Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IRON ORE BRIQUETTING
The present invention is concerned with the
production of iron ore briquettes suitable for transport
and use in iron making processes.
Methods of agglomerating iron ores have been in
development since the late 1800's. However, of all the
available processes only the pelletising and sintering
processes are now of significance, but these suffer from
certain disadvantages.
Pelletising consists of two distinct operations;
forming pellets from moist ore fines and then firing them
at a temperature in the region of 1300°C. It is critical
in order to prepare suitable pellets that the ore be
ground very fine, generally to a size where in the order
of 60% of the ore passes 45 ~.~.m. It is then formed into
pellets in either a horizontal drum or an inclined disc,
generally with the addition of a suitable binder. The
formed pellets are then fired in a process sometimes
referred to as induration in shaft kilns, horizontal
travelling grates, or a combination of travelling grates
and rotary kilns. Pelletising is a practicable and
commercially attractive method of agglomerating fine
concentrates, but requires substantial grinding in order
to achieve the required particle sizing which is an energy
intensive process. Pellets made from goethite-hematite
ores require extended induration times, affecting p~'ocess
economics. Solid fuel, in the form of coke, is often added
to reduce induration time which results in the production
of noxious emissions (including dioxins, NOX and SOX).
Sintering consists of granulating moist iron ore
fines and other fine materials with solid fuel, normally
coke breeze, and loading the granulated mixture onto a
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permeable travelling grate. Air is drawn downwards
through the grate as the temperature is raised. After a
short ignition period, external heating of the bed is
discontinued and as the solid fuel in the bed burns a
narrow combustion zone moves downwards through the bed,
each layer a.n turn being heated to approximately 1300°C.
Bonding takes place between the grains during combustion,
and a strong agglomerate is formed. However, traditional
sintering processes result in high levels of noxious
emissions, particularly sulfur oxides and dioxins, and
therefore the process is undesirable and unsustainable on
environmental grounds.
Briquetting is a process in which there was
commercial interest in the late 1800's and early 1900's,
but production of iron ore briquettes for use as a blast
furnace feed material never reached any significant
levels, decreased after 1950, and had ceased by about
1960. The process as practised involved the pressing of
ore fines into a block of some suitable size and shape,
and then indurating the block. A wide range of binders
such as tar and pitch and/or other additives such as
organic products, sodium silicate, ferrous sulfate,
magnesium chloride, limestone and cement were tested.
However, the earliest briquetting process, the Grondal
process, simply involved mixing iron ore with water and
pressing into oblong blocks the size of building bricks.
These were then hardened by passing them through a tunnel
kiln heated to 1350°C.
While developments in briquetting processes have
been generally directed towards the development of
suitable binders, JP 60-243232 describes briquettes that
have a flat shape in order to provide for stable
distribution in a blast furnace. Specifically, the
Japanese specification discloses that the flat-shaped
briquettes are much more easily reduced at higher
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temperatures than conventional spherical pellets. The
briquettes are made with a volume between 2 and 30cc in
order to balance a relatively high compression strength
against an inferior rotary or tumble strength and impact
resistance with increasing size. The Japanese
specification discloses that larger briquettes are less
easily reduced in a blast furnace. However, aside from
the size and shape of the briquettes there is no other
factor described as critical, and, indeed, there is no
detailed description of any other aspect of the production
of the briquettes.
The applicant has carried out extensive research
work into the production of briquettes from iron ore and
has invented a method that can produce briquettes that
have suitable properties for use in blast furnaces and
other direct reduction vessels.
One of the significant issues that the applicant
has addressed in the research work is that a commercially
viable iron ore briquette plant must be able to process a
substantial throughput of material. In order to do this,
the applicant believes that briquette presses would have
to be able to process of the order of 70-100 tonnes of
iron ore per hour per press. The applicant found in the
research work that it was possible to operate briquette
presses at surprisingly low roll pressures and produce
green briquettes having sufficient green strength to
withstand subsequent handling. This was a surprising
finding because information provided by briquette press
manufacturers indicated that considerably higher roll
pressures than those found by the applicant to be suitable
pressures would be required. The finding that low roll
pressure operation is possible is significant because low
pressure operation makes it possible to use wider presses
and thereby have higher production rates on the presses.
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The present invention is concerned with the
selection of briquette forming parameters.
According to the present invention there is
provided a method of producing an iron ore briquette that
is suitable for use as a blast furnace or other direct
reduction furnace feedstock which includes the steps of:
(a) mixing ore and a flux to form an ore/flux
mixture;
(b) pressing the ore/flux mixture into a green
briquette using a low roll pressure; and
(c) indurating the green briquette to form a fired
briquette.
Low pressure operation for iron ore briquetting
described in step (b) above is significant and makes it
possible to achieve high production rates by the use of
wide rolls on the briquetting machine up to 1.6m in
length.
Preferably the low roll pressure is generated by
a roll pressure force that is sufficient to produce
briquettes having a green compressive strength of at least
2kgf .
Preferably the green compressive strength is at
least 4 kgf.
More preferably the green compressive strength is
at least 5kgf.
More preferably the green compressive strength is
5-30kgf .
