WO2001073138A2

WO2001073138A2 - Steelmaking

Info

Publication number: WO2001073138A2
Application number: PCT/GB2001/001333
Authority: WO
Inventors: Stephen David Bray; Keith Harris
Original assignee: Qual Chem Ltd
Current assignee: Qual Chem Ltd
Priority date: 2000-03-24
Filing date: 2001-03-26
Publication date: 2001-10-04
Anticipated expiration: 2002-09-24
Also published as: WO2001073138A3; GB0007073D0

Abstract

Molten ferrous metal for use in a steelmaking process is poured under gravity to produce a substantially free falling flow of the molten metal; a stream of conveying gas carrying divided additive is directed to impinge with the falling flow.

Description

Steelmaking

The present invention relates to iron and steelmaking and particularly to introducing additives to molten iron prior to conversion to steel .

Conditioning of steel at various stages in steelmaking processes often requires the introduction at relevant process stages of various additives (such additives are often known as conditioning agents because they "condition",- or change the properties and/or the composition of, the resulting steel) . In conventional arrangements, additives for desulphurisation or dephosphorisation may be introduced to molten iron. Further desulphurisation may occur as a final trimming upon leaving a steelmaking converter. This may be done in various ways including by gravity feed (by flow of the additive from a hopper or the like placed above the molten metal) , or for example by direct injection into molten steel, for example by means of a lance arranged vertically above the hot steel. Typically, desulphurising of the molten metal is carried out by adding additive material including, for example, aluminium or a source of lime (CaO) in mixture form to .a ladle containing the molten material discharged from a steelmaking convertor, then agitated by use of gas bubbling and mechanical stirring to disperse the additive mixture through the molten material . An improved process and apparatus have been devised whereby ferrous metal feedstock is treated before presentation to the steel making converter by introducing at least one finely divided additive into molten ferrous metal as it is poured.

According to one aspect, the present invention relates to the treatment of molten ferrous metal feedstock for use in a steelmaking process, which treatment includes the steps of pouring molten ferrous metal under gravity to produce a substantially free falling flow of said molten metal, and directing a stream of conveying gas carrying divided additive to impinge with the falling flow.

In a preferred embodiment of the invention, the falling flow of molten metal is collected in a receptacle or vessel positioned beneath the falling flow. The arrangement takes advantage of kinetic energy available during pouring to disperse the additive (introduced into the falling flow of molten metal) throughout the molten metal in the collecting receptacle or vessel .

The molten ferrous metal is preferably poured from a pouring outlet at a suitable pouring stage. Such a stage would typically be during metal transfer (for example pouring from a blast furnace or transfer ladle upstream of the steel convertor) .

The stream of conveying gas carrying divided additive is directed to impinge substantially transversely with the falling molten metal flow. It is preferred that the , sprayed stream of additive and conveying gas is minimally diverging, non-diverging or converging, in order to minimise the sprayed additive missing the poured molten metal stream.

The use of a pneumatically conveyed stream provides several benefits, including lower cost, and enhanced dispersal of the additive in the molten metal. In terms of cost, there is no requirement for a specifically designed treatment station, because the relevant outlet nozzles ("guns") can be readily added to an existing plant structure, and an expensive and short-lived lance is not needed.

The falling molten metal produced during pouring is specifically targeted, both in terms of timing of jetting of the conditioning material to coincide with pouring (temporal coincidence) and in terms of ensuring the jetted conditioning material spatially coincides with the falling molten metal flow.

In further embodiments, different nozzles can be used for different applications, so that a widely divergent stream can be provided in some embodiments and a stream which little divergence can be formed in other circumstances.

In most applications of the present invention, it is preferred that the conveying gas will be air, although inert conveying gasses (such as nitrogen) may be preferred in some instances. The additive may be in any suitable particulate form, such as tablets, pellets, briquettes or powder. The density and composition of such tablets, pellets, briquettes and the like may be tailored in order to penetrate to desired extents in the molten ferrous metal reaching the collection receptacle. The delivery pressure and velocity of the conveying gas can be tailored to effect desired passage of the additive into the falling molten metal . Typically, the dispensing pressure of the conveying gas will preferably be substantially in the range of 7 bars plus or minus 20%. The discharge rate of dispensed material is preferably substantially in the range'0.5 to 15m³ per hour.

