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PRINCIPLES OF

CERAMICS

PROCESSING

Second Edition

JAMES

S

REED

New York State College of Ceramics

Alfred University

Alfred, New York

A Wiley-Interscience Publication

JOHN WILEY SONS, INC.

New York Chichester Brisbane Toronto Singapore



This text

is

printed on acid-free paper.

Copyright

1995 by John Wiley Sons, Inc.

AIl rights reserved. Published simultaneously

in

Canada.

Reproduction

or

translation

of

any part

of

this work beyond

that permitted

by

Seetion

107 or 108 of

the 1976 Uníted

States Copyright Ael without the permission

of

the copyright

owner is unlawful. Requests for permission or further

information should be addressed to the Permissions Department,

John Wiley Sons, Inc., 605 Third Avenue, New York,

NY

10158-0012.

library 0 Congress CataJoging in Publication Data:

Reed, James Stalford, 1938-

Principies of ceramics processing I James S Reed.-2nd cd.

p cm.

Rev. cd. of: Introduction to lhe principies of ceramic processing.

1988.

A

Wiley-Interscience publicatíon."

Ineludes bíbliographical references and índex.

ISBN 0-471-5972 l-X

1

Ceramics.

I.

Títle.

II

Series: Reed, James Slalford, 1938

Introduclion to lhe principies of ceramíc processing.

TP807.R36 1994

666-dc20

94-20838

Printed in lhe Unitcd States of America

10 9 8 7 6 5 4 3 2 1



PART II

CERAMIC RAW MATERIALS

ln studying ceramics processing

it is

necessary to be familiar with the types

of

r w

materiais available. Clay minerais which provide plasticity when mixed

with water; feldspar which acts as a nonplastic filler on forrning and a fluxing

liquid on firing; and sílica which is a filler that resists fusion have been the

backbone of the traditional ceramíc porcelains. Other silicate mineraIs are used

in

whitewares such as ceramíc tile therrnal shock-resistant cordierite products

and steatite electrical porcelains.

Silica aluminosilicates tabular aluminium oxide magnesium oxide cal

cium oxide and mixtures

of these mineraIs have long been used for structural

refractories. Alumina magnesia and aluminosilicates are now used in some

advanced structural ceramics. Silicon carbide and sílicon nitride are used for

refractory abrasive electrical and structural ceramics. Finely ground alumina

titanates and ferrites are the backbone of the electronic ceramics industry.

Stabilized zirconias are used for advanced structural and electrical products and

zircon zirconía and other oxides doped with transition and rare-earth metal

oxides are widely used as ceramic pigments. These materiais are commonly

prepared by calcining partic1e mixtures but some are now produced using

special chemícal techniques.

ln Chapter 3 the more common ceramic materiais produced

in

large tonnage

and widely used

in

ceramics are considered. Special materiais

of

exceptional

purity and homogeneity which are being developed for research and some very

advanced products are discussed

in

Chapter 4.



CHAPTER 3

COMMON RAW MATERIALS

ln this chapter we briefly consider the nature o the starting materiaIs, tradi

tionally called raw materiaIs, that can be purchased from a vendor and received

at a manufacturing site. These materiaIs can vary widely

in

nominal chemical

and mineral composition, purity, physical and chemical structure, particle size,

and price. Categories o

raw materiais include

(1)

nonunjform crude

m a t ~ r i a l

froPlIlª ral deposits, (2)

r( fined

industrial mineraIs lhat have been benefiç,Íªted

til_remove m i n ~ a I i t n P u r i ~ É ' to sígnific3 ntly

i ~ r e a s e

the mineral purity ª. lJJ

p h y ' s i c a t ~ º l ' l s i s t t l . 0 , and (3) hjgh-tonnage industrial inorganic c h ~ _ r n i c ª l ~ J h ª t

llave

u l d e r g o n ~

extensive

c h e t n í ( ; _ ª L Q r º - t : ~ . s _ ~ Í l g . ª . 1 L ( n l 1 e m e n t

to significantly

~ g ~ . ' l < ~ ~ h e _ . chemical purityand i r n p ~ º - v ~ ~ h . e _ p.hysi.cª characterisliçs.

