MXPA97002040A

MXPA97002040A - Procedure to reduce a material containing oxide and plant to carry out the procedimie

Info

Publication number: MXPA97002040A
Application number: MXPA/A/1997/002040A
Authority: MX
Inventors: Werner Kepplinger Leopold; Zimmerbauer Karlheinz; Zeller Siegfried; Hubert Whipp Roy Jr
Original assignee: Brifer International Ltd; Voestalpine Industrieanlagenbau Gmbh
Priority date: 1994-09-27
Filing date: 1997-03-18
Publication date: 1997-06-01

Abstract

The present invention relates to: invention relates to a process for reducing granular material that contains oxidation, by means of fluidization, wherein the material that contains oxide, is maintained in a reducing gas that flows upwards, in a fluidized bed, and is thus reduced . In order to avoid or considerably reduce the interruptions caused by adhesion or fouling, the velocity of the empty tube of the reduction gas is continuously reduced exclusively above the fluidized bed (24) over the complete unoccupied cross section (31,32,33) of a space on the fluidized bed (24), which prevents the formation of swollen

Description

PROCEDURE TO REDUCE A MATERIAL THAT CONTAINS OXIDES, AND PLANT TO CARRY OUT THE PROCEDURE DESCRIPTION OF THE INVENTION The present invention relates to a process for reducing oxide-containing granular material, particularly in a fine ore, by a fluidized-bed process, wherein the oxide-containing material is maintained in a fluidized bed by means of a reducing gas which it flows from the lower part to the upper part, thus being reduced, as well as a plant to carry out the process. A process of this type is known, for example, from US-A-2,909,423, WO 92/02458 and EP-AO. 571, 358. In this case the reduction of the oxide-containing material, for example of fine ore, is carried out in a fluidized bed maintained by a reducing gas inside a fluidized bed reduction reactor, wherein the reducing gas, which is injected into the fluidized bed reduction reactor by a nozzle grid, flows through the reduction reactor from the bottom to the top, while the oxide-containing material passes through the reduction reactor in approximately a transverse flow relative to the flow of the reducing gas. To maintain the fluidized bed REF: 24320 a certain speed of the reducing gas is required within the fluidized bed zone. Due to the relatively high speed of the reducing gas, a very fine particle entrainment of the oxide-containing material takes place and as the reduction process progresses, the oxide-containing material already reduced from the fluidized bed is entrained, leaving these particles very fines contained in the reducing gas. In order to separate such very fine particles from the reducing gas so that, on the one hand, the partially oxidized reducing gas can be used further, for example, for the above-mentioned reduction reactors and, on the other hand, the material containing already reduced oxide or material that would otherwise be lost -, the reducing gas containing said very fine particles is conducted through dust separators, such as cyclones, the separated dust being recycled to the fluidized bed. The dust separators or cyclones are preferably disposed within the reactors (cf US-A-2,909, 23); but they can also be installed outside the reactors. In practice, it has been shown that partially or completely reduced fine particle size particles of the oxide-containing material tend to adhere to or form cakes with each other and / or adhere to the walls of reactors or cyclones, as well as connecting ducts. and transport conduits. These phenomena are called "adhesion" or "fouling", respectively. Adhesion or fouling depends on the temperature and the degree of reduction of the oxide-containing material. Because the material containing partially or completely reduced oxide adheres to, or is deposited on, the walls of the reduction reactors or other parts of the plant, failures can occur such that it is not possible to operate the plant continuously. an extended period of time without detentions. It has been shown that the plant should be stopped every three to four months. The removal of deposits and cakes is labor-intensive and involves high costs, that is, labor costs and costs that arise from the loss of production from the plant. Frequently, such deposits are automatically released, thus falling to the fluidized side and interfering with the reduction process or - in the case of deposits separating from a cyclone - causing the obstruction of the dust recycling channels leading from the cyclone to the fluidized bed, and thereby making a subsequent dust separation from the reduction gas totally unrealizable. The invention seeks to avoid these disadvantages and difficulties, and aims to provide a