Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02852813 2016-11-02
COMPOSITE BRIQUETTE FOR STEELMAKING OR IRONMAKING
FURNACE CHARGE
Field
100011 The present invention relates generally to ferrous metallurgy and
in
particular, to a composite briquette for a steelmaking or ironmaking furnace
charge,
Background
[0002] In the field of steelmaking, an electric fiamace charge is
typically made
from scrap metal, carbon and fluxes such as lime and/or dolime, all in pieces
having a '
minimum size of 0.5 inch.
[0003] It is known to add specific materials to a furnace charge in the
form of
briquettes. However, carbon, which is an essential part of the mixture of
materials, is
quite slippery in its powdered or comminuted form, Consequently, carbon is
typically
employed in a non-pulverized state, for example as coke. It would be of
advantage to
be able to utilize carbon "fines", for example those recovered from a dust
collector,
and to recycle such fines in their powdered or dust state. A further problem
relates to
the density of carbon, which is quite low compared generally to the metals.
For
example, when carbon is added to the furnace via a charge bucket, it will tend
to float
on top of the. liquid metal, thus decreasing the yield of carbon in solution
in the steel.
[0004] Further, it would also be of advantage to improve the quality of
the
slag through the addition of the briquette.
[00051 Improvements are generally desired. It is therefore an object at
least to
provide a novel composite briquette for steelmaking or ironmaking furnace
charge.
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Summary of the Invention
[0006] In one aspect, there is provided a composite briquette for addition
to a
charge in a steelmaking furnace, the briquette comprising: a quantity of
carbon fines;
a quantity of iron powder, the iron powder densifying the briquette and
suppressing
the slippery nature of the carbon fines; a quantity of magnesium carbonate; a
quantity
of limestone; and a binder, wherein 50 % of the total briquette weight is the
carbon
tines, 25 % of the total briquette weight is the iron powder, and the
remainder of the
total briquette weight, apart from the binder, is magnesium carbonate and
limestone.
[0007] The briquette may comprise from 1 to 10 % by weight of the binder.
[POW The binder may comprisc molasses and lime.
[0009] In one embodiment, there is provided use of the briquette as
addition to
the charge in the steelmaking furnace, the furnace being an electric arc
furnace or a
basic oxygen furnace.
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[00010] In another aspect, there is provided a method of improving a slag-
covered charge in a steelmaking furnace, the slag-covered charge comprising a
charge
covered with slag, the method comprising: making a mixture of: a quantity of
carbon
fines, a quantity of iron powder, a quantity of magnesium oarbonate, a
quantity of
limestone, and a binder; compressing a portion of said mixture in a suitable
mold to
make a briquette, said iron powder densifying the briquette and suppressing
the
slippery nature of the carbon fines, wherein 50 % of the total briquette
weight is the
carbon fines, 25 % of the total briquette weight is the iron powder, and the
remainder
of the total briquette weight, apart from the binder, is magnesium carbonate
and
limestone; and introducing said briquette to the charge below the slag in the
steelmaking furnace so that said iron powder contained in the briquette causes
the
briquette to sink into the charge.
[00011] The mixture may comprise from 1 to 10 % by weight of the binder.
1000121 - The binder may comprise molasses and lime.
1000131 Upon introducing the briquette to the charge, CO2 may be generated
such that the CO2 foams the slag from underneath.
[00014] = The furnace may be an electric arc furnace or a basic oxygen
furnaoe,
[000151 The furnace may be another type of furnace.
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Detailed Description of the Embodiments
[00016] The following is directed to a composite briquette for addition to
the
charge in a steelmaking or ironmaking furnace, and which comprises magnesium
carbonate (MgCO3).
[00017] Magnesium carbonate is known to thermally decompose at a lower
temperature than dolomite (CaMg(CO3)2) and limestone (CaCO3). Specifically,
MgCO3 thermally decomposes into magnesium oxide (MgO) and carbon dioxide
(CO2) at about 402 C, while CaMg(CO3)2 and CaCO3 each thermally decompose
into
their constituent oxides at about 730 C and about 825 C, respectively. As a
result,
when added to the charge in a steelmaking or ironmaking furnace, magnesium
carbonate thermally decomposes more quickly, and more readily, than limestone
or
dolomite.
[00018] Table 1 shows a non-limiting example of a mixture from which a
suitable briquette can be fashioned:
TABLE 1:
Carbon C 50%
Powdered iron Fe 25 %
Magnesium carbonate MgCO3 25 %
Total 100 %
[00019] In the table above, deviations from the indicated percentages may
occur, up to about 5% to either side of the indicated level. The ingredients
may be
combined with a suitable binder, such as for example industrial molasses and
powdered lime, and the binder may make up 1 to 20 %, or more, of the total
weight of
the briquette.
[00020] The example illustrated in Table 1 specifies powdered iron.
However,
this teaching is not intended to be restrictive, as it is possible to use one
or more of
iron, iron oxide, chromium, chromium oxide, nickel, and nickel oxide to
achieve the
same effect. If iron oxide is used, the reaction products will be iron and CO2
gas, as
well as caloric heat that results from burning of the iron oxide. The iron
will revert to
the bath, thus increasing its yield.
