Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02658897 2013-06-28
METHOD AND SYSTEM FOR PRODUCING
METALLIC IRON NUGGETS
[0001] This application claims priority from United States provisional
application serial
no. 60/633,886, filed December 7, 2004
Background of the Invention
[0002] The present invention relates to the reduction iron bearing material,
such as iron
ore, to metallic iron.
[0003] Many different iron ore reduction processes have been described and/or
used in
the past. The processes may be traditionally classified into direct reduction
processes and
smelting reduction processes. Generally, direct reduction processes convert
iron ores into
a solid state metallic form with, for example, use of shaft furnaces (e.g.,
natural gas-based
shaft furnaces), whereas smelting reduction converts iron ores into molten hot
metal
without the use of blast furnaces.
[0004] The conventional reduction processes for production of direct reduced
iron (DRI)
involve heating beneficiated iron ores to below the melting point of iron,
below 1200 C
(2372 F), either by gas-based processes or coal-based processes. For example,
in the
gas-based process, direct reduction of iron oxide (e.g., iron ores or iron
oxide pellets)
employs the use of a reducing gas (e.g., reformed natural gas) to reduce the
iron oxide
and obtain DRI. Methods of making DRI have employed the use of materials that
include carbon such as coal and coke as a reducing agent. A typical
composition of DRI
is 90 to 95% metallization and 2-4% gangue, but has shortcomings for
steelmaking
processes as a replacement of scrap because its oxygen and gangue content
increases
energy usage, increase slag volume, and necessitates the addition of costly
reagents.
[0005] Natural gas-based direct reduced iron accounts for over 90% of the
world's
production of DRI. Coal-based processes are generally used in producing the
remaining
DRI production. However, in many geographical regions, the use of coal may be
more
desirable because coal prices may be more stable than natural gas prices.
Further, many
geographical regions are far away from steel mills that use the processed
product.
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Therefore, shipment of iron units in the form of iron nuggets produced by a
coal-based
direct reduction process may be more desirable than use of a smelting
reduction process.
[0006] Another reduction process in gas-based or coal-based directly reducing
iron
bearing material to metallic nuggets is often referred to as fusion reduction.
Such fusion
reduction processes, for example, generally involve the following processing
steps: feed
preparation, drying, preheating, reduction, fusion/melting, cooling, product
discharge,
and metallic iron/slag product separation. These processes result in direct
reduction of
iron bearing material to metallic iron nuggets and slag. Metallic iron nuggets
produced
by these direct reduction processes are characterized by high grade reduction,
nearing
100% metal (e.g., about 96% to about 97% metallic Fe).'
[0007] Unlike conventional direct reduced iron (DRI), these metallic iron
nuggets have
low oxygen content because they are metallic iron and have little or no
porosity. These
metallic iron nuggets are also low in gangue because silicon dioxide has been
removed as
slag. Such metallic iron nuggets are desirable in many circumstances such as
use in place
of scrap in electric arc furnaces. These metallic iron nuggets can be also
produced from
beneficiated taconite iron ore, which may contain 30% oxygen and 5% gangue. As
a
result, with such metallic iron nuggets, there is less weight to transport
than with
beneficiated taconite pellets and DRI, as well as a lower rate of oxidation
and a lower
porosity than DRI. In addition, generally, such metallic iron nuggets are just
as easy to
handle as taconite pellets and DRI.
[0008] Various types of hearth furnaces have been described and used for
direct
reduction of metallic iron nuggets. One type of hearth furnace, referred to as
a rotary
hearth furnace (RHF), has been used as a furnace for coal-based direct
reduction.
Typically, the rotary hearth furnace has an annular hearth partitioned into a
preheating
zone, a reduction zone, a fusion zone, and a cooling zone, between the supply
location
and the discharge location of the furnace. The annular hearth is supported in
the
furnace to move rotationally. In operation, raw reducible material comprising
a mixture
of iron ore and reducing material is charged onto the annular hearth and
provided to the
preheat zone. After preheating, through rotation, the iron ore mixture on the
hearth is
moved to the reduction zone where the iron ore is reduced in the presence of
the reducing
Percents (%) herein are percents by weight unless otherwise stated.
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material and fused into metallic iron nuggets, using one or more heat sources
(e.g., gas
burners). The reduced and fused product, after completion of the reduction
process, is
cooled in the cooling zone on the rotating hearth, preventing oxidation and
facilitating
discharge from the furnace.
[0009] One exemplary metallic iron nugget direct reduction process for
producing
metallic iron nuggets is referred to as ITmk3 by Kobe Steel. In such a
process, dried
balls formed using iron ore, coal, and a binder are fed to a rotary hearth
furnace. As the
temperature increases in the furnace, the iron ore concentrate is reduced and
fuses when
the temperature reaches between 1450 C to 1500 C. The resulting products are
cooled
and then discharged. The intermediate products generally are shell-shaped,
pellet-sized
metallic iron nuggets with slag inside, from which the metallic iron can be
separated.
[0010] Another direct reduction process for making metallic iron nuggets has
also been
reportedly used. See United States Patent No. 6,126,718. In this process, a
pulverized
anthracite coal layer is spread over a hearth and a regular pattern of dimples
is made
therein. Then, a layer of a mixture of iron ore and coal is placed over the
dimples, and
heated to 1500 C. The iron ore is reduced to metallic iron, fused, and
collected in the
dimples as iron pebbles and slag. Then, the iron pebbles and slag are broken
apart and
separated.
[0011] Both of these direct reduction processes for producing metallic iron
nuggets have
involved mixing of iron-bearing materials and a carbonaceous reductant (e.g.,
pulverized
coal). Either with or without first forming dried balls, iron ore/carbon
mixture is fed to a
hearth furnace (e.g., a rotary hearth furnace) and heated to a reported
temperature of 1450
C to approximately 1500 C, to form metallic iron nuggets and slag. Metallic
iron and
slag can then be separated, for example, with use of mild mechanical action
and magnetic
separation techniques.
[0012] A particular problem with the metallic iron nuggets formed by these
previous
direct reduction processes was the sulfur content of the nuggets. Sulfur is a
major
impurity in direct reduced metallic iron nuggets. In the past, carbonaceous
reductants
utilized in direct reduction processes of iron ore have generally resulted in
metallic iron
nuggets with at least 0.1% or more by weight sulfur. This high level of sulfur
has made
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the metallic iron nuggets made by direct reduction undesirable in many
steelmaking
processes, and particularly in the electric arc furnace processes.
[0013] Attempts have been made to form metallic iron nuggets with low sulfur
content in
these previous direct reduction processes using large amounts of additives
containing
MgCO3 or MgO. Problems, such as increased energy consumption and increased
refractory wear, have occurred with fusing these nuggets due to the increases
in slag
melting temperature caused by MgO in the slag. See EP 1 605 067.
Summary of the Invention
[0014] A method and system are disclosed that provide for various advantages
in the
reduction processes in the production of metallic iron nuggets. The method and
system
results in a marked higher percent of the sulfur in the slag without the use
of large
amounts of Mg compounds, and a marked lower percent of the sulfur in the
metallic iron
nuggets. A novel intermediate metallic nugget/slag product having a ratio of
percent
weight sulfur in the slag to sulfur in the metallic nugget of at least 12, at
least 15 or at
least 30, without large amounts of MgO within the slag, which may result in
nuggets with
less than 0.05% sulfur content. A novel metallic nugget having a sulfur
content less than
0.03% by weight may also be produced by the disclosed process.
[0015] A method for use in production of metallic iron nuggets is disclosed
that
comprises providing a hearth refractory material, providing a hearth material
layer
comprising at least carbonaceous material on the refractory material,
providing a layer of
reducible mixture comprised of at least reducing material and reducible iron
bearing
material arranged in a plurality of discrete portions over at least a portion
of the hearth
material layer, providing a layer of coarse carbonaceous material over at
least a portion of
the discrete portions of reducible mixture, and heating the reducible mixture
to form one
or more discrete portions into an intermediate product of metallic iron
nuggets and slag,
and after separation, metallic iron nuggets. The step of heating the reducible
mixture
may form singular metallic iron nuggets with separate slag portions from a
majority of
the discrete portions. The overlayer is generally provided prior to heating,
but may be
provided after devolatilization of carbonaceous material occurs and before
completion of
solid state reduction.
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[0016] The coarse carbonaceous material of the overlayer has an average
particle size
greater than an average particle size of the hearth layer. In addition or
alternatively, the
overlayer of coarse carbonaceous material may include discrete particles
having a size
greater than about 20 mesh or greater than about 6 mesh, and in some
embodiments, the
overlayer of coarse carbonaceous material may have discrete particles with a
size
between about 20 mesh or about 6 mesh and about 1/2 inch (12.7mm). The coarse
carbonaceous material may be coke, non-caking coal, char, or a combination of
one or
more of these. In the alternative, the overlayer of coarse carbonaceous
material may have
discrete particles with a size between about 3/8 inch (9.7 mm) and about 1/2
inch (12.7
mm) or between about 3 mesh (6.7 mm) and about 3/8 inch (9.7 mm).
[0017] In addition or in the alternative, the discrete particles of the hearth
layer may have
a particle size less than 4 mesh, and in some embodiments a particle size
between 100
and 20 mesh or 6 mesh. Particle sizes less than 100 mesh should be avoided
because
these particles sizes tend to have more ash content. The thickness and
particle size of the
carbonaceous and other material in the hearth layer should be selected so that
the hearth
layer protects the hearth refractory from slag and molten metal formed during
reduction
of the reducible mixture, while avoiding production of excess ash. The hearth
layer may
have a particle size between a range of -6 to -20 mesh to a range of +65 to
+150 mesh.
The carbonaceous material in the reducible mixture is also different in
particle size from
those of the coarse overlayer, but for the different considerations. In the
reducible
mixture a consideration is the surface area for rapid reaction of the
carbonaceous material
with the reducible iron bearing material in commercial production. Less than
65 mesh or
less than 100 mesh particle size of carbonaceous material in the reducible
mixture is
effective for efficient reduction of the iron oxide to produce metallic iron
nuggets.
[0018] The overlayer of coarse carbonaceous material may provide between 50%
and
100% coverage of the discrete portions of reducible mixture and may be about
1/2 inch
(12.7mm) in thickness. Further, in some embodiments of the method, the
coverage of the
overlayer of coarse carbonaceous material may be between about 0.5 lb/ft2
(2.44 kg/m2)
and about 1 lb/ft2 (4.88 kg/m2) of coarse carbonaceous material, or between
about 0.75
lb/ft2 (3.66 kg/m2) and about 1 lb/ft2 (4.88 kg/m2) of coarse carbonaceous
material over
the reducible mixture.
