Effect of metal fluorides in the electrolyte on the electrolytic production of NF 3

Abstract

Electrolysis of the melts of NH 4F·2HF with and without metal fluorides such as LiF, NaF, KF, CsF, MgF 2, and AIF 3 was conducted with a nickel anode. The mixed gas composed of NF 3 and N 2 with a small amount of N 2F 4, N 2F 2, N 2O, and O 2 was liberated at the anode by electrolysis at 25 mA/cm 2 and at 120 °C. The addition of LiF into the melt was most effective for increasing the NF 3 current efficiency and for minimizing the consumption of nickel anode. In contrast, KF in the melt decreased the current efficiency for NF 3 and other constituents in the anode gas as well as hydrogen generated a: the cathode. It also stimulated the consumption of Ni anode. The concentration of nickel ion in the molten KF-NH 4F-HF system was low compared with that in the melts of NH 4F · 2HF with and without alkali metal fluorides such as LiF and CsF because KNiF 3 was deposited on the cathode and the cell bottom. The SEM observation and the XPS and XRD analyses revealed that the oxidized layer formed on nickel in molten NH 4F · 2HF with and without LiF, CsF, and 0.3 mol% NaF was composed of NiF 2 with a small amount of Ni oxides of divalent and trivalent states and highly oxidized nickel fluorides. The surface layer formed on nickel in molten NH 4F · 2HF with 6.7 mol% NaF was mainly composed of NiF 2 and NaNiF 3. On the other hand, the oxidized layer in the melt containing KF was composed of only KNiF 3, and was very brittle. Therefore, it is concluded that KF is detrimental to the nickel anode, depending on the composition of molten fluoride.