EP0363749A1 - Process for dephosphorizing molten metals by treating with alkali metals and alkaline earth metals - Google Patents

Abstract

For dephosphorising molten metals having an iron content of at least 50 per cent by weight and contents of nickel, cobalt and copper of at most 30 per cent by weight in total, a particulate mixture of at least one alkali metal and/or alkaline earth metal and of at least one halide of the same alkali metal or alkaline earth metal is blown into the melt by means of an inert gas. The mixing ratio is here selected such that, under the reaction conditions, a minimum of the alkali metal or alkaline earth metal vapourises. The unconverted components of the mixture (alkali metal and alkaline earth metal and the halide or halides of these metals) and the phosphides, formed by the reaction, of the alkali metals and alkaline earth metals are collected in a top slag located above the melt, further reactions between the alkali metals and alkaline earth metals and the iron-containing molten metal being effected by the bath movement. Finally, the slag with the reaction products is separated from the molten metal, kept with exclusion of moisture and treated with oxygen or oxygen-containing compounds until the reaction products have been completely converted into oxygenated phosphorus compounds. <IMAGE>

Description

The invention relates to a process for removing phosphorus from metallic melts by treating the melt with at least one metal from the group of the alkali and alkaline earth metals in a treatment tank in the presence of slag.

By the article by Tokumitsu / Nakamura / Kajioka / Ishikawa "Production of highly purified austenitic stainless steel by ESR process using Ca-CaF₂ flux", published in "Proc. 6th International Metallurgy Conference on Special Melting", April 1979, pages 670 to 683 , it is known that sulfur and oxygen can be removed from chromium-containing melts (stainless steel) by the ladle method and by the electroslag remelting method (ESR method). At the same time, however, it is pointed out that it is difficult to remove sulfur and oxygen at the same time to very low values. In particular, the removal of phosphorus regularly leads to high oxidation losses on chromium. The authors state that it is using the ESR process and using a slag made of calcium and calcium fluoride is able to remove a large number of impurities from the melt, including numerous metals, but especially phosphorus, oxygen and sulfur. However, the ESR process is an extremely slow process with a high energy requirement and consequently only suitable for smaller quantities of metal. With regard to the energy requirement, it must be taken into account that electrodes must first be cast from the steel to be cleaned, which electrodes must subsequently be remelted continuously into blocks. Since this process must take place in a water-cooled mold, high energy losses to the cooling water are inevitable.

Through the article by Bode / Engell / Schwerdtfeger "Removability of phosphorus, arsenic, antimony, tin and nitrogen from chrome-nickel steel with calcium using excess pressure", published in "Stahl und Eisen", 1983, pages 211 to 215, it is known to apply a top slag made of calcium fluoride to the melt of chrome-nickel steel and to introduce calcium in a tubular steel capsule. However, the process suffers from a high evaporation rate of calcium, which could not be significantly reduced even by a drastic increase in pressure. The process therefore has an extraordinarily poor efficiency with regard to the calcium addition.

From DE-PS 823 219 and US-PS 2 406 582 it is known to treat melts that consist entirely or predominantly of nickel with calcium in order to To remove sulfur. With such melts, the problems of the evaporation of calcium only arise to a minor extent because calcium and nickel form an alloy in any ratio. The removal of phosphorus is not addressed.

It is known from US Pat. No. 4,435,210 to use a finely divided mixture of two components, one of which consists of an alloy of calcium and at least one of the metals aluminum and silicon, for the dephosphorization of molten steel in a ladle, and the other component consists of a slag consisting essentially of calcium and aluminum oxide. Although it is stated that the reaction mixture can contain between 0 and 30 percent of calcium fluoride, this also only insignificantly reduces the evaporation of the calcium. It can be assumed that the reaction mixture with the calcium and the other slag formers ascends and evaporates from the top slag before the calcium has had the opportunity to react with the phosphorus in the steel melt. An interaction between the calcium and its fluoride could not be recognized, probably because the oxidic slag formers at least suppress such an interaction.

Apart from the high evaporation rate of the alkali or alkaline earth metals, there is still another problem which is based on the fact that the phosphide formed in the reaction of these metals with the phosphorus develops a very dangerous or toxic gas in connection with water vapor and / or atmospheric moisture, the phosphine.

It is known from US Pat. No. 4,541,863 to isolate the slag with the reaction product of calcium and phosphorus from the metal melt and to treat it with an oxidizing agent, so that a pure oxygen compound of the phosphorus is thereby formed. The preferred oxidizing agents are the carbonates of sodium, magnesium and potassium. The document in question leaves it completely open in what way and in what relative amount the calcium is introduced into the molten metal.

