IMPROVEMENTS IN OR RELATING TO THE PRODUCTION OF IRON

Abstract: Process for the production of modified iron comprising the steps of containing the iron in molten form, in a prepared state for modifying, in a containing vessel having a sliding gate valve mounted in a wall thereof at or adjacent the base thereof, the sliding gate valve having an opening in the sliding gate connected to a source of modifying agent containing material and selectively movable into and out of alignment with an orifice through the wall of the vessel and comprising the steps of moving the gate so that modifying agent containing material is injected entrained in an inert gas through said opening therein into the orifice in the wall of the vessel for the formation of modified iron therein.

Description: Improvements in or relating to The Production of Iron This invention relates to the production of iron, for casting and more particularly, the production of quasi-fla e and spheroidal graphite iron. Such quasi flake and spheroidal graphite irons are hereinafter referred to as "modified iron". It is known to form modified iron by introducing into liquid iron, graphite modifying elements, alloys or mixtures^, containing singly or in combination, the elements magnesium, cerium, calcium, sodium, yttrium and lithium. Any such additive is hereinafter referred to as a "modifying agent". In addition, an innoculating element, usually silicon, may be contained in the modifying agent or added separately, in which case it is hereinafter referred to as an "inoculating agent". In the case of quasi-flake, this is usually preceded by the addition to the molten iron of a material such as a ferrotitanium alloy. The separate and distinct characteristics of quasi flake iron and SG iron render their production for a multiplicity of uses to be a highly desirable operation. However, the addition of a modifying agent, which is usually characterised by a low boiling point relative to liquid iron temperatures (eg o magnesium at 1107 C) and/or often high reactivity,to molten iron does present, in practice, serious difficulties due to the potentially violent reaction of the modifying agent with the molten iron. Thus, for example, proposals to add magnesium in the form of a briquette from above to the molten iron in a containing vessel usually involves a most violent reaction adjacent the surface in which a very high percentage (frequently more than 50Ϊ) of the magnesium does not, in fact, become absorbed in the iron but is lost by the severity of the reaction through the surface of the melt to the atmosphere. Another frequently used method of modification treatment is the overpour method where iron is tapped onto a modifying agent. This method also has the disadvantages of low and variable yields and dross formation. It is an object of the present invention to provide a process for the production of modified iron which overcomes or at least substantially reduces the above mentioned disadvantages. In accordance with the invention, there is provided a process for the production of modified iron comprising the steps. of containing the iron in molten form, in a prepared state for modifying, in a containing vessel having a sliding gate valve mounted in a wall thereof at or adjacent the base thereof, the sliding gate valve having an opening in the sliding gate connected to a source of modifying agent containing material and selectively movable into and out of alignment with an orifice through the wall of the vessel and comprising the steps of moving the gate so that modifying agent containing material is injected entrained in an inert gas through said opening therein into the orifice in the wall of the vessel for the formation of modified iron therein. The process may incorporate a sliding gate valve intended for the discharge of metal from the vessel therethrough in which case at least two openings pass through the sliding gate of the valve for separate alignment with the orifice through ^ the wall of the vessel to enable discharge of metal from the vessel through one opening in addition to the injection of the modifying agent containing material through the other opening. Alternatively the process may incorporate a sliding gate valve used solely for the purpose of gas and gas-solids injection, discharge of the metal being by means well known to the art, for example, lip or teapot spout teeming or stopper rod teeming. One known problem with sliding gate valves in the base of melt vessels, is that the orifice from the vessel above the valve in practice usually extends from a recess or well within the base, and because of its small volume this tends to encourage the solidification of molten metal at this point. This can lead to difficulties in full utilisation of the valve either for the injection of materials or for the pouring of metal therethrough on discharging the vessel. Accordingly, the process of the present invention may include, as an initial step, the provision of a mass of powdered