DEVICE FOR COLOR OPTIMIZATION OF ACTIVATED TONES

Description

[0001] The invention relates to a device and a method for color optimization of activated tones.

[0002] The cement industry is a major emitter of carbon dioxide. A significant factor is the carbon dioxide released from the limestone. To reduce these emissions, clinker substitutes are used, which ideally do not release carbon dioxide during activation. Activated clays are therefore an important product. However, a problem with clays is that they often contain iron, which is typically (at least partially) oxidized during activation under oxidizing conditions, resulting in a strong red color (Fe III). This is generally unacceptable to customers. To achieve a cement- or clinker-like color that is acceptable to customers, the activated clay is often treated under reducing conditions to reduce the oxidized iron and thus give the activated clay a cement-like color.

[0003] From DE 10 2016 104 738 A1 a method and a device for the thermal treatment of granular solids are known.

[0004] From DE 10 2008 020 600 B4 a process and a plant for the heat treatment of fine-grained mineral solids are known.

[0005] A clinker substitute is known from DE 10 2011 014 498 A1.

[0006] A method for the production of synthetic pozzolans is known from US 2012 / 160 135 A1.

[0007] A color optimization in the production of activated clays is known from WO 2021 / 224 055 A1.

[0008] A clinker substitute based on calcined clay is known from US patent 2014 / 0000491 A1.

[0009] Energy recovery during the cooling of color-optimized activated clays is known from DE 10 2020 211 750 A1.

[0010] US Patent 4,573,908 A discloses a method and apparatus for the production of white cement clinker.

[0011] However, two problems arise during color optimization. Firstly, a longer exposure time of the already activated tones at high temperature is detrimental, as it can lead to deactivation. This effect is also dependent on particle size. Deactivation occurs more quickly with smaller particles. Secondly, the reduction, and thus the color optimization, is also dependent on particle size. The smaller the particles, the faster the decolorization; the larger the particles, the longer the reduction takes. This means that an optimum must be found where the smallest particles are not deactivated and the largest particles are still decolorized. This optimum is achieved by selecting the narrowest possible particle size distribution, i.e., by minimizing the difference between the smallest and largest particles.This in turn is complex and energy-intensive, which creates a new potential source of carbon dioxide, as renewable energy is only available to a limited extent for the processes to produce a narrow grain band.

[0012] The object of the invention is to provide a device and a method in which color optimization is also possible with a significantly broader particle size distribution.

[0013] This problem is solved by a device with the features specified in claim 1. Advantageous further developments are described in the dependent claims, the following description, and the drawings.

[0014] The device according to the invention serves for the thermal activation of mineral materials, in particular clays. The device comprises a calciner, a reduction unit, and a material cooler. The calciner and the reduction unit are connected to each other via a first connection for the transfer of calcined material. The reduction unit is connected to the material cooler via at least a second connection for the transfer of color-optimized material. According to the invention, the reduction unit is a fluidized bed reactor. This fluidized bed reactor has proven to be extremely advantageous as a reduction device when a broad particle size distribution is present (a wide particle size distribution). On the one hand, extremely small particles are carried away very quickly with the fluidizing gas. On the other hand, the particle size also influences the transport speed in the fluidized bed reactor.The fluidized bed reactor thus enables simple color optimization, as small particles, which are quickly color-optimized, are also discharged more quickly, whereas large particles, which require a longer treatment time, also have a longer residence time. This makes it possible to significantly save energy during comminution and fractionation of the particle size distribution.

[0015] Additionally, a preheater can be installed upstream of the calcinator. This allows the heat carried out of the calcinator with the gas flow to be transferred to the material to be thermally activated.

[0016] According to the invention, the reduction device has a first material outlet and a second material outlet. In particular, the first and second material outlets are arranged at different heights. For example, the first material outlet can be located at the bottom of the fluidized bed. This results in a coarse fraction of the color-optimized material being discharged through the first material outlet. For example, the second material outlet can be located at the top of the fluidized bed. This results in a fine fraction of the color-optimized material being discharged through the second material outlet. In this way, in addition to targeted color optimization, separation by size can also be achieved. The two fractions can then be processed separately or combined.

[0017] According to the invention, the reduction device has a gas outlet. The gas outlet is connected to a gas purification device, for example, and preferably, a filter device, via a fourth connection. In the gas purification device, the finest fraction of the activated and color-optimized material is separated. This fraction is usually also the most active fraction. It can either be recombined with the remaining activated and color-optimized material. However, this finest fraction can also be processed separately, for example, for particularly demanding applications.

