| Desulfurization gypsum is an industrial by-product that is generated when SO2 is absorbed by limestone slurry during the time of sulfur-containing flue gas pass through the desulfurization unit,and its main component is calcium sulfate dehydrate(CaSO4·2H2O).With the rapid development of society and industry,a large number of FGD is widely used,and the ecological cycle of FGD by-product gypsum gets more and more attention.The comprehensive utilization of FGD gypsum,not only can enhance the technology level of flue gas desulfurization and turn waste into treasure,but also optimize the desulfurization industry chain structure much further.Some ways were gradually proposed after years of effort from scholars at home and abroad,which were about comprehensive utilization of desulfurization gypsum:desulphurization gypsum was used as raw material to make wall plaster and a plaster,used to output building materials such as brick and board;used as cement retarder;used as soil conditioner;used to make calcium carbonate and ammonia sulfate etc,but these didn’t reach the industrial applications.The process of preparation calcium carbonate and ammonium sulfate with gypsum is similar to Metz Berger,which is proposed by Farben Chemical Industries(IG Farben).Therefore,the research of this paper has some theoretical basis and foundation.However,in terms of composition,structure,properties,reaction environmental and process control conditions,there are some differences between Metz Berger and this paper.These lead to different reaction mechanisms,so the research of this paper has certain necessity.This paper systematically studied the physical and chemical properties of the raw material desulfurization gypsum and calculated the thermodynamic properties of gypsum and ammonium carbonate system,at the same time it researched the reaction mechanism under the solid membrane diffusion controlled conditions;the experiment was conducted in a reaction separate apparatus to study the characteristics of the chemical reaction and separation about the preparation of calcium carbonate from desulfurization gypsum;this paper also studied the impact additives and stir control on the nucleation growth of calcium carbonate.The results as follows:1.Thermodynamic parameters of preparation calcium carbonate with desulfurization gypsum were calculated by use of the knowledge of thermodynamic and Factsage software;It examined the feasibility of the process theory through the analysis of changes in the reaction phase diagram and the simulation of the phase migration and transformation law.The results showed:the reaction of preparation calcium carbonate with desulfurization gypsum was a spontaneous exothermic reaction,so it was not beneficial for the reaction if increasing the temperature;the reaction rate constant was so big that the tendency of the reaction preparation of ammonium sulfate and calcium carbonate was large;the results of the phase simulation showed that the phase changes met the technical require,so the process has the theoretical feasibility.2.For the reaction of desulfurization gypsum and ammonium carbonate solution,the migration and diffusion of anion CO32-were the key in the entire transformation system,CO32-and Ca2+ within the particles gradually formed insoluble CaCO3.As the diffusion reaction,the un-reacted core(CaSO4 · 2H2O)was gradually reduced and the reaction process can be called the un-reacted shrinking core model.When the reaction time was 2 h,the granularity of desulphurization gypsum was no more than 0.125 mm,and the stirring rate was 250 rpm or higher,the experiment showed that the reaction process of desulphurization gypsum and ammonium carbonate was controlled by solid product layer diffusion substituting chemical reaction.Under the above conditions,the kinetic equation of the reaction and the relationship between reaction rate constant and temperature were obtained.3.Adequate reactions between desulphurization gypsum and ammonium carbonate,and the separation of the solid-liquid phase product(calcium carbonate and ammonium sulfate)can be realized by one step in a reaction separation device,the optimal experimental results as follows:chemical reaction conversion rate is 83.14%,solid and liquid separation efficiency is 81.83%.In order to study chemical reaction characteristics of the device,the growth process of solid phase calcium carbonate was studied:Calcium carbonate crystals gradually grew nucleation from small particles,the sulfur in solid phase was replaced by carbon gradually,amorphous calcium carbonate,including calcite,vaterite and aragonite,gradually transformed to stable single calcite.This paper Used CFD to study the separation characteristics of the reaction separation device.Separation characteristics of reaction separation device were obtained by CFD technology and found:Due to the rotation of the impeller,the liquid/solid phase around the impeller under the effect of centrifugal force.The quality of the solid phase particles is large,so its gravity is bigger too.For solid particles,because the centrifugal force was less than gravity,so solid particles sink gradually and were expelled from the discharge port.However,for liquid phase,gravity was less than the centrifugal force,so the liquid phase out-flowed along the drain tube.Eventually,solid phase and liquid phase were separated.4.Through the research of additives and procedures stir control impact on the growth nucleation process of calcium carbonate,its results as follows:When adding trisodium citrate dehydrate and Cetrimonium Bromide(CTMAB),the product was a single calcite calcium carbonate.However,adding sulfuric acid,the product calcium carbonate was the community of calcite,vaterite and aragonite the three polymorphs.For the three additives,the best solution of agitation program was the third method:in order to improve the rate of diffusion and chemical reaction and strengthen the process of mixing and mass transfer,the stirring rate was 450 rpm,40 min before the start of the experiment.Then system was stirred for 40min with 350 rpm rate,in order to prepare for the nucleation transformation from crystalline.The last 40 min was the time of crystalline calcium carbonate growth nucleation.Reducing the rate of stirring can effectively promote the formation of the completed calcium carbonate polymorphs. |
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