WO1998048059A1

WO1998048059A1 - Furnace with rotating hearth and operating method

Info

Publication number: WO1998048059A1
Application number: PCT/EP1998/002250
Authority: WO
Inventors: Thomas Hansmann; Guido Monteyne; René Munnix
Original assignee: Paul Wurth S.A.
Priority date: 1997-04-18
Filing date: 1998-04-17
Publication date: 1998-10-29
Also published as: AU7644398A

Abstract

The invention relates to a method for heating materials in a furnace (10) having a rotating hearth (12). At least one layer of material is charged onto the rotating hearth (12) and subjected to a hot gas from above. This material layer is furthermore heated from beneath, through the rotating hearth (12). To this end, heating gases are preferably suctioned in the furnace and conducted through channels (38, 40) in the rotating hearth. This method is particularly suitable for direct reduction of iron ore. The invention also relates to a furnace with rotating hearth for carrying out this method.

Description

Oven with a rotating cooker and method for its operation

The present invention relates to a rotary hearth or turntable furnace and a method for its operation. Such an oven with a ring-shaped rotary hearth is used, for example, in the production of direct reduced iron (DRI), the so-called sponge iron. Sponge iron is produced in a direct reduction process by reducing iron oxide with solid or gaseous reducing agents. The solid reducing agent used is, for example, carbon, which reacts with oxygen at higher temperatures and forms the reducing gas CO. Such a process can be carried out, for example, in a rotary hearth furnace, i.e. in a furnace with a rotating ring-shaped stove top, which is covered on the top with refractory material and which is surrounded by a housing. Burners are attached to the top of the housing, which penetrate the housing and heat the interior of the housing to the required reaction temperature of over 1200 ° C. The hot flue gases from the burners are passed through the furnace in a countercurrent process and led out of the furnace through a chimney.

The iron oxide together with the reducing agent is applied to the rotary hearth at a first point of the rotary hearth furnace and passes through the rotation of the rotary hearth into the interior of the housing, where, due to the high temperatures, it reacts with the reducing agent to turn around after one turn of the rotary hearth to have directly reduced iron. The form in which the iron is present depends on the type of process used.

In the traditional process, the iron oxide is pressed together with the reducing agent into pellets before charging into the rotary hearth furnace, which pellets are then subsequently charged onto the rotary hearth of the furnace. Inside the furnace, in a controlled atmosphere, the iron oxide within the individual pellets reacts with the carbon monoxide released by the carbon and is reduced to iron within the pellets. The sponge iron lies thus after the reduction in pellet form, the pellets also containing the residues of the reducing agent (ash) and any impurities such as sulfur. After the reduction process, a further process step is consequently necessary in which the directly reduced iron is separated from the ashes and the impurities.

In an alternative process, fine grain iron oxide and fine grain reducing agent, e.g. Charcoal, charged in separate layers on the hearth of the furnace. It is possible to charge only one layer with iron oxide and one layer with reducing agent, or several layers of the individual materials can be stacked alternately. When passing through the furnace, carbon monoxide is released in the carbon layer or layers, which penetrates through the fine-grained iron oxide layers and reduces them to iron. The reduced iron is consequently present in the pure form after the reduction process in one or more layers lying one above the other, the individual iron layers being separated from one another by layers of reducing agent residues and these ash layers being in loose form.

Regardless of the process used, the iron oxide and the reducing agent must first be heated to the required reaction temperature of about 1200 ° C after charging on the fireproof casing of the rotary hearth before the actual reduction reaction can begin. This takes place in the area of the furnace adjacent to the charging zone of the rotary kiln by heat transfer from the hot exhaust gases of the burners to the charged materials. In view of the relatively low thermal conductivity of the charged materials, especially when charging fine-grained batches, the warm-up phase takes a considerable amount of time until an essentially homogeneous temperature distribution is achieved within the charged material layers. However, the longer the warm-up phase lasts, the lower the throughput of the rotary kiln, since the speed of rotation of the rotary hearth must be reduced accordingly. It is therefore clear that the Warm-up phase must be shortened as possible in view of a high throughput of the rotary kiln.

The object of the present invention is therefore to propose a rotary hearth furnace and a method for its operation, with the aid of which the charged material can be heated to a desired temperature as quickly as possible.

This object is achieved according to the invention by a method for heating materials in a furnace with a rotary hearth, in which at least one layer of material is charged onto the rotary hearth and hot gas is applied from above and in which the material layer is additionally heated from below by the rotary hearth.

