Study On The Dissolution Behavior Of Alumina In Cryolite Electrolyte

This thesis mainly presents the further development and collation of other research results on the dissolution process of smelting grade alumina(SG alumina)in cryolite electrolyte.The dissolution phenomenons of fresh alumina and secondary alumina were observed through a quartz crucible and the dissolution kinetics was discussed.Firstly,the inner correlations between several key physiochemical properties of SG alumina including moisture content,loss on ignition,surface area and phase composition were investigated,as well as the relationships between the dissolution rate and the specifications,bath composition,temperature and superheat.Secondly,the physiochemical properties of the alumina agglomerate/crust formed upon SG alumina feeding and the corresponding dissolution rate were studied using the in-situ gravimetric method combined with the see-through cell.The effects of the temperature and alumina concentration in the electrolyte on the formation and dissolution process of the crust were then examined.Thirdly,the dissolution behaviors of fresh alumina and secondary alumina were comparatively studied.The impacts of possible impurities,including moisture content,chemisorbed hydrogen fluoride(HF),carbon dust and fluoride particulates in the secondary alumina on its dissolution behavior were investigated.In addition,the fluoride desorption processes of secondary alumina during heating were calculated according to the mass and fluorine loss at different temperatures.This thesis develops a computational model to investigate the relative importance of three sources of hydrogen on HF emission of the cell.It is found that the loss on ignition is roughly proportional to its surface area,while in inverse proportion to the alpha phase content for SG alumina produced with the same calcining technology.And the lab prepared alumina sample from static calcination showed the same principle.Larger loss on ignition and surface area facilitate the dissolution rate of fresh alumina significantly.Compared to the minor changes in bath composition and superheat,the impacts of the temperature and alumina concentration in the electrolyte on alumina dissolution rate appear to be much greater.The rate controlling step of the dissolution process changes with alumina concentration in the electrolyte.When the alumina concentration is less than 3mass%,the dissolution rate is limited by the chemical reaction rate,but once the alumia concentration exceeds 3mass%,the dissolution rate depends on the dispersion rate of the solvent ions.Under experimental non-stirring conditions,a large proportion as high as 54-64mass%of the fed alumina participated in the formation of the crust upon the feeding of lmass%fresh alumina at 960-965?.The mass of the crust and its corresponding dissolution rate was significantly affected by the alumina concentration in the electrolyte and the temperature.Other properties of the crust such as volume density and gas-filling fraction were also derived.Distinguished from the fresh alumina,no crust was observed during the dissolution of secondary alumina which also possessed faster dissolution rate.It shows that the carbon dust in secondary alumina oxidizes instantly after addition,offering an effective dispersion and preheating to the alumina particles,while the influences from the moisture content,fluoride particulates and HF absorption is not significant.Fluoride desorbs from secondary alumina during heating.The HF desorption is always combined with H2O escape at approximately below 740?.The rest nearly 50mass%fluorine content desorbs in the form of chiolite(NasAl3F14)at 740-1100?.The relative contribution of hydrogen sources to HF evolution of the cell turns out to be:fresh alumina feeding-35-55mass%,ambient moisture-5-55mass%,anode hydrogen-5-10mass%.Accordingly,it explains the phenomenon that the fluoride emission changes with the alumina manufacturer and climate(ambient humidity),and the possible measures to reduce the HF emission of the cell are also proposed.This thesis also explores the interpolar process of aluminum electrolysis.The voltage distribution between carbon anode and aluminum cathode in cryolite electrolyte saturated with alumina was determined using a scanning reference electrode in both laboratory setup and industrial cells.The results showed that the anode-cathode distance(ACD)is consisted of three parts:a relatively stable cathode boundary layer,bubble-free electrolyte layer and gas-liquid layer near the anode.The aluminum diffusion layer with high electronic conductivity as well as the crystallization of cryolite was observed at the cathode boundary layer.The thickness of the aluminum diffusion layer varied with current density.The average bubble coverage decreases with current density and the average coverage at 0.8A·cm-2 is approximately 50mass%.