Calcined kaolin is a kind of mineral material which is made of natural kaolin which is calcined and dehydrated to remove the volatile matter, which is high in white degree, which is larger than surface area, insulator and thermal stability. Calcined kaolin excellent chemical corrosion resistance, solvent resistance, acid and alkali, and good reinforcing and processing performance, widely used in paint, coating, paper making, rubber and plastic products, cables, ceramics and other industries.

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Calcining the calcining process of kaolin

In the calcining process, the calcining products are different and different from the calcining temperature, resulting in different application direction and application fields. Therefore, calcining temperature is a key factor in producing different kaolin products.

calcined at low temperature (600-1000 ℃), its high activity of kaolin products, used in the synthesis of molecular sieves, aluminum salt chemical industry and functional material of plastic, rubber.

The calcination temperature (1000 ℃ — 1000 ℃), its kaolin products high whiteness, opacity, can be used in papermaking, coating industry, structural pigment replace titanium dioxide. The paper can increase the pore volume and thickness of the paper coating, reducing the brightness and the loss of opacity when pressed.

High temperature calcination (above 1200 ℃), Its kaolin products are mullite and cordierite, which are used to produce sand, high grade refractories and special ceramics. 

According to the different calcining temperature, calcined kaolin can be divided into incomplete calcining kaolin and calcining kaolin.

Incomplete calcined kaolin: calcination temperature is 650-750 ℃, dehydroxylation, density of 2.4 2.5 g/cm ⊃ 3; Can be used for paper filling and coating.

Completely calcined kaolin: Calcination temperature is 1000-1050 ℃, under the electron microscope has been a small amount of mullite crystal, the density is about 2.7 g/cm ⊃ 3;     Whiteness is above 90%. The good performance of calcined kaolin is very important for the production of coated paper, especially for low quantitative coated paper, and its dosage is also expanded in the high-grade cardboard coating.

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Surface modification of calcined kaolin

The surface modification of calcined kaolin is based on the need of the application to change the original physical and chemical properties of the surface.

Mechanism of surface modification, surface chemical methods, the molecules of the organic functional groups on the surface of kaolin particles adsorption or chemical reaction, the particle surface were coated, make the surface of kaolin organic.

Surface modification:

1、To change the properties of kaolin powder interface, improve the affinity of calcined kaolin and organic polymer materials;

2、It can improve the dispersion of organic polymer materials, enhance the various properties of products, and play a functional role.

3、Increase the filling quantity of calcining kaolin, improve product class and reduce the cost of products.

The surface modification of calcined kaolin is a very important and deep processing method, which is also an effective way to expand the application of calcined kaolin and improve the quality of organic polymer products.

The selection and application of surface modifiers are the key technologies of surface modification of kaolin, and the process and equipment of surface modification are also important parts of surface modification techniques.

(1) selection of surface modifier for calcining kaolin

When choosing the surface modification agent, should be based on kaolin needed to fill in to the formulation, processing technology of the organic polymer materials and products such as technical performance requirements, choose different modifying agent and additives.

The surface modification of calcined kaolin is mainly used to select silane coupling agent. For the special requirement of high polymer products, two or more kinds of modifiers are selected, and the surface modifiers of the composite are prepared.

According to the performance of the material and technical requirements, etc., to join a certain amount of additives or other modifier, respectively, with silane coupling agent of calcined kaolin for effective surface modification, the product performance and quality of calcined kaolin can achieve higher requirements.

The dosage of surface modifier can be calculated according to the modified amount of calcined kaolin, the surface area and the minimum covering area. The general experience is about 0.3% – 1.2%.

(2) calcining kaolin surface modification process

First, the calcined kaolin is heated to react with silane coupling agent, and it is required to maintain a certain temperature in the dynamic condition of kaolin. Only in this way, excess water can be removed from the reaction process and after the reaction, so as to combine the coupling agent with kaolin to form a stable and strong covalent bond. The process of surface modification is determined according to the mechanism of surface modification.

In the process of surface modification, calcining kaolin is to be reacted at certain temperature.

