What is the Advantage and Disadvantage of washed kaolin

kaolin, soft white clay that is an essential ingredient in the manufacture of china and porcelain and is widely used in the making of paper, rubber, paint, and many other products. Kaolin is named after the hill in China (Kao-ling) from which it was mined for centuries. Samples of kaolin were first sent to Europe by a French Jesuit missionary around as examples of the materials used by the Chinese in the manufacture of porcelain.

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In its natural state kaolin is a white, soft powder consisting principally of the mineral kaolinite, which, under the electron microscope, is seen to consist of roughly hexagonal, platy crystals ranging in size from about 0.1 micrometre to 10 micrometres or even larger. These crystals may take vermicular and booklike forms, and occasionally macroscopic forms approaching millimetre size are found. Kaolin as found in nature usually contains varying amounts of other minerals such as muscovite, quartz, feldspar, and anatase. In addition, crude kaolin is frequently stained yellow by iron hydroxide pigments. It is often necessary to bleach the clay chemically to remove the iron pigment and to wash it with water to remove the other minerals in order to prepare kaolin for commercial use.

When kaolin is mixed with water in the range of 20 to 35 percent, it becomes plastic (i.e., it can be molded under pressure), and the shape is retained after the pressure is removed. With larger percentages of water, the kaolin forms a slurry, or watery suspension. The amount of water required to achieve plasticity and viscosity varies with the size of the kaolinite particles and also with certain chemicals that may be present in the kaolin. Kaolin has been mined in France, England, Saxony (Germany), Bohemia (Czech Republic), and in the United States, where the best-known deposits are in the southeastern states.

Approximately 40 percent of the kaolin produced is used in the filling and coating of paper. In filling, the kaolin is mixed with the cellulose fibre and forms an integral part of the paper sheet to give it body, colour, opacity, and printability. In coating, the kaolin is plated along with an adhesive on the paper’s surface to give gloss, colour, high opacity, and greater printability. Kaolin used for coating is prepared so that most of the kaolinite particles are less than two micrometres in diameter.

Kaolin is used extensively in the ceramic industry, where its high fusion temperature and white burning characteristics makes it particularly suitable for the manufacture of whiteware (china), porcelain, and refractories. The absence of any iron, alkalies, or alkaline earths in the molecular structure of kaolinite confers upon it these desirable ceramic properties. In the manufacture of whiteware the kaolin is usually mixed with approximately equal amounts of silica and feldspar and a somewhat smaller amount of a plastic light-burning clay known as ball clay. These components are necessary to obtain the proper properties of plasticity, shrinkage, vitrification, etc., for forming and firing the ware. Kaolin is generally used alone in the manufacture of refractories.

Mainly look at the following aspects.

Whiteness Brightness

Kaolin whiteness is divided into natural whiteness and calcined whiteness. For ceramic raw materials, the higher the calcined whiteness is the better the quality. The presence of impurities such as Fe2O3 and MnO2 in kaolin will reduce its natural whiteness.

Particle size distribution

The particle size distribution characteristics of kaolin are of great significance to the selectivity of the ore and the process application, and its particle size, its plasticity, slurry viscosity, ion exchange, molding properties, drying properties, firing properties have great influence. Various industrial sectors have specific particle size and fineness requirements for different uses of kaolin.

Plasticity

Plasticity is the basis of the molding process of kaolin in ceramic blanks, and is also the main technical indicator of the process. The higher the plasticity index, the better the molding performance. The plasticity of kaolin can be divided into four levels, strong plasticity >153.6, medium plasticity 7-152.5-3.6, weak plasticity 1-7<2.5, non-plasticity <1.

Combination

It refers to the performance of kaolin combining with non-plastic raw materials to form a plastic clay mass with certain drying strength. Usually any kaolin with strong plasticity also has strong binding ability.

Viscosity

Viscosity refers to the internal fluid due to internal friction and hinder its relative flow of a characteristic, in the production process, viscosity is of great significance, it is not only an important parameter of the ceramic industry, also has a great impact on the paper industry. Generally montmorillonite content, fine particles, exchangeable cations mainly sodium, its viscosity and thickness coefficient is high.

Drying performance

Drying performance refers to the performance of kaolin clay in the drying process, including drying shrinkage, drying strength and drying sensitivity. Drying shrinkage refers to the shrinkage of kaolin clay material after losing water and drying, generally in 3-10%. The finer the particle size, the larger the specific surface area, the better the plasticity, the larger the drying shrinkage. Drying strength refers to the flexural strength of the clay after drying to constant weight. Generally high drying sensitivity of kaolin is easy to form defects, low in the drying of the safer.

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Sinterability

Sinterability refers to the molded solid powdered kaolin billet heated to close to its melting point (generally more than ℃), the material spontaneously fill the grain gap and densification properties. Sintering temperature and sintering range in the ceramic industry is an important parameter in deciding the billet formula and selecting the type of kiln. A low sintering temperature and a wide sintering range (100-150°C) are preferred for the test material.

Sintering shrinkage

Sintering shrinkage refers to the dried kaolin blanks in the firing process, a series of physicochemical changes (dehydration, decomposition, generation of mullite, etc.), resulting in product shrinkage properties, also divided into line shrinkage and body shrinkage of two kinds. Too much firing shrinkage will easily lead to cracking of the blank.

Fire resistance

Refractoriness is the ability of kaolin to resist high temperature without melting. The refractoriness of pure kaolin is generally about ℃, when the content of hydrous mica and feldspar is more, and the content of potassium, sodium and iron is high, the refractoriness decreases, and the refractoriness of kaolin is at least not less than ℃. The industrial sector stipulates that the R2O content of refractories is less than 1.5-2%, and Fe2O3 is less than 3%.

Suspension

Suspension and dispersion refers to the performance of kaolin dispersed in water difficult to precipitate. Also known as anti-flocculation. Generally the finer the particle size, the better the suspension. Kaolin used in the enamel industry requires good suspensibility. Generally according to the sample dispersed in water after a certain period of time to determine the settling speed of its suspension performance is good or bad.

Selectivity

Selectability refers to the kaolin ore by hand selection, mechanical processing and chemical treatment, in order to remove harmful impurities, so that the quality of industrial requirements of the performance. The selectivity of kaolin depends on the mineral composition of the harmful impurities, the state of existence, the size of the particles and so on.

Adsorption

Kaolin has the performance of adsorbing various ions and impurities from the surrounding medium, and has a weak ion exchange property in solution. The advantages and disadvantages of these properties mainly depend on the main mineral composition of kaolin.

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, the use of this property can be used to make high-frequency ceramics, radio ceramics. The level of electrical insulation performance can be measured by its resistance to electrical breakdown.

Among the above performance indicators, particle size distribution is a very important aspect. Ultrafine kaolin in today's industry is more and more widely used, the higher the fineness, the more significant effect.

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