Iron Production by Blast Furnace Route
Feed
Blast Furnace - 1500°C
Filtration
Concentration
Silicabearing
tailings
Silicabearing
binder
Cooling and
Shipping
Pelletization
Sintering - 1200°C
Introduction to Iron Ore
Agglomeration
Binder
Iron Ore
Pellets used to make iron:
1) Blast Furnaces
2) Direct Reduction furnaces
Fluxes
Mix for 5 min in
Kneader mixer
Iron ore
pellets
Dry 105 °C
overnight
Agglomerate to
1/2 in balls
Induration/firing
1200 - 1350 °C
Common Questions about Iron Ore
Pelletization
•
•
•
•
•
Balling Disc, or Balling Drum?
Straight Grate, or Rotary Kiln?
How should we process low-grade ores?
What is the status of cold-bonding?
Which is better during pellet firing, oxidizing
atmosphere or reducing atmosphere?
• What are the alternatives to bentonite?
– Fly-ashes and other industrial byproducts?
– Organic binders?
– Can improved mixing reduce bentonite
requirements?
S. K. Kawatra, 2013
3
Outline
•
•
•
•
•
•
Bentonite use as a binder
Binder additives
Organic binders
Cold bonding
Pelletization equipment
Pellet firing: Straight grate vs. rotary kiln
What is Bentonite?
Clay material, primarily the mineral
montmorillonite
• Essentially two types
– Western or sodium bentonite is highly
absorbent (900%), expandable (14x), and
disperses readily in water, resulting with
excellent binding properties
– Southern or calcium bentonite swells and
expands much less
S. K. Kawatra, 2013
5
Classical Bentonite Dispersion on
Magnetite
Calcium
bentonite
1
2
3
Sodium
bentonite
Bentonite platelet
Bentonite particle
S. K. Kawatra, 2013
Magnetite particle
6
Bentonite Dispersion
- - - - -
- - - - -
Sodium
Bentonite
- - - - - - - - -
1
2
3
- - - - -
- - - - -
Calcium
Bentonite
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - -
- - - - - - - - H+
H+
O= Water
Na+
S. K. Kawatra, 2013
Ca++
Bentonite
7
Structure of Bentonite
Z
SiO4 tetrahedra
are arranged in
hexagonal rings
+
X
Y
Exchangeable Cations
O2-
H
OH-
Al3+, Fe3+, Fe2+, Mg2+
Si4+ (occasionally Al3+)
S. K. Kawatra, 2013
8
What is a bentonite fiber?
• When bentonite expands, platelets become
lubricated by water and can slide relative to one
another.
• Platelets in a particle remain connected, but the
particle is drawn-out and elongated in one or two
dimensions, like a deck of cards being spread
across a table.
• The connected platelets become fibers and
sheets that can bridge from particle to particle,
binding them strongly together.
S. K. Kawatra, 2013
9
Bentonite Fiber Formation
A
B
Bentonite platelet
C
Bentonite particle
Bentonite fiber
Sand particle
Calcium
bentonite
Sodium
bentonite
Fibrous
bonding
Fibers are formed when bentonite platelets
spread apart like a deck of cards
S. K. Kawatra, 2013
10
Nature of Compressive Shear Mixing
• The objective is not to comminute the particles
being bonded, but only to apply shear to the
bentonite particles that will draw them into
sheets and fibers.
• The device used (a roll press) was set with a
gap significantly larger than the size of the
particles being bonded, and therefore there was
no roll/particle/roll contact that would break
these particles.
• The device used was not a high-pressure
grinding roll. It did not have the necessary power
to cause wholesale breakage of particles by
interparticle pressure.
S. K. Kawatra, 2013
11
Schematic of Denver Type-D laboratory rollpress used for compressive shear mixing
Feed
- Objective was to apply shear
forces to the bentonite platelets.
-Gap between rolls was set wide
enough that particles would not
be comminuted.
- We did not use a high-pressure
grinding roll mill.
6”
10”
350 RPM
S. K. Kawatra, 2013
12
Glass Shot Binding Experiments
• Tests were carried out using glass shot in place
of magnetite, so that binder could be
distinguished from the particles by electron
microscopy.
