Improved aerosol-forming substrate

Abstract

An aerosol-forming substrate comprising, on a dry weight basis 10 to 90 wt % of a mineral compound, 7 to 60 wt% of an aerosol former, 1 to 20 wt% of fibres, and 1 to 10 wt% of a binder; and wherein the mineral compound comprises silicon and / or an alkaline earth metal, and the mineral compound comprises kaolinite, perlite, zeolite, bentonite, calcium sulfate, magnesium sulfate, magnesium carbonate or talc.

Description

[0001]IMPROVED AEROSOL-FORMING SUBSTRATE The present disclosure relates to an aerosol-forming substrate. The present disclosure also relates to a method of making an aerosol-forming substrate, an aerosol-generating article, and an aerosol-generating system. 5 A typical aerosol-generating system comprises an aerosol-generating device and an aerosol-generating article comprising an aerosol-forming substrate. In use, the aerosol- generating device interacts with the aerosol-generating article to heat the aerosol-forming substrate and cause the aerosol-forming substrate to release volatile compounds. These compounds then cool to form an aerosol which is inhaled by a user. Such a system may be 10 referred to as a “heat-not-burn” aerosol-generating system. Known aerosol-forming substrates typically have relatively low thermal conductivities. This may be undesirable, particularly in aerosol-generating systems in which a blade is inserted into the aerosol-forming substrate and heated in order to heat the aerosol-forming substrate. This is because the low thermal conductivity of the aerosol-forming substrate may lead to a relatively 15 large temperature gradient in the aerosol-forming substrate during use. This may mean that portions of the aerosol-forming substrate which are located furthest from the blade do not reach a high temperature and so do not release as many volatile compounds as they would if the aerosol-forming substrate had a higher thermal conductivity. In other words, the low thermal conductivity of the aerosol-forming substrate may undesirably result in a low usage efficiency of 20 the aerosol-forming substrate. Further, known aerosol-forming substrates are typically not heatable to operating temperatures by induction. This means that, for inductive heating, a separate susceptor element is typically required. This can increase costs. In addition, this can lead to the same issues as discussed above. For example, where an inductively heated susceptor element is placed in a 25 central position in the substrate, portions of the aerosol-forming substrate which are located furthest from the susceptor element may not reach a high temperature and therefore may not release many volatile compounds. Attempts have been made to increase the thermal conductivity of aerosol-forming substrates. However, to date, these attempts have been inadequate in one or more respects. 30 It is an aim of the present invention to provide an improved aerosol-forming substrate, for example an aerosol-forming substrate having an increased thermal conductivity. According to the present disclosure, there is provided an aerosol-forming substrate. The aerosol-forming substrate may comprise, on a dry weight basis, 10 to 90 weight percent (wt%) of a mineral compound. The aerosol-forming substrate may comprise, on a dry weight basis, 7 to 35 60 weight percent of an aerosol former. The aerosol-forming substrate may comprise, on a dry weight basis, 1 to 20 weight percent of fibres. The aerosol-forming substrate may comprise, on a dry weight basis 1 to 10 weight percent of a binder. The mineral compound may comprise silicon or an alkaline earth metal. The aerosol-forming substrate may be for use in an aerosol-generating system. Thus, according to a first aspect of the present invention, there is provided an aerosol- forming substrate comprising, on a dry weight basis: 10 to 90 weight percent of a mineral 5 compound, 7 to 60 weight percent of an aerosol former, 1 to 20 weight percent of fibres; and 1 to 10 weight percent of a binder; wherein the mineral compound comprises silicon or an alkaline earth metal, preferably wherein the mineral compound comprises kaolinite, perlite, zeolite, bentonite, calcium sulfate, magnesium sulfate, magnesium carbonate or talc. Advantageously, it has been found that the mineral compound may increase the thermal 10 conductivity of the aerosol-forming substrate. The increased thermal conductivity of the substrate may provide a more even temperature distribution throughout the substrate during use. This may result in a greater proportion of the aerosol-forming substrate reaching a sufficiently high temperature to release volatile compounds, and thus a higher usage efficiency of the aerosol- forming substrate. Further, the increased thermal conductivity of the substrate may allow a heater, 15 for example a heating blade or susceptor configured to heat the substrate, to operate at a lower temperature and thus require less power. Further still, the increased thermal conductivity of the substrate may allow a heater to heat the substrate to a temperature in which volatile compounds are released in less time. Thus, the increased thermal conductivity may reduce the time required to form an inhalable aerosol for a user. 20 Advantageously, the present inventors have found that the thermal conductivity of the substrate comprising the mineral compound is not so increased that unfavourable effects occur, or begin to occur, in the aerosol-generating system, such as: the holder surface temperature becoming too hot for the user during and / or after use; the calibration duration being excessively long during the pre-heating cycle of the device; the energy consumption of the device increasing 25 to a generally unacceptable level, for example, the energy consumption increasing by up to 50 percent; and thermal degradation of the substrate and of any components therein. Thus, the present inventors have achieved a balance between providing an aerosol-forming substrate having the advantages associated with an increased thermal conductivity, and the disadvantages found when the thermal conductivity of the substrate is too high. 30 Advantageously, the present inventors have found that the aerosol-forming substrate comprising a mineral compound, wherein the mineral compound comprises silicon or an alkaline earth metal, may be more efficient in delivering the aerosol former and nicotine to the end user than aerosol-forming substrates known in the art, e.g. tobacco-containing and tobacco-free aerosol-forming substrates. The amount of aerosol former required in the substrate may be 35 reduced compared to known aerosol-forming substrates e.g. tobacco-free aerosol-forming substrates. Advantageously, the need for an expensive and low-environmentally sustainable sealed packaging to avoid excessive moisture uptake by hygroscopic components in the substrate, for example the aerosol-former, is mitigated. Advantageously, the extraction efficiency of the aerosol-former and nicotine from the substrate may be improved, and a smaller plug and susceptor may be used compared to devices known in the art, e.g. comprising tobacco-containing and tobacco-free substrates. For instance, a 11 mm plug and 4 mm width susceptor may be used for substrates according to the present invention, compared to a 12 mm plug and 5 mm width susceptor compared to typical tobacco- containing and tobacco-free substrates in the art. For example, the ratio of plug size used for substrates according to the present invention to plug size for plug sizes used in the art is preferably 0.9:1. For example, the ratio of susceptor width used for substrates according to the present invention to susceptor width used in the art is preferably 0.8:1. Beneficially, the substrate according to the present invention thus may be more environmentally sustainable compared to substrates known in the art, as larger plug and susceptor widths require the use of more semi-volatile organic compounds in manufacturing operations due to the changeovers in the manufacturing lines. Further, advantageously the need for a 7 mm mouthpiece filter to ensure the desired level of aerosol delivery is mitigated. Advantageously, the mineral compound is inert under the operating conditions of the device. For example, the mineral compound does not contribute to the aerosol (e.g. the aerosol does not contain a determinable amount of the mineral compound or any thermal degradation products thereof), but the mineral compound does enhance the aerosolization of components in the substrate due to its inherent thermal conductivity. Advantageously, the mineral compound may therefore be considered to act as an inert carrier material under the operating conditions of the device. For example, the mineral compound may act as an inert delivery material for nicotine and / or flavourant included in the substrate or in a filter capsule. Advantageously, the mineral compound has been shown to exhibit a satisfactory toxicological profile at a preliminary assessment, and thus may be suitable for use in a heated aerosol-generating article, intended to be used by a user. For example, the mineral compound may be a food-grade mineral compound. Advantageously, one or both of the fibres and the binder may increase a tensile strength of the aerosol-forming substrate. The increased tensile strength may allow the production of a sheet of the aerosol-forming substrate which does not easily tear. The increased tensile strength may allow the production of a sheet of the aerosol-forming substrate using existing production machinery. The mineral compound may comprise a thermal conductivity of from 0.1 to 5 W m−1.K−1in at least one direction.For example, the mineral compound may comprise a thermal conductivity of from 0.1 to 5 W m−1.K−1in at least one direction at 25 degrees Celsius. For instance, the mineral compound may comprise a thermal conductivity of from 0.12 to 3.5 W m−1.K−1, preferably 0.14 to 3 W m−1.K−1and more preferably 0.15 to 1.5 W m−1.K−1in at least one direction at 25 degrees Celsius. Preferably, the mineral compound has a thermal conductivity of less than 1.0 W m−1K−1. For example, the mineral compound may have a thermal conductivity of from 0.15 to 0.7 W m−1K−1, preferably of from 0.15 to 0.3 W m−1.K−1. Typically, the mineral compound is crystalline. In some examples, the mineral compound is porous. 5 Typically, in examples, silicon in the mineral compound is present in the form of silica (SiO2). For example, the mineral compound comprises silica. In some examples, the alkaline earth metal is present in the mineral compound as a cation. In some examples, the alkaline earth metal forms part of a salt. Preferably, the alkaline earth metal is selected from beryllium (Be), magnesium (Mg), 10 calcium (Ca), or strontium (Sr). Preferably, the alkaline earth metal is calcium or magnesium. Particularly preferably, the alkaline earth metal is magnesium. In some examples, the mineral compound is a clay mineral. In some examples, the mineral compound consists of kaolinite, perlite, zeolite, bentonite, 15 calcium sulfate, magnesium sulfate, magnesium carbonate or talc. In some examples, the mineral compound comprises or consists of kaolinite, perlite, zeolite, calcium sulfate, magnesium sulfate, magnesium carbonate or talc. In examples, the mineral compound comprises or consists of kaolinite, diatomite, celite, perlite, zeolite, calcium sulfate, calcium carbonate, magnesium sulfate, magnesium carbonate or 20 talc. Preferably, the mineral compound comprises or consists of kaolinite, diatomite, celite, perlite, zeolite or talc. Particularly preferably, the mineral compound is kaolinite. In some examples, the mineral compound is bentonite. 