Process for producing granulate

Abstract

A process of producing a graphene-containing granulate from a graphene powder comprising mixing the graphene powder and a liquid medium to provide a graphene-liquid mixture and forming granules from t

Description

The present invention relates to the production of a granulate. In particular, the present invention relates to a process for producing a graphene-containing granulate from a graphene powder, where the graphene-containing granulate is produced from a grapheneliquid mixture. The present invention also relates to a process for producing a graphene-containing granulate by compaction and a system for producing a graphene-containing granulate from a graphene powder. Background Graphene is considered to be a valuable material with great potential due to its many advantageous properties including exceptional mechanical properties (high strength and elasticity), high electrical and thermal conductivity, impermeability to gases and transparency to light. As a result, graphene has found use across a wide range of applications. There are multiple ways that graphene can be produced. One method is to convert carbon-containing gas into graphene in a plasma reactor. For example, plasma-based system is disclosed in WO 2015 / 189643 A1 and was demonstrated to produce graphene from hydrocarbon gases including methane. Such plasma-based processes can produce graphene efficiently and at a commercial scale. The graphene produced by such processes, where it is formed from materials in the gas phase, is in the form of a fine powder. This material shows beneficial properties such as relatively high specific surface area. However, materials such as graphene in the form of a fine powder can pose multiple challenges when used in industrial processes. For example, fine powders can present handling problems due to the susceptibility of fine powders to static and otherwise sticking to surfaces, as well as presenting challenges for accurate dosing into a process. Fine powders can also pose safety risks that must be accounted for and mitigated. Dispersing graphene in a liquid may address problems of dust suppression. However, this may present additional problems as a fluid suspension of graphene may not be easily integrated into other processes or products in the way that a solid product can, and the liquid may not be compatible with the desired use. In addition, one of the benefits of graphene powder is its specific surface area, which is diminished by suspending the graphene in a liquid. Accordingly, there is a need to address the above-described problems. Summary An aspect of the invention provides a process of producing a graphene-containing granulate from a graphene powder comprising: mixing the graphene powder and a liquid medium to provide a graphene-liquid mixture; and forming granules from the grapheneliquid mixture to provide the graphene-containing granulate. By using a liquid medium in a granulation process for graphene in this way, it has been found that a granulate may be produced that avoids the problems that a fine dust or powder present. It has also been found that such a process can provide a product in which the handleability of the graphene is improved, surprisingly without significantly reducing the specific surface area of the graphene. The process also permits the production of easily handleable graphene granulate with a carrier or binder that is specifically tailored to a particular use case. As referred to herein, a granulate will generally be understood to refer to a solid material in particles have a size of 125 pm or more, and no more than 10 mm as measured by sieving (as described, for example in ASTM D1511-12(2023) or DIN 66165-2:2016). A granulate may therefore suitably refer to a material in which at least 80 wt.% of particles have a particle size, as measured by sieving, of from 125 pm to 10 mm, preferably at least 90 wt.% of particles, for example at least 95 wt.% of particles. While the process produces a graphene-containing granulate, it will be appreciated that there may nonetheless be remnants of finer powder in the product, that may for example be removed by sieving as necessary. The individual granules of the granulate may have any suitable shape and it will be appreciated that a granulate or granules as referred to herein suitably includes pellets, which may have a uniform shape such as spheroidal particles, or may include granules having irregular particle shapes. Preferably, the granulate comprises pellets having a generally spheroidal shape, for example, wherein the granulate is in the form of spheroidal pellets. The process comprises mixing the graphene powder and a liquid medium to provide a graphene-liquid mixture. In some preferred embodiments, the liquid medium comprises a solvent, and the process comprises forming granules from the graphene-liquid mixture and evaporating the solvent to form the graphene-containing granulate. This provides advantages in that a granulated graphene product may be formed in which the solvent used in the process for the production of the granulate is absent from the product. In this way, the graphene may be used in further processes in which the presence of the solvent is not desired or is detrimental, whilst maintaining handling benefits of avoiding graphene in the form of a fine powder. The solvent may be removed by any suitable method. For example, in preferred embodiments, granules are formed from the graphene-liquid mixture, and the granules are dried by application of heat and / or reduced pressure, for example in an oven or other suitable dryer. It has been found that forming granules from a graphene-solvent mixture (preferably a graphene-water mixture) using a mechanical mixer, and then evaporating the solvent, can produce a granulated product having surprisingly high specific surface area, despite compaction of the graphene powder into easily handled granules having relatively high density. It will be appreciated that properties of the graphene-containing granulate are suitably measured after removal of the solvent. The solvent may be any suitable solvent, for example the solvent may comprise 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. In preferred embodiments, the solvent comprises or consists essentially of water and / or an alcohol such as ethanol or isopropanol, preferably water. The graphene-liquid mixture may further comprise a binder. The binder may be any suitable binder that aids the formation of granules from the graphene-liquid mixture. It will nonetheless be appreciated that a