Study On Crystallization Process Of Diammonium Phosphate

Diammonium phosphate（DAP）,is an important part of the industrial products of phosphate.At present,industrial production of DAP products contain more impurities,if the need for additives in the follow-up process for purification.The method of separation and purification is often the crystallization method.However,there were few studies on the crystallization process of DAP.Therefore,the crystallization process of DAP was studied systematically in this thesis.The thermodynamic properties of DAP were studied:the density in water and ethanol-water and the solubility in water,methanol-water and ethanol-water were determined.The volume properties of DAP in mixed solvent were calculated and correlated.The solubility data were fitted by thermodynamic equation,and the thermodynamic parameters of DAP were calculated.The results shown that the volume properties of DAP were positive in the selected solvent,and all increased with temperature increasing.The dissolution of DAP was entropy-driven non-spontaneous endothermic process in the selected solvent.The effects of stirring rate,cooling rate and metal cations(Fe³⁺,Al³⁺and Mg²⁺)on the width of DAP metastable zone were investigated by laser scattering method.The results shown when the stirring rate increased and the rate of cooling decreased,the metastable zone became narrow.The addition of Fe³⁺,Al³⁺and Mg²⁺could increase the width of the metastable zone,and the influence of the Fe³⁺and Al³⁺was close,but the effect of Mg²⁺was more obvious.The crystal growth rate of diammonium phosphate in its static solution,static ethanol-water and flow solution with different supersaturation was studied by single crystal growth method.The crystal growth activation energy E_a of DAP in the stationary system was calculated and the growth rate constant K_G was obtained by fitting the growth rate in the flow solution.And the effects of solvent composition and metal cation(Fe³⁺,Al³⁺and Mg²⁺)on the growth rate of DAP were investigated.It was found that the crystal growth rate of DAP in different crystal directions was as follows:x crystal orientation≈y crystal orientation>z crystal orientation and DAP crystals were generally flaky;with the increasing of the concentration of ethanol and metal ion,the crystal growth rate of DAP slowed;the inhibition degree of Mg²⁺on crystal growth was obviously higher than that of Fe³⁺and Al³⁺.The technological conditions for the production of DAP by cooling-solute coupling crystallization were explored.The effects of seed addition or not,the size and the addition amount,the rate of stirring,the speed of cooling and the rate of addition ethanol were investigated by single factor parallel method.And finally got a reasonable crystallization process of DAP.And the effect of Fe³⁺on the particle size of DAP was studied.It was found that the presence of Fe³⁺in the solution reduced the main particle size of the product and increased the particle size distribution range.Therefore,in the production process of DAP in order to get narrower distribution,crystal integrity and larger particle size of the products,so we must minimize the iron ion impurity content.