H.O. Pierson. Handbook of carbon, graphite, diamond and fullerenes. Properties, processing and applications. 1993

3.

inagaki, M., et al, Carbon, 27(2):253-257 (1989)

4.

Fitzer, E., Carbon, 25(2):163-l 90 (1987)

11.

Walker, P. L., Jr., Carbon, 28(2&3):261-279 (1990)

12.

Eser, S. and Jenkins, R. G., Carbon, 27(6):877-887 (1989)

14.

Eggers, D. F., Jr., Gregory, N. W.,

Halsey, J. D., Jr. and Rabinovitch,

B. S., Physical Chemistry, John Wiley & Sons, New York

(1964)

15.

Manteii, C. L., CarbonandGraphiteHandbook, IntersciencePublishers,

New York (1968)

16.

Kawamura, K. and Bragg, R. H.,

Carbon, 24(3):301-309

(1986)

19.

Cowlard, F. and Lewis. J., J. of Mat. Science 2:507-512 (1967)

Figure

5.1. Graphite electrode.

(Photograph

courtesy of Carbon/Graphite

Group

Inc.,

St. Marys, PA.)

2.0 PROCESSING

OF MOLDED

GRAPHITES

Raw

Materials

Selection.

The

selection

of

the appropriate

raw

(precursor)

materials

is

the

first and

critical step

in

the manufacturing

process.

It determines

to a great degree,

the properties

and the cost of the

final product.

The characteristics

of these raw materials

such as the particle

size and

ash

content

of cokes,

the

degree of carbonization

of pitch,

the

particle

structure

of lampblack,

and the

impurities

and particle size

of

natural graphite

must

be taken

into account.

To use high-grade, expensive raw materials to produce an undemanding

product,

such

as a grounding

anode for

electrolytic

protection, would

be

wasteful

and economically

unsound

since

these

electrodes

do not require

optimum

properties and cost

is the

overriding factor.

On the other

hand,

Molded

Graphite

89

Raw

materials

can

be

divided

into

four

generic

categories:

fillers,

binders,

impregnants,

and

additives.t’)-t4)

Fillers.

The

filler

is

usually

selected

from

carbon

materials

that

graphitize

readily.

As

mentioned

in

Ch. 4,

such

materials

are generally

cokes,

also

known

in industry

as “softfillers”. They

graphitize

rapidly

above

2700°C

(the

graphitization

process

is described

in Sec.

2.4

below).

Other

major fillers

are synthetic

graphite

from recycled

electrodes,

natural

graph-

ite, and

carbon

black

(see Ch.

10).

Petroleum

coke

is the filler of choice

in most applications.

it is a porous

by-product

of the

petroleum

industry

and an almost-pure

solid

carbon

at

room

temperature.

it

is

produced

by

destructive

distillation

without

the

addition

of hydrogen,

either

by a continuous

process

(fluid

coking) or, more

commonly,

by a batch

process

(delayed

coking).

The

batch

process

consists

of

heating

high-boiling

petroleum

feed-

stocks

under pressure

to approximately

430°C

usually

for

several

days.t5j

This

promotes

the

growth

of

mesophase-liquid

polycylic

crystals.

The

material

is then calcined

up to 12OO”C, to remove

almost

all

the

residual

hydrogen,

and

finally

ground

and

sized.

By varying

the

source

of oil and the

process

parameters,

it is possible

to obtain various

grades

of petroleum-coke

filler

with different

properties.

The industry

commonly

uses

three grades:

.

Needle

coke,

a

premium

grade

with

distinctive

needle-

shape

particles,

produced

by delayed

coking from selected

feedstocks

with low concentration

of insolubles.

It is used

in applications

requiring

high thermal-shock

resistance

and

low electrical

resistivity.

. Anode

coke

for less

demanding

applications.

.

Isotropic

coke

in

applications

where

isotropic

properties

and a fine-grained

structure

are

required.

Binders.

The

most

common

binder

is coal-tar

pitch

which

is a hard,

brittle

and

glassy

material,

described

in Ch. 4, Sec. 2.3.

It is a by-product

of metallurgical-coke

production

and

is obtained

by the

distillation

or heat

treatment

of coal-tar.

From

35 to 60

kg of pitch

are produced

from

every

metric ton

of coal.

The

composition

of coal-tar pitch

is complex

and may

vary consider-

ably

since

it depends

on the

degree

of

refinement

of the

available

coke-

oven

tars.

Two

factors

can

noticeably

influence

the

quality

and

graphitiza-

tion characteristics

of the

pitch:

(a) its softening

point

and (b) the

content

of

Molded Graphite

91

Milling

and Sizing.

Filler

and

binder

are ground

or milled

to the

particle-size

requirement

which

may

vary

from 1 pm

(flour) to 1.25

cm. A

batch

usually

consists

of more than one size.

This allows better control of

the packing

characteristics

and

optimizes

the

density

of the final

product.

Table

5.1

lists the grain

size of various

grades

of molded

graphites

and its

effect

on

properties.t3j

Table 5.1.

Particle

Sizes

and

Characteristics

of Graphite

Grades

Grade

Grain

Size

Properties

Medium

grain

Up to

1.25 cm

Low

density

Low

thermal

expansion

Low

strength

High

permeability

Fine

grain

0.05

to 0.15

cm

Medium

density

Medium

thermal

expansion

Medium

strength

Medium

permeability

Micrograin

4

pm

to 75 pm

High

density

High

thermal

expansion

High

strength

Low

permeability

Mixing.

Filler

and binder

are

weighed

in the

proper

proportion

and

blended

with large

mixers

into a homogeneous

mix where

each filler particle

is coated

with

the

binder.

