Patent Publication Number: US-6660221-B2

Title: Rotary hearth furnace and screw thereof for discharging reduced iron

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rotary hearth furnace for manufacturing reduced iron through reduction of a raw material for reduced iron mainly composed of carbonaceous reducing material and iron oxide, and a screw thereof for discharging reduced iron on a hearth out of the rotary hearth furnace from an discharge port. 
     2. Description of the Related Art 
     As well known in the art, a rotary hearth furnace is used for manufacturing reduced iron through reduction of a raw material for reduced iron mainly composed of carbonaceous reducing material and iron oxide. 
     Such a rotary hearth furnace has a screw for discharging reduced iron on a hearth out of the rotary hearth furnace from an discharge port installed at an outer periphery side, that is, at a high-speed side on the furnace rotating about a vertical shaft. The durability of the reduced iron discharging screw is essential in enhancing the operation rate of the rotary hearth furnace or the productivity of reduced iron so that various means have been proposed up to now to enhance the durability of the reduced iron discharging screw. 
     According to an example of the reduced iron discharging screw, a cooling water channel is installed within a screw shaft or a rotary shaft, in which cooling water flowing through the cooling water channel lowers temperature to ensure the strength of the screw shaft, and a hollow portion is provided in a screw blade or a spiral blade, in which cooling water flowing through the hollow portion lowers temperature to ensure the hardness of the screw blade so that the wear resistance of the screw blade can be improved. 
     Further, according to another example of the reduced iron discharging screw, a cooling water channel is installed within a screw shaft or a rotary shaft of the reduced iron discharging screw, in which cooling water flowing through the cooling water channel lowers temperature to ensure the strength of the screw shaft thereby improving the durability of the screw shaft while two screw blades or spiral blades are joined together at a portion subjected to especially heavy abrasion to prolong the lifetime of the screw blades. Also, in order to further enhance the wear resistance of leading ends of the screw blades, an Inconel alloy (nickel 55% and chromium 45%) is built up via welding at both sides of the leading ends of the screw blades. 
     However, even if the strength of the screw shaft of the reduced iron discharging screw is enhanced and the screw blade has prolonged lifetime through improvement of the wear resistance of the screw blade and increase of wearing region, all kinds of reduced iron discharging screws require maintenance processes to detach the screw from the rotary hearth furnace for repair and to assemble the same to the rotary hearth furnace after repair. Such maintenance processes of the reduced iron discharging screw are carried out after stopping the operation of the rotary hearth furnace and the temperature within the rotary hearth furnace is lowered under a temperature which is suitable for the maintenance processes. In the maintenance processes, the reduced iron discharging screw is separated from a mooring equipment and a coupling, and then detached from the upper part through a furnace body cover of the rotary hearth furnace for repair, and assembled to the rotary hearth furnace from the upper part after repair. 
     In the reduced iron discharging screw of the prior art, corrosive gas generated from the raw material for reduced iron coheres to the surface of the screw shaft or the surface of the screw blade which is cooled through flowing of cooling water. Therefore, cohered corrosive gas causes cold corrosion to the surface of the screw shaft or the surface of the screw blade thereby decreasing the lifetime thereof Even in the case iron ores are reduced, sulfur included in coal mixed into staple pellets as a reducing agent generates SO X , which causes corrosion to the surface of the screw shaft or the surface of the screw blade thereby decreasing the lifetime thereof as the raw material for reduced iron. 
     In addition, in the prior art, the screw shaft using carbon steel has a service life of 4 to 10 months until cooling water leaks, and the screw blade has the lifetime of about 5 months. In other words, it can be hardly considered that any of the screw shaft or the screw blade has a sufficient service life and the maintenance operation of the reduced iron discharging screw should be performed frequently so that the operating ratio of the rotary hearth furnace cannot be enhanced Further, even if the screw blade is covered with a hard facing layer, which is constituted through hard facing of an Inconel alloy at both sides of the leading end and a leading portion of the screw blade by welding, sides of the screw blade tend to have welding defects such as an undercut between a base metal and a hard facing portion so that the screw blade may be damaged by a notch effect and accordingly the maintenance operation may be necessary even if other portions are sufficiently available. 
