| Flue gas desulfurization (FGD) gypsum is an industrial solid waste produced by coal-combustion power plants, petroleum&chemical plants, and steel factories. Compared with natural gypsum, the quality of FGD gypsum is very unstable, which has limited its application and caused a large of accumulation. It is quite valuable for the wide utilization of FGD gypsum to prepare high-value gypsum products.In this thesis, high-quality gypsum was obtained via a pretreatment process by using FGD gypsum as raw material. On this basis, calcium sulfate whisker (CSW) was prepared via hydrothermal crystallization by utilizing the pretreated FGD gypsum as raw material and adding a crystal modifier into the reaction solution. The main investigation results of the thesis are as follows:1. Characteristics of FGD gypsum and its pretreatment processThe FGD gypsum used in this thesis was supplied by a power plant in Henan Province. It consistes of tabular, round flake, sphere and irregular particles, and its particle sizes mainly ranged from10to100μm. Dihydrate calcium sulfate (DH) content of the FGD gypsum was lightly above80%, and its impurities were CaCO3and the compound composed of Al and Si. After screening, first ball milling, acid treatment and flotation successively, FGD gypsum could be purified, while its activity could be increased by the second ball milling. The morphology of the purified FGD gypsum was only presented as irregular particles, and its particle sizes were decreased to about1-3μm. Its DH content was close to96%.2. Preparation of calcium sulfate whiskers from the purified FGD gypsumBased on the pretreatment of FGD gypsum, CSWs were prepared through a hydrothermal crystallization by using common inorganic salts as crystal modifiers. The effects of some key parameters, such as the type and dosage of crystal modifier, the reaction temperature and time, H2SO4concentration and the concentration of reaction slurry, etc., on the quality of CSWs have been systematically investigated. The results indicated that both the type and dosage of crystal modifier and the H2SO4concentration were the key factors affecting the preparation of CSWs from the purified FGD gypsum. The appropriate dosage of crystal modifier and H2SO4concentration were distinct for each type of crystal modifier, but the reaction temperature and time and the concentration of reaction slurry were constant. In conclusion, CSWs with a uniform crystal morphology and a relatively big aspect ratio can be obtained at120～140℃for60～90min with the concentration of reaction slurry ranging from3.0%to7.0%when the crystal modifier was used suitably.3. Selection of crystal modifier and its mechanismBy using K2SO4, KCl, MgCl2, CuCl2and AlC13as crystal modifier, the effects of the type, dosage and dissociation characteristics of the crystal modifier on the solubility of the purified FGD gypsum and the crystal morphology of the products prepared by hydrothermal crystallization were discussed. The results showed that the qualities of the CSWs can be improved when the crystal modifier was added into the reaction solution, but the type of crystal modifier has an obviously different effect on the quality of CSWs. When1.5%CuCl2(with respect to the amount of purified FGD gypsum) was used as crystal modifier, the sole morphology of CSWs prepared at optimized conditions was fiber and its surface was very smooth and clear. The obtained CSWs had diameters ranging from1to3μm and lengths from400to600μm, and the distribution of its diameter and length was uniform. Moreover, its aspect ratio reached up to200. In summary, the comprehensive quality of CSWs that were prepared by using CuCl2as crystal modifier is better than that of others.For the preparation of CSWs, common ion effect has a main effect on the crystallization of CSWs when K2SO4was used as crystal modifier. Due to complex salt reactions between KCl and FGD gypsum, the stability of KCl is low in the reaction solution, which caused the worse quality of CSWs. For MgCl2crystal modifier, both common ion effect and MgSO4(0) could affect the crystallization of CSWs. However, the amount of simple substance free cation is a key factor that affects the crystallization of CSWs when using CuCl2or AlCl3as crystal modifier. Through comprehensive investigation on the effects of crystal modifiers on the crystal morphology, average aspect ratio and quality of CSW and the solubility of the purified FGD gypsum, it can be found that the chemical stability, the common ion effect (except that neutral ion can be formed such as MgSO4(0)) and the amount of simple substance free cation of a crystal modifer in the reaction solution have significant influences on the quality of CSWs, and their action laws are presented as following order:chemical stability> common ion effect> amount of simple substance free cation.4. Growth mechanism of CSW under the influence of crystal modifiersThe cations introduced by crystal modifiers could temporarily adsorb with SO42-on the surface of CSWs, and the anions could combine with Ca2+, which would reduce the lattice energy of the adsorbed crystal plane. In a short time, the adsorbed cations and anions introduced by crystal modifiers would be replaced by Ca2+and SO42-respectively to promote a rapid growth of whiskers. For CSWs, the surface density of (200) plane is minimum, and the opposite ions introduced by crystal modifiers could easily adsorb at this plane, which would impede Ca2+and SO42-to combine with (200) plane and cause its growth rate to be the slowest. However, the surface density of (002) plane is maximum, and the opposite results could be obtained. The surface density of (020) plane is between (200) plane and (002) plane, and the effects of the opposite ions on the growth rate of (020) plane are also between (200) plane and (002) plane. Therefore, the interaction between whiskers and crystal modifiers is in the form of adsorption-desorption, and the crystal modifier ions adsorbed on the surface of CSWs could not involve into the crystal lattice of whiskers. Moreover, crystal modifiers could significantly promote the oriented growth of CSWs.For the preparation of CSWs, the growth of CSWs (HH) is always accompanied by the dissolution of the purified FGD gypsum (DH) until the crystallization process is over. The driving force of the nucleation and growth of CSWs was the solubility gap between DH and HH. A large number of nuclei could be formed at a short time, and CSWs could be obtained through the oriented growth of Ca2+and SO42-in the reaction solution. All results indicate that the growth of CSWs undergoes a process of "dissolution-recrystallization". |
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