By heating the charged material layer on both sides, the heat transfer to the material layer can be significantly increased, so that it heats up more quickly than with one-sided heating. In addition, the heating of the material layer is significantly more homogeneous than in the known methods, since a high temperature gradient, which builds up in the material layer from above when heated alone, is reduced.

It follows that the warm-up phase, i.e. the time between charging the cold material and reaching an essentially homogeneous temperature distribution above the starting temperature of the subsequent process is significantly reduced compared to the conventional methods. The subsequent process starts faster, which increases the throughput of the furnace and thus its productivity.

The material layer can be heated from below, for example, by electrical heat generation in the rotary hearth or by passing a heat transfer medium through the rotary hearth. The heat generation or the passage of the heat transfer medium preferably takes place in the heat-resistant casing of the rotary hearth, ie in the uppermost layer of the rotary hearth, which is in direct contact with the material layer. Cooling of the top layer of the rotary hearth after charging the cold material Layer can hereby be prevented and consequently a constant heat transfer to the material layer can be guaranteed.

The heat transfer medium can comprise a hot liquid or a hot gas.

In the conventional methods of operating a rotary kiln, the hot exhaust gases from the burners arranged in the housing of the kiln, together with any process gases, i.e. Gases that arise in the actual process in the rotary hearth furnace, countercurrently passed through the rotary hearth furnace and discharged through a chimney in the area of the charging zone. When these hot gases are removed, they are still at a high temperature and therefore still contain a large amount of heat. In order to use this excess amount of heat, the discharged hot gases can serve as heat carriers.

The hot gases are led through the rotary hearth and give off some of their latent heat here. On the one hand, this improves the thermal efficiency of the rotary hearth furnace and, on the other hand, the cooling of the gases associated with the heat dissipation facilitates their subsequent cleaning.

The hot gases are preferably sucked into the furnace at several locations distributed around the circumference. As a result, the amount of the hot gases which are conducted in countercurrent through the rotary hearth furnace and, consequently, their flow rate are reduced. As a result, better heat transfer from the hot gases to the material layer is achieved. When heating fine-grained materials, the lower flow rate of the hot gases also reduces the risk that charged fine-grained material is entrained by the gas flow and a charged layer profile is thereby destroyed.

If no reduction in the flow rate is desired, the reduced amount of gas in the counterflow enables the furnace housing to be designed in a more compact and therefore more cost-effective manner.

The hot gas is preferably drawn in in the upper region of the furnace, ie in the upper region of the furnace housing. On the one hand is the temperature the hot gases are greatest in this area, on the other hand this avoids that a gas layer required for the actual reaction, which forms in the lower area of the furnace above the material layer, is sucked off and the reduction is thereby disturbed. In the direct reduction of iron oxide, this is, for example, a layer of the reducing gas CO, which is formed from the carbon and accumulates above the material layers. This layer of CO gas serves as a protective layer that prevents the sponge iron from re-oxidizing.

If the hot gas drawn in still contains combustible constituents, the hot gas is advantageously afterburned when the process gas is passed through the rotary hearth. This can be initiated in a simple manner by supplying an oxygen carrier.

In an advantageous embodiment, the furnace is divided into different zones adjacent in the circumferential direction and a heating power supplied to the rotary hearth is regulated individually in each zone of the furnace. In this way, a very large heating power can be set, particularly in the charging zone, so that the heating of the cold material layer to the process temperature takes place as quickly as possible.

The present invention also relates to a furnace with a rotary hearth, the rotary hearth being surrounded on its upper side by a housing, and means being provided in the housing in order to apply a heating gas to a layer of material charged onto the rotary hearth from above. According to the invention, the oven has at least one heating element which is arranged in the rotary hearth. The rotary hearth of the furnace is preferably divided into different zones, with a separate heating element being arranged in each zone. This enables individual control of the heating output for the different zones of the rotary hearth. The advantage of this individual regulation is that a certain zone of the rotary hearth is heated differently depending on its position in the fixed furnace. For example, the zone of the rotary hearth that currently extends through the charging zone of the The furnace rotates, is heated particularly strongly, so that the newly charged material layer reaches the required starting temperature for the subsequent reaction as quickly as possible.

The heating element can comprise, for example, an electric heating element, which is advantageously mounted in the heat-resistant casing of the rotary hearth.

In an advantageous embodiment, the heating element has a line system which is arranged in the rotary hearth and can be acted upon by a heat transfer medium. The line system can have a single pipe that extends radially essentially over the entire width of the rotary hearth or a plurality of pipes that extend either radially or adjacent to one another in the respective zone and which are connected to one another alternately at their opposite ends are.