Kaolin surface modification process: first of all to calcined kaolin for dynamic when heated to a certain temperature (100 ℃), under its high-speed dynamic conditions, adding hydrolysis of silane coupling agent or compound modifier, best with atomization method to join. In the process of coupling agent and kaoline reaction temperature should be kept within 120 ℃, In high – speed dynamic conditions, the duration should be about 3-5 minutes.

In addition, surface modification agent and kaoline reaction process, often produce false knot particles, the product quality is the impact, so must be effective classification, only in this way can fully guarantee the quality of products.

(3) calcining kaolin surface modification equipment

The surface modification equipment of calcining kaolin has high speed kneading machine, continuous modification machine and high frequency vibration grinding machine.

High-speed kneading machine modification: the calcine kaolin powder into the kneading machine, under the high speed rotation of agitating vane, the high speed rotating flow impact on the baffle plate, change the direction of material flow, material can be rotary turning up and down, to strengthen the material mixing and dispersing effect, after the friction heat, add fertilizer, adjust the apparent nature of calcine kaolin. When the temperature continues to rise, the modifier is added in atomization or drip-drip, and then the surface modification can be achieved by the process of high speed stirring.

Continuous modifier modification: the SLG three-cylinder continuous modification machine is to add the kaolin and the compound modifier to a certain proportion, and the screw feeding mode enters into the modified cavity. The kaolin can be heated in this section. After the material enters the modified cavity, it is spun by the rotating rotor, and the materials are fully dispersed and modified.

High frequency vibration grinding machine: the equipment is relying on the vibrator, the cylinder body medium produces high frequency small amplitude of vibration, due to the acceleration, vibration acceleration proportion force makes the impact of the grinding medium of tube to produce high strength and rotation motion, and the material can be rapid heating up, make the material in the strong vibration and certain temperature can effectively break up, mix quickly.

The method can be used to improve the dispersion and surface modification of the kaolin clay after it is burned, and it can be modified in the process of dispersive. First before use the machine to adjust medium grading, after the material input mill vibration friction temperature, compound modifier can add fluid of the grinding tube mouth with atomization way according to the flow of material to join than quantity, also can add in the upper and lower tube grinding fluid inlet, added additives, modifier and so on.

The modification effect of the equipment is not only good, but also can improve the processing efficiency of calcined kaolin and simplify the production process, which reduces the modification cost of kaolin and has good application value.

The selection of surface modification equipment should be based on the process determined by the surface modification mechanism of calcined kaolin.

No matter what surface modification equipment is used, surface modification mechanism and surface modification technology are required to meet the technical requirements of the modified process of calcined kaolin surface. Shall require the surface modification equipment can heated to above 100 ℃ in high-speed dynamic situations, and can eliminate water vapor, heat preservation, surface modification agent can add, have the function of classification, solve the problem of the surface modification of reunion, guarantee the quality of products.

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The problem of surface modification of calcining kaolin is noted

At present, the modification of calcine kaolin is mainly used in organic polymer products industry, the industry and the organic raw materials, formulation, processing technology and the requirements of technical performance, thus to calcine kaolin surface modification raises an important question, namely how to adapt to and meet their requirements, and can show the functionality of the modified kaolin.

Modified kaolin can play a functional role, first of all should be paid attention to all kinds of organic polymer products of raw materials and the technology of production process, whether can complement each other in the processing, phase cancellation, carefully analyse the quality and specifications of raw materials, formulation and process technology parameters and process conditions, such as design, these factors can affect the application effect of modified kaolin.To improve the surface modification of kaolin, it is necessary to understand the main technical requirements of organic polymer products, main raw materials and processing technology. Based on this, it is necessary to select the modifier, adjust the formulation of the modifier, and determine the reasonable processing technology .

The surface modification of kaolin is not only used in coupling agent .It is required to add a certain amount of other functional auxiliaries according to the quality of kaolin and the technical requirements of the organic polymer products. The performance and quality of modified kaolin is higher than that of the modified kaolin.