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More preferably the green compressive strength is
15-30kgf.
Preferably the low roll pressure is generated by
a roll pressing force of 10-140 kN/cm on the mixture of
ore/flux.
More preferably the roll pressing force is 10-
60kN/em.
More preferably the roll pressing force is 10-
40kN/cm.
Preferably step (a) includes mixing ore having a
predetermined particle size distribution of ore particles
and flux particles.
The predetermined particle size distribution of
ore particles that is mixed with flux in step (a) can be
produced without grinding ore.
Preferably the method includes crushing and
screening ore to form the predetermined particle size
distribution that is mixed with flux in step (a).
Preferably the top size of the predetermined
particle size distribution of ore that is mixed with flux
in step (a) ,is 4.0 mm or less.
More preferably the top size is 3.5 mm or less.
More preferably the top size is 3.0 mm or less.
More preferably the top size is 2.5 mm or less.
More preferably the top size is 1.5 mm or less.
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More preferably the top size is 1.0 mm or less.
Preferably the predetermined particle size
distribution of ore that is mixed with flux in step (a)
includes less than 50% passing a 45 E,~m screen.
More preferably the particle size distribution
includes less than 30 % passing the 45E,~m screen.
More preferably the particle size distribution
includes less than 10% passing the 45Etm screen.
Preferably the ore is a hydrated iron ore.
Preferably the hydrated ore is a goethite-
containing ore.
Preferably the flux has a particle size
distribution that is predominantly less than 100 Etm.
Preferably the particle size distribution of the
flux includes more than 95% passing a 250 gum screen.
Preferably the flux is limestone.
Preferably the ore/flux mixture produced in step
(a) is selected so that the basicity of the fired
briquette is greater than 0.2.
More preferably the basicity is greater than 0.6.
The term "basicity" is understood herein to mean
(%Ca0 + %Mg0) / (%Si02 + %A1203) of the fired briquette.
Preferably there is no binder in the ore/flux
mixture.
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Preferably the method includes adjusting the
water content of the ore prior to or during mixing step
(a) to optimise briquette quality and product yield.
Preferably the step of adjusting the water
content of the ore includes adjusting the water content so
that the moisture content of the ore/flux mixture is 2-12%
by weight of the total weight of the ore/flux mixture.
The term "total weight of the ore/flux mixture"
means the total of the (a) dry weight of the ore/flux mix,
(b) the weight of the inherent moisture of the mixture,
and (c) the weight of the moisture (if any) added to the
mixture in the method.
The term "moisture content" is the total of (b)
and (c) above.
Preferably the step of adjusting the water
content of the ore includes adjusting the water content so
that the moisture content of the ore/flux mixture is 2-5%
by weight of the total weight of the ore/flux mixture for
ores that are dense hematite ores.
Preferably step (b) includes adjusting the water
content of the ore so that the moisture content of the
ore/flux mixture is 4-8% by weight of the total weight of
the ore/flux mixture for ores containing up to 50%
geothite.
Preferably step (b) includes adjusting the water
content of the ore so that the moisture content of the
ore/flux mixture is 6-12%by weight of the total weight of
the ore/flux mixture for ores that are predominantly, ie
contain more than 50%, goethite ores.
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Preferably pressing step (c) produces briquettes
that are 10 cc or less in volume.
More preferably pressing step (c) produces
briquettes that are 8.5cc or less in volume.
More preferably pressing step (b) produces
briquettes that are 6.5 cc or less in volume.
Preferably indurating step (c) includes heating
the briquette to a firing temperature with 40 minutes.
Preferably indurating step (d) includes heating
the briquette to a firing temperature within 35 minutes.
More preferably indurating step (d) includes
heating the briquette to the firing temperature within 30
minutes.
More preferably step (c) includes heating the
briquette to the firing temperature within 20 minutes.
More preferably step (c) includes heating the
briquette to the firing temperature within 15 minutes.
Preferably the firing temperature is at least
1200°C.
More preferably the firing temperature is at
least 1260°C.
More preferably the firing temperature is at
least 1320°C.
More preferably the firing temperature is at
least 1350°C.
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More preferably the firing temperature is at
least 1380°C.
Preferably the fired briquette has a crush
strength of at least 200kgf.
Preferably the fired briquette has a crush
strength of at least 250kgf.
Iron ore fines are broadly characterised into
four groups on the basis of petrological characteristics,
such as mineralogy, mineral association and particle
texture, porosity, size distribution and chemistry. The
groups are:
Iron ore fines are broadly characterised into
four groups on the basis of petrological characteristics,
such as mineralogy, mineral association and particle
texture, porosity, size distribution and chemistry. The
groups are:
(a) HC - Dense hematite/magnetite ores;
(b) GC - Ores containing up to 50% goethite; and;
(c) G - Ores containing predominantly goethite, ie
greater than 50% goethite, such as pisolites,
detritals, and channel iron deposits.
The following pages of the specification refer
to two particular sub-groups of GC ores, namely:
(i) HG - goethite-containing ores that are
dominated by hematite; and
(,ii) GH - ores with approximately equal
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amounts of hematite and goethite.
