Climate-friendly puzzolana production

Abstract

The present invention relates to a device for producing a thermally activated clay, wherein the device has a rotary kiln (10) and a hot-gas generating device (20) separate from the rotary kiln (10), wherein the hot-gas generating device (20) and the rotary kiln (10) are connected by a gas line (23) for transferring hot gases generated in the hot-gas generating device (20).

Description

[0001] Climate-friendly pozzolan production

[0002] The invention relates to a device and a method for producing a thermally activated clay, an artificial pozzolan, in a rotary kiln.

[0003] Since the firing process of limestone releases mineral-bound carbon dioxide from the limestone itself, activated clays, also known as artificial pozzolans, are increasingly being used to reduce carbon dioxide emissions in the cement industry, as such carbon dioxide release from the clay does not occur during their activation. Thus, the energy source, and thus the fuel used, is the primary source of carbon dioxide emissions in the activation of clays. To reduce these emissions, alternative fuels are increasingly being used, although these are more demanding to burn.

[0004] Originally, artificial pozzolans were produced by thermally activating clays in a rotary kiln. However, since the use of refuse-derived fuels is not without its problems in a rotary kiln, entrained-flow processes are currently used, which make the use of refuse-derived fuels much easier. Two aspects are important when using refuse-derived fuels. First, safe and complete burnout must be ensured. This requires sufficiently high temperatures and a sufficiently long combustion time with a sufficiently high oxygen supply. Therefore, simultaneous optimization of clay activation and refuse-derived fuel burnout is not possible in a rotary kiln.

[0005] WO 2018 / 195 642 A2 discloses a process for producing pozzolans with color change.

[0006] Energy recovery during the cooling of color-optimized clays is known from WO 2022 / 058 206 A1.

[0007] DE 10 2004 009 689 A1 discloses a plant for producing cement clinker. DE 10 2012 108 295 A1 discloses a device and method for processing refuse-derived fuels.

[0008] From US 2023 / 002 277 A1 a process for producing clinker and a second calcined material is known.

[0009] From US 2023 / 110 304 A1 a method and a device for producing cement is known.

[0010] CN 113 387 367 A discloses a process for the processing of kaolin by recycling municipal construction waste.

[0011] A clinker substitute based on calcined clay is known from US 2014 / 000 491 A1.

[0012] It would be desirable to enable the activation of a clay in a rotary kiln while avoiding emissions, in particular by using alternative fuels, such as biomass, as completely as possible.

[0013] The object of the invention is to provide a device with a rotary kiln for the thermal activation of clays, which does not require the firing of primary raw materials such as oil or gas in the rotary kiln and can thus be operated in a particularly climate-friendly manner.

[0014] This object is achieved by the device having the features specified in claim 1 and by the method having the features specified in claim 13. Advantageous further developments emerge from the subclaims, the following description, and the drawings.

[0015] The device according to the invention serves to produce thermally activated clay. The device comprises a rotary kiln and a hot gas generation device separate from the rotary kiln. This separation enables the process conditions to be separated, so that, for example, substitute fuel can be produced in the hot gas generation device under conditions optimal for the combustion of the substitute fuel, while the process conditions in the rotary kiln can be optimized for the activation of the clay. The hot gas generation device and the rotary kiln are connected via a gas line for transferring hot gases generated in the hot gas generation device. Since only the hot gas is thus transferred through the gas line, this offers the advantage of separating the two thermal processes.On the other hand, it is disadvantageous that energy generation no longer takes place directly in the rotary kiln, and thus the heat transfer from the gas to the clay is limited. This complicates thermal activation without direct heat generation in a rotary kiln. The hot gas generation device is a combustion device for a substitute fuel or an electric gas heater. It is therefore a discrete hot gas generation device whose function is precisely the generation of hot gas and not a process in which hot gas is produced as a waste product. The use of a combustion device for a substitute fuel and / or an electric gas heater enables the CO2-free generation of thermal energy, for example when biomass is used as the substitute fuel or electricity from a renewable source is used for the electric gas heater. A substitute fuel can of course be a mixture within the meaning of the invention.Alternative fuels include waste or biomass or a mixture of these.

