Viðburðir eftir árum


Thesis defense School of Science and Engineering - Caroline Mary Medino

Iceland School of Energy

  • 29.5.2019, 14:00 - 15:00

Author: Caroline Mary Medino

Title: Improving Current Efficiency in Low-Temperature Aluminum Electrolysis with Vertical Inert Electrodes

Date and Location: Wednesday, May 29th, 2019 at 14:00 in M103

Supervisor: Guðrún Sævarsdóttir, HR; Halldór Svavarsson, HR; Guðmundur Gunnarsson NMI

Examiner: Ari Jónasson, PhD, process specialist at ISAL

Abstract:

Primary aluminum production is an energy intensive process with an average electric power consumption between 13 – 14 MWh per tonne of aluminum from the electrolysis process alone. Additional energy consumed in the aluminum plant is derived from the carbon anodes used in the process, equating to 3.8 MWh/t Al and contributing to 1.5 tonnes of carbon dioxide emissions for every tonne of aluminum produced. Most aluminum produced today is derived from non-renewable resources, so the aluminum industry emits approximately 500 million tonnes of carbon dioxide equivalent annually; this constitutes about 1 % of the world’s total CO2 emissions. Countries like Iceland, Norway and Canada use hydroelectric and geothermal power for aluminum production so the only way to achieve substantial reduction in carbon dioxide emissions are to change the anode material. By replacing the carbon-based material with an inert material, oxygen, rather than carbon dioxide, is evolved as the main by-product. Potential materials for inert electrodes have a limited lifetime in the corrosive cryolite electrolyte at 960 °C. This drawback has led to significant research in altering electrolyte composition and lowering bath temperature to improve inert anode stability. The objective of this thesis was to investigate the influence that different operating parameters have on current efficiency in low temperature electrolysis with vertical inert electrodes.

Parameters tested include cell design, anode composition, interpolar distance, electrolyte composition, current density, duration and bath temperature. Fourteen laboratory electrolysis experiments were executed using Cu/Ni/Fe anodes and a TiB2 cathode in a ternary NaF-KF-AlF3 electrolyte system. Cell design was determined to be the most influential parameter, yielding current efficiency values over 80 %. This was achieved when the cathode was extended to the bottom of the reduction cell resulting in polarization of the aluminum product. Small changes in bath temperature and electrolyte composition could result in a layer of frozen electrolyte on the cathode, indicating that inert cells should use higher superheat values than conventional cells. Chemical analysis with scanning electron microscopy showed that iron fluoride corrosion tunnels formed in both anode types and are highly influenced by the number of experiments endured. Base metal corrosion penetration depth was similar for both the homogeneous and as-cast microstructure anodes. The higher copper content anode gave a thicker oxide scale. Lower copper content anodes may be beneficial in creating a thinner oxide scale and increasing corrosion resistance. Trends between current efficiency and the remaining operating parameters could not be determined. The highest aluminum purity was achieved in an extended cathode experiment with 99.69 % purity. The lowest purity was obtained for a boron nitride sleeved cathode experiment in which a high boron impurity concentration co-deposited with the aluminum product.

Further research is needed before implementing inert anode technology on an industrial level, therefore parameter optimization should continue to be the focus during experimentation for improving current efficiency. Future electrolysis experiments at Innovation Center Iceland should consider using the following parameters:

· An extended cathode with an alumina sleeve to limit aluminum reoxidation

· A reasonable superheat value

· A sodium-rich electrolyte with potassium additives

· Lower copper content anodes of homogeneous microstructure which are pre-oxidized prior to experimentation.



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