U.S. patent application number 10/485895 was filed with the patent office on 2005-03-10 for iron ore briquetting.
Invention is credited to Gannon, John, Meakins, Ross Lawrence, Salter, Celeste Julienne, Vining, Keith Richard.
Application Number | 20050050996 10/485895 |
Document ID | / |
Family ID | 3830739 |
Filed Date | 2005-03-10 |
United States Patent
Application |
20050050996 |
Kind Code |
A1 |
Gannon, John ; et
al. |
March 10, 2005 |
Iron ore briquetting
Abstract
A method of producing an iron ore briquette that is suitable for
use as a blastfurnace or other direct reduction furnace feedstock
which includes the steps of: (1) mixing ore and a flux to form an
ore/flux mixture (2) pressing the ore/flux mixture into a green
briquette using a low roll pressure; and (3) indurating the green
briquette to form a fired briquette.
Inventors: |
Gannon, John; (Eastwood,
AU) ; Salter, Celeste Julienne; (Hazelbrook, AU)
; Vining, Keith Richard; (Toowong, AU) ; Meakins,
Ross Lawrence; (Bankstown, AU) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Family ID: |
3830739 |
Appl. No.: |
10/485895 |
Filed: |
October 8, 2004 |
PCT Filed: |
August 2, 2002 |
PCT NO: |
PCT/AU02/01032 |
Current U.S.
Class: |
75/751 |
Current CPC
Class: |
C22B 1/24 20130101; C22B
1/243 20130101; B30B 11/16 20130101; C22B 1/2413 20130101; C22B
1/248 20130101 |
Class at
Publication: |
075/751 |
International
Class: |
C22B 001/24 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 2, 2001 |
AU |
PR 6783 |
Claims
1. 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.
2. The method defined in claim 1 wherein 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 2
kgf.
3. The method defined in claim 2 wherein the green compressive
strength is at least 4 kgf.
4. The method defined in claim 2 wherein the green compressive
strength is at least 5 kgf.
5. The method defined in claim 2 wherein the green compressive
strength is 5-30 kgf.
6. The method defined in claim 2 wherein the green compressive
strength is 15-30 kgf.
7. The method defined in claim 1 wherein the low roll pressure is
generated by a roll pressing force of 10-140 kN/cm on the mixture
of ore/flux.
8. The method defined in claim 7 wherein the roll pressing force is
10-60 kN/cm.
9. The method defined in claim 7 wherein the roll pressing force is
10-40 kN/cm.
10. The method defined in claim 1 wherein step (a) includes mixing
ore having a predetermined particle size distribution of ore
particles and flux particles.
11. The method defined in claim 10 wherein the predetermined
particle size distribution of ore particles that is mixed with flux
in step (a) can be produced without grinding ore.
12. The method defined in claim 10 includes crushing and screening
ore to form the predetermined particle size distribution that is
mixed with flux in step (a).
13. The method defined in claim 10 wherein 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.
14. The method defined in claim 13 wherein the top size is 3.5 mm
or less.
15. The method defined in claim 13 wherein the top size is 3.0 mm
or less.
16. The method defined in claim 13 wherein the top size is 2.5 mm
or less.
17. The method defined in claim 13 wherein the top size is 1.5 mm
or less.
18. The method defined in claim 10 wherein the predetermined
particle size distribution of ore that is mixed with flux in step
(a) includes less than 50% passing a 45 .mu.m screen.
19. The method defined in claim 18 wherein the particle size
distribution includes less than 30% passing the 45 .mu.m
screen.
20. The method defined in claim 18 wherein the particle size
distribution includes less than 10% passing the 45 .mu.m
screen.
21. The method defined in claim 1 wherein the ore is a hydrated
iron ore.
22. The method defined in claim 21 wherein the hydrated ore is a
goethite-containing ore.
23. The method defined in claim 1 wherein the flux has a particle
size distribution that is predominantly less than 100 .mu.m.
24. The method defined in claim 23 wherein the particle size
distribution of the flux includes more than 95% passing a 250 .mu.m
screen.
25. The method defined in claim 1 wherein the ore/flux mixture
produced in step (a) is selected so that the basicity of the fired
briquette is greater than 0.2.