The c h o i ç ~ o f a raw material for a particular product wiI1 depend on material

~ o s t ,

t n - ª r k ~ f a c t ~ r s , '::.( ndor

~ e r y i c e s , technical processing çºº§jdera Í.Qns, and

t b ~ l l t i m a t e performance requirements and market price o the f i n i ~ h ~ r o d u c t

For products in which processing adds considerable dollar value, the cost o

the starting material

is

a relatively small component o the production costs.

Accordingly, a higher-quality and more expensive material may be acceptable

for microelectronics, coatings, fibers, and some high-performance products.

But the average cost

o

raw

materiais for building materiaIs and traditional

ceramics such as tile and porcelain must be relatively low. Cost-benefit con

siderations may suggest substitutions o materiaIs o lower cost that do nol

impair the quality, or altematively, a more expensive material, which may

be

more economically processed and/or which will increase the qualityand per

formance o the product.

5



36


3.1

CRUDE M TERI LS

Many early ceramics industries were based near a natural deposit containing a

combination

of

crude minerais that could be conveniently processed into usable

produets. Construetion materiais such as brick and tile and some pottery items

are historical examples, and many are still identified by the regional name.

Some crude materiaIs are

of

sufficient purity to be used in heavy refraetories:

Crude bauxite, a nonplastic ore containing hydrous alumina mineraIs, clay

minerais, and mineral impurities such as quartz and feme oxides,

is

used in

producing some refractories. Today, however, most ceramics are produced

from more refined minerais.

3.2

INDUSTRI L MINER LS

Industrial minerais are used

in

large tonnages for producing construction ma

teriais, refraetories, whitewares, and some electrical ceramics. They are used

extensively as additives in glazes, glass, and raw materiais for industrial ehem

icals. Common examples are listed in Table 3.1.

Clays are produeed by the weathering

of

aluminosilieate roeks and sedi

mentation. Clay minerais are layer-type hydrous aluminosilíeates whieh ean be

\ e

~ 6 ' \ { o

TABLE 3.1

Starting

Materiais for Ceramics

P

,,-

c

Purity

Category (

Materiais

----------------------------- -------------------------

Crude materiais

Variable Shales, stoneware clay, tile ciay, crude

bauxite, crude kyanite, natural ball

clay, bentonite

Industrial minerais

85-98 Ball clay, kaolin, refined bentonite,

(99

quartz) pyrophyI1ite. tale, feldspar,

nepheline syenite, wollastonite,

spodumene, glass sand, potter s flint

(quartz), kyanite, bauxite, zircon,

rutile, chrome ore, caleined kaolin,

dolomite

Industrial inorganic

98-99.9

Calcined alumina (Bayer process),

chemicals

caleíned magnesia (from brines,

seawater), fused alumína, fused

magnesia, aluminum nítride, silícon

carbíde, silicon nitride, barium

carbonate, titania, calcined titanates,

iron oxide, calcíned ferrites,

zírconia, stabílized zirconia, calcined

zirconates

Special inorganic

>99.9

Various materiais (see Chapter 4)

chemicals



INDUSTRI L MINER LS 7

dispersed into fine particles (Fig. 3.1). Kaolin is a relatively pure, white firing

clay eomposed principally

of

the mineral kaolinite I

2

Si

2

0s(OH)4 but contain-

ing other clay minerais, as indicated in Table 3.2, and a minor amount

of

impurity minerais sueh as quartz

Si0

2,

ilmenite

FeTi0

3, rutile

Ti02

and he-

matite Fe203 Ball day is a sedimentary clay of fine particle size eontaining

Fig. 3.1 Kaolin that is a) aggregated and b) dispersed into platelets.



38 COMMON R W MATERIALS

T ABLE 3.2 Clay Minerais

Mineral

Ideal Chemical Formula

Kaolinite Alz(SizOs)(OH)4

Halloysite

A1

2

(Si

2

0s)(OH)4 .

