WO2000036159A1

WO2000036159A1 - Method and apparatus for the direct reduction of mineral iron with optimized injection of reducing gas

Info

Publication number: WO2000036159A1
Application number: PCT/IB1999/001944
Authority: WO
Inventors: Milorad Pavlicevic; Gianpietro Benedetti; Ljuban Milic
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 1998-12-11
Filing date: 1999-12-06
Publication date: 2000-06-22
Anticipated expiration: 2001-06-11
Also published as: IT1302815B1; AR021610A1; ITUD980216A1; AU1292200A

Abstract

Method and apparatus for the direct reduction of mineral iron inside a vertical reduction furnace (10) of the type with a gravitational flow, wherein the reducing gas flows counter to the material introduced into the furnace, comprising the following steps: the mineral iron is fed into said furnace (10) from above, a mixture of gas at high temperature is injected and the reduced mineral is removed from said furnace (10), said mixture of gas being introduced into said furnace (10) by means of first injection means (20a) located peripherically on the outer wall of said furnace (10) and at the same time said mixture of gas being introduced into said furnace (10) by means of second injection means (20b, 20c, 25) located inside said furnace (10).

Description

"METHOD AND APPARATUS FOR THE DIRECT REDUCTION OF MINERAL IRON WITH OPTIMIZED INJECTION OF REDUCING GAS"

* * * * * FIELD OF THE INVENTION

This invention concerns a method and apparatus for the direct reduction of mineral iron, where the iron is present in the form of oxides. To be more exact, the method consists in bringing the mineral iron, of variable granulometry, into contact with a reducing gas in a reduction furnace, wherein both the material and the gas is fed continuously, to obtain directly reduced iron (DRI) in a continuous manner. Tuyeres are provided to optimize the distribution of the reducing gas inside the reduction furnace. The lower outlet of the reduction furnace may be of the multiple type to encourage the selective and simultaneous discharge of several types of product, that is to say, both hot and cold, so that it can subsequently be stored, sent to a melting furnace, for example of the electric arc type, to produce liquid steel, or converted into hot briquetted iron (HBI) .

BACKGROUND OF THE INVENTION The state .of the art includes direct reduction processes wherein hydrocarbons are injected into the current of reducing gas to allow the reaction of reforming the methane in the furnace with the H₂O and CO₂ in the gas; there are also direct reduction processes wherein hydrocarbons having C>5 are injected directly into the furnace in the zone between the injection of the reducing gas and the upper outlet of the exhaust gas. The state of the art also includes processes wherein the hot mineral iron is produced in a reaction furnace of the shaft type, with a vertical and gravitational flow of the material which is subsequently sent, with a closed system of pneumatic transport in an inert atmosphere, to the melting furnace .

In conventional systems, the reducing gas is normally introduced in a single median zone of the reduction furnace, which is also called reaction zone or reactor, inside which the reaction of the reducing gas with the iron oxides takes place. To be more exact, the reducing gas is injected in correspondence with the lateral walls of the furnace, therefore there is a temperature imbalance inside the reaction zone, with higher temperatures towards the side walls of the furnace and lower towards the center, as shown in Fig. 4.

The patent US-A-4 , 032 , 123 discloses a furnace for the direct reduction of -mineral iron, wherein the reducing gas is introduced both through the side walls and also from the bottom of the furnace, by means of a central element which occupies a large part of the reduction zone of the material, which is thus bulky, inefficient and not very practical.

SUMMARY OF THE INVENTION The method to produce metal iron by the direct reduction of iron oxides and the relative plant according to the invention are set forth and characterized in the respective main claims, while the dependent claims describe other innovative characteristics of the invention. The method according to the invention consists in bringing the mineral iron of variable granulometry into contact with a feed gas in a reduction furnace of the shaft type, wherein both the material and the gas are fed continuously, so that a vertical and gravitational flow of the material is created and the direct reduction of the material occurs. The material can be discharged from the reactor cold or preferably hot, to be sent subsequently to a melting furnace so that it can be converted into hot bricquetted iron (HBI) or cooled and converted into directly reduced iron (DRI) .

