Ultra-fine Calcium Carbonate Composites

Calcium carbonate, a kind of inorganic powder material with the function of increasing volume and decreasing cost, is widely used as the filler in many industries such as rubber, plastic, paper, paint, and so on. Taking account of its poor acid-resistance, the surface modification is needed for improving its performance.In this paper, the self-suction stirred carbonzation method was used to form the calcium carbonate nuclei and then coat silicon dioxide on the surface of CaCO₃. The microcosmic process of carbonization and its optimum technologic conditions were studied. The results showed that, under 20℃, the Ca(OH)₂ slurry of 7%(w_B) was carbonized with the mixed gas of CO₂ (25³0% ((φ_B)) and air. During this course, the ultra-fine crystal nucleus of calcium carbonate were formed. Then, sodium silicate aqueous solution was added, keeping SiO₂/CaCO₃ in theory to be 5%(w_B). The carbonization reaction didn't complete until slurry's pH value was steady. After filtrating, washing and drying, the CaCO₃/SiO₂ compounding product could be obtained, and its acid-resistance and other performances were improved.The properties of CaCO₃/SiO₂ composite products made in different conditions, such as acid-resistance, whiteness, DBP absorption number, etc, were measured. The composite particles made in the optimum conditions were analyzed by scanning electron microscope(SEM), infrared ray analysis(IR), x-ray diffraction (XRD) and differentia thermal analysis(DTA). The results showed that the CaCO₃/SiO₂ composite particle was calcite and belonged to hexagonal. The silicon dioxide was bonded on the surface of CaCO₃ in amorphous configuration. The CaCO₃/SiO₂ composite particles were smaller and more uniform, and its acid-resistance was better than that of uncoated CaCO₃ particles.