| There are many problems in aluminum electrolysis in traditional Hall-Heroult aluminium-electrolytic cells such as high carbon consumption and serious environmental pollution as a result of adopting consumable carbon anodes. Research groups in aluminum industry have been focusing on the investigation of inert anodes because they can overcome the problems mentioned above. NiFe2O4-based inert anodes have become the key study because of its high strength, good stability at elevated temperature, good corrosion resistance to molten salt and so on. Considering the high brittleness and poor thermal shock resistance of NiFe2O4-based cermet inert anode, it has not been used in industrial aluminum electrolysis for it doesn’t meet the requirements of aluminum electrolysis process. The improvement in toughness and thermal shock resistance of NiFe2O4-based inert anode has become the focus of industrializing inert anode. In this paper, NiFe2O4-based inert anode toughened by adding NiFe2O4nanopowder and the preparation technologies of large size NiFe2O4-based inert anode have been researched for the first time.In this paper, NiFe2O4nanopowder was prepared by low-temperature solid-state reaction. The precursor was first prepared by rubbing the reactants FeSO4·7H2O, NiSO4·6H2O, NaOH and dispersant (NaCl) sufficiently at room temperature, and then calcined to obtain nanopowder. The effects, such as preparation technique, content of dispersant, calcining temperature and heat preservation time, on the phase, particle size and morphology were researched emphatically through single-factor experiment. The NiFe2O4nanopowder of high crystallinity and single phase, prepared by first calcining the precursor using20%NaCl as dispersant at800℃for1hour subsequently vacuumizing and filtrating, are mostly polyhedral in shape with the particle size range of30-65nm. Ball-milling assisted low-temperature solid-state reaction has been employed to prepare NiFe2O4nanopowder for industrialization, which can obtain NiFe2O4nanopowder at a relatively low calcining temperature (750℃) for the higher reactivity of the precursor. There are four steps in a typical procedure of solid-state reaction:diffusion, reaction, nucleation and growth. Many factors are favorable to accelerate the reaction rate, such as the crystal water from reactants, grinding and the finer reactants.Two-step sintering process was adopted to prepare NiFe2O4-based inert anode in this research. In the process of synthesizing NiFe2O4spinel base material, Fe2O3and NiO powders as raw materials and MnO2and V2O5as additives were mixed uniformly and synthesized at1000℃. Through crushing and screening, adding NiFe2O4nanopowder, particle gradation and compression molding, the NiFe2O4-based inert anode was gained by second-sintering. The effect of addition level of NiFe2O4nanopowder on the combination properties of NiFe2O4-based inert anode was investigated emphatically. The results showed that the addition of NiFe2O4nanopowder had significantly enhanced the sintering activity of green bodies. The more the NiFe2O4nanopowder added, the higher the sintering activity was, as a result, a more dense sintered bodies were gained. NiFe2O4-based inert anodes had the best comprehensive properties while adding30wt%nanopowder in the particle gradation. The values of porosity and bending strength were3.51%and42.47MPa respectively, the values of fracture toughness and impact toughness were3.12MPa·m1/2and3.31J/cm2respectively. The strength residual rate reached85.36%while static corrosion rate reduced to0.00085g·cm-2·h-1. When the nanopowder addition reached up to40%, the densities of inert anodes were declined for large cracks originating from great shrinkage were initiated easily. The microscopic structure, such as boundary bonding strength and pore, was affected by adding nanopowder, which enhanced the fracture surface energy. As a result, the inert anodes were toughened.The appropriate technological conditions for preparing small-size strip NiFe2O4-based inert anodes were as follows:molding pressure was200MPa, sintering temperature was1400℃and sintering time was6hours. The cylindrical NiFe2O4-based inert anodes with the size of Φ100mm prepared by bidirectional pressing under120MPa and then sintering at1150℃for6hours revealed the superior overall performance. The electrolysis test showed that the connection of metal rod and inert anode through solid-state sintering diffusion bonding possessed proper high-temperature strength and electrical conductivity, which could satisfy the required high-temperature strength, thermal shock resistance, resistance to high temperature oxidation and conductivity property.The cylindrical NiFe2IO4-based inert anodes with the size of0100mm toughened by adding nanopowder under appropriate technological conditions was electrolyzed. The results showed that the anode was slightly corroded after electrolysis for10hours. The anode was still in good condition without tumescence and crack and flaking, which showed that the anode had good corrosion properties in molten cryolite and good thermal shock resistance. Electrolytic corrosion occurred in multi-defects grain boundary and then extended to grains. The existence of pores enlarged the corrosion area, which promoted the corrosion of anodes. It is significant to improve the corrosion resistance of grain boundary and decrease the porosity of anodes.In the preparing process of large size square NiFe2O4-based inert anodes toughened by adding nanopowder, the probability of crazing during sintering large size green bodies can be lessened by presintering under relatively lower temperature, in that the internal stress originated from pressing process can be released effectively during presintering. As well, strict controlling the rates of heating and cooling was beneficial to bring down the probability of crazing. |
PDF Full Text Request