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