The invention is therefore based on the object of specifying a method of the type described at the outset which is distinguished by a high degree of efficiency or utilization rate of the alkali metal or alkaline earth metal used and in which the disposal problem of the slag containing the reaction products is also reliably solved.

The object is achieved according to the invention in the method mentioned at the outset by treating melts with an iron content of at least 50 percent by weight and contents of nickel, cobalt and copper of at most 30 percent by weight in the melt at a distance from the bath surface , which corresponds to at least half of the total bath height, by means of an inert gas, injects a particulate mixture of at least one of the alkali and alkaline earth metals mentioned and at least one halide of the same alkali or alkaline earth metal, the mixing ratio being chosen so that under the reaction conditions a Minimum of alkali or alkaline earth metal evaporates that the unreacted components of the mixture (alkali and alkaline earth metal and the halide or the halides of these metals) and the phosphides formed by the reaction of the alkali and alkaline earth metals in the slag located above the melt collects and thereby carries out further reactions between the alkali and alkaline earth metals and the iron-containing molten metal by bath movement and finally separates the slag with the reaction products from the molten metal and keeps it moisture-free and with oxygen or oxygen-containing compounds until the reaction products are completely converted into oxygen compounds of the phosphorus treated.

It is initially surprising that the savings in alkali and alkaline earth metals also occur in melts with an iron content of at least 50 percent by weight. The alkali and alkaline earth metals in question are practically insoluble in iron and have consequently led to the evaporation losses described in the literature.

The melts in which the process according to the invention can preferably be used include, in addition to pure iron melts with less than 5% by weight of alloying elements, the melts of commercially available stainless chromium or chromium-nickel steels, such as, for example, No. 1.4300 (X 12 CrNi18 8), No. 1.4000 (X 6 Cr 13) and No. 1.4371 (X 12 CrMnNi 18 8 5). Such chromium and / or manganese-containing steels cannot be freed from phosphorus by conventional freshening because due to the greater affinity of these metals for oxygen, a large part of the metals is lost through oxidation or combustion before the phosphorus reacts with oxygen. In the case of steels with less than 5 percent by weight of chromium, phosphorus is usually removed by refining or oxidation.

The particulate mixture used for dephosphorization can either consist of a mixture of granules of the alkali and alkaline earth metals on the one hand and at least one halide of the same alkali or alkaline earth metal on the other hand, or of a granulate of a mixture in which the alkali or alkaline earth metal in the at least a halide of the same alkali or alkaline earth metal is dissolved. The alkali and alkaline earth metals in question can be dissolved in their own halides in any proportions, and the alkali or alkaline earth metals evaporate from this melt far more poorly than if they were added to the iron-containing melt alone.

To put it simply, the halides of the alkali and alkaline earth metals effectively replace the overpressure in the prior art, but in addition the effect is much better than when using pure overpressure.

If chromium-containing melts are treated, the phosphorus content can be reduced with the process according to the invention without noticeable erosion of chromium from the melt.

It is particularly advantageous if the alkali or alkaline earth metal is dissolved in the halide in question before being blown into the iron-containing melt, the solution is allowed to solidify and granulated, and the granules are blown into the iron-containing melt.

The size of the granules is not very critical; it is only necessary to ensure that the granulate is transportable by a gas flow which is fed to the blowing lance.

In a particularly expedient manner, the mixing or solution ratio of alkali and alkaline earth metals to the corresponding halide is chosen between 1: 2 and 2: 1.

In the process according to the invention, drops of the solutions of alkali and alkaline earth metals in their halides rise from the injection point below the molten bath level to the latter due to the given temperatures and in the process interact intimately with the melt. In addition to phosphorus, other contaminating elements such as tin, lead, nitrogen, arsenic, antimony, bismuth, oxygen, sulfur, selenium and tellurium are also removed from the melt.

All reaction products rise to the surface of the molten bath and collect in the top slag located there, which preferably consists of the oxides of calcium and aluminum or contains these oxides in substantial amounts. Is of particular advantage also the presence of an amount of 5 to 15 percent by weight magnesium oxide. It should be emphasized that such a slag in turn does not dissolve any alkali or alkaline earth metal and has no significant influence on the P content of the melt.

If an unreacted portion of the alkali or alkaline earth metal in solution in the halide in question rises up to the top slag, the alkali or alkaline earth metal also continues to participate in the reaction, namely through the inevitable bath movement caused by the blowing gas supplied mixing of the iron-containing melt and the top slag is effected.

