EP1507876B1 - Metallurgical treatment method on a metal bath - Google Patents

Description

Technical field to which the invention relates

The present invention relates generally to a method of metallurgical treatments on a metal bath. It relates more particularly to such a process which comprises a first treatment involving the presence or the formation of an acidic slag on the surface of a metal bath and a second treatment involving the presence or the formation of a basic slag on the surface of this bath. metallic.

State of the art

A method of this type is for example a method for treating the raw steel in a pocket in which the steel bath is heated by aluminothermy before carrying out a desulphurization treatment (that is to say a treatment for lowering sulfur content) and / or dephosphorization (i.e., a treatment for lowering phosphorus content). During aluminothermic heating, aluminum is reacted with oxygen to form an acidic slag of Al 2 O 3 at the surface of the steel bath. However, the desulphurization or dephosphorization treatment, which requires a basic slag on the surface of the steel bath, is inhibited by the presence of an acidic Al 2 O 3 slag on the surface of the steel bath. Therefore, the acid slag of Al2O3 must first be removed before the desulfurization and / or dephosphorination treatment can be started. However, such an intermediate scrubbing significantly increases the total duration of the treatment and is not possible in any metallurgical treatment stand.

In order to increase the yield of an aluminothermic heating of a metal bath in a ladle, it is known to carry out this reheating under a bell (cf for example US-A-4518422). By injection of an inert gas, a "window" is first formed in an initial slag layer covering the metal bath. The bell is then lowered above this "window" until its lower edge is immersed in the metal bath. The aluminothermic reagents, namely aluminum and oxygen, are added below this bell. At the same time, the metal bath is stirred by injecting an inert gas. It will be appreciated that the bell allows heating by aluminothermy under a protected atmosphere and with a minimum of losses to the environment. When the aluminothermic heating is finished, the bell is removed. The slag around the bell mixes with the slag Al2O3 formed below the bell, which gives a slag whose content inhibits a desulfurization treatment and / or subsequent dephosphorization by its high content of Al2O3 (> 40%) .

Another method of the kind defined in the preamble is a method in which a pig iron bath or a ferroalloy bath must undergo both oxygen injection desiliconization (i.e., treatment to lower silicon content) and desulfurization and / or dephosphorization. Desiliconization by oxygen injection produces an acid SiO 2 slag on the surface of the metal bath. However, the subsequent desulfurization treatment requires the presence of a basic slag on the surface of the steel bath and is inhibited by an SiO 2 content greater than 10%. It follows that the acidic slag formed during the desiliconization must be cleared before starting the desulphurization treatment. As already explained, such an intermediate scrubbing substantially increases the duration of the process and is not possible in any metallurgical treatment stand.

Object of the invention

The object of the present invention is to optimize the progress of a metallurgical process in which a first treatment involves the presence or the formation of an acidic slag on the surface of a metal bath and a second treatment involves the presence or the formation of a basic slag on the surface of this metal bath.

Presentation of the invention

According to the invention, this objective is achieved by carrying out the two treatments without intermediate scrubbing simultaneously in two separate zones and ensuring on the surface of the metal bath a physical separation between an acid slag zone and a basic slag zone. In order to save a maximum of time, both treatments will take place simultaneously. It will be appreciated that in all these cases the time necessary for the intermediate slagging is spared and the two treatments can be carried out in a single metallurgical treatment stand which is not necessarily equipped to carry out a slag removal (the final scrubbing can take place do elsewhere).

In a preferred embodiment, one of the two treatments is performed under a deep bell whose lower edge is immersed in the metal bath and the other treatment is performed around this deep bell. This deep bell ensures the physical separation between the two slag areas on the surface of the bath, while allowing one of the two treatments to be carried out under a protected atmosphere, with a minimum of losses to the environment. If we do not want to take advantage of these additional advantages of a deep bell, it is also possible to use a simple separation wall to ensure on the surface of the metal bath a physical separation between an acid slag zone and a basic slag zone. This separation wall can either cooperate with the edges of a metallurgical vessel to divide the surface of the metal bath into two juxtaposed zones, or form a kind of ring to delimit an "island" inside the surface of the metal bath. .

The first treatment is for example a chemical reheating which is carried out under a deep bell under a protected atmosphere and which produces an acidic slag under this bell. By chemical heating is meant here a strongly exothermic oxidation of a generally metallic element, such as that for example aluminum (aluminothermie) or silicon (silicothermie).

The first treatment can also be a desiliconation treatment by oxygen injection, especially in the context of a treatment of cast iron or ferroalloys (such as ferro-nickel) with high levels of silicon. This desiliconation treatment by oxygen injection is also advantageously carried out under a deep bell whose lower edge is immersed in the metal bath.

