Method for treating lime sludge in lime kiln, lime kiln feed head and lime kiln

Abstract

The invention relates to a method for treating lime sludge in a lime kiln (2), in which the lime kiln (2) comprises a rotary kiln shell (10) having an inner space (8) between a first end wall (6) and a second end wall (11), and in which the first end wall (6) of the lime kiln (2) comprises a moving wall portion (6a), which rotates together with the rotary kiln unit (10), and a stationary wall portion (6b), which rotates together with the second end wall (11), and wherein the method comprises the method steps of:-feeding lime sludge into the interior space (8) of the rotary kiln shell (10) or in close proximity to the rotary kiln shell (10) by means of a feeding device (1) at the first end wall (7); -entraining lime sludge in the flue gas flowing out of the interior space (8) of the rotary kiln shell (10) and via the flue gas rising duct (3) to the separating device (4); -pre-treating the lime sludge while the lime sludge is entrained by the flue gas stream; -separating the pretreated lime sludge from the flue gas stream by means of a separation device (4); returning the separated lime sludge to the feeding device (1) and the lime kiln (2); and calcining the separated lime sludge in the lime kiln (2). In the method according to the invention, lime sludge particles adhering and / or deposited on the inner surface of the lime kiln (2) are at least partially detached from the inner surface of at least the stationary wall portion (6b) of the first end wall (6) by a cleaning device (13) arranged at the feed end (20) of the lime kiln. The invention also relates to a lime kiln feed head (20) and a lime kiln (2).

Description

Technical Field

[0001] This invention relates to a method for processing lime sludge in a lime kiln, a lime kiln feed head, and a lime kiln. Background Technology

[0002] The lime kiln is part of the pulp mill's chemical recycling unit, where lime sludge (CaCO3) formed during the chemical recycling cycle is recalcined into calcium oxide (CaO). This calcium oxide is then used to causticize green liquor to produce white liquor, thereby separating the lignocellulose from the lignin.

[0003] In a lime kiln, recalcination occurs within the interior space of the rotary kiln shell, where lime slurry is supplied to the upper part of the shell. During this process, the lime slurry flows slowly downwards through the drying zone, heating zone, and reaction zone. Traditional lime kilns aim to enhance heat transfer from the flue gas and lime slurry in the drying zone by incorporating chains and / or elevators that rotate with the kiln and come into contact with the lime slurry during this rotation. For optimal results, the heat treatment of the lime slurry is typically carried out slowly within the kiln, meaning the kiln must be relatively long and thus require significant floor space or land area.

[0004] One existing technology that allows for the use of shorter kilns is a suspension drying system. In such systems, lime slurry is introduced into a vertical duct through which flue gas generated within the kiln moves upward at a relatively high rate. Most of the lime slurry is entrained in the upward-moving gas and dried through contact with it. The flue gas, now containing particles, is fed to a conventional separator device (such as a cyclone separator), from which it is discharged, while the now-dried lime slurry particles are fed into the inlet of the lime kiln.

[0005] For existing lime kilns equipped with suspension dryers, the temperature of the flue gas exiting the kiln sometimes exceeds a critical temperature of 700°C. When the gas temperature rises above 800°C, lime sludge begins to interfere with normal process operation because it begins to accumulate on the hot surfaces of the lime kiln at such temperatures. Lime sludge not entrained by the flue gas falls and requires additional equipment to transport it back into the rotary lime kiln. Therefore, in known lime kilns, when the flue gas outlet temperature exceeds the aforementioned maximum value, the accumulation and adhesion problems intensify, thus preventing the rotary lime kiln's capacity from further exceeding the maximum value determined by the aforementioned critical temperature.

