The technological properties of kaolin

Whiteness is one of the main parameters of the technological properties of kaolin. High-purity kaolin is white. The whiteness of kaolin is divided into natural whiteness and whiteness after calcination. For ceramic raw materials, the whiteness after calcination is more important. The higher the calcined whiteness, the better the quality. The ceramic process stipulates that drying at 105°C is the grading standard for natural whiteness, and calcination at 1300°C is the grading standard for calcined whiteness. Whiteness can be measured by a whiteness meter. A whiteness meter is a device that measures the reflectance of light with a wavelength of 3800 - 7000 Å (1 Å = 0.1 nm). In the whiteness meter, the reflectance of the sample to be tested is compared with that of the standard sample (such as BaSO₄, MgO, etc.), which is the whiteness value (for example, a whiteness of 90 means that it is equivalent to 90% of the reflectance of the standard sample).

Brightness is a technological property similar to whiteness, equivalent to the whiteness under the irradiation of light with a wavelength of 4570 Å.

The color of kaolin is mainly related to the metal oxides or organic matter it contains. Generally, if it contains Fe₂O₃, it shows rose red or brownish yellow; if it contains Fe²⁺, it shows light blue or light green; if it contains MnO₂, it shows light brown; if it contains organic matter, it shows light yellow, gray, blue, black, etc. The presence of these impurities reduces the natural whiteness of kaolin. Moreover, iron and titanium minerals can also affect the calcined whiteness, causing color spots or fusion scars on porcelain.

Particle size distribution refers to the proportion (expressed as a percentage) of particles in natural kaolin within a given continuous range of different particle size grades (represented by the mesh number of sieve pores in millimeters or microns). The particle size distribution characteristics of kaolin are of great significance for the ore's beneficiability and process applications. The particle size has a great impact on its plasticity, mud viscosity, ion exchange capacity, molding performance, drying performance, and firing performance. Kaolin ores all need to be technically processed. Whether it is easy to process to the fineness required by the process has become one of the criteria for evaluating the quality of the ore. Different industrial sectors have specific requirements for the particle size and fineness of kaolin for different uses. For example, in the United States, for kaolin used as a coating, the content of particles less than 2 μm is required to be 90 - 95%, and for kaolin used as a paper filler, the content of particles less than 2 μm is 78 - 80%.

The property that the clay mass formed by kaolin combined with water can deform under the action of an external force and still maintain this deformation after the external force is removed is called plasticity. Plasticity is the basis of the molding process of kaolin in ceramic green bodies and is also a major technological indicator. The magnitude of plasticity is usually expressed by the plasticity index and the plasticity index. The plasticity index refers to the liquid limit moisture content of the kaolin clay mass minus the plastic limit moisture content, expressed as a percentage, that is, . The plasticity index represents the molding performance of the kaolin clay mass. It can be directly measured by a plastometer to obtain the load and deformation when the clay ball is crushed under pressure, expressed in kg·cm. Generally, the higher the plasticity index, the better the molding performance. The plasticity of kaolin can be divided into four levels.

The bonding property refers to the ability of kaolin to combine with non - plastic raw materials to form a plastic clay mass and have a certain dry strength. The determination of the bonding ability is to add standard quartz sand to kaolin (its mass composition: the particle size grade of 0.25 - 0.15 mm accounts for 70%, and the particle size grade of 0.15 - 0.09 mm accounts for 30%). Its level is judged by the maximum sand content when it can still maintain a plastic clay mass and the flexural strength after drying. The more sand is added, the stronger the bonding ability of this kaolin. Generally, kaolin with strong plasticity also has strong bonding ability.

Viscosity refers to a characteristic of a fluid that hinders its relative flow due to internal friction. Its magnitude is expressed by viscosity (the internal friction acting on a unit area), and the unit is Pa·s. The measurement of viscosity generally uses a rotational viscometer, which is measured by the rotational speed in a kaolin slurry with a solid content of 70%. In the production process, viscosity is of great significance. It is not only an important parameter in the ceramic industry but also has a great impact on the paper industry. According to the data, when using kaolin as a coating abroad, the required viscosity is about 0.5 Pa·s for low - speed coating and less than 1.5 Pa·s for high - speed coating.

Thixotropy refers to the property that the mud that has thickened into a gel and no longer flows becomes a fluid when subjected to force and gradually thickens back to its original state after being stationary. Its magnitude is expressed by the thickening coefficient and is measured using an efflux viscometer and a capillary viscometer.

Viscosity and thixotropy are related to the mineral composition, particle size, and cation type in the mud. Generally, the mud with more montmorillonite content, finer particles, and sodium as the main exchangeable cation has higher viscosity and thickening coefficient. Therefore, in the process, methods such as adding clay with strong plasticity and increasing the fineness are often used to increase its viscosity and thixotropy, and methods such as adding diluting electrolytes and water are used to reduce them.

The drying performance refers to the performance of the kaolin clay mass during the drying process, including drying shrinkage, drying strength, and drying sensitivity.

Drying shrinkage refers to the shrinkage of the kaolin clay mass after losing water and drying. Generally, the kaolin clay mass dehydrates and dries at a temperature of 40 - 60°C, and at most no more than 110°C. As the water is discharged and the distance between particles is shortened, the length and volume of the sample will shrink. Drying shrinkage is divided into linear shrinkage and volume shrinkage, expressed as the percentage change in the length and volume of the kaolin clay mass after drying to a constant weight. The drying linear shrinkage of kaolin is generally 3 - 10%. The finer the particle size, the larger the specific surface area, the better the plasticity, and the greater the drying shrinkage. For the same type of kaolin, the shrinkage is different due to different amounts of added water. The more water added, the greater the shrinkage. In the ceramic process, if the drying shrinkage is too large, the green body is prone to deformation or cracking.

Drying strength refers to the flexural strength of the clay mass after drying to a constant weight.

Drying sensitivity refers to the degree of ease of deformation and cracking tendency of the green body during drying. If the sensitivity is high, it is easy to deform and crack during the drying process. Generally, kaolin with high drying sensitivity (drying sensitivity coefficient K > 2) is prone to forming defects; while that with low sensitivity (drying sensitivity coefficient K < 1) is relatively safe during drying.