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[0019] In some embodiments of the disclosed method, the step of providing a
reducible
mixture over at least a portion of the hearth material layer may comprise
forming at least
a portion of the reducible mixture with a predetermined quantity of reducing
material
between about 70 percent and about 90 percent of said stoichiometric amount of
reducing
material necessary for complete metallization. Said stoichiometric amount may
be
between about 75 percent and about 85 percent of said stoichiometric amount of
reducing
material for complete metallization, or about 80 percent of said
stoichiometric amount of
reducing material for complete metallization. The stoichiometric amount of
reducing
material is the calculated amount of carbonaceous material needed for complete
metallization of iron in the formation of metallic iron nuggets from a
predetermined
quantity of reducible iron bearing material.
[0020] The discrete portions may be formed in situ as mounds, or
alternatively,
preformed as briquettes, balls, extrudates, or other shapes as needed. In any
event, the
discrete portions of reducible mixture may be at least partially surrounded
with nugget
separation fill material comprising at least carbonaceous material. The fill
material may
be placed by depositing the carbonaceous material after the discrete portions
are formed,
or by dropping or pushing preformed discrete portions into the hearth layer.
The nugget
separation fill material may also have an average particle size less than the
average
particle size of the coarse carbonaceous material of the overlayer. However,
the step of
providing an overlayer of coarse carbonaceous material may comprise at least
partially
surrounding the discrete portions of reducible mixture with coarse
carbonaceous material.
In some embodiments, this may be accomplished by placing the coarse
carbonaceous
material over the discrete portions of reducible mixture and allowing some of
the coarse
carbonaceous material to go between the discrete portions of reducible
mixture.
[0021] In some embodiments of the method, the step of heating the layer of
reducible
mixture includes heating the layer of reducible mixture at a temperature of
less than about
1425 C. Also, the step of thermally heating the layer of reducible mixture
may include
heating the layer of reducible mixture at a temperature of less than about
1400 C or less
than 1375 C.
[0022] Also disclosed is a method for use in production of metallic iron
nuggets that
comprises providing a hearth refractory material, providing a hearth material
layer
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comprising at least carbonaceous material on the refractory material,
providing a layer of
reducible mixture comprised of at least reducing material and reducible iron
bearing
material arranged in a plurality of discrete portions over at least a portion
of the hearth
material layer, providing a layer of turbulent gas flow disrupting material
over at least a
portion of the discrete portions of reducible mixture, and heating the
reducible mixture to
form one or more discrete portions into an intermediate product of metallic
iron nuggets
and slag, and after separation, metallic iron nuggets. In this alternative
method, at least
partially surrounding the discrete portions of reducible mixture may be nugget
separation
fill material comprising at least carbonaceous material. Also, in some
embodiments, the
step of providing an overlayer of turbulent gas flow disrupting material may
include
providing coarse carbonaceous material. Further, the overlayer of turbulent
gas flow
disrupting material may include providing between about 0.5 lb/ft2 (2.44
kg/m2) and
about 1 lb/ft2 (4.88 kg/m2) of coarse carbonaceous material, or between about
0.75 lb/ft2
(3.66 kg/m2) and about 1 lb/ft2 (4.88 kg/m2) of coarse carbonaceous material.
100231 Also disclosed is an intermediate product comprising metallic iron
nuggets and
slag having less than 5 % MgO and having a ratio of percent by weight sulfur
in the slag
to sulfur in the metallic nuggets of at least 12, at least 15, or at least 30,
which may
produce nuggets of less than 0.05% sulfur. In addition, a metallic iron nugget
composition having a sulfur content less than 0.03% by weight is disclosed.
The metallic
nugget/slag product and metallic iron nuggets may be produced by the method
steps that
comprise providing a hearth refractory material, providing a hearth material
layer
comprised of at least carbonaceous material on the refractory material,
providing a layer
of reducible mixture comprising at least reducing material and reducible iron
bearing
material arranged in a plurality of discrete portions over at least a portion
of the hearth
material layer, optionally at least partially surrounding the discrete
portions of reducible
mixture with nugget separation fill material comprising at least carbonaceous
material,
providing a layer of coarse carbonaceous material over at least a portion of
the discrete
portions of reducible mixture, and heating the reducible mixture to form the
one or more
discrete portions into the intermediate product of metallic iron nuggets and
slag of said
sulfur slag/nugget ratio, and after separation, metallic iron nuggets. The
slag formed may
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have an iron content of less than about 1%, less than about 0.25%, or
essentially less than
0.1%.
[0024] The carbonaceous material of the hearth layer, the coarse overlayer,
and the layer
of reducible mixture may contain an amount of sulfur in a range from about
0.2% to
about 1.5%, and more typically, in the range of 0.5% to 0.8%. The reducible
mixture
may also contain an amount of additives in a range from about 1% to about 10%.
The
reducible mixture may further include an additive selected from the group
consisting of
Si02, CaF2, Na2CO3, aluminum smelter slag, cryolite, fluorspar and soda ash.
The
additives may be separately added to the reducible mixture in its making, or
may be
naturally part of the reducible iron bearing material and/or the carbonaceous
material
used in making the reducible mixture. Typically 2% of the content of the
reducible
mixture may be additives, but may range between about 1% and about 7% by
weight.
Compounds containing Mg, such as dolomite, should be avoided, and in any event
compounds containing Mg are not added in quantities such that greater than 3%,
or
greater than 4%, or greater than 5% MgO results in the slag.
[0025] The above summary of the present invention is not intended to describe
each
embodiment or every implementation of the present invention. Advantages,
together
with a more complete understanding of the invention, will become apparent and
appreciated by referring to the following detailed description and claims
taken in
conjunction with the accompanying drawings.
Brief Description of the Drawings
[0026] FIG. 1 shows a block diagram of one or more general embodiments of a
metallic
iron nugget process;
[0027] FIG. 2 is a generalized block diagram of a furnace system for
implementing a
metallic iron nugget process such as that shown generally in FIG. 1;
[0028] FIG. 3 is a diagram of a linear hearth furnace that may be used to
carry out one or
more processes described herein, and produce one or more products described
herein;
[0029] FIG. 4 shows a pallet or tray with an arrangement of different feed
mixtures
therein for use in describing one or more tests employing a linear hearth
furnace such as
that shown in FIG. 3;
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[0030] FIG. 5 shows a table giving chemical compositions of one or more
additives that
may be used in one or more embodiments of the metallic iron nugget process
described
generally in FIG. 1, and/or for use in other processes that form metallic iron
nuggets;
[0031] FIGS. 6A and 6B are generally top views showing stages of one
embodiment of a
metallic iron nugget process as shown generally in FIG. 1;
[0032] FIG. 6C is a generalized cross-section view of a hearth and the layers
thereon;
[0033] FIGS. 7A-7D show illustrations of the effect of time on metallic nugget
formation
in a metallic iron nugget process as shown generally in FIG. 1;
[0034] FIG. 8 shows a block diagram of one exemplary embodiment of a reducible
mixture provision method for use in a metallic iron nugget process as shown
generally in
FIG. 1, and/or for use in other processes that form metallic iron nuggets.
[0035] FIG. 9 shows a CaO-Si02-A1203 phase diagram;
[0036] FIGS. 10-12 show tables for use in describing the effect of adding
calcium
fluoride or fluorspar to a reducible mixture in a metallic iron nugget process
such as that
shown generally in FIG. 1, and/or for use in other processes that form
metallic iron
nuggets;
[0037] FIGS. 13, 14 and 15 show a table, an illustration, and another table,
respectively,
for use in showing the effect of Na2CO3 and CaF2 additives to a reducible
mixture with
respect to control of sulfur levels in one or more exemplary embodiments of a
metallic
iron nugget process such as that shown generally in FIG. 1, and/or for use in
other
processes that form metallic iron nuggets;
[0038] FIG. 16 is a graph showing concentrations of CO in various zones of a
linear
hearth furnace such as that shown in FIG. 3 for use in describing one or more
tests
employing such a furnace;
[0039] FIG. 17 is a table showing the effect of slag composition on a
reduction process
for use in describing one or more tests employing a linear hearth furnace
shown in FIG.
3;
[0040] FIGS. 18A and 18B show a pallet with an arrangement of different feed
mixtures
therein for use in describing one or more tests employing a linear hearth
furnace such as
that shown in FIG. 3, and the resulting product from a typical test;
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[0041] FIG. 19 is a table showing analytical results of iron nuggets and slag
for use in
describing one or more tests employing a linear hearth furnace shown in FIG.
3;
[0042] FIGS. 20 and 21 show the effect of use of various coal addition levels
on one or
more exemplary embodiments of a metallic iron nugget process as shown
generally in
FIG. 1, and/or for use in other processes that form metallic iron nuggets;
[0043] FIG. 22 is a table showing analytical results of iron nuggets and slag
for use in
describing one or more tests employing a linear hearth furnace as shown in
FIG. 3;
[0044] FIGS. 23 and 24 show a pallet with an arrangement of different feed
mixtures
covered with different amounts of a coarse coke overlayer therein for use in
describing
one or more tests employing a linear hearth furnace as shown in FIG. 3, and
the resulting
product from a typical test;
[0045] FIGS. 25 and 26 show the separation of products produced from the
different
areas shown in FIG. 24;
[0046] FIG. 27 table showing analytical results of weight distribution of iron
nuggets,
micro-nuggets, +20 mesh magnetic fraction and slag as shown in FIG. 24 for use
in
describing one or more tests employing a linear hearth furnace as shown in
FIG. 3;
[0047] FIG. 28 shows the separation of products from the different areas shown
in FIG.
27;
[0048] FIG. 29 table showing analytical results of weight distribution of iron
nuggets,
micro-nuggets, +20 mesh magnetic fraction and slag for use in describing one
or more
tests employing a linear hearth furnace shown in FIG. 3;
[0049] FIG. 30 is a table showing analytical results of iron nuggets and slag
for use in
describing one or more tests employing a linear hearth furnace shown in FIG.
3;
[0050] FIGS. 31-33 show a tray with an arrangement of briquettes containing
different
levels of feed mixtures with the use of different levels of coarse coke
overlayer therein
for use in describing one or more tests employing a linear hearth furnace as
shown in
FIG. 3, and the resulting product from a typical test;
[0051] FIGS. 34-36 show a tray with an arrangement different levels of feed
mixtures
with the use of different levels of coarse coke overlayer therein for use in
describing one
or more tests employing a linear hearth furnace as shown in FIG. 3, and the
resulting
products from a typical test;
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[0052] FIG. 37 shows the separation of products from different areas from that
shown in
FIG. 36;
[0053] FIG. 38 shows the resulting product after heating a pallet with an
arrangement of
feed mixtures with the use of different levels of coarse coke overlayer
therein for use in
describing one or more tests employing a linear hearth furnace as shown in
FIG. 3;
[0054] FIG. 39 shows the separation of products from different areas shown in
FIG. 44;
[0055] FIG. 40 shows the resulting product after heating a tray with an
arrangement
different levels of feed mixtures with the use of different levels of coarse
coke overlayer
therein for use in describing one or more tests employing a linear hearth
furnace as
shown in FIG. 3;
[0056] FIG. 41 shows the separation of products from different areas shown in
FIG. 40;
and
[0057] FIG. 42 shows a plot of the ratio of percent sulfur in the slag over
percent sulfur
in the metallic iron nuggets for tests with and without the addition of the
coarse
overlayer.