[0006] A known solution to the above problem is disclosed in U.S. Patent Publication No. 8,7803,079 B2, which discloses a method for feeding lime slurry into a lime kiln having a rotary kiln shell with an internal space between its first and second end walls. The method involves feeding wet lime slurry into the interior of the rotary kiln shell or immediately adjacent to it. The flue gas temperature is above 700°C, within the typical temperature range for flue gas in a rotary lime kiln. When the lime slurry is fed into the flue gas stream, the flue gas temperature drops below 700°C, which is also within the typical temperature range. The purpose of this method is to prevent lime slurry from adhering to hot surfaces. Wet lime slurry is used to control the temperature of the flue gas to avoid the temperature range where adhesion occurs.

[0007] While it can be assumed that, based on the above description, such known solutions have been developed for regulating the temperature of lime sludge treatment using flue gas at the feed end of the lime kiln and in the flue gas outlet duct, it remains necessary to further explore a method to reduce lime sludge adhesion and clogging at temperatures above 700°C. This is because it is believed that operating the lime kiln at temperatures above its conventional maximum temperature (i.e., approximately 700°C) will allow for higher output than before without increasing the size of the lime kiln. Summary of the Invention

[0008] Therefore, the object of the present invention is to provide a method for processing lime sludge in a lime kiln, a lime kiln feed head, and a lime kiln, which enable an increase in the capacity of the lime kiln without increasing the size of the lime kiln.

[0009] The aforementioned objective is achieved by the method, the lime kiln feed head, and the lime kiln according to the present invention, because, according to the present invention, the lime kiln is kept clean by means of a cleaning device disposed at the feed head of the lime kiln, which is used to remove adhering and accumulated lime mud particles from the inner surface of the feed head, thereby significantly eliminating blockages in the first end wall region of the lime kiln and in the inlet region of the flue gas riser. More specifically, the method according to the present invention is characterized by what is described in independent claim 1, the lime kiln feed head is characterized by what is described in independent claim 12, and the lime kiln is characterized by what is described in independent claim 23. Dependent claims 2 to 11 describe advantageous embodiments of the method, and dependent claims 13 to 22 describe advantageous embodiments of the lime kiln feed head according to the present invention.

[0010] The advantages of this invention are that it reduces the adhesion and clogging of lime sludge at the feed head, thereby allowing the use of temperatures above 700°C. This allows for a smaller lime kiln size, or higher production capacity, compared to known methods and lime kilns using known equipment for feeding and processing lime sludge. This enables the recalcination of lime sludge in conjunction with green liquor causticization in sulfate pulp mills to be carried out more efficiently and in less space, which in particular reduces the manufacturing cost of plant equipment and thus allows pulp production to be arranged in a more profitable and cost-effective manner.

[0011] In this patent application, "feed head" refers to a portion of a lime kiln, which includes a first end wall (i.e., the end wall at the end of the lime kiln through which lime slurry is fed into the lime kiln) and all equipment within that area. Where applicable, the feed head also includes a portion of a rotary kiln shell located in the area of ​​the first end wall, on which components of a cleaning device are arranged, and thus this portion forms part of the solution of the present invention.

[0012] The lime kiln in this application is a so-called flash kiln (i.e., a lime kiln employing flash drying technology), not an LMD kiln (i.e., a lime kiln in which lime slurry is fed into the kiln primarily in the rising section of the feed channel). A flash kiln is a rotary lime kiln in which lime slurry is fed into the kiln primarily in the horizontal section of the feed channel. In a flash kiln, the flue gas flow velocity at the feed end is higher than in an LMD kiln. Furthermore, the temperature in a flash kiln is also higher, which has the disadvantage of causing clogging problems at the feed end. Such clogging problems of the same degree as those in flash kilns have not been observed in LMD kilns. Therefore, in an LMD kiln, clogging and blockage problems do not correspondingly limit production. Furthermore, in this patent application, the term "lime kiln" refers to a rotary lime kiln, i.e., a lime kiln in which the lime kiln rotates during its use.

