Process And Mechanism Of Phosphorite Slurry Enhanced Absorption For Phosphogypsum Pyrolysis Sulfur-rich Flue Gas

Phosphogypsum（PS）is a bulk solid waste in the production of wet phosphoric acid.There is still a big gap between the comprehensive utilization rate of PS and the overall production of PS in China,so it is urgent to accelerate the resource utilization of PS.This thesis addresses the problem of resource utilization of PS,puts forward the research idea of SO₂and Ca O sulfur extraction under the lignite-CH₄double reduction system,and explores the key factors affecting the decomposition of PS and the process mechanism.Secondly,we choose the phosphate slurry pressurized absorption PS to produce a high concentration of SO₂for the production of phosphoric acid slurry and investigate the optimal experimental parameters.The interaction of multiple parameters was investigated through response surface design,and a quadratic polynomial regression model was established.To address the problem of insufficient oxidizing capacity in the phosphoric acid production system from the phosphoric acid slurry,the oxidant dosage for the optimal conditions of phosphoric acid yield was investigated by adding strong oxidants H₂O₂and KMn O₄to the reaction system to achieve efficient catalytic oxidation of SO₂.In addition,the possible main reaction pathways under this reaction system were clarified by density flooding theory（DFT）calculations,which further revealed the reaction mechanism of high concentration SO₂to phosphorate-containing slurry.The main research conclusions of this thesis are as follows:（1）An experimental study on the decomposition of PS under a lignite-CH₄double reduction system was carried out.The effects of different influencing factors on the decomposition of PS and the mechanism of SO₂production from PS decomposition were investigated.The results showed that there were four stages in the calcination process of PS:Stage 1 was located at 100～170℃,which originated from the volatilization of water;Stage 2 was located at 280～400℃,which originated from the volatilization of volatiles;Stage 3 was located at 700～1000℃,which was attributed to the reduction and decomposition of Ca SO₄to Ca S;and Stage 4 was located at 1000℃under anhydrous air,which is attributed to the oxidation of Ca S and residual lignite.The reduced organic sulfides in the lignite reacted with Ca SO₄and increased the rate of Ca SO₄decomposition.The addition of Fe₂（SO₄）₃promotes the decomposition reaction,but excessive Fe₂（SO₄）₃inhibits it.CH₄can capture more O atoms from Ca SO₄.However,too high CH₄will cause the intermediate product to consume the solid-reducing agent.During the reduction process,Ca SO₄reacted with CH₄and lignite to form CO,CO₂,H₂O,and Ca S.During the oxidation process,Ca S was mainly oxidized to Ca O,SO₂,and elemental S.Meanwhile,the remaining lignite was oxidized to CO and CO₂.（2）Proposed method for a high concentration of SO₂produced by PS decomposition.A method was proposed to produce phosphoric acid slurry by reacting phosphate slurry under enhanced pressure,and an experimental study of phosphoric acid production by pressurized desulfurization of phosphate slurry was carried out.The effect of experimental parameters such as pressure,additive type,reaction temperature,and solid-liquid ratio on phosphoric acid yield was investigated,and the optimal experimental conditions were as follows:inlet SO₂concentration of28570mg/m³（10000ppm）;reactor pressure of 0.2 MPa;particle size of the ore powder of 60-80 mesh;40 m L of 3 wt%Fe（NO₃）₃solution;solid-liquid mass ratio of 15%;oxygen content of 7.5%;reaction temperature 35℃;reaction air velocity 100m L·min^-1/g.At this time the phosphate ion concentration of 1526.39 mg/L.Response surface analysis was used to optimize the pressure,the amount of iron nitrate added and SO₂inlet concentration in the pressurized reaction between SO₂and phosphate slurry.The experimental results showed that the influence of the three reaction parameters on phosphate root concentration in the reaction slurry from the largest to the smallest is pressure>Fe（NO₃）₃addition>SO₂inlet concentration.The results obtained using the DFT calculations showed that the presence of Fe³⁺was able to reduce the maximum reaction energy barriers of different reaction processes（SO₂oxidation,SO₂hydration,SO₃hydration,and H₂SO₃oxidation）in the two reaction pathways,indicating that the participation of Fe³⁺in the catalytic oxidation significantly enhances the absorption and conversion processes of SO₂.SO₂undergoes the catalytic oxidation of SO₂to SO₃in the Fe³⁺-enhanced liquid-phase desulfurization,and then the oxidation of SO₂to H₂SO₄.SO₃to H₂SO₄hydration process.（3）Aiming at the problem of low yield of phosphoric acid and SO₂absorption,conversion and utilization due to insufficient oxidizing capacity in the system,we proposed to use strong oxidizing agent H₂O₂to enhance the capacity of the reaction system to oxidize SO₂and carried out experimental research on the production of phosphoric acid by phosphate slurry-H₂O₂and high concentration of SO₂enhancement.The experimental results showed that only H₂O₂involved in the oxidation-enhanced absorption reaction was unstable.The oxidation capacity and stability of the reaction solution was supplemented with 4wt%Fe（NO₃）₃was greatly improved,and the optimal H₂O₂concentration of 1.5 mol/L.Under these conditions,the maximum phosphate ion in the solution after the reaction was 3454.63 mg/L.DFT calculations were used to illustrate that the presence of H₂O₂was able to reduce the maximum reaction energy barriers of the SO₂oxidation,SO₂hydration,and SO₃hydration processes in the two main reaction pathways,but increased the maximum reaction energy barrier of the H₂SO₃oxidation process.The reactive oxygen species under the phosphorite slurry/H₂O₂composite slurry were explored and the surface properties of the phosphorite slurry before and after the reaction were analyzed.The characterization results indicate that the main reactive oxygen species are the hydroxyl radicals and（-OH）and superoxide radicals（O₂--）attached to the phosphorite.In addition,the XPS results illustrate that surface Fe3+is the key pathway for radical generation under the initial stage.（4）In response to the problem of the fast reaction rate of H₂O₂oxidizer and the difficulty of production regulation,H₂O₂was replaced with KMn O₄for experimental investigation.The difference in the phosphate content of the reaction slurry under different special conditions was also explored.The results showed that the synergistic effect of Fe（NO₃）₃and potassium permanganate could enhance the production of phosphoric acid.The potassium permanganate concentration of 1000mg/L,and the maximum phosphate ion in the solution after the reaction was 3177.04mg/L.DFT calculations were used to illustrate that the presence of Mn O₄^-was able to reduce the maximum concentration of the different reaction processes in the two reaction pathways（SO₂oxidation,SO₂hydration,SO₃hydration,H₂SO₃oxidation）maximum reaction energy barriers.Mn O₄^-can effectively enhance the process of SO₂removal.SO₂undergoes a hydration reaction process from SO₂to H₂SO₃followed by a catalytic oxidation process from H₂SO₃to H₂SO₄in Mn O₄^-enhanced liquid-phase absorption.The concentration of phosphate ions in the slurry after the direct reaction of concentrated sulfuric acid with 15%solid-liquid phosphate slurry is 13939 mg/L.Theoretically,the slurry produced after enhancing the absorption of SO₂can reduce sulfuric acid consumption by up to 25%.At inlet SO₂concentration of 285710mg/m³,the phosphate concentration in KMn O₄slurry increased by 15.75%,sulfate by 155%,and 113%at 35%high solid-liquid ratio.