Know Kaolin

Pure kaolin has high whiteness, soft, easy to disperse suspended in water, good plasticity and high adhesion, excellent electrical insulation properties; has good resistance to acid-soluble, low cation exchange capacity, better The fire resistance and other physical and chemical properties. Therefore, kaolin has become a necessary mineral raw material for dozens of industries such as papermaking, ceramics, rubber, chemicals, coatings, medicine, and defense. Kaolin is widely used in the paper industry. There are two main areas, one is the filler used in papermaking (or papermaking) and the other is the pigment used in the surface coating process.

The use of kaolin

It has been reported that there are applications in Japan that use kaolin instead of steel for cutting tools, lathe bits, and internal combustion engine casing. Especially in recent years, the rapid development of modern science and technology has made the application of kaolin more extensive, and some high-tech fields have begun to use kaolin as a new material, and even high-temperature porcelain components for kaolin ore reactors, space shuttles and spacecrafts. Made of kaolin. At present, the total global kaolin production is about 40 million tons (this data is simply the sum of country-to-country production, which does not include statistics on the volume of ore trade, including more repetitive calculations), of which refined soil is about 23.5 million tons. The paper industry is the largest consumer sector for refined kaolin, which accounts for about 60% of the total consumption of kaolin. According to the data provided by Temanex Consulting of Canada, the total paper and paperboard production in the world in 2000 was approximately 319 million tons, and the total amount of kaolin used in papermaking coatings worldwide was approximately 13.6 million tons. For general culture paper, the amount of filler accounts for 10-20% of the paper weight. For coated paper and cardboard (

It mainly includes light-weight coated paper, coated paper and coated paperboard. In addition to the fillers, it needs pigments. The proportion of kaolin used for filling and pigments is 20-35% of the paper weight. Kaolin used in papermaking, can give good coverage of paper and good gloss coating performance, but also increase the paper's whiteness, opacity, smoothness and printability, and greatly improve the quality of the paper.

Kaolin distribution

At present, there are more than 700 kaolin mines in China, and the proven reserves of 200 mines are 3 billion tons, and the mines are scattered. Among them, 1.67 billion tons of coal-based kaolin are mainly distributed in the Carboniferous-Permian coal series in the northeast and northwest of China, and are found in the form of coals, sillplates, or individual ore layers. China is a big country producing coal. Basically, large-scale coal mines are associated with coal-based kaolin, and coal-based kaolin reserves are very abundant. Non-coal-based kaolin 1996 proved industrial reserves of 1.432 billion tons.

Compared with other non-metallic resources, kaolin is not a dominant resource in China, and it is even more in short supply in terms of per capita consumption. In addition, the distribution of kaolin resources in China is relatively decentralized and its grade is not high. Most of them are coal-based kaolins (seldom abroad) and need to be calcined or modified. It is a natural limitation for paper coating.

Moreover, coal-based kaolin belongs to coal associated mines and is difficult to be exploited on a large scale. In China, non-coal kaolin and kaolin reserves are equivalent, but the vast majority are tubular kaolins, which have high viscosity and cannot be used for paper coating.

According to current information, only the kaolin resources in Guangdong, Guangxi, and Anhui Shahe can be used for papermaking coatings, so resources are very valuable. Hebei Shahe once had fierce competition with Maoming Kaolin in the domestic paper coating market in the mid to late 1990s, but it has been shrinking due to insufficient resources.

Kaolin process characteristics

1. Whiteness and brightness

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

Brightness is a process property similar to whiteness, equivalent to whiteness at 4570 Ã… (Angstrom) wavelength light irradiation.

The color of kaolin is mainly related to the metal oxide or organic matter it contains. Generally containing Fe2O3 rose red, brown yellow; containing Fe2 was light blue, light green; MnO2 was light brown; containing the machine was pale yellow, gray, blue, black and other colors. The existence of these impurities reduces the natural whiteness of kaolin. Among them, iron and titanium minerals also affect the calcined whiteness, causing the appearance of stains or blemishes in porcelain.

2. Particle size distribution

Particle size distribution refers to the proportion (expressed in percent) of particles in natural kaolin, given a range of consecutive different particle sizes (represented by millimeter or micron meshes). Kaolin's particle size distribution characteristics are of great significance to the ore's selectability and process application. 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. Whether it is easy to process to the fineness required by the process has become one of the criteria for evaluating ore quality. Various industrial sectors have specific granularity and fineness requirements for different uses of kaolin. For example, in the United States, the content of kaolin used as a coating is less than 2 μm, accounting for 90-95%, and the papermaking filler less than 2 μm accounts for 78-80%.

3. Plasticity

The clay formed by combining kaolin and water can be deformed under the action of external forces. After the external force is removed, the nature of the deformation can still be plasticity. Plasticity is the basis of the kaolin molding process in the ceramic body, and it is also the main process technology index. Plasticity index and plasticity index are usually used to indicate the size of plasticity. The plasticity index refers to the liquid limit moisture content of kaolin clay minus the plastic limit moisture content, expressed as a percentage, that is, W plasticity index = 100 (W liquidity limit - W plastic limit). The plasticity index represents the forming performance of the kaolin clay material, and can be directly measured by the plastic instrument when the mud ball is crushing. The weight and deformation of the clay ball can be obtained in kg·cm. The higher the plasticity index is, the better the molding performance is. Kaolin plasticity 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

Chemical formula

Al2O3-2SiO2-2H2O

5. Binding

Binding refers to the combination of kaolin and non-plastic materials to form a malleable mud mass and has a certain dry strength properties. The binding capacity was determined by adding standard quartz sand to kaolin (its mass composition is 0.25-0.15 grain size 70%, and 0.15-0.09mm grain size is 30%). Judging by the highest sand content when it can still maintain the plastic clay and the flexural strength after drying, the more sand it incorporates, the stronger the kaolin binding ability. In general, Kaolin with strong plasticity has strong binding ability.

