Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR PRODUCING IRON ORE AGGLOMERATES WITH USE OF
SODIUM SILICATE CONTAINING BINDER
The invention relates to a process for producing iron ore agglomerates.
Such a process is known from US 6,293,994, which discloses a process of
making fired mineral pellets by mixing particulate mineral material with
moisture
and binder comprising substantially water-soluble organic polymer and alkali
metal silicate in a dry weight amount which is either (a) above 0.13% based on
moist mix or (b) above 0.08% based on moist mix and at least three times the
dry weight of substantially water-soluble organic polymer. The preferred
polymer is a synthetic polymer formed of water-soluble ethylenically
unsaturated
monomer or monomer blend. The high amount of alkali metal silicate in the
pellets described in US 6,293,994 generally is undesirable, because silicates
can slow down the reduction process in steel making operations by blocking the
pathways the reducing gases use to permeate the pellet, which leads to an
increase in energy costs. Furthermore, the use of such high amounts of alkali
metal silicate results in green pellets that have a high tendency to deform,
which
in turn may lead to pellets of different size and shape, resulting in an
inefficient
process for preparing fired pellets.
The object of the present invention is to provide iron ore agglomerates with
improved physical properties.
The present invention provides a process for producing iron ore agglomerates
comprising agglomerating fine iron ore particles in the presence of a binder
system wherein the binder system comprises a binder selected from the group
consisting of water-soluble natural polymers and modified natural polymers and
an alkali metal silicate and wherein the alkali metal silicate is present in
an
amount of between 0.0001 to 0.07 percent by weight, based on the total weight
of dry iron ore agglomerate, wherein the binder system is free of synthetic
polymer. The process of the invention leads to iron ore agglomerates with
increased cold compression strength, preheat strength, and dry crush strength
relative to the
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use of conventional binder systems comprising the same binder. Furthermore,
small amounts of the alkali metal silicate are already sufficient to obtain a
significant improvement in the physical properties of the agglomerates.
Moreover, the specified amount of alkali metal silicate causes the
agglomerates
obtained with the process of the invention to have a similar or only slightly
higher degree of deformation than binder systems where alkali metal silicate
is
absent. In contrast, binder systems comprising a larger amount of alkali metal
silicate exhibit a significant increase in the degree of deformation, which is
undesirable. In addition, the use of alkali metal silicate in accordance with
the
invention may enable a reduction of the amount of binder without a significant
loss in physical properties of the obtained agglomerates.
The amount of alkali metal silicate is at most 0.07 percent by weight (wt%),
and
preferably at most 0.06 wt%, based on the total weight of dry iron ore
agglomerate. By"dry iron ore agglomerate"is meant the total of all ingredients
used in the formation of the iron ore agglomerate except water. Preferably,
the
amount of alkali metal silicate is at least 0.02 wt%, and most preferably at
least
0.04 wt%, based on the total weight of dry iron ore agglomerate. It was found
that pellets prepared using a binder system comprising at least 0.04 wt% of
alkali metal silicate generally have a smooth surface and a higher resistance
to
abrasion, whereas pellets prepared using a binder system comprising less than
0.04 wt% of alkali metal silicate generally exhibit a rough surface, which can
lead to the generation of fines or debris during processing of the formed
pellets,
e. g. during transport of the pellets.
The alkali metal silicate usually is a sodium silicate, but other alkali metal
silicates can be used. Examples of sodium silicates are sodium metasilicate
and
the commercially available water glass. In the sodium silicates, the molar
ratio
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Na20:Si02 generally is in the range of 2:1 to 1:5, preferably in the range of
1:1 to
1:4. The amount of alkali metal silicate in the binder system generally is at
least 1
wt%, preferably at least 10 wt%, and most preferably at least 15 wt%, and
generally it is at most 99 wt%, preferably at most 85 wt%, and most preferably
at
most 75 wt%, based on the total weight of the binder system.
The alkali metal silicate preferably is well dispersed in the particles to be
agglomerated. The silicate can be added to the iron ore particles in the form
of a
dry powder, an aqueous suspension, an aqueous solution, etc. Preferably, the
alkali metal silicate is added in the form of an aqueous solution.