While not wishing to be bound by theory, it is
believed that the bonding mechanism in green briquettes
involves a combination of bonds including the mechanical
interlocking of particles, van der Waal's forces, and in
the case of raw material types GC and G, hydrogen bonding
to varying degrees is dependent on the percentage of
hydrated iron species present, e.g. goethite. Several
characteristics of the feed material have been identified
as having a significant influence on the formation of such
bonds that affect the quality and processing performance
of the green and fired briquettes. These characteristics
are the moisture level of the feed material and its flow
characteristics, the chemical composition of the ore, its
size distribution and petrological characteristics and
porosity.
Preferably the feed materials are of the widest
size distribution possible in order to achieve a high
packing density and increased bonding of the ore
particles. As noted above, the bonding mechanism of green
briquettes is believed to be through a combination of
bonds arising from the mechanical interlocking of
particles, van der Waal's forces, and hydrogen bonding in
the cases of raw material types GC and G. Although a
broad size distribution increases the packing density and
improves the strength of the green briquette, it is
possible to briquette closely sized iron ores.
The top size of the particles is determined by
the crushing process but is preferably less than 2.5 mm in
order to produce briquettes of acceptable fired properties
following the induration process. Generally, ore types HC
and HG can be briquetted with coarser top sizes due to the
lower heat requirements of these raw materials to attain
acceptable fired strength. The top size of the raw
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material can be reduced through either crushing or
screening processes. The bottom size of the particles has
no absolute limit, but it is not necessary or desirable,
to grind the ore into very fine particles (as required for
pelletising) as this is an additional economic burden
rendered unnecessary by the present invention. Preferably
less than 10% of the particles pass a 45 Eun sieve.
Advantageously the pocket dimensions of the
briquetting apparatus should be selected on the basis of
the maximum particle size to be briquetted, as well as for
adequate induration performance, to ensure that
satisfactory briquetting can be achieved. Typically the
maximum particle size to achieve satisfactory briquetting
is 25-30% of the minimum pocket dimension. If the maximum
particle size exceeds this specification it may be
necessary to select a larger pocket size.
It is desirable to control feed moisture in order
to optimise green briquette quality and product yield.
Moisture addition should not exceed the level at which
liquid bridging becomes a significant form of inter-
particle bonding. This results in both decreased green
strength and adversely affects thermal stability.
Insufficient moisture can lead to overpressurisation in
the briquette pressing step and adversely affect green
briquette quality and yield.
Depending on the feed characteristics of the ore
to be processed, a moisture content of between 2 and 12 wt
for the feed material is used to optimise green
briquette quality and product yield. Dense hematite
concentrates (HC) have low optimum briquetting moistures,
generally in the range of 2-5 wt %. These concentrates
are often made up of closely sized particles with a smooth
surface texture that generates low strength briquettes
because of decreased interlocking of particles. More
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porous goethite-containing ores with up to 50% goethite
(GC) briquette well in the range of 4-8 wt % moisture and
more porous predominately goethite ores (G) briquette well
,in the range of 6-12 wt% moisture. Such ores have a rough
surface texture and shape enhancing their briquetting
characteristics.
Conventional briquetting apparatus may be used in
the method of the invention. In essence, such apparatus
includes two adjacent rolls with pockets which come
together at a nip zone in order to compress the feed
material into adjacent, aligned pockets to produce
briquettes. In the case of the present invention, the
rolls are preferably horizontally aligned to achieve the
required throughput for economic feasibility.
Although briquetting can be carried out over a
wide range of roll pressures depending on the application,
briquetting of iron ores is preferably conducted at roll
pressing forces of 10-140 kN/cm and more preferably at the
low end of this range, typically from 10-60 kN/cm. As is
indicated above, such low pressure operation for iron ore
briquetting is significant and makes it possible to
achieve high production rates by the use of wide rolls on
the briquetting machine up to 1.6m in length.
Preferably the roll pressure is carefully
controlled within the low pressure range in order to
optimise the briquetting operation. If the roll pressure
is too low, the rolls are forced apart producing a thick
web and distorted briquettes impairing the product yield
and the quality of the briquette, particularly after
induration. If the roll pressure exceeds the optimum,
poor closure of the briquettes occurs because of the
"clamshell" effect on release of the briquettes from the
pocket. The clamshell effect is more pronounced for small
roll diameters and excess roll pressures, which also cause
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pocket binding/jamming. Although the density and crush
strength of the green briquettes will be increased, the
impact resistance of the fired briquettes will be severely
impaired.
Preferably the moisture level is selected to
influence the flow characteristics of the material through
the feed system, and moisture levels of 2-12 wt % for the
feed material are generally suitable. If the moisture
level is too high for the feed system, the feed pressure
is adversely affected resulting in a decreased yield and
some impairment of briquette quality, characterised by a
lower green strength. It the feed material is too low in
moisture for the feed system the resultant feed pressure
will cause clamshelling which may result in decreased
yields, increased wear rates of the roll pockets, and
inferior fired properties.
The briquetting apparatus may be operated with a
pre-compactor feed system or with a gravity feed system.
The latter system is advantageous where high tonnages are
to be briquetted, as in the iron ore industry.