binder, in addition to the solvent, is optional and in preferred embodiments a binder in addition to a solvent may not be used. The at least a portion of the binder may suitably remain as part of the granulate following removal of solvent. The binder may suitably be soluble in the liquid or solvent or may form a suspension, for example the solvent may be selected based on the binder to provide solubility of the binder. The binder may comprise a polymeric binder such as cellulose, lignins (such as lignosulphonates), or derivatives thereof such as hydroxyethyl cellulose, ethylcellulose or carboxymethyl cellulose, polyacrylic acid, polyvinyl alcohol, polyvinylpyrrolidone, phenolic polymers, thermoplastics such as polylactic acid, acrylonitrile butadiene styrene, polyetherimide, or polyamides. In some embodiments, the binder may comprise a tar such as coal tar pitch, petroleum pitch or bio-mass pitch. In this way, after solvent removal, a granulate comprising a mixture of graphene and the tar may be formed and may be particularly suited, for example, for integration into a process for forming carbon electrodes for aluminium smelting. The solvent may be selected to provide a graphene-liquid mixture that is fluid at ambient temperature (e.g. about 20 to 30 °C). The proportion of graphene in the graphene-liquid mixture may be suitably varied depending on the specific process, and it will be appreciated that the proportion of graphene in the graphene-liquid mixture may be varied depending on the liquid being used and, where used, the binder. In preferred embodiments, the graphene-liquid mixture comprises from 35 to 90 wt.% graphene, for example from 40 to 80 wt.% graphene, for example from 45 wt.% to 65 wt.% graphene. For example, the graphene-liquid mixture may comprise at least 30 wt.% graphene, for example at least 40 wt.% graphene, such as at least 45 wt.% graphene. The graphene-liquid mixture may comprise from 10 wt.% to 65 wt.% solvent, for example from 25 to 60 wt.% solvent, such as from 35 to 55 wt.% solvent. As discussed, the graphene-liquid mixture may further comprise a binder, which may for example be present in an amount of from 1 wt.% to 50 wt.%, and it will be appreciated that the quantity of the binder may suitable vary based on the particular binder that is used. Where a binder is used in addition to a solvent, the graphene content in the graphene-liquid mixture may be relatively higher than when a binder is not used, for example the graphene-liquid mixture may comprise at least 60 wt.% graphene, such as at least 70 wt.% graphene. In a preferred embodiment, the process comprises mixing the graphene powder and water to provide the graphene-liquid mixture, for example wherein the graphene-liquid mixture consists essentially of graphene and water. Preferably, the graphene-liquid mixture comprises from 35 to 65 wt.% graphene and from 35 to 65 wt.% water. In preferred embodiments, the graphene-liquid mixture comprises from 45 to 65 wt.% water and from 35 to 55 wt.% graphene, preferably from 55 to 65 wt.% water and from 35 to 45 wt.% graphene. In other preferred embodiments, the graphene-liquid mixture comprises from 45 to 65 wt.% graphene and from 35 to 65 wt.% water. The graphene and liquid medium may be mixed and formed into granules using any suitable apparatus, and it will be appreciated that apparatus forwet granulation of powders are known in the art. For example, the granules may be formed by mechanical mixing of the graphene powder and the liquid medium, for example using a high shear granulator, a fluidised be granulator, a pin mixer or a planetary centrifugal mixer, preferably a pin mixer or a planetary centrifugal mixer. It has been found that the formation of granules using a planetary centrifugal mixer can produce a particularly advantageous product. Planetary centrifugal mixers are known in the art and it will be appreciated that a planetary centrifugal mixer is a mixer in which a vessel holding the mixture to be processed is revolved in the manner of a centrifuge to provide a force towards a bottom end of the vessel, whilst also rotating the vessel about its own axis (e.g. an axis extending from the top to the bottom of the vessel). This produces dual asymmetric centrifuge forces on the mixture from rotation of the vessel itself and the base plate to which it is attached. Suitable planetary centrifugal mixers include mixers such as Speedmixer (RTM) devices as provided by Synergy Devices Ltd or mixers provided by THINKY. In particular, it has been found that forming granules from a graphene-solvent mixture (preferably a graphene-water mixture) using a mechanical mixer, and then evaporating the solvent, particularly using a planetary centrifugal mixer, can produce a granulated product having surprisingly high specific surface area, despite compaction of the graphene powder into easily handled granules having relatively high density. In a preferred embodiment, the granules are formed by mechanical mixing of the graphene powder and the liquid medium with a planetary centrifugal mixer, wherein forming the granules comprises mixing the graphene powder and the liquid medium at a first speed to provide a compacted mixture, followed by mixing the compacted mixture at a second speed, lower than the first speed, to form granules from the compacted mixture. The first speed induces relatively higher forces to compact and wet the graphene and increase density, while the lower second speed allows granules to form. It will be appreciated that the speed at which the mixer is operated can vary depending on the size of the mixer, for example because a larger mixer can apply equivalent forces to a material at a lower rotational speed. In addition, as will be appreciated, the rotation speed of a planetary centrifugal mixer may refer to the centrifugal rotation of the vessel around the machine, while the rotation of the vessel about its own axis will typically be proportional to the centrifugal rotation in a set ratio, for example varying in the range from 0.25 to 4. The process may comprise after forming granules, separating particles having a size larger than a selected upper limit by sieving, and mixing the separated particles again, for example at the first speed, to reduce the particle size before recombining with the granules having a size less than the selected upper limit. By way of example, the first speed may be at least 1600 rpm, for example at least 1800 rpm, such as at least 1900 rpm, for example 2000 rpm or more, such as from 1600 to 2400 rpm, for example from 1800 to 2200 rpm, for example from 1900 