Preparation And Characterization Of Both CaCO₃@SiO₂ And CaCO₃@BaSO₄

The application of calcium carbonate is limited by its poor acid resistance and unsatisfactory dispersity.Therefore,surface coating modification of calcium carbonate is carried out to improve its acid resistance,stability and dispersity and expand its application field.In this paper,ground calcium carbonate（GCC）,precipitated calcium carbonate（PCC）and nano-calcium carbonate（NPCC）are taken as the research objects.CaCO₃@SiO₂ was obtained by coating silicon dioxide with GCC and NPCC.The coating kinetics of GCC was investigated.The intermediate amplification test and 50,000 t/a industrial trial production of the coating for NPCC were carried out.PCC is coated with barium sulfate on its surface to obtain CaCO₃@BaSO₄ by a co-precipitation-precipitation transformation method.The main research work and conclusions are as follows:（1）GCC coated by Silicon dioxide（G-CaCO₃@SiO₂）:Using sodium silicate and sulfuric acid as raw materials,the surface of heavy calcium carbonate particles is coated with silicon dioxide by parallel flow method to obtain G-CaCO₃@SiO₂.Through single-factor experiments,the effects of seven single factors of calcium carbonate slurry solid content,temperature,coating amount,pH value,stirring speed,coating time,and aging time on the coating effect of heavy calcium carbonate were explored,and the best results were obtained.The coating conditions are as follows:the solid content of calcium carbonate slurry is 100g/L,the temperature is 90℃,the coating amount is 6%,pH=9.5,the stirring speed is 250r/min,the coating time is120min,and the aging time is 36h.The acid solubility of heavy calcium carbonate is 17.86%.The obtained coated samples were characterized by field emission scanning electron microscopy（SEM）,X-ray diffraction（XRD）,infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）,X-ray energy dispersive spectroscopy（EDS）,and transmission electron microscopy（TEM）analysis.The surface of the calcium carbonate is coated with an amorphous silica coating layer with a thickness of about 8 nm,the silica and the heavy calcium carbonate are combined through Si-O-Cabonds.The kinetic analysis of the coated GCC at different pH shows that the silica as the coating layer has a faster growth rate at pH=10,which is nearly double the difference from pH=7,indicating that the pH is significantly different from the two.The growth of the silicon oxide cladding layer has a greater impact.（2）NPCC coated by Silicon dioxide（nano-CaCO₃@SiO₂）:Using sodium silicate and sulfuric acid as raw materials,using countercurrent method,silicon dioxide was coated on the surface of nano calcium carbonate particles to obtain nano-CaCO₃@SiO₂.Through single factor experiment,the influences of acidification degree of nano-CaCO₃ suspension,water bath temperature,droplet acceleration rate of sulfuric acid,pH of solution at the end of reaction and rotational speed on the coating effect of CaCO₃ were explored respectively.Using SEM and XRD analysis,the best coating conditions are:the pH of NPCC slurry was 9.5,the water bath temperature was 90℃,the drop rate of sulfuric acid was 2m L/min,the pH of the solution was 7.5 at the end of the reaction,and the rotational speed was500r/min.FTIR,XPS,TEM and EDS analysis of the obtained NPCC coated samples show that the surface of the NPCC particles is covered with a silica coating layer with a thickness of about 5 nm.After repeating the best experimental conditions,after material and energy balance calculation,an intermediate scale-up test was carried out with 5kg NPCC slurry as raw material,and the prepared coating product was applied to silicone adhesive and soft PVC,and test the performance of the application product,compared with the blank NPCC preparation sample.The application results show that in the silicone rubber,the coated NPCC has a significant improvement in tensile strength and modulus compared with the samples before coating.In the soft PVC calendered film,the coated NPCC.It is better than uncoated nano calcium carbonate in terms of processing fluidity and color difference index.After the intermediate amplification test,the industrial trial production of50,000 t/a was also carried out.（3）PCC coated by Barium sulfate（CaCO₃@BaSO₄）:PCC particles were prepared by the double decomposition method.By co-precipitation-precipitation transformation method,CaCO₃@BaCO₃ was prepared first,and then barium sulfate coated precipitated calcium carbonate CaCO₃@BaSO₄ was prepared.Through single-factor experiments,the effects of CaCl₂ solution and Na₂CO₃ solution concentration,co-precipitation rate,rotational speed,co-precipitation starting point,co-precipitation temperature,and coating molar ratio on the coating effect of barium carbonate and calcium carbonate during the co-precipitation process were respectively explored.The optimal preparation conditions for the co-precipitation process are as follows:the concentration of CaCl₂ solution and Na₂CO₃solution is0.1 mol/L,the co-precipitation rate is 2 m L/min,the rotation speed is 800 r/min,and the starting point of co-precipitation is that the remaining percentage of calcium ions is 30%,the co-precipitation temperature is 30℃,and the coating molar ratio is n（CaCO₃）:n（BaSO₄）=10:1.Using the gas-solid conversion method,sulfur trioxide gas was introduced into the barium carbonate-coated PCC particles to prepare barium sulfate-coated precipitated calcium carbonate,and the samples were analyzed by FTIR,XPS,TEM,and EDS.Characterization shows that calcium carbonate is coated with a layer of barite barium sulfate coating of about 80nm.