With regard to briquetting presses, a roll
diameter is selected in order to ensure that briquette
quality is obtained at an economic production rate. Large
diameter rolls increase production rates, however they
also increase the area of the nip zone. Careful control
of the nip zone facilitates formation of quality green
briquettes and avoids formation of briquettes with an
excessively thick web. Alterations in roll diameter may
also alter the optimum moisture level for feed material
where increased roll diameters represent increases in feed
moisture. Roll diameters typically vary from 250 mm - 1200
mm. In order to maximise production, preferably the rolls
are operated at the fastest speed possible whilst
maintaining briquette quality. However, a very low roll
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speed may be used if productivity is of a secondary
concern.
Typically, roll speeds in the range of 1 rpm to
20 rpm are employed. It is desirable in order to maintain
quality, particularly at high roll speeds, that the feed
material be presented to the rolls at a rate that matches
the briquette production rate and with a nip zone area
that produces the forces required to form quality
briquettes.
Any suitable roll width may be selected provided
that it is within the pressure capabilities of the
briquetting machine. As briquetting of iron ores is a low
pressure operation, wide rolls are preferred, increasing
the capacity of the machine. The rolls are preferably
horizontally aligned to allow for use with a gravity feed
system. The flow characteristics of iron ores, whether
HC, GC (including HG and GH), or G, are suitable for
gravity feeding at the moisture ranges specified above for
each classification.
The pocket shape should not generally be of a
sharp angular nature, but be more smooth and rounded to
improve handling characteristics. By way of example, a
length/width and width/depth ratio of approximately 0.65
is suitable. Pocket shapes also have specific release
angles, 110-120° that combat the tendency for sticking in
the pockets.
The pocket size can be optimised according to the
requirements for the induration process and the raw
material top size and the iron making blast furnace.
Typically the briquettes have a volume of between 2 and 30
cc. Preferably the volume is 10 cc or less. More
preferably the volume is 8.5 cc or less. More preferably
the volume is less than 6.5 cc.
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A staggered pocket configuration is preferred as
this makes the optimum use of the available space on the
face of the rolls, and hence maximises throughput.
Preferably the induration method and conditions
are selected having regard to the complex relationship
between raw material characteristics and the influence of
the briquette dimensions.
Consideration of the relationship between
briquette volume, shape and the petrological
characteristics of the raw material is required. The
chemical composition of the feed material will have a
significant influence on the properties of the fired
briquette. Apart from moisture. the feed material
includes the iron ore made up of iron oxide and gangue
minerals, with the required flux added to give the
required basicity level in the fired briquette. Test
results have shown that the flux should preferably be
finely sized, typically >95% passing 250 ~Cm, in order to
achieve the required properties in the fired briquette.
While not wishing to be bound by theory, it is
believed that the bonding mechanism for fired briquettes
involves diffusion bonding and re-crystallisation of the
iron oxide particles as well as slag bonding at higher
flux levels. Therefore, flux level and firing temperature
and, to a certain extent, firing time have a strong
influence on briquette properties. Elevated basicity
levels may improve reduced strengths as well as indurated
strengths as higher flux levels encourage the formation of
bonding phases which resist deformation under reducing
conditions.
Induration may be carried out using a straight
grate, grate-kiln or a continuous kiln type process.
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It has been found that green briquettes produced
under optimised conditions are thermally very stable
compared to pellets prepared from the same material. The
feed ore for pelletising must be ground to a fine size,
typically up to 60% passing 45 ~.m, and the pellets dried
slowly at low temperatures, typically <200°C to avoid
spalling. In contrast, as indicated above, the feed ore
for the present invention that can be indurated
successfully can be much coarser, with top sizes
preferably up to 2.5 mm, and hence does not need grinding
to the same extent as is required to produce pellets.
This characteristic represents major capital cost
reductions for briquetting operations over traditional
pellet production plants.
An important characteristic of the briquette of
the present invention is an ability to withstand high
temperatures on heating at fast rates, such as heating to
a firing temperature within 30 minutes, more preferably
within 20 minutes. This is a.n direct contrast with
conventional understanding of how goethitic ores respond
in induration situations, where is has been shown that
they spall when heated too fast through the
dehydroxylation and free water removal zones.
As is indicated above, the thermal stability of
the briquettes of the present invention has been found to
be much greater than pellets and they may be heated at
much faster rates than pellets without spalling. This
allows a much shorter heating cycle. Consequently,
briquette productivity can be significantly higher than
for pellets using the same material. For instance,
briquette productivities potentially in the order of
30t/m2.day in a straight grate kiln can be achieved,
compared to pellet productivities of 16t/ma.day for HG ores
in the same kiln.
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It will be clearly understood that, although
prior art publications are referred to herein, this
reference does not constitute an admission that any of
these documents form part of the common general knowledge
in the art, in Australia or in any other country.
