A method for treating lime mud in a lime kiln, a lime kiln feed head and a lime kiln
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- ANDRITZ OY
- Filing Date
- 2024-07-11
- Publication Date
- 2026-06-10
AI Technical Summary
Existing lime kilns face challenges with lime mud sticking and plugging at temperatures higher than 700°C, which limits the capacity of the rotary lime kiln and requires additional equipment for transporting unentrained lime mud.
A method involving a cleaning device at the lime kiln feed head to detach stuck and build-up lime mud particles from the inner surfaces, combined with a flue gas flow accelerating element to reduce sticking and plugging, allowing higher temperature operation without increasing kiln size.
The method effectively reduces lime mud sticking and plugging, enabling operation at higher temperatures, which increases the lime kiln's capacity without enlarging it, thus improving efficiency and reducing costs.
Smart Images
Figure FI2024050390_06022025_PF_FP_ABST
Abstract
Description
[0001] A METHOD FOR TREATING LIME MUD IN A LIME KILN, A LIME KILN
[0002] FEED HEAD AND A LIME KILN
[0003] Field of the invention
[0004] The present invention relates to a method for treating lime mud in a lime kiln, a lime kiln feed head, and a lime kiln.
[0005] Background of the invention
[0006] A lime kiln is part of the pulp mill chemical recovery plant wherein lime mud (CaCOs) formed in chemical recovery cycle is reburned into calcium oxide (CaO). The calcium oxide is then used for causticizing green liquor to produce white liquor for separating wood fibers from lignin.
[0007] At the lime kiln the reburning takes place in the interior space of a rotary kiln shell, into the upper end of which the lime mud is supplied. During the process the lime mud flows slowly downwardly through drying, heating and reaction zones. Conventional lime kilns desirably enhance the heat transfer from the flue gases and the lime mud in the drying zone by providing chains and / or lifters, which rotate with the kiln and come into contact with the lime mud during that rotation. To obtain proper results, the heat treatment of lime mud typically takes place slowly in the kiln, meaning that the lime kiln must be long and therefore takes up significant floor space or land area.
[0008] One prior art method to allow a shorter kiln is a suspension-type drying system. In such systems lime mud is introduced into a vertical conduit through which the flue gas from the kiln moves upwardly at a relatively high rate of speed. Most of the lime mud is entrained in the upwardly moving gas, and dried by contact therewith, and the flue gas with entrained particles is fed to a conventional separator device (such as a cyclonic separator), the flue gas being discharged and the lime mud particles which now have been dried being fed to the inlet of the lime kiln.
[0009] With existing lime kilns having a suspension dryer, the temperature of the flue gas leaving from the lime kiln may sometimes exceed the critical temperature of 700°C. In case when the gas temperature rises over 800° C the lime mud begins to disturb the normal operation of the process, since at such temperatures the lime mud starts to build up on the hot surfaces of the lime kiln. Lime mud which is not entrained in the flue gas falls and requires additional equipment to transport it into the rotary lime kiln. Therefore, in the known lime kilns build-up and stickiness problems increasing at the flue gas exit temperatures exceeding the above-described maximum prevents rising the capacity of the rotary lime kiln further from the maximum determined by the above-described critical temperature.
[0010] A known solution to tackle the above-mentioned problem is disclosed, in US patent publication No. 8,7803,079 B2 which discloses method of feeding lime mud into a lime kiln with a rotary kiln shell having an interior between its first end wall and second end wall. The method comprises feeding moist lime mud interior of the rotary kiln shell or in close proximity. Flue gas temperature is over 700°C which is within typical temperature range of the flue gas in a rotary lime kiln. The flue gas temperature falls under 700°C which is typical temperature range when lime mud is fed to flue gas flow. An aim of the method is to avoid sticking of the lime mud onto hot surface. Moist lime mud was used to control temperature of flue gases to avoid temperature area where stickiness appears.
