Preparation Of Functionalized Calcium Carbonate From Calcium Nitrate By-product Of Nitrophosphate Fertilizer And Its Applications

The development of the nitrophosphate fertilizer industry has made great contributions to agricultural production.However,the nitrophosphate fertilizer process produces a large amount of by-product Ca（NO₃）₂,which causes resource waste and environmental pollution.The development of the nitrophosphate fertilizer industry could be promoted by realizing the resource utilization of Ca（NO₃）₂.Ca CO₃is widely used in environmental treatment and polymer reinforcement because of its excellent properties.It will be an effective way to realize the comprehensive utilization of Ca（NO₃）₂if Ca（NO₃）₂is converted into Ca CO₃with higher value.Ca CO₃particles directly prepared by the precipitation method often could not meet the application requirements.Therefore,spherical calcite mesocrystals with high specific surface area and Ca CO₃whiskers with high aspect ratio and fine-diameter were prepared by controling the addition of additives in the production process of the precipitation method and the improved recrystallization method based on the direct precipitation method,respectively.The synthesized products were used to the adsorption of Congo red and the toughening polyvinyl alcohol（PVA）.The dispersant polyvinylpyrrolidone（PVP）and the crystal control agent citric acid were added to synergistically control the morphology of the product,and spherical calcite mesocrystals with high specific surface area were prepared by direct precipitation method.The synthesis conditions were optimized from the aspects of dispersant,crystal control agent and reaction temperature,and the formation mechanism of the calcite mesocrystals was illustrated.The synthesized spherical calcite mesocrystals having particle size of 5μm exhibit a specific surface area of 84.44 m²/g,which is higher than that in most studies.It is noted that they feature a specific surface area similar to that of certain vaterite,meanwhile overcome the intrinsic instability of the vaterite.The negatively charged carboxyl groups in citric acid are preferentially adsorbed on the（001）surface of the calcite crystal nucleus composed of only calcium ions,inhibiting the growth of the crystal along the c-axis direction,resulting in the formation of the nanosheet building blocks.Under the dispersing effect of PVP,the nanosheet assemble into spherical calcite mesocrystals structure with good dispersion.The maximum adsorption capacity of the mesocrystals for Congo red is23.04 mg/g.The comprehensive utilization of the mother liquor after synthesizing calcite mesocrystals was designed.The vaterite was prepared by rapidly pouring K₂CO₃solution into Ca（NO₃）₂solution,and then the synthesized vaterite is converted into Ca CO₃whiskers with high aspect ratio and fine-diameter in an aqueous solution at 80℃ by recrystallization.The effects of magnesium ion addition,reaction time and solid-liquid ratio on the synthesized whiskers are investigated.The aspect ratio and diameter of the prepared whiskers are compared with that synthesized by the conventional direct precipitation method.The formation mechanism of Ca CO₃whiskers with high aspect ratio and fine-diameter is studied.The slow dissolution of vaterite in water keeps the calcium ions and carbonate ions in the system at a low supersaturation,which is conducive to the preferential growth of the nuclei along c-axis in the sufficient space.Ca CO₃whiskers were mixed with PVA to form PVA/Ca CO₃composite films.The effects of Ca CO₃whiskers amount on the mechanical properties of the synthesized PVA/Ca CO₃composite films were investigated.When the amount of Ca CO₃was 2 wt.%,the synthesized PVA/Ca CO₃composite film exhibits the best mechanical performances.The elongation at break and tensile strength of the synthesized PVA/Ca CO₃composite film are increased by 244%and 12%,respectively,and the water absorption properties is increased by 10% compared with pure PVA.