Jomar Thonstad

Nickel ferrite as inert anodes in aluminium electrolysis: Part I Material fabrication and preliminary testing

Dense, sintered samples of nickel ferrite/nickel oxide–copper cermets were produced and characterized. Three compositions were chosen, each with different NiO content. A new method of powder preparation involving no use of water as dispersant resulted in a well-dispersed ceramic phase with smaller metal grains than previously reported. The individual phases in the cermets were analysed quantitatively using energy-dispersive X-ray microanalysis, and significant differences between the materials were found. The electrical conductivity of the materials was measured, showing interesting properties not previously described. Reaction sintering was tried and found to lead to a microstructure different from that obtained with pre-calcined powder. The nickel contamination level in the electrolyte did not reach steady state after 4h of electrolysis. Iron and copper did seem to reach steady state after some irregular behaviour early in the tests. The total contamination level of anode constituents in the deposited metal was as low as 0.116wt%. These encouraging results seemed to be partly related to the cell configuration giving very slow mass transfer from the electrolyte into the metal.

Oxidation of manganese(II) and reduction of manganese dioxide in sulphuric acid

Oxidation of Mn2+ to MnO2 and reduction of MnO2 on Pt electrode in sulphuric acid was investigated by cyclic voltammetry as a function of the Mn2+ concentration, acid concentration, scan rate, temperature and convection. It was observed that oxidation of Mn(II) to Mn(IV) was mass transfer controlled. High temperature and stirring enhanced the oxidation rate of Mn(II) to Mn(IV). Decreasing acid concentration favours MnO2 deposition while high acid concentrations inhibit the formation of MnO2. The mechanism of MnO2 deposition from Mn(II) is suggested to be an ECE sequence with the formation of an insulating intermediate (MnOOH). Small amounts of deposited MnO2 may be reduced electrochemically at the electrode surface.

Nickel ferrite as inert anodes in aluminium electrolysis: Part II Material performance and long-term testing

The behaviour of three different compositions based on nickel ferrite–nickel oxide–copper cermets was investigated as anode materials in laboratory electrolysis tests for 50h in a conventional cryolite-based electrolyte. The corrosion of the anodes was assumed to be mass transfer controlled and the transfer of impurities into the electrolyte and subsequently into the cathodically deposited metal was studied. The results indicate that the materials corroded in a controlled manner. Mass transfer coefficients of species from the anode to the electrolyte were of the order of 10−4ms−1 while the mass transfer coefficients for transfer of the species from the electrolyte into the deposited metal were of the order of 10−6ms−1. Nickel exhibited significantly lower mass transfer coefficients than those of iron and copper. The extrapolated corrosion rates of the anode ranged 1.2–2.0 cm year−1, which is acceptable from an industrial perspective. The contamination of the deposited aluminium with respect to Ni and Cu was, however, too high to meet current specifications for commercial grade metal. Post-electrolysis examination of the anodes showed that a reaction layer of approximately 50μm thickness was formed on the anodes. This layer did not contain any metal grains and seemed to prevent preferential corrosion of the metal phase in the underlying cermet.

The voltage of alumina reduction cells prior to the anode effect

Alteration in cell voltage of industrial cells after each feeding of fresh alumina is initially small and accelerates to become extremely fast immediately prior to the incipience of the anode effect. Estimates of the components of the cell voltage on the basis of a mathematical model particularly taking account of the action of the gaseous phase underneath the anode are compared with experimental data from industrial cells. The fundamental agreement (in spite of inevitable model insufficiencies) supports the view that the anode effect is induced as the actual anodic current density approaches the limiting one.

Cyclic and linear voltammetry on Ti/IrO2–Ta2O5–MnOx electrodes in sulfuric acid containing Mn2+ ions

Abstract The anodic current density for oxygen evolution was studied on Ti/IrO 2 –Ta 2 O 5 electrodes where some of the Ta 2 O 5 was replaced by MnO x ( x ≤2) in sulfuric acid and in sulfuric acid containing Mn 2+ ions. Deposition of MnO 2 on the anode surface reduced the anodic current density for oxygen evolution for all coatings. Scanning electron microscopy images of the Ti/IrO 2 –Ta 2 O 5 electrode during voltammetric experiments showed that the MnO 2 deposit was reduced completely during the cathodic scan.