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[000211 The magnesium carbonate could be combined with limestone and/or
dolomite, each of which will produce CO2 gas with the same effect as above.
Dolime,
lime, and/or magnesium oxide may also be included.
1000221 The ironmaking furnace may be, for example, a blast furnace. The
steelmaking furnace may be, for example, an electric arc furnace, a basic
oxygen
furnace, and the like.
[00023] In use, the briquette is added to the charge in a steelmaking or
ironmaking furnace, in such a manner that it is immersed within the charge.
The
briquette dissolves and reacts with the contents of the charge. The powdered
iron
reverts to the bath, thus increasing its yield. The magnesium carbonate
thermally
decomposes into magnesium oxide (MgO) and carbon dioxide (CO2). The
magnesium oxide (MgO) produced is absorbed by the slag. The CO2 produced has
the effect of foaming the slag from underneath, as the location where the CO2
is
generated is buried within the charge.
[00024] As will be appreciated, the low decomposition temperature of
magnesium carbonate advantageously allows the slag thickness to be increased
more
rapidly than, and with less energy consumption than, other substances such as
limestone, dolomite, and the like. As will be understood, the rapid formation
of a
thick slag decreases the amount of oxidation of iron in the bath, which
improves of
the yield of the reaction. Additionally, if the steelmaking furnace is an
electric arc
furnace, the increased thickness of the slag advantageously causes the arc to
be more
localized within the bath and under the slag, which improves efficiency of the
electric
arc furnace and thereby allows melt times to be shortened. These performance
characteristics help mitigate the environmental impact of steelmaking and
ironmaking
operations, and conserve resources.
1000251 As will be appreciated, the accompanying production of CO2 gas
that
occurs upon decomposition of magnesium carbonate causes bubbling under the
surface of the bath, which advantageously causes mixing and improves the
quality of
the slag, and namely the foaminess, consistency and stability of the slag.
1000261 As will be appreciated, the addition of MgO to the slag
advantageously
results in formation of a protective layer of MgO on the walls of the furnace.
As will
be understood, as the melt is being drained from the furnace, the slag
contacts the wall
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surfaces of the furnace and deposits a layer of MgO thereon. As a result, a
new
protective refractory coating is automatically deposited on the walls of the
furnace
with each use, which eliminates the need for separate application of a
protective wall
coating that would otherwise form part of routine furnace maintenance.
[00027] The briquette is not limited to the composition described above,
and in
other embodiments, the briquette may alternatively have other compositions.
For
example, in another embodiment, magnesium carbonate may be added to the charge
of a steelmaking or ironmaking furnace for improving the quality of the slag.
[00028] For example, powdered magnesium carbonate ore may be combined
with a suitable binder, such as for example industrial molasses and powdered
lime,
and compressed in a suitable mold to make a briquette. The binder may make up
1 to
20 c1/0, or more, of the total weight of the briquette.
[00029] The magnesium carbonate could be combined with one or more other
substances. Such substances may comprise, for example, limestone and/or
dolomite,
each of which will produce CO2 gas upon decomposition, and/or any of dolime,
lime,
and magnesium oxide. Still other substances may be combined with the magnesium
carbonate.
1000301 The ironmaking furnace may be, for example, a blast furnace. The
steelmaking furnace may be, for example, an electric arc furnace, a basic
oxygen
furnace, and the like.
[00031] In use, the briquette is added to the charge in a steelmaking or
ironmaking furnace, in such a manner that it is immersed within the charge.
The
briquette dissolves and reacts with the contents of the charge. The magnesium
carbonate thermally decomposes into magnesium oxide (MgO) and carbon dioxide
(C07). The magnesium oxide (MgO) produced is absorbed by the slag. The Ca)
produced has the effect of foaming the slag from underneath, as the location
where
the CO, is generated is buried within the charge.
1000321 In another embodiment, magnesium carbonate ore, in absence of a
binder, may be added in powdered or granular form to the charge of a
steelmaking or
ironmaking furnace for improving the quality of the slag.
[00033] The following examples illustrate various applications of the
above-
described embodiments.
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[00034] EXAMPLE 1
[00035] In this example, a briquette having the composition
shown in Table 2
was made:
TABLE 2:
Carbon 43.7%
Fe 22.5%
CaO 12.2%
MgO 6.6%
S 2.9%
L.O.I. 12.1 %
[00036] The dolime could be replaced with magnesium
carbonate, which will
produce CO2 gas, with the foaming effect described above.
[00037] The L.O.I. is mainly attributed to the decomposition
of the dolomite
and the binder used. The layer of CO and CO2 produced will protect the bath
from
oxidation and enhance the carbon yield.
[00038] The manufacturing process by which the briquette is
formed has the
effect of densification, with the following typical values: loose carbon prior
to
compression has a density of approximately 0.63 to 0.65 g/cm3. If a briquette
is
manufactured from the loose carbon only, the density can be raised into the
range of
1.6 to 1.75 grams/cc. However, utilizing the formulation given at the
beginning of
this example, and compressing the formulation, will yield a density in the
range of 2.4
to 2.6 grams/cc.