The second treatment is for example a desulfurization and / or dephosphorization treatment using a basic slag, formed for example by adding lime, sodium carbonate, magnesium etc. This treatment can be carried out around the deep bell under which the first treatment is performed.

In the context of a desiliconization treatment by oxygen injection, the desulphurization and / or dephosphorination treatment advantageously comprises the addition of limestone, in particular of limestone, to the metal bath. It is a cheap and highly effective desulfurizing agent, but its decomposition in the metal bath gives rise to a highly endothermic reaction which tends to cool the metal bath. However, in combination with desiliconization by oxygen injection, this cooling effect is hardly a problem, because the desiliconation reaction, which is highly exothermic, anyway produces an excess of heat.

When a deep bell is used to ensure a physical separation on the surface of the metal bath between an acidic slag zone and a basic slag zone, the following procedure is advantageously performed: firstly, by injection of an inert gas, a "window" in an initial slag layer covering the surface of the metal bath; this "window" is covered with a deep bell whose lower edge is immersed in the metal bath; one of the two treatments is carried out under the deep bell and the other around the deep bell, simultaneously mixing the metal bath by injecting an inert gas; and at the end of both treatments, stop the brewing, remove the deep bell and clean up immediately after the two slags. Stopping the stirring before removal of the deep bell prevents the two slags from mixing too much, which could be detrimental to the outcome of the process.

Other features and characteristics of the method according to the invention will emerge from some examples presented below by way of illustration, also referring to FIG. 1 attached, which shows a schematic illustration of the implementation of the implementation of a method according to the invention.

Detailed description of some advantageous embodiments of the invention

Fig. 1 is used to further describe, by way of illustration of the present invention, the course of a metallurgical process which comprises a pocket desulfurization treatment of a crude steel bath, preceded by a chemical warming in pocket of this steel bath.

In FIG. 1 shows a metallurgical pocket 10 in a metallurgical treatment stand during the implementation of the aforementioned method. In the initial state, this pocket 10 contained a crude steel bath 12 from the converter or electric furnace, as well as a basic residual slag layer covering the steel bath. In the metallurgical treatment stand, a window 14 has initially been formed by injection of an inert gas into the residual slag layer, that is to say that an area of the surface of the bath has been released. at least partially of the residual slag which covered it. Above this window 14, a deep bell 16 has then been positioned, so that its lower edge 18 is immersed in the metal bath 12 by at least 20 cm (the sparging of the metal bath 12 is important, the deeper the depth of the lower edge of the bell 16 will be important). It remains to be noted that a possible embodiment of such a deep bell 16 is for example described in the patent application WO 98/31841, while specifying, however, that the bell used in the present process need not necessarily be a bell. rotating bell.

Below the bell 16, the steel bath is heated by aluminothermy. To this end, aluminum is added and oxygen is blown under the bell 16, as indicated schematically by the arrows 18 and 20. At the same time, the metal bath 12 is stirred with a gas inert, which is injected, preferably with the aid of a lateral lance 22, into the metal bath 12. The aluminum reacts in a strongly exothermic reaction with oxygen. This reaction results in the formation of an acid slag of Al 2 O 3 below the bell 16. In FIG. 1 this acid slag Al2O3 is identified by the reference 24.

According to the state of the art, the bell 16 was raised at the end of the chemical reheating in order to carry out a slag removal of the highly contaminated residual slag with the slag Al2O3 formed under the bell 16. Thereafter, the desulphurization treatment was carried out on the bath. steel released from slag. Indeed, it is known that in order to perform a desulfurization treatment and / or dephosphorization using a basic slag, it is necessary that the Al2O3 content of this slag is less than 40%.

According to the present invention, the desulfurization and / or dephosphorization treatment is carried out around the bell 16 without performing an intermediate slag removal. For this purpose, using a lance 26, a basic slag forming agent 28 is injected into the metal bath 12 around the bell 16. This agent for forming a basic slag 28 may, for example, be lime, limestone, limestone, soda ash, magnesium etc. The bell 16 prevents the acid slag of Al2O3 formed under the bell 16 from mixing with the basic slag surrounding the pocket 16, which allows perform both treatments simultaneously without intermediate scrubbing. Preferably the aluminothermic heating is started first and the desulfurization and / or dephosphorization treatment is started as soon as the steel bath has reached a sufficient temperature.

At the end of the desulfurization and / or dephosphorization treatment, one stops any brewing of the metal bath 12 before raising the bell 16. Then we cleanse the two slags together.

