Gypsum Thermal Decomposition Characteristics Of The Experimental And Mechanistic Studies

Gypsum (main component CaSO2H2O) is the main by-product of flue gas desulfurization (FGD) with calcium-based sorbents (such as CaO,Ca(OH)2,CaCO3). With the development of industrial and environmental protection, more and more gypsum will be produced from FGD process. Serious pollutions of soil and groundwater and waste of land will be taken place due to unreasonable utilization of gypsum. Thermal chemical treatment technology is promoting technology for utilization of gypsum. Gypsum was decomposed to recyclable CaO and SO2 based on this technology. It is very important to deal with the characteristics of gypsum decomposition at high temperature for development and application of the thermal treatment technology of gypsum. In this paper, the decomposition characteristics of four kinds of gypsum (including analytical gypsum, natural gypsum, dry FGD gypsum and Wet FGD gypsum) were carried out by experimental and theoretical method. The detail research work and conclusion are listed as follows:Physical and chemical characteristics of gypsum were studied by using instruments of Malvern Laser particle size analyzer (Mastersizer 2000), XRD, XRF and scanning electron microscope-energy dispersive spectrometer (SEM-EDS). The results indicated that CaSO4·2H2O and CaSO4·0.5H2O are the main components of analytical gypsum (AG); CaSO4 is the main component of natural gypsum (NG); CaSO4·0.5H2O is the main components of dry FGD gypsum (DG); and CaSO4·2H2O is the main component of wet FGD gypsum (WG). XRF shows the contents of CaO and SO3 are approximately 30% and in the range of 40%-50% for all four kinds of gypsum, respectively.Thermodynamic parameters (such as△G,△H) of decomposition reaction of CaSO4 and system of CaSO4/CO were calculated by using software of HSC Chemistry. Results of thermodynamic calculation indicated that decomposition of CaSO4 can not be taken place at the temperature of less than 1700℃. The initial decomposition temperature and reaction enthalpy can be decreased significantly due to the addition of CO into the reaction system. The results of thermodynamic calculations also suggested that the optimum temperature of reductive decomposition of FGD gypsum was in the range of 800℃-1200℃.Decomposition characteristics and kinetics of analytical gypsum, natural gypsum, dry FGD gypsum and wet FGD gypsum were studied in the nitrogen atmosphere by using thermal analyzer. The results indicated that:1) initial decomposition temperature of FGD gypsums was about 1000℃, which is much lower than that of analytical gypsum and natural gypsum (1200℃) and indicates impurities bound in FGD gypsums have a benefit on the decomposition reaction of CaSO4; 2) the higher reaction temperature is, the more decomposition ratio of gypsum is. Rule of decomposition of gypsum accords with the model of nucleation and growth of gas-solid reaction mechanism; 3) kinetics parameters (such as activation energy E and frequency factor A) of thermal decomposition of AG, NG, DG and WG were calculated based on TG-DTA curves and Coast-Redfern method. Activation energy of FGD gypsums was much lower than that of AG and NG. The order of activation energy was:AG>NG> FGD gypsums.The reductive decomposition characteristics of AG, NG, DG and WG were studied in a reduction atmosphere of CO/N2 by using thermal analyzer. The results indicated that the factors of final reaction temperature and reaction atmosphere have significant influences on the decomposition of gypsum. All gypsums can be decomposed at tested condition and get the total decomposition rate of 40.69%-90.03%; the highest total decomposition rate of AG is 90.03% at the condition of 1050℃,2%CO. In the condition of 850℃-1050℃, the total decomposition rate of four gypsums increases with increasing of final reaction temperature, and the time-consumption of reaching decomposition equilibrium of four gypsums is shortened with increasing of final reaction temperature. In the condition of 0%-5%CO, the total decomposition rate of all four tested gypsums increases with increasing of CO volume fraction, and the time-consumption of reaching decomposition equilibrium of four gypsums is shortened with increasing of CO volume fraction. However, the reductive composition rate of different gypsums behaves different patterns at experimental conditions. Further studies need to be done in the future.Reaction mechanism of high-temperature decomposition of gypsum was discussed based on the above research. The effects of reaction temperature, reaction atmosphere, and catalytic components on decomposition of gypsums were analyzed. The results indicated that mineral components (such as Fe, Si, etc) bound in gypsums have important catalytic effects on decomposition of different kinds of gypsums. Doping of Fe2O3 in analytic gypsum not only increases the total decomposition rate, but also improves reaction rate at reasonable doping amount. AG with 5% Fe2O3 doping is more effective based on the total decomposition rate; however, AG with 10% Fe2O3 doping is slightly better on the basis of the decomposition reaction rate. So, in order to increase decomposition rate of FGD gypsum, it is very important to select reasonable reaction parameter and operation condition based on characteristics of different FGD gypsums.