Magnesia-stabilised zirconia solid electrolyte assisted electrochemical investigation of iron ions in a SiO₂–CaO–MgO–Al₂O₃molten slag at 1723 K

Abstract

Production of metallic iron through molten oxide electrolysis using inert electrodes is an alternative route for fast ironmaking without CO2 emissions. The fact that many inorganic oxides melt at ultrahigh temperatures (>1500 K) challenges conventional electro-analytical techniques used in aqueous, organic and molten salt electrolytes. However, in order to design a feasible and effective electrolytic process, it is necessary to best understand the electrochemical properties of iron ions in molten oxide electrolytes. In this work, a magnesia-stabilised zirconia (MSZ) tube with a closed end was used to construct an integrated three-electrode cell with the "MSZ | Pt | O2 (air)" assembly functioning as the solid electrolyte, the reference electrode and also the counter electrode. Electrochemical reduction of iron ions was systematically investigated on an iridium (Ir) wire working electrode in the SiO2-CaO-MgO-Al2O3 molten slag at 1723 K by cyclic voltammetry (CV), square wave voltammetry (SWV), chronopotentiometry (CP) and potentiostatic electrolysis (PE). The results show that the electroreduction of the Fe 2+ ion to Fe on the Ir electrode in the molten slag follows a single twoelectron transfer step, and the rate of the process is diffusion controlled. The peak current on the obtained CVs is proportional to the concentration of the Fe 2+ ion in the molten slag and the square root of scan rate. The diffusion coefficient of Fe 2+ ions in the molten slag containing 5 wt% FeO at 1723 K was derived to be (3.43 ± 0.06)×10 -6 cm 2 s -1 from CP analysis. However, a couple of following processes, i.e. alloy formation on the Ir electrode surface and interdiffusion were found to affect the kinetics of iron deposition. An ECC mechanism is proposed to account for the CV observations. The findings from this work confirm that zirconia-based solid electrolytes can play an important role in electrochemical fundamental research in high temperature molten slag electrolytes.

Electrochemical behaviour of titanium-bearing slag relevant for molten oxide electrolysis

“…The reduced metal dissolves in the surface of the iridium WE creating a chemical potential gradient that drives solid state diffusion, lowering the activity of the deposited metal towards the thermodynamically stable state of the alloy. A similar mechanism was proposed by Gao et al [10] in the recovery of iron from a silicate melt using an Ir WE. Alloying of the metal produced on a platinum-group metal electrode has been observed before with oxide melts [10,17,29].…”

“…All the potentials measured are referred to this pseudo-reference electrode, which was found to be stable throughout multiple measurements. While some have successfully used oxide / oxygen reference electrodes based on solid ZrO 2 electrolyte separators [10], in this system, the small size of the molten oxide droplet limits the ability to use this type of reference electrode. Furthermore, as ZrO 2 is soluble in this oxide melt [24], using this type of reference electrode would contaminate the system.…”

“…For each ton of steel produced, 1.83 tons of CO 2 are produced [7], which makes steelmaking one of the largest industrial source of the greenhouse gas. Research in this field has been mainly focused on the extraction of iron from FeO containing melts [7][8][9][10]. For the more complex, chemically-aggressive, Ti-bearing oxide melts, such as TiO 2 − SiO 2 − Al 2 O 3 − MgO − CaO slags, thermodynamic data is only available for narrow ranges of temperature and composition [11].…”

Molybdenum modified Montmonrillonite clay as an efficient catalyst for low temperature NH3‐SCR

“…After t = 10 min, the adsorbed NH 3 was consumed completely for both samples. For sample blank, new bands at 1570 cm −1 corresponding to the NO 2 adsorbed on metal oxides and bidentate nitrate species, appeared since t = 5 min and glowed stronger. Peak at 1260 cm −1 appeared since t = 20 min, which would be ascribed to the bidentate nitrate and/or bridged nitrate species .…”

Yttria-Stabilized Zirconia Aided Electrochemical Investigation on Ferric Ions in Mixed Molten Calcium and Sodium Chlorides

“…With multiple functions such as the container for molten slags, and the electron insulating but ion conducting membrane to separate the anolyte and catholyte, the MSZ tube with a closed end was successfully applied to the electrochemical analyses in molten slags containing FeO or NiO at ultra-high temperatures. [43,44] It is known that the O 2ion conductivity of YSZ is usually higher than that of MSZ at the same temperature. Thus, it was thought that YSZ could offer a sufficiently high O 2ion conductivity at…”