EP0032343B1 - Method for the gaseous stirring of a molten metal bath - Google Patents

Abstract

A process for periodically and pneumatically stirring a bath of molten metal, wherein during the periods in which no stirring effect is required a fluid in gaseous state is injected through injection devices below the surface of the bath of molten metal in an amount sufficient to prevent blockage of the devices, whereas during the periods in which a stirring effect is required a fluid in liquid state which will vaporize quickly upon contact with the liquid metal is injected to provide an optimum amount of mixing gas with a minimum of injection devices. The process is advantageously used in converters for pneumatically converting pig iron into steel and in which oxygen is blown from above onto the bath of molten metal.

Description

The present invention relates to the operations of pneumatic stirring of a bath of molten metal by injecting a gas under the surface of the bath.

It is known that such operations find application in various industrial sectors and in particular in the steel industry, where it has already been proposed (French utility certificate no. 2,322,202) to extend the metallurgical possibilities of the processes for pneumatically converting cast iron. by combining, in the converter, a blowing of oxygen from the top by means of an emerged lance, with a blowing of inert gas in the metal bath during and after the refining with oxygen by means of injection devices emerging below the surface and implanted in the wall or more generally in the bottom of the container.

A handicap of this technique results from the fact that very often pneumatic stirring is useful or sought after, for the treatment of metal, only temporarily while the injection of gas, which provides this stirring, is necessary permanently to avoid that the molten metal clogs the injection device by solidifying it.

In this regard, if, taking into account the pressure available in the installation, the injection device is calibrated so as to obtain an optimal gas flow rate for stirring, the minimum flow rate necessarily to be observed outside of the stirring periods for protecting the injection device against the risk of blockage constitutes a substantial economic penalty which may not be justified for metallurgical reasons.

Conversely. if the injection device is calibrated so as to minimize the protection flow, it is often difficult, if not impossible, to then achieve the desired gas flow rates for efficient stirring. We can then think of increasing the number of injection devices, but in doing so, we are brought back to the previous problem.

The object of the present invention is to remedy these difficulties.

To this end, the subject of the invention is a process for pneumatically stirring a bath of molten metal by introducing an inert fluid by means of an injection device opening out under the surface of the bath, according to which the effect brewing is only sought temporarily and characterized in that, outside the brewing periods, the injection device is supplied with an inert fluid in the gaseous state, and in that, during the brewing periods, the inert gaseous fluid is replaced with an inert fluid in the liquid state capable of vaporizing easily on contact with the molten metal.

The fluids injected respectively in gaseous form and in liquid form can be either of the same chemical nature or of different chemical natures.

According to a variant, the fluid injected in liquid form has a density / molecular mass ratio of high value.

The method of the invention can be implemented both with an injection device constituted by nozzles opening out under the surface of the molten metal bath as with an injection device constituted by a plurality of refractory parts, selective and oriented internal permeability, incorporated into the bottom of the container containing the molten metal bath. Such permeable refractory parts have been described in patent application EP-0 021 861, in the name of the applicant.

As can be understood, the invention therefore consists, in its essential characteristics, of modifying during operation the physical state of the stirring fluid - and possibly also its chemical nature - so as to inject a protective gas into the device. injection when the stirring of the bath is not necessary and to inject a stirring liquid when this need occurs.

Thus, the invention provides an elegant solution to the problem posed by making it possible to reconcile the apparently contradictory imperatives of minimizing the protection flow and optimizing the stirring flow by means of the same injection device.

The invention will be better understood in its various aspects and advantages thanks to the description below of two examples of application of inert gas mixing during a pneumatic refining operation of the cast iron in a blown converter. oxygen from above.

Example 1

We consider an LD type converter with a capacity of 60 t and the bottom of which has been fitted with an injection device consisting of steel nozzles having an internal diameter of 5 mm. The nozzles are four in number uniformly distributed in the bottom and connected to a nitrogen supply system whose maximum allowable pressure is limited to 10 ⁵ Pascal.

In Figure 1 attached, there is shown at 1 the converter with the lance 2 for blowing oxygen from above. The bottom 3 of the converter is provided with nozzles 4. The nozzles 4 consist of a central tube 5, through which liquid or gaseous nitrogen passes, and of an external tube 6 concentric with the tube 5 and defining with the tube 5 an annular space into which a cooling fluid is injected. The tubes 5 are connected to a manifold 7 supplied, either with liquid nitrogen coming from the source 8, or with gaseous nitrogen coming from the source 9. The placing of the manifold 7 in communication with the source 8 or with the source 9 takes place using the three-way valve symbolized at 10. The annular spaces defined between the tubes 5 and 6 are connected to a source 11 coolant through a line 12.

