Kaolin process characteristics and main uses (1)

( 1 ) Process characteristics

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. Ceramic process specified drying 105 °C Calcination for grading standards for natural whiteness 1300 °C It is the grading standard for calcination whiteness. The whiteness can be measured by a whiteness meter. The whiteness meter is measured against 3800 — 7000 Ã… A device for the reflectivity of wavelength light. In the whiteness meter, the reflectance of the sample to be tested is compared with the reflectance of the standard sample ( such as BaSO ₄ , 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, equivalent to 4570 Whiteness under Å wavelength illumination.

    The main color of kaolin and its metal oxide contained in or related to organic matter. Generally, Fe ₂ O ₃ is reddish brown and brownish yellow; Fe ²⁺ is light blue and light green; MnO ₂ is light brown; and organic matter is light yellow, gray, blue 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

The particle size distribution refers to natural kaolin particles in given successive different grain size (in microns or mm sieve mesh indicates) the proportion of the range (expressed as percentages). 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. U.S. content of less than 2 μ m is used as a coating kaolin requirements accounted 90--95% papermaking filler accounts for less than 2 μ m 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 plasticity meter . 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, as shown in Table 1 .

 

Table 1 Kaolin plasticity grade

 

4 . Combination

Binding refers to the combination of kaolin and non-plastic materials to form plastic clay and have a certain drying strength. The binding capacity is determined by adding standard quartz sand to the kaolin ( the mass composition is 0.25 - 0.15 , 70% , 0.15 - 0.09mm The grain size accounts for 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.

5 . Viscosity and thixotropy

Viscosity is a characteristic of the inside of a fluid that hinders its relative flow due to internal friction. Its viscosity is expressed in terms of its internal friction ( acting on 1 unit area ) , and the unit is 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 is used as a coating abroad, which 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.

6 . 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 at 40 — 60 °C Not more than 110 °C Dehydration occurs at a temperature and is dried. As the water is discharged, 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 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 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.

7 . Sinterability

Sinterability refers to heating a shaped solid powdered kaolin body close to its melting point ( generally exceeding 1000 °C ), The substance spontaneously fill the gap and grain densification performance. 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 It is advisable to control the sintering temperature and the sintering range by blending the fluxing raw materials and mixing different types of kaolin in proportion.

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