     In the reduced iron discharging screw having two screw blades joined together, the screw blades shaped as spiral blades are joined together at regions subjected to heavy abrasion to increase thickness thereby prolonging the lifetime of the screw blades. However, in this case, it is difficult to manufacture the two screw blades in high precision and the cost of the reduced iron discharging screw is inevitably increased because the screw blades are formed in three-dimensional shape. 
     Further, in the maintenance operation of the reduced iron discharging screw, since the cover of the furnace body should be detached at least as wide as the projected area of the reduced iron discharging screw and the cover of the furnace body is large, a radiation scheme is required in a large scale. Also, in regard to layout of the equipments, the equipments such as staple pins are frequently arranged at the upper position of the reduced iron discharging screw so that there is a problem that time and endeavor are required in a great amount to detach the reduced iron discharging screw according to arrangement among the equipments. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a rotary hearth furnace for enabling a reduced iron discharging screw to be easily detached and assembled, and a reduced iron discharging screw of a rotary hearth furnace excellent in durability. 
     The present invention has been accomplished in view of the aforementioned circumstances, and to solve the foregoing objects, it is provided a rotary hearth furnace including a reduced iron discharging screw, which is rotationally supported via an axial end of a rotary shaft by a pair of supporting devices with the axial end of said rotary shaft passing through-holes formed in both side walls at one and the other sides of a furnace body and has a spiral blade at the outer periphery side of said rotary shaft for discharging reduced iron on a hearth out of the furnace body, from an discharge port formed at an outer periphery side of the rotating hearth, wherein the through-holes in said both side walls are set in a size for allowing the spiral blade of the reduced iron discharging screw to pass the same, and closed by a hatch member externally mounted to the axial end of said rotary shaft in a removable fashion; wherein the inner screw supporting device including a screw supporting metal member capable of reciprocating with a leading end detachably connected to the axial end of said rotary shaft, and a metal member dragging means for dragging the screw supporting metal member out of the furnace body is provided in the outside at an inner periphery side of said hearth; and wherein the outer screw supporting device including screw dragging means for removing said reduced iron discharging screw out of the furnace body, and a screw supporting bogie capable of reciprocating for supporting the removed reduced iron discharging screw is provided in the outside at an outer periphery side of said hearth. 
     To solve the foregoing objects, there is further provided a reduced iron discharging screw of a rotary hearth furnace for discharging reduced iron on a hearth out of a furnace body from an discharge port formed at an outer periphery side, which has a spiral blade at the outer periphery of a rotary shaft with the inside being cooled through flowing of cooling water; wherein a corrosion protective refractory layer is formed at the outer periphery of said rotary shaft. 
     To solve the foregoing objects, there is further provided a reduced iron discharging screw, which further comprises an elongated groove thinner than the thickness width of the spiral blade at the leading end of said spiral blade, wherein the elongated groove is filled with a hard facing layer. 
     To solve the foregoing objects, there is further provided a reduced iron discharging screw wherein said spiral blade has threads in a greater number at an outer periphery side of the hearth than at an inner periphery side of said hearth. 
     To solve the foregoing objects, there is further provided a reduced iron discharging screw wherein the axial end of said rotary shaft is supported via buffer members to be elevated and maintained at a certain height by a supporting device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a sectional constitution in an arranging position of the reduced iron discharging screw of the rotary heart furnace according to the embodiment of the invention; 
     FIG. 2 is a detailed view of a part A shown in FIG. 1; 
     FIG. 3 is a detailed view of a part B shown in FIG. 1; 
     FIG. 4 is a detailed view of a part C shown in FIG. 1; 
     FIG. 5 is a sectional view of a rotary shaft of the reduced iron discharging screw according to the embodiment of the invention; 
     FIG. 6 is a sectional view of a spiral blade according to the embodiment of the invention; and 
     FIG. 7 illustrates a side constitution of the reduced iron discharging screw according to the embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a rotary hearth furnace and a reduced iron discharging screw of the rotary hearth furnace according to a preferred embodiment of the present invention will be described below in reference to the accompanying drawings, in which: FIG. 1 illustrates a sectional constitution in an arranging position of the reduced iron discharging screw of the rotary hearth furnace; FIG. 2 is a detailed view of a part A shown in FIG. 1; FIG. 3 is a detailed view of a part B shown in FIG. 1; FIG. 4 is a detailed view of a part C shown in FIG. 1; FIG. 5 is a sectional view of a rotary shaft of the reduced iron discharging screw; FIG. 6 is a sectional view of a spiral blade: and FIG. 7 illustrates a side constitution of the reduced iron discharging screw. 
