Composite briquettes for electric furnace charge, and in their method of use

Abstract

For use with an electric steelmaking furnace, a briquette is provided which incorporates a suitable binder, a first quantity of carbon fines, and a second quantity of a material in powdered form, the latter material being selected from the group consisiting of lime, dolime, limestone, dolomite, magnesia, magnesium carbonate, other suitable carbonates, and mixtures thereof. The second quantity being large enough to suppress the slippery nature of the carbon fines, and to density the briquette to the extent necessary to cause it to sink into the furnace melt.

Description

This invention relates to an improved composition for a densified carbon briquette for use in charging electric furnaces, and an improved method for using such a briquette.

BACKGROUND OF THIS INVENTION

In a conventional steelmaking shop using electric furnaces, an electric furnace charge is typically made from a mixture of scrap metal, carbon and a suitable flux such as lime and/or dolime. The scrap metal is usually added in pieces having a minimum dimension of about ½ inch.

It is also known to add specific materials to a furnace charge in the form of briquettes, in which the components are present in pulverized or comminuted form. Carbon is an essential part of the charge, but is quite slippery in its pulverized form (often called “carbon fines”). Consequently, carbon is typically added in a non-pulverized, non-briquetted state, for example as solid pieces of coke.

Another problem had to do with the density of carbon, which is quite low compared generally to the metals. As a result, when carbon is added to the furnace via a charge bucket, it tends to float on top of the liquid metal, thus decreasing the yield of carbon in solution in the steel.

Because it would be of advantage to be able to make direct use of carbon fines, for example those recovered from a dust collector, I developed a composite briquette which includes, along with a first quantity of carbon fines, a second quantity of a material in powdered form, selected from the group consisting of iron, chromium, nickel, iron oxide, chromium oxide, nickel oxide, and any mixtures thereof, the second quantity being present in sufficient quantity to suppress the slippery nature of the carbon fines and to densify the briquette sufficiently to keep it submerged in a steel melt. The development just described is covered by my U.S. Pat. No. 5,916,827, issued Jun. 29, 1999, and entitled “Composite Briquette for Electric Furnace Charge”, and Canadian Application 2,241,574 allowed Sep. 18, 2003.

GENERAL DESCRIPTION OF THIS INVENTION

Briquette Constituents

I have now discovered that a briquette for transporting carbon fines into a melt can be manufactured in which the “second quantity” mentioned above (iron, chromium, etc.) is omitted or substantially omitted, so long as there is present (along with the carbon) a sufficient quantity of a material selected from the group consisting of lime, dolime, magnesia, limestone, dolomite, magnesium carbonate, and any mixtures thereof. It appears that the compounds just mentioned are able to suppress the slippery nature of the carbon fines, and to densify the briquette sufficiently to cause it to sink into the melt. Corollary to the above, the usage of raw iron ore for densification will suppress the slippery nature of the carbon fines, as the ore contains iron oxide, and also by-pass the traditional preparation of iron ore (washing off the gangue) as well as the costly phase of reducing the iron ore into iron via the blast furnace. In the blast furnace coke is added to the iron ore in order to reduce the oxygen contained in the ore (iron oxide). Hence the carbon briquette densified with iron ore is equivalent to a mini blast furnace providing a new method for direct reduction of iron ore into iron.

Method of Addition

Traditionally the carbon addition is placed in the first charge bucket within the scrap charge, either mixed with it or layered, or in the second charge bucket, or both.

Several tests were run during which the carbon was added:

• • in the form of coke:
  • in the first charge bucket within the scrap charge as per the traditional practice;
  • in the second charge bucket within the scrap charge as per the traditional practice;
  • in both the first and second charge buckets.

In the form of a. Densified Carbon Briquette (DCB):

• • the nominal amount specified by he steel plant practice was added as per the above;
  • double the nominal amount was also tested as per the above.

All of the above combinations showed results similar to those attained following standard practice using straight coke addition, namely:

• • same “tap carbon” (the amount of carbon in solution in the bath at tap), although the DCB contained less carbon, hence showing its superiority;
  • same amount of electrical energy required to melt the scrap charge.

Adding the DCB prior to he scrap charge, or at the very bottom of the first charge bucket, so that the DCB is the first item falling into the furnace, repeatedly showed a reduction in the total electrical energy required to melt the same charge.

This test was applied to regular coke and to DCB.

