Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SELF-REDUCING IRON OXIDE AGG~O~ERATES
.
FIELD OF THE INVENTlON
This invention relates to a process for
producing high-strength agglomerates from finely-
divided, iron oxide-containing materials and
containing a sufficient amount of internal carbon
to reduce all the iron oxide therein to metallic
iron.
PRIOR ART
It is known to form finely-divided, iron
oxide-containing materials, such as iron ore
concentrates and steel plant waste dusts, into
pellets containing internal carbon for the purpose
of accelerating the rate of reduction of the iron
oxide metallic iron when the pellets are charged to
a steel making furnace. Such processes are exempli-
fied in U.S Patents 2,793,109 (Huebler et al),
2,806,779 (Case) 3,264,092 (~an) 3,333,951 (Ban),
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3,386,816 (English), 3,770,416 (Goksel) and 3,938,987
(Ban) and Canadian Patent 844,592 (Volin et al).
It is generally recognized that the presence
of a carbonaceous material in an amount sufficient
to reduce all the iron oxide to metallic iron tends
to adversely affect crush resistance or compressive
i stength of the pellets. In this regard, the Case
Patent 2,806,779 and the Ban Patent 3,264,092 teach
the use of an agglomerating type coal and then
heating the pellets to a temperature of about
1600-2300F to destructively distill the coal and
thereby produce a char bond for the pellets. The
Ban Patent 3,938,987 teaches that, when non-agglo-
merating coals, such as lignite, sub-bituminous
coals, anthracite coal and coke breeze, are used as
the carbonaceous material, the amount must be about
40-80% of that required to reduce iron oxide to
metallic iron in order to produce pellets having
adequate strength for use in a steel making furnace.
The English Patent 3,386,816 teaches that pellets
containing as little as 8% coke have a compressive
strength of 58 lbs. which is generally considered
unacceptably low for use in most steel making
processes.
SU~IMARY OF THE INVENTION
The principal object of the invention is
to provide a low cost process for forming finely-
divided, iron oxide-containing materials into
hardened agglomerates containing an amount of
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carbonaceous material at least sufficient to reduce
all the iron o~ide to ~etalli. iron and yet having
high compressive strengths.
Another object of the invention is to provide
S a process for producing such aggolomerates which
are suitable as a charge for steel making furnaces.
Other aspects, advantages and objects of the
invention will become apparent to those skilled in
the art upon reviewing the following detailed
description and the appended claims.
i In accordance with the invention, hardened
. agglomerates having a compressive strength of at
least about 100 lbs. and containing a major portion
of iron oxide and a sufficient amount of a carbo-
naceous material to reduce all the iron oxide to
- metallic iron are produced by utilizing a natural or
pyrolyzed carbonaceous material having a volatile
matter (on dry basis) content of about 20 weight % or
less, such as bituminous coal char, anthracite coal,
lignite char, coke, wood char, and graphite and the
like. Hy~rot~ermally hardened, iron oxide agglomerates
containing carbonaceous materials, having a high
volatile matter content, in an amount sufficient to
~ reduce all the iron oxide to metallic iron exhibit
; 25 crush resistance or compressive strengths which are
= unacceptably low for many uses. Quite unexpectedly,
it has been found that the compressive strengths of
such agglomerates can be increased significantly by
using a natural or pyrolyzed carbonaceous materials
having a volatile matter (on dry basis) content of
about 20 weight V/o or less.
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More specifically, the process of invention
includes the steps of preparing a moistened mixture
of a finely divided iron oxide-containing material,
a finely-divided, natural or pryolyzed carbonaceous
material having a volatile matter (on dry basis)
content of about 20 weight lc or less in an amount at
least sufficient to reduce all the iron oxide to
metallic iron, about 1 to about 30 weight ~O of a
bonding agent selected from a group consisting of
oxides, hydroxides and carbonates of calcium and
magnesium and mixtures thereof, and 0 to about 3
weight % of a siliceous material (as available
SiO2); forming the resulting mixture into discrete
green agglomerates; and hydrothermally hardening the
green agglomerates by contacting them with steam for
a time period sufficient to form them into hardened,
integral bonded masses.