25 Preferably, the mineral compound does not comprise carbon. Preferably, the mineral compound is not silicon carbide. Typically, the mineral compound comprises a pH of from 4 to 6 in water. For example, the pH of the mineral compound is measured by dispersing a 0.5 g sample of the mineral compound in 5 grams of water, agitating the dispersion and then measuring the pH of the dispersion using a 30 pH electrode. Advantageously, the acidic pH of the mineral compound can enable a reduction in the amount of acid added to the substrate in order to protonate nicotine and avoid nicotine losses during drying processes. The mineral compound may be in the form of particles. Typically, the mineral compound particles may each have a “particle size”. The meaning of 35 the term “particle size” and a method of measuring particle size is set out hereinafter. Optionally, each of the mineral compound particles has a particle size of at least 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, 100, 200, or 500 microns. Optionally, each of the mineral compound particles has a particle size of no more than 1,000, 500, 300, 200, 100, 50, 20, 10, 5, 2, 1, 0.5, or 0.2 microns. It may be particularly preferable for each of the mineral compound particles to have a particle size of at least 1 micron. 5 Alternatively, or in addition, it may be particularly preferable for each of the mineral compound particles to have a particle size of no more than 300 microns. Particles smaller than 1 micron may be difficult to handle during manufacturing. In addition, particles smaller than 1 micron may be more likely to pass through a filter in an aerosol-generating article comprising the aerosol-forming substrate. Particles greater than 300 microns may take up 10 a rather large amount of space in the substrate which could be used for aerosol-forming material. Thus, it may be particularly advantageous for each of the mineral compound particles to have a particle size of at least 1 micron, or a particle size of no more than 300 microns, or both. Optionally, each of the mineral compound particles has three mutually perpendicular dimensions, a largest dimension of the three dimensions being no more than 10, 8, 5, 3, or 2 15 times larger than a smallest dimension of the three dimensions. Optionally, each of the mineral compound particles has three mutually perpendicular dimensions, a largest dimension of the three dimensions being no more than 10, 8, 5, 3, or 2 times larger than a second largest dimension of the three dimensions. Optionally, each of the mineral compound particles is substantially spherical. 20 Advantageously, the orientation of substantially spherical particles may not affect the thermal conductivity of the substrate as much as the orientation of non-spherical particles. Thus, the use of more spherical particles may result in less variability between different substrates where the orientations of the particles is not controlled. In addition, substantially spherical particles may be more easy to characterise. 25 The mineral compound may be characterised by a particle size distribution. A particle size distribution may be characterised by volume D10, D50 and D90 particle sizes. The volume D10 particle size is defined such that 10% of the sum of the volumes of all of the particles is accounted for by the sum of the volumes of the particles having a particles size less than or equal to the volume D10 particle size. 30 Similarly, the volume D50 particle size is defined such that 50% of the sum of the volumes of all of the particles is accounted for by the sum of the volumes of the particles having a particles size less than or equal to the volume D50 particle size. The volume D90 particle size is defined such that 90% of the sum of the volumes of all of the particles is accounted for by the sum of the volumes of the particles having a particles size 35 less than or equal to the volume D90 particle size. Typically, the mineral compound has a particle size distribution having a volume D10 particle size of from 0.1 to 20 microns. For example, the volume D10 particle size may be of from 0.2 to 18 microns, more preferably 0.4 to 16 microns, even more preferably 0.5 to 15 microns. Typically, the mineral compound has a particle size distribution having a volume D50 particle size of from 1 to 200 microns. For example, the volume D50 particle size may be of from 5 to 180 microns, preferably 10 to 150 microns, more preferably 15 to 130 microns, and even more preferably 20 to 100 microns. 5 In examples, the mineral compound has a particle size distribution having a volume D90 particle size of from 3 to 200 microns. For example, the volume D90 particle size may be from 4 to 150 microns, from 5 to 100 microns, from 6 to 80 microns and from 7 to 60 microns. Advantageously, it has been found that a volume D90 particle size within these ranges can enable the substrate to be formed as a sheet having a high bulk density and an increased tensile strength 10 compared to substrates known in the art. In some examples, the aerosol forming substrate comprises, on a dry weight basis, 20 to 80 wt% of the mineral compound, preferably 25 to 75 wt%, and even more preferably 30 to 70 wt % of the mineral compound. In some examples, the aerosol-forming substrate comprises, on a dry weight basis, 10 to 15 50 wt% of the aerosol former, preferably 12 to 40 wt% of the aerosol former, more preferably 15 to 30 wt% of the aerosol former. The aerosol-former typically comprises or consists of one or more of: polyhydric alcohols, such as propylene glycol, polyethylene glycol, triethylene glycol, 1, 3-butanediol and glycerine; esters of polyhydric alcohols, such as glycerol mono-, di- or tri-acetate; and aliphatic esters of 20 mono-, di- or poly-carboxylic acids, such as dimethyl dodecanedioate and dimethyl tetradecanedioate. For example, the aerosol former is selected from the group consisting of propylene glycol, triethylene glycol, 1,3-butanediol, glycerine, glycerol monoacetate, glycerol diacetate, glycerol triacetate, dimethyl dodecanedioate, and dimethyl tetradecanedioate. Optionally, the aerosol- 25 forming substrate comprises one or both of glycerine and propylene glycol. Preferably, the aerosol-forming substrate comprises glycerine. In some examples, the aerosol-forming substrate comprises, on a dry weight basis, 1.5 to 18 wt% of fibres, preferably 2 to 15 wt% of fibres, preferably 3 to 12 wt% fibres and more preferably 5 to 10 wt% fibres. Advantageously, the inclusion of fibres in the substrate may 30 increase the tensile strength of the substrate. In some examples, the fibres comprise cellulose fibres. Advantageously, cellulose fibres are not overly costly and can increase the tensile strength of the substrate. Optionally, each of the fibres has three mutually perpendicular dimensions, a largest dimension of the three dimensions being at least 1.5, 2, 3, 5, 10, or 20 times larger than a smallest 35 dimension of the three dimensions. Optionally, each of the fibres has three mutually perpendicular dimensions, a largest dimension of the three dimensions being at least 1.5, 2, 3, 5, 10, or 20 times larger than a second largest dimension of the three dimensions. Typically in examples, the aerosol-forming substrate comprises, on a dry weight basis, 1.5 to 8 wt% of a binder, preferably 2 to 6 wt% of a binder, more preferably 4 to 5 wt% of a binder. Suitable binders are well-known in the art and include, but are not limited to, natural pectins, such as fruit, citrus or tobacco pectins; guar gums, such as hydroxyethyl guar and hydroxypropyl 5 guar; locust bean gums, such as hydroxyethyl and hydroxypropyl locust bean gum; alginate; starches, such as modified or derivitized starches; celluloses, such as methyl, ethyl, ethylhydroxymethyl and carboxymethyl cellulose; tamarind gum; dextran; pullalon; konjac flour; xanthan gum and the like. It may be preferable for the binder to be or comprise guar. It may be particularly preferable for the binder to comprise or consist of one or both of carboxymethyl 10 cellulose or hydroxypropyl cellulose or a gum such as guar gum. In some examples, the fibres and the binder are different. The fibres and the binder may have a different composition and / or a different form. When the fibres and the binder have a different composition, then they may comprise different compounds in the substrate. For example, the fibres and the binder are mutually exclusive compounds in the substrate. 15 In some examples, the fibres and the binder comprise different forms in the substrate. In some examples, the fibres are non-binding. In some examples, the binder is non-fibrous. In some examples, the binder is in a particulate form. For example, the binder may be ground. 20 For example, the binder may be granulated. For example, the binder may be in a powder form. In some examples, the aerosol-forming substrate further comprises an acid. Typically, the substrate comprises, on a dry weight basis, from 0.1 to 5 wt % acid, preferably 0.5 to 4 wt % acid, more preferably 1 to 2 wt% acid. 25 Typically, the acid comprises a carboxylic acid. In examples, the acid is selected from the group consisting of fumaric acid, lactic acid, benzoic acid and levulinic acid. In some examples, the aerosol-forming substrate further comprises nicotine. Typically, the substrate comprises, on a dry weight basis, from 0.01 to 5 wt% nicotine, 30 preferably 0.5 to 4.5 wt% nicotine, more preferably 1 to 4 wt% nicotine, more preferably 1.5 to 3.5 wt% nicotine, more preferably still 2 to 3 wt% nicotine. In some examples, the bulk density of the substrate is of from 1.0 to 2.0 g / cm3. For example, the bulk density of the substrate may be of from 1.1 to 1.5 g / cm3, preferably 1.2 to 1.4 g / cm3and more preferably around 1.3 g / cm3. 