The reduction furnace is equipped with means to feed the mineral iron and means to discharge the reduced metal iron, and is equipped with at least two inlet collectors to inject the reducing gas in correspondence with various reduction zones inside the furnace to ensure a larger reduction zone.

The reducing gas sent to the reactor contains hydrocarbons injected into the current before or after the partial combustion of the hydrogen and carbon monoxide with the oxygen.

The direct reduction of the iron oxides is achieved in two different continuous stages inside the reduction reactor.

In a first stage, defined as the pre-heating and pre- reduction stage, the fresh iron oxides, that is, those just introduced into the furnace, come into contact with a mixture of reducing gas consisting of partly burnt gas arriving from the underlying part of the furnace, and fresh hot gas, that is to say, introduced from outside, arriving from a collector which brings fresh reducing gas and possibly CH₄ or other natural gas. This first stage occurs in a corresponding first zone arranged in the highest part of the furnace.

In a second stage, the reduction stage proper, the complete reduction of the iron oxides takes place, due to the action on said oxides, which are already partly reduced in the first stage, of a mixture of reducing gas based on H₂ and CO and at least a hydrocarbon, preferably natural gas, injected into the median zone of the reduction reactor. This second stage occurs in a corresponding second zone arranged below the first zone.

The two inlets of the furnace through which the gas is introduced can be regulated independently by mixing means both in the flow of fresh reducing gas and also in the addition of natural gas in the current introduced.

Moreover, the inlet temperature of the two currents of reducing gas may be regulated independently by injecting O₂ and CH₄ before the gas enters the reduction reactor with a final inlet temperature between 800°C and 1150°C.

The oxidation reaction needed to raise the temperature of the gas induces a change in the oxidation level of the gas itself, from usual values of 0.04-0.08 to 0.06-0.15.

By oxidation level of the reducing gas, the following ratio is intended:

Nox = (H₂O+CO₂) / (H₂0+C0₂+H₂+CO) .

In the second reaction zone of the furnace, wherein the reduction of the iron oxides is completed, and thanks to the introduction of natural gas, a gas with a high content of H₂ , CO and CH₄ is in any case generated. Once it has abandoned the second reaction zone, this gas enters the first reaction zone, arranged at a higher position, and mixes with the hot gas injected into this first zone to preheat and pre-reduce the iron oxides . The gas emerging from the reduction reactor is partly recircled and partly used as fuel.

The recircled gas has a volume composition within the following fields of composition: H2=20-41%, CO=15-28%, C0₂=15-25%, CH₄=3-10%_, N₂=0-8%, H₂0=2-7%. In accordance with one characteristic of the invention, the reducing gas is introduced both laterally by means of tuyeres located along the circumference of the furnace and also in the central zone of the reduction furnace, that is to say, in the core of the material inside the reduction furnace.

It has also been provided, to cooperate with the reducing gas entering laterally into the reduction zone of the furnace, that reducing gas is also introduced into the core of the reduction furnace, through injection means such as for example hollow- tubular elements, water cooled and protected by refractory material, so that the gas can arrive uniformly inside the reduction furnace. One advantage is that it is possible to introduce reducing gas simultaneously from several points, and to regulate the delivery of reducing gas.

A further, considerable advantage is that it is possible to reduce the oxides in a uniform manner along the whole transverse section of the reduction furnace, creating the best fluido-dynamic conditions for the distribution of the reducing gas. In this way, we have a uniform temperature distribution over the whole section, unlike in conventional furnaces where the hot gas laps the material towards the peripheral part better, and enters into the core of the reactor only with difficulty.

Another advantage is that the direct reduction becomes uniform and therefore it is possible to have higher productivity and also a lower consumption of gas and of energy, since the gas itself is used in the best possible way.

With the method and apparatus according to the invention we therefore have: faster reduction times; better thermal profile along the column of material, optimum thermal profile over the whole section of the reduction furnace; homogeneous conditions in the reduction furnace; reduced consumption due to the better exploitation of the reducing gases; higher productivity.

In accordance with another characteristic of the invention, the lower outlet of the furnace is of the multiple type, to encourage the simultaneous discharge of several types of product.

The multiple outlet encourages the distribution of the reducing gas inside the furnace and a better distribution of the material inside the furnace, preventing preferential channels which occur in furnaces with a single outlet cone.