[0016] In a further embodiment of the invention, the device comprises a first temperature sensor. The first temperature sensor is an infrared sensor. The first temperature sensor is located in the outlet area below the rotary kiln. This special arrangement enables contactless (wear-free) measurement of the temperature of the activated clay. With conventional measurement from above, a reliable temperature measurement is not possible because dust interferes with the measurement. Measuring the exiting material from below, however, has proven to be very reliable.

[0017] In a further embodiment of the invention, the device comprises a second temperature sensor. The second temperature sensor is arranged in or on the rotary kiln. For example, the second temperature sensor can be arranged on the outside of the rotary kiln. In this case, only the external temperature of the rotary kiln is measured. The advantage is that this is low-maintenance and wear-free. The disadvantage is that the measurement is comparatively inaccurate. Alternatively, the second temperature sensor can be arranged in a refractory protective tube inside the rotary kiln. This allows measurement of the material in the rotary kiln. The second temperature sensor can, for example, be surrounded and protected on three sides by a metallic guard.

[0018] In a further embodiment of the invention, the gas line is long enough for the gas to stay in the gas line for between 0 and 10 seconds, preferably between 0.5 and 10 seconds. The gas line can therefore be shorter, for example, but with a larger internal diameter, or longer and with a smaller internal diameter. The gas line is preferably long enough for the gas to stay in the gas line for between 1 and 5 seconds. Further preferably, the gas line is long enough for the gas to stay in the gas line for between 1.5 and 2.5 seconds. This residence time in the gas line can ensure safe burnout if this has not already occurred in the hot gas generating device. This therefore enables the use of a hot gas generating device which itself does not, for example, comply with the legal requirements for safe burnout.In addition, other measures, such as the addition of ammonia or urea to remove potential nitrogen oxides, can be used. Appropriate reaction times are also useful for these.

[0019] In a further embodiment of the invention, the device comprises a material cooler. The rotary kiln is connected to the material cooler for transferring the thermally treated clay. The material cooler is connected to the hot gas generation device for transferring gas preheated in the material cooler. This not only cools the finished product, but also returns the heat to the process. A dust filter is preferably arranged between the material cooler and the hot gas generation device. The dust separated in this way is preferably fed to the finished product. In a further embodiment of the invention, the material cooler is a drum cooler. Preferably, the gas flow in the drum cooler is conducted countercurrently to the material flow. The advantage of the drum cooler is that the particle size of the activated clay is less relevant compared to entrained flow heat exchangers.Therefore, the requirements for the clay and the energy required for crushing are correspondingly low.

[0020] In a further alternative embodiment of the invention, the material cooler is a walking floor, two-layer cooler or grate cooler.

[0021] In another alternative embodiment of the invention, the material cooler is a cyclone or fluidized bed cooler. The advantage is that much faster cooling can be achieved. However, the disadvantage is that the maximum particle size of the activated clay is limited to a flyable particle size (e.g., less than 2 mm).

[0022] In a further embodiment of the invention, a comminution device is arranged between the rotary kiln and the material cooler. The comminution device is, for example, a roller crusher. The comminution device can, in particular, be cooled, preferably air-cooled. This is particularly preferred if the material cooler is a cyclone or fluidized-bed cooler.

[0023] In a further embodiment of the invention, a screen, in particular a drum screen, is arranged between the rotary kiln and the material cooler. Particularly preferably, a crusher is arranged behind the coarse outlet of the screen, in particular the drum screen. This allows the amount of hot material to be crushed, which is applied to the crusher, to be significantly reduced, so that heat is less of a problem. The screen can also be designed as a static screen, in particular as a screen with an inclination of 30° to 60°. This enables purely gravity-driven transport, thus eliminating the need for active conveying.

