FURNACE FOR THE DIRECT REDUCTION OF IRON OXIDES

Abstract: Gravitational type furnace for the direct reduction of mineral iron comprising a median reaction zone (16) in which the reactions to reduce the mineral iron occur, means (12, 13) to feed the mineral iron to said reaction zone (16), distribution means (35, 36) to introduce reducing gas into said reaction zone (16), and a discharge zone (18) to discharge the reduced metal iron, above said median reaction zone (16) there being provided an upper zone of pre-heating and pre-reduction (15) into which an additional distributor (31) of reducing gas is inserted.

Description: FURNACE FOR THE DIRECT REDUCTION OF IRON OXIDES * * * * * FIELD OF THE INVENTION This invention concerns a furnace for the production of metal iron by means of the direct reduction of mineral iron, where the iron is present in the form of oxides. The furnace is of the gravitational type and is provided with an upper container from which the mineral iron, coarse or in the form of pellets, is introduced, and a discharge zone from which the directly reduced iron (DRI) is discharged. In an intermediate reduction zone, the furnace is provided with at least a circumferential distributor, provided with nozzles, through which reducing gas is injected. BACKGROUND OF THE INVENTION The state of the art includes furnaces of the gravitational type, or shaft furnaces, for direct reduction processes comprising an upper loading zone, a central part, substantially cylindrical or in the shape of a truncated cone, in which the reduction reaction occurs, means to inject reducing gas into the central zone, and a lower discharge zone, tapered, with the taper facing downwards. To achieve acceptable working conditions in the direct reduction of iron oxides, it is necessary, in the load column, to create conditions of uniform distribution of the reducing gas, both in the peripheral zones and also in the central zones of the load volume. On the contrary, in conventional direct reduction furnaces, filled with mineral iron, the currents of reducing gas prevalently affect the peripheral zone of the load column, so that in every transverse section of the furnace there is a different reducing potential which diminishes the iron reduction process in the whole load volume. The maximum gradient of reducing potential is in the upper part of the furnace, where the highest iron oxides dominate (Fe 2 U 3 , Fe 3 0 ) • A uniform distribution of the gas in the different transverse sections of the furnace, at different heights, depends on the hydraulic resistance of the layer of material loaded, the method used to inject the reducing gas and the intake of the gas in the upper part . Irregularities in the reduction process in the whole volume of the furnace lead to a worsening in the quality of the final product of the directly reduced iron (DRI) and a reduced production. From document FR-A-1.223.457 is known a furnace of the gravitational type wherein, in addition to the nozzles, all disposed in the peripheral wall of the furnace, through which the reducing gas is introduced in the central zone of the furnace, further additional nozzles are provided to introduce air into the furnace. Such additional nozzles are disposed both in the peripheral wall of the furnace and possibly also in one or more central elements disposed in the central zone of the furnace. However, also this solution does not solve the above-mentioned problems. In other devices of conventional type, the upper intake of the gas mainly occurs through peripheral areas of the working space of the furnace, which excludes a stable process of the reducing gas in the central zone of the furnace. This also entails an obstruction of the intake conduit due to the impurities which develop. The problems described above limit the diameters of conventional furnaces and consequently their productivity. Moreover, in direct reduction processes of iron oxides, it is necessary to break up the load of mineral in order to avoid high temperature sticking, in the transition zone between the zone where reduction occurs and the underlying discharge zone. During the reduction process, zones with different temperatures are formed in the furnace: from 300 to 400°C in the upper part and from 850 to 1000°C in the lower layers of the reduction zone. The zone of maximum temperature is critical, because in this zone sticking can limit the overall productivity of the reduction process. The state of the art includes load feeders which operate continuously and which only manage to break the load a little, instead of breaking it completely into fragments, without improving the permeability of the load by the reducing gas, with a consequent reduction in the total efficiency of the reduction process. Moreover, load feeders do not intervene upstream of the sticking, in order to prevent it, since they do not cause the reciprocal movement of the particles of the load, but only carry out a partial action of detachment of the parts of the load which have already stuck. The present Applicant has devised, planned and embodied the furnace for the direct reduction of iron oxides according to the invention to overcome these shortcomings, and to solve the problems which have not yet been resolved regarding an increase in the permeability of the reduction gas in the load, the uniform distribution of the reducing gas in the load volume in order to increase the interaction surface, the reduction of the quantity of gas, the intensification of the heat and volume exchange and the prevention of the sticking of the material introduced inside the furnace . SUMMARY OF THE INVENTION The furnace to produce metal iron by the direct reduction of iron oxides according to the invention is set forth and characterized in the main claim, while the dependent claims describe other innovative characteristics