ES2907003T3 - New material and its production for use as a storage medium in a sensitive energy storage system in the low, medium or high temperature sector - Google Patents

Abstract

Modified red mud, containing the following components: - hematite, - corundum, - rutile and/or anatase, - quartz, - optionally (CaTiO3) perovskite and - optionally (Fe3+,Fe2+)2(Ti,Fe3+) pseudobrookite O5 (Na ,K) nepheline [AlSiO 4] and/or hauynite (Na,Ca)4-8[Al6Si6O24(SO4)], where the modified red mud is substantially free of Na2O and/or glass.

Description

DESCRIPTION

New material and its production for use as a storage medium in a sensitive energy storage system in the low, medium or high temperature sector

The present invention relates to a modified red mud or modified bauxite residue, also referred to as ALFERROCK®, and also to processes for its production and uses thereof.

With the Renewable Energy Act (EEG), Germany has established that nuclear power and power generation from coal will be phased out by 2038. As substitutes, wind farms, solar installations and biogas installations should constitute the energy supply for Germany.

In particular, however, wind farms and solar installations have the disadvantage that no energy is generated in calm air and in the dark. Therefore, these renewable energy sources are not capable of providing baseload power for customers, particularly industrial customers, who rely on a constant power supply.

To avoid this systemic disadvantage, energy generation and consumption must be separated. Only energy stores that are connected between the generators and the energy consumers make it possible to provide a constant flow of energy in the necessary manner and thus make renewable energy sources capable of providing base load energy.

Therefore, there is a demand for suitable energy accumulators and storage media that can remedy these disadvantages in energy production from renewable energy sources and that can also be obtained in a cost-effective, resource-conserving manner.

Red mud is known to be produced as a waste product in the Bayer process for the extraction of aluminum hydroxide (ATH) from bauxite. In the following description, red mud (LR) is understood to be the Bayer process residue produced in the extraction of ATH from bauxite. A large proportion of the bauxite residue or red mud produced in aluminum mining is not further processed or put to economically viable use, but is disposed of at disposal sites.

From the prior art it is already known that bauxite residue is suitable as an adsorbent due to its large internal surface area.

Furthermore, it is known that, due to its chemical composition, modified bauxite residue can be used as a halogen-free inorganic flame retardant in plastics of all types, which are provided as a compound or also in foamed form (WO 2012/ 126487 A1).

In addition, modified bauxite residue is known to have a high density, so this composition can also be used as a soundproofing medium or for weighting drilling mud or for radiation shielding (WO 2014/114283 A1) .

Furthermore, a porous particulate material comprising a bauxite residue for fluid treatment and contaminant removal is known from WO 2005/061408 A1. Contaminants include, for example, heavy metals, anions, and gases.

In experiments with chromate-deficient modified bauxite residues that have been heated to temperatures between 120°C and 250°C, the present inventors surprisingly found that the cooling took place unexpectedly and slowly. In light of this observation, the inventors of the present invention carried out extensive studies on the behavior of bauxite residues (optionally chromate deficient) or red mud, when subjected to heat treatment, and, in this case, carried out observations by comparing chemical, mineralogical, physical and mechanical parameters, in particular thermal parameters. Therefore, the inventors came to a new type of modified red mud which can be used as a storage medium, in particular as a heat storage medium.

The heat treatment influences the parameters mentioned above in a crucial way and thus has a significant influence on the characteristics of the heat storage medium in general, where, in particular, its behavior during cyclic exposure to heat (stability of thermal cycling, resistance to thermal shock) is important. Based on the chemical composition of the red mud used as starting material and the temperature ranges that were run during the heat treatment, they can arise in each case different chemical/mineralogical species, which, in turn, have different chemical, physical, mineralogical, mechanical and thermal characteristics. The sum of these characteristics determines the characteristics of the storage mechanism. This allows to influence the characteristics of the storage mechanism in any way, that is, in a specific way. Important features include, for example,

- thermal capacity

- Thermal conductivity

- electric conductivity

- density

- hardness

- ductility

- porosity

- thermal shock resistance

- stability in the thermal cycle

- thermal coefficient of expansion

- chemical stability, among other things

Therefore, the present invention relates to a modified red mud as defined in one of claims 1 and 4, a method for producing a modified red mud having the characteristics of claim 5, a storage medium comprising a modified red mud, a heat storage medium comprising a storage medium and numerous uses of a modified red mud as a storage medium, particularly in a heat storage medium, as well as methods of optimal application.

In one embodiment, a modified red mud contains the following components:

- hematite (Fe2O3),

- corundum (A^O3),

- rutile (TO2) and/or anatase (TO2),

- quartz (SO2),

- optionally perovskite (CaTiO3) and

- optionally pseudobrookite (Fe3+,Fe2+)2(Ti,Fe3+)O5) nepheline ((Na,K)[AlSiO4]) and/or hauynite ((Na,Ca)4-8[AlaS¡ ⁶ O ² 4(SO ⁴ )]).

Therefore, the modified red mud contains less than 0.5% by weight of Na2O and less than 0.5% by weight of glass, such as soda glass and/or potassium soda glass. The modified red mud is preferably substantially free of K2O and/or CaO and/or other alkali and alkaline earth oxides.

Red mud is the insoluble proportion that remains after alkaline leaching of aluminum from bauxite. The red mud is rinsed in the alkaline medium at the disposal sites. Thus, the red mud contains an indefinite amount of alkali in the form of free caustic soda solution or in the form of sodium carbonate, which is produced by carbonization with CO2. Since this free alkaline content actively but arbitrarily intervenes in the formation of the mineralogical phases, in order to create defined and controllable conditions, this alkaline content, i.e. Na2O, but also K2O and the alkaline earth content, is ie, CaO and MgO, should be at least predominantly, preferably completely, removed by washing or neutralization. Therefore, the modified red mud is substantially free of Na2O and K2O; therefore, soda glass and/or potash glass cannot be formed during tempering.

In the context of this application, "substantially free" can be understood, in particular, as a content of less than 0.5% by weight, in particular less than 0.2% by weight, in particular less than 0.1% by weight. weight, in particular less than 0.05% by weight, in particular less than 0.03% by weight, in particular less than 0.01% by weight.

Therefore, the modified red mud can contain, in particular, the (crystalline) components hematite (Fe2O3), corundum (A^O3), rutile (TO2) and/or anatase (TO2) and quartz (SO2) or it can consist substantially on these components. Other components may be included, but are not required. In particular, perovskite (CaTiO 3), pseudobrookite ((Fe3+,Fe2+)2(Ti,Fe3+)Os), nepheline ((Na,K)[AlSiO 4]) and/or hauynite ((Na,Ca)4, 8[AlaSiaO24(SO4)]) may be mentioned as additional (optional) components. In this case, however, the modified red mud does not contain substantially Na2O (likewise K2O and CaO) and/or glass.