rpm to 2100 rpm. The planetary centrifugal mixer may be operated at the first speed for from 30 seconds to 10 minutes, preferably from 1 minute to 5 minutes, for example from 90 seconds to 3 minutes. In some preferred embodiments, the planetary centrifugal mixer may be operated at the first speed for at least 90 seconds, preferably at least 2 minutes, for example 3 minutes or more. It has been found that an initial high-speed compaction for such time periods can provide a granulate having a larger proportion of particles in the size range of 425 to 850 pm, which avoids overly fine powder whilst also avoiding large granules that may reduce flowability. The second speed may be less than 1800 rpm, for example less than 1700 rpm, such as 1600 rpm or less. In preferred embodiments, following processing at the second speed, the process comprises mixing the graphene powder and the liquid medium at a third speed, lower than the second speed. The third speed may be less than 1600 rpm, preferably less than 1400 rpm, such as 1200 rpm or less. Similarly, the process may optionally comprise further mixing the graphene powder and the liquid medium at a fourth speed, lower than the third speed and a fifth speed lower than the fourth speed, and so on. In some preferred embodiments, the process comprises mixing the graphene powder and the liquid medium at first, second and third speeds. The process may comprise, after mixing at the second speed, separating particles having a size larger than an upper limit by sieving, and mixing the separated particles again at the first speed to reduce the particle size before recombining with the particles having a size less than the upper limit, and optionally mixing the recombined mixture at the third speed. At each speed during and after mixing at the second speed, the mixing may be performed for from 30 seconds to 3 minutes, preferably from 1 minute to 2 minutes. The mixing can cause heat generation in the mixture, and so it has been found that multiple mixing steps can avoid the temperature becoming too high during mixing. It will nonetheless be appreciated that multiple mixing steps may not be used in some instances. In a preferred embodiment, the graphene-liquid mixture comprises from 45 to 65 wt.% water and from 35 to 55 wt.% graphene, and the granules are formed by mechanical mixing of the graphene powder and the water with a planetary centrifugal mixer, wherein forming the granules comprises mixing the graphene powder and the liquid medium at a first speed, followed by mixing at a second speed, wherein the second speed is less than 60% of the first speed, for example approximately 50%. For example, mixing the graphene powder and the liquid medium at a first speed of from 1500 to 1700 rpm, followed by mixing at a second speed of from 700 to 900 rpm. The mixing at the first speed and at the second speed may each comprise mixing for from 30 seconds to 120 seconds, for example from 45 seconds to 90 seconds, such as from 50 to 70 seconds. It has been found that mixing for increased time at the second speed can increase granule size and so the granule size may be adjusted based on the mixing time at the second, lower, speed. In some instances, mixing the graphene powder and the liquid medium at a first speed to provide a compacted mixture may be performed using a different mixer to the step of mixing the compacted mixture at a second speed to form granules. For example, in some preferred embodiments, a first mixer may be used to mix the graphene powder and the liquid medium to provide a compacted mixture, followed by mixing the compacted mixture using a second mixer to form granules. The second mixer is preferably a batch mixer such as a planetary centrifugal mixer. The first mixer is preferably a continuous mixer (i.e. a mixer that mixes graphene and the liquid medium as a continuous process), for example, a pin mixer. Such a process may increase efficiency by providing a higher density mixture from the continuous mixer for use in the batch mixer. The higher density can allow higher loading of the material into the batch mixer than if the batch mixer is used for both compaction and granulation steps. A graphene-containing granulate produced by mechanical mixing, followed by removal of solvent, particularly using a planetary centrifugal mixer, has been found to advantageously provide a granulate having a bulk density of from 300 to 800 kg / m3, preferably from 400 to 600 kg / m3, more preferably from 450 to 550 kg / m3, and / or a specific surface area of at least 100 m2 / g, preferably at least 105 m2 / g, for example at least 107 m2 / g. Specific surface area may suitably be measured according to ISO 9277:2022 or ASTM D6556-21, and bulk density may suitably be measured according to ISO 697:1981. It will be appreciated that the specific surface area may vary based on the nature of the graphene used in the process, and so the specific surface area of the graphene-containing granulate may be at least 85% of the specific surface area of the graphene prior to granulation, preferably at least 90% of the specific surface area of the graphene prior to granulation. It has been surprisingly found that a graphene-containing granulate retaining a high level of the original surface area may be formed by mechanical mixing of a graphene-solvent mixture, followed by removal of solvent. The graphene-containing granulate may also have a tap density of at least 300 kg / m3, for example at least 350 kg / m3, such as at least 400 kg / m3 as measured by ISO 3953:2011 and / or may have a hardness of at least 20 gF, for example at least 30 gF as measured by ASTM D5230-12. Thus, a further aspect provides a graphene-containing granulate, wherein the granulate comprises 90 wt.% or more graphene, preferably 95 wt.% or more graphene, for example 99 wt.% or more graphene. The granulate preferably comprises at least 80 wt.% of particles, preferably at least 90 wt.% of particles, more preferably at least 95 wt.% of particles having a particle size, as measured by sieving, of from 100 pm to 10 mm, preferably from 125 pm to 6 mm, for example from 200 pm to 5 mm, such as from 250 pm to 4 mm; a bulk density of from 300 to 800 kg / m3, preferably from 400 to 600 kg / m3, more preferably from 450 to 550 kg / m3 (measured according to ISO 697:1981); and / or a specific surface area of at least 100 m2 / g, preferably at least 105 m2 / g, such as at least 107 m2 / g. The graphene-containing granulate may also have a tap density of at least 300 kg / m3, for example at least 350 kg / m3, such as at least 400 kg / m3 as measured by ISO 3953:2011 and / or may have a hardness of at least 20 gF, for example at least 30 gF as measured by ASTM D5230-12, where hardness may preferably be in the range of from 20 gF to 60 gF, such as 30 gF to 50 gF, for example 35 gF to 45 gF. It has been found that a granule hardness in this range is particularly beneficial for the granulate. As will be appreciated, the graphene-containing granulate may be formed by the process described herein. In some preferred embodiments, the liquid medium is heated to form the graphene-liquid mixture. 