Study On Mechanism Of Preparation Of Calcium Carbonate By Phosphogypsum Ammonia Method For CO₂ Capture

Phosphogypsum is a by-product of wet smelting of phosphoric acid,and its main component is?CaSO₄·2H₂O?.It is classified as fluorine-containing hazardous solid waste because of its corrosiveness and volatility.The soluble impurities such as fluorine,phosphorus,organic acids and organic substances contained in it are washed by rainwater and immersed in water and soil,causing great harm to surrounding animals and plants and human health.Carbon dioxide is one of the greenhouse gases,and climate warming in recent decades has been caused by massive emissions of greenhouse gases.Among them,emissions account for 30%of the world,and reducing carbon emissions is an urgent task.The abundant calcium resources in phosphogypsum provide the possibility of CO₂capture,and can also achieve CO₂ capture fixation and utilization of phosphogypsum.In this paper,the phosphogypsum ammonia method is used to capture CO₂,and the phosphogypsum is used to absorb carbon dioxide in an alkaline solution to prepare calcium carbonate.The abundant calcium resources in phosphogypsum can be used as an absorbent for CO₂,and its absorption effect is good,high temperature resistance and large storage capacity make phosphogypsum an excellent carbon dioxide absorber.With the idea of“treatment of wastes with processes of wastes against one another”,solid waste is used to solve gas waste.According to the theoretical knowledge,this mechanism research experiment,and proposed a new solution for the resource utilization of phosphogypsum and the storage of carbon dioxide.?1?Research on carbonization reaction.Phosphogypsum can obtain the optimal conditions of the reaction by controlling the various conditions of the phosphogypsum ammonia method:the phosphogypsum particle size is 0.12 mm,the ammonia water volume is 25 ml,the time is 90 min,and the temperature is 30°C.Under the reaction,the reaction rate is sufficient,and the conversion rate of calcium sulfate can reach 99.3%.The study of the change of pH and SO₄^2-concentration in the carbonization reaction shows that increasing the CO₂ flow rate can increase the concentration of SO₄^2-in the solution,that is,promote the dissolution of calcium sulfate,and increase the rate of carbonization reaction.XRD and SEM analysis of the solid product calcium carbonate showed that the crystal form of the product calcium carbonate was related to Ca²⁺.When the concentration of Ca²⁺was low,calcium carbonate with a crystal form of calcite was more likely to be produced;and when the concentration of Ca²⁺was high,It is more conducive to the formation of thermodynamically unstable vaterite calcium carbonate.?2?The mechanism of the CO₂ reaction of phosphogypsum ammonia method was studied.The thermodynamics software Factsage7.1 was used to capture the CO₂ of the phosphogypsum ammonia method.The results show that the reaction of the phosphogypsum ammonia method to capture CO₂ to prepare calcium carbonate is a self-heating reaction,and the temperature will be lowered by some means.It will be beneficial to the reaction;the kinetic study of this reaction concluded that the substantially dissolved Ca²⁺reacted with CO₃^2-in the solution to form a more insoluble calcium carbonate precipitate,which reduced the Ca²⁺concentration in the solution.And promote the dissolution of calcium sulfate dihydrate.The migration and diffusion of CO₃^2-in the whole system is the key,and CO₃^2-gradually forms CaCO₃ which is insoluble with Ca²⁺.As the diffusion reaction proceeds,the unreacted phosphogypsum?CaSO₄·2H₂O?gradually shrinks,and the reaction belongs to the shrinkage unreacted core model.It has been found through experiments that the intrinsic kinetic equation of the reaction and the relationship between the reaction rate constant and the temperature can be obtained under the control of chemical conditions.?3?The effects of impurities in phosphogypsum on carbonization reaction and calcium carbonate crystal form were studied.The results show that the impurities Na⁺,Mg²⁺,P reduce the carbon sequestration rate of gypsum capture CO₂ and the conversion rate of calcium sulfate.The increase in the content of impurities and the amount of carbon sequestration also increased,but when F^-exceeded 2%,both the conversion rate and the amount of carbon fixation decreased.The impurities Na⁺and P have little effect on the crystal form of the product.The addition of a small amount of Mg²⁺promotes the change of calcium carbonate from the vaterite type to the calcite type;when more than5%Mg²⁺is added,it changes from the calcite type to the vaterite type.When more than0.5%of the impurity F^-is added,the calcium carbonate crystal form changes from the vaterite type to the calcite type.Impurity F^-can make the form of calcium carbonate basically regular,but it will also make the particle size different and produce agglomeration.Impurity P makes its morphology extremely irregular and causes severe agglomeration.A small amount of Mg²⁺allows the crystal form of the product to be a square calcite type and has a regular shape.10%Mg²⁺makes the calcium carbonate crystal form agglomerated severely and the particles become large with many fine particles.The mechanism of the CO₂ reaction of phosphogypsum ammonia method was studied.The thermodynamics software Factsage7.1 was used to capture the CO₂ of the phosphogypsum ammonia method.The results show that the reaction of the phosphogypsum ammonia method to capture CO₂ to prepare calcium carbonate is a self-heating reaction,and the temperature will be lowered by some means.It will be beneficial to the reaction;the kinetic study of this reaction concluded that the substantially dissolved Ca²⁺reacted with CO₃^2-in the solution to form a more insoluble calcium carbonate precipitate,which reduced the Ca²⁺concentration in the solution.And promote the dissolution of calcium sulfate dihydrate.The migration and diffusion of CO₃^2-in the whole system is the key,and CO₃^2-gradually forms CaCO₃ which is insoluble with Ca²⁺.As the diffusion reaction proceeds,the unreacted phosphogypsum?CaSO₄·2H₂O?gradually shrinks,and the reaction belongs to the shrinkage unreacted core model.It has been found through experiments that the intrinsic kinetic equation of the reaction and the relationship between the reaction rate constant and the temperature can be obtained under the control of chemical conditions.