Studyon Recovery Of Phosphate From Acetylene Cleaning Waste Water Via Vivianite Crystallization

Acetylene cleaning wastewater from PVC production by calcium carbide process contains a large amount of phosphate.If it is not been recycled,it will cause waste of phosphorus resources and will pollute the surrounding water environment,causing eutrophication of water body.In this paper,the effect of different SI values on the crystallization of vivianite was studied.The induction time and secondary nucleation rate of the crystallization process were measured.The possible effects of other ions on the thermodynamics of crystallization were simulated by using the PHREEQC software.Continuous-flow experiments were been conducted in a fluidized bed reactor to further explore the environmental factors affecting the crystallization of vivianite,providing a theoretical basis for optimizing the technology of phosphate recovery from vivianite crystallizationThe study on the basic crystallization process of vivianite shows that the better SI range of crystallization is 7-10,in which the reaction rate is faster and larger crystals will be produced.The higher the supersaturation is,the longer the induction time of crystallization will be,and long induction time will not be conducive to the reaction of crystallization.The critical supersaturation value of homogeneous and heterogeneous nucleation is 26,which homogeneous nucleation tends to occur when the supersaturation is greater than that,and heterogeneous nucleation when the supersaturation is less than that.When solute exists in the supersaturated solution,secondary nucleation will occur.Under this experimental eondition,the eritical relative supersaturation value for secondary nucleation is 24.If the value is not reached,secondary nucleation will not occur,When the relative supersaturation is greater than this value,the secondary nucleation rate increases with the increase of relative supersaturation.From the simulation results of the PHREEQC software,it can be seen that NaCI needn't to be removed in engineering.From the results of continuous-flow experiments,the crystal size was dispersed in the initial stage of reactor operation.With the long-term operation of the reactor,the crystalline size gradually normalized.In addition,the bigger the reflux ratio,the higher the mixing degree of the crystal in the reactor,the more intense the collision between the crystal and the crystal,and the larger the grain size of the crystal growth.For different hydraulic retention time,when the hydraulic retention time is too short and the reaction is too late,the crystal will be washed out of the reactor before it grows,which may not be conducive to the formation of larger crystals,but also lead to the low removal rate of phosphate.When the hydraulic retention time is reasonable,the vivianite crystal grows better and the phosphate removal rate is higher.