High temperature metal production in conjunction with carbon recovery via carbon dioxide electrolysis to solid carbon

Abstract

A process and the apparatus for at least substantially emission-free molten metal production from a solid oxide feed material involving at least some oxidation of carbon-containing material to CO2. A first step comprises forming a metal from a metal-containing material via electrolysis or electric smelting of the metal-containing material. A second step comprises converting CO2 produced in the first step to solid carbon via molten carbonate or NASA-type electrolysis of CO2.

Description

[0001] HIGH TEMPERATURE METAL PRODUCTION IN CONJUNCTION WITH

[0002] CARBON RECOVERY VIA CARBON DIOXIDE ELECTROLYSIS TO SOLID

[0003] CARBON

[0004] TECHNICAL FIELD

[0005] The present invention relates to an at least substantially CO2 emission-free operation of a high temperature metal production process and apparatus involving at least some oxidation of carbon, wherein carbon dioxide generated in the process and apparatus is collected and converted back into solid carbon via an electrolysis process.

[0006] BACKGROUND

[0007] Climate change is driving a fundamental re-evaluation of future options for energy-intensive industries such as power generation, steel and cement production. Renewable energy is seen as key part of this - according to a recent the IEA report, installed capacity of wind and solar PV will become dominant over all other forms of power generation by about 2024 (1).

[0008] The intermittent nature of wind and solar PV power generation is a problem that will need to be managed. Large-scale energy storage systems (e.g., pumped hydro, static batteries etc) will contribute significantly but are unlikely to provide anything approaching complete coverage. This means renewable energy will, at times, be produced in excess relative to demand (including satiated local energy storage). An agile, scalable technology that is able to utilise this off-peak (hence low cost) power is therefore highly attractive (at least in principle).

[0009] One option under intense study (particularly in Europe) is hydrogen production and utilisation. The concept is that off-peak green power can be used to electrolyse water, and the resulting hydrogen used for a variety of tasks including green steel production via the DRI- EAF route (2). Water electrolysis plants can (in general) ramp up and down in a few minutes and are therefore agile enough to follow the availability of off-peak renewable energy. However, on the hydrogen consumption side, DRI plants (in particular) run at elevated temperatures and need to operate in a steady-state mode. To bridge this, large amounts of hydrogen buffer storage are needed. Hydrogen storage as a pressurised cryogenic liquid is difficult, though not impossible (e.g., LH2 storage site at Kobe Port, Japan, 3). Extremely low temperatures are needed (within about 20-40 K of absolute zero), implying significant energy losses and relatively high cost. Alternatives involving pressurised hydrogen storage in depleted gas reservoirs and salt caverns are currently preferred, although this necessarily implies a need for favourable local geological storage structures. There is little doubt that hydrogen produced via electrolysis using off-peak power will become a significant part of the future energy landscape. However, hydrogen storage issues are challenging - this will most likely limit its use to certain parts of the world where favourable geological structures are available.

[0010] The concept of an electrolysis cell for CO2 (similar in principle to an electrolysis cell for making hydrogen from water) is known. Information exists relating to the production of either formic acid or carbon monoxide gas via electrolysis of CO2 (4-6). Whilst the production of synthetic fuels and chemicals via this route will no doubt contribute, the applicant’s view is that this does not represent a scalable solution to the overall problem. In particular, the demand for synthetic chemicals is likely to dominate how much can be produced. The synthetic fuel route is more open in terms of demand-side tonnage, but CO2 will still ultimately be released into the atmosphere. This approach, when based on fossil carbon dioxide, is one of “re-purposing” CO2 to use it a second time and delay its release, rather than actually dealing with the core problem.

[0011] The applicant is not aware of any industrially established technology for electrolysing CO2 directly to solid carbon and gaseous oxygen.

[0012] A significant body of fundamental (lab-scale) work has shown that molten carbonates operating in the range 450-900 °C can, at least in a chemical sense, electrolyse CO2 to O2 and solid carbon with high faradic efficiency (7, 8). Carbon is typically deposited on a solid cathode for removal and product recovery.

[0013] A long-standing problem (sometimes perceived as a roadblock) is carbon fouling of cathodes. Since the product of the reaction is a solid, it is clear that electrode fouling needs to be managed by some non-standard technique. US Patent 9,290,853 B2 (9) discusses various options and electrolysis reactor types including scraped electrodes and the like. The carbon product is generally described as “graphitic” and is intended for higher-value applications such as electrode manufacture.