[0024] Preferably, the sieve has a mesh size of 2 mm so that particles of less than 2 mm can pass through and particles of more than 2 mm are retained.

[0025] In a further embodiment of the invention, the device has a product return line. The product return line is designed to return product cooled in the material cooler between the rotary kiln and the material cooler. This is particularly preferred if a comminution device is arranged between the rotary kiln and the material cooler, since this allows rapid cooling and thus reduced thermal stress on the comminution device by mixing hot activated clay coming from the rotary kiln with cold, returned activated clay. For further details, see DE 10 2020 211 750 A1.

[0026] In a further embodiment of the invention, the device comprises a pre-cooler. The pre-cooler is arranged between the rotary kiln and the material cooler. The pre-cooler has direct or indirect water cooling. With direct water cooling, water is applied, in particular sprayed, directly to the activated clay. This enables extremely rapid and efficient cooling, which is particularly advantageous for preventing a color change in the activated clay after thermal treatment, in which the activated clay comes into contact with oxygen at high temperatures. With indirect water cooling, the water is sprayed onto the outside of the pre-cooler, thus significantly cooling the pre-cooler itself. The disadvantage of the process is the low efficiency of heat recovery.

[0027] In a further embodiment of the invention, the rotary kiln has internals for vertically conveying the clay. The internals for vertically conveying the clay serve to lift the clay, causing it to fall through the gas flow inside the rotary kiln, thus optimizing heat transfer from the gas flow to the clay. The internals can be straight, angled, or curved. They can be honeycomb or cross internals, or they can be chain elements. It is important that the clay is at least partially lifted by the internals and thus at least partially falls back down again through the gas flow. Since wear of the internals is highly temperature-dependent, these internals are preferably only installed on the side of the rotary kiln facing away from the hot gas generation device, i.e. in areas of the rotary kiln where the temperature of the clay is below 650°C, possibly below 500°C.

[0028] In a further embodiment of the invention, the rotary kiln has retaining rings. This increases the filling level in the rotary kiln, increases the residence time, and thus improves the heat transfer from the gas phase to the clay to be activated. The rotary kiln preferably has one or more retaining rings.

[0029] In a further embodiment of the invention, the gas line has a nitrogen supply for removing nitrogen oxides. For example, ammonia, aqueous ammonia solution, urea solution, or the like can be supplied via the nitrogen supply, so that the supplied nitrogen synproportionates with nitrogen oxides, thus removing the nitrogen oxides. Further possible embodiments can be found in particular in DE 10 2022 209 826.

[0030] In a further embodiment of the invention, an induced draft fan is arranged downstream of the rotary kiln in the gas flow direction. Due to the high temperatures in the gas line between the hot gas generation device and the rotary kiln, a fan cannot be arranged there, so that the conveying capacity is preferably completely provided by the induced draft fan. This in turn leads to the pressure in the hot gas generation device being higher than the pressure in the rotary kiln. Due to the usually very slight negative pressure in the rotary kiln, this leads to the hot gas generation device having virtually no negative pressure and possibly even a slight positive pressure. Such operation is unusual for the combustion of refuse-derived fuels. In a further embodiment of the invention, a dust filter is arranged downstream of the rotary kiln in the gas flow direction. The dust filter is connected to the rotary kiln to recirculate the separated dust.Due to the indirect heating and the associated higher gas flow through the rotary kiln, a higher level of dust development and thus a discharge of very fine material with the gas flow from the rotary kiln is to be expected.

[0031] In a further embodiment of the invention, the rotary kiln has a larger diameter in the central region. In particular, the central region between the first and second races is widened. The central temperature range is also located in the central region. This widening reduces the flow velocity in this region, which in turn reduces the discharge of fines and improves the heat exchange between gas and clay in this region.

