Abstract:
The inventive multi-level furnace comprises a furnace wall delimiting a cylindrical space having a vertical axis, a plurality of beds defining the levels inside said cylindrical space and at least one scraping arm which is provided with a wall scraper and associated with the bed in such a way that it is rotatable about the vertical axis of the furnace. During scraping arm rotation, said wall scraper defines a scraped area on the internal surface of the furnace wall which comprises a plurality of wall cavities forming a row of access openings in the scraped area, thereby making it possible to avoid the formation of a hardened crust adhered to the internal surface of the furnace wall and to develop braking shocks in the scraping arm.

Description:
FIELD OF THE INVENTION 
   The present invention concerns a multiple-hearth furnace. 
   BACKGROUND OF THE INVENTION 
   A multiple-hearth furnace comprises a furnace wall delimiting a cylindrical space with a vertical axis. A plurality of soles positioned one above the other delimit the hearths of the furnace within this space. In each hearth, rabble arms rotated by means of a central shaft coaxial with the vertical axis of the furnace are provided. These rabble arms are equipped with sole scrapers which turn over the material under treatment on the sole and displace it on a first type of sole toward the periphery and on a second type of sole toward the center of the sole. The first type of sole is provided with peripheral drop holes through which the material under treatment falls onto a sole of the second type in the stage below. The second type of sole is provided with a central drop hole through which the material under treatment falls onto a sole of the first type in the stage below. 
   It is also a known practice to equip at least one rabble arm in each stage of the furnace with a wall scraper. The function of this wall scraper is to recover the material that accumulates in the immediate vicinity of the furnace wall so as to push it into the peripheral drop holes on the first type of sole and, on the second type of sole, to redirect it into the flow of material being displaced toward the center of the furnace. When the furnace starts, there is a radial clearance between the wall scraper and the inner surface of the furnace wall. However, as the furnace operates, this functional clearance is quickly clogged with material under treatment. A layer of material forms on the inner surface of the wall which the wall scraper progressively compacts by a “pasting” process, eventually forming a very hard crust that adheres to the inner surface of the wall. The wall scraper rubs against this peripheral crust, generating a by no means insignificant additional braking moment on the rabble arm. It should be noted that the situation is aggravated by the fact that hardness and resistance of the peripheral crust are not usually uniform. The modulus of the braking force exerted on the wall scraper thus varies irregularly, causing jerking of the rabble arm. This results in dynamic stresses which generate fatigue effects that are the source of numerous rabble arm fractures. 
   The object of the present invention is to propose a multiple-hearth furnace which reduces the abovementioned effects. According to the invention, this objective is achieved by a multiple-hearth furnace according to Claim  1 . 
   BRIEF SUMMARY OF THE INVENTION 
   A multiple-hearth furnace according to the present invention comprises, in a manner that is known per se, a furnace wall delimiting a cylindrical space with a vertical axis, a plurality of soles which delimit the hearths within this cylindrical space and at least one rabble arm with a wall scraper. This wall scraper is associated with one of the soles, where it is rotated about the vertical axis of the furnace. During the rotation of this rabble arm about its vertical axis, its wall scraper defines a scraped zone on the inner surface of the furnace wall. According to the present invention, the furnace wall comprises a plurality of wall cavities which form a succession of access openings into the zone scraped by the wall scraper. It will be appreciated that these wall cavities greatly reduce the risk of formation of a crust of hardened material adhering to the inner surface of the furnace wall. Through these access openings in the scraped zone, the wall cavities become filled with material, but a “pasting” compaction effect, which is the origin of the formation of a hardened crust adhering to the inner surface of the furnace wall, scarcely occurs. The material that accumulates in the wall cavities remains relatively soft and results in substantially jerk-free braking. 
