An extraction and precipitation process for the removal of Ca and Mg from ammonium sulfate rare earth wastewaters

Abstract

Softening ammonia sulfate wastewater is one of the vital issues in the rare earth industry. In this article, a novel softening process using [4,4′-isopropylidenebis (phenoxyacetate)] (H2IPOAA) for Ca2+ and Mg2+ (M2+) is developed. The effect factors, i.e., equilibrium time, pH value of aqueous phase and saponification degree are investigated. The extraction mechanism is proposed to be cation exchange. With a saponification degree of 85%, H2IPOAA is successfully used for the precipitation of Ca2+ and Mg2+ from ammonia sulfate wastewater within 10 min. The process can form precipitated particles with an average size of 35 μm, which are much larger than the precipitates formed by H2C2O4, Na2C2O4 and Na2CO3. The precipitated particles can be easily separated from the raffinate. In addition, H2IPOAA is repeatedly used for five times with a little loss. To a considerable extent, the reuse of H2IPOAA contributes to lowering the cost and reducing the pollution. The concentrations of Ca2+ and Mg2+ in the raffinate extracted by H2IPOAA are up to the MVR standard for wastewater treatment. The economical and efficient softening process meets the need of green chemistry, which is expected to replace traditional industrial processes for water softening based on H2C2O4, Na2C2O4 and Na2CO3.