Kaolin is a kind of non-metallic mineral, is a kind of clay and clay rock dominated by kaolinite clay minerals. Because of white and delicate, also known as dolomite. Named after Gaoling Village, Jingdezhen, Jiangxi Province.

The pure kaolin is white and soft, with good plasticity, fire resistance and other physical and chemical properties. Its mineral composition is mainly composed of kaolinite, Eloite, hydromica, illite, montmorillonite, quartz, feldspar and other minerals. Kaolin application is very extensive, mainly used for paper making, ceramics and refractory materials, secondly used in paints, rubber filler, enamel glaze and white cement raw material, a few used in plastics, paint, paint, grinding wheel, pencil, daily cosmetics, soap, pesticide, medicine, textile, oil, chemical industry, building materials, national defense and other industries.

composition

Kaolinite minerals are composed of kaolinite group minerals such as kaolinite, dickaite, pearlite and Eloite, and the main mineral component is kaolinite.

The crystal formula of kaolinite is 2SiO2·Al2O3·2H2O, and its theoretical chemical composition is 46.54% SiO2, 39.5% Al2O3, 13.96% H2O. Kaolin minerals belong to 1:1 layered silicates, and the crystals are mainly composed of silicon oxygen tetrahedron and aluminum hydroxide octahedron. The silicon oxygen tetrahedron is connected to form hexagonal grid layer along the two-dimensional direction in the way of common apex Angle, and the oxygen on the tip of each silicon oxygen tetrahedron is facing to one side. A 1:1 unit layer is composed of the apex oxygen of the common silicon oxygen tetrahedral layer and the aerobic octahedral layer

Physical and chemical properties

Properties: more matte, pure white and delicate, such as containing impurities with gray, yellow, brown and other colors. The appearance can be loose soil block or dense rock block according to different genesis.

Density: 2.54-2.60g /cm3.

Melting point: about 1785℃.

Malleable, wet soil can be shaped into various shapes without breaking and can remain unchanged for long periods of time

genesis

Kaolin is a common and very important clay mineral in nature. It is formed by weathering of feldspar or other silicate minerals in igneous and metamorphic rocks in acidic media lacking alkali metals and alkaline earth metals.

Soil classification

The minerals contained in kaolin in nature are mainly divided into clay minerals and non-clay minerals. The clay minerals mainly include kaolinite minerals and a small amount of montmorillonite, mica and chlorite. Non-clay minerals mainly include feldspar, quartz and hydrothermal compounds, as well as some iron minerals such as hematite, siderite, limonite, titanium minerals such as rutile and organic matter such as plant fiber. Clay minerals mainly determine the properties of kaolin.

Genetic classification

Based on the genesis of kaolin deposits, according to the differences in metallogenic geology, geographical conditions, deposit size, ore body morphology and occurrence characteristics, ore material composition and other aspects reflected by different mineralization, the "Code for Geological Exploration of Kaolin Deposits" in China is divided into three types and six sub-types.

1. Weathering type: it can be divided into weathering residual type and weathering eluvial type;

2. Hydrothermal alteration: it can be divided into hydrothermal alteration subtypes and modern hydrothermal alteration subtypes;

3. Sedimentary type: it can be divided into sedimentary and sediment-weathering subtypes and kaolinite clay rock subtypes in coal-bearing strata.

Industrial type

They are divided into three types according to their texture, plasticity and sandy quality fraction:

1, hard kaolin: hard, no plasticity, after grinding with plasticity.

2, soft kaolin: soft, strong plasticity, sandy mass fraction <50%;

3, sandy kaolin: soft, weak plasticity, sandy mass fraction >50%

The resource distribution

China's distribution

China's kaolin mineral resources rank among the top in the world, with 267 proven mineral sites and 2.91 billion tons of proven reserves. Among them, China's non-coal kaolin reserves rank fifth in the world. The proved reserves are 1.468 billion tons, mainly distributed in Guangdong, Shaanxi, Fujian, Jiangxi, Hunan and Jiangsu provinces, accounting for 84.55% of the total reserves in China. The reserves of coal-bearing kaolin (Kaolinite) occupy the first place in the world, with proved reserves of 1.442 billion tons, mainly distributed in Datong, Huairen, Shuozhou, Zhungeer, Wuda, Anhui huaibei, Hancheng and other places in Inner Mongolia, among which zhungeer coal field in Inner Mongolia has the most resources.