DES CR I PT I ON OF THE PREF ERRE D EM B O D I M EN T S
The process can be used to produce hardened
agglomerates from iron ore concentrates and so-called
"steel plant waste oxides", or iron-rich (e.g.,
30-80% iron) solid particulates or fines recovered
as by-products from steel making processes, includ-
ing dust collected from the fumes of BOF, open
hearth, blast, and electric furnaces, mill scale
fines, ~rit chamber dusts, fines separated from
pelletized iron ore, etc. As used herein, the term
"iron oxide-containing material" encompasses iron
ore concentrates, steel plant waste oxides or
mixtures thereof. The process is particula~ly
suitable for producing high strength agglomerates
....
from iron ores, such as hematite and magnetite,
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preferably in the form of high purity ores or
concentrates containing about 45-70'~ iron and the
balance gangue and oxide. Accordingly, the process
will be described with an iron ore concentrate being
used as a starting material.
A starting mixture is first prepared by
thoroughly blending together an iron ore concentrate,
a carbonaceous material, a bonding agent, a siliceous
material and a sufficient amount oE water to form a
moistened mixture capable of being formed into
discrete agglomerated masses or peliets.
The carbonaceous material can be either
naturally occuring or pyrolyzed so long as it has a
volatile matter (on dry basis) content of about 20
weight % or less, preferably about 10 wei~ht % or
less. Pyrolyzed carbonaceous materials generally are
preferred because of their lower volatile content.
Representative suitable natural carbonaeous
materials include low volatile anthracite coal,
graphite and the like.
The term "pyrolyzed carbonaceous material" as
used herein means a solid product produced by heating
a naturally occurring, high carbonaceous material to
elevated temperatures in the absence of oxygen to
?5 drive off a substantial portion of the volatile
matter, primarily organic matter. Representative
suitable pyrolyzed carbonaceous materials include
chars produced from non-coking bituminous, sub-
bitumlnous and anthracite coals, lignite char, wood
char, coke produced from bituminous coal, coke
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breeze, petroleum and coal tar pitch, and mixtures
thereof. Of these, bituminous coal char, lignite
char and coke breeze are preferred because of their
lower cost.
Suitable bonding agents include the oxides,
hydroxides, and carbonates of calcium and magnesium
r and mixtures thereof. Burned lime (CaO) and hydrated
lime (Ca(OH)2) are preferred because, in addition
to functioning as a bonding agent, they can assist in
slag formation and sulfur removal when the agglomerates
are used in a steel making process.
The amount of bonding agents used is about
O.l to about 30 weight %, based on the total weight
of the dry solids in the starting mixture. When less
than about O.l weight % is used, the hardened pellets
do not have sufficient crush resistance or compressive
strength to withstand the loads normally imposed
thereon during handling, storage and transportation.
On the other hand, amounts of the bonding agents in
~0 excess of about 30 weight % do not appreciably
inceaSe the compressive strengths, can dilute the
concentration of iron oxide in the final agglomerates
to an undesirable level and can cause formation of
excessive amounts of slag during melting. The
preferred amount of bonding agent is about 2 to about
10 weight %.
If the iron oxide-containing material con-
tains an appreciable amount (e.g., about 0.5 weight
% or more) of available SiO2 capable of reacting
with the bonding agent to formed silicate or hydro-
silicate bonds therewith during the conditi-ons of
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hydrothermal hardening, hardened pellets having
compressive strengths up to about 200 lbs. can be
obtained without adding a siliceous material to the
starting mixture. For higher purity iron ore
concentrates containing relatively small amounts
of available SiO2, an amount of natural or artificial
siliceous material containing up to 3 weight %
available SiO2, based on the total weight of the
dry solids, is added to the starting mixture. The
iO total available SiO2 in the mixture, whether as
part of the iron oxide containing material or added
with the siliceous material, should be at least 0.5
weight %.
Representative suitable siliceous materials
include finely ground quartz, silica sand, bentonite,
diatomaceous earth, fuller's earth, sodium, calcium
magnesium, and aluminum silicates, pyrogenic silica,
various hydrated silicas and mixtures thereof. Of
these, finely ground quartz and silica sand are
preferred.
