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Phosphogypsum (PG) is the byproduct of wet-process phosphoric acid (WPA),producing one tonne phosphoric acid will produce about five tons PG. The output of PGis more than5000tons, while the effective utilization rate is less than10%, whichseriously affect the sustainable development of phosphorus chemical industry. Poorcrystal form, harmful impurities, volatility of the quality and lack of quality assessmentstandards of PG are important reasons restricting PG used as building materials. Thecurrent resource utilization technologies for PG are focus on improving its quality bypretreatment and modified, the effectiveness is not remarkable. The quality of PGmainly depends on the phosphoric acid process. In this paper, the relationship betweenthe dihydrate WPA production process and the crystalline form, water-soluble P2O5,co-crystallized P2O5of PG was explored by simulating dihydrate WPA production inlaboratory, improving and optimizing the phosphoric acid process to realize thepromotion of PG quality and the effective recovery of phosphorus resource. Theinfluencing rule and mechanism of impurities and crystalline form on performances ofphosphorus building gypsum were deeply studied, on this basis, an assessment systemfor PG quality was studied and proposed combining the quality of domestic PG, thendiscussed the technical ways for PG to prepare building plaster and wall plaster, theseprovide a strong theoretical basis for regulating emissions and resource utilization ofPG.The content of water-soluble P2O5in PG mainly depends on the filtration washingprocess of PG in phosphoric acid production. The crystalline form, washing watertemperature, washing liquid to solid ratio and washing times mainly affect the washingrate. When the crystalline form of PG changed from the fine needle and thin sheet to thethick columnar and rhombic, the washing rate increased by about3%, and the content ofwater-soluble P2O5reduced by42%. The washing water temperature raised from40℃to80℃, the washing rate increased by1.6%, and the content of water-soluble P2O5reduced by36%. The washing liquid to solid ratio improved from1.5:1to2.5:1, thewashing rate increases by2.2%, and the content of water-soluble P2O5reduced by47%.The washing times increased from1to3, the washing rate increased by1.8%, and thecontent of water-soluble P2O5reduced by32%. When the filtration washing process was optimized through experiments, the water-soluble P2O5content in PG reduced by40%compared with the existing plant process.The degree of crystallization supersaturation of dihydrate gypsum is the mainfactor influencing the content of co-crystallized P2O5. The content of co-crystallizedP2O5is positively correlated with the degree of crystallization supersaturation. Raisingthe reaction temperature, increasing the SO3concentration in liquid phase or decreasingthe P2O5concentration in liquid phase can reduce the degree of crystallizationsupersaturation. When the degree of crystallization supersaturation is controlled below1.4, the content of co-crystallized P2O5of PG will reduce to below0.4%.The degree of crystallization supersaturation in extraction reaction process mainlyaffects the crystalline shape of PG. The extraction of WPA production process wasstudied, the results showed that:(1)The fineness of ground phosphate rocks, SO3andP2O5concentration in liquid phase affect both crystal shape and crystal size. When theSO3concentration in liquid phase increased, the crystallization supersaturationdecreased, the crystal shape changed from thin sheet into thick columnar and rhombic,finally changed to polycrystal, the average size of crystal became larger. When thespecific surface area of ground phosphate rock increased, the P2O5concentration inliquid phase increased, the crystallization supersaturation increased, the crystal shapechanged from thick plate into thick columnar, finally changed to thin needle, theaverage size of crystal became smaller.(2)The reaction temperature, liquid-solid ratio ofslurry, crystal growing time and crystal growing temperature, only impact crystal size.To improve the reaction temperature, reduce liquid-solid ratio of slurry, extend crystalgrowing time and improve crystal growing temperature, the crystallizationsupersaturation decreased, crystal size became larger, crystalline form was stillcolumnar or plate.The water-soluble P2O5reduces the dehydration temperature of PG, especially forthe first dehydration temperature. During the hydration of the building gypsum, thesetting time was delayed, the crystallization supersaturation of dihydrate gypsumdecreased, the gypsum crystal became thick, the strength of the hardened paste becamelower. Among the three forms of water-soluble P2O5, the effect on the gypsumproperties arranged in this order: H3PO4>H2PO4->HPO42-. The co-crystallized P2O5didnot change in the process of calcination, still existed in the gypsum crystal lattice, but inthe hydration process, the co-crystallized P2O5continuously dissolved from the crystal lattice, changed into water-soluble HPO42-, which ionized to H+and PO43-, PO43-quickly binded with Ca2+in the solution, changed to Ca3(PO4)2which covered on thecrystal surface, hindered the gypsum further hydration, resulted in the reduction ofhardened paste strength, and the surplus of H+led to decrease in pH value of the slurry.The crystal shape and crystal size of PG seriously influence the physical and mechanicalproperties of its cementitious materials. Thick rhombic plate crystal has the worstperformance on phosphorus building gypsum, prismatic crystal with largerlength-diameter ratio is superior, short columnar crystal with smaller length-diameterratio is the best. When the crystal shape is rhombic tabular, the larger the aspect ratio,the greater the thickness, the more close to the prismatic, the better the phosphorusbuilding gypsum properties.In this paper,“whether PG needs to be pretreated so as to produce the qualifiedproduct " is used as the classification principle, which is different from the existingstandards according to the grade to classify PG, adding the content of water-solubleP2O5, water-soluble fluoride, co-crystallized P2O5, organic matter, pH value andfineness to the quality evaluation index system. Based on the quality situation ofdomestic PG and the influence rule of the impurities, pH value and fineness on theperformance, set up the corresponding limit value of the evaluation index, divided PGinto two grades. The first level PG produce the qualified building gypsum directly, thesecond level needs non-washing pretreatment. According to the classification indexsystem, about75%of domestic PG reached to the standard of the second level andabove, of which about20%conformed to the first level. The PG production enterprises,which produced the remaining25%PG not conformed to the index system requirements,need to improve the phosphoric acid production process, and the production can alsomeet the index system requirements.The calcination system for preparation of phosphorus building gypsum is differentfrom natural gypsum. The calcination temperature of PG decreased with the increase ofthe water-soluble P2O5content. The high-class building gypsum can be prepared usingthe first level PG when controlling the calcination temperature at150℃to160℃and keeping constant temperature for1.5h to2.0h. To produce the qualified productusing the second level PG, the calcination temperature should be decreased by20℃to30℃. The phosphogypsum-based plaster gypsum can be produced by adding binder,retarder and water-retaining agent to the building gypsum prepared with the second level PG, its performance is excellent and it has a good prospect.
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