Research On Factors Affecting The Quality And Quality Evaluation Index System Of Phosphogypsum

Phosphogypsum （PG） is the byproduct of dihydrate wet-process phosphoric acid（WPA）, producing one ton phosphoric acid will produce about five tons PG. Thissituation of PG which with unstable crystal form and instability quality, contains kindsof harmful impurities and lacks of targeted quality assessment standards affects itsresource process severely. The accumulated storage capacity of PG is more than200million tons, while the effective utilization rate is just about20%. PG was piled uplong-term as solid waste in the residue field, which not only takes up valuable landresources, but also has serious impact on the surrounding ecological environment andhas seriously impact on the sustainable development of phosphorus chemical industry.In this paper, the effect of dihydrate WPA production process on the crystalline of PGwas explored by simulating dehydrate WPA in laboratory. The influencing rule andmechanism of impurities and crystalline form on performance of phosphorus buildinggypsum were studied, on the basis of which and research results of other scholars thequality evaluation index system of PG used to produce building gypsum was proposed.The extraction process affects crystal form of PG greatly: The liquid-solid ratio ofslurry and reaction temperature only affect crystal size, SO₃and P₂O₅concentration inliquid phase affect both crystal shape and crystal size. The crystal shape remainscolumnar and the average particle diameter becomes smaller when the liquid-solid ratioof slurry increased. The crystal shape remains columnar and the average particlediameter becomes larger when the reaction temperature improved. The crystal shapechanges from flaky to coarse columnar to polycrystalline and the average particlediameter becomes larger when the SO₃concentration in liquid phase increased. Thecrystal shape changes from plate to columnar to fine acicular and the average particlediameter becomes smaller when the P₂O₅concentration in liquid phase increased.The crystal growing condition such as crystal growing time and crystalgrowing temperature affect crystal size of PG greatly: The crystal shape essentially thesame but the crystal size becomes larger when the crystal growing time extend or thecrystal growing temperature improved, but the calcium sulfate dihydrate will dehytrateinto hemihydrate or anhydrous and the crystal size will becomes smaller when thecrystal growing temperature is too high.Water-soluble P₂O₅exists in the form of H₃PO₄lets dehydration temperature reduced significantly in the process of dehydrate gypsum dewatering, but fluorine hasno effect on dehydration temperature. The performance of building plaster made fromPG with thick rhombic tabular crystals is the worst. The performance of building plastermade from PG with prismatic crystals of larger aspect ratio is better. The performanceof building plaster made from PG with short columnar crystals of smaller aspect ratio isthe best. Different forms of water-soluble P₂O₅will extend the setting time and reducethe strength of building gypsum, but the impact is in this order: H₃PO₄>H₂PO₄^->HPO₄^2-.NaF will shortened the setting time and reduce the strength of building gypsum.This paper proposes the quality evaluation index system of PG used toproduce building gypsum by limited the grade, content of main impurities and otherindicators based on the principle as "whether PG needs to be deal with simplenon-washing pretreatment or not to produce qualified building gypsum", and based onthe content of this study and research results on PG of many other scholars. PG isdivided into two levels in this standard, the first level of PG can be used to producequalified building gypsum directly, the second level of PG need to be treated by simplenon-washing pretreatment such as adding lime to neutralize the acid or screening toproduce qualified building gypsum.