In one embodiment, the modified red mud may contain

- 48 to 55 % by weight, in particular 49 to 54 % by weight, in particular 50 to 53 % by weight, of hematite (FeAi),

- 13 to 18% by weight, in particular 14 to 17% by weight, in particular 15 to 16% by weight of corundum (AhOa),

- from 8 to 12% by weight, in particular from 9 to 11% by weight, of rutile (TO2) and/or anatase (TO), and

- 2 to 5% by weight, in particular 3 to 4% by weight, of quartz (SO2), and

- less than 0.03% by weight, in particular less than 0.01% by weight, of Na2O and/or less than 0.1% by weight, in particular less than 0.05% by weight, of glass.

In one embodiment, a modified red mud can be obtained by heating substantially washed red mud free of Na2O (or neutralized) and having a mineral composition of

- from 10 to 55%, in particular from 10 to 50% by weight of iron compounds,

- 12 to 35% by weight of aluminum compounds,

- from 3 to 17% by weight, in particular from 5 to 17% by weight of silicon compounds,

- from 2 to 12% by weight, in particular from 2 to 10% by weight of titanium dioxide,

- 0.5 to 6% by weight of calcium compounds, and

- optionally other unavoidable impurities,

at a temperature of at least 800 °C, in particular at least 850 °C, in particular at least 900 °C, in particular at least 950 °C, preferably at least 1000 °C, for example in a range between 1100 and 1200 °C, for example 1150 °C. A modified red mud that can be obtained in this way can also be designated as quenched or sintered red mud or hematite ceramic.

The red mud, which remains as an insoluble proportion after alkaline leaching of aluminum from bauxite, generally contains significant amounts of Na2O or other alkali and alkaline earth oxides, which may also be present as hydroxides or as carbonates. During heating of said (unwashed) red mud at temperatures higher than 800 °C, in particular higher than 1000 °C, these alkaline constituents cause the (crystalline) SiO2 also contained in the red mud to turn into glass, just as , for example, soda glass and/or soda potash glass, which is a poor conductor of heat, if not even a thermal insulator, and thus highly detrimental to or even diametrically opposed to the intended use of modified red mud under the invention for storing heat from the stream (heat-energy coupling) or for converting heat into stream (heat-energy coupling). Other substances which are undesirable according to the invention may be formed during heating of the unwashed red mud due to the high reactivity of alkali and alkaline earth oxides and the like. Furthermore, the presence of said reactive substances in the modified red mud according to the invention is also detrimental to the intended use in a heat storage medium, which brings about continuous heating and cooling of the modified red mud in the heat storage medium. heat, as a chemical stability of the modified red mud (i.e. no chemical reaction during repeated heating and cooling in the selected working temperature range from room temperature to about 1000 °C), and a physical stability (different dimensions thermal shocks, or contraction of the mineral phases present with an effect on thermal shock resistance and thermal cycle stability) for the useful life of the heat storage medium according to the invention, is crucial.

Therefore, according to the invention, it is necessary to wash the red mud before heating, so that it is substantially free of Na2O (and other alkali and alkaline earth oxides, such as K2O and/or CaO) and preferably also free of organic constituents that may have a reducing effect during heating. Since Na2O, and also the other alkali and alkaline earth oxides, are alkaline (i.e. basic) substances, washing substantially free of Na2O can also be designated as neutralization or a corresponding washed red mud can be designated as neutralized red mud. The washing is advantageously carried out by means of water, to which an acid or an acidic substance, such as iron(II) chloride, may advantageously be added.

In one embodiment (after quenching), the modified red mud may be substantially free of one, two, three, or all four of the following components:

- gibbsite [Al(OH)s],

- goethite [FeO(OH)],

- boehmite [AlO(OH)],

- cancrinite [Na6Ca2[(CO3)2|Al6Si6O24]].

In one embodiment, the modified red mud may be substantially free of one, two, three, four, or all five of the following components:

- aluminum titanate (AI2TIO5),

- iron (elemental) (Fe)

- mayenite (Ca12Ah4O33),

- ulvospinell (Fe2TiO4),

- andradite [Ca3Fe2(SO4)3].

These undesirable components can occur when the red mud has not been sufficiently and carefully washed, among other things, also freed from organic components and/or heated or sintered in a reducing atmosphere.

The heating can take place at the indicated temperature, in particular for a period of time from 5 minutes to 36 hours, in particular from 5 minutes to 24 hours, in particular from 5 minutes to 12 hours, in particular from 5 minutes to 6 hours, in particular, from 5 minutes to 2 hours, in particular, from 5 minutes to 1 hour, in particular, from 5 minutes to 30 minutes.

In one embodiment, the modified red mud may have a porosity of less than 15%, in particular in the range of 5 to 12%. In this case, the pore sizes are small. Due to the modification of the red mud according to the invention, such a comparatively low porosity is easily achievable and is particularly suitable for achieving an advantageous high thermal capacity for the intended use of the red mud modified according to the invention and typical thermal conductivity for inorganic substances (due to the low scattering of phonons at boundary surfaces due to the comparatively low porosity). The porosity can be determined in particular by means of gas adsorption isotherms according to the BJH method.

In one embodiment, the modified red mud may have a density in the range of 3.90 to 4.0 g/cm3, in particular 3.91 to 3.95 g/cm3, in particular 3.92 to 3, 94 g/cm3, in particular about 3.93 g/cm3. Due to the modification of the red mud according to the invention, such a comparatively high density can also be easily achieved and is particularly suitable for achieving a high thermal capacity advantageous for the intended use of the red mud modified according to the invention and the typical thermal conductivity.

In one embodiment, the modified red mud may have an average d50 particle size in the range 3 to 10 µm, in particular 5 to 8 µm. The average particle size d50 can be determined in particular by means of laser diffraction or laser diffraction (MALVERN) according to ISO 13320 (2009).

In one embodiment, the modified red mud may have a d10 particle size in the range 0.5 to 2.5 µm, in particular 1.0 to 2.0 µm, and/or a d90 particle size in the range 0.5 to 2.5 µm. range from 15 to 50 jm, in particular from 20 to 40 jm. The mean particle size d10 can be determined in particular by laser diffraction and the particle size d90 can be determined by laser diffraction (Ma Lv ERN) according to ISO 13320 (2009).

In one embodiment, the modified red mud may have a specific heat capacity at 20°C in the range of 0.6 to 0.8 kJ/(kg*K), in particular 0.65 to 0.75 kJ/( kg*K), and/or a specific heat capacity at 726.8 °C in the range from 0.9 to 1.3 kJ/(kg*K), in particular from 0.95 to 1.2 kJ/( kg*K). The specific heat capacity can be determined, in particular, according to DIN EN ISO 11357-4.

In one embodiment, the modified red mud may have a specific thermal conductivity in the range of 3 to 35 W/(m*K), in particular 5 to 20 W/(m*K), in particular 8 to 12 W/(m*K). (m*K). The specific thermal conductivity can be determined in particular by means of plate-shaped test objects in a lambda meter according to DIN ISO 8302.