Through the final process steps to separate the slag with the reaction products from the molten metal, keep it under moisture and finally treat it with oxygen or oxygen-containing compounds, the phosphine formed is converted into the oxide in question, at the same time forming phosphorus pentoxide. When using calcium as a cleaning metal, the following reaction occurs, for example:
Ca₃P₂ + 4 0₂ = 3 Ca0 + P₂0₅.
Ca0 and P₂0₅ can in turn be converted to calcium phosphate, which can be used as a fertilizer in agriculture.

It is particularly advantageous if the separated slag is kept under an inert gas atmosphere until the oxygen treatment.

A device for carrying out the method according to the invention and two exemplary embodiments of the method are explained in more detail below.

• Figur 1 die erste Stufe des Verfahrens, nämlich die Behandlung der eisenhaltigen Metallschmelze mit einem Alkali- oder Erdalkalimetall,
• Figur 2 die zweite Stufe des Verfahrens, nämlich die Trennung der eisenhaltigen Metallschmelze von der Kopfschlacke unter Luftabschluß, und
• Figur 3 die letzte Stufe des Verfahrens, nämlich die Oxidation der Reaktionsprodukte.

Show it:

• 1 shows the first stage of the process, namely the treatment of the ferrous metal melt with an alkali or alkaline earth metal,
• Figure 2 shows the second stage of the process, namely the separation of the ferrous metal melt from the top slag in the absence of air, and
• Figure 3 shows the last stage of the process, namely the oxidation of the reaction products.

FIG. 1 shows a melting unit 1, which is of no interest here and in which the melt to be cleaned is produced. From this melting unit, the melt is poured into a pan 2, which is provided with a bottom outlet 3, which can be closed by a slide valve 4, and with an inlet 5 for a purge gas (for example argon).

During the tapping, slag 6 (so-called head slag) based on Ca0-Al₂0₃ is added. After the tapping has ended, the pan 2 is closed by a lid 7 with a blowing lance 8. The immersion depth of the blowing lance in the molten metal is at least 50% of the total bath height from the pan bottom to the melt / slag interface. The blow lance 8 stands via a flexible line with a reservoir, not shown here, in connection with a granulate which is a mixture of calcium and calcium fluoride. Furthermore, the blowing lance 8 is connected to an argon source, the connection being selected in such a way that the argon introduces a corresponding amount of the mixture of calcium-calcium fluoride into the metal melt 9.

The gas space above the slag 6 is connected to the environment via a filter, but this is not shown here for the sake of simplicity.

After the cover 7 has been put on, argon is let in through the inlet 5 in order to produce a flushing and stirring action in the melt. At the same time a mixture of Ca-CaF₂ is introduced through the blowing lance 8, which is suspended in an argon flow. The gas purging continues until the end of the reaction. The rising gas bubbles and metal drops are only hinted at in FIG.

According to Figure 2, the gate valve 4 is opened after cleaning, so that the molten metal 9 flows as a melt jet 9a in a further pan 10. The process is interrupted before the slag 6 (now provided with the reaction products of the impurities) has the opportunity to exit through the bottom outlet 3. It is important that the gas space 11 remains shut off from the atmosphere. In order to replace the volume released by the melt, the gas space above the slag can either through the blowing lance 8 or through a special line 13 with an inert gas, for example, argon, too.

The pan 10 with the melt 9b can now be fed to a further treatment station, not shown here, in which a treatment such as vacuum degassing or vacuum oxygen decarburization is carried out.

Figure 3 shows the state in which the pan 2 essentially contains only the slag 6. The oxidation treatment of the slag described above is now carried out. It is important that the gas space 11 above the slag is still sealed off from the atmosphere. For oxygen treatment, dry oxygen is blown into the pan 2 through the blow lance 8 until all reaction products have been converted into the pure oxygen compounds of the impurities. As soon as the oxygen treatment of the slag has ended, the still molten slag is discharged through the bottom outlet 3 into a slag bucket 12, in which it can easily solidify in the ambient air. The slag can now be brought to a disposal site.

A converter, an arc furnace or an induction furnace, for example, can serve as the melting unit 1.

A mixture of a production gas and the reactive mixture of the alkali and alkaline earth metals and their halides can also through the bottom inlet 5 be introduced.