6. Viscosity and thixotropy

Viscosity is a characteristic of the internal flow of a fluid that hinders its relative flow due to internal friction. Viscosity is used to indicate its size (internal friction acting on one unit area), and the unit is Pa·s. Viscosity is measured using a rotational viscometer, which is measured as the rotational speed in a 70% solids kaolin slurry. In the production process, the viscosity has important significance. It is not only an important parameter of the ceramic industry, but also has a great impact on the paper industry. According to the data, kaolin is used as a coating material in foreign countries and requires a viscosity of about 0.5 Pa·s at low-speed coating and less than 1.5 Pa·s at high-speed coating.

Thixotropy refers to the fact that a mud that has thickened into a gel and no longer flows becomes fluid after being stressed, and gradually thickens to its original shape after standing. The size is expressed by a thickening factor and measured with an outflow viscometer and a capillary viscometer.

Viscosity and thixotropy are related to the mineral composition, particle size and cation type in the mud. In general, the montmorillonite content is large, the grain is fine, and the exchangeable cation is mainly sodium, which has high viscosity and thickening coefficient. Therefore, the process is often used to increase the viscosity and thixotropy of plastic clay, increase the fineness and other methods to increase the dilution of electrolytes and water and other methods to reduce it.

7. Drying performance

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

Dry shrinkage refers to the shrinkage of kaolin clay produced after dehydration and drying. Kaolin clay generally dehydrates and dries at a temperature of between 40-60°C and no more than 110°C. Due to moisture loss, the particle distance is shortened, and the length and volume of the specimen shrink. Drying contraction line shrinkage and body shrinkage are expressed as percent change in length and volume after the kaolin clay slurry is dried to constant weight. Kaolin drying line shrinkage 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 difference in blending water. In the ceramic process, the drying shrinkage is too large, and the body is easily deformed or cracked.

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

Drying sensitivity refers to the ease with which the green body may be deformed and cracked when dried. High sensitivity, easy to deform and crack during drying. Kaolin with a high dry sensitivity (dryness sensitivity K>2) is prone to defects; the lower one (dryness sensitivity K<1) is safer in drying.

8. Sinterability

The sinterability refers to the ability of the material to spontaneously fill the interstices and densify when the shaped solid kaolin clay body is heated to a temperature close to its melting point (typically in excess of 1000[deg.] C.). The state where the porosity is reduced to the lowest value and the density reaches the maximum value is called the sintering state, and the corresponding temperature is called the sintering temperature. As the heating continues, the liquid phase in the sample increases and the sample begins to deform. At this point, 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 formulation of the blank and to select the type of kiln. The sample is suitable for low sintering temperature and wide sintering range (100-150°C). The sintering temperature and sintering range can be controlled by mixing and mixing raw materials and different types of kaolin in proportion.

9. Shrinkage

Firing shrinkage refers to a series of physical and chemical changes occurring in the dried kaolin material during the firing process (dehydration, decomposition, formation of mullite, melting of fusible impurities, formation of glassy phases, filling of voids between particles, etc.) The properties that lead to the shrinkage of the product are also divided into two types: line shrinkage and body shrinkage. As with the dry shrinkage, the firing shrinkage is too large, which can easily lead to 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 (triangular → hexagonal) to make its volume expand, and it will also produce anti-shrinkage.

10. Fire resistance

Fire resistance refers to the ability of kaolin to withstand high temperatures without melting. The temperature is called refractoriness when it softens and begins to melt under high temperature operation. It can be measured directly with a standard cone or high temperature microscopy, or it can be calculated using the MA Bibelkin's empirical formula.

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

Where: 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 the refractoriness calculated by this formula is within 50°C.

The refractoriness is related to the chemical composition of kaolin. The refractoriness of pure kaolin is generally around 1700°C. When the hydrous and feldspar content is high and the potassium, sodium, and iron contents are high, the refractoriness decreases, and the refractoriness of kaolin is not less than 1500°C. The industrial sector specifies that the R2O content of refractory materials is less than 1.5-2%, and Fe2O3 is less than 3%.

11. Suspension and dispersion

Suspension and dispersability refer to the difficulty of precipitation of kaolin dispersed in water. Also known as deflocculation. The finer the general particle size, the better the suspension. Kaolin used in the enamel industry requires good suspensibility. It is generally based on the speed of sedimentation of a sample dispersed in water to determine its suspension performance.

12. Optional

Optional refers to the kaolin ore hand-selected, mechanical processing and chemical treatment to remove harmful impurities, so that the quality of the industry performance requirements. The choice of kaolin depends on the mineral composition of the harmful impurities, the state of occurrence, the particle size, and the like. Quartz, feldspar, mica, iron, and titanium minerals are all harmful impurities. Kaolin beneficiation mainly includes sand removal, iron removal, and removal* projects.

13. Ion adsorption and exchangeability

Kaolin has the ability to adsorb various ions and impurities from the surrounding media and has weak ion exchange properties in solution. The pros and cons of these properties depend mainly on the main mineral components of kaolin, see Table 8.

Table 8 Cationic Exchange Capacity of Different Types of Kaolin

Mineral composition characteristics Cation exchange capacity

Kaolinite is mainly 2-4mg/100g

Ello stone is mainly 13mg/100g

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

14. Chemical stability

Kaolin has strong acid resistance, but its alkali resistance is poor. This property can be used to synthesize molecular sieves.

15. Electrical insulation

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