Electrolysis of carbon dioxide to solid carbon using a molten carbonate-containing electrolyte

Abstract

A electrolysis-based process and an apparatus comprises producing solid carbon and gaseous oxygen from carbon dioxide via electrolysis in an electrolyser using a molten lithium carbonate-containing electrolyte and a solid or liquid metal cathode. Electrolyser oxygen from the electrolyser is treated so that no more than 5% of a carbon dioxide feed to the electrolyser escapes the process and apparatus. Carbon from the electrolyser is purified to have no more than 0.5% carbonate by weight on a dry basis. Purified carbon, and optionally treated oxygen, is used an ironmaking process and apparatus that produces iron that is suitable for producing green steel, using renewable electricity as a primary energy source (in place of coal).

Description

[0001] ELECTROLYSIS OF CARBON DIOXIDE TO SOLID CARBON USING A MOLTEN CARBONATE-CONTAINING ELECTROLYTE TECHNICAL FIELD The present invention relates to a process and an apparatus for producing solid carbon and gaseous oxygen from carbon dioxide via electrolysis in an electrolyser. The present invention relates more particularly, although not exclusively, to an electrolysis- based process and an apparatus that produces solid carbon and gaseous oxygen from carbon dioxide via electrolysis in an electrolyser using (i) a lithium carbonate-containing electrolyte, (ii) a solid or liquid metal cathode at operating at temperatures in a range of 450-900 °C, (iii) electrolyser oxygen treatment steps for ensuring that no more than 5% of a total carbon dioxide feed to the electrolyser escapes the process and the apparatus, and (iv) product carbon treatment steps for purifying the carbon product from the electrolyser to no more than 0.5% weight percent of total carbonate on a dry basis. The present invention also relates to an ironmaking process and an apparatus that comprises an iron ore smelting process and apparatus, such as a HIsarnaTMprocess and apparatus, and an electrolysis-based process and an apparatus that produces solid carbon and that meets the requirements for producing iron in the iron ore smelting process and apparatus. The term “solid carbon” is understood herein to refer to carbon in the solid state that may contain some residual oxygen (generally < 20% by weight on a dry basis). This oxygen comes from the original carbon dioxide from which the solid carbon was produced. The term “liquid metal” is understood herein to refer to any metallic substance that is partially liquid at a selected operating temperature. BACKGROUND 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). 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). 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) start and stop 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 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. The concept of an electrolysis cell for CO₂ (similar in principle to an electrolysis cell for making hydrogen from water) is not new. A large body of 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, 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. There is currently no industrially established technology for electrolysing CO₂ directly to solid carbon and gaseous oxygen. 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. A long-standing problem (sometimes perceived as a roadblock) is carbon fouling of solid cathodes. It is clear that electrode fouling needs to be managed by some technique. US patent US 9,290,853 B2 (9) discusses various options and electrolysis reactor types including scrapers and other physical removal options for electrodes. The carbon product is generally described as “graphitic” and is intended for higher-value applications such as electrode manufacture. 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 CO₂-containing flue gas directly with molten carbonate electrolyte (for CO₂ absorption) is also described. The methods and products described in the above patents may be applied to closed-loop green steel manufacture wherein (i) carbon is used as a reductant for iron ore in an iron ore smelting furnace and (ii) the resulting CO₂ is captured, converted back to carbon (using renewable energy) and returned as reductant to the smelting furnace (14). Typical process configurations for producing molten iron as part of closed-loop green steel manufacture using HIsarnaTMfurnaces and blast furnaces are shown in Figures 1 and 2 and are described below. The above description of the Background and the following description of the invention is not an admission of the common general knowledge in Australia or elsewhere. SUMMARY OF THE DISCLOSURE In broad terms, the invention provides an electrolysis-based process comprising producing solid carbon and gaseous oxygen from carbon dioxide via electrolysis in an electrolyser using a molten lithium carbonate-containing electrolyte and a solid or liquid metal cathode operating at temperatures in a range of 450-900 °C, recovering carbon dioxide from electrolyser oxygen from the electrolyser so that no more than 5% of a carbon dioxide feed to the electrolyser escapes the process, and purifying carbon from the electrolyser to comprise no more than 0.5% carbonate by weight on a dry basis. In broad terms, the invention also provides an electrolysis-based process for producing solid carbon and gaseous oxygen from carbon dioxide that is characterised in that: (a) an electrolysis step is conducted in an electrolyser comprising at least one anode / cathode pair at temperatures in a range of 450-900 °C using a molten electrolyte containing a minimum of 20% and a maximum of 95% combined lithium carbonate and lithium oxide and produces solid carbon and gaseous oxygen; and (b) solid carbon is collected from the electrolyser and cleaned by removing entrained carbonate and produces a final solid carbon product containing no more than 0.5% carbonate (by weight on a dry basis); and (c) oxygen from the electrolyser is treated so that no more than 5% of a total carbon dioxide feed to the electrolyser escapes the process. The selection of a maximum of 0.5% carbonate (by weight on a dry basis), typically a maximum of 0.4% carbonate (by weight on a dry basis), more typically a maximum of 0.3% carbonate (by weight on a dry basis), in the final carbon product is suitable if the final carbon product is required for a direct smelting furnace. The solid carbon product produced by the process may have no more than 0.1% (by weight on a dry basis) carbonate, typically a maximum of 0.08% carbonate (by weight on a dry basis) if the final carbon product is required for a blast furnace. As is described further below, the morphology of the solid carbon product produced by the process is inherently suitable, as a consequence of the process steps, for use as a feed material to iron ore smelting furnaces, either directly (as dried, finely divided solid) or after processing to form coke or other agglomerates that are suitably bonded carbon aggregates. The present invention is based on a realisation that molten lithium carbonate-containing electrolysis conversion of carbon dioxide to solid carbon in an electrolysis-based process and apparatus may be advantageously combined with an ironmaking process and apparatus in such a way that production of green steel becomes possible, using renewable electricity as a primary energy source (in place of coal) in the combined process and apparatus. The molten lithium carbonate-containing electrolyte may contain a minimum of 30% and a maximum of 95% combined lithium carbonate and lithium oxide. The molten lithium carbonate-containing electrolyte may contain other carbonates such as sodium carbonates, potassium carbonates or caesium carbonates. If a pure lithium-based system is used, carbonate