| Currently, the only way in industry to produce metal Li is by electrowinning from the molten LiCl-KCl melt at about450℃. The obtained metal Li has the purity of98-99wt%with1-2wt%impurities including Na, K, Al, Ca, Mg, Si, etc, which are derived from the corresponding metal chloride compounds or metal oxide compounds existing in the LiCl and/or KC1raw materials. These impurities have to be removed by vacuum distillation in a stainless steel reactor under600-800℃, and sometimes followed by zone smelting. It consumes52kWh/kg-Li to refine the primary lithium from a purity of98.5to99.9wt%, accompanying with a serious corrosion to the reactor. It is necessary to explore a low-cost method to remove these impurities from the primary lithium.In this dissertation, the electrochemical methods have been employed to investigate the reduction mechanism and removal rate of race impurity ions in LiCl-KCl. Several obtained results including:(1) During the investigation of trace impurity K, the0.3V of reduction potential difference between Li+and K+in LiCl-KCl (1:1mol) have been obtained by electrochemical methods, which decreased to0.16V with the LiCl-KCl mol ratio increased to3:7. And the reduction potential of Li+was positively shifted0.15-0.2V with the temperature increasing from450to600℃. The content of impurity K can be reduced after electrolysis from lower mol ratio LiCl-KCl, lower temperature and current density.(2) In order to remove trace impurity AlCl3from LiCl-KCl melts before Li electrolysis, the Al+reduction potential on a tungsten electrode was determined by cyclic voltammetry (CV) and square wave voltammetry (SWV). The reduction potential difference between Al3+and Li+was1.8V, which satisfied the electrochemical removal of trace Al3+from LiCl-KCl melts. The constant potential electrolysis at-1.6V on both solid Fe and liquid Zn cathodes was performed to remove Al3+impurity from the LiCl-KCl-AlCl3melts. The results showed that96.11%of Al3+were removed on a Fe cathode and99.90%on a Zn cathode through10h electrolysis, respectively. The activity coefficient of Al in Al-Fe alloy was lower than1, while it was slightly higher than1in Al-Zn alloy, which means the Fe cathode may remove more impurity Al. The difference between theoretical calculation and the experiment was attributed to the thermodynamics and kinetics of alloy formation. While stirring the melts by argon gas,99.21%of Al3+was separated from the melts by4h electrolysis at450℃, which effectively expedited the Al3+electrochemical reduction rate and shortened the electrolysis time.(3) In order to remove trace impurity MgCl2from LiCl-KCl melts before Li electrolysis, MgCl2reduction processes in LiCl-KCl-MgCl2melt were investigated by cyclic voltammetry (CV), square wave voltammetry (SWV), chronoamperometry (CP) and chronopotentiometry (CA). The results showed that Mg2+was reduced in one step with two-electron transfer and its reduction potential was well defined. The reduction potential difference of Mg2+with Li+underpotential deposition was0.25V; therefore, in order to prevent Li+from codeposition with Mg, accurately controlling Mg2+reduction potential during the potentiostatic electrolysis was necessary. The liquid lead was recommended as cathode to remove trace impurity Mg2+according to the binary alloy phase diagram and magnesium activity coefficient calculation. Then potentiostatic electrolysis was carried out and Mg+was reduced onto a liquid lead cathode from the LiCl-KCl-MgCl2melt at the Mg deposition potential, which showed the Mg2+theoretical removal rates increased with the decreasing of MgCl2concentration and the extending of electrolysis time. About93-99%of removal rate of MgCl2was achieved after8-12h electrolysis when liquid lead was used as cathode. But a small amount of Li was found in cathode, which means the Li+codeposition with Mg during electrolysis.(4) In order to investigate the possibility of electroseparation of CaCl2from CaCl2-LiCl-KCl melts prior to Li+reduction, the theoretical reduction potential difference of0.078V between Ca2+and Li+ions was calculated by Nernst equation. But this potential difference was0.3V by cyclic voltammetry (CV) study in CaCl2(18mol%)-LiCl(18mol%)-KCl(64mol%) system. The potential difference between Ca2+and Li+in CaCl2(0.5wt%)-LiCl-KCl melts was0.2V at680℃analyzed by CV. More than0.2V reduction potential difference between Ca2+and Li+ions was observed on chronopotentiograms (CP) recorded on a tungsten electrode in CaCl2(2.0wt%)-LiCl-KCl melts either at450-680℃, which met the requirements of total electroseparation. Tin electrode can shift the reduction potentials of Ca2+and Li+to more positive ones and can improve their reduction potential difference to0.6V. After3h electrolysis on solid tungsten at-3.4V and liquid tin cathode at-3.0V, the Ca/Li (weight ratio) in the melts changed from3/25before electrolysis to2.6/25and0.76/25respectively, which tin cathode improved. Ca2+removal rate. Under stirring, even higher removal rates of83.65%and94.66%, corresponding to0.54/25and0.16/25of Ca/Li have been obtained for3h and6h electrolysis at-3.0V on a tin electrode. Clearly, stirring availably increases the Ca2+ions diffusion rate and its electrochemical reaction rate.(5) In order to investigate the possibility of electroseparation of NaCl from NaCl-LiCl-KCl melts prior to Li+reduction,-0.05V of theoretical reduction potential difference between Na+and Li+ions was calculated by Nemst equation. But this potential difference was higher than0.2V in LiCl(18mol%)-KCl(64mol%) with NaCl(18mol%) system studied by cyclic voltammetry (CV), which provided the possibility for electrochemical removal. The potential difference between Na+and Li+was0.16-0.2V on a tungsten electrode in LiCl-KCl with0.5-2.0wt%NaCl at450℃analyzed by cyclic voltammetry (CV), square wave voltammetry (SWV) and chronopotentiograms (CP). The liquid lead was recommended as cathode to remove trace impurity Na+. About0.2V reduction potential difference between Na+and Li+was observed on CV recorded on a lead electrode at450℃in LiCl-KCl-NaCl(2.0wt%), which was smaller than Ca2+and Li+potential difference on tin electrode. During the process of Na+impurity removal, the practical removal rates of Na+were increased from47.67%to75.71%with the electrolysis potential shifted from-2.6V to-3.0V. Except for Na+reduction, some Li+were reduced to Li at-3.0V for consumption of LiCl. With the electrolysis time increased to6h, the Na+impurity removal rate was reached to90.12%, meanwhile, the Na/Li (mass ratio) increased to4.25/1, which further proved the metal Li promoting Na+removal rate.(6) Comparing with the condition of three leaf blade impeller and without impeller electrolysis, more impurity removal rates have been achieved by electrolysis in LiCl-KCl-NaCl(2.0wt%)-CaCl2(2.0wt%)-MgCl2(2.0wt%)-AlCl3(2.0wt%) system on a lead cathode for10h with the four leaf blade impeller. The uniform mixing time of impurity ions free diffusion were13.764s and14.394s respectively in three and four leaf blade electrolytic cell by Fluent numerical simulation. Comparing with impurity ions free diffusion, the impellers could improve the ions diffusion and shorten mixing time to3orders of magnitude. And the mixing time for four leaf blade impeller was smaller than the three leaf blade impeller; but this superiority gradually decreased with the rotating speed increasing. Therefore, the four leaf blade with40rpm can shorten the mixing time for impurity ions from bulk solution to the electrode surface solution, which was in accordance with experimental results. The Fluent numerical simulation also showed the components transportation model can be employed to simulate the ions diffusion during electrochemical reaction.
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