| Phosphogypsum is a solid waste which is produced during the "wet process" phosphoric acid production in phosphate fertilizer industry and the main mineral composition of Phosphogypsum is CaSO4·2H2O. It brings in environmental hazards and restricts phosphate fertilizer industry for massive production, the lower use ratio and environmental harmfulness. It is one of important influential utilization approaches for the technology with the highest potential to produce cement clinker and sulfuric acid by phosphogypsum decomposition. However, because of the complex mechanism of phosphogypsum decomposition and many by-products during in the decomposition progress, the technology of phosphogypsum decomposition was limited. CaS was one of most important by-products it had many disadvantageous effects for the yield of SO2, CaO and the quality of cement. With the continuous development of cement industry, the government put forward a series of policy in the cement industy, developing large new dry cement process, promoting structure adjustment and industrial upgrading of cement industry. Taking a new industrialization development road has become the leader of the cement industry. The utilization of phosphogypsum is largely restricted in the cement industy.In the previously research of our group, reaction mechanism and influence factors analysis for CaS generation in the process of phosphogypsum decomposition in a nitrogen atmosphere at different conditions had been studied with high sulfur concentration coal as reducer. We put forward a new method of phosphogypsun decomposition slag reacting with CO2to produce CaCO3. In this study, the decomposition experiment of phosphogypsum for producing CaS in a tube furnace with high sulfur concentration coal as reducer and the carbonic acid reaction of phosphogypsum decomposition slag in a three-phase fluidized bed at different conditions were analyzed. The solid products of phosphogypsum desulfurization can instead of natural limestone for the preparation of calcium products such as CaO and CaCO3, so as to seek a new way of phosphogypsum utilization. The main research contents are as follows:Firstly, the analog calculation of the decomposition characteristics of phosphogypsun with high sulfur concentration coal as reducer was carried out with Reaction and Equilibrium module of chemical thermodynamic calculation software Factsage6.1. The reaction thermodynamic calculation results show that reaction mechanism for CaS generation in the process of phosphogypsum decomposition include gas-solid phase and solid-solid phase reaction module; the reactions of CaS generation all can happen in less than500℃. The thermodynamic equilibrium calculation results show that the major products of phosphogypsum decomposition are CaS, CaO, CO and SO2; Increasing C/CaSO4molar ratio and reducing atmosphere are advantageous to the reactions of CaS generation in the process of phosphogypsum decomposition.Secondly, through high temperature pyrolysis experiment in a tube furnace, the influence factors analysis for CaS generation in the process of phosphogypsum decomposition in a nitrogen atmosphere at different conditions had been studied with high sulfur concentration coal as reducer. The single factor experiment results of CaS generation in the process of phosphogypsum decomposition show that coal particle size, reaction temperature, C/CaSO4molar ratio and reaction time are important influence factors for the reaction. The orthogonal experiment results show that the progressing of various factors on the influence of the experiment is C/CaS04molar ratio>reaction temperature>reaction time; After verification tests, the optimum process conditions for CaS generation in the process of phosphogypsum decomposition were determined:high sulfur coal particle size is140mesh, reaction temperature is1050℃, C/CaSO4molar ratio is6:1, reaction time is30min. Under the optimum process conditions, the decomposition experiments of phosphogypsun, gypsum and CaSO4-2H2O were compared. The content of CaS in solid product of phosphogypsun decomposition is96.08%, which is19.3%and12%higher than the content of CaS in solid product of gypsum and CaSO4-2H2O. The comparison results prove that the optimum process conditions are appropriate for CaS generation in the process of phosphogypsum decomposition.Thirdly, the thermodynamic calculation of the possible conversion reactions of CaS and CaO in the carbonic acid process of phosphogypsum decomposition slag were carried out with Reaction module of chemical thermodynamic calculation software Factsage6.1. The reaction mechanism of CaS conversion in the carbonic acid process of phosphogypsum decomposition slag may be gas-liquid phase or solid-liquid phase reaction module. The reaction thermodynamic calculation results show that the reaction mechanism of CaS conversion is gas-liquid phase reaction module; the liquid phase reaction of CaS conversion follows the Ca(OH)2-H2O-CO2reaction system like CaO do. The carbonic acid process took place in liquid, can be clarified with Whitman’s double film theory.Fourthly, through the carbonic acid reaction experiment in a three-phase fluidized bed, the influence factors analysis for the reaction of CaS conversion to CaCO3in the carbonic acid process of phosphogypsum decomposition slag at different conditions had been studied. The single factor experiment results of the carbonic acid reaction show that CO2gas flow, liquid-solid ratio and reaction time are important influence factors for the reaction; Reaction temperature almost has no effect. The orthogonal experiment results show that the progressing of various factors on the influence of the experiment is reaction time>CO2gas flow>liquid-solid ratio. After verification tests, the optimum process conditions for CaS conversion to CaCO3were determined: CO2gas flow is300ml/min, liquid-solid ratio is6ml/g, reaction temoerature is25℃, reaction time is40min. Under the optimum process conditions, the carbonic acid reactions took place in the three-phase fluidized bed and tank reactor. The conversion rate of CaS in the three-phase fluidized bed reactor was97.34%, and the conversion rate of CaS in the tank reactor was86.32%,11%difference. Comparing with tank reactor, three-phase fluidized bed reactor is more advantageous to the carbonic acid reaction of phosphogypsum decomposition slag. |
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