There are five major kaolin mines in China:

(1) Jiepai Town, Hengyang County, Hunan Province is rich in mineral resources. Kaolin, feldspar, potash feldspar, quartz stone reserves of 200 million tons, the existing mining and mineral products processing enterprises nearly 40. Da dai Ling mining area (mainly kaolin ore) single ore reserves of the crown in Asia up to 80 million tons. The annual mining capacity of the town is more than 500,000 tons, which supplies hundreds of ceramic factories across the country.

(2) Kaolin in Maoming Area. Kaolin ore in Maoming Basin belongs to the subtype deposit of weathering residual sedimentary rock, and its sandy content such as quartz is more than 50%, so it is called sandy kaolin ore. In terms of origin, Maoming kaolin has undergone weathering and residual accumulation -- transportation and self-grinding; -- After weathering three stages, kaolin is completely weathered, and the wafers are mainly single sheet with fine particle size. Mainly for paper coating raw materials.

(3) Longyan Kaolin, a weathered residual kaolin deposit. Because the content of iron is less than 0.3%, titanium is less than 0.02%, and contains a certain amount of low temperature solvent element (Li2O), it is the ideal raw material for electric porcelain, high-end daily use, art porcelain.

(4) Suzhou Yangshan Kaolin, the deposit is hydrothermal alteration kaolin. The pure texture of Suzhou Yangshan mud, its chemical composition is very close to the theoretical composition of kaolinite, Al2O3 content can be as high as 39.0%, white color, fine particles. Mainly used for catalyst carrier and chemical raw materials.

(5) Hepu Kaolin: it belongs to weathered residual kaolin deposit. Mainly used for building ceramic materials.

(6) Northern coal measures kaolin: sedimentary kaolin rock, mainly distributed in China's coal regions, can be used for construction, paint, paint and paper coating - coal measures soil.

The above six production areas account for more than 80% of China's output and are also representative of the main types of resources.

Foreign distribution

Kaolin resources are abundant and widely distributed in the world. The United States, The United Kingdom, Brazil, India, Bulgaria, Australia, Russia and other countries have high-quality kaolin resources. At present, the world has identified about 20.9 billion tons of kaolin resources.

Main kaolin deposits at home and abroad

(1) Georgia-South Carolina kaolin ore belt is the largest kaolin deposit and producing area in the United States. The deposits in this area are secondary sedimentary deposits. The kaolin deposit is characterized by its homogeneity, as kaolin is naturally graded according to size during natural sedimentation.

(2) The Kaolin deposit in Cornwall, UK, is a hydrothermal altered primary deposit with very low iron content and excellent whiteness. These special metallogenic conditions make kaolin produced in Britain famous in the world

Process characteristics

The white degree of 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 calcined whiteness. For ceramic materials, the whiteness after calcination is more important, the higher the whiteness, the better the quality. The classification standard of natural whiteness is 105℃ drying and 1300℃ calcination. Whiteness can be measured with a whiteness meter. A whiteness meter is a device for measuring the reflectivity of light at the wavelength of 3800 -- 7000A (ie, 1 angstrom =0.1 nanometers). In the whiteness meter, the albedo of the sample to be measured is compared with that of the standard sample (such as BaSO4, MgO, etc.), that is, the whiteness value (such as 90 whiteness means 90% of the albedo of the standard sample).

Brightness is similar to whiteness, equivalent to 4570A; The whiteness of light at (angstrom) wavelength.

The color of kaolin is mainly related to the metal oxides or organic matter it contains. Generally containing Fe2O3 rosy red, brown yellow; Fe2+ is light blue, light green; MnO2 containing light brown; Organic matter is light yellow, gray, green, black and other colors. The existence of these impurities, reduce the natural whiteness of kaolin, iron, titanium minerals will also affect the whiteness of calcination, so that the porcelain spots or melting scar.

Particle size distribution

Particle size distribution refers to the proportion (expressed in percentage) of particles in natural kaolin within a given continuous range of different particle sizes (expressed as a mesh of millimetres or microns). The particle size distribution of kaolin is of great significance to the beneficiability and process application of ores, and its particle size has great influence on its plasticity, mud viscosity, ion exchange capacity, molding performance, drying performance and firing performance. Kaolin ore needs 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. Each industrial sector has specific requirements for particle size and fineness of kaolin for different uses. Such as the United States for coating kaolin requirements less than 2μm content accounted for 90-95%, paper stuffing less than 2μm accounted for 78-80%.