melting temperature demands operation above 723 °C. At temperatures at or above this, corrosion control (of the anode in particular and the system in general) can become problematic. Adding carbonates of other types described in the preceding paragraph can decrease the melting temperature, thereby allowing operating conditions that are more favourable in terms of corrosion control. It is for this reason that at least 5% non-lithium carbonates are considered important in the electrolyte. The present invention is also based on a realisation that bulk storage of both carbon dioxide (as a cryogenic liquid) and carbon (as a solid) can be used to accommodate the intermittent nature of renewable green power. It is noted that the invention is not limited to the use of renewable green power. It is possible to use firm power at any time, noting that this is likely to add to costs. Solid carbon collected from the electrolyser may be cleaned to remove carbonate from solid carbon by treatment steps comprising: (a) forming a slurry of carbonate and solid carbon and at least partially converting entrained lithium carbonate to lithium bicarbonate, and (b) removing solid carbon from the slurry by washing with a wash water. The treatment steps may comprise at least partially recovering carbonate from the wash water. The treatment steps may comprise returning recovered carbonate to the electrolysis step. Oxygen from the electrolyser may be treated so that no more than 4%, typically no more than 3%, more typically no more than 2%, of the carbon dioxide feed to the electrolyser escapes the process. Oxygen from the electrolyser may be treated by either being: (a) hot-scrubbed with lean, i.e., depleted, electrolyte from the electrolyser to remove carbon dioxide from the hot oxygen, or (b) cooled, compressed, and subjected to physical or cryogenic separation of carbon dioxide and oxygen. The terms “lean” and “depleted” in the context of electrolyte from the electrolyser are understood herein to mean electrolyte discharged from the electrolyser after being processed in the electrolysis step. As described in relation to embodiments of the invention shown in the Figures, a melt containing 10-15% Li₂O (balance Li₂CO₃) may be considered “depleted”, i.e. lean. The process may comprise a crushing / milling step to enhance access to entrained carbonate in carbon collected from the electrolyser. The cathode may be a solid cathode or a liquid metal cathode, with carbon product either being deposited into the electrolyte as particulates or physically removed from the cathode (either in situ or via physical electrode removal and scraping). In broad terms, the invention also provides an electrolysis-based apparatus for producing solid carbon and gaseous oxygen from carbon dioxide comprising: (a) an electrolyser having at least one anode / cathode pair and configured to contain a molten electrolyte containing a minimum of 20% and a maximum of 95% combined lithium carbonate and lithium oxide and to operate at a temperature in a range of 450- 900 °C to produce solid carbon and gaseous oxygen, and (b) a treatment unit for recovering carbon dioxide from gaseous oxygen from the electrolyser so that no more than 5% of a carbon dioxide feed to the electrolyser escapes the process, and (c) a treatment unit for purifying solid carbon from the electrolyser so that it comprises no more than 0.5% carbonate by weight on a dry basis. More particularly, the invention provides an apparatus for producing solid carbon and gaseous oxygen from carbon dioxide comprising: (a) an electrolyser having at least one anode / cathode pair and is configured to contain a molten electrolyte containing a minimum of 20% and a maximum of 95% combined lithium carbonate and lithium oxide and to operate at a temperature in a range of 450- 900 °C to produce solid carbon and gaseous oxygen, (b) a treatment unit for recovering carbon dioxide from gaseous oxygen from the electrolyser so that no more than 5% of a carbon dioxide feed to the electrolyser escapes the process, (c) a carbon dioxide saturator (which may be either internal or external to the electrolyser) for regenerating depleted molten carbonate electrolyte from the electrolyser with carbon dioxide to be returned to the electrolyser, (d) a carbonate dissolution unit for at least partially converting lithium carbonate in carbon from the electrolyser to lithium bicarbonate; (e) a washing unit for washing lithium bicarbonate from carbon with a wash water, thereby forming a final carbon product containing no more than 0.5% carbonate by weight on a dry basis; and (f) a carbonate recovery unit for recovering carbonate from the wash water. The carbon dioxide treatment unit may comprise a scrubber for hot scrubbing carbon dioxide from oxygen product from the electrolyser using depleted electrolyte in order to decrease the final carbon dioxide content in an oxygen product to a level equivalent to a loss relative to carbon dioxide feed of 5% or less. The carbon dioxide treatment unit may be configured to cool carbon dioxide-containing oxygen. The carbon dioxide treatment unit may be configured for cryogenic recovery of carbon dioxide or pressure swing adsorption or other physical separation to recover carbon dioxide to allow carbon dioxide to be returned to the apparatus and keep net carbon dioxide losses below 5% of the amount fed to the electrolyser. The present invention is also based on a realisation that the following requirements are important for an integrated molten carbonate electrolysis process / apparatus that can be advantageously combined with an ironmaking process / apparatus, with iron produced in the ironmaking process / apparatus being suitable for use as a feed material in a steelmaking process / apparatus, such as a green steelmaking process / apparatus: (a) no more than about 5%, typically no more than 4%, typically no more than 3%, more typically no more than 2%, of the carbon dioxide feed to the electrolyser can be allowed to escape from the electrolysis step as a part of an oxygen product from the electrolyser to achieve an efficient, recycle system with reasonable carbon retention, and (b) solid carbon product from the electrolyser can contain no more than 0.1% of entrained electrolyte if it is to be used in a blast furnace and no more than 0.5% if it is to be used in a direct smelting furnace, such as an HIsarnaTMfurnace. The present invention also comprises an ironmaking process and an apparatus that comprises: (a) a process and apparatus for smelting iron ore and producing iron, such as a HIsarnaTMprocess and apparatus, and (b) the above-described electrolysis-based process and apparatus for producing solid carbon and optionally oxygen that meet the requirements for smelting iron ore. Hot oxygen product from the electrolyser is potentially dangerous and typically needs to be cooled in order to render it safe. Standard oxygen storage usually requires temperatures no greater than 50 °C, and oxygen compatibility with many non-metal plant elements such as primers and valve seals typically require temperatures below about 150 °C (18). In practical terms, this means wet-scrubbing the hot oxygen in a venturi-type or other suitable device to temperatures below about 100 °C (typically 40-50 °C) to render it safe. By way of context for the invention, if there is no carbon dioxide recovery step, any carbon dioxide that is mixed with the oxygen product from the electrolyser will be effectively lost from the system, requiring carbon dioxide or carbon makeup from elsewhere. Some degree of carbon dioxide loss is inevitable, but this loss (as a percentage of carbon dioxide electrolysed) needs to be maintained at a sufficiently low level (from a cost perspective). This may be difficult to achieve in the