   The furnace wall generally comprises an external shell and a refractory inner liner. The wall cavities mentioned above are made in the refractory liner, and in a preferred embodiment, the shell is equipped with cleaning openings through which the wall cavities are accessible. It is thus easy to obtain access to the wall cavities in order to push back the material that has accumulated in the wall cavities onto the sole. It is even possible to clean the sole through these cleaning openings over a certain radial depth which depends on the tools employed. With tools having their ends bent back by a certain angle, it is also possible to clean the inner surface of the refractory liner through the cleaning openings. 
   For reasons of stability, leak-tightness and thermal insulation of the furnace wall, the cleaning opening associated with a wall cavity will be substantially smaller in cross section than the access opening formed by the wall cavity in the scraped zone. For the same reasons, the cross section of the wall cavity preferably diminishes progressively in the direction of the cleaning opening. 
   Preferably, the circumferential extent of the residual surface between two successive access openings is smaller than the circumferential extent of such an access opening. Ideally, two successive access openings would be separated by a sharp edge, but for reasons of wear and stability, a residual surface will generally be provided between two access openings. The circumferential extent of this residual surface is preferably smaller than 50% of the circumferential extent of one of the access openings that it separates. In the vertical direction, the access openings extend slightly beyond the upper limit of the scraped zone. 
   The wall cavities can easily be cleaned through the cleaning openings in the external shell by workers equipped with special tools. However, it is also possible to envisage equipping one or more or even all the wall cavities with a fluid injection device so as to be able to eject the material accumulated in the wall cavity onto the sole by means of the liquid injected. Alternatively, one or more or even all of the wall cavities can be equipped with a mechanical pusher, so as to be able to push the material accumulated in a wall cavity onto the sole. 
   Each of the cleaning openings can also advantageously have associated with it a plugging device comprising a steel blind flange fixed to a companion flange of the external shell mentioned above and a central core made of refractory material that penetrates into the cleaning opening. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Further specific features and features of the invention will become apparent from the detailed description of some advantageous embodiments which are described below, by way of illustration, with reference to the attached drawings. These show the following: 
       FIG. 1 : A cross section through a multiple-hearth furnace at the level of a first type of sole; 
       FIG. 2 : A cross section through a multiple-hearth furnace at the level of a second type of sole; 
       FIG. 3 : A vertical cross section along the line  3 - 3 ′ shown in  FIG. 2 ; 
       FIG. 4 : A vertical cross section along the line  4 - 4 ″ shown in  FIG. 1 ; 
       FIG. 5 : A three-dimensional view of an annular element of a furnace wall of a multiple-hearth furnace according to the invention; and 
       FIG. 6 : A vertical cross section through the furnace wall at the level of a wall cavity with a cleaning opening equipped with a plugging device. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a first cross section through a multiple-hearth furnace according to the invention. A furnace wall  10  radially delimits a cylindrical space with a vertical axis  11  (perpendicular to the plane of the drawing). Inside this space, a plurality of soles positioned one above the other delimit the stages of the furnace in the vertical direction.  FIG. 1  shows a first type of sole  12 . This is a sole  12  with peripheral drop holes  14 . Associated with this sole  12  are two rabble arms  16 ,  16 ′ which are driven in rotation about the vertical axis  11  by a drive shaft  17 . Each of the rabble arms  16 ,  16 ′ carries a series of sole scrapers  18 ,  18 ′ oriented so that they turn over the material under treatment on the sole  12  and displace it toward the periphery of the sole  12 , where it falls through the peripheral drop holes  14  onto a peripheral surface of a lower sole. The references  20 ,  20 ′ denote wall scrapers, whose function is to recover the material accumulating in the immediate proximity of the furnace wall  10  and push it into the peripheral drop holes  14 . 
     FIG. 2  shows a second type of sole  22 . This is a sole  22  with a central drop hole  24  surrounding the drive shaft  17 . Associated with this sole  22  are two rabble arms  26 ,  26 ′ which are similarly rotated by the drive shaft  17 . Each of the rabble arms  26 ,  26 ′ carries a series of sole scrapers  30 ,  30 ′, this time oriented so that they turn over the material under treatment on the sole  22  and displace it toward the central region of the sole  22 , where it falls through the central drop hole  24  into the central region of a lower sole. The reference  32  denotes a wall scraper  26  whose purpose is to recover the material accumulating in the immediate proximity of the furnace wall  10  and push it into the flow of material being displaced toward the center of the sole  22 . 
