Download - Direct smelting and alternative processes for the production of iron and steel

INTERNATIONAL JOURNAL OF ENERGY RESEARCH, VOL. 20,1103-1128 (1996)

DIRECT SMELTING AND ALTERNATIVE PROCESSES FOR THE PRODUCTION OF IRON AND STEEL

T. ZERVAS, J. T. MCMULLAN AND B. C. WILLIAMS

Energy Research Centre, Uniuersiy of Ulster, Coleraine, Co. Londondeny, BT 52 1SA U.K.

SUMMARY

Processes for the direct smelting of iron and steel are presented together with a discussion of alternatives based on plasmas and the direct production of iron carbide. The advantages and disadvantages of each technology are presented and their energy efficiencies are discussed.

KEY WORDS: iron production; direct reduction; direct smelting; energy in steel production; iron carbide production; plasma technology

1. INTRODUCTION Over the past century, considerable research and development effort has been expended in developing methods for producing iron that could substitute for pig iron from blast furnaces (Strassburger, 1969). Much of this work was stimulated by a need to use lower-grade iron ores and non-coking coals that are unsuitable for the coke oven unit in an integrated coke oven and blast furnace plant. This has led to renewed interest in direct reduction and direct smelting processes. Direct reduction processes are those which produce iron by the reduction of iron ore at temperatures below the melting point of the product; the product is referred to as ‘direct reduced’ iron. Processes which produce a molten product similar to blast-furnace hot metal directly from the ore are defined as ‘direct smelting’ processes. If the objective is to produce liquid steel directly from the ore, the term ‘direct smelting’ processes. If the objective is to produce liquid steel directly from the ore, the term ‘direct steelmaking’ is often used. These definitions are clearly based on the characteristics of the product, though further treatment steps may be added to produce special grades of steel. As has been discussed by Zervas et al. (1996a, 1966b and 1966~1, the coke oven and blast furnace

integrated plant is expected to continue to be the world’s main source of raw material (pig iron) for steelmaking production, as long as supplies last of suitable coke for the coke ovens, and are available at a competitive price. Currently, direct reduction iron contributes a small portion of total world iron-making capacity with about 30 million tonnes per year of direct reduction iron capacity having been installed by June 1994 (Scott, 1994).

The direct reduction iron process has not been commercialized at the expected rate. In 1976 it was estimated that direct reduction iron capacity would reach 45 million tonnes per year by 1985. Instead, plant capacity had only reached 30 Mtonne per year by 1994, as mentioned above. The main reasons have been the lower-than-expected demand for iron in the developing world and the consequent reductions in plant capacity and utilization. The world-wide utilization of direct reduction iron plant has been about 60%.

Other research and development efforts in the reduction of iron ore have concentrated on the development of the direct smelting of iron. The main effort here is to develop a molten iron-making process that does not depend on metallurgical coal. Many processes are under development in the U.S.A., Australia, Japan and Europe (Fruehan, 1993). In Austrialia, CRA Ltd and Midrex are cooperat- ing in the development of HIsmelt process in which non-coking coal and pre-reduced iron ore are

CCC 0363-907X/96/121103-26 0 1996 by John Wiley & Sons, Ltd.

Received 11 December 1995

1104 T. ZERVAS, J. T. McMULLAN AND B. C. WILLIAMS

injected into a smelting reactor, and post-combustion of the off-gas with air is possible. In Japan, an association of the steel industry and public organizations are collaborating in the direct iron ore smelting (DIOS) process. In the U.S.A. the steel industry is responding to this challenge through a collaboration in research and development for a process sponsored by the America1 Iron and Steel Institute (AISI) and the U.S. Department of Energy (DOE) which involves a number of U.S.A. and Canadian steel firms and academic research centres.

So far, only two technologies have come to full commerical application (see Table 1). These are the Plasmadust variation of the Plasmasmelt process developed by SKF in Sweden, where it is used to process 70ktonne per year of allowed steelworks dust, and the Corex process which was developed in South Africa by Korf Engineering and Voest Alpine with a capacity fo 300 ktonne of hot metal per year. The ideal concept of a direct-smelting reduction process to convert iron ore and coal to steel has not yet been realized.

The generalized iron and steel metallurgical complex is shown in Figure 1 and the underlying chemical reactions are given in equations (1)-(12) (Davis et al., 1982; Lankford et al., 1985; Stephenson 1980).

