| Phosphogypsum is the byproduct of wet process producing phosphoric acid. Because of its large output and low utilization, which seriously affect the sustainable development of phosphorus chemical industry. Considering the environmental problems faced by the treatment of phosphogypsum and economic benefits, taking into account the characteristics, harm and comprehensive utilization method of phosphogypsum, this paper mainly proposes a two-step method for the phosphogypsum decomposition of reaction mechanism to ahieve CaO. The thermal decomposition process of phosphogypsum decomposition was studied, including the thermodynamics analysis and kinetics study of the oxidation reaction mechanism and influence factors of the intermediate product of CaS. The fluidization process of cold state experiment was carried out. The main research results of this paper were as follows:Firstly, the calculation results of Fact Sage 6.1 showed that phosphogypsum decomposition in CO as a reducing atmosphere will be reduced to CaS below 900℃, and it can be reduced to CaO when temperature above 900℃; while the oxidation reaction between CaS and O2 to get CaSO4 and CaO is a parallel reaction.What is more, CaS was mainly oxidized into CaO when the temperature above 1000℃, and the CaS is oxidized to CaSO4 instead when the temperature is lower than 1000℃.Secondly, the process of phosphogypsum decomposition with two-step method has been studied. Phosphogypsum was reduced in CO atmosphere, after that the residue was oxidized to CaO in air atmosphere. It is reasonable and economic for using air as oxidizing atmosphere. There is not apparent effect in N2 atmosphere for phosphogypsum decomposition and the formation of CaO. The existence of CO2 can promote the generation of CaO, and there is not adverse effect on the phosphogypsum decomposition. At the same time, phosphogypsum decomposition by circulating various atmosphere two times in reducing atmosphere and oxidizing atmosphere to achieve CaO is feasible when the temperature over 1000℃ proved by thermogravimetric analysis, XRD and XRF results. Moreover, the conversion rate of CaO achieved 90.75% when the temperature reached 1100℃.Last but not least, the phosphogypsum fluidized in the air for cold state experiment was also studied. After measuring rotermeter with soap film experiment, the incipient fluidizing velocity of phosphogypsum is 0.665 m/s, and the maximum flow velocity 1.055m/s. The kinetic mechanism model of CaS oxidization was established using the thermal analysis technique and the Kissinger, Flynn-Wall-Ozawa (FWO) and the Coats-Redfem methods. The kinetic parameters of reduction product from CaS oxidation reaction was calculated. The activation energy values was 257.35 kJ/mol,pre exponential factor was 25.37 and the most likely mechanism model was three-dimensional spherical symmetry with Jander equation. What is more, the integral function, differential function and relational expression between mass conversion ratio and time in the thermal decomposition for CaS oxidation was established. Besides, the analysis of the kinetic parameters of pure CaS oxidation reaction of activation energy values was 243.60 kJ/mol, pre exponential factor was 22.76. The mechanism model is contracting sphere (spherical symmetry). And the integral function, differential function and relational expression between mass conversion ratio and time in the thermal decomposition for pure CaS oxidation was also calculated. |
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