35 Advantageously, it has been found that a bulk density within the aforementioned range can enable a well-formed substrate having an improved tensile strength, for example, compared to known substrates in the art. Optionally, the mineral compound is substantially homogeneously distributed throughout the aerosol-forming substrate. Optionally, the aerosol former is substantially homogeneously distributed throughout the aerosol-forming substrate. Optionally, the fibres are substantially homogeneously distributed throughout the aerosol- forming substrate. Optionally, the binder is substantially homogeneously distributed throughout the aerosol- forming substrate. Advantageously, a homogenous distribution of components of the substrate may result in the substrate have more spatially uniform properties. Advantageously, a substantially homogeneously distributed mineral compound may result in the substrate having a substantially uniform thermal conductivity. As another example, substantially homogeneously distributed binder or fibres may result in the substrate having a substantially uniform tensile strength. In some examples, the substrate further comprises carbon particles. For example, the carbon particles are selected from the group consisting of graphite, expanded graphite, graphene, carbon nanotubes, charcoal, and diamond. Typically, the thermal conductivity of the carbon particles is of from 1 W m−1.K−1to 250 W m−1.K−1. For example, the thermal conductivity of the carbon particles is of from 1 W m−1.K−1to 250 W m−1.K−1at 25 degrees Celsius. In some examples, the thermal conductivity of the carbon particles is of from 5 W m−1K−1to 230 W m−1K−1at 25 degrees Celsius. In some examples, the thermal conductivity of the carbon particles is of from 10 W m−1.K−1to 220 W m−1.K−1at 25 degrees Celsius. In some examples, the thermal conductivity of the carbon particles is of from 15 W m−1K−1to 210 W m−1.K−1at 25 degrees Celsius. In some examples, the thermal conductivity of the carbon particles is of from 20 W m−1.K−1to 200 W m−1K−1at 25 degrees Celsius. In some examples, the thermal conductivity of the carbon particles is higher than the thermal conductivity of the mineral compound. Advantageously, including carbon particles in the substrate may further increase the thermal conductivity of the substrate. Therefore, the aforementioned advantages of increasing the thermal conductivity of the aerosol-forming substrate may be enhanced. Advantageously, it is possible to tailor the thermal conductivity of the substrate using carbon particles. For example, it is possible to increase the thermal conductivity of the substrate by including carbon particles in the substrate. In some examples, the carbon particles comprise or consist of graphite. “Graphite” in the present invention is considered to be “non-expanded graphite” or “natural graphite”. In other examples, the carbon particles comprise or consist of expanded graphite. In the present invention, “expanded graphite” refers to a graphite-based material, or a material having a graphite-like structure. Expanded graphite may have carbon layers (similar to natural graphite, for example) with spacing between the carbon layers greater than the spacing found between carbon layers in natural graphite. Expanded graphite may have carbon layers with elements or compounds intercalated into spaces between the carbon layers. Typically, the aerosol-forming substrate comprises, on a dry weight basis, 1 to 50 wt% of carbon particles. The aerosol-forming substrate may comprise, on a dry weight basis, 3 to 40 wt% 5 of carbon particles, preferably 5 to 35 wt % of carbon particles, more preferably 10 to 30 wt% of carbon particles, and even more preferably 15 to 25 wt% carbon particles. Typically, the substrate further comprises a flavourant. For instance, the flavourant may be present as a coating on a surface of the aerosol-forming substrate. 10 For example, the flavourant may be present as a layer on a surface of the aerosol-forming substrate. In some examples, the flavourant is distributed homogenously throughout the substrate. In some examples, the flavourant is sprayed onto a surface of the aerosol-forming substrate. 15 In some examples, the substrate further comprises a tobacco-containing material. For example, the tobacco-containing material is selected from tobacco leaf, fragments of tobacco ribs, reconstituted tobacco, homogenised tobacco, extruded tobacco and expanded tobacco. The substrate may comprise, on a dry weight basis, less than 10, 5, 3, 2, or 1 wt % tobacco. 20 In other examples, the substrate is a tobacco-free aerosol-forming substrate. A tobacco- free aerosol-generating substrate does not comprise tobacco. Thus, the tobacco-free aerosol- forming substrate is substantially free of tobacco (for example, the tobacco-free aerosol-forming substrate contains from less than 1 weight percent of tobacco, preferably of from less than 0.5 weight percent of tobacco, and even more preferably of from less than 0.1 weight percent of 25 tobacco) or does not contain any determinable amount of tobacco. In some embodiments, the tobacco-free aerosol-forming substrate may comprise a nicotine- formulation. Preferably, the nicotine-formulation is not derived from a tobacco-containing material. For example, the aerosol-forming substrate may comprise a nicotine formulation which 30 comprises a gel composition that includes nicotine, at least one gelling agent and an aerosol former. The substrate may comprise or may be in the form of cut-filler, powder particles, granules, pellets, shreds, spaghettis, strips, threads, ribbons or sheets. Typically, the aerosol-forming substrate comprises, or is in the form of, one or more sheets 35 or strips. Optionally, the or each sheet or strip has a thickness of at least 5, 10, 20, 50, 100, 150, or 200 microns. Optionally, the or each sheet or strip has a thickness of no more than 2000, 1000, 500, 400, 300, or 250 microns. Optionally, the or each sheet or strip has a thickness of between 5 and 400, 20 and 350, 50 and 300, 100 and 200 microns. Optionally, the or each sheet or strip has a width of at least 20, 30, 50, 100, or 200milimeters. Optionally, the or each sheet or strip has a width of no more than 500, 400, 300, 250, 200, 100 or 50 millimetres. For example, the or each sheet or strip has a width of from 20 to 500 mm, preferably 30 to 300 microns, more preferably 50 to 200 mm. Optionally, the or each sheet or strip has a length of at least 1 millimetre, 2 millimetres, 5 millimetres, 8 millimetres, 10 millimetres, or 12 millimetres. Optionally, the or each sheet or strip has a length of no more than 20 millimetres, 18 millimetres, 16 millimetres, 14 millimetres or 12 millimetres. Typically, the or each sheet or strip has a length of between 1 and 18 millimetres, preferably 5 and 15 millimetres, more preferably 10 and 12 millimetres. In particularly preferred examples, the or each sheet or strip has a length of 11 millimetres. Advantageously, the thicknesses, widths and lengths set out above have been found to facilitate the production of a suitable quantity of an aerosol with desirable properties upon heating by a heating element of an aerosol-generating device. Optionally, the or each sheet or strip has a grammage of at least 20, 50, 100 g / m2, 150 or 200 g / m2. Optionally, the or each sheet or strip has a grammage of no more than 300 g / m2. Optionally, the or each sheet or strip has a grammage of from 20 and 300 g / m2, preferably 50 and 250 g / m2, more preferably 100 and 225 g / m2, and more preferably still 150 and 220 g / m2. Typically, the substrate is in the form of a sheet. The sheet may be crimped and gathered to form a rod. The rod may be for use in an aerosol-generating article. A susceptor element may be located within the rod of aerosol-forming substrate. The susceptor element may be an elongate susceptor element. The susceptor element may extend longitudinally within the rod of aerosol-forming substrate. The rod may be substantially cylindrical, for example right cylindrical, in shape. The susceptor element may be positioned in a radially central position within the rod of aerosol-forming substrate. The susceptor element may extend along a central, longitudinal axis of the rod of aerosol- forming substrate. The susceptor element may extend all the way to a downstream end of the rod of aerosol- forming substrate. The susceptor element may extend all the way to an upstream end of the rod of aerosol- forming substrate. 5 The susceptor element may have substantially the same length as the rod of aerosol- forming substrate. The susceptor element may extend from the upstream end to the downstream end of the rod of aerosol-forming substrate. The susceptor element may be in the form of a pin, rod, strip or blade. 10 The susceptor element may have a length of between 5 and 15 millimetres, preferably 6 and 12 millimetres, more preferably 8 and 11 millimetres. In particularly preferred examples, the susceptor element has a length of 11 millimetres. The susceptor element may have a width of between 1 and 5 millimetres. For example, the susceptor element may have a length of around 4 millimetres, for example 4 millimetres. 15 The susceptor element may have a thickness of between 0.01 and 2, 0.5 and 2, or 0.5 and 1 millimetres. Suitable susceptor materials include but are not limited to: carbon, carbon-based materials, graphene, graphite, expanded graphite, molybdenum, silicon carbide, stainless steels, niobium, aluminium, nickel, nickel-containing compounds, titanium, and composites of metallic 20 materials. Suitable susceptor materials may comprise a ferromagnetic material, for example, ferritic iron, a ferromagnetic alloy, such as ferromagnetic steel or stainless steel, ferromagnetic particles, and ferrite. A suitable susceptor material may be, or comprise, aluminium. A susceptor material preferably comprises more than 5 percent, preferably more than 20 percent, more preferably more than 50 percent or more than 90 percent of ferromagnetic or paramagnetic 25 materials. Preferred susceptor materials may comprise a metal, metal alloy or carbon. Particularly preferred susceptor materials may be, or comprise, carbon, carbon-based materials, graphene, graphite, or expanded graphite. Advantageously, such materials have relatively high thermal conductivities, relatively low densities, and may be inductively heated. In some examples, there is no susceptor materials present in the aerosol-forming substrate 30 or in the rod of aerosol-forming substrate. Preferably, the aerosol-forming substrate does not comprise a susceptor element. For example, the aerosol-forming substrate may be heated with an external heater. The external heater may be a resistive heater. In other examples, the aerosol-forming substrate may be heated using dielectric heating. 35 For example, the aerosol-forming substrate may be heated using microwave electromagnetic radiation. Dielectric heating, also sometimes referred to as electronic heating, radio frequency (RF) heating, high-frequency (HF) heating, or microwave heating is a heating method where a dielectric material is subjected to high-frequency electric field or microwave electromagnetic radiation to thereby heat the dielectric material. This heating is caused by molecular dipole rotation of the dielectric material (for example an aerosol-forming substrate that has dielectric properties), caused by the alternating polarization of the dielectric material stimulated by the alternating electric field. This dipole rotation causes the molecules to push, pull, and collide with 5 other molecules, distributing the energy to adjacent molecules of the dielectric material, and thereby causing heating of the material. According to a second aspect of the invention, provided is an aerosol-generating article. The article may comprise an aerosol-forming substrate as described above, for example the 10 aerosol-forming substrate according to the first aspect. Optionally, the article is in the form of a rod and comprises a plurality of components, including the aerosol-forming substrate, assembled within a wrapper or casing. Optionally, the aerosol-generating article comprises a front plug. Optionally, the aerosol-generating article comprises a cooling section. For example, the 15 cooling section may comprise a first hollow tube, for example a first hollow acetate tube. The cooling section may comprise a second hollow tube, for example a second hollow acetate tube. Optionally, the second hollow tube comprises one or more ventilation holes. Optionally, the aerosol-generating article comprises a mouthpiece filter. Typically, the cooling section is arranged downstream of the aerosol-generating substrate 20 and upstream of a mouthpiece filter. Optionally, the aerosol-generating article comprises wrapper, for example a paper wrapper. Optionally, the front plug is arranged at the most upstream end of the article. Optionally, the aerosol-forming substrate is arranged downstream of the front plug. 25 Optionally, the first hollow tube is arranged downstream of the aerosol-forming substrate. Optionally, the second hollow tube is arranged downstream of the first hollow tube. Optionally, the mouthpiece filter is arranged downstream of one or both of the first hollow tube and the second hollow tube. Optionally, the mouthpiece filter is arranged at a most downstream end of the article. 