[0014] A series of Chinese patents from University of Wuhan (10-13) describe electrolysis of CO2 in a lithium-containing molten carbonate electrolyte for making carbon in various forms including nanotubes. An option for contacting CCh-containing flue gas directly with molten carbonate electrolyte (for CO2 absorption) is also described.

[0015] An alternative approach considered by the applicant that is at least theoretically capable of yielding the same overall result is based on NASA development of an oxygen recovery system (from CO2) for space travel (14). This process uses a two-step combination wherein CO2 is first electrolysed to CO and O2 at about 800-900 °C using an oxygen-conducting membrane system based on partially stabilized zirconia. In a second step, product CO is reacted via the Boudouard reaction (with the aid of a suitable catalyst) to form solid carbon and CO2 (for recycle to the first step). In principle, this type of approach may be used in place of the molten carbonate process.

[0016] High temperature electrolysis process such as aluminium smelters traditionally use carbon anodes. AI2O3 is electrolysed to aluminium metal at about 1000 °C and, at the anodes, carbon is consumed whilst alumina-bound oxygen reports to offgas as CO2. Much effort has been devoted to the development of inert anodes over the past 30 years or so, with nominal success reported. The aim is to electrolyse alumina using a non-consumable anode, with oxygen reporting to offgas as O2. Many different anode types have been explored (15), including coating layers involving materials such as titanium nitride and titanium boride. Green aluminium (produced using inert anodes in conjunction with green power) attracts a significant market premium, but there appear to be some product quality issues. It seems inert anodes can also lead to product quality issues, limiting the use of green aluminium to “thick” product types such as casings and structural components. Although it is theoretically possible to use carbon anodes in conjunction with CO2 collection and carbon capture and storage (CCS), this would require an unusual geographic combination of CCS resources in sufficiently close proximity to aluminium smelters and sources of low-cost green power. It is also perhaps not the most favoured option because the resulting aluminium would be considered “blue” rather than “green”.

[0017] Another type of high temperature electrolysis process is molten oxide electrolysis of iron ore to steel. This approach, currently championed by Boston Metal (16), aims to commercialise direct steel production using electrolysis cells like those in the inert anode version of an aluminium smelter, albeit at significantly higher temperatures (1500-1600 °C). Materials challenges for inert anodes in such an environment are extreme to say the least - it may or may not be possible to achieve this in any practical large scale operation. Having said this, Boston Metal announced in March 2025 that it had successfully commissioned a multi-inert anode Molten Oxide Electrolysis (MOE) industrial cell for clean steel production that is operational at its Woburn, Massachusetts facility.

[0018] A further type of clean steel production facility is one which uses green hydrogen to produce carbon-free DRI, which is then smelted in an electric smelting furnace (ESF) to produce molten pig iron (17). Molten pig iron contains around 3-4.5% carbon and this needs to come from somewhere. If fossil carbon (anthracite coal, coke breeze or the like) is added in the ESF, there will typically be a significant amount of fossil CO in the fuel gas generated by the ESF. This fuel gas is would normally be used as fuel in the main DRI plant process gas heater (PGH). One potential option considered by the applicant is for flue gas from this heater to be scrubbed to collect CO2, or alternately the PGH could be operated with oxygen combustion and flue gas recycle to produce high-concentration CO2 directly. Either way, in this option considered by the applicant, CO2 could then be collected, stored and converted back to solid carbon via molten carbonate or NASA-type electrolysis using low-cost intermittent green power. Recovered carbon could then be utilised as the carbon source for the ESF, via new carbon electrodes and / or as a direct carburisation agent in the smelter.

[0019] A variation is a DRI process which uses (at least some) natural gas to carburise the DRI, thereby reducing the need for a carburisation agent in the ESF. The carbon balance in such a system will be different, given that natural gas represents a net inflow of fossil carbon into the system. The applicant has determined that if collected CO2 were to be electrolysed back to carbon, there would most likely be an excess of carbon roughly equivalent in quantity to the fossil carbon contained in the natural gas. This excess could be sequestered as a solid, thereby neutralising CO2 emissions associated natural gas and restoring green credentials.

