Process for producing electrode material

Abstract

A process of producing a carbon electrode material for aluminium smelting comprising the following steps: providing an electrode precursor mixture of carbon-based dry aggregate and a binder; providing

Description

The present invention relates to the production of carbon electrode materials. In particular, the present invention relates to a process for producing a carbon electrode material for aluminium smelting, where a coating composition comprising graphene is applied to the surface of the carbon electrode material. Background Aluminium smelting is the process by which aluminium metal extracted from alumina (al oxide). Aluminium smelting involves electrolysis of aluminium oxide, for example dissolved in cryolite. The electrodes used in such processes are typically carbon-based electrodes and large amounts of energy is expended in the process to overcome the electrical resistance of the electrodes. As such, aluminium smelting is highly energy intensive and small improvements to efficiency can be significant. The carbon electrodes are also typically consumable in the process due to reactions with, for example, carbon dioxide and oxygen formed during the electrolysis process. Electrodes should also be of sufficient strength in order to minimise damage to the electrodes, for example during transit as well as during the smelting process. Accordingly, there is a need to provide improved electrodes for aluminium smelting to address the above problems, whilst maintaining the practical functionality of the electrodes and balancing against expense due to consumable electrodes being used. Summary An aspect of the invention provides a process of producing a carbon electrode material for aluminium smelting comprising: providing an electrode precursor mixture of carbon-based dry aggregate and a binder; providing a coating composition comprising graphene; and baking the electrode precursor mixture to form the carbon electrode material; wherein the coating composition is applied to the electrode precursor mixture prior to baking or applied to the carbon electrode material after baking the electrode precursor mixture. By producing a carbon electrode material having a coating comprising graphene, the resistance of the electrode material to reacting with gases formed during a smelting process may be improved. Without wishing to be bound by any particular theory, it is believed that the presence of graphene can provide a coating over the core electrode material that is less prone to degradation and can reduce degradation of the core electrode material by inhibiting the passage of air or carbon dioxide. As the material is added as a coating, this can be integrated with well-established electrode production processes without substantial changes in electrode composition. Graphene also permits conductivity in the coating that may avoid problems of reduced electrical conductivity that could arise with different protective electrode coatings. In the production of carbon electrodes for aluminium smelting, the electrode is typically formed by production of an electrode precursor mixture that comprises dry aggregate and a binder, and compressing the electrode precursor mixture in a mould to provide a compacted electrode precursor mixture that has been formed into the desired shape, but not yet baked. The precursor mixture after being formed / pressed into shape but before baking is also be referred to as the “green electrode”. The moulded electrode precursor material is then baked to form the carbon electrode material. The baked carbon electrode material may in some instances be referred to as a “prebaked” electrode, as the electrode is baked before being used in a smelting process (the terms “baked electrode” and “prebaked electrode” are used interchangeably herein). The coating composition comprising graphene may suitably be provided on the electrode at various stages. Suitably, the coating composition is applied to the electrode precursor mixture prior to baking, and / or is applied to the carbon electrode material after baking the electrode precursor mixture. In some instances, the coating composition may be coated onto the green electrode prior to baking, and a further coating subsequently provided onto the baked electrode. The process may suitably comprise compacting the electrode precursor mixture in a mould prior to baking, wherein the coating composition is applied to the mould surface before introducing the electrode precursor mixture. In this way, when the electrode precursor mixture is added to the mould and compacted, the coating composition is efficiently provided on the surface of the compacted green electrode. The coating composition may be applied to the mould in any suitable way, and it will be appreciated that this will vary depending on the form of the coating composition. Where the coating composition comprises graphene dispersed in a liquid medium, or a semi-liquid medium such as a paste, the coating composition may for example be applied to the mould by spraying or otherwise distributing the coating composition evenly onto the mould surface. Where the coating composition is a solid composition, for example a powder (such as pure graphene powder), this may be applied to the mould by spraying the powder onto the mould or applying the powder to the mould by a buffing process. In some embodiments, a solid material (such as graphene powder) may be compacted to form a film or sheet that is then applied to the mould surface. Alternatively, or additionally, the coating composition may be applied to a compacted electrode precursor (i.e. a green electrode) from the mould. A compacted electrode precursor is typically sufficiently strong in holding its shape that a coating composition may be applied to the outside of the compacted electrode precursor prior to baking. For example, the coating composition may be applied to the surface of the compacted electrode precursor in the same ways described herein in relation to applying the coating composition to the mould surface, and it will be appreciated that the method used may vary depending on the nature of the coating composition. For example, the coating composition may be applied by spraying the coating composition in the form of a liquid (such as a suspension of graphene), a semi-liquid (such as a viscous material or a paste) or a solid powder, or by a buffing process using a solid material such as where the coating composition is graphene powder or a compacted form of graphene powder. As will be appreciated, the coating composition may be in a form that is changed by the baking step, for example wherein volatiles such as solvents present as carriers are evaporated, or wherein the coating composition is set by baking, as is the case for the electrode material of a green electrode. Alternatively, or additionally, the coating composition may be applied to the carbon electrode material after baking the electrode precursor mixture. The coating composition may be applied to the baked electrode by any suitable method, such as those discussed previously herein. It will be appreciated that the coating method may vary based on the form of the coating composition, and by way of example the coating composition may be applied to the baked electrode by spraying the coating composition in the form of a liquid (such as a suspension of graphene), a semi-liquid (such as a viscous material ora paste) or a solid powder, or by a buffing process using a solid material, such as where the coating composition is graphene powder or a compacted form of graphene powder. It will be appreciated that, due to the high temperatures during an aluminium smelting process, the coating composition will be subject to elevated temperatures and, depending on the nature of the coating composition, the initial smelting process may act as a further baking step on the coating composition. Thus, a coating composition comprising graphene may be applied to a mould and / or to a green electrode prior to baking, and / or may be applied to the carbon electrode material (the “prebaked” or baked electrode) after the baking step. It will be appreciated that a coating may be applied in more than one stage of the process. For example, a coating may be applied before baking via application to the mould and / or to the green electrode, in addition to applying the coating composition to the carbon electrode material after baking. The coating composition may suitably be different or the same in each step, for example a coating composition may be applied to a mould before baking, and a different composition may be applied to the green electrode after compression in a mould and / or after the baking step. By way of example, a spray coating of a liquid coating composition (e.g. a graphene suspension such as a suspension of graphene in a molten material) may be provided to a mould and / or to a green electrode prior to baking, and then a further coating, such as a buffing process with solid graphene (e.g. in powder or compressed form) may be applied to the electrode material after baking. Where a coating is applied after baking, the coating process may in some embodiments be advantageously located separately from the electrode production process. For example, this may permit a process, by which graphene and hydrogen are produced from hydrocarbons such as natural gas / methane, to be disposed in the vicinity of an aluminium smelting process so as to provide the graphene fora post-baking coating of the electrodes, whilst also providing hydrogen that may be used to contribute to energy production for aluminium smelting (such as by a hydrogen fuel cell or hydrogen combustion process). In addition, where graphene and hydrogen are produced using a mobile system, for example a containerised system, the step of coating using graphene may conveniently be located at any point along the supply chain. For example, where carbon electrodes are in transit, a process for the production of graphene and coating the electrodes may be disposed along a shipping route from a location where the electrodes are produced to a location in which the aluminium smelting process takes place. As discussed, the coating composition may be in any suitable form, for example a liquid (such as a graphene suspension) or a solid such as a powder or other solid graphenecontaining material that can be buffed onto a surface. The coating may be applied in any suitable thickness, and it will be appreciated that the thickness of the coating in the coated product may vary depending on the nature of the coating composition and its method of application. By way of example, the coating may be applied with a thickness in the range of from 10 pm to 500 pm. Typically, the coating composition is applied to a mould and / or a green electrode material, or a baked electrode, so as to provide a substantially uniform coating across the surface of the material or mould. For example, the coating is typically applied so as to provide a coating across substantially the entire surface area of the mould or the electrode material, for example across at least 70% of the surface