[0013] In this patent application, "cleaning device" refers to a mechanical device used during normal operation of a lime kiln and / or when the lime kiln is suspended but not in a state where manual maintenance can be performed (i.e., during other periods of non-maintenance shutdown). The cleaning device may include scrapers, rods, blades, tips, hammers, or other mechanical tools capable of mechanically removing deposits from the inner surface of the lime kiln feed head. Operation of the cleaning device may be arranged by an external actuator, and / or it may utilize the rotation of the rotary kiln shell to achieve appropriate movement relative to the surface to be cleaned. Attached Figure Description

[0014] In the following, the invention will be described in more detail with reference to the accompanying drawings, wherein:

[0015] Figure 1 A main view of a lime kiln, seen from one side, is shown, the kiln having a feed head constructed according to the present invention;

[0016] Figure 2 A principal view, taken from one side, shows an embodiment of a lime kiln feed head according to the invention, which is adapted to... Figure 1 In the lime kiln;

[0017] Figure 3 A principal view, taken from one side, shows another embodiment of a lime kiln feed head according to the invention, which is adapted to... Figure 1 In the lime kiln. Detailed Implementation

[0018] Figure 1 This is an overall view showing a side view of a lime kiln 2 having a feed head 20 according to the invention. Figure 2 and Figure 3 The feed head 20 shown can be applied to this type of lime kiln 2. Therefore, Figures 1 to 3 The lime kiln 2 shown can be used for decarbonation treatment of lime sludge formed during the causticization process of green liquor in a pulp mill. They include a rotary kiln shell 10 having a first end wall 6 at its first end and a second end wall 11 at its second end, thereby forming an internal space 8 between the first end wall 6 and the second end wall 11.

[0019] Figure 1 The lime kiln 2 shown also has a feeding device 1, a blowing device 15, and a separation device 4.

[0020] The feeding device 1 is an apparatus arranged to feed lime slurry, to be fed into the lime kiln 2, through the first end wall 6 into the internal space 8 of the rotary kiln shell 10, or to feed lime slurry to a position adjacent to the rotary kiln shell 10 of the lime kiln 2. For example, the feeding device 1 is a screw conveyor arranged at the first end wall 6 such that the screw conveyor extends from the internal space 8 of the rotary kiln shell 10 through the first end wall 6 to the outside of the rotary kiln shell 10, as it does in… Figure 1 As shown in the diagram. Therefore, it is able to transport lime slurry from the lime slurry feed area outside the first end wall 6 into the rotary kiln shell 10.

[0021] In these types of lime kilns 2, the first end wall 6 includes a movable wall portion 6a and a stationary wall portion 6b. The movable wall portion 6a rotates together with the rotary kiln shell 10. The feed device 1 is arranged to pass through the first end wall 6 at the stationary wall portion 6a.

[0022] The blowing device 15 is an apparatus arranged to form a flue gas stream within the internal space 8 of the lime kiln shell 10. The flue gas flows out from the internal space 8 of the rotary kiln shell 10 via a flue gas riser duct 3. The formed flue gas stream carries lime sludge out of the internal space 8 of the rotary kiln shell 10 and through the flue gas riser duct 3. The flue gas riser duct 3 then guides the flue gas to a separation device 4 located at its end. Due to the heat provided by the hot flue gas, the lime sludge is pretreated within the rotary kiln shell 10 and the flue gas riser duct 3 while being carried by the flue gas stream. The blowing device 15 is preferably a fan, or other type of blowing device capable of forming a sufficiently strong flue gas stream within the rotary kiln shell 10, the flue gas riser duct 3, and the separation device 4. In various embodiments of the invention, the blowing device 15 is arranged to provide a flow velocity of at least 60 m / s, and preferably at least 70 m / s, as the flue gas stream passes through the first end wall 6 of the lime kiln 2. This high velocity is intended to prevent hot lime slurry particles from adhering to the first end wall 6, the rotary kiln shell 10, the inner surface of the riser duct 3, and the upper part of the separation device 4.

