(194b) Controlled Crystallization of Nesquehonite by the Reaction of MgCl2 with Na2CO3

The design and fabrication of inorganic materials with well controlled morphology and size have recently attracted a lot of interest. Nesquehonite is an important intermediate industrial raw material. But little attention is paid to the research of it. A facile preparation method of rod-like MgCO₃?3H₂O crystals with different size distributions without the introduction of additives has been proposed. The method is achieved via stirring-induced batch crystallization assisted by a certain aging time using MgCl₂ and Na₂CO₃ as raw material. The core of it is to separate nucleation and growth. The structure and morphology of the samples synthesized are examined using XRD and SEM. All experiments were taken at the molar ratio Mg²⁺:CO₃^2-=1:1.Amorphous precipitate is formed first by mixing magnesium chloride and sodium carbonate solutions. Under continuous stirring, it is gradually transformed into rod-like MgCO₃?3H₂O . For the case of 0.50M reactant concentration at 293.2K, it slightly decreases before 10min (S1?abbreviation of stage1) because of more generation of amorphous material. There is a stage (S2) during 10～30min that the concentration is almost unchanged. A rapid decline appears after 35min (S3) and achieves equilibrium after about 60min (S4) (Fig.1). Although the magnesium ions concentration between 10～30min remains constant, substantial changes has occurred?nucleation and growth of nesquehonite. From the view of microscope, many micro-scale rod-like crystals or aggregates appear at about 14^thmin. Amorphous material is sufficient to maintain the supersaturation during this period due to thermodynamic instability. At about 34^thmin, there is no longer adequate "magnesium source" provision and comes the supersaturation elimination stage. When magnesium ions in the solution have grown onto the nesquehonite crystals, the system reaches the phase equilibrium. Although it is a nonclassical case in crystallization, the effects of stirring can be interpreted from the classical nucleation and growth mechanisms. The growth units generate and combine into embryos to reach the critical nuclei size termed as nucleation, after that, the nuclei grow larger by self-assembly of amorphous precipitation. It can be inferred that the presence of stirring increases nucleation frequency factor significantly, and promotes the transformation from amorphous to crystalline phase. The nuclei generate continuously with the stirring and then grow. As the crystals are "born" at difference time, the "development" level is different. So the nucleation and growth can be separated by controlling string conditions during S2. If the stirring stops during this period, no new nuclei will be produced ignoring second nucleation. It is noteworthy that amorphous substance still exists in the slurry to achieve the ordering process. It serves as growth material depot to provide magnesium source. A small amount and particle size distribution crystals are produced by stirring to serve as seed crystals. Simultaneously, the amount of amorphous substance is determined. After the pause, during the aging process, amorphous substance grows only on the surface of these crystals due to the low energy barrier, resulting in an increase of crystal size but not the amount. Based on the viewpoint, in the actual preparation process of nesquehonite, Na₂CO₃ solution is introduced into MgCl₂ solution quickly and put aside under static conditions for appropriate aging time after a certain stirring time to insure the amorphous substance transformation complement. Stirring time (products with controlled size are shown in Fig.2), stirring intensity, reactants concentration as well as reactive temperature were investigated. Nesquehonite micro-rods with required morphology and particle size distribution can be synthesized by controlling these factors, which greatly facilitates the production process.