Although the coke addition did show an improvement in the efficiency of the furnace (a saving of 2% in electrical energy), the DCB showed an even greater improvement by saving from 4% to over 7% in electrical energy.

A further test was conducted showing that substantially the same increase in density could be achieved by using limestone, dolomite, magnesium carbonate or other carbonate, instead of iron or iron oxide. It was decided to use magnesium carbonate (MgCO₃), a stone similar to limestone and dolomite (all of which are carbonates in their natural form). Magnesium Carbonate was also selected for its low temperature of decomposition (406° C. vs. over 815° C. for limestone). In what follows, the briquette utilizing Magnesium Carbonate is identified as “Car-Mag”.

Two briquettes were produced according to the following formulae: The briquette with iron ore was produced only to confirm the density

Standard DCB			Car-Mag
65%	Coke	55%	40%
25%	Iron
	Iron ore	35%
	MgCO3		50%
10%	Binder	10%	10%
100%		100%	100%
2.0 to 2.1	Density g/cc 2.0 to 2.1	2.0 to 2.1

The density of iron is 7.0 g/cc, the one for iron, ore is 5.0 g/cc, compared to 2.0 g/cc for MgCO₃. It is therefore understandable that more iron ore and even more magnesium carbonate must be used to achieve the same density as that of the standard DCB, the latter being adequate to improve the efficiency of carbon as a fuel.

In order to determine whether the use of magnesium carbonate produces any negative effect (for example, an unacceptable increase in electric power consumption due to the energy required to calcine the carbonate), magnesium carbonate briquettes were made according to the following formulation:

MgCO3 75%
MgO   10%
Water 15%
      100%

Note that fine, soft-burned MgO was used due to its high reactivity with water, creating a primitive cement as a binder.

During tests, it was shown that these briquettes, when added into the furnace, decomposed rapidly (within minutes), producing CO2 which foamed the slag present as well as splashing it over the walls of the furnace, and coating them with a protective layer rich in MgO.

The foaming action resulted in a more efficient electric arc, hence a saving in electrical energy. The negative energy required to calcine MgCO3 was overcompensated by the improved electric arc efficiency due to the earlier foaming of the slag. For best efficiency the electric arc should be buried in the slag.

In the formulation next above, some of the magnesium carbonate could be replaced partially or totally with used crushed MgO brick, or any other form of MgO (such as metallurgical slag, which contains CaO, MgO, Alumina, Silica, etc.). For instance:

	Magnesium Carbonate	75%	50%	25%
	Used MgO		25%	50%
	Soft burnt MgO	10%	10%	10%
	Water	15%	15%	15%
		100%	100%	100%

I have discovered that, for best results with the briquette herein disclosed, the carbon addition has to be either prior to the first scrap bucket, or in the first charge bucket but at the very bottom of it. The reason for this is that, to be effective, any carbon addition has to be in solution in the bath. When carbon of any type is added within the scrap charge (standard method), it is dispersed within it, and combustion occurs “up in the air”.

When coke was added according to the method herein disclosed, a 2% improvement in energy consumption was noted This was not as efficient as the DCB addition, because the densities are different. The DCB is 4 times more dense than coke. Thus the more dense DCB penetrates into the bath, whereas the coke will float on top of the bath, burning up in the air (and not in the bath where energy is required).

The following table of test results illustrates the importance of adding DCB as early as possible. It shows the electrical energy used in the same plant, with the same furnaces and the same scrap charge.

	Before the test	Test	After the test
	Addition as per	addition as	Addition as per
	practice in place	per patent	practice in place
	Std carbon	DCB	DCB	Std carbon
Av. Kwh	258.97	266.88	239.1	253.86
(×1000)
Std	5.9	8.8	53	6.0

The amount of energy available in the DCB can be further improved by adding a metal deoxidizer such as aluminum, using as densifier metallic oxides such as those of iron, manganese, etc. This allows the recycling of the metallic dust generated in the steel plant, recuperating the metallics.

The utilization of a carbonate as a densifier, more specifically magnesium carbonate, will result in achieving the same adequate density and will also improve further the saving in electrical energy by foaming the slag as early as possible.

Energy Saving in Kwh/ton
BFSC-90
			Standard MgO
	Standard MO addition	MgCO3	addition after
	Before MgCO3 briquette	briquette	MgCO3 briquette
Average	427.0	420.0	428.0
Std. Dev.	24.1	10.0	14.4

Improved protection of the refractory at the slag line occurs due to a higher MgO content in the slag, and the splashing of the slag during foaming, hence improving the life of the refractories, hence producing an additional saving due to less repair.

