The Study Of Using Raffinate Acid To Produce Industrial-grade Monoammonium Phosphate

With energy problem prominent increasingly,it is inevitable trend that wet process phosphoric acid which consumes less energy would replace hot-process phosphoric acid. Wet-process phosphoric acid contains a lot of impurities and many countries are studying how to purify wet-process phosphoric acid.Currently, the only industrial application of purifying wet-process phosphoric acid is the mean of solvent extraction.Raffinate acid is inevitable by-product of the mean of solvent extraction purifying wet-process phosphoric acid and how to use raffinate acid reasonably has became an important development direction of many phosphoric acid purification enterprises.In our country,most of raffinate acid is used to produce ammonium phosphate fertilizer (MAP, DAP) whose value is low.it can improve raffinate acid’s economic value by producing industrial-grade monoammonium phosphate(MAP).The research content of this paper includes three parts.Firstly,the precipitation conditions of impurity ions in raffinate acid were measured when using ammonia neutralizes raffinate acid.At the same time, the effect of impurity ions concentration on the product purity were studied.Secondly, crystallization process of industrial-grade MAP was researched by measuring solubility,metastable zone width, crystal suspension density and degree of supersaturation and calculating crystallization heat.Last,optimize the process of using raffinate acid to produce industrial-grade monoammonium phosphate and analyse influence of different factors on the quality and yield of monoammonium phosphate.The main conclusions are following:1.The separation rate of impurities increases with pH rising when ammonia neutralizes raffinate acid.When pH is 4,the separation rate of Fe2O3 and Al2O3 are 99.73% and 99.16% respectively.The separation rate of MgO is 91.24% when pH is 4.5 and 98.12% when pH is 5.The separation rate of SO42- is 66.83% when pH is 4.5.The three kinds of metal impurity ions Mg2+、Fe3+、Al3+ in raffinate acid would affect the quality of monoammonium phosphate,which mainly reduces the N content of MAP. The content of Mg2+ is the most high impurity which has the biggest influence on the process and whose content should be controlled under 3% when using raffinate acid to produce industrial-grade MAP.2.The solubility of Industrial-grade MAP in water is the function of temperature. MAP can be gained by cooling crystallization in the industrial purification and production because its solubility increases distinctly with temperature rising.The metastable zone width of industrial-grade MAP gradually becomes narrow with the increasing of temperature in water and ranges from 10℃ to 15.7℃ when temperature’s range is 35 to 60℃.With the prolonging of crystallization time,the yield of MAP crystal increases and remains almost unchanged after 40min, therefore the optimal crystallization time is 40min.3.The optimal experimental conditions of producing industrial-grade MAP with raffinate acid is that dilution ratio isl.75,the temperature of the first neutralization and stewing is 70℃,the time of the first stewing is3.5h,the temperature of the second neutralization and stewing is 60℃,the time of the second stewing is 3h,relative density of concentration is 1.37,crystallization temperature is 40℃ and crystallization time is 40min.The yield of MAP is 32.32% and the purity of product is 98.79% under the optimal conditions of process.