electrolyser itself, because any carbon monoxide by-product (produced via electrolysis) will, at molten carbonate temperatures, most likely back-react with oxygen to form carbon dioxide. From a safety perspective, it is important to ensure this does indeed occur, since otherwise a potentially explosive mixture will be present in the system. This may (in turn) dictate a lower boundary for electrolyser operating temperatures, below which safety is potentially compromised because in-situ carbon monoxide combustion cannot be guaranteed. Further possible sources of carbon dioxide in product oxygen include (i) carbon dioxide released from the electrolyte for any reason, (ii) products of any carbon-oxygen reaction that may occur in the electrolyser, and (iii) if carbon dioxide is bubbled directly into the electrolyser, some of it could report to a top-space of the electrolyser and form part of product oxygen. The applicant has realised that the net effect may be (potentially) unacceptably high carbon dioxide concentrations in product oxygen. If, for example, a hot electrolyser oxygen product contained 25% carbon dioxide from all sources combined (as listed above) then, in a closed- loop application such as green steel manufacture wherein the oxygen product was vented, around 25% of the carbon would potentially be lost. Under such conditions a makeup stream from elsewhere would be needed (e.g. coal or bio-char). If coal were used as the makeup carbon source, this would amount to roughly 25% loss of the “green” nature of the combined process (since fossil carbon dioxide emissions would increase from essentially zero to about 0.50 t CO₂ per tonne of hot metal). Two, but not the only two, options identified by the applicant to manage this are as follows. 1. An additional hot contacting step between outgoing carbon dioxide-laden oxygen product and depleted molten carbonate electrolyte from the electrolyser. Since depleted molten carbonate electrolyte has a high affinity for carbon dioxide, this allows carbon dioxide to be removed from outgoing hot oxygen and returned to the process for electrolysis. This hot contacting step may take many forms, including that of a counter current scrubbing step directly in a top space of the electrolyser. By such means, carbon dioxide losses to product oxygen can be maintained below 5% (as a percentage of carbon dioxide feed to the electrolyser). 2. Quench-cool the carbon dioxide-laden oxygen product from the electrolyser, then compress and separate carbon dioxide from oxygen in a physical or cryogenic processing unit. Separation options include both liquid-phase carbon dioxide recovery based on boiling point differences and other possibilities such as pressure-swing adsorption based on different affinities for adsorption materials. Recovered carbon dioxide could then be returned to the molten carbonate re-carbonation device for re- use in the process. Carbon product may be removed from the electrolyser either as (i) a solid adhering to solid cathodes (requiring some type of external physical removal device, such as a scraper for removal) or (ii) as carbon particulates in the electrolyte that are carried out of the electrolyser. These particulates may be deposited in the electrolyte via internal physical removal devices, such as scraping devices, or by release from the surface of solid or liquid metal cathodes, optionally assisted by one or more of electrolyte convection, pulsing, sonification, vibration or other suitable means. The morphology and particulate size of the carbon product can vary widely as a function of electrolysis conditions described above. Initial carbon collection may entail a degree of electrolyte entrainment (within cavities, pores etc). After initial bulk separation by physical means (such as hot drainage, pressure filtration, centrifugation etc), entrained electrolyte in the carbon product may still be too high to meet the specifications described for feed into an iron ore smelting furnace. The applicant has realised that a simple water wash may not suffice because lithium carbonate is only sparingly soluble in water. Rather, the applicant has realised that a carbon product clean-up unit involving a bicarbonate method (16,17) may be provided as a first chemical cleaning step because this option allows fairly easy recovery of electrolyte for re-introduction to the main electrolysis unit. Depending on the type of iron ore smelting furnace, a single bicarbonate step may or may not be sufficient to meet the carbon product purity requirement (0.1% electrolyte for a blast furnace, 0.5% for a direct smelter). More bicarbonate steps may be used if further clean-up is needed, optionally in conjunction with carbon milling to improve accessibility to entrained electrolyte (if, for example, electrolyte zones are partly or fully surrounded by carbon). One but not the only alternative to a second bicarbonate leaching step is to use an acid to promote electrolyte removal from carbon. This may not be a preferred alternative in many situations, because recovery of electrolyte from the resulting leach liquor typically involves several steps and additional reagents (albeit at small tonnages relative to the main process streams). The resulting (clean) carbon product may be at least partially in the form of fine dust, even without use of a milling step. This is not a problem for PCI injection in blast furnaces and in direct smelting furnaces such as HIsarnaTMfurnaces. For coke replacement in a blast furnace, some type of binding / densification step may be needed. The applicant has realised that micro-scale morphology of a carbon product may be important when considering green steel applications. Lower temperature electrolysis as described above is expected to produce a more amorphous (randomly disordered) type of carbon, whereas higher temperature electrolysis is inclined to produce a more graphitic product. For PCI injection in a blast furnace, amorphous carbon with a higher gas reactivity is preferred. For HIsarnaTMfurnaces and for synthetic coke in a blast furnace, a more graphitic form of carbon with greater reactivity in relation to carbon dissolution in hot metal is preferred. It follows from the above that the electrolysis process of the invention is well-suited to be tuned to production of carbon that is best suited to ironmaking and green steelmaking end- users. The invention also provides a process for producing iron comprising: producing solid carbon and gaseous oxygen in accordance with the electrolysis-based process described above, supplying iron ore, gaseous oxygen, and a source of carbon to a direct smelter and direct smelting iron ore to molten iron and producing an off-gas containing carbon dioxide, with the carbon source for the direct smelter including solid carbon, and using carbon dioxide in the off-gas from the direct smelter in the electrolysis-based process. The process may include using gaseous oxygen from the electrolysis-based process as at least a part of the gaseous oxygen for direct smelting iron ore in the direct smelter. The invention also provides an apparatus for producing iron that includes: (a) a direct smelter for producing molten iron and an off-gas containing carbon dioxide, and (b) the above-described electrolysis-based apparatus for producing solid carbon and gaseous oxygen from carbon dioxide produced in the direct smelter, and (c) equipment for transferring solid carbon produced in the electrolysis-based apparatus to the direct smelter. The ironmaking apparatus may include equipment for transferring gaseous oxygen produced in the electrolysis-based apparatus (after carbon dioxide removal) to the direct smelter. The invention also provides a process for producing iron comprising: producing solid carbon and gaseous oxygen in