     The reference numeral  1  shown in FIG. 1 designates a rotary hearth furnace which comprises a furnace body  2  having a reduced iron discharging screw  4  configured as follows for discharging reduced iron on a hearth  3  rotating about a vertical rotary center not shown in the left in FIG. 1 to an discharge port  3   a  installed at an outer periphery side in the right of the hearth  3  The reduced iron discharging screw  4  has a rotary shaft  41  with ends, which are installed in both side walls of an insulating housing  2   a  for partially constituting the furnace body  2  which covers the upper part of the hearth  3  and inserted with a play into through-holes  2   b , which are set in a such dimension that a spiral blade  42  of the reduced iron discharging screw  4  can pass through the same. At an inner periphery side of the hearth  3  of the rotary shaft  41  is externally fitted an inner bearing  5  constituted by connection of a grease feed pipe  51  into the axial end of the rotary shaft  41 , and at the outer periphery side is externally fitted an outer bearing  5 ′ constituted by connection of the grease feed pipe  51  into the axial end. 
     The inner bearing  5 , as shown in FIG. 2, is supported via buffer members  6  made of an elastic member such as a rubber sheet on upper and lower surfaces of flanges attached to the inner bearing  5  to be elevated by a hydraulic inside supporting device  7 ′. Also, the outer bearing  5 ′ is supported via buffer members not shown to be elevated by a hydraulic outside supporting device  7 ′. In this case, the inner and outer supporting devices and  7 ′ are adapted to uniformly maintain the distance between the axial center of the reduced iron discharging screw  4  and the hearth  3 , the buffer members  6  are adapted to maintain the contact pressure of a leading end of the spiral blade  42  of the reduced iron discharging screw  4  to the surface of the hearth  3  at or under a certain value because for example, unevenness exists on the surface of the hearth  3  (it is impossible to remove all). 
     In addition, since the inside and outside supporting devices  7  and  7 ′ are hydraulically operated, they may be configured to displace the reduced iron discharging screw  4  upward when the contact pressure of the leading end of the spiral blade  42  is at or under the certain value. Since the supporting devices  7  and  7 ′ are configured as above, there is an effect that damage of the reduced iron discharging screw  4  or the hearth  3  caused by clogging of foreign materials can be prevented. 
     Each of the through-holes  2   b ,  2   b  is externally provided to the axial end of the rotary shaft  41  in a detachable fashion, and is closed by each of inner and outer hatch members  8  and  9  having the following constitution. 
     That is, the inner hatch member  8  at an inner periphery side of the hearth  3 , namely the left side of the FIG. 1, as shown in FIG. 3, is constituted by a flange member detachably fixed to a side wall of the insulating housing  2   a  for closing the through-hole  2   b , a seal cover  81  integrally constructed by a cylindrical member externally fitted into the rotary shaft  41 , and a seal flange  82  externally fitted to a side wall side of the inner bearing  5  or the outside of the seal cover  81  to be fixed to a cylindrical member and having a grease chamber  82   a  in the inner side for storing grease supplied from a grease feed pipe  82   b.    
     Also, the outer hatch member  9  at an outer periphery side of the hearth  3 , as shown in FIG. 4, is constituted by a flange member detachably fixed to the side wall of the insulating housing  9 a for dosing the through-hole  2   b , a seal cover  91  integrally constructed by a cylindrical member externally fitted to the rotary shaft  41 , and a seal flange  92  externally fitted to the outside of the seal cover  91  at a side wall side of the outer bearing  5 ′ for being fixed to the cylindrical member and having a grease chamber  92   a  in the inner side for storing grease supplied from a grease feed pipe  92   b . In other words, the outer hatch member  9  has the substantially same constitution as the inner hatch member  8 . Further, the rotary hearth furnace  1  comprises an inner screw supporting device  10  and an outer screw supporting device  20  configured as follows, which are used when the reduced iron discharging screw  4  is detached from the furnace body  2  of the rotary hearth furnace  1  and when the reduced iron discharging screw  4 , which is detached and repaired, is assembled to the furnace body  2 . 
