CN1665942A - Metallurgical treatment of metal baths - Google Patents

Abstract

The present invention relates to a method for the metallurgical treatment of a molten metal bath. The method comprises a first treatment involving the presence or formation of an acidic slag on the surface of the molten metal bath and a second treatment involving the presence or formation of a basic slag on the surface of the molten metal bath. According to the present invention, the two treatments are performed without intermediate slagging, thereby achieving a physical separation between an acidic slag zone and a basic slag zone on the surface of the molten metal.

Description

Metallurgical treatment of metal baths

Technical field related to the present invention

The present invention generally relates to a method of metallurgical treatment of a bath of molten metal. More particularly to a method comprising a first treatment comprising the presence or formation of an acidic slag on the surface of the molten metal bath, and a first treatment comprising the presence or formation of an alkaline slag on the surface of the molten metal bath the second treatment.

current technology

This type of method is, for example, a method of processing raw steel in ladle tanks. Wherein, the molten steel bath is firstly heated by the aluminothermic method before desulfurization treatment (ie treatment to a lower sulfur content) and/or dephosphorization treatment (ie treatment to a lower phosphorus content). During aluminothermic heating, aluminum _reacts with oxygen to form acidic _Al2O3 slag on the surface of the molten steel bath. Actually, the desulfurization treatment and dephosphorization treatment of alkaline slag are required respectively on the surface of the molten steel bath, and the dephosphorization treatment is inhibited by the presence of acidic Al ₂ O ₃ slag on the surface of the molten steel bath. Therefore, the acidic Al ₂ O ₃ slag must first be skimmed (slagging) before starting the desulfurization and/or dephosphorization treatment. In fact, this intermediate slagging greatly prolongs the overall time of treatment and is not feasible in every case of metallurgical treatment.

In order to increase the efficiency of the thermite heating of molten metal baths in molten steel tanks, it is known how to perform heating under the lid (see examples in US 4518422A). By injecting an inert gas, a "window" is first formed in the initial slag layer covering the molten metal bath. The lid is lowered over this "window" until its bottom edge is submerged in the molten metal bath. Under this cap is added the thermite reactants, namely aluminum and oxygen. By injecting inert gas while stirring the molten metal bath. The preferred cover allows the thermite to be heated under a protective atmosphere with minimal damage to the environment. When the thermite heating is complete, remove the cap. The slag around the lid mixes with the _Al2O3 slag _formed under the lid to produce _a slag whose _Al2O3 content (>40%) inhibits subsequent desulfurization and/or dephosphorization treatments.

Another process of the type mentioned in the introduction is one in which a bath of molten primary cast iron or a bath of molten iron alloy has to be desiliconized (ie treated to a lower silicon content) and desulphurized and/or dephosphorized by oxygen injection. Desilication by oxygen injection produces an acidic SiO ₂ slag on the surface of the molten metal bath. In fact, the subsequent desulfurization treatment requires the presence of basic slag on the surface of the molten steel bath, which is inhibited by a _SiO2 content of more than 10%. The acidic slag formed during desiliconization must thus be skimmed off (slagging) before commencing the desulphurisation process. As already explained, this intermediate slagging greatly prolongs the process time and is not feasible in every case of metallurgical treatment.

purpose of invention

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

Description of the invention

According to the present invention, this is achieved by carrying out the two treatments in two separate zones simultaneously or continuously without intermediate slagging, and by providing a physical separation at the surface of the molten metal bath between the acidic slag zone and the basic slag zone. Purpose. In order to save the most time, it is preferable to carry out the two treatments at the same time. In some cases, it may be advantageous to first terminate or start a first process before starting a second process, or vice versa. In any case, it is advantageous to save time for intermediate slagging and to carry out both treatments in a separate metallurgical treatment station which is ideal for performing slagging operations (slagging can be done anywhere) unnecessary.

In a preferred implementation one of the two treatments is performed under the thick cover whose bottom edge is immersed in the bath of molten metal, and the other treatment is performed around this thick cover. This thick cover provides physical separation between the two slag regions on the bath surface while allowing one of the two to be processed under a protective atmosphere with minimal damage to the environment. A separate partition wall may also be used to provide physical separation of the molten metal bath surface between the acidic slag zone and the basic slag zone if exploiting the other advantages of the thick material cover is not necessary. Such a partition either incorporates the edge of the metallurgical vessel to divide the surface of the molten metal bath into two juxtaposed regions, or forms a ring to define an "island" within the surface of the molten metal bath.

The first treatment is, for example, chemical heating which takes place under a heavy cover under a protective atmosphere and produces acidic slag under the cover. Chemical heating here refers to the highly exothermic oxidation of common metallic elements such as aluminum (aluminotherm) or silicon (silithermal).

