Synthesis of Calcium Carbonate by Semicontinuous Carbonation Method in the Presence of Dextrans

Precipitated calcium carbonate (PCC) was prepared by means of semicontinuous carbonation of Ca(OH)(2) suspension, at 35 and 45 degrees C and in the presence of non-ionic dextran and dextran sulfate. The carbonation process was regulated at different predetermined values of electrical conductivity, that corresponded to different concentrations of dissolved Ca(OH)(2): 0.5 mS cm(-1) < kappa(25) < 5.0 mS cm(-1). The results of physical-chemical characterization of the product showed that calcite was the only polymorphic modification obtained in the whole range of experimental conditions investigated. In addition, it was found that morphology, crystal size distribution and specific surface area of PCC strongly depended on the electrical conductivity / concentration of dissolved Ca(OH)(2), at which the process was performed: predominantly scalenohedral crystals of higher surface area (about 5 m(2) g(-1)) were produced at higher electrical conductivity, while at lower electrical conductivity predominantly rhomohedral calcite crystals of relatively low specific surface area were obtained. In the system of the highest electrical conductivity, kappa(25) = 5.0 mS cm(-1), and at 35 degrees C, the addition of non-ionic dextran significantly influenced the process by preventing the regulation. The crystals that appeared were in the form of irregular aggregates of high specific surface area, S approximate to 29 m(2) g(-1). FT-IR and TG analyses indicated that the non-ionic dextran was adsorbed onto the calcite surface, most probably by relatively strong and specific interactions between oxygen from the hydroxyl groups of dextran molecules and calcium ions from the crystal surface. On the contrary, the anionic dextran (dextran sulfate) exerted minor effects in the course of semicontinuous carbonation process and in the properties of the final product, PCC. However, the analysis of the precipitate indicated that dextran sulfate was adsorbed at the surfaces, most probably by the weak and non-specific electrostatic interactions.