In one embodiment, the modified red mud may be present in the form of a compressed solid material.

The procedure for producing a modified red mud comprises washing and drying the red mud with a mineral composition

- from 10 to 55%, in particular from 10 to 50% by weight of iron compounds,

- 12 to 35% by weight of aluminum compounds,

- from 3 to 17% by weight, in particular from 5 to 17% by weight of silicon compounds,

- from 2 to 12% by weight, in particular from 2 to 10% by weight of titanium dioxide,

- 0.5 to 6% by weight of calcium compounds, and

- optionally other unavoidable impurities, and subsequently heating the washed (neutralized) red mud to a temperature of at least 800 °C, in particular at least 850 °C, in particular at least 900 °C, in particular at least 950 °C, preferably at least 1000 °C, for example in a range between 1100 and 1200 °C C, for example 1150 °C.

The washing of the red mud serves in particular so that, for the reasons stated above, the red mud to be heated is substantially free of Na2O (and other alkali and alkaline earth oxides, such as K2O and/or CaO) and preferably also free of constituents organics that may have a reducing effect during heating. For this purpose, the washing can take place in particular by means of water, to which an acid or an acidic substance, such as iron(II) chloride, can advantageously be added.

In one embodiment, the heating can take place at the indicated temperature, in particular, for a period of time from 5 minutes to 36 hours, in particular from 5 minutes to 24 hours, in particular from 5 minutes to 12 hours, in particular from 5 minutes to 6 hours, in particular from 5 minutes to 2 hours, in particular from 5 minutes to 1 hour, in particular from 5 minutes to 30 minutes.

In one embodiment, the heating of the washed and dried red mud can be carried out in a non-reducing (neutral) atmosphere (gas). As a result, an (unwanted) reduction of the components in the red mud, in particular iron compounds, can be avoided.

In one embodiment, the red mud is heated only once (ie no more times). Heating of the red mud several times, such as presintering, is as a rule not necessary in the process according to the invention, so that this additional process step (which is unnecessary according to the invention) can advantageously be omitted.

In one embodiment, the process may also comprise a preceding granulation of the red mud (sintered or quenched) after heating and subsequently a compression of the granulated red mud or of the granules. A granulation of the warm red mud simplifies (or even facilitates) a compression within a moisture range that is stated to be very narrow.

In one embodiment, a modified red mud according to the invention can be produced in the process for the production of a modified red mud.

The storage medium comprises a modified red mud according to the invention. In the context of the present application, a "storage medium" can be understood, in particular, as the active (or actual) storage material. For example, in the case of a heat storage medium, the storage medium may be a heat storage material which must have a corresponding (or suitable) thermal capacity and thermal conductivity.

The storage medium may optionally contain additional components in addition to the modified red mud.

In one embodiment, the storage medium may further comprise one or more of the following components:

- an agent to prevent air inclusion (absorption of air inside the storage medium) and/or adsorption of air (accumulation of air on the surface). (For example, the matrix for the current/heat storage device can be treated by means of a twin screw extruder with the addition of 5 to 10% by weight of polydimethylsiloxane or polydiphenylsiloxane polymer and with the simultaneous use of degassing). under vacuum with maximum output (for example, rotary vane vacuum pumps connected in series) so that all air inclusions are removed by intensive dispersion in the polysiloxane system. The material obtained is kneadable).

- an agent for improving thermal conductivity, in particular selected from the group consisting of metal colloids, metal powders, graphite and silicon-containing substances,

- an agent for the formation of a thixotropic composition (in particular by adding polypentaerythritol and a carboxylic acid (C18, for example) to the heat carrier matrix before treatment in the twin-screw extruder, the matrix can be adjusted thixotropically , for example, in an extended range In the current/heat storage medium activation procedure, i.e. slow heating, in addition to polysiloxane, these agents can also be thermally decomposed to carbon, where the resulting carbon has no negative effects on the desired characteristics of the medium of the current/heat storage device, but improves thermal conductivity.

In one embodiment, the storage medium may be substantially free of softening agents ("plasticizers"). Softeners are usually not needed in the red mud according to the invention, so these softeners (which are unnecessary according to the invention) can advantageously be omitted.

The heat storage medium comprises a storage medium according to the invention. In the context of the present application, a "heat storage medium" can be understood, in particular, as a device that contains the storage medium as an active heat storage material and, in addition, optionally also has other apparatus elements .

In one embodiment, the heat storage medium may be current/heat storage. In the context of the present application, a "current/heat storage" can be understood, in particular, as a storage device that can convert electrical energy into thermal energy (heat energy) and/or can convert thermal energy into electrical energy, and which, in particular, can convert electrical energy into thermal energy and also thermal energy into electrical energy.

In one embodiment, the heat storage medium may also comprise devices for loading and unloading the heat storage medium. The devices for loading and unloading the heat storage media may, for example, contain mechanical components and/or other components, such as, for example, conductive contacts or connections, in particular electrically conductive contacts or connections and/or contacts. or heat-conducting connections.

In one embodiment, the heat storage means may be a current/heat store which also comprises means for converting current to heat, such as heating elements or resistance wires or heating wires. As a result, a conversion of electrical energy into thermal energy is possible. In this case, these media can be in indirect or direct contact with the storage medium, for example embedded in it.

In an implementation phase, the heat storage means may be a heat/current storage comprising means for converting heat into current, such as equipment for generating steam or hot air, turbines and generators. As a result, a conversion of heat into current is possible. The heat storage media according to the invention can be configured for cogeneration both by energy/heat coupling and by energy/heat/energy coupling. In other words, the heat storage media according to the invention can be used for cogeneration by energy/heat coupling (or current (energy)/heat coupling) and also by energy/heat/energy coupling (or current (energy)/heat/current (energy) coupling). All the properties and characteristics necessary for this, such as density p, specific thermal conductivity Cp, working temperature range AT, thermal conductivity within substance A, intrinsic thermal energy transmission capacity in the storage device (thermal diffusivity ), porosity, grain or particle size distribution, simultaneously high hardness and ductility, as well as chemical stability, can be provided by the modified red mud according to the invention due to its chemical composition or its modification due to manufacturing tolerances.

Therefore, the mode of action of heat storage media can be divided into two stages:

1st stage: cogeneration by energy/heat coupling

2nd stage: cogeneration by heat/energy coupling.

Both stages can be used in each case independently or in combination as energy/heat/energy coupling cogeneration.

Cogeneration by power/heat coupling works with an efficiency of 95 to 100%. Therefore, the heat storage medium according to the invention is an ideal medium for transmitting heat to heat-requiring systems, in other words, it is a heat transmitter of an ideal type. This includes, for example, buildings (floor and wall heating systems), also technical equipment of all kinds, such as boilers, rotary kilns, distillation equipment, pipes, coffee machines and many more. The heat transfer (energy/heat coupling) to all such systems is more attractive, since a procedure with an efficiency of 95-100% is used. Currently used heat transfer systems typically operate at 25-35% efficiency. Therefore, power/heat coupling cogeneration reduces the energy demand and, therefore, energy costs to about 1/3. Thus, huge amounts of CO2 can be saved from burning fossil fuels on a global scale. Heat storage media for heat-energy coupling cogeneration can be designed in such a way that, apart from the heat transfer side, all other surfaces are thermally insulated. This applies both to spaces and to metal surfaces of technical equipment of all kinds.

