US2988445A - Method for making briquettes for the treatment of molten metals and alloys - Google Patents

Description

Unite States Patent Ofifice 2,988,445 Patented June 13, 1961 2,988,445 lvIETHOD FOR MAKING BRIQUETIES FOR THE TREATMENT OF MOLTEN METALS AND ALLOYS Fredrik Jtirgen Ordiug Hurum, Riis Alle 32, Slemtlal, Oslo, Norway No Drawing. Filed Nov. 19, 1956, Ser. No. 622,820 Claims priority, application Sweden May 29, 1952 2 Claims. (Cl. 75-58) The introduction of magnesium to molten metals and alloys, particularly to iron, nickel and the different types of steel and cast iron, has in recent years gained considerable importance for the reason that magnesium is a powerful deoxidizer and desulphurizer and is able to give a clean metal and remove dissolved gases.

In my prior application Serial No. 322,394, filed November 25, 1952, now abandoned, of which this application is a continuation-in-part, I have described a method for submerging a briquette below the surface of a molten metal bath for the purpose of introducing a solid element which is easily evaporated or oxidized.

The eifect of magnesium as a deoxidizer and cleansing agent is attributed to its ability to destroy dissolved oxides of the iron group and finely dispersed silicates and in turn form insoluble magnesium oxides and magnesium silicates. It is furthermore well known that magnesium attacks the soluble sulphides of the iron group and forms an insoluble magnesium sulphide which will rise to the surface where it can be removed.

It should be borne in mind that magnesium does not form an alloy with iron and is practically insoluble in the molten metal. Magnesium will boil at a temperature below the melting point of cast iron and as the temperature increases the boil will become so violent that the burning vapor may involve serious hazards and the efiect will rapidly get lost. Accordingly magnesium should be added to the ladle shortly before the metal is poured.

In my application Ser. No. 322,394, referred to above, I have described a method for submerging briquettes below the surface of the metal bath for the purpose of introducing such elements as are easily evaporized or oxidized. I have found this method well adapted to the treatment of both cast iron and steels with magnesium and the present application teaches how this method is to be used for the introduction of magnesium under the varying conditions prevailing in the metallurgy of metals belonging to the iron group.

It has been found that this method permits introducing magnesium to the interior of the bath with good and certain etfect without the use of such expensive alloying metals as nickel or copper. The metal treated will not be contaminated by slag particles since the method may be so practiced that no slag will be formed inside the briquette which subsequently could be carried away with the magnesium vapor and lodged in the metal. It has in fact been found that briquettes consisting of finely powdered CaO or MgO mixed with metallic magnesium or a magnesium alloy in the powdered state, will not collapse or disintegrate during the submersion even when the content of magnesium is as high as 25% by weight. This behaviour of my briquettes may be attributed to the fact that the highly refractory oxides of calcium and magnesium will make the briquettes retain the full porosity so that the escape of magnesium vapor is not obstructed by slag formations inside the surface of the briquettes leading to the bursting of the briquette under the vapor pressure.

The magnesium will begin to evaporate from the surface of the briquette and this process will progressively proceed towards the interior so that in the course of a very short time, usually ranging from 1 to 5 minutes all v the magnesium will have evaporated while the briquettes remain undamaged in shape and appearance.

As the magnesium escapes from the indefinite number of pores on the surface of the briquettes, a very intimate contact between the magnesium vapor and the metal to be treated is assured.

While burnt lime is everywhere available at a low price, it has the drawback that its purity varies and it also readily absorbs carbon-dioxide and moisture from the air and accordingly briquettes containing burnt lime do not always give fully reliable results.

The same may be said of burnt dolomite which also contains impurities in varying amounts and is exposed to the action of the atmosphere and the hazards of storage.

I prefer to use dead-burned magnesium oxide as a carrier or moderator in my briquettes containing magnesium. This material is now available at a reasonable price and of a very high purity such as prepared for the electrolytic production of magnesium. Dead-burned magnesium oxlde may be stored without harmful effect for long times due to its chemical indifference towards the atmosphere. Such briquettes will not crack or disintegrate even when plunged into a steel bath at 1600 C. They will not form any slag and can be removed from the melt without loss of any magnesium oxide. They have accordingly proved suitable for cleaning steels with magnesium vapor. Such a treatment may be carried out in the ladle with very little loss of temperature, since no heat is required for chemical reactions or the fusion of any slag. I have found that 0.01% magnesium added to a steel by means of these briquettes may efiect valuable improvements.

Magnesium oxide is a relatively good conductor of heat and for that reason the evaporation of the metallic magnesium is rapid without being violent and the Whole treat ment may be carried out in the course of less than 5 minutes.

While my briquettes consisting of magnesium and magnesium oxide may be used successfully for the treatment of cast iron and blast furnace metal for such purposes as desulphurizing, deoxidation and the formation of spherulitic graphite, the conditions of treatment are in these cases less severe because the metal is practically saturated with carbon and the temperature range is lower. Under these circumstances I have found that I can substitute a low grade cheap coke for the more expensive MgO without changing the principles and method of treatment. A mixture consisting of metallic magnesium or a magnesium alloy in the powdered state and finely crushed coke can be pressed to briquettes suitable for submersion in the molten iron. I have found that such briquettes give satisfactory service when containing one part of magnesium to 2 /2 parts by weight of coke.

