Kai Grjotheim

Formation of metal fog during molten salt electrolysis observed in a see-through cell

Visual observations of several molten salt electrolysis processes were made in a two-compartment, see-through quartz cell. The electrolyses of aluminium, magnesium, lead, zinc, sodium and potassium were studied. The colour of the melt in the anode compartment was pale yellow for fluoride-chloride melts and red for chloride melts, caused by the presence of dispersed anode gases during electrolysis. In the cathode compartment, streamers of metal fog were formed. The colours of the metal fog were purple for aluminium, grey for magnesium, lead and zinc, blue for sodium and green for potassium.The metal fog tended to sink to the bottom of the cell, which indicated that it had a higher density than that of the melt. The metal fog also penetrated into the anode compartment, probable due to convection and diffusion in the melt. The most probable explanation of the nature of the metal fog is that it consisted ofdispersed metal particles. This chemically unstable phase dissolved easily in the melt and was oxidized quickly by the anode gases.

Recent studies of convection in commercial aluminum reduction cells

Convection rates and patterns in the bath electrolyte and the cathode metal of 150 kA prebaked anode aluminum reduction cells have been studied by means of radioactive tracers. Gamma radiation probes have been used for a continuousin situ determination of the radiotracer activity in the bath and liquid metal. The measurements are compared with predicted convection patterns from model magnetodynamic calculations for both magnetically compensated and uncompensated reduction cells. The measurements give evidence of a strong coupling between the convection currents in the metal and bath phases. The recorded activity vs time data further allow an estimation of the degree of turbulence in the reduction cells. From these investigations attempts are made to correlate data from the convection studies with current efficiency, as determined by the method of isotope dilution. The same method has also been used to determine liquid bath volume and the melting-out of solid bath electrolyte during periods with anode effect.

Impact of Alternative Processes for Aluminum Production on Energy Requirements

Increasing prices and the shortage of large blocks of electrical energy have given greater impetus to the search for viable alternative processes for aluminum production. These include electrolysis of aluminum chloride, sulfide, and nitride; carbothermal reduction of either the ore or alumina; and disproportioning reactions of either aluminum sulfide or the monochloride route. Common to all these processes are the starting material—an ore containing aluminum oxide—and the final product—the metal. Thus, the thermodynamic cycle will invariably dictate similar theoretical energy requirements for the three processes. In practice, however, the achievable efficiencies and, more noticeably, the proportion of electrical to carbothermal energy required for the various stages of operation can vary.

Transport numbers in molten fluorides—I. Sodium fluoride☆

Abstract The external transport number of Na + in molten NaF was determined to be 0·64 ± 0·05 by a tracer technique. A boron-nitride crucible was used as reference; it also served as a diaphragm separating the anolyte and the catholyte. Viscous flow could not be avoided completely, but was corrected for by performing several experiments and extrapolating to zero charge.

Application of the activity concept in the physical chemistry of slags

In metallurgical literature, single ion activity coefficients are often used erroneously. The present analytical study deals with the introduction of the activity concept in the treatment of slag equilibria. It leads to the conclusion that the use of single ion activity coefficients is confusing. They may be used only in an analogous manner as in dilute aqueous solutions of electrolytes,i.e. when there is a large excess of one component which can be considered as the solvent. In this case, by convention, the activity coefficients are expressed in terms of the ideal dilute solution as standard state (Henrian activities). By studying systems (concentrated solutions) with all components having as standard states their pure liquid compounds (Raoultian activities), the use of single ion activities and single ion activity coefficients lead to obvious contradictions and should be avoided.

Influence of lithium carbonate addition to carbon anodes in a laboratory aluminium electrolysis cell

Abstract The possible influence of the addition of lithium carbonate to the carbon anode in a laboratory aluminium electrolysis cell was studied. The counter-electromotive force was determined by a steadystate current interruption technique. No measurable effect of the lithium carbonate addition was found on the anodic overvoltage or on the rate of the anodic reaction.

Determination of metal inventory and current efficiency in commercial aluminum reduction cells

A method for the determination of the metal inventory in commercial aluminum reduction cells, based on radioisotope dilution, has been developed, employing a198Au tracer. By taking a large number of samples from the cells, the current efficiency is evaluated from the slope of the cumulative metal inventory vs time data. The method allows a determination of the current efficiency over a period of 2 to 3 days with a relative precision of approximately 1 pct. The paper gives a thorough discussion of the sources of error inherent in the procedure suggested.

Key improvements to Hall-Héroult since the end of World War II

During the last 50 years, some of the most important developments in the Hall-Héroult process include improvements to bath chemistry, alumina feeding, anode and cathode quality, and process control. In addition, progress has been made in cell size and design, production capacity, energy consumption, and magnetic field modeling.

Technological developments for aluminum smeltinq as the industry enters the 21st century

The capital-intensive nature of aluminum smelting, with its low productivity per unit reactor and high electrical consumption rates, has motivated the search for alternative smelting processes to replace the aging Hall-Héroult technology. Optional routes include carbothermic reduction of alumina, chlorination followed by electrolysis of aluminum chloride, and electrolytic decomposition of alumina using inert electrodes. Still in need of some fundamental innovation, the alternative techniques are limited by unsatisfactory materials performance and reactor design constraints. There have been, however, significant advances in the process efficiencies and scale of both the Bayer process and the Hall-Héroult cells. As a result, the basic Hall-Héroult technology will continue as the dominant aluminum smelting process for at least the next 50 years.

Continuous bath temperature measurements in Al electrolysis cells

This article describes the importance of temperature monitoring in aluminum electrolysis cells. The challenge of getting long-term performance from thermocouples is discussed, and appropriate examples are given of information of value to cell operations derived from bath temperature measurements.