Effect of silicon on properties of causticized calcium carbonate produced in wheat straw soda pulping

Green liquor in the chemical recovery process of wheat straw pulping was treated with carbon dioxide to precipitate silicon. We report on the properties of causticized calcium carbonate (CCC) with different silicon contents, as well as its impact on paper quality when used as filler. The research results showed that silicon content had no significant effect on the crystal form of CCC, and all crystals were formed as calcite. Calcium silicate itself did not polymerize to form crystal in the course of causticization. Instead, silicon existed as a solid solution in CCC. The morphology of CCC changed gradually from amorphous to square with the decrease of silicon content. The brightness of CCC increased slightly and the average particle size tended to grow after removal of silicon. When 97% of silicon was removed and the average particle size reached 6.84 μm, the specific surface area and sedimentation volume decreased gradually as the desilication rate increased. When CCC was used as filler and silicon in CCC decreased, opacity remained nearly the same and brightness increased slightly; however, the Cobb value decreased significantly and the sizing efficiency obviously improved. When the silica content decreased to 3.2% the Cobb value was 27.7 g/m2, which reached the standard for fine paper sizing. The tear strength increased gradually, while the tensile strength decreased with the reduction of silicon.