[0023] A separation device 4 located at the end of the riser duct 3 is arranged to separate the pretreated lime sludge from the flue gas stream. The separation device 4 is, for example, a cyclone separator, in which the gas stream rotates such that the heavier solid lime sludge particles, which are mixed with the gaseous portion of the incoming flue gas stream, are separated from the gaseous portion and fall into the return duct 5 through the lower cone of the separation device 4.

[0024] The return pipe 5 is a conduit connected to the feed device 1, thereby returning the separated lime mud particles to the feed device 1. The feed device 1 then transports these recycled lime mud particles to the internal space 8 of the rotary kiln shell 10, where the lime mud is calcined.

[0025] Furthermore, according to embodiments of the lime kiln 2 of the present invention (for example, such as...) Figure 2 and Figure 3 The diagram shows a cleaning device 13 arranged to detach at least a portion of the attached lime particles from the stationary wall portion 6b of the first end wall 6. Therefore, the cleaning device 13 is preferably located in a region close to the first end wall 6. This is because it has been observed that most of the attached lime particles tend to accumulate on the inner surface of the area surrounding the inlet opening of the riser pipe 3 located at the stationary portion 6a of the first end wall 6. Therefore, in Figure 2 and Figure 3In the illustrated embodiment, the cleaning device 13 is preferably configured to clean the stationary portion 6a of the first end wall 6. However, in some embodiments, it may also be arranged to clean other portions of the first end wall 6 in that area.

[0026] exist Figure 2 In the illustrated embodiment, the cleaning device 13 includes at least one scraper 13a having its own axis of rotation 17a and rotating about it. To rotate the scraper, it has a separate actuator 16 connected to a shaft 17 that passes through a stationary wall portion 6b of the first end wall 6 and coincides with the axis of rotation 17a. In such embodiments, two or more scrapers 13a may be provided, each having its own axis of rotation. Alternatively, in such embodiments, only one actuator may be provided, connected to each axis of rotation via a suitable drive system. Alternatively, each scraper may have its own actuator. The actuator may, for example, be an electric motor. The shaft and / or drive system may be arranged such that the device can be placed in a suitable location sufficiently away from the hot surface of the first end wall of the lime kiln.

[0027] In addition, Figure 3 In the illustrated embodiment, the cleaning device 13 includes a scraper 13b fixed inside the rotary kiln shell 10 such that it abuts against the stationary portion 6a of the first end wall. The cleaning device 13 includes a rod, beam, or tube, preferably fixed between the inner surfaces of the rotary kiln shell 10, such as... Figure 3 As shown in the diagram. Therefore, when the scraper 13b rotates relative to the stationary part along with the rotary kiln shell, the scraper 13b scrapes the inner surface of the stationary part 6b of the first end wall.

[0028] Furthermore, the scraper of the cleaning device 13 may include at least one tool 14. Figure 2 and Figure 3 The illustrated embodiment includes a device 14, which is arranged at the portion of the scraper that abuts against the inner surface of the stationary portion of the first end wall 6. Therefore, in Figure 2 and Figure 3 In one embodiment, the device 14 detaches the attached lime sludge from the stationary portion 6b of the first end wall. Such a device 14 can be detachably attached to the edge of the scraper and abut against the stationary portion 6b of the first end wall.

[0029] Because in Figure 2In the illustrated embodiment, the operation of the cleaning device is independent of the operation of the lime kiln; therefore, the scraper 13a can be arranged to perform cleaning operations continuously or intermittently during suitable cleaning periods. Thus, in such embodiments, the operation can also be arranged to occur during periods of inactivity in the rotary kiln shell 10. Furthermore, in such embodiments, to control cleaning, the control system of the lime kiln 2 can be arranged to detect the temperature of the first end wall, the pressure inside the rotary kiln shell, and / or, for example, the flow velocity of flue gas in the riser duct 3, using suitable sensor technology. Therefore, with such a control device, the operation of the cleaning device 13 can be controlled based on the formation of deposits in the area surrounding the inlet opening of the riser duct 3, thereby avoiding blockages and other harmful effects caused by lime sludge adhesion.

