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The formation and decomposition of tetrafl uoroborate ions (BF 4 − ) in H 3 BO 3 -Al 3+ -F − solutions were investigated via experiments and thermodynamic calculations. The concentration of the formed BF 4 − increased with decreasing pH, raising the total fl uoride 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 fl uoroaluminate ions by adding an aluminum compound under acidic conditions. A method for removing fl uoride in the form of BF 4 − is proposed whereby fl uoroaluminate 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 fl ue gas desulfurization plants, and resulted in a satisfying level of reduction in the range of the fl uoride emission limit of 8 mg/l.

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