Blending

is usually

carried

out

at 160

- 170°C

although

temperatures

may

reach

as high as 315°C

on

occasion.

When

mixing

at lower

temperatures

(belowthe

melting point of the

binder),

volatile

solvents

such

as

acetone

or

alcohol

are often

added to

promote

binder

dispersion.

The

final

properties

of the molded

product

are

controlled

to

a great

degree

by

the

characteristics

of the

filler-binder

paste

such

as:

(a)

the

temperature

dependence

of

the

viscosity,

(b) the

general

rheological

behavior,

and

(c) the

hydrodynamic

interaction

between

filler

particles.pj

Forming Techniques.

Three

major techniques

are used

to form

the

graphite

mix:

extrusion,

compression

(uniaxial

loading),

and

isostatic

pressing.

They are

shown

graphically

in Fig. 5.3.

Figure 5.3. Forming techniques

for molded graphites.

The

selection

of a given

technique

has

a great

influence

on the

final

properties

of the

molded

product

as shown

in Table

5.2.

Extrusion.

Extrusion

is a major

technique

which

is favored

for

the

production

of

parts having

a constant

cross-section,

such as

electrodes.

The

mix

is cooled

to just

above the

softening

point

(approximately

125”C),

then

extruded

through steel

dies,

cut to length and

rapidly

cooled to solidify

the

pitch

before distortion

occurs.

The resulting

shape

is known

in

the

industry

as a “green shape”.

Molded Graphite

93

Table 5.2. Characteristics

of Forming Techniques

Technique

Characteristics

Extrusion

Anisotropic

properties

Non-uniformity

of cross-section

Presence

of flow lines and laminations

Limited

to

parts

of constant

cross-section

Production

of large parts possible

Low cost

Compression

Non uniformity

Edge effect

Presence

of flow lines and

laminations

Medium

cost

lsostatic

Isotropic

properties

Uniformity

No flow lines or laminations

High cost

Extrusion

pressures

are

on the

order of

7 MPa (100 psi).

Some

alignment

of the

coke-filler

particles

takes

place

which

imparts

anisotropy

to the properties

of the finished

product.

This

anisotropy

can be controlled

to some

extent

by

changing

the

mix

formulation

and

the

extrusion

geom-

etry.t4) The

center

of the

extruded

material

is usually

of lower quality

than

the

material

near

the outside

edge and defects such as

flow

lines

and

laminations

are

difficult

to

avoid.

On

the

plus

side,

it is

the lowest-cost

technique

which

is

satisfactory

for most large parts, such as furnace

electrodes.

It represents

the largest

tonnage

of molded

graphite.

Compression

(Unlaxlal)

Molding.

The

mix

in compression

molding

is usually

a fine

powder

(flour)

as opposed

to the

coarser

material

used in

extrusion.

Tungsten

carbide

dies

are frequently

used with

pressures

on the

order of 28 to 280 MPa (4000

to 40,000

psi).

Complex

shapes

can

be

produced

by this

process (Fig.

5.4).[*)

However,

die-wall

friction

and

die

edge effect may cause non-uniformity

in the

density and other properties

of

the

finished

product.

Molded Graphite

95

in a material

with great

uniformity,

isotropic

properties,

and generally

with

few

defects.

However,

the molding

process

is expensive

and cost is higher

than

extrusion

or compression molding.

Carbonization.

Carbonizing

(also known as baking) the green shape

is the next step (see Ch. 4, Sec. 2).

Carbonization

takes place in a furnace

in an inert or reducing

atmosphere.

The process

may last from a few days

to several weeks

depending

on the constituents,

and the size

and geometry

of the

part.

The

temperature

is raised

slowly

to 600°C

at which

stage

the

binder

softens,

volatiles

are

released

and the

material

begins

to shrink

and

harden.

Typical

shrinkage

is 6%. The

parts must

be supported

by a packing

material

to

prevent

sagging.

The temperature

is then

raised

to 760

to

980°C

(or

up to

1200°C

in

special

cases).

This

can

be

done faster than the first temperature

step,

since

most

of

the

volatiles

have

by

now

been

removed,

the

material

is

already

hard,

and

sagging

is no longer

a problem.

Impregnation.

After

the

carbonization

stage,

the

material

has

a high

degree

of porosity.

To further

densify

it, it is necessary

to impregnate

it with

coal-tar

pitch or a polymer such

as phenolic.

Impregnation

is usually

carried

out in a high-pressure

autoclave

and the carbonization

process

is repeated.

In special,

limited-use

applications,

non-carbon

impregnating

materials

such

as

silver

and

lithium

fluoride

impart specific

characteristics,

particu-

larly

increased

electrical

conductivity.t6)

Graphitization.

During

graphitization,

the

parts

are

heated

up to

3000°C

(see Ch. 4,

Sec.

3).

The

temperature

cycle

is

shorter

than

the

carbonization

cycle

and

varies

depending

on the

size

of the

parts,

lasting

from

as

short

as

a few

hours

to

as

long

as

three

weeks.

It

is

usually

performed

in

a

resistance

furnace

(the original

Acheson

cycle)

or

in

a

medium-frequency

induction

furnace.

Graphitization

increases

the

resistance

of

the

material

to thermal

shock

and

chemical

attack.

It also

increases

its

thermal

and

electrical

conductivities.

Puffing.

Puffing

is an irreversibleexpansion

of molded

graphite

which

occurs

during

graphitization

when

volatile

species,

such

as sulfur from

the

coke,

are released.

Puffing

is detrimental

as

it causes

cracks

and

other

structural

defects.

It can be eliminated

(or at least

considerably

reduced)