WO2000047780A2

WO2000047780A2 - Method and apparatus for preheating of direct reduced iron used as feed to an electric arc furnace

Info

Publication number: WO2000047780A2
Application number: PCT/IB2000/000607
Authority: WO
Inventors: Juan A. Villarreal-Trevino
Original assignee: Hylsa SA de CV
Current assignee: Hylsa SA de CV
Priority date: 1999-02-02
Filing date: 2000-02-01
Publication date: 2000-08-17
Anticipated expiration: 2001-08-02
Also published as: AU4139100A; WO2000047780A3

Abstract

A method and apparatus for preheating of direct reduced iron or DRI (10) prior to its introduction to a metallurgical furnace like an electric arc furnace (70) by contacting the DRI (10) with a non-oxidizing, reducing or carburing gas (44) at a high temperature so that electric demand in the furnace (70) can be reduced, the tap-to-tape time can be decreased, whereby the productivity of the furnace (70) as well as the carbon content of the DRI to be charged can be increased.

Description

METHOD AND APPARATUS FORPREHEATING OF DIRECT REDUCED

IRON USED AS FEED TO AN ELECTRIC ARC FURNACE

FIELD OF THE INVENTION The present invention relates to a method and apparatus for preheating direct reduced iron, DRI (sometimes called sponge iron), or the like, used as feed material for a metallurgical furnace, typically an electric arc furnace. The majority of the DRI produced worldwide leaves the direct reduction reactor cold, especially when the DRI is to be transported or stored. To preheat said materials without re-oxidation while preserving or incrementally adjusting carbon content, a hot inert, reducing, or carburizing atmosphere is used prior to charging the materials to the electric arc furnace (EAF).

The development of new processes and improvements to established techniques and apparatus has always been related to the motivation to discover cheaper processes or apparatus, with lower costs, lower investment, environmental safety, and improved quality. Although economic savings have been the primary goal during the past years, quality requirements have become increasingly important and indispensable for all steel producers. Within this scope and new vision, the present invention describes an improvement in steelmaking processes, specifically an improved method and apparatus for preheating the direct reduced iron (DRI), sponge iron, or the like, fed to an EAF.

U.S. Patent No. 4,290,800 to Sensis et al. describes preheating the cold DRI by providing a stream of oxidizing hot gas. This and the other patents cited herein are incorporated by reference. This method ignites the carbon present in the DRI, thereby heating the ore material but causes a decrease in the carbon content of said DRI. To avoid this problem, the patentees suggest saturating the DRI with oil. The hydrocarbons contained in the oil take over the protective function of the carbon to avoid re-oxidation of the sponge iron. The saturating agents used are preferably heavy oils which are produced in the refining of crude oil or petroleum. Since the characteristics and properties of the steel depend on its composition, sulfur is one of the most important elements to watch to avoid an excess. Heavy oils are well know to contain large quantities of sulfur compounds that could cause higher sulfur content in the steel composition.

U.S. Patent No. 3,929,459 to Tress & Hunter; U.S. Patent No. 4,385,889 to Yasukawa; U.S. Patent No.4,280,836 to Maeda and some others, show the utilization of the heating potential of the effluent gas from the EAF to preheat the feed materials. These are examples of methods and apparatus that are commonly used in EAF melt shops for the preheating of iron scrap, because they are not so expensive and diminish the energy required by the EAF to heat and melt the solid material. Although EAF effluent gas is useful to preheat iron scrap, it is not suitable to preheat the DRI. The oxidizing atmosphere created by the effluent gas of the EAF would re-oxidize the metallic iron content of the DRI (counteracting the direct reduction which produced the DRI).

U.S. Patent No. 4,642,048 to Kim shows utilization of the heating potential of the effluent gas from the EAF to preheat DRI as the feed material, but does not address the problem of re-oxidation that occurs from exposing the DRI to the hot oxidizing EAF effluent gas.

