Challenges facing the aluminum industry

J. Electroanal. Chem., 118 (1981) 375--380

375

Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

CHALLENGES

W.E. Alcoa

FACING

THE A L U M I N U M

INDUSTRY

HAUPIN Laboratories,

New Kensington,

Today

Aluminum

PA

Company

(U~S.A.)

is a c h a l l e n g i n g

a time of opportunity~

time

for the a l u m i n u m

The a l u m i n u m

efficient

in the use of energy,

petroleum

coke.

proving

It must

as new t e c h n o l o g y

society.

that have

industry

to combat

not to c o m p r o m i s e In this paper the b u s i n e s s

to p r o m o t e

inflation.

one goal I will

because

concentrate

this

is where

Indeed,

the need to improve

Let's

through

industry

Each a d d i t i o n a l

is well

known

Productivity

improvement,

constructed. increases

to the square

pro-

part of

lies.

This

face no problems.

and p r o d u c t i v i t y

In the past

comes more

inflation density

extends

But today

struction

so high

that

with o l d e r plants

while

power heat

comes at the expense

provided

capital

has p u s h e d

at much

than the It

efficiency. generation

Therefore,

cells or larger

it is d i f f i c u l t

built

increases

the

in half.

difficult

density.

new technology

inflation

forty years

nearly

is an inhibitor.

density,

of c u r r e n t

the a d d i t i o n a l

and equipment.

compete

industry

requirements

to c u r r e n t

additional

In the past,

costs

increase

on the s m e l t i n g

efficiency

by this route

either

that e x c e e d e d

but

current

is p r o p o r t i o n a l

Thus,

to The

must be careful

of my e x p e r i e n c e

of the

however,

go further,

power e f f i c i e n c y

ductivity.

benefits

fabrication.

that l o w e r i n g

is p r o p o r t i o n a l creased

energy

It must

the i n d u s t r y

a

uses

fit this category.

mainly most

has cut its e n e r g y

We can still

and e c o n o m i c

those

another.

talk about e n e r g y efficiency.

aluminum

last.

sectors

and im-

It must m a i n t a i n

recycling~

Finally,

in a c h i e v i n g

is not to imply that other

from m i n i n g

social

more

energy

the environment,

It must promote

uses p a r t i c u l a r l y

but also

must become

this possible.

the g r e a t e s t

must continue

industryt

electrical

to p r o t e c t

environment.

Transportation

ductivity

makes

in d u s t r y

particularly

continue

safe and h e a l t h y w o r k i n g of a l u m i n u m

of A m e r i c a

in-

of pro-

cells must be productivity

cost of n e w plants equipment

and con-

for a new p l a n t

lower costs.

to

Improvements,

376

therefore,

that can be r e t r o f i t t e d to the present process,

avoiding

the high capital cost of e n t i r e l y new e q u i p m e n t and/or new plants, are highly desirable~ There is much that can be done along this line.

Just m a i n t a i n i n g

the e l e c t r o l y t e c o m p o s i t i o n and t e m p e r a t u r e at known o p t i m u m conditions could increase power e f f i c i e n c y at least 3-5%. as it may seem,

Incredible

the a l u m i n u m industry r o u t i n e l y operates its s m e l t i n g

cells w i t h o u t automatic t e m p e r a t u r e control;

and yet every degree

Celsius above the o p t i m u m o p e r a t i n g temperature costs at least onehalf p e r c e n t in current efficiency.

It is not that the a l u m i n u m

industry is in the dark ages; there is no known t e m p e r a t u r e sensor that will survive for long in the a g g r e s s i v e e n v i r o n m e n t of the smelting cell.

Solving this p r o b l e m is an important challenge.

A n o t h e r o p p o r t u n i t y is i m p r o v e d alumina c o n c e n t r a t i o n control. Most plants today use some form of a u t o m a t i c

feeding of alumina,

and there are many schemes based on the cell v o l t a g e - c u r r e n t - t i m e r e l a t i o n s h i p to sense the alumina c o n c e n t r a t i o n in the electrolyte. The c a l c u l a t i o n s g e n e r a l l y require the use of a computer.

