Recovering chemical sludge from the zero liquid discharge system of flue gas desulfurization wastewater as flame retardants by a stepwise precipitation process

In this study, a five-stage stepwise precipitation process, including pre-sedimentation, magnesium removal, gypsum precipitation, ettringite precipitation and calcium removal, was proposed as a softening pretreatment for zero liquid discharge system for flue gas desulfurization wastewater. Batch tests and long-term bench-scale experiment showed that magnesium, sulfate and calcium were efficiently removed with efficiencies all above 98.0%, leaving a clean effluent majorly containing NaCl and NaOH. The precipitated CaSO4, CaCO3, Mg(OH)2 and ettringite were completely separated by stepwise precipitation, and the purity of Mg(OH)2 and ettringite were further enhanced by washing and soaking treatment. CaSO4 and CaCO3 can be directly recycled as gypsum product and desulfurizing agent within the power plant, while Mg(OH)2 and ettringite presented proper particle size and excellent thermal properties as a synergistic flame retardant. The flame retardancy of ethylene vinyl acetate copolymer were greatly improved when blended with recovered Mg(OH)2 and ettringite, and possessed better performance by blending them together because ettringite could act as a dispersing and compatible agent of Mg(OH)2, and relieve the intensity of smoke releasing. Chemical sludge recovery compensates the total cost of the five-stage process by 45.0%, and makes the process technically versatile, economically beneficial and environmentally friendly without solid waste production. [Display omitted] •A five-stage process was proposed for stepwise removal of Mg2+, SO42- and Ca2+.•Bench system efficiently removed Mg2+, SO42- and Ca2+ by 98.8%, 98.4% and 98.0%.•Precipitate CaSO4, CaCO3, Mg(OH)2 and ettringite were fully separated and recovered.•Blending Mg(OH)2 with ettringite had better flame retardancy and smoke depressing.•Sludge recovery compensates softening cost by 45.0% without solid waste production.