     The inner screw supporting device  10 , as shown in FIGS  1  and  3 , is arranged at the outside of the inner periphery of the hearth  3 . Also, the inner screw supporting device  10  is provided with a rod-shaped screw supporting metal member  11  which reciprocates as guided by a guide roller  14  adjustable in height installed over a frame at a certain interval and has the leading end being detachably connected to the leading end at an inner periphery side of the hearth  3  of the rotary shaft  41  through attachment/detachment of a bolt. The screw supporting metal member  11  is adapted to support the leading end of the reduced iron discharging screw  4  at the inner periphery side of the hearth  3  when the reduced iron discharging screw  4  is detached from the furnace body  2  and when the reduced iron discharging screw  4 , which is detached and repaired, is assembled to the furnace body  2 . Further, the inner screw supporting device  10  has an inner winch  12  as metal member dragging means for dragging the screw supporting metal member  11  to be displaced outward of the inner periphery side of the hearth  3  by the winding of a metal member dragging rope  13 . 
     In addition, within the screw supporting metal member  11  is installed a cooling water channel which has a constitution that cooling water is flown through the same to enable water cooling. 
     The outer screw supporting device  20 , as shown in FIG. 1, is arranged at the outside of an outer periphery side of the hearth  3 . Also, the outer screw supporting device  20  is provided with a screw supporting bogie  21  which reciprocates as guided by a guide rail  24  constructed over the frame and supports the leading end of the reduced iron discharging screw  4  at the inner periphery side of the hearth  3 . 
     Further, the outer screw supporting device  20  has an outer winch  22  as a screw dragging means for dragging the reduced iron discharging screw  4  out of the outer periphery side of the hearth  3  so that the reduced iron discharging screw  4  can be removed from the furnace body  2  by the winding of a screw dragging rope  23 . 
     As can be apparently understood from the foregoing description, according to the inner screw supporting device  10  and the outer screw supporting device  20 , when the reduced iron discharging screw  4  is removed from the furnace body  2 , the leading end of the screw supporting metal member  11  is connected to the leading end of the rotary shaft  41  of the reduced iron discharging screw  4  at the inner periphery side of the hearth  3 . Also, the screw dragging rope  23  is connected to the leading end of the hearth  3  at the outer periphery side and wound through the operation of the outer winch  22  while a metal member dragging rope  13  is unwound from the inner winch  12  so that the reduced iron discharging screw  4  can be removed from the furnace body  2  outward of the outer periphery side of the hearth  3 . 
     Meanwhile, on the contrary to the above, the metal member dragging rope  13  is wound through the operation of the inner winch  12  while the screw dragging rope  23  is unwound by the outer winch  22  so that the reduced iron discharging screw  4  can be displaced toward the inner periphery side of the hearth  3  to be assembled to the furnace body  2 . 
     Then, referring to FIG.  1  and FIGS. 5 to  7 , a detailed constitution of the reduced iron discharging screw  4  will be described. A swivel joint  4 a configured as an elbow is provided to the leading end of the rotary shaft  41  of the reduced iron discharging screw  4  at the outer periphery side of the hearth  3 , and a cooling water channel  41   a  (refer to FIG. 5) defined within the rotary shaft  41  is supplied with cooling water from a cooling water introducing pipe  4   b  connected to the swivel joint  4   a . Of course, cooling water heated through cooling the rotary shaft  41  of the reduced iron discharging screw  4  is drained via a cooling water drain pipe  4   c  connected to the swivel joint  4   a.    
     Further, at the outer periphery of the rotary shaft  41  of the reduced iron discharging screw  4 , is formed a refractory layer  43  as shown in FIG.  5 . The refractory layer  43  is formed on the outer periphery of the rotary shaft  41  like this for the purpose of preventing the contact of corrosive gas to the outer periphery of the rotary shaft  41  to prevent corrosion of the rotary shaft  41 . In this case, the temperature of the refractory layer  43  is maintained higher than that of the outer periphery of the rotary shaft  41 , which is successively water-cooled, resultantly having an effect of restricting cohesion of corrosive gas. 