Preferred embodiments of the present invention
will now be described, by way of example only, with
reference to the accompanying drawings, in which:
FIG. 1 is a schematic illustration of suitable
apparatus with 250 mm diameter rolls and a precompacted
feed system for conducting the process of the present
invention;
FIG. 2 is a schematic illustration of suitable
apparatus with 450 mm diameter rolls and a gravity feed
system for conducting the process of the present
invention;
FIG. 3 is a schematic illustration of suitable
apparatus with 650 mm diameter rolls and a gravity feed
system for conducting the process of the present
invention;
FIG. 4 is a plot of yield of whole briquettes
versus feed moisture for HG material on 450 mm rolls with
6 cc almond forms and 4 cc elongate almond pockets;
FIG. 5 is a plot showing the effect of feed
moisture on green briquette strength for HG material on
450 mm rolls with varying pocket dimensions;
FIG. 6 is a plot showing the effect of feed
moisture on green briquette strength for HG material using
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650 mm rolls and 7.5 cc 'pillows';
FIG. 7 shows the effect of roll pressing force on
briquette properties; thickness, green strength and green
density on 450 mm rolls and 9 cc almond forms;
FIG. 8 is a plot showing the effects of roll
pressing on green strength for HG material using 650 mm
rolls and 7.5 cc 'pillows';
FIG. 9 is a plot showing the effect of roll
pressing force on green strength for GH material using 650
mm rolls and 7.5 cc 'pillows';
FIG. 10 shows the effect of roll speed on
briquette properties; thickness, green strength and green
density for a rolls pressure of 90kg/cm2 and a feed
moisture of 6 wt % using 450 mm rolls and 9 cc almond
forms;
FIG. 11 is the operating window for a briquetting
machine with a pre-compactor, 250 mm rolls, 4 cc almond
forms and HG material;
FIG. 12 shows temperature profiles for briquette
induration in a 500 mm deep bed;
FIG. 13 shows temperature profiles for briquette
induration that produced briquettes at high productivities
and a typical temperature profile for pellet induration
that produced pellets at a lower productivity;
FIG. 14 is a plot showing the effect of average
bed temperature on briquettes made with GH material using
650 mm rolls and 7.5 cc 'pillows' at the end of a grate
cycle in a~batch grate kiln;
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FIG. 15 is a plot showing the effect of average
bed temperature on briquettes made with GH material using
650 mm rolls and 7.5 cc 'pillows' at the end of a grate-
kiln firing cycle in a batch grate kiln;
FIG. 16 is a plot showing the effect of time at
firing temperature (1380°C) on briquettes made with GH
material using 650 mm rolls and 7.5 cc 'pillows' during a
test cycle in the batch grate kiln;
FIG. 17 is a plot showing the effect of time at
firing temperature (1380°C) on briquettes made with GH
material using 650 mm rolls and 7.5 cc 'pillows' during a
test cycle in the batch grate kiln;
FIG. 18 is a plot showing the effects of
residence time on 7.5 cc GH briquettes in the kiln during
a test cycle in the kiln only.
FIG. 19 is a plot showing the effect of bed
height and grate firing profile on briquettes made with GH
material using 650 mm rolls and 7.5 cc 'pillows' during a
test cycle in the batch grate kiln;
FIG. 20 is a plot showing the effect of bed
height arid grate firing profile on briquettes made with GH
material using 650 mm rolls and 7.5 cc 'pillows' during a
test cycle in the batch grate kiln;
FIG. 21 shows the effect of basicity and firing
temperature on the fired crush strength of briquettes made
with HG material, 250 mm rolls and 4 cc almond forms;
FIG. 22 shows the effect of basicity on the
briquette reduced properties; swell, crush strength after
reduction (CSAR) and reducibility index of briquettes made
with HG material, 250 mm rolls and 4 cc almond form;
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EXAMPLE 1
Briquetting was performed using three different
roll presses with varying roll diameter, width and feed
systems.
Initial testing was conducted using a Taiyo K-
102A double roll press, which has a nominal capacity of
300 kg/hr. This machine has 250 mm diameter rolls of 36
mm width and features a screw-type precompactor. A
schematic showing its main components can be seen in
Figure 1.
The briquettes produced were pillow-shaped with
nominal dimensions of 13x19x28 mm and a volume of 4 cc.
There was a single row of 30 pockets around the
circumference of each roll.
Of the two rolls, one was fixed whilst the other
"floating roll" was held against the fixed roll by an oil
and gas filled ram. The oil in the ram was pressurised to
provide the desired load force between the rolls.
Briquetting was also performed using a Komarek
BH400 double roll press, with a roll diameter of 450 mm
and a roll width of 75 mm. Feed material was gravity fed
into the nip zone from a feed hopper located above the
rolls. A schematic of its main components can be seen in
Figure 2.
Briquettes of varying dimensions were produced
with the following details:
(1) Nominally 17.5x28x34.3 mm with a volume of
8.9 cc. There was a double row of 48 pockets arranged in
staggered alignment around the circumference of each row
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( 9 cc Almond forms ) .
(2) Nominally 14.5x22.1x33.9 mm with a volume
of 6.3 cc. There was a double row of 60 pockets arranged
in a staggered alignment around the circumference of each
roll(6 cc Almond forms).
(3) Nominally 15.2x21.7x22.9 mm with a volume
of 3.9 cc. There was a triple row of 58 pockets arranged
in a staggered alignment around the circumference of each
row (4 cc spherical).