• Cylindrical test specimens were produced from
the glass shot-bentonite mixtures so that the
compressive strengths could be accurately
determined in pounds per square inch.
• Compressive strength test results were
correlated with photomicrographs of the
specimens to determine how the morphology of
the bentonite relates to dry strength.
S. K. Kawatra, 2013
13
Sand Compaction Device
RegistrationMark
Specimen
Cylinder
Weight
Cam
Finished
Specimen
Crank
Specimen
Cylinder
S. K. Kawatra, 2013
14
Compression Tester for
Cylindrical Sand Specimens
Test
Specimen
S. K. Kawatra, 2013
15
Dry compressive
strength, psi
Bentonite Binder Study - Muller
250
200
150
100
50
0
0
60 120 180 240 300 360
Mulling time, seconds
A conventional muller mixer produced
cylindrical test specimens with strengths of
about 200 psi.
S. K. Kawatra, 2013
16
Dry Compressive
Strength, psi
Bentonite Binder Study - Roll Press
450
400
350
300
250
200
150
100
50
0
Experiment 1
Repeat
Maximum strength
after mulling 5 minutes
0
10
20
30
Roll Press Passes
A roll press improved the bentonite effectiveness
more than the muller, reaching over 350 psi
S. K. Kawatra, 2013
17
Measuring Bentonite Fibers - SEM
0 passes
2 passes
5 passes
S. K. Kawatra, 2013
10 passes
20 passes
18
Measuring Bentonite Fibers - SEM
0 passes
20 passes
Roll pressing significantly improved the distribution of bentonite and
the formation of bentonite fibers and sheets.
S. K. Kawatra, 2013
19
Bentonite Fibers - SEM
• Bentonite fibers were qualitatively analyzed
by imaging with the SEM.
• Quantitative analysis was attempted by pointcounting and image analysis
• Bentonite and glass shot had overlapping
grayscale ranges with all available imaging
techniques.
• Bentonite could not be distinguished well
enough from the glass shot for quantitative
analysis to work.
S. K. Kawatra, 2013
20
Quantifying Bentonite Fibers - BET
• BET measures specific surface area by nitrogen gas
adsorption/desorption on the surface of a material.
• It was expected that bentonite fibers could have a
measurably higher surface area than the bentonite
particles as shown in the SEM pictures.
• Can BET be used to measure and quantify bentonite
fibers?
• The samples shown in the previous SEM pictures
were analyzed by BET gas adsorption.
• Surface areas of the glass shot and bentonite alone
were also determined for comparison.
S. K. Kawatra, 2013
21
Surface Area,
m^2/gram
BET - Surface Area of Bentonite
on Glass Shot - 6.0% Bentonite
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
Passes through the roll press
The bentonite alone had a specific surface area of 8.65 m2/gram
The glass shot alone had a specific surface area of 0.027
m2/gram
S. K. Kawatra, 2013
22
Nitrogen Adsorption on Bentonite
(as coated on a mineral surface)
Bentonite platelets
Mineral surface
Nitrogen
molecules
It was expected that bentonite particles would
absorb more nitrogen after fibers were developed.
However, the difference was negligible.
This can be explained because the nitrogen was adsorbed on the
platelets that compose the bentonite particle and not just over the
surface of the particle.
S. K. Kawatra, 2013
23
Quantifying Bentonite Fibers - XRD
• It was hypothesized that bentonite platelets
composing the fibers would have a measurably
different spacing
• Then, XRD could be used to quantify the difference
• Samples of the glass shot coated with bentonite by
roll pressing various times were analyzed with
XRD
• The diffraction patterns were the same for the
samples
• XRD did not show a difference between the
bentonite particles remaining in the glass shot (no
roll press passes) and the bentonite fibers (20 roll
press passes)
S. K. Kawatra, 2013
24
Conclusions - Bentonite Fibers
• Bentonite binder effectiveness is markedly
improved when a compressive shear mixing
method is used rather than traditional mixing.
• Microscopic examination of bentonite-bonded glass
shot showed that the strength improvement was not
due to breakage of particles.
