| Past reports reveal that proper magnetic field treatment results in decrease of crystallization time, promotion of both quantity and quality of the precipitate. Calcium silicate releases Ca2+ and alkalinity required for phosphorus crystallization in solution and induces phosphorus crystallization as calcium phosphate on its surface, which gives it more advantages compared with steel slag, ceramsite or other materials as seed crystal. In this study calcium silicate was used as seed crystal for the treatment of simulated phosphorus-containing waste water in static experiments, and the optimum conditions of induced crystallization(IC) were tested. Effect of magnetic field on IC process and the structure and properties of the product was studied. Finally the enhancement of magnetic field on the effect of IC of phosphorus by calcium silicate was tested in a fixed bed crystallization reactor.Conclusions are as follows from researches above:(1)Efficiency of phosphorus removal through IC by calcium silicate relied on the seed crystal size, dosage, initial concentration, reaction time, initial pH and temperature, among which the former four factors affected the IC more than the latter two ones did, especially temperature. The removal efficiency was promoted with the increase of dosage, initial pH, reaction time and temperature in a certain range, while inhibited with the raise of initial concentration and seed crystal size. The optimum conditions were: solution volume of 100 mL with initial phosphorus concentration of 20 mg/L, seed crystal size of 4-6 mesh, dosage of 1 g, reaction time of 2 h, initial pH value of 7, and room temperature. The IC process conformed to second order kinetics equation, especially under high concentration. But the variations of the crystallization kinetics constant indicated that low concentration favored the IC process, which meant calcium silicate had a better treatment performance with phosphorus containing waste water of low concentration. Crystallization rate increased little with the temperature increment. Activation energy was low in this reaction according to the calculation result, meaning it took place rapidly.(2)Effect of magnetic field on calcium phosphate crystallization relied on magnetic field intensity, treatment duration, initial concentration of the solution, stirring speed, reaction temperature, and the solution being treated. Results showed that the magnetic field treatment efficiency was strengthened with the increase of magnetic field intensity, duration, stirring speed in a certain scale while weakened with the raise of initial concentration of the solution and reaction temperature. Treating K2HPO4 solution with magnetic field favored the process compared with the same treatment of CaCl2 solution. The optimum conditions were: magnetic field intensity of 2400 mT, duration of 25 min, K2HPO4 solution initial concentration of 0.7 mmol/L, stirring speed of 150 r/min and reaction temperature of 25℃, treating K2HPO4 solution. Characterization of XRD and EBSD showed difference in the structure between the productswithout and with magnetic field treatment, which was Ca3(PO4)2 of small size and low purity and Ca3(PO4)2 of large size and high proportion of phosphorus element, which benefitted phosphorus removal from the solution.(3)Phosphorus removal efficiency and treatment cycle of the crystallization reactor was promoted with the increase of hydraulic detention time, decrease of inflow concentration and increase of seed crystal packing height. Reactor with magnetic field process showed better performance in phosphorus removal efficiency and treatment cycle under the same conditions compared with the one without magnetic field process. |
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