(4) Nominally 11.2x17.3x32.1 mm with a volume
of 3.9 cc. There was a double row of 72 pockets arranged
in a symmetrical alignment around the circumference of
each roll (4 cc elongate).
Of the two rolls, one was fixed whilst the other
"floating roll" was held against the fixed roll by an oil
and gas filled ram. The oil in the ram was pressurised to
provide the desired specific pressing force between the
rolls.
Briquetting was also conducted using a ICoppern
52/6.5 double roll press with a diameter of 650 mm and a
roll width of 130 mm. Feed material was gravity fed into
a nip zone from a hopper located above. Nip zone area was
controlled through use of a 'nip zone adjuster'. A
schematic of its main components can be seen in Figure 3.
The briquettes produced were 'pillow'shaped with
nominal dimensions of 30x24x16 mm and forms a volume of
7.5 cc. There were four rows of 77 pockets arranged
symmetrically across the face of the roll.
Of the two rolls, one was fixed whilst the other
"floating roll" was held against the fixed roll by an oil
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and gas filled ram. The oil in the ram was pressurised to
provide the desired specific pressing force between the
rolls.
EXAMPLE 2
The effect of feed moisture content was
investigated.
Figure 4 illustrates that feed moisture had a
significant effect on the yield of 6 cc and 4cc briquettes
produced by the briquetting press with 450 mm rolls as
described in Example 1. The feed material was gravity fed
to the rolls while the rolls operated at a fixed roll
speed of 20 rpm and a roll pressure of 90kg/cm2.
Feed moisture control is also important as
variation a.n moisture content affects green properties
such as green strength, abrasion resistance and shatter
strengths. This is illustrated in Figures 5 and 6.
Figure 5 shows the relationship between feed
moisture level and strength for briquettes made with HG
using the 450 mm rolls, a gravity feed system, and a
variety of pocket sizes.
Figure 6 shows the same relationship for
briquettes made with the 650 mm rolls and 7.5 cc pockets
for HG material.
Green strength tended to increase to a maximum
for the optimum moisture content of approximately 6%. At
moisture levels exceeding 7.5% the green strength was'
unacceptably low.
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Feed moisture had less of an influence on shatter
strength and the green abrasion resistance of the
briquettes.
EXAMPLE 3
As is indicated above, although briquetting
operations can be carried out over a wide range of rolls
pressures, it is preferred that briquetting be carried out
at low pressures. Such low pressure operation for iron
ore briquetting is significant and opens up the
possibility of achieving high production rates with wide
rolls on a briquetting machines.
However, as is indicated above, roll pressure
should be carefully controlled within this low pressure
range if the briquetting operation is to be optimised. If
roll pressure is too low and nip gone area is not
carefully controlled, the rolls are forced apart producing
a thick web and distorted briquettes impairing the product
yield and the quality of the briquette, particularly after
induration. If roll pressure exceeds the optimum, poor
closure of the briquettes occurs because of the
"clamshell" effect on release of the briquette from the
pocket. Although the density and crush strength of the
green briquette will be increased, the impact resistance
of the fired briquette will be severely impaired.
Figure 7 shows the effect of roll pressure on
briquette thickness and quality (measured in terms of
crush strength) for raw material HG produced in a gravity
fed machine with 450mm diameter rolls with nominal 9 cc
pockets. The figure shows that acceptable green strength
was obtained at roll pressures as low as 60 kg/cm2.
Figures 8 and 9 show the effect of pressing force
and resultant green strength that was obtained using the
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650 man diameter rolls. The work was carried out on HG and
GH raw material types and illustrates a similar
relationship between roll pressure and green strength as
with the 450 mm work. Specifically, the figures show that
acceptable green strengths were obtained at pressing
forces of 20 kN/cm.
Pressing force was also found to exert a
significant influence on the shatter strength and the
green abrasion resistance of the briquettes, with both
variables increasing in response to increased roll
pressure.
EXAMPLE 4
Roll speed was also investigated.
Roll speed, measured in rpm, was found to exert
an influence on the amount of pressure applied to feed
materials.
Increased roll speeds result in shorter residence
time in the nip zone of the rolls and hence lower pressure
is exerted for a longer period of time. Roll pressure can
be used primarily to control the amount of pressure
exerted on feed material and roll speed can be altered to
maximise the production rate. However, it is important to
consider the effects of roll speed on briquette thickness
and green strength when optimising the green briquetting
operation.
The affect of roll speed on briquette thickness
and quality (measured in terms of crush strength) for raw
material HG is shown in Figure 10 for a gravity fed
machine with 450 mm diameter rolls.
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The Figure shows that thickness and green
strength decreased as roll speed increased.
EXAMPLE 5
The process variables of the briquetting machine
as described a.n Example 1, ie, roll speed, precompactor
speed and roll pressure, and the briquette density were
used to determine an operating window for this particular
system of briquetting.
The diagram shown in Figure 11 is an example of
an operating window for briquetting with 250 mm rolls to
form nominally 4 cc briquettes out of HG material on the
Taiyo press.
To simplify the curves, roll pressure was fixed
at 150 kg/cmz and precompactor speed was fixed at 20 rpm.
