What is the use of kaolin?

KAOLIN has a wide range of uses. It is mainly used in papermaking, ceramics and refractory materials, followed by coatings, rubber fillers, enamel glazes and white cement raw materials, and a small amount is used in plastics, paints, pigments, grinding wheels, pencils, Daily cosmetics, soap, pesticides, medicine, textiles, petroleum, chemicals, building materials, national defense and other industrial sectors.

 

Process characteristics of kaolin:

Brightness

Whiteness is one of the main parameters of kaolin process performance. Kaolin with high purity is white. The whiteness of kaolin is divided into natural whiteness and whiteness after calcination. For ceramic raw materials, the whiteness after calcining is more important. The higher the calcining whiteness, the better the quality. Ceramic technology stipulates that drying at 105°C is the classification standard for natural whiteness, and calcination at 1300°C is the classification standard for calcining whiteness. The whiteness can be measured with a whiteness meter. The whiteness meter is a device that measures the reflectance of light with a wavelength of 3800-7000 angstroms; (that is, angstroms, 1 angstrom = 0.1 nanometers). In the whiteness meter, compare the reflectance of the sample to be tested with the standard sample (such as BaSO4, MgO, etc.), that is, the whiteness value (for example, a whiteness of 90 means 90% of the reflectance of the standard sample).

Brightness is a process property similar to whiteness, equivalent to 4570 angstroms; (angstroms) whiteness under irradiation of wavelength light.

The color of kaolin is mainly related to the metal oxide or organic matter it contains. Generally, Fe2O3 is rose red and brownish yellow; Fe2+ is light blue and light green; MnO2 is light brown; organic matter is light yellow, gray, blue, black and other colors. The presence of these impurities reduces the natural whiteness of kaolin. Among them, iron and titanium minerals will also affect the whiteness of calcination, causing stains or melting scars on porcelain.

Particle size distribution

The particle size distribution refers to the proportion (expressed as a percentage) of the particles in the natural kaolin in a given continuous range of different particle sizes (expressed by the mesh of millimeters or micrometers). The particle size distribution characteristics of kaolin are of great significance to the selectability of ore and process applications. Its particle size has a great influence on its plasticity, mud viscosity, ion exchange capacity, molding performance, drying performance, and firing performance. Kaolin mines require technical processing, and 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. Various industrial sectors have specific requirements for particle size and fineness of kaolin for different purposes. For example, the United States requires 90-95% of kaolin used as coatings with a content of less than 2μm, and 78-80% of paper fillers less than 2μm.

plasticity

The mud formed by the combination of kaolin and water can be deformed under the action of external force. After the external force is removed, it can still maintain this deformation property, which is plasticity. Plasticity is the basis of the molding process of kaolin in the ceramic body, and it is also the main technological index. Plasticity index and plasticity index are usually used to express the size of plasticity. The plasticity index refers to the liquid limit water content of the kaolin clay minus the plastic limit water content, expressed as a percentage, that is, W plasticity index = 100 (W liquid limit-W plastic limit). The plasticity index represents the molding performance of kaolin clay. It can be obtained by directly measuring the load and deformation of the clay ball when it is crushed under pressure with a plasticizer. It is expressed in kg·cm. The higher the plasticity index, the better the molding performance. The plasticity of kaolin can be divided into four levels.

Plasticity Strength Plasticity Index Plasticity Index

Strong plasticity>153.6

Medium plasticity 7-152.5-3.6

Weak plasticity 1-7<2.5

Non-plasticity <1

Associativity

Combination refers to the combination of kaolin and non-plastic raw materials to form a plastic mud mass and have certain dry strength properties. The binding capacity is determined by adding standard quartz sand to kaolin (its mass composition is 0.25-0.15 grain size accounting for 70%, and 0.15-0.09mm grain size accounting for 30%). The maximum sand content and the flexural strength after drying can be used to judge its height. The more sand mixed, the stronger the binding capacity of this kaolin. Generally, kaolin with strong plasticity has strong binding capacity.

viscosity

Viscosity refers to a feature that hinders the relative flow of the fluid due to internal friction. The viscosity is used to express its size (the internal friction acting on a unit area), and the unit is Pa·s. Viscosity is generally measured by a rotational viscometer, measured by the rotational speed in a kaolin mud containing 70% solid content. In the production process, viscosity is of great significance. It is not only an important parameter of the ceramic industry, but also has a great impact on the paper industry. According to data, when kaolin is used as coating in foreign countries, the 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 characteristic that the mud that has been thickened into a gel and no longer flows becomes fluid after being stressed, and then gradually thickens into its original state after being stationary. The size is expressed by the thickening coefficient, and measured by an outflow viscometer and a capillary viscometer.

Viscosity and thixotropy are related to the mineral composition, particle size and cation type in the mud. Generally, the content of montmorillonite is high, the particles are fine, the exchangeable cations are mainly sodium, and its viscosity and thickening coefficient are high. Therefore, in the process, methods such as adding strong plastic clay and increasing the fineness are commonly used to increase its viscosity and thixotropy, and to reduce it by increasing diluted electrolyte and moisture.