of the mould or electrode material, for example at least 80%, at least 90%, or even at least 95% such as at least 99%. In some preferred embodiments, the coating composition is provided over only a portion of the electrode surface, which may increase efficiency by only coating the parts of the electrode that are more susceptible to reactions with air. For example, the coating may be applied to the surface area of an electrode that is exposed to air during smelting and are not submerged in the smelter. The electrode may, for example, have an upper face that is generally not submerged in the smelter in use and a lower face that is submerged in the smelter in use, and one or more side faces that are partially submerged in the smelter in use. The process may therefore comprise applying the coating to the upper face and the one or more side faces, or the upper face and at least the portion of the side face(s) that would be exposed to air during smelting (e.g., leaving the lower face uncoated). In preferred embodiments, the coating composition comprises the graphene, and optionally a binder, dispersed in a liquid medium. For example, the coating composition may comprise a suspension of graphene in a liquid medium. Where the coating composition comprises graphene dispersed in a liquid medium, the coating composition may be applied by any process suitable for applying a liquid to a green or baked electrode or a mould, such as spray coating, dip coating, pouring or painting, and it will be appreciated that the application process may vary based on the properties of the coating composition, such as viscosity, adhesion strength to the electrode material or mould, for example. The liquid medium may suitably comprise a solvent, for example water or and organic solvent, for example an alcohol such as ethanol or isopropanol, n-methylpyrrolidone, xylene, methyl ethyl ketone, toluene, dimethyl acetamide, cyrene, dimethylformamide or dimethylsulfoxide. Preferably, where the liquid medium comprises a solvent, the solvent comprises or consists essentially of water or an alcohol such as ethanol or isopropanol, preferably water. The coating composition may suitably comprise the solvent and from 0.1 wt.% to 30 wt.% graphene. The coating composition may comprise a binder with the liquid medium, for example a solvent. The binder may be any binder suitable for enhancing binding of the graphene to the electrode material, before or after baking, or the surface of a mould, and / or may aid handling by increasing viscosity of the coating composition (or decreasing viscosity where appropriate). For example, a binder may modify the viscosity and / or the adhesive properties of the coating composition so as to aid retention of the coating composition on the surface of a green electrode material or the surface of a mould before baking, or to the surface of a baked electrode material. As will be appreciated the amount of binder in the coating composition will vary depending on the binder used and the desired use of the coating composition. The coating composition may for example comprise from 1 wt.% to 50 wt.% of a binder together with graphene and a liquid medium such as a solvent. The binder may be any suitable material, and in preferred embodiments, the binder comprises a polymeric binder, such as an organic polymeric binder, for example cellulose or cellulose derivatives such as ethyl cellulose, lignin-based polymers, a phenolic polymer, polyetherimide, polyimide or polybenzimidazole. Thus, in some preferred embodiments, the coating composition comprises or consists essentially of a solvent and from 0.1 wt.% to 30 wt.% graphene, and optionally from 1 wt.% to 50 wt.% of a binder. In some preferred embodiments, the liquid medium comprises the binder. For example, the liquid medium may comprise a liquid that has sufficient viscosity and / or adhesive properties so as to itself provide both the liquid medium in which the graphene is dispersed, as well as providing binding of the coating composition to the surface of a mould ora green electrode material, or a baked electrode material. The liquid medium may for example comprise a molten material where the liquid medium is provided in liquid form by heating, but at lower temperatures (e.g., ambient temperature of 20 to 30 °C) the material forming the liquid medium solidifies. The liquid medium may, for example, comprise a tar such as coal tar pitch, petroleum pitch or bio-mass pitch, or a liquid or molten polymer or prepolymer (that is, a mixture that will at least in part polymerise after mixing with the graphene), for example a thermosetting polymer such as epoxy, polyester or phenolic, or a molten thermoplastic polymer such as polyamide or polyetherimide. The binder may be selected so as to provide a high char (carbon) residue following carbonisation, which may occur in the baking step or during the high temperatures of the smelting process. In preferred embodiments, the liquid medium comprises or consists essentially of coal tar pitch or petroleum pitch. It will be appreciated that coal tar pitch or petroleum pitch are typically used as a binder in the production of carbon electrodes for aluminium smelting, and so a coating composition comprising these materials with graphene can provide a coating composition that is particularly suited to integration with the surface of the carbon electrode, as well as providing an efficient process in which materials already being used for electrode formation can also be used in the coating composition. Where the liquid medium comprises the binder, for example wherein the coating composition consists essentially of the binder and graphene, the coating composition may suitably comprise from 1 wt.% to 75 wt.% graphene and from 25 wt.% to 99 wt.% of the binder, preferably from 1 wt.% to 50 wt.% graphene and from 50 wt.% to 99 wt.% of the binder. The graphene may be mixed with the liquid medium in any suitable way and it will be appreciated that generally mixing a liquid component with a solid powder to form a suspension is well-known to the skilled person. For example, high shear mixers, impeller mixers, rotor stator mixers and the like may be used. Where the liquid medium is a molten material, such as a tar or pitch, this may be melted separately then combined with the graphene, orthe graphene may be mixed with the solid form of the liquid medium, followed by melting and mixing in situ. In some embodiments, the coating composition comprises dry graphene powder. It will be appreciated that a dry powder may itself have fluid-like properties and may be applied to, and stick to, a surface by spraying, for example. In other instances, a coating composition comprising dry graphene powder may be a compressed sheet or similar formed from compacted graphene powder. The graphene powder may be applied to the surface of the carbon electrode material, to the surface of a mould in which the electrode precursor mixture is compacted, or to the surface of the electrode precursor mixture following compaction but before baking. Graphene in solid form may be applied in any suitable way, for example by buffing the surface with graphene, spraying the graphene powder onto the surface, or compressing the graphene powder into the form of a sheet and applying the sheet to the surface (such sheets being formed, for example, by a roller compaction process). It will be appreciated that the graphene powder may be treated with modifiers whilst still remaining a powder or solid formed from the compacted powder. For example, the graphene powder may comprise a binder applied to the powder to enhance adhesion in the electrode material, whilst retaining the graphene in solid or powder form. The graphene used in the processes and products described herein may be any suitable graphene and may be produced by any suitable process. Preferably, the graphene is produced by operation of a plasma reactor system configured to produce graphene from a carbon-containing process gas. It has been found that graphene produced in this way is well suited to incorporation into carbon electrodes. For example, it has been found that such graphene can contain polycyclic aromatic hydrocarbons and polyacetylene chains adsorbed or bonded to / on its surface, which can allow the graphene to more effectively disperse in binders having similar chemistry, such as coal tar pitch for example, which is largely constituted of polycyclic aromatic hydrocarbons. These surface groups may also aid in providing cross-linking of the graphene into the electrode structure. By way of example, polycyclic aromatic hydrocarbons may be identified by gas chromatographymass spectroscopy (GC-MS), and polyacetylenes may be identified by Raman spectroscopy. By way of example, plasma reactor systems are described in WO2015 / 189643 and WO2024 / 013488. Preferably, the process comprises providing said graphene by providing a carbon-containing process gas, for example a process gas comprising hydrocarbons, to a plasma reactor system configured to produce hydrogen and graphene from the process gas. The hydrocarbons may be any suitable materials, in preferred embodiments the process gas comprises or consists essentially of methane, for example the process gas comprises or consists essentially of natural gas. As will be appreciated, natural gas is well known in the art and typically comprises more than about 80 %v / v of methane, and smaller amounts of other hydrocarbons such as ethane, propane and butane, and may also contain small amounts of non-hydrocarbon gases such as nitrogen, CO2, argon or other noble gases, and sulfur-containing gases such as H2S, where in some instances the nonhydrocarbon and / or the non-methane hydrocarbons may have been removed or reduced by refining prior to being provided to the apparatus in the feed stream. In some embodiments, undesired gases or particulates such as aerosols in the feed stream may be removed by one or more filters arranged to filter the carbon-containing gas. Preferably, the plasma reactor system comprises a reaction chamber, a plasma nozzle coupled to the reaction chamber, and means for supplying the process gas to the plasma nozzle. The plasma reactor system may comprise means for providing radio frequency radiation, preferably microwave radiation, to the process gas within the plasma nozzle so as to produce a plasma within the plasma nozzle, and thereby cause cracking of hydrocarbons in the process gas within the plasma nozzle to provide cracked hydrocarbon species, wherein the plasma nozzle is arranged such that an afterglow of the plasma extends into the reaction chamber, the cracked hydrocarbon species also pass into the reaction chamber and recombine within the afterglow to provide graphene and hydrogen in the reaction chamber. The plasma nozzle configuration is described in more detail elsewhere herein, and an example of a plasma reactor comprising a plasma nozzle coupled to a reaction chamber that may be used to produce graphene and hydrogen is described in WO 2015 / 189643 A1. The plasma nozzle is preferably shaped and configured so as to cause at least one vortex to be formed in the process gas within the plasma nozzle, said