[0011] Even though it may be considered that based on the above descriptions such known solutions have been already developed being suitable for regulating the temperature in lime mud treatment with flue gases at the feed end of the lime kiln and in the flue gas outlet duct, there is still a need to find method to reduce lime mud sticking and plugging at higher temperature than 700°C further. This is because it has been considered that operating a lime kiln at temperatures higher than their usual maximum temperatures (i.e., around 700°C) would allow higher production rates than before without increasing the size of the lime kiln. Summary of the invention
[0012] Thus, the aim of the present invention is to provide a method for treating lime mud in a lime kiln, lime kiln feed end and a lime kiln which enable to increase capacity of the lime kiln without enlarging the size of the lime kiln.
[0013] The above-described aim is reached by the present method, the lime kiln feed head and the lime kiln because according to the invention the lime kiln is kept clean by means of a cleaning device arranged at the feed head of the lime kiln to detach sticked and build-up lime mud particles from inner surfaces of the feed head which remarkably eliminates plugging at the first end wall region of the lime kiln and at the inlet area of the flue gas riser duct. More specifically the method according to the present invention is characterized by what have been described in the independent claim 1 , the lime kiln feed head what have been described in the independent claim 12 and the lime kiln what have been described in the independent claim 23. Dependent claims 2-11 describe advantageous embodiments of the method, and dependent claims 13-22 advantageous embodiments of the lime kiln feed head according to the invention.
[0014] An advantage of the present invention is that it reduces sticking and plugging of the lime mud at the feed head and thereby allows using higher temperatures than 700°C thus reducing the lime kiln size or to get more capacity without increasing the size of the lime kiln compared to the known methods and the lime kilns applying known equipment for feeding and treating lime mud in the lime kiln. This causes that lime mud reburning in connection with the green liquor causticizing in sulphate pulp mills can be made more effectively and less space requiring manner which, among others, reduces the manufacturing costs of the plant equipment and hence enables to arrange pulp production more profitably and cost-effectively.
[0015] In this patent application by the “feed head” is meant a part of the lime kiln including the first end wall (i.e., the end wall at the end of the lime kiln through which the lime mud is fed into the lime kiln) and all the equipment in that region. When applicable the feed head also includes a portion of the rotary kiln shell at the region of the first end wall to which the parts of the cleaning device have been arranged and which thereby forms a part of the inventive solution.
[0016] The Lime kiln in this application is, so called flash kiln (i.e., lime kiln applying flash drying technology) and not LMD kiln (i.e., a lime kiln wherein lime mud is fed into lime kiln essentially in a rising portion of the feeding channel). Flash kiln is a rotary lime kiln wherein lime mud is fed into the lime kiln essentially in a horizontal portion of the feeding channel. In flash kiln the flue gas flow velocity is higher at the feed end than in LMD kiln. Also, the temperature is higher in flash kiln is higher which drawback is that it causes plugging problems at the feed end. In LMD kilns such plugging problems have not been observed in respective extent as in flash kilns. Thus, plugging and blocking problems do not limit the production corresponding manner in LMD kilns. Furthermore, in this patent application it is meant with the term “lime kiln” a rotary lime kiln i.e., a lime kiln wherein the lime kiln rotates during the use of the lime kiln.
[0017] Cleaning device is meant in this patent application a mechanical device which is used during the normal operation of the lime kiln and / or during the periods when the lime kiln is suspended but not in a state where manual maintenance operations can be carried out (i.e. during the other periods than maintenance shutdowns). The cleaning device may comprise a scraper, bar, blade, peak, hammer, or other mechanical tool that is able to mechanically disengage the said depositions from the internal surfaces of the lime kiln feed head. Operation of the cleaning device may be arranged by means of an external actuator and / or it may utilize rotation of the rotary kiln shell to accomplish appropriate movement in respect of the surfaces to be cleaned.
[0018] Brief description of the drawings
[0019] In the following, the invention will be described in more detail with reference to the appended drawings in which
[0020] Figure 1 shows a principal drawing of a lime kiln having a feed head configured according to the invention seen from a side, Figure 2 shows a principal drawing of an embodiment of the lime kiln feed head according to the invention being applicable in the lime kiln of figure 1 seen from a side, and
[0021] Figure 3 shows a principal drawing of another embodiment of the lime kiln feed head according to the invention being applicable in the lime kiln of figure
[0022] 1 seen from a side.