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, which is plasticity. Plasticity is the base and main technical index of kaolin forming in ceramic body. Plasticity index and plasticity index are usually used to indicate the size of plasticity. Plasticity index refers to the liquid limit water content of kaolin clay minus 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 mud, which can be obtained by directly measuring the load and deformation size of mud ball when crushing under pressure with plasticizer, 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

Binding property refers to the combination of kaolin and non-plastic materials to form plastic mud and has a certain drying strength. The binding ability was determined by adding standard quartz sand into kaolin (the mass composition of 0.25-0.15 particles accounted for 70%, 0.15-0.09 mm particles accounted for 30%). It can be judged by the highest sand content when it can still keep the plastic mud and the flexural strength after drying. The more sand added, the stronger the binding ability of this kind of kaolin. Usually kaolin with strong plasticity also has strong binding ability.

Viscous

Viscosity refers to the internal friction of a fluid that hinders its relative flow. Its size is expressed by viscosity (internal friction acting on 1 unit area) in Pa·s. Viscosity is generally measured by rotational viscometer in a kaolin slurry with a 70% solid content. Viscosity plays an important role in the production process. It is not only an important parameter in ceramic industry, but also has a great influence on papermaking industry. According to the data, kaolin used as coating abroad, at low speed coating required viscosity of about 0.5Pa·s, high speed coating required less than 1.5Pa·s.

Thixotropy refers to the characteristic of a slurry that has been thickened to gelatinous and no longer flowing to become fluid under stress and then gradually thickened back to its original state when at rest. The thickness coefficient was used to represent the size of the viscosity, which was measured by outflow viscometer and capillary viscometer.

Viscosity and thixotropy are related to mineral composition, particle size and cation type in mud. Generally, the content of montmorillonite is much, the particle size is fine, and the exchangeability cation is mainly sodium, and its viscosity and thickness coefficient are high. Therefore, it is commonly used to increase the viscosity and thixotropy by adding clay with strong plasticity and improving fineness, and reduce the viscosity and thixotropy by increasing dilute electrolyte and moisture.

Drying performance

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

Drying shrinkage refers to the shrinkage of kaolin clay after dehydration and drying. Kaolin mud generally in 40-60 ℃ at most not more than 110℃ at the temperature of dehydration and drying, because of water discharge, particle distance is shortened, the length and volume of the sample will shrink. The linear shrinkage and volume shrinkage are expressed by the percentage change in length and volume of 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, the greater the drying shrinkage. The same type of kaolin, because of the different water, its shrinkage is different, more, the shrinkage is large. In ceramic process, the body is prone to deformation or cracking due to excessive drying shrinkage.

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

Drying sensitivity refers to the degree of difficulty of deformation and cracking tendency when the body is dry. High sensitivity, easy to deform and crack during drying. Generally, kaolin with high drying sensitivity (drying sensitivity coefficient K>2) is easy to form defects. Low (drying sensitivity coefficient K<1) is safer in drying.

Sintering sex

Sinter property refers to the properties of densification when the solid powdery kaolin body is heated to close to its melting point (generally more than 1000℃). The state in which porosity drops to the lowest value and density reaches the maximum value is called sintering state, and the corresponding temperature is called sintering temperature. When heating continues, the liquid phase in the sample increases and the sample begins to deform. At this time, the temperature is called the transformation temperature. The interval between sintering temperature and conversion temperature is called sintering range. Sintering temperature and sintering range are important parameters to determine billet formulation and kiln type in ceramic industry. Low sintering temperature and wide sintering range (100-150 ℃) are suitable for test materials. The sintering temperature and sintering range can be controlled by mixing mixing materials and mixing different types of kaolin in proportion.

Sintering shrinkage

Sintering shrinkage refers to a series of physical and chemical changes (dehydration, decomposition, mullite generation, fusible impurities melting to form glass phase to fill the gap between particles, etc.) in the sintering process of dried kaolin billet, resulting in product shrinkage performance, also divided into linear shrinkage and volume shrinkage. Like drying shrinkage, burning shrinkage is too large, easy to lead to cracking of the body. In addition, when roasting, if there is a large amount of quartz mixed in the billet, it will undergo crystal transformation (tripartite → hexagonal), so that its volume expansion, but also produce reverse shrinkage.

Fire resistance

Fire resistance refers to the ability of kaolin to resist high temperatures without melting. The temperature at which softening occurs and melting begins at high temperature is called refractoriness. It can be directly measured by standard thermometric cone or high temperature microscopy, or by M.A. Betzbelodov empirical formula for calculation.

