Kaolin is a kind of clay and clay rock dominated by clay minerals of the kaolinite group. It is a non-metallic mineral and is named after Gaoling Village, Jingde Town, Jiangxi Province. Kaolin is divided into non-coal construction kaolin and coal series kaolin (rock). At present, most of the kaolin produced in the world belongs to the former.
As an important mineral raw material, kaolin is widely used in many fields. In the ceramic industry, kaolin is mainly used as a ceramic raw material for making various ceramics; in the refractory and cement industries, higher grade kaolin can be used to make optical glass, glass fiber crucibles and laboratory crucibles; kaolin with lower purity It can be used to make refractory materials such as refractory bricks and refractory mud; in the paper industry, kaolin is used as filler and coating for paper; in the rubber industry, kaolin is used as a reinforcing agent and filler.
Many application fields have special requirements on the surface or interface properties of kaolin. In order to meet the application requirements, it must be surface modified. Surface modification is one of the very important deep processing modification methods of kaolin. It refers to the physical, chemical or mechanical treatment of the surface of kaolin according to the needs of the application, so as to improve the whiteness, brightness, surface activity or improve the polymerization of kaolin. Compatibility and other purposes.
Surface modification of kaolin
There are mainly the following methods for surface modification of kaolin: calcination, coupling agent, adsorption, and surface coating.
Calcined modification
Calcining modification refers to calcining kaolin at high temperature to remove part or all of the hydroxyl groups on the surface of kaolin to obtain special surface properties, which makes the crystal structure of kaolin change, from ordered sheet crystal structure to disordered kaolin .
When calcining kaolin, attention should be paid to the selection of temperature. When calcined at a lower temperature, the activity of kaolin is greater; when calcined at a higher temperature, aluminum spinel can be formed, and mullite will be produced at a certain temperature. The activity is small and cannot meet the needs of some polymer material products.
Surface coating method modification
The surface coating method uses physical adsorption or chemical adsorption to coat a layer of organic matter on the surface of kaolin to achieve the purpose of modification. Commonly used modifiers are stearic acid, glyceryl trimethacrylate, trimethoxypropane triglycidyl ether, low molecular polyethylene wax and so on.
The advantage of this method is that after kaolin is modified, the stability of the material structure and catalyst activity can be enhanced, the degree of powder aggregation is weakened, and the dispersion and fluidity are improved.
Coupling agent modification
Coupling agent treatment uses the interaction between the active groups on the surface of kaolin and the coupling agent to achieve the purpose of changing the surface properties of kaolin. Coupling agent treatment usually has two treatment methods: wet treatment and dry treatment. Wet processing is to immerse the kaolin powder in a solution with a coupling agent, act at a certain temperature, and then separate the solvent from the kaolin powder, and then dry the powder. Compared with dry processing, wet processing has the advantage of uniform mixing of powder and coupling agent, but wet processing has more complicated processing technology, large solvent loss and high cost.
Dry processing is to put the kaolin powder into a high-speed stirring mixer, stir and dry at a certain temperature, slowly add the solvent and auxiliary agent dissolved in the coupling agent, and after a certain period of stirring treatment, it can be prepared Obtain surface modified kaolin filler.
Modification by intercalation
The intercalation modification method uses the weak bonding force between the layers of the layered structure powder particle crystal layer or the interlayer contains exchangeable cations, and uses chemical reactions or ion exchange methods to change the interlayer and interface properties of the powder.
Liquid-phase intercalation method is a commonly used method of kaolin intercalation modification, and its application range is relatively wide. The reaction of the intercalant in the emulsion or solution state is a manifestation of the liquid phase intercalation method.
The evaporation solvent intercalation method is a kind of liquid phase intercalation, and its principle is: firstly the small molecules evaporate the solvent to complete the concentration and mixing process, and then enter the kaolin interlayer to continue the intercalation process.
The principle of the mechanochemical intercalation method is that mechanical energy is applied to the intercalation agent by the action of mechanical grinding, stirring, shearing and suction filtration to induce physical or chemical reactions, and then The interaction between kaolin and intercalation agent promotes the smooth realization of kaolin intercalation modification.
Conclusion
Starting from the 21st century, the organic polymer product industry has entered a stage of industrial transformation and opening. Transition from a quantity-oriented type to a quality, variety, and benefit-oriented industrial structure in order to achieve the goal of industrial opening up.
For this reason, the kaolin industry must also face the reality. It should focus on the development direction of the organic polymer product industry and closely integrate with the production technology of its products, so that the surface modification of kaolin should be high-quality, high-tech, high-value-added and multi-functional. Sexual transformation improves the surface modification of calcined kaolin to a higher level.
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