2H

2

0

Pyrophyllite Alz{Si

2

0sh(OHh

Montmorillonite

(Ali 67Nau33Mgo33)(SizOs)z(OHh

Mica

A1

2

K(Si ,.sAlosOsMOHh

Illite

Al

2

xMg,K,-x-

Source: After W. D. Kíngery, lntroduction

t

Ceramics 1st ed., Wiley

Interscíence, New York, 1960.

complex organic matter ranging down to a submicron size. BentonÍte

is

a

complex clay containing a relatively high proportion of the clay mineral mont

morillonite. Clays are used

in

whiteware formulations and aluminosilicate re

fractories to produce plasticity

in forming and resistance to deformation when

partial fusion occurs during firing.

Other layer-type hydrous silicates are Mg

3

Si

4

0

IO

(OHh

and pY Qphyllite

AI

Si

4

0

IO

(OHh, which are used extensively in compositions for ceramic tile,

cordierite, and steatite porcelain. Commercial grades contain impurities such

as calcite CaCO) or dolomite (Mg,Ca)C0

3

and other mineral impurities that

depend on the source.

Crushed and milled quartz SiO

z

derived from relatively pure deposits

of

sandstone

is

a granular silicate mineral used extensively

in

whitewares, refrac

tories, and glaze compositions (Fig. 3.2). Eelds.Qars composed of the minerais

albite NaAlSi

3

0

g

and microcline or orthodase KAlSi

3

0 g and nepheline syenite

containing albite, microcline, and nephelite KosNa15(Al,Si)20g are the prin

cipal fluxes used

in

whitewares and silicate glazes. Wollastonite

CaSi0

3

is used

in

some tile compositions and glazes. Petatite

lIAlSi

4

0

lO

and spodumene

LiAlSi

2

0

6

are used as a secondary flux and to redu ce the thermal expansion

of

the fired material.

Chrome ores composed principally

of

a complex solid solution of spinels

(Mg,Fe)(AI,Cr,Feh04 and impurities such as dolomite and magnesium silicates

are used

in

combination with calcined magnesia MgO

in

basic refractories.

~ ~ l l ~ CaQ produced by calcining limestone

CaC0

3

and calcined dolo

mite (Ca, Mg)O is bonded with tar and used for lining basic oxygen steel

fumaces. Beneficiated kyanite

AI

2

SiO

s

, bauxite, and zircon SiZr04 are also

used in refractory compositions. Milled zircon is also used as an opacifier in

glazes and

in

producing zircon pigments and is a precursor for zirconia

Zr02'

Calcined kaolin (Fig. 3.2) is used as a nonplastic filler in refractory mixes and

mortars.

The beneficiation

of

industrial mineraIs begins with crushing and grinding

to a smãiT enough size t liberate undesired mineral phases. Further beneficia

tion may indude settling and flotation to segregate mineraIs

by

density or size,

the separation of magnetic minerais using powerful electromagnets, blending

of different processing runs for consistency, and perhaps particle size classifi-





40


DragUne

Sllo\lel

Water

Portable

Blunger

Pump

Wet

Screening

~ r i t

J

Chemical

Leachingor

Magnetlc

Centrifugai

Claaaiflcatlon

Slurry

Blending

and Storage

Slurry

Irom

Other Deposita

Separation

l

Grit

Surlace

Modification

Rotary

Vacuum

Filtration

pron

Drying

Pulverizing

1

Slurry

Spray

Hopper

Bagging

Tank Car Drylng

Car

BOl Car

or Truck

Fig. 3.3 Processing flow diagram for the beneficiation

of

kaolin.

Concentrated soIids are usually dried using a rotary

or

belt dryer

or

by spray

drying. Some materiaIs are calcined, and a hard aggregate is fonned. Dried

cake or calcined materiaIs may be pulverized or ground and then sized or air

elutriated before bagging or loading in hopper cars. Many fine materiais are

loaded and unloaded using pneumatic ftuidization and are stored at the plant

site in large sílos.