The invention further relates to the use of a modified red mud according to the invention as a storage medium, in particular in a heat storage medium.

In one embodiment, the modified red mud can be used to store heat at a temperature of up to 1000°C, in particular at a temperature of over 100°C up to 1000°C. However, use for heat storage at a temperature above 80 °C, in particular above 90 °C, is possible. In one embodiment, the modified red mud can be used as a storage medium that can be heated and cooled simultaneously. In this way a kind of control is provided, by means of which, for example, the storage medium does not heat up above 500 °C during operation, so that a (trigonal) a quartz inversion can be avoided. quartz 13 (hex). That is, a heating of the storage medium above a predetermined threshold value, such as for example 500 °C, is counteracted by simultaneously emitting a corresponding amount of heat to a different medium and corresponding cooling.

This condition constitutes a revolutionary development of previous storage technologies. In the past, all storage devices could be charged or discharged (in succession), such as batteries or pumped storage facilities or energy/heat reservoirs, in which hot air is first generated by means of current and then the stones are heated (loading operation). In the 2nd stage (unloading operation) the hot stones heat cold air into hot air, which then makes steam to drive turbines and generators. In both the charging operation and the discharging operation, temperature ranges from about 100°C to, for example, 1000°C or parts thereof are traversed from side to side.

A significant problem in the described 2-stage process is the cubic expansion capacity or the contraction capacity of the crystalline substances contained in the storage substance in the temperature ranges that occur during loading and unloading. Since all contained substances have different expansion coefficients in the respective temperature range, the substance (eg natural stones) is systematically broken and thus destroyed, i.e. thermal shock resistance and stability. of the thermal cycle are not provided to the extent necessary.

If charging and discharging can take place simultaneously, it is no longer necessary to run through relatively large temperature ranges. Operation of the storage device can now take place at an arbitrary and desired temperature, or within a narrow temperature range. The energy supplied during the charging operation is extracted to the same extent during the discharging operation. In other words: with an approximately constant temperature, the coefficient of expansion remains stable and does not change. Therefore, the destructive force of the expansion coefficients that change specifically in each contained substance does not occur. The thermal shock resistance and thermal cycle stability are provided, that is, the heat storage media can be operated for a long time. For example, if 500 °C is not exceeded, no decompositions other than hydroxides, oxide hydrates, and carbonates occur, and sintering processes, etc. do not occur. The storage substance remains stable in all parameters, and changes such as decompositions, phase transformations, sintering are ignored. The exclusion of the free caustic sodium solution or of the sodium carbonate resulting from the carbonization prevents the alkali or alkaline earth oxides from influencing the formation of chemical-mineralogical species in the temperature ranges they cross. Only in this way can clearly defined substances be formed with defined parameters in all respects. For example, only the following substances are contained in the storage substance according to the invention: hematite, alpha-Al2O3, anatase, rutile, perovskite, cancrinite and quartz.

If maximum quantities of current to be stored occur, the working temperature range of the storage device must be extended, for example, at 1000 °C, the chemical-mineralogical components of the storage material change (cf. Table 1 "residue of mineral phase bauxite"), thermal cycle stability and thermal shock resistance are maintained, since loading and unloading operations take place at a narrow temperature or temperature range, i.e. self-destruction of the charging device. storage induced coefficient of expansion cannot occur.

In one embodiment, the modified red mud can be used as a storage medium in a power/heat store.

In one embodiment, the storage medium can be heated by means of electrical current and/or cooled while generating electrical current.

In one embodiment, the modified red mud can be used for the storage of electrical energy obtained from renewable energy sources. In the context of the present application, "renewable energy sources" can be understood to be, in particular, wind energy, hydroelectric energy, tidal energy, solar energy, geothermal energy and biomass, but preferably renewable sources. renewable energies for which the time scale is largely removed from human influence and for which the possibility of effective storage is therefore very important, such as, in particular, wind power, tidal power and solar energy.

In one embodiment, the current/heat storage may comprise resistance wires, which are heated by electrical current and thus heat the storage medium.

In one embodiment, the thermal energy stored in the storage medium can be transferred to another medium, preferably a fluid medium, and thus the storage medium can be cooled (discharged), where the other medium or also the exchange medium of heat is selected in particular from the group consisting of water, (steam), molten salt, such as for example ionic liquids, and thermal oil or gas.

The storage medium may have the other medium or heat exchange medium flowing through it, for example in the manner of a heat exchanger. A corresponding configuration of the storage medium as a heat exchanger is possible due to the at least initial free formability of the storage medium. In this case, for example, meandering pipes for the corresponding fluid media can be provided in a storage medium element/corresponding storage medium elements. On the other hand, however, the geometrically block-shaped storage medium elements can also be arranged so as to produce a corresponding pipe system with a rectangular, polygonal or round cross-section. Alternatively, the other medium or heat exchange medium could also flow or sweep over corresponding blocks of the storage medium when this other medium or heat exchange medium flows over the surface(s) of the storage medium at a preselectable flow rate.

In this case, the control of the cooling (unloading) of the storage medium can take place, for example, by means of the temperature and the flow rate of the other medium or heat exchange medium. As a result, heating and/or cooling of the storage medium above or below a predetermined threshold value can be prevented.

In this case, the generation of electric current can take place, for example, by means of steam turbines or by means of gas turbines, where in the latter case the storage medium could, for example, take over the function of the " combustion chamber" to heat the working gas. In this case, air can be used as the working gas.

In one embodiment, the storage medium and a download device may be provided integrally or as separate elements. Here, for example, installations can be conceived in which the storage medium elements, the regulating device and the current generation assembly form a system that, for example, is integrated and installed in the region of a building with an installation. solar as a closed set. In one embodiment, the current/heat storage can be used for the transport of electrical energy after cogeneration has taken place without the presence of line systems. In this connection, for example, storage medium elements with a low mass, for example three to five kilograms, could be provided, which can then be transported in thermally insulated boxes safely and with low heat loss on wide sections. Once arrived at the destination, the electrical energy can then be recovered again from the thermal energy.

In one embodiment, the current/heat store can generate electrical current again after cogeneration by means of heat/current coupling. In this connection, gas or steam turbines can be used, but also thermoelectric generators, based on the thermoelectric or Seebeck effect.

In one embodiment, the current/heat store can be used to supply power to isolated power consumers.

In one embodiment, the isolated power consumers can be supplied with thermal energy and electrical energy. In other words, a so-called isolated operation is possible here, regardless of a connection to the power grid.