[0020] If a conventional natural gas-based DRI-ESF process was installed with a view to shifting it across to green hydrogen at a later stage, it may be that availability and / or cost of green hydrogen becomes problematic.

[0021] One potential alternative option considered by the applicant is to convert CO2 to solid carbon using molten carbonate or NASA-type electrolysis, with solid carbon sequestration neutralising natural gas fossil carbon as described above. In such a situation molten carbonate or NASA-type electrolysis with solid carbon sequestration could provide a type of flexibility that is currently missing, thereby allowing a combination of green hydrogen (possibly supply or cost limited) to function in conjunction with ongoing natural gas consumption (all whilst retaining strong green credentials and avoiding CO2 sequestration).

[0022] A further potential alternative option considered by the applicant involves electrolytically recovered carbon displacing carbon from natural gas in the DRI. Given the flexibility of molten carbonate or NASA-type electrolysis to produce different types of carbon, it may be possible to produce a carbon with low gas reactivity but good dissolution properties in molten iron. Recovered carbon could then be incorporated into iron ore pellets and, on passing through the DRI process, it would be relatively well-preserved and would impart the necessary carbon to the DRI. On entering the ESF a portion of it would then dissolve into the metal to produce molten pig iron with the necessary level of carbon, whilst the rest reports to CO-rich offgas for use in the process has heater (PGH) as described above. CO2 could then be collected and electrolysed to the appropriate form of carbon which would in turn be added into the green balling stage of the pellet plant.

[0023] In all the above potential options considered by the applicant the net result is green metal production (a) without recourse to carbon dioxide sequestration and (b) using intermittent renewable power (via molten carbonate electrolysis) to close the carbon loop.

[0024] The above description is not an admission of the common general knowledge in Australia or elsewhere.

[0025] In particular, the considerations of the applicant on possible variations of known technologies to address shortcomings of the technologies is not an admission of the common general knowledge in Australia or elsewhere.

[0026] SUMMARY OF THE DISCLOSURE

[0027] The present invention is based on the above considerations of the applicant in the above “Background”.

[0028] In particular, the present invention is based on a realisation that molten carbonate or NASA- type electrolysis conversion of CO2 to solid carbon may be advantageously coupled with molten metal-producing processes such as those based on electrolysis and / or electric DRI smelting such that:

[0029] • Use of carbon as required for practical, robust and high-quality molten metal production is possible.

[0030] • The resulting CO2 is effectively recycled or sequestered as solid carbon in order to close the loop and restore and maintain green credentials.

[0031] • There is no need for CO2 sequestration with its associated geological requirements and constraints.

[0032] The molten carbonate version of the electrolysis process mentioned above may be along the lines described in patent applications in the name of the applicant including but not limited to International patent application PCT / AU2025 / 050290 and the disclosure in the specification of this application is incorporated herein by cross-reference.

[0033] Commercially viable high temperature molten metal-producing processes will, in general, need to operate in a steady-state mode. This is driven by the need to (i) utilise capital as efficiently as possible, and (ii) avoid downtime which could lead to serious practical issues such as those related to heat losses and unwanted cooling / solidification. It is for this reason that green-metal plants of this type (other than ones heavily reliant on natural gas) are generally located in regions where firm power is cheap. As environmental pressure increases, the need for low-cost firm green power will become stronger and limit potential plant locations even further.

[0034] The invention takes advantage of CO2 being able to be collected (one way or another) from an offgas of such molten metal-producing processes. This may be achieved via amine scrubbers, PSA or other known techniques, and the invention is not confined to these techniques. Collected CO2 can be stored, for example, in liquid form, either under mild cryogenic conditions (e.g., 10 bar and -30 °C) or at ambient temperature and about 70 bar pressure. Either way, large amounts of CO2 can be stored using standard engineering techniques. This allows the molten carbonate or NASA-type electrolysis process for carbon recovery to operate in accordance with the invention only when low-cost renewable (wind / solar) power becomes available.

[0035] The molten carbonate or NASA-type process may be arranged in accordance with the invention in such a way that stop-start operation is readily accommodated without significant detrimental impacts. This provides an opportunity for highly selective operation in relation to renewable power cost, operating only when excess power supply is available, and cost is below a given benchmark.

