EFFECT OF METAL FLUORIDES IN THE ELECTROLYTE ON THE ELECTROLYTIC PRODUCTION OF NF3

Electrolysis of the melts of NH4F . 2HF with and without metal fluorid es such as LiF, NaF, KF, CsF, MgF2, and AlF3 was conducted with a nick el anode. The mixed gas composed of NF3 and N-2 with a small amount of N2F4, N2F2, N2O, and O-2 was liberated at the anode by electrolysis a t 25 mA/cm(2) and at 120 degrees C. The addition of LiF into the melt was most effective for increasing the NF3 current efficiency and for m inimizing the consumption of nickel anode. In contrast, KF in the melt decreased the current efficiency for NF3 and other constituents in th e anode gas as well as hydrogen generated at the cathode. It also stim ulated the consumption of Ni anode. The concentration of nickel ion in the molten KF-NH4F-HF system was low compared with that in the melts of NH4F . 2HF with and without alkali metal fluorides such as LiF and CsF because KNiF3 was deposited on the cathode and the cell bottom. Th e SEM observation and the XPS and XRD analyses revealed that the oxidi zed layer formed on nickel in molten NH4F . 2HF with and without LiF, CsF, and 0.3 mol% NaF was composed of NiF2 with a small amount of Ni o xides of divalent and trivalent states and highly oxidized nickel fluo rides. The surface layer formed on nickel in molten NHI4F . 2HF with 6 .7 mol% NaF was mainly composed of NiF2 and NaNiF3. On the other hand, the oxidized layer in the melt containing KF was composed of only KNi F3, and was very brittle. Therefore, it is concluded that KF is detrim ental to the nickel anode, depending on the composition of molten fluo ride. (C) 1998 Elsevier Science S.A.