or granular refractory material in the well in the wall of the vessel above the orifice and the valve prior to the filling of the vessel with the molten iron, and blowing a gas into the well prior to the injection of the modifying agent containing material such as to displace the refractory filler therefrom. The filler material may be blown from the well in the wall of the vessel by means of a gas through a melt discharge opening of the sliding gate valve or through the inlet opening for the modifying agent containing material. Alternatively, injection of the gas to clear the filler material may be by means of a tuyere extending within the thickness of a fixed plate of the sliding gate valve into the relevant well space. Yet again, a gas channel can be provided through a refractory brick of the wall of the metal vessel into the opening. It has been found that, by use of the mass of refractory filler material, the formation of a solidified plug of metal within the well and the orifice is prevented, whilst by the blowing of the gas into the well immediately prior to injecting the modifying agent containing material, the mass of refractory material can be displaced from the well and dissipated in the melt within the vessel so that it does not cause any blockage of the valve upon operation thereof. Apart from the operation of a gas blow to clear refractory material from the well above the orifice through the base of the vessel, gas bubbling through the bottom orifice of a kind often used for stirring and other purposes in molten metal processes, is, with the operation of the invention, to be avoided or at least reduced to a minimum during at least the majority of the operation, since such bubbling will tend to take the entrained modifying agent containing material to the surface of the melt where it can be lost to the atmosphere. Nevertheless it has been found advantageous to gas bubble for an optimum period of one half to two minutes at up to 50 Nl/min to effect separation of desulphurisation-deoxidation products, if temperature permits. With a tuyere of internal diameter of the order of 6mm, for example, the modifying agent can be pneumatically conveyed into the liquid iron with low gas flow rates because of the attainment of high gas velocities and, hence, powder velocities which prevent metal ingress into the tuyere during the injection. Typical gas flow rates used in this case would be 300 - 450 Nl/min, depending on the required powder flow rate. In a preferred embodiment of the invention, the efficiency of the operation of the invention is ensured by maintaining a slow injection of the modifying agent containing material whereby the potentially violent reaction of the modifying agent with the iron can be contained without creating unduly disturbed conditions within the melt and therefore loss through the surface of the modifying agent. We have found that, where magnesium is a modifying agent, magnesium injection rates in the range 3-6kg/minute can be satisfactory. Such rates compare with injections into steel for desulphurisation , for example, through a lance injection system of 50kg/minute. It is to be noted that the gas/solid ratio can be directly comparable to injection for desulphurisation, which is known in the art. Again, because of the violent nature of the reaction, the iron melt may be held in its containing vessel at a relatively low temperature thereby reducing the violence of the reaction. A typical injection commencement temperature for large section o o castings would be in the range 1290 C to 1340 C, an o injection termination temperature range of 1270 to o o 1280 , and then a teaming temperature of 1270 C to o 1280 C. For the production of small section thickness castings or where the containing vessel capacity is small, injection temperatures may need to be greater o than 1450 C. Where the modifying agent is magnesium, the magnesium containing material may be of any convenient nature such as a magnesium-lime granular mixture or magnesium-alumina-aluminium mix, or an alloy in granular form typical of those presently used for S G iron production, e.g. an Fe-Si-Mg alloy. As is known in the art, for injection processes the modifying agent whether based on magnesium, calcium, sodium, yttrium or lithium is preferably in finely divided form, e.g. with a tuyere having an internal diameter of the order of 6mm, a powder grain size of no greater than 1.4mm has been shown to be desirable. There are considerable advantages of using an injection technique. Thus more than one addition of a modifying agent material can be injected if required. Again it is possible to inject other reagents such as carbon simultaneously or sequentially with the injection of a modifying agent material. Yet again it is possible to bubble gas (i.e. during the modifying agent material injections) and thereby improve the temperature and compositional control, and to increase carbon pick-up from additions made during tapping or to the full