Electrolysis of Molten Iron Oxide with an Iridium Anode: The Role of Electrolyte Basicity

Abstract

Molten oxide electrolysis (MOE) is a carbon-free, electrochemical technique to decompose a metal oxide directly into liquid metal and oxygen gas. From an environmental perspective what makes MOE attractive is its ability to extract metal without generating greenhouse gases. Hence, an inert anode capable of sustained oxygen evolution is a critical enabling component for the technology. To this end, iridium has been evaluated in ironmaking cells operated with two different electrolytes. The basicity of the electrolyte has been found to have a dramatic effect on the stability of the iridium anode. The rate of iridium loss in an acidic melt with high silica content has been measured to be much less than that in a basic melt with high calcia content. Electrolysis is being investigated by the steel industry as a carbon-lean route that copes with the potential environmental constraints on emissions.1-3 Of all the new methods under consideration, only molten oxide electrolysis (MOE) produces liquid metal, 4,5 which occurs by the decomposition of iron oxide dissolved in an appropriately designed solvent melt according toThe reduction mechanism of MOE is similar to that of the HallHéroult process for aluminum production, which consists of the electrolytic decomposition of aluminum oxide dissolved in a molten fluoride solvent comprising cryolite. However, the two processes are fundamentally different with regards to the compensating oxidation reaction at the anode. In the Hall-Héroult cell, oxidation requires the attendant consumption of the carbon anode resulting in the generation of carbon dioxide. In MOE the compensating reaction is the generation of oxygen, which is predicated on the existence of a so-called inert anode whose development is nontrivial given the extreme conditions in the cell including:-temperatures in excess of the melting point of iron (1538 C) -high solubilizing power of a multicomponent oxide melt -evolution of pure oxygen gas at atmospheric pressure.Furthermore, to meet the production requirements of an industrial process, the anode must sustain high current densities, potentially exceeding 1 A cm À2 . Under these conditions, most metals are poor candidates due to the oxidizing atmosphere surrounding the anode and the extreme anodic potential to which the electrode is subjected. Furthermore, passivating oxide layers, which would normally protect a metallic surface, are dissolved by the molten oxide electrolyte resulting in unabated oxidation of the metal. 6,7 Previous work in this laboratory demonstrated that iridium can serve as an oxygen-evolving anode. 4,8 Furthermore, the anodic current density and, hence, the rate of oxygen evolution was found to increase with the optical basicity of the electrolyte at a given value of potential. The focus of the present study is the assessment of the chemical stability of iridium as a function of electrolyte composition. While the cost and scarcity of this metal make it unsuitable for industrial applications, it has a role to play in laboratory-scale studies ...

A new anode material for oxygen evolution in molten oxide electrolysis

“…The iron metal, which has 20 alloyed with the molybdenum cathode substrate, contains almost no carbon (<25 ppm), a small 21 amount of chromium (<500 ppm), and an amount of oxygen in agreement with that predicted by 22 thermodynamics for carbon-free iron at 1600°C (<4000 ppm). At the cathode the corresponding 23 faradaic efficiency is 34%, in close agreement with previous investigations 12,13 . Along with these coverage by electrolyte (so-called drag-out) that has adhered to the surface.…”

“…The fact that the oxygen content of the outlet 13 gas gradually reaches values comparable to those observed with a noble-metal anode in the same 14 cell 13 is evidence of successful production of oxygen with this new anode material. Its measured 15 faradaic efficiency based on the recovered oxygen and taking into account typical consumption by 16 oxidation of the inactive parts of the furnace (molybdenum current collectors mainly) is around 17 24%, similar to that previously obtained with an iridium anode.…”

“…2a), in agreement 11 with both the thermodynamic prediction and the experimental measurements of chromium oxide 12 chemical solubility in an oxide melt of this specific composition (supplementary discussion A). 13 The composition and morphology of the oxide scale was found to be pretty much independent of 14 the composition of the base alloy, which influenced mainly the extent of chromium depletion 15 Figure S2). In the case of pure chromium, the inner layer proved to contain a 16 higher concentration of aluminium, and the outer layer exhibited a non-negligible concentration of 17 calcium.…”

“…The mass fraction of iron in the alloy (x) 2 has been varied between 0 and 30%, the iron-rich limit corresponding to an alloy melting point equal 3 to the process temperature (1600°C). The selected electrolyte composition is very basic (optical 4 basicity of 0.75), which is advantageous for its physico-chemical properties, e.g., electrical 5 conductivity and viscosity, but challenging in terms of anode materials compatibility 13 . 6 The electrolysis cell operated in constant current mode (Fig.…”

“…First, the successful extraction of metal and 24 oxygen obtained with an affordable alloy-based anode material at the bench-scale will enable the 25 design of a reactor of sufficiently large scale that longer term MOE performance can be properly 26 assessed, e.g., specific energy consumption and cell service lifetime. This is especially important in The original set-up described elsewhere 13 has been adapted for the investigation of a non-noble 2 metal anode as presented in the Methods Section. In summary, a vertical tube furnace constantly 3 purged with helium is used to maintain the cell temperature.…”