a review of binders in iron ore pelletization.pdf

8/20/2019 A review of binders in iron ore pelletization.pdf

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AREVIEWOFBINDERS IN IRONORE

PELLETIZATION

T. C. EISELE ANDS. K. KAWATRA

Department of Chemical Engineering, MichiganTechnological

University, Houghton, Michigan, USA

The majority of iron ores must be ground to a fine particle size to allow the

iron oxides they contain to be concentrated, and the concentrate must then be

agglomerated back into large enough particles that they can be processed in

blast furnaces. The most common agglomeration technique is pelletization,

which requires the use of binders to hold the iron oxide grains together so that

the agglomerates can be sintered into high-strength pellets. Although bento-

nite clay is the most commonly used binder, there are many other possibilities

that could be competitive in a number of situations. This article reviews the

We would like to thank the following sponsors for providing the generous financial

support for this work: EVTAC Mines, particularly Bob Anderson; Hibbing Taconite, espe-

cially Steven G. Rogers; ISPAT Inland Mining; LTV Steel Mining; National Steel Pellet,especially Jim Wennen, Sarah Blust, and Dennis Murr; Northshore Mining; Svedala;

Minnesota Department of Natural Resources, particularly Peter Clevenstine; USX-MN

Ore Operations, especially Bob Strukel; and Cleveland Cliffs Iron Co, particularly Paul

Rosten, Dick Kiesel, Bob Thiebault, and Ted Seppanen. We would also like to thank Chris

Glenn and Franz Reisch of American Colloids, John Engesser of the Coleraine Minerals

Research Laboratory, and Dr. Ron Weigel for their invaluable technical advice. Thanks

are also due to S. Jayson Ripke of Northshore Mine for his critical analysis, Henry Walqui

and Basak Anameric of Michigan Tech, and the following undergraduate students: Katy

Marten, Kari Buckmaster, Karla Andrade, Gabriella Ramirez, Toby Lee, Frank Perras,

Elise Anderson, and Jamie Krull.

Address correspondence to S. K. Kawatra, Dept. of Chemical Engineering, Michigan

Technological University, 1400 Townsend Drive, Houghton, Michigan 49931-1295 USA.

E-mail: [email protected]

Mineral Processing & Extractive Metall. Rev., 24: 190, 2003

Copyright#Taylor & Francis Inc.

ISSN: 0882-7508 print

DOI: 10.1080/08827500390198190

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numerous types of binders (both organic and inorganic) that have been

considered for iron ore pelletization, including discussion of the binding

mechanisms, advantages and limitations of each type, and presentation of

actual pelletization results, so that the performance of the various types of

binders can be compared and evaluated.

Typical iron ores contain a great deal of gangue minerals, particularly

silicates, and the iron oxides must be concentrated from these ores before

they can be used by the steel industry. In the process of concentrating the

iron oxides, the ore is ground into a fine particle size that is not suitable

for use in ironmaking, and thus the ore must be agglomerated into larger

particles before it is used. The most common agglomeration technique is

pelletization, which requires that a small amount of binder be added to

the powdered ore to control balling rates and hold the pellets together

until they are hardened by sintering.

A variety of binders are possible, with the most commonly used

being bentonite clay; however, the bentonite contributes silica and other

undesirable elements to the ore, and so there is considerable interest in

developing binders that have the good qualities of bentonite at a

comparable or lower cost without contributing any harmful con-

taminants to the ore.

This review covers the various types of binders that have been

investigated for use in iron ore agglomeration. The binding mechanisms,

chemistry, pelletization results, and advantages=drawbacks of each are

discussed.

AGGLOMERATIONTECHNIQUES

The feed to a blast furnace should form a permeable bed of material,permitting gas to flow through it uniformly at a high rate. Powdered

iron ore concentrates are not suitable in their as-produced form, both

because fines tend to pack into a nonpermeable bed and because the

fine particles are likely to be carried away as dust by the high gas

flowrates. The powdered ore must therefore be agglomerated into larger

particles that will improve permeability of the furnace burden, increase

the rate of reduction, and reduce the amount of material blown out of

the furnace as dust. There are four basic methods that have beendeveloped for agglomerating iron ores: sintering, nodulizing, briquet-

ting, and pelletizing. These processes are briefly described below (AISE

1985).

2 T. C. EISELE AND S. K. KAWATRA


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Sintering

The sintering process consists of combining iron-bearing fines with a solid

fuel and igniting the mixture on a traveling grate with a downdraft of air.

As the fuel burns, the temperature in the bed increases to about 1300C

to 1480C, sintering the fines into a porous, clinker-like material that is

suitable for use as blast furnace feed. The bonding between the particles is

by recrystallization and partial melting, and so no additional binder

needs to be added in this process. Sinter performs well in the blast fur-

nace, particularly if it is made with fluxes added before sintering and sized

to 25 mm 6 mm before charging to the furnace.

Because the sinter product is subject to breakage and abrasion duringhandling, this process is mostly used for processing ores from mines that

are very close to the blast furnace operation and for recycling iron-

bearing fines, such as furnace dusts and mill scale.

Nodulizing

Like sintering, nodulizing does not require the addition of binders. The

process works by charging iron-bearing fines to a rotary kiln and heating

to the point of incipient melting. As the charge is tumbled in the kiln, it

forms into nodules that are bonded together by the liquified portion of

the partially melted fines. The process does have a few advantages, such

as insensitivity to feed moisture and particle size and high strength of the

nodules; however, its disadvantages of high fuel consumption, operating

and control difficulties, nonuniform nodule size, and poor nodule redu-

cibility in the blast furnace have tended to make this process uncompe-

titive, and it is no longer in general use.

Briquetting

Briquetting consists of compressing fines into lumps of regular shape

using rolls, punches, extruders, and similar devices. Although it is used

routinely for many materials, briquetting of unheated iron ores has not

been successful because the available binders do not develop sufficientstrength. Briquetting is used for some direct reduction processes because

the metal produced is ductile enough to bond together by mechanical

deformation without the need for binder.

A REVIEW OF BINDERS IN IRON ORE PELLETIZATION 3


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This process is generally more expensive than other agglomerating

processes due to wear of the briquetting surfaces and the energy required

to compress the briquettes.

Pelletizing

Pelletizing differs from the other agglomeration techniques listed in that

the powdered ore is first formed into a ‘‘green’’ pellet or ball, which is

then dried and hardened in a separate step, usually by heating. Green

pellets are made by combining moist ore with a binder and rolling it into

balls using either a pelletizing disc or a pelletizing drum. The pellets are

then dried, preheated, and finally heated to approximately 1300C to

harden them. This temperature is lower than the melting point of iron

oxides, and the pellets harden by recrystallization across the particle grain

boundaries.

Pelletizing has the following requirements:

The ore being pelletized must have a sufficiently fine particle size dis-

tribution.

Sufficient moisture is needed to make the ore sticky enough to pelletizebut not so much moisture that the ore becomes ‘‘muddy.’’

A binder is necessary to hold the particle grains together after the pellet

is dried and before it is finally hardened.

The pelletization process is very widely used, particularly when the ore

must be shipped great distances between the mine and the blast furnace,

because the fired pellets are durable and easy to handle. The pellets also

perform very well in the blast furnace, with good bed permeability andreducibility.

The pelletization process is the primary consumer of binders in the

iron ore industry. The selection of a proper binder type and dosage is of

critical importance in producing good quality pellets at a reasonable price.

FUNCTIONSOFABINDER

Out of the four basic agglomeration techniques, pelletization is the onethat is both widely used in the iron ore industry and requires a binder to

be added to the ore in order to work properly. This review will therefore

concentrate on the suitability of various binders for pelletization.