of the invention. The furnace according to the invention is of the gravitational or shaft type, wherein both the material and the gas are fed continuously, so as to create a vertical and gravitational flow of the material and so that the direct reduction of the mineral occurs . The reduction furnace is equipped with means to feed the mineral iron and means to discharge the reduced metal iron, and is provided with at least a reducing gas distributor arranged in correspondence with a reduction zone of the furnace. One purpose of the invention is to achieve a reduction furnace wherein, before entering the reaction zone proper, the material introduced is subjected to a pre-heating and pre-reducing process, so that it is possible to obtain a high productivity and a better quality of directly reduced iron (DRI) . In accordance with this purpose, the reduction furnace according to the invention is provided with an upper pre- heating and pre-reducing zone, arranged above the median reaction zone and into which an additional reducing gas distributor is inserted. According to one characteristic of the invention, the additional distributor comprises a vertical terminal arranged at the center of the upper zone and provided with radial nozzles. According to another characteristic of the invention, the vertical terminal is connected to a conduit of reducing gas which enters the upper zone substantially from a central part of the roof of the furnace. Another purpose of the invention is to achieve a reduction furnace in which there is a stable and uniform distribution of the reducing gas throughout the volume of the furnace. In accordance with this purpose, the reduction furnace according to the invention comprises a gas intake device, with an upper chamber having a toroidal grid arranged in proximity with the upper roof of the furnace and associated with the intake conduit . BRIEF DESCRIPTION OF THE DRAWINGS These and other characteristics of the invention will become clear from the following description of a preferred form of embodiment, given as a non-restrictive example, with reference to the attached drawings wherein: Fig. 1 is a sectioned side view of a furnace for the direct reduction of iron oxides according to the invention; Fig. 2 is an enlarged detail of Fig. 1. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT With reference to Figs. 1 and 2, a furnace 10 for the direct reduction of iron oxides according to the invention comprises a vertical container 11, shaped substantially like a truncated cone converging downwards, an upper loading tank 12 from which, through distribution tubes 13, the mineral load (iron oxides) is suitable to be introduced, an upper pre-heating and pre-reducing zone 15, wherein a first, partial reduction of the iron oxides takes place, a median reaction zone 16, or reactor, wherein the reduction reaction of the iron oxides takes place, and a lower zone or discharge zone 18. The vertical container 11 is indicatively about 30 m high and about 9 m wide in its upper part. In the upper part of the container 11, in proximity with the roof 20, an intake device 21 is provided, comprising a chamber 22 with a toroidal grid 23 and a conduit 24. The toroidal grid 23 is made with radial elements separated from each other with a variable pitch, so that the pitch has a minimum value in correspondence with the intersection between the chamber 22 and the intake conduit 24, and a maximum value in a zone diametrically opposite the chamber 22. At the center of the roof 20 there is inserted a conduit 30 having a vertical terminal part 31 provided with radial nozzles 32, through which a mixture of reducing gas is suitable to be injected into the pre-reduction zone 15, in direct contact with the load of material introduced therein from above . In correspondence with the reactor 16, the furnace 10 is provided with two circumferential distributors 35 and 36, each equipped with nozzles 40 through which a mixture of reducing gas, arriving from corresponding conduits 38 and 39, is suitable to be introduced. There are thus two reduction stages, upper and lower, created in the reduction zone 16. The mixture of reducing gas and the plant upstream of the conduits 30, 38 and 39 can be of any known type, for example of the type described in the PCT international publication No. WO-A-00/36156. The furnace 10 as described heretofore functions as follows: The mineral iron, for example in pellet form, is introduced into the furnace 10 from the upper container 12 through the distribution tubes 13, while the reducing gas is introduced from the conduits 30, 38 and 39 and distributed in the pre-heating and pre-reducing zone 15, through the vertical feeder 31, and also in the two stages of the reactor 16 through the circumferential distributors 35 and 36. The intake device 21 takes in the reducing gas upwards and encourages the diffusion of the latter and its distribution from the periphery towards the center of the reactor 16. The intake device 21 also creates a cyclone effect in the upper zone of the furnace, near the roof 20, which further encourages the uniform and homogeneous distribution of the reducing gas inside the pre-reducing zone 15 and the reduction zone 16. In said zones 15 and 16, various reactions take place in sequence, from the highest to the lowest iron oxides and, in the end, to metal iron. It is obvious that modifications and additions can be made to the furnace 10 for the direct reduction of mineral iron as described heretofore, but these shall remain within the field and scope of the invention. It is also obvious that, although this invention has been described with reference to specific examples, a person of skill in this field will certainly be able to achieve many other forms of equivalent furnaces, but these shall all come within the field and scope of this invention.

