Crystallization Characteristics Of Magnesium Ammonium Phosphate And Recycling Technology From Wastewater Containing Phosphorus

The crisis of phosphorus resource is depleted in China,meanwhile,the excessively high phosphorus content in domestic sewage and industrial wastewater were also a problem.However,at present,it is difficult to effectively meet the standards for the treatment and recycling of phosphorus-containing wastewater,which has caused serious environmental pollution and huge resources waste.Among many phosphorus removal technologies,magnesium ammonium phosphate（MAP）crystallization is a low-cost,effective method that can recover phosphorus resources from wastewater.But there are still problems such as the crystallization characteristics and laws of MAP are unclear,the related parameters are incomplete and etc.Therefore,further studying the thermodynamic properties and understanding the kinetic parameters of MAP has important practical significance for the treatment and the recovery of phosphorus resources from phosphorus-containing wastewater.In this paper,in the perspective of crystallization chemistry,the crystallization characteristics and resource utilization of MAP are studied.The main research results are as follows:First,the range of MAP metastable zone was measured.The effects of p H,impurities,additives and seed crystals on the super-solubility of MAP were systematically studied.It was found that the super-solubility of MAP increased with the decrease of p H.When it decreased from 8.5 to 8,the super-solubility increased obviously.SO₄^2-,NO₃^-,Cu²⁺,Ca²⁺,urea and anionic surfactant will increase the super-solubility of MAP.CO₃^2-,Ni²⁺,Zn²⁺and ethanol will reduce its super-solubility.The presence of seed crystals can obviously reduce the super-solubility of MAP,and as the particle size of the seed crystal decreases and the dosage increases,the super-solubility of MAP decreases.XRD and SEM characterization analysis of the obtained crystal nucleus showed that Ca²⁺,urea and anionic surfactant would have an influence on the composition and crystal morphology of MAP.Then,the crystallization kinetics of MAP was studied.By fitting the nucleation and growth kinetic parameters during the crystallization of MAP,the nucleation and growth rules of MAP crystals were proved.The results showed that the initial concentration of phosphorus promotes the nucleation and linear growth rate of MAP.The reaction time can increase the average particle size of MAP crystal,and decrease it nucleation and linear growth rate.If p H is higher,the average crystal particle of MAP will be smaller,and it result in decreasing linear growth rate and increasing the nucleation rate.Under 20%excess Mg²⁺conditions,the crystal particle size produced will be larger.Finally,the fluidized bed reactor（FBR）crystallization was constructed to investigate the relationship between condition control and phosphorus removal effect.The effects of different p H,ion molar ratio and reflux rate on the removal efficiency of 100 mg/L simulated phosphorus containing wastewater were studied.Under the optimal conditions（p H=9.5,P:Mg:N=1:1.4:1.2,reflux rate=12 L/min）,the phosphorus removal rate and recovery rate can reach to95.57%and 93.34%,respectively.And stable processing under continuous injection conditions in 20%.The results show that the above research can provide effective basic data and technical methods for efficient crystallization of phosphorus in phosphorus-containing wastewater,moreover achieving phosphorus removal and resource utilization.