Abstract
Wastewater from rare earth smelteries contains large amounts of ammonium nitrogen (NH4+-N), which causes severe environmental problems. In this contribution, the desalination efficiency of reverse osmosis (RO) was investigated in the treatment of NH4Cl or NaCl solutions from 0.1 to 40 g/L under different operating pressures with a commercial RO membrane. Experimental results showed that when an operating pressure above 30 bar is applied to the 5 g/L NH4Cl solution, the permeate was found to meet the discharge standards of NH4+-N. Compared to NH4Cl, the permeate fluxes of NaCl solutions were higher due to the higher net driving force and lower propensity to membrane fouling. Theoretical models indicate a linear relationship between water flux and the net driving force for both NH4Cl and NaCl solutions. On the contrary, a power function between the salt flux and concentration difference correlated well with the experimental data for salt transport. The equations for water and salt transport obtained by this work would provide a facile and practical means for predicting the membrane performance in design and optimization of RO processes for the treatment of wastewater from the rare earth industry.
Highlights
In the past two decades, the demand for rare earth elements (REEs) underwent an explosion due to the widespread application of REEs in daily life, such as cell phones, computer memory, rechargeable batteries, fluorescent lighting, magnets, etc [1]
The good agreement between literature data and predicted data from this study proves the efficiency and accuracy of the transport model described in Equation (6) for calculating the water flux during reverse osmosis (RO)
Commercial RO membranes were used for the desalination of highly concentrated NH4Cl and NaCl solutions simulated as saline wastewater from the rare earth industry in this work
Summary
In the past two decades, the demand for rare earth elements (REEs) underwent an explosion due to the widespread application of REEs in daily life, such as cell phones, computer memory, rechargeable batteries, fluorescent lighting, magnets, etc [1]. China has abundant rare earth resources, which contribute an average share of over 85% to the global REEs supply in the last twenty years [2]. In the extraction stage, ammonium hydroxide (NH4OH) is added to saponify the acidic extractants, resulting in an annual discharge of ten million tons of wastewater with a high content of ammonium nitrogen (NH4+-N), which generally ranges from 300 to 40,000 mg/L in different stages (e.g., extraction, precipitation and washing stages) and is far above the national discharge standards in the rare earth industry (GB26451-2011) [3,4,5]. It is reported that the washing wastewater from the REEs precipitation process mainly contains NH4+-N, while other parameters (e.g., REEs, heavy metals, COD, TP) are lower than the discharge standards [6,7]. Developing reliable technologies for NH4+-N discharge in the rare earth industry is of great concern for sustainable water reuse and cleaner production from the economic and environmental perspectives
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