During the oxygen refining phase, mixing with nitrogen is not necessary since the decarburization itself provides sufficient agitation of the metal bath. It is therefore, during this period, to inject the minimum amount of gas per nozzle, necessary to avoid clogging thereof. It is known that the generally adopted criterion consists in obtaining a sonic velocity of the gas at the nose of the nozzle. Under these conditions, the minimum protection flow is close to 10 1 of nitrogen per second and this flow is obtained under a pressure of 2 · 10 5 Pascal.

At the end of the oxygen refining period, the treatment of the bath begins with stirring with nitrogen. Nitrogen in the liquefied state is then substituted for the previously blown nitrogen gas. Under the maximum pressure of 15. 105 Pascal, the flow of liquid nitrogen passing through the nozzle is 0.2 1 / sec. approximately which, on contact with the molten metal in the converter vaporizes rapidly, providing a gas flow rate close to 120 IIsec., which corresponds to the value sought for efficient stirring.

By way of comparison, if one continues to supply the nozzles with nitrogen gas after the refining period, the maximum gas flow rate that can be reached per nozzle is only 60 l / sec. about.

As such a flow rate may be insufficient to ensure satisfactory stirring, it is then necessary either to double the number of nozzles, hence an investment and additional operating costs, or to use nozzles of larger internal diameter, this which in any case leads to double the consumption of nitrogen necessary for the protection of the nozzles.

It can be seen that the process according to the invention makes it possible, in the case of nitrogen mixing, to double the mixing flow at a given protection flow, or, expressed differently, to reduce the protection flow for half for a flow of brewing given.

Of course, these proportions are valid for nitrogen and are generally modified with the nature of the gas used. Thus, if liquefied argon is used, the maximum flow rate will be 65 l / sec. approximately, therefore much more unfavorable than in the case of nitrogen.

Example 2

We consider an LD type converter with a capacity of 60 t. Five permeable refractory pieces are incorporated into the masonry at the bottom of the converter. Each of these parts is made up of an ordered and joined assembly of non-porous refractory plates, juxtaposed without a material seal between them; the tightening and cohesion of these plates is ensured by shrinking, by means of a metal casing; a closing plate completes the metal envelope so as to seal each part vis-à-vis the outside of the converter; a fluid supply pipe is tightly fixed on this closure plate and opens into a fluid distribution channel formed inside the assembly of the refractory plates each constituting the permeable refractory parts.

In Figure 2 attached, there is shown at 1 the converter with the lance 2 for blowing oxygen from above. In the bottom 3 of the converter, the permeable refractory pieces 13 are placed. Each piece 13 is constituted by an assembly 14 of refractory plates maintained laterally by a metal casing 15 completed by a closure plate 16. The pieces 13 are connected by conduits 17 to a nurse 7 supplied, either with liquid nitrogen coming from the source 8, or with gaseous nitrogen coming from the source 9. The nurse 7 is placed in communication with the source 8 or with the source 9 using of valve 10.

During the oxygen refining phase, nitrogen gas is injected at a minimum flow rate of less than 1 l / sec. ; this flow rate is obtained at a pressure of approximately 10 ⁵ Pascal. At the end of the oxygen refining period, nitrogen is then injected in the liquid state at a rate of approximately 0.3 l / sec. ; this flow rate is obtained at a pressure of approximately 6 · 10 ⁵ Pascal. On contact with the molten metal, the liquid nitrogen vaporizes quickly and provides a gas flow close to 200 l / sec.

According to a variant of the invention, the gaseous and liquid fluids are of different chemical nature. For example, in a pig iron refining operation, the shielding gas can be argon and the brewing liquid liquid nitrogen, whose cost price is lower than that of liquid argon and whose injection after the refining period, that is to say in a highly oxidized metal bath, no longer presents a great risk of nitriding of the latter. Furthermore, this passage from argon to liquid nitrogen makes it possible to achieve a flow rate of vaporized gas which, as we have seen, could not be obtained with liquid argon.

However, the use of fluids of the same kind has an appreciable practical advantage, namely the possibility of implementing the invention with a single source of liquefied gas.

