Method and apparatus for the production of cast iron, cast iron produced according to said method

Abstract

A method for the production of cast iron starting from pre-reduced iron ore (DRI) with an electric arc furnace includes the steps of preparing a charge of pre-reduced iron ore DRI having a metallization higher than 90% and containing over 2.8% by weight of carbon, wherein at least 80% of the carbon is combined with the iron to form iron carbide Fe.sub.3C; charging the charge of pre-reduced iron ore into the electric arc furnace; and melting the DRI charge to form liquid cast iron having at least 80% by weight of actual carbon content deriving from the carbon in the charge of pre-reduced iron ore, the melting step being in a reducing atmosphere and in a melting chamber of the electric arc furnace subjected to a positive internal pressure generated by the gases produced by reduction reactions that develop during melting.

Claims

1. A method for production of cast iron starting from pre-reduced iron ore (DRI) with an electric arc furnace (EAF), comprising the following steps: a. preparing a charge of the pre-reduced iron ore (DRI) having a metallization higher than 90% and containing up to 6.5% by weight of carbon, wherein at least 80% of said carbon is combined with iron to form iron carbide (Fe.sub.3C); b. charging the charge of the pre-reduced iron ore (DRI) into the electric arc furnace (EAF) without adding free carbon; and c. melting the DRI charge to form liquid cast iron, wherein said liquid cast iron has a predetermined actual content of carbon, at least 80% by weight of said actual carbon content of the liquid cast iron deriving from the carbon in the charge of the pre-reduced iron ore (DRI), and wherein step c. is carried out in a reducing atmosphere and in a melting chamber of the electric arc furnace (EAF) subjected to a positive internal pressure generated by gases produced by reduction reactions that develop in step c.

2. The method according to claim 1, wherein at least 90% of said carbon in the charge of the pre-reduced iron ore (DRI) is combined with iron as the iron carbide (Fe.sub.3C).

3. The method according to claim 1, wherein in step a said DRI charge is charged into the electric arc furnace at a temperature higher than 400° C.

4. The method according to claim 1, further comprising: d. discharging contents of the EAF furnace downstream of step c into a transfer ladle or container.

5. The method according to claim 1, further comprising: e. granulating the liquid cast iron, or f. casting the liquid cast iron into ingots.

6. The method according to claim 1, wherein the cast iron is grey cast iron or nodular cast iron.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The structural and functional characteristics of the invention, and its advantages with respect to the known art, will be apparent from the following description, referring to FIG. 1 enclosed, which is a schematic block diagram illustrating a non-limiting embodiment of the invention showing the main operations of the method.

[0040] With the intent of first describing the method in its general features, this is destined for the production of cast iron starting from pre-reduced iron ore (DRI) using an apparatus comprising an electric arc furnace (EAF).

[0041] The term “cast iron” refers herein and in the following claims to any type of cast iron.

[0042] The method of the invention characteristically comprises the following steps:

[0043] a. preparing a charge of pre-reduced iron ore (DRI) having a metallization higher than 90% and containing over 2.8% by weight of carbon, wherein at least 80% of said carbon is combined with the iron to form iron carbide Fe.sub.3C,

[0044] b. charging the charge of pre-reduced iron ore (DRI) into the electric arc furnace (EAF),

[0045] c. melting the DRI charge to form liquid cast iron

[0046] wherein said liquid cast iron has a predetermined target content of carbon, at least 80% by weight of said target carbon content of the cast iron deriving from the carbon in the charge of pre-reduced iron ore (DRI), wherein step c. is effected in a reducing atmosphere condition and in a melting chamber of the electric arc furnace (EAF) subjected to a positive internal pressure generated by gases produced by reduction reactions that develop in step c.

[0047] An upper limit of the carbon content by weight of the charge of pre-reduced iron ore (DRI) is preferably 6.5% by weight.

[0048] A large percentage, normally higher than 90% by weight of carbon in the charge of pre-reduced iron ore (DRI) is preferably combined with the iron in the form of iron carbide Fe.sub.3C; this avoids having carbon in the form of graphite which would be mostly lost in the slag.

[0049] In particular, the method is carried out in an apparatus which implements the method and comprises an electric arc furnace (EAF) equipped with a melting chamber in which the electrodes are active.

[0050] The melting chamber of the apparatus in which the DRI charge is melted, is subjected to slightly positive pressure to prevent or in any case limit the inlet of air from the outside, avoiding the oxidation of the carbon present in the metal bath.

[0051] According to an advantageous optional feature, step a. provides the heating of said DRI charge to a temperature higher than 400° C., completely favoring energy saving in the melting process.

[0052] According to other variants, an additional step is optionally provided

[0053] b1—for the addition, to the pre-reduced iron ore charge (DRI), of a carbonaceous material in the electric arc furnace (EAF) in order to regulate the carbon content,

[0054] said step b1 being effected between steps b. and c. either contemporaneously with step b. or contemporaneously with or downstream of step c.

[0055] According to other variants, an additional step d. is optionally provided, for discharging the contents of the EAF furnace—downstream of (after) step c. into a transfer ladle or container.

[0056] The additional step is also optionally provided

[0057] d1. of adding a carbonaceous material to said transfer ladle.

[0058] Said carbonaceous material, of step b1 or d1, is generally selected from the group of coal, coke, graphite, or mixtures thereof.