In furnaces with a single outlet cone, in fact, the finer material tends to arrange itself in the middle, and this encourages the reducing gas to flow in the outer part, so that it reduces the iron oxides nearest the wall more, and has difficulty in penetrating the core of the solid bed of material, and thus the iron oxides are reduced with more difficulty and in any case the process takes a longer time.

The whole reaction zone of the furnace works at a more uniform temperature, and especially at a constant temperature along the whole section of the reduction furnace, encouraging a higher speed of reaction, with consequent- reduction of consumption and increase in productivity.

The extractors are very flexible to use, and by varying the outlet flow, the formation of bridges in the furnace is prevented. The reduced metal iron is discharged preferably hot through the multiple outlet, preferably with 3 or 4 cones, which are able to discharge the material in a controlled and independent manner.

The big advantage of being able to discharge simultaneously from several points is that it is possible to regulate the flow of material at outlet by varying the speed of extraction of the individual discharge systems .

Another advantage is that this movement helps to make the material descend from the upper zone in a uniform manner, with a perfect mixing of the larger particles and the finer particles, creating a continuous movement of the material and reducing the possibility of the material sticking.

A further, considerable advantage is that it is also possible to simultaneously discharge hot material destined for different uses : one part can be introduced directly into a melting furnace, for final melting; one part can be briquetted; and one part can be cooled outside in a silo and sent for storage.

Another advantage is that it is in any case possible to discharge all the hot material into the melting furnace to produce steel, reducing energy consumption to a minimum.

All the material can also be briquetted hot or cold and stored.

Unlike conventional furnaces equipped with a single outlet, the possibility of having several outlets allows to empty the furnace in the event that one of the outlets is blocked. In this case it is possible to act on the other outlets to empty the furnace almost completely, except for the part of the cone with the blocked outlet; once the rest of the furnace has been emptied it is possible to carry out maintenance work more easily, and to unblock the material. This operation is not possible in conventional furnaces . 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 schematic view of a reduction furnace used in an apparatus for the direct reduction of mineral iron according to the invention; Fig. 2 shows a transverse section along the line from A to A in Fig. 1; Fig. 3 shows a transverse section of Fig. 2, wherein the jets of gas introduced into the furnace of Fig. 1 are shown schematically; Fig. 4 is a diagram showing the temperature inside a furnace of the state of the art, wherein the distance "d" from one side wall of the furnace is indicated on the x coordinate; Fig. 5 is a diagram showing the temperature inside the furnace of Fig. 1, wherein the distance "d" from one side wall of the furnace is indicated on the x coordinate ,- Fig. 6 is a schematic view, partly in section, of a variant of a furnace used in the apparatus according to the invention;

Fig. 7 shows a transverse section, enlarged, along the line from B to B of Fig. 6; Fig. 8 shows a transverse section, enlarged, along the line from C to C of Fig. 6; Fig. 9 shows a transverse section, enlarged, along the line from D to D of Fig. 6. DETAILED DESCRIPTION OF TWO PREFERRED FORMS OF EMBODIMENT With reference to Fig. 1, an apparatus for the direct reduction of iron oxides according to the invention comprises a shaft type reduction furnace, also called reduction reactor 10, comprising in turn an upper mouth 11 for feeding material from above, through which the mineral (iron oxides) is suitable to be introduced, a first upper pre-heating and pre-reducing zone 12, a second zone, or median zone 14 wherein the final reduction reaction of the iron oxides takes place, and a lower zone or discharge zone 15, shaped like a truncated cone and terminating downwards with a lower aperture 16 for the removal of the iron.

The iron-based metal oxides are introduced into the reduction furnace 10 in the form of pellets or coarse mineral of an appropriate size; the iron contained therein is normally between 63% and 68% in weight.

At the end of the process according to the invention, the iron contained in the reduced material emerging from the furnace 10 is normally between 80% and 90% in weight.

In correspondence with the two zones 12 and 14 of the furnace 10 there are two independent inlets 17, respectively 18, through which a mixture of reducing gas is suitable to be introduced.

