The Research On Phosphogypsum's Decomposition With Fe-based Addition In Multiple-atmosphere Control

Phosphogypsum?PG?is an industrial by-product of producing phosphoric acid from a phosphate rock and sulfuric acid,and the output is huge.Untreated PG is mainly disposed of by land dumping and sea dumping,resulting in a range of pollution such as soil,water and air.With the increasing awareness of environmental protection and the potential economic value of PG,the application of PG as a raw material in various fields has become a hot research topic.At present,the utilization of calcium and sulfur in PG is mainly reductive decomposition of PG into CaS,or directly to decompose PG into CaO,but both of which have lower decomposition rates and higher decomposition temperatures.In order to make full use of the calcium and sulfur resources in the PG,the decomposition rate of the PG is increased and CaO is produced.In this thesis,PG was used as the research object.Fe-based additive was added by FeCl₃ solution impregnation method,and brown coal was applied as reductive agent to decompose intermediate product CaS in N₂ atmosphere.Then the reaction atmosphere was switched to air atmosphere,and CaS was further Oxidized to the target product CaO.Firstly,the equilibrium analysis of C+O+Ca+S system is combined with the calculation of Factsage theory.The existence of elements in the reaction process are studied with the thermodynamics.The thermodynamics,phase diagram and auxiliary effect of Fe-based additives in different reaction processes are investigated.This study may provide theoretical basis for the cycle control atmosphere of decomposing PG with Fe-based additives.Secondly,the effects of different control factors on the circulation controlled decomposition of PG were discussed.The study found that in the N₂ atmosphere,when the temperature rose to 80°C,the free water in the PG began to volatilize from 140°C to 220°C,the PG lost the water of crystallization;when the temperature was between 600-1100°C,the lignite reacted with the PG at the same time of pyrolysis,and the gas and solid reaction between CO and PG occured.The main product is CO,CO₂,SO₂,CaO and CaS.After the decomposition reaction is completed,the N₂ atmosphere is switched to an air atmosphere,and the intermediate product CaS was oxidized to CaO and a part of CaSO₄.During the reaction,since the outer shell of the sample is covered by the product layer,the oxidation reaction is finally stopped due to the increase of the diffusion resistance in the particles,and the output of CaO needed to be maximized by multiple atmosphere cycles.In the thermogravimetric and tube furnace experiments,the reaction temperature was raised from room temperature to 1100°C and constant,the pressure was normal pressure,the mass ratio of PG and lignite was 10:1,and the FeCl₃ solution was 0.8 mol/L.After one hour,the atmosphere was cycled four times,the CaO content and purity in the product were the highest,and the SO₂ yield was higher.According to the experimental data,the reaction kinetics were calculated for the whole process of the reaction.Thirdly,the reaction mechanism of the whole reaction was explored.The results showed that the decomposition reaction of PG in N₂ atmosphere,the solidification reaction of lignite and PG was the main reaction,the gas-solid reaction was the secondary reaction;Fe and its oxide participate in the PG.The low temperature decomposition reduced the reaction temperature.In the air atmosphere,the intermediate product CaS was mainly oxidized to CaSO₄ at 700-900°C,and the main oxidation product of CaS at 910-1100°C was CaO;the oxide of Fe was not directly involved in the reaction,but changed the formation mode of the surface layer of the particles.The activation causes the reaction product to form a dense outer shell,changing the microscopic pore structure of the surface layer and the near surface layer,causing the crystal to migrate and recombine,creating conditions for the reaction to extend to the deep layer.The whole process of the atmosphere cycle control reaction conformed to the unreacted shrinkage model.Finally,the reaction characteristics of the cycle control decomposition of PG in fluidized state were studied.The finite element method was used to numerically simulate the process of decomposition and decomposition of phosphorus gypsum atmosphere in fluidized state.The PG was discussed in combination with the energy consumption of cement clinker rotary kiln.The atmosphere recycle controlled the energy consumption of the decomposition reaction system.In the fluidization experiment,the atmosphere circulation could be reduced to two times except for the experimental conditions in the tube furnace.The ideal gas-solid ratio was 0.04 m³·?h?g?^-1.The decomposition rate of PG in the fluidized state was about 2%higher than the decomposition rate of PG in the tube furnace.The chemical reaction engineering module of Comsol Multiphysics was used to simulate the process of decomposition of PG in fluidized state.The quality and heat transferring characteristics of the decomposition were studied.The results showed that the particles were evenly heated during the reaction.The surface layer formed the product layer.Based on the finite element analysis,the error between the boundary conditions of the heat transfer equation and the experimental conditions could be effectively reduced by numerical calculation,which may provide some guidance for the design of the industrial equipment of PG circulation decomposition.