[0036] Scenarios described above (which do not use natural gas) typically relate to a true closed loop system wherein CO2 becomes a carbon carrier and is never released to the atmosphere in any significant amount.

[0037] Whilst CO2 is normally present in (for example) the baghouse of an aluminium smelter at relatively low concentrations, it has been demonstrated that, at least in principle, amine scrubbers can capture the bulk of this (17). It may be that, for various practical reasons, it is preferable to capture CO2 that would otherwise have been expelled to atmosphere at a different location and use that to recover carbon for new aluminium smelting anodes. The CO2 loop could then be closed in accordance with the invention in a virtual sense, although actual CO2 emissions from the aluminium smelting step might still occur.

[0038] For the purpose of the invention, it does not matter which version of the CO2 closed loop is established, actual or virtual - the two are regarded as equivalent. Selection criteria and preferences will depend on a multitude of local factors.

[0039] In broad terms, the present invention provides a process and an apparatus for at least substantially emission-free molten metal production from a solid oxide feed material involving at least some oxidation of carbon-containing material to CO2 via electrolysis or electric smelting of a metal-containing material to form the metal, wherein carbon-containing material is used as an electrode and / or as a reductant, and CO2 is emitted, collected, and converted to solid carbon for use in the process and apparatus or for sequestration or otherwise via molten carbonate or NASA-type electrolysis of CO2.

[0040] Basically, the process and the apparatus described in the preceding paragraph is a two-step process and apparatus, comprising:

[0041] (a) a first step comprising forming a molten metal from a metal-containing material via electrolysis or electric smelting of the metal-containing material; and

[0042] (b) a second step comprising converting CO2 produced in the first step to solid carbon via molten carbonate or NASA-type electrolysis of CO2.

[0043] The process and the apparatus may comprise forming anodes from solid carbon produced in the second step and using the anodes in the electrolysis of the metal-containing material in the first step.

[0044] The process and the apparatus may comprise sequestering solid carbon produced in the second step.

[0045] The metal-containing material may be a metal oxide-containing material in situations where the first step of the process and the apparatus is an electrolysis process and apparatus.

[0046] The metal-containing material may have been formed from a metal oxide-containing material and have a high metallisation (e.g. greater than 90%, typically greater than 95%) in situations where the first step of the process and the apparatus is an electric smelting process and apparatus. The metal in the metal-containing material may be iron.

[0047] The metal in the metal-containing material may be aluminium.

[0048] The molten carbonate electrolysis of the second step of the process and the apparatus may be along the lines described in patent applications in the name of the applicant including but not limited to International patent application PCT / AU2025 / 050290 and the disclosure in the specification of this application is incorporated herein by cross-reference.

[0049] The NASA-type electrolysis of the second step of the process and the apparatus may be a two-step process wherein CO2 is electrolysed in a first step to CO and O2 at about 800-900 °C using an oxygen-conducting membrane system based on partially stabilized zirconia and product CO is reacted via the Boudouard reaction (with the aid of a suitable catalyst) in a second step to form solid carbon and CO2 (for example, for recycle to the first step). An example of a NASA-type electrolysis process and apparatus is described in International application in the name of the applicant PCT / AU2025 / 050298 and the disclosure in the specification of this application is incorporated herein by cross-reference.

[0050] The invention also provides a process and an apparatus for at least substantially emission-free molten metal production from a solid oxide feed material involving at least some oxidation of carbon-containing material to CO2, typically wherein there is no requirement for geological or other permanent storage of CO2, that is characterised in that:

[0051] • carbon in the carbon-containing material is in any form, including solid carbon, coal or hydrocarbon fuel such as natural gas;

[0052] • carbon is oxidised, typically substantially oxidised, and reports at least mainly as CO2 in an offgas (apart from a component which may be dissolved in molten metal product);

[0053] • CO2 is collected, typically at least substantially collected, from the offgas and converted to solid carbon using a single or double-step electrolysis process with electrolysis occurring at a temperature of 600-900 °C, such as molten carbonate or NASA-type electrolysis, typically using intermittent renewable power; and • solid carbon from the single or double-step electrolysis process is either re-used (for example, as carbon anodes in a high temperature molten metal electrolyser or as a carburiser in the molten metal furnace) or sequestered in solid form or otherwise such that emissions from introduced fossil carbon in the metal production process are effectively neutralised.

