【作者】 付海陆；

【导师】 吴忠标； 官宝红；

【作者基本信息】 浙江大学 ， 环境工程， 2013， 博士

【摘要】 钙基脱硫剂脱除烟气中的SO2时,生成亚硫酸钙(calcium sulfite, SH),经强制氧化,则转化为二水硫酸钙(calcium sulfate dihydrate, DH).以亚硫酸钙和二水硫酸钙为主要成分的脱硫副产物称为脱硫石膏。在过去的十五年中,我国脱硫石膏成为著名的大宗工业固体废物,目前年排放量大约为6900万吨,从环境保护和资源利用角度而言,急需规模化消纳和资源化利用。本课题针对脱硫石膏高附加值资源化利用,研究亚硫酸钙和二水硫酸钙制备α-半水石膏(a-calcium sulfate hemihydrate, a-HH)以及它们之间的相变关系和结晶规律。在合理配制的Ca-Mg-Mn氯化物溶液中,亚硫酸钙氧化和α-半水石膏结晶同时发生,亚硫酸钙氧化是反应限速步骤。在确定的温度和其他条件下,α-半水石膏的结晶路径随CaCl2浓度变化而变化。在2.50-3.50m CaCl2体系中,亚硫酸钙经由DH向α-半水石膏转化(亚硫酸钙→二水石膏→α-半水石膏),二水石膏的存在时间随着氯化钙浓度的升高而缩短；在4.00m CaCl2体系中,没有发生亚硫酸钙向二水石膏转化的现象,即二水石膏的存在时间缩短为零,实现亚硫酸钙→α-半水石膏的直接转化。据此,提出了亚硫酸钙在常压盐介质中直接制备α-半水石膏的工艺。为了阐明在CaCl2溶液中亚硫酸钙制备α-半水石膏结晶路径的变化规律,研究了均相CaCl2体系中硫酸钙多相的竞争成核,依据经典成核理论(classical nucleation theory, CNT)模拟α-半水石膏的相对成核速率。在α-半水石膏亚稳定相区和二水石膏亚稳定相区,自发成核初始结晶中的优势相随着过饱和度增加的顺序分别为：①不稳定二水石膏→亚稳定α-半水石膏→不稳定二水石膏和②亚稳定二水石膏→不稳定α-半水石膏→亚稳定二水石膏。α-半水石膏和二水石膏分子的体积(v0)、界面能(γ)和过饱和度(S)对成核影响力的消长导致二者竞争成核,从而出现选择性结晶；当二者成核速率相当时,产生共结晶。低过饱和度条件下在α-半水石膏亚稳定区生成二水石膏,在二水石膏亚稳定区生成α-半水石膏,都是由非均相成核造成；高过饱和度条件下,生成热力学稳定性低的相态符合多相结晶的Ostwald规则。初始结晶相中α-半水石膏的含量随着CaCl2浓度和温度的升高而增大,主要是因为α-半水石膏和二水石膏成核过饱和度之比(SHH/SDH)随着CaCl2浓度和温度的升高而增加,α-半水石膏成核竞争力随之增强。二水石膏向α-半水石膏的转化速率随着体系温度和CaCl2浓度的升高和二水石膏粒径的减小而加快,该转化是成核-生长控制过程,符合加速反应分散动力学方程。提高温度和CaCl2浓度,则增大了二水石膏和α-半水石膏二者的溶度积比值(Ksp,DH/Ksp,HH)、降低了水活度(aw),使得α-半水石膏成核-生长的过饱和度增大,这是熵增过程,加快了α-半水石膏的成核和平均生长速率。减小二水石膏粒径,则增大比表面积和晶格缺陷数目,这是焓降、熵增过程,通过表面成核促进了α-半水石膏的成核速率,但是对平均生长速率没有显著影响。研究表明,亚硫酸钙在CaCl2溶液中向α-半水石膏转化,二水石膏和α-半水石膏的竞争成核导致了亚硫酸钙到α-半水石膏相变路径的变化,α-半水石膏的成核及生长是影响二水石膏向α-半水石膏转化的关键步骤。研究结果为脱硫石膏高附加值资源化利用提供理论指导、工艺路线和重要的参数,也深化了对硫酸钙多相结晶的认识,为溶液中的无机矿物相变热力学和动力学控制提供了研究方法。更多还原