It should be noted that the treatment carried out under the bell could for example also be a desilicon treatment of cast iron or ferroalloys, especially ferro-nickel, by oxygen injection. In this case the silicon reacts with the blown oxygen below the bell to form an acid SiO 2 slag under the bell. Around the bell can then perform a desulphurization treatment and / or dephosphorization as described above. The bell prevents the acid SiO 2 slag formed under the bell 16 from mixing with the basic slag surrounding the pocket 16, which makes it possible to perform the two treatments simultaneously without intermediate scrubbing. In fact, for an effective desulfurization and / or dephosphorization treatment, the SiO 2 content of the basic slag must not be greater than 10%.

Example 1

This example relates to a pocket treatment of the crude steel converter with the objective of desulphurizing at 80% of this steel.

Initial state: A metallurgical ladle contains 160 t of crude steel converter and 600 kg of residual slag. The analysis results are as follows: 0.04% C, 600 ppm O, 0.010% S. The temperature of the steel bath is 1600 ° C. 200 kg of deoxidizing Al and 600 kg of CaO were added to the casting.

Heating by aluminothermy: The first treatment is an aluminothermic heating which is carried out, as described with reference to FIG. 1, under a deep bell positioned above an area of the steel bath previously released from its residual slag layer. An increase in temperature of the steel bath of about 90 ° C is obtained by injection of 530 kg of aluminum and 350 m3 of oxygen in 7 minutes (flow rate of 50 m3 / min of O 2). Brewing below the bell is done by injecting argon using a side lance with a flow rate of 0.2 m3 / min.

Desulfurization: The second treatment is an 80% high desulfurization that takes place around the bell. As desulfurizing agent is used a powder composed of 60% CaO and 35% Al2O3, the remainder being impurities. The addition of Al2O3 is intended to regulate the fluidity of the slag obtained. Other slag agents can also be added.

The desulfurizing agent is injected using a submerged head lance, using argon as the carrier gas. Before starting the injection of the desulfurizing agent, the injection lance is used to perform a prior stirring of the steel bath. For this purpose the injection lance is fed for 5 minutes with a flow rate of approximately 0.5 m3 / min of argon, the feed of the desulfurizing agent being cut off. This preliminary stirring makes it possible in particular to homogenize the temperature of the steel bath before its desulfurization. 960 kg of the above-mentioned desulphurizing agent (solid flow rate about 80 kg / min) are then injected over a period of about 12 minutes with a flow rate of about 1 m 3 / min of argon as the carrier gas. The treatment is terminated by carrying out with the same lance for 5 minutes an intense stirring with a flow rate of approximately 1 m3 / min of argon, the supply of desulfurizing agent being cut again. Then we stop all brewing and we go up the bell.

Final state:

Steel: 0.04% C, 0.002% S, temperature approximately 1600 ° C.

Dairy: about 1000 kg of Al2O3 formed under the bell, plus about 2500 kg of desulfurizing slag around the bell.

Note :

If only moderate desulphurization of the steel is desired, it may not be necessary to inject a desulfurizing agent into the bath using a lance. Indeed, the residual slag, which is found around the bell, can already contain a sufficient amount of desulfurizing agents to obtain a moderate desulphurization of the steel. It is then sufficient to stir the steel bath around the bell to react with the residual slag floating on its surface and to add, if necessary, more slag agents to adjust in particular the consistency of the slag.

Example 2

This example concerns a raw pig pocket treatment with the aim of desilicating and desulfurizing the cast iron.

Initial state: A metallurgical ladle contains 100 t of pig iron, the analysis results of which are as follows: 4.5% C, 0.8% Si, 0.10% S. The temperature of the melt bath is 1350 ° C . The cast iron is covered with a residual slag layer of basic character.

Desiliconization treatment : A desiliconization treatment by oxygen injection is carried out, as described above, under a deep bell positioned above a zone of the bath previously released from its residual slag layer. 450 m3 of oxygen are injected under the bell in 10 minutes (flow rate 45 m3 / min of 02). Brewing below the bell is done by injecting argon using a side lance with a flow rate of 0.2 m3 / min.

Desulfurization: Desulphurization takes place around the bell. As desulfurizing agent is used a powder composed of 70% CaCO3 and 30% Na2CO3. Other slag agents can also be added.

The desulfurizing agent is injected using a submerged lance, using argon as a carrier gas. About 1000 kg of the above-mentioned desulfurizing agent (solid flow rate about 50 kg / min) are injected over a period of about 20 minutes with about 1 m3 / min of argon as the carrier gas. After stopping all brewing, you can put the bell up and clean the two slags together.

