In the application process of kaolin, modification, as an important deep processing method, is based on kaolin active groups (including aluminum alcohol group, silanol functional group, etc.), and changes the process characteristics of kaolin through mechanical, physical and chemical methods to meet its application requirements in various fields and industries.
Kaolin is a typical layered aluminosilicate mineral that is connected by hydrogen bonds. Through some special methods, certain substances can be inserted into the interlayer void by overcoming the interlaminar hydrogen bond, and the layer spacing can be increased without destroying the layered structure of kaolin. Due to the strong hydrogen bonding between kaolin layers and no replaceable ions, there are not many small organic molecules that can be directly interpolated, mainly including dimethyl sulfoxide, hydrazine, formamide, acetamide, potassium acetate, etc.
In addition, although some molecules cannot directly enter the interlayer, they can indirectly enter the interlayer of kaolin through substitution and entrainment, such as methanol, benzamide, fatty acid salt, etc. Common intercalation methods include: liquid phase intercalation, evaporative solution intercalation, mechanical chemical intercalation, and some new microwave radiation intercalation and ultrasonic intercalation, etc., usually, the intercalation modification method can expand the layer spacing of kaolin from 7.2Å to more than 10Å.
Calcination is a common method of kaolin processing, different purposes of use of kaolin calcination temperature is different, calcination should be medium and low temperature, that is, 450~925 °C, on the one hand to achieve kaolin dehydroxylation, on the one hand, to make kaolin maintain high activity metakaolin and avoid the transformation to spinel and mullite. At the same time, medium and low temperature calcination can also increase the pore size, pore volume and specific surface area of kaolin, which is conducive to adsorption. The synthesis of molecular sieve and aluminum salt chemical industry, the metakaolin generated by low temperature calcination has high activity, which is conducive to the synthesis of molecular sieve of aluminosilicate, and the calcination temperature is about 700 °C; PVC cable material batching, increase kaolin porosity, enhance the electrical insulation performance of composite materials, calcination temperature < 850 °C; Papermaking filler and coating add components to remove impurities carbon in kaolin, increase whiteness, increase porosity and enhance its oil absorption performance, calcination temperature of about 1000 °C; Refractory filler, FRP reinforced filler, ceramic kiln furniture and high-grade ceramic blanks ingredients and precision casting models, high temperature after the transition to stable mullite, calcination temperature 1300~1525 °C.
Generally divided into acid modification and alkali modification, the function of acid modified kaolin is mainly to dissolve part of Al in metakaolin, and then adsorb to the surface of the residue, thereby increasing the specific surface area and forming a new active site. Neither kaolin nor calcined kaolin contain B acid and L acid center, while acid-modified kaolin contains L acid center but does not contain B acid center and therefore high activity. The function of alkali-modified kaolin is mainly to dissolve part of Si in calcined kaolin, which is more difficult for Al to dissolve. Kaolin studies have shown that when reacting with NaOH solution, soluble Si and Al are first leached, and then soluble ions also react to form precipitates such as nepheline and sodalite. Different from acid modification, the specific surface area of the alkali modified product is reduced and the number of acid centers is less than that of the acid modified product, but the acid center strength is higher than that of the acid modified product. The research on acid-base modification to improve the high temperature adsorption performance of kaolin is still in the initial stage, and there is a lack of extensive research on the modification process and its parameters, and it is necessary to deeply explore the influence mechanism of modification on adsorption to provide theoretical guidance for the modification method.
The adsorption capacity of kaolin polymerized hydroxyferric modified kaolin is much greater than that of the original kaolin. The adsorption temperature has a greater effect on the polymeric hydroxyfer-modified kaolin, and when the temperature is higher, the adsorption capacity will gradually increase. In the polyhydroxyiron solution, the interlayer aluminum in kaolin has ion exchange and isocrystalline substitution with the iron in the solution, and under the influence of sodium carbonate, the aluminum in kaolin will be removed in large quantities, thereby increasing the silicon content and iron content in kaolin, thereby greatly improving the adsorption performance of polyhydroxyiron modified kaolin.
In the process of compounding organic substances and inorganic substances, coupling agent materials are usually used as coupling materials between the two, and the surface of kaolin substances can react with coupling agents, and kaolin materials can have better compatibility with organic phases after coupling agent modification. The more commonly used coupling agents are silane coupling agents and titanate coupling agents. One end of the coupling agent can chemically bind to the Si-O and Al-O bonds in kaolin, while the other end is exposed to the outside, which ensures that the surface structure of kaolin has better compatibility with organic matter. However, in the current research on the modification of kaolin by coupling agents, only the modification mechanism of silane coupling agent and titanate coupling agent is clear, and the modification mechanism of some other coupling agents is still not in-depth enough.