vortex being subjected to said radio frequency radiation. Preferably, the plasma nozzle is shaped and configured so as to cause multiple vortices to be formed in the process gas within the plasma nozzle, said multiple vortices being subjected to said radio frequency radiation. The use of multiple vortices in this manner increases the time for which the process gas is exposed to the radiation, thereby increasing the efficiency of the plasma cracking process. It can also allow better plasma stability by generating an area of lower pressure inside a third vortex where the plasma is more confined. Thus, in a preferred embodiment, three vortices are formed within the plasma nozzle. The multiple vortices may suitably be concentric vortices. Preferably, the plasma is generated at substantially atmospheric pressure. For example, the process gas may be provided to the plasma nozzle at atmospheric pressure or at a slight positive pressure, such as a pressure from about 1 to 1.5 bar absolute. It will nonetheless be appreciated that the process gas may be provided to the plasma nozzle at pressures higher than atmospheric pressure, for example up to about 3 bar absolute or up to about 10 bar absolute. For the sake of completeness, we note here that some parts of the plasma may not be at atmospheric pressure when it is formed at the core of a vortex where its pressure is likely to be lower than atmospheric pressure. However, there may suitably be no further system used to change the pressure of the plasma in the nozzle other than the fluid mechanics induced by the nozzle design. The plasma nozzle may comprise: one or more inlets to receive a stream of the process gas, that forms a first vortex in use; an open end in communication with the reaction chamber; and a vortex-reflecting end opposite the open end; wherein the nozzle is internally tapered towards the open end; such that, in use, a second vortex is created by the vortex-reflecting end, and a third vortex is produced by reflection of the second vortex from the vortex-reflecting end. Advantageously, the second vortex is created by the vortexreflecting end which “sucks” the first vortex (by virtue of the Coanda effect) and then reflects it to form the third vortex. It will be appreciated that an open end of the plasma nozzle in communication with the reaction chamber will have a cross-section that is smaller than the dimensions of the reaction chamber, for example such that the nozzle provides an opening through a wall of the reaction chamber. Preferably, the exit of one or more plasma nozzles coupled to the reaction chamber (which may include means for actively cooling the afterglow) is flush with the wall of the chamber, so as to provide a flat surface that may for example be cleaned by a scraper system. The plasma nozzle may be coupled to the reaction chamber so as to direct flow from the plasma nozzle towards the centre of the reaction chamber, for example wherein the plasma nozzle provides a flow (and afterglow) radially inwards into the reaction chamber. In other embodiments, the plasma nozzle may be disposed at least partially at an angle from the perpendicular to the wall of the reaction chamber to which it is coupled. For example, the plasma nozzle is disposed so as to provide a flow into the reaction chamber at an angle of less than 90 degrees from the wall of the reaction chamber (e.g. a tangent to the wall at the point at which the plasma nozzle is disposed). The means for supplying radio frequency radiation may comprise a microwave generator (for example operating at 2.45 GHz, although other frequencies are also usable). A waveguide may be arranged to direct the radio frequency, for example microwave, radiation to the nozzle, for example to coincide with the vortex(es) of the process gas. As those skilled in the art will appreciate, the expression “radio frequency radiation” encompasses the full extent of microwave frequencies, together with a range of nonmicrowave frequencies. In some embodiments, the radio frequency radiation is terahertz radiation, for example radio frequency radiation in the range of from 0.3 THz to 3 THz. The radio frequency radiation may be suitably provided at various power levels according to the requirements of a particular system. As a general rule, higher power of the plasma system gives better cracking efficiency and allows processing of higher process gas flow rates. Scaling and increasing the power of the plasma system may be achieved by combining several nozzles around a reaction chamber or by increasing the power of the microwave generator and providing a larger nozzle. The power of the microwave generator may be between 1 and 30 kW, for example between 1 and 20 kW. However, there is no restriction of scale on the nozzle and so the power may be varied accordingly. For example, in some embodiments the power may be up to 100 kW or up to 1 MW. Process gas flow through the plasma nozzle may for example be in the range of about 20 L / min to 150 L / min depending on the precise scale of the nozzle, however it will be appreciated that the flow rate may suitably be varied according to a particular nozzle and reactor setup and desired conversion efficiency of hydrocarbons in the process gas. For example, when the power provided for producing plasma in the plasma nozzle is increased then the flow rate may also be increased whilst maintaining the conversion efficiency of the process gas. The pressure at which the process gas is provided to the plasma nozzle will suitably depend on the desired pressure in the reaction chamber, and may suitably for example be higher than the reaction chamber pressure, for example about 0.1 bar to 0.5 bar higher than the pressure in the reaction chamber, such as 0.2 to 0.4 bar higher than the pressure in the reaction chamber. The plasma reactor system may in some embodiments comprise a plurality of plasma nozzles coupled to the reaction chamber. A plurality of plasma nozzles may be distributed in any suitable way around the reaction chamber and in preferred embodiments the plasma nozzles are distributed so as to minimise interference between the respective afterglow exiting each plasma nozzle, for example the plasma nozzles may be distributed evenly around the periphery of the reaction chamber, for example circumferentially around the reaction chamber (e.g. 180 degrees apart for 2 nozzles, 120 degrees apart for 3 nozzles, 90 degrees apart for 4 nozzles and so on). The plasma nozzles may be distributed vertically within the reaction chamber, for example plasma nozzles may be separated vertically on the walls of the reaction chamber (for example separated by at least about 5 cm, such as at least about 8 cm or at least about 10 cm), and may also be distributed around the periphery of the chamber as previously described (e.g. circumferentially) or may be disposed vertically above or below another plasma nozzle. The number of plasma nozzles coupled to the reaction chamber may be selected based on the reaction chamber volume, so as to limit the temperature within the reaction chamber to avoid degradation of graphene in the reaction chamber (for example to limit the temperature of gases in the reaction chamber to no more than about 200 °C). Each plasma nozzle may be orientated radially towards the centre of the reaction chamber or may be disposed at an angle to the wall of the reaction chamber as described previously. Due to the increase in volume when passing from the plasma nozzle into the reaction chamber, the afterglow from the plasma and the cracked species present in the afterglow may be passively cooled upon entering the reaction chamber. In some preferred embodiments, the reaction chamber incorporates means for actively cooling the afterglow on exiting the plasma nozzle, such as direct cooling by introducing a flow of gas (at a lower temperature than the afterglow) to mix with species in the afterglow, or indirect cooling by a heat exchanger carrying a refrigerant or a heat pipe system configured to draw heat away from the afterglow exiting the plasma nozzle. Preferably, in use, the afterglow within the reaction chamber has an operating temperature lower than 3500°C. More preferably, in use, the afterglow within the reaction chamber has an operating temperature lower than 1000°C. For instance, the operating temperature of the afterglow within the reaction chamber can be as low as 300°C. Preferably, in use, the temperature just outside the nozzle, at the carbon formation point within the afterglow, is in the range of 800°C to 1200°C. Particularly preferably this temperature is in the range of 900°C to 1000°C. Nonetheless, it will be appreciated that the gas temperature in the reaction chamber will be significantly lower than this outside of the afterglow. The plasma generated in the nozzle is preferably a non-equilibrium plasma (also referred to as a non-thermal plasma) such as is known in the art, for example a plasma in which the electron temperature is greater than the temperature of heavier species (ions and neutral species) in the plasma. In some embodiments, the process gas may comprise a buffer gas, such as argon, nitrogen or helium, that is blended with the feed stream to provide the process gas. For example, the apparatus, e.g. the regulator system, may be configured to blend a buffer gas with a hydrocarbon feed to form the process gas. It will be appreciated in this context that a buffer gas refers to an inert gas, such as argon or nitrogen, added to dilute the hydrocarbons in the process gas and not to inert gases present in the feed stream itself that is provided to the apparatus (for example small amounts of nitrogen, argon and so on that may be present in natural gas) or to blending of hydrocarbon gases with the feed stream. If a buffer gas is used, the ratio of hydrocarbon species to buffer gas in the process gas is preferably 50:50 or less, for example around 20:80 or less. Preferably, no buffer gas is provided in the process gas. The plasma reactor system may be a containerised system configured to produce the graphene in the plasma reactor system within the container, for example wherein the container comprises a shipping container. The container with which the apparatus is provided may be any suitable container for allowing the apparatus to be transported between locations. For example, the apparatus may comprise an intermodal container such as are commonly used for transporting goods, such as a shipping container. For example, the container may comprise an intermodal container conforming to an international standard size for shipping, such as ISO standard 668:2020. Such a containerised system may be conveniently disposed at a location where carbon electrodes are produced as part of a system or facility for producing carbon electrodes for aluminium smelting. Thus, an additional aspect provides a system or a facility for producing carbon electrodes for aluminium smelting comprising a plasma reactor system as described for providing the graphene (which may or may not be a containerised system). The system or facility may suitably be configured for producing carbon electrodes as described herein (for example electrodes