[0032] In a further embodiment of the invention, the device comprises a preheater and / or a dryer. The preheater or dryer is arranged downstream of the rotary kiln in terms of gas flow and upstream of the rotary kiln in terms of material flow. Although drying and heating can also take place entirely and exclusively in the rotary kiln, due to the larger gas flow caused by the external generation of the hot gas flow, it is advantageous if the heat transfer takes place in an additional dryer and / or preheater. The preheater can be designed as a cascade of cyclone preheaters or as a riser heat exchanger with a longer residence time and a separation cyclone. A drum dryer is preferably used as the dryer. The advantage of the drum dryer is that it can also work with very coarse particles (down to the cm range).A mill or crusher followed by a riser dryer with a separation cyclone can also be used as a dryer.

[0033] In a further embodiment of the invention, the hot gas generation device is a combustion device for a substitute fuel. Particularly preferably, the combustion device is designed for burning biomass. Alternatively, it can be an electrically operated superheater, which would also be climate-neutral when operated with renewable electricity.

[0034] In a further embodiment of the invention, the combustion device has a gas-tight solids discharge. This allows operation of the combustion device even at overpressure.

[0035] In a further embodiment of the invention, the combustion device is a reverse-acting grate, a rotary kiln, a direct current grate or a rotary hearth.

[0036] In a further embodiment of the invention, the rotary kiln has an auxiliary combustion device. When using substitute fuels, such as biomass, fluctuations in calorific value and humidity typically occur, causing the temperature generated in the combustion device to fluctuate. While these fluctuations can be mitigated by controlling the supply of substitute fuel, this is often slow. Therefore, it may be advantageous to provide an auxiliary combustion device that uses a fuel that is easy to dose and burns quickly and easily. Typically, this can be a gas burner, to avoid carbon dioxide emissions, for example, in the case of biogas. This allows for a very rapid response to these fluctuations, and the temperature in the rotary kiln can be kept particularly constant.

[0037] In a further embodiment of the invention, the device comprises a mixing device. The mixing device is designed to mix the clay with a reducing agent. The mixing device is arranged upstream of the rotary kiln in the direction of material flow. The mixing device can be arranged, for example, between a dryer and the rotary kiln. Coal, for example, can be used as the reducing agent. Unburned carbon residues from the combustion device can also be used, for example. The mixing creates a locally reducing atmosphere around the clay to be activated in the rotary kiln, which in turn prevents the clay from taking on an undesirable color, for example due to the oxidation of iron. It is essential, however, that only a small amount of reducing agent is used so that no reducing agent ends up in the finished product and does not have a disruptive effect.

[0038] In a further embodiment of the invention, the rotary kiln has at least one laterally arranged feed device for a reducing agent. The laterally arranged feed device can, for example, be a so-called scoop feeder. This allows the reducing agent to be fed directly, particularly within the correct temperature range.

[0039] In a further embodiment of the invention, the rotary kiln has at least one laterally arranged air inlet. Oxygen-containing gas, in particular, can be supplied through the air inlet. This enables complete combustion of, for example, a reducing agent or its gaseous reaction products. At the same time, an extremely low-oxygen environment can be maintained, especially in the highest-temperature region.

[0040] In a further aspect, the invention relates to a method for operating a device according to the invention. The clay is thermally activated in a rotary kiln by means of hot gas generated outside the rotary kiln. As already explained, the spatial separation of hot gas generation and thermal activation is essential, as both can be optimized separately. In particular, the hot gas generation device can be operated using a substitute fuel at sufficiently high temperatures with sufficient oxygen for a sufficiently long time, whereas the thermal activation is carried out in particular at lower temperatures and preferably with a very low oxygen content. The hot gas is fed to the rotary kiln at a temperature of 600 to 1800 °C. When using substitute fuels, a temperature of 800 to 1200 °C is preferred, and 600 to 1800 °C is preferred with an electric gas heater.The highest temperatures are preferably achieved when combining a combustion device for a substitute fuel and an electric gas heater. This temperature range is sometimes significantly higher than that used for the thermal treatment of clay. However, since the temperature drops during thermal treatment due to energy consumption, the clay does not heat up to the full temperature of the hot gas, which would deactivate it. Therefore, this temperature window has proven sufficient and not too high; good activation occurs without deactivation due to excessively high temperatures. This is important because the amount of heat supplied in this way must be sufficient for activation, as activation is activated exclusively by the hot gas supplied from outside; therefore, no additional combustion or other energy input takes place within the rotary kiln.

