GB2158379A - Improvements in or relating to the operation of sliding closures below melt openings of liquid-metal containing vessels - Google Patents

Abstract

A mass of powdered or granular refractory filler material is placed in a well in the base of the vessel above the opening and above the fixed and the sliding closure plates of sliding gate valve with the valve closed; the vessel is filled with molten metal; and subsequently, a gas is blown into the well such as to displace the refractory filler therefrom. Thereafter molten metal may be discharged through the basal opening via the sliding gate valve; or another material e.g. a treatment agent in a carrier gas, may be injected into the molten metal via the sliding gate valve and basal opening. Arrangements for carrying out the procedure can be embodied in various ways.

Description

SPECIFICATION Improvements in or relating to the operation of sliding closures for below melt openings of liquid metal containing vessels This invention relates to the operation of sliding closures, commonly called sliding gate valves, of the linear or rotary displacement types, for below melt basal openings in liquid metal containing vessels such as ladles or furnaces, where the openings are used as outlets for metal discharge or as inlets for gas and/or solids injection.

Sliding gate valves for use in molten metal vessels such as casting ladles or tundishes for continuous casting are of course well known.

They are advantageous over previously used conventional closure facilities (such as stopper and stopper rod assemblies) in that for example refractory wear problems commonly associated with stopper and stopper rod assemblies caused at least in part by the high metal temperatures currently used, do not occur to anything like the same extent with sliding gate valves. Additionally, of course, attack from chemically active slag is less harmful with regard to sliding gate valves than with stopper rods and stoppers. Similarly sliding gates have been proposed for inlets for the provision of gas and/or solids injection.

One problem, however, with sliding gate valves for outlets, is that the recessed outlet opening above the valve, because of its small volume, encourages solidification of molten metal prior to pouring, so that commencement of pouring through the outlet can at times be hindered and even prevented by the presence of a plug of solidified metal in the vessel well above the outlet valve.

In an attempt to overcome this problem there is often provided in the well of the metal vessel, above the sliding gate valve, prior to filling the vessel with molten metal, a mass of powdered refractory material.

This mass of "filler" material located in the well of the outlet opening prevents molten metal from entering whilst the vessel or ladle is standing prior to pouring, and therefore prevents freezing. However, it has been found that upon opening the sliding gate valve with such filler material in place, there is a possibility-that the refractory filler itself will block the outlet opening and require to be removed, usually by oxygen lancing.

However, this practice degrades the quality of steel in the ladle by the formation of oxidation products and is also inconvenient when the sliding gate delivers metal through a protective shroud.

Again, where, as has been previously proposed, solid powdered granular reagents are injected into the outlet of a melt containing vessel, via a tuyere through the sliding gate valve (for desulphurisation for example), injection cannot commence until the filler material has been displaced.

Similarly sliding gate valves for inlets for gas and/or solids injection do not solve the problem that the inlet opening through the vessel wall tends to become blocked with solidified metal, thereby preventing injection of the gas and/or solid when the sliding gate valve is moved to align the vessel tuyere or nozzle with the gate valve tuyere.

It is an object of the present invention to overcome or at least substantially reduce the above mentioned problems.

According to the present invention, there is provided a method of operating a sliding gate valve for a below melt basal opening of a molten metal containing vessel in connection with the passage of material therethrough comprising the steps of placing a mass of powdered or granular refractory filler material in a well in the base of the vessel above the opening and above the fixed and the sliding closure plates of sliding gate valve with the valve closed; filling the vessel with molten metal; blowing a gas into the well such as to displace the refractory filler therefrom; and passing material through the basal opening via the sliding gate valve.

As previously intimated the passage of material through the sliding gate valve may comprise the discharge of molten metal from the vessel, which may be preceded by the ancillary injection of material therethrough into the vessel. In this case the gas may be blown into the well just prior to operating the sliding gate for discharge or injection as the case may be.

Alternatively the passage of material through the sliding gate valve may solely comprise the injection of gas and/or solid material (such as granular or powdered additives) into the vessel. In this case the gas may be blown into the well upon opening the valve.

The mass of filler material placed in the well may be allowed to fill the inlet or outlet opening (as the case may be) in the base of the vessel.

We have found that by means of the invention, where the basal opening is for the injection of gas and/or solid material into the vessel the provision of the filler material is able to prevent blocking of the opening by solidified metal.

We have also found that by means of the invention, where the basal opening is for the discharge of molten metal from the vessel, and the formation of a solidified plug of metal within the well and opening is prevented by the mass of refractory filler, and also, by the inventive feature of blowing a gas into the well/inner nozzle assembly in the base of the metal containing vessel above the sliding gate closure plates immediately prior to pouring metal through the gate valve, the mass of refractory filler can be displaced from the well and dissipated in the melt within the vessel immediately prior to injection or pouring so that it cannot cause any blockage of the outlet upon operation of the sliding gate valve for either injection or pouring.

