WO2006107256A1

WO2006107256A1 - A method for separating metallic iron from oxide

Info

Publication number: WO2006107256A1
Application number: PCT/SE2006/000395
Authority: WO
Inventors: Joachim Von Scheele
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2005-04-08
Filing date: 2006-04-03
Publication date: 2006-10-12
Anticipated expiration: 2007-10-08
Also published as: ZA200708105B; RU2007135253A; RU2403289C2

Abstract

A method for converting low density sponge iron fines into high density agglomerates of increased metallic iron content suitable as charge material for steel making comprises the following steps: supplying fines to the flame of an oxy-fuel burner having a power intensity ensuring total melting of the fines, thereby allowing separation of the metal containing fines into a metallic part and an oxidic part; separating the two phases into a liquid slag and a liquid iron bath in a suitable furnace or vessel; and converting the liquid iron into high density agglomerates. With the method the metallic part of fines containing both metal and metal oxide can be separated from the rest and be recovered in suitable form for use as a raw material in steel-making, in foundries etc.

Description

A METHOD FOR SEPARATING METALLIC IRON FROM OXIDE

FIELD OF INVENTION

The present invention relates generally to a method for separating metallic iron from oxide of fines containing both metallic iron reduced from iron oxide and remaining iron oxides, particularly pyrophoric fines, so called sponge iron fines resulting from the production of Direct Reduced Iron.

BACKGROUND

The dominating iron raw material for steel-making is the blast furnace hot metal turned into steel in basic oxygen converters . The main alternative to hot metal is so called DRI (Direct Reduced Iron) produced by solid state reduction of iron ore. In recent years, there has been a great progress in production of DRI. The DRI is supplied to the steel making shop (internal or external) either as so called sponge iron, which is the form that the DRI obtains directly after reduc- tion, or as briquettes e.g. HBI (Hot Briquetted Iron).

The sponge iron has a porous structure resulting from the removal of oxygen from the ore feed. Thus the sponge iron is a low-density product with a large internal metallic surface which makes it prone to self-ignition that can start by small amounts of moisture. Oxidation of metallic iron by moisture creates hydrogen which makes storing and transportation of sponge iron very hazardous, in particular sea-borne shipping, due to the risk of explosions and exothermal reactions. Therefore special measures are to be taken for the transportation of sponge iron to external users . An alternative is the HBI which means that the DRI is hot discharged from the reduction furnace and compressed at discharge temperature to a high density non-pyrophoric product .

In the DRI process, a by-product is sponge iron in the form of fines. In the production of DRI, depending on the type of process, and whether the sponge iron is briguetted or not, various amounts of sponge iron fines are obtained as a by-product. These sponge iron fines, having a metallic iron content that varies between 5-90%, constitute a problem. The fines are divided into several quality classes and fines having a relatively high metal content can be sold on the open market to be used as a raw material in steel- making however, at a reduced price due to its size and with the same transportation and storing problem as for ordinary size sponge iron. Fines having a low metallic iron content are often stored unprocessed in large heaps . This of course constitutes an environ- mental hazard and the pressure from public authorities is increasing. This waste of metal containing material is also uneconomical.

A known solution to the transport problem is cold briguetting of the fines. However, this introduces an extra cost for a separate briquetting plant, a binder agent is usually required which further dilutes the metallic iron content and adds to the costs, and, moreover, the briquetting process in itself generates fines . To summarize, sponge iron fines generally has the following properties :

^■ too low a size to suit the ordinary handling and feeding methods in the steel making furnace

^■ often lower metallisation (metallic iron content) than ordinary DRI resulting in higher energy requirement in the steel making process resulting in higher cost and reduced productivity.

^■ similar or worse self ignition tendency as ordinary size sponge iron.

As a result of the combined effect of these negative properties connected with sponge iron fines , the fines are either sold at a low price for other uses than as a steel furnace charge, e.g. as an iron-bearing additive in the production of blast furnace sinter, or, as already mentioned, just being stored in heaps due to their low value .

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for converting low density, sponge iron fines into high density, non-pyrophoric agglomerates with high contents of metallic iron and of suitable size for use as a charge material in steel making by separating, in liquid state, metallic iron from oxide of fines containing both metallic iron and iron oxide, and subsequently quenching the liquid metallic iron to form high density agglomerates. The method thus present a solution to the problems connected with sponge iron fines as described above.

The invention is based on the realisation that the fines containing both metallic iron and iron oxide can be fed directly to and melted in the flame of a burner whereby separation is effected.

According to a first aspect of the present invention there is provided a method for separating metallic iron from oxide as defined in claim 1.

With the method the metallic part of fines containing both metallic iron and iron oxide can be separated from the rest and be recovered in suitable form for use as a raw material in steel-making, in foundries etc

BRIEF DESCRIPTION OF DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a plant used with the method according to the invention;

FIG. 2 is a sectional view of a burner used with the method according to the invention; and

FIG. 3 is a schematic front view of the burner shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION In the following, a detailed description of the method according to the invention will be given. In the following description the term fines is used for the material supplied to the plant. By the term fines is meant products with a diameter less than approximately 10 millimeters, predominantly less than 6 mm. These fines are by-products from DRI processes, related briquetting processes, etc. Also, in the preferred embodiment the metal content of the fines is at least 5%, more preferably at least 20%, and most preferably at least 50% by weight.