Detailed Description of the Embodiments
[0058] Certain embodiments of a process for the production of metallic iron
nuggets are
described with reference to FIGS. 1-3. Various other embodiments of the
process for the
production of metallic iron nuggets and examples supporting such various
embodiments
are also described with reference to the other Figures as described below. The
method
and system for producing metallic iron nuggets as will be described in further
detail by
way of example, together with one or more of the resulting benefits and
features. As
explained in detail hereinafter, the disclosed process permits the control of
the amount of
sulfur to produce a novel intermediate slag/metallic nugget product, and with
separation,
novel metallic iron nuggets.
[0059] FIG. 1 shows a block diagram of one or more generalized illustrative
embodiments of a metallic iron nugget process 10. The metallic iron nugget
process 10
shown in the block diagram shall be described with further reference to a more
detailed
embodiment shown in FIG. 3. One skilled in the art will recognize that one or
more of
the process steps described with reference to the metallic iron nugget process
10 may be
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optional. For example, blocks 20, and 26 are labeled as being optionally
provided. As
such, it will be recognized that the metallic iron nugget process 10 is an
illustrative
embodiment, and that the present invention is not limited to any specific
process
embodiments described herein, but rather as described in the accompanying
claims.
[0060] As shown in block 12 of FIG. 1, a hearth 42 is provided as shown in
FIG. 6C.
The hearth 42 may be any moving hearth suitable for use with a furnace system
30 (e.g.,
such as that shown generally in FIG. 2) operable for use in carrying out the
metallic iron
nugget process 10, or another metallic nugget processes that incorporate one
or more
features described herein. Generally, hearth 42 includes a refractory material
upon which
reducible material to be processed (e.g., feed material) is received. Hearth
42 may be a
hearth suitable for use in a rotary hearth furnace, a linear hearth furnace
(e.g., as shown in
FIG. 3), or any other furnace system operable for implementation for direct
reduction of
metallic iron nuggets. The refractory material may be, for example, refractory
board,
refractory brick, ceramic brick, or a castable refractory.
[0061] Further, a combination of refractory board and refractory brick may be
selected to
provide maximum thermal protection for an underlying substructure. In one or
more
embodiments, the hearth may include a supporting substructure that carries a
refractory
material (e.g., a refractory lined hearth) forming hearth 42. The supporting
substructure
may be formed from one or more different materials, such as, for example,
stainless steel,
carbon steel, or other metals, alloys, or combinations thereof that have the
required high
temperature characteristics for furnace processing.
[0062] With reference to block 14 of FIG. 1, a hearth material layer 44 is
provided on
hearth 42. The hearth material layer 44 includes at least carbonaceous
material.
[0063] As used herein, carbonaceous material refers to any carbon-containing
material
suitable for use as a reductant with the iron-bearing material. According to
one or more
particularly advantageous embodiments, the hearth material layer 44 includes
anthracite,
coke, char, or mixtures thereof. For example, anthracite coal, low volatile
bituminous
coal, medium volatile bituminous coal, high volatile bituminous coal, sub-
bituminous
coal, coke, graphite, or other sub-bituminous char materials may be used for
the hearth
layer 44. Some low, medium, and high volatile bituminous coals may not be
suitable for
use as hearth layers by themselves, but may be used as make-up materials to
pulverized
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bituminous char. Also, coke materials such as coke breeze may be used. The
carbonaceous material of the hearth layer may contain an amount of sulfur in a
range
from about 0.2% to about 1.5%, and more typically, in the range of 0.5% to
0.8%.
[0064] The hearth material layer 44 is of a thickness sufficient to prevent
slag from
penetrating the hearth material layer 44 and contacting refractory material of
hearth 42.
For example, the carbonaceous material may be ground or pulverized to an
extent such
that it is fine enough to prevent the slag from such penetration, but
typically not so fine as
to create excess ash. As recognized by one skilled in the art, contact of slag
with the
hearth 42 during the metallic iron nugget process 10 produces undesirable
damage to the
refractory material of hearth 42. A suitable particle size for the hearth
layer is less than 4
mesh2 and desirably between 4 and 100 mesh, with a reasonable hearth layer
thickness of
about 1/2 inch or more, is effective protection for the hearth 42 from
penetration of the
slag and metallic iron during processing. Carbonaceous material less than 100
mesh is
generally high in ash and also may result in entrained dust that is difficult
to handle in
commercial operations.
[0065] Further, referring to block 18 of FIG. 1, a layer of reducible mixture
46 is
provided on the underlying hearth material layer 44. The layer of reducible
mixture
includes at least a reducible iron-bearing material and reducing material for
the
production of iron metal nuggets.
[0066] As used herein, iron-bearing material includes any material capable of
being
formed into metallic iron nuggets via a metallic iron nugget process 10 as
described with
reference to FIG. 1. For example, the iron-bearing material may include iron
oxide
material, iron ore concentrate, taconite pellets, recyclable iron-bearing
material, pellet
plant wastes and pellet screened fines. Further, such pellet plant wastes and
pellet
screened fines may include a substantial quantity of hematite. In addition,
such iron-
bearing material may include magnetite concentrates, oxidized iron ores, steel
plant
wastes (e.g., blast furnace dust, basic oxygen furnace (BOF) dust and mill
scale), red mud
from bauxite processing, titanium-bearing iron sands and ilmenites,
manganiferous iron
ores, alumina plant wastes, or nickel-bearing oxidic iron ores. Also, less
expensive iron
ores high in silica may be used. Other reducible iron bearing materials may
also be used
¨13¨
CA 02658897 2013-06-28
for making the reducible mixture for producing metallic iron nuggets used in
the
processes described herein to produce metallic iron nuggets. For example,
nickel-bearing
laterites and garnierite ores for ferronickel nuggets, or titanium bearing
iron oxides such
as ilmenite that can be made into metallic titanium iron nuggets (while
producing a titania
rich slag), or iron rich oxides which contain manganese oxides can be used to
produce
manganese iron nuggets.
[0067] At least in some embodiments, such iron-bearing material may be ground
to less
than 65 mesh (i.e., -65 mesh) or less than 100 mesh (i.e., -100 mesh) in size
for
processing according to the disclosed processes. The various examples
presented herein
use iron-bearing material ground to 100 mesh and less unless otherwise
specified.
However, larger size particles of iron-bearing material may also be used. For
example,
pellet screened fines and pellet plant wastes are generally approximately 3
mesh (about
0.25 inches, about 6.7 mm) in average size. Such material may be used
directly, or may
be ground to -65 or -100 mesh (0.21 mm to 0.15 mm) to provide larger surface
contact of
carbonaceous reductant with the iron bearing material during processing. The
reduction
process is generally more effective to efficiently produce metallic iron
nuggets with
increased surface area with more finely divided material.
[0068] The carbonaceous material for the reducible mixture may be ground to
100 mesh
or less in size for processing. In another embodiment, such carbonaceous
material is
provided in the range of -65 mesh to -100 mesh. However, carbonaceous material
in the
range of -200 mesh to -8 mesh (0.074 mm to 2.4 mm) may also be used. The use
of
coarser carbonaceous material (e.g., coal) may require increased amounts of
coal in the
reducible mixture for carrying out the reduction process. Finer ground
carbonaceous
material may be more effective in the reducible mixture. Even larger size
carbonaceous
material may also be used. For example, carbonaceous material of less than
about 6 to 7
mesh (e.g., about 0.13 inch to about 0.11 inch, about 3.3 mm to 2.8 mm) in
average size
may be used. Such larger size material may be used directly, or may be ground
to -65 or
-100 mesh for better contact and more efficiently react with the iron-bearing
reducible
material during processing. The various examples presented herein use
carbonaceous
material ground to -100 mesh unless otherwise specified. When other additives
are also
2 The mesh size of the discrete particles is measured by Tyler Mesh Size for
the
¨14¨
CA 02658897 2013-06-28
added to the reducible mixture, such additives may also ground to -100 mesh or
less in
size.
[0069] Various carbonaceous materials may be used in providing the reducible
mixture
of reducing material and reducible iron-bearing material. For example, eastern
anthracite
and bituminous non-caking coals may be used as the carbonaceous reductant in
at least
one embodiment. However, in some geographical regions, such as on the Iron
Range in
Northern Minnesota, the use of western sub-bituminous non-caking coal offers
an
attractive alternative, as such coals are more readily accessible with the
rail transportation
systems already in place, plus they are generally lower in cost and lower in
sulfur levels.
As such, western sub-bituminous coals may be used in one or more processes as
described herein. Further, an alternative to the direct use of sub-bituminous
coals may be
to carbonize it, e.g., at 900 C, prior to its use. In any case, the
carbonaceous material in
the reducible mixture may contain an amount of sulfur in a range from about
0.2% to
about 1.5%, and more typically, in the range of 0.5% to 0.8%.
[0070] The amount of reducing material in the mixture of reducing material and
reducible iron bearing material will depend on the stoichiometric quantity
necessary for
complete metallization of the iron in the reducing reaction in the furnace
process. As
described further below, such a quantity may vary depending on the furnace
used and the
furnace atmosphere in which the reducing reaction takes place. In one or more
embodiments, the quantity of reducing material necessary to carry out the
reduction of
the iron-bearing material is between about 70 percent and 90 percent of the
stoichiometric quantity of reducing material theoretically necessary for
carrying out the
reduction to completely metallize the iron. Such carbonaceous material may be
used at
different stoichiometric levels (e.g., 70 percent, 80 percent or 90 percent)
of the
stoichiometric amount necessary for reduction of the iron-bearing material. In
one
embodiment, for compacts containing coal at 80% of the stoichiometric amount
to
completely reduce the iron oxide, the mounds of reducible material have a
density of
about 1.8-1.9, balls have a density of about 2.1, and briquettes or extrudates
have a
density of about 2.1. This feature of the invention is described in more
detail below.
measurements given herein.
¨15¨
CA 02658897 2013-06-28
[0071] The reducible mixture 46 may have a thickness of more than 0.25 inches
(6.35
mm) and less than 2.0 inches (50.8 mm). In some embodiments, the reducible
mixture 46
may have a thickness of less than 1 inch (25.4 mm) and more than 0.5 inches
(12.7 mm).
In other embodiments, the reducible mixture 46 may have a thickness of about
0.5 inches
or less (12.7 mm or less). The thickness of the reducible mixture is generally
limited
and/or dependent upon the effective heat penetration therein. Increased
surface area of
iron bearing material and carbonaceous material in the reducible mixture
allows for
improved heat transfer and reduction activity.