     Further, at the leading end of the spiral blade  42  of the reduced iron discharging screw  4  is installed an elongated groove  45  thinner than the thickness width of the spiral blade  42 . The elongated groove  45  is filled with a hard facing layer  46 . A hard facing material adopted in the hard facing layer  46  employs a Fe-based material which is a eutectic of chromium carbide into austenitic stainless steel. This can prevent deficiency of a hard facing portion caused by a welding defect such as an undercut created between a base metal and the hard facing portions at sides of the leading end of the screw blade as in the screw blade according to the prior art in which the leading end and the both sides are covered by the hard facing layer. 
     Again, since the hard facing layer is not formed at the sides of the leading end of the spiral blade  42  according to the embodiment of the invention, it can be considered that the sides of the leading end of the spiral blade  42  may be worn in an early stage. However, according to experiences of the inventors, it has been observed that the sides of the leading end are not heavily worn even if the leading end of the spiral blade  42  is heavily worn. So, this can be understood that the leading end and the both sides thereof of the screw blade according to the prior art are covered with the hard facing layer, nevertheless, since the hard facing layer tends to be peeled off from the leading end due to a shearing force created during rotation when the hard facing layer is formed only at the leading end. 
     Further, in the reduced iron discharging screw  4 , the spiral blade has threads in a greater number at the outer periphery side of the hearth  3  or at the side of the discharge port  3   a  than at the inner periphery side of the hearth  3 . 
     More particularly, as shown in FIG. 7, in the outer periphery of the reduced iron discharging screw  4  at the discharge port  3   a  side of the rotary shaft  41  and between spiral pitches of the spiral blade  42 , an intermediate spiral blade  44  (a portion with the whole section being printed in FIG. 7) is provided with a length of ⅓ of the whole length of the spiral blade  42 . Of course, at the leading end of the intermediate spiral blade  44 , as in the spiral blade  42 , is installed an elongated groove thinner than the thickness width of the intermediate spiral blade  44 , which is filled with a hard facing layer. The reduced iron discharging screw  4  is configured as above for the purpose that reduced iron on the hearth  3  can be moved in a direction to the discharge port  31   a  without escaping from the spiral blade as well as the reduced iron discharging screw  4  is rotated in low speed to reduce abrasion of the spiral blade  42  and the intermediate spiral blade  44   
     As well known, as the hearth  3  has a rotational speed increasing as advancing radially outward, the hearth  3  is relatively faster than the spiral blade  42  when contacting the spiral blade  42 . Further, in order to reliably displace reduced iron toward the discharge port  3   a  while preventing reduced iron from escaping from the spiral blade when reduced iron on the upper surface of the hearth  3  is discharged out of the furnace body  2 , it is necessary to rotate the reduced iron discharging screw  4  at a rotational speed sufficient to capture reduced iron moving in the highest speed or positioned on the outermost outer periphery of the hearth  3 . Therefore, the reduced iron discharging screw  4  is rotated in high speed so that the spiral blade  42  is worn in a short time period and thus the reduced iron discharging screw  4  is necessarily short lived. Therefore, the intermediate spiral blade  44  is installed to prevent reduced iron from escaping even the reduced iron discharging screw  4  is rotated in a low rotational speed while reliably displacing reduced iron toward the discharge port  3   a  as well as enabling the lifetime of the spiral blade to be prolonged 
     In this case, while the length of the intermediate spiral blade  44  is set ⅓ of the whole length of the spiral blade  42  as above, the length of the intermediate spiral blade  44  is not limited to ⅓ of the whole length of the spiral blade  42  but can be suitably determined according to a relative speed of the spiral blade and the surface of the hearth  3 . Moreover, even if it is so configured that the whole length of the intermediate spiral blade  44  is the same as that of the spiral blade  42  and the intermediate spiral blade  44  is placed between the whole spiral pitches of the spiral blade  42 , the same effect of reducing abrasion of the spiral blade can be obtained as in the reduced iron discharging screw  4  having the foregoing constitution. However, it is not preferable in the manufacturing cost of the reduced iron discharging screw  4  since the intermediate spiral blade  44  is arranged excessively even though the rotational speed at the inner periphery side of the hearth  3  is lower than at the outer periphery side and the spiral blade itself serves sufficiently. 