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Binders accomplish two very important functions in iron ore pelle-

tization:

The binder makes the moist ore plastic, so that it will nucleate seeds

that grow at a controlled rate into well-formed pellets.

During drying, the binder holds the particles in the agglomerates to-

gether while the water is removed and continues to bind them together

until the pellet is heated sufficiently to sinter the grains together.

The suitability of a binder is determined by how well it can carry out each

of these functions while at the same time not causing contamination orsintering problems.

An additional feature of bentonite binder that is helpful in pelletization

is its ability to absorb several times its own weight in water. This makes it

possible to control the free moisture content of the pelletization feed by

simply adjusting the bentonite addition rate. This is a valuable feature

because pelletization works over a fairly narrow range of feed moisture

contents. Because not all ore concentrates will filter to the same moisture

content, this capability of bentonite gives a relatively inexpensive method

for making small adjustments of feed moisture content after filtration.

CLASSIFICATIONSOFBINDINGSYSTEMS

Binders are in general anything that can be used to cause particles to

adhere together into a mass. Since binders can accomplish this in a

number of different ways, they cannot all be used in all possible appli-

cations. It is therefore useful to categorize binders in some systematicway. Several different classifications have been proposed, with one of the

most useful being the division of binders into the following five groups

(Holley 1982):

1. Inactive Film: The binder forms a sticky layer on the particles which

bind them together. The film can bind by capillary

forces or through adhesional or cohesional forces.

The binding is typically reversible.2. Chemical Film: The binder forms a film on the particle surface, which

then undergoes a chemical reaction and hardens. The

binding is typically irreversible.



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3. Inactive Matrix: The binder forms a more-or-less continuous

matrix that particles are imbedded in. Often the

binder is a material such as a tar, pitch, or waxthat is heated or emulsified to make it fluid and

then hardens upon cooling or drying. Binders of

this type often require high compaction forces

and high binder dosages. Binding may be re-

versible upon heating.

4. Chemical Matrix: The binder forms an approximately continuous

matrix, which undergoes a chemical reaction that

causes it to harden. Binding by this mechanismusually is irreversible.

5. Chemical Reaction: The binder actually undergoes a chemical reac-

tion with the material that it is binding, resulting

in a very strong bond. This is specific to parti-

cular types of material, and binders of this type

have not been developed for iron ores.

It should be noted that sometimes a single binder can be classified in

different ways, depending on its dosage and the details of its application;

examples of each of these types of binder are shown in Table 1. Iron ore

pelletization currently uses ‘‘inactive film’’ binders because they are

generally effective at low dosages, bind the particles rapidly without the

need to wait for a chemical reaction to be completed, and typically do not

require large compaction pressures in order to work.

PELLETQUALITYMODELS

A number of pellet quality models have been developed for predicting

various properties of pellets. These models assume that the pellet prop-

erties are dependent only on the physical characteristics of the particles

being pelletized, the viscosity and surface tension of the fluid phase, and

the bond strength of the binder.

Wet tensile strength is related to the wet crushing strength of a pellet.

Rumpf’s formula for the tensile strength of moisture-filled agglomerates

(wet balls) is as follows (Rumpf 1962):

st ¼ C 1 e

e

g

d cos y; ð1Þ



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T a b l e 1 . E x

a m p l e s o f v a r i o u s b i n d i n g s y s t e m s

I n a c t i v e f i l m

C h

e m i c a l f i l m

I n a c t i v e m a t r i x

C h e m i c a l m a t r i x

C h e m i c a l r e a c t i o n

W a t e r

S o d i u m s i l i c a t e þ

C O 2

C o a l t a r p i t c h

Q u i c k l i m e þ

w a t e r

W a t e r ( p a r t i a l l y

A l c o h o l


D i l u t e a c i d

P e t r o l e u m a s p h a l t

H y d r a t e d l i m e þ

C O 2

d i s s o l v e s o r r e a c t s

O i l s


L i m e

C a r n u

b a w a x

L i m e þ

m o

l a s s e s

w i t h m a n y m a t e r i a l s ) ;

B e n t o n i t e c l a y

P a r a f fi n

P o r t l a n d c e

m e n t þ

D i l u t e s u l p h u r i c a c i d

A t t a p u l g i t e

c l a y

S l a c k

w a x

w a t e r

( r e a c t s w i t h

S o d i u m s i l i c a t e

W o o d

t a r s

P l a s t e r o f P

a r i s þ

s e m i s o l u b l e a l k

a l i s ) ;

P o t a s s i u m s

i l i c a t e

G i l s o n i t e

w a t e r

D i l u t e p h o s p h o r i c

S o d i u m l i g n

o s u l f o n a t e

R e s i n s

a c i d ( r e a c t s w i t h

C a l c i u m l i g n o s u l f o n a t e

B e n t o

n i t e c l a y

s e m i s o l u b l e a l k

a l i s ) ;

A m m o n i u m

l i g n o s u l f o n a t e

A t t a p u l g i t e c l a y

M a g n e t i t e a n d

w a t e r

P o l y v i n y l a l c o h o l

C o r n

s t a r c h

( m a g n e t i t e o x i d i z e s

M o l a s s e s

D r y s u g a r s

a n d r e c r y s t a l l i z e s ) ;

C o r n s t a r c h

D r y l i g n o s u l f o n a t e s

M a g n e s i u m c h

l o r i d e

T a p i o c a s t a

r c h

C o l l o i d a l a l u m i n a

a n d w a t e r ( m a g n e s i u m

W h e a t fl o u r

C o l l o i d a l s i l i c a

h y d r o l i z e s t o o

x i d e )

P o t a t o s t a r c h

M e t a l s t e a r a t e s

C a s e i n

G l u c o s e

D e x t r i n

S a l t s

S u l f a t e s

A l g i n a t e s

G l u e s

G u m a r a b i c

S o d i u m b o r

a t e

F u l l e r ’ s e a r t h

F o r t h e ‘ ‘ i n a c t i v e fi l m ’ ’ b i n d e r s , a l l e x c e p t t h e w a t e r , a l c o h o l , a n d o i l r e

q u i r e s o m e l i q u i d ( u s u a l l y w a t e

r ) t o b e a d d e d t o c o m p l e t e t h e

b i n d i n g

s y s t e m ( H o l l e y , 1 9 8 0 , 1 9 8 1 ) .

7


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where

st ¼ tensile strength of an agglomerateC ¼ a constant

e ¼porosity

g ¼ surface tension

d ¼mean particle size of grains in the agglomerate

y ¼ contact angle at the air=water=solid interface

Impact fracture resistance is viscosity controlled because of the high rate

of deformation upon impact. This is predicted using Wada’s viscocapil-lary model (Kater and Steeghs 1984). The dependence of impact fracture

on fluid viscosity means that the impact resistance can easily be affected

by the binder type, as binders can strongly affect the fluid viscosity.