30 Optionally, the most downstream end of the article, which may be referred to as a proximal end of the article, may be configured for insertion into a mouth of a user. A user may be able to inhale on, for example directly on, the proximal end of the article. Optionally, the front plug, the aerosol-forming substrate, one or both of the first hollow tube and the second hollow tube, and the mouth plug filter are circumscribed by a wrapper, for example 35 a paper wrapper. Optionally, the front plug has a length of between 2 and 10, 3 and 8, or 4 and 6 mm, for example around 5 mm. Optionally, the aerosol-forming substrate has a length of from 1 to 18 mm, preferably 5 to 15 mm and more preferably of from 10 to 12 mm. In some preferred examples, the aerosol- forming substrate has a length of 11 mm. Optionally, the first hollow tube has a length of between 2 and 20, 5 and 15, or 5 and 10 5 mm, for example around 8 mm. Optionally, the second hollow tube has a length of between 2 and 20, 5 and 15, or 5 and 10 mm, for example around 8 mm. Optionally, the mouthpiece filter has a length of between 5 and 20, 8 and 15, or 10 and 15 mm, for example around 12 mm. 10 The lengths of one or more of the front plug, the aerosol-forming substrate, the first hollow tube, the second hollow tube, and the mouth plug filter may extend in a longitudinal direction. According to a further aspect of the invention, provided is an aerosol-generating system. The system may comprise an aerosol-generating article, and an electrical aerosol- generating device for heating the aerosol-forming substrate of the aerosol generating article. The 15 aerosol-generating article may that as described above. Optionally, the electrical aerosol-generating device is configured to resistively heat the aerosol-generating article in use. Optionally, the electrical aerosol-generating device is configured to inductively heat the aerosol-generating article, for example the aerosol forming substrate when a susceptor material 20 is present, of the aerosol-generating article, in use. According to the present disclosure, there is provided a method of forming an aerosol- forming substrate, for example a substrate as described above such as the substrate according to the first aspect. The method may comprise forming a slurry comprising one or more or all of the mineral 25 compound, the aerosol former, the fibres, and the binder. The method may comprise casting and drying the slurry to form the aerosol-forming substrate or a precursor for forming into the aerosol- forming substrate. Thus, according to a further aspect of the invention, provided is a method of forming an aerosol-forming substrate according to the first aspect of the invention, the method comprising: 30 forming a slurry comprising the mineral compound, the aerosol former, the fibres, and the binder; and casting and drying the slurry to form the aerosol-forming substrate or a precursor for forming into the aerosol-forming substrate. Advantageously, provided is a straightforward method of forming the aerosol-forming substrate. Beneficially, the method according to the invention can enable the substrate to be 35 produced in a variety of forms, for example, cut-filler, powder particles, granules, pellets, shreds, spaghettis, strips or sheets. Typically, the slurry comprises water. For example, the slurry may comprise from 40 to 90 wt % water, optionally 40 and 85 wt % water, optionally 50 and 80 wt % water, optionally 60 and 80 wt % water, or optionally 60 and 75 wt % water. Typically, the slurry comprises an acid. Optionally, the acid comprises a carboxylic acid. 5 For example, the acid may be selected from the group consisting of fumaric acid, lactic acid, benzoic acid, and levulinic acid. In some examples, the slurry comprises nicotine. Typically, the slurry comprises a flavourant. Optionally, forming the slurry comprises forming a first mixture. The first mixture may 10 comprise the aerosol former. The first mixture may comprise the fibres. The first mixture may comprise water. The first mixture may comprise the acid. The first mixture may comprise the nicotine. Forming the slurring may comprise forming a second mixture. The second mixture may comprise the mineral compound. The second mixture may comprise the binder. Forming the slurry may comprise adding the second mixture to the first mixture to form a combined mixture. 15 Thus, forming the slurry may comprise: forming a first mixture comprising the aerosol former, the fibres, water, optionally, the acid, and optionally, the nicotine; forming a second mixture comprising the mineral compound and the binder; and adding the second mixture to the first mixture to form a combined mixture. 20 The combined mixture may subsequently be formed into the slurry, for example by mixing. Optionally, forming the first mixture comprises providing the aerosol former or a solution comprising the aerosol former and the nicotine. Optionally, forming the first mixture comprises adding the acid to the aerosol former or the solution comprising the aerosol former and the nicotine to form a first pre-mixture. 25 Optionally, forming the first mixture comprises adding the water to the aerosol former or the solution comprising the aerosol former and the nicotine, or to the first pre-mixture, to form a second pre-mixture. Optionally, forming the first mixture comprises adding the fibres to the second pre-mixture. Optionally, forming the second mixture comprises mixing the mineral compound and the 30 binder. Optionally, the method, for example the step of forming the slurry, comprises a first mixing of the combined mixture. Optionally, the first mixing occurs under a first pressure of no more than 500, 400, 300, 250, or 200 mbar. 35 Optionally, the first mixing occurs for between 1 and 10, 2 and 8, or 3 and 6 minutes, for example for around 4 minutes. Optionally, the method, for example the step of forming the slurry, comprises, after the first mixing, a second mixing. Optionally, the second mixing occurs under a second pressure which is less than the first pressure. Optionally, the second pressure is no more than 500, 400, 300, 200, 150, or 100 mbar. Optionally, the second mixing occurs for between 5 and 120, 5 and 80, 5 and 40, or 10 and 30 5 seconds, for example around 20 seconds. Optionally, casting the slurry comprises casting the slurry onto a flat support, for example a steel flat support. Optionally, after casting the slurry and before drying the slurry, the method comprises setting a thickness of the slurry, for example setting a thickness of the slurry to between 100 and 10 1200, 200 and 1000, 300 and 900, 500 and 700 microns, for example around 600 microns. Optionally, drying the slurry comprises providing a flow of a gas such as air over or past the slurry. Optionally, the flow of gas is heated. Optionally, the flow of gas is heated to a temperature of between 100 and 160, or 120 and 15 140 degrees Celsius. Optionally, the flow of gas is provided for between 1 and 10 or 2 and 5 minutes. Optionally, drying the slurry comprises drying the slurry until the slurry has a moisture content of between 1 and 20, 2 and 15, 2 and 10, or 3 and 7 wt %. Optionally, drying the slurry forms the precursor for forming into the aerosol-forming 20 substrate, the precursor being a sheet of aerosol-forming material. Optionally, the method comprises cutting the sheet of aerosol-forming material. In some examples, a flavourant is sprayed onto the aerosol-forming substrate after it has been cast and dried. Alternative methods for forming the aerosol-forming substrate are possible. 25 For example, the aerosol-forming substrate may be formed using a “dough process”. Typically, a dough process for forming the aerosol-forming substrate comprises mixing a mineral compound, an aerosol former, fibres, and a binder with water to form a mixture. The volume of water typically used in the dough process is less than that typically used when preparing the aerosol-forming substrate using a slurry process e.g. as described above. For example, the 30 mixture may comprise from 25 to 35 wt % water, based on the weight of the mixture. The mixture can be kneaded e.g. in a dough mixer, to form a dough-mixture having a dough-like consistency. The dough-mixture can be flattened e.g. using rollers, to form a sheet, which is dried to form of the aerosol-forming substrate. As referenced, the term “aerosol-forming substrate” refers to a substrate capable of 35 releasing an aerosol or volatile compounds that can form an aerosol. Such volatile compounds may be released by heating the aerosol-forming substrate. An aerosol-forming substrate may comprise an aerosol-forming material. An aerosol-forming substrate may be adsorbed, coated, impregnated or otherwise loaded onto a carrier or support. An aerosol-forming substrate may conveniently be part of an aerosol-generating article. As referenced, the term “heated aerosol-generating article” refers to an aerosol-generating article for producing an aerosol comprising an aerosol-forming substrate that is intended to be heated rather than combusted in order to release volatile compounds that can form an aerosol. As referenced, the term “aerosol former” refers to a component that can be volatilized and convey a desired substance, for example, nicotine and / or flavouring, in an aerosol when the aerosol-generating material is heated above the specific volatilization temperature of the aerosol former. An aerosol former may be any suitable compound or mixture of compounds that, in use, facilitates formation of a dense and stable aerosol and is substantially resistant to thermal degradation at the operating temperature of the heated aerosol-generating article. Different aerosol formers vaporize at different temperatures. Thus, an aerosol former may be chosen based on its ability to remain stable at or around room temperature but volatize at a higher temperature, for example from 40 to 450 degrees Celsius. In the present invention, the term “mineral compound” refers to a compound which is naturally occurring or non-naturally occurring (for example, “man-made”), is crystalline or non- crystalline, is solid at room temperature (25 degrees Celsius) and atmospheric pressure (1.01 x105Pa), and contains silicon and / or an alkaline earth metal. The term “alkaline earth metal” refers to the elements contained in group 2 of the periodic table. Specifically, the elements in group 2 of the periodic table refers to beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra). In the context of the present invention, the term “alkaline earth metal” refers to beryllium (Be), magnesium (Mg), calcium (Ca) or strontium (Sr). As referred to, the term “clay mineral” refers to a compound that is a hydrous aluminium phyllosilicate. As referred to, “kaolinite” is a naturally occurring layered silicate with a tetrahedral sheet of silica (SiO4) linked through oxygen atoms to an octahedral sheet of alumina (AlO6). Kaolinite has the chemical composition Al2Si2O5(OH)4. As referred to, “diatomite” is a naturally occurring compound having silica (SiO2) as its principal constituent. A typical dried chemical composition of diatomite is 80-90 % silica, 2-4 % alumina and 0.5 to 2 % iron oxide. As referenced, “celite” is a naturally occurring compound having silica as its main constituent. A typical chemical composition of celite is 80–90% silica, with 2– 