accordance with the electrolysis-based process described above, producing molten iron and an off-gas containing carbon dioxide in a blast furnace, using carbon dioxide in the off-gas from the blast furnace in the electrolysis-based process, and using solid carbon produced in the electrolysis-based process as a carbon source for the blast furnace. The process may include mixing solid carbon from the electrolysis-based process and a binder, such as bio-oil or tar, and forming lumps of solid carbon, processing the lumps to coke, and supplying the coke to the blast furnace. The process may include supplying solid carbon from the electrolysis-based process to the blast furnace, for example, as a substitute for pulverised coal injection into the blast furnace. The process may include using gaseous oxygen from the electrolysis-based process in the blast furnace. The invention also provides an apparatus for producing iron that includes: (a) a blast furnace for producing molten iron and an off-gas containing carbon dioxide, and (b) the above-described electrolysis-based apparatus for producing solid carbon and gaseous oxygen from carbon dioxide produced in the blast furnace, and (c) equipment for processing solid carbon produced in the electrolysis-based apparatus for use as a feed material in the blast furnace. The apparatus may include equipment for transferring gaseous oxygen produced in the electrolysis-based apparatus (after carbon dioxide removal) to the blast furnace. The invention also provides a process and an apparatus for producing steel that includes converting iron produced as described above into steel. The process may include using gaseous oxygen from the electrolysis-based process in a steelmaking vessel, such as a BOF. BRIEF DESCRIPTION OF THE DRAWINGS The present invention is described further by way of an example with reference to the following drawings, of which: Figure 1 is a schematic diagram of a closed-loop green ironmaking process and apparatus that includes (a) a HIsarnaTMfurnace producing molten iron and a carbon-dioxide- containing off-gas and (b) an electrolysis-based process and apparatus in accordance with the invention producing carbon and oxygen from carbon-dioxide-containing off-gas from the HIsarnaTMfurnace and supplying carbon and oxygen to the HIsarnaTMfurnace; Figure 2 is a schematic diagram of a closed-loop green ironmaking process and apparatus that includes (a) a blast furnace producing molten iron and a carbon-dioxide- containing off-gas and (b) an electrolysis-based process and apparatus in accordance with the invention producing carbon and oxygen from carbon-dioxide-containing off-gas from the blast furnace and supplying carbon and oxygen to the blast furnace, and Figure 3 is a diagram of one embodiment of an electrolysis-based process and apparatus in accordance with the invention for producing carbon and oxygen that are suitable for use in the embodiments of the closed-loop green ironmaking process and apparatus in accordance with the invention shown in Figures 1 and 2. DESCRIPTION OF EMBODIMENT The following description initially describes basic considerations of the applicant that underpin the invention, including the embodiment of the electrolysis-based process and apparatus in accordance with the invention shown in Figure 3 and the embodiments of the closed-loop green steel manufacturing process in accordance with the invention shown in Figures 1 and 2. The description then describes the embodiments shown in the Figures. The applicant has realised that for a typical iron smelting process / apparatus to work effectively with carbon and oxygen that are produced in an electrolyser operating with a molten carbonate electrolyte, there are several key requirements, including the following requirements: (a) the morphology of the carbon produced in the electrolyser should be suitable for use in an iron ore smelting furnace, (b) carbon produced in the electrolyser must contain sufficiently low levels of carbonate electrolyte that could otherwise bleed through into the iron ore smelting furnace; and (c) loss of CO₂ in gaseous product from the electrolyser (essentially an oxygen gas product) must be minimised in order to achieve a high closed-loop carbon recycle factor in the electrolyser. Use of intermittently available green power is envisaged as a preferred primary energy source for embodiments for ironmaking and steelmaking by the applicant, noting that the invention is not confined to the use of green power. This implies a need for large-scale carbon dioxide storage (most likely as a liquid in cryogenic tanks, but not confined to this option) and similarly large-scale storage of carbon product (in stockpiles or in submerged liquid ponds, if this is required for safety). This in turn means that electrolyser feedstock for the carbon dioxide reduction step is typically essentially pure carbon dioxide. Molten carbonate electrolysis of carbon dioxide to carbon may be conducted in a lithium carbonate-containing electrolyte (i.e. up to 95%) at temperatures around 750-850 °C. As carbon dioxide is removed from lithium carbonate, lithium carbonate is converted to lithium oxide. A melt containing 10-15% Li₂O (balance Li₂CO₃) may be considered “depleted”, i.e. lean, and is re-carbonated by an appropriate means to allow ongoing operation. Since depleted electrolyte is an excellent absorbent for carbon dioxide, it is possible to re-carbonate the melt via (i) external recirculation through a dedicated re-carbonation device or (ii) internally via addition of carbon dioxide directly into the electrolyser. The lithium carbonate-containing electrolyte may contain mixtures of lithium carbonate and other carbonates of elements such as sodium, potassium, caesium, barium and the like. Such formulations will generally allow operation at lower temperatures (compared to formulations that have high concentrations of lithium carbonate in the electrolyte), but this may also entail a higher voltage requirement to overcome reduced electrical conductivity at various points in the electrolyser. If carbon dioxide is added to the electrolyser directly, some of it may report to a top space and mix with product oxygen. Alternatively, if (for any reason) some of the electrolysis product is carbon monoxide rather than solid carbon, it is expected that this carbon monoxide will react with oxygen in the top space of the unit to produce carbon dioxide. In addition, product oxygen may react with product carbon to produce additional carbon dioxide. All of these mechanisms (combined) result in a certain carbon dioxide loading in oxygen electrolysis product. Given the importance of retaining carbon dioxide in the system and minimising losses, the applicant has recognised that some method for preventing bulk contamination of product oxygen with carbon dioxide is necessary. Solid carbon produced in the electrolyser may be in a form of (i) solid carbon adhering to a solid cathode, which may be removed, for example by being scraped off, in situ or externally of the molten electrolyte, or (ii) carbon in the form of carbon particulates deposited into the liquid electrolyte from a solid or liquid metal cathode. Either way, the applicant has realised that this solid carbon material needs to be collected and separated (to a high degree) from entrained electrolyte. Any significant “bleed” of electrolyte into an iron smelting furnace could have a deleterious effect on hearth refractory life and is to be avoided for this reason. The applicant has also realised that in blast furnaces it is also possible that lithium hydroxide could form and volatilise in a lower part of the shaft, then be condensed higher up to become “bottled” (in much the same way as and sodium and potassium). This makes the green steel applications of