In one embodiment, the current/heat store can be used to power machines or mobile devices such as vehicles. This is suitable, for example, for vehicles that have a gas turbine drive, in which the working medium can be heated by means of a storage medium element.

A heat storage medium that can be charged and discharged simultaneously is always, in the sense of energy/heat/energy coupling cogeneration, a thermal power plant consisting of a current generator such as renewable energy sources (and therefore free of CO2), a storage device, a turbine and a generator.

However, such a heat storage medium can also ideally be used as a heat exchanger with other systems. This is heat/energy coupling cogeneration, for example in the field of appliance technology. It is no longer necessary that heating, for example, of boilers, rotary kilns or technical installations of all kinds take place, for example, with steam or hot flue gases, but it is possible to heat the storage substance, for example, by means of energy (EE current) and transmit it by direct contact, for example, on metal, ceramic or other substances. Since the efficiency of power/heat coupling cogeneration is 95-100%, heating systems with similar efficiency rates is possible. These advantages will significantly reduce energy costs in the future and simplify technical constructions. This includes, for example, the heating of buildings by elements that can be heated and contain the storage medium.

Other objects and advantages of embodiments of the present invention are described with reference to the following detailed description and accompanying drawings.

Figure 1 shows a particle size distribution of dry conventional bauxite residue.

Figure 2 shows the density characteristics of a test sample during heating of red mud from 100 °C to 1000 °C in an atmosphere of oxygen (O2) or nitrogen (N2).

Figure 3 shows a particle size distribution of a warm red mud at 1000°C according to an exemplary embodiment of the invention.

Figure 4 is a graphical representation of the series of measurements shown in Table 3 for the specific heat capacity of ALFERROCK® according to an exemplary embodiment of the invention.

Additional details of the present invention and additional embodiments thereof are described below. However, the present invention is not limited to the following detailed description, but merely serves to illustrate the teaching according to the invention.

It should be noted that the features that are described in connection with an exemplary embodiment or an exemplary subject may be combined with any other exemplary embodiment or with any other exemplary subject. In particular, the features described in relation to an exemplary embodiment of a modified red mud according to the invention may be combined with any other exemplary embodiment of a modified red mud according to the invention, as well as any exemplary embodiment of a method for production of a modified red mud, of a storage medium, of a heat storage medium and uses of a modified red mud, and vice versa, unless explicitly stated otherwise.

If a term is designated by an indefinite or definite article, such as "a", "an" and "the", in the singular, this also includes the term in the plural, and vice versa, provided that the context does not specify this. unequivocally contrary. The term "comprise" or "have", as used herein, includes not only the meaning of "contain" or "include", but can also mean "consist of" and "substantially consist of".

For the studies carried out within the context of the present invention, the material to be studied was first characterized at room temperature, and in particular the chemical and mineralogical composition was determined. In addition, this material was slowly heated to 1000 °C, and in this case every 100 °C the mineralogical phases were determined, as well as the density and specific heat capacity.

The characterization of the material to be studied:

1. Chemical composition (typical for bauxite waste)

• 10 to 50% by weight of iron compounds

• 12 to 35% by weight of aluminum compounds

• 5 to 17% by weight of silicon compounds

• 2 to 10% by weight of titanium dioxide

• 0.5 to 6% by weight of calcium compounds

2. Mineralogical composition

In the initial state of the study, the following mineral phases were determined radiographically:

• hematite

• goethite

• anatase

• rutile

• perovskite

• boehmite

• gibbsite

• cancrinite

• quartz

3. Particle size

The particle diameters (|jm) are shown in Figure 1. According to this, the substance is very fine and has 3 maxima. With a good distribution it was expected that the substance would have a high density, since the very fine crystals can be inserted into cavities in the medium-fine crystals and the latter can be inserted into cavities in the coarser crystals. The measured density of 3.63 (g/cm ³ ) confirms this evaluation.

By adding thermally stable and chemically inert substances with arbitrary particle size distribution, any cavities still present can be reduced with an effect on mechanical, electrical and thermal characteristics. This constitutes a further optimization of the storage mechanism in the context of the invention.

4. Carrying out the test

Samples of the test substance were heated in stages under oxygen and under nitrogen up to 1,000 °C. Samples were taken in each case at 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C and the changes in mineralogical composition as well as density.

The specific heat capacity was measured in the temperature range from room temperature (30.26 °C) to 584.20 °C.

5. Interpretation of the results

5.1 Mineral phases

The mineralogical composition of the substance changes according to temperature (see Table 1 below).

At about 300 °C gibbsite decomposes, at about 400 °C goethite decomposes and at about 500 °C boehmite decomposes. At 573 °C, alpha quartz transforms into beta quartz.

Above 600 °C, CO2 emission from cancrinite Na6Ca2 [(AlSiO 4)6 occurs substantially from hematite (Fe2O3) and corundum (AhOa) and, in smaller proportions, from TO 2 , cancrinite and perovskite.

At 1000 °C, cancrinite and the two TiO2 phases anatase and rutile convert to the minerals pseudobrookite [(Fe3+)2Ti]O ⁵ and nepheline [(Na,K)[AlSiO4].

Table 1: Bauxite residue mineral phases (apparent density 0.944 g/cm3)

T[°C] Mineral phases Bauxite residue GEA Density

100 Hematite, Goethite, Anatase, Rutile, Perovskite, Boehmite, Gibbsite,

cancrinite, quartz _3.63

200 Hematite, Goethite, Anatase, Rutile, Perovskite, Boehmite, Gibbsite,

cancrinite, quartz _3.64

300 ^{Hematite, Goethite, Anatase, Rutile, Perovskite, Boehmite,} 3.74 _{Cancrinite, Quartz, a-Al2O3}

400 ^{hematite, goethite, anatase, rutile, perovskite, boehm}

_{cancrinite, quartz, a-Al2O3} ^ite, 3.81

500 ^{Hematite, anatase, rutile, perovskite, boehmite, cancrinite}

_{quartz, a-Al2O3} ^, 3.81

600 hematite, anatase, rutile, perovskite, cancrinite, quartz, a-AhO3 3.89

700 hematite, anatase, rutile, perovskite, cancrinite, quartz, a-AhO3 3.60

800 hematite, anatase, rutile, perovskite, cancrinite, quartz, a-AhO3 3.71

900 hematite, anatase, rutile, perovskite, cancrinite, quartz, a-AhO3 3.73

1000 hematite;, anatase, rutile, perovskite, quartz, a-Al2O3, nepheline,

pseudobrookite ^3.93

5.2 Density

As can be seen in Figure 2, the density develops as a function of temperature from 3.63 (g/cm3) at 100°C to 3.93 (g/m3) at 1000°C. The decomposition of the mineral phases with the elimination of water and CO2, as well as the sintering procedures, reduce the density between 600 °C and 700 °C, to then reach 1,000 °C to rise again to a value of 3.93 (g/cm3).