4% alumina (attributed mostly to clay minerals), and 0.5–2% iron oxide. As referenced, “perlite” refers to the naturally occurring material having a composition with silicon dioxide as its principal constituent. For example, the typical chemical composition of perlite is 70-75 % silicon dioxide, 12-15 % aluminium oxide, 3-4 % sodium oxide, 3-5 % potassium oxide, 0.5-2% iron oxide, 0.2-0.7 % magnesium oxide and 0.5-1.5 % calcium oxide. As referred to, “zeolite” refers to a group of crystalline aluminosilicate materials mainly containing silicon, aluminium and oxygen, and having the general formula of Mn+1 / n(AlO2)(SiO2).yH2O, where Mn+1 / n is a metal ion (e.g. Na+) or H+. As referenced, “calcium sulfate” refers to the naturally occurring salt having the chemical formula CaSO4 and its associated hydrates. Calcium sulfate naturally exists in three levels of hydration corresponding to different crystallographic structures. Calcium sulfate exists as CaSO4, which is known as “anhydrite”, CaSO4.2H2O, known as “gypsum” and CaSO4.1 / 2H2O, known as “bassanite”. As referred to, “calcium carbonate” refers to the naturally occurring salt having the chemical formula CaCO3, and its associated hydrates. Calcium carbonate exists as three different polymorphs known as calcite, aragonite and vaterite. As referred to, “magnesium sulfate” refers to the naturally occurring salt having the chemical formula MgSO4 and its associated hydrates. Magnesium sulfate can crystallise as several different hydrates, including MgSO4.7H2O, which is also known as “epsomite” or “epsom salt”. As referred to, “magnesium carbonate” refers to the naturally occurring salt having the chemical formula MgCO3, and its associated hydrates. As referred to, “talc” refers to a hydrated magnesium silicate having the chemical formula Mg3Si4O10(OH)2. As referred to, “thermal conductivity” is a measure of a material’s ability to conduct heat, measured in W·m−1·K−1. The term “tensile strength” is used throughout the specification to indicate a measure of the force required to stretch a sheet of homogenised plant material until it breaks. More specifically, the tensile strength is the maximum tensile force per unit width that the sheet material will withstand before breaking and is measured in the machine direction or cross direction of the sheet material. It is expressed in units of Newtons per meter of material (N / m). Tests for measuring the tensile strength of a sheet material are well known. A suitable test is described in the 2014 publication of the International Standard ISO 1924-2 entitled “Paper and Board – Determination of Tensile Properties – Part 2: Constant Rate of Elongation Method”. As used herein, the term “grammage" is equivalent to the “basis weight” of the sheet of solid aerosol-generating substrate and is defined as equal to the mass per unit area of the sheet. The grammage may be determined in accordance with ISO Standard 536:2012. As used herein, the term “thickness” is used to describe the smallest dimension of the sheet of solid aerosol-generating substrate. The thickness is measured perpendicular to the length and width of the sheet. The thickness may be determined in accordance with ISO Standard 534:2012. As referred to, “bulk density” is a material property defined as the mass of the particles of the material divided by the bulk volume. Bulk density of the aerosol-generating substrate can be calculated by measuring the total weight of the aerosol-generating substrate and dividing this by the volume of the segment of aerosol-generating substrate. As used herein, the term “particle size” may refer to a single dimension and may be used to characterise the size of a given particle. The dimension may be the diameter of a spherical particle occupying the same volume as the given particle. All particle sizes and particle size distributions herein can be obtained using a standard laser diffraction technique. Particle sizes 5 and particle size distributions as stated herein may be measured using a commercially available sensor, for example a Sympatec HELOS laser diffraction sensor. As used herein, the term “rod” may refer to a generally cylindrical, for example right cylindrical, element of substantially circular, oval or elliptical cross-section. As used herein, a “sheet” may refer to a generally planar, laminar element having a width 10 and a length which are substantially greater than, for example at least 2, 3, 5, 10, 20 or 50 times, its thickness. As used herein, the term “length” refers to the major dimension in a longitudinal direction of the aerosol-generating device, of an aerosol-generating article, or of a component of the aerosol-generating device or of an aerosol-generating article. 15 As used herein, the term “width” refers to the major dimension in a transverse direction of the aerosol-generating device, of an aerosol-generating article, or of a component of the aerosol- generating device or of an aerosol-generating article, at a particular location along its length. A “gathered sheet” may refer to a sheet of an aerosol-forming substrate or aerosol- generating article that is convoluted, folded, or otherwise compressed or constricted substantially 20 transversely to a longitudinal axis of the aerosol-forming substrate, or aerosol-generating article. The term “crimped” may refer to a sheet having one or more ridges or corrugations. The ridges or corrugations may be substantially parallel. When present in a component of an aerosol- generating article, the ridges or corrugations may extend in a longitudinal direction with respect to the aerosol-generating article. 25 Aerosol-generating articles according to the present invention are suitable for use in an aerosol-generating system comprising an electrically heated aerosol-generating device having an internal heater element for heating the aerosol-generating substrate. For example, aerosol generating articles according to the invention find particular application in aerosol-generating systems comprising an electrically heated aerosol-generating device having an internal heater 30 blade which is adapted to be inserted into the rod of aerosol-generating substrate. Aerosol generating articles of this type are described in the prior art, for example, in European patent application EP-A-0822670. As referenced, the term “aerosol-generating device” refers to a device comprising a heater element that interacts with the aerosol-generating substrate of the aerosol-generating article to 35 generate an aerosol. The invention is defined in the claims. However, below there is provided a non-exhaustive list of non-limiting examples. Any one or more of the features of these examples may be combined with any one or more features of another example, embodiment, or aspect described herein. Example EX1. An aerosol-forming substrate comprising, on a dry weight basis: 10 to 90 wt % of a mineral compound; 7 to 60 wt% of an aerosol former; 1 to 20 wt% of fibres; and 1 to 10 wt% of a binder; and wherein the mineral compound comprises silicon and / or an alkaline earth metal, preferably wherein the mineral compound comprises kaolinite, perlite, zeolite, bentonite, calcium sulfate, magnesium sulfate, magnesium carbonate or talc. Example EX2. The aerosol-forming substrate according to Example EX1, wherein the mineral compound comprises a thermal conductivity of from 0.1 to 5 W m−1.K−1in at least one direction. Example EX3. The aerosol-forming substrate according to any of Examples EX1 to EX2, wherein the mineral compound comprises a thermal conductivity of from 0.12 to 3.5 W m−1.K−1in at least one direction. Example EX4. The aerosol-forming substrate according to any of Examples EX 1 to 3, wherein the mineral compound comprises a thermal conductivity of from 0.14 to 3 W m−1.K−1in at least one direction. Example EX5. The aerosol-forming substrate according to any of Examples EX 1 to 4, wherein the mineral compound comprises a thermal conductivity of from 0.15 to 1.5 W m−1.K−1in at least one direction, preferably of from 0.15 to 0.7 W m−1.K−1, more preferably of from 0.15 to 0.3 W m−1K−1. Example EX5A. The aerosol-forming substrate according to any of Examples EX 1 to 5, wherein the mineral compound comprises a thermal conductivity of less than 1.0 W m−1.K−1. Example EX6. The aerosol-forming substrate according to any of Examples EX 1 to 5A, wherein the alkaline earth metal is present as a cation. Example EX7. The aerosol-forming substrate according to any of Examples EX 1 to 6, wherein the alkaline earth metal is present as a salt. Example EX8. The aerosol-forming substrate according to any of Examples EX 1 to 7, the alkaline earth metal is selected from beryllium (Be), magnesium (Mg), calcium (Ca), or strontium (Sr). Example EX9. The aerosol-forming substrate according to any of Examples EX 1 to 8, wherein the alkaline earth metal is calcium or magnesium, preferably wherein the alkaline earth metal is magnesium. Example EX10. The aerosol-forming substrate according to any of Examples EX 1 to 9, wherein the mineral compound is a clay mineral. Example EX11. The aerosol-forming substrate according to any of Examples EX 1 to 10, wherein the mineral compound comprises or consists of kaolinite, diatomite, perlite, zeolite, calcium sulfate, calcium carbonate, magnesium sulfate, magnesium carbonate or talc. Example EX11A. The aerosol-forming substrate according to any of Examples EX1 to EX11, wherein the mineral compound consists of kaolinite, perlite, zeolite, bentonite, calcium sulfate, magnesium sulfate, magnesium carbonate or talc. EX11B. The aerosol-forming substrate according to any of Examples EX1 to EX11A, 5 wherein the mineral compound comprises or consists of kaolinite, perlite, zeolite, calcium sulfate, magnesium sulfate, magnesium carbonate or talc. Example EX12. The aerosol-forming substrate according to any of Examples EX 1 to 11B, wherein the mineral compound comprises or consists of kaolinite, diatomite, perlite, zeolite or talc. 10 Example EX13. The aerosol-forming substrate according to any of Examples EX 1 to 12, wherein the mineral compound is kaolinite. Example EX13A. The aerosol-forming substrate according to any of Examples EX 1 to 12, wherein the mineral compound is bentonite. Example EX13B. The aerosol-forming substrate according to any of Examples EX1 to 15 EX13A, wherein the mineral compound does not comprise carbon. Example 13B. The aerosol-forming substrate according to any of Examples EX1 to EX13B, wherein the mineral compound is not silicon carbide. Example EX14. The aerosol-forming substrate according to any of Examples EX 1 to 13B, wherein the mineral compound comprises a pH of from 4 to 6 in water. 20 Example EX15. The aerosol-forming substrate according to any of Examples EX 1 to 14, wherein the mineral compound has a particle size distribution having a volume D90 particle size of from 3 to 200 microns. Example EX16. The aerosol-forming substrate according to any of Examples EX 1 to 15, wherein the mineral compound has a particle size distribution having a volume D90 particle 25 size of from 4 to 150 microns. Example EX17. The aerosol-forming substrate according to any of Examples EX 1 to 16, wherein the mineral compound has a particle size distribution having a volume D90 particle size of from 5 to 100 microns. Example EX18. The aerosol-forming substrate according to any of Examples EX 1 to 17, 30 wherein the mineral compound has a particle size distribution having a volume D90 particle size of from 6 to 50 microns Example EX19. The aerosol-forming substrate according to any of Examples EX 1 to 18, wherein the mineral compound has a particle size distribution having a volume D90 particle size of from 7 to 25 microns. 