| Hall-Héroult molten salt aluminum electrolysis is currently an industrial aluminum production method.Excessive electrolysis temperature leads to high energy consumption,high carbon consumption and large CO2 emissions.By reducing the electrolysis temperature,the problems caused by conventional aluminum electrolysis can be effectively alleviated.However,the current industrial electrolyte is a Na3AlF6-AlF3 system with a cryolite ratio of about 2.6.Lowering the electrolysis temperature can decrease electrical conductivity,current efficiency,produce crust at the bottom of the cathode,and increase pressure drop.Therefore,it is imperative to develop a new aluminum electrolysis system for aluminum electrolysis.The eutectic temperature of Na5Al3F14-AlF3 system is 695℃ and the peritectic temperature is 734℃.The liquidus temperature of Na5Al3F14-AlF3 system is much lower than the liquidus temperature of industrial electrolyte.The liquid phase region in this region is wider,and changes of cryolite ratio don’t cause dramatic changes in liquidus temperature.However,the low liquidus temperature will decrease the solubility of alumina and the electrical conductivity.By adding KF and LiF can effectively improve the solubility of alumina and conductivity.Therefore,electrolysis Na5Al3F14-KF-LiF-AlF3-Al2O3 system is studied.In this paper,the liquidus temperature and cathode penetration of Na5Al3F14-KF-LiF-AlF3-Al2O3 system were studied.The liquidus temperature was determined by cooling curve method and differential thermal analysis method.Cathode penetration mainly is studied by ash method and analyzed by quantitative calculation.The liquidus temperature of Na5Al3F14-KF-LiF-AlF3-Al2O3 system with CR=(NaF+KF+LiF)/AlF3=1.3 and CR=1.4 was measured,and the effect of the addition of KF or LiF and the co-addition of KF and LiF to the liquidus temperature were investigated.KF and LiF can significantly reduce the primary crystal temperature of the Na5Al3F14-KF-LiF-AlF3 system in a certain region,and then the liquidus temperature will increase.The primary crystal temperature of the Na5Al3F14-KF-LiF-AlF3-Al2O3 system with CR=1.3,KRKF=KF/(KF+NaF+LiF)=0.2,KRLiF=LiF/(KF+NaF+LiF)=0.21.The empirical formulas for the primary crystal temperature of Na5Al3F14-KF-LiF-AlF3 system with CR=1.3 and CR=1.4,x[KF]=0～20%,x[LiF]=0～20%were fitted,and R2 is 0.9981 and 0.9767.The isothermal curves of CR=1.3,KRLiF=0～0.5,KRKF=0～0.99 and CR=1.4,KRLiF=0～0.35,KRKF=0～0.35 were drawn.The effects of KF and LiF on the solid solution transformation of Na5Al3F14-AlF3 system were investigated.Some Na+of Na5Al3F14 will replaced by K+/Li+ in a certain concentration range of KF and LiF to form Na5Al3F14 solid solution.A simple phase diagram of the partial system of Na5Al3F14-KF-LiF-AlF3 with CR=1.3 was drawn,and the variation of phase composition with the concentration of KF and LiF was given.The effects of KF and LiF addition alone and in combination on the morphology of the quenched phase of the electrolyte were discussed.A high-K worm-like phase and a flocculent phase existed when KF and LiF were added,respectively.The electrolysis experiments with different concentrations of KF and LiF were carried out in Na5A13F14-KF-LiF-AlF3-Al2O3 system.And the parameters of current efficiency,electrolyte loss rate,cathode density change,and alkali metal content in aluminium beads were characterized successively.When x[LiF]=16%,x[KF]=16%,x[KF]=16%or x[LiF]=8%,the electrolysis at constant temperature has the highest superheat due to the lowest primary crystal temperature,so it has higher current efficiency.Quantitative analysis was made on the cathode carbon blocks after electrolysis in Na5Al3F14-KF-LiF-AlF3 system containing different concentrations of KF and LiF.The effects of KF and LiF addition alone and in combination on the electrolyte permeation in cathode carbon blocks were discussed.KF could promote the penetration of electrolyte,however,LiF would inhibit effectively the penetration of electrolyte. |
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