[0072] As shown by block 20 of FIG. 1, additives may optionally be provided to
the
reducible mixture, for one or more purposes, in addition to the reducing
material (e.g.,
coal or char) and reducible iron-bearing material (e.g., iron oxide material
or iron ore).
For example, additives may be provided (i) for controlling slag basicity, (ii)
for binders to
provide binder functionality (e.g., lime can act as a weak binder in a micro-
agglomerate
configuration when wetted), (iii) for controlling the slag fusion temperature,
(iv) to
reduce the formation of micro-nuggets, and/or (v) for further controlling the
content of
sulfur in resultant iron nuggets formed by the metallic iron nugget process
10. The table
of FIG. 5 shows the chemical compositions of various additives to the
reducible mixture
46. That includes, for example, chemical compositions such as A1(OH)3,
bauxite,
bentonite, Ca(OH)2, lime hydrate, limestone, and Portland cement. Other
additives may
also be used such as CaF2, Na2CO3, fluorspar, soda ash, aluminum smelter slag,
cryolite,
and Si02= One or more of such additives, separately or in combination, may
provide for
beneficial results when used in the metallic iron nugget process 10. These
additives and
their impact particularly in reducing sulfur levels in the metallic iron
nuggets is explained
in more detail below. Some of the illustrated additives contain trace amounts
of Mg, as
shown. Mg, in compounds such as dolomite, should be avoided and in any event
is not
used in quantities that will produce 5% mass or more MgO in the resulting
slag.
[0073] The reducible mixture 46 is then formed into discrete portions
(compacts) either
in situ as explained in detail in U.S. Patent Publication No. 2006/0150774,
filed
December 7, 2005, or preformed into briquettes or extrudates for use in the
disclosed
process of forming metallic iron nuggets. Compacts refer to any compacted
reducible
mixture or other feed material that has pressure applied thereto to form in
situ desired
¨16¨
CA 02658897 2013-06-28
discrete portions on the hearth layer. For example, compaction or pressure to
form
discrete portions as mounds in situ on the hearth layer or to provide one or
more discrete
portions of different profiles in a layer of reducible material. Discrete
portions or
compacts may also be preformed compacted balls or shaped reducible mixtures
such as
briquettes or extrudates, which are preformed using compaction or pressure. It
should
also be noted that different pressurization during formation of the compacts
may result in
different processing characteristics as desired for the particular embodiment
of the
present process.
[0074] Where the discrete portions 59 (See FIG. 6A) are formed in situ, a
channel
definition tool 35 (See FIG. 2) may then be used to create a plurality of
channel openings
50 that extend at least partially through the layer of the reducible mixture
to define the
plurality of nugget forming discrete portions 59 of reducible material for
forming the
metallic iron nuggets. The channel definition tool 35 may be any suitable
apparatus (e.g.,
channel cutting device, mound forming press, etc.) for creating the channel
openings 50
in the layer of reducible mixture (e.g., forming the discrete portions 59,
pressing the
reducible mixture, cutting the openings, etc.). The channel definition tool 35
may include
one or more molds, cutting tools, shaping tools, drums, cylinders, bars, and
the like. The
disclosed process for forming metallic iron nuggets is not limited to any
specific
apparatus for creating the channel openings 50 in the formation of the
discrete portions
59 of nugget forming reducible material.
[0075] With reference to FIG. 1, as shown in optional block 26, areas
surrounding the
discrete portions 59 of reducible mixture are at least partially filled with
nugget
separation fill material. The nugget separation fill material 58 includes at
least
carbonaceous material. For example, in one or more embodiments, the
carbonaceous
material includes pulverized coke, pulverized char, pulverized anthracite, or
mixtures
thereof. In some embodiments, at least a portion of the discrete portions of
reducible
mixture are dropped onto or pushed into a portion of the hearth material layer
to form the
nugget separation fill material 58. The reducible mixture may be formed into
briquettes
or extrudates for use in the process of the producing metallic iron nuggets.
In any case,
the size of the particles of carbonaceous material provided to surround the
discrete
¨17¨
CA 02658897 2013-06-28
portions 59, whether formed in situ or preformed, may be the same size as the
particles
used for the hearth layer.
[0076] Such pulverized material used to fill the areas surrounding the
discrete portions of
reducible mixture may be ground to -4 or -6 mesh (4.7 mm or 3.3 mm) in size
for
processing according to the disclosed process. In at least some embodiments,
such
pulverized material used to fill the areas surrounding the discrete portions
of reducible
material is -20 mesh (0.83 mm). Finer pulverized material of -100 mesh (0.15
mm) also
may be used for the fill surrounding the discrete portions, but a balance
should be found
to avoid an increase in the amount of micro-nugget formation. Larger size
materials may
also be used. For example, carbonaceous material of about 1/4 inch (6 mm) in
average
size may be used for the fill surrounding the discrete portions.
[0077] With reference to FIG. 1, as shown in block 22, a layer containing
coarse
carbonaceous material 49 is provided over at least some of the discrete
portions of the
reducible mixture. The coarse carbonaceous material of the overlayer has an
average
particle size greater than an average particle size of the hearth layer. In
addition or
alternatively, the overlayer of coarse carbonaceous material may include
discrete
particles having a size greater than about 4 mesh or about 6 mesh and in some
embodiments, the overlayer of coarse carbonaceous material may have discrete
particles
with a size between about 4 mesh or 6 mesh and about 1/2 inch (about 12.7 mm).
There
may be of course some discrete particles less than 4 mesh or 6 mesh in size
when discrete
particles greater than 4 mesh or 6 mesh size are desired, but the majority of
the discrete
particles will be greater than 4 mesh or 6 mesh where a particle size greater
than 6 mesh
is desired. The coarse carbonaceous material may be coke, coal, char, or a
combination
of one or more of these.
[0078] With the formed discrete portions 59 of reducible mixture 46 provided
on the
hearth material layer 44, and with nugget separation fill material 58 and the
carbonaceous
overlayer 49 in place, a reducing furnace 34 (shown in FIG. 2) is provided to
thermally
directly reduce the layer of reducible mixture 46 to produce one or more
metallic iron
nuggets 63 in one or more of the plurality of discrete portions 59. The
reducing furnace
34 may include any suitable furnace regions or zones for providing the
appropriate
conditions (e.g., drying/heating, reducing, fusion and cooling zones) for
processing the
¨18¨
CA 02658897 2013-06-28
reducible mixture 46 of the discrete portions 59 to form one or more metallic
iron
nuggets 63. For example, a linear hearth furnace, or any other furnace capable
of
performing the thermal treatment (block 24 of FIG. 1) of the reducible mixture
46 may be
used.
[0079] As further shown in FIG. 6B, resultant slag 60 on hearth material layer
44 is
shown with the one or more metallic iron nuggets 63. That is, slag beads on
hearth
material layer 44 are separated from the iron nuggets 63, or attached thereto.
With
reference to block 28 of FIG. 1, the metallic iron nuggets 63 and slag 60
(e.g., attached
slag beads) are discharged from hearth 42, and the discharged metallic nuggets
are then
separated from the slag 60 (block 29).
[0080] As further shown in FIGS. 6A and 6B, metallic iron nuggets formed by
the
process described with reference to FIG. 1 is shown. Resultant slag 60 on
hearth material
layer 44 is shown with the one or more metallic iron nuggets 63. Slag beads on
hearth
material layer 44 are shown separated from the iron nuggets 63, and attached
thereto.
With further reference to block 28 of FIG. 1, the metallic nuggets 63 and
attached slag 60
are discharged from hearth 42, and the discharged metallic nuggets are then
separated
from the slag 60 (block 29).
[0081] The presence of CO in the furnace atmosphere accelerated the fusion
process
somewhat as compared to a N2 only atmosphere; the presence of CO2 in furnace
atmospheres adjacent the reducible material slowed the fusion behaviors of
metallic iron
nuggets. A presence of CO2 in furnace atmospheres during iron nugget formation
starting at about 1325 C (2417 F), wherein the temperature was on the verge
of forming
fused iron nuggets, has been observed to inhibit the formation of the metallic
iron
nuggets. The effect of CO2 became less pronounced at higher temperatures and,
in fact,
the effect became virtually absent over 1400 C (2552 F) because of CO2
forming 2 CO
at above such temperature. This effect is shown by the plot set forth as FIG.
16. This
finding is observed mainly in the N2 and CO atmosphere in the tube furnace or
the box
furnace. Also, the presence of carbon near the hot reduced iron will allow the
iron to
pick up carbon in solution. This carbonizing of the iron reduces the melting
point of the
iron and in turn lowers the process temperature for full fusion of the
metallic iron.
¨19¨
CA 02658897 2013-06-28
[0082] Whether preformed or formed in in situ, the compacts positioned on the
hearth
layer may have the areas surrounding the discrete portions 59 of reducible
material filled
at least partially with nugget separation fill material (e.g., carbonaceous
material) (block
26) as described herein. With use of such areas surrounding the discrete
portions of
reducible material and nugget separation fill material 58 therein,
substantially similar-
sized metallic iron nuggets 63 may be almost always, if not always, uniformly
formed in
each discrete portion 59, which the areas surrounding the discrete portions of
reducible
material assist in defining. This process of formation of the metallic iron
nuggets is
markedly improved by the overlayer 49 of coarse carbonaceous material, and
markedly
improves the partitioning of the sulfur in the slag of the intermediate
slag/metallic nugget
product and lowers of the sulfur levels in the metallic iron nuggets without
large amounts
of MgO in the slag. As formed, the carbonaceous material of the coarse
overlayer may
contain an amount of sulfur in a range from about 0.2% to about 1.5%, and more
typically, in the range of 0.5% to 0.8%.
[0083] Metallic iron nugget processes that differ from that described with
reference to
FIG. 1 (e.g., the ITmk3 process, the Hi-QIP process) also can be adapted to
practice the
process described herein and to produce the novel intermediate slag/nugget
product with
high sulfur partitioning into the slag. In these embodiments, the same
reducing material
and same iron bearing materials may be used (i.e., type of composition), but
the form of
the reducible mixture on the hearth may be different. For example, the form
that the
reducible mixture takes may be preformed green balls using a binder, or may be
filled
dimples in a pulverized carbonaceous hearth layer, rather than briquettes or
other type of
compacts to form the discrete portions. As such, the process may be used to
form novel
intermediate products with ratios of sulfur in slag to sulfur in nuggets of
greater than 12,
or 15 or 30, and novel metallic iron nuggets with less than 0.03% sulfur, and
not just with
the process described above with reference to FIG. 1. In other embodiments of
the
disclosed process, depressions are formed in a portion of the hearth material
layer
followed by the placement of the reducible mixture into the depressions.