     Hereinafter, description will be made about an operational aspect of the rotary hearth furnace  1  and the reduced iron discharging screw  4  configured as above. First, describing the operational aspect of the rotary hearth furnace  1 , the reduced iron discharging screw  4  is rotated via a chain sprocket  41   d  from a driving unit  1   a  installed on the furnace body  2  together with the hearth  3  through the operation of the rotary hearth furnace  1 . 
     The reduced iron discharging screw  4  is gradually worn out through continued rotation and when the reduced iron discharging screw  4  is removed from the insulating housing  2   a  of the furnace body  2  for repair when the amount of wear reaches a preset standard value, however, a preparatory operation is performed to remove the reduced iron discharging screw  4  before a removal operation. 
     The grease feed pipe  51  is detached from the inner bearing  5  supporting the end of the reduced iron discharging screw  4  at the inner periphery side of the hearth  3  and the grease feed pipe  82   b  is detached from the seal flange  82  of the inner hatch member  8 . Then, an end plate  41   b  for determining a longitudinal position of the reduced iron discharging screw  4 , a spacer  41   c  and the inner bearing  5  detached from the inner supporting device  7  are detached from the end of the rotary shaft  41  of the reduced iron discharging screw  4  at the inner periphery side of the hearth  3  while the inner hatch member  8  constituted by the seal cover  81  and the seal flange  82  is detached from the insulating housing  2   a , so that the end of the rotary shaft  41  at the inner periphery side of the hearth  3  becomes free. 
     Then, a screw hole where the end plate  41   b  was attached is utilized to connect the leading end of the screw supporting metal member  11  of the inner screw supporting device  10  to the end of the rotary shaft  41  at the inner periphery side of the hearth  3  so that the end of the rotary shaft  41  at the inner periphery side is maintained at a certain height by the guide rollers  14 ,  14  as well as the cooling water feed and cooling water drain pipes not shown are connected to the screw supporting metal member  11 . 
     Then, the grease feed pipe  51  is detached from the outer bearing  5 ′ supporting the end of the reduced iron discharging screw  4  at the outer periphery side of the hearth  3 , the grease feed pipe  92   b  is detached from the seal flange  92  of the outer hatch member  9 , and then the outer bearing  5 , is detached from the outer supporting device  7 ′. Further, the outer hatch member  9  constituted by the seal cover  91  and the seal flange  92  is detached from the insulating housing  2 a and a chain is detached from the sprocket  41   d  to set free the end of the rotary shaft  41  at the outer periphery side of the health  3 . Then, the preparatory operation for removing the reduced iron discharging screw  4  from the furnace body  2  is completed. The inner hatch member  8  and the outer hatch member  9  are removed in such a preparatory operation so that the reduced iron discharging screw  4  can be removed via the through-holes  2   b ,  2   b.    
     When such a preparatory operation is completed, an operation is performed for removing the reduced iron discharging screw  4  from the furnace body  2 . In other words, the screw dragging rope  23  is wound using the outer winch  22  of the outer screw supporting device  20  while the free end of the rotary shaft  41  at the outer periphery side of the hearth  3  is supported as suspended via a wire rope not shown using a hook F to remove the reduced iron discharging screw  4  via the through-holes  2   b  of the insulating housing  2   a . Then, the removed reduced iron discharging screw  4  is loaded on the screw supporting bogie  21 , which is displaced to a certain repairing site on the frame of the outer screw supporting device  20  thereby to finish the operation of removing the reduced iron discharging screw  4 . In addition, when the reduced iron discharging screw  4  is removed from the furnace body  2 , a simple insulation operation is preferably performed to protect the reduced iron discharging screw  4  in order to reduce damage of the reduced iron discharging screw  4  in passing through the furnace body  2 . 
     Then, the reduced iron discharging screw  4  is repaired, and then the metal member dragging rope  13 , which was unwound in removing the reduced iron discharging screw  4 , is wound using the inner winch  12  of the inner screw supporting device  10  to assemble the reduced iron discharging screw  4  via the through-holes  2   b  of the insulating housing  2   a . Portions of the components at the outer periphery side of the hearth  3  and the components at the inner periphery side of the hearth  3  are attached to corresponding sites according to a sequence reverse to the above. 