coated with a coating composition comprising graphene). As will be appreciated, the system is also configured to produce hydrogen, and the system may be configured to use the hydrogen to generate power for the electrode production process (for example to provide power for the preparation or baking of electrode material, or to output power to an electrical grid to offset power used in the process). The graphene used in the present process may have a relatively small lateral flake size. Preferably, at least 50 %, preferably at least 70 %, more preferably at least 90%, of the graphene provided for mixing with the fluid medium has a lateral flake size of less than 500 nm, for example less than 400 nm or less than 300 nm. Preferably, at least 50 %, preferably at least 70 %, more preferably at least 90%, of the graphene has a lateral flake size of 100 nm or more. It will be appreciated that lateral flake size will be a distribution of different sizes and can be characterised by that distribution. The lateral flake size suitably relates to the largest measurement of a graphene flake as measured by SEM (scanning electron microscopy) and measuring the size across a large sample of flakes. The specific surface area of the graphene may be from 80 m2 / g to 500 m2 / g, preferably from 100 m2 / g to 350 m2 / g. Specific surface area may suitably be measured according to ISO 9277:2022. The Raman spectrum of the graphene may suitably exhibit one or more of: a D peak intensity to G peak intensity ratio, l(D) / l(G), of less than 1;aD’ peak intensity to G peak intensity ratio, l(D’) / l(G), of less than 0.5; a G peak having a full width at half maximum of from 25 to 50 cm-1; a 2D peak intensity to G peak intensity ratio, l(2D) / l(G), of greater than 0.65; and a single-component 2D peak. Raman spectra can be suitably measured using an excitation laser wavelength of 532nm at a power <0.25mW under a 50x objective lens. As will be appreciated, the peaks referred to are known in the art in relation to Raman analysis of graphene and refer to D peak (around 1350 cm’1), G peak (around 1580 cm’1), 2D peak (around 2690 cm’1), D’ peak (around 1620 cm’1). The graphene suitably comprises at least 90 % carbon by weight, preferably at least 95 % carbon by weight, for example at least 98 % carbon by weight. The graphene provided in the electrode precursor mixture may have a sulfur content of at least 10 ppm, for example at least 50ppm, for example from 10 ppm to 1 wt.%. In some embodiments, the graphene may be doped with sulfur during production of the graphene. The inclusion of sulfur in the graphene may be advantageous where the graphene is coated on the electrode so that there is a degree of localisation of sulfur in the graphene at the surface of the electrode, which may reduce reactivity of the binder at the surface towards oxygen and reduce electrode deterioration. The production of carbon electrodes from a carbon-based dry aggregate and a binder is well-known in the art and it will be appreciated that any suitable carbon electrode material, produced by any suitable process may be coated in accordance with the present disclosure. The carbon-based dry aggregate (the general term “dry aggregate” as used herein will be understood to refer to the “carbon-based dry aggregate”) may be any suitable material and it will be appreciated that the use of such materials in the production of carbon electrodes is well-known to the skilled person. Preferably, the carbon-based dry aggregate comprises coke and / or recycled carbon solids. For example, carbon solids may be recycled carbon electrode materials. Such electrode materials are known as “butts”, and these materials may be recycled by processing into a granulate, and may form all or preferably at most a portion of the carbon-based dry aggregate used in the process. The coke may be any suitable coke such as petroleum coke or coal coke. Suitably, the carbon-based dry aggregate has a carbon content of at least 90 wt.%, preferably at least 95 wt.%, for example at least 97 wt.%. The dry aggregate may typically also contain an amount of sulfur from 1 to 3 wt.% and other impurities such as vanadium or sodium at an amount of less than 1 wt.%, for example less than 0.1 wt.%. The dry aggregate may be in any suitable form, and it will be appreciated that size distribution of the dry aggregate may vary. The dry aggregate may comprise a fines portion having a Blaine specific surface area of at least 2000 cm2 / g, for example at least 2500 cm2 / g or at least 3000 cm2 / g, such as about 3500 cm2 / g. For example, the fines portion may have a Blaine specific surface area of from 2000 cm2 / g to 7000 cm2 / g, for example from 2500 cm2 / g to 5000 cm2 / g, such as from 3000 cm2 / g to 4000 cm2 / g. The fines portion may have a particle size of less than 0.25 mm. The fines portion may make up from 10 wt.% to 50 wt.% of the dry aggregate, for example 20 wt.% to 40 wt.%, such as from 25 wt.% to 35 wt.%. The dry aggregate may comprise a granulate in which at least 90% of the particles have a size of less than 10 mm, preferably less than 8 mm, preferably at least 95% of the particles, such as at least 98% of the particles. The dry aggregate may comprise at least 50 wt.% of particles having a size of less than 1 mm. The dry aggregate may have a particle size distribution in which the number of particles of a given size generally increases with decreasing particle size. In some instance, the dry aggregate may also comprise larger material, for example where recycled butts material is used, this may exceed 10 mm, and may be as large as 20 mm or 30 mm. As will be appreciated, the particle size or the size distribution as referred to herein may be measured by sieving, for example by passage through a square mesh. The binder used in producing the carbon electrode material is not particularly limited and may be any suitable binder. It will be appreciated that a person of skill in the art will be familiar with the use of a binder and a dry aggregate in forming carbon electrode materials. The binder is preferably a tar or pitch produced (for example by destructive distillation) of organic materials, for example from coal, biomass (such as wood), petroleum, or peat. The binder may in some instances comprise a phenolic resin. Preferably, the binder comprises or consists essentially of coal tar pitch and / or petroleum pitch, more preferably coal tar pitch. Suitably, the binder is a material that is substantially liquid at elevated temperatures, for example about 150 °C. Typically, the binder may be a solid at ambient temperature (about 20 to 30 °C) which aids ease of handling but is melted when mixed with the dry aggregate. The binder may suitably have a Mettler softening point (as measured by ISO 5940-2:2007) of 150 °C or less, for example 135 °C or less, such as 120 °C or less. The binder may suitably have a viscosity at 160 °C of at least 1400 cP, for example at least 1700 cP (as measured by ASTM D5018-18). The binder may suitably have a coking value of from 40 % to 80 % (as measured by ISO 6998:1997), for example from 50 % to 70 %. The binder may suitably have a real density in helium (as measured by ISO 21687:2007) of from 0.6 to 2.0 kg / dm3, for example from 0.9 to 1.7 kg / dm3, such as from 1.2 to 1.4 kg / dm3. The binder may suitably have a quinoline insoluble content (as measured by ISO 6791:1981) of from 2 to 15 %, for example from 3 to 10 %, for example from 4 to 8 %. The binder may suitably have a toluene insoluble content (as measured by ISO 6376:1980) of from 10 to 40 %, for example from 15 to 35 %, for example from 20 to 30 %. The binder may suitably have an ash content (as measured by ISO 8006:1985) of up to 5 %, for example up to 1 %, for example up to 0.3 %. The binder may suitably have a sulfur content (as measured by ISO 12980:2000) of less than 2 %, for example less than 1 %. As described, carbon electrodes for use in aluminium smelting are typically made from a baked mixture of carbon-based dry aggregate and binder. The baking step removes volatiles from the mixture to leave a solid structure of relatively pure carbon. The baking step can increase crystallite size in the electrode material, which can increase conductivity. However, crystallites of increased size can also cause detrimental effects such as reduced thermal shock resistance. Independent of whether a coating is provided, it has also been surprisingly found that by integrating graphene into the precursor mixture before baking the electrode material, its properties may be significantly improved, without increasing the measured crystallite size within the baked material. For example, it has been found that typically when the binder content (e.g. coal tar pitch) in the electrode mixture is increased, the electrical resistance can be reduced but this leads to an increase in sticking of packing materials and loss of density following baking (i.e., a reduction in baked apparent density as compared to green apparent density as measured by ISO 12985-1:2018). However, the addition of graphene has been surprisingly found to reduce the sticking of packing materials as well as the loss of density following baking. This can allow lower electrical resistance for an electrode whilst maintaining density and ease of handling. In addition, the inclusion of graphene has also been found to improve thermal conductivity, as well as resistance to air and CO2. Thus, in some preferred embodiments, the electrode precursor mixture comprises a mixture of carbon-based dry aggregate, a binder, and graphene. It will be appreciated that the graphene used in the electrode material is preferably graphene produced by a plasma reactor system as described herein in relation to graphene provided in a coating composition. Nonetheless, while graphene may be included in the electrode precursor material to which the coating composition is applied, this is optional, and the coating composition may be applied to any electrode precursor material that may be produced in a conventional way without the inclusion of graphene. It is also beneficial that, where a coating is provided, the coating may contain a higher graphene loading than the electrode material itself, which may increase resistance to air or CO2, without significantly changing the properties of the bulk electrode material. Thus, where the presence of graphene is mentioned in relation to the electrode precursor mixture or carbon electrode material that is coated, it will be appreciated that the graphene may be omitted. It will also be appreciated that by providing a coating composition comprising graphene on the carbon electrode material, the graphene at the surface of the electrode material may integrate with the electrode material (formed from dry aggregate and pitch) and benefits to sticking of packing materials and chemical resistance may still be obtained at the surface of the electrode, whilst also providing a suitably conductive coating. The process for producing the carbon electrode material, such as mixing the dry aggregate, binder and optionally graphene may be carried out in any suitable way. Prior to the baking step to produce the carbon electrode