| The current aluminum electrolysis technology still adopts Hall-Heroult process. There are some defects such as high energy consumption and serious environmental pollution using this traditional aluminum electrolysis and producing greenhouse gases (CO2), fluorocarbons (CF4, C2F6) and sulfurous gases (SO2, CS2, H2S). It contradicts to the economic theme of developing low carbon economy in current society. Inert anode has become a hot research topic because it does not participate in electrochemical reaction in the electrolysis process and can resolve these questions mentioned above. NiFe2O4 spinel, which is regarded as a kind of promising material for inert anode, has low solubility in fluoride melt, but its industrial application in aluminum electrolysis has been restricted by the poor resistance to thermal shock and bad electrical conductivity. In this paper, ZrO2 particles and metal powder (Cu-Ni) have been added into NiFe2O4 to improve its thermal shock resistance and electrical conductivity, respectively. Industrial simulation electrolysis corrosion test was conducted in cryolite-alumina melts and the corrosion behavior of samples was investigated elementarily.The effect of the NiO content on microstructure and mechanical properties of the NiFe2O4 composite ceramics and NiFe2O4 cermet inert anodes was investigated. The results show that the NiO content is not beneficial to densification of NiFe2O4 composite ceramics when the addition of the NiO is more than 5%. The minimum flexural strength of 27MPa can be obtained for sample with 40%NiO doping. The value of flexural strength for sample with 40%NiO doping was reduced by 60.87% as against that of the value of sample with 5%NiO doping. The addition of NiO can promote the sintering of 17%(Cu-Ni)/NiFe2O4 cermet inert anodes, the maximum bending strength of 219MPa and cycles number of 17 times can be obtained with 20%NiO doping.NiFe2O4,20%NiO-NiFe2O4 and 17%(Cu-Ni)/(20NiO-NiFe2O4) cermet inert anode were corroded by molten cryolite for 8h, thermal corrosion rates of samples with NiFe2O4, 20%NiO-NiFe2O4 and 17%(Cu-Ni)/(20NiO-NiFe2O4) cermet inert anode are 0.5312mg/(cm2·h)、0.6187mg/(cm2·h) and 0.7875mg/(cm2·h), respectively. The effect of operating parameters on the corrosion rate of 17%(Cu-Ni)/(20NiO-NiFe2O4) cermet inert anode was investigated. The results is that the corrosion rate of 17%(Cu-Ni)/(20NiO-NiFe2O4) cermet inert anode can be greatly reduced by enhancing alumina concentration, controlling the CR between 2.0～2.2 and current density between 0.5～1 A/cm2.The wettability process between carbon anodes or NiFe2O4 spinel based ceramic inert anode and electrolyte with chemical composition 2.2NaF·AlF3+5wt%CaF2+5wt%Al2O3 were investigated. The results showed that the wetting angles between electrolyte and carbon anodes is above 90°, it proved to be nonwetting between them. The initial wetting angles for NiFe2O4,17%Cu-doped,17%Ni-doped and 17%(Cu-Ni)-doped samples are76.98°, 75.30°,94.28° and 82.30°, it proved that NiFe2O4 spinel based ceramic inert anode has good wettability with electrolyte. The molten electrolyte spread quickly on the surface of NiFe2O4 spinel inert anodes, finally it disappeared thoroughly and the wetting angle is 0°.A dense layer about 45 μm was formed at the lateral side of the inert anode according to SEM micrographs of the 20%NiO-NiFe2O4 inert anode after electrolysis for 8h. It can be proved that new phase (Fe0.847Al0.153) (Fe0.153Al1.847)O4 is created in the electrolysis for the 20%NiO-NiFe2O4 inert anode.The anode sample for 17%(Cu-Ni)/(20NiO-NiFe2O4) cermet inert anodes sintering in air are seriously corroded by molten cryolite for 8h. A porous layer is observed on the bottom of the samples sintering in controlled atmosphere. There is a common point for the both of the two samples that the metal phase in the surface of the anode was dissolved firstly.The effect of sintering temperature on phase, microstructure, relative density and mechanical properties of the ZrO2/NiFe2O4 composite ceramics was investigated. The results show that monoclinic peaks decreased in intensity at 1200℃ and vanished completely in intensity at 1300℃, while t-ZrO2 phase is increased as temperature increasing. The maximum value of the flexural strength and the fracture toughness can reach 192MPa and 3.38MPa·m1/2 sintering at 1400℃, respectively. The static corrosion rate of the samples sintering in 1400℃ (0.5000mg·cm2·h-1) was decreased by 76.64% than that of the samples sintering in 1100℃. The inert anodes were electrolyzed by molten cryolite for 8h, but still in regular shapes without tumescence, crack and flaking, which show that the anodes have good anticorrosion properties in molten cryolite. |
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