| Flue Gas Desulfurization gypsum (called FGD gypsum for short) was the industrial by-product gypsum of flue gas desulphurization to coal-fired power plant. Along with the emphasis on the quality of atmospheric enwironment, the annual emissions of gypsum increased dramatically, and its utilization was imminent. The charistics of high content of gypsum didydrate gypsum,in powder and the wet discharge of FGD gypsum were ideal for the preparation of high-strength gypsum in hydrothermal method.Theα-hemihydrate gypsum was prepared by aqueous salt solution method at atmospheric pressure in this issue, and the influence of technological conditions and crystal modifier on the preparation and crystal morphology ofα-hemihydrate gypsum was mainly studied. The content mainly contained the following aspects:1) The influence of solution pH and gypsum impurities on preparation ofα-hemihydrate gypsum was studied deeply. The results showed that: pH had little influence on the conversion rate of FGD gypsum and the crystal morphology ofα-hemihydrate gypsum in the blank reaction without crystal modifier. The main impurities contained in FGD gypsum were calcium carbonate and calcium sulfite, and calcium sulfite greatly reduced the conversion rate ofα-hemihydrate gypsum, while the calcium carbonate less affected the preparation ofα-hemihydrate gypsum.2) The growth habit ofα-hemihydrate gypsum was needle-like, which demanded much water for standard consistency and so resulted in low strength. Adding crystal modifier to the reaction solution to change crystal morphology ofα-hemihydrate gypsum from needle into short column was core technology in preparation of high-strengthα-hemihydrate gypsum in liquid method. Crystal modifier generally divided into inorganic salts, multiple organic acids, macromolecules type and surfactant type four categories. Its function and effect law were deeply explored from angles of crystal morphology of the products,dehydration rate,liquid ion concentration and so on. The results showed that multiple organic acids crystal modifier had the most significant effect and could prepare out of ideal short prismatic crystal ofα-hemihydrate gypsum with aspect ratio closed to 1:1. 3) Then multiple organic acids crystal modifier was discussed in more depth. The effect of its molecular structure and the pH of solution on its crystal transformation role were studied. The results showed that organic acid with three C-C bond length between adjacent carboxyl had good crystal transformation function, and pH was one of the most sensitive factors of affecting the role of crystal transformation. The mechanism of multi-organic acids modifier was due to its carboxylic acid ion ionized out in solution. It could change the relative growth rate of different crystal faces by selective adsorption at different crystal surface and complexion with calcium ion and so as to achieve the purpose of changing the crystal morphology. At different solution pH, the complexion stability between carboxylic acid ion ionized out from multiple organic acid and calcium ion on crystal surface was with a large difference, and therefore the crystal transformation effect of multiple organic acid was very different.4) In addition, the complex mixed effect of crystal modifiers was also studied in this subject. It was initially proposed that complex mixed behavior could not produce ultra-fold effect. In the multiple complex of organic acids and inorganic salt, the substance playing the role of transferring crystal morphology was still multi-organic acids.In the conditions of crystal transfer agent citric acid content of 0.01% and solution pH 4.6, an ideal short column ofα-hemihydrate gypsum with aspect ratio closed to 1:1 was prepared, and in the water requirement ratio of 41%, its hardened intensity reached 30.25MPa.
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