The Transmission And Transformation Of The Reductive Decomposition Process Of Phosphogypsum And The Impacts On The Phase Diagram

Phosphogypsum is a kind of big bulk industrial solid wastes stemming from phosphate fertilizer industry.With big production,low comprehensive utilization rate,hazardous properties,the resource utilization of phosphogypsum has become a problem to be solved urgently.Based on the previous work of our group,simulation compution was done on the equilibrium compounds of the systems of coal and gypsum under different atmosphere,with or without SiO₂,Al₂O₃ and Fe₂O₃,thus the possible reactions in those processes could be deduced.Next experiments were done in a thermal gravimetric analyzer and a tube furnace to throw light on the transmission and transformation of CaSO₄,CaO,CaS,S as well as the effect and mechanisms of SiO2,Al₂O₃ and Fe₂O₃ on the reductive decomposition of phosphogypsum.The conclusions have been drawn as follows:The transformation of CaS to CaSO₄ can be inhibited by high temperature?above 973.15 K?or non-oxidative atmosphere,and thus the decomposition of phosphogypsum will be promoted.The following simulation compution of the situation in the air also proves that carbon doesn't react as a reductant of the decomposition of phosphogypsum,while under nitrogen atmosphere,CaSO₄ will become CaO through the production of the temporary compounds CaCO₃ and sulphur.By conducting the simulation compution of mechanisms of SiO2,Al₂O₃ and Fe₂O₃ on the reductive decomposition of phosphogypsum,we found that:with the addition of SiO₂,the product Ca₃SiO₅ will be generated via the formation of the intermediates like CaSiO₃,CaSiO₃,Ca₃Si₂O₇ and Ca₂SiO₄;Ca₃Al₂O₆ will be generated via the formation of the intermediates like CaAl₁₂O₁₉,CaAl₄O₇ and CaAl₂O₄ with the addition of Al₂O₃;Fe₂O₃ will transform to Ca₂Fe₂O₅ through the formation of the intermediates like FeS₂,FeS and Fe₃O₄.The experiments of the reductive decomposition of gypsum under nitrogen atmosphere in the thermal gravimetric analyzer and the tube furnace reveals that the primary product CaO is generated through the reaction CaSO₄+3/2C=CaO+S+3/2CO₂T.The results of the experiments of the reductive decomposition of gypsum with the impurities in phosphogypsum under nitrogen atmosphere in the thermal gravimetric analyzer and the tube furnace show that:a)the decomposition mechanism of gypsum is very close to the theoretical situation and SiO₂ has little impact on the decomposition process;b)Al₂O₃ will slightly decrease the decomposition temperature of CaSO₄ and the intermediates CaAl₁₂O₁₉,CaAl₄O₇ or CaAl₂O₄ won't be formed which is different from the simulation results,and Ca₄Al₆O₁₂?SO₄?will be generated from Al₂O₃ and CaO;c)CaSO₄ will be transformed to CaO via reaction 4CaSO₄+xCaCO₃+?2-x?CaS+2Fe₃O₄+CO₂=3Ca₂Fe₂O₅+?1+x?CO+?5+x?SO₂+?0.5-x?S2 after the generation of CaCO₃ and CaS by reaction CaSO₄+3/2C=CaCO₃+S+1/2CO₂? and CaSO₄+12CaCO₃+12FeS=13CaS+4Fe₃O₄+12CO₂T,and the decomposition temperature will greatly drop with the addition of Fe₂O₃.With the aid of the comparasion between the theoretical phase diagram and the experimental results,quiet a lot of discrepancies of the phase regions were found which could lay firm foundations for the drawing of the practical phase diagrams.