Mechanism of Preparing Calcium Sulfate Dihydrate Whiskers from Phosphogypsum under Atmospheric
Phosphogypsum(PG)is a bulk solid waste produced during wet phosphoric acid process,which is largely stocked and can cause seriously environmental pollution.Therefore,it is urgent to remove impurities,realizing its high valuable utilization as a resource.In this study,PG was extracted and purified by thermolysis purification using hydrochloric acid to obtain high quality and high lengh-diameter ratio calcium sulfate dihydrate whisker.Besides,the growth mechanism of calcium sulfate dihydrate whisker was simulated based on molecular dynamics.The results show that at 80 ℃,reaction time of 1.5 h and 4 mol/L hydrochloric acid,the mass fraction of calcium sulfate dihydrate in PG increases from 82.50％to 99.80％after thermolysis purification.In addition,after thermolysis purification,the content of P2O5 decreases from 1.78％(mass fraction,the same below)to 0.03％,the mass fraction of SiO2 decreases from 7.37％to 0.02％,and fluorine is almost completely removed.Finally,calcium sulfate dihydrate whisker products are obtained.The simulation calculation results exhibit that calcium sulfate dihydrate is more soluble in hydrochloric acid solution than that in pure water at 298 K.In addition,the solubility of calcium sulfate dihydrate decreases at increased temperature.Moreover,the(020)plane of calcium surfate dihydrate presents the lowest interplanar spacing,in which calcium sulfate ions are more closely arranged and conducive to the formation of orderly and complete crystals.The growth mechanisms of whisker indicate that,firstly,calcium sulfate dihydrate gradually dissolve into hydrochloric acid.At higher temperatures,the solution begins to supersaturate,calcium sulfate crystals precipitate to form crystal nuclei.Subsequently,the crystals grow along the spiral dislocation of(020)plane of calcium surface dihydrate,thereby forming calcium sulfate dihydrate whisker.
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