Drying performance

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

Drying shrinkage refers to the shrinkage of kaolin clay after drying without water. Kaolin mud is generally dehydrated and dried at a temperature of 40-60°C up to 110°C. Due to the discharge of water, the distance between particles is shortened, and the length and volume of the sample will shrink. Drying shrinkage is divided into line shrinkage and body shrinkage, expressed as the percentage change of the length and volume of the kaolin mud after drying to constant weight. The drying line 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. The same type of kaolin shrinks differently due to different blending water, and more shrinkage. In the ceramic process, the drying shrinkage is too large, and the green body is prone to deformation or cracking.

Dry strength refers to the flexural strength of the mud after drying to constant weight.

Drying sensitivity refers to the degree of difficulty with which the green body may deform and crack when it is dried. High sensitivity, easy to deform and crack during the drying process. Generally, kaolin with high drying sensitivity (drying sensitivity coefficient K>2) is easy to form defects; the lower one (drying sensitivity coefficient K<1) is safer during drying.

Sinterability

Sinterability refers to the ability of the formed solid powdery kaolin body to be heated to close to its melting point (generally more than 1000°C), the material spontaneously fills the gap between the grains to densify. The state where the porosity drops to the minimum and the density reaches the maximum is called the sintering state, and the corresponding temperature is called the sintering temperature. When heating continues, the liquid phase in the sample increases and the sample begins to deform. The temperature at this time is called the transformation temperature. The interval between the sintering temperature and the conversion temperature is called the sintering range. The sintering temperature and sintering range are important parameters for determining the blank formulation and choosing the type of furnace in the ceramic industry. The sample is suitable for low sintering temperature and wide sintering range (100-150℃). In the process, the sintering temperature and sintering range can be controlled by mixing flux materials and mixing different types of kaolin in proportion.

Firing shrinkage

Firing shrinkage refers to a series of physical and chemical changes (dehydration, decomposition, formation of mullite, melting of fusible impurities to form a glass phase filling the voids between the particles, etc.) during the firing process of the dried kaolin blank , And the properties that cause product shrinkage are also divided into two types: linear shrinkage and body shrinkage. Like the drying shrinkage, the firing shrinkage is too large, which will easily cause the green body to crack. In addition, during firing, if a large amount of quartz is mixed in the billet, it will undergo a crystal transformation (three-sided → six-sided), which will cause its volume to expand and reverse shrinkage.

Fire resistance

Fire resistance refers to the ability of kaolin to resist high temperatures without melting. The temperature at which it softens and begins to melt under high temperature operation is called refractoriness. It can be directly measured using standard temperature measuring cones or high-temperature microscopy, or calculated using M.A Bezbelov’s empirical formula.

Refractoriness t (℃)=[360+Al2O3-R2O]/0.228

In the formula: Al2O3 is the mass percentage of Al2O3 when the sum of SiO2 and Al2O3 analysis results is 100; R2O is the mass percentage of other oxides when the sum of SiO2 and Al2O3 analysis results is 100.

The error of calculating the refractoriness by this formula is within 50℃.

The refractoriness is related to the chemical composition of kaolin. The refractoriness of pure kaolin is generally around 1700℃. When the content of hydromica and feldspar is high, and the content of potassium, sodium, and iron is high, the refractoriness will decrease. 1500°C. The industrial sector stipulates that the R2O content of refractory materials is less than 1.5-2%, and Fe2O3 is less than 3%.

Suspension

Suspension and dispersibility refer to the property of kaolin that is difficult to settle when dispersed in water. Also known as anti-flocculation. Generally, the finer the particle size, the better the suspension. Kaolin used in the enamel industry requires good suspension. Generally, the suspension performance of the sample dispersed in water is determined by the sedimentation speed of a certain period of time.

Optionality

Optionality means that kaolin ore is manually selected, mechanically processed and chemically treated to remove harmful impurities and make the quality meet the performance required by the industry. The selectivity of kaolin depends on the mineral composition, occurrence state and particle size of harmful impurities. Quartz, feldspar, mica, iron and titanium minerals are all harmful impurities. Kaolin beneficiation mainly includes projects such as sand removal, iron removal, and sulfur removal.

Adsorption

Kaolin has the ability to adsorb various ions and impurities from the surrounding medium, and has weak ion exchange properties in the solution. The pros and cons of these properties mainly depend on the main mineral components of kaolin, and the cation exchange capacity of different types of kaolin.

Features of mineral composition Cation exchange capacity

Kaolinite is mainly 2-5mg/100g

Halloysite-based 13mg/100g

Contains organic matter (ball soil) 10-120mg/100g

Chemically stable

Kaolin has strong acid resistance, but its alkali resistance is poor. Use this property to use it to synthesize molecular sieves.

Electrical insulation

High-quality kaolin has good electrical insulation, and it can be used to make high-frequency porcelain and wireless porcelain by using this property. The level of electrical insulation performance can be measured by its resistance to electrical breakdown.

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