Abstract:
A rotary hearth furnace for use in the iron and steel industry comprises a furnace ( 12, 112 ) with plan in the shape of an annulus, closed at the bottom by a rotary hearth ( 14, 114 ), lined at the top with refractory material ( 15, 115 ), and a base ( 28, 128; 30, 130 ) of the furnace ( 12, 112 ). Said hearth ( 14,114 ) comprises a plurality of sectors of an annulus ( 17, 117; 17′, 117′ ), all the same as one another and connected to form an annulus, complementary to that of the internal plan of the furnace ( 12, 112 ), which rotate around the central axis of the annulus, by means of two concentric sets of wheels ( 26, 126 ) arranged according to two circumferences, set at equal instances, with supports ( 25,125 ), fixed to the base ( 28 ) or below the hearth ( 114 ), complementary to two circular rails ( 20, 120 ), fixed respectively below the hearth ( 14 ) or the base ( 128 ). According to the invention, both said sets of wheels ( 26, 126 ) and said two rails ( 20, 120 ) are positioned in such a way as to have an equal load distribution.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERAL SPONSERED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention refers to a rotary hearth furnace for use in the iron and steel industry. 
   2. Description of Related Art 
   Rotary hearth furnaces have been used for a long time, particularly in the iron and steel sector. 
   Their uses are very varied. For example they are used to heat metals in ingots, slabs or blooms, before rolling; or for the heat treatment of materials, such as metal parts, glass or graphite; or again for processing loose or agglomerated raw materials such as coal or alumina, or mixtures of raw materials such as iron ore with carbon materials, or waste rich in iron with carbon materials. 
   Rotary hearth furnaces are built in different shapes and with a diameter varying from a few metres to more than 50 m, with width even larger than 6 m. 
   The hearth rotation speeds are also variable. Large heating furnaces rotate at even less than one revolution per hour, while small rotary furnaces, for calcination or for processing raw materials, reach for example fifteen revolutions per hour. 
   The hearth is generally in the shape of an annulus and rotates through two circumferential sets of wheels. These are located on the circumferences close to the ends of the annulus. 
   The wheels run on rails, and two different construction solutions are possible. 
   A first solution is to make wheels integral with the frame of the rotary hearth and rails fixed to the ground, mounted on very rigid structures, often made of reinforced concrete. 
   The second solution is to make rails integral with the frame of the rotary hearth and wheels fixed to the ground, mounted on very rigid structures, often made of reinforced concrete. 
   In the latter case the rotary hearth must be planned and built considering a fatigue stress in the metal structure, and consequently in its refractory lining, due to the continuous changing of the points of contact between the wheels and the rails during rotation of the hearth. 
   This fatigue stress may be very critical for the life of the refractory and so the furnaces are planned with wheels positioned on the two diameters, internal and external, on the same radii, so as to be able to divide the above metal structure into sectors having the same angular spacing as the wheels. In doing this, the deflection of the hearth structure due to the changing of the position of the point of contact of the wheels with the rails applied on the structure generates limited stresses and eliminates or minimises the cyclical movements of the refractory. 
   New processes have recently been developed in the field of the treatment of iron ore which require rotary hearth furnaces with very large hearth areas, and in some cases with very high rotating speeds, even more than 15 revolutions per hour. 
   These rotary hearth furnaces require hearth surfaces larger than those built up till now, with diameters even larger than 50 m and hearth widths larger than 6 m, even over 10 m. 
   In these furnaces certain problems, which are not important in the traditional applications, become critical when the dimensions and the rotating speeds are increased so considerably. The main problems to be tackled are the wear of the coupling between wheels and rails, and the curving of the hearth panels due to the difference in temperature in the panel supporting structure. 
   Normally the wheels and the rails are generally positioned on two circumferences very close to those of the ends of the hearth. Due to the geometry of the system, the wheels on the outer surface are more loaded than the wheels on the internal circumference. With the increase in the width of the hearth, this load difference is increased and consequently there may be great differences in the wear of the wheels and of the rails, which are on the two internal and external circumferences. The behaviour described above may be compensated, for example, by changing the size of the wheels and of the rails. 
   In these furnaces, also the planarity of the hearth is of primary importance. As is known, the supporting beams of the refractory hearth are subject to heating due to heat conduction through the hearth and at the same time they are cooled by irradiation and conduction with the environment below. This normally generates, in these supporting beams, a difference in temperature between the top and the bottom of the hearth, giving rise to a phenomenon of curving of the hearth when it reaches the working temperature. 
   When the width of the hearth increases, the traditional construction with frames having panels as wide as the hearth itself becomes critical on account of the strains due to said thermal effects and the consequent stresses or strains which may be generated in the refractory structure. 
   Moreover, since some new rotary hearth furnaces need to be installed at a considerable height above ground level, even higher than 20 metres, the high supports of the furnace, which must support the wheels or the rails of the hearth, increase the flexibility of the structure. In particular, if the structure, under the load of the hearth, generates different deflections from point to point, and particularly in an asymmetric manner, the induced stresses may be very critical, provoking functional complications and phenomena of fatigue during operation of the furnaces. 