[0041] In a further embodiment of the invention, the pressure in the combustion device is higher than the pressure in the rotary kiln. This operating mode is preferred in order to be able to operate the gas flow downstream of the rotary kiln using only an induced draft fan. The pressure in the hot gas generation device can be selected to be around or even above ambient pressure.

[0042] In a further embodiment of the invention, the temperature in the combustion device is selected to be higher than the temperature in the rotary kiln. This allows for the safe combustion of alternative fuels, such as biomass, without negatively impacting product quality, since excessively high temperatures lead to deactivation of the clay.

[0043] In a further embodiment of the invention, the temperature in the rotary kiln is regulated by supplying substitute fuel to the combustion device. The advantage is that no additional, usually primary, fuels need to be used, thus minimizing carbon dioxide emissions from non-renewable fuels.

[0044] In a further embodiment of the invention, the clay is fed into the rotary kiln together with a reducing agent. The reducing agent can be, for example, coal, wood, or other biogenic materials, preferably incompletely combusted residues from the combustion of a refuse-derived fuel. By introducing them together, a local reducing atmosphere is created in the area of ​​the clay to be activated in the rotary kiln. This atmosphere prevents, for example, iron in the clay from oxidizing, which would cause the clay to take on an undesirable color during activation. This further supports the separation between the conditions during hot gas generation and activation, as the oxygen concentration can thus be selected more freely.

[0045] In a further embodiment of the invention, oxygen or an oxygen-containing gas mixture is introduced into the rotary kiln at a process gas temperature range of 400 to 850 °C, preferably 500 to 650 °C. This is preferably done to ensure the complete burnout of incompletely converted combustion products of the organic constituents of the feed material (clay) and / or the incompletely converted reaction products of a reducing agent.

[0046] In a further embodiment of the invention, the oxygen content in the gas line downstream of the hot gas generation device is selected to be below 2 vol.%, preferably below 1 vol.%. This minimizes the oxidation of a clay and thus any unwanted color change.

[0047] In a further embodiment of the invention, the hot gas generating device is electrically heated. The use of green electricity thus also makes it possible to avoid carbon dioxide emissions.

[0048] In a further embodiment of the invention, a heat energy of 300 to 1200 kcal per kg of product is supplied to the rotary kiln via the hot gas. The amount of heat supplied is determined by the heat capacity of the hot gas as well as the temperature at the inlet of the hot gas into the rotary kiln and the temperature at the outlet from the rotary kiln, i.e., the amount of heat actually released by the hot gas in the rotary kiln.

[0049] In a further embodiment of the invention, 1 to 5 standard cubic meters of hot gas are fed to the rotary kiln per kg of product.

[0050] In a further embodiment of the invention, the residence time of the clay in the rotary kiln is selected between 15 and 90 minutes. This residence time is longer than with combustion in a rotary kiln because the temperature gradient is greater and, on the other hand, heat transfer from radiant heat, as with an open flame, is eliminated. Therefore, only direct heat transfer from the gas to the clay is possible, which is why thorough mixing can lead to a shortening of the residence time. The residence time in the rotary kiln is adjusted primarily by speed, inclination, and the installation of internals, especially retention devices.

[0051] In a further embodiment of the invention, the activation is controlled so that the hot gas leaves the rotary kiln at the kiln inlet at 100 to 600 °C.

