碳酸铈在NaCl-H_(2)O体系中的相平衡热力学模型

Thermodynamic model for the phase equilibrium of cerium carbonates in the NaCl-H_(2)O system

碳酸铈是生产CeO的重要前驱体,对其性质具有决定性影响。碳酸铈的结晶特征取决于反应结晶过程中过饱和度的控制,其在NaCl-H_(2)O体系中的相平衡数据是关键基础数据。本工作首先在298.15~363.15 K温度范围内合成了碳酸铈,XRD分析结果显示,323.15 K 及以下得到的产品为八水碳酸铈[Ce_(2)(CO_(3))_(3)·8H_(2)O], 343.15 K 及以上得到的产品为碱式碳酸铈[CeCO_(3)OH]。本工作采用经典等温法测定了这两种碳酸铈化合物在 NaCl-H_(2)O 体系中的相平衡数据,并利用 Aspen Plus 平台的ELEC-NRTL 方程建立了可准确预测 Ce_(2)(CO_(3))_(3)·8H_(2)O 和 CeCO_(3)OH 在 NaCl-H_(2)O 体系中相平衡数据的热力学模型。在无限稀释假设的基础上,通过回归Ce_(2)(CO_(3))_(3)·8H_(2)O 和 CeCO_(3)OH 在水中的溶解度数据,确定了这两种化合物的溶度积。采用赋存形态分析方法,将CeCO_(3)~+, CeOH, CeHCO等组分引入热力学模型。利用实验数据获得了新的离子对(Ce-HCO~-和Ce-Cl~-)参数,提高了新模型的预测能力,所建立的热力学模型预测值与实验数据吻合较好。

Cerium carbonates are important precursors for the production of CeO, which have a decisive effect on the properties of CeO. The crystal characteristics of cerium carbonate compounds depend on the control of supersaturating in reactive crystallization, and the solubility of cerium carbonate compounds in the NaCl-H_(2)O system are important basic data. In this work, cerium carbonates were synthesized by H_(2)Omogeneous precipitation metH_(2)Od at the temperature range of 298.15~363.15 K. The generated solids were characterized by X-ray diffractometer(XRD), scanning electron microscope(SEM) and thermogravimetric analyzer(TG). The results sH_(2)Owed that Ce_(2)(CO_(3))_(3)·8H_(2)O was obtained below 323.15 K and transferred to CeCO_(3)OH above 343.15 K. The solubility of the two cerium carbonate compounds in the NaCl-H_(2)O system were determined by the classical isothermal metH_(2)Od. The results sH_(2)Owed that the solubility of Ce_(2)(CO_(3))_(3)·8H_(2)O increased with the increase of temperature and concentration of NaCl, while the solubility of CeCO_(3)OH increased with the increase of concentration of NaCl, and decreased with the increase of temperature. The solubility of Ce_(2)(CO_(3))_(3)·8H_(2)O in water at 298.15 K was 4.08×10mol/kg, which was of the same order of magnitude as the solubility of cerium carbonate [Ce_(2)(CO_(3))_(3)] that was 2.17×10~2.17×10mol/kg at 298.15 K. The results indicated the reliability of the experimental data measured in this work. A thermodynamic model for predicting the solubility of Ce_(2)(CO_(3))_(3)·8H_(2)O and CeCO_(3)OH in the NaCl-H_(2)O system was finally established by using the ELEC-NRTL equation embedded in the Aspen Plus platform. The solubility products of Ce_(2)(CO_(3))_(3)·8H_(2)O and CeCO_(3)OH were obtained by regression of their solubility data in water on the basis of an infinite dilution assumption. The species such as CeCO_(3), CeOH, CeHCOwere introduced into the thermodynamic model on basis of the speciation metH_(2)Od and their equilibrium constants were calculated by Van’t H_(2)Off isothermal formula. The new ion pair parameters of Ce-HCO-and Ce-Cl-were obtained by regression of the experimental data to improve the predictive ability of the new model. The predicted values of the established thermodynamic model were well in agreement with the experimental data.