On the use of liquid metals as cathode in molten fluorides

• The electrochemical behaviour of liquid electrodes (Bi, Sb, Sn, Ga, Pb) was performed in fluoride salts.
• Then, in situ generated Bi-reducing agent (Li, Ca, Na) electrode were produced.
• General relationship between logarithm of global molar fraction of reducing agent and the electrolysis potential was found.

The aim of this article is to investigate the behaviour of liquid metallic electrodes in pyroprocesses. Thus, studies of electrochemical properties of various liquid metallic electrodes (Bi, Sb, Sn, Pb, Ga) were performed in LiF–NaF (750 °C) and LiF–CaF2 (850 °C). Thanks to linear sweep voltammetry technics, electroactivity domains were determined as well as a reactivity (Sb > Bi > Pb > Sn > Ga > Mo) and a nobility scale (Ga < Sn < Pb < Sb < Bi < Mo), valid in both solvent: from these data, the extraction yield of an element can be evaluated. Then the preparation of alloyed cathodes for reductive extraction process, corresponding to a spontaneous reaction between a reducing agent contained in the liquid metal and the element to be removed from the molten salt, was examined. The in situ generation of liquid Bi–Li, Na or Ca (reducing agents) reactive electrodes was performed in LiF–NaF and LiF–CaF2 solvent by galvanostatic electrolyses. Elementary analyses showed that Li+ and Ca2+ were reduced on Bi at the same reaction rate in LiF–CaF2, whereas Na was preferentially inserted into Bi, compared to Li, in LiF–NaF. Linear relationships between the electrolysis potential and the logarithm of the global mole fraction of reducing agent were found. As a consequence, by fixing the potential at the end of the electrolysis, Bi reactive cathodes with a controlled quantity of reducing agent can be produced in galvanostatic conditions.