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

Abstract

A containerless approach was used to investigate the electrochemical behaviour of TiO 2 -SiO 2 -Al 2 O 3 -MgO -CaO slags in their molten state. Iridium was used in a three-electrode configuration to perform a combination of electrochemical techniques inside a modified thermal imaging furnace. The real-time visualisation during experimentation and the post-mortem microscopy analysis confirmed the direct production of an Ir−Ti−Si alloy and the evolution of oxygen during electrolysis. Thermodynamic properties of the slag were predicted with FactSage and were consistent with experiment. The results justify the use of this method to better characterize the potential of these systems as a secondary source of materials.

Electrochemical Study on the Reduction of Si and Ti from molten TiO2–SiO2–Al2O3–MgO–CaO Slag

“…The experiments were conducted in a strong reducing atmosphere due to the graphite crucible, and the slag turned black after both electrochemical and control experiments, suggesting a high concentration of Ti 3+ consistent with high electronic contributions to electrical conductivity [30]. This explanation is validated by experiments on related TiO 2 − SiO 2 − Al 2 O 3 − MgO − CaO slags in a floating zone furnace in argon with p O 2 = 10 −6 : a current efficiency of (29 ± 6) % for electrolysis of a slag with 21 wt% TiO 2 at 1903 K was achieved considering just reduction of Ti 4+ [11], and the fraction of total current borne by electronic carriers was determined to be 0.24 at 1823 K in a slag with 33 wt% TiO 2 [12].…”

“…The final compositions of the samples were measured by x-ray fluorescence spectroscopy (XRF, Siemens/Bruker SRS3000), and are reported in Table 1. Sample production has been described previously for these mixtures [11].…”

“…At standard conditions, the Gibbs energy required for the reaction of a pure molten oxide into the most stable metallic phase can be obtained from the Ellingham Diagram. At temperatures in the vicinity of its liquidus temperature [11], TiO 2 − SiO 2 − Al 2 O 3 − MgO − CaO slags are chemically stable and no reduction reactions are predicted to occur spontaneously for any pure constitutive oxide. Upon the application of a potential, any iron oxide will be reduced first as it requires the minimum thermodynamic cell voltage [12].…”

“…The non-ideal properties of the molten slag system will shift the decomposition voltages predicted [13]. The small electrochemical window of operation between the decomposition of silica and titania [11], along with the high concentration (> 20 wt%) of titania commonly present in ironmaking slags [14] have attracted research attention [15,16]. Recently, we electrochemically produced Ti-Si alloys from molten TiO 2 − SiO 2 − Al 2 O 3 − MgO − CaO slag [11].…”

“…The small electrochemical window of operation between the decomposition of silica and titania [11], along with the high concentration (> 20 wt%) of titania commonly present in ironmaking slags [14] have attracted research attention [15,16]. Recently, we electrochemically produced Ti-Si alloys from molten TiO 2 − SiO 2 − Al 2 O 3 − MgO − CaO slag [11]. Our proof of concept study showed that reduction of Ti and Si was possible, but it did not elucidate the thermodynamic conditions for the reduction.…”