   BRIEF SUMMARY OF THE INVENTION 
   The general aim of the present invention is to improve, in a rotary hearth furnace, the performances of the wheels and of the rails, and to decrease the stresses on the structures of the rotary hearth and of its refractory lining. 
   Another aim is to overcome the above-mentioned existing inconveniences of the conventional construction technique in an extremely simple, economic and particularly functional manner. 
   In view of the above aims, according to the present invention, it has been intended to realise a rotary hearth furnace for use in the iron and steel industry, having the characteristics stated in the enclosed claims. 
   The structural and functional characteristics of the present invention and its advantages with respect to the prior art will be even more clear and evident from an examination of the following description, referring to the enclosed drawings, which show a rotary hearth furnace for use in the iron and steel industry realised according to the innovative principles of the invention. 

   
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  shows a layout view from below of only a metal bearing structure of a rotary hearth for a furnace, according to a first realisation of a furnace of the present invention; 
       FIG. 2  is a section taken according to the plane II—II of  FIG. 1 , that is according to a radial plane, of a first realisation of a rotary hearth furnace for use in the iron and steel industry, the rotary hearth of which is shown in  FIG. 1 ; 
       FIG. 3  is a side elevation section enlarged and developed on the plane, partly showing means for rotating the hearth according to the realisation in  FIG. 2 , with wheels placed on a fixed structure; 
       FIG. 4  is a side elevation section, made according to a radial plane of half the furnace, of a second realisation of a rotary hearth furnace for use in the iron and steel industry; 
       FIG. 5  is a side elevation section enlarged and developed on the plane, partly showing means for rotating the hearth according to the realisation in  FIG. 4 , with wheels placed on the mobile structure of the furnace. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference above all to  FIGS. 1 ,  2  and  3 , a rotary hearth furnace for use in the iron and steel industry according to the invention, in a first possible example of an embodiment, is indicated overall as furnace  12 , placed on a support structure  16  and equipped with a rotating hearth  14 . 
   The furnace  12  has a plan in the shape of an annulus lined with refractory material and it is closed at the side and at the top by walls  13  lined on the inside with refractory material. Instead, at the bottom the furnace  12  is closed by the hearth  14 , also in the shape of an annulus but rotating around a central vertical axis of the annulus. This hearth  14  is lined at the top with refractory material  15 , for example with refractory panels. 
   The hearth  14  is composed of a series of annulus sectors  17 . As well as this circumferential division of the sectors  17 , there may be, as shown in  FIG. 1 , a radial division of the annulus of the hearth  14 , breaking the sectors  17  into two semi-sectors  17 ′ along arcs  18  of an intermediate circumference between the two end circumferences, internal and external, of the annulus. 
   The intermediate circumference which comprises the arcs  18  is such as to divide the sector  17  into two semi-sectors  17 ′ of the same weight. Due to geometric considerations on the areas subtended by semi-sectors of an annulus, it is therefore larger than the median circumference of the annulus of the hearth  14 . 
   The division into semi-sectors  17 ′ of the hearth allows a considerable decrease of the hearth level variations due to the thermal curving of a support structure of the sectors  17  when the hearth reaches the normal working temperatures. 
   The semi-sectors  17 ′ are shown above a reticular structure comprising cross members  22 , uprights  23  and  23 ′, and possibly tie rods or stiffening struts  24 . 
   As may be seen in  FIGS. 2 and 3 , the reticular structure has two annular bars  19  at the bottom, concentric with the annulus of the hearth  14 . These bars  19  are placed on circumferences coinciding with the centre of gravity of the sectors  17  and  17 ′, so that the weight of these sectors is discharged directly on the system below. 
   Below these bars  19  there are two rails  20 , having the same section. 
   In the example in  FIG. 2  these bars  19  are placed in a position such that the weight of the hearth  14 , bearing down on the two bars  19  is almost identical. 
   The support structure  16  comprises a base  28 , placed on circumferential sets of columns  30 . Fixed on this base  28  are two sets of supports  25  for wheels  26 , placed along a circumference, so that the wheels  26  are complementary to and operatively aligned with the two rails  20  of the hearth  14 . 
   The two sets of wheels  26  are placed in such a way that the pairs of wheels  26  are positioned on the same radii of two concentric circumferences, internal and external, these radii being spaced at equal distances on the same circumferences. The number of these pairs of wheels  26  is equal to those of the sectors  17 . In this way the stresses due to the changing of position, during rotation of the hearth  14  itself, of the forces applied by the wheels  26  on the bars  19  and therefore on the sectors  17  and on the refractory material  15 , are minimised. In fact the semi-sectors  17  and  17 ′ are principally supported by the uprights  23  which join them vertically to one of the two annular bars  19 . These uprights  23 , as may be seen in  FIG. 2 , are in fact located in an area close to the centre of gravity of the semi-sectors  17 ′. 
   Each sector  17  is thus equipped with two uprights  23 , one for each semi-sector  17 ′, connected to the two bars  19 . The two bars  19  are connected to the cross members  22 , placed in a radial position. 
