Study On Phosphorus Recovery From Wastewater By Synthetic Tobermorite Through Seeded Crystallization And Phosphorus Recycle

Phosphorus is not only an indispensable nutrient element for all the living forms, a nonrenewable and irreplaceable resource, but also an important factor which causes eutrophication of water bodies. Synthetic tobermorite contains neither heavy metals nor phosphorus and can giving out OH^- and Ca²⁺ itself, and also it can promote the crystallization of phosphate. So, Synthetic tobermorite is an ideal seed for the phosphorus recovery from wastewater. Meanwhile, phosphorus fertilizer can increase the production of the crop. 84%-90% of the phosphorite in the world is used for producing every kind of phosphorus fertilizer, so phosphorite is the main material for producing phosphorus fertilizer in industry. We are short of phosphorus and it can not renewable. Moreover, Ca₁₀(PO₄)₆F₂ is the main component in phosphorite and it is hard to dissolve in water, and also it will cause the pollution of fluorine. Thus, in order to reduce the amount of the exploitation of phosphorus and prolong the time we could use phosphorus, now what we should do is not only to recover phosphorus from wastewater, but also we should find a good way to recycle it.The reaction of recovering phosphorus from wastewater by synthetic tobermorite can reach the equilibrium at about 24h. Thermodynamic study shows thatâ–³G﹤0, the value ofâ–³S is positive andâ–³Hï¹¥0 during the process of phosphorus recovery. These parameters demonstrate that the process of crystallization is endothermic and the phosphorus adsorption and crystallization by synthetic tobermorite as seed are favorable. Kinetics study shows that the rate constant increases with the increase of temperature, the reaction order decreases with the increase of temperature.Our research group have got the optimized conditions of phosphorus recovery by synthetic tobermorite, those are pH=8, Ca/P=2 (molar ratio), seed contents 2～2.4g/L, surge speed 140r/min. Based on those optimized conditions, this research increases the amount of the seed and the wastewater. After repeat the recovery experiment for 55 times, the rate of dephosphorization is 82% and the production contains 35.02% phosphorus. This production is already can be the raw material for the phosphorus nitrate production as the high quality phosphorite. XRD,FTIR,SEM and EDS have been employed in testing recovery seed, the results showed that the main recovered product was hydroxyapatite with lower crystallization degree. The results of recovering phosphorus from the wastewater from hoggery show that when pH=8, Ca/P=2 (molar ratio), seed contents 12g/L, the rate of dephosphorization can reach 89.08%. So, synthetic tobermorite has a good quality for the phosphorus recovery from real wastewater.The experiment of producing phosphorus nitrate by refrigeration shows that low concentration of HNO₃ will not infect the amount of phosphorus which is dissolved by HNO₃. And HNO₃ can dissolve most of the phosphorus within a short time under the room temperature. The result shows that under the room temperature, 55% HNO₃ can dissolve 98.28% phosphorus by reacting for 2 hours; 40.6% HNO₃ can dissolve 95.23% phosphorus by reacting for 1hour. All the results demonstrate that the phosphorus which is recovered by synthetic tobermorite can be easily recycled. After more research, the optimized conditions for dissolving by HNO₃ are the concentration of HNO₃ is 55%, the dosage of HNO₃ is 290% compare to the theoretic dosage, and the time for reaction is 2 hours. And the optimized conditions for the refrigeration process are the time for freezing is 6 to 8 hours, and the temperature should be -5℃.