Solvent extraction desalination applied to desulphurization wastewater towards zero liquid discharge: Parameters analysis and energy evaluation

Abstract

With massive impurities, desulphurization wastewater is difficult to achieve zero liquid discharge (ZLD). The present paper proposes a novel ZLD technology based on solvent extraction desalination (SED) to avoid the intensive use of high-grade electrical energy and steam. Three amines are compared for the water extraction and salt precipitation performance from desulphurization wastewater simulated by NaCl and Na2SO4 solution. Results show that with the same volume ratio, diisopropylamine exhibits the optimized extraction performance, while N,N-dimethylcyclohexylamine has the highest ions removal efficiency of over 98 %. Diisopropylamine is selected as the target solvent for further evaluation regarding its feasibility. The detailed analysis indicates that for 100 mL of wastewater, the corresponding solvent volume to achieve ZLD is 300 mL, 400 mL and 600 mL when the extraction temperature is 5°C, 10°C and 15°C, respectively. High water recovery efficiencies and salt removal efficiencies within five repeated ZLD tests highlights the reusability performance of SED. In addition, a novel SED-ZLD system with the flue gas before desulphurization as heat source is proposed. Thermodynamic calculation results show that for 5 t/h of wastewater, less than 50,000 m3/h of flue gas is sufficient for heating. Energy evaluation suggests that although SED-ZLD system consumes more energy, the exergy values of the heat source is much lower than that of multiple effect distillation or reverse osmosis, highlighting the superiority in waste heat utilization.