The Preparation And Properties Of NiFe₂O₄/M Based Cermet Inert Anode For Primary Aluminum Smelting

As an innovative technology for aluminum electrolysis, inert anode can greatly reduce production costs, cut down energy consumption and is environmental friendly. NiFe₂O₄/M cermet, which is regarded as a kind of promising material for inert anode, has the difficulty of achieving stable electrical connection, the poor conductivity and resistance to thermal shock hindering its industrial application, although the advantages of low solubility in the fluoride melt. Funded by the National High Technology Research and Development Project (863 project) and the State Key Project of Fundamental Research (973 project), the aim of current work is to develop large scale prototype inert anode samples through investigating the thermal debindering and sintering processing, the mechanical and electricity properties, the thermal shock resistance of NiFe₂O₄/M cermet.The main works are as follows:(1) A simple inert anode model with graded structure was designed by the functional graded materials design methodology and finite element analysis. This model had a reasonable thermal residual stress distribution and good processing feasibility.(2) The non-defect thermal debindering process for large scale NiFe₂O₄/M green body was achieved. The method based on the calculation of the kinetic of polymer thermal degradation and gas pressure builting up inside the green body, was gained to predict the critical heating rate for the non-defects binder removal cycle.(3) The driving force of pore shrinkage and the kinetic criterion for the stability of pore shrinkage were researched to show the solid state sintering mechanism of NiFe₂O₄. The best sintering condition was gained by investigating the influence of the addition of NiO,Ni,Cu on the sintering behaviors of NiFe₂O₄-based cermet. The cup shape inert anode samples with 100 mm in-radius and 120 mm in-height were obtained.(4)The NiFe₂O₄/M cermet with well electric conductivity (27.84 S·cm^-1 at 960℃) was gained by well controlled the oxygen partial pressure in sintering atmosphere and the content of Ni and NiO. The electric conduction mechanics of NiFe₂O₄/M cermet was shown by investigating the activation energy of electrical conductivity of NiFe₂O₄-based cermet with different content of Ni and NiO.(5) The bending strength, fracture toughness, and the thermal shock resistance behavior of NiFe₂O₄/M were tested. The addition of metal increased both the bending strength and fracture toughness property, with the highest bending strength 157Mpa for NiFe₂O₄-10NiO/17Ni, andthe highest fracture toughness value 5.11 MPa·m^1/2 forNiFe₂O₄-10NiO/10Ni. The results of the residual strength measurement and thermal shock recycle test indicated that NiFe₂O₄-10NiO/xNi (x=10～17) has both high quenching residual strength and good thermal shock resistance.(6) The stable graded electric connection between the inert anode and the anode steel rod was achieved by the diffusion welding process. A key technique problem in development of inert anode was solved.The prototype inert anode samples, protected by the consumable can, were conducted to preheating-replacement-electrolysis test at a 150 kA industrial cell. The results of these tests indicated that the prototype inert anode samples have both good thermal shock resistance and firmed connection structure.The researches in this paper provided some important technical supports for the first industrial test of inert anode in China.