material, the components may be mixed in any suitable way. For example, an impeller mixer or similar may be used to mix the dry aggregate, binder and optionally graphene. In some embodiments, the dry aggregate, optionally including the graphene, may be pre-heated before mixing, for example in order to dry the dry aggregate prior to mixing. The optionally preheated dry aggregate may be mixed with the binder whilst heating the mixture, for example to a temperature above the Mettler softening point of the binder, for example at least 40 °C above the softening point, such as about 60 °C above the softening point of the binder. The mixing may be any suitable length of time to increase the temperature of the binder and to provide a homogeneous mixture, for example at least 5 minutes, or at least 10 minutes. The electrode precursor mixture may be moulded into the required shape in a mould and typically under elevated pressure in a press, which may be pre-heated. The mixture may for example be moulded in a press at a pressure of about 400 bar. The moulded electrode precursor material is then baked to form the carbon electrode material. The baked carbon electrode material may in some instances be referred to as a prebaked electrode, as the electrode is baked before being used in a smelting process. The precursor mixture after being formed / pressed into shape but before baking may also be referred to as a “green electrode”. The step of baking the mixture to provide the carbon electrode material may be performed in any suitable way and the production of carbon electrodes is well known to the skilled person. For example, the electrode precursor material may be baked in any suitable furnace or oven. The step of baking the mixture to provide the carbon electrode material suitably comprises heating at a temperature of from 1000 °C to 1300 °C, for example from 1100 °C to 1200 °C. Such temperatures are typically used for the formation of carbon anodes for aluminium smelting, however, other temperatures may be used. For example, as will be appreciated, to provide a cathode material for aluminium smelting, a higher temperature of around 2000 to 3000 °C may be used. The step of baking the mixture to provide the carbon electrode material may therefore generally comprise heating at a temperature of from 1000 to 3000 °C. The step of baking the mixture to provide the carbon electrode material may comprise heating the mixture for at least 16 hours, for example for at least 24 hours, such as at least 36 hours, and it will be appreciated that such times refer to the time at the baking temperature applied for the majority of the baking process. As will be appreciated, the time may vary depending on the scale of the process, and at large scales, the baking step may be conducted over at least 100 hours or more, for example 200 hours or more. In addition, the temperature may suitably be ramped from ambient up to the baking temperature over a period of time of around 24 to 36 hours, for example ramping relatively quickly to 150 °C at 100 °C / h, then up to 300 °C at 10 °C / h, then up to around 1100 °C (or the chosen final baking temperature) at 50 °C / h. Where provided in the electrode precursor mixture, the graphene may be included in the mixture in any suitable quantity. Preferably, the graphene is present in an amount of from 0.05 wt.% to 10 wt.% of the electrode precursor mixture, for example from 0.08 wt.% to 5.0 wt.%, such as from 0.2 wt.% to 2.0 wt.% or from 0.4 wt.% to 1.0 wt.%. It has been found that even amounts of graphene below 1 wt.% can significantly improve the properties of the electrode material. It will be appreciated that the binder content may be varied depending on the binder used and the other components present. The binder may suitably be present in an amount of from 5 wt.% to 25 wt.% of the electrode precursor mixture, for example from 8 wt.% to 20 wt.%, such as from 10 wt.% to 16 wt.%. It will be appreciated that the binder content may be varied depending on the specific process and may, for example, be varied based on the scale of the process. For example, ata large scale (i.e. an industrial scale rather than a lab scale) the binder may be present in an amount of from 10 to 14 wt.%, for example 11 to 13 wt.%. The carbon-based dry aggregate typically makes up a majority of the content of the electrode precursor mixture. The carbon-based dry aggregate may suitably be present in an amount of from 60 wt.% to 90 wt.% of the electrode precursor mixture, for example from 70 wt.% to 90 wt.%, such as from 80 wt.% to 85 wt.%. In total, the carbon-based dry aggregate, the binder and optionally graphene may suitably comprise at least 90 wt.% of the electrode precursor mixture, preferably at least 95 wt.%, such as at least 98 wt.%. For example, the electrode precursor mixture may consist essentially of the carbon-based dry aggregate, the binder and optionally graphene. It will be appreciated that the mixing of each component may be performed in various ways. For example, the binder may be melted (or naturally in a liquid form) prior to mixing with the dry aggregate, or the binder may be a solid at ambient temperature and may be melted after mixing with the dry aggregate. As will also be appreciated, where included, the graphene may be added into the mixture in various ways and at different stages. For example, the binder may be melted / liquid, and the graphene mixed with the binder prior to mixing with the dry aggregate. Such a process may provide the graphene more localised within the binder, which may aid stabilisation and resistance of the portions of the mixture comprising the binder following baking. For example, the binder may after baking provide weak points in the electrode to reaction or corrosion by air, carbon dioxide or oxygen produced in the smelting process, and localising the graphene in the binder may help to mitigate such reactions. As will be appreciated, even where the graphene is not pre-mixed with the binder, it will nonetheless be at least in part disposed in the binder by mixing before baking. The graphene may for example, be added to the binder in an amount of from 0.01 wt.% to 20 wt.% of the binder. Pre-mixing the graphene with the binder may also aid in the handling of the graphene. For example, graphene as a fine powder may cause handling difficulties. By mixing the graphene with the binder, a more easily handled mixture (for example a substantially liquid mixture) may be obtained before mixing with the dry aggregate. It will be appreciated that where the graphene is mixed with binder prior to mixing with the dry aggregate, this may be only a portion of the binder, and additional binder may be provided separately to that mixed with the graphene. The graphene may be mixed with the dry aggregate prior to mixing with the binder, or may be mixed with the binder in solid form and dry aggregate prior to melting the binder. The graphene may for example, be added to the dry aggregate in an amount of from 0.01 wt.% to 10 wt.% of the dry aggregate mass. The graphene may be mixed with a specific portion of the dry aggregate prior to mixing with the remaining components. For example, the graphene may be mixed with a fines portion of the dry aggregate prior to mixing with the remaining components (for example, a fines portion as described previously herein). In this way, the components comprising fine powders may be combined for efficiency of handling, for example to minimise the number of different components that need to be handled in a particular way due to their form. The graphene may for example, be added to the fines in an amount of from 0.1 wt.% to 20 wt.% of the fines portion. As discussed, the carbon electrode material is for aluminium smelting. The carbon electrode material may be produced in the form of an electrode or the carbon electrode material may be further processed to provide an electrode that can be used in a smelting process. The carbon electrode material may suitably be an anode material for aluminium smelting. The carbon electrode material may instead be a cathode material for aluminium smelting, in which, as a person of skill in the art will appreciate, slightly different form of dry aggregate may be used. For example, when making an anode, the dry aggregate may comprise a more amorphous form of coke to prevent the formation of large crystalline domains, while for a cathode a coke that more easily forms larger graphitic crystals may be used, such as needle coke. A carbon electrode material comprising graphene material may, as discussed previously, provide advantageous properties as compared to an electrode that does not comprise graphene. For example, the electrode material may have: a specific electrical resistance, measured according to ISO 11713:2000, of 58 pQm or less, preferably 56 pQm or less; and / or a compressive strength, measured according to ISO 18515:2014 of 42 MPa or more, preferably 44 MPa or more; and / or a static elasticity modulus, measured according to ISO 18515:2014, of 4.3 GPa or more, preferably 4.5 or more; and / or a thermal conductivity, measured according to ISO 12987:2004, of 3.0 W / mK or more; and / or an air reactivity residue, measured according to ISO 12989-1:2000, of 73 % or more, preferably 75% or more, for example 77% or more; and / or an air reactivity dust value, measured according to ISO 12989-1:2000, of less than 7 %, for example 6 % or less, or 5 % or less; and / or an air reactivity loss, measured according to ISO 12989-1:2000, of less than 18 %, for example 17 % or less, such as 16 % or less; and / or a CO2 reactivity residue, measured according to ISO 12988-1:2000, of 95.25 % or more, preferably 95.5 % or more, for example 96.0 % or more; and / or a CO2 reactivity dust value, measured according to ISO 12988-1:2000, of less than 0.6 %, for example 0.5 % or less, or 0.4 % or less; and / or a CO2 reactivity loss, measured according to ISO 12988-1:2000, of less than 4.2 %, for example 4.1 % or less, such as 4.0 % or less; and / or a baked apparent density, measured according to ISO 12985-1:2018, of 1.560 kg / dm3 or more. ln addition, the carbon electrode material can provide a decrease in the sticking of packing materials, as well as an increase in the static elasticity modulus compared to an electrode without graphene. The air permeability may also be reduced by including graphene in the electrode materials, which may help to reduce degradation of the electrode material by reactions with air or other gases. It will be appreciated that by providing a coating composition comprising graphene, some of the benefits of integrating graphene into the electrode material may be obtained locally at the surface of the electrode material. This may be particularly efficient in that a conventional carbon electrode may be provided with only a coating comprising graphene (which is a relatively expensive material to include in the bulk electrode material), which can reduce air and CO2 reactivity at the exposed surface, as well as reducing air permeability, whilst also providing a coating having advantageous conductivity to avoid increased energy consumption due to a coating increasing