EP1029089B1 - Method for continuous smelting of solid metal products - Google Patents

Description

The present invention relates to a continuous product melting process solid metal.

The process relates more particularly to the melting of metallic solids, such as than solid iron, solid cast iron, scrap iron or cast iron, pre-reduced, etc., which are used with the possible addition of cast iron liquid, eg for steel production. The process can take place indoors a reactor comprising, for example, an electric furnace, in which the energy necessary for melting is produced by an electric arc and / or a burner furnace gas, oil or coal and / or a plasma torch oven.

Continuous melting of solid products is generally done in a reactor which includes two adjacent zones, namely a fusion zone and a zone metallurgical treatment. Solid products are loaded into the fusion of the reactor and then melted under the effect of a considerable energy supply. The molten metal is transferred progressively to the second zone and undergoes metallurgical treatment therein in order to adjust its chemical composition. This metallurgical treatment generally includes a refining of the liquid metal during which refining gases are injected, such as eg oxygen, in the metal bath using blowing lances in order to reduce the carbon and silicon content of the steel produced. Such a process is described for example in French patent application FR-A-1,482,929.

However, in addition to carbon and silicon, the molten metal contains other impurities which have negative effects on the physical and mechanical properties of steel product. Among these impurities, mention is made in particular of sulfur, which reduces between others the resilience of steel, its resistance to fatigue, its resistance to corrosion and its weldability.

Unfortunately, sulfur cannot be removed at the same time as carbon since desulphurization requires very operating conditions different from decarburization.

Indeed, during decarburization, we are in the presence of a slag which is oxidizing due to the insufflation of large quantities of oxygen. Now, the desulphurization is all the more effective the slag is reducing.

In the refining area of the ovens as described in the patent application French FR-A-1,482,929, the conditions are such that we barely reach remove 30% of the sulfur initially contained in the metal charge in the oven. This means that a secondary metallurgical treatment must always be carried out. in a container placed downstream of the melting reactor, eg an oven poached.

The object of the present invention is therefore to propose a process for the fusion continuous solid metal products which also allows a reduction of the carbon content also a lowering of the sulfur content of molten metal.

a) affiner les produits fondus dans un milieu de laitier oxydant,

b) séparer le laitier de la zone de traitement métallurgique du laitier de la zone de fusion,

c) transformer le laitier oxydant en un laitier réducteur,

d) désulfurer les produits fondus dans un milieu de laitier réducteur,

e) couler le métal fondu.

According to the invention, this objective is achieved by a process for the continuous melting of solid metal products for the production of steel according to claim 1. It is a process for the continuous melting of solid metal products in a reactor with two distinct zones, a melting zone and a metallurgical treatment zone, in which in the melting zone, the solid metal products are continuously heated until the melting of the solid products and the molten products thus obtained are transferred to the gradually in the metallurgical treatment zone. According to the invention, the method comprises in a same container of the metallurgical treatment zone a succession of steps consisting in

a) refining the products melted in an oxidizing slag medium,

b) separating the slag from the metallurgical treatment zone from the slag from the melting zone,

c) transforming the oxidizing slag into a reducing slag,

d) desulfurizing the molten products in a reducing slag medium,

e) pour the molten metal.

The solid metal is continuously melted in the melting zone. In this area, we are in the presence of an oxidizing slag which allows the elimination of a large part of the phosphorus contained in the liquid metal. Indeed, phosphorus, which among other things, reduces the ductility and weldability of the steel, is transferred to the oxidizing slag by an exchange reaction with this slag. As you go its fusion, the liquid metal is transferred to the second zone in which the actual metallurgical treatment takes place.

Arrived in the second zone, the metallurgical treatment of the molten metal is done in two phases. In the first phase, we mainly carry out a reduction of carbon and silicon contents of the metal bath in oxidizing conditions. This refining is done by injecting oxygen into the bath metallic and by adding, for example, CaO to form slag. The contents in carbon and silicon of the metal bath can thus be reduced to predetermined values, which are preferably between 0.05% and 0.1% for carbon.

After this oxidative refining, the conditions in the treatment zone are modified to pass from an oxidizing medium to a reducing medium. This transformation conditions is achieved by adding aluminum Al or / and silicon Si and / or carbon C in the slag. The slag is thus calmed and passes from a more oxidizing slag to a more reducing slag. It should be noted that silicon and / or carbon are added so as not to increase their contents in the metal bath, which would reduce the effect of the previous refining, but so as only to reduce the FeO in the slag and to lower the content oxygen in the metal.

