Mechanism Analysis Of Reductive Decomposition Of Phosphogypsum Under Hydrogen Sulfide Atmosphere

Phosphogypsum is a solid industry by-product from wet producing process of phosphoric acid and the main chemical component of phosphogypsum is CaSO4·2H2O. As a kind of solid waste, the quantity of phosphogypsum is the largest in the phosphorus chemical industry. Because of the huge amount of phosphogypsum, the hazardous effects on the environment and the lower utilization rate, the comprehensive utilization of phosphogypsum is imminent and the environmental pressure of phosphorus chemical enterprises is becoming more and more heavily with the improvement of environmental requirements. Thus, reclaiming the sulfur and calcium resources from the phosphogypsum decomposition residue which are produced in the decomposition process of phosphogypsum is becoming one of the most promising way in using phosphogypsum. And many scholars have done some researches on the process of producing cement from the production process of sulfuric acid by using phosphogypsum, and some progress has also been made. However, the process is restricted by the problem of higher temperature and energy-consumption on the decomposition of phosphogypsum. In order to reduce the decomposition temperature of phosphogypsumln, obtain a decrease in the cost of energy consumption of using phosphogypsum and provide a new way and theoretical reference to the comprehensive utilization of phosphogypsum, the decomposition reaction of phosphogypsum was investigated by using H2S as the reducing gas and oxides as the additives. And the target product was CaS in the decomposition process of phosphogypsum.In this paper, the thermodynamic behavior of the decomposition reaction of phosphogypsum in the reducing atmosphere of hydrogen sulfide was determined by using the thermodynamic software named Factsage6.1. And the molar gibbs free energy change (⊿rGm) and molar enthalpy change (⊿rHm) of those reactions probably occurred in the decomposition process of phosphogypsum in the reducing atmosphere of H2S were calculated using the Reaction module. It can be seen from the calculation results that most of the reactions occurred in the decomposition process of phosphogypsum were endothermic reaction and the initial decomposition temperature of phosphogypsum was reduced from1250℃which is the theoretical temperature of direct pyrolysis of phosphogpsum to800℃. In addition, the simulation of components produced in the decomposition process of phosphogypsum was calculated using the Equilib module, and the result showed that the decomposition of phosphogypsum was not a single reaction of hydrogen sulfide and calcium sulfate in phosphogpsum but the coupling between two or more reaction. Some complex mutual transformation between those reactants and products or product and product were occurred and the main reaction of decomposition of phosphogypsum in the reducing atmosphere of H2S was listed in Eqs.(1)-(2). CaSO4(s)+H2S-�'CaS(s)+H2O(g)+SO2(g)(1)3CaSO4(s)+CaS(s)�'4CaO(s)+4SO2(g)(2)The effects of the different factors on the decomposition of phosphogypsum in the reducing atmosphere of hydrogen sulfide were studied by using the thermal analyzer. The TG curve showed that the second weight loss step of phosphogypsum was occurred at850℃at which CaSO4has begun to decompose. It can be also found from various factors that the fastest rate and best effect of decomposition of phosohogyosum were obtained under the condition of the particle size of100～200of phosphogypsum, the temperature of1000℃, the heating rate of5℃per minute and the content of5%Vol of hydrogen sulfide.Amplification experiments on the results obtained by thermal analysis experiment were carried out in the tubular furnace and the chemical analysis on the phosphogypsum decomposition slag produced in the amplification experiments. The results showed that the decomposition effect of phosphogypsum was ineffective at850℃and the main components of decomposition slag were CaS and CaSO4undecomposed completely in the decomposition of phosphogypsum. The decomposition rate reached to89%and the main product in the slag is CaS which accounted for45%when the temperature increased to950℃. And the decomposition rate increased from89%to94%at1050℃, and meanwhile, the changes on the components in the decomposition slag were obvious. The content of calcium sulfide decreased to30%and calcium oxide was determined in the slag. It can be interpreted that the parallel competing reaction between calcium sulfide and calcium oxide occurred at1050℃led to the changes. As a result, lower decomposition temperature of phosphogypsum and a quantity of calcium sulfide can be obtained easily when the particle size of phosphogypsum is100-200mesh, the reaction temperature is950℃and the heating rate is5℃per minute.The effect of the additives CaCl2、Fe2CO3and MgO to the reaction system of phosphogypsum in the reducing atmosphere of hydrogen sulfide were studied in this paper. The result shows that, the decomposition temperature of the reaction system were significantly reduced(from950℃to850℃) when CaClor Fe2O3were added, respectively. However, there were no significant phenomenon were observed when MgO were added.