electrical resistance of the electrode. A further aspect provides a process of producing a carbon electrode material for aluminium smelting comprising: providing a carbon electrode material; and applying a coating composition comprising graphene to the carbon electrode material. As will be appreciated, the graphene and / or the carbon electrode material may be as defined elsewhere herein. For example, the process of this aspect may comprise providing a carbon electrode material (i.e. a baked electrode material) that was produced separately, such as in a different location, and then applying a coating composition to the carbon electrode material (which may be done as described elsewhere herein). As will also be appreciated, while a further baking step may in some instances be performed, use of the coated electrode in an aluminium smelting process will bake the coated electrode in situ due to the elevated temperatures inherent to an aluminium smelting process. A further aspect provides a carbon electrode material for aluminium smelting comprising a baked electrode precursor mixture of carbon-based dry aggregate and a binder, and a coating comprising graphene. As will be appreciated, the coating and / or the electrode precursor mixture may be as described elsewhere herein and may be produced by the processes described herein. A further aspect provides an aluminium smelting system comprising an anode or cathode made from a carbon electrode material as described herein. A further aspect provides the use of graphene as a coating to enhance the corrosion resistance and / or the electrical resistance of a carbon electrode for aluminium smelting. A will be appreciated, the graphene may be in the form of a coating composition as described elsewhere herein. Brief description of figures The invention will now be further described, by way of example only, and with reference to the following Examples and the drawings in which: Figure 1 is a graph showing the specific electrical resistance of the carbon electrode material for examples having varying graphene and pitch content. Figure 2 is a graph showing the compressive strength of the carbon electrode material for examples having varying graphene and pitch content. Figure 3 is a graph showing the sticking of packing materials for the carbon electrode material for examples having varying graphene and pitch content. Figure 4 is a graph showing the baked apparent density for the carbon electrode material for examples having varying graphene and pitch content. Figure 5 is a graph showing the specific electrical resistance of the carbon electrode material for examples having varying graphene and optimised pitch content. Figure 6 is a graph showing the compressive strength of the carbon electrode material for examples having varying graphene and optimised pitch content. Figure 7 is a graph showing CO2 reactivity residue of the carbon electrode material for examples having varying graphene and optimised pitch content. Figure 8 is a graph showing air reactivity residue of the carbon electrode material for examples having varying graphene and optimised pitch content. Figure 9 shows a graph of lateral flake size as measured by SEM for graphene produced by a plasma reactor system. Figure 10 shows a Raman spectrum measured for graphene produced by a plasma reactor system. Examples Samples of a carbon electrode material were produced in which graphene was introduced. The Examples show the effect of the presence of graphene when combined with the materials that are typically used to prepare carbon aluminium smelting electrodes. For example, this can be used to demonstrate the properties of a material at the surface of a carbon electrode when graphene is used to provide a coating at the surface of a carbon electrode formed from carbon-based dry aggregate and binder. Dry aggregate The dry aggregate used in the following Examples was a mixture of two petroleum cokes and a butts comprising recycled carbon anode material. The size distribution of the particles in the dry aggregate are shown below in Table 1. Coke 1 had a sulfur content of 2.5 %, Coke 2 had a sulfur content of 1.2 % and the Butts had a sulfur content of 1.8 %. The coke and butts also contained 100 to 200 ppm vanadium and the butts contained 350 ppm sodium. Table 1: Dry aggregate Fraction [mm] Coke 1 [%] Coke 2 [%] Butts [%] Total [%] 8-4 2.7 1.3 10 14 4-2 5.3 2.7 6 14 2-1 6.7 3.3 4 14 1-0.5 7.3 3.7 3 14 0.5-0.25 8 4 2 14 Fines (3500 Blaine) 20 10 0 30 Total 50 25 25 100 Binder 5 The binder used was a standard coal tar pitch having properties as shown in Table 2 below. The coal tar pitch was crushed to a particle size of <4mm and used in the solid state. Table 2: Coal tar pitch Method Unit Result Softening Point Mettler ISO 5940-2 °C 113.5 Viscosity at 160°C ASTM D5018 cP 1895 Real Density in Helium ISO 21687 kg / dm3 1.313 Coking Value ISO 6998 % 58.2 Quinoline Insoluble ISO 6791 % 6.8 Toluene Insoluble ISO 6376 % 26.9 Ash Content ISO 8006 % 0.19 XRF Analysis S ISO 12980 % 0.61 Si PPm 184 Fe PPm 97 Al PPm 100 Na ppm 150 Ca ppm 50 Pb ppm 170 Zn ppm 241 10 Graphene The graphene powder used in the following examples was provided by operation of a plasma reactor system as described previously herein. The process gas fed to the system was methane and graphene samples were separated from hydrogen by filtration and collected. Figures 7 and 8 show SEM and Raman spectroscopy analysis of the graphene which was produced by conversion of methane in a plasma reactor system. As can be seen, in Figure 7, the graphene produced and measured by SEM shows a lateral flake size distribution with no substantial presence of flakes larger than 500 nm, and where the majority of flakes are larger than 100 nm. Figure 8 shows a Raman spectrum of the graphene. The Raman spectra were measured using an excitation laser wavelength of 532nm at a power <0.25mW under a 50x objective lens. The strong 2D peak in the spectra indicates a high crystalline structure and low defect density is confirmed by the D peak. General procedure The specified amount of graphene (0.00 wt.% to 1.00 wt.%) was added to a 4kg dry aggregate portion, where the percentage of graphene is relative to the total dry aggregate (i.e. 1 wt.% graphene is 40.4 g of graphene with 4 kg of dry aggregate). The graphene content as a percentage of the total electrode precursor mixture is accordingly slightly lower than the graphene content as referred to in Tables 3 to 8 below when the binder is also accounted for. This dry mixture was heated at 200 °C in a steel container overnight in a drying cabinet. The hot dry aggregate was transferred to an intensive Eirich (RTM) impeller mixer of 10 litres capacity and the specified amount of the coal tar pitch was added and mixed for 10 minutes with the dry aggregate and graphene, in order to raise the pitch temperature to about 173 °C. Portions of about 320 g were pressed in a pre-heated (100 °C) floating mould press for 30 seconds at a specific pressure of 400 bar to produce cylinders with approximately 50 mm diameter and 100 mm length. The cylinders were baked in an electrically heated furnace (5 kW) in a steel recipient and covered by packing material pre-sized to 1-2 mm. The heating was ramped from ambient to 150 °C at a rate of 100 °C / h, 150 to 300 °C at 10 °C / h, 300 to 1100 °C at 50 °C / h, followed by baking at 1100 °C for 20 hours. Samples were produced at graphene levels of 0.00 wt.% (comparative - no graphene), 0.10 wt.%, 0.25 wt.%, 0.50 wt.%, and 1.00 wt.% (relative to the dry aggregate). Each sample was also produced with a pitch content of 14.5 wt.%, 15.0 wt.%, 15.5 wt.%, 16.0 wt.% and 16.5 wt.% (relative to the total dry aggregate / graphene and binder). The samples were analysed and the results shown in Tables 3 to 7 below. The column 5 labelled “n” shows how many different samples were measured and averaged for each measurement. It is noted that baking loss, baking shrinkage and sticking of packing material were measured by internal methods, where the same method was used for each test to provide a fair comparison. The sticking of packing materials may for example measured by weighing the electrode material after baking, including packing material stuck 10 to it, and then weighing again after removing the packing material. Baking loss is the mass lost between the “green” electrode before baking, and the baked weight after removing packing material. Baking shrinkage is the dimensional change in the electrode material after baking. Table 3 shows a comparative example where no graphene was added into the electrode material. 15 -29- 0% Graphene Addition Pitch content 14.5% 15.0% 15.5% 16.0% 16.5% Method Unit AVG 2STD AVG 2STD AVG 2STD AVG 2STD AVG 2STD n Green Apparent Density ISO 12985-1 kg / dm3 1.647 0.007 1.651 0.004 1.656 0.004 1.667 0.005 1.672 0.004 10 Baking Loss % 5.5 0.2 5.7 0.1 6.0 0.2 6.2 0.3 6.6 0.2 10 Baking Shrinkage % -0.2 0.2 -0.1 0.2 -0.6 0.2 -1.0 0.2 -1.5 0.3 10 Sticking of Packing Material % 0.3 0.3 0.5 0.2 1.0 0.6 4.0 1.6 7.1 1.6 10 Baked Apparent Density ISO 12985-1 kg / dm3 1.553 0.003 1.555 0.004 1.555 0.004 1.548 0.005 1.539 0.004 10 Specific Electrical Resistance ISO 11713 pOm 61.9 2.7 58.8 1.4 54.8 1.4 54.1 0.9 53.4 0.6 10 Compressive Strength ISO 18515 MPa 38 3 41 0 45 2 46 2 48 1 3 Static Elasticity Modulus ISO 18515 GPa 3.7 0.2 4.2 0.1 4.4 0.3 4.4 0.5 4.8 0.4 3 Coefficient of Thermal Expansion ISO 14420 10'6 / K 4.16 - 4.03 - 4.01 - 4.20 - 4.17 - 1 Thermal Conductivity ISO 12987 W / mK 2.8 0.5 2.8 0.3 2.9 0.3 2.8 0.6 3.0 0.7 4 Real Density in Xylene ISO 9088 kg / dm3 2.083 - 2.081 - 2.081 - 2.080 - 2.085 - 1 Crystallite Size Lc ISO 20203 A 32.3 - 32.5 - 32.2 - 31.7 - 32.8 - 1 Air Permeability ISO 15906 nPm 0.96 0.24 0.71 0.12 0.98 0.29 1.17 0.08 1.43 0.27 4 CO2 Reactivity Residue ISO 12988-1 % 95.2 0.1 95.2 0.2 95.1 0.2 95.3 0.5 95.1 0.5 3 Dust % 0.6 0.0 0.6 0.0 0.6 0.1 0.6 0.2 0.7 0.4 3 Loss % 4.2 0.1 4.2 0.2 4.2 0.1 4.1 0.4 4.2 0.3 3 Air Reactivity Residue ISO 12989-1 % 72.2 3.1 71.7 0.9 72.7 2.4 74.7 3.4 73.6 0.6 3 Dust % 8.5 0.7 8.3 0.4 7.6 0.3 7.6 0.7 7.7 0.2 3 Loss % 19.3 2.5 19.9 0.7 19.8 2.0 17.8 2.6 18.8 0.8 3 XRF Analysis S ISO 12980 % 1.81 - 1.72 - 1.68 - 1.70 - 1.69 - 1 V ppm 158 - 149 - 146 - 144 - 147 - 1 Ni ppm 124 - 117 - 116 - 114 - 116 - 1 Si PPm 172 - 102 - 108 - 98 - 104 - 1 Fe PPm 195 - 175 - 195 - 173 - 198 - 1 Al PPm 249 - 91 - 100 - 101 - 98 - 1 Na PPm 200 - 163 - 170 - 168 - 199 - 1 Ca PPm 79 - 70 - 72 - 72 - 76 - 1 P PPm 6 - 6 - 7 - 8 - 6 - 1 -os- 0.10% Graphene Addition Pitch content 14.5% 15.0% 15.5% 16.0% 16.5% Method Unit AVG 2STD AVG 2STD AVG 2STD AVG 2STD AVG 2STD n Green Apparent Density ISO 12985-1 kg / dm3 1.643 0.003 1.655 0.003 1.661 0.003 1.666 0.003 1.672 0.004 10 Baking Loss % 5.3 0.1 5.4 0.2 5.7 0.1 5.9 0.3 6.1 0.2 10 Baking Shrinkage % 0.6 0.2 -0.3 0.2 -0.1 0.1 -0.4 0.2 -0.5 0.2 10 Sticking of Packing Material % 0.3 0.2 0.2 0.2 0.3 0.3 0.7 0.3 2.8 1.7 10 Baked Apparent Density ISO 12985-1 kg / dm3 1.564 0.003 1.562 0.004 1.565 0.004 1.562 0.004 1.563 0.003 10 Specific Electrical Resistance ISO 11713 pQm 60.7 1.2 58.7 1.3 55.9 1.2 53.8 1.1 52.7 0.6 10 Compressive Strength ISO 18515 MPa 39 1 43 2 44 1 46 2 