In addition to the wet ball properties, it is important to predict

whether the pellets will remain intact upon drying and heating. The main

mode of failure of pellets during heating is thermal spalling, where

pressure buildup inside the pellet due to fluid evaporation causes the

outer layers of the pellet to flake off. The Kozeny-Karman equation for

thermal spalling is as follows (Kater and Steeghs 1984):

D p ¼ K Z

d 2ð1 eÞ2

e3 Lv; ð2Þ

where

Dp ¼pressure drop resulting from the flow of a fluid through a porous

system of equal-sized spheresK ¼Kozeny-Karman constant

Z ¼ viscosity of the liquid phase

d ¼particle diameter

e¼ porosity

L¼depth of the agglomerate

v ¼ velocity of fluid flow (drying rate)

Once the pellet has dried, the relevant strength parameter is the drystrength, which is a function of the type of binder used and the grain

morphology inside the pellet. The strength is related to the dry tensile

strength, which can be estimated as follows (Rumpf 1962):



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st ¼ C 1 e

e

H

d 2 ð3Þ

where

st ¼ tensile strength of an agglomerate

C¼ a constant

e¼porosity

H¼mean strength of an interparticle bridge

d¼mean particle size of grains in the agglomerate

IMPORTANTBINDERCHARACTERISTICS

There are a number of property tests that have been used, either his-

torically or currently, to measure the quality of bentonite binders

(Wakeman et al. 1980). Many of these tests are also applicable to other

binder types. These tests are

Batch balling

Enslin water absorption

Alumina plate water absorption

Grit content

Moisture content

Size distribution

Marsh funnel

Gel strength

Colloid content

Chemical analysis Methylene blue uptake

Free swell

Exchangeable cations by atomic absorption spectroscopy (AAS)

Glycolated layer expansion by X-ray diffraction (XRD)

BatchBalling

This is the most basic measure of the quality of a binder for iron orepelletization, as it produces green pellets that can be directly measured to

determine quality. For this test, bentonite and moist iron ore concentrate

are mixed and pelletized in a small balling drum, disc, or tire to produce



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green balls. The resulting green balls are then sized to between 7=16 inch

(1.11 cm) and 1=2 inch (1.27 cm) and evaluated for drop number, wet

strength, and dry strength. If desired, the balls can also be sintered formore advanced testing. Results are reported as three values: average

number of drops from a height of 18 inches (45.7 cm) before failure (drop

number); average wet compression strength at failure, expressed in force

per pellet; and average dry compressive strength at failure expressed in

force per pellet. The drop test, wet compression test, and dry compression

test are repeated for a number of pellets, usually 20, in order to provide

good statistics. Pellet crushing tests should be carried out in accordance

with ASTM standard method E 382, ‘‘Standard Test Method forDetermination of Crushing Strength of Iron Ore Pellets’’ (ASTM 1997).

Pellet strengths are frequently reported in units of pounds=pellet by

industry in the U. S., and in kilograms=pellet elsewhere. It should be noted

that since the Newton is the recognized unit of force in SI, it is technically

more correct to express crushing strengths as Newtons=pellet (N=pellet)

or dekanewtons=pellet (daN=pellet). All of these units can be found used

in the pelletization literature. The conversions between these units are

1 lb ¼ 0:4536 kg; 1lbf ¼ 4:448N; 1kgf ¼ 9:807N; 1daN ¼ 10 N:

Unfortunately, while there are standard methods for evaluating pellets

after they are made, there is no generally agreed-upon standard method

for producing pellets in the first place. Each iron ore producer uses their

own ore, apparatus, and procedures to produce pellets, and therefore it is

virtually impossible to make meaningful comparisons of results from

different laboratories. The closest approach to a standard procedure was

the procedure developed by the Bentonite Users Committee (1982), whichwas as follows:

1. Equipment:

6.00-6 airplane tire (approximately 16 inches (40.6 cm) 6 inches

(15.24 cm)), rotating at 52 rpm.

Model No. 1 Cincinnati Muller (12 inch (30.48 cm)). Screens: 4 mesh (4.75 mm), 6 mesh (3.35 mm), 13.2 mm, 12.5 mm, and

either 10 mesh (2.0 mm) or 12 mesh (1.7 mm).

Atomizer filled with distilled water.



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Balance accurate to 0.1 g with 3 kg capacity.

Means to remove balls from tire.

Sealable containers for seeds and balls.

2. Concentrate Sample:

2500 g of ore concentrate (dry weight) at 8.5% moisture. Of this ma-

terial, 700 g is used for seed production.

3. BentoniteAddition=Mixing:

Weigh out appropriate amount of bentonite (the Bentonite UsersCommittee used a level of 0.7% bentonite in their reproducibility

studies).

Spread concentrate uniformly in the muller.

Distribute bentonite uniformly over top of concentrate.

Mix for three minutes.

If necessary, moisture content can be adjusted by slowly adding water

ahead of the muller wheel after 1 minute of mixing.

Screen mixed product through a 2.0 mm screen.

4. Seed Ball Preparation:

Start with 700 g of the feed material.

Add a small portion of feed to the rotating balling tire and use ato-

mizer to spray the material with distilled water to initiate seed for-

mation.

When top size of seeds approaches 4 mesh (0.187 inches (0.475 cm)),

remove from tire and screen at 4 and 6 mesh (0.132 inches (0.335 cm)).Discard the þ4 mesh material. Save the 4=þ6 mesh seeds in a sealed

container.

Return the 6 mesh material to the balling tire and add additional feed

and water spray until size approaches 4 mesh again.

Screen seeds and repeat procedure until a sufficient amount of 4=þ6

mesh seeds have been produced (approximately 34 g).

5. Green Ball Preparation:

Place 34 g of seeds into balling tire and add concentrate by handfeeding

over a 6-minute period. Add water spray as required.



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After forming balls, allow one additional minute of re-rolling without

additional water spray.

Screen balls at 13.2 mm and 12.5 mm; retain the 713.2=þ12.5 mm ballsfor testing.

In the experiments that were carried out with the above procedure by the

Bentonite Users Committee to determine the reproducibility of the test,

concentrate from the Minntac plant (Mountain Iron, Minnesota) was

used as a standard feed material. The definition of this procedure was not

completely successful in producing reproducible results between labora-

tories, and so it was never made into a formal standard (Bentonite UsersCommittee 1980d1982b).

EnslinWaterAbsorption

The Enslin test is a measure of water absorption capacity of the binder,

which was originally designed for soil testing (Seger and Cramer 1984). In

this test, 0.2 g of binder are placed on a glass frit connected to a buret and

allowed to absorb water from the buret for a set period of time of up to24 hours. The volume of water absorbed is measured, converted to a

weight, and the results are reported as percentage weight gain.

Alumina PlateWaterAbsorption

Like the Enslin test, the plate water absorption Test (PWAT) is used to

measure the water absorption capacity of the binder. This test was

developed by the Bentonite Users Committee (19781980f) and wasspecifically designed for the iron ore industry to evaluate binders. The

procedure is as follows. First, accurately weigh 12 g of binder onto a

circle of filter paper of specified size. This is placed onto a porous ceramic

plate that is nearly immersed in water and allowed to absorb water for a

specified time of up to 24 hours. At the end of this time, the filter paper

and binder are removed from the plate, weighed, and the percentage

weight gain from absorbed water is calculated and reported. The com-

plete procedure is described by ASTM standard E 946 (ASTM 1992b),but it has since been discontinued by ASTM. A problem in this test is that

there is apparently variability of the results when different alumina plates

are used, and so calibration is a concern.



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Grit Content

Grit content of bentonite is the fraction of the material that is retained on

a 325 mesh screen during wet screening. The active clay minerals in

bentonite disintegrate when wet, and the inactive minerals remain as

particles coarse enough to be captured by the screen. Because coarse

mineral particles do not contribute to binding, the grit test provides a

measure of the amount of inactive material in the binder.

Moisture Content

Because binders absorb moisture readily, they will often contain a sig-

nificant amount of moisture, even when in a nominally ‘‘dry’’ state.

Binders are sold by weight, and so this moisture represents material that

is being paid for but does not directly benefit the user. Moisture is

determined by drying the material at 105C until it reaches a constant

weight.

SizingThis is a measure of the fineness of grind of the material as received from

the supplier, and consists of dry screening of the material on a 200-mesh

screen. The more finely ground the material is, the more rapidly it will

disperse or dissolve in water.