[0018] According to the invention, the gas purification device has a solids outlet. The material cooler has at least one first cooling material inlet, one second cooling material inlet, one cooling material outlet, one cooling gas inlet, and one cooling gas outlet. The first cooling material inlet is arranged closer to the cooling gas inlet than the second cooling material inlet in terms of fluid flow. The solids outlet is connected to the first cooling material inlet via a third connection for transferring the material separated in the gas purification device. The first material outlet and / or the second material outlet is connected to the second cooling material inlet via the second connection for transferring color-optimized material.

[0019] This brings the fine material from the gas cleaning device into contact with the cooler cooling gas, resulting in rapid cooling. This is advantageous because the fine fraction from the gas cleaning device is most susceptible to reoxidation and should therefore be cooled particularly quickly.

[0020] The two fractions from the reduction unit can either be combined first and then fed together to the material cooler through the second cooling material inlet, or separately through separate cooling material inlets. It is also possible to combine the material discharged from the fluidized bed material bed (middle fraction) with the fines from the gas cleaning unit before they enter the cooler.

[0021] In a further embodiment of the invention, the material cooler has a third cooling material inlet. The first cooling material inlet is arranged closer to the cooling gas inlet than the third cooling material inlet, from a fluid dynamics perspective. The second material outlet is connected to the third cooling material inlet via a fifth connection for transferring color-optimized material. This allows the middle fraction and the coarse fraction to be fed separately, with the third cooling material inlet preferably being arranged between the first and second cooling material inlets.

[0022] In a further embodiment of the invention, the first material outlet and the second material outlet are each connected to the second cooling material inlet of the material cooler for transferring color-optimized material. Furthermore, the solids outlet of the gas purification device is connected to the first cooling material inlet of the material cooler. Alternatively, the first material stream from the first material outlet and the second material stream from the second material outlet are introduced into the material cooler at different points. This allows for customized cooling, particularly if the first and second material streams have different particle size distributions.

[0023] In a further embodiment of the invention, the device includes a deagglomeration unit. The deagglomeration unit is connected to the calcinator for transferring deagglomerated material. Preferably, the deagglomeration unit is connected to the calcinator via a preheater. For example, and more preferably, the deagglomeration unit is connected to the calcinator for transferring deagglomerated material via a riser tube dryer. At least one preheater is preferably arranged between the riser tube dryer and the calcinator. More preferably, the deagglomeration unit is a hammer mill, vertical roller mill, impact mill, pendulum roller mill, or agitator mill. A hammer mill is particularly preferred.

[0024] In a further embodiment of the invention, the device includes a color detection device. The color detection device is designed to detect the color of the product. The color detection device is arranged in or along the product stream downstream of the material cooler. By detecting the color, reduction and thus color optimization can be carried out to the necessary extent, or limited to the necessary extent, thereby saving reducing agents.

[0025] In a further embodiment of the invention, the device includes a reducing agent supply. The reducing agent supply is connected to the reduction device for supplying the reducing agent. Various gaseous, liquid, or solid substances can be used as reducing agents. Typical gaseous reducing agents are methane, hydrogen, or carbon monoxide. Examples of liquid reducing agents are liquid hydrocarbons. An important example of a solid reducing agent is coal, in particular coal dust or biomass. If the reducing agent is a fuel, combustion usually occurs with a deficiency of oxygen, so that a reducing atmosphere is created, for example, and in particular, carbon monoxide. The device further includes a control device. The control device is designed to regulate the amount of reducing agent supplied by the reducing agent supply.The control device is connected to the reducing agent supply in a manner appropriate to transmit control commands or to directly control, for example, actuators. The control device is further designed to regulate the amount of reducing agent depending on the color detected by the color detection device. For this purpose, the control device has, in particular, a data connection to the color detection device to obtain the color of the product detected by the device. The control device also includes, for example, a color threshold. If the color threshold is exceeded, i.e., if the sample is too reddish, the supply of reducing agent is increased to achieve greater decolorization. If the color threshold is not reached, the supply of reducing agent is reduced to avoid unnecessary consumption.For example, the color threshold can also be specified as a range. Alternatively, the control device can have an assignment table in which the amount of reducing agent is specified for different color ranges. Overall, this allows the use of the minimum amount of reducing agent required to achieve sufficient decolorization and thus market acceptance.