3Fe,O3 + H, = 2Fe30 + H,O (1)

3Fe,03 + CO = 2Fe30, + CO, (2)

Fe,O, + H, = 3FeO + H,O (3)

Fe30, + CO = 3FeO + CO, (4)

FeO + H, = Fe + H,O ( 5 )

FeO + CO = Fe + CO, (6)

3Fe + CO + H, = Fe,C + H,O (7)

3Fe + 2CO = Fe,C + CO, (8)

CO, + C = 2CO (Boudouard reaction) (9)

H,O + C = CO + H , (10)

FeO + C = Fe + CO

3Fe + C = Fe,C (11)

(12)

At temperatures below about 1000" C, the dominant reactions are given by equations (1)-(6). Metallic iron will also absorb carbon according to equations (7) and (8). At temperatures above 1000"C, the additional reaction of carbon, CO, and H,O begins to play a part as shown in equations (9) and (10). CO and H, are produced, so increasing the reducing potential of the gas. The resultant net reaction of equation (11) is important for processes that produce DRI directly from solid coal without prior gasification of its fixed carbon. Above about 1200" C, metallic iron absorbs any carbon present, according to equation (121, leading to fusing or melting of the solid even though the melting point of pure iron is 1530" C. Thus, 1200" C represents the watershed between direct reduction and direct smelting processes. In practice, direct smeling processes operate above 1300" C to ensure that carbon is absorbed rapidly and liquid hot metal is produced.

Regardless of the reduction mechanism as discussed above, a necessary precursor to any iron oxide reduction process is the generation of a suitable reductant gas (H,,CO). This can be generated in a one-step process within the reductant reactor or it can be produced externally in a two-step process (Meyers, 1984). The main chemical reactions involved in gasification are as follows.

Corn bustion

C + O , = AH= -394kJmol-' (13)

Tabl

e 1.

Dire

ct r

educ

tion

plan

ts in

stal

led

thro

ugho

ut th

e w

orld

(Ste

ffen

and

Lun

gen,

198

8).

Cou

ntry

C

ompa

ny a

nd lo

catio

n

Nom

inal

Pl

ant

Red

uctio

n

Yea

r N

o. o

f (G

: gas

; (k

tonn

e se

rvic

e ag

ent

capa

city

ou

t of

Proc

esse

s co

mm

issi

oned

un

its

K: c

oal)

pe

r ye

ar)

Indi

a

Indo

nesi

a

Iraq

Iran

Can

ada

Egyp

t A

NS,

El

Dik

heila

Arg

entin

a A

cind

ar, V

illa

cons

truct

ion

Dal

min

e Si

derc

a, C

ampa

na

Aco

s Fi

nos,

Pira

tini

Port0

Ale

gre

Usi

ba, S

alva

dor

Stat

e ow

ned

com

pany

i B

razi

l

Bur

ma

( Man

dala

y

( H

ambu

rger

Sta

hlw

erke

, Ham

burg

Fe

dera

l Rep

ublic

Ger

man

y

Gre

at B

rita

in

Briti

sh S

teel

, Hun

ters

ton

Spon

ge Ir

on I

ndia

Ltd

Pa

lonc

ha

BSIL

, Cha

ndil

Oris

sa S

pona

ge Ir

on

Pala

sapu

nga

Ipita

ta, J

oda

Sunf

lag,

Bha

ndar

a Es

sar,

Hiz

ira

Kra

kata

u St

eel,

Cile

gon

Sodc

o, K

hor

El Z

ubei

r

Nisc

o, A

hwaz

N

isco

, Ahw

az

Nis

co, A

hwaz

N

isco

, Mob

arak

eh

Sidb

ec D

osco

, C

ontr

ecoe

ur 1

C

ontr

ecoe

ur 2

I i

Mid

rex

Mid

rex

Mid

rex

SL/R

N

HYL

Kin

glor

- M

etor

Mid

rex

Mid

rex

SL/R

N

SL/R

N

Acc

ar

TD

R

Cod

ir

Mid

rex

HyL

/HyL

Ill

HYL

HYL

Puro

fer

Mid

rex

Mid

rex

Mid

rex

Mid

rex

1987

1978

/83

1976

1973

19

74

1981

19

84

1972

1979

1980

/85

1987

1983

19

86

1988

19

90

1978

/82

1979

1977

(?

) (?

) (?

)

1973

19

77

1 1 1 1 1 1 1 1 2 2 1 1 1 1 2 4 2 1 3 2 5 I 1

G

G

G

K

G K

K

G

G

K

K K

K

K

G

G

G

G

G

G

G

G

G

720

600

330 65

3 25 20

20

400

800 60

150

150 90

150

840

2240

543

360

1100

80

0 32

00

400

600

5 rn n z > =! < rn n \ rn

W - P Y m

r 8 3 P F1

X

U

C

W n rn

v1

v1 c1

X

X

X

Tabl

e 1.

