The Preparation And Electrochemical Performance Of NiFe₂O₄ Inert Anodes

There are some defects for adopting consumable carbon anodes in aluminum electrolysis of traditional Hall-Heroult aluminium-electrolytic cells, such as high energy consumption, high carbon consumption, high cost, high renew frequency of anodes and serious environmental pollution. Inert anode does not participate in electrochemical reaction in the electrolysis process with almost non-consumption, so research groups in aluminium industry have been focusing on the investigation of inert anodes. A lot of research work and trials have been done to seek appropriate inert anode materials, which can be used in production aluminium. The research on structure and performance in solid-phase synthesis process of NiFe2O4 spinel, and the behavior of bubble evolved from NiFe2O4 inert anode during aluminum electrolysis was conducted in detail.The structure and performance of prepared NiFe2O4 inert anodes were studied. It is found that porosity decreased with increasing sintering temperature, bending strength increased by decreasing porosity. The densification of matrix can not be promoted by prolonging holding time at lower temperature. In the process of synthesizing NiFe2O4 spinel (excess 15%(mass)NiO), additive of MnO2 was added to Fe2O3 and NiO powder as raw materials. The effects of the additive on sintering and properties of NiFe2O4 spinel were investigated. The results show that additives can promote sintering and improve the properties of the materials.The average porosity of 1.0wt%MnO2-doped samples sintered at 1100? and 1400? with different heat preservation time was 33.85% and 1.82%, respectively. The bending strength for the both kinds of samples is 12.45MPa and 46.47MPa, respectively.The wetting angles between electrolyte with chemical composition of Na3AlF6+ 3wt%CaF2+5wt%Al2O3 and carbon anodes are 103.1°. The initial wetting angle between electrolyte and NiFe2O4 anode is 76.9°. It can be seen that the molten electrolyte spread quickly on the anode surface, and 140s later, the wetting angles changed into 23.4°. The initial wetting angles for 1.0wt% MnO2-doped sample is 56.86°. And about 50s later, the wetting angle came down below 15°.Bubbles releasing process was researched with low current density. The results showed that bubbles releasing was a dynamic process. Several bubbles formed on the bottom of the anode surface, then grew and coalesced to form a single large bubble, and it finally released from anode surface. Bubble diameter when release is smaller with increasing current density, and the bubble size produced by NiFe2O4 anodes is smaller than that produced by carbon anodes. The detention time of bubbles on bottom surface of inert anodes is shorter than that on carbon anodes. The speed of producing bubbles was accelerated with high current density. Bubble size is smaller for accelerated electrolysis, so it is difficult to measure bubble diameter.