3.3

INDUSTRI L INORG NIC CHEMIC LS

Important industrial ceramic chemicals include tabular and calcined aluminas,

magnesium oxide, silicon carbide, sílicon nitride, alkaline earth titanates, soft

and hard ferrites, stabilízed zirconia, and inorganic pigments. Extensive chem

ical beneficiation redu ces the content

of

accessary mineraIs and may increase

the chemicaI purity up to about 99.5 .

For

many materiaIs, the scale of op

eration is extremely Iarge, which aids in lowering the unit processing costs and

selling price.

Alumina AI

2

0

3

is the most widely used inorganic chemical for ceramics

(Table 1.2) and is produced worldwide in tonnage quantities for the aluminum

and ceramics industries using the Bayer processo The principal operations in

the Bayer process are the physical beneficiation of the bauxite, digestion (in



INDUSTRIAL INORGANIC CHEMICALS

the presence of caustic soda NaOH at an eIevated temperature and pressure),

clarification, precipitation, and calcination, followed by crushing, milling, and

sizing (see Fig. 3.4). During the digestion, most of the hydrated aIumina goes

into soIution as sodium aIuminate:

ImpuritY SOlid) +

AI OHh solid)

+

NaOH solnl

(3.1)

Na+ solo) +

AI OH)4 solnl

+

ImpuritY sOlid)

and insoluble compounds of iron, silicon, and titanium are removed by settling

and filtration. After cooling, the filtered sodium aIuminate solution

is

seeded

with very fine gibbsite AI(OHh, and at the lower temperature the aluminum

hydroxide refonns as the stable phase. The agitation time and temperature are

carefully controlled to obtain a consistent gibbsite precipitate. The gibbsite

is

continuously classified, washed to reduce the sodium content, and then cal

cined. Material calcined at lIOO-1200°C is crushed and ground to obtain a

range of sizes (Fig. 3.5). Tabular aluminas are obtained by calcining to a higher

temperature, about 1650°C.

Magnesium oxide MgO of greater than 98% purity is prepared

by

precipi

tating magnesium hydroxide in a basic mixture of treated dolomite and natural

brines or seawater containing MgCl

2

and MgS

4

, followed by washing, filtra

tion, drying, and calcination.

Zirconia

Zr 2 of

99 purity is obtained by the caustic fusion

of

zircon

ZrSi

4

:

Chemical dissolving

of

the silicate

in

water simultaneously hydrolyzes the

sodium zirconate to hydrated zirconia. Zirconia is also produced

by

hot chlo

rination

of

zircon in the Presence of carbon, and the hydrolysis of the zirconium

tetrachloride product to fonn ZrOC1

2

• The ZrOCl

2

can be calcined directly

or

reacted with a base in water to fonn hydrous zirconia. Zircon may also be

dissociated to Zr 2 +

Si

2

by heating above 1750°C and the zirconium sep

arated by leaching with sulfuric acid:

Sílicon carbide SiC

is

produced in large tonnages using the Acheson process

by

reacting a batch consisting principally of high-purity sand and low-sulfur

coke at 2200-2500°C in an electric arc furnace.

Si

2

+

3C SiC

+

2CO gas)

(3.4)

The crystalline product is crushed, washed

in

acid and alkali, and then dried

after iron has been removed magnetically. Granular material is used

in

refrac

tories and bonded abrasives. Milled material chernically treated to remove

4



I \ )

""

o

Fig. 3.4

The Bayer process for chemically refining bauxite into alumin

Inc., Pittsburgh, PA.)






impurities introduced in millíng is used industrially for structural ceramics.