A series of curves are shown for feed moisture from 4 wt
to 12 wt %. Each represents conditions that resulted in
the formation of whole briquettes.
To the right of the curves there is a region of
low feed pressure where pockets are not filled or the
briquettes are weak and split readily. To the left of the
curves there is a region where the pressure on the feed is
too high. Briquettes shear and pocket blockage occurred.
Across the strength range, below 6 kgf, the briquettes
were too weak to withstand pocket release and either
remain in the pockets or split on release. Above 30 kgf,
further compaction could not be achieved. The briquettes
were thick and began to 'clam shell'. The strength range
of 6 to 30 kgf defined the outer limits within which whole
briquettes could be formed with the sample material and
the Taiyo briquetting machine.
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To determine the operating window certain product
and quality parameters including yield, density, crush
strength and drop/shatter strength need to be considered.
Once these properties are taken into consideration, a
smaller region can be defined which is the operating
region of the briquetting process.
In Figure 11, this region occurs at rolls speeds
between 5 and 9 rpm and green strengths between 6 kgf and
18 kgf.
EXAMPLE 6
Green briquettes produced under optimised
conditions were found to be thermally very stable compared
to pellets formed from the same material. This is shown
in Figures 12 and 13.
Figure 12 shows the temperature profiles for the
inlet and outlet gas and three positions within the bed of
briquettes during laboratory-scale induration trials
simulating a straight grate process.
The bed temperatures were measured by
thermocouples placed at 100, 250 and 500 mm from the top
of the bed.
The briquettes were found to be be thermally
stable when heated at fast rates shown in the figures.
The excellent drying performance allowed the inlet gas
temperature to be raised from ambient to 1340°C in ten
minutes without spalling the briquettes.
Figure 13 shows the temperature profiles for
briquette induration that produced nominal 4 cc briquettes
of HG ore at productives of 32 t/m2. d and 25 t/m2 .d. The
figure also shows, by way of comparison, a typical
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induration temperature profile for pellets. The pellet
profile was an optimised profile so that pellet spalling
was minimised and fired properties were maximised. The
pellet profile produced pellets with a productivity of
16t/m2.d, which is considerably lower than the
productivities of the briquettes. The briquettes and the
pellets were made from the same ore type.
The high productivities for the briquettes was
due to the thermal stability of the green briquettes which
enabled the briquettes to be heated at fast rates.
The thermal stability of the briquettes was found
to be not exclusive to one induration method and to one
ore type.
EXAMPLE 7
A pilot scale grate-kiln system was used to
determine the properties of briquettes as they exited a
grate prior to entry to a kiln.
The equipment consisted of a pot grate and a
batch kiln. To simulate the travelling grate a LGP gas
burner was used to generate the flame temperature. The pot
grate was capable of up and down draught gas flow. The
temperature of the material was measured throughout the
bed using thermocouples set into and through the wall of
the pot. These measurements were assumed to be the
briquette temperature during the firing cycle. Due to the
size of the briquettes tested, it may be that the
temperature measurement shows the external briquette
temperatures and not the internal temperatures. The
temperature measured is most likely a mixture of briquette
outside temperature and gas temperature at that location
in the bed.
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Figure 14 shows how the temperature of the
briquettes made from GH material (d95 = lmm) with a green
nominal size of 7.5cc initially increased to a maximum at
approximately 300-400°C average bed temperature, and then
fell to a minimum temperature at 700°C. At higher
temperatures the strength then increased again. The
strength fell to a minimum value at 700°C, which is lower
than the green strength. This a.s a critical factor for
transport of the material from the grate-to the kiln. As
the strength was lowest at this temperature range, the
maximum amount of degradation could be expected if the
firing profile included transfer from the grate to the
kiln at this temperature.
For a straight grate process, the bed height
selected for the induration process was found to be not
critical and not inhibited by gas permeability generally
selected to avoid deformation of the briquettes at the
lower parts of the bed while achieving a reasonable
productivity. In addition, at briquette volumes exceeding
6 cc, permeability of the bed was not greatly compromised
by bed height. Consequently, the induration process is not
restricted by this variable as is the case with
pelletising operations. Green briquette bed depth can be
selected to optimise productivity without compromising
quality.
A grate-kiln process may offer certain advantages
in terms of producing a better fired product compared to
products obtained from other induration processes. It
also heats the briquettes more uniformly through high
temperature ranges in a way that reduces temperature
gradients within the briquette and avoids differential
shrinkage of the briquette that,may lead to cracking.
Also, as all the briquettes are subject to similar firing
temperatures and time in the rotating kiln, briquette
quality is more uniform compared to the straight grate
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process.
Possibilities also exist for the production of
briquettes suitable for direct reduction processes,
providing a raw material of a suitable grade is used.
EXAMPLE 8
Firing temperature was investigated.
Briquettes of GH material (d95 = 1mm) 7.5cc were
fired in the grate-kiln pilot rig, all using the same
firing profiles for the grate section. After transfer to
the kiln, the same profile was applied for firing, except
that the firing temperature reached was altered as shown.
The results are shown in Figure 15.