The sliding gate valve may be of any conventional configuration, such that the sliding plate may move on a linear or rotary path or alternatively the sliding plate may be successively displaced and replaced by replacement plates. In one embodiment, the invention is applied with respect to a sliding gate valve of the kind disclosed in and defined by our United Kingdom Patent No. 1 554104.

In that patent specification a sliding gate valve assembly is disclosed in which the valve incorporates an inlet tuyere or pipe fixed to and passing through the sliding plate, and offset from the pouring nozzle of the sliding plate such that the inlet pipe or tuyere may be aligned with the opening and well in the base of the vessel for melt treatment such as desulphurisation. With such an arrangement, operation of the present invention may be such that prior to injection of gas powder mixes into the vessel a gas is blown through the inlet pipe or tuyere into the well, to displace the filler material. The pipe is then used to provide a period of gas stirring to ensure that the well and inner nozzle is completely free of filler material and to prevent solidification of steel in the well and inner nozzle.Injection of entrained solid material via the gas, in accordance with the 1 5 provisions of UK Patent Specification No. 1554104 then takes place.

The invention is additionally of value in relation to the pouring of molten steel for example from a ladle by means of a sliding gate valve into the tundish of a continuous casting plant.

In this case it is common for a refractory shroud to be disposed around the stream of metal from the sliding gate downwards into the receiving tundish. The shroud is intended to exclude atmosphere from the stream to reduce oxidation. With this form of application, blockages at the pouring stage can be of very grave concern since the outlet from the ladle once in position is inaccessible and failure to initiate teeming necessitates the use of an oxygen lance which in turn contaminates the steel with additional oxide inclusions.

In an alternative arrangement, injection of the gas to clear filler material from the inner opening and well can be by means of a tuyere extending longitudinally within the thickness of the fixed plate of the sliding gate valve into the relevant opening and well space.

Yet again, a gas channel can be provided through the refractory brick of the base of the metal vessel into the opening.

Where a tuyere or pipe is used for the injection of gas this can be ceramic, metallic or a composite material and can, for example, be of aluminium or stainless steel.

With the tuyere in the injection position it may correspond to the vertical centre line of the opening or may be off centre to provide a non-symmetrical blowing pattern when operated.

The tuyere diameter may be smaller than the diameter of the base opening of the vessel, and may be in the range 1 .0mum to 1 0.Omm. This compares with a typical opening diameter of 65mm. A small tuyere diameter is advantageous in that it provides a high gas velocity which is most effective for the expelling of the filler material from the opening.

The gas used to displace the filler material may be inert such as nitrogen, or argon or can be reactive such as air or oxygen. The gas pressure at the tuyere tip will of course, be chosen such as to be able to displace the filler. The pressure applied must be sufficient to rupture any sintered or fused filler in contact with the liquid metal, to.rupture any solidified metal in contact with the filler, in addition to exceeding the pressure exerted by the liquid steel. The consequent gas flow through the inner nozzle/well must then be sufficient to displace the residual filler. The characteristics cf the filler material, its grain size, size analysis, purity and composition have a significant effect on the gas pressure required to eject the filler.

By way of an example it was found that in a 4 tonne capacity ladle gas flows of between 300-1500 NL/min at a pressure of 1-5 bar have been found necessary to displace the mass of well filler, (2 kg to 1 5 kg).

The invention includes within its scope apparatus for carrying out the methods herein specified.

In order that the invention may be more readily understood, a number of embodiments thereof will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a sectional side elevation of a sliding gate valve assembly intended for operation according to the invention; Figure 2 is a variation on the sliding gate valve of Figure 1; Figure 3 is a further variation on the sliding gate valve arrangement of Figure 1; Figure 4 is yet another variation on the sliding gate valve arrangement of Figure 1; Figure 5 is a still further variation on the sliding gate valve arrangement of Figure 1.

Figure 6 is a sectional side elevation of a further form of sliding gate assembly intended for operation according to the invention; Figure 7 shows the assembly of Figure 6 in a second operational position; Figure 8 shows the assembly of Figure 7 in a third operational position; Figure 9 shows a variant of the assembly of Figure 6; and Figure 10 is a further variant of the assembly of Figure 6.

It will be seen that Figure 1 illustrates a sliding gate valve 1 intended to be disposed at the outlet from a base 7 of a molten steel vessel 10, the valve having a fixed plate 3 and a sliding plate 4 carrying a discharge nozzle 5.

The ladle outlet has an inner nozzle 6 set in its base which is surrounded by a well 8 within the refractory lining 9 of the base of the ladle 10.

The outlet is controlled by the sliding gate valve assembly which is formed of refractory.

