Metal fog

Generally, pure molten salts are transparent and clear. If a piece of metal is introduced into a pure molten salt, a so-called metal fog is observed streaming from the metal into the melt and coloring it. The color intensity usually increases with increasing concentration of dissolved metal. In most cases the properties of the pure molten salt change when a metal dissolves in it, and a true solution is formed. For example, the solubility is higher if the metals dissolve in their own salts, and several binary systems show a complete miscibility between metal and salt above... 

When a piece of aluminum is added to clear, transparent molten cryolite, foglike streamers spread out from the metal and they soon render the melt completely opaque [188-195], It is clear that the metal fog is not a stable chemical phase in equilibrium with the electrolyte, since it dissipates when it rises from the molten... 

Taylor s experiments180 on the decomposition of metal alkyls led him to believe that the active alkyl groups, released by decomposition of the compounds, functioned in the same way as the active metal atoms. Experiments have shown that the presence of hydrogen atoms induces oxidation of ethylene at room temperature. The theory is advanced that the free alkyl radicals act in a manner similar to hydrogen atoms or metal fogs, i.e., as active oxidation centers producing a slow homogeneous combustion of fuel. This action of free radicals may accomit for the effects of non-metallic knock suppressors as aniline, toluidine, etc. 

The nature of the dissolved aluminum is still a subject of discussion intermediate A1(I) and individual A1 in form of so-called metal fog concepts are in use. Both hypotheses are equally acceptable for the mechanism (4.5). 

The so-called metal fog may also be important for the loss in current efficiency. Metal fog consists of small metal droplets which are formed by homogeneous nucleation from a supersaturated solution of dissolved metal, and these droplets are easily consumed by chlorine from the anode. 

Li metal is produced by electrolysis in LiCl-KCl melt [1], but its demand is still limited currently. However, production of Li metal should remarkably increase considering the future development of Al-Li and/or Mg-Li alloy, so that an improvement of the process should be required. There are some obstacles in Li electrolysis, such as reduction of moisture in bath and the so-called metal fog formation. Their control should be valid not only for better Li electrolysis but also for the electrolysis of other active metals, such as Ca. Since the effective production process of Ca metal is desired because of its strong reducing power [2], the results on Li electrolysis should be useful for Ca electrolysis. 

Electrochemical study in molten salt systems is ordinarily completed by evaluating the electrochemical responses and analyzing the reaction products. In situ visual observation of the electrode reaction is sometimes attempted because it gives valuable information on the phenomenon. Although visual observation is rather difficult in a high-temperature molten salt system, it should be strongly required for some cases, such as metal-fog formation. The record of the in situ observation is also helpful to understand the phenomenon in detail. 

The bath around the electrode was slightly coloured blue at a potential below 0.05 V. This indicates that metal fog was formed in precedence over Li metal deposition, but its amount seemed very small. Meanwhile, remarkable metal fog formation was observed below 0 V, that is, after the Li deposition occurred. The metal fog formation following Li deposition tended to occur at bigger cathodic overpotential and higher temperature, as shown in Figure 2.4.3. 

Current oscillation was seen as the fog was formed. Li20 addition in the bath was effective to prevent this metal fog formation though the cathodic current decreased with Li20 addition. These results suggest that this type of metal fog was formed by electrolysis with bigger current density. 

In cyclic voltammetric measurement, metal fog formation following after Li deposition was also seen, and the formation depended on the scan rate this metal fog tended to be formed at a lower scan rate, and the formation was usually accompanied with the current oscillation, as shown in Figure 2.4.4. The dependence of the metal fog formation on the scan rate implied that the formation was affected by the amount of deposited metal or the decrease in Li ion around the electrode. 

Figure 2.4.3 (a-c) Dependence of Li metal fog formation during potentiostatic electrolysis... 

Figure 2.4.4 Change in cyclic voltammograms by metal fog formation at 726 K...

Two types of metal fog formation, namely, the formation in precedence over Li deposition and that following the deposition, were observed in Li electrolysis. The former type was seen in Na electrolysis in NaCl-based melt and in Ca electrolysis in CaCl2-based melt [3], whereas the latter seems unique. It is hard to explain both of them by one mechanism. 

This ratio can be regarded as the short-term cathodic current efficiency of Li deposition, and was close to 1 in the case of the Li deposition without the metal fog. The metal fog formation of both types slightly worsened the ratio, but the influence was small in this study. 

The cathodic phenomena in Li electrolysis in a LiCl-KCl eutectic melt were studied. Two types of metal fog formation were observed, but their influence on the efficiency of Li deposition was limited. It was confirmed that some cathodic reactions of residual moisture in the bath occurred in the bath and gas bubbles of H2 were generated. The reduction of residual moisture was competitive against the Li deposition, so that the efficiency of Li deposition was directly influenced by it. 

Metal dissolution is a general phenomenon in molten salts, and dissolved metals are responsible for the major loss in current efficiency due to their reaction with the anode product. So-called metal fog is a visual phenomenon associated with metal deposition from molten salts. Results from experimental studies have shown that metal fog consists of small metal droplets formed by homogeneous nucleation from a supersaturated solution of dissolved metal [1]. The electrode kinetics for metal deposition reactions are known to be very fast. Therefore, limitations due to nucleation and diffusion are more important for the metal deposition process. Nucleation may be of significance both for solid and liquid metal products. 

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