There is a clear indication in Figure 15 that to
achieve suitable fired strength in briquettes of this size
the firing temperature in the kiln should be at least
1380°C.
Figure 15 also shows that tumble strength (Tumble
Index - TI) and abrasion resistance (Abrasion Index - AI),
improved with firing temperature.
EXAMPLE 9
airing temperature and time at temperature were
investigated.
Briquettes made from GH material (d95 = 1mm) with
a nominal size of 7.5cc were fired in a series of grate-
kiln tests. The grate firing profile was the same, with
only the firing time in the kiln at the firing temperature
being changed from 6 to 9 minutes. The total firing time
in the kiln remained the same, the extra time for the
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firing was taken from the rate of heating in the kiln, so
that the 9 minutes firing time had a quicker heating rate
to 1380° compared to the 6 minutes firing time.
Tests were also conducted with 6.3cc GH
briquettes using the same firing profile as that used for
the 7.5cc case.
Results are illustrated in Figures 16 and 17.
For the nominally 7.5cc size GH briquettes, the
fired strength increased significantly from the longer
firing time in the kiln. This was due to greater heat
penetration of the briquettes during the firing cycle.
The fired properties for the 6.3 cc GH briquettes
were superior to those produced for the 7.5cc case,
inferring that the issue of heat penetration is a
significant issue for fired property generation of the
briquettes. This result also suggests that when heat
penetration in the briquettes is insufficient adequate
strength will not be generated in the fired product.
EXAMPLE 10
The effect of residence time in a grate kiln was
investigated.
Briquettes made from GH material (d95 = 1mm) arid
nominally 7.5cc were fired in a pilot scale batch grate
kiln. They were charged green into a kiln that had been
preheated to either 500 or 1000°C. Firing profiles were
imposed on the briquettes and the total residence time
reported. The results are shown in Figure 18.
Figure 18 shows that the fired properties
improved with increasing residence time, suggesting the
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importance of heating the product thoroughly to achieve
the final properties required.
The effect of rapid heating was not reduced by a
larger bed depth of the grate. This is shown in Figures
19 and 20. The green briquette bed was highly permeable
and did not restrict airflow, as often occurs with
pellets. The maximum bed depth useable has not been
defined, but is likely to be greater than 300mm. This far
exceeded that possible for even the best pellet beds in a
grate-kiln system.
EXAMPLE 11
The effect of the chemistry of briquettes was
investigated.
The effect of basicity and temperature on the
fired briquette properties made from HG material was
determined by firing the briquettes in the muffle furnace
at specific temperatures and times. The results are shown
in Figure 21.
Results for the chemical analyses of the fired
briquettes made at varying basicities produced fired
briquettes which varied in grade from 63.81% Fe at a
basicity of 1.2 up to 65.93% Fe for a basicity of 0.2,
reflecting the level of flux addition.
As can be seen in Figure 21, crush strength
increased with both temperature and as basicity increased
from 0.2 to 0.8. This effect becomes more significant as
the temperature increased across the range studied and it
was possible to achieve 300 kgf at 1295°C for 0.6 basicity
and at 1280°C for 0.8 basicity.
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The explanation for increased basicity levels
resulting in increased strengths is related to changes in
the bonding mechanism. At low basicity levels, bonding of
the particles occurs as a result of recrystallisation of
iron oxide and the formation of iron oxide-iron oxide
bonds. At increased basicity levels, melt formation
occurs at lower temperatures enhancing melting of iron
oxide crystals, and slag bonding becomes more significant
giving higher strengths for the same temperature.
EXAMPLE 12
Reduction testing, using whole briquettes and
standard reduction test methods JIS 8713/IS07215 was
carried out on HG briquettes that were fired at 1300°C for
10 min. The results of reducibility, swell and crush
strength after reduction (CSAR) are shown in Figure 22.
The reducibility index (RI) remained relatively
stable across the range of basicity levels. The RI varied
from 53.8% at a basicity of 0.20 to just over 62.2% at a
basicity of 1.00.
The swell index showed some response and varied
from 11% at the lowest basicity to 14.8% in the mid-
ranges, decreasing to zero at a basicity of 1.20.
The crush strength after reduction (CSAR) showed a large
response to changes in the basicity level, ranging from 22
kgf at 0.20 basicity to 121 kgf at 1.20 basicity. This
change in reduced strength reflects the fired crush
strength results and is again related to variation in the
bonding phases of the fired briquettes. The low basicity
briquettes were predominantly bonded by iron oxide-iron
oxide bonds, which degrade during reduction. At increased
basicity levels, slag bonding becomes more significant.
These bonds are more stable during reduction, accounting
for the higher reduced strengths and little or no swell at
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a basicity of 1.20. Slag bonding also becomes a more
important form of bonding in briquettes made from GH and G
where higher Si02 and A1z03 levels result in increased flux
additions. Such briquettes generally prove stronger after
reduction as the reduction process does not result in the
breakdown of non-ferrous bonding phases. High grade ores,
such as HC, which require low flux addition rely almost
solely on oxide-oxide bonding and hence have lower
strength after reduction values.
Many modifications may be made to the embodiments
of the present invention described above without departing
from the spirit and scope of the invention.
