Study On Anodic Bubble Behaviors In Molten Salts Electrolysis

Molten salts electrolysis is a common method for the extraction of non-ferrous metals,especially for light metals.The electrolysis cell is one of the key equipment in the electrolysis process,where electrochemical reactions and macroscopic multiphase fluid flow exist.The process of molten salts electrolysis have the characteristic of a non-linear and multi-field coupled.One of the most representative process in the cell is the anodic process,which involves the generation of anodic gas on the anode-electrolyte interface,forming a multiphase reaction system.The behaviors of anodic bubbles have a significant impact on the operation of cells.On the one hand,the bubbles nucleated at the surface of the anode cause additional voltage drop to increase energy consumption of the process due to its high electrical resistivity,and even lead to anode effect in extreme condition.On the other hand,the movement of bubbles improves mass and heat transfer in the cell by stirring liquid electrolyte.The development of a new energy-saving technology in the molten salts electrolysis industry is cannot be complete without the basic understanding of the bubble evolution behavior from the anode.However,the morphology and movement of bubbles is very difficult to be directly measured in the commercial cell.Therefore,the investigation of the bubble behavior using high temperature electrolysis model is of great significance to the design and development of the future reduction cell.In this work,with fluoride molten salt and chloride molten salt as research subjects,fundermental research on the behaviors of anodic bubbles evolution are systematically studied using a laboratory high temperature transparent electrolytic cell.With this method,much more phenomena and laws can be revealed.The main research contents and contributions are as follows:(1)The formation,growth,coalescence and release of CO₂ bubbles on carbon anodes in aluminium electrolysis cells was investigated using a laboratory transparent electrolytic cell from both side and bottom observations.The results show that the bubbles grew through gas diffusion and various types of coalescences.Large bubbles play a dominant role in the bubble coverage,which leads to the evolution of gas coverage showing a cyclic pattern.The maximum gas coverage is between 66%and 83%,and the gas coverage decreases with the increase of current density.(2)Based on the analysis of bubble morphology and the structure of anode surface,differences in the behaviors of bubble evolution between industrial carbon and graphite anodes were studied.It is found that the bubbles prefer to appear at several fixed areas on the undersurface of the anode.Compared to graphite anodes,the large-scale bubbles are easily formed on the industrial carbon anode.Variation in the surface structure of anodes is the main reason for different bubble evolution properties.Based on this fact,the mechanism of the nucleation and growth of the bubbles in the process of aluminium electrolysis is proposed.Refer to the crystal structure of the carbon anode,the vertically oriented carbon layer is beneficial for oxygen atoms to permeate into the carbon layer gap and bind with the inner carbon atoms,which makes higher gas formation rate at this position,leading to reach the critical condition for bubble nucleation.When the bubbles are formed,the gas-liquid interface becomes a new nucleation spots.Gas molecules in the molten electrolyte and electrode will enter the bubbles preferentially to promote the growth of bubbles,where new bubbles will are inhibited to nucleate around the bubbles.(3)The influence of anode geometry on bubble behavior during aluminum electrolysis was investigated.With the increase of the inclination angle at the undersurface of the anode,the velocity of bubbles increases,but the size and gas coverage decreases."Fortin" bubble was observed in cryolite-alumina molten salt at the anode inclination angle of 8°.For the slotted anode,it is found that the bubbles cannot move through the slots(width of the gap should be more than 3mm)for further coalescence.The anode with rounded corner contributes to reducing the bubble size and gas coverage,while it slightly increases the bubble releasing frequency.(4)The bubble behavior on the Ni-Fe alloy anode during aluminium electrolysis was observed.Pre-oxidized anode can significantly prevent the electrochemical dissolution and corrosion of anode,which is beneficial for the generation of oxygen gas.In this condition,tiny bubbles escaped continuously from the anode bottom without much coalescence,forming a foam-like bubbles around the Ni-Fe alloy anode,which is very different from the phenomenon observed with a graphite anode.(5)Electrolysis processes in several chloride molten salt systems was observed.The evolution behavior of Cl₂ bubbles on graphite anode was investigated in NaCl,KCl,CaCl₂,KCl-LiCl and MgCl₂-NaCl-KCl melt,respectively.The results show that the behaviors of chlorine bubbles in the electrolysis of NaCl,LiCl-KCl,CaCl₂ and MgCl₂-NaCl-KCl molten salts are similar,where large bubbles release periodically,and the thickness of bubbles ranges from 3.7-4mm.The behavior of bubbles in KCl molten salts is significantly different from that of other chloride molten salts investigated in this work.The bubble size is relatively small,and it does not form large bubbles covering the entire surface of anode bottom.Anodic effect is easy to be observed in the process of NaCl electrolysis.When the anode effect occurs,a layer of gas film covering the whole undersurface of the anode can be seen clearly,which blocks the electrochemical reaction.Compared to the gas coverage on the anode bottom,variation in the thickness of the bubble has little effect on the bubble electrical resistance.