.

The viscosity and Thixotropy are related to the mineral composition, particle size and cation type in the mud. Generally speaking, if the content of Montmorillonite is high, the particles are fine, and the exchangeable cation is mainly sodium, its viscosity and thickness coefficient are high. Therefore, the viscosity and Thixotropy are usually improved by adding highly plastic clay and improving fineness, and the viscosity and Thixotropy are reduced by adding diluted electrolyte and water.

Drying performance

Drying performance refers to the performance of kaolin slurry during the drying process. Including drying shrinkage, drying strength, and drying sensitivity.

Drying shrinkage refers to the shrinkage of kaolin mud after dehydration and drying. Kaolin mud is generally dehydrated and dried at temperatures ranging from 40 to 60 ℃ up to no more than 110 ℃. Due to water discharge and shortened particle distance, the length and volume of the sample will shrink. Dry shrinkage is expressed as the percentage change in length and volume after the kaolin slurry is dried to constant weight, which is divided into linear shrinkage and volumetric shrinkage. The shrinkage of kaolin mainline is generally 3-10%. The finer the particle size, the larger the specific surface area, the better the plasticity, and the greater the dry shrinkage. Due to the difference in mixed water, the shrinkage of the same type of kaolin also varies. In ceramic technology, if the dry shrinkage is too large, the body is prone to deformation or cracking.

Drying strength refers to the flexural strength after drying to constant weight.

Drying sensitivity refers to the difficulty of deformation and cracking of the green body during drying. High sensitivity, easy to deform and crack during the drying process. Generally, kaolin with high drying sensitivity (drying sensitivity coefficient K>2) is prone to defects; Low (drying sensitivity coefficient K<1) makes drying safer.

Sintering property

Sintering refers to the density performance of solid powder kaolin billets formed by heating them to near their melting point (usually exceeding 10000 ℃) when the material spontaneously fills the gaps between particles. When the porosity decreases to the lowest value and the density reaches the maximum value, it is called the sintering state, and the corresponding temperature is called the sintering temperature. As the temperature continues to heat, the liquid phase in the sample continues to increase and the sample begins to deform