Phosphates Recovery through Hydroxyapatite Crystallization from Wastewater Using Converter Slag as a Seed Crystal

With powdered converter slag used as a seed crystal, the effects of slag dosage, pH, temperature, and Ca²⁺ concentration on phosphate removal from synthetic solution through crystallization were investigated in batch experiments. The results indicated that slag concentration of 1.7 g/L, pH value of 9-10, reaction time of 3h , reaction temperature of 20-25°C, Ca/P molar ratio of 2, seemed to be the favorable conditions. Under the optimal reaction condition, the removal efficiencies of the swine farm wastewater and the supernate of concentrated sludge , were 95.36% and 96.54%, respectively. After converter slag as a seed material was used for ten times, the P removal efficiency was still above 80%. The Scanning electron micrograph (SEM) images and X-ray diffraction (XRD) pattern of converter slag before and after phosphorus crystallization confirmed the formation of crystalline hydroxyapatite. Energy dispersive X-ray spectroscopy (EDS) mapping analysis revealed that the molar composition ratio of Ca/P of the crystalline material was 1.72. The Ca/P molar ratio > 1.67 for crystalline substance might result from the presence of CaCO₃ on the crystalline surfaces. The results indicated that total P contents in the generated crystallization products of up to 14.5 % P-tot (33.2 % P₂O₅) were achieved, and was comparable to natural phosphate rock.