新型非氯盐体系介导的脱硫石膏制备α-半水石膏及其转晶过程研究

Flue gas desulfurization (FGD) gypsum is a famous bulk industrial solid waste, and preparation of α-calcium sulfate hemihydrate (α-HH), a kind of superior cementitious material, from FGD gypsum presents a high value added utilization alternative. At present, the common chloride-salts based transformation system is highly corrosive, and the residual chloride ion in the product undermines the quality significantly. Besides, the crystallization process of α-HH is not clearly uncovered. In this project, we construct the calcium sulfate phase-transition diagram in sodium carboxymethyl cellulose (CMC) solution based on the phase-transition thermodynamics, and determine the metastable window of α-HH. The ion addition enhanced delivery effect and its influence on the stock solution stability are comprehensively investigated, and a new non-chlorides transformation system is proposed. The single/coordinated effect of impurities in the FGD gypsum and the effect of particle size on the transformation process and the product quality are investigated. We try to interpret the underlying role from the nucleation and growth view and regulate the transformation dynamically, making preparation of high quality α-HH from FGD gypsum come true. The micro formation mechanism from ion clusters and to α-HH nuclei is explored. The selective nucleation of calcium sulfate polymorphs and the variation of crystallization pathways are elaborated. We also simulate the α-HH nucleation, growth, agglomeration and breakup kinetic process using population balance equation (PBE), and offer important parameters to reactor design. The investigation results will provide a new non-chlorides system for high quality α-HH preparation using FGD gypsum and a new insight into α-HH micro crystallization process coupled with macro crystallization kinetics.

脱硫石膏是我国著名的大宗工业固废，利用其制备优质胶凝材料α-半水石膏代表了高附加值利用方向。目前，常用的氯盐转晶体系腐蚀性强、氯根残留会严重降低产品品质，且α-半水石膏结晶过程尚不明晰。本项目依据硫酸钙相变热力学绘制其在羧甲基纤维素钠溶液中的相变图，确定α-半水石膏亚稳定区间；研究离子添加协同传递效应和对母液稳定性的影响，提出一种新型非氯盐转晶体系。研究脱硫石膏杂质单独/联合作用及粒径对转化过程和产品品质的影响，从成核和生长的角度阐述其影响机理并进行动力学调控，实现脱硫石膏制备高品质α-半水石膏。揭示离子聚集形成晶核的α-半水石膏微观结晶过程，阐述硫酸钙多形体选择性结晶及其路径变化。采用粒数衡算方法模拟α-半水石膏成核、生长、聚结和破碎过程，为反应器设计提供动力学参数。研究成果将为脱硫石膏制备高品质α-半水石膏提供新型非氯盐转晶体系，为α-半水石膏微观结晶过程及宏观结晶动力学提供新的认识。

本项目（51502277）研究工业副产物脱硫石膏资源化制备高附加值α-半水石膏。针对氯盐和无机酸转晶体系的氯根残留和强腐蚀性问题，本项目提出了绿色温和的乙二醇水溶液转晶体系，探究了转晶工艺参数的影响，阐释了外加离子对于转化动力学和晶体形貌的影响，分析了脱硫石膏粒径和杂质对于转晶过程的影响，对硫酸钙微观结晶路径进行了探索。.项目依据饱和蒸气压平衡法，测定不同温度和醇水比条件下水活度，构建了构建了乙二醇-水体系硫酸钙热力学相图，确定了α-半水石膏热力学亚稳定窗口。提高温度、醇水比、固含量和搅拌速率可以通过加快α-半水石膏成核从而加速转化，产品为针状α-半水石膏晶须，长径比在20左右。.项目首次发现了外加非晶格阳离子（Na+）的动力学“促进”-“抑制”作用，非晶格阳离子耦合硫酸根离子增加结晶推动力，而非晶格阳离子在二水石膏表面的附着/掺杂掩盖了α-半水石膏的成核位点，产生抑制作用。晶须长径比主要依赖晶体聚集度和径向尺寸，外加离子作用的晶核架桥以及成核速率共同决定了晶须形貌，较低的外加离子和乙二醇百分比有助于大长径比（100左右）晶须的合成。.较小的石膏粒径可以提供更多的成核位点，缩短诱导时间，促进石膏相变；粒径对于结晶推动力几乎没有影响，对生长影响细微。证实了脱硫石膏中的碳酸钙和亚硫酸钙是抑制转晶的重要因子，其中亚硫酸钙的抑制作用更强（例如，0.4wt%亚硫酸钙延长6.94h转化时间）。原因是脱硫石膏表面亚硫酸根的附着掩盖了α-HH成核位点，同时降低了转化推动力。通过洗涤可以有效去除脱硫石膏中的亚硫酸钙，优化转晶工艺。结晶动力学研究表明，过饱和度、界面能和分子体积等相互交织作用的竞争成核导致了硫酸钙的选择性结晶。相关研究为无机矿化过程提供了支撑，对脱硫石膏高附加值资源化具有重要意义。