Hot gas from the furnace is preferably used as the heat carrier. For this reason, the furnace preferably comprises a device for sucking in hot gas in the furnace and for introducing the hot gas into the line system, this device advantageously forming a suction opening in the upper region of the stove.

Some particularly advantageous embodiments of the invention will now be described below with reference to the accompanying figures. 1 shows a schematic overall view of a rotary hearth furnace for producing sponge iron; Fig.2: a section of a plan view of a rotary hearth of a first

Design of an oven according to the invention; 3 shows a section along the line AA of Figure 2; 4 shows a section along the line BB of FIG. 2; 5 shows a section along the line CC of Figure 2; 6 shows a section along the line CC of another variant of the rotary hearth; 7: a view according to FIG. 2 of a second embodiment of an oven; 8: a section along the line AA of FIG. 7; 9 shows a section along the line BB of FIG. 7; 10 to 12: alternative gas ducts in an oven according to FIGS. 7 to 9; Fig. 13: an alternative course of the pipe system

Fig. 14: an embodiment with an electric heating element

A rotary hearth furnace 10 for producing sponge iron is shown schematically in FIG. The furnace comprises an annular rotary hearth 12 which is rotatably mounted on a foundation and which is surrounded on its upper side by a housing 14 (the housing is shown partly in section for a better understanding). Within the housing 14, the reduction of iron oxide to directly reduced iron takes place in a controlled atmosphere at high temperatures of approximately 1200-1400 ° C. For this purpose, in a first area 16 of the rotary hearth furnace, fine-grained iron oxide and fine-grained coal dust are charged in separate, superimposed layers onto a refractory lining of the rotary hearth 12 by means of a charging device 18. It is possible to charge only one layer with iron oxide and one layer with coal, or several layers of the individual materials can be stacked alternately. Alternatively, the iron oxide and the reducing agent can also be pressed together into pellets and charged as such onto the rotary hearth 12.

After charging, the iron oxide and the coal dust reach the reaction area 20 of the rotary kiln through the rotation of the rotary hearth 12. In this region 20 of the rotary kiln 14 burners are placed in the housing 22 are ^¬ which heat the furnace interior to the required reaction temperature of about 1200-1400 ° C. The hot exhaust gases from the burners 22 are passed through the furnace in a countercurrent process and discharged through a chimney 24. In the inert atmosphere prevailing in the furnace 10, the carbon dust releases carbon monoxide, which reduces the iron oxide to iron. After the reduction in the reduction area 20 of the furnace is completed, the finished sponge iron is either in pure form in one or more superimposed layers or together with the residues of the reducing agent in pellet form. This sponge iron then reaches the decharging area 26 of the rotary kiln, in which the sponge iron is removed from the furnace by means of a decharging device 28.

A first embodiment of an oven according to the invention will now be described with reference to FIGS. 2 to 5. This type of furnace is a so-called turntable furnace, in which the housing 14 is only two-sided in section (see FIG. 3 or 4) and delimits the annular furnace interior 30 upwards and radially outwards. The interior 30 is delimited downwards and radially outwards by the rotary hearth 12. For this purpose, it is designed as a horizontally lying, annular disk which is rotatably mounted on the foundation (not shown) and which has an annular, upwardly extending extension 32 on its inner edge. Since the rotary hearth 12 rotates with the attachment 32 relative to the housing 14 which is mounted in a manner fixed against relative rotation, the interior 30 of the oven is sealed off from the surroundings, for example by water cups 34. In the upper region, the rotary hearth 12 has a plurality of line systems 36 distributed in the circumferential direction, which can be acted upon by a hot gas for heating the heat-resistant cladding of the rotary hearth 12. In the illustrated embodiment, each line system 36 comprises two radially extending pipes 38, 40, which extend essentially over the entire charging width of the rotary hearth 12 and which are connected to one another at their radially outer end by a channel 42. On the radially inner side, the pipeline 38 opens into a feed channel 44 for the hot gas, while the pipeline 40 opens into a discharge channel 46 for the heating gas, which is embodied in the lower region of the rotary hearth 12. Both the feed channel 44 and the discharge channel 46 extend in the circumferential direction over a certain angular range of the rotary hearth 12. All Pipe systems 36 in this angular range are supplied with hot gas via the common feed channel 44, which is then discharged via the common discharge channel 46. The rotary hearth 12 is consequently divided into different zones which can be acted upon independently of one another by hot gas by means of a plurality of feed channels or discharge channels arranged next to one another in the circumferential direction.