Standard MgO	MgCO3 first bucket	MgCO3 second bucket
3500 lb.	3500 lb.	3500 lb.
MgO in slag	MgO in slag	MgO in slag
8.8%	10.7%	8.0%

More particularly, this invention provides, for addition to the charge in a steelmaking furnace, a briquette comprising

a suitable binder,

a first quantity of carbon fines, and

a second quantity of a material in powdered form, selected from the group consisting of lime, dolime, magnesia, limestone, dolomite, magnesium carbonate, iron ore and mixtures thereof, said second quantity being large enough to suppress the slippery nature of the carbon fines, and to densify the briquette to the extent necessary to cause it to sink into the melt.

Further, this invention provides a method for making a briquette for addition to the melt in a steelmaking furnace, the method comprising making a mixture of

1. a suitable binder,

2. a first quantity of carbon fines, and

3. a second quantity of a material in powdered form, selected from the group consisting of lime, dolime, magnesia, limestone, dolomite, magnesium carbonate, iron ore and mixtures thereof, said second quantity being large enough to suppress the slippery nature of the carbon fines, and to densify the briquette to the extent necessary to cause it to sink into the melt, and using a suitable mold for forming the mixture into a briquette shape.

In addition, this invention provides a method for improving the slag-covered melt in a steelmaking furnace, the furnace including a shell for holding the molten steel, and at least one charge bucket for adding, to the shell, a mix which includes at least a quantity of steel scrap, a source of carbon, and flux precursors, the method comprising the steps:

making a mixture of a suitable binder, a first quantity of carbon fines, and a second quantity of a material in powdered form, said material being selected from the group consisting of lime, dolime, magnesia, limestone, dolomite, magnesium carbonate, iron ore and mixtures thereof, said second quantity being large enough to suppress the slippery nature of the carbon fines, and to densify the briquette to the extent necessary to cause it to sink into the molten steel,

compressing a portion of said mixture in a mold to make a briquette therefrom, and

introducing said briquette to the shell prior to adding the contents of the charge bucket.

Further, this invention provides a method for improving the slag-covered melt in a steelmaking furnace, the furnace including a shell for holding the molten steel, and at least one charge bucket for adding, to the shell, a mix which includes at least a quantity of steel scrap, a source of carbon, and flux precursors, the method comprising the steps:

making a mixture of a suitable binder, a first quantity of carbon fines, and a second quantity of a material in powdered form, said material being selected from the group consisting of lime, dolime, magnesia, limestone, dolomite, magnesium. carbonate, iron ore and mixtures thereof, said second quantity being large enough to suppress the slippery nature of the carbon fines, and to densify the briquette to the extent necessary to cause it to sink into the molten steel,

compressing a portion of said mixture in a mold to make a briquette therefrom., and

introducing said briquette to the shell ahead of all other materials by placing the briquette at the inside bottom of the charge bucket.

GENERAL DESCRIPTION OF THE DRAWINGS

One embodiment of the invention is illustrated in the attached drawings, in which like numerals refer to like parts throughout the several views, and in which:

FIG. 1 is a perspective view of the main working components of a dual shell electric furnace to which this invention can apply, including the two furnace shells, the shell covers, and the electrodes cover, and

FIG. 2 is a perspective view of a charge bucket for use in charging the shells pictured in FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

A dual shell electric furnace 10 has the following basic components:

• • 1. a first furnace shell 12 shaped essentially as an open-topped container adapted to hold a steel melt;
  • 2. a second furnace shell 14 substantially similar to the shell 12;
  • 3. a cover 16 swingable between an open position (that shown in FIG. 1) and a closed position in which it substantially closes the open top of the first shell 12;
  • 4. a cover 18 swingable between an open position (that shown in FIG. 1) and a closed position in which it substantially closes the open top of the second shell 14;
  • 5. an electrode cover 20 which can close either of the shells selectively, the electrode cover being movable between the shells, and also being capable of assuming an out-of-the-way location. The electrode cover 20 is illustrated as having three electrodes 21 passing through the cover, although the number of electrodes is not critical. When the cover 20 is in place on either shell, the electrodes 21 extend from under the cover 20 to and through, the slag layer on top of the molten metal, and finally into the melt itself.

The charge bucket 22 illustrated in the figures is somewhat smaller than the furnace shells to which it is adapted to add a mixture of scrap metal, coke and other materials. At the bottom of the charge bucket there are two doors 24, which are normally closed to retain the contents, but can open to allow the charge in the bucket to fall down into whichever of the shells is appropriately positioned.

