Ultra-high temperature electrolysis of molten oxides : titanium extraction from ironmaking slag. (2020)
Type of ContentTheses / Dissertations
Thesis DisciplineChemical Engineering
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
This thesis is a comprehensive study of ironmaking slag produced from titanomagnetite concentrates and its use as a molten electrolyte for titanium metal extraction. The electrolysis of ironmaking slag, the largest by-product of the steelmaking industry, was investigated for its potential as a secondary source of metal whose use, unlike most common practices, would not lead to degradation of the environment and over-extraction of resources. Provided the electricity is generated from renewable resources, the implementation of such electrolytic route can contribute to decarbonization of the metallurgical sector.
The electrolysis of these metal oxide feedstocks is a promising alternative whose study has traditionally been hindered by the ultra-high temperature required for molten state operation. With liquidus temperatures above 1473 K, these complex oxide systems melt at temperatures higher than lava comes out of a volcano. In this work, a laboratory scale, electrolytic cell was designed inside a vertical tube furnace and was used to achieve the extreme temperatures required to melt the slags and perform electrochemical measurements. Additionally, synthetic TiO₂ − SiO₂ − Al₂O₃ − MgO − CaO slags, with different TiO₂ concentrations, were used to study the effect of the continuous extraction of titanium from the slag. A new containerless approach was used to assess the thermochemical and electrical behaviour of these slags, which enabled precise measurement while avoiding containment contamination. A combination of electrochemical measurements, physicochemical techniques and computational thermodynamic calculations were employed to elucidate the potential use of ironmaking slag as a molten oxide electrolyte for the extraction of titanium metal.
The use of ironmaking slag as an electrolyte was first characterized in terms of the potential constraints arising from operating directly from the process. The results illustrated that the removal of any entrained iron in the slag is crucial for the development of an efficient electrolytic process, and for the production of pure titanium metal. A computational thermodynamic model was developed and validated by comparing experimental DSC and in situ high-temperature XRD measurements with the predicted phase relations for the slags. The predicted thermodynamic cell voltage for the possible decomposition reactions within the molten slags indicated a small potential window between the reduction of Si and Ti ions from the melt, suggesting poor selectivity.
Cyclic voltammetry, potentiostatic and galvanostatic electrolysis measurements were performed to investigate the reduction process of the electroactive species in the melt. The electrolytic decomposition to metal and oxygen gas was confirmed by post-mortem microscopy analysis and real-time visualisation during experimentation. Electrolysis experiments showed that titanium and silicon were deposited together, forming an alloy with the working electrode when iridium or molybdenum was used. Changing the amount of TiO2 in the slag composition was not able to yield titanium or silicon as pure metal products. Ti – Si alloys are the main products of the electrochemical reduction of molten TiO2 - SiO2 - Al2O3 - MgO - CaO slags, where pure Ti metal production is unlikely without the prior removal of SiO2. To alter the electrochemical sequence, future work could explore modifying the chemistry of the system, e.g. using a supporting electrolyte.
While producing titanium metal would be the highest added-value product, the electrolytic extraction of Ti – Si alloys could still be a profitable endeavour as these alloys are well recognized as high-temperature structural materials. Electrochemical impedance spectroscopy and stepped-potential chronoamperometry experiments were performed to investigate the electrical behaviour of such a process. The results confirmed that TiO₂ − SiO₂ − Al₂O₃ - MgO - CaO slags are mixed conductors, where current is carried by both ionic and electronic carriers. Since electron hoping between oxidation states had a predominant effect on enhancing electronic conduction, in order to sustain a current efficiency that enables high production rates while minimizing energy consumption, the operating conditions of the cell should be chosen so the ratio of the different valences of the titanium ions present in the slag is minimized.
The electrolysis of molten oxides (MOE) is particularly attractive due to the possibility to electrochemically extract metals directly from their ores while producing only oxygen. For the first time, the use of ironmaking slag for such a purpose has been investigated with a containerless method and an oxygen-evolving counter electrode. The results of this work proved that the electrochemical recovery of metals from molten ironmaking slags is feasible and provides the foundations to develop an electrochemical process to extract valuable metals from ironmaking slag.