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Phosphogypsum to restore the system vulcanizing process optimization and research of calcium

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Tutor: MeiYi
School: Kunming University of Science and Technology
Course: Applied Chemistry
Keywords: phosphogypsum,calcium sulfide,reductive pyrolysis,granulation
CLC: X786
Type: Master's thesis
Year:  2011
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Abstract:
With the development of China¡¯s phosphate fertilizer industry, a large amount of phosphogypsum stacked as waste has been restricting the sustainable and healthy development of the related industry. The recycle of phosphogypsum for sulfur, particularly producing sulfur or sulfuric acid, the most potential technology route which has the most obvious economic benefits, not only can effectively make use of phosphogypsum for sulfur, but also resolve the serious problem of the scarcity of sulfur resources. The production technology of calcium sulfide with phosphogypsum was studied, and the basic experimental data and theoretical guidance for reductive process of phosphogypsum to form calcium sulfide was provided.The basic thermodynamics analysis of reaction mechanism was made, as a conclution, the technology of producing calcium sulfide from phosphogypsum, can efficiently recycle phosphogypsum for calcium sulfide by controlling reaction temperature and maintaining plenty and stable atmosphere of CO in reaction system.The experimental research of reductive process in condition of three different material forms was made. Firstly, Through singe-factor experiment, the influencing factors of the calined reductive process in the condition of naturally stacked raw materials were investigated, such as calcination temperature, calcination time, reactant ratio and particle size. On this basis, orthogonal experiment were carried out and the optimization of process parameters were obtained:the calcination temperature is 900¡æ, the calcination time is 3h, the ratio of reactants is to take excessive C over 25%, the superior reductive rate is 97.30%. Secondly, through single-factor experiment, the influencing factors of the calined reductive process in the condition of compacted raw materials were investigated, such as calcination temperature, calcination time, reactant ratio and particle size. By orthogonal experiment, the optimization of process parameters were obtained:the Calcination temperature is 900¡æ, the calcination time is 2h, the ratio of reactants is to take excessive C over 20%, the superior reductive rate is 98.91%. Obviously, through compacting of raw materials, an optimization of reductive process was achieved, which obtained higher reductive rate in the condition of much lower calcination time and excessive percentage of C. Finally, to realize large-scale industrialization, it was proposed that producing calcium sulfide with Hemihydrates, which were obtained by roasting phosphogypsum and in preparation for granulation, and the preliminary exploration to this advanced reductive process was carried out. The granulation reduction process, was more efficient than the natural stacking reduction process, and the reduction rate was 96.54%under the optimum conditions of the compation reduction system, then, the particles had good strength after granulation, which was conductive to reduction, but the particles was to be powders with no strength in late, which were beneficial to the usage of calcium sulfide, providing a useful reference for producing calcium sulfide with phosphogypsum to realize industrialization.The mechanism of reduction process was discussed and it is the most important that the Combination between solid-solid reaction and gas-solid reaction of C and CaSO4 to realize efficient reduction process in the condition of full atmosphere of CO. The solid-solid reaction of C and CaSO4 lead to the reduction of phosphogypsum and the gas-solid reaction of C and CaSO4 was ingnited with the formation of CO. The close contact between phosphogypsum and coal was done by pressing materials into blocks, which is not only conductive to solid-solid reaction, but also form full CO reducing atmosphere, and then the conversion of CaSO4 to CaS was effectively completed by gas-solid reaction. Therefore, it was stressed that the reductive reaction, calcium sulfate react with CO to produce calcium sulfide, must be fully carried out to achieve high reductive rate.
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