Formation and decomposition of tetrafluoroborate ions in the presence of aluminum

The formation and decomposition of tetrafluoroborate ions (BF (4) (-) ) in H3BO3-Al3+-F- solutions were investigated via experiments and thermodynamic calculations. The concentration of the formed BF (4) (-) increased with decreasing pH, raising the total fluoride concentration and lowering the total aluminum ion concentration. Once formed, BF (4) (-) was stable under neutral and alkaline conditions. Fluoride in the form of BF (4) (-) was converted to fluoroaluminate ions by adding an aluminum compound under acidic conditions. A method for removing fluoride in the form of BF (4) (-) is proposed whereby fluoroaluminate ions formed by the reaction of BF (4) (-) with aluminum are decomposed with calcium ions. This process was applied to the treatment of wastewater from flue gas desulfurization plants, and resulted in a satisfying level of reduction in the range of the fluoride emission limit of 8 mg/l.