In its upper part, above zone 12, the furnace 10 is provided with an aperture 19 through which the exhaust gas exits. In the lower extremity 15 there is an aperture 30 through which, carried by a conduit 31 and regulated by a valve 32, a cooling gas can be introduced, such as for example a gas formed by a mixture of CO, CO₂ , H₂ , H₂O, CH₄, N₂ , of variable quantities and temperature according to the quality of the material desired and the outlet temperature desired.

According to one characteristic of the invention, at least in the reaction zone 14, but advantageously also in the pre- reduction zone 12, the reducing gas is injected inside the reduction furnace 10 by means of a plurality of tuyeres 20 arranged radially and lying on a substantially horizontal plane .

According to a first form of embodiment, the tuyeres 20 comprise: terminal elements 20a (Figs. 2 and 3), for example eight, with their extremities near the side wall of the furnace 10; terminal elements 20b, for example four, with their extremities near the center of the central and vertical axis 21 of the furnace 10, or having a radial depth of between 1/2 and 2/3 of the radius of the furnace 10; and terminal elements 20c, for example four, spaced between an element 20a and an element 20b, or between two elements 20a, and having their extremities in an intermediate zone between the side wall and the central and vertical axis 21 of the furnace 10, or having a radial depth of between 1/3 and 1/2 of the radius of the .furnace 10.

In this way, the jets of reducing gas are arranged uniformly and in an optimum manner over the whole transverse section, thus ensuring a substantially constant temperature over the whole width "d" (Fig. 5) of the pre-reduction zone

12 and the reduction zone 14.

According to a variant, the reducing gas is distributed by means of one or more, preferably three, horizontal tubes 25, made of material resistant to heat and to oxidizing and carburizing agents. The tubular elements 25 are arranged transversely to the furnace 10 and each is provided with a plurality of peripheral apertures or radial tuyeres 26, from which the gas can flow inside the reaction zone 14.

Each tubular element 25 comprises a central tube 27 made of refractory material and is cooled internally by cooling water circulating in a circumferential cavity 28.

Each tubular element 25, moreover, can be through ' and equipped with rotary movement, so that the peripheral apertures 26 can describe a round angle and distribute the reducing gas better and more uniformly inside the reduction zone 14.

According to a variant, as shown in Fig. 6, the discharge zone 15 has three lower extremities, shaped like a cone or a truncated cone, 15a, 15b, and 15c, with the taper facing downwards, each provided with a lower aperture 16a, 16b and respectively 16c, through which the directly reduced iron (DRI) can be selectively discharged in a controlled and independent manner.

Each lower aperture 16a, 16b and 16c is provided with a rotary valve 47 suitable to regulate the flow of material emerging from the furnace 10.

Moreover, further means 48 to inject reducing gas inside the furnace 10 are provided in the zone of intersection of - li ¬

the extremities 15a-15c.

It is obvious that modifications and additions can be made to the method for the direct reduction of mineral iron and the relative apparatus as described heretofore, but these shall remain within the field and scope of the invention.

Claims

1 - Method for the direct reduction of mineral iron inside a vertical reduction furnace (10) of the type with a gravitational load, wherein the reducing gas flows counter to the material introduced into the furnace, comprising the following steps: the mineral iron is fed into said furnace (10) from above, a mixture of gas at high temperature is injected and the reduced mineral is removed from said furnace (10), the method being characterized in that said mixture of gas is introduced into said furnace (10) by means of first injection means (20a) located on the outer wall of said furnace (10) and in that at the same time said mixture of gas is introduced into said furnace (10) by means of second injection means (20b, 20c, 25) located inside said furnace (10) .

2 - Method as in Claim 1, characterized in that the delivery of said reducing gas is controlled in the different injection zones (12, 14) along the length of said furnace (10) . 3 - Method as in Claim 1 or 2 , characterized in that said mixture of gas is introduced into at least two zones (12, 14) of said furnace (10) arranged one above the other in such a manner as to achieve, in a controlled manner, a first pre-heating and pre-reducing stage in the upper part (12) of said furnace (10) and a second stage of final reduction in the lower part (14) of said furnace (10) .

