Crystallization Regulation Of Struvite

Industrial wastewater is always rich in nitrogen and phosphorus elements,whose direct discharge will cause the eutrophication of water body resulting in environmental pollution,and also is a waste of resources.At present,the method of struvite(Mg NH4PO4·6H2O)crystallization is being investigated to treat high-concentration nitrogen and phosphorus wastewater through the addition of magnesium ions,and the obtained struvite can be directly used as agricultural fertilizer.However,the formation of undesirable fine crystals during the process remains a big challenge for struvite crystallization method.These fine particles are very difficult to be separated from the solution,and then precipitated blocking the pipelines in the downstream process.In this work,the growth of struvite crystals and the morphological control was systemat ically studied.Thermodynamic and kinetic data of the crystallization process were accurately measured,based on which a technology of struvite crystallization with the aid of feedback control was developed and optimized to increase the size of the crystals.The details are as follows:The solubility and metastable zone of struvite in water was determined by laser dynamic method,and the critical supersaturation of nucleation was also obtained.The growth rates of crystal planes {0 1 2} and {1 0 1} under supersaturations were measured at different temperatures to establish the growth rate equation,preliminary acquiring the basic data of struvite crystallization.Based on struvite crystal structure,the vacuum habit and simulated habit after aqueous solution modification of struvite were predicted by the adhesion energy theory.Four different experimental habits of struvite were also found,which were prismatic,fusiform,dendritic and needle-like with increasing p H.The morphological changes were explained through diffusion dynamics simulation as follows: The increase in phosphate concentration with increasing pH reduced diffusion rate,which caused the change of crystal morphology from reaction control to diffusion control.According to the DLA model,it was proved that diffusion control leaded to the formation of struvite dendrites.A growth process using pH as driving force to obtain large-sized struvite crystals through feedback control was developed.The process effectively controlled the primary nucleation by adding seed crystals and also inhibited the secondary nucleation through adjusting pH of the solution within the range of metastable region.The crystals continued to gain impetus during growth by the feedback control of supersaturation.Finally,the average size of products obtained by the process reached 193.9 ?m through optimizing supersaturation,control mode,seed ratio and temperature.The settling rate of the products measured by the improved Andreasen method was increased by nearly 10 times compared with the products without feedback control,which proved that the process can improve solid-liquid separation effect of struvite crystallization method effectively.