GB2157713A - Production and/or refining of metal by top blowing in conjunction with a stirring gas - Google Patents

Abstract

The invention provides a method of producing or refining metal of the kind involving successive blowing cycles, each comprising blowing a refining gas into or on to the upper surface of a melt contained in a treatment vessel and injecting a stirring or processing gas directly into the melt through at least one injection element projecting through the wall or base thereof, and tapping metal so produced or refined from the vessel; including the steps of providing the injection element, between successive blowing cycles, with a covering of solidified slag; monitoring pressure in the supply of gas to the injection element in a succeeding blowing cycle; and carrying out further injection element slag covering operations between blowing cycles in dependence upon the monitored pressure.

Description

SPECIFICATION Improvements in or relating to the production andl or refining of metal This invention relates to the production and/or refining of metal and more particularly, although not exclusively, to apparatus for and methods of producing or refining steel in a treatment vessel such as a converter vessel.

It has already been proposed to produce and/or refine metal such as steel in a vessel in a blowing cycle in which a melt of unconverted or unrefined metal is provided in the vessel, a converting or refining gas, such as oxygen, is blown into or on to the upper surface of the melt (by means of an overhead lance for example), and additionally, a stirring or processing gas is passed via one or more injection elements through the base or lower side wall of the vessel such as to inject directly into the melt below the level of the upper surface thereof.

After conversion (production) or refining, the steel is tapped from the vessel so that the steel is poured therefrom into a receiving ladle.

Whilst with such an arrangement excellent control of conversion or refining is obtainable, aided very largely by the injection of stirring or processing gas below the melt in the vessel, problems do arise in that severe erosion can occur at or adjacent the elements in the vessel wall through which the stirring or processing gas is introduced. To overcome this problem, it has been proposed that after tapping the converted or refined melt, remnant slag within the vessel be allowed to cover and solidify over the lower portion of the internal surface of the vessel including the injection element.

This serves to provide a protective covering, after solidification, of the injection element, but does introduce serious control problems of its own.

Thus, it has been found that too little slag cover over the injection element still permits serious erosion of the vessel base to occur, whilst too much slag cover can prevent any injection of gas through the injection element into the melt.

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

According to one aspect of the present invention there is provided a method of producing or refining metal of the kind involving successive blowing cycles, each comprising blowing a refining gas into or on to the upper surface of a melt contained in a treatment vessel and injecting a stirring or processing directly into the melt through at least one injection element projecting through the wall or base thereof, and tapping metal so produced or refined from the vessel; providing the injection element with a covering of solidified slag between successive blowing cycles from retained slag in the vessel; monitoring pressure in the supply of gas to the injection element in a succeeding blowing cycle; and carrying out further injection element slag covering operations between blowing cycles in dependance on the monitored pressure.

The invention is, at least partly, based upon the appreciation that the degree of slag cover over the injection element when metal is present in the vessel, is that this pressure can be used to monitor the degree of slag cover, so that by empirical calculation a pressure range relating to a range of slag cover thickness for optimum element gas flow/refractory performance, can be established.

Having established such a pressure range, the aim of the monitoring or the pressure in the supply of gas to the injection during operation of the vessel is to ensure maintainance of the pressure range by successive between blow injection element slag covering operations.

The injection element slag covering operations may be carried out by tilting or rocking the vessel so that remnant slag therein washes over the element.

Some gas injection may continue during such operations to cool and therefore to assist in the formation of a solid layer of slag over the element.

and also to ensure that gas outlets, perhaps in labyrinth or honeycomb form, are provided through the slag layer from the injection element.

The injection element may comprise a porous plug arrangement of convenient form, or may comprise a tuyere arrangement.

In steel converter operation the invention may be carried out by monitoring the back pressure in the supply to an injection nozzle used in operation of the converter vessel caused by the back pressure of the head of metal in the treatmet vessel. Such back pressure, A P equals the total back pressure of the converter full with charge (P2) less the back pressure of the converter loaded only with scrap (P1).

Alternatively or additionally, A P can be measured as the vessel is turned down to sample or to tap, P2 then being the back pressure at 100 NM/h prior to tilting the converter and P, the back pressure at the same flow of the same gas when the vessel is in the sample position and the elements are clear.

We have found in practice that with one or more fully open injection element passing, typically, gas at 100m3 per hour in a standard refining vessel, a A P value of 1.2 bar is obtained. In contract, should the injection elements be fully covered with slag so that no gas passes into the steel making bath the A P value is, of course, 0. Partially covered elements give A P values between 0 and 1.2 bar depending on the degree of slag cover.

In operation of the present invention we have determined that for minimum bottom refractory wear, A P values between 0.4 and 0.7 are necessary in practice, values within this range permitting a majority of the injection gas to pass into the bath to give good stirring effect whilst substantially reducing refractory wear around the elements.

Monitoring of the A P value during a blow can be used, by appropriate electronic circuitry, automatically to control the length of time during which the vessel is subjected to an injection element slag covering operation at the end of that blow. Thus, a A P value somewhat higher than the optimum value in one blow can be used to cause a longer than slag covering operation at the end of that blow, thereby increasing the degree of slag cover over the injection element, and therefore decreasing the L P value during the next blow.