SettlingTest for Ultrafine Particles

The presence of ultrafine particles in an ore concentrate affects theproperties of the pellets made. Because many binders consist of colloidal

material, a means for measuring ultrafines can give an estimate of how

much binder is actually present in production pellets. One procedure for

measuring ultrafines was devised by Stone and Cahn (1968) as follows:

Weigh material accurately using between 4 and 5 g of material.

Blend with 220 ml of distilled water in a Waring blender at low speed

for 40 seconds. Wash suspension into a 250-ml graduated cylinder and dilute with

distilled water to 250 ml.

Allow the suspension to settle for a predetermined length of time.



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Decant a measured volume of the suspension and accurately record its

pH, temperature, and weight.

Filter the suspension and determine the weight of the solids by stan-dard gravimetric techniques. These solids are the ultrafine fraction.

Calculate the quantity of ultrafine material present in the original

material.

This procedure is not fully standardized. The results will vary depending

on the specific settling time and decantation procedure used.

Diffusibility and Bonding Strength

This is a nonstandard test that has been suggested as a rapid means for

evaluating bentonites. Diffusibility is determined by first dispersing 10 g

of bentonite in 490 ml of water. A flat bed of dried iron ore concentrate is

then prepared, and 1 ml of the dispersed bentonite slurry is then placed

on the bed in each of several designated spots. The wetted spots are then

dried to form ore lumps and their appearance observed. The volume of

each dried lump is determined, and the lump volume in cm3 is the dif-

fusibility. The impregnated ore lumps are then shaken by an automaticsieve shaker on a 20-mesh screen for 30 seconds. The ‘‘bonding strength’’

is the percentage of the ore lump weight that remains behind on the

screen. Although this test is much faster and uses less material than a

complete pelletization test, its reproducibility and relevance to plant

performance is not as good.

Marsh Funnel

The Marsh Funnel is a method for quickly estimating the viscosity of abentonite-water slurry. This method consists of preparing a slurry of the

bentonite in water and determining the amount of time it takes to flow

from a standardized funnel. It is useful for applications where slurry

viscosity is important, such as production of drilling muds, but is not a

good predictor for binding properties. It is therefore of little direct

interest in iron ore pelletization.

Gel Strength

This is a measure of the shear strength of a suspension of bentonite in

water, as measured by a direct-reading viscometer. Again, this is a useful



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measurement for applications where the flow properties are important,

but it has been found to have little relevance to iron ore pelletization.

Colloid Content

Colloidal material in a bentonite suspension is measured by preparing a

suspension of 12% by weight bentonite in water and allowing it to settle

in a graduated cylinder for 1824 hours. The liquid is then decanted off,

weighed, dried, and re-weighed. The weight of solids remaining after

evaporation of the water is then taken as the amount of colloidal material

in the clay.

Chemical Analysis

Chemical analysis is used to monitor deleterious impurities in the binder,

such as phosphorus and sulfur, as well as to determine the important

components present. Analysis can be carried out by any convenient

means, such as AAS, inductively coupled plasma spectroscopy (ICP), or

X-ray fluorescence spectroscopy (XRF).

Methylene Blue Uptake

The quantity of Methylene Blue that can be adsorbed by the clay is a

measure of its ion exchange capacity. The basic procedure is to titrate a

suspension of the clay with a solution of Methylene Blue and determine

the addition level that allows color to appear in the liquid phase

(ASTM 1992a). A typical value for Methylene Blue uptake is

90 milliequivalents=100 g clay.

Swell Index

The swell index is a means of evaluating the degree to which a clay will

swell in contact with water. The test is carried out by drying and grinding

the clay to pass 150 mm and then slowly dusting 2.00 g of the clay over the

surface of 90 ml of water in a 100-ml graduated cylinder. The cylinder is

then carefully washed, the level is brought up to 100 ml, and it is allowedto stand for 16 hours. The volume of the settled material after 16 hours is

then measured (ASTM 1995). A typical value for the swell index is

30ml=2 g.



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ExchangeableCationsbyAtomicAbsorptionSpectroscopy(AAS)

The characteristics of bentonite are controlled by the nature of the

exchangeable cations in the expandable layer. Clays with Naþ as the main

cations in the expandable layer are much more able to expand, disperse,

and absorb water than clays with Caþ2 as the main cations. These types

of clay can be distinguished by atomic absorption spectroscopy.

Glycolated Layer Expansionby X-RayDiffraction (XRD)

It is possible to measure the amount of expansion of an expandable

clay by low-angle XRD. This can be a powerful analytical tool fordistinguishing grades of expandable clays; however, if water is used to

expand the clay, there are many variables that can confound the dif-

fraction results, making them essentially meaningless. It is therefore

much more valuable to use a glycol as a standard liquid, which will cause

the various clay types to have a reproducible, characteristic degree of

expansion.

TYPESOFBINDER

Literally hundreds of materials have been considered for use as binders in

iron ore pellets, with the goal of finding the material that will produce the

highest-quality final pellet at the lowest possible cost, with minimal

introduction of contaminants and with minimal inconvenience in pro-

cessing. These various types of binders can be broadly classified as

Clays and colloidal minerals Organic polymers and fibers

Cements and cementitious materials

Salts and precipitates

Inorganic polymers

Each of these classes of binder have inherent advantages and drawbacks

and all have been investigated to some extent as iron ore pellet binders.

The remainder of this review will cover each of these binder classes indetail.

The strength of a pellet is dependent on the type of bonding pro-

duced by the binder, as shown in Figure 1. Virtually any finely divided



17/90

material can contribute to Van der Waals bonding, but this type of bond

is very weak and of only minor importance. Capillary forces are stronger

but still are not sufficient for finished pellets and additionally require the

presence of liquid in the pellet. Binders that can take advantage of

adhesional or cohesional forces are therefore needed.It should be noted that, in addition to affecting the unfired strength

of the pellets, various additives alter the characteristics of fired pellets.

Studies have been carried out to determine the effects of additives such as

NaCl, KCl, CaCl2, MgCl2, Ca(OH)2, MgO, Al2O3, CaCO3,

CaMg(CO3)2, glucose, ferrous sulphate, and bentonite on the fired pellet

properties (Ball et al. 1974). Some of these additives increase strength up

to a certain point, whereas others have no effect or even cause a strength

decrease. Microstructural studies have shown that these effects are linkedto the degree to which additives cause quartz dissolution and melt for-

mation. In general, electrolytes (NaCl, KCl, CaCl2) and alkali calcium

compounds (Ca(OH)2, CaCO3) tended to cause an increase in fired

Figure 1. Magnitudes of bond strengths for various classes of interparticle bonds in pellets:

(A) van der Waals’, magnetic, or electrostatic forces; (B) capillary forces from the liquidphase; (C) adhesional and cohesional forces; (D) mechanical interlocking; (E) solid bridges

formed by sintering or crystallization of dissolved materials (after Sastry 1996).



18/90

strength, mainly due to an increase in the amount of slag melt that

formed; however, overdosage of any of these additives leads to a decrease

in strength. MgO reduced the fired strength, due to reaction with themagnetite to produce magnetite=magnesioferrite solid solutions, with

relatively little material left over to form a slag. Bentonite tended to

increase the pellet strength due to increased amounts of slag whereas

glucose reduced the strength by increasing the porosity. Ferrous sulfate

had no significant effect on fired pellet strength. The effect of additives

has not been completely studied, therefore it is difficult to be certain in

advance whether a particular binder will have undesirable effects on the

fired pellet quality.

