重晶石在硼硅酸盐玻璃熔体中的分解行为及其玻璃陶瓷的固硫机理

The safe treatment and disposal of high level liquid waste has become one of key factors affecting the sustainable development of nuclear energy. Glass-ceramics immobilization is one of important development trends of glass for the immobilization of high-level liquid waste. Currently, the borosilicate glass for the immobilization of sulfur-bearing high level liquid waste has some drawbacks such as the poor thermal stability of sodium sulfate and low solubility of sulfur in the glass waste forms. In this project, it is proposed that the sulfur will be firstly incorporated in stable barite crystals, and then barite‒borosilicate glass-ceramic waste forms will be prepared by melting method. The relationships among the composition, structure, and physical properties of the glass-ceramic waste forms will be studied systematically. The processing techniques of the glass-ceramic waste forms will be optimized. The effects of coexisting phases and melting conditions (temperature, heating rate, etc.) on the decomposition behavior of barite in the borosilicate melt will be investigated, and the occurrence of sulfur in the barite ‒borosilicate glass-ceramic waste forms will be studied. Based on the above results, the sulfur retention mechanism of the ‒borosilicate glass-ceramics will be revealed. Moreover, the leaching properties of the glass-ceramic waste forms will be evaluated. It is expected that the implementation of this project can provide an alternative new way for the solution to the “yellow phase” of the borosilicate glass waste forms. The research results can provide scientific basis and technical support for the optimization and preparation of high-performance sulfur bearing glass-ceramic waste forms, and lay a good foundation for the applied basic research of glass-ceramics immobilization and the safe disposal of high level liquid waste.

高放废液的安全处理处置已成为影响核能可持续发展的关键因素之一，玻璃陶瓷固化是玻璃固化高放废液的重要发展方向。针对硼硅酸盐玻璃固化含硫高放废液存在硫酸钠高温下易分解、硫在玻璃固化体中溶解度偏低等不足，本项目提出将硫先固定在稳定的重晶石（BaSO4）中，再采用熔融法制备重晶石-硼硅酸盐玻璃陶瓷固化体。系统研究固化体的组成、结构和物理性能之间的关系，优化集成固化体的制备工艺技术；研究共存物相和熔制条件（温度、升温速率等）对重晶石在硼硅酸盐玻璃熔体中分解行为的影响，探明硫元素在重晶石-硼硅酸盐玻璃陶瓷固化体中的赋存状态，揭示玻璃陶瓷的固硫机理，评价固化体的抗浸出性能。本项目的实施，可望为解决硼硅酸盐玻璃固化体中的“黄相”问题提供一条新途径，相关研究成果可为优化和制备高性能含硫高放废液玻璃陶瓷固化体提供科学依据和技术储备，为推进高放废液玻璃陶瓷固化的应用基础研究及其安全处置奠定基础。

当前，我国历史遗留的高放废液急需妥善处理处置。硼硅酸盐玻璃是国内外固化处理高放废液的首选基材。我国现存高放废液的特点是硫和钠含量较高，而硫酸盐在硼硅酸盐玻璃中的溶解度较低（SO3≤1wt%），在玻璃固化过程中通常会产生分离黄色第二相（黄相）。针对玻璃熔制过程中Na2SO4在高温下易分解、硫在玻璃固化体中溶解度偏低等不足，本项目提出了将高放废液中的硫先固定在重晶石（BaSO4）中，再采用熔融法制备重晶石-硼硅酸盐玻璃陶瓷固化体的思路，为解决“黄相”问题提供了一条新途径。. 本项目优化集成了硼硅酸盐基础玻璃配方和玻璃陶瓷制备工艺技术，优选基础玻璃中Si/B比值为3～4，Si/Na比值为12，采用先制得基础玻璃再与BaSO4混合熔制，玻璃陶瓷包容的硫含量达4.45wt%（以SO3计）。探究了BaSO4在硼硅酸盐玻璃熔体中的分解行为，发现低熔点的B2O3和Na2O促进了BaSO4的分解，BaSO4晶体在玻璃熔体中随温度升高逐渐长大，当熔制温度低于1050℃时，玻璃陶瓷中硫含量基本保持不变；用Doyle-Ozawa法和Kissinger法计算得出BaSO4在硼硅酸盐玻璃熔体中的分解活化能分别为1641.93KJ/mol和1702.49KJ/mol。当BaSO4掺量为0~4wt%（以SO3计）时，样品为非晶态结构；当BaSO4掺量为5~6wt%时样品内部开始出现重晶石相，其随着掺量增加重晶石含量逐渐增加；当BaSO4掺量大于或等于7wt%时，样品表面形成由BaSO4组成的白色分离层，样品致密性较差。Ba元素和S元素主要分布在BaSO4晶体中，少量Ba元素均匀分布在玻璃相中。掺入BaSO4后玻璃陶瓷固化体中的Na、B元素的28d归一化浸出率均在10-1g•m2•d-1数量级，比纯硼硅酸盐玻璃高一个数量级，掺入6wt%的玻璃陶瓷样品的SO42-的28d归一化浸出率较低（1.28×10-2g•m2•d-1），比掺入相同含量Na2SO4的样品SO42-的28d归一化浸出率低一个数量级。. 本项目取得的研究成果，可为解决高硫高钠高放废液玻璃固化过程中产生的“黄相”问题提供重要的科学依据和技术支撑，对促进高放废液玻璃固化技术的发展和提升我国高放废液处理水平具有重要的科学意义和学术价值。