Reuse of MgCl_2 as Crystal Controlling Agent in the Synthesis Process of Needle-like Calcium Carbonate

MgCl2 was used as crystal controlling agent for the synthesis of regular needle-like calcium carbonate with diameter of 2.5?5μm and aspect ratio of 10?20.Complexometric titrating method was employed to determine the contents of Ca2+ and Mg2+ in filtrated solution after carbonization and the fresh MgCl2 was added to balance the Mg2+ content as same as the raw solution.The morphology,particle size and polymorphs of calcium carbonate were characterized by SEM and XRD.The results show that the content of aragonite phase CaCO3 decreases with the recycle time increasing.When the content of aragonite phase CaCO3 is lower than 80%,the aspect ratio of need-like calcium carbonate decreases significantly companied with the production of calcite cubic CaCO3 particles,thus the filtrated MgCl2 solution is unsuitable to recycled anymore.The direct recycle filtrate can be used for only once while the reused time by the MgCl2 filtrate can reach 12 by optimizing the recycle process through the addition of Mg2+ and the justification of pH of the filtrate.