【Abstract】 During the flue gas desulfurization (FGD) process, calcium based agent produces calcium sulfite (SH), which converts to calcium sulfate dihydrate (DH) after forced oxidation. The FGD byproduct mainly composed of SH and DH is termed as FGD gypsum. In the past15years, FGD gypsum has become a famous bulk industrial solid waste and the current discharge is about69,000,000ton. It demands urgent consumption as a useful resource in large quantities from the point of environmental protection and untilization. To make high value added use of FGD gypsum, this paper investigates the preparation of α-calcium sulfate hemihydrate (α-HH) from SH and DH and the corresponding phase transition and crystallization rules.In certain Ca-Mg-Mn chloride solution, SH oxidation and α-HH crystallization can be accomplished at the same time, the former of which turns to be the rate limiting step. Crystallization route of α-HH during the oxidation of SH depends heavily on the CaCl2concentration provided temperature and other factors are fixed. In2.50-3.50m CaCl2solutions, α-HH precipitated via intermediate phase DH, namely SH→DH→α-HH, and the existing time of DH shortened with the increase in CaCl2concentration. In a4.00m CaCl2solution, the transformation from SH to DH was not observed, namely, the existing time of DH was reduced to zero, presenting a direct SH→α-HH transformation. Hence, it proposes a direct a-HH preparation method from SH in the salt medium under atmospheric pressure.To interpret the crystallization route evolution during SH-a-HH conversion in CaCl2solution, nucleation competition of calcium sulfate phases in homogenous solutions was investigated and the relative nucleation rate of a-HH (RHH) on the basis of classical nucleation theory (CNT) was simulated. Dominant calcium sulfate initially precipitated presents①unstable DH→metastable α-HH→unstable DH and②metastable DH→unstable α-HH→metastable DH evolution orders depending upon supersaturation in a-HH and DH metastable zones, respectively. The comprehensive effect of molecular volume (v0), interfacial energy (y) and supersaturation (S) of DH and a-HH leads to their competitive nucleation and hence selective crystallization. Concomitant nucleation occurs when the nucleation rates are comparable. The formation of DH at lower supersaturations in α-HH metastable zone and the formation of α-HH in DH metastable zone are attributed to their repsective heterogeneous nucleation. The occurrence of thermodynamically less stable phase at higher supersaturations conforms to the Ostwald’s rule of stages. The mole fraction of α-HH in the initial precipitate increases with the CaCl2concentration and temperature increasing. This is due to the larger supersaturation ratio of α-HH to DH (SHH/SSDH), which makes α-HH nucleation more competitive at higher CaCl2molality and temperature.The transformation accelerates with the temperature and CaCl2molality increment and DH particle size reduction. The transformation is a nucleation-growth limited process, which well fits to the dispersive kinetic model. Increment in temperature and CaCl2molality enlarges the solubility product ratio of DH to α-HH (KsP,DH/Ksp,HH) and lowers down the water activity, respectively, which both enlarges the supersaturation and activationentropy change, expediting the α-HH nucleation and growth. Reduction in DH particle size increases the specific surface area and the lattice deformity number, which lowers down the activation enthalpy and enlarges the activation entropy, and boosts α-HH surface nucleation, but has no evident effect on the growth rate.The study demonstrates SH can be transformed into α-HH in CaCl2solution, and the nucleation competition between DH and α-HH accounts for the transition route variation. Nucleation and growth of α-HH is the key step affecting the DH transformation to α-HH. The results provide theoretical guidance, processing method and important factors for the high value added utilization of FGD gypsum. Also, it deepens the understanding of calcium sulfate multi-phase crystallization and offers a method to investigate the phase-transition thermodynamics and kinetics control of inorganic minerals in aqueous solutions.更多还原