plate water absorption method for bentonite evaluation … · plate water absorption method for...
TRANSCRIPT
PLATE WATER ABSORPTION METHOD FOR
BENTONITE EVALUATION AND ITS
RELEVANCE TO IRON PELLETIZING
PERFORMANCE
Dr. Theodore Karidakis
S&B Industrial Minerals S.A.
OHO Si, Al Al, Fe, Mg
exchangeable cations neutral molecules (H2O)
Tetrahedral-layer
Oktahedral-layer
Tetrahedral-layer
Interlayer
Tetrahedral-layer
d00
1
BENTONITE
A clay mineral with unusual physicochemical / surface properties. These are
attributed to the crystalline structure of montmorillonite which is similar to a
pack consisting of successive platelets.
Crystal structure of montmorillonite
Montmorillonite characteristics
Exceptionally small crystal size (Average grain size 0.5μm).
Negative surface charge.
High cation exchange capacity
High surface area.
High water absorbency.
Binding properties
Swelling capacity.
Thixotropy.
High plasticity.
Ore beneficiation
Bentonite
Additives silo
PELLETIZING DRUMS
Screw mixer
Iron ore, conveyor belt
7-10% moisture
Pellets (green)
FIRING UNIT (INDURATION FURNACE)
Fired pellets to Blast furnace
Bentonite in iron ore pelletizing
BENTONITE EVALUATION METHODS
Montmorillonite content (%)
Cation Exchange Capacity (meq/g)
Swelling test (ml/2g)
Plate test (%)
Enslin test (%)
Rheological properties
Thermogravimetric analysis
Infra Red analysis
What are the most appropriate methods for quality assessment of
bentonite in pelletizing?
The problem
There is a large differentiation in bentonite qualities due to specific genesis conditions of
each deposit.
Bentonite cannot be fully characterized by its currently used evaluation methods due to
the large interactions between its properties and the pelletizing conditions.
Due to the large number of interfering factors and the complexity of interaction effects, it
is often difficult to describe mechanisms that associate bentonite properties with
pelletizing performance.
Problem constraints
The statistical approach
Statistical design of pelletizing tests allows:
Determination of significant effects of bentonite properties on agglomeration
performance.
Determination of significant interactions between factors.
Calculation of experimental errors (quite considerable in pelletizing processes).
The Plate Water Absorption Method
Ceramic plate
semi immersed in
distilled water
bentonite
Plate Water absorption test
Bentonite Ceramic
plate
Plate Water absorption test
Bentonite property Bentonite A Bentonite B Bentonite C
Plate Water Absorption
(4h,0,5g) (%)
691 835 933
Swelling (2g/100ml) 31 38 38
Montmorillonite (%) 73,7 85,0 92,3
Properties of bentonite samples used for agglomeration
Vol fraction (%) Concentrate A Concentrate B
Undersize (μm) Undersize (μm)
10 2.76 1,01
50 36.24 28,41
90 138.74 82,70
Mean diameter (μm) 59.83 36,51
Specific surface (m2/g) 3,8 4,6
Grain size analysis and specific surface of the iron concentrate samples
LEVELS OF
FACTORS
FACTORS
A
Type of iron
ore
B
Bentonite plate
Value
(%)
C
Bentonite
Addition rate
(%)
0
1
2
A
B
691
835
933
0.3
0.5
0.7
Full factorial experiment 2X3X3. Levels of factors
Responses: Green drop No, Green strength (kg/p), Dry strength (Kg/p)
Constant parameters: Moisture level (%)
Gre
en
dro
p N
um
ber 25
5,1
13,91667
±0,9111
Bentoniteplate value
691
933
819,667
Bentoniteaddition rate
0,3
0,7
0,5
Gre
en
dro
p n
um
ber 26,2
7,9
16,78667
±0,4931
Bentoniteplate value
691
933
819,667
Bentoniteaddition rate
0,3
0,7
0,5
Prediction profilers for green drop number versus plate and addition rate of bentonite
Concentrate A
Concentrate B
Results
Dry
str
ength
(Kg/p
)
8,24
2,51
5,13
± 0,254
Bentoniteplate value
691
933
819,667
Bentoniteaddition rate
0,3
0,7
0,5
Dry
str
ength
(Kg/p
)
6,84
1,97
4,382222
±0,4399
Bentoniteplate value
691
933
819,667
Bentoniteaddition rate
0,3
0,7
0,5
Concentrate A
Concentrate B
Prediction profilers for dry strength versus plate and addition rate of bentonite
Results
Plate Water Absorption had a statistically significant effect on green drop of pellets
produced with concentrate A, while had no effect on the pellets of concentrate B.