51 1 3 Static Elasticity Modulus ISO 18515 GPa 3.5 0.1 4.1 0.9 4.1 0.2 4.6 0.9 5.1 0.3 3 Coefficient of Thermal Expansion ISO 14420 lO^ / K 4.03 - 4.12 - 4.04 - 4.24 - 3.99 - 1 Thermal Conductivity ISO 12987 W / mK 3.23 0.30 3.14 0.29 3.32 0.34 3.04 0.49 3.18 0.14 4 Real Density in Xylene ISO 9088 kg / dm3 2.083 - 2.076 - 2.081 - 2.079 - 2.078 - 1 Crystallite Size Lc ISO 20203 A 31.7 - 31.1 - 31.9 - 32.1 - 31.4 - 1 Air Permeability ISO 15906 nPm 0.88 0.33 1.75 1.69 1.04 0.31 1.33 0.89 1.94 0.45 4 CO2 Reactivity Residue ISO 12988-1 % 95.6 1.0 95.3 1.2 95.6 0.4 95.7 0.5 95.9 0.2 3 Dust % 0.5 0.5 0.5 0.5 0.4 0.2 0.3 0.2 0.3 0.1 3 Loss % 4.0 0.5 4.2 0.7 4.0 0.3 4.0 0.3 3.8 0.1 3 Air Reactivity Residue ISO 12989-1 % 77.4 3.7 74.9 5.4 77.7 3.0 79.4 3.5 78.1 4.8 3 Dust % 6.7 1.2 7.8 1.5 6.3 0.8 5.5 1.2 5.8 1.4 3 Loss % 15.9 2.4 17.3 3.9 15.9 2.1 15.0 2.1 16.1 3.4 3 XRF Analysis S ISO 12980 % 1.72 - 1.76 - 1.77 - 1.76 - 1.78 - 1 V PPm 147 - 154 - 150 - 149 - 154 - 1 Ni ppm 118 - 118 - 119 - 118 - 119 - 1 Si PPm 92 - 100 - 94 - 91 - 136 - 1 Fe PPm 181 - 215 - 194 - 195 - 240 - 1 Al PPm 105 - 103 - 107 - 113 - 191 - 1 Na PPm 166 - 157 - 177 - 167 - 161 - 1 Ca PPm 73 - 70 - 74 - 72 - 79 - 1 P PPm 6 - 6 - 6 - 7 - 6 - 1 -31 0.25% Graphene Addition Pitch content 14.5% 15.0% 15.5% 16.0% 16.5% Method Unit AVG 2STD AVG 2STD AVG 2STD AVG 2STD AVG 2STD n Green Apparent Density ISO 12985-1 kg / dm3 1.643 0.005 1.648 0.007 1.663 0.003 1.666 0.004 1.677 0.003 10 Baking Loss % 5.3 0.2 5.5 0.3 5.8 0.2 6.1 0.2 6.4 0.1 10 Baking Shrinkage % 0.7 0.1 0.5 0.5 0.0 0.3 -0.9 0.3 -2.2 0.2 10 Sticking of Packing Material % 0.4 0.2 0.4 0.3 1.1 0.6 2.4 1.6 7.5 2.6 10 Baked Apparent Density ISO 12985-1 kg / dm3 1.567 0.003 1.566 0.006 1.566 0.004 1.550 0.005 1.535 0.005 10 Specific Electrical Resistance ISO 11713 pOm 59.7 2.0 56.6 2.6 54.5 1.4 54.2 1.3 55.0 0.6 10 Compressive Strength ISO 18515 MPa 42 2 46 1 49 1 48 2 48 0 3 Static Elasticity Modulus ISO 18515 GPa 3.9 0.8 4.3 0.6 4.7 0.3 4.6 1.1 4.7 0.7 3 Coefficient of Thermal Expansion ISO 14420 io’7k 4.07 - 4.23 - 4.16 - 4.36 - 4.05 - 1 Thermal Conductivity ISO 12987 W / mK 2.96 0.19 2.91 0.21 3.11 0.40 3.08 0.31 2.84 0.80 4 Real Density in Xylene ISO 9088 kg / dm3 2.072 - 2.078 - 2.083 - 2.077 - 2.075 - 1 Crystallite Size Lc ISO 20203 A 31.6 - 32.3 - 32.1 - 32.3 - 31.3 - 1 Air Permeability ISO 15906 nPm 0.82 0.25 0.75 0.13 0.84 0.09 1.79 1.09 2.23 0.27 4 CO2 Reactivity Residue ISO 12988-1 % 95.2 1.1 95.5 0.4 95.5 0.5 95.4 0.3 95.2 0.3 3 Dust % 0.5 0.4 0.3 0.1 0.3 0.2 0.5 0.0 0.6 0.2 3 Loss % 4.3 0.7 4.2 0.3 4.1 0.2 4.2 0.2 4.2 0.2 3 Air Reactivity Residue ISO 12989-1 % 75.9 1.9 77.8 4.6 80.0 2.6 74.2 2.4 74.3 4.3 3 Dust % 7.4 0.8 6.2 1.9 5.3 1.2 7.3 0.9 7.7 1.0 3 Loss % 16.7 1.1 16.0 2.7 14.6 1.4 18.5 1.5 18.0 3.1 3 XRF Analysis S ISO 12980 % 1.76 - 1.70 - 1.70 - 1.68 - 1.71 - 1 V PPm 152 - 145 - 147 - 142 - 146 - 1 Ni ppm 120 - 114 - 115 - 113 - 116 - 1 Si ppm 105 - 97 - 88 - 103 - 103 - 1 Fe ppm 201 - 182 - 181 - 172 - 182 - 1 Al PPm 111 - 103 - 92 - 104 - 109 - 1 Na PPm 165 - 148 - 150 - 163 - 186 - 1 Ca PPm 73 - 69 - 71 - 75 - 85 - 1 P PPm 6 - 6 - 6 - 6 - 6 - 1 -32 - 0.5% Graphene Addition Pitch content 14.5% 15.0% 15.5% 16.0% 16.5% Method Unit AVG 2STD AVG 2STD AVG 2STD AVG 2STD AVG 2STD n Green Apparent Density ISO 12985-1 kg / dm3 1.648 0.003 1.651 0.003 1.663 0.005 1.667 0.004 1.672 0.005 10 Baking Loss % 5.6 0.2 5.7 0.3 6.0 0.1 6.0 0.2 6.1 0.3 10 Baking Shrinkage % -0.2 0.1 -0.1 0.1 -0.3 0.3 0.0 0.2 -0.5 0.2 10 Sticking of Packing Material % 0.8 0.3 1.8 1.5 2.3 1.2 1.2 0.7 2.9 1.4 10 Baked Apparent Density ISO 12985-1 kg / dm3 1.553 0.003 1.556 0.004 1.559 0.003 1.567 0.003 1.562 0.004 10 Specific Electrical Resistance ISO 11713 pOm 57.8 1.1 55.3 1.1 53.6 0.0 52.9 0.9 52.5 0.9 10 Compressive Strength ISO 18515 MPa 43 1 47 2 49 1 51 4 51 1 3 Static Elasticity Modulus ISO 18515 GPa 4.1 0.2 4.3 0.2 4.5 0.6 4.7 0.3 4.9 0.2 3 Coefficient of Thermal Expansion ISO 14420 io7k 4.09 - 4.10 - 4.21 - 4.30 - 4.13 - 1 Thermal Conductivity ISO 12987 W / mK 3.17 0.30 3.37 0.32 3.08 0.35 3.31 0.54 2.97 0.24 4 Real Density in Xylene ISO 9088 kg / dm3 2.077 - 2.081 - 2.079 - 2.076 - 2.074 - 1 Crystallite Size Lc ISO 20203 A 32.3 - 32.9 - 31.2 - 31.9 - 31.4 - 1 Air Permeability ISO 15906 nPm 0.81 0.21 0.86 0.25 1.20 0.44 0.96 0.29 1.56 0.35 4 CO2 Reactivity Residue ISO 12988-1 % 95.3 0.2 95.3 0.4 95.4 1.1 96.1 0.3 96.0 0.5 3 Dust % 0.6 0.2 0.4 0.3 0.6 0.6 0.2 0.2 0.2 0.2 3 Loss % 4.1 0.0 4.3 0.3 4.0 0.5 3.7 0.3 3.8 0.2 3 Air Reactivity Residue ISO 12989-1 % 76.1 2.0 75.0 2.6 80.1 4.8 79.9 3.9 79.6 7.6 3 Dust % 6.7 0.5 7.0 0.7 5.2 1.8 4.9 1.4 5.2 3.3 3 Loss % 17.2 1.5 18.0 2.0 14.8 2.9 15.2 2.5 15.3 4.3 3 XRF Analysis S ISO 12980 % 1.69 - 1.74 - 1.68 - 1.72 - 1.73 - 1 V ppm 144 - 153 - 144 - 151 - 149 - 1 Ni ppm 115 - 119 - 116 - 118 - 118 - 1 Si PPm 97 - 106 - 104 - 118 - 103 - 1 Fe PPm 184 - 186 - 178 - 179 - 180 - 1 Al PPm 97 - 119 - 95 - 120 - 95 - 1 Na PPm 170 - 198 - 182 - 162 - 158 - 1 Ca PPm 76 - 73 - 70 - 74 - 71 - 1 P PPm 6 - 7 - 7 - 6 - 7 - 1 -33- 1% Graphene Addition Pitch content 14.5% 15.0% 15.5% 16.0% 16.5% Method Unit AVG 2STD AVG 2STD AVG 2STD AVG 2STD AVG 2STD n Green Apparent Density ISO 12985-1 kg / dm3 1.653 0.003 1.656 0.003 1.662 0.004 1.673 0.003 1.677 0.006 10 Baking Loss % 5.5 0.1 5.5 0.1 5.6 0.2 6.0 0.1 6.2 0.2 10 Baking Shrinkage % -0.1 0.2 0.0 0.2 -0.1 0.2 -0.5 0.2 -0.5 0.3 10 Sticking of Packing Material % 0.3 0.1 0.3 0.1 0.4 0.2 0.5 0.3 1.2 0.6 10 Baked Apparent Density ISO 12985-1 kg / dm3 1.559 0.004 1.565 0.004 1.567 0.004 1.565 0.004 1.564 0.003 10 Specific Electrical Resistance ISO 11713 pfim 56.8 1.4 54.8 0.9 55.2 1.1 53.6 0.0 52.4 0.8 10 Compressive Strength ISO 18515 MPa 46 1 49 2 51 1 52 2 55 4 3 Static Elasticity Modulus ISO 18515 GPa 4.3 0.3 4.4 0.5 4.9 0.3 4.7 0.6 4.9 0.2 3 Coefficient of Thermal Expansion ISO 14420 10'7k 4.50 - 4.08 - 4.19 - 4.37 - 4.39 - 1 Thermal Conductivity ISO 12987 W / mK 3.31 0.66 3.77 0.11 3.64 0.11 3.63 0.57 3.79 0.06 4 Real Density in Xylene ISO 9088 kg / dm3 2.076 - 2.073 - 2.069 - 2.072 - 2.070 - 1 Crystallite Size Lc ISO 20203 A 32.3 - 32.4 - 31.3 - 32.6 - 32.2 - 1 AirPermeability ISO 15906 nPm 0.76 0.25 0.60 0.08 0.95 0.10 0.85 0.13 1.41 0.55 4 CO2 Reactivity Residue ISO 12988-1 % 95.3 0.6 95.6 0.7 96.1 0.5 96.0 0.2 96.0 0.5 3 Dust % 0.5 0.2 0.4 0.4 0.3 0.3 0.3 0.2 0.2 0.2 3 Loss % 4.2 0.3 3.9 0.4 3.6 0.2 3.7 0.1 3.8 0.2 3 Air Reactivity Residue ISO 12989-1 % 76.4 4.2 81.6 4.4 83.2 4.7 81.7 3.1 80.3 2.9 3 Dust % 6.1 1.2 4.3 1.6 3.9 1.6 4.1 0.9 4.6 1.2 3 Loss % 17.5 3.0 14.0 2.8 12.9 3.2 14.2 2.1 15.1 1.6 3 XRF Analysis S ISO 12980 % 1.74 - 1.73 - 1.70 - 1.74 - 1.70 - 1 V PPm 152 - 153 - 147 - 151 - 148 - 1 Ni ppm 118 - 116 - 116 - 118 - 116 - 1 Si ppm 98 - 97 - 90 - 94 - 98 - 1 Fe PPm 191 - 183 - 174 - 234 - 181 - 1 Al PPm 111 - 106 - 107 - 92 - 103 - 1 Na PPm 185 - 181 - 170 - 157 - 168 - 1 Ca PPm 72 - 73 - 72 - 71 - 70 - 1 P PPm 7 - 6 - 6 - 7 - 5 - 1 As can be seen from the data in Tables 3 to 7, by the combination of even very small proportions of graphene with the electrode material, multiple properties of the material may be improved. In particular, the specific electrical resistance as shown graphically in Figures 1 and 5 shows a significant decrease upon the inclusion of graphene. While at higher pitch content the electrical resistance of the comparative 0% graphene sample can also be lowered, this results in significantly worse sticking of packing materials as shown graphically in Figure 3, and lower baked apparent density as shown graphically in Figure 4. As shown graphically in Figures 2 and 6, the compressive strength of the material is also improved by the inclusion of graphene. The air and CO2 resistance is also improved as shown by the above data and shown graphically in relation to the residue measurement in Figures 7 and 8. This can not only improve the lifetime of the electrodes, by reducing electrode consumption, but can also maintain quality of the electrode material to benefit its use as a butts material if recycled. The thermal conductivity is also improved, which can be favourable in terms of thermal shock resistance of the electrode. The pitch level in each example was also varied and when balancing binder content with baked apparent density and sticking of packing materials. Table 8 below collates the data from Tables 3 to 7 for each example of a particular graphene content at optimal binder content. From this it can be seen that many properties are improved using graphene when optimised for the binder content. In particular, at a 1 wt.% addition of graphene, as compared to no graphene, the specific electrical resistance is 4.3 pQm lower (around 7% decrease), the thermal conductivity is 0.7 W / mK higher (around 25 % increase), the compressive strength is 10 MPa higher (around 24 % increase), the static elasticity modulus is 0.5 GPa higher (around 12 % increase), the air reactivity residue is 7.4% higher (around 10 % increase), and the CO2 reactivity residue is 0.7 % higher (around 1 % increase, but note that this is an increase from about 95 % to 96 %, which is 20 % of the maximum possible increase going up from 95 % towards 100 %). Figures 5 to 8 show comparisons of some of the properties of the material at optimised pitch content. Table 8 Method Unit Graphene added - % 0 0.1 0.25 0.5 1 Optimum Pitch Content - % 15 15.5 15.5 16 16 Green Apparent Density ISO 12985-1 kg / dm3 1.651 1.661 1.663 1.667 1.673 Baking Loss % 5.7 5.7 5.8 6 6 Baking Shrinkage % -0.1 -0.1 0 0 -0.5 Sticking of Packing Material % 0.5 0.3 1.1 1.2 0.5 Baked Apparent Density ISO 12985-1 kg / dm3 1.555 1.565 1.566 1.567 1.565 Specific Electrical Resistance ISO 11713 pQm 58.8 55.9 54.5 52.9 53.6 Compressive Strength ISO 18515 MPa 41 44 49 51 52 Static Elasticity Modulus ISO 18515 GPa 4.2 4.1 4.7 4.7 4.7 Coefficient of Thermal Expansion ISO 14420 10¼ 4.03 4.04 4.16 4.3 4.37 Thermal Conductivity ISO 12987 W / mK 2.75 3.32 3.11 3.31 3.63 Real Density in Xylene ISO 9088 kg / dm3 2.081 2.081 2.083 2.076 2.072 Crystallite Size Lc ISO 20203 A 32.5 31.9 32.1 31.9 32.6 Air Permeability ISO 15906 nPm 0.71 1.04 0.84 0.96 0.85 CO2 Reactivity Residue ISO 12988-1 % 95.2 95.6 95.5 96.1 96 Dust % 0.6 0.4 0.3 0.2 0.3 Loss % 4.2 4 4.1 3.7 3.7 Air Reactivity Residue ISO 12989-1 % 71.7 77.7 80 79.9 81.7 Dust % 8.3 6.3 5.3 4.9 4.1 Loss % 19.9 15.9 14.6 15.2 14.2 XRF Analysis S ISO 12980 % 1.72 1.77 1.7 1.72 1.74 V ppm 149 150 147 151 151 Ni ppm 117 119 115 118 118 Si ppm 102 94 88 118 94 Fe PPm 175 194 181 179 234 Al ppm 91 107 92 120 92 Na ppm 163 177 150 162 157 Ca PPm 70 74 71 74 71 P PPm 6 6 6 6 7 As will be appreciated, where a graphene-containing coating composition is used, a material formed from a combination of graphene with the dry aggregate and binder will be 5 present at the surface of the electrode. In particular, the improvements shown in the above Examples in relation to CO2 and air reactivity, as well as specific electrical resistance, show that by forming a graphene containing material at the electrode surface, the resistance of the electrode to reactions with the gases produced in the smelting process may be improved whilst maintaining advantageous electrical properties.