The feed channel 44 is supplied with hot gas via one or more intake pipes 48 distributed over the length of the feed channel 44, which are arranged in the neck 32 and form an opening 50 in the upper region of the interior 30 of the furnace. During operation, hot gas is drawn in through the intake pipes 48 in the upper region of the furnace and passed into the feed channel 44. The distribution of the hot gas to the various line systems 36 opening into the feed channel 44 then takes place in the feed channel 44.

The hot gases are drawn in and passed through the line systems 36 advantageously by means of a negative pressure generated in the respective discharge duct 46. The discharge duct 46 is connected to an annular suction duct 52 in the non-rotatable substructure of the furnace. The transition between the discharge duct 46 in the rotary hearth 12 and the suction duct 52 is preferably implemented by means of water cups. A pump 54 connected to the suction channel 52 provides the necessary negative pressure and promotes the sucked-in gases for exhaust gas cleaning.

It should be noted that after passing through the line systems 36 of the rotary hearth 12, the hot gases have given off a large part of their energy and have accordingly experienced a significant cooling. This makes it possible to pump out the gases without complex and expensive special pumps. Exhaust gas cleaning is also made considerably easier.

The suction channel 52 is preferably divided into various subchannels in the circumferential direction of the furnace, each subchannel being connected to its own pump system. By regulating the pump powers for the different subchannels differently, the amount of hot gas drawn in via the line systems 36 can be regulated differently depending on the furnace area become. In this way, it is possible to individually control the heating power of the zone of the rotary hearth 12 located in that particular furnace area by means of the amount of hot gas drawn off in a certain furnace area.

It should be noted that the amount of hot gas drawn off can alternatively be regulated by different cross sections of the suction channel 52 in the different furnace areas. In this case, the suction channel 52 can be designed continuously and can be subjected to a uniform negative pressure.

If the hot gases drawn into the furnace contain combustible constituents, then after-combustion of the hot gases is advantageously initiated when the gases are passed through the line system 36. For this purpose, the feed channel 44 has a gas inlet 56 through which an oxygen carrier, e.g. Fresh air, sucked in or can be blown under pressure into the feed channel 44. The combustible constituents of the hot gases are ignited by the supplied oxygen, the temperature of the hot gases led through the line system 36 is increased and the heat transfer to the charged material layers is improved.

FIGS. 5 and 6 show, in a section along the line CC of FIG. 2, different possibilities for arranging the line systems 36 in the rotary hearth 12. In a first embodiment (FIG. 5), the pipes 38, 40 are firmly installed in the upper layer 58 of the heat-resistant material of the rotary hearth 12. The heat-resistant material is, for example, a refractory concrete that is applied to a base of the rotary hearth 12. The pipes 38, 40 and the connecting channels 42 are placed on the substructure before the refractory concrete is applied and then poured into the concrete. It should be noted that if non-heat-resistant pipelines are used, these can be burned out after the concrete has hardened, so that only the cavities that define the piping system 36 remain in the concrete layer. In a second embodiment (FIG. 6), the heat-resistant cladding of the rotary hearth 12 comprises a layer 60 of heat-conducting material, which is applied to an underlying layer of refractory concrete. In this case, the pipes 38, 40 are placed on the layer of refractory concrete and simply covered with the material. This design of the rotary hearth 12 facilitates later replacement of defective pipes.

A second embodiment of an oven according to the invention is described with reference to FIGS. 7 to 9. This type of furnace is a so-called rotary hearth furnace, in which the housing 14 is formed on three sides in section (see FIG. 8 or 9) and delimits the annular furnace interior 30 upwards and radially inwards and outwards. The interior 30 is delimited at the bottom by the rotary hearth 12 and sealed from the surroundings by means of water cups 34. The line systems 36 'each comprise a pipeline 38' which extends radially outwards from the inner edge of the rotary hearth 12 and is returned radially inwards in an arc. Each pipe 38 'is connected to its own suction pipe 48, which is arranged in the neck 32 of the rotary hearth 12 and forms an opening 50 in the upper region of this neck 32. It should be noted that the approach 32 does not have to take over the function of the furnace limitation in this embodiment. It only serves to accommodate the intake pipes 48 for the various pipes 38 '.

The radially inward end of the pipeline 38 'is led downwards out of the rotary hearth 12 and, analogously to the discharge duct 46 of FIGS. 3 and 4, is connected to a system for generating a negative pressure.