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 biomass pitch, or a liquid or molten polymer or pre-polymer (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, or a wax. The liquid medium may, for example be heated to at least 80 °C, for example at least 100 °C or at least 130 °C for mixing with the graphene, and it will be appreciated that the precise temperature may vary depending on the liquid medium being used. Following mixing of the molten liquid medium with the graphene, the process suitably comprises forming granules from the graphene-liquid mixture and cooling the granules to form the graphene-containing granulate. In this way, graphene-containing granules can be formed that can be used to efficiently integrate graphene into a process whilst avoiding the need to address problems relating to handling of fine powders. For example, a masterbatch of graphene-containing granules may be produced for integration into a further process. The graphene-containing granules may for example be meltable by heating, to provide a fluid mixture that can be readily mixed into other liquid components. In a preferred embodiment, the liquid medium is a tar such as coal tar pitch, petroleum pitch or bio-mass pitch, preferably coal tar pitch. Solid granules can be formed at around ambient temperature (e.g., 20 to 30 °C) as the melting point of pitch may suitably be around 90 to 130 °C. The combination of graphene in granules with a tar or pitch has been found to be particularly useful for integration of graphene into the production of carbon electrodes for aluminium smelting. Carbon electrodes for use in aluminium smelting are typically made from a baked mixture of carbon-based dry aggregate and a binder that is typically a tar or pitch such as coal tar pitch. The baking step removes volatiles from the mixture to leave a solid structure of relatively pure carbon. It has been found that integration of graphene into this material can reduce the electrical resistance of the electrode material whilst maintaining density and ease of handling (such as sticking of packing materials to the electrode material during baking), as well as improving resistance to air and CO2. By providing a graphene-containing granulate formed from graphene and a tar or pitch such as coal tar pitch, the granulate may be readily integrated into the process for production of improved carbon electrodes, avoiding problems relating to safety and dosing of fine powder into the process. Graphene may be combined with a molten liquid medium in any suitable way, and such processes as a used to produce masterbatches are known in the art. For example, the liquid medium may be mixed with the graphene to form a fluid mixture (where the material forming the liquid medium may be in solid form during initial mixing, followed by heating to form a liquid) by high shear mixing or any other suitable process. A fluid mixture of graphene and the molten liquid medium may then be formed into granules by granulation mixing processes as discussed herein, such as using a high shear granulator, a fluidised bed granulator a pin mixer or a planetary centrifugal mixer, operating at sufficiently high temperature to retain the liquid medium in molten form, or where the mixture is sufficiently viscous, by an extrusion process. As will be appreciated, where a molten liquid medium is cooled to form solid granulate, the graphene may be used in lower quantities as compared to granulation using a solvent. Graphene may for example, be mixed with a molten liquid medium in an amount of from 0.01 wt.% to 99 wt.% of the graphene-liquid mixture, for example from 0.1 wt.% to 60 wt.%, such as from 1 wt.% to 20 wt.%, or the graphene may be mixed with the molten liquid medium in a higher proportion, for example from 20 wt.% to 99 wt.%, such as from 40 wt.% to 99 wt.%. 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 with a meltable liquid such as pitch into carbon electrodes, as well as providing a granulate having particularly beneficial surface area when formed using a planetary centrifugal mixer. 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. By way of example, polycyclic aromatic hydrocarbons may be identified by gas chromatography-mass spectroscopy (GC-MS), and polyacetylenes may be identified by Raman spectroscopy. The form of the graphene produced by this process may also aid in the formation of a porous granulate having surprising surface area when granulated using solvent in a planetary centrifugal mixer. 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 non-hydrocarbon 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 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 graphenecontaining granulate is produced or used, for example 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 a graphenecontaining granulate comprising a plasma reactor system as described for providing the graphene (which may or may not be a containerised system) and means for forming a graphene-containing granulate from the 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 granulation process (for example to provide power for the preparation of the granulate, or to output power to an electrical grid to offset power used in the process). Where the plasma reactor system is disposed inside a container such as a shipping container, the steps of mixing the graphene powder and a liquid medium to provide a graphene-liquid mixture and forming granules from the graphene-liquid mixture to provide the graphene-containing granulate may be conducted within the container. Thus, in some embodiments the graphene-containing granulate is produced from the graphene inside the container in which the plasma reactor system is disposed; or the plasma reactor system produces the graphene in a first container, and the graphene-containing granulate is produced from the graphene within a second container configured to couple to and receive the graphene from the first container. For example, a mixing system and a source of a liquid medium may be provided inside the container holding the plasma reactor system, such that graphene from the plasma reactor system may be provided to a granulator for granule formation. In some embodiments, the means for forming a graphene-containing granulate from the graphene may be disposed in a further container (such as a shipping container) that can be coupled with a container having a plasma reactor system. For example, the plasma reactor system may be disposed in a first container, and the steps of mixing the graphene powder and a liquid medium to provide a graphene-liquid mixture and forming granules from the graphene-liquid mixture are conducted within a second container configured to couple to and receive the graphene from the first container. The graphene may be mixed with the liquid medium in the first container and the graphene suspension provided from the first container to the second container for granulation. 