In operation, the various components interact as follows. Assume first that the two shells 12 and 14 are initially empty. A charge is loaded into the charge bucket 22, and can include scrap metal, coke or other source of combustible carbon, fluxing materials, etc. The charge bucket is positioned above shell 12, with the cover 16 swung out of the way or otherwise removed. Then the doors 24 are opened to allow the charge in the charge bucket to fall into the shell 12. This loading procedure is repeated as many times as is necessary to build up the desired quantity of materials in the shell 12, then cover 16 closes shell 12, and side burners in the wall of the shell 12 are ignited to heat the charged contents to a temperature close to that required for melting the iron-containing materials.

When the charge in shell 12 has reach the desired temperature, cover 16 comes off, a second charge is dumped into shell 12, and the electrode cover 20 is put in place over it for the melting phase. Power is then fed to the electrodes 21 to complete the melt.

At this stage, shell 14 receives initial charges as were added to shell 12. Cover 16 closes shell 14 and the pre-heat phase starts.

When the melting phase is completed in shell 12, the melt is emptied into a ladle to carry the melt to the next process phase or to the caster.

The second charge is fed into shell 14 and the electrode cover is then moved from shell 12 to shell 14 when the pre-heat phase has been completed.

While one embodiment of this invention has been illustrated in the attached drawings and described hereinabove, it will be evident to those skilled in the art that changes and modifications may be made thereto without departing from the essence of this invention, as set out in the accompanying claims.

Claims

1. For addition to a charge in a steelmaking furnace, a briquette comprising:

a suitable binder,

a first quantity of carbon fines, and

a second quantity of a material in powdered form, selected from the group consisting of lime, dolime, magnesia, limestone, dolomite, magnesium carbonate, iron ore and mixtures thereof, said second quantity being large enough to suppress the slippery nature of the carbon fines, and to densify the briquette to the extent necessary to cause it to sink into the melt.

2. A method for making a briquette for addition to the melt in a steelmaking furnace, the method comprising making a mixture of:

1. a suitable binder,

2. a first quantity of carbon fines, and

3. a second quantity of a material in powdered form, selected from the group consisting of lime, dolime, magnesia, limestone, dolomite, magnesium carbonate, iron ore arid mixtures thereof, said second quantity being large enough to suppress the slippery nature of the carbon fines, and to densify the briquette to the extent necessary to cause it to sink into the melt, and using a suitable mold for forming the mixture into a briquette shape.

3. A method for improving the slag-covered melt in a steelmaking furnace, the furnace including a shell for holding the molten steel, and at least one charge bucket for adding, to the shell, a mix which includes at least a quantity of steel scrap, a source of carbon, and flux precursors, the method comprising the steps:

making a mixture of a suitable binder, a first quantity of carbon fines, and a second quantity of a material in powdered form, said material being selected from the group consisting of lime, dolime, magnesia, limestone, dolomite, magnesium carbonate, iron ore and mixtures thereof, said second quantity being large enough to suppress the slippery nature of the carbon fines, and to densify the briquette to the extent necessary to cause it to sink into the molten steel,

compressing a portion of said mixture in a mold to make a briquette therefrom,

introducing said briquette to the shell prior to adding the contents of the charge bucket to the shell.

4. A method for improving the slag-covered melt in a steelmaking furnace, the furnace including a shell for holding the molten steel, and at least one charge bucket for adding, to the shell, a mix which includes at least a quantity of steel scrap, a source of carbon, and flux precursors, the method comprising the steps:

making a mixture of a suitable binder, a first quantity of carbon fines, and a second quantity of a material in powdered form, said material being selected from the group consisting of lime, dolime, limestone, magnesia, dolomite, magnesium carbonate, iron ore and mixtures thereof, said second quantity being large enough to suppress the slippery nature of the carbon fines, and to densify the briquette to the extent necessary to cause it to sink into the molten steel,

compressing a portion of said mixture in a mold to make a briquette therefrom, and

introducing said briquette to the shell ahead of all other materials by placing the briquette at the inside bottom of the charge bucket.

5. The method claimed in claim 4, in which said second quantity is at least 5% of the weight of the briquette, and the density of the briquette is at least about 2.0 g/cc.

6. The method claimed in claim 5, in which said second quantity is at least 10% of the weight of the briquette.