The term low-density sponge iron should be interpreted as a product having a porous structure with a large internal metallic surface resulting from the removal of oxygen from the ore feed. Also, high-density agglomerates of iron are a product wherein the density of individual agglomerates is higher than approximately 7.

Starting with Fig. 1, an overall diagram of a plant for separating metallic iron from oxide of fines containing both metallic iron and iron oxide, generally designated 10, is shown. The plant is built around a burner 20 installed in a sidewall of a furnace 30. The burner is a so-called oxy-fuel burner and is thus supplied with fuel through a first feeding line 21 and with oxygen through a second feeding line 22. By oxygen is in this context meant a gas with an O₂ content exceeding 21% and preferably so-called technical oxygen having an O₂ content of approx. 90-100%.

Fines are supplied to the burner through a third feeding line 23. The third feeding line 23 is connected to some kind of supply, generally designated 40. In the exemplary plant shown in Fig. 1, the furnace vessel 30 is a separate unit having a closable outlet 32 near the lower portion thereof for the emptying of the molten iron recovered by the method or alter- natively, a tap hole 38, preferably of siphon type, for continuous tapping of liquid iron. The vessel has a separate tap hole 39 for continuous or batch wise tapping of the liquid slag.

The exemplary plant also comprises a water tank for quenching the liquid iron into high density agglomerates and an arrangement for unloading the product 60 from the quenching tank. To obtain a suitable size and shape of the agglomerates, a device 51 is provided for disintegrating the liquid metallic tapping stream into droplets of corresponding size, this device may be designed according to known published methods .

A first embodiment of the burner 20 known per se will be described in more detail below with reference to Figs. 2 and 3, wherein Fig. 2 is a sectional view of the front portion of the burner and Fig. 3 is a front view. This burner is adapted to be used with fuel in the form of a gas, such as propane, natural gas, or butane, or with oil fuel.

The burner 20 comprises a main portion 24, to which the supply lines 21-23 shown in Fig. 1 are connected. The portion 24 is provided with an essentially circular cross-section, see Fig. 3, in which the configuration of the supply lines 21-23 appears in more detail. Fuel is supplied through the first supply line 21 in the form of six equidistant pipes 21a-f placed at a constant distance from the center axis of the main portion 24. Oxygen is supplied through an annular outer portion 22 and thus surrounds the fuel supplied through the pipes 21a-f . Finally, fines are supplied through the pipe 23, which is co-axially placed in the burner.

As already mentioned, the burner 20 is mounted in the sidewall of the furnace 30. In the preferred embodiment, the burner can be tilted, i.e., can be posi- tioned in different angles relative to the horizontal and the vertical. The different orientations can be used for obtaining desired characteristics for the burning process.

In the following, the method for separating metallic iron from oxide of fines containing both metallic iron and iron oxide will be described in detail.

The fines are transferred from the supply 40 to the burner 20 by means of any suitable means, such as an inert or reducing carrying gas . The rate by which the fines are fed to the burner is determined by the specific application.

The operation of the oxy-fuel burner 20 is controlled by means of the amount of fuel and oxygen supplied through the first and second supply lines 21 and 22, respectively. The supply lines are connected to sources of fuel and oxygen (not shown) , as is conventional .

Fuel, such as oil or gas, is supplied in the feeding pipes 21a-f, see Fig. 3, while an envelope of oxygen is supplied through the annular feeding area 22. The oxy-fuel mixture results in the flame 25 having properties, such as length, temperature etc., that are controlled by the supply rate of fuel and oxygen. The higher the oxygen content, the higher the temperature, resulting in a theoretical flame temperature of approx. 2000⁰C or more. Thus, the fines are injected into the central portion of the flame.

As is seen from Fig. 2, the fines are injected into the flame 25 where they are brought to totally melt, thereby allowing separation of the metal containing fines into a metallic part and an oxidic part. The separation of metal and oxide into two bulk phases as described below is driven by the systems tendency to lower its total surface energy and by the difference in the density of the two phases.

The melting process is controlled by means of several parameters, of which can be mentioned: temperature and velocity of the flame 25, stoichiometry, i.e., the ratio oxidizing gas to added fuel, the oxygen content of the oxidizing gas, the supply rate of oxygen and added fuel, the rate of injection of fines and its characteristics, the travel time of the fines in the flame, and burner characteristics and configuration, such as tilting.

The minimum power requirement for the burner operation is obtained from the formula P=k_min* θ [kw] where k_min is at least 1500 [kW^#s/kg] and preferably Ic_1111n is equal to 2500 [kW^»s/kg], and θ [kg/s] is the mass flow of injected fines. A feature of the burner operation is that the power intensity of the burner flame, defined as the power of the burner divided by the area of the least circle that encloses the flame root, is at least 10 kW/cm^z, and preferably the said power intensity is at least 20 kW/cm².