BENTONITE, OTHERCLAYS, ANDCOLLOIDALMINERALS

Clays are members of a class of minerals known as the Phyllosilicates,

which is derived from the Greek word for ‘‘leaf.’’ They are so named

because their crystalline structure allows them to cleave in one direction,

and many of the members of the group therefore have a plate-like or

flaky appearance. These minerals consist of sheets of SiO4 tetrahedra,

arranged in various ways with a variety of counterions that neutralize the

excess charge on the sheets and bind them together into layers. These

layers are weakly bonded in clays and can be easily separated when wet.

As a result, clays can be readily dispersed in water as either a thick,

plastic mass or as very finely divided platelets. When a slurry of clay

dries, the plates can attach mechanically or electrostatically to surfaces,

acting as a binder. Some clays are more useful for this purpose than

others, depending on details of their structures. It is generally accepted

that a clay that performs well in the plant must have two characteristics:

1. A high degree of dispersion in the plant concentrate

2. A high capacity for absorbing moisture in the balling feed

The clays which possess these properties to the greatest degree are the

bentonite clays, which are the most commonly used clay-type binders.

Structure and Chemistry of Bentonite

Bentonite is formed by hydrothermal alteration of volcanic ash deposits.

It is actually a mixture of clay minerals, with the primary component



19/90

being the smectite class mineral, montmorillonite, which has the ideal

composition: (Na,Ca)0.33(Al1.67,Mg0.33)Si4O10(OH)2 nH2O. The basic

crystal structure of montmorillonite is shown in Figure 2. Isomorphicsubstitution of Alþ3 with Mgþ2 into the tetrahedral SiO4 sheets alters the

Figure 2. Structure of the smectite crystal. Each clay platelet consists of three layers: two

layers of silica tetrahedra and an octahedral alumina=magnesia layer joining them. Platelets

are loosely bonded by counterions (typically sodium or calcium) between them. In the pres-

ence of water, the counterions hydrate, causing the clay to expand (after Grim 1968).



20/90

crystal charge balance and requires surface adsorption of exchangeable

cations (commonly Naþ and Caþ2) to balance the charge. When com-

bined with water, hydration of these exchangeable cations causes the claymineral to swell. The swelling ability of montmorillonite varies depending

on the type of exchangeable cation. Calcium ions have a higher charge

and smaller diameter than sodium ions, and as a result the calcium ions

tend to interact more strongly with the aluminosilicate platelets, making

them less prone to hydration. As a result, sodium montmorillonite

hydrates and expands readily on contact with water whereas calcium

bentonite expands to a much lesser extent.

The expansion of the clay minerals in bentonite when they come incontact with water has three effects that are of interest in pelletization:

1. First, it absorbs water, which can be valuable for controlling the

moisture content of the finished pellets. It also increases the viscosity

of the fluid between the mineral grains in the pellet, leading to a well-

rounded, plastic pellet that can be conveniently handled for sizing and

transport in the plant.

2. Second, the expanded clay is very easily spread through the iron ore

upon mixing. During drying, the clay bonds to the mineral grains and

to each other, giving excellent dry strength to the pellet. This is one of

the most important functions of a pellet binder because in the absence

of a binder, the pellet will disintegrate after it is dried. The effect of

bentonite platelets on pellets during the drying process is illustrated in

Figure 3.

3. During sintering to produce finished high-strength pellets, the sodium

and calcium components of the bentonite act as fluxing agents, re-

ducing the melting point of some of the minerals in the pellet. Thisallows a portion of the pellet to melt before the sintering temperature

is reached. This helps to strengthen the pellets during the preheating

stage, allowing dusting and breakage to be minimized during transfer

to the final firing step.

4. The traditional view of the behavior of clay as a binder is that the

expanded clay disaggregates into submicron platelets, which then at-

tach to the iron ore particles and to each other as they dry. One of the

features of clay minerals that help in this regard is that the edges of theplatelets tend to have an electrostatic charge of the opposite sign from

the faces of the platelets. This causes the clay platelets to bond to each

other quite strongly by electrostatic bonding as the slurry dries (Van



21/90

Olphen 1987); however, it appears that this traditional view may not

be entirely correct, particularly when the moisture content of the

material being bonded is not sufficient to completely disperse the

bentonite (Wenninger and Green 1970; Kawatra and Ripke 2001,

2002a). In low-moisture situations, it has been suggested that, rather

than dispersing, the clay grains expand into a stack of lubricated

platelets. These platelets can slide relative to one another under shear

to form strands, as shown in Figure 4. This effect is consistent with the

fibrous appearance of bentonite binding sand grains, which is shown

in the scanning electron micrographs of Figure 5.

Sources ofBentonite

The best-grade sodium bentonites in North America are mined from

Wyoming, from deposits where beds of volcanic ash reacted with fresh

Figure 3. Traditional view of how bentonite platelets bind mineral grains in a pellet. Plate-lets are initially dispersed in the liquid, and the platelets bond to the mineral grains and each

other as the liquid dries. Bonding is enhanced by the electrostatic attraction between the pla-

telet faces (which have a negative charge) and the platelet edges (which are positively

charged).



22/90

water over time. Lower-grade calcium bentonites that formed from

alteration of volcanic ash by seawater are also available, but these aremuch less suitable as pellet binders because of their lesser expansion

ability.

Wyoming sodium bentonites cost approximately $0.025=lb

($0.055=kg) at the point of production, and so the cost of bentonite is

quite reasonable; however, competition for the highest-grade bentonites

has increased, as it is in great demand for use in ‘‘clumping’’ cat litter

(Rosten 1999). Since the cat litter market is bidding the price of bentonite

up to as high as $0.10=lb ($0.22=kg), these high-grade bentonites arebecoming much less available for use in the comparatively low-value iron

ore pelletization market. In 1997, the value of bentonite sold for use as

cat litter was $840 million and is expected to reach $1 billion by the year

Figure 4. Behavior of bentonite grains that are not completely dispersed in water. The grain

expands when moistened and the platelets are lubricated by the interplatelet water. Under

shear stress, the grain can then spread into a long fiber in an effect similar to spreading a

deck of cards across a table (Ripke and Kawatra 2000a).



23/90

2003, and therefore this trend of reduced availability of high-grade

bentonite is likely to continue. It will therefore be necessary to use lower-

grade bentonite or alternative binders to keep binder costs down in iron

ore pelletization.

Factors Affecting Bentonite Performance

Bentonites from different sources and deposits behave differently, with

considerable variations depending on details of their composition,

structure, and history. The most important parameters for evaluating a

bentonite are as follows

Water Absorption Capacity. Using the plate water absorption test

(PWAT), good-grade bentonites can absorb in excess of 900% of their

weight in water. The best grades of bentonite are those where the main

counterions in the expandable layer are sodium (sodium bentonites), as

these bentonites are highly absorbent, expand to as much as 14 timestheir dry volume on contact with water, and disperse readily in water, all

of which result in excellent binding properties. When the main counter-

ions are calcium (calcium bentonites), the water absorption, swelling, and

Figure 5. Scanning electron micrographs of silica sand and of the same sand after bonding

with bentonite. The bentonite formed strands stretching over and between grains, which is

consistent with the bonding mechanism described in Figure 4 (Wenninger and Green 1970).

(a) Sand grains, AFS Fineness No. 55, 250 magnification. (b) Sand with 6.0% sodium

bentonite, mulled 1.5 minutes with 3.2% water, 250 magnification.



24/90

dispersion is much reduced, and these bentonites are correspondingly

much less effective as binders.