[0026] It is possible to include both a reducing agent and a fuel. For example, biomass is used as the fuel, with approximately stoichiometric combustion. This typically results in a low residual oxygen content after combustion. Subsequently, hydrogen, for example, is added as a reducing agent. As a small molecule, hydrogen exhibits excellent diffusion properties, but as a fuel, it would be more expensive than the biomass.

[0027] In In another embodiment of the invention, a gas-sealing material lock is arranged between the reduction device and the material cooler.

[0028] In In another embodiment of the invention, a gas-sealing material lock is arranged between the calcinator and the reduction device.

[0029] In In another embodiment of the invention, the device has a gas supply to the reduction device. The gas supply is adjustable with respect to volume flow and gas velocity. The device includes a control device. The device further includes a particle size presetting device. The particle size presetting device can, for example, be configured such that the particle size distribution is determined by a particle size measuring device and transferred to the particle size presetting device. Alternatively, the particle size presetting device can be an input device in the conventional sense, in which an operator enters the particle size distribution either directly or via a selection from predefined distributions.The particle size presetting device can also serve as an interface to a central database system, which, for example, contains data from laboratory analyses or information on supplied products and can thus transmit the particle size distribution based on the supplied material. It can also be provided that the particle size is measured at at least two different locations, for example, at a riser tube dryer and upstream of the fluidized bed reactor. Both pieces of information, along with the measurement location, are transmitted via the particle size presetting device to the control device. The control device is designed to regulate the gas supply such that it controls the volume flow rate and gas velocity depending on the particle size of the material specified by the particle size presetting device.The particle size distribution influences the loosening velocity (loosening point), i.e., the point at which the gas flow just begins to fluidize the solid. Since the device is preferably operated with a gas flow fluidization velocity that is 5 to 15 times the loosening velocity, it is helpful to measure a parameter such as the particle size so that changes in the fluidized bed loosening velocity can be easily detected.

[0030] In a further embodiment of the invention, the particle size presetting device includes or is connected to a particle size measuring device. For example, the particle size measuring device can determine the particle size and particle size distribution by means of light scattering in or before the calcinator. Alternatively, samples can be taken before or after the reduction device and fed to the particle size measuring device.

[0031] In a further alternative embodiment of the invention, the particle size preset device is an input device via which the information can be manually entered by the plant personnel. Alternatively, the particle size preset device is an interface via which information on the particle size distribution can be acquired, for example, from an analytical laboratory system.

[0032] In a further embodiment of the invention, the reduction device has guiding elements for directing the material flow. For example, these can be arranged in the lower region of the fluidized bed to lengthen the path of the largest particles and thereby increase the residence time of these largest particles in the reduction device.

[0033] In a further embodiment of the invention, the reduction device has guiding elements for directing the gas flow. This serves, in particular, to retain the gas flow already exiting the fluidized bed and the finest particles carried away with the gas flow within the reduction device for a minimum time, thus ensuring reliable reduction and color optimization. This can be achieved, for example, by means of a labyrinth guide.

[0034] In a further embodiment of the invention, a gas sensor is arranged downstream of the gas outlet, designed to detect the concentration of one or more substances selected from the list including carbon monoxide, carbon dioxide, hydrogen, and methane. These substances can either be used directly as reducing agents or are generated in the reduction device. Therefore, the detected concentration at the outlet can be used as an indicator of an excess of reducing agent. The carbon dioxide concentration is preferably detected together with the carbon monoxide concentration, particularly to determine their ratio.

[0035] In a further embodiment of the invention, the device has a bypass between the calcinator and the material cooler to bypass the reduction unit. This bypass is preferably used exclusively for starting and stopping the device. The activated material from the reduction unit can be transferred directly to the material cooler via the bypass.

[0036] In a further embodiment of the invention, the reduction device has a dip tube for supplying activated material. The dip tube preferably extends into the fluidized bed. This allows the activated material to be introduced directly into the interior of the fluidized bed, thus extending the residence time, particularly for the finest particles, and ensuring color optimization even for the finest particles.

[0037] In another aspect, the invention relates to a method for color optimization of an activated material using a device according to the invention. For color optimization in the reduction device, a broad particle size distribution is selected, characterized by the fact that at least 10 wt.% of the particles are smaller than 50 µm and at least 10 wt.% of the particles are larger than 250 µm. Such a broad particle size distribution is unusual and cannot be processed by existing systems. In existing systems, the very small particles would be deactivated before the very large particles could be color-optimized. However, the use of a fluidized bed reactor allows precisely this broad particle size distribution to be used. This allows for significant energy savings during comminution or deagglomeration and / or eliminates the need for a size separation step.

