Preparation Of Calcium Carbonate Nanoparticles In A Surface-Aerated Stirred Tank

In the past few years,gas-liquid carbonation method has been the most popular way to prepare calcium carbonate nanoparticles.As the reaction rate between carbon dioxide and calcium hydroxide is very fast,it is important to intensify the gas-liquid mass transfer of the process.In recent years,to enhance the mixing and mass transfer performance,many reactors such as high-gravity reactor,microstructure reactor,membrane reactor,etc.have been employed to prepare CaCO₃ nanoparticles.However,there still exists numerous margin for the improvement of the process.In addition,blockage of the equipment,e.g.the orifices on the gas-sparger of the stirred tanks,the micro-channel of the microreactors,still creates big obstacles to the practical applications.Thus,developing a low-cost carbonation process for easy operation control is still the demands of industry.In this study,a surface aerated stirred tank equipped with a novel long-short blades?LSB?impeller was used to prepare calcium carbonate nanoparticles.The change of pH and electrical conductivity were detected.The effect of the surfactant and dispersant on the dispersion of nano calcium carbonate was studied.Then,the effect of different operation conditions including temperature,the partial pressure of carbon dioxide,rotating speed of the impeller,and the concentration of calcium hydroxide on the mean particle size was discussed.The prepared nanoparticles were characterized by TEM,XRD and SEM.The result shows that the calcite nanoparticles are obtained,the average particle size of prepared nano CaCO₃ is small and it disperses well.The mean particle size decreases when the rotating speed increases,whereas it increases when the temperature increases.When CO₂ partial pressure increases or the concentration of Ca?OH?₂ increases,the mean particle size decreases firstly and then increases.And the optimal operation conditions are as follows:the temperature is20?,the rotating speed of the impeller is 300 rpm,the CO₂ partial pressure is 0.25 and the concentration of calcium hydroxide is 4wt%.And the mass transfer of the precipitation process has also been analyzed.The gas-liquid interfacial area was measured by using the transport-reaction equations.In the surface aerated stirred tank,the effective contact area of effective contact area of gas-liquid interface are only determined by rotation rate of impeller when the temperature and total pressure are constant.The effective contact area of effective contact area of gas-liquid interface can be calculated out:???Compared with other carbonation methods,this work provides an easy-operation and high-efficient method for preparing uniform CaCO₃ nanoparticles with low risk of equipment blockage.The effect of different post-processing methods on the dispersion and particle size of nano calcium carbonate powder are studied in this paper.Apart from it,a new drying method called absorbent resin drying was studied to solve the problem that nanomaterials are tend to aggregate in the drying process.And the effect of different operation conditions including water adsorption time,pressure of unit area,the amount of water absorbent resin,and the area of absorbent bag on the water absorption was discussed.Then the prepared nanoparticles were characterized by XRD and SEM.The conclusions were drawn out as follows:Nano calcium carbonation powder materials prepared by different post-processing technique has different mean particle size and dispersibility.The new drying method can shorten the time required for freeze-drying and reduces energy consumption.The purity of the prepared nanopowder material is not affected and the dispersion is relatively good.The mean particle size decreases when the rotating speed increases.The water removal rate increases when the area of absorbent bag increases.When water adsorption time,pressure of unit area or the amount of water absorbent resin increases,the water removal rate increases firstly and then tend to be gently.And the optimal operation conditions are as follows:the water adsorption time is 3 h,the pressure of unit area is 4 g/cm² and the amount of water absorbent resin is 0.08 g/cm².