| 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. 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. Therefore, the research of mechanism of CaS generation in the process of phosphogypsum decomposition has important practical value and theoretical significance and can provide basic data and theoretical direction.Firstly, chemical thermodynamic equilibrium calculation which adopts FactSage6.1 calculation software was carried out for the possible reactions and the decomposition characteristics of phosphogypsum at reducing agent of high-sulfur coal. The results of thermodynamic data showed that the reaction of CaS generation could occur at 500℃in theory, however, the reactions of CaS elimination needed a higher temperature and most reactions occurred above 1200℃. The results of chemical thermodynamic equilibrium calculations of the model system of phosphogypsum showed the products of phosphogypsum decomposition were CaO, CaS, SO2, CO and so on. All analyses indicated reaction temperature, reaction atmosphere and the molar ratio of CaSO4/C were important influence factors. The temperature was 1100-1200℃and the molar ratio of 1.0-2.0 was the best reaction conditions for phosphogypsum decomposition in theory. And the calculative results also indicated that the proper CO or CO2 was benefit for phosphogypsum decomposition. The analysis to the phase diagram of CaSO4-C, CaSO4-CO and CaSO4-C-CO made clear that the system of CaSO4-C-CO was nearly to the real system of phosphogypsum decomposition.Secondly, the mechanism of CaS generation in the process of phosphogypsum decomposition was investigated with Metteler Toledo thermogravimetric analyzer and tube furnace. High-sulfur coal was used as reducing agent in N2 atmosphere. In this process, the chemical reaction between CaSO4 and CO was the key reaction for generating CaS. And CaSO4 was converted to CaS and CaO by parallel reactions in CO atmosphere. The reaction mechanism could be explained by Shrinking core model. It was showed that reaction temperature, coal particle size and heating rate had important influence on CaS generation by gas and solid productions analyses. All analysis indicated that the temperature was 1100℃, heating rate was 5℃/min and the coal particle size of 60-100 mesh were the best reaction conditions for phosphogypsum decomposition in N2 atmosphere.Thirdly, the decomposition characteristics of phosphogypsum in CO2, CO and soaking zone were studied. And these results investigated that Ca(OH)2 and CaCO3 were produced during in phosphogypsum decomposition in CO2, CO or air besides CaS. For controlling CaS generation, the temperature was 1100℃, heating rate was 5℃/min and 20% of CO2 were the best conditions in CO2 for phosphogypsum decomposition. In the CO atmosphere, the mass of CaS increased with the volume of CO increasing, thus, the volume of CO must be under 0.5% to control CaS generation. The results of phosphogypsum decomposition at different atmospheres under soaking zone showed that 4-16% of CO2 had disadvantage effect on controlling CaS generation; however, the proper air was benefit for reducing CaS generation. |
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