Study On Dissolution Process Of Alumina In NaF-AlF₃-based Melts

The dissolution of alumina in NaF-AlF3system is of a major fundamental part of Hall-Heroult aluminum electrolysis theory and technology. Usually, it is believed that the knowledge accumulated over100years is already enough for understanding this process. However, with the rise and development of high or super-high amperage aluminum reduction cell in recent years, it has put forward higher demand for continuous adding alumina and avoiding cathodic deposition. Moreover, the influence of K content coming along with alumina and the technical demand for alumina dissolution rate in the advanced low temperature, energy-saving reduction cell, all have encountered the fundamental problems related to the alumina dissolution. Based on these problems, the present paper is devoted to the study of the dissolution process of alumina in NaF-AlF3-bases melt electrolyte. The basic starting point and the main purpose of this work are to provide new information for enriching the modern theory and technology.of aluminum electrolysis.This work includes four parts:1) the ionic structures and the effect of KF in NaF-AlF3and NaF-AlF3-Al2O3molten salt were studied;2) the dissolution behavior of alumina in the cryolite molted salt were studied, and the effect and the mechanism of ultrasound on the dissolution rate of alumina was discussed;3) the influence of alumina content, carbon cathode materials and the pore structure on interface wetting-spreading behavior;4) the alumina dissolution reaction path was calculated by quantum chemistry calculation method based on density functional theory.Firstly, the ionic structures and the effect of KF in NaF-AlF3and NaF-AlF3-Al2O3molten salt were studied by Raman tests and the quantum chemistry calculation method. The Raman activities of Na3AlF6-AlF3, Na3AlF6-KF-AlF3and Na3AlF6-KF-AlF3-Al2O3melts were tested to obtain the experimental Raman spectra of various ionic complexes. Then, the Raman activity and thermodynamic property of M-Al-F and M-Al-O-F ion clusters was calculated by quantum chemical calculation method and the composition profile vs cryolite ratio was built in MF-AIF3binary system melt. It was found that the stable Al-F ion clusters are AIF4-, AlF52-and AlF63-in the binary system melt. The dissociation reactions occur between the complexes in the molten melts: AlF63-·3M+(?)AlF52-·2M++F-·M+, AlF52-·2M+(?)AlF4-·M++F-·M+. The dissociation equilibrium constant values increase with increased K+ion number. The stable Al-O-F ion clusters are AlOF2-,Al2OF62-and Al2O2F42-in the MF-AlF3-Al2O3ternary system melt, where the dissociation reactions2AlOF2-(?)Al2O2F42-and AlOF2-+A1F4-(?)Al2OF62-occur.Secondly, the quartz transparent cell was adopted to observe the dispersion and dissolution process of alumina in molten melts. A series of experiments were employed to explore the influence of melt components and method of alumina added on the alumina dissolution rate based on the measurement of equilibrium electromotive force of a concentration galvanic cell. The effect and the mechanism of ultrasound on the alumina dissolution rate were discussed. It was found that the dissolution rate of alumina decreases with the dissolved alumina concentration increased in cryolite melt and increase with the increased cryolite ratio. As KF was added, it could improve the alumina dissolution rate. The dissolution rate of alumina is controlled by mass transfer process. The driving force for this diffusion is the differences in concentration of dissolved alumina between that at the surface of the alumina particle (Cs) and that of the bulk of electrolyte (Cb). It is a collaborative relationship between the effect of ultrasound field and the chemical potential field of alumina(V4). Ultrasound enhances the diffusion coefficient, strengthens the mass transfer process in concentration boundary layer of individual alumina particle surface, and as a result accelerated the dissolution rate of alumina.Thirdly, a series of experiments were employed to explore the influence of melt components and carbon cathode materials on the interface behaviors between the cryolite-alumina molten salts and based materials using a modified sessile drop method. The effect of porous structures of cathode materials on the wetting behavior is analyzed by the image analysis method and SEM-EDS technique. The wetting and spreading mechanism was explored for the molten melts on carbon cathod materials based on the wetting molecular dynamics theory. The presence of pores can change the wetting behaviors of the cryolitic melts, which can be described by the Cassie-Wenzel intermediate model in association with the cryolitic melts impregnating into the pores. The apparent contact angles are dependent on pores mean diameter, aspect ratio, porosity and pores’ coordination number, while the wetting balance time is sensitive to the accumulation of pores mean diameter less than600μm. The molar activation free energy for sodium cryolite melt molecules on HC35sample is the largest and that is the smallest on SMH, while it is the largest on HC100sample and that is the smallest on HC35sample for potassium cryolite melt. Thus, it causes the best wettability for sodium cryolite melt on SMH and the worst wettability on HC35and the best wettability for potassium cryolite melt on HC35and the worst wettability on HC100. Graphitization degree increasing, it is increased for the distance between two neighboring adsorption sites and improved for the equilibrium frequency of melt molecular displacement in the three-phase contact line vicinity in the wetting process. This caused the spreading velocity of melt droplet on based sample increased.Fourthly, the adsorption of F" anion on γ-Al2O3(110) non-polar surface has been studied by quantum chemistry calculation method based on density functional theory. We put forward the view that the alumina dissolving process began in adsorption of F" anion on γ-Al2O3(110) non-polar surface in the interface of alumina particles and molten melt. The F-anions tend to adsorb on the unsaturated coordination Al atoms at the surface. Strong chemisorptions occur, and F-Al structures form when F-anions adsorb on the surface. The adsorption behavior is dominated by the interaction between the2s,2p orbit of F-anion and the3p orbit of the base metal Al on the surface. Under these conditions, the F-anions are activated and may have possibility to participate in next step chemical reaction. We considered and calculated three possible reaction paths. The first is that the lattice surface O migrates to leave the alumina lattice surface and dissolved into the molten salt in the form of O2-. The second is that the lattice surface Al migrates to leave the alumina lattice surface and dissolved into the molten salt in the form of OAIF2". The third is that the reaction between lattice O and adsorption F-make O, F and Al dissolved into the molten salt in the form of Al2OF4. The activation F-ions and the F-Al bonding mechanism further induce the reaction of lattice O and adsorption F" the generate the O-Al-F complex. It leads to part of the chemical bond rupture and new chemical bonds reconstruction at alumina lattice surface. Finally, the O-Al-F complex dissolved into the molten melt in the form of OAlF2-.