molten carbonate electrolysis of carbon dioxide particularly demanding because it requires a high degree of removal of electrolyte from carbon product. For a blast furnace, sodium oxide plus potassium oxide content in total feed is expected to be below about 5 kg / thm (15). In ore feed this translates to about 0.3% by weight. Any carbonate electrolyte entrained into the blast furnace would be additive to this. The applicant has recognised that, as a consequence, entrained carbonate electrolyte should comprise no more than about 0.1% of the carbon product. For a direct smelting process, such as the HIsarnaTMprocess, this constraint is not as demanding. Since there is no potential to “bottle” alkalis in the system, it is primarily the impact on slag viscosity and refractory wear that is most relevant. Relatively little is known about the behaviour of lithium oxide in ironmaking slags, and from studies in related fields (mould powders, pottery glazes and the like) the applicant has decided that it may be reasonable to assume a maximum of 1% lithium oxide in slag will not manifestly disturb the process. Given that slag make is unusually low when CO₂-derived carbon is used in such a process (around 0.5 tonne slag per tonne of carbon feed), the applicant has decided that the required carbon product quality specification upper limit in relation to entrained electrolyte becomes 0.5%. Since (i) the presence of at least some lithium carbonate in the electrolyte is essential, and (ii) lithium carbonate is only sparingly soluble in water, there is no simple water-washing strategy that could achieve the necessary electrolyte removal from carbon product. A known method for enhancing lithium carbonate solubility in water is carbonation to form bicarbonate (16, 17). Alternative methods involve use of acid additions, resulting in more complex recovery systems for electrolyte components. With reference to Figure 3, one, but not the only, embodiment of an electrolysis-based process and apparatus in accordance with the invention is configured to produce solid carbon and gaseous oxygen products via electrolysis and downstream treatment steps that meet the requirements for producing iron in an iron ore smelting process and apparatus, such as a HIsarnaTMprocess and apparatus. The electrolysis apparatus shown in Figure 3 includes an electrolyser 101 that includes an electrolysis chamber that, in use, contains a molten carbonate electrolyte and at least one anode / cathode pair for electrolysing CO₂ in the electrolyte and forming solid carbon 121, hot gaseous oxygen 113, and “depleted”, i.e., lean, electrolyte 106. The electrolyser 101 and the components, such as the anode / cathode pair, may be any suitable construction. A skilled person, including a team or teams of skilled persons with an appropriate range of technical skills, would be able to design and construct the electrolyser 101. The electrolyte in this embodiment is predominantly, i.e. up to 95%, lithium carbonate- containing electrolyte, operating at 750-800°C with a lithium oxide content (by weight) of 10- 15% when the electrolyte is depleted and <5% lithium oxide when loaded (carbonated). It is noted that the invention is not confined to these lithium oxide concentrations in the electrolyte. It is also noted the molten lithium carbonate-containing electrolyte may contain other carbonates such as sodium carbonates, potassium carbonates or caesium carbonates in any suitable amounts. With further reference to Figure 3, feed carbon dioxide 103 is dissolved into molten salt electrolyte 119 in a carbon dioxide saturator 102 to yield carbon dioxide-loaded electrolyte 104. This loaded electrolyte passes through pump 105 and is then fed into electrolyser 101. Depleted electrolyte 106 is removed from the electrolyser 101 and pumped via pump 107 to flow splitter 108. A portion of the depleted electrolyte stream 109 is sent to hot oxygen scrubber 112 and the balance 110 is sent to molten salt reservoir 111. The split may be any suitable split given electrolyser operational considerations and downstream operational considerations that may change over time. Split fraction 109 is fed to hot oxygen scrubber 112 (shown here as a separate unit for clarity, but which may also be physically integrated into a top space of electrolyser 101). Hot oxygen 113 from the electrolyser 101 is also supplied to the oxygen scrubber 112, and carbon dioxide in the hot oxygen 113 is scrubbed from the hot oxygen 113 with depleted electrolyte from the split fraction 109, with appropriately selected operating conditions (such as residence time, temperatures and relative amounts of hot oxygen 113 and depleted electrolyte) ensuring that no more than about 5%, typically no more than 4%, more typically no more that 3%, and typically no more than 2%, of the carbon dioxide feed to the electrolyser 101 can escape from the process as a part of an oxygen product. Resulting carbon dioxide-depleted oxygen 114 is then transferred to wet scrubber 115 where it is cooled to about 50°C. Cold carbon dioxide-depleted oxygen 116 from wet scrubber 115 containing < 5% carbon dioxide is safely vented (or optionally utilised in the ironmaking process or another application). It is noted that the carbon dioxide scrubber may not be required in some embodiments. Partially loaded electrolyte 117 from hot oxygen scrubber 112 is transferred to reservoir 111. A small amount of makeup lithium carbonate 118 is added to replace net losses in the system, and the resultant mixed molten salt electrolyte 119 is pumped via pump 120 to saturator vessel 102 for re-carbonisation with feed carbon dioxide 103, as described above. It is evident from the above that the embodiment minimises the amount of carbon dioxide that escapes the process and returns the carbon dioxide separated from hot oxygen from the electrolyser 101 to the electrolyser 101. Carbon 121 (and entrained electrolyte) from electrolyser 101 is removed via a bulk removal system 120. This could take the form of periodic cathode removal with carbon scraping or other removal options from solid cathodes, or in-situ carbon removal from solid or liquid metal cathodes via scraping or other removal options including electrolyte flow, pulsing, sonication and / or any other method suitable for detaching carbon and depositing it into the electrolyte. Initial separation of carbon 121 from entrained electrolyte in the bulk removal system may be achieved via draining, centrifugation, filtration or any other suitable physical method. Carbon 121, after initial separation described in the preceding paragraph, still contains some entrained electrolyte and is treated to remove at least a part of the remaining entrained electrolyte for the reasons mentioned above so that the product carbon is suitable for ironmaking and steel manufacture processes, such as green steel manufacture processes. Carbon 121 from the bulk removal system is cooled and fed into slurry tank 122 together with water 123. In some embodiments, carbon 121 may first be crushed / milled to improve access to carbonate. The resulting slurry 124 is fed into pressurised carbonate dissolution vessel 125, together with recycled CO₂ 126 and makeup CO₂ 127. Lithium carbonate in carbon 121 is at least partially converted into lithium bicarbonate in vessel 125 and the resulting slurry of dissolved lithium bicarbonate and solid carbon 128 is sent to filter 129. Wash water 130 washes lithium carbonate from carbon 121 and cleans final carbon product 131 to a lithium carbonate concentration of < 0.5% lithium carbonate by weight on a dry basis. This concentration is suitable for use in a direct smelting iron ore furnace. Lithium bicarbonate-containing in filtrate 132 is fed to carbonate re-precipitation vessel 133 where bicarbonate is