For dry strength, analysis showed that the effect of plate was statistically significant
for both concentrates but higher on the concentrate A.
Significant interaction between plate and type of concentrate indicates that plate
should be always assessed in combination with the iron concentrate.
Effect of water hardness on plate water absorption of bentonite
789755
849 862878863
950
685
767
867
594
795 826
751
657
0
200
400
600
800
1000
Bentonite 1
(sodium
activated)
Bentonite 2
(sodium
activated)
Bentonite 3
(sodium
activated)
Bentonite 4
(natural sodium)
Bentonite 5
(natural sodium)
Pla
te w
ate
r a
bs
orp
tio
n (
%)
Distilled
water
Water with
100 ppm
Ca+Mg
Water with
300 ppm
Ca+Mg
Effect of calcium and magnesium at 100 and 300 ppm concentration levels
in the pellet feed water on the plate value of various bentonite qualities.
Results
Calcium and magnesium in the process water can replace sodium as exchangeable
cations and thus seriously deteriorate bentonite performance.
Not all bentonite qualities have the same ‘resistance’ to water hardness.
There is no significant difference in water resistance between natural sodium and
activated bentonite.
Effect of bentonite thermal stability on its plate water absorption
789
863 878
755707
768 793
544
735
950
0
200
400
600
800
1000
Bentonite 1 Bentonite 2 Bentonite 3 Bentonite 4 Bentonite 5
pla
te w
ate
r ab
so
rpti
on
ground with
initial
moisture 15%
ground with
initial
moisture 0%
Effect of initial moisture of bentonite prior to milling on the plate values of various
bentonite samples.
‘Dry’ versus ‘wet’ milling of bentonite
Temperature/ °C 100 200 300 400 500 600 700 800 900
HEAT FLOW COR/ µV
-15.0
-12.5
-10.0
-7.5
-5.0
-2.5
0.0
2.5
TG COR/ %
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
DTG/ %/min
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Exo
Fig. : Experiment : B-02410
Mass (mg) : 50,83 11-12-02 Procedure : BLANK 0/1050 C, NORMAL,He, 10C/min (Seq
1)
Atm. : He Crucible : Pt/Rh
TG-DTA 92-16
95C
748C
686C
560C
812C
476C 245C
285C
322C
385C
TG-DTA diagram of bentonite with relatively high thermal stability (sample1-3).
Montmorillonite
dehydroxylation
748oC
Temperature/ °C 100 200 300 400 500 600 700 800 900
HEAT FLOW COR/ µV
-30
-25
-20
-15
-10
-5
0
5
TG COR/ %
-17.5
-15.0
-12.5
-10.0
-7.5
-5.0
-2.5
0.0
2.5
DTG/ %/min
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0 Exo
Fig. : Experiment : Â-02403
Mass (mg) : 67,91 10-31-02 Procedure : BLANK 0/1050 C, NORMAL,He, 10C/min (Seq
1)
Atm. : He Crucible : Pt/Rh
TG-DTA 92-16
110C
240C 429C
448C
498C
612C
853C
TG-DTA diagram of bentonite with relatively low thermal stability (samples 4,5)
Montmorillonite
dehydroxylation
498oC
‘Dry’ grinding of bentonite results in plate decrease that ranges from 80 – 210 units.
Decrease in plate, which may significantly affect pelletizing performance, depends on
thermal stability of bentonite, i.e. temperature of dehydroxylation.
Temperature of montmorillonite dehydroxylation depends on the isomophic substitution
of Al3+ by other cations in montmorillonite lattice and the strength of the Mn+-OH bonds.
It has been found that substitution of Al3+ by Mg2+ results in bentonites with higher
thermal stability than bentonites with isomophic substitution of Al3+ by Fe3+
Conclusions
Plate Water Absorption is a relatively good quality index for bentonite performance
in iron pelletizing.
However, Plate Water Absorption, measured by the conventional method, is not
sufficient as it largely depends on pelletizing conditions.
Due to the large complexity of interfering factors, plate values should be evaluated
taking into consideration iron concentrate characteristics, sensitivity to water
hardness and thermal stability of bentonite.
THANK YOU FOR YOUR ATTENTION