The binder in the binder system of the invention can be an inorganic binder or
an
organic binder, or a mixture thereof. Examples of inorganic binders are
bentonite
and hydrated lime. In the context of the present application alkali metal
silicate is
not considered to be an inorganic binder. Examples of organic binders are
polymers including:
(1) Water-soluble natural polymers, such as guar gum, starch, alginates,
pectins,
xanthan gum, dairy wastes, wood related products, lignin, and the like;
(2) Modified natural polymers such as guar derivatives (e.g. hydroxypropyl
guar,
carboxymethyl guar, carboxymethylhydroxypropyl guar), modified starch (e.g.
anionic starch, cationic starch), starch derivatives (e.g. dextrin), and
cellulose
derivatives, such as alkali metal salts of carboxymethyl cellulose,
hydroxyethyl
cellulose, hydroxypropyl cellulose, carboxymethylhydroxyethyl cellulose,
methyl
cellulose, lignin derivatives (e.g. carboxymethyl lignin), and the like.
The aforesaid polymers may be used alone or in various combinations of two or
more polymers.
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The binder system is free of synthetic polymers. Examples of synthetic
polymers
are polyacrylamides, such as partially hydrated polyacrylamides,
methacrylamide
and polymethacrylamide, polyacrylates and copolymers thereof, polyethylene
oxides, and the like.
A further aspect of the present invention is a process for producing iron ore
agglomerates comprising agglomerating fine iron ore particles in the presence
of a
binder system wherein the binder system comprises carboxymethyl cellulose or a
salt thereof and an alkali metal silicate. The use of the combination of
carboxymethyl cellulose and alkali metal silicate leads to agglomerates with
increased physical properties, such as cold compression strength, preheat
strength, and dry crush strength. In addition, the reducibility of the iron in
the
agglomerates generally is higher than is observed when a binder system
comprising an inorganic binder is used in the agglomeration process.
The invention further concerns a binder system comprising carboxymethyl
cellulose and an alkali metal silicate. The amount of alkali metal silicate in
the
binder system generally is at least 1 wt%, preferably at least 10 wt%, and
most
preferably at least 15 wt%, and generally it is at most 99 wt%, preferably at
most
85 wt%, and most preferably at most 75 wt%, based on the total weight of the
binder system.
The carboxymethyl cellulose or the salt thereof (both are referred to as
"CMC")
preferably is substantially water-soluble. Preferred salts of carboxymethyl
cellulose
are alkali metal salts of carboxymethyl cellulose. Of these alkali metal salts
the
sodium salt is preferred. The CMC used in the present invention generally has
a
degree of substitution (the average number of carboxymethyl ether groups per
repeating anhydroglucose chain unit of the cellulose molecule) of at least
0.4,
preferably at least 0.5, and most preferably at least 0.6, and at most 1.5,
more
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preferably at least 1.2, and most preferably at most 0.9. Generally, the
average
degree of polymerization of the cellulose furnish is at least 50, preferably
at least
250, and most preferably at least 400, and generally it is at most 8,000,
preferably
at most 7,000, and most preferably at most 6,000. It is more preferred to use
5 sodium carboxymethyl cellulose having a Brookfield viscosity in a 1%
aqueous
solution of more than 2,000 cps at 30 rpm, spindle #4. Still more preferred is
sodium carboxymethyl cellulose having a Brookfield viscosity in a 1% aqueous
solution of more than about 4,000 cps at 30 rpm, spindle #4.
A series, of commercially available binders containing sodium carboxymethyl
cellulose especially useful in the present invention is available from Akzo
Nobel,
under the trademark PeridurTM.
The manner in which the binder is added to the particulate material depends on
the
type of material being agglomerated, the type of binder being used, and the
desired results. For example, the binder may be added as a dry powder, an
aqueous suspension, an aqueous solution, an aqueous gel, an aqueous sol
(colloidal system), etc.
The amount of binder employed also varies with the results desired. For
example,
when an organic binder is used, the amount of binder may range from 0.0025 to
0.5 wt.%, based on the weight of the iron ore particles, with a preferred
range
being 0.005 to 0.2 wt.%. In the case of an inorganic binder, the amount of
binder
may range for example from 0.1 to 3 wt.%, based on the weight of the iron ore
particles.
The binder and the alkali metal silicate can be added to the iron ore
particles
together, one after the other, etc. This is not critical, so long as care is
taken to
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ensure that when the agglomeration takes place, the binder and the additive
are
present to perform.
The process of the invention is useful in agglomerating fine iron ore
particles. The
invention, however, is not limited to iron ores and is also useful in the
agglomeration of fine particles of other metal ores. This invention is
particularly well
adapted for the agglomeration of materials containing iron, including iron ore
deposits, ore tailings, cold and hot fines from a sinter process, iron oxides
from
dust collected in systems, or aqueous suspensions of iron ore concentrates
from
natural sources or recovered from various processes. Iron ore or any of a wide
variety of the following minerals may form a part of the material to be
agglomerated: taconite, magnetite, hematite, limonite, goethite, siderite,
franklinite,
pyrite, chalcopyrite, chromite, ilmenite, and the like.