Example 1: Treatment of a 30 ton melt:

The steel melt to be treated contains the following components after casting from the melting unit 1:
1% C; 18% Cr; 8.5% Ni; 0.07% P; 0.04% S.
During the pouring into the pan 2 300 kg of a slag mixture of 150 kg Ca0 and 150 kg Al₂0₃ as top slag are given up. After the lid 7 has been put on, the melt is rinsed vigorously for 3 minutes with an amount of argon of 50 l / min. The freeboard of pan 2 is 1000 mm; the temperature of the steel is 1600 ° C.

After the blowing lance 8 has been lowered to a depth of approximately 800 mm below the bath level (this corresponds to approximately 50% of the total bath height), the Ca-CaF 2 mixture is blown in. With a blowing time of 10 minutes and an average blowing rate of 12 kg / min, a total of 120 kg Ca-CaF₂ are blown into the melt. The blowing of the cleaning substance was stopped and the reaction was completed after a further 5 minutes. The metal was tapped into a second pan 10 at a temperature of 1550 ° C. according to FIG. The analysis of the melt after completion of the reaction led to the following values:
1% C; 18% Cr; 8.5% Ni; 0.025% P; 0.02% S.

The further treatment of the melt took place in a downstream vacuum degassing plant, which could optionally also be operated with an oxygen supply.

The still liquid slag in the first pan 2 was refreshed with the same blowing lance 8 with oxygen. The lower end of the lance was at a distance of approx. 400 mm from the slag. A strong floor flush of approx. 75 l / min led to a quick oxidation. The blowing time was about 7.5 minutes. About one minute after the completion of the freshening, the treated slag could easily be tapped into the slag bucket 12.

Example 2: Treatment of a 50 ton melt:

The timing of the individual process steps corresponded to that in Example 1. The steel melt to be treated contained the following components:
0.8% C; 20% Cr; 10% Ni; 0.06% P; 0.04% S.
The amount of top slag added during tapping was 600 kg and consisted of 50% Ca0 and Al₂0₃. 100 l / min of argon were introduced through the bottom inlet 5 to bring about a gas purge. The initial temperature of the melt was 1600 ° C.

After the lid was put on, the blowing lance 8 was immersed in the melt to such an extent that its lower end was about 1000 mm below the bath level, which also corresponded to 50% of the bath height here. This was constantly Inert gas passed through the blowing lance. A Ca-CaF₂ mixture containing 50% Ca was blown in over a period of 14 minutes with an average amount per unit time of 15 kg / min, so that a total of 210 kg of the treatment mixture got into the melt. The reaction was complete 5 minutes after the blowing had ended and the analysis of the melt gave the following values:
0.8% C; 20% Cr; 10% Ni; 0.020% P; 0.015% S.
The final temperature was 1550 ° C, which guaranteed a subsequent vacuum degassing. The slag was treated in accordance with Example 1.

Claims (5)

1. A process for removing phosphorus from metallic melts by treating the melt with at least one metal from the group of alkali and alkaline earth metals in a treatment tank in the presence of slag, characterized in that in the treatment of melts with an iron content of at least 50 Weight percent and contents of nickel, cobalt and copper of at most 30 weight percent in the melt at a distance from the bath surface which corresponds to at least half of the total bath height, by means of an inert gas, a particulate mixture of at least one of the alkali and alkaline earth metals mentioned and blowing in from at least one halide of the same alkali or alkaline earth metal, the mixing ratio being chosen so that under the reaction conditions a minimum amount of alkali or alkaline earth metal evaporates so that the unreacted components of the mixture (alkali and alkaline earth metal and the Ha logenide or the halides of these metals) and the phosphides of the alkali and alkaline earth metals formed by the reaction in the slag located above the melt and thereby further reactions between the alkali and alkaline earth metals and the iron-containing metal melt by the bath movement and finally the slag separates with the reaction products from the molten metal and keeps it closed with moisture and treated with oxygen or oxygen-containing compounds until the reaction products have been completely converted into oxygen compounds of phosphorus. 2. The method according to claim 1, characterized in that the alkali or alkaline earth metal is brought into solution in the halide in question before being blown into the iron-containing melt, the solution is allowed to solidify and granulated and the granules are blown into the iron-containing melt. 3. The method according to claim 1, characterized in that one chooses the mixing or solution ratio of alkali and alkaline earth metal (s) to the corresponding halide between 1: 2 and 2: 1. 4. The method according to claim 1, characterized in that the separated slag is stored until the oxygen treatment under an inert gas atmosphere. 5. The method according to claim 1, characterized in that after the treatment of the iron-containing melt, the gas space of the treatment container is sealed off from the atmosphere.

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