Claims: CLAIMS 1 - Gravitational type furnace for the direct reduction of mineral iron, comprising a median reaction zone (16) in which the reactions to reduce the mineral iron occur, means (12, 13) to feed the mineral iron to said reaction zone (16), distribution means (35, 36) to introduce reducing gas into said median reaction zone (16), and a discharge zone (18) to discharge the reduced metal iron, characterized in that an upper zone of pre-heating and pre-reduction (15) is provided above said median reaction zone (16) and that an additional distributor (31) of reducing gas is inserted into said upper zone (15) . 2 - Furnace as in Claim 1, characterized in that said additional distributor comprises a vertical terminal (31) arranged at the center of said upper zone (15) and provided with radial nozzles (32) . 3 - Furnace as in Claim 2, characterized in that said vertical terminal (31) is connected with a conduit (30) for reducing gas which enters said upper zone (15) substantially from a central part of the roof (20) . 4 - Furnace as in Claim 1, characterized in that an intake device (21) for the reducing gas is provided in correspondence with said upper zone (15) . 5 - Furnace as in Claim 4, characterized in that said intake device (21) comprises a toroidal grid (23) arranged in an upper chamber (22) of said upper zone (15) . 6 - Furnace as in Claim 5, characterized in that said toroidal grid (23) is arranged coaxially with said additional distributor (31) . 7 - Furnace as in Claim 5, characterized in that said intake device (21) comprises an intake conduit (24) arranged laterally with respect to said upper chamber (22), said toroidal grid (23) being made with radial elements separated from each other with a variable pitch from a minimum value, in correspondence with the intersection between said chamber (22) and said intake conduit (24), and a maximum value, in a zone diametrically opposite said chamber (22) . 8 - Method for the direct reduction of mineral iron by means of a gravitational type furnace having a median reaction zone (16) in which the reactions to reduce the mineral iron occur, comprising the steps of feeding the mineral iron to said reaction zone (16), introducing reducing gas into said median reaction zone (16), and discharging the reduced metal iron from said furnace, characterized in that additional reducing gas is inserted into an upper zone (15) of preheating and pre-reduction, provided above said median reaction zone (16) of said furnace. 9 - Method as in Claim 8, characterized in that said additional reducing gas is distributed by means of a vertical terminal (31) disposed at the center of said upper zone (15) and provided with radial nozzles (32) . 10 - Method as in Claim 9, characterized in that said reducing gas is introduced substantially from a central part of the roof (20) of said furnace through said vertical terminal (31) . 11 - Method as in Claim 8, characterized in that the gas is removed from the furnce through an intake device (21) provided in correspondence with said upper zone (15).

Inventors: PAVLICEVIC MILORAD, POLONI ALFREDO, PAVLICEVIC, MILORAD, POLONI, ALFREDO
Applicants: DANIELI OFF MECC, PAVLICEVIC MILORAD, POLONI ALFREDO, DANIELI & C. OFFICINE MECCANICHE SPA, PAVLICEVIC, MILORAD, POLONI, ALFREDO
Application year: 2000
Main classification: C21B13
IPC codes: C21B13, F27B1, F27D7