[Removal of calcium and high-strength ammonia nitrogen from the wastewater of rare-earth elements hydrometallurgical process by chemical precipitation].

A lot of high-strength ammonia nitrogen wastewater is generated in the ion-type rare-earth elements hydrometallurgical process. Magnesium ammonium phosphate (MAP) precipitation was chosen to remove the ammonia nitrogen from the wastewater after Ca2+ was eliminated using Na2CO3 to generate CaCO3 precipitate, because the wastewater contained a lot of Ca2+, and Ca2+ was an important impact factor for MAP precipitation. Central composite design (CCD) is a principal response surface methodology (RSM) used in experimental design. Response surface methodology (RSM) was used to optimize the factors in MAP precipitation, achieving the optimal conditions and the precipitates under such conditions. Two kinds of precipitates were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that when n (Ca2+): n (CO3(2-) = 1:1.05, mix rate and reaction time were 1500 r x min(-1) and 30 min, respectively, the removal ratio of Ca2+ reached 100%; the optimal condition of MAP precipitation was pH = 9.03, n (Mg): n (N) = 1.20, n (P) : n (N) = 1.1, with a reaction time of 30 min and a mix rate of 1000 r x min(-1), the removal ratio of ammonia nitrogen reached 95.40% and the residual total phosphorus concentration was 5.65 mg x L(-1). SEM and XRD analysis showed that the two kinds of precipitates were pure CaCO3 and MgNH4PO4 x 6H2O, respectively.