[0059] The liquid cast iron is then solidified from the ladle according to one of the following alternative steps:

[0060] e. granulation of the liquid cast iron

[0061] f. casting the liquid cast iron into ingots.

[0062] The cast iron thus obtained is basic cast iron, and comprises, in addition to iron, the following weight percentages of elements:

[0063] Carbon 2.1-4.5%

[0064] Silicon<1.5%

[0065] Manganese 0.5-1.0%

[0066] Sulfur<0.05%

[0067] Phosphorous<0.12%

[0068] In other preferred embodiments, the cast iron obtained is grey cast iron or nodular cast iron.

[0069] The grey cast iron thus produced even more preferably comprises, in addition to iron, the following weight percentages of elements:

[0070] Carbon 3.5-4.5%

[0071] Silicon 1.5-3.5%

[0072] Manganese 0.5-1.0%

[0073] Sulfur<0.05%

[0074] Phosphorous<0.12%

[0075] The nodular cast iron thus produced even more preferably comprises, in addition to iron, the following weight percentages of elements:

[0076] Carbon 3.5-4.5%

[0077] Manganese<0.5%

[0078] Sulfur<0.02%

[0079] Phosphorous<0.04%

[0080] Referring now to FIG. 1, which illustrates a simplified block scheme of a preferred embodiment of a method for the production of cast iron according to the invention, 10 indicates as a whole a supply of pre-reduced iron ore (DRI).

[0081] The latter contains a carbon percentage higher than 2.8% by weight, preferably ranging from 3% to 6% by weight, and more preferably from 4% to 5%.

[0082] The metallization of the charge of pre-reduced iron ore (DRI) is at least 90% by weight, preferably at least 94% by weight.

[0083] The DRI charge is fed to an apparatus 20 according to the invention comprising an electric arc furnace, to be melted.

[0084] The DRI containing carbon can be charged into the melting chamber of the apparatus 20 at a temperature ranging from room temperature to 500° C. or higher (preferably up to 700° C.), the energy consumption of the melting process will obviously be lower with an increase in the charging temperature of the DRI.

[0085] The charging is effected using means known per se in the state of the art, for example by gravity, by means of a pneumatic transport system, by a mechanical conveyor provided with means for maintaining an inert atmosphere in contact with the hot DRI, or in thermally insulated containers (not shown as they are known in the art).

[0086] The DRI charge is then melted in the EAF at a temperature at least higher than 1,350° C., preferably ranging from 1,400° C. to 1,550° C.

[0087] Once the process has been completed, the contents of the furnace are discharged (tapped) (see 26) from the EAF.

[0088] It should be noted that the carbon content in the charge material (DRI) is already close to the target content of the cast iron to be produced.

[0089] The carbon content can optionally be adjusted by the addition of a further carbonaceous material.

[0090] In a first embodiment, this carbonaceous material is mixed with the DRI directly in the melting chamber of the apparatus.

[0091] The carbonaceous material 24 that can be used is, for example but not exclusively, carbon, coke, graphite or mixtures thereof.

[0092] In a second embodiment, alternatively to or combined with the first embodiment, the carbonaceous material 30 is subsequently mixed with the molten DRI, for example in a transfer ladle 28.

[0093] The carbonaceous material 30 that can be used is, for example but not exclusively, carbon, coke, graphite or mixtures thereof.

[0094] These additions are optional and are only necessary when the target carbon content of the cast iron to be produced exceeds the carbon content of the DRI.

[0095] The carbon content of the DRI is preferably equal to the target carbon content of the cast iron to be produced, so that the method expressly excludes the addition of carbon both in the melting chamber and subsequently.

[0096] According to the invention, the carbon content in the DRI charge is combined therein with iron, preferably mostly in the form of carbon carbide Fe.sub.3C. The combined carbon provides a number of advantages in the electric arc furnace (EAF) with respect to the use of “free” carbon which can be added in the form of soot, coal, coke, graphite: soot is in fact easily entrained by the hot gases during the melting phase of the DRI, coal brings numerous impurities, among which sulfur which must be controlled and eliminated in the final composition of cast iron, coke has a high cost and graphite, as high-purity carbon, is even more expensive.

[0097] It is therefore evident that the use of the carbon content in DRI as carbon carbide Fe.sub.3C is economically convenient for the production of a commodity such as cast iron.

[0098] The tapping step 26 of the liquid cast iron thus obtained is set at a temperature which is such as to have a certain degree of overheating, preferably ranging from 1,400° C. to 1,550° C., also in relation to the melting point of the slag that is expected to be produced.

[0099] In this way, there can be sufficient time in the transfer ladle 28 for adjusting the desired final composition to meet the pre-determined chemical analysis of the cast iron in view of its final use.

[0100] Ferroalloys or scorifiers 30 can also be optionally introduced into the transfer ladle 28 containing liquid cast iron.

[0101] Said ferroalloys or scorifiers are known per se in the state of the art and consequently no further reference will be made herein to the same.

[0102] The liquid cast iron 32 is then discharged from the transfer ladle 28 and cast into forming containers 34 in the form of ingots 36, or it can be granulated through processes known in the art, thus forming the final cast iron.

[0103] The objectives of the present invention have therefore been achieved.