Under the reducing conditions thus created, we then proceed to the second metallurgical treatment phase, namely desulfurization of the metal bath. During this desulfurization, the bath is preferably stirred metallic by bubbling inert gas, eg argon, to facilitate the exchange between the metal bath and the slag. The slag being reducing, the much of the sulfur passes into the slag.

Under these conditions, desulfurization of more than 80% becomes possible and happens, depending on the sulfur content of the solid products and depending on the quantity of slag used, at final contents of 0.010% sulfur in the steel poured from the second zone of the reactor.

The proposed process thus allows the production of low-grade steel carbon and sulfur in a two-zone reactor and therefore avoids pocket furnace treatment in the production of mass steels such as reinforcing bars, for which a content of 0.020 to 0.030% of sulfur is aimed at final product.

For new electric steelworks producing high-end steels with scrap substitutes which allow low levels to be achieved copper (cast iron, pre-reduced, iron carbide), low sulfur steels (less than 0.010% sulfur on final product) are difficult to achieve in electric steelworks: indeed, the oxidizing conditions of the electric oven do not allow additionally desulfurize 30%, i.e. that we remove at best 30% of the sulfur stuffs. However, with the exception of desulphurized cast iron, the substitutes for scrap contain much more sulfur than the pure scrap metal they replace: 0.020% S to 0.100% S for pre-reduced (DRI) according to origin and 0.050% to 0.100% S for non-desulfurized cast iron. Massively using a substitute containing more than 0.030% S, we cannot reach the objective of less than 0.020% S when casting the conventional electric oven. The present process which allows for more efficient desulfurization is therefore an important asset for the massive use of pre-reduced or non-desulfurized cast iron as a substitute to scrap. This process therefore widens the range of raw materials usable for the production in the electric oven of high purity products, eg containing less than 0.010% sulfur, which today require no use than very pure scrap or pre-reduced, or desulfurized cast iron.

In current high productivity processes, oxidative refining and refining reducer are done in different places. The present process makes it possible to make the two reactions being mutually exclusive in the same place and successively.

At the same time, it allows the production of steels with a content of nitrogen much lower than the steels produced according to conventional processes. In fact, the two sources are avoided in the metallurgical treatment zone of main nitrogen pollution such as the electric arc and scrap melting late and reducing the residual nitrogen content by bubbling of a neutral gas, in particular argon in the metallurgical treatment zone. In addition, denitriding is facilitated by the low sulfur content of the bath metallic.

For a load of 100% scrap for example, we will produce in an oven conventional electric steel at 70 to 80 ppm N, while with the present process we manage to produce steels with a nitrogen content of around 40 ppm or even less.

According to a first preferred embodiment, the reducing slag is evacuated from the metallurgical treatment zone before, during or after step e), i.e. pouring the liquid metal. It is indeed preferable to eliminate the reducing slag rich in sulfur before refining the new metal charge liquid, this in order to avoid that during the refining the sulfur contained in the slag go back into the liquid metal bath.

Under certain conditions, eg when there is too much slag in the area of fusion or when more slag is needed in the metallurgical treatment zone, it may be advantageous to transfer oxidizing slag contained in the melting zone towards the treatment zone metallurgical before, during or after the refining of the molten products in the metallurgical treatment area.

The reverse, i.e. the transfer of oxidizing slag from the metallurgical treatment zone to the melting zone during or after the refining of the melted products in the metallurgical treatment zone can be interesting. Indeed, in the metallurgical treatment zone, the slag is foaming and contains a lot of iron oxides and metallic iron drops. During the transit of the slag to through the melting zone, the slag is deoxidized on contact with the liquid metal to higher carbon content and the metal drops are decanted there. We thus achieves a countercurrent mass exchange, which minimizes loss of iron.

In addition, in an arc furnace, the foaming slag formed which is transferred to the melting zone has the effect of stabilizing the electric arc and increasing its performance.

At certain points in the process, - p. ex. when casting metal and / or when desulfurization of metal - it may be advantageous to stop completely or to substantially reduce the transfer of molten products to the metallurgical treatment.

Depending on the composition of the raw materials, it may be advantageous to carry out a pre-refining of the molten products in the melting zone.

