1, whiteness and brightness

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

Brightness is a process property similar to whiteness, which is equivalent to the whiteness of 4570 Ã… (angstrom) wavelength light.

The main color of kaolin and its metal oxide contained in or related to organic matter. Generally, Fe2O3 is reddish brown and brownish yellow; Fe2+ is light blue and light green; MnO2 is light brown; and organic matter is yellowish, gray, green and black. The presence of these impurities reduces the natural whiteness of the kaolin. The iron and titanium minerals also affect the whiteness of the calcination, causing stains or melting of the porcelain.

2, particle size distribution

Particle size distribution refers to the proportion (in percent) of particles in natural kaolin that are within a given range of different particle sizes (represented by the mesh of millimeter or micrometer mesh). The particle size distribution characteristics of kaolin are of great significance to the ore selectivity and process application. The particle size has a great influence on its plasticity, mud viscosity, ion exchange capacity, forming property, drying performance and firing performance. Kaolin mines require technical processing and are easy to process to the required fineness of the process. It has become one of the criteria for evaluating ore quality. Each industrial sector has specific particle size and fineness requirements for kaolin for different uses. For example, in the United States, the content of kaolin used as a coating is less than 2 μm, and the content of papermaking filler is less than 2 μm, which is 78-80%.

3. Plasticity

The mud formed by the combination of kaolin and water can be deformed under the action of external force, and the plasticity can be maintained after the external force is removed. Plasticity is the basis of the molding process of kaolin in the ceramic body, and is also the main process technical index. The plasticity index and the plasticity index are usually used to indicate the plasticity. The plasticity index refers to the liquid limited water content of the kaolin clay minus the plastic limit moisture content, expressed as a percentage, that is, the W plasticity index = 100 (W liquid limit - W plastic limit). The plasticity index represents the forming property of kaolin clay. The load and deformation of the mud ball under pressure and crushing can be directly measured by a plasticizer. It is expressed in kg·cm. The higher the plasticity index, the better the forming property. The plasticity of kaolin can be divided into four levels.

Plasticity plasticity index plasticity index

Strong plasticity>153.6

Medium plasticity 7-152.5-3.6

Weak plasticity 1-7<2.5

Non-plasticity <1

4, chemical formula

Al2O3-2SiO2-2H2O

5, combination

Binding refers to the combination of kaolin and non-plastic materials to form plastic clay and have a certain drying strength. The binding ability is determined by adding standard quartz sand to the kaolin (the mass composition is 0.25-0.15, 70%, and 0.15-0.09mm, 30%). The highest sand content and the flexural strength after drying can be judged by the highest sand content and the flexural strength after drying. The more sand is mixed, the stronger the kaolin binding ability is. Generally, kaolin with strong plasticity is also strong.

6, viscosity and thixotropy

Viscosity refers to a feature in the interior of a fluid that hinders its relative flow due to internal friction. The viscosity is expressed in terms of its size (acting on the internal friction of 1 unit area) in units of Pa·s. The viscosity is generally measured by a rotational viscometer in terms of the rotational speed of the kaolin slurry containing 70% solids. 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 influence on the paper industry. According to the data, kaolin used as a coating abroad requires a viscosity of about 0.5 Pa·s at low speed coating and less than 1.5 Pa·s at high speed.

Thixotropy refers to the property that the mud which has thickened into a gel and no longer flows becomes a fluid after being stressed, and gradually thickens into a state after being static. The size is indicated by a 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 montmorillonite content is large, the particles are fine, and the exchangeable cation is mainly sodium, and its viscosity and thickening coefficient are high. Therefore, it is common to add clay with strong plasticity, improve the fineness and the like to improve the viscosity and thixotropy of the process, and reduce it by increasing the dilution of electrolyte and moisture.

7, drying performance

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

Dry shrinkage refers to the shrinkage of kaolin clay after dehydration and drying. Kaolin mud is generally dehydrated and dried at a temperature of 40-60 ° C up to 110 ° C. Due to moisture discharge, the particle distance is shortened, and the length and volume of the sample are contracted. Dry shrinkage line shrinkage and body shrinkage are expressed as a percentage of the length and volume change of the kaolin clay after drying to constant weight. The drying line shrinkage of kaolin is generally between 3 and 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 has different shrinkage due to the different blending water, and many of them have large shrinkage. In the ceramic process, the drying shrinkage is too large, and the green body is liable to be deformed or cracked.

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

Dryness sensitivity refers to the degree of difficulty in deformation and cracking tendency of the green body when it is dried. It has high sensitivity and is easily deformed and cracked 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 in drying.

8. Sinterability

It refers to the performance of solidifying the formed solid powdery kaolin body when it is heated to near its melting point (generally over 1000 ° C), and the material spontaneously fills the intergranular gap. The state in which the porosity decreases to the lowest value and the density reaches the maximum is called the sintered state, and the corresponding temperature is called the sintering temperature. When heating is continued, the liquid phase in the sample increases continuously, and the sample begins to deform. At this time, the temperature is called the conversion temperature. The interval between the sintering temperature and the conversion temperature is called the sintering range. Sintering temperature and sintering range are important parameters in the ceramic industry to determine the billet formulation and the type of furnace. The sample has a low sintering temperature and a wide sintering range (100-150 ° C). The sintering temperature and sintering range can be controlled by blending the fluxing raw materials and mixing different types of kaolin in proportion.

9, firing shrinkage

The firing shrinkage refers to a series of physicochemical changes (dehydration, decomposition, mullite formation, melting of fusible impurities to form a glass phase filled in the space between the particles, etc.) during the calcination of the dried kaolinite. The performance that causes the product to shrink is also divided into two types: line shrinkage and body shrinkage. Like the drying shrinkage, the firing shrinkage is too large, which tends to cause cracking of the green body. In addition, when calcined, if a large amount of quartz is mixed in the billet, it will undergo crystal transformation (trigonal to hexagonal), causing its volume to expand and also causing back shrinkage.

10. Suspension and dispersion

Suspension and dispersibility refer to the ability of kaolin to be dispersed in water and difficult to precipitate. Also known as deflocculation. Generally, the finer the particle size, the better the suspension. Kaolin used in the enamel industry requires good suspension. Generally, the suspension performance of a sample dispersed in water is determined by a settling time for a certain period of time.

11, optional

Optional means that the kaolin ore is hand-selected, machined and chemically treated to remove harmful impurities and to achieve the desired quality of the industry. The selectivity of kaolin depends on the mineral composition, state of occurrence, particle size, etc. of harmful impurities. Quartz, feldspar , mica , iron, titanium minerals, etc. are all harmful impurities. Kaolin mineral processing mainly includes projects such as sand removal, iron removal and sulfur removal.

12. Ion adsorption and exchangeability

Kaolin has the property of adsorbing various ions and impurities from the surrounding medium, and has weak ion exchange properties in the solution. The pros and cons of these properties are mainly determined by the main mineral components of kaolin.

Cation exchange capacity of different types of kaolin:

Mineral composition characteristics cation exchange capacity

Kaolinite is mainly 2-5mg/100g

Halloysite is mainly 13mg/100g

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

13, electrical insulation

High-quality kaolin has good electrical insulation, and it can be used to make high-frequency porcelain and radio porcelain. The electrical insulation performance can be measured by its electrical breakdown resistance.

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