For applications in the thermal range, it is only possible to use substances that are stable as bodies and that in the respective arbitrary temperature ranges do not give off any other gases such as H 2 O or CO 2 and do not undergo any other sintering processes. Oxides such as Fe2O3, A^O3, TO2 or SO2 hardly change significantly in case of temperature increases. A significant feature is revealed by the fact that the density of the substance heated to 1000 °C remained constant during the constant cooling, i.e. no rehydrations occurred.

5.3 Particle size

Tempering of the substance according to the invention up to temperatures of 1000 °C changes the particle diameters to significantly higher values, for example by decomposition of hydroxides, oxide hydrates or carbonates and by sintering processes (cf. Figure 3). . For example •

• d-iü values grow from 0.074 pm to 1.341 pm

• d50 values grow from 0.261 pm to 6.743 pm

• dg0 values grow from 1.692 pm to 28.17 pm

5.4 Specific heat capacity

The specific heat capacity of substances is a function of temperature. As the temperature increases, the specific heat capacity also increases. The following Table 2 shows the corresponding examples.

Table 2: Specific thermal capacity at different temperatures:

In mixtures, the specific heat capacity is the sum of the specific heat capacities of the respective components of the mixtures.

The characterization of the provided substance shows a mixture of different mineral substances. During quenching, parts of the substances decompose and, for example, remove water or CO2 with the formation of oxides or other chemically stable mineral phases. In addition, sintering procedures are carried out. The measurement results of these substances show a value of 0.791 (kJ/(kg * K)) at 30 °C and a value of 1.037 at 584 °C. At 1,000 °C a value of 1.14-1.18 can be assumed by extrapolation (cf. Fig. 4).

Table 3: ALFERROCK® Specific Thermal Capacity Measurement Series

During the cooling of the substance heated to 1000 °C, the values of Cp return to the values that correspond to the respective temperatures. However, since the starting substance has changed due to the decompositions of mineral phases and the formation of other substances and sintering processes have also occurred, after cooling the substance will have different values of the specific heat capacity of the starting substance. starting substance. It is important to establish that, after quenching, a stable substance is available, which can be heated and cooled at any frequency and which in this case does not undergo any further change in the individual substances in the mixture. As already mentioned, this also applies to density.

5.5 Specific thermal conductivity

The thermal conductivity of a system is, among other things, a function of parameters such as pressure, temperature, mineralogical composition, porosity, density, etc.

As described, all thermally unstable components have been decomposed by heating of the given substance. After tempering a substance is produced consisting of corundum (A^Oa), hematite (Fe2O3), rutile and anatase (TO2), as well as flame retardants such as pseudobrookite [(Fe3+)2Ti]O5 or nepheline [(Na,K) [AlSiO4

The following Table 4 shows the values of the thermal conductivity and density of those substances that constitute the most important components of the tempered substance.

Table 4: Thermal conductivity density of individual components

During the quenching process, the particle diameter of the produced substances has increased significantly, and in this case, the surface area has decreased. Therefore, within the primary crystals, the conductivity also increased to the values that are stated in Table 4. In principle, in crystal mixtures, phonons are reflected at the crystal boundaries with a simultaneous reduction in conductivity. thermal, that is, there is a causal relationship between crystal structures and the thermal conductivity of a substance. Air is still contained in the substance mixture and, as a poor conductor of heat, lowers the measured thermal conductivity. To avoid this effect, different procedures are possible, including, for example, the application of pressure, ie pressing the substance to form solid bodies.

In addition, substances can be added, which prevent air inclusions between the crystallites or on the surface of the crystallites and thus allow the production of blocks of solid substances.

These include, for example:

• metal colloids

• metallic powder

• graphite

• Si-based sinterable pyrolyzing substances

In addition to the addition of said substances, pressure and thermal energy can also be used.

It is crucial to be able to produce good building blocks of heat-conducting substances. For the use of the substance obtained after quenching as a heat storage medium, a good thermal conductivity, in particular the prevention of air inclusions, is significant for the charging operation (heating of the substance) and for the discharging operation (transfer of stored heat to systems that, for example, generate steam).

Example:

A mixture of the provided substance consisting of untempered substance and substance tempered up to 1000 °C in a ratio of 1:1 is surface-modified with 5% PDMS (polydimethylsiloxane prepolymer) and introduced into a BUSS kneader or a co-rotating twin screw extruder. The compounding machine has a housing temperature of 135°C and maximum vacuum degassing. Torque is set to 65 - 85% of maximum. The material is removed by means of a cooling conveyor.

The resulting product, free of water and air, is introduced into the insulated container and mechanically compressed. The temperature is then slowly increased to 1,000 °C, and thus the heat storage medium is prepared for operation. Instead of PDMS, other substances can be used, such as metal powders, graphite or salt solutions.

6 . Resume

After washing or neutralization, bauxite/red mud residue, which is largely free of alkalis and alkaline earths, is used as starting material. The goal is to obtain simple and clearly defined substance structures with clear parameters even after tempering at, for example, 1000 °C or higher temperatures.

During tempering up to temperatures of 1,000 °C, all components of the substance mixture that are unstable in this temperature range decompose. These include gibbsite, goethite, boehmite, as well as cancrinite and the TiO2 phases which, where applicable, form pseudobrookite [(Fe3+)2Ti]O5 and nepheline [(Na,K) (AlSiO4)] at 1000 °C.

After cooling, a mixture of substances was formed, consisting of oxides such as Al2O3, Fe2O3, TiO2, SiO2 and optionally substances that are resistant to high temperatures, such as pseudobrookite and nepheline, which did not show any further changes after quenching. renewed at 1000 °C.

With the aforementioned change in the composition of the material, the density also changed from 3.63 (g/cm3) at room temperature to 3.93 (g/cm3) at 1,000 °C. This expected operation was further accompanied by sintering effects. During cooling of the tempered substance mixture at 1000 °C, the density reached at 1000 °C remains unchanged, since the density of oxides such as AhO3, Fe2O3, as well as TO 2 and SO2 does not change in the temperature ranges between 25°C and 1000°C.

These sintering effects and the decomposition of the mineral phases have led to an increase in the diameter of the particles in the mixture of substances. While before quenching, for example, d50 = 0.261 pm and dg0 = 1.692 pm applied, after quenching the following values could be measured: d50 = 6.743 pm and dg0 = 28.17 pm. The enlargement of the particles means a reduction in surface area and better thermal conductivity. The air content (bad conductor of heat) was reduced between the very small crystallites.

The study of the specific thermal capacity of the characterized substance showed an increase in the specific thermal capacity from 0.79 (kJ/(kg K)) at 25 °C to 1.037 (kJ/(kg K)) at 600 °C. At 1000 °C a value of 1.14 1.18 (kJ/(kg K)) by extrapolation.

