Studies were performed investigating the anodic testing of calcium ruthenate for electrowinning in calcium chloride–calcium oxide melts. The results showed that calcium ruthenate may be suitable as an inert anode in calcium chloride containing melts as it exhibited a low rate of corrosion in melts containing a small amount of calcium oxide, capable of producing oxygen on its surface, and did not contaminate the melt. To reduce the amount of ruthenium in the anode, solid solutions of calcium ruthenate in calcium titanate were investigated. At low concentrations, the solid solution is a semiconductor with a relatively low conductivity at room temperature, but at the temperature of operation, 1173 K, the material is an excellent electronic conductor. The other way of reducing the amount of ruthenium is to coat the solid solution onto a substrate. In this way, the substrate would give the mechanical strength while the coating would give the electrical conductivity and corrosion protection. Calcium ruthenate-based anodes can endure long-term use in the laboratory under an applied electrical field with oxygen being liberated on the anode indicating that these materials are candidates for the electrowining in calcium chloride–calcium oxide melts.

Development of an Inert Anode for Electrowinning in Calcium Chloride–Calcium Oxide Melts

Evaluation of Mg2+-substituted NiFe2O4 as a green anode material

During the last few years the development of inert anodes for the primary aluminium industry has advanced considerably. This update reviews results of tests on (1) anodes made of ferrites (Ni, Co and Zn) alone or in combination with nickel oxide and copper leading to the final composition Ni1-x-yFe2-xMyO4, where M is zinc and/or cobalt, x is from 0 to 0.5 and y is from 0 to 0.6; (2) anodes made of oxides (Ni, Li, Sn, Sb, Cu) alone or in combinations; and finally (3) anodes made of combinations of metal powders including Ni, Al, Fe, Cu and Zn. The studies examined the solubility in cryolite alumina melts, but also performance in electrolysis tests from which the results are discussed. Laboratory solubility tests revealed that compositions with limited corrosion (1–2 cm/y) exist and could be successful not only in laboratory but also in industrial cells.

Inert Anodes: An Update

Electrical conductivity and molten salt corrosion behavior of spinel nickel ferrite

Inert anodes demand a fairly high alumina content (>50% saturation) in the electrolyte, requiring a new way to control the alumina concentration. Probably the greatest challenge is how to tackle anode corrosion, which gives rise to metal contamination. All candidate oxide materials that can be used as components of inert anodes have a finite solubility in the electrolyte. Consequently, the anode materials will corrode slowly, and the corrosion products will be reduced into the aluminum produced. Even if the anode corrosion rate is slow (10–20 mm/year), an unacceptable contamination of the aluminum may result. Possible ways to reduce the contamination are discussed.

Cell operation and metal purity challenges for the use of inert anodes

The corrosion rates of inert anodes based on tin oxide and nickel ferrite cermet materials were studied as a function of some operating parameters. To reach a better understanding of the corrosion mechanism, the behavior of the anodes was observed under some specific conditions, such as in pure cryolite, at high current densities, at different potentials, and at varying cathode surface areas. It was confirmed that low alumina concentrations led to catastrophic corrosion of the anodes and that high current densities and high as well as low NaF/AlF3 molar ratios were also detrimental. The corrosion rate of tin oxide based anodes showed a minimum (so-called “normal corrosion”) at anodic potentials of 2.2 to 2.4 V with respect to aluminum. The normal corrosion is due to chemical dissolution of the anode material and reduction of the corrosion products into the cathode metal. The corrosion rate increased with increasing cathode surface area. At potentials higher than ∼2.5 V, the anodes showed catastrophic corrosion. Catastrophic corrosion can be ascribed to decomposition of the anode material by depletion of alumina at the anode surface provoked by low bulk concentration of alumina and/or high current density.

R. Hovland

Papers

Studies on the corrosion and the behavior of inert anodes in aluminum electrolysis