35 Example EX20. The aerosol-forming substrate according to any of Examples EX 1 to 19, wherein the aerosol forming substrate comprises, on a dry weight basis, 20 to 80 wt% of the mineral compound. Example EX21. The aerosol-forming substrate according to any of Examples EX 1 to 20, wherein the aerosol forming substrate comprises, on a dry weight basis, 25 to 75 wt% of the mineral compound. Example EX22. The aerosol-forming substrate according to any of Examples EX 1 to 21, 5 wherein the aerosol forming substrate comprises, on a dry weight basis, 30 to 70 wt % of the mineral compound. Example EX23. The aerosol-forming substrate according to any of Examples EX 1 to 22, wherein the aerosol-forming substrate comprises, on a dry weight basis, 10 to 50 wt% of the aerosol former. 10 Example EX24. The aerosol-forming substrate according to any of Examples EX 1 to 23, wherein the aerosol-forming substrate comprises, on a dry weight basis, 12 to 40 wt% of the aerosol former. Example EX25. The aerosol-forming substrate according to any of Examples EX 1 to 24, wherein the aerosol-forming substrate comprises, on a dry weight basis, 15 to 30 wt% of 15 the aerosol former. Example EX26. The aerosol-forming substrate according to any of Examples EX 1 to 25, wherein the aerosol former is selected from the group consisting of propylene glycol, triethylene glycol, 1,3-butanediol, glycerine, glycerol monoacetate, glycerol diacetate, glycerol triacetate, dimethyl dodecanedioate, and dimethyl tetradecanedioate. 20 Example EX27. The aerosol-forming substrate according to any of Examples EX 1 to 26, wherein the aerosol-forming substrate comprises, on a dry weight basis, 1.5 to 15 wt% of fibres. Example EX28. The aerosol-forming substrate according to any of Examples EX 1 to 27, wherein the aerosol-forming substrate comprises, on a dry weight basis 2 to 10 wt% of 25 fibres. Example EX29. The aerosol-forming substrate according to any of Examples EX 1 to 28, wherein the aerosol-forming substrate comprises, on a dry weight basis 3 to 12 wt% of fibres. Example EX30. The aerosol-forming substrate according to any of Examples EX 1 to 29, 30 wherein the aerosol-forming substrate comprises, on a dry weight basis 5 to 10 wt% of fibres. Example EX31. The aerosol-forming substrate according to any of Examples EX 1 to 30, wherein the fibres comprise cellulose fibres. Example EX32. The aerosol-forming substrate according to any of Examples EX 1 to 31, 35 wherein the fibres comprises or consists of one or both of carboxylmethyl cellulose or hydroxypropyl cellulose. Example EX32A. The aerosol-forming substrate according to any of Examples EX 1 to 32, wherein the fibres and the binder comprise different compounds in the substrate. Example EX32B. The aerosol-forming substrate according to any of Examples EX 1 to 32A, wherein the fibres and the binder comprise different forms in the substrate. Example EX32C. The aerosol-forming substrate according to any of Examples EX1 to EX32B, wherein the fibres are non-binding, or the binder is non-fibrous, or the binder is in a particulate form, or the binder is ground, or the binder is granulated or the binder is in a powder form. Example EX33. The aerosol-forming substrate according to any of Examples EX 1 to 32C, wherein the aerosol-forming substrate comprises, on a dry weight basis, 1.5 to 8 wt% of a binder, preferably 2 to 6 wt% of a binder and more preferably 4 to 5 wt% of a binder. Example EX34. The aerosol-forming substrate according to any of Examples EX 1 to 33, wherein the aerosol-forming substrate further comprises an acid. Example EX35. The aerosol-forming substate according to Example EX 34, wherein the substrate comprises, on a dry weight basis, from 0.1 to 5 wt % acid, preferably 0.5 to 4 wt % acid, more preferably 1 to 2 wt% acid. Example EX36. The aerosol-forming substrate according to any of Examples EX 34 to 35, wherein the acid is a carboxylic acid, preferably selected from the group consisting of fumaric acid, lactic acid, benzoic acid and levulinic acid. Example EX37. The aerosol-forming substrate according to any of Examples EX 1 to 36, wherein the aerosol-forming substrate further comprises nicotine. Example EX38. The aerosol-forming substate according to Example EX 37, wherein the substrate comprises, on a dry weight basis, 0.01 to 5 wt% nicotine, preferably 0.5 to 4.5 wt% nicotine, more preferably 1 to 4 wt% nicotine, more preferably 1.5 to 3.5 wt% nicotine, and more preferably still 2 to 3 wt% nicotine. Example EX39. The aerosol-forming substrate according to any of Examples EX 1 to 38, wherein the bulk density of the substrate is of from 1.0 to 2.0 g / cm3, preferably 1.1 to 1.5 g / cm3, more preferably 1.2 to 1.4 g / cm3, and more preferably still around 1.3 g / cm3. Example EX40. The aerosol-forming substrate according to any of Examples EX 1 to 39, wherein the mineral compound is substantially homogenously distributed throughout the aerosol-forming substrate. Example EX41. The aerosol-forming substrate according to any of Examples EX 1 to 40, wherein the substrate further comprises carbon particles. Example EX42. The aerosol-forming substrate according to Example EX 41, wherein the carbon particles are selected from the group consisting of graphite, expanded graphite, graphene, carbon nanotubes, charcoal, and diamond. Example EX43. The aerosol-forming substrate according to any of Examples EX 41 to 42, wherein the thermal conductivity of the carbon particles is of from 1 W m−1.K−1to 250 W m−1.K−1. Example EX44. The aerosol-forming substrate according to any of Examples EX 41 to 43, wherein the thermal conductivity of the carbon particles is of from 1 W m−1.K−1to 250 W m−1.K−1at 25 degrees Celsius. Example EX45. The aerosol-forming substrate according to any of Examples EX 41 to 44, wherein the thermal conductivity of the carbon particles is of from 5 W m−1.K−1to 230 W m−1.K−1at 25 degrees Celsius. Example EX46. The aerosol-forming substrate according to any of Examples EX 41 to 45, wherein the thermal conductivity of the carbon particles is of from 10 W m−1.K−1to 220 W m−1.K−1at 25 degrees Celsius. Example EX47. The aerosol-forming substrate according to any of Examples EX 41 to 46, wherein the thermal conductivity of the carbon particles is of from 15 W m−1.K−1to 210 W m−1.K−1at 25 degrees Celsius. Example EX48. The aerosol-forming substrate according to any of Examples EX 41 to 47, wherein the thermal conductivity of the carbon particles is of from 20 W m−1.K−1to 200 W m−1.K−1at 25 degrees Celsius. Example EX49. The aerosol-forming substrate according to any of Examples EX 41 to 48, wherein the carbon particles comprise graphite. Example EX50. The aerosol-forming substrate according to any of Examples EX 41 to 49, wherein the aerosol-forming substrate comprises, on a dry weight basis, 1 to 50 wt% of carbon particles. Example EX51. The aerosol-forming substrate according to any of Examples EX 41 to 50, wherein the aerosol-forming substrate comprises, on a dry weight basis, 3 to 40 wt% of carbon particles. Example EX52. The aerosol-forming substrate according to any of Examples EX 41 to 51, wherein the aerosol-forming substrate comprises, on a dry weight basis, 5 to 35 wt % of carbon particles. Example EX53. The aerosol-forming substrate according to any of Examples EX 41 to 52, wherein the aerosol-forming substrate comprises, on a dry weight basis, 10 to 30 wt% of carbon particles. Example EX54. The aerosol-forming substrate according to any of Examples EX 41 to 53, wherein the aerosol-forming substrate comprises, on a dry weight basis, 15 to 25 wt% carbon particles. Example EX55. The aerosol-forming substrate according to any of Examples EX 1 to 54, wherein the substrate further comprises a flavourant. Example EX56. The aerosol-forming substrate according to Example EX55, wherein the flavourant is present as a coating on a surface of the aerosol-forming substrate. Example EX57. The aerosol-forming substrate according to any of Examples EX 55 to 56, wherein the flavourant is distributed homogenously throughout the substrate. Example EX58. The aerosol-forming substrate according to any of Examples EX 1 to 57, wherein the substrate further comprises a tobacco-containing material. Example EX59. The aerosol-forming substrate according to Example EX 58, wherein the tobacco-containing material is selected from tobacco leaf, fragments of tobacco ribs, 5 reconstituted tobacco, homogenised tobacco, extruded tobacco and expanded tobacco. Example EX60. The aerosol-forming substrate according to any of Examples EX 58 to 59, wherein the substrate comprises, on a dry weight basis, less than 10, 5, 3, 2, or 1 wt % of tobacco-containing material. Example EX61. The aerosol-forming substrate according to any of Examples EX 1 to 57, 10 wherein the substrate is a tobacco-free aerosol-forming substrate. Example EX62. The aerosol-forming substrate according to any of Examples EX 1 to 61, wherein the substrate comprises or is in the form of cut-filler, powder particles, granules, pellets, shreds, spaghettis, strips or sheets. Example EX63. The aerosol-forming substrate according to Example EX 62, wherein the 15 substrate is in the form of a sheet or strip. Example EX64. The aerosol-forming substrate according to Example EX 63, wherein the thickness of the sheet or strip is between 5 and 400, 20 and 350, 50 and 300, 100 and 200 microns. Example EX65. The aerosol-forming substrate according to any of Examples EX 1 to 64, 20 wherein the aerosol-forming substrate does not comprise a susceptor element. Example EX66. An aerosol-generating article comprising an aerosol-generating substrate according to any of Examples EX1 to 64. Example EX67. The aerosol-generating article according to Example EX 66, wherein the aerosol-generating article comprises a front plug. 25 Example EX68. The aerosol-generating article according to any of Examples EX66 to EX68, wherein the aerosol-generating article comprises a cooling section, preferably wherein the cooling section comprises a first hollow tube, for example a first hollow acetate tube. Example EX69. The aerosol-generating article according to Example EX69, wherein the 30 cooling section comprises a second hollow tube, for example a second hollow acetate tube. Example EX70. The aerosol-generating article according to Example EX69, wherein the second hollow tube comprises one or more ventilation holes. Example EX71. The aerosol-generating article according to any of Examples EX66 to 35 EX70, wherein the aerosol-generating article comprises a mouthpiece filter. Example EX72. The aerosol-generating article according to any of Examples EX66 to EX71, wherein the aerosol-generating article comprises wrapper, for example a paper wrapper. Example EX73. The aerosol-generating article according to any of Examples EX66 to EX72, wherein the aerosol-generating article comprises a front plug, the aerosol-forming substrate arranged downstream of the front plug, a first hollow tube arranged downstream of the aerosol-forming substrate, a second hollow tube arranged 5 downstream of the first hollow tube, and a mouth plug filter arranged downstream of the second hollow tube. Example EX74. The aerosol-generating article according to Example EX 73, wherein the front plug, the aerosol-forming substrate, the first hollow tube, the second hollow tube, and the mouthpiece filter are circumscribed by a wrapper, for example a paper wrapper. 