Developed by a team at the University of Toronto, the method introduces an oxysulfide electrolyte for electrorefining as an alternative way of removing copper and carbon impurities from molten steel. The process also generates liquid iron and sulphur as by-products.
The method is described in Resources, Conservation & Recycling and is co-authored by Jaesuk (Jay) Paeng, William Judge and Professor Gisele Azimi.
In a statement, Azimi said: “Our study is the first reported instance of electrochemically removing copper from steel and reducing impurities to below alloy level.”
Currently, 25 per cent of steel is produced from recycled material, but the global demand for greener steel is projected to grow as governments endeavour to achieve net-zero targets.
Steel is created by reacting iron ore with coke as the source of carbon and blowing oxygen through the metal produced. Current standard processes generate nearly two tonnes of carbon dioxide per tonne of steel produced, making steel production one of the highest contributors to carbon emissions in the manufacturing sector.
Traditional steel recycling methods use an electric arc furnace to melt down scrap metal. Since it is difficult to physically separate copper material from scrap before melting, the element is also present in the recycled steel products.
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“The concentration of copper adds up as you add more scrap metals to be recycled, and when it goes above 0.1 weight percentage [wt per cent] in the final steel product, it will be detrimental to the properties of steel,” said Azimi, who holds the Canada Research Chair in Urban Mining Innovations.
Copper cannot be removed from molten steel scrap using the traditional electric arc furnace steelmaking practice, so this limits the secondary steel market to producing lower-quality steel product, such as reinforcing bars used in the construction industry.
“Our method can expand the secondary steel market into different industries,” said Paeng. “It has the potential to be used to create higher-grade products such as galvanised cold rolled coil used in the automotive sector, or steel sheets for deep drawing, used in the transport sector.”
To remove copper from iron to below 0.1wt per cent, the team had to first design an electrochemical cell that could withstand temperatures up to 1600°C.
Inside the cell, electricity flows between the cathode and the anode through a novel oxysulfide electrolyte designed from slag, a waste derived from steelmaking that often ends up in cement or landfills.
“We put our contaminated iron that has the copper impurity as the anode of the electrochemical cell,” said Azimi. “We then apply an electromotive force, which is the voltage, with a power supply and we force the copper to react with the electrolyte.”
“The electrolyte targets the removal of copper from the iron when we apply electricity to the cell,” added Paeng. “When we apply electricity on the one side of the cell, we force the copper to react with the electrolyte and come out from iron. At the other end of the cell, we simultaneously produce new iron.”
Looking forward, the team wants to enable the electro-refining process to remove other contaminants from steel, including tin.
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