26. The method defined in claim 25 wherein preferably the basicity
is greater than 0.6.
27. The method defined in claim 1 wherein there is no binder in the
ore/flux mixture.
28. The method defined in claim 1 wherein the method includes
adjusting the water content of the ore prior to or during mixing
step (a) to optimise briquette quality and product yield.
29. The method defined in claim 28 wherein 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.
30. The method defined in claim 28 wherein 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.
31. The method defined in claim 28 wherein 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 4-8% by
weight of the total weight of the ore/flux mixture for ores
containing up to 50% geothite.
32. The method defined in claim 28 wherein 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 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.
33. The method defined in claim 1 wherein pressing step (c)
produces briquettes that are 10 cc or less in volume.
34. The method defined in claim 33 wherein pressing step (c)
produces briquettes that are 8.5 cc or less in volume.
35. The method defined in claim 33 wherein pressing step (b)
produces briquettes that are 6.5 cc or less in volume.
36. The method defined in claim 1 wherein indurating step (c)
includes heating the briquette to a firing temperature with 40
minutes.
37. The method defined in claim 36 wherein indurating step (d)
includes heating the briquette to a firing temperature within 35
minutes.
38. The method defined in claim 36 wherein indurating step (d)
includes heating the briquette to the firing temperature within 30
minutes.
39. The method defined in claim 36 wherein step (c) includes
heating the briquette to the firing temperature within 20
minutes.
40. The method defined in claim 36 wherein step (c) includes
heating the briquette to the firing temperature within 15
minutes.
41. The method defined in claim 36 wherein the firing temperature
is at least 1200.degree. C.
42. The method defined in claim 41 wherein the firing temperature
is at least 1260.degree. C.
43. The method defined in claim 41 wherein the firing temperature
is at least 1320.degree. C.
44. The method defined in claim 41 wherein the firing temperature
is at least 1350.degree. C.
45. The method defined in claim 41 wherein the firing temperature
is at least 1380.degree. C.
46. The method defined in claim 1 wherein the fired briquette has a
crush strength of at least 200 kgf.
47. The method defined in claim 46 wherein the fired briquette has
a crush strength of at least 250 kgf.
Description
[0001] The present invention is concerned with the production of
iron ore briquettes suitable for transport and use in iron making
processes.
[0002] 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.
[0003] Pelletising consists of two distinct operations; forming
pellets from moist ore fines and then firing them at a temperature
in the region of 1300.degree. 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 .mu.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 process 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,
NO.sub.x and SO.sub.x).
[0004] 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 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 in turn
being heated to approximately 1300.degree. 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.
[0005] 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.degree. C.
[0006] 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 temperatures than
conventional spherical pellets. The briquettes are made with a
volume between 2 and 30 cc 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.
[0007] 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.
[0008] 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.
[0009] The present invention is concerned with the selection of
briquette forming parameters.
[0010] 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:
[0011] (a) mixing ore and a flux to form an ore/flux mixture;
[0012] (b) pressing the ore/flux mixture into a green briquette
using a low roll pressure; and
[0013] (c) indurating the green briquette to form a fired
briquette.
[0014] 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 1.6 m in length.
[0015] 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 2 kgf.
[0016] Preferably the green compressive strength is at least 4
kgf.
[0017] More preferably the green compressive strength is at least 5
kgf.
[0018] More preferably the green compressive strength is 5-30
kgf.
[0019] More preferably the green compressive strength is 15-30
kgf.
[0020] Preferably the low roll pressure is generated by a roll
pressing force of 10-140 kN/cm on the mixture of ore/flux.
[0021] More preferably the roll pressing force is 10-60 kN/cm.
[0022] More preferably the roll pressing force is 10-40 kN/cm.
[0023] Preferably step (a) includes mixing ore having a
predetermined particle size distribution of ore particles and flux
particles.
[0024] The predetermined particle size distribution of ore
particles that is mixed with flux in step (a) can be produced
without grinding ore.
[0025] Preferably the method includes crushing and screening ore to
form the predetermined particle size distribution that is mixed
with flux in step (a).
[0026] 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.
[0027] More preferably the top size is 3.5 mm or less.
[0028] More preferably the top size is 3.0 mm or less.