This single source is then connected to the injection device by two parallel circuits, one of which includes an evaporator and which are activated alternately using a “bypass”, depending on whether one wishes to blow the gas or inject the brewing liquid.

It goes without saying that the invention is not limited to liquefied gases, but extends to any fluid in the liquid state under standard conditions of temperature and pressure, but insofar as it is obviously not not harmful to the development of the metal to be brewed. Of course, mixtures of fluids of different chemical natures are also possible.

In view of the above, it is advantageous to use a brewing fluid which. per unit volume in the liquid state, provides the largest volume of gas by vaporization on contact with the molten metal.

Generally speaking. it is therefore advantageous to choose a stirring fluid which has the highest possible density / molecular weight ratio.

In this respect, liquefied gases appear to be entirely suitable.

These considerations should not make us lose sight of others relating to the viscosity of the stirring liquid on which, of course, the flow rate likely to be able to pass through the injection device under a given pressure depends.

This explains why the use of liquid argon which nevertheless presents a density / molecular mass ratio appreciably higher than that of liquid nitrogen (respectively 35 and 29 1 ′ approximately) leads to a flow of vaporized gas. much lower than the latter.

Otherwise. these considerations do not take into account the cooling effects of the bath caused in particular by the vaporization of the stirring fluid, but it is known to counterbalance them if necessary by various additional heat supply means such as preheating of the fluid when this is possible or an extension of the oxygen blowing beyond the actual refining period.

In addition. the method according to the invention makes it possible to reduce the brewing period compared to the known technique. This is an appreciable advantage, in particular on the thermal level. because studies made by the inventors have shown that the cooling of a steel bath in a converter during brewing is less the result of the blowing of the brewing fluid than the calorific losses caused by the expectations for sampling and the duration of agitation of the bath, although these losses diminish when the converter capacity increases.

Otherwise. it should be emphasized that if the stirring liquid can be under standard conditions of temperature and pressure, a gas which has been liquefied prior to its injection, the opposite is quite possible with regard to the shielding gas .

Indeed. according to a variant, the protective gas of the injection device is. under normal conditions of temperature and pressure, a liquid which has been vaporized prior to its passage through the injection device.

We immediately understand the particular advantages of this variant as regards precisely the “thermal” aspect of the operation.

However, in accordance with another variant embodiment, the protection of the injection device with a gas originating from the prior vaporization of a liquid is combined with stirring of the bath by injection of a liquid originating from a previously liquefied gas. , which makes it possible to have a high flow rate of stirring gas, while reducing to a certain extent the cooling of the bath outside the brewing periods.

Finally, the invention is not limited, as regards its applications, to mixing in a converter, but extends to other fields, such as the processing of steel in a ladle, and more generally, to any treatment. pneumatic stirring of a bath of molten metal by injecting a stirring fluid under the surface of the bath.

Claims (9)

1. A process for stirring a bath of molten metal using pneumatic means and by feeding an inert fluid through an injection device opening under the bath surface, according to which the stirring effect is required but temporarily and characterized in that. apart from the stirring periods, the injection device is fed with an inert fluid in the gaseous state and that, during the stirring periods, the inert fluid in the gaseous state is changed into an inert fluid in the liquid state readily vaporizable on coming in contact with the molten metal.

2. A process, according to claim 1, characterized in that the gaseous and liquid fluids are both of same chemical nature.

3. A process, according to claim 1, characterized in that both gaseous and liquid fluids are of different chemical nature.

4. A process, according to claims 1 to 3, characterized in that the gas injected in the liquid state under normal temperature and pressure conditions, has been liquefied previously.

5. A process, according to claims 1 to 4, characterized in that the fluid injected in the gasesous state under normal temperature and pressure conditions, has been vaporized previously.

6. A process, according to any claim 1 to 4, characterized in that the fluid injected in the liquid state features a high density-to-molecular- weight ratio.

7. A process, according to any claim 1 to 6, characterized in that the injection device consists of pipes opening under the surface of the molten-metal bath.

8. A process, according to any claim 1 to 6, characterized in that the injection device consists of a plurality of internal, selective and oriented refractory shapes built into the bottom of the vessel holding the molten-metal bath.

9. A process, according to claims 1 to 6, characterized in that it is applied to conversion, by pneumatic means, of cast iron into steel in a top oxygen-blown converter.

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