Abstract
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.
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