Claims

1. A process of producing a carbon electrode material for aluminium smelting comprising:providing an electrode precursor mixture of carbon-based dry aggregate and a binder;providing a coating composition comprising graphene; andbaking the electrode precursor mixture to form the carbon electrode material;wherein the coating composition is applied to the electrode precursor mixture prior to baking or applied to the carbon electrode material after baking the electrode precursor mixture.

2. A process according to claim 1, wherein the process comprises compacting the electrode precursor mixture in a mould prior to baking, wherein the coating composition is applied to the mould surface before introducing the electrode precursor mixture, and / or wherein the coating composition is applied to the compacted electrode precursor mixture.

3. A process according to claim 1, wherein the coating composition is applied to the carbon electrode material after baking the electrode precursor mixture.

4. A process according to any one of the preceding claims, wherein the coating composition comprises the graphene, and optionally a binder, dispersed in a liquid medium.

5. A process according to claim 4, wherein the liquid medium comprises a solvent, for example water or and organic solvent, for example an alcohol such as ethanol or isopropanol, n-methylpyrrolidone, xylene, methyl ethyl ketone, toluene, dimethyl acetamide, cyrene, dimethylformamide or dimethylsulfoxide.

6. A process according to claim 5, wherein the coating composition comprises the solvent and from 0.1 wt.% to 30 wt.% graphene, and optionally from 1 wt.% to 50 wt.% of a binder.

7. A process according to any one of claims 4 to 6, wherein the binder comprises a polymeric binder, for example cellulose or cellulose derivatives such as ethyl cellulose, lignin-based polymers, a phenolic polymer, polyetherimide, polyimide or polybenzimidazole.

8. A process according to claim 4, wherein the liquid medium comprises the binder, for example wherein the liquid medium comprises a tar such as coal tar pitch, petroleum pitch or bio-mass pitch, or a liquid or molten polymer or pre-polymer, for example a thermosetting polymer such as epoxy, polyester or phenolic, or a molten thermoplastic polymer such as polyamide or polyetherimide.

9. A process according to claim 8, wherein the coating composition comprises from 1 wt.% to 75 wt.% graphene and from 25 wt.% to 99 wt.% of the binder, preferably from 1 wt.% to 50 wt.% graphene and from 50 wt.% to 99 wt.% of the binder.

10. A process according to any one of the preceding claims, wherein the coating composition comprises dry graphene powder, for example wherein the graphene powder is applied to the surface of the carbon electrode material, to the surface of a mould in which the electrode precursor mixture is compacted, or to the surface of the electrode precursor mixture following compaction but before baking, for example by buffing the surface with graphene, spraying the graphene powder onto the surface, or compressing the graphene powder into the form of a sheet and applying the sheet to the surface.

11. A process according to any one of the preceding claims, wherein the graphene is produced by operation of a plasma reactor system configured to produce graphene from a carbon-containing process gas.

12. A process according to any one of the preceding claims, wherein at least 50 %, preferably at least 70 %, more preferably at least 90%, of the graphene has a lateral flake size of less than 500 nm.

13. A process according to any one of the preceding claims, wherein the specific surface area of the graphene is from 80 m2 / g to 500 m2 / g, preferably from 100 m2 / g to 350m2 / g.

14. A process according to any one of the preceding claims, wherein the Raman spectrum of the graphene exhibits one or more of: a D peak intensity to G peak intensity ratio, I(D) / l(G), of less than 1; a D’ peak intensity to G peak intensity ratio, l(D’) / l(G), of less than 0.5; a G peak having a full width at half maximum of from 25 to 50 cm-1; a 2D peak intensity to G peak intensity ratio, l(2D) / l(G), of greater than 0.65; and a single-component 2D peak.

15. A process according to any one of the preceding claims, wherein the graphene comprises at least 90 % carbon, preferably at least 95 % carbon, for example at least 98 % carbon.

16. A process according to any one of the preceding claims, wherein the step of baking the electrode precursor mixture to provide the carbon electrode material comprises heating at a temperature of from 900 °C to 1500 °C, for example from 1000 °C to 1200 °C, for example heating for at least 16 hours, for example for at least 24 hours, such as at least 36 hours.

17. A process according to any one of the preceding claims, wherein the electrode precursor mixture comprises a mixture of carbon-based dry aggregate such as coke, a binder such as coal tar pitch, and graphene.

18. A process according to claim 17, wherein graphene is present in the electrode precursor mixture in an amount of from 0.05 wt.% to 10 wt.%, for example from 0.08 wt.% to 5.0 wt.%, such as from 0.2 wt.% to 2.0 wt.% or from 0.4 wt.% to 1.0 wt.%; and / or the binder is present the electrode precursor mixture in an amount of from 5 wt.% to 25 wt.%, for example from 8 wt.% to 20 wt.%, such as from 10 wt.% to 16 wt.%; and / or the carbonbased dry aggregate is present in the electrode precursor mixture in an amount of from 70 wt.% to 90 wt.%, for example from 80 wt.% to 85 wt.%; for example wherein carbonbased dry aggregate, binder and graphene comprise at least 90 wt.% of the electrode precursor mixture, preferably at least 95 wt.%, such as at least 98 wt.%.

19. A process according to any one of the preceding claims, wherein the carbon electrode material is an anode material for aluminium smelting.

20. A process of producing a carbon electrode material for aluminium smelting comprising:providing a carbon electrode material; andapplying a coating composition comprising graphene to the carbon electrode material.

21. A process according to claim 20, wherein the process, the graphene and / or the carbon electrode material is as further defined in any one of claims 1 to 19.

22. A carbon electrode material for aluminium smelting comprising a baked electrode precursor mixture of carbon-based dry aggregate and a binder, and a coating comprising graphene.

23. An electrode material according to claim 22, wherein the coating and / or the electrode precursor mixture are as defined in any one of claims 4 to 16, 18 or 19.

24. An electrode material according to claim 22 or claim 23, wherein the carbon electrode material is produced by the process of any one of claims 1 to 21.

25. An aluminium smelting system comprising an anode or cathode made from the material of any of claims 22 to 24.

26. Use of graphene as a coating to enhance the corrosion resistance and / or the electrical resistance of a carbon electrode for aluminium smelting.

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