• Compressive shear develops the bentonite grains
into fibers and sheets that produce stronger
bridging between the magnetite grains.
• Strengths of pellets are approximately doubled
when compressive shear mixing is used instead of
conventional mixing methods.
S. K. Kawatra, 2013
25
• While visiting plants to determine the “state of
the art”, we were told the following by
different plant engineers:
– “High PWA bentonite is increasingly used for kitty
litter and other high-demand consumer products,
and will be difficult to get. We do not yet know the
quality of pellets made with low PWA bentonites.”
– “Bentonite PWA has no effect on pellet quality
over the ranges of PWA that are sold for iron ore
pellet binders.”
– “We have always known that there is an optimum
value of PWA around 900 where pellet strength is
highest. When PWA is above 900, the pellet
strength falls.”
• The question was, what is the best value of
PWA for pellet production?
S. K. Kawatra, 2013
26
Effect of Ions on Water Absorption
1000
800
1033
PWA, %
945
850
757
600
631
1033 Mag. Conc. Water
945 Mag. Conc. Water
400
850 Mag. Conc. Water
757 Mag. Conc. Water
631 Mag. Conc. Water
200
0
1
10
100
1000
10000
100000
Time, minutes
S. K. Kawatra, 2013
Presence of calcium
and magnesium
ions greatly reduced
the water absorption
ability of all five
bentonites studied
27
Conclusions - PWA
• PWA is not as important as moisture water
chemistry
• Presence of calcium and magnesium in moisture
significantly reduces bentonite effectiveness
regardless of PWA
• Calcium and magnesium is concentrated into
balling feed moisture during filtration process
S. K. Kawatra, 2013
28
• While visiting plants to determine the “state of
the art”, we were told the following by
different plant engineers:
– “High PWA bentonite is increasingly used for kitty
litter and other high-demand consumer products,
and will be difficult to get. We do not yet know the
quality of pellets made with low PWA bentonites.”
– “Bentonite PWA has no effect on pellet quality
over the ranges of PWA that are sold for iron ore
pellet binders.”
– “We have always known that there is an optimum
value of PWA around 900 where pellet strength is
highest. When PWA is above 900, the pellet
strength falls.”
• The question was, what is the best value of
PWA for pellet production?
S. K. Kawatra, 2013
29
Fly-Ash Binder
• There was no published work with fly-ash as a binder
for this application
• Fly-ash as a binder is commonly used to partially
replace portland cement in concrete
• Advantages
– Low cost - Available from power plants located near steel
mills - low cost (shipping alone can triple the cost of
bentonite)
– Environmental stewardship - 40 million tons of fly-ash is
land filled in the U.S. each year
S. K. Kawatra, 2013
30
Comparison of Bentonite to Fly-ash
CRITERIA
BENTONITE
FLY-ASH
Primary
constituents
Structure
SiO2, Al2O3,
Fe2O3
SiO2, Al2O3,
Fe2O3
Crystalline
amorphous
Dry particle 80% -74m;
60% -325m
size
(-1m crystals)
Expandable/ yes
absorbent/
dispersive
bonding
Physical/gel
80% -50m
no
Chemical/
pozzolanic
S. K. Kawatra, 2013
31
Analyte
Unit 2
Fly-Ash, %
3 8. 6 8
Bentonite,
%
34 . 62
Al2O3
2 2. 8 1
23 . 16
Fe2O3
6. 0 6
5 . 49
6 7. 5 5
68 . 26
1 5. 0 4
9 . 63
MgO
3. 5 4
2 . 11
Na2O
1. 7 0
1 . 06
K2O
0. 9 4
0 . 39
TiO2
1. 1 7
1 . 25
MnO2
0. 0 5
0 . 01
P2O5
0. 9 1
2 . 20
SrO
0. 2 0
0 . 39
BaO
0. 4 8
0 . 53
S O3
1. 1 9
5 . 93
LOI
7. 2 7
13 . 24
SiO2
SiO2+Al2O3
+Fe2O3
CaO
~80% passing
400 microns
quartz
maghemite
mullite
ASTM Classification of Fly
Ashes
ASTM C618-98
Class F
SiO2+Al2O3+
Fe2O3, min.%
SO3, max.%
70.0
50.0
5.0
5.0
3.0
3.0
6.0 (12.0)
6.0
Moisture content, max.%
Loss on Ignition, max.%
Class C
S. K. Kawatra, 2013
33
Fly-Ash Based Binders
• Fly-ash must undergo a pozzolanic reaction to
act as a binder, which requires the presence of
alkali and water.