[0030] exist Figure 3 In the illustrated embodiment, the cleaning device 13 operates continuously during the process of introducing lime sludge into the rotary kiln shell 10. In this type of embodiment, the operation of the cleaning device occurs during the rotation of the rotary kiln shell 10. This is because, as described above, the scraper 13b is fixed to the rotary kiln shell 10. Furthermore, in this type of scraper 13b, the scraper 13b includes a rod, beam, or tube passing through the rotary kiln shell 10, and when the scraper 13b is hollow (i.e., as a tube), the scraper 13b can have internal or external cooling. Internal cooling can be achieved by providing a fan or a corresponding gaseous cooling medium blowing device in the rotary kiln shell to circulate cooling air or other gaseous cooling medium in the hollow portion of the scraper. External cooling can be achieved by installing a fan or a corresponding gaseous cooling medium blowing device outside the rotary kiln shell. When the nozzle of the fan or the corresponding gaseous cooling medium blowing device is aligned with an opening on the scraper extending from the outside of the rotary kiln shell 10 into the hollow portion of the scraper, it blows cooling air or other gaseous cooling medium into the hollow portion of the scraper. The purpose of cooling is to lower the temperature of the scraper so that it does not overheat and cause the scraper material to lose its strength.

[0031] Figures 1 to 3 The lime kiln 2 shown may also include a flue gas flow acceleration element 12, which increases the flue gas flow velocity, at least in the region between the feed device 1 and the flue gas riser duct 3. As previously mentioned, a higher flue gas velocity reduces the adhesion of hot lime sludge (especially lime sludge with a temperature exceeding 700°C). The flue gas acceleration element 12 may include, for example, a narrowing section 12a. This narrowing section 12a is preferably located at or near the inlet opening of the riser duct 3, where the flue gas exits the internal space 8 of the rotary kiln shell 10.

[0032] When using Figure 1 and Figure 2When the lime kiln shown in the invention processes lime sludge according to the method of the present invention, the method includes at least the following steps:

[0033] - The lime mud is fed into the internal space 8 of the rotary kiln shell 10 or a position adjacent to the rotary kiln shell 10 by the feeding device 1 located at the first end wall 6.

[0034] - The lime mud is entrained in the flue gas flowing out from the internal space 8 of the rotary kiln shell 10, and is carried to the separation device 4 via the flue gas rising pipe 3;

[0035] - While the lime sludge is being carried by the flue gas stream, the lime sludge is pretreated.

[0036] - The pre-treated lime mud is separated from the flue gas stream by the separation device 4;

[0037] - The separated lime mud is returned to the feeding device 1 and the lime kiln 2;

[0038] - The separated lime sludge is calcined in the lime kiln 2; and

[0039] - At least a portion of the lime mud particles adhering to the inner surface of the stationary wall portion 6b of the first end wall 6 are removed by the cleaning device 13.

[0040] The method may further include a stage in which the flue gas flow velocity is locally increased in the region between the feed device 1 and the flue gas riser duct 3 by means of at least one flue gas flow acceleration element 12. The flue gas acceleration element may be, for example, a narrowing portion 12a disposed inside the riser duct 3 (preferably at or near the inlet opening of the riser duct 3). This narrowing portion 12a may be symmetrical or asymmetrical. For example, in Figure 2 and Figure 3 In the illustrated embodiment, the narrowing portion 12a is configured such that it further restricts the cross-sectional area of ​​the riser duct 3 from its lower portion. Furthermore, in some embodiments, the gas flow acceleration element may also be formed, for example, by a sill formed at the inlet opening of the riser duct. In such cases, the narrowing portion restricts the cross-sectional area only from the lower portion of the riser duct.