The effects of relatively small variations in water absorbency on

pellet strength can be seen in Table 2. For bentonite from each source,

increases in the water absorbency were accompanied by increases in both

the 18-inch (45.7 cm) drop number and the dried pellet strength. The

bentonites from source ‘‘A’’ also appear to produce higher pellet qualitythan bentonites from source ‘‘B’’ with similar PWAT values. While the

water absorbency does have an effect, it is apparent that other properties

of the bentonite have effects of similar magnitude. As a result, there is

some question about the importance of variations in water absorbency

for iron ore pelletization.

Particle Size Distribution. Fine particles are important for a good binder

because they increase the available surface area for binding. In general,

the finer the clay, the stronger the pellets will be (Ehrlinger et al. 1966). Alow amount of grit and a high quantity of 2mm material are both

correlated with high pellet strength, with the quantity of 2mm material

being the most important (Volzone and Cavalieri 1996).

Calcium=Magnesium=Sodium Content. The most water absorbent and

expandable bentonites are those where the exchangeable cations are

predominantly sodium. The Wyoming bentonites are well known for

their high ratio of sodium to calcium and their resulting high quality.Bentonites which contain more calcium and magnesium as their ex-

changeable cations are much less water absorbent, generally to such a

degree that they are not acceptable as pellet binders.

Table 2. Effects of bentonite source and PWAT value on pellet properties in an

operating ore concentrator

Binder

source

PWAT value,

%

%

moisture Drop no.

Dry strength, Lbf =pellet

(N=pellet)

Supplier A 881 10.4 13.3 9.6 (42.7)

895 10.5 15.5 10.7 (47.6)

1028 10.5 18.8 12.8 (56.9)

Supplier B 916 10.5 13.6 9.5 (42.2)

1035 10.5 15.9 10.6 (47.1)

934 10.3 10.7 9.0 (40.0)



25/90

The properties of bentonite can be changed by combination with

chemicals that alter its exchangeable cations. For example, there are

some cases where adding sodium carbonate to bentonites with poorwater absorption properties causes the properties to improve (Ehrlinger

et al. 1966). This is apparently because the added sodium displaces part

of the calcium from the interplatelet space, converting a calcium bentonite

into a sodium bentonite, although it is also possible that the improved

performance is due to the dispersing action of the sodium carbonate.

Sodium carbonate in solution also helps to prevent the properties of

high-grade bentonite from being degraded by soluble sources of cal-

cium, such as gypsum. The effects of adding soda ash (sodium carbo-nate) on the bonding strengths of high and low gypsum content

bentonites are shown in Table 3. It clearly can be seen that increasing

sodium carbonate dosage greatly increases the bonding strengths of the

two high-gypsum bentonites but has little effect on the low-gypsum

bentonite. Addition of sources of soluble calcium, such as Ca(OH)2 and

CaCl2, are known to cause the binding properties of bentonite to

degrade because the sodium bentonite is being converted into calcium

bentonite; however, the effectiveness of bentonite as a binder is

apparently not affected by addition of insoluble calcium compounds

like limestone (CaCO3), as can be seen from Table 4. In fact, addition

of limestone may increase the pellet strength, probably due to altera-

tions in the particle size distribution.

Table 3. Effect of soda ash on the bonding strength of bentonites with high and low

gypsum contents

Bonding strength, %

% Soda ash addition

High-gypsum

bentonite #1

(1.44% SO3)

High-gypsum

bentonite #2

(1.02% SO3)

Low-gypsum bentonite

(0.46% SO3)

0.00 11.65 15.04 27.51

0.25 10.40 15.13 30.02

0.50 11.00 16.18 31.62

1.00 12.23 21.14 35.182.00 16.06 34.92 30.31

3.00 23.02 31.79 31.90

4.00 35.04 34.99 29.28



26/90

Chemical Environment. Chemistry of the plant water can affect the per-

formance of bentonite binders, sometimes quite markedly. Ions in solution

affect the Zeta potentials of both the iron oxide grains and the bentonite

platelets, and so can affect the binding action. An example of this is shown

in Table 5, where magnetite concentrates from three different sources were

first pelletized with their respective plant process waters and subsequentlypelletized after washing repeatedly with distilled water. In each case, the

washed concentrate produced stronger, tougher pellets, apparently due to

removal of dissolved salts. Further tests using washed concentrates and

additions of pure salts showed that high levels of calcium chloride, mag-

nesium chloride, and acidic pH caused the dry compressive strength to

decrease, as can be seen from Figures 68.

Mixing Effects. The effectiveness of bentonite binder appears to increaseas its mixing with the iron ore improves. This effect can be seen in Table 6.

Increasing the mixing time from 30 seconds to 60 seconds resulted in a

small but consistent increase in the dry crushing strength of the pellets and

Table 4. Physical properties of pellets made from Egyptian Baharia iron ore and Gebel

El-Rifai limestone (after Abouzeid et al. 1985)

Material being pelletized

Water,

% wt.

Bentonite

% wt.

Pellet drop

number

Dry crushing

strength, Kg=pellet

(N=pellet)

Porosity,

% vol.

Iron ore 13.5 0.0 15 4.3 (42.2) 31.1

13.5 0.5 27 4.3 (42.2) 29.7

13.5 1.0 39 5.3 (52.0) 32.9

13.5 1.5 52 6.2 (60.8) 32.9

Limestone 16.5 0.0 31 2.4 (23.5) 27.6

16.5 0.5 42 3.8 (37.3) 27.2

16.5 1.0 54 5.8 (56.9) 25.7

16.5 1.5 65 6.6 (64.7) 25.9

Fluxed pellets, 10% limestone 13.8 0.0 19 4.2 (41.2) 34.6

13.8 0.5 21 5.9 (57.9) 33.7

13.8 1.0 21 7.0 (68.7) 33.3

13.8 1.5 26 7.0 (68.7) 33.4

Fluxed pellets, 20% limestone 14.1 0.0 21 3.3 (32.4) 33.7

14.1 0.5 27 5.0 (49.0) 33.9

14.1 1.0 33 6.1 (59.8) 33.3

14.1 1.5 37 6.1 (59.8) 33.4



27/90

had a similar effect on the wet-drop values, although the wet crushing

strengths showed very little change with the increase in mixing time.

Effects of Bentonite in the PelletizationProcess

The first effect of bentonite in pelletization, which occurs even before

balling begins, is control of the moisture content. Pellets have the highestwet strength when their moisture content approaches the value for

complete saturation of the voids (Nicol and Adamiak 1973). Since ben-

tonite absorbs moisture, it can be used to take up excess water and bring

the moisture content down to the saturation point. Using bentonite in

this way also varies the pellet properties due to variations in the binder

dosage. The effects of varying bentonite dosage on pellet properties is

shown in Table 7.

During the balling process, bentonite tends to slow the growth rate of balls compared to balling without bentonite, as can be seen in Figure 9.

While this does reduce the capacity of the process, the bentonite leads to

a smoother, more uniform ball than would be produced without binder.

Table 5. Effect of soluble impurities on the balling properties of magnetite concentrates

from three Bethlehem Steel Co. iron ore concentrators

Concentrate

source

Concentrate

treatment % moist.

Wet drop

18 in.