[0023] Detailed description of some advantageous embodiments
[0024] Figure 1 is a general drawing showing a side view of a lime kiln 2 having a feed head 20 according to the invention being the kind of which the feed heads 20 shown in the figures 2 and 3 can be applied. Thus, the lime kilns
[0025] 2 depicted in the figures 1 to 3 are applicable for decarbonizing lime mud formed in a green liquor causticizing process at a pulp mill. They comprise a rotary kiln shell 10 with a first end wall 6 at its first end and a second end wall 11 at its second end thereby forming interior space 8 in between the first end wall 6 and the second end wall 11 .
[0026] The lime kiln 2 depicted in the figure 1 has also a feeding device 1 , a blowing device 15, and a separating device 4.
[0027] The feeding device 1 is a device that is arranged to feed the lime mud to be fed in the lime kiln 2 into the interior space 8 of the rotary kiln shell 10 or in close proximity to the rotary kiln shell 10 of a lime kiln 2 through the first end wall 6. The feeding device 1 is, for instance, a screw conveyor that is arranged at the first end wall 6 such that it extends from interior space 8 of the rotary kiln shell 10 through the first end wall 6 to outside the rotary kiln shell 10 as it is shown in the figure 1 . Thus, it can convey lime mud from lime mud feeding area, outside the first end wall 6 into the rotary kiln shell 10.
[0028] In these types of lime kilns 2 the first end wall 6 includes a moving wall-part 6a and a stationary wall-part 6b. Moving wall-part 6a rotates with the rotary kiln shell 10. The feeding device 1 is arranged to go through the first end wall 6 at the stationary wall-part 6a. The blowing device 15 is a device being arranged to form flue gas stream into the interior space 8 of the lime kiln shell 10. The flue gas flows out from the interior space 8 of the rotary kiln shell 10 through a flue gas riser duct 3. The flue gas stream formed entrains the lime mud out from the interior space 8 of the rotary kiln shell 10 through the flue gas riser duct 3. The flue gas riser duct 3 then conducts the flue gas to a separator device 15 located at the end of the flue gas riser duct 3. Because of the heat provided by the hot flue gas, the lime mud is pretreated while it is entrained with the flue gas stream inside the rotary kiln shell 10 and the flue gas riser duct 3. The blowing device 15 is preferably a fan or other kind of blowing device which is able form strong enough flue gas stream inside the rotary kiln shell 10, the flue gas riser duct 3 and the separator device 4. In different embodiments of the invention the blowing device 15 is arranged to provide flow velocity of the flue gas stream through first end wall 6 of the lime kiln 2 that is at least 60 m / s, and preferably at least 70 m / s. The purpose of such high velocity is that hot lime mud particles would not stick to the interior surfaces of the first end wall 6, rotary kiln shell 10, riser duct 3 and to the upper portions of the separating device 4.
[0029] The separating device 4 at end of the riser duct 3 is arranged to separate the pretreated lime mud from the flue gas flow. The separating device 4 is e.g., a cyclone wherein the gas stream revolves so that heavier solid lime mud particles mixed with the gaseous portion of the incoming flue gas stream are separated from gaseous portion and dropped through the lower cone part of the separating device 4 to a returning duct 5.
[0030] The returning duct 5 is a duct which is connected to the feeding device 1 and thereby returns the separated lime mud particles back to the feeding device 1 . The feeding device 1 then conveys these recycled lime mud particles into the interior space 8 of the rotary kiln shell 10 wherein the lime mud is calcinated.
[0031] Furthermore, the embodiments of the lime kiln 2 according to the invention, e.g., such as shown in the figures 2 and 3 comprise a cleaning device 13 arranged to detach at least part of the sticked lime particles at least from the stationary wall-part (6b) of the first end wall (6). Thus, the cleaning device 13 is preferably provided in the region close to the first end wall 6. This is because it has been observed that most of the sticked lime mud particles tend to accumulate inner surfaces of the area surrounding the inlet opening of the riser duct 3 that locates at the stationary portion 6a of the first end wall 6. For this reason, in the embodiments shown in the figures 2 and 3 the cleaning device 13 is preferably configured to clean the stationary part 6a of the first end wall 6. However, in some embodiments it may be arranged to clean also other portions of the first end wall 6 in that region.