Since, as already mentioned, the density has also increased, the product of density and specific heat capacity, as a crucial criterion for applications as heat storage media, reaches values higher than that of water. Water has a density at 20 °C of 998.2 (kg/m3) and an exceptional specific heat capacity of 4.182 (kJ/(kg K). This results in a volumetric heat capacity of 4175 (kJ/m 3 K) On the other hand, the given substance has a density of 3890 (kg/m3) and a specific heat capacity of 1.037 ((kJ/kg K)) and therefore a volumetric heat capacity of 4.034 (kJ/(m3 K)) at approx. of 4,559 (kJ/(m3 K)). This value significantly exceeds the value for water. A substantial difference between water and the specified substance is the temperature at which the storage media can operate. While water ideally works in temperature ranges between 40 °C and 90 °C, i.e. it has an AT of 50 °C, the given substance can function in the temperature range up to 1000 °C, that is That is, the substance can evaporate water above a temperature of 100 °C and therefore can work with an AT of 900 °C. For this reason, the supplied substance can store 15 to 20 times more heat than water (depending on volume).

In storage media, the coefficient of thermal conductivity is more important for the charging operation (heating of the storage device) than for the discharging operation. The thermal conductivity of the oxides substantially contained in the substance is between 3 and 35 (W/(m K). What is crucial for heat storage media is the need to be able to compact the substance used as storage medium to form solid blocks in which thermal energy can flow optimally, i.e. from the heating element to the storage substance, within the storage substance and from the storage substance to systems that consume thermal energy. In this regard, it is advantageous if gases that conduct heat poorly within the substance or on the surface of the substance are removed. In addition to pressure applications, substances can be added by which the primary crystals "glue together". These include , for example, metal colloids, metal powder, graphite, Si-containing sinterable pyrolyzing substances.Above all, it is also crucial that in e The tempering process of the supplied substance decomposes all unstable substances up to 1000 °C and thus makes available a predominantly oxidized and thermally stable storage substance that can be heated and cooled at any frequency without generating gases such as H2O or CO2, which can destroy the storage block.

The charging and discharging of the heat storage media take place simultaneously at an arbitrary temperature or in a narrow temperature range. As a result, a permanent change in the thermal expansion coefficients is avoided and the thermal shock behavior and thermal cycling behavior are stabilized in the sense of a long life expectancy of the energy storage device.

Use of the supplied substance as a storage material for high temperature heat storage media

The storage system

Both water and solid substances (for example, the substance described above) belong to sensitive heat storage systems (sensitive, because the heat from the storage device is perceptible).

The heat storage media can be heated by power/heat coupling by electrical energy from wind farms or solar installations. In still air or in the dark, these heat storage media can, for example, generate steam that drives turbines which, in turn, generate electrical energy (heat/power coupled cogeneration) by downstream generators. Therefore, the heat storage medium assumes the role of emergency generator or, on a large scale, of "replacement power plants". If this procedure is successful, power line systems can also be designed simply and effectively.

The requirements for energy accumulators are set out below:

• High energy density

• High power density

• Low accumulated energy consumption

• Low losses

• Low self-discharge

• Long cycle life

• Long useful life

• Low investment costs

• Low operating costs

The provided substance meets the stated requirements to a large extent.

The substance is

• inorganic

• safe

• with a long service life

• recyclable

• available in very large quantities

• very economical

• works in the temperature range up to 1,000 °C

• can be loaded and unloaded simultaneously

• can be easily manufactured.

In particular, the fact that, as a high-temperature-sensitive storage medium, the provided substance can be simultaneously charged and discharged, enables a continuously operating and controllable storage power plant to be operated. In this way, power generation deficiencies can be compensated or higher demands can be met.

In addition, the heat storage media can be used, in particular, for wind or solar farms, which makes the energy generated there capable of providing base power as a "global solution".

In addition, small heat storage assemblies can be used, for example, for a complete energy source, for example, for residential buildings. These small assemblies are heated, for example, by renewable energy sources, and are then used as a routine replacement for the full power supply, ie thermal power and electricity supply, for residential buildings.

In addition, small heat storage assemblies can be used in machines of all kinds for power supply.

Furthermore, after cogeneration, electrical energy can be made "transportable" in the form of heat storage media without line systems.

Vehicles can also be powered this way. After heat/power coupling cogeneration has occurred, regularly replaced heat storage media such as batteries can operate electric motors, compared to lithium batteries.

The equipment for the conversion of heat into electrical energy may be provided as an integral component of the storage device or in assemblies that are independent of it.

Example

The supplied substance is a filter cake that must first undergo a heat treatment, ie it must be slowly heated up to 1,000 °C. In this case, the water content of the filter cake is first evaporated, then all minerals that are unstable in the high-temperature range are calcined up to 1,000 °C. Next, the substance consists only of oxides, as well as stable inorganic phases such as nepheline or others. This substance cools and forms the storage mass.

Loading (ie heating) of the storage mass is carried out directly by embedded resistance wires or heating elements, ie resistance wires in ceramic sleeves or other systems. By means of corresponding control devices, the storage mass can be constantly adjusted to arbitrary temperatures.

The discharge is carried out by means of a circulation of water passing through the storage mass at a suitable and optimal point in the temperature/vapor pressure range. The water evaporates, the steam drives the turbines, current is generated. Excess steam is guided back into circulating water by means of cooling equipment ("cooling tower").

Optimum conditions can be established by the specific thermal conduction of the heat storage medium between heat supply (hottest point) and heat consumption.

The heat storage material is consolidated with heating media for heat supply and piping system (water) for heat dissipation to form a block. This block is thermally isolated from the outside.

The claim that the heat storage system characterized in this way can be charged and discharged simultaneously is crucial. As a general rule, storage facilities are designed to be loaded or unloaded; see, in this connection, pumped storage facilities. On the other hand, with the possibility of simultaneous charging by renewable energy sources and discharging, it is possible to build stable storage power plants that are capable of providing baseload power.

Currently, the most important heat storage system for sensitive heat storage media is water. This system is characterized by the fact that it works with water ideally in a temperature range of 40 °C - 90 °C, since above 100 °C the water is present as steam. Therefore, water has an AT of 50 °C.

In contrast to this, the heat storage system that works with a storage mass that has been produced from the given substance can work at temperatures up to 1000 °C, i.e. the substance can evaporate water above a temperature of 100 °C and therefore can work with an AT of 900 °C. Therefore, this system is a high temperature storage system.

Example:

Comparison of ALFERROCK®/Water Sensitive Heat Storage Media

Calculation of the amount of heat that can be stored

The amount of heat Q that a storage material can store is calculated according to the following equation:

Q =m * cp * AT = p * cp * V * AT [J]

m = mass [kg]

cp = specific heat capacity

p = density [ Jj

V = volume [m3]

p * cp = volumetric heat capacity

AT = temperature range [K]

Q(1m3) = volumetric thermal capacity * AT [J]

1. Water (for 1 m3)

p = 998.2kg

cp = 4,182k ]

kg*K.

p* cp = 4.175 m 3 K.

