Utilization Of Phosphogypsum For The Preparation Of BSA Cement By Two-step Calcining Process

Phosphogypsum(PG)is a byproduct generated during the production of phosphoric acid by wet-process.Due to the low utilization,the majority of phosphogypsum is piled up.Currently,phosphogypsum has been utilized in many fileds,among which the use of phosphogypsum in building materials is an available way of effective recycling.Phosphogypsum is used as cement retarder,preparation of gypsum cementitious material and preparation of calcium sulphoaluminate cement.However,due to the negative effects of soluble phosphorus and fluorine impurities on the performance,the large consumption of phosphogypsum is limited.In this study,the partially decomposed phosphogypsum is used for the preparation of calcium sulphoaluminate cement.Phosphorus and fluorine impurities can be used as mineralizers for cement clinker formation.The generated CaO can substitute limestone to serve as calxium for the formation of clinker phases,The SO2 is collected for the production of sulfuric acid.the undecomposed phosphogypsum participates the formation of calcium sulfoaluminate.In this way,the goal of large consumption of phosphogypsum is achieved.However,since sulphoaluminate cement has a relatively high demand for aluminum-containing raw materials,not only raw materials are limited,but also increase production costs.Therefore,two-step calcining process of is used for the preparation of belite-sulphoaluminate cement by phosphogypsum,high-aluminaslag and high-iron slag.Phosphogypsum decomposition requires the reduction atmosphere,wile the formation of clinker phase requires the oxidation atmosphere.In this paper,the belite-sulphoaluminate cement clinker is prepared by two-step calcining prosess.Firstly,the decomposition characteristics of phosphogypsum were studied,the decomposition rate of phosphogypsum was increased,resulting a higher concentration of SO2.Based on the process of phosphogypsum decomposition with high decomposition rate,the decomposed phosphogypsum completely replaced limestone.The preparation of belite-calcium sulphoaluminate cement clinker and the hydration of the cement was investigated.The effects of different atmospheres,temperatures and C/Ss(molar ratio of carbon and sulfur)on the decomposition characteristics of phosphogypsum were studied.The results demonstrated that CO2 atmosphere can lower down the decomposition temperature of phosphogypsum.Temperature influences the decomposition products of phosphogypsum.The regulation of the temperature can purify the decomposition product of phosphogypsum.The decomposition of phosphogypsum is promoted in in the reducing C-CO2 atmosphere.The decomposition rate of phosphogypsum reaches 90.5% under the calcination system of CO2-1000 ? with the C/S ratio of 3,but CaS is the main decomposition product.The decomposition characteristics of PG were investigated mainly by two-step calcining process with weak reducing atmosphere in the low temperatureand oxidizing atmosphere in the high temperature.The results showed that the two-step calcining process of CO2-1000 ?-1h / air-1170 ?-1 h could enable the decomposition rate of phosphogypsum reach 91.5%.The decomposition product did not contain CaS.Meanwhile,the SO2 gas could be controlled to intensive release within the temperature range of 1000~1170 ? and the generated CaO could completely replace the limestone.By regulation of the calcining processes,the preparation of belite-calcium sulfoaluminate cement clinker was investigated.Results demonstrated that the optimum calcination process is CO2-950 ? /1h-Air-1000 ? /1h-1200 ? /30 min,and the phase composition of the clinker is almost the same with the designed clinker composition.But some calcium ferrite is present,This is due to the introduction of high-iron waste in the process of batching,part of the Fe2O3 and CaO combined to form part of the calcium ferrite.However,due to the increase in phosphogypsum content,the introduction of more phosphorus impurities,affecting the activity of calcium sulphoaluminate minerals,thus affecting the early hydration of belite-sulphoaluminate cement.But the cement hydration of sulphoaluminate cement after 3d could hardly be affected.The compressive strengths of the belite-sulphoaluminate cement at 3d,7 d and 28 d were 59.85 MPa,68.56 MPa and 72.45 MPa,separately.which is almost the same as the compressive of the belite-calcium sulfoaluminate cement prepared by traditional method.This shows that the use of partially decomposed phospogypsum by two-step calcining process is available.At 1000 ?,the SO2 could be collected,Achieve the goal of the integrated production of belite-calcium sulfoaluminate cement and sulfuric acid by partially decomposed phosphogypsum and the consumption of phosphogypsum.For 1 ton of belite-sulphoaluminate cement clinker,1.66 tons of phosphogypsum is consumed.