Sources of one or more of the following materials may be included in the additive used according to the invention, depending upon user requirements: lime, magnesia, alumina, fluorspar, silicon, aluminium, or the like. Each of these materials is commonly used in steelmaking processes, generally in order to aid process control. Such additives are used according to the invention as divided material

(which may be fine or coarse) .

The invention is particularly suited to desulphurisation or dephosphorisation of molten ferrous material as it is poured, typically into a ladle. -In one embodiment of the invention, the additive may include or consist essentially of lime, which may be in the form of relatively small briquettes. In this embodiment, the additive is typically pneumatically conveyed or gunned into a tapping stream or the like in which molten ferrous metal is tapped from a blast furnace or from a transfer ladle downstream of the blast furnace. The additive is added or gunned into the stream in a tight cone. This can reduce dust in adding the lime additive and can avoid large amounts of the lime additive remaining unreactive on the surface of a ladle or the like.

In a further embodiment of the invention, the additive may be in the form of mixture material (for example formed as small briquettes) containing lime, aluminium and/or soda ash (or any other desulphurising additive) , which can thereby be used as a desulphurising medium for molten ferrous material. Alternatively the • briquettes may be of dephosphorising nature, typically a mixture of a source of lime and iron oxide. In this embodiment, the additive can be fired into a falling flow of the metal (for example, when the latter is being poured from a blast furnace torpedo car into a BOS plant transfer ladle) . The pouring action releases large amounts of kinetic energy and additive material can be drawn and stirred into the molten . ferrous metal without the costs and delays associated with conventional systems.

Desirably a desulphurising additive consisting of a material mixture in self supporting pressed form (briquetted, pelletised, tabletted or composite granular form) is used. This provides environmental advantages in that less dust is evolved. Typically the self supporting bodies or granules are formed of finely divided lime (CaO) or as a blend of finely divided aluminium material and finely divided lime. These materials may be compressed into briquettes or other suitable form. The formed body (briquette or the like) may subsequently beneficially be granulated or crushed into granules . The granules of the mixture have a high surface/reactivity area but remain substantially low in degree of dust evolved which make them particularly suited for the present application. The formation of composite additive granules in this way is particularly advantageous in relation to ^' the use of aluminium and lime for desulphurisation of molten ferrous metal (iron) prior to steelmaking conversion. The aluminium/lime pellets/briquettes/tablets are preferably formed by use of a compressive process and preferably comprise 10 to 14% fine aluminium powder at 120 mesh, the balance being predominantly fine lime (CaO) 90% content at 200 mesh. Typically, the material will be blended together in ribbon mixers and tableted or briquetted to form hard, self-supporting shapes and then converted to granules

(typically in the range 5 to 8mm diameter) . Such granules may be made by granulating or crushing briquettes of blended material . Typically these granules would have a high surface/reactivity area but remain dust free. An alternative procedure would be to utilise mixed granulated materials blended in the required proportions of aluminium and 1ime .

According to another aspect, the invention extends to a process of making steel in which ferrous metal treated according to the -invention is supplied as the principal feedstock to a steelmaking converter. As mentioned above, the additive is pneumatically conveyed into the free falling molten ferrous stream and may, for example, be of dephosphorising or desulphurising nature. Dephosphorising and desulphurising additives may be dispensed concurrently into the stream from separate gunning nozzles or via a common flowpath dispensing apparatus. As an alternative, separate or common flowpath pneumatic conveying apparatus may be used to deliver sequentially additives of different nature. This makes the method and apparatus of the invention highly versatile and efficient. It is believed that such a technique may be novel and inventive more broadly than for use in treating molten iron steelmaking feedstock and is also considered to be novel and inventive for treating a poured stream of molten steel downstream of the steel converter.