ladle. The improved temperature control and homogenisation obtained with bubbling has been found to prevent ladle skulling and dross build-up in the ladle typical of the overpour technique. The invention, utilising sliding gate valves in or adjacent the base of a vessel has considerable advantage over earlier arrangements for injecting modifying agent containing materials into iron for its modification. In particular, it is to be noted that the level of injection is the maximum possible below the surface of the melt, thereby ensuring maximum efficiency capability and minimum loss of the reactive or low boiling point modifying agent. With a 40 tonne ladle the ladle depth is typically 2.0 metres and therefore the lance injection depth will be 1.6-1.9 metres, depending on the type of lance used. #■ With the injection/teeming valve the point of entry of the modifying agent will be typically 2.4 metres below metal level; the additional 0.5 m corresponding to the length of the channel formed by the well block and inner nozzle above the injection point in the sliding gate plate and the injection lance outlet to ladle floor distance. Also, with lance injection techniques the powder flow is usually started before immersion and is maintained until after the lance is withdrawn from the vessel. Using the present invention, with use of correct tuyere diameters (typically 6mm) and gas flows (typically 300 - 450 Nl/min), the powder is injected and terminated under an already established metal head thereby conserving high value injection materials. The applicants believe that their appreciation of the utilisation capability of sliding gate valves with appropriately modified techniques, is a most significant inventive development in the production of modified irons. By way of example, the following table illustrates injection trials into a 30 tonne molten iron ladle - 10- EXAMPLES Example Mg Mg Injection Mg After No Injected Injected time (mins) Injection Kg/tonne (kg) (wt.%) 1 0.8 24.8 7.0 0.033 2 0.8 24.8 9.2 0.0405 3 0.85 24.8 7.2 0.037 4 1.52 47.2 14.2 0.089 5 1.47 45.6 12.5 0.078 amplt 2 Sulphlur Mg Ingot No Before After Efficiency Mould Inject Inject % Quality -ion -ion (wt. % ) 1 0.041 0.002 78 S.G. 2 0.031 0.002 78 Q.F. 3 0.038 0.002 84 Q.F. 4 0.031 0.002 73 S.G. 5 0.049 0.002 68 S.G. It is to be noted that the material resulting in quasi flake iron had a ferro titanium alloy added to the vessel before injection of the magnesium. In all cases, it is to be noted that the absorption efficiency of the injected magnesium is around 70$ or above. The table also illustrates a further advantage of the process in the ability to produce irons with trace sulphur levels (0.002* S) from a high initial sulphur content which is significantly lower than those resulting from many present iron magnesium modification techniques. In the tests illustrated in the table, a magnesium-lime mix containing 80* by weight of magnesium entrained in argon gas was utilised. Nitrogen gas could equally be used for entraining. Examples of apparatus on which the invention can be practised are illustrated in the accompanying drawings in which:- Figure 1 is a side elevation showing a sliding gate valve for modifying agent injection only, prior to injection usage; Figure 2 shows the arrangement of Figure 1 in its injecting mode; Figure 3 is a sectional elevation of an injection and discharging sliding gate assembly mounted in the base of a ladle shown in its injection mode; and Figure 4 is a sectional elevation of a variant of the assembly of Figure 3. Referring now to Figures 1 and 2, it will be seen that the base wall 22 and lining 28 of an iron containing ladle are provided with a refractory upper nozzle insert 23 carrying a tuyere block 24 through which passes an inner tuyere 25. A lower nozzle block 26 is located in the wall 22 and engages with the static plate 27 of the sliding gate valve assembly 12. A sliding plate 29 carries an injection tuyere 30 connecting with a source of injecting modifying agent material incorporating magnesium (not shown). In operation a well 31 formed in the lining 28 and the upper nozzle 23 above the inner tuyere 25 are filled with filler material 32 which also fills the tuyere 25. The inner tuyere 25 and well 31 is filled to overflow with filler material with the gate 12 in the closed position. The ladle is then filled with molten iron. To eject the well filler 32, gas is supplied to the tuyere 30 and then this is indexed in the direction of arrow 13 of Figure 1 so that it aligns with the inner tuyere 25 as shown in Figure 2 and blows out the filler material 32. Following a period of gas blowing to ensure that the inner tuyere 25 is clear of filler material, injection of modifying agent containing material can commence. At the end of such injection, the gate 12 is indexed in the direction of arrow 14 of Figure 2 and thereby closed. The arrangement of Figure 3 operates in a similar fashion to that of Figures 1 and 2 except that in this case the valve 1 combines injection with melt discharge by means of a discharge inner nozzle in the vessel base. After injection of the modifying agent containing material through tuyere 11, the plate 4 is - 1 3- indexed so as to close the orifice for a brief period whilst the iron/modifying agent reaction is completed or alternatively to gas bubble at a very low flow rate (up 50 Nl/min) to effect separation of 5 desulphurisation deoxidation products. The plate is then indexed to align the inner nozzle 6 with the pouring nozzle 5 so that the iron, either in 5 the form of quasi flake or spheroidal graphite is poured from the vessel through the nozzle 5. 