The first treatment may also be a desilication treatment by oxygen injection, in particular as part of the treatment of cast iron or iron alloys such as nickel-iron with high silicon content. The desilication treatment by oxygen injection is also advantageously carried out under a heavy cover whose bottom edge is immersed in a bath of molten metal.

The second treatment is, for example, a desulfurization and/or dephosphorization treatment involving the formation of basic slag by adding eg lime, sodium carbonate, magnesium carbonate, etc. This treatment can be carried out around the heavy material cover where the first treatment was performed.

As part of the desilication treatment by oxygen injection, the desulfurization and/or dephosphorization treatment advantageously comprises the addition of limestone, in particular viteline, to the molten metal bath. This is an inexpensive and very effective desulfurizer, but its decomposition in the molten metal bath causes a highly endothermic reaction, which leads to cooling of the molten metal bath. In fact, in combination with desilication by oxygen injection, this cooling effect hardly causes any problems, since in any case the highly exothermic desilication reaction generates too much heat.

If a thick cover is used to provide a physical separation of the molten metal bath surface between the acidic slag zone and the basic slag zone, the method is advantageously carried out as follows: an inert gas is first injected to form in the initial slag layer covering the molten metal bath surface A "window"; this "window" is covered with a heavy cover whose bottom edge is immersed in a bath of molten metal; one of two processes is carried out under the heavy cover and the other around the thick cover, simultaneously by injecting The molten metal bath is stirred with inert gas; and at the end of the two treatments, the stirring is stopped, the heavy material cover is removed, and the two kinds of slag are immediately skimmed (slagging). Stirring is stopped prior to removal of the thick cover to prevent excessive mixing of the two slags, which is detrimental to the outcome of the process.

Other characteristics and properties of the method according to the invention will emerge from the several embodiments described below, with reference to the accompanying drawing 1, which is a schematic diagram of the implementation of the method according to the invention.

Detailed description of some advantageous embodiments of the invention

To illustrate the invention, Figure 1 is used to describe in more detail a metallurgical process comprising desulfurization of a molten raw steel bath prior to chemical heating of the molten steel tank.

Figure 1 shows a metallurgical ladle 10 in a metallurgical treatment station during the implementation of the aforementioned method. In the original state, the molten steel tank 10 contains a molten bath 12 of raw steel from a converter or electric furnace, and a residual basic slag layer covering the molten steel bath. In the metallurgical processing station, an inert gas is first injected over the residual slag layer to form a window 14, a region of the surface of the molten steel bath 12 from which the molten steel bath is at least partially precipitated from the overlying residual slag. Above the window 14, a thick cover 16 is placed so that its bottom edge 18 is immersed in the molten metal bath 12 to at least 20 cm (the more the molten metal bath 12 splashes, the deeper the bottom edge of the cover 16 is immersed). It can be observed that one possible implementation of such a thick cover 16 is for example described in patent application WO 98/31841, although the cover used in the method need not be a rotary cover.

Below the cover 16, the bath of molten steel is heated by the aluminotherm process. For this purpose, aluminum is added under the lid 16 and oxygen is blown in, as indicated by arrows 18 and 20 . Simultaneously, the molten metal bath 12 is agitated by an inert gas, which is preferably injected into the molten metal bath 12 using a side lance 22 . Aluminum and oxygen react in a highly exothermic reaction. This reaction leads to the formation of acidic Al ₂ O ₃ slag under the lid 16 . In Figure 1, the acidic _Al2O3 slag is identified by number ₂₄ .

In the prior art, the lid 16 is lifted at the end of the chemical heating to skim off the remaining slag, which is highly contaminated by the Al ₂ O ₃ slag formed under the lid 16 . Then desulfurization treatment is carried out in the molten steel bath where the slag is precipitated. In fact, it is well known that in order to use basic slag for desulfurization and/or dephosphorization, _{the Al2O3} content of the slag must be below 40%.

According to the invention, desulfurization and/or dephosphorization is performed around the material cover 16 without intermediate slagging operation. For this purpose, a lance 26 is used to inject a formulation for forming an alkaline slag 28 around the cap 16 in the molten metal bath 12 . The forming agent of the basic slag 28 may be, for example, lime, limestone, vitexine, sodium carbonate, magnesium carbonate, and the like. The cover 16 prevents the acidic Al ₂ O ₃ slag formed under the cover 16 from mixing with the basic slag around the molten steel tank 16, so that it is possible to carry out the two treatments simultaneously or continuously without intermediate slagging. Preferably the aluminothermic heating is started first and then the desulfurization and/or dephosphorization treatment is started when the molten steel bath has reached a sufficient temperature.