process of the initially defined type, as well as a plant to carry out the process, by means of which the reduction of the granular material containing oxide can be carried out for very long periods of time without risk of interruptions of operation caused by adhesion or fouling. According to the invention, this object is achieved when the speed in empty tube of the reducing gas, exclusively above the fluidized bed, is reduced over the entire free cross-section of a space above the fluidized bed, continuously and avoiding eddy formations. With this measure, adhesion and fouling are effectively prevented despite a high speed of the reducing gas within the fluidized bed, decisively reducing the discharge of material containing oxide or partially or completely reduced material, respectively, by middle of the reducing gas. It has been shown that adhesion and fouling are extremely reduced as soon as the amount of particles entrained by the reducing gas falls below a given maximum value, so that the interruptions of operation caused by them will be avoided. It is very important that the flow of reducing gas above the fluidized bed proceeds substantially such that swirling does not occur. Since such eddies would decisively impede the possibility of reduced discharge of very fine particles. Therefore, the principle of the present invention is that the flow velocity of the reducing gas, that is, its velocity in an empty tube, must be decreased over the total free cross-section 5 of the space above the fluidized bed.; there should not be formations of eddies. By reducing the speed in empty tube of the reducing gas, longer and uninterrupted periods of operation can be expected not only from the reduction reactors, but also from the cyclones and from all the other parts of the plant that tend adhesion and fouling (conveyor tubes, etc). A further advantage of the invention can be observed in that the portion of fine granulometry contained in the material ' . Granular containing oxide can be increased without risk of disturbances in the process, thereby providing increased flexibility of the reduction process. Since it has been shown that adhesion and fouling occur to an increased degree when the The oxide-containing material already exhibits a high degree of reduction, is provided, according to a preferred embodiment of the invention, in the stepwise reduction of the oxide-containing material in fluidized beds arranged consecutively, in which an increase in the grade 23 of reduction from stage to stage, to decrease the speed in empty tube of the reducing gas from that stage from which a minimum degree of reduction of 25% is obtained. Preferably, the empty tube velocity of the reducing gas is decreased from that stage from which a minimum degree of reduction of 50% is obtained. A substantial increase in the uninterrupted period of operation of a plant is achieved if the vacuum tube velocity of the reducing gas above the fluidized bed is decreased by at least 25%, preferably by at least 50%, with the velocity in an empty tube of the reducing gas advantageously decreased from 0.8 m / s to 1.5 m / s on the inlet side up to 0.4 m / s at 0.75 m / s on the outlet side. Preferably, the empty tube velocity in the fluidized bed and in a space above that space in which the empty tube velocity of the reducing gas is decreased, is kept substantially constant. A plant for carrying out the process, comprising at least one fluidized bed reduction reactor, is characterized in that the fluidized bed reduction reactor comprises: a lower cylindrical fluidized bed section which houses the fluidized bed and includes a gas distribution bottom, a supply duct for the reducing gas and a supply duct and a discharge duct for the oxide-containing material, provided above the lower gas distribution bottom, • a tapered section that widens conically upward disposed above the fluidized-bed section and thereafter, the inclination of the wall of the tapered section relative to the central axis of the reactor being equal to at most 10 °, and • at least one section of partially cylindrical stabilization following the tapered section, said stabilizing section being closed at its upper part, and from to which comes a reducing gas discharge conduit. Conventionally, the empty tube cross section in the tapered section increases by at least 25%, preferably at least 50%, between the inlet cross section and the outlet cross section of the tapered section. Preferably, at least one dust separator is disposed within the reduction reactor, its inlet port for the powder-laden reducing gas being provided in the region of the stabilizing section, and a powder recycling conduit of the dust separator arrives. to the fluidized bed section.

