Phosphorus Recovery from Wastewater by Red Mud-seeded Crystallization of Calcium Phosphate

To obtain a practical process for phosphorus (P) recovery from wastewater, taking red mud as a seed crystal, the effects of process conditions on the crystallization of calcium phosphate for P recovery from synthetic wastewater were investigated. The scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and powder X-ray diffraction (XRD) were employed for the characterization of the recovered products. The results showed that with red mud mesh of 40-60, a seed concentration of 8 g/L and an initial phosphate concentration of 60mg/L, and under the conditions of stirring speed of 180rpm, stirring time of 30min and precipitation time of 30min, the phosphorous recovery efficiency could reach 74.1%. With different initial phosphate concentrations, the recovery of PO43- and Ca2+ increased with the increase of the pH, but the increase rate decreased with the increase of initial phosphate concentration. The recovery rate of PO43- increased with Ca:P molar ratio, and the recovery rate of Ca2+ had a peak with Ca:P molar ratio increasing. With different Ca:P molar ratio, there has different peaks under different initial phosphate concentrations. The repeated reuse times of seed crystal should not be more than 3 times. The results showed that the main recovered product of P from the wastewater was hydroxyapatite (HAP).