[0032] In an embodiment shown in the figure 2 the cleaning device 13 comprises at least one scraper 13a which has its own rotation axis 17a in respect of which it is rotated. For rotating the scarper it has a separate actuator 16 which is connected to a shaft 17 which is fitted through the stationary wall-part 6b of the first end wall 6, and which coincides with the said rotation axis 17a. In this type of embodiments there may be two or more scrapers 13a which each have their own rotation axes. Furthermore, in such embodiments there may be only one actuator that is connected to each rotation axes by a suitable transmission system. Alternatively, each scraper may have their own actuator. Actuators may be, for instance, electric motors. Shafts and / or transmission systems may be arranged such that this equipment can be placed in suitable position far away enough from the hot surfaces of the first end wall of the lime kiln.
[0033] Also, in an embodiment shown in the figure 3 the cleaning device 13 comprises, a scraper 13b that is fixed inside the rotary kiln shell 10 such that it abuts against the stationary portion of the first end wall 6a. The cleaning device 13 comprises a bar, beam or tube that is preferably fixed between inner surfaces of the rotary kiln shell 10 as it is shown in the figure 3. Thus, when the scraper 13b rotates with the rotary kiln shell in respect of the stationary portion it scrapes the inner surface of the stationary portion of the first end wall 6b.
[0034] Furthermore, the scraper of the cleaning device 13 may include at least one implement 14. In the embodiment shown in the figures 2 and 3 there is an implement 14 arranged at a portion of the scraper that abuts against interior surface of the stationary portion of the first end wall 6. Thus, the implement 14 in the embodiments of figures 2 and 3 detaches the sticked lime mud from the stationary portion of the first end wall 6b. Such implement 14 may be detachably attached to the edge of the scraper abutting against the stationary portion of the first end wall 6b.
[0035] Since in the embodiment shown in the figure 2 the cleaning device operates independently from the operation of the lime kiln scraper 13a may have been arranged to take place continuously or occasionally in suitable cleaning periods. Thus, in such embodiments the operation may be arranged to happen also during the stationary periods of the rotary kiln shell 10. Furthermore, in such embodiments, for controlling the cleaning the control system of the lime kiln 2 may have been arranged to detect temperature of the first end wall, pressure inside the rotary kiln shell and / or e.g., flow velocity of the flue gas in the riser duct 3 by applying suitable sensor technology. Thus, with such control arrangement it is possible to control the operation of the cleaning device 13 according to formation of the depositions at the region surrounding the inlet opening of the riser duct 3 such that blockages and other harmful effects caused by the sticking lime mud are avoided.
[0036] In the embodiment of figures 3 the cleaning device 13 operates continuously during introduction of the lime mud in the rotary kiln shell 10. In case of this embodiment, operation of the cleaning device takes place during the rotation of the rotary kiln shell 10. This is because the scraper 13b is fixed to the rotary kiln shell 10 as it is described above. Furthermore, in such scraper 13b wherein the scraper 13b comprises a bar, a beam or a tube extending through the rotary kiln shell 10 and when it is hollow (i.e., as a tube) it may have internal or external cooling. Internal cooling may be arranged by providing a fan or corresponding gaseous cooling medium blowing device in the rotary kiln shell that circulates cooling air or other gaseous cooling medium in the hollow parts of the scraper. External cooling may be arranged by providing a fan or corresponding gaseous cooling medium blowing device outside the rotary kiln shell such that it blows cooling air or other gaseous cooling medium into the hollow portions of the scraper in the moment when a nozzle of the fan or corresponding gaseous cooling medium blowing device is aligned with a hole of the scraper that extends from outside the rotary kiln shell 10 into the hollow portions of the scraper. The purpose of the cooling is to cool down the scraper such that it does not heat up too much so that its material would lose its strength.
[0037] The lime kiln 2 shown in the figures 1 to 3 may comprise also a flue gas flow accelerating element 12 to increase the flue gas flow velocity at least in the area between the feeding device 1 and the flue gas riser duct 3. As described-above the higher flue gas velocity reduces sticking of the hot lime mud (especially that of having temperature over 700°C). The flue gas accelerating element 12 may comprise e.g., a narrowing 12a. The narrowing 12a locates preferably at the inlet opening of the riser duct 3 or proximity of it where the flue gas exits from the interior space 8 of the rotary kiln shell 10.