     Then, after the screw supporting metal member  11  is detached from the end of the rotary shaft  41  at the inner periphery side of the hearth  3  and the metal member dragging rope  13  is wound again by the inner winch  12  to retract the screw supporting metal member  11  in a direction facing away from the end of the rotary shaft  41  at the inner periphery of the hearth  3 , the end plate  41   b , the spacer  41   c  and the inner bearing  5  are attached while the grease feed pipe  51  and the grease feed pipe  82   b  are respectively attached to the inner bearing  5  and the seal flange  82  of the inner hatch member  8  to finish an operation of recovery. 
     Therefore, according to the rotary hearth furnace  1  related to the present embodiment, it is not necessary to detach a portion of furnace body cover corresponding to the projected area of the reduced iron discharging screw as in the prior art, and the opening area of the through-holes  2   b ,  2   b  is smaller than the projected area of the reduced iron discharging screw, so that a large scale of radiation scheme is not necessary. Further, in regard to layout of the equipments, even if the equipments such as the staple pins are arranged at the upper position, the reduced iron discharging screw  4  is removed and assembled in a lateral direction so that the reduced iron discharging screw  4  can be detached/assembled from/to the furnace body  2  in a time period much more shorter and with endeavors less than in the prior art. Thus, there is a remarkable effect that the maintenance cost of the reduced iron discharging screw  4  can be reduced. 
     Hereinafter, description will be made about an operational aspect of the reduced iron discharging screw  4  of the rotary hearth furnace according to this embodiment. In the reduced iron discharging screw  4 , at the outer periphery of the rotary shaft  41  is formed the refractory layer  43  which is continuously maintained at a temperature higher than that of the outer periphery of the rotary shaft  41  which is continuously water cooled in the operation. Then, in addition that cohesion of corrosive gas is restricted, contact of corrosive as to the outer periphery of the rotary shaft  41  is prevented even if corrosive gas is cohered so that corrosion of the rotary shaft  41  is effectively restricted. 
     Further, the elongated groove  45  installed in the leading end of the spiral blade  42  of the reduced iron discharging screw  4  is filled with the hard facing layer  46  to avoid the welding defect such as the undercut between the base metal and the hard facing portion at the sides of the leading end of the screw blade as in the screw blade in the prior art in which the leading end and the both sides are covered by the hard facing layer thereby preventing loss of the spiral blade due to the welding defect. 
     Further, the intermediate spiral blade  44  is circumferentially provided with the length of ⅓ of that of the spiral blade  42  at the outer periphery at the discharge port side  3   a  of the rotary shaft  41  of the reduced iron discharging screw  4  and between the spiral pitches of the spiral blade  42  so that reduced iron on the hearth  3  can be displaced toward the discharge port  3   a  without escaping from the spiral blade  42  and the reduced iron discharging screw  4  can be rotated in a low speed. Accordingly, the spiral blade  42  and the intermediate spiral blade  44  have the reduced amount of abrasion so that the lifetime of the spiral blade  42  and the intermediate spiral blade  44  are prolonged in a great amount compared to those in the prior art. Also, since the bearing externally fitted to the reduced iron discharging screw  4  is supported via the buffer member  6 , even if the distance between the shaft center of the reduced iron discharging screw  4  and the surface of the hearth  3  is changed slightly, the contact pressure of the leading end of the spiral blade  42  of the reduced iron discharging screw  4  to the surface of the hearth  3  can be maintained at or under a certain contact pressure thereby serving to restrict abrasion of the spiral blade by a large margin. 