In addition to the bonding agent and the
siliceous material, other strengthening additive can
be included in a starting mixture to further increase
the strength of the hardened agglomerates. For
examples, oxides, hydroxides, carbonates, bicarbonates,
sulfates, bisulfate, and borates of alkali metals
~e.g. potassium and sodium) and mixtures thereof can
be added in amounts ranging to about 3 weight %. Of
these, sodium hydroxide, sodium carbonate, and sodium
bicarbonate are preferred. The presence of some of
these strengthening additives might be considered
undesirable when the hardened agglomerates are used
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as a charge for blast furnaces. In those cases, such
additives can be omitted without significantly
reducing the strength of the agglomerates. When
used, the preferred amount of the strengthening
additives is about 0.15 to about 1 weight %.
The amount of water included in the starting
mixture varies, depending on the physical properties
of the materials and the particular agglomeration
technique employed. For example, when a pelletizing
process employing a balling drum or disc is used Eor
form spherical pellets, the total amount of water in
the moistened starting mixture generally should be
about 5 to about 20 weight ~/~, preferably about 10 to
about 15 weight %. On the other hand, when a briquetting
press is used, the amount of water in the ~oistened
starting moisture generally should be about 3 to about
15 weight %, preferably from about 5 to about 10
weight %.
The average particle size of the various
solid materials included in the starting mixture
generally can range from about 10 to about 325 mesh
with all preferably being less than about 200 mesh.
Particle sizes coarser than about 100 mesh make it
difficult to obtain a homogeneous mixture of the
constituents and, in some cases, produce insufficient
surface area to obtain the requisite high strength
bond in the hardened agglomerates. Also, it is
difficult to form pellets from mixtures containing
coarser pellets. Preferably, at least half of all
solid materials in the starting mixture have an
average particle size less than about 200 mesh for
pelletizing. Briquettes can be produced wi.th coarser
particles.
Many low volatile, naturally occurring and
pyrolyzed carbonaceous materials have small capillary-
like pores or cavities which tend to absorb water
during the mixing step. This free internal moisture
tends to be converted to steam during the hydrothermal
hardening step, causing a reduction in the compressive
strength and sometimes cracking or bursting when
excessive amounts are present in the pores or cavities.
This can be minimized by allowing the moistened
mixture to rest or stand a sufficient time for a
substantial portion of the free internal moisture in
the carbonaceous material to migrate from the pores
or cavities to the surface.
The time and conditions for this holding or
standing step can vary considerably depending primarily
on the particular type of carbonaceous material and
bonding agent being used. Removal of excess internal
moisture from the pores or cavities in the carbonaceous
material can be accelerated by heating the moistened
mixture to an elevated temperature. When burned lime
and/or magnesium oxide is used as the bonding agent,
they react with the moisture present to form hydrates.
This exothermic hydration reaction tends to accelerate
migration of the free internal moisture to the
particle surface, resulting in a shortening of the
standing time required without external heating.
As a general guide, the moistened mixture,
prior to agglomeration, is allowed to stand for about
0.5 to about 48 hours, preferably about 2 to about 3
hours, at a temperature of about 60 to about 90C.
~ligher temperatures and pressures can be used, but
are less desirable because of the higher opèrational
1~84~2
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costs. When burned lime or magnesium oxide is used
as the bonding agent, the moistened mixture preferably
is placed in a closed, thermally insulated container
to take advantage of the exothermic hydration reaction.
The moistened mixture is next formed into
green agglomeration of the desired size and shape for
the intended end use by a conventional agglomeration
technique, such as molding, briquetting, pelletizing,
extruding and the like. Pelletizing with a balling
disc or drum is preferred because of the lower
operating costs.
When in the form of spherical pellets, the
green agglomerates generally have a diameter of about
5 to about 25mm, preferably about 10 to about 20mm.
When briquetting is used, the agglomerates preferably
are in a spherical-like or egg shape and have a major
diameter ranging up to about 75mm. Larger pellets
and briquettes can be used if desired.
The crush resistance or compressive strength
of the hardened agglomerates can be increased by
drying the green agglomerates to a free moisture
content of about 5 weight % or less, preferably about
3 weight % or less, prior to the hydrothermal hardening
step. This drying can be accomplished by conventional
means, such as by placing the green agglomerates in an
oven or by blowing a heated gas thereover, using
drying temperatures up to the decomposition temperature
of the carbonaceous material. The time required to
reduce the free moisture content to about 5 weight %
or less depends upon the drying temperatures used,
the moisture content of the green agglomeraLes, flow
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rate of the drying gas, the level to which the
moisture content is reduced, size and shape of the
green agglomerates, etc.