[0038] When using the lime kiln shown in the figures 1 and 2 for treating the lime mud according to the method of the invention it comprises at least the following method steps:
[0039] -feeding lime mud into the interior space 8 of the rotary kiln shell 10 or in close proximity to the rotary kiln shell 10 by a feeding device 1 at the first end wall 7,
[0040] -entraining the lime mud in flue gas flowing out from the interior space 8 of the rotary kiln shell 10 to a separating device 4 through a flue gas riser duct 3,
[0041] -pretreating the lime mud while the lime mud is entrained with the flue gas stream,
[0042] -separating the pretreated lime mud from the flue gas stream by the separating device 4,
[0043] -returning the separated lime mud back to the feeding device 1 and to the lime kiln 2, -calcining the separated lime mud in the lime kiln 2, and
[0044] -detaching at least part of the lime mud particles being sticked at least on the inner surfaces of the stationary wall-part 6b of the first end wall 6 by the cleaning device 13.
[0045] The method may further comprise a phase wherein the flue gas flow velocity is increased locally at the area between the feeding device 1 and the flue gas riser duct 3 by means at least one flue gas flow accelerating element 12. Flue gas accelerating element may be e.g., a narrowing 12a provided inside the riser duct 3, preferably at the inlet opening of the riser duct 3 or its proximity. The narrowing 12a may be symmetric or asymmetric. For instance, in the embodiment shown in the figures 2 and 3 the narrowing 12a is configured such that it limits the area of the cross section of the riser duct more from lower portion of the riser duct 3. Furthermore, in some embodiment gas flow accelerating element may be formed e.g., by a sill that is formed at the inlet opening of the riser duct. In such case the narrowing limits the area of the cross section only from the lower part of the riser duct.
[0046] When using lime kiln 2 according to the method of the invention the flue gas velocity is at least in one position inside the rotary kiln cell 10 and / or in the riser duct 3 preferably at least 60 m / s and more preferably at least 70 m / s. However, in some embodiments of the method the velocity of the flue gas does not exceed 60 m / s during the whole or during some sub-periods of the treatment.
[0047] In the embodiments shown in the figures 2 and 3 the stationary portion of the first end wall 6a may has additionally a gas flow regulating element 15. The purpose of the gas flow regulating element 15 is to control the air that leaks into the interior space 8 of the lime kiln 2 through the seal 7 in between the moving wall-part 6a and the stationary wall-part 6b of the first end wall 6. During the operation there is always pressure difference between the interior and exterior of the lime kiln 2 and since the sealing 7 between moving wall-part 6a and the stationary wall-part 6b cannot be made absolutely gastight some leaking of air and / or flue gas occurs through the seal 7. The effect of the gas flow regulating element 15 is that it improves mixing of the leak air with the hot flue gas. Thus, it reduces local cooling of the interior surfaces at the first end wall 6 and the rotary kiln shell 10 caused by the said leaking. This is because, the gas flow regulating element 15 aids rising temperature of the leak air before it reaches the interior surfaces of the first end wall 6 and the rotary kiln shell 10 at the feed head 20. Furthermore, it promotes maintaining constant lime mud temperature at these areas in the lime kiln 2. These both, effects of the gas flow regulating element 15 reduce formation of depositions on the interior surfaces at the said areas. Such formation is caused because hot lime particulate tends to stick on the interior surfaces of the feed head and rotary kiln shell being slightly cooled by the leak air flowing through the sealing 7 between the rotary wall-part 6a and the rotary wall-part 6b at the first end wall 6. This phenomenon has been found to be most frequently present at the stationary surfaces i.e., in case of the lime kiln 2 on the surfaces at the stationary wall-part 6b of the first end wall 6. Furthermore, it has been observed that continuous temperature variation in these surfaces tends to further strengthen this phenomenon. The gas flow regulating element 15 reduces such temperature variation and thus prevents this phenomenon also because of that.