[0084] The metallic iron nugget process 10 may be carried out by a furnace
system 30 as
shown generally in FIG. 2. The furnace system 30 generally includes a charging
apparatus 36 operable to provide a layer of reducible mixture 46 on at least a
portion of
¨20¨
CA 02658897 2013-06-28
hearth material layer 44. The charging apparatus may include any apparatus
suitable for
providing a reducible mixture 46 onto a hearth material layer 44. A
controllable feed
chute, a leveling device, and a feed direction apparatus may be used to place
such
reducible mixture on the hearth 42.
[0085] The furnace system 30 further may include a nugget separation fill
apparatus 37
operable to at least partially fill the areas surrounding the discrete
portions 59 of
reducible mixture with nugget separation fill material 58. Any suitable fill
apparatus 37
for providing such nugget separation fill material 58 into the areas
surrounding the
discrete portions of reducible mixture may be used for manual or automatic
operation
thereof. Apparatus 37 can also be used to provide the coarse carbonaceous
overlayer 49
over the discrete portions 59, which may also partially fill the areas
surrounding the
discrete portions 59.
[0086] Further as shown in FIG. 2, the furnace system 30 includes a discharge
apparatus
38 used to remove the metallic iron nuggets 63 and the slag 60 formed during
processing
by the furnace system 30 and discharge such components (e.g., metallic iron
nuggets 63
and slag 60) from the system 30 after the metallic iron nuggets are cooled and
solidified.
The discharge apparatus 38 may include any number of various discharge
techniques
including gravity-type discharge (e.g., tilting of a tray including the
nuggets and slag) or
techniques using a screw discharge device or a rake discharge device. One will
recognize
that any number of different types of discharge apparatus 38 may be suitable
for
providing such discharge of the nuggets 63 (e.g., iron nugget 63 and slag bead
60
aggregates). Further, a separation apparatus may then be used to separate the
metallic
iron nuggets 63 from the slag beads 60. Any method of breaking and separating
the iron
nugget and slag bead aggregates may be used, e.g., tumbling in a drum,
screening, or a
hammer mill. However, any suitable separation apparatus may be used (e.g., a
magnetic
separation apparatus).
[0087] In the absence of any other information of the furnace gas composition
of iron
nugget processes, most of the laboratory tests in a box furnace described
herein were
carried out in an atmosphere of 67.7% N2 and 33.3% CO, assuming that CO2 in a
natural
gas-fired burner gas would be converted rapidly to CO in the presence of
carbonaceous
reductants and hearth layer materials by the Boudouard (i.e., carbon solution)
reaction
¨21¨
CA 02658897 2013-06-28
(CO2+C=2C0) at temperatures higher than 1000 C, and a CO-rich atmosphere
would
prevail at least in the vicinity of the reducible materials largely by reason
of the presence
of the coarse overlayer. In these tests, carbon dioxide often predominated and
could
reach levels of over 8%. The use of the coarse carbonaceous overlayer,
however, enabled
production of metallic iron nuggets even under these adverse conditions.
[0088] One or more different reducing furnaces may be used according to the
disclosed
processes depending on the particular application of the disclosed processes.
For
example, in one or more embodiments herein, laboratory furnaces were used to
perform
the thermal treatment. One will recognize that from the laboratory furnaces,
scaling to
mass production level can be performed and the present processes contemplate
such
scaling. As such, one will recognize that various types of apparatus described
herein may
be used in larger scale processes, or production equipment necessary to
perform such
processes at a larger scale may be used.
[0089] For example, a linear hearth furnace such as that described in U.S.
Provisional
Patent Application No. 60/558,197, entitled -Linear hearth furnace system and
methods,"
filed 31 March 2004, published as US 2005/0229748A1, may also be used. A
summary
of the linear hearth furnace described therein is as follows. One exemplary
embodiment
of such a linear hearth furnace is shown generally in FIG. 3 and, may be, a
forty-foot long
walking beam iron reduction furnace 712 including three heating zones 728,
730, 731
separated by internal baffle walls 746, and also including a final cooling
section 734. As
described herein, various tests were also run using this linear hearth furnace
and results
thereof are described with reference to the Figures.
[0090] Zone 728 is described as an initial heating and reduction zone. This
zone may
operate on two natural gas-fired 450,000 BTU (113,398 Kcal) burners 738
capable of
achieving temperatures of 1093 C. The burners are typically operated sub-
stoichiometrically to minimize oxygen levels.
[0091] Zone 730 is described as the reduction zone. This zone may operate on
two
natural gas-fired 450,000 BTU (113,398 Kcal) burners 738 capable to achieve
1316 C.
The reduction of the feed mixture occurs in this zone 730.
[0092] Zone 731 is described as the melting/fusion zone. This zone may operate
on two
natural gas-fired 1,000,000 BTU (251,995 Kcal) burners 738 capable to sustain
this zone
¨22¨
CA 02658897 2013-06-28
at 1426 C. The function of this zone is to complete the reduction, fusing the
iron into
metallic iron nodules or "nuggets". In the event that this furnace is being
used to make
direct reduced iron or sponge iron, the temperatures in this zone would be
reduced where
complete reduction would be promoted without melting or fusion.
[0093] The walking beam 724 transports trays 715 to the opposite end 722 of
the furnace
where they are discharged onto a similar platform (roller ball plate) elevator
754. A
safety mechanism has been installed to monitor the position of the hot trays
at the
discharge of the furnace. Discharge rollers drive the trays onto the platform
elevator
where they can be removed or re-inserted back into the furnace. The discharge
rollers
will not function unless trays are in position for discharge, platform
elevator is in the
"up" position, and the walking beams have been lowered to prevent hot trays
from
accidental discharge. Tiered conveyor rollers are located at the discharge of
the furnace
to remove and store sample pallets until cool. A controller 718 coupled to
walking beam
mechanism 724 controls the furnace through a PC interface.
[0094] The exhaust gas system 747 is connected to an exhaust fan 753 with a
variable
flue damper controlled by the furnace PLC. Because the exhaust fan 753 is
oversized for
this application, a manually controlled in-line damper or pressure control 755
is used to
reduce the capacity of the exhaust fan 753 to improve zone pressure control.
As a safety
precaution, a barometric leg into a level controlled water tank is installed
between the
common header and exhaust fan to absorb any sudden pressure changes. Exhaust
gases
are discharged from the fan 753 to a forty-foot exhaust stack 757. The exhaust
ducts are
refractory lined to the exterior walls of the furnace where they transition to
high
temperature stainless steel, fitted with water spray nozzles 749, used to cool
the waste
gases.
[0095] The sample trays or pallets 715 (as shown in FIG. 4) have 30 inch
square (762
mm square) refractory lined pans with a flat bottom to be conveyed through the
furnace
by the walking beam mechanism 724. The trays framework may be made from a 303
stainless steel alloy or carbon steel. They may be lined with high temperature
refractory
brick or ceramic fiberboard with sidewalls to contain the feed mixture.
[0096] The above described furnace systems are given to further illustrate the
nugget
formation process 10, and has provided certain aspects in testing and the
results reported
¨23¨
CA 02658897 2013-06-28
herein. However, any suitable furnace system capable of carrying out one or
more
embodiments of a metallic iron nugget formation process described herein may
be used.
[0097] As shown in FIGS. 7A-7D, each of the one or more metallic iron nuggets
includes
a maximum cross-section. One or more of the metallic iron nuggets includes a
maximum
length across the maximum cross-section that is greater than about 0.25 inch
(about 6.35
mm) and less than about 4.0 inch (about 101.6 mm). A maximum length across the
maximum cross-section is greater than about 0.5 inch (about 12.7 mm) and less
than
about 1.5 inch (about 38.1 mm). Note that these iron nuggets were produced
without the
use of a coarse carbonaceous overlayer.
Control of Stoichiometric Amount of Reducing Material
[0098] In previous metallic iron reduction processes, such as those using
dried balls
described in the Background of the Invention above, carbonaceous reducing
materials are
typically added to the reducible mixture in an amount greater than the
theoretical
stoichiometric amount required to complete reduction the iron oxides. This is
done to
promote carburizing of metallic iron in order to lower the melting point and
the reduction
temperature of the reducible mixture to metallic iron. The amount of
carbonaceous
reductant in the balls includes an amount required for reducing iron oxide
plus an amount
required for carburizing metallic iron and for loss associated with oxidation.
[0099] As discussed previously, in certain furnaces (e.g., such as natural gas
fired
furnaces with high CO2 and highly turbulent gas atmospheres), added
carbonaceous
material (e.g., coal) in feed mixtures (e.g., such as those reducible mixtures
described
herein) is lost by the carbon solution (Boudouard) reaction in certain zones
of the furnace
(e.g., pre-heating and reduction zones). To compensate for this loss, it may
be necessary
to add reducing material (e.g., carbonaceous material) in excess of the
stoichiometric
amount theoretically necessary for complete metallization. However, also as
described
herein, such an addition of reducing material (e.g., coal) in excess of the
stoichiometric
amount may lead to formation of larger amounts of micro-nuggets, i.e., nuggets
that are
too large to pass through a 20 mesh screen (+20 mesh material) and less than
about 1/8"
(about 3 mm).
¨24¨
CA 02658897 2013-06-28
[001001 As previously described, in some embodiments of the disclosed
processes,
the reducible mixture includes a predetermined quantity of reducing material
(e.g.,
carbonaceous reductant) between about 70 percent and about 90 percent of the
stoichiometric amount necessary for complete metallization thereof. As seen in
FIGS.
20-21, the addition of about 70% to about 90% of the theoretical amount
minimized the
formation of micro-nuggets. Carbon needed for further reduction and
carbonizing molten
metal came from, for example, CO in the furnace atmosphere from oxidization of
the
carbonaceous material of the coarse carbonaceous overlying layer 49 and
underlying
carbonaceous hearth material layer 44. The sub-stoichiometric carbon levels in
the
reducible mixtures are believed to assist in controlling the nucleation sites
and inhibiting
formation of small metallic nuggets that do not consolidate in larger nuggets.
The
stoichiometric requirements in carbon for complete reduction of the iron in
the metallic
nuggets are satisfied from the carbon in the hearth layer, the nugget
separation fill and/or
the overlayer. These sources are believed to also provide the additional
carbon needed
for dissolved carbon in the iron phase of the metallic iron nuggets. The
availability of
carbon from the hearth layer, nugget separation fill and overlayer for
solubilization into
the reduced iron lowers its melting point, and in turn reduces the processing
temperature
needed for metal/slag separation.
1001011 Use of compacts may alleviate any need to use nugget separation
material
as described with reference to FIG. 1. For example, control of pressure,
temperature and
gas diffusion in a briquette, extrudates, or other type of preformed compact
may provide
such benefits.
