Study On Mechanism Of Preparation Of Uniform Vaterite-type CaCO₃ Via CO₂ Mineralization By Flue Gas Desulfurization Gypsum

China is a major emitter of carbon dioxide(CO₂)in the world.China is facing big pressure to reduce CO₂ emissions.To build a CO₂ reduction technology system with a low cost,energy consumption and environmental risk is a major strategic requirement to solve the problem of CO₂ emissions in China.Mineral carbonation of industrial solid wastes containing calcium and magnesium to sequester CO₂is a“win-win”strategy because it can not only achieve CO₂ reduction,but also realize the resource utilization of solid wastes and achieve the goal of treating waste with wastes,which has practical significance for the establishment of CO₂ emission reduction technology in Chinese industry.Flue gas desulfurization gypsum(FGDG)is a waste discharged from limestone-gypsum wet desulphurization process,which has leaded to serious environmental problems duo to lack of effective and environmental friendly disposal methods.Considersing this facts,a novle idea of mineral carbonation of FGDG for CO₂ sequestration to prepare vaterite was proposed in this thesis.The feasibility of vaterite preparation from CO₂ mineralization by CaSO₄·2H₂O,vaterite growth mechanism in the gas-liquid-solid system,influence mechanism of impurity on vaterite generation from CO₂ mineralization by FGDG and study on mechanism of preparation of uniform vaterite Ca CO₃ from CO₂ mineralization by FGDG using crystalline controlling agent were systematically investigated in this work.The main research contents are as follows:(1)Pure CaSO₄·2H₂O was used as the model mineral of FGDG to study the feasibility of preparing vaterite via CO₂ mineralization by FGDG.The thermodynamic parameters of CO₂ mineralization by CaSO₄·2H₂O in ammonia medium were calculated.The effects of NH₄⁺/Ca²⁺molar ratio,CO₂ flow rate,solid-liquid ratio and stirring method(magnetic stirring and ultrasonic probe vibration)on mineralization process and products were studied.The process conditions of CO₂ mineralization by CaSO₄·2H₂O to prepare vaterite were optimized.The results show that under the optimal conditions of NH₄⁺/Ca²⁺molar ratio of2.4,CO₂ flow rate of 200 m L/min,solid-liquid ratio of 10 wt%and reaction time of 60 min,the mineralization product is pure and uniform vaterite.Additionally,CaSO₄·2H₂O conversion efficiency increased with increaseing molar ratio of NH₄⁺/Ca²⁺.When NH₄⁺/Ca²⁺molar ratio is 2.4,the conversion efficiency was about 98%.The polymorph and morphology of mineralization product(vaterite)were almost unaffected by CO₂ flow rate and solid-liquid ratio.Ultrasonic probe vibration did not change the polymoph of vaterite,but it significantly affected the morphology of vaterite.Under magnetic stirring condition,the average particle size of vaterite was about 10?m,and it was about 3?m under ultrasonic probe vibration condition.The stability study of vaterite shows that it was stabilized in aqueous solution for 2 hours,and then gradually transformed into calcite.Meanwhile,vaterite was stabilized below 400°C.(2)Vaterite growth mechanism in gas-liquid-solid system of CaSO₄·2H₂O-NH₃-CO₂-H₂O was explored.The results show that p H of magnetic stirring system decreased first and then tended to be stable.The concentration of SO₄^2-in magnetic stirring system gradually increased first,then increased to the maximum value and finally kept constant value.It took20 min when SO₄^2-concentration reached to the maximum value.The conversion efficiency of CaSO₄·2H₂O gradually increased with mineralization time and then remained stable,and it was 98%after reaction of 20 min.In the growth process of vaterite,spherical vaterite particles were generated after reaction for 5 min.With the extension of reaction time,it gradually accumulated and grew into larger spherical particles.Although the p H change of ultrasonic probe vibration system is slightly faster than that of magnetic stirring system,it took 30 min when SO₄^2-concentration reached to the maximum value.Ultrasonic amplitude significantly affected the conversion efficiency of CaSO₄·2H₂O.CaSO₄·2H₂O was almost completely converted into vaterite,and it was about 98%under the amplitude of 50%.For system of ultrasonic probe vibration,spherical vaterite was generated after reaction for 5min.It gradually becomes irregular particles,and finally accumulated into larger irregular particles with reaction time.The growth of vaterite was a complex process mainly including three stages.In the first stage,low ratio of Ca²⁺/CO₃^2-contributed to vaterite formation.In the second stage,the synergistic effect of low ratio of Ca²⁺/CO₃^2-and high concentration of SO₄^2-resulted in the formation of vaterite.In the third stage,high concentration of SO₄^2-contributed to the formation of vaterite.(3)The influence mechanism of impurity on the formation of vaterite from CO₂mineralization by FGDG was studied.The results show that the main phase of FGDG was CaSO₄·2H₂O.The impurities in FGDG were mica and dolomite.FGDG was rhombic crystal,and its purity is about 97.4%.EPMA results show that there were 8 main elements(Ca,O,S,Si,Al,Fe,K,and Mg)in FGDG.The mineralization product is a mixture of vaterite(?60%)and calcite(?40%),but the product from CO₂ mineralization by CaSO₄·2H₂O was vaterite under same conditions.It was found that when muscovite is added into the system of CO₂ mineralization by CaSO₄·2H₂O,the mineralization product is uniform vaterite.It is clear that muscovite does not affect the growth of vaterite.However,when dolomite was added to the system of CO₂ mineralization by CaSO₄·2H₂O,the mineralization product was a mixture of vaterite and calcite,and the content of calcite increased first and then decreased with increasing dolomite concentration.When dolomite concentration is 1.0 wt%,the content of calcite reached the maximum value(?27%).The results show that the hydrophilicity and surface negative charge of dolomite particles were the important factors for inducing CO₂ mineralization by CaSO₄·2H₂O to produce calcite.(4)The preparation of uniform vaterite Ca CO₃ via CO₂ mineralization by FGDG using crystalline control agent was studied.It is found that sodium stearate inhibited the formation of vaterite.The CO₂ mineralization by FGDG was not completed within 60 min in the presence of sodium polyacrylate because it prolongs the induction period of Ca CO₃crystallization.Also,FGDG was not completely converted to Ca CO₃ under acidic amino acids(glutamic acid and aspartic acid)due to carboxylic acid group in amino acids reacted with ammonia resulting in insufficient ammonia content.Glycine significantly increased the content of vaterite in mineralization products.Vaterite content firstly increased to the maximum and then kept constant with increasing of glycine concentration.The content of vaterite reached the maximum value of about 97%under 20 wt%glycine concentration.The formation of calcium glycinate and the adsorption of glycine played an important role in vaterite growth.