[0048] As can be seen from the figures 2 and 3 the gas flow regulating element 15 comprises in these embodiments a visor 15a. The visor 15a is preferably placed above the feeding device 1 . This is because due to the weight of the rotary kiln shell 10 most of the leaking air coming through the sealing 7 between the stationary wall-part 6a and the moving wall-part 6a comes through the upper portions of the seal 7.
[0049] Alternatively, or in addition, the gas flow regulating element may comprise other kind of element than a visor. In some other embodiments it may comprise a protuberance, shoulder or ridge, or other type of portion at the first end wall that protrudes to direction of the second end wall of the lime kiln in suitable manner such that the leaking air coming through the seal between the stationary wall-part and the moving wall-part and the flue gas is mixed and uniformly distributed within in the interior space at the proximity of the first end wall before it flows in to the inlet opening of the riser duct.
[0050] Furthermore, the feed head 20 of the lime kiln 2 may comprise a particle control element 18 configured to prevent migration of lime mud particles from kiln bed to the feed end and to the feeding device 1. The particle control element may comprise an additional a sill 18 at the inlet of the riser duct 3 (as it is shown in the figure 2) and / or it may be formed of part of the flue gas accelerating element 12, such as narrowing 12a by forming the narrowing asymmetric such that it narrows the riser duct more from its lower side and forms suitable protuberance 18 (as it is shown in the figure 3) to prevent migration of lime mud particles from kiln bed to the first end wall region and from there to the feeding device 1 .
[0051] It should be also notified that the invention is not limited to the embodiments described above. As described above, the cleaning device may be other kind of mechanical scraping arrangement, i.e., it may be formed of bar, blade, peak, hammer, or other mechanical tool that is able to mechanically disengage the said depositions from the internal surfaces of the lime kiln feed head. Also, the flue gas accelerating element may be arranged in other way than in the example embodiment described above. It may be formed e.g., of an “adjustable narrowing” i.e., an adjustable flap being arranged into the riser duct at the region of its inlet opening. Thus, in such embodiment flue gas velocity can be adjusted to reach optimal flue gas velocity to the surface areas that are critical in view of the formation of the depositions.
[0052] Thus, the method, lime kiln feed head and the lime kiln are not limited to the embodiments described above but can be varied within the scope of the appended claims.
Claims
Claims1. A method for treating lime mud in a lime kiln (2) wherein the lime kiln (2) comprises a rotary kiln shell (10) having an interior 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 moving wall-part (6a) that rotates with the rotary kiln cell (10) and a stationary wall-part (6b), and wherein the method comprises method steps of:-feeding lime mud into the interior space (8) of the rotary kiln shell (10) or in close proximity to the rotary kiln shell (10) by a feeding device (1 ) at the first end wall (6),-entraining the lime mud in flue gas flowing out from the interior space (8) of the rotary kiln shell (10) to a separating device (4) through a flue gas riser duct (3),-pretreating the lime mud while the lime mud is entrained with the flue gas stream,-separating the pretreated lime mud from the flue gas stream by the separating device (4),-returning the separated lime mud back to the feeding device (1 ) and to the lime kiln (2), and-calcining the separated lime mud in the lime kiln (2), characterized in that the lime mud particles being sticked and / or deposited on the inner surfaces of the lime kiln (2) are detached at least partly from inner surfaces of at least the stationary wall-part (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 increased locally at the area between the feeding device (1 ) and the flue gas riser duct (3) by means at least one gas flow accelerating element (12).
3. The method according to claim 2, wherein the flue gas flow velocity is increased by the flow accelerating element (12) reducing the area of the cross section at the riser duct (3).
4. The method according to any of claims 1 to 3, wherein the flue gas velocity at least in one position inside the rotary kiln cell (10) is at least 60 m / s, and preferably at least 70 m / s.
5. The method according to any of the preceding claims, wherein leaking air flowing into the rotary kiln shell (10) through the seal (7) at the first end wall (6) is controlled by a flue gas regulating element (15) at the first end wall (6) of the lime kiln (2).
6. The method according to any of the preceding claims, wherein the cleaning device (13) operates continuously.
7. The method according to any 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 on the basis of temperature measurement of the first end wall (6), pressure measurement of the interior space of the lime kiln (2) and / or riser duct (3) and / or flow velocity measurement of the flue gas flowing in the riser duct (3).
