Simultaneous removal of sulfate and nitrate from real high-salt flue gas wastewater concentrate via a waste heat crystallization route

Abstract

Flue gas wastewater concentrate was acidic and highly saline, and generated from the evaporator of desulfurization wastewater in power/heating plant. Its conventionally sprayed into flue gas pipeline and completely evaporated to generate hazardous halite into flue gas ash. Herein, a facile low-temperature thermal route was developed to treat the concentrate without the generation of any hazardous waste. The results showed that the concentrate was at pH 0.2 and contained 59 g/L sulfate, 13.1 g/L nitrate and 16.4 g/L chloride. When it was thermally treated at 150 °C (similar to the temperature of waste heat in power/heating plant), more than 99% sulfate and 97% nitrate were effectively removed with the addition of aluminum subacetate reagent. Without aluminum subacetate reagent, the removal of sulfate and nitrate only achieve by 2.8% and 9.1%. During the thermal treatment, the sulfate reacted with aluminum ion to form regular hexahedron natroalunite, whilst the nitrate was eliminated as nitrogen gas via the redox reaction between nitrate and organics (e.g., subacetate). The analysis of Gibbs free energy revealed that the formation of natroalunite took placed at the temperature > of 67 °C and accelerated at high temperature, but the reduction of nitrate was pH-dependent and accelerated at the initial pH < of 0.4. With the formation of natroalunite, the decrease of chloride from 16.4 g/L to 14.7 g/L were also observed, and the treated concentrate can be reused in the flue gas desulfurization triple-chambered process. In summary, this method showed merits to not only reuse the waste heat in power/heating plant for the effective removal of sulfate and nitrate from the flue gas wastewater concentrate, but also produce the recycled water and the value-added natroalunite particles.