   The cross members  22  may be placed corresponding to the two uprights  23  of each sector  17 . An upright  23 ′ is added to the two uprights  23 , in the area of the separating arc  18  between the two semi-sectors  17 ′, having the aim of absorbing any vertical force that could be generated if the position of the uprights  23  were not exactly in the centre of gravity of said semi-sectors  17 ,  17 ′ and also to give stability to the semi-sectors  17 ,  17 ′ themselves. 
   It should be noted that the best condition is the one in which the uprights  23  are in the centre of gravity of the semi-sectors  17 ′. In this case the vertical stresses in the arc  18  that divides the two semi-sectors  17 ′ are cancelled and the division may be used as a thermal expansion joint. Moreover, the fact that the cross member  22  is not stressed by the loads transmitted to the hearth corresponding to the upright  23 ′ allows the avoidance of possible phenomena of deflection of the beam, thus optimising the work of the wheels  26  on the rails  20 . 
   In the  FIG. 2  are shown circumferential sets of columns  30  (in this case four) in such a way that the columns  30  are aligned in groups on the same radii of concentric circumferences, where these radii are spaced at equal distances on the same circumferences. The number of these groups of columns  30  is equal to that of the sectors  17 . 
   More particularly, with reference to  FIGS. 2 and 3 , two of the four circumferences, on which the columns  30  are placed, may be equal to the circumferences on which are placed the supports  25  for the wheels  26 , and these supports  25  are located on the base  28  corresponding to each column  30 . 
   As the columns  30  are designed in such a way as to have all the same vertical deflection under the load of the hearth  14 , the stresses on the hearth  14 , due to changing of the points of application of the load of the bars  19  on the wheels  26  during rotation, are thus minimised. 
   Instead, if the columns  30  cannot be positioned as described, the support structure  16  must be designed in such a way that the vertical deflection of the wheels  26  is as identical as possible. 
     FIGS. 4 and 5  illustrate a further possible practical embodiment of the invention, where the components equal to and/or equivalent to those illustrated in  FIGS. 1 ,  2  and  3  are marked with the same reference numbers, increased by 100. 
   This second embodiment differs from the first only in the fact that the reciprocal position between the supports  25  of the wheels  26  and the rails  20  indicated in the first embodiment is inverted. 
   As may be seen in  FIGS. 4 and 5 , in this embodiment the rails  120  are fixed to the base  128 . 
   Instead the supports  125  are anchored below the annular bars  119 . More precisely, the supports  125  are paired and positioned on the same radii of the two concentric circumferences of the bars  119 , spaced at equal distances on the circumferences themselves. The number of these pairs of supports  125  is equal to that of the sectors  117 . Moreover the supports  125  are fixed corresponding to the uprights  123  of the reticular structure that holds up the hearth  114 . 
   In so far as regards the support structure  116 , the precaution is always taken to position the circumferential sets of columns  130  in such a way that they are aligned, in groups, on the same radii of the concentric circumferences, where the radii are spaced at equal distances on the circumferences themselves. The number of these sets of columns  130  is equal to that of the sectors  117 . 
   In this way the wheels  126  of the hearth  114 , spaced at equal distances in the same way as the columns  130 , do not exert any differential stress on the hearth  114  when this is being rotated. The hearth is in fact subject to a uniform lifting and lowering movement due to the deflection of the rails  120  and of the underlying support structure  116 . It does not produce stress on the structure of the hearth  114  and does not induce movements in the refractory material  115  placed above it. 
   Finally, a rotary hearth furnace for use in the iron and steel industry according to the invention, where the structure that supports the hearth has a median radius equal to the one by which the hearth is divided into two concentric annuli having the same load, stresses the wheels of the two internal and eternal circumferences in an identical or very similar manner. 
   In this way the behaviour of the wheels is the same and the construction can be simplified by using wheels and rails of the same size. 
   In addition to simplifying the construction, and therefore also the maintenance, there are other advantages, such as an improvement in accessibility to the furnace seals. 
   Moreover, with maximum benefit when the width of the furnace is considerable, it is possible to divide the hearth structure not only radially into sectors, but also by splitting it along the circumference into semi-sectors; this is facilitated by positioning the wheels almost on the centre of gravity with respect to the two semi-sectors and thus minimising stresses in the area where the sectors are divided into semi-sectors and torsional stress on the annular bars. 
   These semi-sectors generate various benefits. Above all, the torsional effect on the support structures of the rails or wheels mounted on the hearth is eliminated or minimised. Then there is a reduction of the maximum radial heat expansion on the sectors of the structure because this is now divided into two semi-sectors, anchored in the centre. Lastly, dividing the sectors into semi-sectors, the total bending value of the hearth sectors, due to the differences in temperature between the top and the bottom of the metal structures which make up said sectors, is reduced with respect to the solution without semi-sectors. 
   From the above description with reference to the figures, it appears evident that a rotary hearth furnace for use in the iron and steel industry with large dimensions according to the invention is particularly useful and advantageous. The aims mentioned in the introduction to the description are thus achieved. 
   The forms of the rotary hearth furnace for use in the iron and steel industry according to the invention can of course be different from the one shown purely as an example without limitation in the drawings, just as the materials may be different. 
   The area of protection of the invention is therefore defined by the enclosed claims.