[0054] The invention also provides a process and an apparatus for at least substantially emission-free molten metal production from a solid oxide feed material involving at least some oxidation of carbon-containing material to CO2, typically wherein there is no requirement for geological or other permanent storage of CO2, that is characterised in that:

[0055] • carbon in the carbon-containing material may be in any form, including solid carbon, coal or hydrocarbon fuel such as natural gas;

[0056] • carbon is oxidised, typically substantially oxidised, and reports mainly as CO2 in an offgas (apart from a component which may be dissolved in molten metal product);

[0057] • fossil CO2 from a different process (that may or would otherwise be emitted) is collected and converted to solid carbon using a single or double-step electrolysis process with electrolysis occurring at a temperature of 600-900 °C, such as a molten carbonate or NASA-type electrolysis, typically using intermittent renewable power; and

[0058] • solid carbon from the single or double-step electrolysis process is either used in the high temperature metal production process (for example, as carbon anodes in a high temperature electrolyser or as a carburiser in the molten metal furnace) or sequestered in solid form or otherwise such that emissions from introduced fossil carbon in the metal production process are effectively neutralised.

[0059] In one aspect, the process and an apparatus for molten metal production of the invention is a high temperature electrolytic process and apparatus for producing a liquid metal product from an associated metal-containing oxide at 900-1700 °C, wherein the anode is at least substantially carbon-based and is progressively consumed to give CO2 as a by-product in electrolyser offgas. This aspect may further comprise collecting CO2, typically a bulk, i.e., at least a substantial amount, of the CO2, optionally storing it, and finally converting it back into solid carbon via a single or double-step electrolysis process with electrolysis occurring at a temperature of 600- 900 °C, such as a molten carbonate or NASA-type electrolysis.

[0060] The single or double-step electrolysis process may operate intermittently such that low-cost renewable power is used as the primary energy source.

[0061] Product carbon may then be used, optionally in conjunction with one or more suitable binding agents, to manufacture new carbon anodes for re-use in the high temperature electrolysis operation.

[0062] In this manner it becomes possible to retain practical benefits associated with use of carbon anodes in high temperature molten metal electrolysis, whilst at the same time operating a truly green overall process without recourse to CO2 sequestration.

[0063] Product carbon may also be used in any other suitable applications.

[0064] In another, although not the only other, aspect, the process and an apparatus for molten metal production of the invention is a single or multi-step molten metal production process and apparatus wherein a carbon-containing material (typically natural gas and / or coal) is used in conjunction with a clean reductant such as green hydrogen.

[0065] In this aspect, the process comprises oxidation of the carbon-containing material followed by CO2 collection and optional storage.

[0066] Collected carbon dioxide is then reduced to solid carbon using a molten carbonate or NASA- type electrolysis unit in the second aspect (as per the first aspect). In this aspect, product carbon from the CO2 electrolyser may be sequestered in solid form, thereby neutralising the atmospheric effect of carbon from fossil fuel, whist avoiding the need for CO2 sequestration.

[0067] BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The present invention is described further by way of example only with reference to the accompanying Figures, of which:

[0069] Figure 1 is an embodiment of an electrolytic aluminium production process and apparatus in accordance with the invention;

[0070] Figure 2 is an embodiment of a molten oxide electrolysis process and apparatus for steel production in accordance with the invention; and

[0071] Figure 3 is an embodiment of a DRI-ESF process and apparatus for molten pig iron production in accordance with the invention.

[0072] Although all three examples use the molten carbonate version of the process for CO2 conversion to solid carbon for illustrative purposes, it is understood that the NASA-type two- step process could equally well have been used (and the overall result is essentially the same).

[0073] DESCRIPTION OF EMBODIMENTS

[0074] It is noted that the molten carbonate electrolysis process operated in the electrolysers 113, 211, and 317 described in the Figures 1-3 embodiments may be along the lines described in patent applications in the name of the applicant including International patent application PCT / AU2025 / 050290 and the disclosure in the specification of this application is incorporated herein by cross-reference.