     Therefore, according to the reduced iron discharging screw  4  of the rotary hearth furnace related to the embodiment of the present invention, the refractory layer  43  is formed at the outer periphery to prevent corrosion of the rotary shaft  41  caused by corrosion gas, the hard facing layer  46  is filled in the elongated groove  45  installed in the leading end of the spiral blade  42  to prevent damage, the intermediate spiral blade  44  is arranged at the outer periphery at the discharge port side  3   a  of the rotary shaft  41  and between the spiral pitches of the spiral blade  42  to reduce the rotation number and thus restrict abrasion of the spiral blade, and the buffer member  6  supports the leading end of the spiral blade  42  to prevent increase of the contact pressure to the hearth  3  surface. These incur a synergy effect that the lifetime of the reduced iron discharging screw  4  is prolonged in a greater amount than in the prior art and the frequency of repairing the reduced iron discharging screw  4  is decreased. As a result, remarkably excellent effects can be obtained that the operating ratio of the rotary hearth furnace  1  can be enhanced by a large margin and the cost of reduced iron can be reduced in a great amount. Moreover, the lifetime of the reduced iron discharging screw  4  can be prolonged when only one of the foregoing means is implemented. 
     As described above, in the rotary hearth furnace according to the present invention, the inner hatch member and the outer hatch member are attached/detached and the reduced iron discharging screw is supported using the inner screw supporting device and the outer screw supporting device so that the reduced iron discharging screw can be laterally removed from the furnace body via the through-holes and laterally assembled to the furnace body via the through-holes on the contrary. 
     Therefore, in the rotary hearth furnace according to the present embodiment, it is not necessary to detach the furnace body cover as wide as at least the projected area of the reduced iron discharging screw, the radiation scheme in a large scale is not necessary due to small opening area of the through holes, and the reduced iron discharging screw is removed laterally as above even if the equipments such as the staple pins are arranged in regard to layout of the equipments so that the reduced iron discharging screw can be detached in a much shorter time period than in the prior art regardless of the arrangement between the equipments. Accordingly, the maintenance cost of the reduced iron discharging screw can be reduced in a great amount. 
     In the reduced iron discharging screw of the rotary hearth furnace according to the present invention, the corrosion protective refractory layer is formed at the outer periphery of the rotary shaft of the reduced iron discharging screw, in which the temperature of the refractory layer is continuously maintained higher than that of the outer periphery of the rotary shaft which is continuously water cooled during the operation. Accordingly, cohesion of corrosive gas is restricted and the contact of corrosive gas to the outer periphery of the rotary shaft is prevented even if corrosive gas is cohered to restrict corrosion of the rotary shaft thereby enabling the lifetime of the rotary shaft to be prolonged in a great amount. 
     In the reduced iron discharging screw of the rotary hearth furnace according to the present invention, the elongated groove installed in the leading end of the spiral blade of the reduced iron discharging screw  4  is filled with the hard facing layer so that the welding defect such as the undercut may not occur between the base metal and the hard facing portion at the sides of the leading end of the screw blade unlike the screw blade of the prior art, in which the leading end and the both sides are covered with the hard facing layer, thereby preventing damage of the spiral blade caused by the welding defect. 
     In the reduced iron discharging screw of the rotary hearth furnace according to the present invention, the spiral blade has more threads at the outer periphery side of the hearth than at the inner periphery of the hearth so that reduced iron which is being displaced in high speed can be displaced toward the discharge port without escaping from the spiral blade and the reduced iron discharging screw can be rotated in low speed to reduce abrasion of the spiral blade thereby prolonging the lifetime of the spiral blade in a greater amount compared to the prior art. 
     In the reduced iron discharging screw of the rotary hearth furnace according to the present invention, the reduced iron discharging screw is supported via the buffer member. Then, even if the distance between the shaft center of the reduced iron discharging screw and the surface of the hearth is changed slightly, the contact pressure of the leading end of the spiral blade of the reduced iron discharging screw to the surface of the hearth is maintained at or under a certain contact pressure thereby serving in a great amount to restrict abrasion of the spiral blade. 
     In the reduced iron discharging screw of the rotary hearth furnace of the present invention, the refractory layer is formed at the outer periphery to prevent corrosion caused by corrosion gas, the elongate groove of the leading end of the spiral blade is filled in the hard facing layer, the spiral blade has more threads at the discharge port side to reduce the rotation number thereby restricting abrasion of the spiral blade, and the buffer member prevents increase of the contact pressure of the leading end of the spiral blade to the hearth surface. Then, a synergy effect is incurred that the lifetime of the reduced iron discharging screw is prolonged in a great amount, the maintenance frequency of the reduced iron discharging screw is reduced to enhance the operating ratio of the rotary hearth furnace in a great amount, and the cost of reduced iron can be saved.