[0029] More preferably the top size is 2.5 mm or less.
[0030] More preferably the top size is 1.5 mm or less.
[0031] More preferably the top size is 1.0 or less.
[0032] Preferably the predetermined particle size distribution of
ore that is mixed with flux in step (a) includes less than 50%
passing a 45 .mu.m screen.
[0033] More preferably the particle size distribution includes less
than 30% passing the 45 .mu.m screen.
[0034] More preferably the particle size distribution includes less
than 10% passing the 45 .mu.m screen.
[0035] Preferably the ore is a hydrated iron ore.
[0036] Preferably the hydrated ore is a goethite-containing
ore.
[0037] Preferably the flux has a particle size distribution that is
predominantly less than 100 .mu.m.
[0038] Preferably the particle size distribution of the flux
includes more than 95% passing a 250 .mu.m screen.
[0039] Preferably the flux is limestone.
[0040] Preferably the ore/flux mixture produced in step (a) is
selected so that the basicity of the fired briquette is greater
than 0.2.
[0041] More preferably the basicity is greater than 0.6.
[0042] The term "basicity" is understood herein to mean (% CaO+%
MgO)/(% SiO.sub.2+% Al.sub.2O.sub.3) of the fired briquette.
[0043] Preferably there is no binder in the ore/flux mixture.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] The term "moisture content" is the total of (b) and (c)
above.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] Preferably pressing step (c) produces briquettes that are 10
cc or less in volume.
[0052] More preferably pressing step (c) produces briquettes that
are 8.5 cc or less in volume.
[0053] More preferably pressing step (b) produces briquettes that
are 6.5 cc or less in volume.
[0054] Preferably indurating step (c) includes heating the
briquette to a firing temperature with 40 minutes.
[0055] Preferably indurating step (d) includes heating the
briquette to a firing temperature within 35 minutes.
[0056] More preferably indurating step (d) includes heating the
briquette to the firing temperature within 30 minutes.
[0057] More preferably step (c) includes heating the briquette to
the firing temperature within 20 minutes.
[0058] More preferably step (c) includes heating the briquette to
the firing temperature within 15 minutes.
[0059] Preferably the firing temperature is at least 1200.degree.
C.
[0060] More preferably the firing temperature is at least
1260.degree. C.
[0061] More preferably the firing temperature is at least
1320.degree. C.
[0062] More preferably the firing temperature is at least
1350.degree. C.
[0063] More preferably the firing temperature is at least
1380.degree. C.
[0064] Preferably the fired briquette has a crush strength of at
least 200 kgf.
[0065] Preferably the fired briquette has a crush strength of at
least 250 kgf.
[0066] 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:
[0067] 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:
[0068] (a) HC--Dense hematite/magnetite ores;
[0069] (b) GC--Ores containing up to 50% goethite; and;
[0070] (c) G--Ores containing predominantly goethite, ie greater
than 50% goethite, such as pisolites, detritals, and channel iron
deposits.
[0071] The following pages of the specification refer to two
particular sub-groups of GC ores, namely:
[0072] (i) HG--goethite-containing ores that are dominated by
hematite; and
[0073] (ii) GH--ores with approximately equal amounts of hematite
and goethite.
[0074] 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.
[0075] 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.
[0076] 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 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
.mu.m sieve.
[0077] 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.
[0078] 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.
[0079] 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 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.
[0080] 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.
[0081] 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 1.6 m in length.
[0082] 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 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.
[0083] 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.
[0084] 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.
[0085] 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 speed may be used if productivity is of a secondary
concern.
[0086] 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.
[0087] 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.
[0088] 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.degree. that combat the tendency
for sticking in the pockets.
[0089] 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.
[0090] 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.
[0091] 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.
[0092] 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 .mu.m, in order to achieve the
required properties in the fired briquette.
[0093] 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.
[0094] Induration may be carried out using a straight grate,
grate-kiln or a continuous kiln type process.
[0095] 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 .mu.m, and
the pellets dried slowly at low temperatures, typically
<200.degree. 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.
[0096] 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 in 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.
[0097] 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 30
t/m.sup.2.day in a straight grate kiln can be achieved, compared to
pellet productivities of 16 t/m.sup.2.day for HG ores in the same
kiln.
