Formation and decomposition of tetrafluoroborate ions in the presence of aluminum

Abstract

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.

Determination of Total Fluoride in Boron-containing Solutions

“…The present authors have investigated the formation and decomposition of BF4 -in H3BO3-Al 3+ -F -solutions via experiments and thermodynamic calculations, and have shown that the fluoride in BF4 -can be converted to fluoroaluminate by adding an aluminum salt solution under acidic conditions. 10 In addition to this study on the formation and decomposition of BF4 -, we have also shown that total fluoride content in solutions containing a large amount of aluminum can be determined by ISE analysis using a tartrate-and TRIS-based TISAB solution. 3 By combining these characteristics of BF4 -and fluoride, a novel pretreatment method before determining total fluoride in solutions containing BF4 -with ISEs is proposed.…”

“…According to a thermodynamic calculation, 10 the fraction of free fluoride was as high as 64% when a 200 mg L -1 BF4 -solution was in an equilibrium state at pH 1. These results demonstrate that the total fluoride ion concentration can be determined in sample solutions containing both F -and BF4 -, in amounts comparable to those in pre-scrubber wastewater, using the ISE method with the addition of a tartrateand TRIS-based TISAB solution.…”

“…Note that BF4 -no longer exists, and boron exists exclusively as H3BO3. 10 Upon adding the resulting reaction mixture to a sodium hydroxide solution to obtain an alkaline solution having a pH value of about 11, not only the decomposition, but also formation of BF4 -does not proceed any more. 10 On the subsequent addition of tartrate-and TRIS-based TISAB, the pH is adjusted to 8.4, and aluminum and fluoride exist exclusively as tartrate complexes (concentration: C2,Al-tartrate) and free F -(C2,F -), respectively.…”

“…10 To examine the effects of the reaction temperature on the decomposition process, experiments were also performed at 23, 35, 50, and 70 C. The relationship between the fluoride detection efficiency and the reaction temperature is shown in Fig. 4.…”

“…The point is that the detection degree of fluoride depends only the decomposition conditions of BF4 -, and the ISE analysis is not affected by the presence of boron at pH 8.4, since the decomposition and the formation of BF4 -do not occur under these conditions. 10 Based on the equilibria estimated by a thermodynamic calculation, 10 the conversion degree of BF4 -to boric acid was higher than 99.9% at pH 1 for a mixed solution shown in the standard procedure.…”