Effect of NaCl solution concentration on transcrystallization behavior of α-hemihydrate gypsum from phosphogypsum
In order to improve the utilization rate and added value of phosphogypsum by-products discharged from wet phos-phoric acid process,α-hemihydrate phosphogypsum materials were prepared in NaCl solution by the atmospheric pressure hydrothermal synthesis method,and the effects of NaCl solution concentration on crystal water content,crystallization be-havior,mineral phase composition and microscopic morphology of α-hemihydrous phosphogypsum were studied.The results showed that the crystalline water content of phosphogypsum was gradually decreased with the prolongation of reaction time in 10.0％～20.0％NaCl solution at a temperature of 95 ℃ and leveled off after decreasing to about 6％.Combined with XRD and SEM analysis,it was concluded that phosphogypsum could be dehydrated and recrystallized to form stable α-hemihy-drous gypsum with hexagonal columnar shape in 10.0％～20.0％NaCl solution at 95℃.At the same time,the dehydrated transcrystallization rate of phosphogypsum was increased slowly at first,then rapidly,and finally gradually tended to be gentle,which was a typical growth-nucleation dynamic process.With the increase of NaCl concentration,the induced nucle-ation time and crystal growth time of α-hemihydrous phosphogypsum were gradually accelerated,and the total crystalliza-tion time was gradually shortened.Among them,the completely dehydrated transcrystallization time of phosphogypsum in 20.0％NaCl solution was only 0.5 h,and which was shortened by 1.38 h compared with 15.0％NaCl solution.Therefore,it was concluded that the increase of NaCl solution concentration accelerated the dehydration and crystallization efficiency of phosphogypsum,which could provide guidance for the actual large-scale production of α-hemihydrate gypsum.
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