[0075] It is also noted that the molten carbonate electrolysis process operated in the electrolysers 113, 211, and 317 described in the Figures 1-3 embodiments may be any other suitable molten carbonate electrolysis process in these or any other suitable electrolysers. With reference to Figure 1 , in an embodiment of an electrolytic aluminium production process, AI2O3 (alumina) feed 101 and electrolyser offgas 105 enter a fluoride scrubber 102 wherein fresh incoming alumina is partially heated, electrolyser offgas 105 is partially cooled, and gas-phase fluorides are partially captured by the heated alumina leaving the fluoride scrubber 102.

[0076] Resulting preheated alumina 103 is fed to electrolyzer 104 for electrolysis at about 1000 °C to produce molten aluminium metal.

[0077] Consumable carbon anodes 104A are used in the electrolyser 104, where they are oxidized to CO2. The electrolyzer 104 may be any suitable electrolyzer.

[0078] Electrolyser offgas 105 contains this CO2, together with an amount of ingress air. The CO2 concentration is typically around 3-10% in this stream.

[0079] After fluoride scrubbing in scrubber 102, fluoride-depleted offgas 106 passes to de-dusting unit 107 which may comprise either a wet or a dry dust removal system.

[0080] De-dusted offgas 108 then enters CO2 scrubber 109 which typically comprises an amine system.

[0081] CCE-depleted offgas 110 is vented, whilst collected CO2 111 is stored in a liquid-phase cryogenic storage system 112 or any other suitable option.

[0082] At a time when low-cost intermittent green power becomes available, CO2 is withdrawn from storage 112 and fed into molten carbonate electrolysis unit 113 where it is converted to solid carbon. Conditions in electrolyser 113 may be adjusted such that the morphology of produced carbon is compatible with carbon anode production. Collected carbon 114 is then fed, together with one or more binding agents 116 such as bio-oil, to carbon anode production plant 115. Product anodes are then used in electrolyser 104.

[0083] With reference to Figure 2, in an embodiment of a molten steel production process, iron ore feed 201 enters pre-treatment plant 202 wherein ore may be simply dried, or alternately processed to remove impurities (depending on the needs of the molten slag electrolysis step).

[0084] Pre-treated iron ore 203 is fed to electrolyzer 204 which contains carbon anodes 204A. Iron oxide is electrolysed to steel and iron oxide-bound oxygen reacts with carbon anode 204A and reports to offgas 205 as CO2.

[0085] Offgas 205 is cooled and cleaned in unit 206 and from there it passes to CO2 scrubber 207. CO2 is extracted from offgas in amine scrubber 207, and CO2 depleted offgas 208 is vented. Collected CO2 209 is fed to CO2 storage unit 210.

[0086] At a time when low-cost intermittent green power becomes available, CO2 is withdrawn from storage 210 and fed into molten carbonate electrolysis unit 211 where it is converted to solid carbon. Conditions in electrolyser 211 may be adjusted such that the morphology of produced carbon is compatible with carbon anode production.

[0087] Collected carbon 212 is then fed, together with one or more binding agents 214 such as biooil, to carbon anode production plant 213. Product anodes are then used in electrolyser 204.

[0088] With reference to Figure 3, in an embodiment of a molten pig iron production process, iron ore pellets 301 are fed to DRI shaft furnace 302 where they are reduced to DRI using hot (about 900 °C) hydrogen 303 which has been preheated in process gas heater (PGH) 304. In a bottom section of the shaft furnace 302 an amount of natural gas 305 is added. This natural gas partially cracks and carburises DRI product such that it contains around 3-4% carbon at the bottom of the shaft.

[0089] Hot DRI 306 is then fed to electric smelting furnace (ESF) 313 where it is smelted to produce molten pig iron containing 3-4% carbon using clean electric power in conjunction with a small additional feed of coke breeze 319 as reductant. Offgas from ESF 313 is cooled and cleaned in gas cleaner 314 and the resulting CO-rich gas 315 is used as fuel gas in PGH 304.

[0090] Top gas from shaft furnace 302 passes to gas cleaning and cooling step 307 and from there to amine scrubber 308.

[0091] CCh-depleted process gas 309 from the amine scrubber 308 is mixed with fresh green hydrogen 310. Mixed gas 311 is heated in PGH 304 to about 900 °C prior to being fed as hot hydrogen 303 to shaft furnace 302 as reductant.

