| Phosphogypsum?PG?,main component of which is calcium sulfate dihydrate,is the main industrial by-product of phosphate chemical industry.However,the comprehensive utilization rate of PG is very low,and more and more PGs have been stockpiling on the open-air yard that resulted in worse environmental pollution.Therefore,manufacturing the high value-added?-hemihydrate calcium sulfate whisker products from PGs as raw material instead of natural gypsum has become the key direction of"Twelfth Five-Year Plan"for industrial by-product gypsums comprehensive utilization,which is more effectively to realize the comprehensive utilization of PG and it has a wide application prospect to turn waste into treasure.In this paper,the preparation of?-hemihydrate calcium sulfate whisker from PGs was studied in detail,which will provide the theoretical basis and technical support for the industrial utilization of PGs.The direct utilization ratio of PGs is very low due to the various and complex composition of impurities,especially phosphorus and fluorine.The HCl-H2O solution method in atmosphere was employed to pretreat PGs,and the effects of pH,reaction time?,temperature,solid-liquid ratio,etc.,on the disposal efficiency of soluble impurities were studied by single factors analysis and orthogonal experimental design,and then the optimum process parameters were achieved as follows:pH=1,reaction time was 2h,solid-liquid ratio was 1:3 g·ml-1,and temperature was 103?.After the pretreatment of PGs,the effective removal ratios of P2O5 and F-were 88.73%and 93.77%,respectively.And the pretreated PG crystals are financially feasible with better stability and better whiteness in color.The quality of pretreated phosphogypsum meets the national standard of GB/T23456-2009,which provides basic data for the re-utilization of PGs.The effects of crystal modifiers of inorganic acid,inorganic salt and inorganic acid-inorganic salt compoundings,temperature,suspension density,etc.,were on the crystal polymorphic form,reaction time,crystal morphology,aspect ratio,etc.,were stuieded in the preparation of?-CSHW from PGs.As a result,H2SO4-CaCl2-MgCl2is the optimum crystal modifier,the average length of?-CSHW is 234.14?m and the average aspect ratio is over 120,the purity of sample is over 97%with uniform size and smooth surface,and the?-CSHW is single fibrous crystal.The transformation process from PGs to?-hemihydrate calcium sulfate whiskers confirms the mechanism of dissolution-crystallization,i.e.under the induction of crystal modifier,CaSO4·2H2O in the salt solution will first partially dissolve?supersaturation?crystal transformation to?-CaSO4·0.5H2O.And the importance of each factor in the crystal transition process is as follows:addition of H2SO4>the crystal modifier concentration>solid-liquid ratio.The crystal growth of the?-hemihydrate calcium sulfate whiskers from PGs corresponds to the mechanism of the screw dislocation growth,and the crystal transformation of?-hemihydrate calcium sulfate whiskers under different concentrations can meet the empirical kinetic equation which is ln?/?1-??=kt. |
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