• Pozzolanic reactions tend to be slow (several
days), and binder applications require fast
binding
• Fly-ash based binders are made from fly-ash,
calcium hydroxide, and accelerator (soluble
calcium salts)
S. K. Kawatra, 2013
34
Iron Ore Pellet Production
• Pellets are produced by combining iron
ore concentrate at approximately 10%
moisture with a small amount of binder,
and tumbling in a pelletizing drum or
disk. Pellets are then fired at 1200 C .
• Pellets are approximately 1.2 cm in
diameter, and before firing they must
have a “green strength” > 22 N/pellet
S. K. Kawatra, 2013
35
Carbon Content in Pellets
• Carbon content of the fly-ash does
not reduce the strength of pellets, and
can even increase the strength slightly.
• Carbon will act as a supplemental fuel
during pellet firing, and will therefore be
beneficial for pellet production.
S. K. Kawatra, 2013
36
Pelletization Procedure
•
•
•
•
Kneader mix binder into 3kg
concentrate at 350 rpm&150rpm orbital
motion for various times
Delump with 2.4mm (8mesh) screen
Add some feed and make-up water
spray to balling drum, form seeds,
periodically screen, and grow into 12.7
x 11.2 mm (1/2 x 7/16 in.) Diameter
pellets
Test 20 pellets each for wet knock and
wet compressive strength
°
•
Dry at 105 C
•
Metallographic preparation for SEM
Counter-clockwis
Rotation @ 25rpm
Balling Drum
Schematic
Testing Procedures
Test
Procedure
Use of Data
WetKnock
Undried pellets were dropped
repeatedly from 46 cm (18 in.)
onto a steel plate. The number
of drops required for fracture
was recorded.
Undried pellets were crushed
using an Instron compression
machine, with a crosshead
speed of 40 mm/min. The
fracture load was recorded.
Pellets dried at 105°C (221°F)
for at least 1 hour were then
crushed using an Instron 4206
compression test machine.
Ultimate strength was recorded.
Measures the
ability of the wet
pellet to remain
intact during
handling.
Measures the
ability of the wet
pellet to remain
intact during
handling.
Measures the
ability of dried
pellets to survive
handling and firing
(> 22.2 newtons).
WetCrush
DryCrush
S. K. Kawatra, 2013
38
Why Add Activator and Accelerator to
Fly-ash?
• Lime (CaO) or Ca(OH)2 is added as an activator to
induce pozzolanic reactivity (cementitious)
• Compounds, such as calcium acetate, calcium nitrate,
or calcium chloride were added to accelerate the
pozzolanic reaction because:
– The activator has higher solubility in the CaCl 2
solution than in water alone
– CaCl2 adds calcium ion, increasing formation of
the bonding calcium silicates
– CaCl2 is hygroscopic (absorbs/retains water),
allowing the reaction to proceed longer
S. K. Kawatra, 2013
39
Accelerators for Fly-Ash Binders
S. K. Kawatra, 2013
40
Dry crush strength,
newtons
Strength of dry iron ore pellets at varying mixtures
of fly-ash based binder (FBB) and bentonite (B).
50
40
30
20
10
0
0.66%
bentonite
0
25
50
75
FBB/(FBB+B), %
The dashed line represents the minimum
acceptable dry strength of 22.2 newtons (5 lbf).
S. K. Kawatra, 2013
100
0.66% fly-ash+
+1.0% Ca(OH)2
+0.2% CaCl2
41
Compatibility of Fly-Ash-Based Binder with
Bentonite Binder
• Fly-ash based binder gives strengths only slightly
lower than a similar dosage of bentonite.