Con

finic

ed

c

!e

Cou

ntry

C

ompa

ny a

nd lo

catio

n

Red

uctio

n N

omin

al

Plan

t

Yea

r N

o. o

f (k

tonn

e se

rvic

e (G

: gas

; Pr

oces

ses

com

mis

sion

ed

units

K

coa

l)

agen

t ca

paci

ty

out o

f

per y

ear)

Qua

tar

Lybi

a

Mal

aysi

a

Mex

ico

New

Zea

land

Nig

eria

Peru

Saud

i Ara

bia

Sovi

et U

nion

Sou

th A

fric

a

Tri

nida

d an

d To

bago

US

A.

Ven

ezue

la

Qua

tar

Stee

l, D

oha

Stat

e ow

ned

com

pany

, Mis

urat

a

Saba

h G

as In

d. L

abua

n Tr

engg

anu

Stee

l, Tr

engg

anu

Hyl

sa, M

onte

rrey

1 M

H

ylsa

, Mon

terr

ey 2

M

Hyl

sa, M

onte

rrey

3M

H

ylsa

, Pue

bla

1 P

Hyl

sa, P

uebl

a 2P

Si

carts

a. L

as T

ruch

as

Tam

sa, V

era

Cru

z

New

Zea

land

Ste

el 1

, Gle

nbro

ok

New

Zea

land

Ste

el 2

, Gle

nbro

ok

Del

ta S

teel

, War

ri

Side

rper

u, C

him

bote

Saud

i lro

n&St

eel,

Al J

ubai

l

OEM

K, K

ursk

Dun

swar

t Ste

el, B

enon

i Is

cor,

Van

derb

ijlpa

rk

Isco

r, Pr

etor

ia

Scaw

Met

als,

Ger

mis

ton

Uni

on S

teel

Cor

p. V

eree

nigi

ng

Dav

stee

l, Zo

nden

vate

r

Isco

tt, P

oint

Lis

as

( Geo

rget

own

Stee

l, G

eorg

etow

n

Min

orca

, Pue

rto O

rdaz

Fi

or d

e V

enez

uela

, Mat

anza

s Si

dor,

Mat

anza

s 1

Sido

r, M

atan

zas

2 Si

dor,

Mat

anza

s 3

Sido

r, M

atan

zas

4 Si

vens

a, M

atan

zas

Mid

rex

Mid

rex

Mid

rex

NSC

HYL

HyL

111

H

yL 1

11

HYL

HYL

HYL

HyL

111

SL/R

N

SL/R

N

Mid

rex

SL/R

N

Mid

rex

Mid

rex

Cod

ir SL

/RN

C

orex

D

RC

Fl

uor/H

uls

Rot

ary

Kiln

Mid

rex

Mid

rex

Mid

rex

Fior

H

yL 1

M

idre

x H

yL 1

M

idre

x M

idre

x

1978

1988

1984

19

85

1957

19

60/7

9 19

74/8

2 19

69

1977

19

88

1967

1970

19

86

1982

/83

1980

1982

/83

1983

/87

1973

19

84/8

5 19

88

1983

19

85

1985

1980

/82

1971

1972

/89

1976

19

77

1977

19

81/8

7 I9

79

1990

I 2 1 1 1 1 1 1 1 4 1 1 4 2 3 2 4 1 4 1 1 1 1 2 1 1 1 1 1 3 3 1

G

G

G

G

G

G

G

G

G

G

G

K

K

G K G

G

K

K K K G K G

G

G

G

G

G

G

G

G

400

1 100

650

600

100

250

550

335

700

2000

28

0

150

700

1000

100

800

1650

150

600

300 75

250 35

840

400

750

350

400

400

1920

I2

75

400

X

9

N

icr < rn

-$

ALTERNATIVE IRON / STEEL PRODUCTION PROCESSES

Fhul coldrollhy, hot row -- mBtdb&&C.

1107

- i " t C-hUO-

-g -

I-

- Gasification

c + ;02 = co A H = - l l l k Jmol - ' (14)

(15)

(9)

C O + H,O(g) = C O , + H, A H = -41kJmol-' (16)

C O + 3H, = C H , + H,O(g) A H = -206kJmol-' (17)

2CO + 2H2 = CH, + C O , A H = -247kJmol-' (18)

C + H,O(g) = C O + H ,

c + C O , = 2 c o

AH = + 135.6 kJ mol- ' AH = + 172.46 kJ mol- '

Shifr

Methanation

and the general process can be represented by

C , H , + ( n / 2 ) 0 , = nCO + ( m / 2 ) H 2 (19)

The combustion part of the process is exothermic, as shown by the heats of combustion, whereas most of the gasification reactions are endothermic. The water gas shift reaction takes place in the gasifier and helps to adjust the hydrogen-to-carbon monoxide ratio. It is also catalytically induced in a separate methanation step to form SNG.