The size of the material being agglomerated varies according to the desired
results. For example, when the particulate material being agglomerated is iron
ore,
100% of the particles may be less than 80 mesh, preferably, 90% are less than
200
mesh, and most preferably, 75% are less than 325 mesh.
It is also envisaged to use conventional additives, for instance a base such
as
sodium hydroxide, soda, or other additives such as sodium citrate, sodium
oxalate,
etc. These additives, their purpose, and their use are known to the skilled
person.
Many processes for the agglomeration of particles, especially metal-based
particles, are known in the art. Examples of such processes are pelletization,
briquetting, sintering, etc. The binder system used in accordance with the
invention
is particularly suitable for pelletization. In the mining industry it is
common practice
to agglomerate or pelletize finely ground beneficiated mineral ore concentrate
to
facilitate processing and handling/shipping of the ore. After the mineral ore
has
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been mined, it is frequently wet ground to liberate and separate unwanted
gangue
minerals from the desired material, e.g. iron in the case of iron ore. The
processed
wet ground ore is screened to remove large particles, which can be recycled
for
further grinding. The screened fines are then vacuum filtered to reduce the
moisture content to an acceptable range for pelletization. The filtered
mineral ore is
known in the art as "concentrate". A second process involves "dry grinding"
and
beneficiation of the mineral ore, in which case the moisture required for
pelletization is added afterwards.
After beneficiation, a binding agent is added to the wetted mineral ore
concentrate
and the binder/mineral ore composite is conveyed to a balling drum or other
means
for pelletizing the ore. The binding agent serves to hold or bind the mineral
ore
together, so that the individual agglomerates can be transported without
losing
their integrity en route to further processing and induration.
Following the balling drum operation, the pellets are formed, but they are
still wet.
These wet pellets are commonly referred to as "green pellets" or "green
balls".
These green pellets are thereafter transported to a kiln and heated in stages
to an
end temperature of about 1,300-1,350 C. In the pelletizing process, the wet
green
pellets are loaded into the furnace for further processing. The moisture in
the
pellets is removed by induration at temperatures normally between 400-600 C.
Following drying in the furnace, the pellets are transported to the preheat
zone.
This is an additional heating stage to further increase the pellets' hardness
before
they are transported to the kiln and/or final firing stage. Heating generally
occurs at
900-1,200 C to bind the pellets together (e.g. to oxidize magnetite or
crystallize
hematite). From the preheat zone, the pellets are dropped 10-15 feet from the
grate to the kiln. This is where the preheat strength is needed to prevent the
pellets
from chipping and breaking apart into dust particles. Finally, the preheated
pellets
are fired at a temperature of between 1,300 and 1,350 C.
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The ability of the pellets to withstand breakage throughout processing can be
approximated by performing standard tests that measure the strength the
pellets
will need at each stage of processing. (e.g. wet crush strength, dry crush
strength,
preheat strength, and cold compressive strength).
The present invention is illustrated in the following Examples.
Examples
In the following Examples green pellets of iron ore comprising various
compounds
in the amounts indicated in Table 1 were prepared. The green pellets were
prepared by agglomerating iron ore concentrate in the presence of a binder and
a
binder additive. The amounts of binder and/or sodium silicate (in percent by
weight) shown in Table 1 are based on the total weight of the iron ore
concentrate.
The iron ore concentrate employed in the Examples of Table 1 was Brazilian
hematite ore. The binder is Peridur 330 (ex Akzo Nobel), which comprises
sodium
carboxymethyl cellulose and sodium carbonate, and the sodium silicate
(Na20:Si02 is 1:3.3) used in these experiments is supplied by PQ Corporation.
The process of making agglomerates are generally known to the skilled person.
The process is described in detail in US 6,071,325, which discloses a process
of
making agglomerates of 2,500 grams in a rotating airplane tire (approximately
40
cm diameter).
