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 pHe 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 pHe that transformed the crystalline phases of granular pellets (0.5-2 mm in diameter) from metal hydrogen phosphate (MeHPO4) to hydroxyapatite (Me-5(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. (C) 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.