The melted droplets fall to the bottom of the furnace 30, wherein they are added to the molten charge. As can be seen in Fig. 1, the charge is divided into an upper layer 34 and a lower layer 36. In the upper layer, the oxidic part of the material fed to the furnace is accumulated as a liquid slag while the liquid metallic part is accumulated in the lower layer 36. Thus, both the metallic part and the oxidic part can be removed in separate tap streams from the furnace 30 for separate subsequent treatments. This subsequent treatment preferably involves granulation of the metallic part by means of quenching iron droplets in a water bath resulting in high density pieces wherein at least 90 % of the produced high density pieces have a preferred size of 10-40 mm. The quenching is preferably done by using the exemplary equipment shown in Fig. 1, but for the person skilled in the art, it is obvious that a similar granulation result can be achieved also with other means such as dry granulating.

The method enables the slag, leaving the process in liquid form, to be adapted to its subsequent use, which may be use as iron-bearing raw material to be up-graded or directly used as sinter feed, use as slag former, use as road filling material etc., the adap- tation including measures such as granulating, slow cooling, or mixing the slag with e.g. lime/limestone to make a slag forming product etc.

In its basic form, the method according to the invention does not involve any active measures in order to produce a chemical conversion. Thus, the method basically separates metallic iron from oxide in fines containing both metallic iron and iron oxide. However, it is perfectly possible to add a carbon- bearing material together with the fines and/or to run the melting process sub-stochiometrically, resulting in a reduction of the iron oxide content of the fines . The carbon-bearing material can also be added either to a "hot heel" of liquid iron which remains in the furnace after emptying the metallic part of the charge or be injected/charged in suitable form separately into the process vessel. This reduction process increases the iron yield of the total process. It should be noted that, depending on the operation conditions and the properties of the fines material, the addition of carbon-bearing material may be used also to increase the carbon content of the metallic product which adds to its value .

The fines may have a certain carbon content, sometimes up to approximately 4%. This carbon is normally found in the form of iron carbide (Fe₃C). Naturally the needed input of carbon-bearing material when applying the method is adapted to the level of carbon contained in the fines . A preferred embodiment of the method according to the invention has been described. The person skilled in the art realizes that this can be varied within the scope of the appended claims. Thus, also the fines have been described as being essentially dry, in an alternative embodiment the fines form part of a slurry that may consist of sponge iron fines mixed with an organic liquid. Furthermore, a separate furnace unit 30 has been shown. The method according to the invention is equally applicable to other kinds of furnaces, such as electric arc furnaces, induction furnaces, reverberatory furnaces, electrically heated furnaces, blast furnaces, cupola furnaces, rotary furnaces, and converters etc.

Also, in the described embodiment, the burner is positioned in a sidewall of a furnace. However, it is realized that other suitable positions are possible, such as in the upper part of the furnace. Also, a configuration with more than one burner is possible. It is then possible to inject the fines between three burner flames, for example.

A specific burner configuration has been shown. It is also appreciated that any suitable burner configuration having different number of pipes etc can be used.

Claims

1. A method for converting low density sponge iron fines into high density agglomerates of increased metallic iron content suitable as charge material for steel making, the method comprising the following steps :

supplying fines to the flame of an oxy-fuel burner having a power intensity ensuring total melting of the fines, thereby allowing separation of the metal containing fines into a metallic part and an oxidic part;

separating the two phases into a liquid slag and a liquid iron bath in a suitable furnace or vessel; and

converting the liquid iron into high density agglomerates .

2. The method according to claim 1, comprising the additional step of adapting the liquid slag to its subsequent use.

3. The method according to claim 1 or 2 , wherein the liquid iron is converted into high density agglomerates by disintegrating the tap stream of liquid iron into droplets of suitable size and subsequently quenching these droplets in a cooling medium.

4. The method according to claim 3 , wherein at least 90 % of the produced high density pieces have a size of 10-40 millimeters.

5. The method according to any of claims 1-4, wherein the fines have a diameter less than approximately 10 millimeters and preferably less than 6 millimeters .

6. The method according to any of claims 1-5, wherein the fines are by-products from DRI processes.

7. The method according to any of claims 1-6, wherein the metal content of the fines is at least 5%, more preferably at least 20%, and most preferably at least 50% by weight.

8. The method according to any of claims 1-7, wherein the method involves no active measures to produce a chemical conversion.

9. The method according to any of claims 1-7, wherein a carbon-bearing material is added together with the fines to increase the iron yield and/or the carbon content of the metallic product.

10. The method according to any of claims 1-9, wherein the burner generates a power P of at least P=^_1n* θ [kW], where Ic_1111n is at least 1500 [kWs/kg] and preferably Ic_1111n = 2500 [kW^»s/kg], and θ [kg/s] is the mass flow of injected fines.

11. The method according to any of claims 1-10, wherein the power intensity of the flame, defined as the power of the burner divided by the area of the least circle that encloses the flame root, is at least 10 kW/cm², and preferably at least 20 kW/cm².