According to another aspect, the invention provides apparatus comprising : a) a pouring station at which a stream of molten ferrous metal flow is allowed to fall freely under gravity; and b) conveying apparatus arranged to direct a stream of finely divided conditioning material conveyed in a conveying gas to impinge with the substantially free falling flow.

Apparatus according to the invention preferably includes positioning means for positioning the conveying apparatus in-line with the falling flow. The positioning means may comprise movable apparatus performing other functions (such as protective doors or shields) . The conveying apparatus may be provided with demountable mounting means for mounting in position, providing ease of de-mounting for maintenance and repair. The mounting means may comprise a magnetic mount .

Desirably the apparatus includes control means for controlling: a) the positional orientation of the conveying apparatus for spraying (co-ordinating spatial coincidence of the falling molten metal and the gas conveyed conditioning material) ; and/or b) the timing of operation of the conveying apparatus to coincide with the falling molten metal (temporal coincidence) .

In certain embodiments the conveying apparatus may be movable automatically or mechanically from a stowed position to an operative position in which the conveying apparatus is in line targeting the arc of fall of the molten metal.

According to a further aspect, the invention provides a steelmaking process comprising: i) producing molten ferrous material in a blast furnace; ii) directing a stream of conditioning material conveyed in a conveying gas to impinge with free falling molten ferrous material poured following process stage i) ; and iii) transferring material from process stage ii) to a steel converter and converting to steel. The invention will now be further described in two specific embodiments, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is a schematic side view of an first embodiment of the invention for desulphurisation of molten ferrous metal;

Figure 2 is a plan view of the embodiment Figure 1 in a first configuration;

Figure 3 is a plan view, similar to the view in Figure 2, of the embodiment in a second configuration; and

Figure 4 is a schematic view of a second embodiment in which lime is gunned into a pouring stream.

Referring to the drawings, there is shown means for treatment of molten ferrous material feedstock for use in a steelmaking process which treated feedstock is intended for supply as the principal feedstock to a steelmaking converter.

Referring to Figures 1 , 2 and 3 , the molten ferrous metal for use in primary steelmaking is poured from a furnace 1 into a transfer ladle 2 by tipping about a rotational axis such that the falling molten ferrous metal is poured from spout 3 to follow an arcuate path 4 into transfer ladle 2. While in flight, the falling molten metal is impinged by a substantially ^• horizontal diverging stream 15 of desulphurising pellets from gun 5. In accordance with the present invention, a nozzle 5 of a pneumatic conveying system is aligned to target the falling molten ferrous metal 4; preformed pellets, briquettes or granules of an aluminium/lime mixture are conveyed by a pneumatic conveying gas directly into the falling molten metal flow 4.

The aluminium/lime pellets/briquettes/tablets employed are formed in a compressive process and comprise, typically, 10 to 14% fine aluminium powder at 120mesh, the balance being predominantly fine lime (CaO) 90% content at 200 mesh. The material was blended together in ribbon mixers and tableted or briquetted to form hard, self-supporting shapes. Granules (typically in the range 5 - 8mm diameter) were made by granulating or crushing briquettes of blended material. Typically these granules have a high surface/reactivity area but remain dust free.

The pellets, briquettes or granules conveyed into the falling molten metal flow 4 take advantage of the kinetic energy of the falling flow 4 entering into the transfer ladle 2 to fully disperse the pelletised, briquetted or granulated desulphurising material throughout the molten ferrous metal in the ladle. The requirement to physically stir the molten iron in the transfer ladle 2 is therefore dispensed with or, at least, the degr.ee of stirring required limited. This gives to the overall steelmaking process a process time benefit, and also lowers process costs by dispensing with the use of consumable/high maintenance items such as apparatus for injecting additives or gas bubbling directly into the molten ferrous metal in -lithe ladle. The requirement to move the transfer ladle to a conventional desulphurisation station is also avoided, which results in considerable process time and energy saving .

