A novel sulfate removal process by ettringite precipitation with aluminum recovery: Kinetics and a pilot-scale study

A novel sulfate removal process via ettringite precipitation was developed by dissolving ettringite and recycling Al³⁺ under low pH condition. Effects of solid to liquid ratios, pH and temperature on ettringite dissolution, Al recovery and transformation of precipitates were investigated by batch experiments. The optimum condition for Al recovery is pH =3.0, suspended solid of 9.8 g/L and temperature below 303 K. Ettringite dissolution consists of two stages, (i) rapid but inconsistent dissolution with the fastest release of sulfate, followed by calcium, and then Al(OH)₆³⁻; (ii) slow dissolution of Al(OH)₆^3- core and gypsum precipitation. Dissolution of Al(OH)₆^3- core follows the first-order kinetics with activation energy of 41.18 kJ/mol, while gypsum re-precipitation follows the second-order kinetics with activation energy of 26.36 kJ/mol. Long-term results of pilot-scale systems for treatment of real flue gas desulfurization wastewater showed that the process achieved sulfate removal of 98.3%–99.5% and Al recovery above 98.4%, and converted 98.8% sulfate in ettringite to CaSO₄, which resulted in 66.0% of sludge reduction and improved sludge dewaterability. Economic evaluation shows that the process with Al recovery reduces cost of ettringite precipitation by 35.1%, and is highly feasible and cost-effective for industrial application of high-sulfate content wastewater treatment.