EP0062548B1 - Process for gas-stirring a metal bath - Google Patents

Abstract

A process of pneumatic stirring of a bath of molten metal according to which a stirring fluid is injected through permeable refractory elements arranged at the bottom of a crucible containing the bath, in which the used stirring fluid is, at least temporarily, an organic liquid such as hydrocarbon. The process is advantageously used on a bath of molten pig iron during refining of the same by oxygen, and in which the organic liquid is replaced immediately after decarburization of the bath by an inert gas such as argon or nitrogen. The process thus permits an efficient stirring of the bath of molten metal without the risk of polluting the metal either by hydrogen or nitrogen.

Description

The present invention relates to the pneumatic mixing of a metal bath. It applies more particularly to the stirring of a bath of cast iron during refining in a converter to oxygen blown from above.

It is known to extend the metallurgical possibilities of refining techniques with oxygen blown from above by stirring the bath with a gaseous fluid injected through permeable refractory elements housed in the bottom of the converter (certificate French utility No. 2322202 - IRSID, Luxembourg patent No. 42419 - CNRM).

The fluid used is generally a chemically inert gas vis-à-vis the bath, such as argon -or preferably nitrogen due to its very low cost price.

This new technique, whose recent developments have brought it to the attention of the public under the name of “LBE” Process (Lance-Brassage-Equilibre), tends, as its name suggests, to achieve balance between metal and slag, thereby allowing the respective advantages of the conventional top-blown and bottom-blown oxygen refining processes to be offset. This technique results in particular in better control of the conduct of refining operations by providing greater flexibility both in the choice of the time, as well as the duration or, to a certain extent, the intensity of the gas injection. .

However, it encounters technological constraints which limit the possibilities of introducing as much gas per element as one would like.

It has already been proposed to replace the stirring gas to be injected with a liquefied gas vaporizing on contact with the bath. It was thus possible to obtain, at a given unit flow rate, much larger mixing gases (EP-A-0032343 - IRSID).

However, it appeared that, depending on the type of permeable elements used, the latter were more or less resistant to thermal shocks caused by repetitive injections of cryogenic fluid.

The object of the present invention is to achieve high mixing gas flow rates while avoiding the difficulties encountered in known practices.

To this end, the subject of the invention is a process for pneumatically stirring a metal bath contained in a crucible, according to which a stirring agent consisting, at least, is injected through permeable refractory elements mounted in the bottom of the crucible. temporarily, with an organic fluid and characterized in that said fluid is a hydrocarbon in the liquid state.

By “liquid” is meant an organic fluid in the liquid state under standard temperature and pressure conditions or easily liquefiable under low pressure, in this case under a pressure lower than the injection pressure in the permeable elements. (3 to 6 bars approximately).

In the case of the most common saturated aliphatic hydrocarbons, these are therefore those of the family C _n H _{zn + 2} where n is equal to or greater than 3 (propane, butane, etc.).

In this way, the advantages linked to an injection of a stirring fluid in the liquid state are obtained without subjecting the refractory elements permeable to thermal shocks detrimental to their behavior.

It is also possible, in accordance with a preferred implementation variant of the invention, to eliminate any risk of this kind simply by preheating the organic liquid before injecting it.

• - une protection des éléments réfractaires perméables et des régions avoisinantes par dépôt d'une couche de carbone de décomposition du liquide organique injecté,
• - l'absence de tout risque de nitruration du bain. On connait en effet les difficultés que l'on a pour éliminer l'azote d'un bain métallique brassé à l'aide de ce gaz.

Furthermore, the invention has other interests among which we can notably mention:

• - protection of permeable refractory elements and surrounding regions by depositing a layer of carbon decomposing the injected organic liquid,
• - the absence of any risk of nitriding the bath. We know in fact the difficulties we have in removing nitrogen from a metal bath stirred with this gas.

It should be noted, however, that the use of an organic fluid may cause some hydrogenation of the bath which, in some cases, may be undesirable.

In accordance with an implementation of the invention, it is possible to overcome this difficulty.

To do this, we start by injecting a brewing fluid consisting of an organic liquid, for example propane, then, some time before the desired end of brewing - for example a few minutes before - the organic liquid is replaced by an inert gas, such as nitrogen or argon, depending on the maximum allowable nitrogen contents for the target shade.

In this way, stirring with inert gas provides a "washing" of the bath, which is thus freed from the dissolved hygrogen accumulated during initial stirring by the injection of organic liquid.

This particular implementation of the invention is entirely suitable for refining cast iron by conversion to oxygen with stirring with argon or nitrogen.

In this case, the moment when one passes from the injection of the organic liquid to that of the nitrogen must be at the earliest, the end of the decarburization of the bath. Indeed, it is only from the end of the decarburization period that the metallurgical conditions become clearly unfavorable for a dissolution of the nitrogen, so that the choice of this element as downstream mixing gas decarburization of the bath, does not cause excessive nitriding of the latter, at least within the tolerances usually accepted.

In this embodiment, the invention therefore provides an efficient stirring of the bath during the refining process and this without risk of pollution of the metal by both hydrogen and nitrogen.

Claims (5)

1. Process of pneumatic stirring of a metal bath contained in a crucible, according to which a stirring agent constituted, at least temporarily by an organic fluid is injected through permeable refractory elements set in the bottom of the crucible, characterized in that said fluid is an hydrocarbon in the liquid state in standard conditions of temperature and pressure, or liquefiable under a pressure lower than its injection's pressure in the permeable refractory elements.

2. Process according to claim 1, characterized in that an inert gas is substituted for the liquid hydrocarbon before the end of stirring.

3. Process according to claim 2, characterized in that the inert gas is argon.

4. Process according to claim 2, characterized in that the inert gas is nitrogen.

5. Process according to claim 2, characterized in that, the metal bath being a liquid pig iron bath during refining by an oxygen converter, an inert gas is substituted for the liquid hydrocarbon the sooner at the end of the period of decarburation of the bath.