   The soles of the multiple-hearth furnace are alternately of the first type shown in  FIG. 1  and of the second type shown in  FIG. 2 . The material under treatment that falls into the central region of a sole  12  of the first type is displaced by the rabble arms  16 ,  16 ′ into the peripheral region of this sole  12 , where it falls through the peripheral drop holes  14  onto the peripheral region of a sole  22  of the second type. Here, the material under treatment is taken up by the rabble arms  26 ,  26 ′ of this sole  22 . These rabble arms  26 ,  26 ′ displace the material under treatment into the central region of the sole  22 , where it falls through the central drop hole  24  onto another sole of the first type shown in  FIG. 1 . 
     FIG. 3  shows a vertical cross section through the furnace wall  10  at the level of the sole  22  in  FIG. 2 , the reference  42  identifying the inner surface and the reference  44  the outer surface of the furnace wall  10 . This furnace wall  10  comprises, in a manner known per se, an external shell  46  made of steel and a refractory inner liner  48 .  FIG. 3  also shows the end of the wall scraper  26  with its wall scraper  32 , displaying a terminal blade  50 . As the wall scraper  26  rotates about the vertical axis  11 , the terminal blade  50  passes at a distance “x” from the inner surface  42  of the furnace wall  10 . This distance “x” must be calculated so as to avoid any direct contact between the wall scraper  32  and the refractory inner liner  48 , even when the wall scraper  26  and the furnace wall  10  undergo thermal expansions or contractions of different amplitudes. If a projection is made of the two ends of the terminal blade  50  rotating about the vertical axis  11  onto the inner surface  42  of the furnace wall  10 , two circles are defined on this surface  42  delimiting an annular zone  52  which represents the scraped zone  52  of the furnace wall  10  at the level of the sole  22 . 
   According to the present invention, the furnace wall  10  comprises a plurality of wall cavities  54  which form a succession of access openings  56  in the scraped zone  52 . It will be appreciated that these wall cavities  54 , which are formed in the refractory inner liner  48 , greatly reduce the risk of formation of a crust of hardened material adhering to the inner surface  42  of the furnace wall  10  and offering resistance to the passage of the wall scraper  32 . Through these access openings  56  in the scraped zone  52 , the wall cavities  54  in the wall  10  become progressively filled with material. However, the “pasting” compaction effect, which is the origin of the formation of a peripheral crust of very hard material adhering to the inner surface of the furnace wall, scarcely occurs. The material that accumulates in the wall cavities  54  is scarcely compacted by the passage of the wall scraper  32 . It remains relatively soft and thus results in substantially jerk-free braking. 
   Cleaning openings  58  in the external shell  46  provide access to the wall cavities  54 . Through these cleaning openings  58 , it is easy to introduce from the outside bars, lances or other cleaning devices in order to push the material accumulated in the wall cavities  54  back onto the sole  22  or even to clean the sole over a certain radial depth which depends on the tools employed. With tools with their tips bent back through a certain angle, it is also possible through the cleaning openings  58  to clean the inner surface  42  of the refractory liner around an access opening  56 . 
   For reasons of stability, leak-tightness and thermal insulation of the furnace wall  10 , the cleaning opening associated with a wall cavity  54  will be substantially smaller in cross section than the access opening  56  formed by this wall cavity in the scraped zone  52 . The cross section of the wall cavity  54  thus diminishes gradually in the direction of the cleaning opening. In the preferred embodiment shown in the drawings, the wall cavities  54  are, for example, pyramidal in shape, and the cleaning openings are cylindrical in shape and are formed on the apex axis of the pyramid (see  FIGS. 2 and 3 ). The pyramidal wall cavities  54  will most frequently be rectangular or square in cross section. However, their cross section may also be triangular or polygonal and, in general, be of a shape to fit other objects incorporated into the furnace wall, for example openings for burners, gas ducts, probes, etc. It is also possible to give the wall cavities the shape of an axisymmetric cone and then to make the cleaning opening  58  on the apex axis of this axisymmetric cone. 