[0038] In a further embodiment of the invention, the particle size distribution is selected such that all particles are smaller than 2 mm.

[0039] In a further embodiment of the invention, the particle size distribution is selected such that at least 5 wt.% of the particles are larger than 900 µm.

[0040] In a further embodiment of the invention, the particle size distribution is selected such that at least 5 wt.% of the particles are smaller than 20 µm.

[0041] In a further embodiment of the invention, at least 10 wt.% of the material stream from the reduction device is discharged via the gas stream through the gas outlet and separated in the gas purification device. This means that the finest fraction, with its particularly small particle size, constitutes at least 10 wt.%. Under a conventional method, this fraction would very likely lose activity again in the reduction device. In a fluidized bed reactor according to the invention, however, this fraction is discharged very rapidly via the fluidizing gas, so that color optimization occurs without deactivation. This fluidized bed effect simultaneously separates the finest fraction.

[0042] In a further embodiment of the invention, the fluidization rate in the reduction device is selected such that it is 5 to 15 times the loosening rate. The loosening rate, or loosening point, is the flow velocity of the fluidizing gas at which fluidization of the material occurs, i.e., the theoretically lowest velocity required to form the fluidized bed. This higher fluidization rate significantly enhances the varying residence times of the particles depending on their size. This means that the finest particles, which are rapidly color-optimized and would therefore lose activity just as quickly, are carried away very rapidly by the gas, thus exhibiting a comparatively short residence time.

[0043] In In another embodiment of the invention, the fluidizing gas in the reduction device has a temperature of at least 700 °C. For example, and in particular, exhaust gas with an oxygen content of less than 15% by volume can be used. The lower the oxygen content and the higher the temperature of the exhaust gas used, the less it needs to be heated further, thus requiring less fuel (and therefore reducing agent). Preferably, however, the exhaust gas has an oxygen content of more than 0.5% by volume to enable combustion and thus energy release. The color optimization process, for example the reduction of trivalent iron, is endothermic, so the temperature would decrease. In addition, the reduction device radiates heat.In particular, in order to compensate for these two effects in the reduction device and to prevent cooling in the reduction device, the gas preferably has a corresponding residual oxygen content.

[0044] In In another embodiment of the invention, a reducing agent is introduced directly into the reduction device. For example, coal, in particular pulverized coal, or biomass can be introduced into the reduction device via a direct feed. This generates the reducing atmosphere directly in the fluidized bed, and the heat is also directly supplied through combustion, thus maintaining a constant temperature. Alternatively, hydrogen or methane, for example, can be introduced directly into the reduction device.

[0045] In In another embodiment of the invention, a reducing agent is introduced into the reduction device with the gas stream. Gaseous reducing agents such as hydrogen and methane are particularly suitable for this purpose, but liquid reducing agents, for example liquid hydrocarbons, are also possible.

[0046] In a further embodiment of the invention, a reducing agent is introduced into the reduction device with the material stream. This is preferred for solid reducing agents, such as coal, in particular coal dust. Here, the reducing agent is mixed with the activated material before being introduced into the reduction device, so that the reducing atmosphere is the same for all particles from the outset.

[0047] In a further embodiment of the invention, in iron-containing minerals, in particular clays, with an Fe 2 O 3 content above 20 wt.%, a reduction of at least 80% of the iron is carried out.

[0048] In a further embodiment of the invention, a reduction of 50 to 80% of the iron is carried out in iron-containing minerals, in particular clays, with an Fe₂O₃ content between 10 wt.% and 20 wt.%. Here, the proportion of iron to be reduced is increased from 50% at 10 wt.% to 80% at 20 wt.%. This can be done, for example, linearly or in stages.

[0049] In a further embodiment of the invention, a reduction of up to 50% of the iron is carried out in iron-containing minerals, in particular clays with an Fe 2 O 3 content below 10 wt.%.

[0050] The device according to the invention is explained in more detail below with reference to an embodiment shown in the drawings. Fig. 1 first embodiment Fig. 2 second embodiment Fig. 3 third embodiment

[0051] In Fig. 1 A first embodiment of a device according to the invention for the production of color-optimized activated material, in particular clays, is shown.