Still,

even the best c o n t r o l l e d cells frequently e x p e r i e n c e anode effects-a c o n d i t i o n i n d i c a t i n g i n s u f f i c i e n t alumina concentration. effect,

At anode

c e l ~ voltage rises 6 to 7-fold, w a s t i n g energy and g e n e r a t i n g

carbon tetrafluoride.

A l t h o u g h carbon t e t r a f l u o r i d e is nontoxic and

does not cause ozone depletion,

as c h l o r o f l u o r o c a r b o n s

do, it still

r e p r e s e n t s a loss of fluoride.

On the other hand, e x c e s s i v e

feeding

causes some alumina to settle u n d i s s o l v e d under the m o l t e n a l u m i n u m at the b o t t o m of the cell, where it acts as an e l e c t r i c a l raising the cell voltage.

insulator,

Improved alumina sensing and feeding

m e c h a n i s m s that will handle equally well alumina from d i f f e r i n g sources,

offer c h a l l e n g e s with big p o t e n t i a l payoffs.

The c a l c i u m fluoride c o n c e n t r a t i o n and ratio of sodium fluoride to a l u m i n u m fluoride in the e l e c t r o l y t e change slowly enough that p e r i o d i c sampling,

coupled w i t h a u t o m a t e d analysis,

good control of these variables.

However,

can p r o v i d e

good sampling p r a c t i c e

must be e s t a b l i s h e d and adhered to, and a u t o m a t e d a n a l y t i c a l equipment must be checked frequently. R e p l a c i n g the p r e s e n t carbon c a t h o d e s with r e f r a c t o r y h a r d metals, such as t i t a n i u m diboride,

is a r e t r o f i t we think could save up to

25% of the electrical energy p r e s e n t l y used by H a l l - H e r o u l t cells. This b e n e f i t results from r e d u c i n g the cathode voltage loss and by stabilizing the cathode surface,

thereby a l l o w i n g closer anode-

377

cathode novel

spacing.

cathode

needed.

designs

Refractory

very easily. this

Refractory

hard metals

are r e q u i r e d

hard m e t a l s

Improved

to reduce

the amount

also are brittle

fabricating

techniques

hence,

of m a t e r i a l

and tend to crack

are n e e d e d

to o v e r c o m e

limitation.

Inert anodes greater

have p o t e n t i a l

potential

trode,

fluoride

decomposition effect

potential

of carbon

0.5 volt

for carbon

in area

by the

increase greater

full cross

as they are consumed.

share

While

overall

of the load.

energy

energy--and

process.

represents

I mentioned

current

cathode.

higher enters

in Figure cathode,

surface

an anode. mately

cases,

i.

the r e m a i n d e r

strides

anode

efficiency.

at a terminal these

two terminal

flows

of each plate

becomes

a cathode

a cell with

ii b i p o l a r

as much a l u m i n u m

a number

plates

intermediate and the

plates

is one

electrode

at a terminal

from terminal

these

it

problems.

electrodes

isolated

through

Hence,

literature

for the Hall-

unsolved

electrodes,

it passes

12 times

the elec-

in recent years

and exits

but e l e c t r i c a l l y As c u r r e n t

can be used

In a b i p o l a r

anode

is petro-

and i m p l e m e n t i n g

that the use of b i p o l a r energy

it is an

represents

inert anode

still

of

are p l a c e d

anode

to

plates. second

would produce

as a c o n v e n t i o n a l

their

spacing.

carbon

Patent

been made

this d e v e l o p m e n t with many

carry

plant producing

processes. have

is

of the i n c r e a s e d

oxygen w h i c h

of the power

area

standoff,

of carbon

because

inert

decrease

a process

anode-cathode

energy

consumption source

no e f f e c t i v e anodes,

about

since

anodes

the new anodes

of a s a t i s f a c t o r y

conductive

terminal first

great

earlier

as shown

is com-

area,

carbon

carbon

also p r o d u c e

challenge

In b e t w e e n

electrically

cold before

other c o m b u s t i o n

Refining

another

way to achieve cell,

energy

the d e v e l o p m e n t

Heroult

voltage

0.i volt versus

anode

while

in an e l e c t r i c a l

efficiency

indicates

average

for by n a r r o w e r

Inert anodes

boiler

or improve

(Ref~. 1-8) toward

In most

saving because

leum derived.

tricity

result

a critical

to increase

the

caused by inert anodes,

Additionally,

setting of large

can be c o m p e n s a t e d

this w o u l d

elec-

raise

the d e p o l a r i z i n g

(about

section

that can take eight or more hours

voltage

into a b i p o l a r

inert anodes

increased

overvoltage

0.45 volt

their

lost by the daily

W hi l e

some of this

anodic

is r e c o v e r e d

maintain

cell.

but even

anodes).

