Two-stage sulfate removal from reject brine in inland desalination with zero-liquid discharge

This research proposes a two-stage process for the removal of sulfate from a nano-filtration (NF) reject stream. In Stage 1, calcium chloride is used to partially remove the majority of sulfate as calcium sulfate precipitate while the remaining sulfate is removed in Stage 2 as calcium sulfoaluminate precipitate. Kinetics of the removal process in both stages was investigated and the results indicated that the removal kinetics is not a limiting factor for the practicability of the process. Equilibrium characteristics of sulfate precipitation were evaluated and optimum conditions for maximum sulfate removal were obtained. Sulfate removal in Stage 1 was independent of the pH while efficient sulfate removal in Stage 2 was found to be above pH 11. Efficient sulfate removal was observed at practical ranges of lime and sodium aluminate doses. An equilibrium model was developed to describe the chemical behavior in Stage 2. A valid explanation for the mechanism of sulfate removal in Stage 2 was the formation of a solid mixture consisting of ettringite and monosulfate as confirmed by X-ray diffraction (XRD) analysis. This approach is expected to reduce the volumes of the overall precipitated solids and in turn improve the overall efficiency of the ZLD treatment system. © 2015 Elsevier B.V. All rights reserved.

The bench scale two-stage process showed effective removal of sulfate. Kinetics experiments for Stage 1 and Stage 2 of the process showed fast removal of sulfate, being almost complete within 2 h. Results showed that high sulfate removal in Stage 1 using CaCl2 as a reagent was limited by the solubility of calcium sulfate, resulting in the need of a Stage 2 for efficient sulfate removal via the precipitation of sulfate as calcium sulfoaluminate. Lime and aluminum doses at ratios of 100% and 67% of the initial sulfate concentration, respectively, provided optimum conditions for efficient sulfate removal. The removal of sulfate in Stage 2 greatly depended on the pH, and effective sulfate removal was observed above pH 11. An equilibrium model was developed using PHREEQC to depict the chemical behavior in the two-stage process and to predict the final sulfate concentrations and solids formed at the Fig. 10. Fractions of solids in the mixture in Stage 2.