Karen Sende Osen

Anode Processes on Carbon in Chloride-Oxide Melts

Alkali and alkaline earth metals such as lithium, sodium, magnesium and calcium are produced by electrowinning in chloride melts. Direct cathodic reduction of metal oxides in chloride melts has been proposed for the FFC process. Fundamental data for the anodic process on carbon in mixed chloride/oxide melts are lacking. Anodes of graphite and vitreous carbon (VC) were studied in NaCl-NaO, and in binary melts of the type NaCl-MCl2-MO with 0-7 mol% oxide, (M = Ca, Sr and Ba), using linear polarization, CV and EIS. The gaseous products were Cl2, CO and CO2 and the exit concentration of carbon oxides were analysed by gas chromatography. In this paper the emphasis is on the NaCl-CaCl2-CaO system.

Correlation between Moisture and HF Formation in the Aluminium Process

Hydrogen fluoride (HF) emission to the working atmosphere is still a problem in the aluminium industry. Moisture in secondary alumina fed to the cell and humidity in the ambient air reacts with fluorides in the bath and fluoride vapours to form hydrogen fluoride. The relation between the various sources of water and the resulting HF emission is still not well understood. In this work, industrial measurements have been done to determine where HF escapes from the bath. The quantities of HF and moisture at the specific sites have also been determined. Measurements were done in the duct during normal operation as well as during anode change, above the feeder hole and above an open hole in the crust. A strong correlation between feed cycle and HF levels was measured. Increased HF emissions were also recorded during anode change.

HF Measurements Inside an Aluminium Electrolysis Cell

HF emissions to the working atmosphere may still be a problem for the aluminium industry. The objective in the present work was to study how the HF evolution is distributed between feeder holes, other openings in the crust, gases diffusing through the crust, fumes from the secondary alumina residing on top of the crust etc. A movable “gas sniffer” connected to a Tunable Diode Laser was used to measure the HF concentrations at the above mentioned locations. The stationary HF level in an open flaming feeder hole was approximately 9000 ppm, when measured a few cm above the bath surface. In comparison, when the probe was positioned 5–10 cm above a crust area with good integrity, the HF concentration was in the range 5–10 ppm. The results support the notion that most of the HF evolves from open feeder holes and the tapping hole.

Electrochemical Behavior of Dissolved Titanium Species in Molten Salts

INTRODUCTION Titanium has many attractive properties and the use of titanium and its alloys can be greatly increased if a new and less expensive process for the production of pure titanium were developed. Electrolysis in a molten salt electrolyte is a possible way to produce titanium, and many attempts to develop an electrolysis process have been reported. Large cells and pilot plants have been run, but no commercial electrolysis production has been achieved. Of importance is the work by Ginatta [1] which resulted in the successful development of a pilot cell for the electrowinning of titanium from TiCl4 dissolved in a molten NaCl based electrolyte. A more recent approach is due to Fray et al. [2] who suggested a new electrolysis method, later known as the FFC Cambridge method. Similar methods have been proposed by Suzuki et. al [3] and Okabe and Waseda [4].

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).

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.