First the binder was mixed into the dry concentrate and homogenized. Then, the
alkali metal silicate was mixed with the required amount of water (moisture
content
between 8 and 9 wt%) and subsequently thoroughly mixed with the concentrate
and the binder (using a Mullen Mixer model No. 1 Cincinnati Muller,
manufactured
by National Engineering Co. or the like). Pellet "seeds" were formed by
placing a
small portion of the concentrate in the rotating tire and adding atomized
water to
initiate pellet growth. Seed pellets with a size between 3.5 and 4 mm were
retained
and kept apart for the formation of pellets with the desired size of 11.2 and
12.5
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mm. Finished green pellets were produced by placing 165 grams of seed pellets
described above in the rotating tire and adding a portion of the remaining
concentrate mixture over a 3-minute growth period. Atomized water was added if
necessary.
Table 1
Comparative Peridurlm. Sodium silicate
Example (wt%) (wt%)
1 0.03
2 0.03 0.20
Example .
1 0.03 0.03
2 0.03 0.05
3 0.03 0.06
4 0.03 0.08
The moisture content, the drop number, and the wet and dry compressive
strengths of the obtained green pellets were measured.
Wet drop number
The Wet drop number was determined by repeatedly dropping a green pellet
having a size between 11.2 and 12.5 mm from a height of 46 cm onto a
horizontally placed steel plate until a visible crack formed in the pellet
surface. The
number of times the pellet was dropped up to the point of fracture/cracking
was
determined. The average number of times averaged over 20 green pellets is
referred to as the 'Wet drop number".
Wet compressive strength
20 wet green pellets having a size of between 11.2 and 12.5 mm were stored in
an
airtight container. One by one the pellets were removed and placed in a
standard
measuring device in which a plunger of a scale was lowered onto the green
pellet
at a speed of 25 mm per minute. The maximum applied force at which the pellet
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cracked was determined. The average force averaged over 20 green pellets is
referred to as the Wet compressive strength.
Deformation
5 A minimum of 20 wet green pellets having a size of between 11.2 and 12.5
mm
were stored in an airtight container. One by one the pellets were removed and
placed in a standard measuring device in which a plunger of a scale was
lowered
onto the green pellet at a loading rate of 25 mm per minute. The machine
(Model
Lloyd Texture Analyser TA-Plus, controlled by PC with Nexygen version 4.5
10 software) is equipped with a 50 N load-cell and has a probe diameter of
10 mm.
The deformation/deflection of the green pellet is recorded while increasing
the
force. The deformation is defined as the change in diameter of the green
pellet at a
force of 1 N, provided that the pellet is not ruptured at this point.
Dry compressive strength
green pellets having a size of between 11.2 and 12.5 mm were dried in an oven
at 105 C for a minimum of two hours. Following drying, the dried pellets were
placed one by one in a standard measuring device in which a plunger of a scale
was lowered onto the green pellet at a speed of 25 mm per 10 seconds. The
20 maximum applied force at which the pellet cracked was determined. The
average
force averaged over 20 green pellets is referred to as the Dry compressive
strength.
The values obtained for the above parameters are tabulated in the Table below.
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Table 2
Comparative Moisture Wet drop Deformation
Appearance Dry
Example content number (mm) green pellet compressive
(%) strength
(kg/pellet)
1 8.6 2.7 0.22 rough, non- 1.0
sticking
2 8.6 8.3 0.28 smooth, 2.0
sticky
Example
1 8.2 3.1 0.20 rough, non- 1.2
sticking
2 8.3 4.1 0.24 smooth, non- 1.9
sticking
3 8.3 5.6 0.23 smooth, non- 2.9
sticking
4 8.1 4.9 0.24 smooth, non- 3.3
sticking
From the above Table it can be deduced that the pellets of Examples 1-4, which
are in accordance with the invention, show an increased dry compressive
strength
compared to the pellets obtained using a binder system comprising only the
Peridur binder (Comparative Example 1). At the same time the pellets of
Examples
1-4 show an improvement in wet drop number and only a slight increase in
deformation, whereas the pellets of Comparative Example 2 reveal a
significantly
higher deformation and wet drop number. Consequently, the pellets of
Comparative Example 2 will be deformed in the steel making process to a much
higher extent than the pellets of the invention, rendering the process for
preparing
fired pellets less efficient compared to processes using the pellets of the
invention.
It is further noted that the appearance of the green pellets of Examples 2-4
is
smooth and non-sticking, whereas the green pellets of Comparative Example 1
are
rough. The pellets of Examples 2-4 will generate a lower amount of fines or
debris,
e.g. during transport of these pellets, compared to the pellets of Comparative
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Example 1. Although the green pellets of Comparative Example 2 are smooth,
they
are sticky, causing undesirable clustering of the pellets during processing.