[0041] When the lime kiln 2 is used according to the method of the present invention, the flue gas velocity is preferably at least 60 m / s, and more preferably at least 70 m / s, inside the rotary kiln unit 10 and / or at least in one location in the riser duct 3. However, in some embodiments of the method, the flue gas velocity does not exceed 60 m / s throughout the entire processing period or some sub-periods.

[0042] exist Figure 2and Figure 3 In the illustrated embodiment, the stationary portion 6b of the first end wall may additionally be provided with a gas flow regulating element 15. The purpose of the gas flow regulating element 15 is to control the leakage of air into the internal space 8 of the lime kiln 2 through the seal 7 between the moving wall portion 6a and the stationary wall portion 6b of the first end wall 6. During operation, a pressure difference always exists between the inside and outside of the lime kiln 2, and because the seal 7 between the moving wall portion 6a and the stationary wall portion 6b cannot be absolutely airtight, some leakage of air and / or flue gas occurs through the seal 7. The effect of the gas flow regulating element 15 is that it improves the mixing of leaked air with hot flue gas. Therefore, it reduces the localized cooling of the inner surfaces of the first end wall 6 and the rotary kiln shell 10 caused by the leakage. This is because the gas flow regulating element 15 helps to raise the temperature of the leaked air before it reaches the inner surfaces of the first end wall 6 and the rotary kiln shell 10 at the feed head 20. In addition, it helps to maintain a constant temperature of lime slurry in these areas of the lime kiln 2. Both of these effects of the gas flow regulating element 15 reduce the formation of deposits on the inner surfaces in the area. This formation occurs because hot lime particles tend to adhere to the inner surfaces of the feed head and the rotary kiln shell, which are slightly cooled by air leaking through the seal 7 between the rotating wall portion 6a and the stationary wall portion 6b at the first end wall 6. This phenomenon has been found to occur most frequently at the stationary surfaces, i.e., on the surface of the stationary wall portion 6b at the first end wall 6 in the case of lime kiln 2. Furthermore, it has been observed that persistent temperature fluctuations on these surfaces tend to further exacerbate this phenomenon. The gas flow regulating element 15 reduces such temperature fluctuations and thus also prevents this phenomenon from occurring due to these fluctuations.

[0043] As from Figure 2 and Figure 3 As can be seen in these embodiments, the gas flow regulating element 15 includes a baffle 15a. This baffle 15a is preferably positioned above the feed device 1. This is because, due to the weight of the rotary kiln shell 10, most of the leaked air that passes through the seal 7 between the stationary wall portion 6b and the moving wall portion 6a leaks from the upper part of the seal 7.

[0044] Alternatively, or otherwise, the gas flow regulating element may include other types of elements besides baffles. In some other embodiments, it may include a protrusion, shoulder, or ridge, or other types of portions disposed on the first end wall that extend suitably toward the second end wall of the lime kiln, such that leaked air from the seal between the stationary and moving wall portions mixes and is evenly distributed in the interior space near the first end wall before flowing into the inlet opening of the riser pipe.

[0045] Furthermore, the feed head 20 of the lime kiln 2 may include a particle control element 18, configured to prevent lime slurry particles from migrating from the kiln bed toward the feed end and the feed device 1. The particle control element may include an additional sill 18 at the inlet of the riser pipe 3 (e.g., Figure 2 (as shown), and / or it can be formed by a portion of the flue gas accelerating element 12, such as a narrowing portion 12a, i.e., by forming the narrowing portion 12a asymmetrically, making it narrower more from the lower side of the riser pipe, and forming a suitable protrusion 18 (as shown). Figure 3 (as shown in the diagram) to prevent lime mud particles from migrating from the kiln bed to the first end wall region and from there to the feed device 1.

