Abstract:

Lansfordite (MgCO₃·5H₂O) was synthesized via the reactive crystallization of MgCl₂ with Na₂CO₃ under the regulation of NaHCO₃. The effects of crystallization parameters on the crystallization process and products were experimentally investigated by means of SEM, XRD, TG techniques and taking into consideration the change of magnesium ion concentration during the crystallization process. MgCO₃·3H₂O was easy to generate as by-product in the synthesis process. Process conditions of pure MgCO₃·5H₂O preparation were as follows: NaHCO₃ amount was ≥3.00 g·L^-1, Na₂CO₃ feeding speed was 2—50 ml·min^-1, reactive temperature was 0—5.0℃. The as-synthesized lansfordite crystals displayed prismatic morphology with smooth faces and a size of 20—160 μm. Analysis showed that crystallization kinetics was promoted with the addition of NaHCO₃.?HCO₃^- group created the steric effect inhibiting nesquehonite growth but was beneficial for lansfordite crystallization. Therefore, pure lansfordite could be synthesized under the regulation of NaHCO₃. Changes in process parameters could make the crystallization process shift between thermodynamic control and kinetic control, affecting crystal polymorph and morphology.

Key words: crystallization, lansfordite, kinetically control, particulate processes, chemical reaction

摘要：

在NaHCO₃调控作用下, 通过MgCl₂和Na₂CO₃反应结晶合成了五水碳酸镁(MgCO₃·5H₂O)。利用SEM、XRD和TG等技术, 结合反应结晶过程中镁离子浓度变化, 研究了工艺参数对结晶过程和产物影响。实验结果表明, 合成五水碳酸镁过程中极易生成MgCO₃·3H₂O副产物。纯MgCO₃·5H₂O制备工艺条件为:NaHCO₃加入量≥3.00 g·L^-1, 碳酸钠加料速率2～50 ml·min^-1, 合成温度0～5.0℃。所合成的五水碳酸镁呈棱柱状形貌, 晶体表面光滑, 尺寸20～160 μm。分析表明, 添加碳酸氢钠促进了结晶动力学, 同时 基团对三水碳酸镁晶体的形成具有位阻效应, 但有利于五水碳酸镁晶体形成, 产生晶型调控的协同效果。工艺参数的改变将使得结晶过程发生热力学和动力学控制的转换, 影响结晶产物的晶型晶貌。

关键词: 结晶, 五水碳酸镁, 动力学控制, 颗粒过程, 化学反应