[0052] The material first enters a hammer mill 10 and is deagglomerated there. This results in a relatively broad particle size distribution. The deagglomerated material is then lifted and dried in a riser tube dryer 20. The material then passes through a preheater 30 into a calciner 40, where it is thermally activated. During activation under oxidizing conditions, however, the oxidation of iron to Fe33 occurs, causing the material to turn reddish. This oxidation is not necessarily quantitative; that is, not all iron atoms are necessarily oxidized to Fe33. To reverse this, the thermally activated material is introduced into a fluidized bed reactor 50 via a first connection 1.To fluidize the fluidized bed, fluidizing gas is supplied to the fluidized bed reactor 50 from a fluidizing gas supply 100 via a combustion chamber 90. In this combustion chamber, for example, pulverized coal is supplied in excess to the oxygen supplied with the fluidizing gas via a reducing agent supply 110, resulting in the formation of carbon monoxide in the combustion chamber. The fluidizing gas can be, for example, oxygen-depleted exhaust gas from a process, such as from the preheater 30, or from the exhaust gas after the gas cleaning device. The fluidized bed reactor 50 is operated, for example, such that the fluidization rate corresponds to 5 to 15 times the loosening rate.This ensures that the finest particles, which also reduce most rapidly and are thus color-optimized, are quickly discharged through the gas outlet 53 and pass through the gas outlet 53 and the subsequent fourth connection 4 into the gas purification device 60, where they are then separated. Furthermore, the fluidized bed reactor 50 has two outlets for the color-optimized material. The first material outlet 51 is located at the bottom and serves to remove the largest particles, while the second material outlet 52 is located in the upper region of the fluidized bed and serves to remove the medium particle fraction. All three fractions of the color-optimized material are fed to a material cooler 70. For this purpose, the solids outlet 61 of the gas purification device 60 is connected to the first cooling material inlet 71 via a third connection 3.The first material outlet 51 is connected to the second cooling material inlet 72 via a second connection 2, and the second material outlet 52 is connected to the third cooling material inlet 73 via a fifth connection 5. Cooling gas is supplied to the material cooler 70 via the cooling gas inlet 75 and discharged via the cooling gas outlet 76. The cooled product can be removed via the cooling material outlet 74. The first cooling material inlet 71 is located closest to the cooling gas inlet 75, ensuring that the finest fraction is cooled most quickly and thus best protected against reoxidation. After cooling, the color is determined in the color detection device 80. This allows the addition of reducing agent via the reducing agent supply 110 to be regulated so that as little reducing agent as possible is added, while still achieving sufficient decolorization.

[0053] Fig. 2 shows a second embodiment, which differs from the first in Fig. 1 The illustrated embodiment differs in that the reducing agent is mixed with the activated material before entering the fluidized bed reactor 50 and is introduced into the reactor together with the activated material. For example, pulverized coal is supplied via the reducing agent feed 110. This also allows the heat to be generated directly in the fluidized bed of the reactor 50 by combustion. This embodiment is preferred when the fluidizing gas supplied via the fluidizing gas feed 100 is already close to (just below) the ignition temperature of the reducing agent.

[0054] In Fig. 3 A third embodiment is shown, which differs from the first in Fig. 1The illustrated embodiment differs in that the fractions from the first material outlet 51 and the second material outlet 52 are first combined and then fed together to the second cooling material inlet 72. A gas sensor 120 is arranged downstream of the gas outlet 53, so that, in addition to color detection in the color detection device 80, the reducing components still present in the exhaust gas can also be used as a control variable. For example, the gas sensor 120 can be a carbon monoxide sensor. Additionally, the finest material fraction is separated from the gas purification device 60. This fraction often exhibits the highest reactivity and can therefore be used for particularly high-quality products. Reference sign

[0055] 1 First connection 2 Second connection 3 Third connection 4 Fourth connection 5 Fifth connection 10 Hammer mill 20 Riser dryer 30 Preheater 40 Calcinator 50 Fluidized bed reactor 51 First material outlet 52 Second material outlet 53 Gas outlet 60 Gas cleaning device 61 Solids outlet 70 Material cooler 71 First cooling material inlet 72 Second cooling material inlet 73 Third cooling material inlet 74 Cooling material outlet 75 Cooling gas inlet 76 Cooling gas outlet 80 Color detection device 90 Combustion chamber 100 Fluidizing gas supply 110 Reducing agent supply 120 Gas sensor