[00039] The densification due to compression has the effect
of increasing the
efficiency of the carbon addition, since the carbon is allowed to penetrate
the bath,
rather than simply floating on top of the bath.
1000401 EXAMPLE 2
[00041] In this example, a briquette ("Briquette A") having
the post-calcination
composition shown in Table 3 was made:
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TABLE 3:
MgO 92.19%
CaO 2.46%
A1203 0.85 %
Si02 2.58 %
TiO2 O.14%
Fe203 0.71 %
Cr203 0.02 %
MnO O.05%
< 0.001 %
Moisture 1.0%
Total 100%
[00042] The briquette was formed by providing a mixture of powdered
magnesium carbonate ore and a binder, combined in a weight ratio of 90:10, and
compressing the mixture in a suitable mold. The binder was a mixture of
industrial
molasses and powdered lime, combined in a weight ratio of 3:2.
[00043] The briquette had a generally square shape and a size of 40 mm per
side, with a density of 2.18 g/cm3 and a white colour. The briquette had a
L.O.I.
value of 35.0 %, which is mainly attributed to the decomposition of the
magnesium
carbonate and the binder. Notably, the L.O.I. value of the briquette is lower
than the
L.O.I. value of the powderized mixture of Example 3.
[00044] The briquette was used during reactions in a 125 tonne electric
arc
furnace. A summary of the performance of the briquette ("Briquette A") during
the
reactions is shown in Table 4. For comparison, a summary of the performance of
a
standard conventional additive, namely crushed brick ("standard practice"),
during the
reactions is also shown:
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TABLE 4:
Standard Briquette A difference
Practice
Number of Heats 44 11
Quantity added (lbs) 3500 3500
Actual MgO added (lbs) 3220 2100 -34.78 %
Average MgO in solution (%) 8.79 1.75 9.20 1.88 +4.66 %
Briquette A with lst charge (%) 10.69 1.80
Briquette A with 2nd charge 7.95 0.62
(%)
[00045] As may be seen, the use of Briquette A results in a reduction of
the
actual MgO added by about 35 /0, while advantageously increasing the average
MgO
in the slag by about 4.5 %. The amount of Mg0 in the slag is about 34% higher
when
the Briquette A was added with the first charge (i.e. when little or no slag
layer
previously existed) than when the Briquette A was added with the second
charge.
[00046] The decomposition of magnesium carbonate within Briquette A
produces fine, active MgO particles, which are absorbed by the slag. It was
observed
that when Briquette A was added and the briquettes penetrated the slag so as
to be
buried in the charge, tiny bubbles of CO2 were seen to form.
[00047] The average composition of the slag after the reactions, by weight
percent, is shown in Table 5:
TABLE 5:
Standard Briquette A difference
Practice
CaO 36.07 3.72 36.41 + 3.04 + 0.93 %
A1203 6.99 1.98 7.68 0.99 + 9.87 `)/0
Si02 11.83 3.75 13.23 1.44 + 11.83%
Fe203 27.71 7.32 24.59 5.53 - 11.26 %
Mn203 5.46 1.03 5.31 0.37 - 2.82 %
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[000481 As may be seen, the use of Briquette A results in a reduction of
the
iron content of the slag by more than 11%, as compared to standard practice.
This
may be attributed to the ability of the magnesium carbonate to rapidly
decompose and
contribute to or form the slag, which allows a protective barrier to more
quickly form
on the bath surface. As a result, less of the iron in the bath is oxidized
during the
reaction, which advantageously increases the yield of the reaction.
1000491 During the test, 22 heats were carried out using crushed brick,
followed by 11 heats carried out using Briquette A, followed by 22 heats
carried out
using crushed brick. The operational performance of the 125 tonne electric arc
furnace before, during, and after the addition of Briquette A is shown in
Table 6:
TABLE 6:
Standard Standard Standard Briquette
A
Practice Practice Practice
(before test) (after test) (avg)
Power usage 427.0 24.1 428.0 14.4 427.5 420.0 9.9
(KWh/T)
[00050] As may be seen, the amount of power required for the reaction is
lower
when Briquette A is used, as compared to standard practice.
1000511 EXAMPLE 3
1000521 Magnesium carbonate may alternatively be added to the charge in
powderized form. A powderized mixture having the post-calcination composition
shown in Table 7 was used :
TABLE 7:
MgO 97.0%
CaO 2.0%
A1203 0.2 %
Si 02 0.3 %
Fe203 0.5 %
Total 100 %
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[00053] The powderized mixture had a density of 2.28 g/cm3 and a white
colour.
[00054] The powderized mixture was used during a reaction in a 125 tonne
electric arc furnace.
[00055] The powderized mixture had a L.O.I. value of 51.1 %. Notably, the
L.O.I. value of the powderized mixture is greater than the L.O.I. value of the
briquette
of Example 2.
[00056] Although embodiments have been described above, those of skill in
the
art will appreciate that variations and modifications may be made without
departing
from the scope thereof as defined by the appended claims.
