Kaolinite Activation By Citric Acid And Its Application In Ceramics Manufacture

Kaolinite is an important raw material in the manufacture of traditional ceramic. Becauseof large bond energy of Si-O bonds, the sintering temperature of ceramics is very high, whichconsumes a great deal of enegy and makes a heavy burden on the environment.In this work, kaolinite and ceramic body are treated with citric acid. Dissolution behaviorof Si and framework of acid-activated kaolinite are characterized by XRF, TG-DSC, XRDand SEM techniques, and phase constitution and microstructure changes in the sinteringprocess are studied in order to explore the feasibility of decreasing sintering temperature bythe activation of citric acid, a low-molecular-weight organic acid.The results indicate that the silicon of kaolinite can be dissolved after being treated withcitric acid because of its proton attack and complexation interaction, and both the SiO₂contentand Al₂O₃content in the treated kaolinite decrease and continue to decrease with the increaseof citric acid concentration. The particle diameter of activated kaolinite which is treated with1M citric decreases from3.184μm to2.974μm and the surface area increases from8.18m²/gto31.2m²/g. So the reaction between citric acid and kaolinite is firstly exfoliation of layers,and then part of aluminium-oxygen octahedron and silicon-oxygen tetrahedron latticestructure deform or disintegrate, which promotes the cleavage of Si-O bonds and Al-O bondsin kaolinite. The citric acid-activated kaolinite becomes rather reactive. Therefore, sinteringactivated kaolinite forms mullite phase at980℃, and the needle-shaped mullite has highaspect ratio and its grain size reaches nanometer scale. Nanometer mullite and micrometerquartz mutually compose network structure, enhancing the property of framework.In addition, because the main composition of ceramic body is kaolinite, ceramic body istreated with citric acid similarly. Then using activated ceramic body as sintering additive istested. Compared with the raw ceramic body sintered at1350℃, when the content of additiveis2%and sintering temperature is1280℃, the quartz phase disappears and the mullite phasehas good crystallinity and distribution, and the crystal phase structure, microstructure,mechamical property and ceramic-forming property are better, which indicates the sinteringtemperature can be reduced by70℃. Therefore, citric acid activated ceramic body can be used as sintering additive applied in ceramic manufacture.All these results indicate that citric acid has evidently influence on kaolinite and ceramicbody, promotes earlier formation and growth of mullite phase, decreases the sinteringtemperature, and realizes saving energy and reducing consumption.