decomposed back into carbonate-containing slurry 134 and carbon dioxide 135. Stream 135 is then recycled via gas compressor 136 to give carbon dioxide stream 126. Carbonate-containing slurry 134 is filtered in vessel 137, and recovered cold, solid carbonate 138 is fed (optionally) via dryer 139 to produce anhydrous lithium carbonate stream 140 which is then returned to carbon dioxide saturator 102. In use of the above-described embodiment of the apparatus shown in Figure 3, a solid carbon product and a gaseous oxygen product are produced from carbon dioxide via electrolysis, with the process comprising: (a) conducting an electrolysis step at 450-900 °C in the electrolyser 101 using a molten electrolyte containing at least 20% lithium carbonate and lithium oxide and up to 95% lithium carbonate and lithium oxide, producing carbon and hot gaseous oxygen; and (b) collecting and cleaning carbon from the electrolyser 101 by removing entrained carbonate by one or more treatment steps involving wet formation of lithium bicarbonate, subsequent lithium bicarbonate removal via water washing producing the solid carbon product containing no more than 0.5% carbonate by weight on a dry basis, at least partial carbonate recovery from a wash water, and return of carbonate to the electrolysis step, and (c) treating hot oxygen from the electrolyser by scrubbing and removing carbon dioxide from the hot oxygen with depleted electrolyte so that no more than about 5% of the carbon dioxide feed to the electrolyser 101 escapes from the electrolyser 101 as a part of the gaseous oxygen product. As described above, the resultant solid carbon product (which is a purified electrolyser carbon product) and the gaseous oxygen product (which is a purified oxygen product) that are produced via the treatment steps of the embodiment shown in Figure 3 are suitable for use in the production of molten iron for green steel manufacture in terms of the carbonate concentration in the carbon product and the concentration of carbon dioxide in the oxygen product. In addition, as described above, the morphology of the resultant solid carbon product can be formed to be suitable for use for the production of molten iron for green steel manufacture. The embodiment of the electrolysis apparatus described in relation to Figure 3 is well-suited be combined with an iron ore smelting apparatus, such as a HIsarnaTMapparatus or a blast furnace, for smelting iron ore to iron with the solid carbon product and the oxygen product from the Figure 3 electrolysis-based process and apparatus being used as feed materials for the iron ore smelting apparatus, with the molten iron produced in the smelting apparatus being used in a green steel making process / apparatus. Figures 1 and 2 show two possible embodiments of a combination of an iron ore smelting process and apparatus and a green ironmaking process and apparatus in accordance with the invention in this regard. Figure 1 is a closed-loop green ironmaking process and apparatus that includes (a) a HIsarnaTM furnace producing molten iron and a carbon-dioxide-containing off-gas and (b) an electrolysis-based process and apparatus in accordance with the invention producing carbon and oxygen from carbon-dioxide-containing off-gas from the HIsarnaTM furnace and supplying carbon and oxygen to the HIsarnaTM furnace. Figure 2 is a schematic diagram of a closed-loop green ironmaking process and apparatus that includes (a) a blast furnace producing molten iron and a carbon-dioxide-containing off-gas and (b) an electrolysis-based process and apparatus in accordance with the invention producing carbon and oxygen from carbon-dioxide-containing off-gas from the blast furnace and supplying carbon and oxygen to the blast furnace. With reference to Figure 1, in use, technical-grade oxygen 9, solid carbon 11, and blast furnace-quality iron ore fines 13 are supplied to a HIsarnaTMsmelter 3 and a HIsarnaTMprocess processes these feed materials and produces hot metal 5 and an off-gas 7 with >90% carbon dioxide. The hot metal 5 can be used advantageously as a feed material in a steelmaking process and apparatus, such as a green steelmaking process and apparatus. The carbon dioxide-containing off-gas 7 is compressed and cooled (with removal of non- condensable gas species) into a liquid and is stored as a liquid in storage 23. The liquid carbon dioxide-containing off-gas 7 is transferred from storage 23 to the embodiment of an electrolysis-based apparatus of the invention shown in Figure 3, identified by the numeral 17 in Figure 1. The embodiment of the electrolysis-based process shown in Figure 3 converts carbon dioxide in the off-gas 7 into carbon 19 and technical-grade oxygen 21 in the electrolysis-based apparatus 17. When intermittent green power is available from source 25, the electrolysis-based apparatus 17 can operate with green power. As mentioned above, the invention is not confined to the use of green power and extends to the use of other sources of power. The carbon 19 is transferred from the electrolysis-based apparatus 17 to carbon storage 15. The technical-grade oxygen 21 is transferred from the electrolysis-based apparatus 17 to oxygen storage 27. As required, carbon 19 and technical-grade oxygen 21 are transferred from storage 15, 17 and used as feed materials in the HIsarnaTMsmelter 3. It is evident from the above that the Figure 1 embodiment is a closed loop process and apparatus with respect to carbon and carbon dioxide. The embodiment of a process and an apparatus for producing molten iron shown in Figure 2 is based on the use of a blast furnace 31. The Figure 2 embodiment is similar in many respects to the Figure 1 embodiment and the same reference numerals are used to describe the same features in the Figures. A key difference between the embodiments in Figures 1 and 2 is that solid coke is needed in the blast furnace in order to maintain shaft porosity. This requirement means that the Figure 2 embodiment converts at least a portion of solid carbon 19 from the electrolysis-based apparatus 17 into briquettes with suitable strength, using a binding agent such as bio-oil, tar or other suitable material, in a briquette plant 33 and then converts the briquettes 35 to synthetic coke 39 in coke ovens 37. With reference to Figure 2, in use, the blast furnace 31 converts iron ore 13 and synthetic coke 39 from coke ovens 37 into hot metal 5. A top gas 41 from blast furnace 31 is captured in carbon dioxide scrubber 43, and from there is transferred to carbon dioxide tank storage 23. When intermittent green power 25 becomes available from source 25, the electrolysis-based apparatus of the embodiment of Figure 1 can operate with green power and convert carbon dioxide into oxygen 21 and solid carbon 19. It is evident from the above that the Figure 2 embodiment is a closed loop process and apparatus with respect to carbon and carbon dioxide. The molten iron produced in the HIsarnaTMsmelter 3 in Figure 1 and the blast furnace 31 in Figure 2 and oxygen 21 from the oxygen storage tanks 27 in the Figures can be transferred directly to a green steelmaking plant (not shown) and the iron processed to produce green steel and the oxygen used in the steelmaking process. It will be appreciated by persons skilled in the art that numerous variations and modifications may be made to the above-described embodiment, without departing from the scope of the following claims. The present embodiment is therefore to be considered in all respects as illustrative of the scope of protection, and not restrictively. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, a limited number of the exemplary methods and materials are described herein. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. 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.