10 Example EX75. The aerosol-generating article according to any of Examples EX 67 or EX 68 to Ex 74 when dependent on Example EX 67, wherein the front plug has a length of between 2 and 10, 3 and 8, or 4 and 6 mm, for example around 5 mm. Example EX76. The aerosol-generating article according to any of Examples EX66 to EX75, wherein the aerosol-forming substrate has a length of from 1 to 18 mm, preferably 15 5 to 15 mm, more preferably of from 10 to 12 mm, particularly preferably 11 mm. Example EX77. The aerosol-generating article according to any of Examples EX 68 or EX 69 to Ex 76 when dependent on example Ex 68, wherein the first hollow tube has a length of between 2 and 20, 5 and 15, or 5 and 10 mm, for example around 8 mm. Example EX78. The aerosol-generating article according to any of Examples EX 69 or EX 20 70 to Ex 77 when dependent on Example EX 69, wherein the second hollow tube has a length of between 2 and 20, 5 and 15, or 5 and 10 mm, for example around 8 mm. Example EX79. The aerosol-generating article according to any of Examples EX 71 or EX 72 to Ex 78 when dependent on Example EX 71, wherein the mouthpiece filter has a length of between 5 and 20, 8 and 15, or 10 and 15 mm, for example around 12 mm. 25 Example EX80. An aerosol-generating system comprising an aerosol-generating article according to any of Examples EX66 to EX79, and an electrical aerosol-generating device for heating the aerosol-forming substrate of the aerosol generating article. Example EX81. The aerosol-generating system according to Example EX 80, wherein the electrical aerosol-generating device is configured to resistively heat the aerosol- 30 generating article in use. Example EX82. The aerosol-generating system according to any of Examples EX 80 to EX 81, wherein the electrical aerosol-generating device is configured to inductively heat the aerosol-generating article, for example the aerosol forming substrate when a susceptor material is present, of the aerosol-generating article, in use. 35 Example EX83. A method of forming an aerosol-forming substrate according to any of claims 1 to 65, the method comprising: forming a slurry comprising the mineral compound, the aerosol former, the fibres, and the binder; and casting and drying the slurry to form the aerosol-forming substrate or a precursor for forming into the aerosol-forming substrate. Example EX84. The method according to claim Example EX83, wherein the slurry comprises water. Example EX85. The method according to Example EX84, wherein the slurry comprises from 40 to 90 wt % water, preferably 40 and 85 wt % water, 50 and 80 wt % water, 60 and 80 5 wt % water, 60 and 75 wt % water. Example EX86. The method according to any of Examples EX83 to 85, wherein the slurry comprises an acid. Example EX87. The method according to any of Examples EX83 to 86, wherein the slurry comprises nicotine. 10 Example EX88. The method according to any of examples EX83 to Ex 87, wherein forming the slurry comprises: forming a first mixture comprising: the aerosol former; the fibres; 15 water; optionally, the acid; and optionally, the nicotine, forming a second mixture comprising: the mineral compound; and 20 the binder, and adding the second mixture to the first mixture to form a combined mixture. Example EX89. The method according to Example EX 88, wherein forming the first mixture comprises providing the aerosol former or a solution comprising the aerosol former and the nicotine. 25 Example EX90. The method according to Example EX89, wherein forming the first mixture comprises adding the acid to the aerosol former or the solution comprising the aerosol former and the nicotine to form a first pre-mixture. Example EX91. The method according to any of Examples EX 88 to EX 90, wherein forming the first mixture comprises adding the water to the aerosol former or the solution comprising 30 the aerosol former and the nicotine, or to the first pre-mixture, to form a second pre-mixture. Example EX92. The method according to any of Examples EX88 to EX91, wherein forming the first mixture comprises adding the fibres to the second pre-mixture. Example EX93. The method according to any of Examples EX88 to EX92, wherein forming the second mixture comprises mixing the mineral compound and the binder. 35 Example EX94. The method according to any of Examples EX88 to EX93, wherein the method comprises a first mixing of the combined mixture. Example EX95. The method according to Example EX94, wherein the first mixing occurs under a first pressure of no more than 500, 400, 300, 250, or 200 mbar. Example EX96. The method according to Example EX94 or EX95, wherein the first mixing occurs for between 1 and 10, 2 and 8, or 3 and 6 minutes, for example for around 4 minutes. Example EX97. The method according to any of Examples EX94 to EX96, wherein the method comprises, after mixing the first mixing, a second mixing. 5 Example EX98. The method according to Example EX97, wherein the second mixing occurs under a second pressure which is less than the first pressure. Example EX99. The method according to Example EX98, wherein the second pressure is no more than 500, 400, 300, 200, 150, or 100 mbar. Example EX100. The method according to Example Ex 97 or Ex 98 or Ex99, wherein the 10 second mixing occurs for between 5 and 120, 5 and 80, 5 and 40, or 10 and 30 seconds, for example around 20 seconds. Example EX101. The method according to any of Examples EX 83 to Ex 100, wherein casting the slurry comprises casting the slurry onto a flat support, for example a steel flat support. Example EX102. The method according to any of Examples EX 83 to Ex 101, wherein after 15 casting the slurry and before drying the slurry, the method comprises setting a thickness of the slurry, for example setting a thickness of the slurry to between 100 and 1,000, 200 and 900, 300 and 800, 500 and 700 microns, for example around 600 microns. Example EX103. The method according to any of Examples EX 83 to EX102, wherein drying the slurry comprises providing a flow of a gas such as air over or past the slurry. 20 Example EX104. The method according to Example EX103, wherein the flow of gas is heated. Example EX105. The method according to Example EX104, wherein the flow of gas is heated to a temperature of between 100 and 160, or 120 and 140 degrees Celsius. Example EX106. The method according to any of Examples EX103 to EX105, wherein the flow of gas is provided for between 1 and 10 or 2 and 5 minutes. 25 Example EX107. The method according to any of Examples EX83 to EX106, wherein drying the slurry comprises drying the slurry until the slurry has a moisture content of between 1 and 20, 2 and 15, 2 and 10, or 3 and 7 wt %. Example EX108. The method according to any of Examples EX83 to EX106, wherein drying the slurry forms the precursor for forming into the aerosol-forming substrate, the precursor 30 being a sheet of aerosol-forming material. Example EX109. The method according to Example EX108, wherein the method comprises cutting the sheet of aerosol-forming material. Example EX110. The method according to any of Examples EX83 to 103, wherein the slurry further comprises a flavourant. 35 Example EX111. The method according to any of Examples EX83 to 110 wherein a flavourant is sprayed onto the aerosol-forming substrate after it has been cast and dried. Example EX112. A method of forming an aerosol-forming substrate according to any of claims 1 to 65 using a dough-process. Example 1 – Formation of aerosol-forming substrate A slurry was formed using a lab disperser capable of mixing viscous liquids, dispersing powders through liquids, and removing gas from a mixture (for example by applying a vacuum or other suitably low pressure). In this embodiment, a commercially available lab disperser from PC 5 Labor system was used. To form the slurry, a first mixture was formed by adding to the lap disperser around 7.11 grams of the aerosol former, then around 157.5 grams of water, then around 1.57 grams of the fibres. Then, these first ingredients were mixed at 25 degrees Celsius for 5 minutes at 600-700 rpm to ensure a homogeneous mixture and to hydrate the fibres. Then, a second mixture was 10 formed by manually mixing around 32.95 grams of the mineral compound and around 0.92 grams of the binder. In this embodiment, the mineral compound was kaolinite (obtained from Sigma- Aldrich). This mixing of the second mixture avoided the formation of lumps in the lab dispersion. Then, the second mixture was added to the first mixture to form a combined mixture. Then, the combined mixture was mixed at 5000 rpm for 4 minutes at 25 degrees Celsius and a first reduced 15 pressure of around 200 mbar. The reduced pressure helped to ensure that the mineral compound was homogeneously dispersed in the mixture and that there was little trapped air and few lumps in the combined mixture. Then, the combined mixture was mixed at 5000 rpm for 20 second minutes at 25 degrees Celsius and a second reduced pressure of around 100 mbar. This second reduced pressure helped to remove any remaining air bubbles. A slurry was thus formed for 20 casting. The slurry was then casted and dried using a suitable apparatus. In this embodiment, a commercially available Labcoater Mathis apparatus was used. This apparatus included a stainless steel, flat support, and a coma blade for adjusting a thickness of slurry cast onto the flat support. 25 The slurry was cast onto the flat support and a gap between the coma blade and the flat support was set at 0.6 millimetres. This ensured that a thickness of the slurry was no more than 0.6 millimetres at any given point. The slurry was then dried with hot air between 120 and 140 degrees Celsius for between 2 and 5 minutes. After this drying, a sheet of the aerosol-forming substrate was formed. 30 Various properties of the aerosol-forming substrate were then measured in triplicate, and are shown in Table 1 below. The methods used for the testing are those defined hereinbefore. Measurement Thickness Grammage (g / m2) Bulk Density (g / cm3) number (micrometres) 1 150.0 206.1 1.374 2 166.7 203.6 1.222 3 153.3 204.5 1.334 Average 156.7 204.7 1.310 Standard Deviation 8.8 1.241 0.079 The present inventors have found an aerosol-forming substrate having an increased thermal conductivity, for example compared to known aerosol-forming substrates. The composition of the aerosol-forming substrate was found to provide desirable properties for 5 forming an aerosol-forming substrate for use in a heat-not-burn device. For example, the presently found aerosol-forming substrate was found to have an increased thermal conductivity, and thus has the advantages outlined earlier in this specification. The composition according to the invention enabled an aerosol-forming substrate to be formed which was robust and had an increased grammage and bulk density compared to known aerosol-forming substrates. 10 For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Also, all ranges include the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein. In this context, therefore, a number A is 15 understood as A ± 10 % of A. Within this context, a number A may be considered to include numerical values that are within general standard error for the measurement of the property that the number A modifies. The number A, in some instances as used in the appended claims, may deviate by the percentages enumerated above provided that the amount by which A deviates does not materially affect the basic and novel characteristic(s) of the claimed invention. Also, all 20 ranges include the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.