At the end of the desulfurization and/or dephosphorization treatment, all agitation of the molten metal bath is stopped before lifting the lid 16 . Then skim off the two kinds of slag together.

It should be noted that under-cap treatments are also possible, for example desiliconization by oxygen injection of cast iron or iron alloys, especially ferronickel. In this case, the silicon reacts with the oxygen blown in under the cover to form acidic SiO ₂ slag under the cover. Then carry out the above-mentioned desulfurization and/or dephosphorization treatment around the material cover. The material cover prevents the acidic SiO ₂ slag formed under the material cover 16 from mixing with the basic slag around the molten steel tank 16, so that the two treatments can be carried out simultaneously or continuously without intermediate slagging. In fact, for effective desulfurization and/or dephosphorization treatment, the _SiO2 content of the basic slag cannot exceed 10%.

Example 1

This example relates to a ladle treatment of raw converter steel for the purpose of 80% desulfurization of the steel.

Initial state : Metallurgical steel ladle containing 160 tons of raw converter steel and 600 kg of residual refining slag. The analysis results were as follows: 0.04% C, 600 ppm O, 0.010% S. The temperature of the molten steel bath was 1600°C. Add 200kg of deoxidized Al and 600kg of CaO during perfusion.

Aluminothermic heating : The first treatment is aluminothermic heating, as shown in Figure 1, which occurs under a heavy cover above the molten steel bath area where the residual slag layer was previously precipitated. By injecting 530 kg of aluminum and 350 m ³ of oxygen (at a rate of 50 m ³ /min O ₂ ) within 7 minutes, the temperature of the molten steel bath was increased by about 90 °C. Stirring under the lid was induced by injecting argon using a side lance at an injection rate of 0.2 m ³ /min.

Desulfurization : The second treatment is an intense 80% desulfurization that takes place around the cap. The desulfurizer used is a powder composed of 60% CaO and 35% _Al2O3 , and the rest _is impurities. The addition of Al ₂ O ₃ is to adjust the fluidity of the obtained slag. Other slag agents can also be added.

Using argon as a carrier gas, the desulfurization agent is injected by means of a nozzle submerged in the head. The molten steel bath is pre-stirred using the injection nozzle before starting to inject the desulfurizer. For this purpose, the injection nozzle was supplied with argon gas at a rate of about 0.5 m ³ /min for 5 minutes, and the supply of the desulfurizing agent was stopped. This preliminary stirring makes it especially possible to homogenize the temperature of the molten steel bath before desulfurization. Next, 960 kg of the above-mentioned desulfurizing agent was injected together with argon at a flow rate of about 1 m ³ /min (solid feed rate of about 80 kg/min) as a carrier gas at intervals of about 12 minutes. The treatment was terminated by vigorous stirring with argon at a rate of about 1 m ³ /min for 5 minutes using the same nozzle, and stopping the supply of the desulfurizing agent. Then stop stirring and lift the lid.

final state

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

Slag: About 1000kg of Al ₂ O ₃ formed under the material cover and about 2500kg of desulfurization slag around the material cover.

comments :

If only moderate steel desulfurization is required, it is not necessary to inject desulfurization agent into the bath using the lance. In fact, the residual slag around the charge cap already contains a sufficient amount of desulfurization agent to achieve moderate steel desulfurization. The bath of molten steel is then stirred sufficiently around the lid to allow it to react with the residual slag floating on its surface and, if necessary, to further add a slag agent to specifically adjust the consistency of the slag.

Example 2

This example concerns a ladle treatment of primary cast iron for the purpose of desiliconization and desulfurization of cast iron.

Original state : The metallurgical steel ladle contains 100 tons of raw cast iron, the analysis results of which are as follows: 4.5% C, 0.8% Si, 0.10% S. The temperature of the molten cast iron bath was 1350°C. The cast iron is covered by a layer of residual alkaline slag.

Desiliconization treatment : As mentioned above, the desilication treatment by oxygen injection is carried out under a thick cover located above the bath area from which the residual slag layer was pre-precipitated. 450 m ³ of oxygen (at a rate of 45 m ³ /min O ₂ ) were injected under the hood within 10 minutes. Stirring was performed under the lid by argon injection at a rate of 0.2 m ³ /min using a side lance.

Desulfurization : Desulfurization occurs around the material cover. The desulfurizer used is a powder composed of 70% CaCO ₃ and 30% Na ₂ CO ₂ . Other slag agents may also be added.

Using argon as a carrier gas, the desulfurization agent is injected by means of a submerged nozzle. Using about 1 m ³ /min of argon as a carrier gas, inject about 1000 kg of the above-mentioned desulfurizer (solid flow rate is about 50 kg/min) within about 20 minutes. After all stirring was stopped, the lid was lifted and both slags were skimmed together.

