Study On The Kinetics Of Crystallization Of Wet Process Industrial Monoammonium Phosphate Under Temperature Gradient

Industrial monoammonium phosphate products has a broad market domestic andoverseas,the economic benefits are remarkable, in view of this, many domestic scientificresearch units and phosphate manufacturers are actively engaged in technical research. Thequality of Industrial-grade monoammonium phosphate was not only affected by the purityof the product, and also influenced by the form and the shape of crystal. There are a lot ofzwitterions in the wet-process phosphoric acid, temperature and stirring speed has distincteffect on the crystallization process of industrial monoammonium phosphate, all these willresult in different particles crystal size and irregular shape, directly affect the filterperformance and the purity of the product, and also have great influence on the export ofproducts. Only by further study of crystallization behavior of industrial monoammoniumphosphate and changing the quality of industrial monoammonium phosphate can wepromote the rapid development of the industry. At present, the research for thecrystallization of monoammonium phosphate is rarely, especially the industrial research.Although the domestic research on industrial monoammonium phosphate isothermalcrystallization process, but the crystallization of the ammonium phosphate in the process ofactual production usually under the non-isothermal, it is difficult to directly applied inindustrial crystallization. It is inevitable trend for theoretical research to establish the studyon crystallization under temperature gradient of industrial monoammonium phosphate,undoubtedly.This paper has two parts, first of all, Measurement on sulfate ion、solubility andmetastable zone width of industrial grade monoammonium phosphate can help us to studythe influence of temperature gradient and stirring speed on metastable zone width,it canalso be used to study the Crystallization kinetics of industrialgrade monoammoniumphosphate under the temperature gradient. Secondly,the experiment invest inindustrial-grade monoammonium phosphate which comes from wet-process phosphoricacid,purification of the wet-process phosphoric acid and then the investigation of thereaction temperature and reaction time and the amount of actual addingamount,lastly,inspection of the influence of temperature gradient and stirring speed on the form and the shape of crystal within production and the influence on particle sizedistribution.All of these can make us to work out the best process conditions.This experiment for the first time to fill solubility data of industrial coolingcrystallization within mainly operating temperature (40~80℃), it makes the solubilityfitting equa tion is more accurate, and by solving the mathematical model of thecrystallizer we will get more reliable kinetic parameters. The width of metastable zonegradually become narrow with the increase of temperature, and widen with lowertemperature, between35to70℃temperature, metastable zone width range from5.3℃to18.4℃. The effect of sulfate ion、temperature gradient and the stirring speed on itsmetastable zone width, and According to relationship between monoammonium phosphateconcentration with the changing time, we can estimation of the relevant parameters ofcrystallization kinetics equation, the value of g is from0to3, b value from0.5to2.5, kgand kb are known to be positive.The best experimental conditions of pretreatment of wet process phosphoric acid,temperature is90℃, dosage of potassium carbonate is120%of theoretic dose, and rockphosphate is150%. The reaction time is3h, defluorization rate can reach98%, anddesulfurization rate reaches73.04%. Deep desulfurization of wet-process phosphoric acidexperimental conditions: removal for60min, temperature of50℃, the actual amount is1.1times, this can meet the requirements of industrial grade ammonium dihydrogenphosphate which aimed at98%purity, after deep desulfurization,desulfurization rate canreach96.97%. Best crystallization process condition is: the stirring speed in250~350r/min，the temperature gradient is0.2~0.4℃/min when the particle size distribution isrelatively concentrated, and with complete crystal forms, the purity of product above98%.