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 more preferably from 110 m2 / g to 330 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; 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. 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 particularly advantageous where the graphene is used in the formation of carbon electrodes for aluminium smelting, where the sulfur content may reduce reactivity towards oxygen and reduce electrode deterioration. A further aspect provides process of producing a graphene-containing granulate from a graphene powder comprising compacting the graphene powder to form the graphene containing granulate, wherein the graphene is produced by operation of a plasma reactor system configured to produce graphene from a carbon-containing process gas. The graphene and / or its production may be as defined elsewhere herein. The graphene that is compacted is suitably in the form of a dry powder. As will be appreciated, the granulate may be formed solely from the graphene powder. Thus, the granulate may comprise at least 90 wt.% compacted graphene powder, for example at least 95 wt.%, for example the granulate may consist essentially of compacted graphene. The graphene powder may be compacted in any suitable way. For example, the graphene powder may be compacted by applying mechanical pressure to the graphene powder, such as by a roller-compactor or by any other suitable method. For example, a granulate may be formed by passing the graphene powder through rollers at a pressure of from 50 to 150 bar, for example from 75 to 125 bar, and with a roller gap of from 0.5 mm to 5 mm, for example from 1 mm to 2 mm. Flakes produced by roller compression may then be formed into granulate by breaking up the compressed flakes, for example by crushing and / or by pushing the material through a mesh for example a mesh having a mesh size of from 1 to 2 mm. Without wishing to be bound by any particular theory, it is believed that graphene produced by such a plasma reactor system can be particularly suited to dry compaction and granulation as a result of the graphene containing polycyclic aromatic hydrocarbons and polyacetylene chains adsorbed or bonded to / on its surface, which can aid self-adhesion and the formation of a stable granulate. The graphene-containing granulate formed by compaction of the dry graphene powder may suitably have a bulk density of from 200 kg / m3 to 500 kg / m3, preferably from 230 kg / m3 to 400 kg / m3, for example from 250 kg / m3 to 300 kg / m3. As measured by sieving, the granulate may comprise at least 50 wt.%, preferably at least 75 wt.%, for example at least 90 wt.% of particles having a size of from 200 pm to 2 mm, preferably containing at least 40 wt.% of particles having a size of from 200 pm to 1 mm. As will be appreciated, the plasma reactor system may be a containerised system as described previously herein and the step of compacting the graphene powder to form the graphene containing granulate may be carried out in the container housing the plasma reactor system. Alternatively, the plasma reactor system produces the graphene in a first container, and the graphene-containing granulate is produced from the graphene within a second container configured to couple to and receive the graphene from the first container. A further aspect relates to a system for producing a graphene-containing granulate from a graphene powder comprising: a plasma reactor system configured to produce graphene from a carbon-containing process gas; and means for processing the graphene into a graphene-containing granulate by mixing the graphene with a liquid medium to provide a graphene-liquid mixture and forming granules from the graphene-liquid mixture, or by applying mechanical pressure to the graphene powder to form the graphene containing granulate. As discussed previously herein, 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. Preferably, the means for processing the graphene into a graphene-containing granulate are disposed inside the container in which the plasma reactor system is disposed; or the plasma reactor system is disposed in a first container, and the means for processing the graphene into a graphene-containing granulate are disposed within a second container configured to couple to and receive the graphene from the first container. In this way, a self-contained or modular system may be provided that is highly mobile and deliverable for the production of a graphene granulate at a source of carbon-containing process gas (such as natural gas), and / or at a location where the graphene-containing granulate is integrated into a further process, such as a process for producing carbon electrodes, or any other process where the introduction of graphene in granule form is desirable. In particular, where the system is disposed at a source of carbon-containing process gas such as a source of natural gas, where the graphene may need to be transported to a different point of use or sale, the integration of granulation of the graphene advantageously provides a solid product that is easier to transport and handle than graphene in the form of a fine powder. The means for processing the graphene into a graphene-containing granulate may comprise a source of the liquid medium and optionally a binder, and a mixer, such as high shear granulator, a pin mixer or a planetary centrifugal mixer configured to receive the graphene and the liquid medium. As will be appreciated, the graphene, liquid medium and mixing process and apparatus may be as defined previously herein. In some preferred embodiments, the system further comprises a dryer for evaporating solvent from granules obtained from the mixer, for