(45.7cm)

Wet comp.

lbs=pellet

(N=pellet)

Dry comp.

lbs=pellet

(N=pellet)

Pea Ridge Set 1 — unwashed 9.2 6.6 1.4 (6.2) 5.5 (24.5)

Set 1 — washed 9.5 15.1 2.7 (12.0) 10.5 (46.7)

Set 2 — unwashed 9.4 6.3 1.1 (4.9) 4.5 (20.0)

Set 2 — washed 9.6 13.3 2.6 (11.6) 9.8 (43.6)

Grace Set 1 — unwashed 8.9 4.3 2.0 (8.9) 6.3 (28.0)

Set 1 — washed 9.1 6.8 2.1 (9.3) 9.2 (40.9)

Set 2 — unwashed 9.0 5.4 1.9 (8.4) 6.3 (28.0)

Set 2 — washed 9.4 8.4 2.1 (9.3) 9.7 (43.1)

Cornwall Set 1 — unwashed 9.0 3.8 1.6 (7.1) 6.6 (29.4)

Set 1 — washed 9.1 5.8 1.9 (8.4) 9.5 (42.2)

Set 2 — unwashed 8.9 3.8 1.7 (7.6) 6.0 (26.7)

Set 2 — washed 9.1 5.2 2.0 (8.9) 11.0 (48.9)

In each case, the binder was Wyoming bentonite with 78% colloid content and 87% pas-

sing 200 mesh, and was added to the concentrate at a dosage of 12 lb=ton (0.54%) (Rice and

Stone, 1972).



28/90

Figure 6. Effect of magnesium chloride concentration on the drop number, wet compressive

strength, and dry compressive strength of pellets made using Wyoming bentonite and

washed Bethlehem Steel Pea Ridge concentrate. The magnesium chloride was dissolved in

the moisture contained in the pellet (Rice and Stone 1972).

Figure 7. Effect of calcium chloride concentration on the dry compressive strength of pellets

made using Wyoming bentonite and washed Bethlehem Steel Cornwall concentrate (Rice

and Stone 1972).



29/90

Drying rates are reduced by the presence of bentonite, and the amount of

moisture that can be removed without spalling is reduced, both of

which are undesirable; however, bentonite also increases the drying

temperature that can be used without spalling, which tends to cancel out

the drying limitations (Nicol and Adamiak 1973).

OtherClayTypes

One of the major cost items in the use of bentonite clay is the cost of

shipping. This cost is typically higher than the actual price of the ben-

tonite at the mine because iron ore concentrators are a considerable

distance from the sources of high-grade bentonite. This cost could be

greatly reduced if the clay binders could be produced closer to the iron

ore producer, and so there is always interest in developing binders from

clay deposits near the mines.

ExpandingClays. Clays that contain large percentages of montmorillonite

and other expanding clay minerals can form from other sources than the

volcanic ashes that give rise to bentonites. The Illinois Geological Survey

Figure 8. Effect of solution pH on the dry compressive strength of pellets made using

Wyoming bentonite and washed Bethlehem Steel Cornwall concentrate. The pH was ad-

justed by adding of HCl and KOH.



30/90

carried out a study of a number of different ‘‘accretion-gleys,’’ which are

clay deposits formed on the surface of glacial till (Ehrlinger et al. 1966).

These clay deposits are common throughout the Midwest and are con-

veniently located relative to the iron ore pelletization plants. The clayswere dried, disaggregated, screened, and added to an unidentified iron ore

concentrate at a dosage of 16 lbs of clay per ton of concentrate (0.8%).

Three test series were carried out: Series A, with clay screened to pass 325

mesh (45 mm); Series B, which was the same as Series A but with sodium

carbonate also added at a rate of 2 lbs per ton (0.1%); and Series C, with

Table 6. Effect of mixing time and bentonite dosage on the properties of hematite pellets

Bentonite

dosage

lbs=long ton (%)

Mixing

time

(seconds) % moist.

Wet drop

18 in.

(45.7 cm)

Wet crush

lbs=pellet

(N=pellet)

Dry crush

lbs=pellet

(N=pellet)

2 (0.09) 30 10.84 9 2.9 (12.9) 13.9 (61.8)

2 (0.09) 60 10.51 11 3.1 (13.8) 17.6 (78.3)

6 (0.27) 30 10.36 16 4.5 (20.0) 17.8 (79.2)

6 (0.27) 60 10.32 13 4.2 (18.7) 19.4 (86.3)

10 (0.45) 30 10.72 23 4.0 (17.8) 19.5 (86.7)

10 (0.45) 60 10.72 20 3.9 (17.3) 19.9 (88.5)

16 (0.71) 30 10.78 38 3.4 (15.1) 28.5 (126.8)

16 (0.71) 60 10.78 53 3.4 (15.1) 29.2 (129.9)

20 (0.89) 30 11.07 40 3.8 (16.9) 27.3 (121.4)

20 (0.89) 60 11.38 62 3.4 (15.1) 30.4 (135.2)

Table 7. Effects of bentonite dosage on green ball properties

Bentonite,

% moisture

Wet drop Wet crush Dry crush

lbs=long ton

(%) Feed

Feed þ

Bentonite

Finished

pellets

18 in

(45.7 cm)

lbs=pellet

(N=pellet)

lbs=pellet

(N=pellet)

18 (0.80) 9.90 9.47 10.55 15.4 2.41 (10.7) 15.0 (81.7)

15 (0.67) 9.90 9.64 10.23 10.6 2.74 (12.2) 13.4 (73.0)

12 (0.54) 9.90 9.68 10.30 9.8 2.31 (10.3) 10.3 (56.1)

9 (0.40) 10.11 9.65 10.01 6.8 2.43 (10.8) 8.1 (44.1)

6 (0.27) 10.11 9.87 9.75 5.4 2.49 (11.1) 6.8 (37.0)

3 (0.13) 10.07 9.85 9.65 5.1 2.21 (12.0) 4.6 (25.1)

0 (0) 10.07 10.04 9.95 4.0 1.47 (8.0) 1.2 (6.5)

18 (0.80) 10.07 10.04 10.53 14.8 2.48 (13.5) 16.0 (87.2)



31/90

clay screened to pass 20mm and no sodium carbonate addition. The results

of the pelletization experiments are given in Table 8. Unfortunately, all of

the pellets made in these experiments tended to be too dry, as indicated by

their low and variable moisture contents and their very low drop numbers;

however, some conclusions can be drawn. First, nearly all of the clays

produced markedly higher dry strengths when sodium carbonate was

added. Smaller, but still significant, increases in the dry strength were seen

when the size of the clay was reduced. All of the clays produced lower drystrengths than the bentonite sample, but, interestingly, they tended to

produce higher fired strength than the bentonite.

Attempts to upgrade substandard Wisconsin expanding clays was

carried out by Clum et al. (1977). The clay studied had a high quartz

content in its natural state and had a low Na=Ca ratio that tended to

reduce its expansive properties. This clay was first treated by sedimenta-

tion to remove a portion of the quartz, followed by washing with various

sources of Na and K cations. While NaOH, KOH, NaCl, and KCl wereineffective in improving the properties of the clay, it was found that

the clay could be made into an effective binder by washing with an

18.2% solution of sodium carbonate or potassium carbonate; however,

Figure 9. Effect of bentonite on batch balling kinetics (after Sastry 1996). While bentonite

results in a stronger final pellet, the presence of bentonite also causes the pellets to grow

more slowly during the balling process.



32/90

T a b l e 8 . P r o p e r t i e s o f i r o n o r e p e l l e t s m a d

e u s i n g a w i d e r a n g e o f g l a c i a l c l a y s f r o m I l l i n o i s a n d a t y p i c a l w e s t e r n b e n t o n i t e f r o m W y o m

i n g

C l a y

s o u r c e

%

M

% M x

C l a y s i z e a n d

a d d i t i v e s

% m o i s t .

D r o p

n o .