A tuyere 11 is disposed within the moving plate 4 alongside the pouring nozzle 5.

The sliding gate is actuated by a hydraulic set such as to be moveable in a horizontal plane between three positions. In the first position the outlet is closed by the refractory sliding gate 4, in the second position the outlet is in alignment with the tuyere 11, and in the third position the inner nozzle 6 is in alignment with the discharge nozzle 5.

In operation, the well 8 and inner nozzle 6 are filled with a mass of refractory filler material (not shown) such as chrome sand-graphite. The ladle then receives liquid steel from a B.O.S. vessel or electric arc furnace with the outlet closed by the sliding gate in the above mentioned first position.

Alternatively the vessel may be a steel melting and/or refining vessel containing a sliding gate valve such as a top or bottom blown basic oxygen furnace or electric arc furnace.

Such vessels may incorporate a sliding gate valve to control the discharge of liquid metal and slag.

The sliding gate is subsequently moved to the above mentioned second position and an inert gas such as argon blown through the tuyere 11 to clear the inner nozzle and well of filler material. Gas pressure is preferably applied to the tuyere before moving the plate to register the tuyere with the inner nozzle. Gas is continuously blown through the well for a substantial period to ensure all filler material is removed. The sliding plate 4 may then be moved to allow pouring through nozzle 5 or alternatively an entrained solid in powder form, such as a desulphurant may be blown via the tuyere into the liquid metal through the inner nozzle 6. After an appropriate period of time the entrained solids supply may be shut off whilst a reduced gas flow is maintained to prevent freezing of the liquid steel within the inner nozzle and well.

Finally, after only a brief delay to allow for the process reaction products to separate from the metal aided by continuous gas injection, the sliding gate may be moved to the above mentioned third position whereupon metal is discharged through the nozzle 5 into, for example, a continuous casting tundish or ingot moulds.

In an alternative arrangement illustrated in Figure 2 it is to be seen that the tuyere 1 2 for ejecting the filler material from the well 8 is separate and distinct from the tuyere 1 3 used for solids injection. This is because in some circumstances where powders are to be introduced at the rate of, for example, 50 to 100 kg/minute, a tuyere size of greater than 1 2 mm may be required, whilst as is discussed above, the optimum size for the tuyere through which gas for removing filler material from the well 8 might be considerably smaller than this. It will be appreciated, however, that where powdered reagents can be injected at low flow rates, a single small bore tuyere as exemplified in Figure 1 can be utilised.Alternatively, the arrangement of Figure 1 can be utilised without injection of solid material where the well filler is to be ejected from the well 8 and inner nozzle 6 for teeming from the ladle into a tundish for subsequent continuous casting. It is to be noted that the tuyere 1 2 extends alongthe fixed plate 3 through the thickness thereof.

Figure 3 illustrates two further alternative arrangements in which a tuyere 14 or 1 5 for the gas for displacing the well filler and preventing freezing in the inner nozzle and well passes through the sliding plate 4 of the valve, but at a different location to the tuyere used for solids injection. It will be seen that two alternative dispositions are illustrated, in one of which tuyere 1 5 passes directly through the sliding plate, and in the other of which tuyere 1 4 passes along plate 4 through the thickness thereof.

It will be appreciated that a second tuyere necessitates a four position sliding gate assembly i.e. a shut-off position, registration of the small bore well filler displacement tuyere with the inner nozzle, registration of the gassolids injection tuyere and a teeming position.

In any of these arrangements the tuyere used for displacing the well filler may be a multi-hole tuyere 1 9 as shown in Figure 5.

The advantage of this arrangement being the reduced gas flow rates necessary to prevent wetting of the tuyere. This latter feature is important in minimising excessive turbulence in the melt during gas injection and to minimise unwanted slag/metal reaction or atmosphere/liquid steel reactions.

Figure 4 illustrates yet another alternative arrangement in which there is provided a gas channel 1 6 for the gas for displacing the well filler which passes through the base of the ladle itself, through the ceramic lining bricks and into the inner nozzle 6 itself.

Figures 6 to 8 illustrate a sliding gate valve assembly 20 for use with an injection tuyere assembly 21 for gas and/or solids injection into a molten metal ladle having a base wall 22. and lining 28. In this case the sole function of the valve assembly is for injection treatments and excludes metal discharge.

The wall and lining is 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 20.

A sliding plate 29 carries an injection tuyere 30 connecting with a source of injection material (not shown).

A well 31 formed in the lining 22 and upper nozzle 23 above the inner tuyere 25 is 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 as shown in Figure 6, with the gate 20 in the closed position.

The ladle is then filled with molten metal. To eject the well filler 32 gas is supplied to the tuyere 30 and then this is indexed so that it aligns with the inner tuyere 25 as shown in Figure 7 and blows out the filler material 32.