U.S. Patent No. 3,163,520 to Collin et al. discloses a process wherein a non- oxidizing gas is used for preheating a charge of iron ore and a carbonaceous material such as coke or coal. Even though the Collin teaches that this process can be advantageously used for preheating the charge without any or negligible pre- reduction, the object of the process is to provide the heat for increasing the temperature of the ore pellets and to supply the heat of reduction that will take place in a further step as in an electric pig-iron furnace or in an EAF. In the present application, the EAF charge is not iron ore, but DRI with a degree of metallization of at least 85%, thus a further reduction of the charge is not intended to be performed in the melting furnace.

The present invention has the advantage of using a reducing or inert atmosphere to preheat the DRI, avoiding the re-oxidation of the feeding charge and achieving a more cost-effective way to produce steel, such as by reducing the electric demand in an electric arc furnace (using instead a less expensive heat source for at least the initial heating) and even more importantly, decreasing the heating time and the overall tap-to-tap time thereby increasing the productivity of the EAF.

Although DRI has been known for most of this century and has been produced on a large commercial scale increasingly since the late 1950's (with energy considerations being of particular importance since at least the energy crises starting in the early 1970's); nevertheless, the pre-heating of cold DRI prior to feeding to an EAF in a non-oxidizing or carburizing atmosphere (so as not to re-oxidize the reduced iron) is believed never to have been thought of nor practiced prior to the inventive insight of the applicant. In fact, patent 4,290,800 teaches just the opposite (using oxidizing gas to pre-heat the sponge iron feed to the EAF). SUMMARY OF THE INVENTION

The present invention describes a method for producing steel in a metallurgical furnace, more specifically a method for preheating and carburizing direct reduced iron (DRI), sponge iron, or the like with a non-oxidizing gas and feeding said preheated material to a metallurgical furnace.

Since the cost of producing electric energy seems to increase almost day by day, several methods for using alternative processes for decreasing the electric demand of energy by the metallurgical furnaces have been proposed (especially since the 1970's, showing a long-felt want). With this necessity and with the object to avoid detrimental re-oxidation of the DRI at the time of preheating and preferably of also advantageously carburizing the DRI to be charged, the present invention disclose methods and apparatus for achieving these objectives. The method herein described comprises charging the DRI at ambient temperature to a container or heating vessel, contacting a non-oxidizing and preferably carburizing gas stream at a high temperature with the DRI. After the DRI has reached a desired temperature, it is discharged and fed to said furnace. This method can be performed in a continuous or batch wise mode, preferably in the continuous mode.

The preferred embodiment of the invention comprises the preheating of said DRI with a hot non-oxidizing gas; which may be of a reducing and/or of a carburizing nature, or also may be an inert gas (like nitrogen) or a hydrocarbon-containing gas (such natural gas). One of the preferred embodiments of the present invention comprises maintaining the preheating system at a pressure above the atmospheric pressure in order to decrease the volume of heating gas flow through the system. The use of lock hopper means before and after the heating vessel is illustrated and described below with respect to the accompanying drawings as a means for isolating the reactive DRI from the oxygen in the external atmosphere, but other isolation means, such as gas seals, can be utilized for this purpose. The DRI having at least a metallization of 85% and a temperature lower than the desired feeding temperature to the EAF is fed to a heating vessel, that can be in the form of a hopper or a vertical shaft, and is contacted with a stream of hot non- oxidizing and/or carburizing gas within said vessel to increase the temperature of the DRI to the desired feeding temperature of the DRI to the EAF. When the preheating is continuous, the DRI flows downwardly through the vessel preferably with the hot gas flowing counter-currently. When it is desired to feed the hot DRI through the upper part of said furnace, one or more hoppers and conduit means could be used to feed it through the roof of said furnace. When it is desired to feed the hot DRI through the walls of said furnace one or more screw feeders can be used for this purpose. The metallurgical furnace is most commonly an electric arc furnace, but other furnaces such as induction furnaces can be used.

The non-oxidizing gas is preferably a reducing and/or carburizing gas, which is produced by a suitable a source and then is heated to a temperature in the range of about 400°C to about 1000°C, but it is to be understood that the higher the temperature the more beneficial is the utilization of the present invention. The reducing gas is preferred to comprise hydrogen, carbon monoxide and methane as its main components. This reducing gas can be produced by the reformation of natural gas with heat and steam, by coal gasification, by partial combustion, or by other methods known in the art, such as the thermal cracking of natural gas.