S i l i c o l ' t : l i ~ r i d e Si

3

N

4

is prepared by reacting silicon metal powder with nitrogen

or a mixture of silica and carbon powders with nitrogen at a high temperature:

(3.5)

Silicon nitride and aluminum nitride powders may be produced by the car

bothermal process:

3Si0

2

+ 6C + 2N

2

(gas)

---

Si

3

N

4

+ 6CO gas)

(3.6)

Al

2

0

3

+ 3C + N

2

(gas) --- 2AIN + 3CO gas)

(3.7)

Aluminum nitride

is

also formed by dírect nitridation:

2AI solíd) +

N

2

(gas) ---

2AIN

(3.8)

The oxynitride SIALON is produced by the reaction of mixed powders of

silicon nitride, aluminum nitride, and alumina at a high temperature; the re

action is

(3.9)

The production of mixed metal oxides for e1ectronic ceramics such as barium

titanate BaTi0

3

, ferrites such as M11o.sZ11o.sFez04 and BaFe12019, mixed metal

oxide resistors, and ceramic colors such as doped zirconia involves the batching

and reaction of industrial inorganic chemicals, as is shown for the ferrite in

Fig. 3.6. The concentration of chemical dopants is carefully controlled. Soluble

material

is

sometimes removed

by

filtering before drying. Precursor industrial

chemicals for these compounds are commonly powders finer than a few microns

in size. Barium carbonate BaCO} and titania

Ti0

2

are commonly used for

preparing the titanates, and manganese carbonate

MnC0

3

,

zinc oxide ZnO,

hematite Fe203, and barium carbonate for the ferrites.

Titania

Ti0

2

is produced by the sulfate or chio ride processo

ln

the sulfate

process, ilmenite

FeTi0

3

is treated wíth sulfuric acid at 150-180°C to form

the soluble titanyl sulfate

TiOS0

4

:

After removing undissolved solids and then the iron sulfate precipitate, the

titanyl sulfate is hydrolyzed at

90°C

to precipitate the hydroxide TiO(OH)z:

(3.11 )

The titanyl hydroxide is ca1cined at about

lOOO°C

to produce titania

Ti0

2

• ln

the chloride process, a high-grade titania ore is chlorinated in the presence of



RAW

MATERIALS ORYING OVEN

VACUUM CONVEYOR

CYCLONE

COLLECTOR

DIAPHRAGM

SLURRY PUMP

~ - A

BALL MILL

FAN

FILrER

AIR

LURRY

FEED

T N ~

AIR

..

HEATER

+

PRODUCT

-

PROOUCT

CART

Fig. 3.6 Preparatíon

of

calcíned manganese zínc ferrite

~

and spmy dried powdcr for processing. [From E.J. Moytl,

est E/ec. Eng. 7, 3-11 (1963).]



46 COMMON RAW MATERIALS

carbon at 900-1000°C and the chloride TiCI

4

fonned

is

subsequently oxidized

to

Ti0

2

•

Barium carbonate BaCO) is the primary source

of

barium oxide BaO for

ceramícs. Barite ore, nominally BaS04, is reduced at a high temperature to

barium sulfide BaS which

is

water soluble. The reaction

of

an aqueous sulfide

solution with sodium carbonate Na2C03

or

carbon dioxide CO

2

produces a

barium carbonate precipitate which

is

then washed, dried, and ground.

The commercial iron oxide hematite ex-Fe20) used for preparing ferrites is

producedfrorn tilé-tlíennal decomposition

of

hydrated ferrous sulfate FeS04 .

7H

2

0 or by the precipitation

of

hematite and goethite ex-FeZ03 . H

2

0 from an

oxygenated sulfate solutíon containing dispersed iron metal. The size and shape

of the ultimate crystals of FeZ 3 are very dependent on the pH, temperature,

time, and impurities during precipitation. Zinc oxide ZnO

is

produced by roast

íng a concentrate

of

the mineral sphalerite ZnS in air. Manganese carbonate

MnC0

3

is derived from manganese sulfate MnS04

When thennally reacting titanates and ferrites, the temperature, time, and

atmosphere musibe adequate pennlTdecomposition ofihe carbonate and

promote interditfusion

of

the reactants through the reaction product that may

be several microns

in

thickness (Fig. 3.7). The time dependence of the relative

amount x

of

reactant

A

of radius rA transfonned into reaction product is given

by the Carter equation:

Kt

[I

+

(z

l xf í3 + (z

1)(1

X 2í3

=

Z + 2(1

z) ---, (3.12)

rÃ

where K is the apparent rate constant and z is the volume

of

product fonned

from a unit volume of reactant A. * The effect of temperature T is commonly

as

Out

Mixed Powder

Reaction

Fig. 3 7 Model for mixed powder reaction when particles are well dispersed, indi

cating maximum diffusion length for reaction

is

controlled by radius of reactanl particle.

*H. Schmalzried,

Solid State Reactions,

Academic Press. New York, 1974.



INDUSTRIAL INORGANIC CHEMICALS 7

expressed by the Arrhenius relation and

o

exp

T

(3.13)

where

o

is the limiting rate constant that depends on the diffusion path length

and

is

the apparent activation energy for diffusion. The reaction

is

a function

of

both time and temperature, but temperature has a greater influence on the

rate. The time for total reaction eliminating the reactants varies directly with

the maximum size of agglomerates

or

segregated material in the batch. When

reacting micron-size oxide powders, thermal processing at a temperature in

excess

of

1200°C is commonly requisite, as is shown in Fig. 3.8 for the

formation

of

spinel

MgA1

2

0

4

•

Q t h ~ r i n l 1 ' ) Q I ª ~ ~ , : : a r i a b l e s

affecting solid-state reactions during calcining are

the

º a r t i c l ~ ) ; i z e

distributions of the reactants, the mixedness of the reactants,

the comJX)sition and flowQLgases-l the depth

a n d t u r n . o v ~ Q f _ l . a t e . , i : l l ,

and

e n d - º 1 ~ f 1 1 i c

a ~ : t ~ ~ ~ ~ ~ r m i c e _ f f e c t s . Sintering during calcination produces par

100

75

*

c:

o

50

~

c:

I I)

(.J

c:

o

u

25

OL-____ ____

900 1000 1100 1200

1300

1400

Temperature OC)

Fig. 3 8 The fonnation of spinel MgAl

2

0

4

from the solíd-state reaction of micron

size MgO and

o:-AI

2

0]

powder as a function of the reactíon temperature for a constant

reaction time of 8 h.





SUGGESTED READING

Radiation

! ! ! ! !

Hot Gas Flow .....

Mlxlng

r--.. ....:.. ...-<c

l :><r<o I>

- T ra n ala

tlon

Refractory

Support

Thermal

Conductlon

Fig. 3.10 Thermal transport, gas flow, and particle movement during rotary calcina

tion and solid-state reaction.

SUMM RV

Few ceramics are produced today using crude raw materiais. Industrial minerais

are refined physically to reduce the concentration of undesirable mineral im

purities and to produce a particular particle size distribution; water-soluble

impurities are removed by washing. Industrial inorganic chemicals used to

produ ce the majority of technical ceramics are chemically processed on a large

scale to improve both the chemical and the mineral purity; the calcined product

containing hard aggregates is commonly milled to disperse the aggregates and

obtain a product

of

controlled size distribution. Mixed-oxide industrial chem

icals are commonly produced by calcining a mixture of these industrial chem

icals. The completeness

of

the reaction and unifonnity

of

the product depend

on the particle size and mixedness of the reactants and the time, temperature,

and atmosphere and their unifonnity during calcination. Different lots

of

pro

cessed materiais are blended to maintain a higher levei

of

unifonnity.

SUGGESTED RE DING

1.

Magnus Ekelund and Bertil Forslund, "Carbothermal Preparation

of

Silicon Ni

tride: Influence of Starting Material and Synthesis Parameters," J Am. Ceram.

Soc. 75(3), 532-539 (1992).

2. Julie M. Schoenung, "Analysis

of

the Economics

of

Silicon Nitride Powder Pro

duction,

Am. Ceram. Soco Buli. 70(1), 112-116 (1991).