Following a period of gas stirring to ensure that the inner tuyere 25 is clear of filler material solids injection can commence if reqired. After injecting the required amount a further period of gas stirring is possble using the inner tuyere 25.

Gas stirring may be maintained until the ladle has been teemed so that the inner tuyere 25 can be reused. If plant logistics do not allow continued gas stirring then the gate is closed, so resulting in metal penetration and sealing of the inner tuyere, as shown in Figure 8 at 33. A new inner tuyere must thereafter be installed or the solidified metal plug removed from the tuyere.

An alternative arrangement is shown in Figure 9 where, instead of one tuyere, a multituyere block 34 is used. Care will have to be taken to ensure that sufficient filler material is covering the unused tuyeres to maintain their integrity.

In Figure 9, as only one tuyere is used at a time, as the moving plate 29 moves from right to left, inner tuyere 35 will be used first.

To reduce misalignment inaccuracies with the moving plate the bore of the inner tuyere adjacent to the moving plate may be tapered (not shown). The operating parameters of the dedicated system are similar to that of the dual function system, but with the injecticn tuyere system the inner tuyere size is fixed.

The particular plant processing may determine the tuyere arrangement, i.e. number of tuyeres, diameter of tuyere.

Thus, if the tuyere is to be only used for gas stirring then tuyere diameters of < 6 mm may be used, the exact tuyere diameter being dependent on the required minimum and maximum gas flow rates.

On the other hand, for solids injection rates of 100 kg/minute a multiport-tuyere arrangement 36 (Figure 10) or a large diameter tuyere ( > 1 2 mm) may be necesary.

The tuyere material and gas compositions used in relation to the embodiments of Figures 1 to 5 can be used in the embodiments of Figure 6 to 10.

By means of the invention we have provided an arrangement whereby displacement of the filler material prior to an injection process or pouring from containing vessels of molten metal, is practised.

The same arrangement also ensures, with a pouring arrangement, that freezing within the vessel opening and well is prevented after completion of the injection process and allows immediate opening of the gate. The arrangement may be applied to pouring for the ingot or continuous casting routes and may be subjected to various vacuum degassing and secondary steel making operations.

The arrangement may also be applied to metal containing vessels such as top or bottom blown basic oxygen furnaces and arc furnaces where sliding gate valves may be incorporated to facilitate control of slag discharge or to facilitate bottom tapping.

Claims (14)

1. A method of operating a sliding gate valve for a below melt basal opening of a molten metal containing vessel in connection with the passage of material therethrough, comprising the steps of placing a mass of powdered or granular refractory filler material in a well in the base of the vessel above the opening and above the fixed and the sliding closure plates of sliding gate valve with the valve closed; filling the vessel with molten metal; blowing a gas into the well such as to displace the refractory filler therefrom; and passing material through the basal opening via the sliding gate valve.

2. A method as claimed in Claim 1 wherein the vessel basal opening is an outlet arranged for the discharge of metal therefrom.

3. A method as claimed in Claim 2 wherein the gas is blown into the well just prior to operating the sliding gate for discharging molten metal.

4. A method as claimed in Claim 2 wherein gas and additives are injected through the opening prior to discharge of metal.

5. A method as claimed in Claim 4 wherein the gas is blown into the well just prior to operating the sliding gate for injection purposes.

6. A method as claimed in Claim 1 wherein the vessel basal opening is an inlet arranged solely for the injection of gas or gas with entrained solid material.

7. A method as claimed in Claim 6 wherein the gas is blown into the well upon opening the valve.

8. A method as claimed in any one of the preceding claims wherein the gas blown into the well such as to displace the refractory filler therefrom is passed into the well via a pipe specially provided for that purpose.

9. A method as claimed in any one of Claims 1 to 7 wherein the gas blown into the well such as to displace the refractory filler therefrom is passed from a pipe located in the sliding gate assembly for the injection of gas or gas with entrained solid material.

1 0. A method as claimed in any one of the preceding claims wherein the vessel basal opening is filled with granular refractory filler material at the same time as the well, and the filler material is displaced therefrom at the same time as from the well by blowing of a gas.

11. A method as claimed in any one of the preceding claims wherein the gas used to displace the filler material is inert or oxidising.

1 2. A method as claimed in any of the preceding claims wherein the pressure applied to the blowing gas is sufficient to exceed the ferrostatic pressure of liquid steel above the well in addition to being sufficient to rupture any filler-liquid steel interface, the gas flow rate additionally being sufficient to remove all residual filler from the well.

1 3. A method of operating a sliding gate valve substantially as herein described with reference to the accompanying drawings.

14. Apparatus for carrying out the methods as claimed in any one of the preceding claims.

1 5. Sliding gate valve apparatus substantially as shown in and as hereinbefore described with reference to the accompanying drawings.