example wherein the dryer is configured to provide a heated gas to dry the granulate, for example wherein the dryer comprises an oven, a rotary dryer or a fluid bed dryer. In some embodiments, the dryer is integrated into the mixer. For example, the system may comprise a mixer such as a planetary centrifugal mixer, which may be heated and / or subject to vacuum in order to remove solvent from the mixture as granules are formed. In some preferred embodiments, the system comprises a heater for heating the liquid medium prior to mixing with the graphene, for example a heater configured to melt a polymer or a tar such as coal tar pitch in order to provide the liquid medium for mixing with the graphene. The heater may be integrated into the mixer, for example where the mixer is configured to receive graphene and a solid material that, when melted, forms the liquid medium, wherein heating the mixture provides a mixture of the graphene and the liquid medium. As will be appreciated, the molten liquid medium and the means for mixing this with graphene and forming the granulate may be as defined previously herein. Brief description of Figures Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which: Figures 1A and 1B are graphs showing size distribution and density for granules produced in Example 1; Figures 2A and 2B are graphs showing size distribution and density for granules produced in Example 2; Figures 3A and 3B are graphs showing size distribution and density for granules produced in Example 3; Figure 4 shows a graph of lateral flake size as measured by SEM for graphene produced by a plasma reactor system; and Figure 5 shows a Raman spectrum measured for graphene produced by a plasma reactor system. Examples 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 4 and 5 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 4, 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 5 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. Wet granulation The following Examples 1 to 5 were performed using a planetary centrifugal mixer, specifically a Synergy Speedmixer (RTM) (1 litre cup size, model DAC 1200-500). In each case, graphene and water are loaded into the vessel (mixing cup) of the mixer and are mixed to form the granulate in a series of mixing steps. Generally, granules are formed by mixing at an initial rotation rate (the rotation rate referring to centrifugal rotation, with a 1:1 rotation ratio such that the vessel simultaneously rotates at the same rate) to provide compaction, followed by one or more further stages of mixing at a reduced rotation rate to achieve granulation / pelletisation. The granulate formed is then dried to remove the water before sieving and characterisation. Example 1 Three separate samples of granulate were produced using 100g graphene powder and different weight ratios of graphene:water. The first sample was produced using 50:50 graphene:water, the second sample was produced using 54:46 graphene:water, and the third sample was produced using 58:42 graphene:water. For each sample, an initial compaction stage was performed in the planetary centrifugal mixer at 2000 rpm for 90 seconds (3 x 30 seconds), followed by a second stage at 1800 rpm for 90 seconds (3 x 30 seconds), a third stage at 1600 rpm for 90 seconds (3 x 30 seconds), and a fourth stage at 1400 rpm for 90 seconds (3 x 30 seconds). Following the fourth stage, particles above 850 pm were separated by sieving and subjected to mixing in the planetary centrifugal mixer at 2000 rpm for 1 minute, the resulting particles were recombined with the remainder of the mixture and subjected to a final fifth mixing stage for 90 seconds (3 x 30 seconds) at 1200 rpm for the 50 wt.% graphene sample, 1400 rpm for the 54 wt.% graphene sample, and 1600 rpm for the 58 wt.% graphene sample. Due to water evaporation during the process, the graphene content in these samples of granulate increased from 50 wt.% to 52 wt.% in the first sample, 54 wt.% to 57 wt.% in the second sample and from 58 wt.% to 60 wt.% in the third sample. In Figures 1Aand IBand Table 1 below, the samples are labelled according to this adjusted graphene content. As can be seen from Figures 1A and 1B, where less water is used, a larger proportion of smaller granules are produced. The density shows some variation, however this may be a result of the different mixing speed in the final stage, which leads generally to more compaction observed in the sample with higher graphene content. The samples were analysed to determine their specific surface area (BET surface area -ISO 9277:2022) and the results are shown in Table 1 below. As can be seen the surface area compares surprisingly well with the uncompacted graphene powder in view of the compaction achieved. Table 1 Sample Specific surface area (m2 / g) Comparative 115.1 60 wt.% graphene 108.2 57 wt.% graphene 108.2 52 wt.% graphene 109.5 Example 2 Three separate samples of granulate were produced using 100g graphene powder and different weight ratios of graphene:water. The first sample was produced using 54:46 graphene:water, the second sample was produced using 56:44 graphene:water, and the third sample was produced using 58:42 graphene:water. For each sample, an initial compaction stage was performed in the planetary centrifugal mixer at 2000 rpm for 120 seconds (4 x 30 seconds), followed by a second stage at 1800 rpm for 90 seconds (3 x 30 seconds), and a third stage at 1600 rpm for 90 seconds (3 x 30 seconds). Following the third stage, particles above 850 pm were separated by sieving and subjected to mixing in the planetary centrifugal mixer at 2000 rpm for 1 minute, the resulting particles were recombined with the remainder of the mixture and subjected to a final fourth mixing stage at 1150 rpm for 90 seconds (3 x 30 seconds). Due to water evaporation during the process, the graphene content in these samples of granulate increased from 54 wt.% to 57 wt.% in the first sample, 56 wt.% to 58 wt.% in the second sample and from 58 wt.% to 61 wt.