[0046] It should also be noted that the present invention is not limited to the embodiments described above. As previously mentioned, the cleaning device can be other types of mechanical scraping arrangements, i.e., it can be formed by rods, blades, tips, hammers, or other mechanical tools capable of mechanically detaching deposits from the inner surface of the lime kiln feed head. Furthermore, the flue gas accelerating element can also be arranged differently from the example embodiments described above. For example, it can be formed by an "adjustable narrowing section," i.e., an adjustable baffle arranged in the riser pipe at the area of ​​the inlet opening of the riser pipe. Therefore, in such embodiments, the flue gas velocity can be adjusted to achieve an optimal flue gas velocity in the surface area critical to deposit formation.

[0047] Therefore, the method described, the lime kiln feed head, and the lime kiln are not limited to the embodiments described above, but can be modified in various ways within the scope of the appended claims.

Claims

1. A method for processing lime sludge in a lime kiln (2), wherein the lime kiln (2) comprises a rotary kiln shell (10) having an internal space (8) between a first end wall (6) and a second end wall (11), and wherein the first end wall (6) of the lime kiln (2) comprises a movable wall portion (6a) and a stationary wall portion (6b), the movable wall portion (6a) rotating together with the rotary kiln unit (10), and wherein the method comprises the following method steps: - The lime mud is fed into the internal space (8) of the rotary kiln shell (10) or a position adjacent to the rotary kiln shell (10) by the feeding device (1) at the first end wall (6); - The lime mud is entrained in the flue gas flowing out from the internal space (8) of the rotary kiln shell (10) and carried to the separation device (4) via the flue gas rising pipe (3); - While the lime sludge is being carried by the flue gas stream, the lime sludge is pretreated. - The pre-treated lime mud is separated from the flue gas stream by the separation device (4); - The separated lime sludge is returned to the feeding device (1) and the lime kiln (2); and - The separated lime mud is calcined in the lime kiln (2). Its features are, Lime mud particles adhering to and / or deposited on the inner surface of the lime kiln (2) are at least partially detached from the inner surface of at least the stationary wall portion (6b) of the first end wall (6) by a cleaning device (13) provided at the lime kiln feed head (20).

2. The method according to claim 1, wherein the flue gas flow velocity is locally increased in the region between the feeding device (1) and the flue gas riser (3) by means of at least one gas flow acceleration element (12).

3. The method according to claim 2, wherein the cross-sectional area of ​​the rising pipe (3) is reduced by the flow acceleration element (12), thereby increasing the flow velocity of the flue gas.

4. The method according to any one of claims 1 to 3, wherein the flue gas velocity at at least one location inside the rotary kiln unit (10) is at least 60 m / s, and preferably at least 70 m / s.

5. The method according to any one of the preceding claims, wherein leaked air flowing into the rotary kiln shell (10) via a seal (7) at the first end wall (6) of the lime kiln (2) is controlled by a flue gas regulating element (15) at the first end wall (6).

6. The method according to any one of the preceding claims, wherein the cleaning device (13) operates continuously.

7. The method according to any one of the preceding claims, wherein the cleaning device (13) operates periodically.

8. The method according to claim 7, wherein the operation of the cleaning device (13) is controlled based on temperature measurements of the first end wall (6), pressure measurements of the internal space of the lime kiln (2) and / or the riser pipe (3) and / or the flow velocity measurements of the flue gas flowing in the riser pipe (3).

9. The method according to any one of the preceding claims, wherein at least a portion of the stationary wall portion (6b) is scraped by a cleaning device (13) including scrapers (13a, 13b).

10. The method according to claim 9, wherein the stationary wall portion (6b) is scraped by a scraper (13a) that abuts against the stationary wall portion (6b) and rotates about its own axis of rotation (17a) independently of the rotation of the rotary kiln shell (10).

11. The method according to claim 9 or 10, wherein at least a portion of the stationary wall (6b) is scraped by a scraper (13b) that is fixedly attached to the rotary kiln shell (10) and rotates together with the rotary kiln shell (10).