FIGS. 10 to 12 show alternative hot gas guides in a rotary hearth furnace, the rotary hearth 12 of which does not have an extension 32 which extends upwards. The simplest variant of such a gas guide is shown in FIG. 10. Each pipe system 36 comprises a pipe 38 ", which extends radially over the entire width of the rotary hearth 12 and opens on both sides into the radial boundary surfaces of the rotary cooker 12. In between, the pipe 38" is connected to a suction nozzle 64 which leads downward out of the rotary hearth 12. As described above, a vacuum is applied to the pipeline 38 "via the suction connection 64, so that the hot gases are sucked in via the radial annular gaps 62 between the rotary hearth 12 and the respective radial housing wall. As described above, the hot gases are preferably in the upper one In a further variant (FIG. 11), the furnace therefore has a suction pipe 48 'which is arranged, for example, in the radially inner furnace wall 66 and forms an opening 50' in the upper region of the interior 30 of the furnace at the other end, the suction pipe 48 'opens into a feed channel 68 which is connected to the pipeline 38'".

The pipeline 38 '"extends radially essentially over the entire width of the charging surface of the rotary hearth 12 and is guided at its ends downwards out of the rotary hearth 12. Below the rotary hearth 12, the connection to the feed channel 68 is made on the one hand and to a discharge channel 70 on the other hand, the sealing between the rotating ends and the non-rotatable channels 68 and 70 takes place in a known manner.

An alternative embodiment of the gas transfer between the non-rotatable gas supply and discharge is shown in FIG. 12. The rotary hearth 12 has on its radial boundary surfaces in each case an annular extension 72 or 74 which engages in a corresponding annular channel 76 or 78 in the adjacent furnace wall. The pipeline 38 "" opens on both sides of the charging surface in the radial boundary surface of the respective annular extension 72 or 74. The hot gases are then supplied via the ring channel 76, into which the suction pipes 48 'open, while the suction takes place via the ring channel 78. It goes without saying that the configurations described above represent only a few examples of possible gas flows into the furnace. It will be clear to the person skilled in the art that the configurations described above can also be combined with one another. Furthermore, it is clear that, as an alternative to the essentially radial gas ducts described, line systems can be provided which extend in serpentine lines essentially in the circumferential direction. Such an embodiment is shown for example in FIG. 13.

Another variant for heating the rotary hearth 12 is shown in FIG. 14. In this embodiment, the heating device comprises electrical heating elements 80 which e.g. extend in the radial direction over the rotary hearth 12. The advantage of such an electrical heating device lies in the more precise regulation of the heating power supplied.

Claims

claims

1. A method for heating materials in an oven with a rotary hearth, wherein at least one layer of material is charged onto the rotary hearth and a hot gas is applied from above, characterized in that the material layer is additionally heated from below through the rotary hearth.

2. The method according to claim 1, characterized in that the heating of the material layer is carried out from below by electrical heat generation.

3. The method according to claim 1, characterized in that the heating of the material layer is carried out from below by passing a heat transfer medium through the rotary hearth.

4. The method according to claim 3, characterized in that the heat transfer medium comprises hot gas.

5. The method according to claim 4, characterized in that the hot gas is sucked into the furnace.

6. The method according to claim 5, characterized in that the suction of the hot gas takes place in the upper region of the furnace.

7. The method according to any one of claims 4 to 6, characterized in that when the process gas is passed through the rotary hearth, the hot gas is afterburned.

8. The method according to any one of the preceding claims, wherein the furnace is divided into different adjacent zones in the circumferential direction, characterized in that a heating power supplied to the rotary hearth is individually controlled in each zone of the furnace.

9. furnace with rotary hearth, the rotary hearth of its upper side being surrounded by a housing, and wherein means are provided in the housing in order to apply a layer of material to the rotary hearth from above with a heating gas, characterized by at least one heating element which in the Rotary hearth is arranged.

10. Oven according to claim 9, characterized in that the rotary hearth is divided into different zones and that a heating element is arranged in each zone.

11. Oven according to one of claims 9 or 10, characterized in that the heating element comprises an electric heating element.

12. Oven according to one of claims 9 or 10, characterized in that the heating element has a line system arranged in the rotary hearth, which can be acted upon with a heat transfer medium.

13. Oven according to claim 12, characterized in that the heat transfer medium comprises a hot gas.

14. Oven according to claim 13, characterized by a device for drawing in hot gas in the oven and for introducing the hot gas into the line system.

15. Oven according to claim 14, characterized in that the device for sucking in the hot gas has a suction opening in the upper region of the

Oven.

16. Use of a furnace according to one of claims 8 to 15 for the direct reduction of iron oxide.