Of the r e m a i n i n g 0.2 volt

application

about one volt because

is lost,

for by lower

anodes

for r e t r o f i t

if they can be i n c o r p o r a t e d

electrolysis

pensated

full

are very expensive;

The

surface approxi-

cell w i t h m o n o p o l a r

378

II F i g u r e 1 - B i p o l a r E l e c t r o d e Cell Used in A l c o a S m e l t i n g Process, U.S. Pat. 4133727

electrodes,

o p e r a t i n g at the same current.

of such a cell

the anodes

should not be consumable.

In d e s i g n i n g the A l c o a

S m e l t i n g Process,

c h l o r i d e e l e c t r o l y t e and the e l e c t r o l y s i s o r d e r to avoid c o n s u m a b l e anodes. at the anode.

Obviously,

we chose a less a g g r e s s i v e

of a l u m i n u m c h l o r i d e

Aluminum chloride produces

in chlorine

E l e c t r i c a l l y c o n d u c t i v e m a t e r i a l s r e s i s t a n t to hot

c h l o r i n e are available.

However,

this r e q u i r e d that the a l u m i n a

first had to be c o n v e r t e d to a l u m i n u m c h l o r i d e b e f o r e b e i n g fed to the A l c o a

S m e l t i n g P r o c e s s cell.

Chlorine

from the cell is r e c y c l e d to an a d j a c e n t c h e m i c a l p l a n t

to p r o d u c e m o r e a l u m i n u m c h l o r i d e

for the cell.

This cell

saves

379

over thirty percent of e l e c t r i c a l energy r e q u i r e d by our best HallHeroult cells.

Also, being c o m p l e t e l y enclosed,

it is e n v i r o n m e n t a l l y

more acceptable than H a l l - H e r o u l t cells. A n o t h e r p o s s i b l e way to c i r c u m v e n t the need for scarce p e t r o l e u m coke is through the use of solvent refined coal. fications,

the SRC process is able to produce not only anode q u a l i t y

coke, but also a solid, sulfur,

With various modi-

sulfur-free

fuel, both light and heavy oils,

and fuel gas.

Alcoa, w i t h U.S. g o v e r n m e n t assistance,

is looking at direct carbo-

thermic r e d u c t i o n of low grade bauxite and clay as another p o s s i b l e method for p r o d u c i n g both fuel gas and aluminum.

It is a n t i c i p a t e d

that little e l e c t r i c a l power will be r e q u i r e d by the process and, if successful,

it will produce large q u a n t i t i e s of carbon m o n o x i d e

for fuel or other chemical processes,

a l u m i n u m - s i l i c o n alloys suitable

for castings,

ferrosilicon which can be used

and a l u m i n u m - c o n t a i n i n g

by the M a g n e t h e r m Process to produce m a g n e s i u m from dolomite. The greatest energy and raw m a t e r i a l s saving, however, able through recycling.

is avail-

Recycled a l u m i n u m requires less than 5% of

the energy needed to produce virgin metal. to be d o w n g r a d e d w i t h r e p e a t e d recycling.

The metal,

to restore the metal to its original purity. challenge to the a l u m i n u m industry.

however,

This r e p r e s e n t s another

Much of the world,

larly the United States, has become a t h r o w a w a y society. tant but n o n - m e t a l l u r g i c a l

challenge

tends

Techniques are being studied

and particuA concomi-

is to work to generate a cultural

change from a t h r o w a w a y society to a c o n s e r v i n g society that will recycle its scrap. E n v i r o n m e n t a l concern is not new to the a l u m i n u m industry.