As an alternative, or in addition, to controlling the degree of slag cover by the length of time of the slag cover operation, the degree of slag cover can also be controlled such that when the immediately preceding A P reading is high, a low flow is maintained through elements during the slag cover operation to encourage the formation of a solid slag cover over the injection elements, whilst when the immediately preceding L P reading is low (below 0.5 bar for example) a high gas flow through the injection element through the slag cover operation of a gas exit into the vessel through the relevant injection element.

The periods of variable flow just described can be adjusted to suit particular refining vessel or converter operations, and can be used during all tilting operations, and periods when only slag is in the vessel, and, indeed to increase metered flow so as to compensate for restrictions on gas flow thereby maintaining storring efficiency during refining operations.

Remnant slag within the treatment vessel after tapping can be 'thickened' and cooled with materials containing a source of magnesium oxide to give a more adherent coating of slag over the injection element. Dolomite can profitably be used for this operation, the vigourous calcining reaction affording a measure of protective cover to the lower sidewalls of the converter and assisting in giving a porous layer over the elements The invention includes within its scope apparatus for carrying out the method as hereinabove defined.

In order that the invention may be more readily understood, one embodiment thereof will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of the operation of the invention in connection with a steel converter vessel; and Figure 2 is a graphical representation of the operation of the invention through a number of 'blows' of the converter vessel of Figure 1.

Referring now to the drawings, it will be seen that a converter vessel 1 of standard configuration having a basic brick lining 12 and mounted in usual manner upon trunnions 3 for tilting action is provided in its base with gas pervious injection element 4, and an overhead oxygen lance 5.

The same vessel is illustrated in two phases of the cyclical blowing operation of the converted On the left hand side it is shown after tapping, by tilting, of the converter with a slag layer only across the base of the vessel above the injection element.

As will be seen a slag layer is illustrated above the injection element. On the right hand side the same vessel is shown with a charge of molten metal and with the oxygen lance in place, the injection element passing, stirring and processing the gas through the base of the vessel directly into the melt. The pressures P1 and P2 of the gas flowing through the injection element with slag cover over the element, and when full charge of molten metal is present respectively, are fed to an electronic circuit 6 which computes a value for A P during the conversion blow illustrated on the right. The L P value can then arrange to control, via circuit 7, the flow rates for the gas through the stirring element during the blow operation, and is arranged also to control, via circuit 8, the blow rates and time applied for the slag cover operation between successive blows.

Figure 2 is a graph of G P values and slag cover operation times against heat or blow numbers.

Reference to Figure 2 shows quite clearly the utilisation of slagging operations between successive blows of the converter of Figure 1 (illustrated by straight vertical lines 9) and the associated A P values (indicated by the dotted line 10 across the graph).

Periodical examination of the injection elements of the converter vessel by means, for example, of an AGA laser scanning method established that there had been a very considerable reduction in wear of refractory adjacent thereto during conversion operations when using injection element slag cover control based upon use of A P values in the manner illustrated in Figure 2. At the same time consistent conversion to steel is achieved with the vessel by utilising A P values to control flow rates of stirring gases to maintain adequate and correct stirring injection through the injection elements.

The A P values can be measured before a heat to assess the success of the slag coating, and/or at turn down, for example, or indeed during tapping to give a clearer indication for the slagging operations to follow. In fact G P can also be used to determine the frequency of slagging operations.

By means of the invention, a satisfactory control of the wear associated with below melt injection elements can be achieved without detracting in any way from satisfactory steel conversion.

Claims (11)

1. A method of producing or refining metal of the kind involving successive blowing cycles, each comprising blowing a refining gas into or on to the upper surface of a melt contained in a treatment vessel and injecting a stirring or processing gas directly into the melt through at least one injection element projecting through the wall or base thereof, and tapping metal so produced or refined from the vessel; including the steps of providing the injection element, between successive blowing cycles, with a covering of solidified slag; monitoring pressure in the supply of gas to the injection element in a succeeding blowing cycle; and carrying out further injection element slag covering operations between blowing cycles in dependence upon the monitored pressure.

2. A method as claimed in claim 1 wherein the injection element slag covering operations are carried out by tilting or rocking the vessel so that remnant slag therein washes over the element.

3. A method as claimed in claim 3 wherein gas injection, at least at a reduced rate, is continued during the injection element slag covering operations.

4. A method as claimed in any one of the preceding claims for the refining of steel in a steel converter vessel wherein pressure in the supply of gas to the injection element is carried out by monitoring the back presure A P, in the gas supply to an injection nozzle used in the operation of the vessel, where such back pressure, A P, equals the total back pressure of the converter fully charged less the back pressure of the converter loaded only with scrap.

5. A method as claimed in claim 4 wherein injection element slag covering operations are conducted such as to maintain A P values between 0.4 and 0.7.

6. A method as claimed in claims 4 or 5 wherein monitoring of G P during a converter blow is used automatically to control the length of time during which the vessel is subjected to the injection element slag covering operation at the end of that blow.

7. A method as claimed in claim 4, 5 or 6 wherein monitoring of A P during a converter blow is used automatically to control the rate of flow of gas through the injection element during the injection element slag covering operation at the end of that blow.

8. A method as claimed in claim 4, 5, 6 or 7 in which monitoring of A P is used to determine the frequency of slagging operations.

9. Apparatus for carrying out the method as claimed in any one of the preceding claims.

10. A method of refining metal substantially as hereinbefore described with reference to the accompanying drawings.

11. Apparatus for refining metal substantially as shown in and as hereinbefore described with reference to the accompanying drawings.