• Mixtures of fly-ash and bentonite do not perform as
well as either bentonite alone, or fly-ash alone.
• Incompatibility is apparently due to the difference in
binding mechanism between the two binders
(physical vs. pozzolanic)
S. K. Kawatra, 2013
42
Cornstarch as a Binder
S. K. Kawatra, 2013
43
Common problems with iron ore binders
• All iron ore binders have drawbacks or problems
associated with their use
• Bentonite clay
– Is a sodium/calcium alumino-silicate clay
– Common problems:
• Increases pellet silica content
• Variable properties depending on sodium/calcium ratio
• Cornstarch
– Is a widely available natural polymer
– Common problems:
• Combusts before pellets are strengthened at high T (°C)
• Low levels of inorganic material in starch structure
S. K. Kawatra, 2013
44
Can binders be combined to minimize
their negative effects?
• Binders may be combined together to minimize
negative effects during agglomeration
• Not all binder combinations will be successful
– Certain binder combinations are not synergistic
– Binding mechanisms should be understood
• Physical gel
• Pozzolanic reactions
• Inactive matrix
• Can cornstarch be used as a partial bentonite
replacement?
S. K. Kawatra, 2013
45
Starch Benefits and Characteristics
• Starch is a good choice for bentonite replacement
because it is:
– Easily modifiable
– Easy to access, widely available and renewable
– Able to increase the viscosity of the binding liquid which
aids in balling
• Consists of amylose (28 %) and amylopectin (72 %)
Amylose
S. K. Kawatra, 2013
Amylopectin
46
Iron Oxide Pellets’ Strength
(Force required to fracture pellet)
Compressive
Strengths
Procedure
Wet Drop Number
A freshly made wet pellet is dropped from 18” onto a steel plate
until it fractures. This value is recorded as the wet drop number
(usually 5 drops is accepted)
Wet Crush Strength
A freshly made wet pellet is compressed by compression
testing machine with a crosshead speed of 40 mm/min until it
breaks and the reading is recorded (usually 9 newtons (2 lbf) is
accepted)
Dry Compressive
Strength
After drying at 1050C for 3 – 24 hours, a single pellet is
compressed until it fractures, and the reading is recorded
(usually 23 newtons (5 lbf) is accepted)
Dry Compressive
Strength of Sintered
Pellets
Dried pellets are indurated at temperatures 1200-1300°C, a
single pellet is compressed until it fractures and the reading is
recorded (usually 1780 newtons (400 lbf) is accepted)
All the test are repeated with 20 pellets and average is taken
Wet drop number improves with increasing
starch solubility
25
High Solubility, High Viscosity Starch
Low Solubility, Low Viscosity Starch
Acceptable Wet Drop Number (5 Drops)
Wet Drop Number
20
15
10
5
100 % bentonite
100 % starch
0
0%
20%
S=starch B=bentonite
40%
60%
S/(S+B), %
80%
100%
0.66 % w/w binder
Wet drop number was unnaffected by bentonite; however, the
addition of high solubility starch yielded significant improvements
Pellet quality improves using mixtures of
high viscosity cornstarch & bentonite
Wet/Dry Compressive Strength (N)
350
300
250
Dry Strength High Solubility Starch
Dry Strength Low Solubility Starch
Wet Strength High Solubility Starch
Wet Strength Low Solubility Starch
Minimum Dry Strength (23
(22.24
N) N)
Minimum Wet Strength (9
(8.89
N) N)
Low Viscosity, Low Solubility Starch
200
High Viscosity, High Solubility
Minimum Strength
150
20%
40%100
60%
80%
S/(S+B) %100 % bentonite
100%
100 % starch
50
0
0%
20%
S=starch B=bentonite
40%
60%
S/(S+B) %
80%
100%
0.66 % w/w binder
Linear increase in green-ball dry strength with increasing
quantity of cornstarch.
S. K. Kawatra, 2013
49
Compressive Strength (N)
4000
Ball strength after cornstarch combustion
3000
improves with bentonite addition.