3. Martin R. Houchin, David H. Jenkins, and Hari N. Sinha, "Production

of

High

Purity Zirconia from Zircon, " Am. Ceram. Soco Buli. 69(10),1706-1710 (1990).

4.

L.

D. Hart, Alumina Chemicals The American Ceramic Society, Westerville, OH,

1990.

49



50 COMMON RAW MATERIALS

5.

H. Okada, H. Kawakarni, M. Hashiba, E. Miura, Y. Nurishi, and T. Hibino,

"Effect of Physical Nature

of

Powders and Firing Atrnosphere on ZnAI

2

0

4

For

rnation," J.

Am. Ceram. Soe.,

68(2), 58-63 (1985).

6. Materiais o Advaneed Ceramies and Tradítíonal Ceramics, Cerarnic Industry

Magazine, Corcoran Publishers, Solon, OH, 1985.

7.

Proeess Míneralogy o Ceramic Materiais, Wolfgang Baurngart et aI., Eds., EI

sevier, New York, 1984.

8. Kirk-Othmer, Eneyclopedia o Chemieal Teehnology, Wiley-Interscience, New

York, 1983.

9. Betty L Milliken, "Color Control in a Pigrnent Manufacturing

Plant,"

Am. Ce

ramo

Soe. Buli., 62(12), 1338-1340 (1983).

10. T. Nornura and T. Yarnaguchi, "Ti0

Aggregation and Sintering of BaTiO] Ce

rarnics,"

Am. Ceram. Soe. Buli.,

59(4), 453-455, 458 (1980).

II. F. H. Norton, Fine Ceramies, Krieger, Malabar, FL, 1978.

12. W.

D. Kingery, H.

K.

Bowen, and D. R. Uhlrnann,

Introduetion 10 Ceramics,

2nd ed, Wiley-Interscience, New York, 1976.

13.

W. E. Worrall, Clays and Ceramie Raw Materiais, Halsted Press Div., Wiley

Interscience, New York, 1975.

14. Rex W. Grirnshaw, The

Chemistry and Physies

o

Clays and Other Ceramie Ma

t e r i a L ~ Wiley-Interscience, New York, 1971.

15.

Annual Ceramie Industry Data Book, Cahners, Boston, MA.

16.

Ceramic Source,

Arnerican Cerarnic Society, Colurnbus, OH.

PROBLEMS

3.1

Categorize the materiais in the following pairs of starting materiais as

crude material, industrial mineral,

or

industrial inorganic chemical and

expIain your assignment: bauxite-Bayer process alumina, kaolin-calcined

kaolin, calcined alumina-calcined kaolin, silicon carbide-silicon nitride,

and titania-barium tÍtanate.

3.2 When fonning a compound by a solid-state reaction, does the completion

of

the reaction depend on the average or the maximum particle size?

Explain. Does the aggregation

of

the starting materiais influence the re

action?

3.3 Write the chemical reaction for the hydrolysis

of

sodium zirconate which

folIows the reaction in Eq.

(3.2).

3.4 State several reasons why a pigment calcined in an open crucible in a

gas-fired kiln may differ in color from the sarne pigment batch fired in a

closed sagger in an electric fumace.

3.5

Construct a processing f10w diagram for the formation

of

BaTiO) powder.



EXAMPLES

3.6

Why are hard powder aggregates commonly formed in a solid-state re

acted batch of material?

3.7 Calculate the amount

of BaC0

3

and

Ti0

2

required to produce 1 kg

of

BaTi0

3

·

3.8 Calculate the amounts of

Si0

2

, C

and N

2

required to form 1 kg of SbN

4

.

3.9

Write the nominal reaction equation for the formation of doped barium

titanate Bal xMxTi03 on heating a mixture of BaC0

3

•

MS0

4

• and Ti0

2

powders. Illustrate the diffusion paths.

3.10 Substitute the Arrhenius relation into the Carter equation and comment

on the dependence

of

amount reacted on increasing time and increasing

temperature.

