In recent times, a large amount of steel has been produced using carbon, and steel production is responsible for around 10% of global CO₂ emissions. It is, therefore, very important to develop new processes for producing steel without CO₂ emissions to decrease worldwide CO₂ emissions. One such method is the hydrogen reduction of iron ore, which can be used instead of carbon reduction. Hydrogen reduction is widely studied, and many researchers and companies consider this method as a strong alternative of carbon reduction. However, this method requires hydrogen to be generated from carbon-free technologies such as water electrolysis by the electricity of renewable energies.

Some researchers consider that producing steel using hydrogen produced by electrolysis is inefficient, and it may be better to produce steel directly by electrolysis. A research team of the Department of Materials Science and Engineering (MSE) took up this idea and has developed an electrolysis method to produce CO₂-free steel by electrolysis of Fe oxides using molten oxide electrolytes. The research team is composed of the research groups of three professors, Yi Kyung-woo, Nam Ki Tae, and Jung In-Ho, of the department, and the steelmaking company, POSCO, has supported the research.

There are various methods of making iron by electrolysis, which can be classified according to the type of electrolyte. Electrolytes in iron electro-winning studies include aqueous solutions, molten hydroxide, molten salt, and molten oxide.

Electro-winning in aqueous solutions is carried out at around a temperature of 110°C using an aqueous sodium hydroxide solution. This method has the advantage of low working temperature and easy handling. However, the solubility of iron ore in the aqueous solution is low, and the hydrogen generation reaction is competitively generated so that current efficiency may be lowered and the oxygen concentration obtained is high.

Methods that use molten hydroxide electrolytes usually adopt molten NaOH, and this method performs electrolytic work at a higher speed than aqueous solutions due to its higher charge density and higher working temperature (~500°C) compared to the aqueous solution. However, current efficiency may be lowered because the hydrogen generation reactions may occur at the anode. Furthermore, the solubility of iron ore is low, so it is difficult to increase the current density.

Various molten salts can be used as electrolytes. If the iron oxides are converted to iron fluoride or chloride and mixed into the electrolytes, high speed and high current density operation can be possible. However, other processes to convert from oxide to chloride or fluoride may increase the cost and generate toxic halogen gases. To avoid this inconvenience, oxide ore can be directly added into the electrolyte. In this case, the solubility of oxides in the electrolytes is very low.

To overcome the problem of the low solubility of Fe oxide in the electrolytes, molten metal oxides have been used as the electrolytes. This method is usually called molten oxide electrolysis (MOE). Because the ore of the Fe is oxides, the solubility of the oxides in the molten oxide is usually higher than other electrolytes. The first report to use MOE for Fe production adopts molten SiO₂-CaO-MgO-Al₂O₃ compound oxides as electrolytes. Because the melting temperature of the compound is very high, the operating temperature of the electrolysis is as high as 1600 ℃. This high temperature brings merit to the electrolysis method: the reduced Fe is obtained as melt, which can be used as the cathode for further electrolysis. Liquid metal electrodes are a great advantage because they can eliminate worries about dendrite generation, which is a big problem in the metal electrolytic winning process. However, this method suffers from the disadvantage that the working temperature is too high and the ion conductivity in the electrolyte is not high.

The research team of three MSE professors has developed a new electrolysis method that uses a new oxide system, B₂O₃-Na₂O, as the electrolyte. This electrolyte has several merits compared to previous studies. First, the solubility of Fe oxides in the electrolyte system is very high. When the ratio of Na₂O:B₂O₃ is 1:2, the solubility of Fe₂O₃ is more than 15% at 900 ℃ and reaches 25% at 1000 ℃. At various conditions, the solubility is at least 10%. This value is higher than any other reported electrolysis method of Fe oxides. The experiment of the electricity transfer of this electrolyte shows fast speed, and this can be explained by the high concentration of ions and mobility of Na+ ions. Another merit of this system is its relatively low temperature compared to other MOE methods. The working temperature of this new method is about 900~1000 ℃. This temperature is higher than the working temperatures of other electrolysis methods except for MOE and is far lower than other MOE methods. If the operating temperature is about 1000 ℃, the flexibility of the equipment, such as electrodes, crucibles, and sensors, increases. Moreover, energy loss during the process is far lower. The energy consumption of the present method is far smaller than other MOE techniques, as shown in the Figure below.

Prof. Yi noted, “we expect this method to be a possible alternative to produce CO₂-free steel if coupled with low carbon electricity sources, such as solar cell, wind or nuclear.”