Reclamation of phosphorus from aqueous solutions as alkaline earth metal phosphate in a fluidized-bed homogeneous crystallization (FBHC) process

This investigation studies the crystallization of alkaline earth metal phosphates (Me-PO₄, Me=Mg, Ca, Sr, Ba) to recover phosphorus (1000 ppm-P) in a fluidized-bed reactor without the addition of seeds. Experimental results showed that the effluent pH_e critically determined the phosphorus removal rate (PR%) and crystallization ratio (CR%). The PR%s that were achieved by the generation of Mg, Ca, Sr and Ba-phosphates with an inlet molar ratio [Me]_in/[P]_in of 2 were 77.8%, 99.4%, 97% and 98.9% at the pH_e values exceeded 7.5, 6.6, 7.2 and 6.4, respectively. However, the CR% was strictly optimized by a pH_e that transformed the crystalline phases of granular pellets (0.5-2mm in diameter) from metal hydrogen phosphate (MeHPO₄) to hydroxyapatite (Me₅(PO₄)₃(OH)), and maximum CR%s of 54.5%, 78.8%, 79.0% and 89.0% were obtained by the generation of Mg, Ca, Sr and Ba-phosphates. According to estimates of aquatic chemistries and supersolubility behaviors in the effluent, a successful FBHC process operated under a supersaturation that was close to the metastable zone. Finally, a reasonable mechanism of the removal of phosphorus by converting the aqueous phosphorus into highly pure metal phosphate pellets was inferred.