In a preferred embodiment, the gases released during the refining of the products molten are transferred to the melting zone in order to heat the products solids in this area. Indeed, the refining of the steel bath is accompanied by the formation of abundant amounts of CO (near the half of the CO released by the process is produced during refining). energy contained in this CO gas can be used to heat the solid products in the melting zone as well as the solid products in a product preheater possible solids either in counter-current or in partial co-current. We can thus recover the energy contained in hot gases to increase efficiency energy of the reactor.

In another preferred embodiment, the fusion zone is loaded with continuous solid products. Loading the melting zone into products solids being continuous, the melting zone permanently contains solid products and the energy efficiency of the melting zone can be maximized.

Solid products are advantageously preheated before loading using hot reactor gases. In this way, the gases released during the melting and refining can be recovered to increase the temperature solid products before loading them into the oven. Solid products therefore more quickly reach their melting temperature and the fusion is significantly shortened. This leads to an increase in yield overall thermal performance of the reactor, and possibly its productivity. The preheating is carried out for example in a preheater which can be executed in the form of a vertical or inclined hopper extending the melting zone or in the form of an inclined rotating drum.

It should be noted that the heating and / or melting of the solid products can (can) be carried out either using an electric arc or using gas burners gas, oil or coal either using a combination of these different means.

In addition to the advantages described above, the method of the present invention has other advantages over conventional fusion processes. In effect, since the melting zone can operate continuously and the pouring in discontinuous is carried out from the metallurgical treatment zone, the time out of power caused by loading and pouring in conventional ovens are eliminated and the drop in usable power in the final period called refining and overheating is no longer necessary.

Fig.1:

une coupe longitudinale d'un four électrique de fusion en continu de produits solides pendant la phase de fusion / affinage - surchauffe,

Fig.2:

une coupe longitudinale d'un four électrique de fusion en continu de produits solides pendant la phase de fusion / réduction du laitier et désulfuration de l'acier,

Fig.3:

une coupe longitudinale d'un four électrique de fusion en continu de produits solides pendant la phase de coulée et de décrassage du laitier riche en soufre,

Other features and characteristics of the invention will emerge from the detailed description of some advantageous embodiments presented below, by way of illustration, with reference to the accompanying drawings. These show:

Fig.1:

a longitudinal section of an electric furnace for continuously melting solid products during the melting / refining - overheating phase,

Fig.2:

a longitudinal section of an electric furnace for continuously melting solid products during the melting / reduction of slag and desulphurization of steel phase,

Fig.3:

a longitudinal section of an electric furnace for continuously melting solid products during the casting and cleaning phase of the sulfur-rich slag,

Fig. 1 shows a section through a continuous product fusion reactor 10 solids, such as solid iron, solid cast iron, scrap iron or cast iron, pre-reduced (DRI), etc. which are used eg for production steel. The reactor 10 is produced as an electric furnace, in which the energy required for fusion is produced by an electric arc and by burners 12 mounted in the lower lateral part of the furnace 10.

The electric oven 10 comprises a hearth 14 made of a refractory material, surmounted a tank 16 and a vault 18. At least one electrode 20 mounted on a mast (not shown) via an arm (not shown) is introduced into the oven 10 through an opening 22 made in the vault 18. The arm can slide on the mast so that it can go up and down the electrode 20.

The electric oven 10 is subdivided into two separate zones. The first area, called melting zone 24, is charged, preferably continuously, with scrap 25 using a vertical hopper 26 fitted above the area in this melting zone 24, the scrap 25 is melted using electrodes 20 passing through the vault 18 of the oven 10. To increase the speed of scrap 25, an additional supply of energy is made to using the burners 12 in the side wall of the oven 10.

Advantageously, in the melting zone, even when the source main energy is an electric arc, an additional 10 to 20 kg of carbon per tonne of steel produced. This carbon is provided either in the form of coke or anthracite, either in the form of a carbide metal, eg of cast iron. The about half of this carbon is removed by oxygen injection using nozzles or nozzles immersed so that the molten metal transferred in the metallurgical treatment zone has an intermediate carbon content in the range of 0.5 to 1%. Because of the oxygen injection, we are in the presence an oxidizing slag, which allows the elimination of a large part of the phosphorus in the melting zone.

As the scrap melts, liquid metal accumulates in the sole 14 of the melting zone 24 and when this has reached a certain level, it pours over a weir 27 into the second zone of the furnace called metallurgical treatment zone 28.

In the first phase of the process, the liquid metal in the treatment zone is subjected to conventional refining operations by gas injection such as oxygen by means of a lance 32 in order to adjust the chemical composition of the liquid metal. During such refining operations, the carbon content of the steel can be reduced from about 1% by weight to about 0.1%.