% in the third sample. In Figures 2A and 2B, the samples are labelled according to this adjusted graphene content. As can be seen from Figures 2A and 2B, this process produced a large proportion of granules in the 425-850 pm range. In addition, where relatively more water is used in the sample, a larger proportion of granules in the 425-850 pm range are produced. The samples using lower water content also showed increased bulk density in the granulate produced. Example 3 Two separate samples of granulate were produced using 100g graphene powder and a weight ratio of graphene:water of 54:46. In each case the length of the initial high speed compaction stage was varied. For each sample, an initial compaction stage was performed in the planetary centrifugal mixer at 2000 rpm. For the first sample this initial compaction stage was for 180 seconds (6 x 30 seconds), while for the second sample this was for 90 seconds (3 x 30 seconds). This was followed by a second stage at 1600 rpm for 120 seconds (4 x 30 seconds) Following the second stage, particles above 850 pm were separated by sieving and subjected to mixing in the planetary centrifugal mixer at 2000 rpm for 1 minute, the resulting particles were recombined with the remainder of the mixture and subjected to a final third mixing stage at 1150 rpm for 90 seconds (3 x 30 seconds). As can be seen from Figures 3A and 3B, the increased initial compaction time produced a large proportion (91%) of granules in the 425-850 pm range, and generally increased the bulk density of the granulate. Example 4 12 granule samples were separately prepared, each using 100g graphene powder and from 80 to 130 g of water. An initial compaction step was performed at 2000 rpm for 90 seconds, followed by a pelletisation step for 4 minutes at a fixed rate between 800 and 1790 rpm. The dried pellets produced were found to have an average specific surface area of 109 m2 / g (with 50% of samples showing at least 110 m2 / g), an average hardness of 48 gF, and an average tap density of 414 kg / m3 (generally increasing with decreasing water:graphene ratio). Sieving analysis of the samples showed on average 67% of pellets were in the range of 0.2 to 2.0 mm and on average around 6 wt.% fines below 0.2 mm were present. 17 further samples were prepared, each using 100g graphene powder and from 85 to 150 g of water. An initial compaction step was performed at between 1600 and 1800 rpm for 90 seconds, followed by a pelletisation step for 4 minutes at a fixed rate of 800 rpm. The pellets were dried before characterisation. The pellets produced were found to have an average specific surface area of 108 m2 / g, an average hardness of 38 gF, and an average tap density of 395 kg / m3 (generally increasing with decreasing water:graphene ratio). Sieving analysis of the samples showed on average 73% of pellets were in the range of 0.2 to 2.0 mm and on average around 4 wt.% fines below 0.2 mm were present. Example 5 A granulate / pellet sample was prepared using 60 wt.% water and 40 wt.% graphene. An initial compaction step was performed at 1600 rpm for one minute, followed by a granulation step at 800 rpm for one minute. The pellets formed were dried before characterisation. The specific surface area of the pellets was 114.3 m2 / g, which demonstrates only a very small reduction in surface area compared to the uncompacted graphene (see Comparative in Table 1 of Example 1). The pellets had a bulk density of 313.5 g / cm3, and a hardness of 37 gF. Example 6 - Pin mixer pelletisation 5 separate pellet samples were prepared by feeding graphene powder (5 kg) and water into a Mars Mineral Pin Mixer (model 8D32L, 8’ inch (20.32 cm) diameter, 32 inch (81.28 cm) length, 0.28 m3 / h capacity) at a feed rate of from 0.25 to 0.41 kg / min (water:graphene ratio of from around 55 to around 60), and mixing at a speed of between 1250 and 1550 rpm. 5 granule samples were prepared by mixing approximately 5 kg of graphene powder with water in a water:graphene ratio of from around 55 to 60. The pellets were dried before characterisation. Across the 5 samples, the pellets produced were found to have an average specific surface area of 106 m2 / g, an average hardness of 22 gF, and an average tap density of 349 kg / m3. Sieving analysis of the samples showed on average 87% of pellets were in the range of 0.25 to 2.0 mm and on average around 2 wt.% fines below 0.25 mm were present. Example 7 - Dry granulation The dry graphene powder was compacted in a roller compactor at pressures of from 50 bar to 130 bar and with a roller gap of from 1.0 mm to 2.0 mm, where the graphene was fed by a screw system from a stirred hopper. The rolled flake material was then mechanically crushed and passed through a 2.0 mm or a 1.0 mm mesh to form granulate. The granulate was found to have a bulk density in the range of from 250 kg / m3 to 300 kg / m3 Using a 2.0 mm mesh to form the granulate produced a granulate having around 44 to 46 wt.% of granules in the 200 pm to 1000 pm range and over 30 wt.% granules in the 500 to 1000 pm range. In addition, around 38 wt.% of particles were in the 1000 to 1600 pm range, with less than 10 wt.% particles larger than 1600 pm. Using a 1.0 mm mesh to form the granulate produced a granulate having around 80 wt.% of granules in the 200 pm to 1000 pm range and around 50 wt.% granules in the 500 to 1000 pm range. In addition, around 20 to 25 wt.% of particles were in the sub 200 pm range. Particle sizes were measured by sieving (DIN 66165-2). Example 8 As described herein, graphene-containing granulate produced by mixing graphene with molten coal tar pitch provides a granulate that is particularly useful for the formation of carbon electrodes comprising graphene. Samples of carbon electrode material comprising graphene were produced by mixing graphene in an amount of from 0.1 wt.% to 1.0 wt.% with carbon-based dry aggregate (coke) and coal tar pitch and mixed with an impeller mixer to raise the pitch temperature to about 173 °C. 320 g portions of the mixture were pressed in a mould for 30 seconds at 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. The samples were analysed and compared to material without graphene and it was found that the addition of graphene to the carbon electrode material, multiple properties of the material may be improved. In particular, the specific electrical resistance showed a significant decrease upon the inclusion of graphene and the compressive strength of the material is also improved by the inclusion of graphene. The air and CO2 resistance of the material was also improved. This can not only improve the lifetime of the electrodes in an aluminium smelting process, by reducing electrode consumption, but can also maintain 5 quality of the electrode material to benefit its use as a butts material (to substitute for coke) if recycled. The thermal conductivity is also improved, which can be favourable in terms of thermal shock resistance of the electrode. Integrating graphene into granules with pitch provides an improved process for achieving 10 these surprising advantages by avoiding problems relating to handling and dosing of graphene into the process as a fine powder.

