Evolution Of Harmful Impurities In Phosphogypsum During Calcination And Their Effect On Properties Of Dehydrated Phase

Phosphogypsum（PG）,a by-product of industrial smelting phosphoric acid,is a relatively high-grade gypsum resource,but harmful impurities（mainly refers to phosphorus and fluorine,hereinafter named as impurities）limit its utilization as building materials.The calcination method can comprehensively and efficiently remove impurities and also prepare PG-based cementitious materials.However,the evolution of impurities in PG during the calcination process and their effect on the properties of the dehydrated phase of PG are still unclear.Therefore,elaborating the evolution of impurities in PG during calcination and the influence on the properties of dehydration phase is the basis and key to promote the PG calcination to remove impurities and prepare cementitious materials.To address this problem,this thesis studied the evolution mechanism of different form of soluble phosphorus,soluble fluorine,eutectic phosphorus,and organic matters in PG calcination process based on the composition and structure of impurities.Furthermore,the influence of soluble phosphorus and soluble fluorine impurities on the properties ofβ-hemihydrate PG prepared from 150℃ and anhydrite prepared from 800℃ were investigated.Through reaction thermodynamic calculations and experimental test verification,this thesis studied the distribution and types,solid phase reaction,composition and content of impurities in PG.At low calcination temperatures（150℃～400℃）,H₃PO₄ was dehydrated to H₄P₂O₇ and then converted to HPO₃.Ca（H₂PO₄）₂·H₂O and CaHPO₄·2H₂O were dehydrated to Ca（H₂PO₄）₂ and CaHPO₄·2H₂O,respectively.At high calcination temperatures（500℃～800℃）,HPO₃ was transformed into Ca（PO₃）₂,Ca（H₂PO₄）₂ mainly into Ca₂P₂O₇,CaHPO₄ into Ca₂P₂O₇,and eutectic phosphorus was completely removed at 800℃.Na₂SiF₆was first decomposed into NaF and volatile SiF₄ at 350℃-500℃.NaF transformed into CaF₂at 600℃-800℃.The flotation organic matter were mainly phenol,carboxylic acid,silane,sulfide,and amide substances,which decomposed at 120℃,179.8℃,317.9℃ and 355.6℃ in turn.The polyacrylamide began to crack into alkanes above 200℃.At low calcination temperatures,the composition and structure of impurities did not obviously change.The content of soluble and eutectic phosphorus had no changed,the content of soluble fluorine slight decreased,and the organic matters is difficult to remove effectively.At high calcination temperatures,the composition and structure of impurities slightly changed.The content of soluble phosphorus,eutectic phosphorus,and soluble fluorine significantly reduced.The organic content was 0%at 500℃.Through comparative experiments in the properties of untreated PG and water-washed PG calcined at 150℃-800℃,this thesis studied the effect of impurities evolution on the properties of the PG dehydrated phase.At low calcination temperatures,impurities could increase the water requirement,shorten the setting time,and reduce the strength.The crystal aspect ratio decreased,and the interconnection became weaker.At high calcination temperatures,impurities could promote the setting and hardening of anhydrous PG and increase the strength The hardened untreated PG calcined at 800℃ achieved the maximum strength at whole calcination process with the maximum compressive and flexural strength of17.1 MPa and 6.1 MPa.The crystal developed fine and lapped tightly.Specifically,this thesis studied the effects of soluble fluorine（F^-）and different forms of soluble phosphorus(H₃PO₄,H₂PO₄^-,and HPO₄^2-)impurities on the properties of hemihydrate PG（β-CaSO₄·0.5H₂O）prepared at 150℃.The results show that H₃PO₄ and H₂PO₄^-could promoted the dissolution of CaSO₄·0.5H₂O and the nucleation of CaSO₄·2H₂O,while HPO₄^2-prolonged CaSO₄·2H₂O nucleated,but all of them promoted the overall hydration process ofβ-CaSO₄·0.5H₂O.During hydration process,only CaSO₄·2H₂O was precipitated in the liquid phase.HPO₄^2-was adsorbed on the CaSO₄·2H₂O surface,replaced SO₄^2-and generated CaHPO₄·2H₂O,which is conducive to the nucleation of CaSO₄·2H₂O due to its similar crystal structure to CaSO₄·2H₂O.The active site Caon the CaSO₄·2H₂O surface could capture electrons from functional groups contained P in free P,thereby strongly adsorbed free P and made the deprotonation of H₃PO₄,H₂PO₄^-and HPO₄^2-,resulting in the difference between adsorbed P and free P species and distribution.Thus,different kinds and contents of soluble phosphorus have different effects on the setting time.The adsorption amount and deprotonation of free P on the CaSO₄·2H₂O surface were two opposite factors that affected the 2h strength of hardened PG.After drying,the connection method of the CaSO₄·2H₂O grains affected the dry strength.Soluble fluorine could accelerate the dissolution of CaSO₄·0.5H₂O and the nucleation of CaSO₄·2H₂O.The larger the dosage,the more obvious effect was.CaF₂ precipitated more easily in the liquid phase than CaSO₄·2H₂O and thus provided nucleation site,which accelerated the hydration process of CaSO₄·0.5H₂O,but had no effect on the final hydration degree.The active site Caof CaSO₄·2H₂O made the free F and CaF₂ in the liquid phase adsorb on the（111）crystal plane,resulting in the decrease of the crystal aspect ratio,the increase of the porosity,and the decrease of the strength.Specifically,this thesis studied the effects of different compositions of phosphorus and fluorine on the crystal structure and hydration and hardening properties of anhydrous PG prepared at 800℃.The results show that phosphorus and fluorine impurities could improve the early hydration,refine the pore size,increased the strength of anhydrous PG.The early properties of anhydrite prepared from PG with H₃PO₄ is similar to that from Ca（H₂PO₄）₂·H₂O.The crystal developed fine and stacked in a plate shape.The anhydrite prepared from PG with NaF showed a higher hydration degree,but the early strength decreased with the increase of content from 0.5%to 1.5%due to the low saturation index and the adsorption of CaF₂ on the crystal surface,which resulted in the reduced crystal size and increased porosity.Remarkably,all the samples,regardless of impurities contents,source,and storage time,presented similar strength,hydration degree,microstructure,and pore structure in the late hydration stage.P and F atoms enter into the lattice by replacing the S and O atoms in CaSO₄,respectively,leading to the increase of point defects and crystal activity.Moreover,the impurities reacted with CaSO₄ to form the sulfate activator and converted calcium phosphate adsorbed on the facet to form more nucleation site,which promoted the dissolution of anhydrous PG and the nucleation and precipitation of CaSO₄·2H₂O crystal,resulting in the improvement in the early hydration degree and strength.