[0052] The device according to the invention is explained in more detail below using an embodiment shown in the drawings.

[0053] Fig. 1 first example

[0054] Fig. 2 second example

[0055] Fig. 3 third example

[0056] Fig. 4 fourth example

[0057] Fig. 1 shows a first example of a device according to the invention. The clay to be activated is fed via a clay feed into a dryer 40, for example a drum dryer. From there, the clay is transferred to a rotary kiln 10 and thermally treated there. The activated clay is transferred from the rotary kiln 10 to a material cooler 30 and cooled there. During the transfer, the temperature of the clay is detected by a temperature sensor 11, which is arranged below the rotary kiln 10 and can thus measure the temperature contactlessly using infrared and without excessive dust exposure. The cooled, activated product is removed from the material cooler 30 via the product outlet 32. This is the material flow through the device. The gas flow runs countercurrently. The cold gas is fed to the material cooler 30 via the gas feed 31 and heated there. The heated gas is fed to a dust filter 50.The separated dust is fed to the product at product outlet 32. The dust-free, heated gas is fed to the hot gas generation device 20. In the hot gas generation device 21, a substitute fuel, for example, biomass, is introduced via a substitute fuel feed 21 and combusted safely and completely there, for example, at 1100°C for at least 20 seconds. Residues of the substitute fuel are discharged via a lock 22 so that the hot gas generation device 21 can be operated under a slight overpressure. The hot gas generated in the hot gas generation device 21 is fed into the rotary kiln 10 via a gas line 23, which is long enough for the hot gas to remain in the gas line 23 for approximately 5 seconds. There, the heat is transferred to the clay to be activated, thus activating the clay.Since heat generation does not occur within the rotary kiln 10, it can be assumed that part of the heat is transferred indirectly, i.e., first from the gas to the rotary kiln 10 and then from the rotary kiln 10 to the clay, since the surface area of ​​the rotary kiln 10 is larger than the surface area of ​​the clay in the rotary kiln 10. The gas, which has been significantly cooled in the rotary kiln 10, is led from the rotary kiln 10 into the dryer 40. The dryer 40 is designed as a drum dryer. The gas is then discharged from the dryer 40 via the exhaust gas 42.

[0058] Fig. 2 shows a second example of a device according to the invention. The clay is fed via the clay feed 41 into a preheater 43, which is designed as a cascaded entrained-flow heat exchanger. The material preheated in the preheater 43 is transferred to the rotary kiln 10 and thermally activated there. The material activated in the rotary kiln 10 is fed past the temperature sensor 11 onto a screen 60. For example, particles larger than 2 mm are fed into a crusher 61, and the free material smaller than 2 mm from the screen 60, together with the material crushed in the crusher 61, is fed into a mixer 62, where it is mixed with cold, recycled material, and then cooled in a material cooler 30 in the form of an entrained-flow heat exchanger.The activated and cooled material leaving the material cooler 30 is divided, with part of it being returned to the mixer 62 via the product return line 33 and the other part being discharged via the product outlet 32. The gas flow is conducted analogously to the first example, except that a dust filter 50 is arranged downstream of the preheater 43 to transfer the finest material back into the rotary kiln 10 via the preheater 43 and thus ultimately into the finished product. The third example shown in Fig. 3 differs from the example shown in Fig. 2 in that the temperature sensor 11 is arranged below the screen 60.

[0059] The fourth example shown in Fig. 4 differs from the second example shown in Fig. 2 in that the screen 60 is designed as a drum screen and is arranged on the same level as the rotary kiln 10. Therefore, the gas line 23 of the hot gas generating device 20 is connected to the screen 60, so that the hot gas is fed through the drum screen to the rotary kiln 60. Therefore, the temperature sensor 11 is also arranged behind the screen 60.