Alcoa

started i n s t a l l i n g fluoride p o l l u t i o n control e q u i p m e n t at its smelting plants improved, equipment.

four decades ago.

Many of our older plants,

as t e c h n o l o g y

are in the second or third g e n e r a t i o n of fluoride control The o r i g i n a l equipment,

e l e c t r o s t a t i c precipitators,

c o n s i s t i n a of wet scrubbers and

has been largely r e p l a c e d by dry scrubbers

that are both more e f f i c i e n t and return the c a p t u r e d fluoride to the smelting cells.

Fluxing of metals,

once a p o l l u t i o n generator,

been c o n v e r t e d to fumeless fluxing methods.

has

Alcoa'has developed a

"Thermopure" water t r e a t m e n t system that uses waste heat to p u r i f y waste water by low temperature distillation.

The industry is con-

t i n u a l l y striving to do a better, more e f f i c i e n t job in e n v i r o n m e n t a l control. dust.

A l c o a is w o r k i n g on sealed ore buckets to reduce fugitive

We are also trying to improve our methods for used p o t l i n i n g

380

disposal.

Fluoride

emission

of the cost of an a l u m i n u m cost,

based on a cost

Institute

(Ref.

Some people much e l e c t r i c

control

smelting

study by the

have c r i t i c i z e d

quired

in the d e s i g n energy

to produce

aluminum

I would

of an a u t o m o b i l e

vehicles

the v e h i c l e s

the a l u m i n u m

produced

like

lifetime

inflation,

plants

modernization centive

make one

to modernize.

a cost e f f e c t i v e resist

efficiency governments

the easy route

to balance

the vicious

required

my concern

has

forced

lifetime

of re-

the a l u m i n u m

used

save more

to make

When

break

all of

their budgets

to incosts

for the more that

is little

cycle.

We must

in-

We must

incorporate

into all m o d e r n i z a t i o n . productivity

and

stop d e f i c i t

Perhaps

our

financing

Together this

We

to gain e n e r g y

We must also e n c o u r a g e

productivity.

cycle.

adds

in advance

there

this vicious

productivity.

control.

control

inflation.

new c o n s t r u c t i o n

one knows

increase

about

no longer pay

of s a c r i f i c i n g

inflationary

substituted

last year.

while we do our part by i n c r e a s i n g break

In fact,

less competitive,

productivity

it uses

the energy

last year will

gain will

We must

because

times

and e n v i r o n m e n t a l

to increase

or e n v i r o n m e n t a l

i0).

than was

in turn,

and equipment.

will

all work d i l i g e n t l y

must

8-12%

an energy bank

of a l u m i n u m

three

by r e s t a t i n g

so high that the p r o d u c t i v i t y efficient

(Ref.

in the U.S.

to c o n c l u d e

industry

saves over the

to over

in the U.S.

The high cost of both e n e r g y flation while

about

Primary A l u m i n u m

represents

Each pound

equivalent

the a l u m i n u m

in land and marine energy over

International

the a l u m i n u m

Actually

that can be used over and over. for steel

represents

and 1-3% of its o p e r a t i n g

9).

power.

the a u t o m o b i l e

alone p la n t

we can

is our g r e a t e s t

challenge.

REFERENCES 1 2 3 4 5 6 7 8 9

B. Marincek, U.S. Patent 3,562,135 (1971). B. Marincek, U.S. Patent 3,692,645 (1972). K. Yamada et al, German Patent a p p l i c a t i o n 2,547,168 (1976). K. Yamada et al, British Patent 1,461,155 (1977). H. Klein, U.S. Patent 3,718,550. H. Alder, U.S. Patent 3,960,678 (1976). H. Alder. U.S. Patent 3,930,967 (1976). H. Alder, U.S. Patent 4,057,480 (1977). I n t e r n a t i o n a l P r i m a r y A l u m i n u m Institute E n v i r o n m e n t a l C o m m i t t e e Report, "Fluoride E m i s s i o n Control: Costs for New A l u m i n u m R e d u c t i o n Plants," April 1975. i0 A l u m i n u m A s s o c i a t i o n Report TI2, "Use of A l u m i n u m in A u t o m o b i l e s - Effect on the Energy Dilemma," April 1980.