12. A feed head (20) for a lime kiln (2), wherein the lime kiln (2) includes a rotary kiln shell (10) having an internal space (8) between a first end wall (6) and a second end wall (11), and wherein the first end wall (6) at the feed head (20) has a stationary wall portion (6b) and a rotating wall portion (6a), the rotating wall portion (6a) rotating together with the rotary kiln unit (10), and wherein the feed head (20) comprises: - Feeding device (1), which is arranged to feed lime mud into the internal space (8) of the rotary kiln shell (10) or adjacent to the rotary kiln shell (10) through the first end wall (7); - Flue gas riser (3), which extends from the interior space (8) of the rotary kiln shell (10) through the stationary wall portion (6b) of the first end wall (6); - A blowing device (15) is arranged to form a flue gas stream from the internal space (8) of the rotary kiln shell (10) to the flue gas riser pipe (3) to entrain lime sludge in the flue gas flowing out from the internal space (8) of the rotary kiln shell (10), thereby pretreating the lime sludge while it is being entrained by the flue gas stream. - Separation device (4), located at the end of the flue gas riser pipe (3), for separating the pretreated lime mud from the flue gas stream; - Return pipe (5), the return pipe (5) is used to return the separated lime mud to the feed device (1) and then to the lime kiln (2) through the feed device (1) for calcination of the lime mud in the lime kiln (2). The feed head (20) is characterized in that it further includes a cleaning device (13) disposed at the lime kiln (2) and the cleaning device (13) is arranged to detach at least a portion of the attached and / or deposited lime particles from at least a portion of the inner surface of the stationary wall portion (6b) of the first end wall (6).

13. The feed head (20) according to claim 12, wherein the feed head (20) includes a flue gas flow acceleration element (12) for increasing the flue gas velocity at least in the region between the feed device (1) and the flue gas riser (3).

14. The feed head (20) according to claim 13, wherein the flue gas accelerating element (12) is a narrowing section (12a) that reduces the area of ​​the cross section in the rising pipe (3).

15. The feed head (20) according to any one of claims 12 to 14, wherein the lime kiln (2) includes a flue gas regulating element (15) at the first end wall (6) of the lime kiln (2), the flue gas regulating element (15) being used to control leaked air flowing into the rotary kiln shell (10) via the seal (7) at the first end wall (6).

16. The feed head (20) according to claim 15, wherein the flue gas regulating element (15) includes a baffle (15a).

17. The feed head (20) according to any one of claims 12 to 16, wherein the cleaning device (13) is configured to clean at least a portion of the stationary wall portion (6b) of the first end wall (6).

18. The feed head (20) according to claim 17, wherein the cleaning device (13) is a scraper (13a, 13b) arranged inside the rotary kiln shell (10), and the scraper (13a, 13b) is configured to scrape at least a portion of the stationary wall portion (6b) of the first end wall (6).

19. The feed head (20) according to claim 18, wherein the scraper (13b) is fixed to a portion of the rotary kiln shell (10) at the feed head (20) and / or the rotating wall (6a), and is arranged to scrape at least a portion of the stationary wall (6b) when the rotary kiln unit (10) is rotated.

20. The feed head (20) according to claim 18, wherein the scraper (13a) has its own axis of rotation, and wherein the scraper (13a) includes an actuator (16) for rotating the scraper (13a) relative to its own axis of rotation (17a) independently of the rotation of the rotary kiln shell (10).

21. The feed head (20) according to any one of claims 17 to 19, wherein the scraper (13) comprises at least one device (14) abutting against the inner surface of the stationary wall portion (6b) of the first end wall (6).

22. The feed head (20) according to claim 21, wherein the at least one device (14) is a rod, fork, rake, knife or scraper that is detachably attached to the scraper (13a, 13b) and extends close to the inner surface of the stationary wall portion (6b) of the first end wall (6).

23. A lime kiln, comprising a lime kiln feed head according to any one of claims 12 to 22.

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