[0060] Reference symbol

[0061] 10 rotary kilns

[0062] 11 Temperature sensor

[0063] 20 Hot gas generation device

[0064] 21 Alternative fuel supply

[0065] 22 Lock

[0066] 23 Gas pipeline

[0067] 30 material coolers

[0068] 31 Gas supply

[0069] 32 Product outlet

[0070] 33 Product recirculation

[0071] 40 dryers

[0072] 41 Sound supply

[0073] 42 exhaust

[0074] 43 preheaters

[0075] 50 dust filters

[0076] 60 sieve

[0077] 61 crushers

[0078] 62 mixers

Claims

Patent claims 1. A device for producing a thermally activated clay, the device comprising a rotary kiln (10) and a hot gas generating device (20) separate from the rotary kiln (10), the hot gas generating device (20) and the rotary kiln (10) being connected via a gas line (23) for transferring hot gases generated in the hot gas generating device (20), the hot gas generating device (20) being a combustion device for a substitute fuel and / or an electric gas heater.

2. Device according to claim 1, characterized in that the device has a first temperature sensor (11), wherein the first temperature sensor (11) is an infrared sensor, wherein the first temperature sensor (11) is arranged in the outlet area below the rotary kiln (10).

3. Device according to one of the preceding claims, characterized in that the device has a second temperature sensor, wherein the second temperature sensor is arranged in or on the rotary kiln (10).

4. Device according to one of the preceding claims, characterized in that the gas line (23) has a length such that the gas within the gas line (23) has a residence time of between 0.5 and 10 s.

5. Device according to one of the preceding claims, characterized in that the device has a material cooler (30), wherein the rotary kiln (10) is connected to the material cooler (30) for transferring the thermally treated clay, wherein the material cooler (30) is connected to the hot gas generating device (20) for transferring gas preheated in the material cooler (30).

6. Device according to claim 5, characterized in that a dust filter (50) is arranged between the material cooler (30) and the hot gas generating device (20).

7. Device according to one of the preceding claims, characterized in that the rotary kiln (10) has internals for the vertical conveyance of the clay.

8. Device according to one of the preceding claims, characterized in that the rotary kiln (10) has baffle rings.

9. Device according to one of the preceding claims, characterized in that the rotary kiln (10) has a larger diameter in the central region.

10. Device according to one of the preceding claims, characterized in that the combustion device has a gas-tight solids discharge.

11. Device according to one of the preceding claims, characterized in that the combustion device is a reverse-acting grate, a rotary kiln, a direct current grate or a rotary hearth.

12. Device according to one of the preceding claims, characterized in that the rotary kiln (10) has an auxiliary combustion device.

13. A method for operating a device according to one of the preceding claims, characterized in that the clay is thermally activated in a rotary kiln (10) by means of hot gas generated outside the rotary kiln (10), the hot gas being supplied to the rotary kiln (10) at a temperature of 600 to 1800 °C.

14. The method according to claim 13, characterized in that the pressure in the combustion device is higher than the pressure in the rotary kiln (10).

15. Method according to claim 13, characterized in that the temperature in the rotary kiln (10) is regulated by supplying substitute fuel to the combustion device.

16. The method according to any one of claims 13 to 15, characterized in that oxygen or an oxygen-containing gas mixture is introduced into the rotary kiln (10) in a temperature range of 400 to 850 °C, preferably 500 to 650 °C.

17. Method according to one of claims 13 to 16, characterized in that after the hot gas generating device (20) in the gas line (23) the oxygen content is below 2 vol.%, preferably below 1 vol.%.

18. Method according to one of claims 13 to 17, characterized in that a heat energy of 300 to 1200 kcal per kg of product is supplied to the rotary kiln (10) via the hot gas.

19. Process according to one of claims 13 to 18, characterized in that 1 to 5 standard cubic meters of hot gas are fed to the rotary kiln (10) per kg of product.

20. Process according to one of claims 13 to 19, characterized in that the residence time of the clay in the rotary kiln (10) is selected between 15 and 90 minutes.

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