| Precipitated calcium carbonate(PCC)is used in various industrial fields as additives to medicines,foods,papers,plastics,printing ink,etc.And it has three kinds of crystal polymorphs:calcite,aragonite,and vaterite,with hexagonal,orthorhombic and rhombohedral structures,respectively.They can be transformed into each other under certain conditions.In nature,control of the polymorphous structure is very precise,however,polymorphic control of the crystals is very difficult in industrial processes,which the synthesized route used is bubbling of carbon dioxide through a calcium hydroxide slurry,due to the exist of high content impurity ions such as Mg2+ and Al3+ in the circulating water.Therefore,it is very important to study the effect of impurity ions on the preparation of PCC and to eliminate its effect in theory and industrial area.This article focuses on the effect of Mg2+ and Al3+ on the morphology,polymorphs and particle size distribution of PCC with a simulation study method by the bubbling carbonation route.The analysis results showed that,Mg2+ exerted a significant influence on PCC precipitation and can either be incorporated within the calcite lattice when the Mg/Ca molar ratio in solution was low,or induce aragonite precipitation when the magnesium concentration was sufficiently high,and it was also worthy to note that the carbonation temperature and concentration of calcium hydroxide slurry affected the influence degree of Mg2+.For example,the aragonite has been formed at Mg/Ca molar ratio 0.6 under the condition of 80 ? and 0.325mol/L.While Al3+ has no effect on the polymorphs of PCC as the carbonation temperature increased with low Al/Ca molar ratio(0?0.15),but the morphology and particle size distribution of PCC were changed.The crystal orientation agents of sugars,alcohols,organic acids,complexing agents,salts,organic polymers and surfactants were used to eliminate the effect of Mg2+ during the PCC preparation.The results showed that,the types,the molecular structure and the functional group of crystal orientation agent were important factors to determine its ability to eliminate the effect of Mg2+.And the monosaccharides,straight-chain organic acids,complexing agents,chelate salts,acidic copolymer with low molecular weight,the anionic,amino acid-type and certain zwitterionic surfactants could eliminate the effect of Mg2+.The effective crystal orientation agents were glucose,EDTA-2Na,citric acid and sodium polyphosphate,and the appropriate added amount of them was 1.5wt%,respectively.However,the mechanism of Mg2+ influences the crystallization behavior has not been clarified sufficiently.Especially the mechanism of the crystal orientation agent effect is obscure.In this article the crystallization mechanism under different conditions was investigated lastly through the analysis of samples generated at different stages of the carbonization process by XRD,FE-SEM and FT-IR,etc.The results showed that,Mg2+ induced the formation of aragonite by adsorbing onto the surface of growing calcite crystals to inhibit the growth of the calcite,on the other hand,the dehydration of the Mg2+ ions prior to reduce the degree of supersaturation,thus contributing to the formation of aragonite nuclei.The mechanism of glucose eliminating was that the electrostatic interaction between the hydroxyl of glucose and Ca2+ and the adsorption of the glucose molecules on the surface of the calcite nuclei.The mechanism of EDTA-2Na and sodium polyphosphate eliminating was both due to their strong complexing abilities.While the mechanism of citric acid to regulate calcite calcium carbonate nucleation and growth mainly through two pathways,one was that the complexation occurred between citric acid and Ca2+,which promoted the calcite nucleation with large amount,the other was that citric acid molecules adsorbed on the surface of calcite nuclei and inhibited the growth of the calcite.In the carbonization process,there had no interactions between citric acid and Mg2+ directly. |
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