Final state:

Pretreated iron: 4.3% C, 0.4% Si, 0.02% S, temperature around 1400 ° C.

Dairy: about 860 kg of SiO2 formed under the bell, plus about 700 kg of desulfurizing slag around the bell.

Note about the treatment of cast iron:

In the conventional desulphurization of melting in a single step, a Mg-CaCl 2 or Mg-CaO mixture is most often used as desulfurizer. These are very effective desulphurizers, but also very expensive. They are mainly used because they produce a limited cooling of the metal bath. However, the combination of desulphurization with highly exothermic desiliconization makes it possible to use a more cooling but cheaper desulfurizing agent, such as, for example, limestone (CaCO3) or limestone. The decomposition of CaCO3 or Na2CO3 in the steel bath also generates oxygen, which contributes to the desiliconization of the melt (1 kg CaCO3 or Na2CO3 reduces the desilicon oxygen requirement by approximately 0.1 m3). Furthermore, it is preferable to use a CaCO3 + Na2CO3 mixture in order to obtain a more fluid slag and thus to limit losses by entrainment of iron during the scrubbing process. Nevertheless, the use of Na2CO3 also requires limiting the temperature to 1400 ° C to prevent a loss of Na2CO3 by vaporization.

Remarks about a ferroalloy treatment:

It will be appreciated that a ferroalloy bath, in particular a ferro-nickel bath, may also be subjected to a combined desiliconization and desulphurization treatment, as presented in Example 2 for melting.

However, in the case of ferro-nickel, taken as an example, it is generally sought to achieve much greater desilication (lowering the Si content by more than 1%). Desilicating with a gaseous oxygen jet, one would obtain in the absence of an effective coolant, a heating of the order of 300 ° C or more.

As is done in some processes for the desiliconization of cast iron, ore or iron oxide obtained as a by-product in the manufacture of steel may be used as cooling. However, with the proposed process that combines desiliconization and desulphurization, it is particularly appropriate to use limestone (CaCO3) and / or sodium carbonate (Na2CO3) as desulfurizing agent, since these are both very cooling and Effectively desulphurize as long as it is not diluted with silica (SiO2).

Apart from the quantitative aspect (Si decrease of 1 to 2% instead of 0.2 to 0.4% for blast furnace cast irons), the proposed process for smelting applies in a similar way to ferro- alloys, by properly adjusting the proportions of oxygen and cooling / desulfurizing.

Claims (10)

1. A method for metallurgical treatment on molten metal bath comprising

   a first treatment involving the presence or the formation of an acidic slag on the surface of said molten metal bath; and

   a second treatment involving the presence or the formation of a basic slag on the surface of said molten metal bath;

   characterized in that the two treatments are carried out without intermediate slagging, simultaneously in two separate zones, and by providing a physical separation at the surface of said molten metal bath between an acidic slag zone and a basic slag zone.
2. The method as claimed in claim 1, in which one of the two treatments is carried out under a deep bell the bottom edge of which is immersed in said molten metal bath and the other treatment is carried out around said deep bell.
3. The method as claimed in claim 2, wherein said first treatment is a chemical heating treatment which is carried out under said deep bell
4. The method as claimed in claim 3, wherein said chemical heating treatment is an alumino-thermal or silico-thermal process
5. The method as claimed in any one of claims 1 to 4, wherein said second treatment is a desulfurization and/or dephosphorization treatment based on a basic slag
6. The method as claimed in claim 1, wherein said first treatment is a treatment for desiliconizing cast iron or ferroalloys, particularly ferronickel, by oxygen injection.
7. The method as claimed in claim 6, wherein said desiliconizing treatment by oxygen injection is carried out under a deep bell the bottom edge of which is immersed in said molten metal bath and said second treatment is a desulfurization and/or dephosphorization treatment carried out around said deep bell.
8. The method as claimed in either one of claims 6 or 7, wherein said second treatment is a desulfurization and/or dephosphorization treatment based on lime
9. The method as claimed in claim 8, wherein said second treatment comprises the addition of limestone, particularly of castine, to said molten metal bath
10. The method as claimed in any one of claims 1 to 9, wherein:

    at the start of said method, the surface of said molten metal bath is covered with a layer of residual slag;

    a window is formed in said layer of residual slag by the injection of an inert gas;

    said window is covered using a deep bell the bottom edge of which is immersed in said molten metal bath;

    one of the two treatments is carried out under said deep bell and the other around said deep bell, while simultaneously agitating the molten metal bath by injection of an inert gas; and

    at the end of the two treatments, said agitation is stopped, said deep bell is removed, and the two slags are then immediately skimmed

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