The hot gases released during the refining of the molten products are transferred in the melting zone 24 and are then sucked by the hopper 26 supplying the scrap 10 furnace 25. A large part of the energy contained in these gases can be used to heat the scrap 25 in the melting zone 24 as well as the solid products contained in the preheater hopper 26. We get closer here a countercurrent heat exchanger which has thermal efficiency optimal.

Lime (CaO) is added in the melting zone and in the treatment zone metallurgical to form a slag. Different additives like e.g. fondants can also be added.

In the metallurgical treatment zone 28, the slag is foaming 34 and contains a lot of iron oxides and metallic iron drops during the refining phase.

The slag contained in the two zones is separated by a slag dam 36, possibly removable, installed between the two zones at the weir 27. This barrier prevents the passage of slag from the melting zone 24 into the metallurgical treatment zone 28.

Separating the slag from the two zones is especially important in the second phase of the process, the desulfurization phase. During the phase refining and overheating, the slag contained in the melting zone 24 and that contained in the metallurgical treatment zone 28 have properties neighboring chemicals, namely that the slags of the two zones are slags oxidants. For this reason, it is not necessary to separate them during this first phase. The barrier 36 can therefore be removed entirely or partially to allow a slag exchange of these two zones. Through against, during the desulfurization phase, the chemical properties of slag contained in the melting and metallurgical processing zones are different and incompatible.

While the chemical composition of slag from melting zone 24 remains unaffected, aluminum and / or silicon are added and optionally carbon in metallurgical treatment zone 28 in order to transform the oxidizing slag into reducing slag. Of course, the oxygen lance 32 is stopped during this desulfurization phase.

To guarantee a good homogenization of the liquid metal bath, we inject in the liquid steel bath a neutral gas (argon) through one or more porous block (s) 38 through the bottom of the metallurgical treatment zone 28. The eddies created by this gas injection improve the contact between the steel liquid to be treated and the slag reducing so that the desulfurization takes place in the best conditions.

During this desulfurization phase, we can reduce or even close completely the arrival of liquid metal from the melting zone 24 in order to limit the quantity of unrefined metal in the treatment zone metallurgical 28 during desulphurization.

Fig. 3 shows the last phase of the process, the pouring of the liquid metal. After refining and desulfurization, the sulfur-rich reducing slag is discharged through the scouring door 40 and the molten metal is poured from the metallurgical treatment zone 28 by a taphole 30 while retaining a background of liquid metal in this area. This bath foot is used to reduce wear of the refractory lining. It should be noted that the tap hole 30 can as well be fitted in the side wall of the treatment area metallurgical 28 than in the bottom of this area.

It is advantageous to remove most of the reducing slag at the end of the desulfurization and in any case before the next refining to avoid sulfur contained in the reducing slag does not pass back into the liquid steel at the time of refining of the next batch.

Although in the reactor described above the metallurgical treatment zone 28 operates in discontinuous mode, it should be noted that the fusion zone 24 operates continuously. Downtime caused by loading and pouring procedures in conventional ovens are therefore removed and the drop in usable power in the so-called final period refining and overheating is no longer necessary.

Material flows and energy flows from the furnace can be summed up by the as follows: scrap 25 is introduced into the oven through the hopper 26, it crosses the fusion zone 24 and is then drawn off by the metallurgical treatment 28.

The gas flow passes through the furnace in the opposite direction. Indeed, the gases are injected or formed in the metallurgical treatment zone 28 and the melting zone 24 to be aspirated by the hopper 26.

The slag contained or formed in the metallurgical treatment zone 28 is evacuated by the scouring door 40 located in this area while the slag contained in the melting zone 24 can be evacuated by a scouring 42 located in the melting zone 24.

The efficiency of the process is subsequently illustrated with the aid of two examples.

In the first example, we compare the efficiency of sulfur removal and phosphorus for a charge of ordinary scrap in an electric furnace conventional and in a two-zone oven implementing the method according to the present invention. In both cases, consider an amount of 100 kg of slag per tonne of steel.

In the conventional oven, the slag is very oxidizing and contains <0.1% by weight of carbon and about 25% by weight of FeO. The partition coefficient for the sulfur i.e. the ratio of sulfur content in the slag / sulfur content of the metal is less than 5 and that for phosphorus is about 50. We manage therefore to eliminate 70% to 80% of the phosphorus initially contained in the metal and about 25% to 30% of the sulfur. The conventional oven therefore makes it possible to obtain from of steel scrap with a very reduced phosphorus concentration but with a non-negligible sulfur concentration.