W

e t

s t r e

n g t h

O z

( N )

D r y

s t r e n g t h

L b f ( N )

F i r e d

s t r e n g t h

L b f ( N )

F u n k h o u s e r E a s t

6 1

3 2

7 4 5 m m

4 . 8

2 . 3

1 2 . 8

( 3 . 5 6 )

1 . 9 ( 8 . 4 )

1 3 7

2 ( 6 1 0 2 )

7 4 5 m m þ

N a 2 C O 3

7 . 9

2 . 0

1 6 . 3

( 4 . 5 3 )

3 . 0 ( 1 3 . 3 )

1 3 7

2 ( 6 1 0 2 )

7 2 0 m m

6 . 7

2 . 0

1 6 . 1

( 4 . 4 8 )

2 . 4 ( 1 0 . 7 )

1 4 3

3 ( 6 3 7 4 )

P a n a m a A

6 6

2 5

7 4 5 m m

7 . 0

2 . 0

1 3 . 1

( 3 . 6 4 )

1 . 0 ( 4 . 4 )

1 4 3

0 ( 6 3 6 0 )

7 4 5 m m þ

N a 2 C O 3

4 . 5

2 . 1

1 2 . 5

( 3 . 4 8 )

3 . 6 ( 1 6 . 0 )

1 4 2

6 ( 6 3 4 3 )

7 2 0 m m

6 . 3

2 . 0

1 5 . 9

( 4 . 4 2 )

2 . 4 ( 1 0 . 7 )

1 4 7

9 ( 6 5 7 8 )

R o c h e s t e r

5 5

3 1

7 4 5 m m

5 . 6

2 . 0

1 7 . 0

( 4 . 7 3 )

2 . 4 ( 1 0 . 7 )

1 4 9

6 ( 6 6 5 4 )

7 4 5 m m þ

N a 2 C O 3

5 . 8

2 . 2

1 3 . 8

( 3 . 8 4 )

3 . 9 ( 1 7 . 3 )

1 2 0

7 ( 5 3 6 9 )

7 2 0 m m

5 . 8

2 . 0

1 3 . 3

( 3 . 7 0 )

1 . 3 ( 5 . 8 )

1 3 4

3 ( 5 9 7 4 )

Z i o n C h u r c

h

4 3

2 9

7 4 5 m m

5 . 7

2 . 0

1 2 . 3

( 3 . 4 2 )

3 . 0 ( 1 3 . 3 )

1 2 0

1 ( 5 3 4 2 )

7 4 5 m m þ

N a 2 C O 3

6 . 4

2 . 0

1 0 . 6

( 2 . 9 5 )

4 . 8 ( 2 1 . 4 )

1 0 0

5 ( 4 4 7 0 )

7 2 0 m m

6 . 5

2 . 0

1 3 . 9

( 3 . 8 6 )

3 . 2 ( 1 4 . 2 )

1 2 1

3 ( 5 3 9 5 )

L i e r l e C r e e k

6 1

2 7

7 4 5 m m

6 . 4

1 . 9

1 5 . 0

( 4 . 1 7 )

3 . 2 ( 1 4 . 2 )

1 4 0

4 ( 6 2 4 5 )

7 4 5 m m þ

N a 2 C O 3

8 . 3

2 . 0

1 6 . 1

( 4 . 4 8 )

5 . 7 ( 2 5 . 4 )

1 4 4

2 ( 6 4 1 4 )

7 2 0 m m

5 . 8

2 . 0

1 0 . 1

( 2 . 8 1 )

2 . 1 ( 9 . 3 )

1 2 3

1 ( 5 4 7 5 )

A k e r s S c h o o l

6 1

2 8

7 4 5 m m

6 . 5

1 . 7

1 2 . 5

( 3 . 4 8 )

2 . 9 ( 1 2 . 9 )

1 2 8

9 ( 5 7 3 3 )

7 4 5 m m þ

N a 2 C O 3

5 . 3

2 . 0

1 5 . 3

( 4 . 2 5 )

3 . 4 ( 1 5 . 1 )

1 0 8

3 ( 4 8 1 7 )

7 2 0 m m

5 . 3

1 . 7

9 . 9

( 2 . 7 5 )

2 . 7 ( 1 2 . 0 )

1 0 4

4 ( 4 6 4 4 )

W o o d l a n d ( l o w e r )

6 9

1 7

7 4 5 m m

6 . 2

2 . 0

9 . 5

( 2 . 6 4 )

0 . 8 ( 3 . 6 )

9 2

2 ( 4 1 0 1 )

7 4 5 m m þ

N a 2 C O 3

5 . 9

2 . 0

1 2 . 4

( 3 . 4 5 )

1 . 5 ( 6 . 7 )

8 1

9 ( 3 6 4 3 )

7 2 0 m m

7 . 2

1 . 8

1 0 . 8

( 3 . 0 0 )

2 . 2 ( 9 . 8 )

9 0

5 ( 4 0 2 5 )

32


33/90

W o o d l a n d ( u p p e r )

5 3

3 7

7 4 5 m m

5 . 2

2 . 0

1 1 . 7

( 3 . 2 5 )

1 . 8 ( 8 . 0 )

9 9

4 ( 4 4 2 1 )

7 4 5 m m þ

N a 2 C O 3

8 . 7

2 . 0

1 2 . 2

( 3 . 3 9 )

1 . 5 ( 6 . 7 )

7 3

0 ( 3 2 4 7 )

7 2 0 m m

7 . 1

2 . 0

1 2 . 5

( 3 . 4 8 )

2 . 4 ( 1 0 . 7 )

1 0 1

2 ( 4 5 0 1 )

H i p p l e S c h o o l

5 5

2 7

7 4 5 m m

6 . 5

2 . 0

1 2 . 0

( 3 . 3 4 )

1 . 2 ( 5 . 3 )

1 1 6

2 ( 5 1 6 8 )

7 4 5 m m þ

N a 2 C O 3

6 . 8

2 . 0

1 2 . 8

( 3 . 5 6 )

3 . 0 ( 1 3 . 3 )

1 0 7

2 ( 4 7 6 8 )

7 2 0 m m

6 . 4

2 . 0

1 4 . 3

( 3 . 9 8 )

1 . 0 ( 4 . 4 )

1 0 4

1 ( 4 6 3 0 )

F o r r e s t o n

6 5

2 3

7 4 5 m m

7 . 4

1 . 9

1 3 . 3

( 3 . 7 0 )

0 . 7 ( 3 . 1 )

1 1 9

0 ( 5 2 9 3 )

7 4 5 m m þ

N a 2 C O 3

6 . 8

2 . 0

1 6 . 8

( 4 . 6 7 )

3 . 9 ( 1 7 . 3 )

1 5 2

4 ( 6 7 7 9 )

7 2 0 m m

6 . 3

1 . 9

1 4 . 7

( 4 . 0 9 )

3 . 6 ( 1 6 . 0 )

1 3 1

1 ( 5 8 3 1 )

M t . M o r r i s

5 2

1 3

7 4 5 m m

6 . 3

2 . 0

1 3 . 5

( 3 . 7 5 )

0 . 9 ( 4 . 0 )

1 0 9

5 ( 4 8 7 0 )

7 4 5 m m þ

N a 2 C O 3

6 . 4

2 . 0

1 3 . 9

( 3 . 8 6 )

2 . 0 ( 8 . 9 )

9 8

6 ( 4 3 8 6 )

7 2 0 m m

6 . 1

1 . 9

1 3 . 8

( 3 . 8 4 )

1 . 0 ( 4 . 4 )

1 2 7

6 ( 5 6 7 6 )

C e d a r v i l l e E

a s t

3 1

6 2

7 4 5 m m

6 . 2

2 . 0

1 4 . 6

( 4 . 0 6 )

0 . 8 ( 3 . 5 )

1 0 4

9 ( 4 6 6 6 )

7 4 5 m m

a review of binders in iron ore pelletization.pdf

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