[0092] A small proportion (typically 2-4%) of CCh-depleted process gas 309 may be removed and fed to PGH 304 as additional fuel (not shown). The primary purpose is avoidance of uncontrolled build-up of inert gas species (such as nitrogen).

[0093] CO2 recovered from amine scrubber 308 is fed to CO2 storage system 316 which comprises a set of cryogenic CO2 storage tanks, sufficiently large to allow CO2 electrolysis only when renewable power is available below a benchmark power price.

[0094] CO2 is also collected from the offgas stream leaving PGH 304 in unit 308A. CO2 is then fed to CO2 storage 316. This CO2 collection step may comprise either an amine scrubber or use of oxygen combustion in conjunction with flue gas recycle to achieve a sufficiently high CO2 concentrations.

[0095] Molten carbonate electrolyzer 317 draws CO2 from storage unit 316 and is operated periodically using low-cost renewable power. CO2 is converted to solid carbon 318 which is then buried locally (e.g., in a disused local coal mine) as a form of permanent sequestration.

[0096] The embodiments described in relation to Figures 1-3 are effective embodiments of the invention.

[0097] In this regard, the embodiments described in relation to Figures 1-3 are closed carbon loop systems wherein CO2 becomes a carbon carrier and is never released to the atmosphere in any significant amount. In addition, the net result is green metal production (a) without recourse to carbon dioxide sequestration and (b) using intermittent renewable power (via molten carbonate electrolysis) to close the carbon loop. Many modifications may be made to the embodiments of the process and apparatus described in relation to the Figures without departing from the spirit and scope of the invention.

[0098] By way of example, the invention is not confined to the molten carbonate electrolysis process operated in the electrolysers 113, 211, and 317 described in the Figures 1-3 embodiments and extends to any other suitable molten carbonate electrolysis process and any other suitable electrolysers.

[0099] By way of further example, whilst the embodiments described in relation to the Figures include the use of molten carbonate electrolysis to convert CO2 to solid carbon, the invention also extends to other suitable options for converting CO2 to solid carbon.

[0100] One option is to use NASA-type electrolysis described above. NASA-type electrolysis is a two-step combination wherein CO2 is electrolysed in a first step to CO and O2 at about 800- 900 °C using an oxygen-conducting membrane system based on partially stabilized zirconia. In a second step, product CO is reacted via the Boudouard reaction (with the aid of a suitable catalyst) to form solid carbon and CO2 (for recycle to the first step). As noted above, an example of a NASA-type electrolysis process and apparatus is described in International application in the name of the applicant PCT / AU2025 / 050298 and the disclosure in the specification of this application is incorporated herein by cross-reference.

[0101] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

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[0115] 12. Molten salt electrolyte for capturing CO2 to prepare multiple products and application of molten salt electrolyte, CN114262904A • 2022-04-01 • UNIV WUHAN Molten salt electrolyte for capturing CO2 to prepare multiple products and application of molten salt electrolyte, CN114262904A • 2022-04-01 • UNIV WUHAN A. S. Yasinskiy et al, An update on inert anodes for aluminium electrolysis, Non-ferrous Metals. 2020. No. 1 15 Light Metals, Carbon Materials 1. pp. 15-23 Green et al, Development Status for a Combined Solid Oxide Co-Electrolyzer and Carbon Formation Reactor System for Oxygen Regeneration (nasa.gov), American Institute of Aeronautics and Astronautics, 2016 Boston Metal: https: / / www.bostonmetal.com / general Hydrogen-based Steel: https: / / www.europarl.europa.eu / RegData / etudes / BRIE / 2020 / 641552 / EPRS BRI(2 020)641552 EN.pdf Udara S. P. R. Arachchige et al, Simulation of Carbon Dioxide Capture for Aluminium Production Process, International Journal of Modeling and Optimization, Vol. 4, No. 1, February 2014

Claims

CLAIMS1. A process and the apparatus for at least substantially emission- free molten metal production from a solid oxide feed material involving at least some oxidation of carbon-containing material to CO2, comprising:(a) a first step comprising forming a metal from a metal-containing material via electrolysis or electric smelting of the metal-containing material; and(b) a second step comprising converting CO2 produced in the first step to solid carbon via molten carbonate or NASA-type electrolysis of CO2.