AT = 50 KQ = 4,175 [ lm -^ 3 K 1 ] * 50 K * 1 m3 Q = 208.7 * 103 kJ

Converted to Wh:

1 J =

QWater = 57.88 kWh

2. ALFERROCK® (for 1 m3)

P = 3,930 i §

Cp=1.16k ^]

k g * K

P * Cp = 4,558.8 \ _, £ _{m 3} ~ _K.

AT = 900K

Q = 4,558.8 [ L m -^ 3K 1 i * 900 K * 1 m3

Q = 4102.9 * 103 [ lm -^ 3 K 1 ]

Converted to Wh:

Q ^alferrock ® - 1.1397 MWh

3. Comparison ALFERROCK®/water

Q • ALFERROCK 1.1397 MWh

Q • water 57.88 kWh ^19.7

ALFERROCK® can store 19.7 times the amount of heat at an operating temperature of up to 1,000°C.

ALFERROCK ® high temperature heat storage medium can also exceptionally be used at lower temperatures as a heat storage medium, heat exchanger and thermostat. It should be noted that during quenching of the given substance, the increase in density from 3.63 cm 3 at 100 °C to 3.93 cm 3 at 1000 °C does not decrease, but remains constant at 3.93 cm 3 . Therefore, the p * cp value is increased by 9%.

In the following Table 5 the storable quantities of heat in the region of approximately 200 °C, 300 °C, 400 °C, 500 °C and 600 °C are presented and they present very attractive values.

T l : R i i r m l r n r í

Claims (14)

1.- Modified red mud, which contains the following components: - hematite, - corundum, - rutile and/or anatase, - quartz, - optionally (CaTiO3) perovskite and - optionally (Fe3+,Fe2+)2(Ti,Fe3+) pseudobrookite O5 (Na,K) nepheline [AlSiO 4] and/or hauynite (Na,Ca)4-8[AlaSiaO ² 4(SO ⁴ )], wherein the modified red mud is substantially free of Na2O and/or glass. 2. Modified red mud according to claim 1, containing - 48 to 55% by weight of hematite, - 13 to 18% by weight of corundum, - 8 to 12% by weight of rutile and/or anatase, - 2 to 5% by weight of quartz, and - less than 0.03% by weight of Na2O and/or less than 0.1% by weight of glass, and/or where the modified red mud contains less than 0.5% aluminum titanate, iron, mayenite, ulvospinell and/or andradite. 3. - Modified red mud according to claim 1 or 2, wherein the modified red mud can be obtained by heating substantially washed red mud with a mineral composition - 10 to 55% by weight of iron compounds, - 12 to 35% by weight of aluminum compounds, - 3 to 17% by weight of silicon compounds, - 2 to 12% by weight of titanium dioxide, - 0.5 to 6% by weight of calcium compounds, - less than 0.5% by weight of Na2O and - optionally other unavoidable impurities, at a temperature of at least 800 °C, preferably at least 1000 °C. 4. - Modified red mud according to one of the preceding claims, where the modified red mud has a porosity of less than 15%, in particular in the range from 5 to 12%; I where the modified red mud has a density in the range of 3.90 to 4.0 g/cm3, in particular about 3.93 g/ ^cm3 ; I where the modified red mud has an average particle size d50 in the range of 3 to 10 pm, in particular 5 to 8 pm; I where the modified red mud has a d10 particle size in the range from 0.5 to 2.5 pm, in particular from 1.0 to 2.0 pm, and/or a d90 particle size in the range from 15 to 50 pm, in particular from 20 to 40 pm; and/or where the modified red mud has a specific heat capacity at 20 °C in the range of 0.6 to 0.8 kJ/(kg*K), in particular 0.65 to 0.75 kJ/(kg *K), and/or a specific heat capacity at 726.8 °C in the range from 0.9 to 1.3 kJ/(kg*K), in particular from 0.95 to 1.2 kJ/(kg *K); I where the modified red mud has a specific thermal conductivity in the range from 3 to 35 W/(m*K), in particular from 5 to 20 W/(m*K), in particular from 8 to 12 W/(m* K); I where the modified red mud is present in the form of a compressed solid material. 5. Process for the production of a modified red mud, in particular a modified red mud according to one of the preceding claims, comprising the following steps: washing and drying of red mud with a mineral composition - 10 to 55% by weight of iron compounds, - 12 to 35% by weight of aluminum compounds, - 3 to 17 % by weight of silicon compounds, - 2 to 12% by weight of titanium dioxide, - 0.5 to 6% by weight of calcium compounds, and - optionally additional unavoidable impurities, such that the washed red mud contains less than 0.5 wt% Na2O, and subsequent heating of the washed red mud to a temperature of at least 800°C, preferably at least 1000°C or 1150°C. 6. Process for the production of a modified red mud according to claim 5, wherein the washing of the red mud takes place by means of iron (II) chloride; I where the heating of the washed and dried red mud is carried out in a non-reducing atmosphere; I where the heating of the washed and dried red mud is carried out in a non-reducing atmosphere; I which also includes the following stages: granulate the red mud after heating, and subsequently compress the granulate. 7. Storage medium comprising a modified red mud according to one of claims 1 to 4. 8. Storage medium according to claim 7, further comprising one or more of the following components: - an agent to prevent air entrainment and air adsorption, - an agent for improving thermal conductivity, in particular selected from the group consisting of metal colloids, metal powder, graphite and silicon-containing substances, - an agent for the formation of a thixotropic composition. 9. Heat storage means, comprising a storage medium according to one of claims 7 to 8. 10. Heat storage media according to claim 9, further comprising devices for loading and unloading the heat storage media; I further comprising means for converting current to heat or heat to current; I that are configured for power/heat coupling or power/heat/power coupling cogeneration. Use of a modified red mud according to one of Claims 1 to 4 as a storage medium, in particular in a heat storage medium. 12. Use according to claim 11 for heat storage at a temperature of up to 1000 °C, in particular at a temperature of more than 100 °C up to 1000 °C, and/or as a storage medium that can be repeatedly heated and cooled; I as a storage medium that can be heated and cooled simultaneously; I where the thermal energy stored in the storage medium is transferred to another medium and thus the storage medium is cooled, where the other medium is selected in particular from the group consisting of water, steam, molten salt, thermal oil or gas; I where the storage medium and a download device are provided integrally or as separate elements. Use according to claim 11 or 12 as a storage medium in a heat current storage, in particular where the storage medium is heated by means of electric current and/or cooled by generation of electric current. 14. Use according to claims 11 to 13 as a heat transfer medium in heating systems in buildings, as well as in technical devices of all kinds. Use according to claim 13 or 14 for the storage of electricity generated from renewable energy sources; I where the current-heat storage comprises resistance wires that are heated by electric current and thus heat the storage medium; I where the current/heat store is used for the transport of electrical energy after the cogeneration by current/heat coupling has taken place without the presence of line systems; and/or where the current/heat storage system generates electrical energy again after heat-electricity coupling, in particular where isolated energy consumers are served with thermal energy and electrical energy; I where the heat-energy storage system is used to supply power to isolated power consumers.