1001021 In addition, the control of the amount of reducing material in the
reducible
mixture based on the stoichiometric amount theoretically necessary to complete
the
metallization process, applies not only to the methods described with
reference to FIG. 1,
but also to other direct reduction processes for forming metallic nuggets. The
coarse
overlayer together with the underlying hearth layer described herein reduces
the
formation of micronuggets formed in the reduction process.
¨25¨
CA 02658897 2013-06-28
Tests With Additives Without a Coarse Overlayer.
[00103] As described previously with reference to FIG. 1, the reducible
mixture 46
for use in the metallic iron nugget process 10 may include one or more
additives in
combination with the reducing material and the reducible iron-bearing material
(e.g.,
reducible iron oxide material). One such method 200 for providing the
reducible mixture
46 with optional additives as shown in the block diagram of FIG. 8. A mixture
of at least
reducing material of carbonaceous material such as coal, coke or charcoal and
reducible
iron oxide material are provided (block 202). Optionally in addition, calcium
oxide or
one or more compounds capable of producing calcium oxide upon thermal
decomposition
thereof (block 204) may be added to the reducible mixture. Further, in
addition or
alternatively, sodium oxide or one or more compounds producing sodium oxide
upon
thermal decomposition may be provided (block 206), in combination with the
other
components of the reducible mixture. Also, one or more fluxing agents may
optionally
may be provided for use in the reducible mixture (block 208). The fluxing
agents that
may be provided for use with the reducible mixture (block 208) may include any
suitable
fluxing agent. For example, an agent that assists in the fusion process by
lowering the
fusion temperature of the reducible mixture or increases the fluidity of the
reducible
mixture may be included. The additives may be naturally part of the reducible
iron
bearing material used as a source for the iron oxide, and typically may be 2%
of the
content of the reducible iron bearing material but may range from about 1% to
about 7%
by weight. In some embodiments, calcium fluoride (CaF2) or fluorspar (e.g., a
mineral
form of CaF2) may be used as the fluxing agent. Alternatively, Si02, borax,
NaF, soda
ash (Na2CO3), or aluminum smelting industry slag or cryolite, may be used as
the fluxing
agent. With respect to the use of fluorspar as the fluxing agent, about 0.5%
to about 4%
by weight of the reducible mixture may be fluorspar.
[00104] Use of fluorspar, for example, as well as one or more other
fluxing agents,
lowers the fusion temperature of the slag phase during formation of the
metallic iron
nuggets, and at the same time reduces the generation of micro-nuggets.
Fluorspar has
been found to lower not only the nugget formation temperature, but also to be
uniquely
effective in decreasing the amount of micro-nuggets generated. It is believed
that the
¨26¨
CA 02658897 2013-06-28
lower temperature slag allows for removal of slag from the reducing iron and
formation
of the metallic iron nuggets.
[00105] In an attempt to improve sulfur removal capacity of slag, as shall
be
described further herein, the level of lime or one or more other compounds
capable of
producing calcium oxide may also be increased beyond a composition (L), as
shown on
the CaO-Si02-A1203 phase diagram of FIG. 9 that indicates the slag
compositions of (A),
(L), (Li), and (L2). Composition (L) is located in the low fusion temperature
trough in
the CaO-Si02-A1203 phase diagram. The slag compositions are abbreviated by
indicating
the amounts of additional lime used in percent as a suffix, for example, (Li)
and (L2)
indicate lime addition of 1% and 2%, respectively, over that of Composition
(L). The
amount of chemical CaF2 (abbreviated to CF) added in percent was also
indicated as a
suffix, for example, (L0.5CF0 25), which represents that 0.25% by weight of
CaF2 was
added to a feed mixture with Slag Composition of (Lo 5).
[00106] It is common practice in the steel industry to increase the
basicity of slag
by adding lime to slag under a reducing atmosphere for removing sulfur from
metallic
iron, for example, in blast furnaces. However, increasing lime from Slag
Composition
(L) to (Li 5) and (L2) may lower sulfur but increase the fusion temperature
and the
amount of micro-nuggets generated as described herein. In the present process,
the use of
fluxing additives that lower the slag fusion temperature, such as fluorspar,
may be used to
(i) lower the temperature of iron nugget formation, (ii) decrease sulfur in
the iron
nuggets, and, (iii) decrease the amount of micro-nuggets formed in processing.
For
example, addition of certain additives, such as fluorspar to the feed mixture
may reduce
the amount of micro-nuggets produced during processing of the reducible feed
mixture.
[00107] Although fluorspar is reported to be a not particularly effective
desulfurizer in steelmaking slag, we have found that with increasing fluorspar
addition,
sulfur in iron nuggets was found to be lowered more effectively at Slag
Compositions
(Li.5) and (L2) than at (Li). Therefore, the use of fluorspar not only lowered
the operating
temperature and further lowered the sulfur in iron nuggets, but has also been
found to
have the unexpected benefit of minimizing the generation of micro-nuggets in
the
metallic iron nuggets. It is believed that the melting temperature for the
slag components
is lower when fluorspar is employed. An increased amount of liquid slag is
thus
¨27¨
CA 02658897 2013-06-28
available to interact with the sulfur in the iron nugget and extract the
sulfur into the slag.
If lime is present as an additive, the slag volume is increased and the
fluorspar is more
effective in increasing sulfur levels in the slag and decreasing sulfur levels
in the metallic
iron nuggets.
[00108] With reference to FIG. 8, calcium oxide, and/or one or more
compounds
capable of producing calcium oxide upon thermal decomposition may also be used
(block
204). For example, lime may be used as an additive to the reducible mixture.
Increased
use of lime decreased sulfur in iron nuggets from 0.084% to 0.05%. Increased
use of
lime, however, requires increasingly higher reduction temperatures and longer
time at
reduction temperature for forming fully fused metallic iron nuggets. As such,
a
substantial amount of lime is not desirable, as higher temperatures also
result in less
economical production of metallic iron nuggets, and reduces yields with
increased
formation of micronuggets. Yet, further decreases in sulfur content may be
accomplished
by use of the coarse overlayer of carbonaceous material as explained more
fully herein.
1001091 Also shown in FIG. 8, sodium oxide, and/or one or more compounds
capable of producing sodium oxide upon thermal decomposition, may be used in
addition
to lime (block 206) to lower sulfur in the formed metallic iron nuggets. Soda
ash,
Na2CO3, NaHCO3, NaOH, borax, NaF and/or aluminum smelting industry slag, may
be
used to lower sulfur in the metallic iron nuggets (e.g., used in the reducible
mixture).
However, without the use of a coarse overlayer of carbonaceous material, the
sulfur
levels in the metallic nuggets by use of these additives has been found to
range from
0.083% to 0.018% by weight.
[00110] The table of FIG. 15 shows the effect of temperature on analytical
results
of iron nuggets formed from reducible feed mixtures. The reducible feed
mixture
included a 5.7% Si02, magnetic concentrate, a Slag Composition (LI 5FSiSCI),
and
medium-volatile bituminous coal at 80% of the stoichiometric requirement for
metallization. The reducible feed mixture was heated in the tube furnace at
the listed
temperatures for 7 minutes in a N2-00 atmosphere. As shown in the table of
FIG. 15,
sulfur in the iron nuggets decreased markedly with decreasing temperature from
0.029%
S at 1400 C to 0.013% S at 1325 C. An addition of Na2CO3 together with 1-2%
CaF2
not only lowered sulfur in the metallic iron nuggets to well below 0.05%, but
also
¨28¨
CA 02658897 2013-06-28
lowered the operating temperature and minimized the generation of micro-
nuggets.
Lowering the process temperature, therefore, is an additional advantage with
the use of
these additives, and the attendant lowering energy cost and maintenance, with
lower
sulfur in the metallic iron nuggets.
[00111] A furnace atmosphere with a minimum of 75% CO and a maximum of
25% CO2 may be useful in producing metallic iron nuggets with less than 0.05%
sulfur.
[00112] Generally, FIG. 10 shows the effect of fluorspar addition on
analytical
results of iron nuggets formed from feed mixtures that included a 5.7% Si02
magnetic
concentrate, medium-volatile bituminous coal at 80% of the stoichiometric
requirement
for metallization and slag composition (Li), (L15), and (L2). The samples in a
2-segment
pattern in boats were heated at 1400 C for 7 minutes in a N2-CO atmosphere.
[00113] The table of FIG. 13 shows the effect of Na2CO3 and CaF2 additions
on
sulfur analysis of iron nuggets at different levels of lime addition, the iron
nuggets
formed from feed mixtures that included a 5.7% Si02 magnetic concentrate,
medium-
volatile bituminous coal at 80% of the stoichiometric requirement for
metallization, and
slag composition (LoiCF1 or LmFSI). The feed mixtures were heated in the tube
furnace
at 1400 C for 7 minutes in a N2-CO atmosphere.
[00114] An addition of Na2CO3 without CaF2 decreased sulfur in iron
nuggets as
effectively as, or even more effectively than the CaF2, but the amount of
micro-nuggets
generated increased, as shown in FIGS. 14A-14C. When CaF2 was used along with
Na2CO3, the sulfur content in iron nuggets decreased even further and the
amount of
micro-nuggets remained minimal at about 1%. Another point of note was that the
effect
of CaF2 in lowering the fusion temperature of iron nuggets was more pronounced
at Slag
Compositions (Li), (L15), and (L2) than at Slag Compositions L and L05. This
analytical
data shows that at least in this embodiment decrease in sulfur was more
pronounced with
soda ash than with increased addition of lime.
[00115] Although fluorspar is reported to be not particularly an effective
desulfurizer in steelmaking slag, FIG. 10 shows that with increasing fluorspar
addition,
sulfur in iron nuggets was lowered more effectively at Slag Compositions (Li
5) and (L2)
than at (Li). At Slag Compositions (Li 5) and (L2), iron nuggets analyzed
including
0.058% by weight sulfur and 0.050% by weight sulfur, respectively, while
sulfur
¨29¨
CA 02658897 2013-06-28
decreased steadily to as low as 0.013% and 0.009% by weight, respectively, at
fluorspar
addition of 4%. Therefore, the use of fluorspar not only lowered the operating
temperature and the sulfur in iron nuggets, but also showed an unexpected
benefit of
minimizing the generation of micro-nuggets as shown in FIGS. 11 and 12.
[00116] Concentrations of CO, expressed as percentages of CO+CO2, were
plotted
in the equilibrium concentration diagrams of iron oxide reduction and carbon
solution
(Boudouard) reactions as shown in FIG. 16. The CO concentration in Zone 1
(1750 F
(954 C)) was in the stability region of Fe304, and those in Zones 2 (2100 F
(1149 C))
and Zone 3 (2600 F (1427 C)) were in the low range of the stability region
of FeO. All
the points were well below the carbon solution reaction, supporting a view
that added
coal was rapidly lost in the linear hearth furnace. The gas sampling ports of
the linear
hearth furnace were located on the furnace wall at about 8 inches above pallet
surfaces.
