Influences On The Performance And Effective Mechanism Of Phosphogypsum Modified With Various Pretreatment Processes

Discharged by the phosphate fertilizer enterprise during the production process and main ingredient is the dihydrate of calcium sulfate, the precipitate, phosphogypsum, is the main industrial by-products. Because it contains some acidic substances and heavy metals, which can severely contaminate the land, water and so on, phosphogypsum has been listed as a hazardous waste.In this thesis, various pretreatment methods were involved in different phase composition of phosphogypsum, especially focusing on studying the modification mechanism, sorting out the optimal process in the same phase composition and comparing the advantages and disadvantages of different methods. By using the characteristics that separating phrases of phosphogypsum had the same hydration products, multiphase phosphogypsum was modified with mixture methods, therefore achieving the purposes of utilizations of phosphogypsum massively, economically and efficiently.Research shows:(1) Washed with water and neutralized with modifier could significantly improve the pH value and reduce the content of soluble impurities of phosphogypsum but washing consumes loads of water. Regarding to soluble phosphorus and fluoride, washed with water and cured with modifier could both decrease70%of the soluble fluoride content with same efficiency. On the other hand, cured with modifier was ahead of the washing process in coping with soluble phosphorus.(2) Weighed according to the mass proportion,25:15:3, phosphogypsum, fly ash and cement were mixed with3%modifier and then grinded for25min. In the light of this way, cementitious materials were prepared, which compressive strength and softening coefficient after hydrated for28d was up to27.9Mpa and86%, respectively, without wastewater which might prevent second time pollutions.(3) Doped with2%modifier, grinded for15min and calcined for2h,(3semi-hydrated phosphogypsum was prepared, which initial setting time, final setting time, flexural strength and compressive strength was7min,11min,2.7Mpa and4.9Mpa, respectively, meeting the requirements of " GBT9776-2008building gypsu"(4) Added phosphogypsum powders which dried at40℃to constant weight into24%CaCl2salt solution heated to95℃under atmospheric pressure and waited until the completion of whole reaction, a semi-hydrated phosphogypsum powders were produced, which pH values were close to neutral without soluble phosphorus and fluorine. However, heated at low temperature might cause incomplete crystal transformation of dihydrate phosphogypsum, while heated at high temperature might give birth to II anhydrous phosphogypsum which hardly hydrated in pure water.(5) Mixed phosphogypsum with3%modifier, calcined at800℃for2h after grinding for l0min and then added with1%of K2SO4and4%β semi-hydrate phosphogypsum, cementitious materials can be prepared, which the initial setting time, the flexural strength and compressive strength after hydrated for3d and28d were4.7h,2.02Mpa,7.6Mpa,5.9Mpa,18.1Mpa, respectively.