3500
Low Solubility/Viscosity Corn Starch
2500
High Solubility/Viscosity Corn Starch
2000
Minimum Dry Strength (23 N)
1500
Low Viscosity, Low Solubility Starch
High Viscosity, High Solubility
1000
Minimum Strength
500
0
0%
20%
40%
60%
S/(S+B) %
80%
100%
Due to differences
in agglomeration
or combustion
behavior?
100 % starch
100 % bentonite
S=starch B=bentonite
0.66 % w/w binder
pellets weaker after cornstarch combusts. Balls burnt at 500 °C
for 30 min. Cornstarch performance improves with bentonite
addition.
S. K. Kawatra, 2013
50
Sintered compressive strength is unnaffected by
changing starch or bentonite
4500
4500
Compressive Strength (N)
4000
3500
3000
2500
2000
1500
1000
500
Compressive Strength (N)
4000
3500
3000
2500
2000
1500
1000
100 % bentonite
Low Viscosity, Low Solubility Starch
High Viscosity, High Solubility
Minimum
Minimum Sintered
Strength Compressive
Strength (1780 N) 100 % starch
500
Low Viscosity, Low Solubility Starch
High
0%
20% Viscosity,
40% High Solubility
60%
80%
100%
S=starch B=bentonite
0.66 % w/w binder
S/(S+B) %
Minimum Strength
0
Pellets
made with varying doses of bentonite and corn starch
0
0%
60%compressive
80%
100%
showed
no20%
change40%
in sintered
strength
S/(S+B) %
High temp strength may be affected by starch
expansion and loss of inorganic binder mass
Starch expands during
combustion (275 °C).
120%
Total Mass Loss During Ashing %
100%
Viscosity)
Binder mass G46S
lost(Low
after
combustion
G242 (High Viscosity)
at 500ºC
Low
viscosity
starch
80%
60%
40%
100 %
bentonite
100 %
starch
20%
High
viscosity
starch
0%
100% Starch
0% Bentonite
100%
S=starch B=bentonite
High viscosity starch
expanded 2x more than low
viscosity starch.
75%
50%
S/(S+B), %
25%
0.66 % w/w binder
Total mass loss during combustion at 500ºC
increases with cornstarch content. Less
material is available for binding.
S. K. Kawatra, 2013
52
Summary of cornstarch-bentonite binders
• Cornstarch-bentonite binders have stronger dry
strengths than bentonite alone
• Pellet strength affected by cornstarch viscosity
– Higher viscosity led to stronger dry strengths
– Higher viscosity led to weaker balls after binder combustion
– Higher viscosity cornstarch expanded 2x more than low
viscosity starch
– Difference in strengths after combustion are likely due to
combustion behavior, as shown by expansion
• Ball strengths can be tailored using mixtures of
cornstarch and bentonite as binder
S. K. Kawatra, 2013
53
MTU Cold Bonding Process Flow
Diagram
Typical dry pellet content
(wt %), excluding reductant
Mixed Fe-oxide 90.05
Burnt Lime, CaO
7.00 Mix & age
Pelletize
(10 % H2O)
Dry to 3 %
Silica flour, SiO2 2.95
Also add H2O + reductant
Burnt lime hydrates
Example cold-bonded
pellet properties
Crushing strength 400 lbs
- 28 Mesh (tumble)
Hardened
pellets
Autoclave
(125 psig, 220 °C)
4.98 %
- 14 Mesh (Linder)
4.83 %
Reduction (1000 °C) 81.78 %
Various C-S-H phases
Minerals dissolve and
precipitate & bond pellet diffuse in steam phase
Cold Bonding Comments & Observations
• Cold bonding uses high quantities of binder
– Reduces energy consumption from firing but does not
reduce silica
• Can we use fly ash as a cold bonding binder?