When the hot reducing gas is contacted within the heating vessel with the DRI, it transfers its heat potential through convective means to the DRI. The use of conductive means to preheat the DRI is not desired, since it is known in the art that the DRI is a low heat conductor. In this specification and in the accompanying drawings, the preferred embodiment of the invention is shown and described. Various alternatives and modifications thereof have been suggested, but it is to be understood that these are not intended to be exhaustive, and that many changes and modifications can be made within the scope of the invention. The suggestions herein described are selected and included for illustrative purpose only, in order that others skilled in the art will more fully understand the invention and the principles thereof and will thus be enabled to modify it in a variety of forms, each as may be best suited to the conditions of a particular use. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a preferred embodiment of the present invention. Figure 2 shows basically the embodiment illustrated in figure 1, but in lieu of lock hoppers, instead combines two valve-isolated preheating units which share the same source of hot gas. DETAILED DESCRIPTION OF THE INVENTION

Direct reduced iron (DRI), sponge iron, briquettes (HBI), or the like is fed as stream 10 at ambient temperature in a manner known in the art to an optionally open hopper 12. The DRI passes through a valve means 14 to a closed hopper 16, which is able to be pressurized, and then passes through valve 18 to preheater 20. The combination of the valve 14, hopper 16 and valve 18 forms the first lock hopper means. See U.S. Patent Nos. 3,710,808 and 4,498,498. A hot inert, reducing or carburizing gas is injected as stream 44 into preheater 20, where it passes through the bed of DRI to be pre-heated. After contacting the DRI, the gas stream exits the preheater 20 as an exhaust gas stream 22, then passes through the cooling and cleaning system 24, typically a water quench cooler, forming the cleaned gas stream 26. The clean gas stream 26 passes through compressing means 28 and forms the pressurized gas stream 30. Nitrogen, natural gas, carbon monoxide, hydrogen, combinations thereof or another suitable gas is fed as make-up gas 34 from a suitable source 32. Make-up gas 34 is combined with said pressurized gas 30 to form stream 36, which passes through heater 38 to form said the hot inert, reducing or carburizing gas 44. Heater 38 is supplied with a suitable combustible gas 40 through burner 42. As a supplementary fuel, the fumes produced by the EAF 70 can be quenched (removing water therefrom), and the resulting cooled gas fed to the same burner 42. The hot DRI is discharged through discharging means 50 and valve means 52, into hopper 54 (which with valves means 52 and 56 forms a system to pressurize and de- pressurize the discharge hopper 54 as the second lock hopper means). The pressure of the system can be from atmospheric pressure to about 3 Kg/cm but it can be performed at higher pressure depending on the economic factors that exist in a predetermined facility. If it is desired that the operating pressure is near the atmospheric pressure, the charge and discharge system can be greatly simplified by gas seals or other means as best suited to the operating conditions. The stream of hot DRI passing through valve 56 can be split to feed into two or more hoppers 60 and 64 respectively via conduit means 62 and 66 so as to be fed to different areas in the electric arc furnace 70; or could be introduced through screw feeder 68 through the walls of the EAF 70. See U.S. Patent No. 5,218,617.

In the second embodiment in figure 2, the two preheater units 100 and 100A are alternately pressurized so that lock hopper feed and discharge combinations are not needed. Instead, while the direct reduced iron is being heated in preheater 100 A by a hot gas flow therethrough, the preheater 100 is fed from an optionally open accumulating hopper 94 via an open isolating feed valve 96 as stream 98, and after first being discharged as stream 104 via a discharging-isolating valve 102 means or combination. Meanwhile, the direct reduced iron already charged into preheater vessel 100 A is isolated by closed valves 96 and 102 and heated by gas flowing in via the open valve in stream 46 and out through the open valve in stream 22. When preheater vessel 100 has been charged, the valves for streams 22 A and 46 A are closed so that it can be pressurized by opening the valves of gas streams 22 and 46. Vessel 100A can then be depressurized and discharged via discharging valve means 102 A and then recharged via open valve 96A. It will be recognized that since preheater 100 A mirrors preheater 100, the similar attachments of the one are identified by the same relative reference numbers as used in connection with the other, but differentiated by the addition of "A" to the respective reference numbers.