10 In the variant of Figure 3 shown in Figure 4 a second injection tuyere 40 is provided, so that if blockage or wear of tuyere 11 occurs, the tuyere 40 can be used in its stead. In this arrangement the sliding gate assembly needs to be a four position 15 gate. In general terms it is to be noted that any convenient tuyere configuration can be used, e.g. multi-hole, multiple or concentric. In some cases, it may be practicable to discharge 20 the metal from the ladle by lip teeming or stopper rod, with the sliding gate valve being in the shut position or gas bubbling employed at a very low gas flow rate to maintain an open inner nozzle/well block. Once the metal has been lip teemed the discharge 25 nozzle of the sliding gate valve can be indexed and used as a lancing hole to re-open the inner nozzle/well block. The ladle may incorporate a lid with facilities for fume extraction or, alternatively, the lid may provide a pressure tight seal to allow modifying agent vapour to increase pressure above the melt thereby increasing modifying agent recovery or efficiency as is well known in the art.

Claims: CLAIMS 1 A process for the production of modified iron comprising the steps of containing the iron in molten form, in a prepared state for modifying, in a containing vessel having a sliding gate valve mounted in a wall thereof at or adjacent the base thereof, the sliding gate valve having an opening in the sliding gate connected to a source of modifying agent containing material and selectively movable into and out of alignment with an orifice through the wall of the vessel and comprising the steps of moving the gate so that modifying agent containing material is injected entrained in an inert gas through said opening therein into the orifice in the wall of the vessel for the formation of modified iron therein. 2 A process as claimed in claim 1 wherein the sliding gate valve is used for the discharge of metal from the vessel therethrough, and at least two openings pass through the sliding gate of the valve for separate alignment with the orifice through the wall of the vessel to enable the discharge of metal from the vessel through one opening in addition to the injection of the modifying agent containing material through the other opening. 3 A process as claimed in claim 1 wherein the sliding gate valve is used solely for the purpose of gas and gas-solids injection.# ■ 4 A process as claimed in any one of the preceding claims wherein as an initial step a mass of powdered or granular refractory material is provided in a well in the wall of the vessel above the orifice and valve prior to the filling of the vessel with molten iron, and blowing a gas into the well prior to the injection of the modifying agent containing material such as to displace the refractory filler therefrom. 5 A process as claimed in claim 4 wherein the filler material is blown from the well by means of a gas through a melt discharge opening of the sliding gate valve or through the inlet opening for the modifying agent containing material or through a separate tuyere extending within the thickness of a fixed plate of the valve into the well space or through a brick of the wall of the metal vessel into the well. 6 A process as claimed in any one of the preceding claims including the step of gas bubbling through the bottom orifice for a few minutes to effect separation of desulphurisation-de-oxidation products. 7 A process as claimed in any one of the preceding claims wherein the injection of the modifying agent containing material is conducted at a slow rate. 8 A process as claimed in claim 7 wherein the modifying agent is based on magnesium and the injection is at a rate in the range of 3-6 kg/minute. 9 A process as claimed in any one of the preceding claims wherein the modifying agent containing material is a magnesium-lime mix or a magnesium-alumina- aluminium mix. 10 A process as claimed in any one of claims 1 to 8 wherein the magnesium containing modifying agent is a magnesium containing alloy in granular form. 11 A process for the production of modified iron substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: KENNEDY JAMES, STAMP DAVID WILLIAM, KENNEDY, JAMES, STAMP, DAVID, WILLIAM
Applicants: BRITISH STEEL CORP, BRITISH STEEL CORPORATION
Application year: 1987
Main classification: C21C1
IPC codes: B22D1, B22D41, C21C1