Effect of Addition of Alkali Metal Fluoride to a Molten NH_4F-HF System on Current Efficiency for NF_3 Formation and Nickel Anode Consumption

摘要

The current efficiency for NF_3 formation and the current loss caused by Ni dissolution were investigated in electrolysis of the NH_4F•2HF melts with and without alkali metal fluorides such as CsF, KF, and LiF. The addition of CsF to the melt was most effective for increasing the NF_3 current efficiency. In contrast, the addition of KF to the melt decreased the current efficiency for NF_3 formation and increased the current loss caused by Ni dissolution. The SEM observation and XRD analysis revealed that the oxidized layer formed on nickel in the melt containing LiF or CsF was composed of NiF_2 with highly oxidized nickel fluoride. 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 and highly oxidized nickel fluoride may relate to the NF_3 current efficiency.