[0002] References 1. IEA Renewables 2020 Fuel Report (November 2020) Hyperlink: 2. SEI, 2019. Hydrogen Breakthrough Ironmaking Technology (HYBRIT) is a ground-breaking effort to reduce CO₂ emissions and de-carbonise the steel industry. Hyperlink: 3. Hydrogen Storage, Wikipedia Hyperlink: 4. J Kaczur et al, Process for High Surface Area Electrodes for the Electrochemical reduction of carbon Dioxide, US Patent 8,858,777 B2, Oct 14, 2014 (Liquid Light Inc, NJ) 5. C A Oloman and H Li, Continuous Electrochemical Reduction of Carbon Dioxide, Canadian patent 2,625,656, October 2006 (Mantra Energy Alternatives, CA) 6. N B Jakobsson et al, Process for Producing CO from CO2 in a Solid Oxide Electrolysis Cell, US Patent 10,494,728 B2, Dec 2019 (Haldor Topsoe) 7. E Laarsonen et al, Insights into carbon production by CO2 reduction in molten salt electrolysis in a coaxial-type reactor, J Env Chem Eng, 10 (2022) 106933, 8. J Ren et al, The Minimum Electrolytic Energy Needed To Convert Carbon Dioxide to Carbon by Electrolysis in Carbonate Melts, J. Phys. Chem. C 2015, 119, 41, 23342–23349, 9. R A Elgammal et al, electrolytic generation of graphite. US Patent 9,290,853 B2 filed July 24th2013 10. Molten carbonate electrolyte for capturing CO2 and application thereof, CN114262905A • 2022-04-01 • UNIV WUHAN 11. Method for electrochemically preparing spiral carbon nanotube by adopting molten salt, CN112030177A (B) • 2020-12-04 • UNIV WUHAN 12. Molten salt electrolyte for capturing CO2 to prepare multiple products and application of molten salt electrolyte, CN114262904A • 2022-04-01 • UNIV WUHAN 13. Molten salt electrolyte for capturing CO2 to prepare multiple products and application of molten salt electrolyte, CN114262904A • 2022-04-01 • UNIV WUHAN 14. M J Dry and R J Dry, Steel from Uranium? Uranium 2020 Conference (Virtual), 14-15 October 2020 M Geerdes et al, Modern Blast Furnace Ironmaking, Third Edn, 2015 IOS Press BV (Delft University) p163 Tiihonen et al, Method for recovering lithium carbonate, US Patent US 9.255.012 B2, Metso-Outotec, Priority March 2012 Ariyoshi et al, method for dissolving lithium publication classification compound, method for manufacturing lithium carbonate, and method for recovering lithium from lithium ion secondary cell scrap, US Patent US 2020 / 0248283 A1, Nippon Mining & Metals, Priority Aug 2018 M S Sohal and J S Herring, Oxygen Handling and Cooling Options in High Temperature Electrolysis Plants, INL, 2008, DOE / ID-Number (inl.gov)