   In  FIG. 2 , it can be seen that the circumferential extent of the residual surface  60  between two successive access openings  56   1 ,  56   2  in the scraped zone  52  is much smaller than the circumferential extent of such an access opening  56 . In the example in  FIG. 2 , the circumferential extent of the residual surface  60  between two successive access openings  56   1 ,  56   2  in the scraped zone  52  only represents, for example, 20% of the circumferential extent of an access opening  56 . The smaller the circumferential extent of the residual surface  60 , the lower the risk of forming of a peripheral crust adhering to the inner surface  42  of the furnace wall  10 . In an extreme case, two successive access openings  56   1 ,  56   2  in the scraped zone  52  may even be separated by a sharp edge, so that in the scraped zone  52  there is practically no surface left on which a hardened crust of material could form. Moreover, in the vertical direction, the access openings  56  extend slightly beyond the upper circumference delimiting the scraped zone  52 . 
     FIG. 4  shows a vertical cross section through the furnace wall  10  at the level of the sole  12  in  FIG. 1 . The reference  52 ′ indicates the extent of the “scraped zone” of the furnace wall  10  at the level of this sole  12 . As in the case of the scraped zone  52  at the level of the sole  22 , the scraped zone  52 ′ is also subdivided by a succession of access openings  56 ′ formed by wall cavities  54 ′ in the refractory liner  48 . The only significant difference is that at the level of the peripheral drop holes  14  in this sole  12 , there is a wall depression  70  in the refractory liner  48 , the purpose of which is to enlarge the cross section of a peripheral drop hole  14 . Since this wall depression  70  in the furnace wall extends a little way beyond the lower circumference delimiting the scraped zone  52 ′, the access opening  56 ′ does not extend as far as the lower circumference delimiting the scraped zone  52 ′, but stops above the upper edge  72  of the depression  70 . 
   The way in which the access openings  56 ,  56 ′ are arranged in the inner surface of the refractory liner will be better understood by reference to  FIG. 5 , which shows a three-dimensional view of an annular element of the furnace wall  10 . No soles are shown in  FIG. 5 . The hatched rectangles  74  indicate the positions of support blocks for a sole of the type in  FIG. 1 , that is to say a sole with peripheral discharge holes  14 . The wall depressions  70  between the support blocks  74  are plainly visible. In the assembled multiple-hearth furnace, a sole with a central discharge opening will be arranged immediately below the lower edge of the annular element depicted. The upper row of access openings  56 ′ is then the succession of access openings associated with a sole  12  with peripheral discharge holes  14 , while the lower row of access openings  56  is the succession of access openings associated with a sole  22  with central discharge opening  24 . On the side where the external shell  46  is visible, the cleaning openings  58 ′ giving access to the wall cavities  54 ′ and the cleaning openings  58  giving access to the wall cavities  54  can be seen. 
     FIG. 6  shows, in a vertical cross section, a detail of a wall cavity  54  with a cleaning opening hermetically sealed by means of a leak-proof plugging device  90 . The cleaning opening proper comprises a hole  92  in the external shell  46 . This hole  92  opens into a metal sleeve  94  which extends a certain distance into the refractory liner  48 . The leak-proof plugging device  90  comprises a steel blind flange  96  fixed to a companion flange  98  of the external shell  46 , and a central core  100  made of refractory material that penetrates into the metal sleeve  94 . A refractory ring  102  surrounds the central core  100 . The blind flange  96  is fixed onto the companion flange  98  by means of keys mounted on pivots, so that the blind flange  96  can be removed and refitted quickly. A hand-grip  104  is provided for easy handling of the leak-proof plugging device  90 .