Controllable Preparation Of Functional Calcium Carbonate And Surface Modification

As an environmental-friendly inorganic material,calcium carbonate has been widely used in rubber,paper,paints,inks and other departments,since its wide variety of sources,low price and high brightness.Ordinary calcium carbonate only had the role of increasing volume and reducing cost,when it used as filler.However,functional calcium carbonate can also improve the performances of the compound materials.Controllable preparation technologiesof functional calcium carbonate products,which have specific morphology and size,have become a hot research topic in the field of materials.In this paper,an improved carbonation method has been adopted to synthesizing the functional calcium carbonate,the cube nanometer calcium carbonate and calcium carbonate whiskers.In addition,surface modification of functional calcium carbonate has been investigated on the basis of preparing process.(NaPO3)6 additive has been adopted to preparing cubic nanometer calcium carbonate as controlling agent.Many operational conditions,which have significant effects on properties of product,were thoroughly investigated to achieve the optimum conditions,such as carbonation temperature,the initial concentration of calcium hydroxide,the injection flow rate of carbon dioxide,additive dosage and the stirring rate.The results revealed that Ca(OH)2 slurry concentration of 3.Owt.%,carbonization temperature of 10℃,the CO2 flow rate of 300 mL-min-1,stirring speed of 800 r-min-1,adding 2%controlling agent five minutes after reaction are beneficial to the formation of cubic nanometer calcium carbonate.The products with average particle size of 35 nm,size distribution of 20～80 nm,specific surface area of 62.8 m2.g-1 were successfully synthesized.The products were reproducible,and its performances have reached the national standard.Controlling agent(NaPO3)6 had an important influence on particle size and morphology of calcium carbonate products.Nevertheless,it did not affect polymorphs of products.Whether adding or not,the products are of calcite crystal only.(NaPO3)6 ionized into anions,and they selectively adsorbed on the surface of crystalline grain,resulted in the differences between the grains of adsorption and exclusion.It is the differences that affected crystalline grain growth rate in different crystal surface,thus to achieve the role of morphology control.With MgCl2 as controlling agent,calcium carbonate whiskers with high aspect ratio has been synthesized by only one carbonization step.Without co-equilibrium by heating MgC12 and Ca(OH)2 together,the carbonization reaction conducted directly after adding MgC12 solution into Ca(OH)2 slurry.Calcium carbonate whiskers with the diameter of 1.0～1.6 μm and length of 25-35μm have been prepared at the condition carbonization temperature of 75℃,magnesium to calcium R of 1.0,the CO2 flow rate of 100 mL-min"1,stirring speed of 800 r-min-1.The aspect ratio of whisker was up to 26.Aragonite content of the whiskers is 95.7%.In addition,the products also contained small amounts of calcite and magnesium,the mass percent of magnesium is about 1/50 of calcium.The whiskers were uniformity overall without cracks and other defects.Wet surface modification of calcium carbonate was carried out with different surfactants,modification effects were characterized with activation index and oil absorption value.The result showed that sodium hydroxide co-modification with stearic acid was better than other surfactants.Activated calcium carbonate with activation index of 99.9%and oil absorption value of 45g/100g were obtained at the optimum conditions stearic acid dosage of 2.5%,modification temperature of 85℃,slurry concentration of 8 wt.%,modification time of 60min,sodium hydroxide to stearic acid of 2:1,stirring speed of 800 r-min-1.FT-IR results showed that chemical reaction has conducted between stearic acid and calcium carbonate.And the carbon chain of stearic acid was successfully introduced to the surface of calcium carbonate,forming a strong chemical adsorption.SEM results revealed that agglomeration significantly improved after modification.And the dispersion of products became better.