In the arrangement specifically shown in .the drawings, the nozzle 5 is mounted to one of a pair of safety doors 6, 7 which slide from respective retracted positions (shown in Figure 2) to respective closed positions (figure 3) in which doors 6,7 lie immediately adjacent one another and nozzle 5 is in line with spout 3 of mixer 1 targeting the falling metal flow 4.

It is important that the conveying gas from nozzle 5 targets accurately the falling molten metal flow 4 and also that the stream of pneumatic conveying gas- does not diverge to an extent such that pneumatically conveyed conditioning/additive material misses the falling flow of molten metal 4. The pneumatic conveying gas therefore diverges relatively little with distance from nozzle 5.

The pneumatic conveying system includes a hopper store for additive or conditioning material in tablet, briquette, pellet or granule form (or the like) , which is introduced into the pneumatic flow and fed via pneumatic conveying line 8 to the nozzle 5.

Referring to Figure 4 (in which like parts are denoted by like reference numerals) , there is shown a schematic view of an alternative embodiment, in which a vessel 13 is arranged to pour molten ferrous material in the form of a failing flow 4. While in flight, the molten metal is impinged by a stream 15 of lime, directed from pneumatic gun 5.

The small lime briquettes are fired into the poured flow of molten ferrous metal during tapping. The briquettes are dragged down into the ladle 2 and mix with the molten metal where the lime can mix efficiently.

Typically, in either of the embodiments discussed above, the system of spraying the desulphurisation material from nozzle 5 is automated to be controlled by a process control system which ensures that the molten ferrous metal is not poured from furnace 1 until the doors 6, 7 are in a closed position and/or the- nozzle 5 is co-aligned with the pouring path of the falling molten ferrous metal 4. The nozzle 5 may be demountably mounted (preferably by magnetic mounting arrangement) to the retractable/advanceable positioning element (such as door 6) . This allows for efficient maintenance procedures.

The arrangement is suitable for directing dephosphorising or desulphurising material to impinge with the free falling poured molten ferrous material . Dephosphorising and desulphurising additive may be conveyed either by separate nozzles 5 or by a common nozzle 5 which may dispense the different additives contemporaneously (concurrently) or in sequence .

Claims

Claims ;

1. A method of treatment of molten ferrous metal for use in a steelmaking process, which method includes the steps of pouring molten ferrous metal under gravity to produce a substantially free falling flow of said molten metal, and directing a stream of conveying gas carrying divided additive to impinge with the free falling flow.

2. A method according to claim 1, wherein the falling flow of molten metal is collected in a receptacle or vessel positioned beneath the falling flow.

3. A method according to claim 1 or 2 , wherein the stream of conveying gas carrying divided additive is directed to impinge substantially transversely with the poured molten metal flow.

4. A method according to claim 3, wherein the stream conveying the additive impinges with the free falling flow at a position substantially above the upper level or rim of a receptacle positioned below the falling flow.

5. A method according to any of claims 1 to 4, wherein the sprayed stream is substantially non-diverging, or converging .

6. A method according to any preceding claim, wherein the additive comprises a desulphurising additive.

7. A method according to any preceding claim, wherein the additive comprises a dephosphorising additive.

8. A method according to any preceding claim wherein distinct additive materials are dispensed via a common conveying apparatus .

9. Apparatus for use in steelmaking comprising: a) a pouring station at which molten ferrous metal is caused to fall under gravity; and b) conveying apparatus arranged to direct a stream of finely divided conditioning material conveyed in a conveying gas to impinge with the substantially free falling metal.

10. A process of making steel, in which ferrous metal treated according to any one of claims 1 to 4 is supplied as a principal feedstock to a steelmaking converter.

11. A steelmaking process comprising: i) producing molten ferrous material in a blast furnace ; ii) directing a stream of conditioning material conveyed in a conveying gas to impinge with free falling molten ferrous material poured following process stage i) ; and iii) transferring material from process stage ii) to a steel converter.