Henrik Gudbrandsen

PFC evolution characteristics during aluminium and rare earth electrolysis

In addition to aluminium electrolysis, the electrolysis of rare earth (RE) metals from fluoride melts is a significant source of perfluorocarbon (PFC) emissions to the atmosphere. These processes have many similarities, they are both based on molten fluoride salt electrolysis at temperatures around 1000 °C, and are utilizing carbon materials as the anode. Although PFC emissions from aluminium industry and rare earth electrolysis have similar overall reactions, they are often reported to have different characteristics. In order to get a better understanding of these differences and similarities, different laboratory experiments focusing on anode reactions and gas compositions in Al2O3 saturated cryolite and REF3-LiF melts during aluminium and rare earth metal electrolysis were studied. The results obtained, combined with thermodynamic data analysis allowed to better understand onset, evolution and termination behaviour of PFC evolution in molten fluoride systems of different chemistries.

Electrochemical Processing of Rare Earth Alloys

The light rare earth metals Nd, Pr, La, Ce as well as some alloys with Fe, are today produced in China by electrolysis in molten fluorides using oxide raw materials. A major challenge is to obtain a good cell operation without de-composing the electrolyte leading to emissions of perfluorinated carbon (PFC) green-house gases to the atmosphere. This work is focused on understanding the fundamental requirements to run the electrolysis cells for DyFe alloy production in an efficient and environmental friendly way. Electrolysis experiments was carried out in DyF3-LiF melts at 1050 °C. A Fe rod was used as consumable cathode and the (consumable) anode was made of graphite. To establish at which anode potential PFC occurred and thus enabling optimisation of the oxide batch feed rate, analysis of the anode gases was performed with Fourier Transform Infrared Spectrometer (FTIR). The produced DyFe alloy was characterised by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS).

Field Study of the Anodic Overvoltage in Prebaked Anode Cells

The anodic overvoltage in aluminum electrolysis represents 10 – 15% of the total cell voltage, and possible means of reducing the overvoltage should then be studied. In the present work the anodic overvoltage was measured in two prebake cells, 180 kA and 235 kA, the latter having some anodes with slots, and in one 145 kA HSS cell. A special reference electrode was used. At normal current densities the overvoltage varied from 0.72 V at 5 wt% Al2O3 to 0.86 V at 2 wt%. At comparable anodic current densities and alumina concentrations, the overvoltage for the anodes with one transverse 1.5 cm groove were 0.11 V less than for the conventional anodes. For the HSS cell the overvoltage was similar.

Direct Method for Producing Scandium Metal and Scandium-Aluminium Intermetallic Compounds from the Oxides

The electrochemical de-oxidation process, also called FFC-Cambridge process, has been proposed previously to produce reactive metals and their alloys through reduction of their metal oxides. The process works by introducing metal oxides into a molten salt bath where it is electrolysed to form metal powders offering both economic and environmental benefits over the traditional metal production methods. Within the frame of the EU-financed project SCALE (GA 730105), SINTEF is investigating the optimal parameters of the direct electrolytic reduction of Sc2O3 and Sc2O3–Al2O3 precursors (dross from Al-Sc alloy production), giving Sc and Al-Sc metallic powders, respectively, in a molten CaCl2-based electrolyte at a working temperature of ca. 900 °C. The influence of the applied cathodic potential in the reduction mechanism and in the metal product has been studied.

Current Efficiency in Hall-Héroult Cells: The Role of Mass Transfer at the Cathode

The current efficiency (CE) in aluminium cells is governed by transport of dissolved metal (mainly sodium) across the boundary layer at the cathode. The transport takes place by ordinary mass transfer, but since solutions of alkali metals and their salts show electronic conductivity, the CE is also influenced by loss of electrons. The dependence of convection is not necessarily the same for the two loss mechanisms. A laboratory experiment was designed, where the mass transfer coefficient in the so-called Sterten-Solli laboratory cell for measuring CE was varied in a controlled manner by means of a mechanical stirrer. The effect of stirring on the mass transfer coefficient (k) was first surveyed by recording the limiting current density for potassium ferri- and ferrocyanide in an aqueous electrolyte as a function of the stirring rate, followed by measuring the CE in cryolitic melts at different stirring rates. It turned out that plots of the CE versus the mass transfer coefficient produced straight lines that extrapolated back to 99% CE at k = 0. This means that predictions of the CE can be made by using equations for ordinary mass transfer.