Claims

CLAIMS 1. An aerosol-forming substrate comprising, on a dry weight basis: 10 to 90 wt % of a mineral compound; 7 to 60 wt% of an aerosol former; 5 1 to 20 wt% of fibres; and 1 to 10 wt% of a binder; and wherein the mineral compound comprises silicon and / or an alkaline earth metal, and the mineral compound comprises kaolinite, perlite, zeolite, bentonite, calcium sulfate, magnesium sulfate, magnesium carbonate or talc.

2. The aerosol-forming substrate according to claim 1, wherein the mineral compound comprises a thermal conductivity of from 0.1 to 5 W m−1.K−1in at least one direction.

3. The aerosol-forming substrate according to any of claims 1 to 2, wherein the mineral compound is a clay mineral.

4. The aerosol-forming substrate according to any of claims 1 to 3, wherein the mineral compound consists of kaolinite, perlite, zeolite, bentonite, calcium sulfate, magnesium sulfate, magnesium carbonate or talc.

5. The aerosol-forming substrate according to any of claims 1 to 4, wherein the mineral compound is kaolinite.

6. The aerosol-forming substrate according to any of claims 1 to 5, wherein the mineral compound comprises a pH of from 4 to 6 in water.

7. The aerosol-forming substrate according to any of claims 1 to 6, wherein the mineral compound has a particle size distribution having a volume D90 particle size of from 3 to 200 microns.

8. The aerosol-forming substrate according to any of claims 1 to 7, wherein the aerosol-forming substrate further comprises an acid.

9. The aerosol-forming substrate according to any of claims 1 to 8, wherein the aerosol-forming substrate further comprises nicotine.

10. The aerosol-forming substrate according to any of claims 1 to 9, wherein the bulk density of the substrate is of from 1.1 to 1.5 g / cm3.

11. The aerosol-forming substrate according to any of claims 1 to 10, wherein the substrate further comprises carbon particles.

12. The aerosol-forming substrate according to any of claims 1 to 11, wherein the substrate comprises, on a dry weight basis, 1.5 to 18 wt% of fibres, preferably 2 to 15 wt% of fibres, preferably 3 to 12 wt% fibres and more preferably 5 to 10 wt% fibres.

13. The aerosol-forming substrate according to any of claims 1 to 12, wherein the substrate is a 5 tobacco-free aerosol-forming substrate.

14. An aerosol-generating article comprising an aerosol-generating substrate according to any of claims 1 to 13.

15. An aerosol-generating system comprising an aerosol-generating article according to claim 14, and an electrical aerosol-generating device for heating the aerosol-forming substrate of the aerosol generating article.

16. A method of forming an aerosol-forming substrate according to any of claims 1 to 13, the method comprising: forming a slurry comprising the mineral compound, the aerosol former, the fibres, and the binder; and casting and drying the slurry to form the aerosol-forming substrate or a precursor for forming into the aerosol-forming substrate.