A 100 mm diameter cup-shaped inert anode for aluminum electrolysis consisting of cermet 17Ni/83(10NiO-90NiFe2O4) was prepared and the operating performance was evaluated in a laboratory cell with the electrolyte CR2.3 and Al2O3 concentration 7.43% (mass fraction). The results indicate that no major operational difficulties are encountered during the testing which lasts for 101.5 h and the inert anode exhibits good general performances. The steady-state average concentration of impurity Ni in the bath is close to the solubility, however, the Fe concentration is lower than its solubility. The contents of the main contaminants for aluminum produced are Ni 0.128 8%, Fe 1.007 4%. The corrosion rate of inert anode under electrolysis conditions based on the content of impurity Ni in metal aluminum is approximately 8.51 mm/a.
NiFe2O4-based cermet inert anodes with metallic phase compositions of Cu, Ni and 85Cu15Ni were prepared by cold pressing-sintering. Their corrosion resistance was also investigated in Na3 AIF6-Al2O3 melts. The resuits show that the metallic phase species in cermets have no effect on the concentration of impurities in bath during electrolysis, the total steady-state concentration of impurities is almost the same, i.e. between 4.12 × 10^-4- 4.80 × 10^-4. There exists metal preferential corrosion for the cermet inert anode with metal Ni as metallic phase. For NiFe2 O4-based cermets, the cermet with metal Cu as metallic phase exhibits better corrosion resistance than the others.
(85Cu-15Ni)/(10NiO-NiFe2O4) cermets were prepared with Cu-Ni mixed powders as toughening metallic phase and 10NiO-NiFe2O4 as ceramic matrix. The phase composition, microstructure of composite and the effect of metallic phase content on bending strength, hardness, fracture toughness and thermal shock resistance were studied. X-ray diffraction analysis indicates the coexistence of (Cu-Ni), NiO and NiFe2O4 phases in the cermets. Within the content range of metallic phase from 0% to 20% (mass fraction), the maximal bending strength (176.4 MPa) and the minimal porosity (3.9%) of composite appear at the metallic phase content of 5%. The fracture toughness increases and Vickers’ hardness decreases with increasing metal content. When the thermal shock temperature difference (△t) is below 200 ℃, the loss rate of residual strength for 10NiO-NiFe2O4 ceramic is only 8%, but about 40% for (85Cu-15Ni)/(10NiO-NiFe2O4) cermets. As △t is above 200 ℃, the residual strength sharply decreases for sample CN0 and falls slowly for samples CN5-CN20.
Ball mixing and electroless plating were respectively used as the adding methods of metallic phase to prepare Ni/(90NiFe2O4-10NiO) cermets for the inert anode in aluminum electrolysis. The microstructure and thermal shock resistance of cermet samples were studied. The results show that, for the samples prepared by ball mixing method, aggregation of metallic phase is found in either the green blocks or sintered samples and the extent of aggregation increases with the increase of metal content. For 6.5Ni/(90NiFe2O4-10NiO) cermets prepared with electroless plating method, the homogeneous and fine metallic particles are found in either the green compacts or sintered samples, but the relative density and thermal shock residual strength decrease by 3% and 28%-58% respectively, compared with samples prepared with ball mixing method.
Ni/ （10NiO-NiFe2O4） cermets were fabricated by using cold pressing-sintering method. The phase composition and effect of metallic content on the mechanical properties such as bending strength, Vickers’ hardness, fracture toughness and thermal shock resistance were studied. The results show that the cermets consist of Ni, NiO and NiFe2O4. Within the range of metallic content from 0 to 17%（mass fraction）, the relative density decreases with the increase of metallic content and the decrease of sintering temperature, Vickers’ hardness decreases from 7097MPa to 4814MPa and the bending strength increases from 110MPa to 157MPa, and the fracture toughness reaches the optimal value of 5.11MPa·m 1/2 at the metallic content of about 10%. The residual strength after thermal shock testing falls sharply as the thermal shock temperature difference is above 200℃.The cermets samples, whose metallic content is 10% and 15%, respectively, exhibit promising property of thermal shock resistance at 960℃ with six cycles of heating and quenching testing.
The effects of CaO content in the range from 0 to 4.0%, and sintering temperature on the phase composition, relative density and electrical conductivity of 10NiO-NiFe2O4 composites doped with CaO were studied. The results show that there is no change of structure for NiO or NiFe2O4; there is apparent oxygen absorbing and releasing behavior during the heating process in air for 10NiO-NiFe2O4 composites. Introduction of CaO can accelerate the densification of 10NiO-NiFe2O4 composites. The maximum value of relative density is 98.75% for composite doped with 2.0% CaO and sintered at 1 200 ℃, which is beyond about 20% for the undoped composites. The sintering activated energy of sample containing 2% CaO decreases by 15.87 kJ/mol, compared with that of the undoped sample.
The electrical conductivity of cermet 83(90NiFe2O4-10NiO)-17Ni at different temperatures was measured in air, the operating performance of inert anode was evaluated in a laboratory electrolysis cell with various electrolyte compositions. The results indicate that the electrical resistivity of cermet studied has negative temperature coefficient, which is the characteristic of semi-conducting material. The proper addition of AlF3 in the bath can improve the corrosion resistance of cermet inert anode, but excess adding amount will cause the catastrophic corrosion. Post-examination of anodes shows that metal Ni leaches preferentially on the anode surface. Chemical dissolution, electrolyte penetration as well as electrochemical dissolution serve as major corrosion mechanisms.
The thermal stresses relaxation of Ni/NiFe2O4 system functionally graded cermet inert anode for aluminum electrolysis was optimally designed. The transient thermal stresses of the inert anode under complex boundary condition during high-temp （955℃） electrolysis were calculated using the finite-element software ANSYS, the influence of different parameters on the distribution of the thermal stresses were analyzed. The results showed that, during the process of thermal shock, the thermal hoop tensile stress on the surface of the anode is very large, which is possibly the major cause of anode crack; when the radius of the anode is between 0.05-0.15m, a range that can be realized by recent manufacturing technology, the optimum composition distribution exponent p is 0.25; The hoop tensile stresses reduce with the decrease of anode scale and also decrease with the decrease of the convection coefficient between the electrolyte and the anode.