Claims

1. A device for the thermal activation of mineral materials, wherein the device comprises a calciner (40), a reduction device and a material cooler (70), wherein the calciner (40) and the reduction device are connected to one another via a first connection (1) for the transfer of calcined material, wherein the reduction device is connected to the material cooler (70) at least via a second connection (2) for the transfer of color-optimized material, characterized in that the reduction device is a fluidized bed reactor (50), wherein the reduction device comprises a first material output (51) and a second material output (52), wherein the first material output (51) and the second material output (52) are each connected to one another to the material cooler (70) for the transfer of color-optimized material, wherein the reduction device comprises a gas outlet (53), wherein the gas outlet (53) is connected to a gas purification device (60) via a fourth connection (4), wherein the gas purification device (60) comprises a solids outlet (61), wherein the material cooler (70) comprises at least a first inlet (71) for material to be cooled, a second inlet (72) for material to be cooled, an outlet (74) for cooled material, a cooling gas inlet (75) and a cooling gas outlet (76), wherein the first inlet (71) for material to be cooled is arranged fluidically closer than the second inlet (72) for material to be cooled to the cooling gas inlet (75), wherein the solids outlet (61) is connected to the first inlet for material to be cooled via a third connection (3) for the transfer of the material precipitated in the gas purification device (60), wherein the first material output (51) and / or the second material output (52) is connected to the second inlet (72) for material to be cooled via the second connection (2) for the transfer of color-optimized material.

2. The device as claimed in claim 1, characterized in that the material cooler (70) comprises a third inlet (73) for material to be cooled, wherein the first inlet (71) for material to be cooled is arranged fluidically closer than the third inlet (73) for material to be cooled the cooling gas inlet (75), wherein the second material output (52) is connected to the third inlet (73) for material to be cooled via a fifth connection (5) for the transfer of color-optimized material.

3. The device as claimed in one of the preceding claims, characterized in that the first material output (51) and the second material output (52) are each connected to the second inlet (72) for material to be cooled of the material cooler (70), and the solids outlet (61) of the gas purification device (60) is connected to the first inlet (71) for material to be cooled of the material cooler (70).

4. The device as claimed in one of the preceding claims, characterized in that the device comprises a color recording device (80), wherein the color recording device (80) is configured to record the color of the product, wherein the color recording device (80) is arranged in or along the product flow downstream of the material cooler (70).

5. The device according to claim 4, characterized in that the device comprises a reducing agent supply, wherein the reducing agent supply is connected to the reduction device for the delivery of reducing agent, wherein the device comprises a control device, wherein the control device is configured to regulate the amount of reducing agent supplied by the reducing agent supply, wherein the control device is configured to regulate the amount of reducing agent as a function of the color recorded by the color recording device (80).

6. The device as claimed in one of the preceding claims, characterized in that a gas-sealing material lock is arranged between the reduction device and the material cooler (70).

7. The device as claimed in one of the preceding claims, characterized in that the device comprises a gas supply to the reduction device, wherein the gas supply can be regulated in respect of volume flow rate and gas velocity, wherein the device comprises a control device, wherein the control device is configured to drive the gas supply, wherein the control device controls the volume flow rate and gas velocity as a function of the particle size of the material.

8. The device as claimed in one of the preceding claims, characterized in that a gas sensor (120) is arranged fluidically downstream of the gas outlet (53), wherein the gas sensor (120) is configured to record the concentration of one or more substances selected from the list consisting of carbon monoxide, carbon dioxide, hydrogen, methane.

9. A method for the color optimization of activated material with a device as claimed in one of the preceding claims, wherein a wide particle size distribution is selected for the color optimization in the reduction device, the wide particle size distribution being distinguished in that at least 10 wt% of the particles are smaller than 50 µm and at least 10 wt% of the particles are larger than 250 µm or at least 5 wt% of the particles are larger than 900 µm.

10. The method as claimed in claim 9, characterized in that at least 10 wt% of the material flow is extracted from the reduction device via the gas flow through the gas outlet (53) and precipitated in the gas purification device (60).

11. The method as claimed in one of claims 9 to 10, characterized in that the fluidization velocity in the reduction device is selected so that the fluidization velocity is from 5 times to 15 times the minimum fluidization velocity.

12. The method as claimed in one of claims 9 to 11 characterized in that a reducing agent is introduced directly into the reduction device.

13. The method as claimed in one of claims 9 to 12, characterized in that a reducing agent is introduced with the gas flow into the reduction device.