[0098] 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.
[0099] Preferred embodiments of the present invention will now be
described, by way of example only, with reference to the
accompanying drawings, in which:
[0100] 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;
[0101] 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;
[0102] 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;
[0103] 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;
[0104] 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;
[0105] FIG. 6 is a plot showing the effect of feed moisture on
green briquette strength for HG material using 650 mm rolls and 7.5
cc `pillows`;
[0106] 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;
[0107] 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`;
[0108] 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`;
[0109] FIG. 10 shows the effect of roll speed on briquette
properties; thickness, green strength and green density for a rolls
pressure of 90 kg/cm.sup.2 and a feed moisture of 6 wt % using 450
mm rolls and 9 cc almond forms;
[0110] FIG. 11 is the operating window for a briquetting machine
with a pre-compactor, 250 mm rolls, 4 cc almond forms and HG
material;
[0111] FIG. 12 shows temperature profiles for briquette induration
in a 500 mm deep bed;
[0112] 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;
[0113] 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;
[0114] 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;
[0115] FIG. 16 is a plot showing the effect of time at firing
temperature (1380.degree. 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;
[0116] FIG. 17 is a plot showing the effect of time at firing
temperature (1380.degree. 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;
[0117] 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.
[0118] 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;
[0119] FIG. 20 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;
[0120] 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;
[0121] 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;
EXAMPLE 1
[0122] Briquetting was performed using three different roll presses
with varying roll diameter, width and feed systems.
[0123] 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 FIG. 1.
[0124] The briquettes produced were pillow-shaped with nominal
dimensions of 13.times.19.times.28 mm and a volume of 4 cc. There
was a single row of 30 pockets around the circumference of each
roll.
[0125] 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.
[0126] 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 FIG. 2.
[0127] Briquettes of varying dimensions were produced with the
following details:
[0128] (1) Nominally 17.5.times.28.times.34.3 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 (9 cc Almond
forms).
[0129] (2) Nominally 14.5.times.22.1.times.33.9 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).
[0130] (3) Nominally 15.2.times.21.7.times.22.9 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).
[0131] (4) Nominally 11.2.times.17.3.times.32.1 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).
[0132] 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.
[0133] Briquetting was also conducted using a Koppern 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 FIG.
3.
[0134] The briquettes produced were `pillow` shaped with nominal
dimensions of 30.times.24.times.16 mm and forms a volume of 7.5 cc.
There were four rows of 77 pockets arranged symmetrically across
the face of the roll.
[0135] 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.
EXAMPLE 2
[0136] The effect of feed moisture content was investigated.
[0137] FIG. 4 illustrates that feed moisture had a significant
effect on the yield of 6 cc and 4 cc 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 90
kg/cm.sup.2.
[0138] Feed moisture control is also important as variation in
moisture content affects green properties such as green strength,
abrasion resistance and shatter strengths. This is illustrated in
FIGS. 5 and 6.
[0139] FIG. 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.
[0140] FIG. 6 shows the same relationship for briquettes made with
the 650 mm rolls and 7.5 cc pockets for HG material.
[0141] 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.
[0142] Feed moisture had less of an influence on shatter strength
and the green abrasion resistance of the briquettes.
EXAMPLE 3
[0143] 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.
[0144] 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 zone 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.
[0145] FIG. 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 450 mm diameter
rolls with nominal 9 cc pockets. The figure shows that acceptable
green strength was obtained at roll pressures as low as 60
kg/cm.sup.2.
[0146] FIGS. 8 and 9 show the effect of pressing force and
resultant green strength that was obtained using the 650 mm
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.
[0147] 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
[0148] Roll speed was also investigated.
[0149] Roll speed, measured in rpm, was found to exert an influence
on the amount of pressure applied to feed materials.
[0150] 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.
[0151] The effect of roll speed on briquette thickness and quality
(measured in terms of crush strength) for raw material HG is shown
in FIG. 10 for a gravity fed machine with 450 mm diameter
rolls.
[0152] The Figure shows that thickness and green strength decreased
as roll speed increased.
EXAMPLE 5
[0153] The process variables of the briquetting machine as
described in 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.
[0154] The diagram shown in FIG. 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.