The intermittency of the flow, and thus the formation of eddies of the reducing gas within the tapered section, is safely avoided if the inclination of the wall of the tapered section relative to the central axis of the reactor is 8o at the most, the inclination of the wall of the tapered section relative to the central axis of the reactor conveniently varying between 6 ° and 8 °. If the plant comprises a plurality of fluidized bed reduction reactors arranged consecutively and connected by means of reducing gas conduits and by means of conduits transporting the oxide-containing material from one reactor to another reactor, merely the reactors disposed at the end, viewed in the direction of flow of the oxide-containing material, preferably only the last disposed reactor, is provided with a tapered section. Next, the invention will be explained in more detail by means of the drawings, wherein Figure 1 illustrates a process diagram according to a first embodiment, and Figure 2 shows a cross section through a fluidized bed reduction reactor of according to FIG. 1 along line II-II of FIG. 1. In FIG. 3, the stepwise arrangement of four reduction reactors according to another embodiment is illustrated schematically.

The plant according to the invention comprises four fluidized bed reduction reactors 1 to 4, arranged consecutively in series, wherein a material containing iron oxide, such as fine ore, is supplied to the first fluidized bed reduction reactor 1. through a feed pipe of ore 5, and it is conducted from a fluidized bed reduction reactor to another fluidized bed reduction reactor via conveyor conduits 6, and the completely reduced material (sponge iron) is hot briquetted in a Briquetting arrangement 7. If required, the reduced iron is protected from reoxidation during briquetting by an inert gas system not illustrated. Before introducing the fine ore to the first reduction reactor 1, it is subjected to a preparation of ore, such as drying and screening, not illustrated in detail. The reduction gas is conducted in countercurrent to the ore flow from a reduction reactor 4 to another reduction reactor 3 to 1, and is extracted from the last reduction reactor 1, seen in the direction of the gas flow, as a gas of head through an overhead gas discharge conduit 8, and it is cooled and purified in a wet scrubber 9. The production of reducing gas is effected by reforming in a reformer 10 fed with natural gas through a conduit 11 and desulfurized. in a desulphurisation plant 12. The reformed gas formed from natural gas and steam consists essentially of H2. CO, CH4, H20 and C0. This reformed natural gas 13 flows to different heat exchangers 14, in which it is cooled, thus condensing the water of the gas. The reformed gas duct 13 opens into the upper gas discharge duct 8 after the overhead gas has been compressed by means of a compressor 15. The mixed gas thus formed is conducted through a C02 scrubber 16. and released from C02, then being available as a reducing gas. This reducing gas driven by a reducing gas supply conduit 17, is heated to a gas reducing temperature of about 800 ° C in a gas heater 18 arranged after the CO2 scrubber 16, and is fed to the first bed reactor fluidized 4, visualized in the direction of gas flow, where it reacts with fine ores to produce directly reduced iron. The 4 to 1 reduction reactors are connected in series; the reduction gas passes from one reduction reactor to another reduction reactor through the connection conduits 19. A portion of the overhead gas is set aside from the gas circulation circuit 8, 17, 19 in order to avoid enrichment of inert gases, such as N2. The overhead gas is fed through a branch 20 to the gas heater 18 to heat the reduction gas, and is burned there. The possible energy shortages are supplemented by natural gas supplied through a supply conduit 21. The sensible heat of the reformed gas leaving the reformer 10, as well as the reformer smoke gases is used in a recuperator 22 to preheat the Feeding gas (= mixture of natural gas - water vapor) during its passage and to produce the steam required for the reformation. The combustion air supplied to the reformer 10 is also preheated. According to the embodiment of the reduction plant illustrated in FIG. 1, the two reduction reactors disposed at the last 3 and 4, visualized in the transport direction of the fine ore, are constructed in the following manner (cf. 2): Each of the reduction reactors 3, 4 comprises a lower cylindrical fluidized bed section 25 which houses a fluidized bed 24, and provided at a predetermined level with a gas distribution bottom designed as a grid of nozzles 26. to supply and distribute the reducing gas uniformly. The reducing gas flows through the reduction reactor 3 and 4, respectively, from the lower part to the upper part starting from the nozzle grid 26. Above the nozzle grid 26, even inside the cylindrical fluidized-bed section 25, conveyor conduits 6 (supply and discharge conduits) enter. for fine ore. The fluidized bed 24 has a bed height 27 from the nozzle grid 26 to above the level of the supply and discharge ducts 6 for the fine ore. A tapered section 28, which conically widens upwards, follows the cylindrical section of fluidized bed 25, the inclination of the wall 29 of this tapered section 28, relative to the center axis 30 of the reactor, reaching up to 10 ° at sumo, preferably 6th to 8th. Due to the gradual enlargement of the cross-section 31, an increasingly gradual decrease in the speed in the empty tube of the reducing gas flowing upwards takes place in that region. The enlargement of the cross-sectional space must be carried out in such a way that the empty tube cross section within the tapered section 28, from the inlet cross section 32 to the outlet cross section 33, is at least 25%, preferably at least 50%. Due to the only slight inclination of the wall 29 of the tapered section 28, it is feasible to obtain a flow without swirling and separation of the stream from the wall 29 despite the cross section 31 which widens in that tapered section 28. In this way, the formation of eddies, which would cause a local increase in the speed of the reducing gas, is avoided. Therefore, the uniform and continuous reduction of the speed in empty tube of the reducing gas is ensured in the total cross section 31 of the tapered section 28 at any level thereof. On the upper end of the tapered section 28 follows a stabilizing section 35, which is provided with a cylindrical wall 34 and is closed upward with a partially designed spherical roof 36. On the roof 36 'of the reactor located above of the roof 36, an opening 37 is centrally provided for extracting reducing gas, said reducing gas being fed to the preceding reduction reactor 3 and 2, respectively, through the connection conduit 19 that follows the opening 37. Cyclones 38 are provided inside the reduction reactor, arranged in the cylindrical part of the stabilizing section 35 which serve to separate the powder of the reducing gas. Dust recycling ducts 39 emerging from the cyclones 38 are directed vertically downwards, meeting the fluidized bed 24. The gas discharge ducts of the cyclones 38 reach the space provided between the roof 36 and the roof 36 'of the _ J reactor.

