In this study, phosphate was removed from aqueous solution and recovered as struvite (MAP, MgNH4PO4 center dot 6H(2)O) by fluidized-bed crystallization (FBC) process. The effects of influent pH and Mg/P molar ratio on MAP crystallization were investigated. Thermal analysis and characterization of the MAP crystals collected from the fluidized-bed reactor (FBR) were performed. The kinetics and thermodynamics of the MAP decomposition reaction were determined using the Coats-Redfern method. Experimental results show that the total concentration of phosphate in the fluidized-bed reactor (FBR) increased with influent pH and Mg/P molar ratio. Thermal analysis of MAP decomposition at different temperatures revealed two distinct stages: (1) removal of H2O from the crystal surface and (2) removal of NH3. H2O and NH3 removal were highly endothermic, non-spontaneous and thermodynamically unstable. Low pH and high Mg/P ratio favored the production of more stable MAP crystals. The FT-IR and XRD results verified the formation of magnesium hydrogen phosphate (MgHPO4) by the evolution of ammonium from MAP at temperatures between 200 and 500 degrees C, and its further conversion to magnesium phosphate oxide (Mg2P2O2) at temperatures greater than 800 degrees C. (C) 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Journal of the Taiwan Institute of Chemical Engineers, v.45 n.5, pp.2395-2402