– Significant source of both CaO and SiO 2
– Fly ash used as a supplement in cement industry
• Mechanical issues with autoclave
– Good quality pellets can be made, but
– Batch process
S. K. Kawatra, 2013
55
Disc Pelletizer
• Relatively light weight
• Highly adjustable
• Product classified by
size
• Limited capacity
• Require close control
Scraper
Pellets
Moist Feed
+ Binder
Disk Agglomeration
Iron ore
concentrate
Sprays
Green balls
Rotating Disk
1”
2
S. Komar Kawatra - September 1,
Disc Pelletizer
Pellets
Disc Pelletizer
Drum pelletizer
• High capacity, tight control of product size by screen
• Prone to “surging”, throughput difficult to control
Moist Feed
+Binder
Spray Water
& Cutter
Screen
Pellets
Undersize Pellets
Drum Agglomeration
Iron ore
concentrate
Sprays Inside
+ 16”
7
Pelletizing
Drum
1” 7”
to
16
2
Green balls
Roller
Screens
- 12 ”
S. Komar Kawatra - September 1,
Straight-Grate Furnace
A Simplified Model
Exhaust gas Drying gas
Oxidizing gas
Firing gas
Exhaust gas Exhaust gas
Moist pellets
from balling
drum/disc
Pellet Travel
Drying gas
Downdraft
Drying
Preheat
Fire
Cool
Cool 2
Fired pellets to
iron-making
Exhaust gas
Exhaust gas
Exhaust gas
Cooling gas
• Straight-Grate Furnaces:
– Convert weak, moist pellets into physically strong
agglomerates suitable for blast furnace & DRI feed
– Pellets are dried and heat-hardened by hot,
oxygen-rich gases
S. Komar Kawatra - September 1,
1250 °C
Cooling gas
Furnace Temperature
Updraft
Recycled Drying
pellet
hearth
300 °C
Grate-Kiln Furnace
Drying gas
Drying gas
Oxidizing gas
Exhaust gas
Moist pellets
from balling
drum/disc
Pellet Travel
Downdraft
Drying
Downdraft
Drying
Preheat
Cool
Cool 2
Fired
pellets
Exhaust gas Exhaust gas Exhaust gas
Cooling gas
Cooling gas
– More prevalent than Straight-Grate furnaces
• Tumbling action in the kiln provides more uniform pellet
firing
• Tumbling action may reduce dust on fired pellets
• More efficient (no recirculating pellet hearth, less burners)
S. Komar Kawatra - September 1,
Furnace Temperature
• Grate-Kiln Furnaces:
1250 °C
300 °C
Fines Generation Rates for Five Plants
Fines Generation Rate vs Pellet Charge
15 min residence, constant generation rate.
Fines = -35Mesh, 4 Trials
Fines Generation Rate
(g/min)
1.0
Plant A (Straight Grate Kiln)
Plant B (Grate Kiln Cooler)
Plant C (Grate Kiln Cooler)
Plant D (Grate Kiln Cooler)
Plant E (Grate Kiln Cooler)
0.8
0.6
0.4
0.2
0.0
0
200
400
600
800
Pellet Charge (g)
1000
1200
Highlights of Straight grates and Rotary kilns
• Fines production
– Straight grate  increased fines during transportation and handling;
less fines produced within the furnace
– Grate kiln  more fines produced within the furnace; less fines
produced during shipping and handling
• Bed Depth
– Straight Grate has deeper pellet bed  higher pressure drop,
increased fan power consumption
• Ore Type
– Both straight grate and grate kiln furnaces can be used to process either
magnetite or hematite
• Furnace manufacturers
– Straight grate suppliers say Straight grate furnaces superior
– Rotary kiln suppliers say Rotary kiln furnaces superior
• Buy whichever furnace is most economical
S. Komar Kawatra - September 1,
Summary
•
Sodium bentonite is far superior to calcium bentonite as a binder
–
–
•
•
•
When adding additives to binders, it is necessary to ensure that additives
are compatible with the binder
Organic binders reduce silica content of the iron ore pellet
Cold bonding has tremendous potential
–
–
•
•
Presence of calcium in iron ore concentrate makes sodium bentonite less effective
Amount of ions, particularly sodium ions, in iron ore concentrate is more important than PWA
Physics and chemistry of cold bonding works
It is necessary to overcome mechanical problems
Disc pelletizers and balling drum pelletizers both work well
Straight grate and rotary kiln furnaces both work well
–
It is necessary to conduct pot grate tests before choosing furnace type