It is also be understood that, if desired, one embodiment of the present invention could feed hot DRI to one, two, or more metallurgical furnaces.

Claims

WHAT IS CLAIMED IS:

1. A method for producing steel in a metallurgical furnace, comprising the steps of feeding DRI to a vessel, preheating said DRI with a stream of non- oxidizing hot gas, and feeding said DRI at a high temperature to said metallurgical furnace.

2. A method according to claim 1, further comprising feeding said DRI to a hopper, passing said DRI through a first pressurizing lock hopper, feeding said DRI into said vessel under pressure, contacting said DRI with said stream of non-oxidizing hot gas within said vessel, discharging said DRI at a high temperature through a depressurizing second lock hopper, and feeding said depressurized DRI at high temperature to said metallurgical furnace.

3. A method according to claims 1 or 2, further comprising introducing said DRI at high temperature into said metallurgical furnace through its roof by means of a hopper feed system.

4. A method according to any one of the preceding claims, further comprising feeding said DRI at high temperature into said metallurgical furnace by means of a screw feeder through a wall of said metallurgical furnace.

5. A method according to any one of claims 1 to 4, wherein said high temperature of said DRI preheated is in a range from about 400°C to about 800°C.

6. A method according to any one of claims 1 to 4, wherein said non- oxidizing hot gas is a reducing gas comprising carbon monoxide, hydrogen and methane.

7. A method according to any one of the preceding claims, wherein said non-oxidizing hot gas is nitrogen.

8. A method according to any one of claims 1 to 6, wherein said non- oxidizing hot gas is natural gas.

9. A method according to claim 8, wherein said natural gas is cracked inside said vessel and carburizes partially the DRI.

10. A method according to any one of the preceding claims, further comprising feeding said stream of non-oxidizing hot gas to said vessel, contacting said DRI with said stream of non-oxidizing hot gas in said vessel, withdrawing said non-oxidizing hot gas from said vessel as a top gas, cooling said top gas, passing said cooled top gas through a compressor, adding additional non-oxidizing gas as a make up to said compressed stream of cooled top gas, heating said compressed stream to produce said stream of hot non-oxidizing hot gas.

11. A method according to any one of the preceding claims, wherein said metallurgical furnace is an electric arc furnace.

12. An apparatus for the production of steel which comprises: a) a source of hot non-oxidizing hot gas; b) a source of DRI; c) a heating vessel for containing DRI; d) a charging conduit connecting said source of DRI to said heating vessel; e) a metallurgical furnace; f) a discharging conduit connecting said heating vessel to feed said metallurgical furnace; g) isolating means for isolating said vessel during flow of said gas through DRI in said vessel; and h) gas conduit means connecting said source of hot non-oxidizing hot gas to and from said vessel.

13. An apparatus according to claim 12, further comprising: a) a first hopper functioning as said source of DRI; b) first lock hopper means for pressurizing batches of DRI to feed under pressure to said heating vessel and positioned in the charging conduit connecting said first hopper to said vessel; and c) second lock hopper means for depressurizing batches of DRI to feed under pressure from said heating vessel and positioned in the discharging conduit connecting said heating vessel to said metallurgical furnace.

14. An apparatus according to claims 12 or 13, further comprising: a) a gas heater for heating the non-oxidizing gas from said source; b) first gas conduit means connecting said heater and said heating

vessel; c) cooling means; d) second gas conduit means connecting said heating vessel and said cooling means; e) third gas conduit means connected to said source of non-

oxidizing gas for feeding to said gas heater; and f) pumping means connected between said cooling means and said gas heater.

15. An apparatus according to any one of claims 12 to 14, wherein said discharging means include a set of hoppers connected to the roof of said metallurgical furnace.

16. An apparatus according to any one of claims 12 to 14, wherein said discharging means comprises a screw feeder connected to said metallurgical furnace.

17. An apparatus according to any one of claims 12 to 16, wherein said metallurgical furnace is an electric arc furnace.