9. The method according to any of the preceding claims, wherein at least part of the stationary wall-part (6b) is scraped by a cleaning device (13) comprising a scraper (13a, 13b).
10. The method according to claim 9, wherein the stationary wall-part (6b) is scraped by a scraper (13a) abutting against the stationary wall-part (6b) and being rotated in respect of its own rotation axis (17a) independently from the rotation of the rotary kiln shell (10).
11. The method according to claim 9 or 10 wherein at least part of the stationary wall-part (6b) is scraped by scraper (13b) attached fixedly to the rotary kiln shell (10) and which rotates with the rotary kiln shell (10).
12. A lime kiln (2) feed head (20) wherein the lime kiln (2) comprises a rotary kiln shell (10) having an interior space (8) between a first end wall (6) and a second end wall (11 ), and wherein the first end wall (6) being at the feed head (20) has a stationary wall-part (6b) and a rotary wall-part (6a) rotating with the rotary kiln cell (10) and wherein the feed head (20) comprises:-a feeding device (1 ) arranged to feed the lime mud into the interior space (8) of the rotary kiln shell (10) or in close proximity to the rotary kiln shell (10) through the first end wall (7),-a flue gas riser duct (3) extending out from interior space (8) of the rotary kiln shell (10) through the stationary wall-part (6b) of the first end wall (6),-a blowing device (15) arranged to form flue gas stream interior space (8) of the rotary kiln shell (10) and into the flue gas riser duct (3) to entrain the lime mud in flue gas flowing out from the interior space (8) of the rotary kiln shell (10) to pretreat the lime mud while the lime mud is entrained with the flue gas stream,-a separating device (4) at the end of the flue gas riser duct (3) for separating the pretreated lime mud from the flue gas stream,-a returning duct (5) to return the separated lime mud to the feeding device (1 ) and therethrough to the lime kiln (2) to calcinate the lime mud in the lime kiln (2), characterized in that the feed head (20) further comprises a cleaning device (13) provided at the lime kiln (2) and arranged to detach at least part of the sticked and / or deposited lime particles from at least part of the interior surfaces of the stationary wall-part (6b) of the first end wall (6).
13. The feed head (20) according to claim 12, wherein the feed head (20) comprises a flue gas flow accelerating element (12) to increase the flue gas velocity at least in the area between the feeding device (1 ) and the flue gas riser duct (3).
14. The feed head (20) according to claim 13, wherein the flue gas acceleration element (12) is a narrowing (12a) reducing the area of the cross section in the riser duct (3).
15. The feed head (20) according to any of claims 12-14 wherein the lime kiln (2) comprises flue gas regulating element (15) at the first end wall (6) of the lime kiln (2) to control leaking air flowing into the rotary kiln shell (10) through 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) comprises a visor (15a).
17. The feed head (20) according to any of claims 12-16, wherein the cleaning device (13) is configured to clean at least part of the stationary wallpart (6b) of the first end wall (6)18. The feed head (20) according to claim 17, wherein the cleaning device (13) is scraper (13a, 13b) arranged inside the rotary kiln shell (10) and configured to scrape at least part of the stationary wall-part (6b) of the first end wall (6).
19. The feed head (20) according to claim 18, wherein the scraper (13b) is fixed to the portion of the rotary kiln shell (10) at the feed head (20) and / or rotary wall-part (6a) and arranged to scrape at least part of the stationary wall-part (6b) when the rotary kiln cell (10) is rotated.
20. The feed head (20) according to claim 18 wherein the scraper (13a) has its own rotation axis and wherein the scraper (13a) comprises an actuator (16) for rotating the scraper (13a) in respect of its own rotation axis (17a) independently from the rotation of the rotary kiln shell (10).
21. The feed head (20) according to any of claims 17 to 19, wherein the scraper (13) includes at least one implement (14) abutting against the interior surface of the stationary wall-part (6b) of the first end wall (6).
22. The feed head (20) according to claim 21 , wherein the at least one implement (14) is a stick, fork, rake, knife or scraper being detachably attached to the scraper (13a, 13b) and extending close to the internal surface of stationary wall-part (6b) of the first end wall (6).
23. A lime kiln, comprising lime kiln feed head according to any of claims 12-22.