Enhancing recovery of ammonia from rare earth wastewater by air stripping combination of microwave heating and high gravity technology

Currently, a larger amount of NH₃-N wastewater is generated in the rare earth manufacture process. Air stripping is one of the effective technologies for ammonia removal, but low separation efficiency and huge stripping tower rendering the process techno-economically inefficient. Hence, microwave heating and high gravity separation are employed to intensify air stripping process. Microwave dielectric characteristics for this solution indicate strong absorption, which can reduce the heating time rapidly. The effect of main operating parameters such as the rotational speed (ω), liquid flow rate (Q_L), gas flow rate (Q_g), and stripping temperature (T)) on the volumetric liquid mass transfer coefficient (K_La) and stripping efficiency (η) are assessed. Results show that K_La increase significantly with increasing gas flow rate, followed by rotating speed and liquid flow rate. However, the η decrease with increase in Q_L due to reduction in the liquid hydraulic retention time. Under the optimal conditions, K_La and η could reach 0.0061 s⁻¹ and 99.3%, respectively. Dimensionless empirical correlations are developed relating process parameters with the K_La and η which shows a good agreement of the model equation with the experimental data. In addition, high gravity air stripping and conventional technologies are compared.