Intensification of electrochemical properties of the molten chloride electrolytes of the cerium subgroup lanthanides

Abstract

Abstract. The electrical conductivity of molten chloride electrolytes of the cerium subgroup lanthanides increases with rising electric field strength and strive to achieve the limiting high voltage values (Wien effect). On exposure of the high-voltage microsecond pulsed fields, the melts are transited into a prolonged non-equilibrium state with increased electrical conductivity and electrolyze current density. During the relaxation processes in non-equilibrium melts, increased electrical conductivity tends to restore the values that are specific to equilibrium systems. IntroductionIn various modern industries there is an increasingly widespread use of rare earth metals (REM). In an industrial scale mainly REM alloys and relatively pure lanthanum, cerium and neodymium are produced. REM and their alloys are advantageously prepared by electrolysis of anhydrous molten chloride mixtures Ln-MCl (Ln = REM and M = Na, K). The structures of the molten electrolytes, as well as the nature and distribution their structural species, determine their observed physicochemical properties, the mechanisms and kinetic pathways that decrease energetic efficiency of metals production technologies [1]. For better understanding the structure of equilibrium molten electrolytes and improving the chemical and electrochemical technologies, their properties should be studied in a non-equilibrium state, which can be achieved by various external influences, for example, by the action of strong electric pulses [2]. We have found that electrical conductivity of molten alkaline-earth chlorides and their mixtures with alkaline chlorides increase with increasing external electric field strength (EFS) and strive to achieve the limiting high-voltage values in the fields of the order of 1 MV/m [3], in the same way as in the Wien effect in the aqueous electrolyte solutions.After the high-voltage pulsed discharges in the electrolytes having been completed, their low voltage conductivity (measured by usual AC Bridge) turns out to be increased [4] i.e., in them the phenomenon of activation (the "memory effect") is observed. The activation degree of electrical conductivity reaches up to 20 % in the case of individual alkali earth metals chlorides and to 55 % in the case of their mixtures with potassium chloride. These observations were interpreted as a consequence of the stimulated dissociation of complex formations. The activation of the melts is followed by the recombination of complex ions during prolonged relaxation processes in the nonequilibrium melts.

Structural Relaxation in the Activated Molten Chloride Electrolytes of Polyvalent Metals

“…The activation degree of conductivity also rises with increasing number of pulses and their voltage amplitude, and tends to the saturation. In Figure 7, as examples, these dependences are presented for a magnesium electrolyte Similar results were obtained for other compositions of molten mixtures [7][8][9][10][11][12][13][14][15]. The results obtained in Sections 3.1 and 3.3 are collected in Table 1.…”