[0155] To simplify the curves, roll pressure was fixed at 150
kg/cm.sup.2 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.
[0156] 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.
[0157] 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.
[0158] In FIG. 11, this region occurs at rolls speeds between 5 and
9 rpm and green strengths between 6 kgf and 18 kgf.
EXAMPLE 6
[0159] 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 FIGS. 12 and 13.
[0160] FIG. 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.
[0161] The bed temperatures were measured by thermocouples placed
at 100, 250 and 500 mm from the top of the bed.
[0162] 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.degree. C. in ten minutes without spalling the
briquettes.
[0163] FIG. 13 shows the temperature profiles for briquette
induration that produced nominal 4 cc briquettes of HG ore at
productives of 32 t/m.sup.2. d and 25 t/m.sup.2 .d. The figure also
shows, by way of comparison, a typical 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 16 t/m.sup.2.d, which is considerably lower than the
productivities of the briquettes. The briquettes and the pellets
were made from the same ore type.
[0164] 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.
[0165] The thermal stability of the briquettes was found to be not
exclusive to one induration method and to one ore type.
EXAMPLE 7
[0166] 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.
[0167] 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.
[0168] FIG. 14 shows how the temperature of the briquettes made
from GH material (d95=1 mm) with a green nominal size of 7.5 cc
initially increased to a maximum at approximately 300-400.degree.
C. average bed temperature, and then fell to a minimum temperature
at .about.700.degree. C. At higher temperatures the strength then
increased again. The strength fell to a minimum value at
.about.700.degree. C. which is lower than the green strength. This
is 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.
[0169] 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.
[0170] 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 process.
[0171] 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
[0172] Firing temperature was investigated.
[0173] Briquettes of GH material (d95=1 mm) 7.5 cc 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 FIG. 15.
[0174] There is a clear indication in FIG. 15 that to achieve
suitable fired strength in briquettes of this size the firing
temperature in the kiln should be at least 1380.degree. C.
[0175] FIG. 15 also shows that tumble strength (Tumble Index--TI)
and abrasion resistance (Abrasion Index--AI), improved with firing
temperature.
EXAMPLE 9
[0176] Firing temperature and time at temperature were
investigated.
[0177] Briquettes made from GH material (d95=1 mm) with a nominal
size of 7.5 cc 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 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.degree. compared to the 6 minutes firing time.
[0178] Tests were also conducted with 6.3 cc GH briquettes using
the same firing profile as that used for the 7.5 cc case.
[0179] Results are illustrated in FIGS. 16 and 17.
[0180] For the nominally 7.5 cc 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.
[0181] The fired properties for the 6.3 cc GH briquettes were
superior to those produced for the 7.5 cc 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
[0182] The effect of residence time in a grate kiln was
investigated.
[0183] Briquettes made from GH material (d95=1 mm) and nominally
7.5 cc 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.degree. C. Firing profiles were imposed on the briquettes and
the total residence time reported. The results are shown in FIG.
18.
[0184] FIG. 18 shows that the fired properties improved with
increasing residence time, suggesting the importance of heating the
product thoroughly to achieve the final properties required.
[0185] The effect of rapid heating was not reduced by a larger bed
depth of the grate. This is shown in FIGS. 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 300 mm. This far
exceeded that possible for even the best pellet beds in a
grate-kiln system.
EXAMPLE 11
[0186] The effect of the chemistry of briquettes was
investigated.
[0187] 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 FIG. 21.
[0188] 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.
[0189] As can be seen in FIG. 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.degree. C. for 0.6 basicity and at 1280.degree. C. for 0.8
basicity.
[0190] 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
[0191] Reduction testing, using whole briquettes and standard
reduction test methods JIS 8713/IS07215 was carried out on HG
briquettes that were fired at 1300.degree. C. for 10 min. The
results of reducibility, swell and crush strength after reduction
(CSAR) are shown in FIG. 22.
[0192] 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.
[0193] 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 a basicity of 1.20. Slag bonding also becomes a more
important form of bonding in briquettes made from GH and G where
higher SiO.sub.2 and Al.sub.2O.sub.3 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.
[0194] Many modifications may be made to the embodiments of the
present invention described above without departing from the spirit
and scope of the invention.
* * * * *