2. A process and an apparatus for at least substantially emission-free molten metal production from a solid oxide feed material involving at least some oxidation of carbon-containing material to CO2 is characterised in that:• carbon in the carbon-containing material is in any form, including solid carbon, coal or hydrocarbon fuel such as natural gas;• carbon is oxidised and reports at least mainly as CO2 in an offgas (apart from a component which may be dissolved in molten metal product);• CO2 is at least substantially collected from the offgas and converted to solid carbon using a single or double-step electrolysis process with electrolysis occurring at a temperature of 600-900 °C, such as a molten carbonate or NASA-type electrolysis, typically using intermittent renewable power; and• solid carbon from the single or double-step electrolysis process is either reused (for example, as carbon anodes in a high temperature molten metal electrolyser or as a carburiser in a molten metal furnace) or sequestered in solid form such that emissions from introduced fossil carbon in the metal production process are effectively neutralised.

3. A process and an apparatus for at least substantially emission-free molten metal production from a solid oxide feed material involving at least some oxidation of carbon-containing material to CO2 is characterised in that:• carbon in the carbon-containing material is in any form, including solid carbon, coal or hydrocarbon fuel such as natural gas;• carbon is oxidised and reports mainly as CO2 in an offgas (apart from a component which may be dissolved in molten metal product);• fossil CO2 from a different process is collected and converted to solid carbon using a single or double-step electrolysis process with electrolysis occurring at a temperature of 600-900 °C, such as a molten carbonate or NASA-type electrolysis, typically using intermittent renewable power; and• solid carbon from the single or double-step electrolysis process is either used in the high temperature metal production process (for example, as carbon anodes in a high temperature electrolyser or as a carburiser in a molten metal furnace) or sequestered in solid form such that emissions from introduced fossil carbon in the metal production process are effectively neutralised.

4. An aluminium smelting process according to any one of the preceding claims wherein carbon anodes are used, and CO2 is collected and converted to back to solid carbon via molten carbonate electrolysis, and recovered carbon is either used to manufacture new carbon anodes or sequestered in solid form (for example by simple burial) to offset emissions from new imported fossil-based anodes.

5. A molten oxide electrolysis process according to any one claims 1 to 3 for direct conversion of iron ore to steel (or pig iron), wherein carbon anodes are used, CO2 from anode oxidation is collected and converted back to solid carbon via molten carbonate electrolysis, and recovered carbon is either used to manufacture new carbon anodes or sequestered in solid form (for example by simple burial) to offset emissions from new imported fossil-based anodes.

6. A green hydrogen-DRI-ESF process according to any one of claims 1 to 3 for production of molten pig iron wherein carbon-containing materials such as natural gas are introduced and report partly as dissolved carbon in metal, and partly as CO2, the latter is recovered from offgas and stored, such that low-cost intermittent renewable power can be used to electrolyse CO2 to solid carbon via molten carbonate electrolysis, and the resulting carbon is sequestered in solid form (forexample by simple burial) to offset CO2 emissions from imported fossil-based materials into the process.

7. A green hydrogen-DRI-ESF process according to any one of claims 1 to 3 for production of molten pig iron wherein carbon-containing materials are introduced and report partly as dissolved carbon in metal and partly as CO2, the latter is recovered from offgas and stored, such that low-cost intermittent renewable power can be used to electrolyse CO2 to solid carbon via molten carbonate electrolysis, and the resulting carbon is re-used in the process through one or more of the following:(i) re-use as a carburising agent in the ESF;(ii) re-use as a feed material for ESF electrode manufacture; and(iii) re-use as a solid carbon additive in the pellet green balling stage.

8. A process and an apparatus for at least substantially emission-free molten metal production from a solid oxide feed material involving at least some oxidation of carbon-containing material to CO2 via electrolysis or electric smelting of a metalcontaining material to form the metal, wherein carbon-containing material is used as an electrode and / or as a reductant, and CO2 is emitted, collected and converted to solid carbon for use in the process and apparatus via molten carbonate electrolysis CO2.

9. The process and the apparatus defined in claim 8 wherein the metal-containing material is a metal oxide-containing material in situations where the process and the apparatus are an electrolysis process and apparatus.

10. The process and the apparatus defined in claim 8 wherein the metal-containing material has a high metallisation (e.g. greater than 90%) in situations where the process and the apparatus are an electric smelting process and apparatus.

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