Content of sodium and lithium in aluminium during electrolysis of cryolite-based melts

The content of sodium and/or lithium in polarized and nonpolarized aluminium in contact with cryolite melts was determined in a laboratory cell. The cryolite-based melts contained 0 to 20mass% excess AlF3 and 0 to 2.5mass% LiF. The cathodic current density ranged from 0 to 0.5Acm−2. The lithium content in aluminium increases linearly with increasing concentration lithium fluoride in the␣melt. It also increases with increasing cathodic current density and decreasing cryolite ratio. On the other hand the sodium content decreases with increasing concentration of LiF in the melt. This effect is more notable at higher current densities.

An Impedance Study of the Kinetics and Mechanism of the Anodic Reaction on Graphite Anodes in Saturated Cryolite‐Alumina Melts

The anodic reaction on spectrally pure graphite (SPG) and pyrolytic graphite was studied in the potential range from 1.2 to 1.8 V vs. an aluminium reference electrode in alumina-saturated cryolite melts at 1000°C by means of electrochemical impedance spectroscopy. It has been found that the total electrode reaction can be interpreted by a two-step two-electron charge-transfer process with an intermediate adsorption. The experimentally recorded impedance spectra were interpreted in terms of an equivalent circuit approach and the kinetic theory of Bai and Conway, giving the double-layer capacitance, electrode coverage, effective rate constants, and the charge needed for the coverage of the electrode by a monolayer. The process is faster on SPG anode, and on both anodes the second step is faster than the first one.

Influence of the Sulphur Content in the Carbon Anodes in Aluminium Electrolysis — a Laboratory Study

The chemistry of sulphur in carbon anodes is not fully understood, especially its influence on the electrolysis parameters. The results of this study are indicative of an important link between the sulphur content in the anode material and the carbon consumption as well as the current efficiency during aluminium electrolysis. By performing a laboratory scale investigation of different carbon anodes with sulphur contents ranging from 1.97 to 3.82 wt% S in addition to graphite anodes with sulphur content close to zero, it was found that increasing sulphur content contributes significantly to a decrease in the current efficiency and a rise in the carbon consumption. When going from 0 to 3.82 wt% S, the current efficiency decreased from 92 to 85% (1.8 % per 1 wt% S), and the carbon consumption rose from 108 to 128% (5.2 % per 1 wt% S).

Cathodic overvoltage and the contents of sodium and lithium in molten aluminium during electrolysis of cryolite-based melts

Abstract The cathodic concentration overvoltage in Na 3 AlF 6 –AlF 3 –Al 2 O 3 (sat.) melts was investigated at the temperature of 1000°C. The melts contained 10 and 20 wt.% of AlF 3 . Experimental data on the cathodic overvoltage agreed well with values calculated from the content of sodium in aluminium. The dependence of the cathodic overvoltage on temperature was studied as well. It was shown that the addition of 5 wt.% CaF 2 or 5 wt.% MgF 2 does not influence the overvoltage significantly, while the addition of 2 wt.% LiF decreases the overvoltage by 20 mV at a cathodic current density of 0.25 A cm −2 and by 50 mV at 0.75 A cm −2 . These results are consistent with the influence of these additions on the content of sodium in aluminium.

Mass Transfer of Protons during Electrodeposition of Cobalt in Chloride Electrolytes

In acid cobalt chloride electrolytes, the electrodeposition of cobalt and the evolution of hydrogen take place simultaneously. On a rotating platinum disk electrode, the current efficiency for cobalt deposition was calculated based on the anodic charge needed for stripping of the cobalt deposits divided by the total cathodic charge. The results showed a decrease in current efficiency with increasing rotation rate. From the partial current density for hydrogen evolution, the diffusion coefficient for protons was calculated. The derived diffusion coefficients varied with overpotential and pH but were always 1 order of magnitude lower compared to what was expected from literature values. This was attributed to water drag when transporting cobalt ions toward the electrode. The variations in the calculated diffusion coefficients were explained by the change in the transfer number when the proton concentration was changed and in the potential gradient when the potential was varied.