Claims

1. A process of producing a graphene-containing granulate from a graphene powder comprising:mixing the graphene powder and a liquid medium to provide a graphene-liquid mixture; andforming granules from the graphene-liquid mixture to provide the graphenecontaining granulate.

2. A process according to claim 1, wherein the liquid medium comprises a solvent, and the process comprises forming granules from the graphene-liquid mixture and evaporating the solvent to form the graphene-containing granulate.

3. A process according to claim 2, wherein the graphene-liquid mixture further comprises a binder.

4. A process according to claim 3, wherein the binder comprises a polymeric binder such as cellulose, lignin, or derivatives thereof such as hydroxyethyl cellulose, ethylcellulose or carboxymethyl cellulose, polyacrylic acid, polyvinyl alcohol, polyvinylpyrrolidone, phenolic polymers, thermoplastics such as polylactic acid, acrylonitrile butadiene styrene, polyetherimide, or polyamides.

5. A process according to any one of claims 2 to 4, wherein the graphene-liquid mixture comprises from 35 to 90 wt.% graphene; from 10 wt.% to 65 wt.% solvent and optionally from 1 wt.% to 50 wt.% of a binder.

6. A process according to claim 1, wherein the liquid medium is heated to form the graphene-liquid mixture, 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 prepolymer, for example a thermosetting polymer such as epoxy, polyester or phenolic, or a molten thermoplastic polymer such as polyamide or polyetherimide, or a wax.

7. A process according to claim 6, wherein the process comprises forming granulesfrom the graphene-liquid mixture and cooling the granules to form the graphene-containing granulate.

8. A process according to any one of the preceding claims, wherein the granules are formed by mechanical mixing of the graphene powder and the liquid medium, for example by a high shear granulator, a pin mixer or a planetary centrifugal mixer.

9. A process according to claim 8, wherein the granules are formed by mechanical mixing of the graphene powder and the liquid medium with a planetary centrifugal mixer, preferably wherein forming the granules comprises mixing the graphene powder and the liquid medium at a first speed to provide a compacted mixture, followed by mixing the compacted mixture at a second speed, lower than the first speed, to form granules from the compacted mixture.

10. 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.

11. 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.

12. 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 350 m2 / g, more preferably from 110 m2 / g to 330 m2 / g.

13. 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, l(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, I(2D) / I(G), of greater than 0.65; and a single-component 2D peak.

14. 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.

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

16. A process according to claim 15, wherein the graphene-containing granulate is produced from the graphene inside the container in which the plasma reactor system is disposed; or wherein the plasma reactor system produces the graphene in a first container, and the graphene-containing granulate is produced from the graphene within a second container configured to couple to and receive the graphene from the first container.

17. A process of producing a graphene-containing granulate from a graphene powder comprising compacting the graphene powder to form the graphene containing granulate;wherein the graphene is produced by operation of a plasma reactor system configured to produce graphene from a carbon-containing process gas as defined in any of claims 10 to 13.

18. A process according to claim 17, wherein the graphene powder is compacted by applying mechanical pressure to the graphene powder, such as by a roller-compactor.

19. A graphene-containing granulate, wherein the granulate comprises 90 wt.% or more graphene, the granulate comprising at least 80 wt.% of particles having a particle size, as measured by sieving, of from 100 pm to 10 mm, a bulk density of from 300 to 800 kg / m3 and a specific surface area of at least 100 m2 / g.

20. A system for producing a graphene-containing granulate from a graphene powder comprising:a plasma reactor system configured to produce graphene from a carbon-containingprocess gas; andmeans for processing the graphene into a graphene-containing granulate by mixing the graphene with a liquid medium to provide a graphene-liquid mixture and forming granules from the graphene-liquid mixture, or by applying mechanical pressure to the graphene powder to form the graphene containing granulate.

21. A system according to claim 20, wherein the plasma reactor system is 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.

22. A system according to claim 21, wherein means for processing the graphene into a graphene-containing granulate are disposed inside the container in which the plasma reactor system is disposed; or wherein the plasma reactor system is disposed in a first container, and the means for processing the graphene into a graphene-containing granulate are disposed within a second container configured to couple to and receive the graphene from the first container.

23. A system according to any one of claims 20 to 22, wherein the means for processing the graphene into a graphene-containing granulate comprises a source of the liquid medium and optionally a binder, and a mixer, such as high shear granulator, a pin mixer or a planetary centrifugal mixer configured to receive the graphene and the liquid medium.

24. A system according to claim 23, wherein the system further comprises a dryer for evaporating solvent from granules obtained from the mixer, for example wherein the dryer is configured to provide a heated gas to dry the granulate, for example wherein the dryer comprises an oven, a rotary dryer or a fluid bed dryer, optionally wherein the dryer is integrated into the mixer.

25. A system according to claim 23 or 24, wherein the system comprises a heater for heating the liquid medium prior to mixing with the graphene, for example a heater configured to melt a polymer or a tar such as coal tar pitch in order to provide the liquid medium for mixing with the graphene.

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