The green agglomerates are introduced into a
reaction chamber or pressure vessel, such as an
autoclave, wherein they are heated to an elevated
temperature in the presence of moisture to effect a
hardening and bonding of the individual particle into
an integral, high strength mass. The compressive
strength of the hardened agglomerates produced by
this hydrothermal hardening step depends to some
extent upon the temperature, time, and moisture
content of the atmosphere use.
The application of heat to the green agglomerates
can be achieved by any one of a number of methods.
The use of steam is preferred because it simultaneously
provides a source of heat and moisture necessary for
the hydrothermal reaction. Either saturated steam or
substantially saturated steam be used. Superheated
steam tends to produce hardened agglomerates having
reduced strengths. Therefore, steam at temperatures
and pressures at or close to that of saturated steam
is preferred. Temperatures generally ranging from
about 100 to about 250C, preferably 200 to about
225C, can be satisfactorily employed to achieve the
desired hardening of the green agglomerates within a
reasonable time period.
Autoclaving pressures substantially above
atmospheric pressure are preferred in order to
decrease the hardening time and to improve the
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strength of the hardened agglomerates. Generally,
economic conditions dictate that the maximum pressure
should not exceed about 35 atmospheres and a pressure
of about 10 to about 25 atmospheres is preferred.
The retention time of the pellets in the
reaction chamber or pressure vessel depends upon
several process variables, such as pressure, temperature,
and atmosphere of the chamber, size and composition
of the pellets, etc. In any case, this time should
be sufficient for the bonding agent to form silicate
and/or hydrosilicate bonds in the available SiO2
and bond the individual particles into a hardened,
high strength condition. When higher temperatures
and pressures are used, the time for the hydrothermal
hardening generally is about 5 minutes to about 15
hours, preferably about 30 to about 60 minutes.
The hardened agglomerates are removed from
the reacting chamber and, upon cooling, are ready for
use. The hot, hardened agglomerates usually contain
up to about 1.5% free moisture and have compressive
strength characteristics suitable for most uses. The
compressive strength of the hardened agglomerates can
be increased by rapidly drying them, preferably
immediately after removal from the reaction chamber
and before appreciable cooling as occurred, to remove
substantially all of the free moisture therefrom.
This drying can be accomplished in a convenient
manner.
The minimum compressive strength of hardened
agglomerates produced by the process of the invention
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varies depending on the size of the agglomerate. For
example, spherical pellets with a diameter of 12-15mm
have a compressive strength of at least 100 lbs. and
those with a diameter of about 30 mm have a compressive
strength in the neighborhood of about 200 lbs. or
more.
Without further elaboration, it is believed
that one skilled in the art, using the preceding
description, can uitilize the invention of its
fullest extent. The following example is presented
to illustrate the invention and should not be
construed as a limitation thereto.
EXA~IPLE
A series of tests was run to evaluate the
crush resistance or compressive strength of hardened
magnetite pellets containing different types of
carbonaceous materials in amounts sufficient to
reduce all the iron oxide to metallic iron. The
ingredients making up the starting mixture were
blended together in a roller or intensive mixer for a
sufficient time to obtain a uniformly moistened
blend. Green, spherically-shaped pellets (15mm) were
prepared from the mixtures in a conventional balling
device. The green pellets were dried to a moisture
content of about 0-3 weight % and then placed in a
high pressure steam autoclave. The autoclave was
heated and maintained at a temperature of 210~C and a
pressure of 22 atm for one hour. After cooling, the
compressive strength of the pellets was measured with
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a Dillon tester. Results from these tests are
summarized in Table I.
From these results, it can be seen that
pellets containing a carbonaceous material having a
low volatile matter content (anthracite) or which was
pyrolyzed (bituminous char char, lignite char and
coke), had compressive strengths in excess of 100
v lbs. Whereas those containing a carbonaceous material
having a high volatile matter content (bituminous
coal and lignite) had substantially lower compressive
strengths. The use of chars from non-coking bituminous
coal, lignite and other low grade carbonaceous materials
is particularly advantageous because of the low cost of
these materials and the volatiles driven off during
the pyrolyzing process can be burned and used as a
heat source.
From the foregoing description, one skilled
in the art can easily ascertain the essential
characteristics of the invention and, without departing
from the spirit and scope thereof, can make various
changes and modifications to adapt the invention to
various usages and conditions,