3.11 Make a sketch similar to Fig. 3.7 for the formation

of

ZnFe204 from the

reaction

of

Fe203 and ZnO.

EXAMPLES

xample

3 1

Contrast the mineralogícal structure of the kaolin and mont

morillonite families

of

clay minerais.

Solution The kaolin minerais include kaolinite, nacrite, dickite, and halloy

site, and kaolinite is the most abundant and important. The kaolin minerais are

two-Iayer mineraIs. The disilicate layer with the composition Si

2

0

S

has Si

tetrahedrally coordinated with

O

bonds. The second layer has the composition

AI

2

(OH)6 and is called the gibbsite layer. The

AI

atoms are octahedrally co

ordinated with

O

bonds common to both layers and -OH within the layer.

Partial substitution of AI

H

for Si

4

in the octhedral layer and Mg

2

and Fe

2

in the tetrahedral layer commonly occurs when formed and the basal plane is

negatively charged. Chemisorbed alkalis and

Ca2+

occur for charge neutrality.

The resultant octahedrallayer is distorted and this weakens the bonding between

the structural units. Slight differences of the stacking of the units produces the

particular types of clay minerais.

The montmorillonites are three-Iayer minerais having a gibbsite sheet sand

wíched between two disilicate layers. The parent mineral is pyrophyllite. Iso

morphous cation substitution produces related minerais. Partial substitution of

Mg

2

for

A1

3

in

the octahedrallayer with surface adsorbed alkali for charge

neutrality produces montmorillonite. One-fourth substitution of AI

H

for Si

4

in the tetrahedral layer causes the surface alkali to e strongly bonded, and the

material mica is produced. Significant substitution in both layers, as is indicated

in

Table 3.2, produces the mineral iIIite. When Mg

2

completely replaces AI

H

in the gibbsite layer, the mineral is talco

5



52 COMMON

R W

MATERIALS

~

E

Water

'arers

o

c1

O ~ 1 4 1

~ I ~ 1 1 1 y;tl oo

~ 1 1 5

o \

~

I

J - ~

.

aOo ~

d ~ D

H a f l o y ~ . t e

MOOimçr O'-'ite

Ch one

Hyórated) (Hydril ed}

@(OH) () Si-AI

l i)

AI-Mg

Example 3.1 Layer structures of clays and related mineraIs (spacing indicated is in

angstroms). [From W. D. Kingery, H.

K.

Bowen, and D. R. Uhlmann, lntroduction

t Ceramics

2nd ed., Wiley-Interscience, New York, 1976; after

R

E. Newnham and

G

W Brind1ey,

Acta Cryst. 9

759-764 (1956); 10, 88 (1957).]

fx mple 3.2

Compare and contrast the processing

of

ceramic grade sílica

and Bayer process aIumina.

Solution The operations

of

crushing, grinding, and classification are invoIved

in the production

of

both materiaIs. The big difference in the processing

of

the

alumina is major chemical refining: the chemicaI dissoIving

of

the aluminum

constituents

of

the bauxíte, the chemical precipitation

of

gibbsite, the washing

of the precipitate, and the calcination

of

the precipitate in the Bayer processo

fx mple 3.3

Explain why the dispersion of aggregates and aggIomerates in

reactant materiais and the uniform mixing

of

reactants are essentiaI

to

obtain a

uniform product from a solid-state reaction technique.

Solution As is indicated by the Carter equation and Fig. 3.7, the time to

produce a particular amount

of

reaction product depends directly on

r K



EXAMPLES 5

Dispersion

of

aggregates will reduce the maximum reaCtant size r

A

which is

especially important because the dependence is on the square

of

the size. In-

creasing temperature increases the rate constant

K.

A higher temperature or time will be required to complete the reaction in

poorly mixed microscopic regions where the effective

rA

is large. Aggregate

in

precursors may not be completely reacted and may have a different grind-

ability than the majority of the product.

Documents

CAPITULO 3 do livro do Reed