Development Of NiFe₂O₄ Based Cermet Inert Anode And Engineering-oriented Technique For Aluminum Smelting

The new technology for aluminum smelting based on inert anode and wettable cathode, which can greatly reduce production cost, cut down energy consumption and is environmentally friendly, means a technological revolution of the traditional aluminum electrolysis industry. Funded by the State Key Project of Fundamental Research (973 project) and National High Technology Research and Development Project (863 project), and with the aim of developing satisfying inert anode material, the current work investigated in details NiFe₂O₄ based cermet. NiFe₂O₄ based cermet inert anode samples with high density were prepared by cold pressing-sintering process, the performances of high temperature anti-oxygen, thermal shock resistance, electrical conductivity and corrosion resistance were studied systematically. At the same time, the engineering-oriented techniques for cermet inert anode such as the bonding between NiFe₂O₄ based cermet and metal rod and its preheating-baking were also studied. The main conclusions and results are summarized as follows:1) The contravention existing in the densification of material and overflow of metal phase or uneven distribution was resolved, and the NiFe₂O₄ based cermets with the eligible phase compositions and high density (more than 95%) were obtained by general investigation on the preparation technics such as the adding style of metal, sintering atmosphere, sintering temperature and holding time. The influence of composition of cermet on its performance, which includes high temperature anti-oxygen, thermal shock resistance, electrical conductivity and corrosion resistance to cryolite melts during electrolysis, was also studied. The composition of cermet inert anode with ideal phase was determined and the content of NiO in ceramic phase is 10%. The NiFe₂O₄ based cermet inert anode with good general performance was also prepared: the metal mass percent oxided is less than 1.540% after 720min at 1000℃in air, the bending strength resistance at room temperature reaches 157MPa, the electrical conductivity at 960℃reaches 65.22 S.cm^-1, the corrosion rate of the figuration dimensionφ100mm×l20mm is 8.51mm.y^-1 based on the results from electrolysis experiment lasting 100 hours with electrolyte CR2.3, Al₂O₃ concentration saturation, overheating temperature 10℃and current density 0.75～1.1A/cm².2) The phenomenon that the components of NiFe₂O₄ based cermet inert anode corroded into electrolyte distribute unevenly was observed and proofed during electrolysis, and was also explained by the electromigration characteristic of impurity ions. The discovery and the reasonable explanation are beneficial to the investigation further on the behavior of inert anode corrosion and its components transferring in electrolyte during electrolysis. It also has instructional function on optimizing the electrolyte system and the cell configuration to decrease the corrosion rate of inert anode.3) The molten metal infiltration technique was introduced to the field on studying inert anode for aluminum electrolysis. The electric bonding between the NiFe₂O₄ based cermet and metal rod at high temperature was also accomplished. The metallurgy bonding between them can be realized by optimizing the condition of techincs and the bonding strength at room temperature reaches 17.45Mpa. Compared with the mechanical connection and the diffusion jointing, it can settle successfully the problem of cracks because of the difference of heat dilatability between the materials during using.4) A new technique named "preheating-baking in-phase" for inert anode and electrolysis cell was put forward, which adopts pre-covering the cermet inert anode in carbon anode and conventional coke baking method. The results from simulate experiments indicate that the most temperature difference between the top and the bottom of inert anode is less than 35℃by adjusting the baking process when the new technique of "preheating-baking in-phase" is applied. The phenomenon of crack and rupture for inert anode, which is caused by the formidable thermal shock impact during the "preheating-replacing" process adopted usually, can be avoided availably. It provides resultful technical support for the practical application of cermet inert anode in engineering.