Simultaneous removal of Ca2+ and SO42- from desulfurization wastewater by sodium aluminate coagulation

The wet flue gas desulfurization (FGD) wastewater treated by traditional three-tank process is characterized by high alkalinity, high hardness, and high sulfate, which often leads to severe scaling and clogging in subsequent membrane treatment facilities. To address this challenge, sodium aluminate (NaAlO2) could react with Ca2+ and SO42- in FGD wastewater to generate indissoluble ettringite precipitates for their simultaneous efficient removal. The effects of NaAlO2 dosage〔n(Ca2+)∶n(Al3+)〕, stirring speed, reaction time and reaction temperature on the simultaneous removal of Ca2+ and SO42- from FGD wastewater were investigated through single-factor experiments. The optimum reaction conditions for simultaneous removal of Ca2+ and SO42- by NaAlO2 were determined. The results showed that Ca2+ and SO42- could be effectively removed by adding NaAlO2 to the raw water (initial solution pH=11.9) in the ratio of n(Ca2+)∶n(Al3+)=2∶1 at room temperature with stirring at 300 r/min for 30 min, and the removal rates of Ca2+ and SO42- could reach (95.7±0.1)% and (81.1±2.5)%, respectively. Additionally, the study found that adding 0.2 g/L of hydrocalumite as a seed crystal to the system significantly shortened the settling time of flocs. XRD and SEM analyses showed that after the addition of seed crystals, the columnar ettringite crystals were attached to the hydrocalumite seeds to form dense heteroepitaxial encapsulation, which effectively improved its settling performance. Compared to the traditional sodium carbonate softening process, the use of NaAlO2 for the simultaneous removal of Ca2+ and SO42- in FGD wastewater reduced the chemical cost by 25.97% and alleviated the treatment burden on subsequent nanofiltration processes. The coagulation process for simultaneous removal of Ca2+ and SO42- was a more economical and efficient treatment approach.