Because of the high turbulence of furnace gases, the CO concentrations of 4%
should
represent a well mixed value. The arrow at 2600 F (1427 C) in FIG. 16
indicates the
increase in CO with time in Zone 3.
[00117] Analytical results of iron nuggets and slags of linear hearth
furnace Tests
14 and 17 are given in FIG. 17, along with such results for another Test 15.
In linear
hearth furnace Test 15, a tray having an arrangement of feed mixtures in domes
was used,
such as generally shown in FIG. 18A to provide the nuggets shown in FIG. 18B.
The
feed mixture of Test 15 included medium-volatile bituminous coal at 115% and
110% of
the stoichiometric amount and at Slag Compositions (Li 5FSI), placed on a
layer of-1O
mesh coke. No overlayer of coarse carbonaceous material was used during these
tests.
[00118] As shown in FIG. 17, sulfur in the iron nuggets ranged 0.152 to
0.266%,
or several times to even an order of magnitude higher than those in iron
nuggets formed
in the laboratory tube and box furnaces with the same feed mixtures as shown
and
described previously with reference to FIG. 10. The slags were analyzed to
confirm that
they were indeed high in lime. Though the CaO/Si02 ratios ranged from 1.48 to
1.71, it
was noted that the slags were high in FeO ranging from 6.0 to 6.7%. The FeO
analyses
of slags in the laboratory tube and box furnaces under identical slag
compositions
analyzed less than 1% FeO. The formation of high FeO slags was apparently
responsible
for higher sulfur in iron nuggets by interfering with de-sulfurizing. The use
of an
¨30¨
CA 02658897 2013-06-28
increased percentage of coal as well as the use of high sulfur coke (0.65% S)
as a hearth
layer as compared to low sulfur coke (0.40% S) in the laboratory tests might
also have
contributed to high sulfur in the iron nuggets. The ratio of sulfur in the
slag over sulfur,
(S)/[S], in the metallic iron nuggets by weight was only 0.64 and 1.40.
[00119] In FIG. 19, analytical results of iron nuggets and slag of linear
hearth
furnace Tests 14, 15, and 17, along with additional Tests 21 and 22 are shown.
Carbon
and sulfur in iron nuggets and iron, FeO and sulfur in slags for such Tests
are
summarized. In linear hearth furnace Tests 21 and 22, a pallet having an
arrangement of
different feed mixtures in 6-segment domes was used, such as generally shown
in FIG.
18A. The feed mixture included medium-volatile bituminous coal in the
indicated
percentages of the stoichiometric amount as shown in FIG. 19 and at the
indicated Slag
Compositions as shown in FIG. 19, placed on a -10 mesh coke layer. The
temperature in
Zone 3 was 25 F (13.9 C) higher at 2625 F (1441 C) in Tests 21 and 22.
[00120] As shown in FIG. 19, the FeO in slags was halved when a fluorspar
addition was increased to 2% with attendant decrease in sulfur in iron
nuggets. In view
of the results of Test 17 with a fluorspar addition of 2%, the lower FeO might
have been
the results of a higher temperature of 2625 F (1441 C).
1001211 As these tests show, novel metallic iron nuggets with less than
0.05%
sulfur can be produced with the addition of additives to the reducible
mixture. However,
these additives add to the expense of producing the metallic iron nuggets of
the disclosed
process.
Tests with Coarse Overlayer In the Linear Hearth Furnace
[00122] The products resulting from the linear hearth furnace tests were
tested for
the amount of sulfur in the metallic iron nuggets formed by the process and
the amount of
micro-nugget formation. These linear hearth furnace tests revealed that
unexpectedly
high CO2 levels and highly turbulent furnace gas adjacent the reducible feed
mixture
during the reduction process consumed much of the carbonaceous material (e.g.,
added
reducing material in the reducible iron bearing mixture) in Zones 1 and 2, and
not enough
reducing material was left for carburizing and melting the metallic iron in
the high
¨31¨
CA 02658897 2013-06-28
temperature zone (Zone 3). Use of coal in the amount of 105 to 125 percent of
the
stoichiometric amount was found necessary for forming fully fused metallic
iron nuggets.
[00123] The tests were run using a 40-ft. long (12.19 m), natural gas-
fired linear
hearth furnace including three heating zones and a cooling section like that
described
with reference to FIG. 3. The heating schedule of feed samples in the furnace
was
modified to eliminate the baffle between the reduction zone (Zone 2) and the
fusion or
high temperature zone (Zone 3). No Mg was deliberately added beyond trace
amounts or
impurities found in the materials used. Hydrated lime or limestone may be
added to
adjust the C/S ((Ca0)/(Si02)) ratio to the range of 1.40 to 1.60, or to 1.43
to 1.48.
[00124] Sample trays 223 (or pallets) as illustrated in FIG. 4 were used
in the tests.
The trays were made from a 30 inch square carbon steel framework and were
lined with
high temperature fiber board (with sidewalls) to contain samples (i.e., the
reducible
mixture and products resulting after completion of reduction processing. The
trays 223
were conveyed through the furnace by a hydraulically driven walking beam
system as
described with reference to FIG. 3. The arrow 229 in FIG. 4 indicates the
direction of
tray movement through the furnace. A 1/2" (12.7 mm) layer of anthracite char
of particle
size between 6 and 100 mesh was used in each of the tests described below in
this
section, unless otherwise stated.
[00125] The sample tray 223 traveled through Zone 1 at 1800 F (982 C)
for 3
minutes without stopping, then through Zone 2 at 2400 F (1316 C) by moving
one
stroke of 5.5" (140 mm) every 16 seconds for a total time of 5 minutes. Then,
the tray
was moved to the center of Zone 3 (in 55 seconds) for a total time of 10
minutes. The
tray was held in Zone 3 at 2600 F (1427 C) for long enough time to visually
ascertain
fusion of the mounds or briquettes, and then moved into the cooling zone
without
stoppage. The tray was held in the cooling zone for 20 minutes and then
discharged.
[00126] Note that the LHF 22 test shown in FIGS. 22 through 27 and
discussed
below are different tests than test LHF 22 shown in FIG. 19.
[00127] Test LHF 22 are mounds with coarse coke overlayer. Three rows of
reducible feed mixtures in mounds, consisting of 5.7% Si02, magnetic
concentrate, Slag
Composition Li 5FS2, and medium-volatile bituminous coal at 85, 90 and 95% of
the
stoichiometric amount, were placed over a hearth layer of anthracite char
between 6 and
¨32¨
CA 02658897 2013-06-28
100 mesh in particle size, as shown in FIG. 23. The feed mixtures were covered
with
coarse coke overlayers of 1.0, 0.5 and 0.25 lb/ft2 (100, 50 and 25% coverage,
respectively, 4.88, 2.44 and 1.22 kg/m2 respectively) of between 1/4" (6.35
mm) and 1/2"
(12.7 mm) particle size. A sheet of paper was placed over the feed mixtures to
prevent
accidental mixing with coarse coke when the coke was distributed over the feed
mixtures.
The tray was held at 1400 C (2552 F) for 24 minutes in Zone 3.
[00128] The products are shown in FIG. 24. Most of the outermost rows and
columns were not fused because coarse coke particles of the overlayer rolled
off around
the periphery, and the mounds were exposed to the furnace gas and oxidized.
Therefore,
the outermost pieces were excluded from the samples from each section for
weight
measurements and chemical analyses. The products in each section, excluding
these
outermost pieces, are shown in FIGS. 25 and 26.
[00129] FIG. 25 shows the products from the reducible feed mixtures with
85, 90
and 95% stoichiometric coal, overlayered with coarse coke at 1.0 lb/ft2 (100%
coverage,
4.88 kg/m2), gathered from Row (a) in FIG. 23. The weights of products are
shown in
FIG. 27, and the analytical results of the iron nuggets and slag in FIG. 22.
The amount of
micro-nuggets at 85% stoichiometric coal was 1.4% and increased to 3.3% as the
amount
of added coal increased to 95% of the stoichiometric amount.
[00130] The metallic iron nuggets analyzed about 0.02% S. This level of
sulfur is
below the desired level of less than 0.05%. Further optimization as to size
consist,
coverage density, proximate analyses of reductant coal as well as hearth layer
materials,
and optimum temperature as well as time at temperature in lowering the sulfur
in the iron
nuggets, become of interest. Fe and FeO in slag analyzed notably lower than
0.2 and
0.3%, respectively, and the sulfur in the slag analyzed at about 1%.
[00131] FIG. 26 shows the products from the reducible feed mixtures with
85, 90
and 95% stoichiometric coal, overlayered with coarse coke at 0.5 lb/ft2 (50%
coverage,
2.44 kg/m2), gathered from Row (b) in FIG. 23. It was determined that the
coarse coke
coverage of 0.5 lb/ft2 (50% coverage) produced iron nuggets just as
effectively at 90% of
the stoichiometric coal. The weight distributions of products, shown in FIG.
27, indicate
again that the amounts of micro-nuggets increased from 0.8% with 85%
stoichiometric
¨33¨
CA 02658897 2013-06-28
coal to 2.9% with 95% stoichiometric coal. Hence, the lower stoichiometric
amount of
coal was found to reduce the amount of micro-nugget generation.
[00132] It was found in previous tests that the coarse coke overlayer
lowered the
amount of micro-nuggets to 1 to 2%, as compared to 10-15% without a coarse
coke
coverage. In the previous tests, the coal addition of 85% of the
stoichiometric amount
again generated lower amounts of micro-nuggets.
[00133] The sulfur content in the metallic iron nuggets analyzed at about
0.04%. It
is apparent that coarse coke overlayer of 50% (0.5 lb/ft2, 2.44 kg/m2)
resulted in doubling
the sulfur content over the previous test with an overlayer of 1.0 lb/ft2
(4.88 kg/m2) yet
remained below 0.05% S. An increase in reductant coal in the reducible mixture
from 85
to 95% of the stoichiometric amount appeared to increase the % S in the iron
nuggets,
suggesting that much of the sulfur came from reductant coal. Fe and FeO
content of the
slag were in the same range as in the test with full coverage (1.0 lb/ft2,
4.88 kg/m2), but
the sulfur content of the slag was somewhat lower than 1%.
[00134] Shown in FIG. 28 are the products from the reducible feed mixtures
with
85, 90 and 95% stoichiometric coal, overlayered with coarse coke at 0.25
lb/ft2 (25%
coverage, 1.22 kg/m2), gathered from Row (c) in FIG. 23. Here, all sections
resulted in
large amounts of square pieces, indicating that the coarse coke overlayer of
0.25 lb/ft2
(25% coverage, 1.22 kg/m2) was not sufficient. With these large amounts of
insufficiently fused iron nugget-slag mixtures, no weight distributions were
recorded.