In the process according to the present invention, in the melting zone, a medium oxidizing slag which contains less than 10% by weight of FeO and which has a basicity of about 2.5. The partition coefficient in such conditions is 5 to 10 for sulfur and about 25 for phosphorus. In assuming that 32 kg of CaO is used to form 80 kg of slag / t of steel in the melting zone, we manage to eliminate inside the melting zone fusion between 30% and 40% of the sulfur and between 60% and 70% of the phosphorus.

The metal with these reduced sulfur and phosphorus contents is then transferred in the metallurgical treatment area. In this area, we train 20 kg slag per t of steel by adding 12 kg of CaO per tonne of steel and possibly fondants.

After the oxidative refining, the slag is made reducing by adding either the aluminum is silicon and / or carbon. The FeO content of the slag is reduced to 0 and the basicity of the resulting slag is approximately 3. The treatment area metallurgy is subjected to strong mixing with argon. when uses aluminum, the partition coefficient for sulfur is around 500 while it is only around 100 when the slag is deoxidized by silicon. Under the above conditions and using aluminum for calm the slag, we manage to eliminate between 80 and 90% of the sulfur contained in the steel supplying the metallurgical treatment zone. When using the silicon to deoxidize the slag, eliminates between 60% and 70% of the sulfur contained in the steel supplying the metallurgical treatment zone.

The overall reduction in sulfur content per 100 kg of slag / t of steel is therefore 86% by weight when using aluminum and 72% in the case of silicon. The overall reduction in phosphorus content is 60% in weight.

The present process therefore makes it possible to obtain much sulfur contents. lower than in conventional processes while having performance comparable for phosphorus.

We can do the same analysis for a metal oven with a significant proportion of pre-reduced or non-desulfurized cast iron; such oven is used to produce quality end products requiring low copper and sulfur contents.

The table below summarizes the metallurgical performance of a process of conventional fusion to those of a fusion process according to the present invention.

In the case of the production of concrete rods from scrap, we see that the process according to the present invention makes it possible to avoid an additional step of desulfurization in the bag.

When producing wire for drawing, the present process makes it possible to use cheaper raw materials or else when the same raw materials are used, it makes it possible to dispense with a second desulfurization step.

Claims (12)

1. Method for the continuous melting of solid metallic products for the production of steel in a reactor with two separate zones, the reactor comprising a melting zone and a metallurgical treatment zone,in which in the melting zone the solid metallic products are continuously heated until the solid products melt and the melted products so obtained are progressivly transferred into the metallurgical treatment zone, said method comprising, in a same receptacle of the metallurgical treatment zone, a succession of steps consisting in

   a) refining the melted products in an environment comprising an oxidising slag,

   b) separating the slag of the metallurgical treatment zone from the slag of the melting zone,

   c) converting said oxidising slag into a reducing slag,

   d) desulphurising the melted products in an environment comprising a reducing slag,

   e) pouring the molten metal.
2. Method according to Claim 1, characterised by the evacuation of the reducing slag from the metallurgical treatment zone before, during or after step e).
3. Method according to one of Claims 1 or 2, characterised by the transfer of the oxidising slag from the melting zone to the metallurgical treatment zone before, during or after step a).
4. Method according to one of Claims 1 or 2, characterised by the transfer of the oxidising slag from the metallurgical treatment zone to the melting zone before, during or after step a).
5. Method according to one of Claims 1 to 4, characterised by the interruption of the transfer of melted products to the metallurgical treatment zone during step d).
6. Method according to one of the preceding claims, characterised by a pre-refining of the melted products in the melting zone.
7. Method according to one of the preceding claims, characterised in that the gases released in the metallurgical treatment zone during the refining of the melted products are transferred to the melting zone in order to heat up the solid products present in this zone.
8. Method according to one of the preceding claims, characterised in that the melting zone is charged continuously with solid products.
9. Method according to one of the preceding claims, characterised by a charging of liquid pig iron into the reactor.
10. Method according to one of the preceding claims, characterised in that, before their charging, the solid products are pre-heated using hot gases from the reactor.
11. Method according to one of Claims 1 to 10, characterised in that the solid products are heated by means of an electric arc and/or burners and/or plasma torches.
12. Method according to one of the preceding claims, characterised by a supply of energy to the metallurgical treatment zone during the initial stage of the method.

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