The reduction grades of the fine ore in the reduction reactor 1 are approximately 8%, in the reduction reactor 2 they are approximately 31%, in the reduction reactor 3 they are approximately 72% and in the reduction reactor 4 they are approximately 95%. %. By reducing the speed of the reducing gas in the two reduction reactors 3 and 4 from 1.2 m / s in the fluidized-bed section 25 to 0.6 m / s in the upper end of the tapered section 28, the dust loading of the cyclones 38 can be reduced from about 3000 g / m3 to about 650 g / m3. As a result, an average extension of more than six months can be achieved in the period in which the plant can remain in operation without interruption. According to the embodiment shown in Fig. 3, from one part of the reduction plant, three reduction reactors 1 to 3 are again arranged to be connected consecutively, although only the reduction reactor 4 disposed last in the flow direction of the fine ore is provided with a section tapered 28. Although that reduction reactor disposed to the latter has a metallic outer wall designed to be tapered to the bottom of the reactor, a fluidized bed section 25 is also provided there, which is bounded by a cylindrical wall, as it is. evident from the dashed lines indicating the inner wall 40 of the reduction reactor 4. The degree of reduction of the fine ore introduced to the last reduction reactor is more than 72%. Also in that case, an average extension of six months could be reached in the period of uninterrupted operation of the plant. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims

1. A method for reducing a granular material containing oxide, in particular a fine ore, in the fluidized bed method, wherein the oxide-containing material, by means of a reducing gas flowing from the lower part to the upper part, is maintained in a fluidized bed, being thus reduced, characterized in that the speed in empty tube of the reducing gas, exclusively above the fluidized bed, is decreased in the total free cross-section of a space present above the fluidized bed, continuously and avoiding the swirl formations.

2. A method according to claim 1, characterized in that the reduction of the oxide-containing material is carried out stepwise in fluidized beds arranged consecutively and under an increase of the degree of reduction from stage to stage, the empty tube velocity of the gas being Reduced reducer from that stage from which a minimum degree of reduction of 25% is obtained.

3. A method according to claim 1, characterized in that the speed in empty tube of the reducing gas is decreased from that stage from which a minimum degree of reduction is obtained. fifty%.

4. A method according to one or more of claims 1 to 3, characterized in that the empty tube velocity of the reducing gas above the fluidized bed is decreased by at least 25%, preferably by at least 50%.

5. A method according to one or more of claims 1 to 3, characterized in that the speed in empty tube of the reducing gas is decreased from 0.8 m / s to 1.5 m / s on the input side up to 0.4 m / s at 0.75 m / s on the output side.

6. A method according to one or more of claims 1 to 5, characterized in that the speed in empty tube in the fluidized bed and in a space above that space in which the speed in empty tube of the reducing gas is decreased, is maintained substantially constant.

7. A plant for carrying out the process according to one or more of claims 1 to 6, comprising at least one fluidized bed reduction reactor, characterized in that the fluidized bed reduction reactor comprises: • a lower cylindrical section fluidized bed that accommodates the fluidized bed and includes a gas distribution bottom, a supply conduit for the reducing gas and in addition a supply conduit and a discharge conduit for the oxide-containing material, provided above the distribution bottom of gas, • a tapering section that widens conically upwards disposed above the fluidized bed section and thereafter thereof, the inclination of the wall being of the tapered section, relative to the central axis of the reactor, being equal to 10 ° at the most, and • at least one partially cylindrical stabilizing section following the tapered section, is said stabilizing section closed on its upper part, and from which a reducing gas discharge conduit emerges.

8. A plant according to claim 7, characterized in that the cross-section of empty tube in the tapered section increases by at least 25%, preferably at least 50%, between the input cross section and the output cross section of the tapered section.

9. A plant according to claim 7 or 8, characterized in that at least one dust separator is arranged inside the reduction reactor, its inlet opening for the powder-laden reducing gas in the region of the stabilizing section, and a dust recycle duct of the dust separator extends to the fluidized bed section.

10. A plant according to one or more of claims 7 to 9, characterized in that the inclination of the wall of the tapered section relative to the central axis of the reactor is 8 ° at the most.

11. A plant according to claim 10, characterized in that the inclination of the wall of the tapered section relative to the central axis of the reactor varies between 6 ° and 8 °.

12. A plant according to one or more of claims 7 to 11, characterized in that the plant comprises a plurality of fluidized bed reduction reactors arranged consecutively, connected by means of reducing gas conduits and by means of conveyor conduits carrying the material which contains oxide from one reactor to another reactor, and in which merely the reactors disposed to the latter, seen in the flow direction of the oxide-containing material, preferably only the reactor disposed last, are provided with a tapered section.