Sulfate removal from wastewater using ettringite precipitation: Magnesium ion inhibition and process optimization

Removal of High-Concentration Sulfate Ions from the Sodium Alkali FGD Wastewater Using Ettringite Precipitation Method: Factor Assessment, Feasibility, and Prospect

“…In addition, based on the literatures and experimental results [19,21], the following chemical equilibrium reactions can be used to describe the SO 4 2− removal reaction process:…”

“…One of the reasons is that Ca(OH) 2 solubility decreased with the increase of solution pH, resulting in lower Ca 2+ concentration in the solution and less ettringite production. Another reason is that the amount of CO 2 absorbed by the solution increased with increasing alkalinity of the solution, resulting in an increase of CO 3 2− concentration in the solution; thus CO 3 2− can react with ettringite to form hydrated carbonated calcium aluminate (3CaO·Al 2 O 3 ·CaCO 3 ·11H 2 O) [19,23], leading to decrease in SO 4 2− removal e ciencies. erefore, it is necessary to control the wastewater pH in practical engineering applications in the range of 5.0∼9.0 to achieve a higher SO 4 2− removal.…”

“…However, these methods are not very suitable for the treatment of high concentration of SO 4 2− wastewater, except the chemical precipitation method. e chemical precipitation method mainly includes lime precipitation [6], barium chloride precipitation [17,18], and ettringite (Ca 6 Al 2 (OH) 12 (SO 4 ) 3 · 26H 2 O) precipitation methods [19].…”

“…e limestone precipitation method is widely used in the field of water treatment; however, due to the relatively high solubility of gypsum [19], the SO 4 2− removal efficiency is low. e barium chloride precipitation method can reach high SO 4 2− removal efficiency, but a large number of corrosive chloride ions and toxic barium ions will be introduced into the water; besides, barium chloride is more expensive than lime, so the technology is rarely used in the field of flue gas desulfurization wastewater treatment.…”

“…In this method, lime and aluminum salts are added into the wastewater to react with sulfate to form insoluble ettringite (pKsp � 111.6) [20], and then sulfate is effectively removed. e ettringite precipitation method has become a preferred method due to its high removal efficiency and cost-effectiveness, and many studies have used this method to treat industrial wastewaters, such as aluminum anodizing, textile industries, and mine water [19][20][21]. However, in existing researches the sulfate concentrations in simulated or actual wastewaters were usually lower; hence, anions and heavy metal ions were not considered in the removal of sulfate; meanwhile, there exists little information on the removal of SO 4 2− from the sodium alkali (NaOH) FGD wastewater using the method in the literatures.…”