The Study Of Influence Of Sulfrate Ion On Crystal Growth And Morphology Of Ammonium Dihydrogen Phosphate

Ammonium dihydrogen phosphate is an important kind of fine phosphate,which has a wide range of applications in agriculture and industry.Crystallization is the ultimate control step of the ammonium dihydrogen phosphate production process,and can effectively control the purity,crystal morphology and particle size distribution of the final product,thus meeting the user's requirements for product quality.In general,the crystal of ammonium dihydrogen phosphate has a tetrahedral shape and a particle size of 480?m.However,in the course of the production of ammonium dihydrogen phosphate in the wet route,a considerable amount of impurity ions such as Mg²⁺and SO₄^2-,existing in the crystallization mother liquor,will be embedded in the crystal lattice of the ammonium dihydrogen phosphate during its crystallization,thus affecting the crystal morphology and particle size distribution of the product.Unconventional"X-type"crystals often appear in actual industrial production,therefore,this article focuses on the effect on the crystallization process of ammonium dihydrogen phosphate,product crystal type and particle size distribution under different crystallizing conditions and cooling conditions in the presence of sulfate ion.The specific research contents include:?1?The solubility and metastable zone width of ammonium dihydrogen phosphate have been tested by on-line laser measurement system,and SO₄^2-is added to the solution of ammonium dihydrogen phosphate to study the influence on them.The results show that solubility and metastable zone width of ADP both become narrower in the presence of sulfate ion.At the same time,the effects of stirring rate,cooling rate and saturation temperature on the metastable zone width are investigated.The higher the temperature,the more narrow the metastable zone width,the greater the cooling rate,the wider the metastable zone width,and the appropriate increase in the stirring speed,the metastable zone width will be reduced accordingly.?2?A set of program-controlled crystallization system is built to realize ammonium dihydrogen phosphate crystallization at a constant cooling rate.It is also used to study the effects of stirring rate,cooling rate,the saturation temperature and sulfate ion doping concentration on ammonium dihydrogen phosphate crystal size distribution and morphology during the cooling crystallization process.The results show that adding sulfate ion in the cooling crystallization process only changes the extent of crystal surface,but not change the crystal shape.The average particle size of crystal reaches the largest at 868.22?m and crystal surface is relatively at a smooth level when the stirring speed is 200 r/min,the cooling rate is 0.2?/min,the saturation temperature is 50?and the sulfate ion doping concentration is1%.?3?The crystallization behavior of ammonium dihydrogen phosphate is also studied during the condensing crystallizing.The effects of concentration density and sulfate doping concentration on the particle size distribution and morphology of the concentrated crystalline product are determined experimentally,and the same study for the apparent linear growth rate of the crystal.The experimental results show that X-type crystals may be produced during the process of concentrating crystallization;which turns out that sulfate ion is also one of the influencing factors of crystal growth and morphology control of ammonium dihydrogen phosphate.Simultaneously with the presence of sulfate ions,the morphology of ammonium dihydrogen phosphate will switch from the regular tetrahedron to the X-type when the density is greater than 1.40 during the crystallization process.At the same time,increasing either the solution concentration density or the sulfate ion doping concentration will increase the apparent linear growth rate of the crystal.The results of this study have deepened our understanding of the influence of impurity ions,especially sulfate ions,on the crystallization process of ammonium dihydrogen phosphate,and provided useful theoretical guidance for improving the control of the actual crystallization process of ammonium dihydrogen phosphate.