Research On Preparation And Surface Modification Of Nanometer Calcium Carbonate

Calcium carbonate is being used more and more widely as an important chemical material. In this article, technical parameters in the process of the preparation, mass transfer mechanism in the carbonation process and the surface modification of the nanometer calcium carbonate were studied because of the flaws existing in the current carbonation process.In this article, a stirrer and a gas distributor were introduced into the traditional bubble tower to improve the mass transfer process. Effects of experimental factors on the particle size and morphology of synthesized precipitated nano-CaCO₃ in the carbonation process were investigated, which included reaction temperature, concentration of Ca（OH）₂, flow rate of CO₂, stirring speed and shape control agents. The CaCO₃ crystal samples were characterized by SEM and XRD. The results showed that the concentration of Ca（OH）₂ and the shape control agent determined the morphology of the nano-CaCO₃ crystal, and the flow rate of CO₂, stirring speed and reaction temperature control the diameter of nano-CaCO₃ particle. The cubic nano-CaCO₃ particulates with diameter of 30⁸0nm were prepared, and the optimal operation parameters for preparing calium carbonate of cube form is determined as following: the reaction temperature is 15³0℃, the concentration of Ca（OH）₂ is 6%¹1%, the flow rate of CO₂ is 0.08⁰.15 L/min and the stirring speed is 200⁴00r/min. These results may be useful for industrial carbonation process to improve the quality of nano-CaCO₃.The unit processes such as the dissolution of calcium hydroxyl, the absorption of carbon dioxide, the crystallization of calcium carbonate were systematically investigated in the paper. A reaction and mass transfer mechanism has been put forward on the controlled growth of calcium carbonate nanocrystal. when t<θc （θc is the time while the dissolution rate of calcium hydroxyl is equal to the production rate of calcium carbonate）, the calcium carbonate crystallization was the rate-controlling step of the entire carbonation crystallization. when t>θc, the dissolution of calcium hydroxyl was the rate-controlling step of the entire carbonation crystallization. The carbonization process which has transferred the CaCO₃ crystallization controlled to Ca（OH）₂ dissolution controlled. With the ZnSO₄ added the transfermation can be prolonged, This is used for keeping a high supersaturation and used for the nucleation which make the size of nano-CaCO3 smaller; The low supersaturation in the later period conducive to crystal growth, the formation of calcium carbonate crystals with regular morphology. During the entire carbonation, the mass transfer resistance of carbon dioxide can be neglected. According to the mass transfer mechanism to optimize the carbonization process by feeding many times, and prepare the smaller nano-CaCO₃.The surface of nanometer calcium carbonate powder was modified with sodium sterrate by the active rate. The optimum conditions for surface modification include sodium sterrate dosage of 3%, slurry density of 10%, modification temperature of 80℃, modification time of 60 minutes. The active rate of the modified nanometer calcium carbonate is over 96%. IR and TG analyses showed that the sodium sterrate is attached to calcium carbonate by chemical bond.