Basic Research On Reduction Mechanism Of The Flue Gas Desulfurized Gypsum And The Co-Disposal Of Reduction Products And Smelting Acidic Wastewater

Flue gas desulfurization（FGD）gypsum is produced in the process of removing SO₂ from flue gases.After 2020,the amount of FGD gypsum emitted in China each year exceeds 10⁸ tons,of which the metallurgical industry contributes to about 10～20%.The storage of FGD gypsum has become a growing problem,not only encroaching on numerous lands,but also causing serious harm to the environment.At present,FGD gypsum is mainly used as a mixture of ingredients for building materials.However,the addition of FGD gypsum in building materials has the maximum limit to maintain the performance of building materials.The utilization of FGD gypsum is a major issue in the smelting industry and environmental protection.In this paper,the reduction roasting behavior of FGD gypsum is studied.The reduction mechanism and S transformation are studied by laboratory test,isothermal thermogravimetric kinetics calculation,XRD and XPS.Based on the reduction mechanism,a new process of recycling of FGD gypsum is proposed.Ca and S in FGD gypsum are effectively recovered.An environmentally friendly technology that utilizes FGD gypsum to treat smelting acid wastewater are investigated.The main conclusions are as follows:（1）The thermodynamic analysis of Ca SO₄ reduction roasting show that it is feasible to completely reduce Ca SO₄ to Ca S with the C:Ca SO₄molar ratio of 2～3 and temperature at 700°C～900°C.The results of reduction roasting of Ca SO₄·2H₂O and FGD gypsum show that the increase of temperature and carbon content can effectively increase the content of Ca S,and promote the growth of Ca S particles in roasted products.However,higher temperatures cause Ca SO₄ and Ca S/C occur to form Ca O,which adversely affect the reduction behavior.The reduction rate of Ca SO₄·2H₂O is 90.67%,and the content of Ca S in the roasting product is 95.61%under the optimal conditions（carbon content of 30%,roasting temperature of 900°C and roasting time of 2 h）.Under the same conditions,the reduction rate of FGD gypsum is 97.89%,and the content of Ca S is 90.42%.Due to the poor crystal form in FGD gypsum,it is more conducive to the occurrence of reduction behavior in the high temperature process.This is more conducive to actual industrial production.（2）The reduction of Ca SO₄ particles occurs gradually from the outside to the inside:first,the reduction reaction occurs at the boundary of the particles and gradually shrinks to the core.Finally,the roasting product has a hollow-out structure.The reduction process conforms to the typical shrinking-core reaction model(1-（1-α）^1/3=kt),and the apparent activation energy of calcium sulfate reduction is 141.71 k J·mol^-1,which is mainly controlled by the phase boundary.The reduction process of Ca SO₄ to Ca S is a complex multi-step reduction process:the conversion process of element S is SO₄^2-→SO₃^2-→S₂^2-→S^2-,of which the reduction of calcium sulfate(SO₄^2-)to calcium sulfite(SO₃^2-)is the main control step of the whole reduction process.（3）An environmentally friendly technology that utilizes FGD gypsum to treat smelting acid wastewater is proposed.The reduction roasted product（Ca S）can react with acid to form H₂S gas.H₂S is used to vulcanize the heavy metals（HMs）in smelting acid wastewater without introducing other impurity ions,and the intermediate product（calcium sulfate）can be recycled.The results of the equilibrium composition of H₂S and Ca S show that Ca S can continuously generate H₂S under acidic conditions.The gas yield of H₂S is 95.52%under the optimal conditions（acid dosage of 1.5times the theoretical value,room temperature,liquid-solid ratio of 1:10 and time of 20 min）.The gas production process is mainly controlled by internal diffusion,and the apparent activation energy of the reaction is3.2086 k J·mol^-1.The kinetic reaction equation is 1-2/3α-（1-α）^2/3=0.028C_H+^2.6034（S/L）^1.97305e^-384/Tt.Under the optimized conditions,H₂S gas can effectively vulcanize the HMs in smelting acid wastewater.The removal efficiency of As,Cd,Pb and Sb are 99.99%,99.98%,99.93%and99.91%,respectively.And the residual ion concentrations of As,Cd,Pb and Sb are 0.022 ppm,0.106 ppm,0.003 ppm and 0.223 ppm,respectively.All of which meet the emission standards.There are 64 figures,30 tables and 210 references.