4 - Method as in Claim 1, wherein said mixture of gas consists of reducing gas and at least a hydrocarbon, preferably natural gas, characterized in that each hydrocarbon mixed with said reducing gas is proportioned and controlled in an independent manner in the different injection zones (12, 14) along the length of said furnace (10) . 5 - Method as in Claim 4, characterized in that before being mixed with each hydrocarbon, said reducing gas is heated to a temperature of between 800°C and 1150°C.

6 - Method as in Claim 4 or 5 , characterized in that said reducing gas is heated in an independent manner in the different zones (12, 14) along the length of said furnace (10) .

7 - Method as in any claim from 4 to 6 inclusive, characterized in that said reducing gas is heated by making it partly react with O₂.

8 - Method as in any claim hereinbefore, characterized in that the reduced material is removed from said furnace (10) in a controlled and independent manner from at least two extremities (15a-15c) , preferably shaped like a cone or truncated cone with the taper facing downwards, each provided with a corresponding lower aperture (16a-16c), through which said reduced metal iron can be selectively discharged in a controlled and independent manner.

9 - Apparatus for the direct reduction of mineral iron comprising a vertical reduction furnace (10) of the type with a gravitational load, to achieve therein reactions to reduce the mineral iron, means (11) to feed the mineral iron from above into said furnace (10), means (20) to inject a mixture of gas at high temperature inside said furnace (10) and means (15) to remove the reduced mineral from the lower part of said furnace (10) , the apparatus being characterized in that said injection means (20) comprise first injection means (20a) arranged on the circumference on the outer wall of said furnace (10) and second injection means (20b, 20c, 25) located inside said furnace (10).

10 - Apparatus as in Claim 9, characterized in that said second injection means comprise a series of tuyeres (20b, 20c) which penetrate into the transit zone of the material being reduced to differentiated depths in the section of said furnace (10) .

11 - Apparatus as in Claim 10, characterized in that said injection means (20) comprise at least a tubular element (25), suitably cooled, provided with radial apertures or tuyeres (26) for the passage of the reducing gas from the hollow zone to the inside of the reaction zone of said furnace (10) .

12 - Apparatus as in any claim from 9 to 11 inclusive, characterized in that said furnace (10) is provided with at least two zones (12, 14) arranged distanced in a vertical direction, into each of which said mixture of gas at high temperature is suitable to be introduced to achieve said reduction reactions in a controlled manner. 13 - Apparatus as in Claim 12, characterized in that means to control the delivery of said mixture of gas at high temperature are provided in said zones (12, 14).

14 - Apparatus as in Claim 12 or 13, characterized in that said injection means (20) are arranged in correspondence with said at least two zones (12, 14) in order to achieve the reduction reactions inside said furnace (10) in a controlled manner.

15 - Apparatus as in claim 12, 13 or 14, wherein said mixture of gas consists of reducing gas and at least a hydrocarbon, preferably natural gas, characterized in that means to mix said reducing gas and hydrocarbons are provided upstream of said injection means (20) so that each hydrocarbon mixed with said reducing gas is proportioned and controlled in an independent manner in the different injection zones (12, 14) along the length of said furnace (10) .

16 - Apparatus as in Claim 15, characterized in that means to heat said reducing gas are provided upstream of said mixing means to heat said mixture to a temperature of between 800°C and 1150°C.

17 - Apparatus as in Claim 16, characterized in that said heating means are suitable to heat said reducing gas in an independent manner in the different zones (12, 14) along the length of said furnace (10) .

18 - Apparatus as in Claim 16, characterized in that said heating means are suitable to heat said reducing gas by making it partly react with O₂. 19 - Apparatus as in any claim from 9 to 18 inclusive, characterized in that said removal means (15) comprise at least two extremities (15a-15c) preferably shaped like a cone or a truncated cone with the taper facing downwards, each provided with a corresponding lower aperture (16a-16c) , through which said reduced metal iron can be selectively discharged in a controlled and independent manner. 20 - Apparatus as in Claim 19, characterized in that each of said lower apertures (16a-16c) is provided with a rotary valve (47) . 21 - Apparatus as in Claim 19, characterized in that further means to inject the reducing gas inside said furnace (10) are provided in the zone of intersection of said extremities (15a-15c) .