Claims

CLAIMS 1. An electrolysis-based process for producing solid carbon and gaseous oxygen from carbon dioxide, characterised in that: ^ an electrolysis step is conducted in in an electrolyser comprising at least one anode / cathode pair at 450-900 °C using a molten electrolyte containing a minimum of 20% and a maximum of 95% combined lithium carbonate and lithium oxide and produces solid carbon and gaseous oxygen; ^ solid carbon is collected from the electrolyser and cleaned by removing entrained carbonate and produces a final carbon product containing no more than 0.5% carbonate by weight on a dry basis; and ^ oxygen from the electrolyser is treated so that no more than 5% of a total carbon dioxide feed to the electrolyser escapes the process.

2. The process according to claim 1 wherein solid carbon collected from the electrolyser is cleaned to remove carbonate from solid carbon by treatment steps comprising: (a) forming a slurry of carbonate and solid carbon and at least partially converting entrained lithium carbonate to lithium bicarbonate, and (b) removing solid carbon from the slurry by washing with a wash water.

3. The process according to claim 2 wherein the treatment steps comprise at least partially recovering carbonate from the wash water.

4. The process according to claim 3 wherein the treatment steps comprise returning recovered carbonate to the electrolysis step.

5. The process according to any one of the preceding claims wherein oxygen from the electrolyser is treated so that no more than 2% of the carbon dioxide feed to the electrolyser escapes the process.

6. The process according to any one of the preceding claims wherein oxygen from the electrolyser is treated by either being:(a) hot-scrubbed with lean, i.e. depleted, electrolyte to remove carbon dioxide from the hot oxygen, or (b) cooled, compressed, and subjected to physical or cryogenic separation of carbon dioxide and oxygen.

7. The process according to any one of the preceding claims comprises a crushing / milling step to enhance access to entrained carbonate in solid carbon collected from the electrolyser.

8. The process according to any one of the preceding claims wherein the cathode is a solid cathode or a liquid metal cathode, with carbon product either being deposited into the electrolyte as particulates or physically removed from the cathode (either in situ or via physical electrode removal and scraping).

9. An electrolysis-based apparatus for producing solid carbon and gaseous oxygen from carbon dioxide, the apparatus comprising: (a) an electrolyser having an anode / cathode pair and is configured to contain a molten electrolyte containing a minimum of 20% and a maximum of 95% combined lithium carbonate and lithium oxide and to operate at a temperature in a range of 450-900 °C to produce solid carbon and gaseous oxygen, (b) a treatment unit for purifying solid carbon from the electrolyser so that it comprises no more than 0.5% carbonate by weight on a dry basis, and (c) a treatment unit for recovering carbon dioxide from electrolyser gaseous oxygen from the electrolyser so that no more than 5% of a carbon dioxide feed to the electrolyser escapes the process.

10. An apparatus for producing solid carbon and gaseous oxygen from carbon dioxide, the apparatus comprising: (a) an electrolyser having at least one anode / cathode pair and is configured to contain a molten electrolyte containing a minimum of 20% and a maximum of 95% combined lithium carbonate and lithium oxide and to operate at a temperature in a range of 450-900 °C to produce solid carbon and gaseous oxygen,(b) a treatment unit for recovering carbon dioxide from gaseous oxygen from the electrolyser so that no more than 5% of a carbon dioxide feed to the electrolyser escapes the process, (c) a carbon dioxide saturator for regenerating depleted molten carbonate electrolyte from the electrolyser to be returned to the electrolyser, (d) a carbonate dissolution unit for at least partially converting lithium carbonate in carbon from the electrolyser to lithium bicarbonate; (e) a washing unit for washing lithium bicarbonate from carbon with a wash water, thereby forming a final carbon product containing no more than 0.5% carbonate by weight on a dry basis; and (f) a carbonate recovery unit for recovering carbonate from the wash water.

11. The apparatus defined in claim 9 or claim 10 wherein the treatment unit for recovering carbon dioxide from electrolyser oxygen comprises a scrubber for hot scrubbing carbon dioxide from oxygen from the electrolyser using depleted electrolyte in order to decrease the final carbon dioxide content in an oxygen product to 5% or less.

12. The apparatus defined in claim 11 wherein the treatment unit is configured for cooling and then compressing carbon dioxide-containing oxygen.

13. The apparatus defined in claim 9 or claim 10 wherein the treatment unit for recovering carbon dioxide from electrolyser oxygen is configured for cryogenic recovery of carbon dioxide or pressure-swing adsorption to recover carbon dioxide for re-use in the apparatus, such that the final carbon dioxide loss is no more than 5% of the carbon dioxide fed to the electrolyser.

14. A process for producing iron comprising: producing solid carbon and gaseous oxygen in accordance with the electrolysis-based process defined in any one of claims 1-8, supplying iron ore, gaseous oxygen and a source of carbon to a direct smelter and direct smelting iron ore to molten iron and producing an off-gas containing carbon dioxide, with the carbon source for the direct smelter including solid carbon produced in the electrolysis process, and supplying carbon dioxide in the off-gas from the direct smelter to the electrolysis process.

15. A process for producing iron comprising: producing solid carbon and gaseous oxygen in accordance with the electrolysis-based process defined in any one of claims 1-8, producing molten iron and an off-gas containing carbon dioxide in a blast furnace, with carbon dioxide in the off-gas from the blast furnace being used in the electrolysis-based process, and supplying solid carbon produced in the electrolysis-based process as a carbon source for the blast furnace.

16. An apparatus for producing iron that includes: (a) a direct smelter or a blast furnace for producing molten iron and an off-gas containing carbon dioxide, and (b) the electrolysis-based apparatus for producing solid carbon and gaseous oxygen from carbon dioxide produced in the direct smelter or the blast furnace defined in any one of claims 9-13, and (c) equipment for transferring solid carbon produced in the electrolysis-based apparatus to the direct smelter or the blast furnace.

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