Fire resistance T (℃) =[360+ Al2o3-R2O]/0.228

Where, Al2O3 is the mass percentage of Al2O3 when the sum of the analysis results of SiO2 and Al2O3 is 100; R2O is the mass percentage of other oxides when the sum of analysis results of SiO2 and Al2O3 is 100.

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

Refractoriness is related to the chemical composition of kaolin. The refractoriness of pure kaolin is generally about 1700℃. When the content of hydromica and feldspar is much, and the content of potassium, sodium and iron is high, the refractoriness is reduced, and the refractoriness of kaolin is not less than 1500℃. The industrial sector stipulates that the content of refractory R2O is less than 1.5-2%, Fe2O3 is less than 3%.

Suspended

Suspension and dispersion refer to kaolin dispersion in water is difficult to precipitate properties. Also known as deflocculation. Generally, the smaller the particle size, the better the suspension. Kaolin used in the enamel industry requires good suspension. Generally, the suspension performance of dispersed samples in water is determined by the sedimentation velocity over a certain period of time.

Optional sex

Separability refers to the fact that kaolin ores are manually selected, machined and chemically treated to remove harmful impurities and achieve the quality required by industry. The selectivity of kaolin depends on the mineral composition, occurrence state and particle size of harmful impurities. Quartz, feldspar, mica, iron, titanium minerals are harmful impurities. Kaolin mineral processing mainly includes sand removal, iron removal, sulfur removal and other items.

Adsorption

Kaolin has the ability to adsorb various ions and impurities from the surrounding medium and has weak ion exchange properties in solution. These properties depend on the main mineral composition of kaolin, as shown in Table 8.

Table 8 Cation exchange capacity of different types of kaolin

Mineral composition characteristics cation exchange capacity

Kaolinite is mainly 2-5mg /100g

Eloshi is mainly 13mg/100g

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

Chemical stability

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

Electrical insulation

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

Processing way

In terms of the current processing methods of kaolin, there are two methods: mechanical crushing and airflow crushing. And mechanical crushing generally crushing to 300 mesh -1000 mesh, but its crushing processing for mechanical way, so the grinding fine powder has increased iron content and other impurities, for the application of higher purity requirements of the industry, there are defects; Airflow grinding due to the use of materials and materials between the collision and shear, no crushing medium, so as to effectively ensure the purity of the material, so as to meet the requirements of high purity industry application efficiency, at the same time airflow grinding fineness can reach 5000 mesh (fineness range adjustable 1000-5000 mesh)

Application

Industrial USES

Kaolin has become a necessary mineral raw material for dozens of industries such as papermaking, ceramics, rubber, chemical industry, coating, medicine and national defense.

The ceramic industry is the earliest and most applied kaolin. The general dosage is 20% ~ 30% of the formula. The role of kaolin in ceramics is to introduce Al2O3, which is conducive to the formation of mullite, improve its chemical stability and sintering strength, in the firing kaolin decomposition of mullite, forming the main frame of body strength, can prevent the deformation of products, make the firing temperature wider, but also make the body has a certain whiteness. At the same time, kaolin has certain plasticity, cohesiveness, suspension and bonding ability, endowing porcelain clay and porcelain glaze with good formability, so that the ceramic clay is conducive to rolling blank and grouting, easy to form. If used in wire, can increase the insulation, reduce its dielectric loss.

Ceramics not only have strict requirements on the plasticity, bonding, drying shrinkage, drying strength, sintering shrinkage, sintering properties, refractoriness and post-burning whiteness of kaolin, but also involve chemical properties, especially the presence of chromic elements such as iron, titanium, copper, chromium and manganese, which reduce the post-burning whiteness and produce spots.

The particle size of kaolin is generally required to be as fine as possible, so that the porcelain clay has good plasticity and drying strength. However, the particle size of ingredients should be improved for the casting process requiring fast casting, accelerating the speed of grouting and dehydration. In addition, the difference of kaolinite crystal degree in kaolin will also significantly affect the process performance of the porcelain billet. If the degree of crystallization is good, the plasticity and bonding ability will be low, the drying shrinkage will be reduced, the sintering temperature will be high, and the impurity content will also be reduced. On the contrary, its plasticity is high, drying shrinkage is large, sintering temperature is low, the corresponding impurity content is also high.