Final state :

Pre-treated cast iron: 4.3% C, 0.4% Si, 0.02% S, temperature about: 1400°C.

Slag : about 860 kg of SiO ₂ formed under the lid, plus about 700 kg of desulfurization slag around the lid.

Comments on the cast iron treatment :

In traditional once-through cast iron desulfurization, a mixture of Mg-CaC ₂ or Mg-CaO is usually used as a desulfurizer. These are highly efficient desulfurizers, but are also very expensive. They are used primarily because they provide limited cooling of the molten metal bath. Indeed, desulfurization combined with highly exothermic desilication makes it possible to use cooler but cheaper desulfurization agents such as limestone (CaCO ₃ ) or viteline. The decomposition of CaCO ₃ or Na ₂ CO ₃ in the molten metal bath also produces oxygen, which is beneficial to the desilication of cast iron (1 kg of CaCO ₃ or Na ₂ CO ₃ reduces the oxygen requirement for desilication by about 0.1 m ³ ). Also, it is preferred to use a mixture of CaCO ₃ +Na ₂ CO ₃ in order to obtain a more fluid slag and thus limit losses due to iron entrainment during slagging. However, the use of _Na2CO3 also requires limiting the temperature to 1400°C to prevent loss of _{Na2CO3 through evaporation} .

Comments on ferroalloy processing :

For molten iron alloy baths, especially molten nickel-iron baths, a combination of desiliconization and desulfurization as in Example 2 for cast iron is also carried out.

However, in the case of nickel-iron, by way of example, the purpose is usually to achieve a more intensive desilication (to reduce the Si content by more than 1%). Desilication by oxygen injection, in the absence of an effective coolant, can cause a temperature increase of 300°C or more.

Iron ore or oxides can be used as coolants, as is done in some cast iron desiliconization processes, obtained as a by-product of steel manufacturing. However, using the proposed method of combining desilication and desulfurization, it is particularly advantageous to use viteline (CaCO ₃ ) and/or sodium carbonate (Na ₂ CO ₃ ) as desulfurization agents, since these products are strong coolants and effective Desulfurizing agents, as long as they are not diluted by adding silicon dioxide (SiO ₂ ).

Apart from the quantitative aspects (1-2% reduction of Si content instead of 0.2-0.4% for blast furnace cast iron), the proposed method for cast iron can be similarly applied by adjusting the ratio of oxygen and required cooling/desulfurizing agent in iron alloys.

Claims (10)

1. a method that is used for the metallurgy processing of bath of molten metal comprises:

First handles, and it is included in and exists on the described bath of molten metal surface or the formation acid slag; With

Second processes, and it is included in and exists on the described bath of molten metal surface or the formation caustic slag;

It is characterized in that providing physical separation not have two processing of middle slagging by the described bath of molten metal surface between acid slag district and caustic slag district.

2. the method for claim 1, wherein in two processing carries out under its bottom margin immerses the thick material lid of described bath of molten metal, and another processing is carried out around described thick material lid.

3. method as claimed in claim 2, wherein said first processing are that the chemical heat of carrying out under described thick material lid is handled.

4. method as claimed in claim 3, it is aluminothermy or silicothermic process that wherein said chemical heat is handled.

5. as arbitrary described method among the claim 1-4, wherein said second handles desulfurization and/or the dephosphorization treatment that is based on basic slag.

6. the method for claim 1, wherein said first to process be to cast iron or ferroalloy, particularly ferronickel, processes by the desiliconization of oxygen injection.

7. method as claimed in claim 6, it is to carry out under its bottom margin immerses the thick material lid of described bath of molten metal that wherein said desiliconization by oxygen injection is handled, and described second to handle be to carry out desulfurization and/or dephosphorization treatment around described thick material lid.

8. as claim 6 or 7 described methods, wherein said second handles desulfurization and/or the dephosphorization treatment that is based on lime.

9. method as claimed in claim 8, wherein said second handles and to comprise in described bath of molten metal and add Wingdale, particularly castine.

10. as arbitrary described method among the claim 1-9, wherein:

When the beginning of described method, the surface of described bath of molten metal is covered by the remaining slag of one deck;

By inert gas injecting, in described remaining slag layer, form a window;

Described window is covered by the thick material lid that its bottom margin immerses described bath of molten metal;

Under described thick material lid, carry out in two processing, around thick material lid, carry out another, bathe by the injecting inert gas stirring molten metal simultaneously; With

When two processing finish, stop described stirring, remove described thick material lid, skim immediately then two kinds of slags.

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