The following are species of briquettes for treatment of iron and steel in parts by weight:

These briquettes will behave like the briquettes of Mg/MgO and the magnesium will rapidly become evaporated because the coke is a good conductor of heat. The

coke will not dissolve in the metal and the briquettes may be removed from the melt with shape and appearance undamaged after the magnesium has become completely evaporated. This treatment may be carried out in any ladle and the time required may be in the neighborhood of 5 minutes. The loss of temperature is insignificant because the treatment is rapid and no heat is required for fusing any slag or performing any chemical reactions. As will be understood the briquettes will not contaminate the metal with any slag inclusions and this is a desirable feature when the metal is subsequently inoculated with ferrosilicon in order to precipitate spherulitic graphite.

The addition of ferrosilicon to a cast iron in the ladle at ordinary temperature requires energetic stirring and considerable time in order to give complete and uniform solution. Ferrosilicon, when submerged in cast iron will absorb considerable quantities of iron in solid solution and form the compound FeSi with a high melting point. The absorption of further quantities of iron will give silicoferrite in a pasty condition. The difiusion of still further iron into the silico-ferrite will give a silico-austenite which will finally melt. (F. Hurum: A Study on the Formation of Nodular Graphite, American Foundrymens Society, col. 62.)

For the above reasons briquettes of ferrosilicon have not proved useful as an addition to cast iron in the ladle.

I have found, however, that briquettes when used according to my US. patent application Ser. No. 322,394 may successfully be used for adding ferrosilicon to a ladle containing cast iron whenever this treatment is combined with the addition of magnesium. Ferrosilicon has in fact proved a very satisfactory carrier of magnesium. The evaporation of magnesium is in this case more rapid than when using cokeor magnesium oxide as acarrier or moderator and it does not appear safe to use more than magnesium in such briquettes. Contrary to expectation the ferrosilicon will dissolve rapidly and completely even at temperatures as low as 1200 C. This is due to the fact that the magnesium vapor formed in the interior of the porous briquette will exert a pressure on the outer layer consisting of silico-ferrite and silico-austenite in the process of fusion so that this layer will break up and be carried away with the magnesium vapor thereby exposing the briquette to a progressive and rapid solution. The magnesium treatment thus effected has proved very successful for the production of spherulitic graphite and the ferrosilicon has vice versa given a very rapid and energetic inoculation under complete freedom of slag inclusions or finely dispersed impurities so harmful to a proper nucleation. This treatment may be accomplished in the course of about lminute and .the loss of temperature is accordingly insignificant. The required amount of magnesium to produce spherulitic graphite by this treatment may be reduced to less than 0.1% of the weight of the cast iron treated.

My observations have made it evident that this good result may be attributed to the fact that the magnesium is evaporated before the ferrosilicon is dissolved. If on the other hand an easily fusible carrier-alloy had been used, this alloy would have fused and risen to the surface before the evaporation of magnesium had been completed and the effect would have been strongly reduced.

The success of this treatment is due to the use of magnesium in such a state that it will become completely evaporated and the use of a carrier which will not consume any magnesium in chemical reactions and which will not contaminate the metal with inclusions. It is also an advantage for the heat economy and elficiency that the carrier will not fuse, although the carrier may dissolve in the metal provided it does not fuse before the magnesium is evaporated.

While in principle metallic magnesium in the state of a powder appears best suited for my magnesium treatment, it may be advantageous for the purpose of crushing to fine powder and mixing to use an alloy or an intermetallic compound of magnesium such as a magnesium-silicide, the compounds of magnesium with nickel or copper, or such alloys as magnesium-ferrosilicon, magnesium-silicon or magnesium-calcium-silicon. These compounds and alloys are easily crushed to a fine powder suitable for mixing and briquetting. While dead-burned magnesium oxide is best suited as a carrier for the treatment of steel, it would be within the claim of my invention to use such irreducible substitutes as burnt dolomite and burnt lime.

I claim:

1. A method for treating molten metals and alloys of the iron group with magnesium to effect deoxidation, desulfurization, degassing, cleaning, and the formation of spherulitic graphite in cast iron which comprises submerging in said molten metal a pressed briquet containing 25% of metallic magnesium powder and 75% of an alkaline earth metal oxide, whereby the briquet releases magnesium vapor directly into the molten metal withoutundue violence.

2. A method for treating molten metals and alloys of the iron group with magnesium to efiect deoxidation, desulfurization, degassing, cleaning, and the formation of spherulitic graphite in cast iron which comprises submerging in said molten metal a pressed briquet containing 30% of metallic magnesium powder and of coke, whereby the briquet releases magnesium vapor directly into the molten metal without undue violence.

References Cited in the file of this patent UNITED STATES PATENTS 2,574,581 McKinney et al. Nov. 13, 1951- 2,675,308 Millis et a1 Apr. 13, 1954 2,726,152 Bash Dec. 6, 1955

Claims (1)

1. A METHOD FOR TREATING MOLTEN METALS AND ALLOYS OF THE IRON GROUP WITH MAGNESIUM TO EFFECT DEOXIDATION, DESULFURIZATION, DEGASSING, CLEANING, AND THE FORMATION OF SPHERULITIC GRAPHITE IN CAST IRON WHICH COMPRISES SUBMERGING IN SAID MOLTEN METAL A PRESSED BRIQUET CONTAINING 25% OF METALLIC MAGNESIUM POWDER AND 75% OF AN ALKALINE EARTH METAL OXIDE, WHEREBY THE BRIQUET RELEASES MAGNESIUM VAPOR DIRECTLY INTO THE MOLTEN METAL WITHOUT UNDUE VIOLENCE.