Perfluorocarbon Formation During Rare Earth Electrolysis

A challenge during rare earth (RE) electrolysis is to avoid emissions of perfluorocarbon (PFC) green-house gases. The objective of this work was to study how to operate the RE electrolysis process with neither PFC formation nor anode effect. Linear Sweep Voltammetry was carried out at 1050 and 1100 °C, and electrolysis was performed in REF3-LiF melts at ca. 1050 °C during on-line off-gas analysis. To avoid anode effect, the current density values must be strictly less than 0.43 and 0.68 A cm−2 at working temperatures of 1050 and 1100 °C, respectively. The optimal REO batch feed rate for avoiding PFC formation could be established by correlating the onset of PFC with the values and the changes that occurred in the anode potential.

Gas Anodes Made of Porous Graphite for Aluminium Electrowinning

One of the major downsides of the current aluminium production process is the high CO2 emission. One alternative is to replace the consumable carbon anodes with inert anodes so that oxygen evolves instead of CO2. Also PFC emissions will be eliminated by using inert anodes. However, so far a sufficiently inert anode has not been found. Another option is to utilize natural gas through porous anodes in order to change the anode process. This will decrease CO2 emission remarkably and also eliminate PFC emissions and anode effect. The porous anode could be made of carbon or it can be inert. However, the as-mentioned problem still exists regarding porous inert anodes. Therefore, at the moment porous carbon anodes seem to be the best practical option. In this study, porous anodes made of different grades of graphite were used for electrolysis experiments in a laboratory cell. Also, off-gas analysis was performed to get an insight of the ongoing reactions. Our results show that for some types of graphite anodes, methane participates effectively in the anodic reaction.

A Laboratory Study of Electrochemical Removal of Noble Elements from Secondary Aluminium

At present, recycled aluminium is “diluted” by primary metal to keep the impurity concentrations at acceptable levels. Future increase in recycling will require a new process for efficient removal of all impurities, including noble elements, from secondary aluminium. In the current preliminary laboratory study, an electrochemical method based on the well-known three-layer refining cell was explored. Metal with purity comparable to primary metal could be obtained from highly alloyed scrap feeds containing elements more noble than aluminium, such as iron, copper, manganese, and zinc. The energy consumption was as low as 8.5 kWh/kg Al, indicating that the process can be expected to be less expensive than for production of primary metal, but with higher costs than re-melting.

Evaluation of Gas Composition from Laboratory Scale Electrolysis Experiments with Anodes of Different Sulphur Content

With the anticipated decrease in available anode raw materials, future anodes are expected to contain higher levels of impurities. Currently the implications of this are being investigated through evaluation of electrochemical performance of anodes and the environmental aspects through gas evolution studies. In this work, four experiments of aluminum smelting have been conducted with anodes of varying sulphur content. The aim of this work was to evaluate the speciation of sulphur in the off-gas. It was found that COS is the main sulphur species in the inert argon furnace atmosphere. The gas composition was further evaluated as a function of set anode voltages in potentiostatic mode. The significance levels of gas composition were evaluated through repeated experiments. As an FTIR multicomponent analyzer was used, less commonly discussed gaseous constituents such as CH4 and HCl were evaluated with respect to given impurity levels in the anode.

High Temperature Electrolysis for Liquid Iron Production

Introduction The production of iron and steel is by far the largest metal production industry in the world. One major concern is the massive emissions of CO2 which contributes to almost 10 % of the global emissions. A possible alternative to reduce or eliminate the CO2 emissions is to develop an electrolysis process with inert oxygen evolving anodes. Attempts to develop a high temperature electrowinning process to produce liquid iron at temperatures above 1535 C have been reported [1-3]. The main feature of this technique is to dissolve iron oxide in a mixture of other molten oxides and use this as an electrolyte during electrolysis at high temperatures. At such a temperature and in such an environment, the choice of materials is important and very difficult. In particular the selection of an oxygen evolving anode is challenging.