[00135] The conclusion of this series of tests is that the use of a coarse
coke
overlayer of 0.5 to 1.0 lb/ft2 (50 to 100% coverage, 2.44 to 4.88 kg/m2)
enabled the
formation of fully fused metallic nuggets and lowered the sulfur in iron
nuggets to below
0.03%.
[00136] Test LHF 26 are dry briquettes with a coarse coke overlayer. To
investigate the effect of coarse coke overlayer over dry briquettes, two
columns of dry
briquettes at 80% and 110% stoichiometric amount of coal without a binder were
arranged as shown in FIG. 31. The briquettes were provided with a coarse coke
overlayer between 1/4" (6.35 mm) and 1/2" (12.7 mm) particle size at 1.0 and
0.75 lb/ft2
(4.88 and 3.66 kg/m2) in Rows (a) and (b), respectively. A sheet of paper was
placed
over the feed mixtures to prevent accidental mixing with coarse coke when the
coke was
¨34¨
CA 02658897 2013-06-28
distributed over the reducible feed mixture. The coke-overlayered feed is
shown in FIG.
32. The tray was held at 2552 F (1400 C) for 20 minutes in Zone 3.
[00137] As shown in FIG. 33, the products of the briquettes were 100%
fused into
metallic nuggets. Most of the iron nuggets in the periphery, particularly at
0.75 lb/ft2
which is Section (b), were associated with black covered slag, while the slag
associated
with the iron nuggets in the interior was essentially white. This difference
was attributed
to the fact that the coarse coke particles rolled off around the periphery and
the fused iron
nuggets and slag were exposed to the turbulent furnace gas and oxidized at the
periphery.
The analytical results are shown in FIG. 30. The metallic iron nuggets
analyzed 0.030%
S and 3.68% C, while the slag analyzed low in iron and 1.39% S. The ratio of
sulfur in
the slag over sulfur in the metallic nuggets by weight, (S)/[S], was
calculated to be 46.
[00138] Test LHF 27 were mounds of reducible mixtures with different
degrees of
coarse coke overlayer. To investigate the effect of different degrees of
coverage by
coarse coke, two trays of reducible feed mixtures in mounds with 80%
stoichiometric
coal were prepared, as shown in FIG. 34. In both trays, the mounds were
divided into
three equal rows and the rows were overlayered with coarse coke of between
1/4" (6.35
mm) and 1/2" (12.7 mm) particle size at 1.25, 1.0 and 0.75 lb/ft2 (6.1, 4.88
and 3.66
kg/m2) in Rows (a), (b), and (c), respectively. A sheet of paper was placed
over the feed
mixtures before spreading the coarse coke, as shown in FIG. 35. The tray was
sent
through the furnace according to the standardized heating schedule of these
tests and
heated to 2600 F (1427 C) for 20 minutes in Zone 3.
[00139] The products are shown in FIG. 36. With the coke overlayer, the
products
could not be seen, but iron nuggets associated with the black-overlayered slag
are seen in
the outside columns on both sides, overlayered with 0.75 and 1.0 lb/ft2 (3.66
and 4.88
kg/m2) of coarse coke and in the top row. After the coke overlayer was
removed, and the
products were separated into nuggets, micro-nuggets, +20 mesh mag. and slag
fractions
and shown in FIG. 37. The weights and analytical results of the products are
shown in
FIGS. 29 and 30, respectively.
[00140] Sulfur analyses of the iron nuggets increased from 0.020% to
0.030% with
decreasing coke coverage. Iron analyses of slag were low, 0.03 to 0.3% Fe and
0.27 to
¨35¨
CA 02658897 2013-06-28
0.55% FeO. Ratio of sulfur in the slag over sulfur in the nuggets by weight,
(S)/[S],
ranged from 55 to 35, again decreasing with decreasing coarse coke coverage.
[00141] Sample (b) was the iron nuggets and associated black-colored slag
from
the outer columns at the periphery where the coarse overlayer slid off. Iron
analysis of
the slag was notably higher than those from the interior, which were 0.82% Fe
and 1.20%
FeO. Sulfur analysis of the iron nuggets was 0.076% S with the ratio, (S)/[S],
of 10.8.
The results show that an exposure of the products to the furnace atmosphere
was
detrimental to the removal of sulfur from iron oxides into the slag.
[00142] Test LHF 28 were mounds with different degrees of coarse coke
overlayer. Another tray of reducible feed mixtures in mounds with 80%
stoichiometric
coal, covered with coarse coke of between 1/4" (6.3 mm) and 1/2" (12.7 mm)
particle
size at 1.25, 1.0 and 0.75 lb/ft2 (6.1, 4.88 and 3.66 kg/m2)in Rows (a), (b),
and (c),
respectively, was sent through the furnace according to the standardized
heating schedule
for these tests, but the temperature of Zone 3 was lowered to 2550 F (1399
C) and kept
at that temperature for 20 minutes.
[00143] The products are shown in FIG. 38. The products were hidden from
view
by the coke overlayer. The products were separated and shown in FIG. 39. More
than
one half of the iron products were reduced, but not fused into metallic
nuggets, as seen in
the mounds to the right of the fused iron nuggets. With a coarse coke
overlayer coverage
of 0.75 lb/ft2, the amount of fully fused iron nuggets approached one half of
the product.
The weights of the products are shown in parentheses in FIG. 29.
[00144] Two iron nuggets and associated slag were selected from Section
(c) with
0.75 lb/ft2 coke coverage, and analyzed. The analytical results are shown in
FIG. 30.
The sulfur analyses of the iron nuggets was 0.034% S and the iron analyses of
the slag
were 0.26% Fe and 0.54% FeO. The ratio of the sulfur in the slag over the
sulfur in the
nuggets by weight, (S)/[S], was 33.
[00145] Test LHF 29 was briquettes with different degrees of coarse coke
overlayer at a lower temperature. In Test LHF 26, the products formed at 1427
C (2600
F) were all fused into metallic iron nuggets. In this test, an identical tray
of dry
briquettes overlayered with coarse coke, as shown in FIG. 31, was sent through
the
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CA 02658897 2013-06-28
furnace according to the standardized heating schedule, but with the
temperature of Zone
3 lowered to 1399 C (2550 F) and kept at that temperature for 20 minutes.
[00146] The products are shown in FIG. 40 and the separated products are
shown
in FIG. 41. All of the briquettes were fully fused into metallic iron nuggets.
The iron
nuggets, however, were notably smaller than those from the mounds as the
briquettes
were only half the size and weight of the mounds. The weights of the metallic
nuggets
are shown in FIG. 29.
[00147] Micro-nuggets at 80% stoichiometric amount of coal were low, 1.0
and
0.5%, for coarse coke overlayers of between 1/4" (6.35 mm) and 1/2" (12.7 mm)
particle
size at 1.0 and 0.75 lb/ft2(4.88 and 3.77 kg/m2) coverage, respectively.
Meanwhile, those
at 110% stoichiometric amount of coal were notably higher, 5.0 and 3.5%, for
coarse
coke overlayers of 1.0 and 0.75 lb/ft2(4.88 and 3.77 kg/m2) coverage,
respectively.
Therefore, again, as previously observed, the generation of micro-nuggets was
less at
80% stoichiometric amount of coal in the reducible mixture. It is also noted
that the
lower coverage by coarse coke overlayer generated less micro-nuggets.
[00148] The analytical results are shown in FIG. 30. The results showed
that the
iron nuggets had undergone substantially complete metallization. It was
determined that
the iron nuggets analyzed were 0.016 to 0.029% S, while the slag analyzed
essentially no
Fe and 0.23 FeO. The ratio of sulfur in the slag over sulfur in the nuggets by
weight,
(S)/[S], ranged from 45 to 82.
[00149] Referring to FIG. 42, the relationship (i.e., the ratio) of sulfur
in the slag
over sulfur in the metallic nuggets, (S)/[S], is plotted as a function of the
percent by
weight sulfur in the nuggets for the various tests that have been done. The
filled squares
(M) are for the LHF tests on reducible feed mixtures using 5.3% SiO2 taconite
concentrate, a Slag Composition LI 5FS2, and 80% stoichiometric Fording
Standard coal.
The open squares (0) are for box furnace tests on reducible feed mixture using
3.6%
Si02 taconite concentrate, a Slag Composition LI 5FS2, and 80% stoichiometric
Jim
Walter coal.
[00150] The data points for ratio (S)/[S] below 12 were for tests without
the coarse
coke overlayer and the others are for tests with the coarse coke overlayer
with no
deliberately added Mg producing levels between 5% and 13% of MgO in the slag.
As
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CA 02658897 2013-06-28
shown by the plotted data in FIG. 42, sulfur is increased in the slag by the
use of the
coarse carbonaceous overlayer, and that when the ratio (S)/[S] is 30 or more,
sulfur
content in the metallic iron nuggets that are produced was 0.03% or less.
These latter
metallic iron nuggets are particularly useful in steelmaking processes such as
the electric
arc furnace, because the iron nuggets may be substituted for scrap in the
charge.
[00151] The coarse coke overlayer enabled carburizing both from the hearth
layer
and the coarse coke overlayer. The availability of carbon from the coarse
overlayer is
advantageous in lowering the overall processing temperature requirements,
while creating
the necessary reduction conditions to allow effective separation of sulfur
into the slag.
[00152] In view of the above, in some embodiments of the present process,
the use
of a reducible feed mixture that includes a reducible mixture, on the hearth
layer, that has
a predetermined quantity of reducing material between about 70 percent and
about 90
percent of the stoichiometric amount of reducing material and with a coarse
carbonaceous
material over at least a portion of the layer of the reducible mixture results
in complete
metallization thereof, and also reduce the potential for formation of micro-
nuggets. The
result was reproduced with the box and tube furnaces. In other words, a sub-
stoichiometric amount of reducing material (e.g., coal) may be used with the
overlayer to
obtain almost complete metallization and formation of metallic iron nuggets
from a
predetermined quantity of reducible iron bearing material, the reducing
material (e.g.,
coal) and the iron bearing material providing a reducible feed mixture for
processing
according to one or more embodiments described herein.
[00153] One will recognize that various shapes of the compacts preformed
and
formed in situ may be used, and still maintain the benefit of having a feed
mixture with a
sub-stoichiometric amount of reducing material (e.g., coal) next to the hearth
layer to
minimize micro-nugget formation. The configurations of the discrete portions
(compacts) described herein are given for illustration only.
[00154] This invention has been described with reference to illustrative
embodiments and is not meant to be construed in a limiting sense. It will be
apparent to
one skilled in the art that elements or process steps from one or more
embodiments
described herein may be used in combination with elements or process steps
from one or
more other embodiments described herein, and that the present invention is not
limited to
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CA 02658897 2013-06-28
the specific embodiments provided herein but only as set forth in the
accompanying
claims. Various modifications of the illustrative embodiments, as well as
additional
embodiments to the invention will be apparent to persons skilled in the art
upon
reference to this description.
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