2. REACTORS

A reactor is a unit of equipment designed to carry out certain reaction processes. As the majority of the reaction processes in metallurgy are heterogeneous, the main purpose of the reactor is to bring the reacting phases together under favourable conditions of pressure and temperature, to supply the necessary energy and to allow the separation of the product phases.

1108 T. ZERVAS, J. T. McMULLAN A N D B. C. WILLIAMS

Although the reactors are similar, there are differences and the reactors can be classified as continuous or discontinuous, series or parallel, etc. (Rosenqvist, 1983).

2.1 Vertical shaft reactor The vertical shaft reactor (Stephenson and Smaller, 1980) is a moving bed reactor in which iron ore moves under gravity in counter current to a reductant gas stream. The shaft reactor has been always a favoured reactor for the production of iron from iron ore and it has been commercially adopted as the preferred reduction unit in a number of direct reduction processes. As iron oxide requires higher concentrations of reductant to drive the reactions, a counter current

reactor is paticularly suited to the reduction of iron oxide. It also permits efficient heat transfer between the gas and solids. As the vertical shaft furnace is simple in construction, and existing and proven designs can be used

with a minimum of modification, it is attractive for the application of direct reduction processes. The disadvantage of the shaft reactor is that it is not possible to use ore fines without agglomeration

because of the need to ensure adequate space within the bed to permit access and passage of the reductant gas.

The development of mechanical devices to discharge the direct reduction iron from the shaft reactor is essential for its successful application in the reduction processes.

The shaft is usually fitted with slowly oscillating charge feeders situated just below the reductant gas, which helps to maintain the inflow of material into the reduction zone. The inflow of the charge and the uniform distribution of reductant gas across the vertical shaft reactor are essential to assure uniform metallization in the product. The vertical shaft reactor is designed to operate at ambient pressure so that the feed and discharge legs can be sealed with a dynamic gas seal using neutral gas. This avoids the capital costs associated with the mechanical sealing systems required for high pressure operation. The high pressure process operates at 4-5atm and therefore requires sealing valve systems at the top and bottom of the shaft, involving considerable complexity. Some shafts have no mechanical feeders but depend on furnace geometry and good temperature control to avoid charge sticking and to maintain the inflow.

2.2 Fluidized bed reactors The main advantage of fluidized bed reactors is their ability to accept iron ore fines as the charge material without agglomeration. Another advantage is that the rate of reaction is higher. These advantages over the vertical shaft reactor are, however, offset by some disadvantages.

Pellets Lump ore ""r1

.rc-------- Reducing gas I +

Sponge iron

Figure 2. Simplified diagram of the vertical shaft reactor (shaft furnace)

ALTERNATIVE IRON /STEEL PRODUCTION PROCESSES 1109

As the main principle of operation of a fluidized bed reactor is good mixing of the charge, it is necessary to employ two or more beds in series to obtain an approximation of counter current operation, with associated problems of charge transfer between the beds (Stephenson and Smaller, 1980).

The reductant gas velocity must not exceed the sedimentation velocity of the smallest fines in the bed so as to avoid carry over of charge to the exit gas stream. Competing with this, the heat and reduction requirements of the process dictate that a minimum mass flow of gas must be maintained. In order to satisfy both requirements, fluidized bed systems must operate at elevated pressure.

Another disadvantage is that operation of fluidized beds becomes difficult at high levels of metallization as the sticking tendency of the material tends to interfere with fluidization. In practice, the particle size distribution of iron ore fines may be far from ideal for good fluidization, and the bed will operate best with fines having fairly narrow particle size range. Operation is more difficult with materials having high levels of superfine particles.

3. MELTING REACTOR PROCESSES

3.1 Fluidized bed melter gasifier

In this melter (Smith and Corbet, 19881, coal is charged into the top end of the melter where it is dried and devolatilized by contact with gases produced during the melting process. The coal char creates a bed which is fluidized by injected oxygen. Prereduced iron is charged by gravity from the prereducer reactor into the fluidized bed, where it is reduced and melted, creating a pool of slag and metal under the fluidized bed which is periodically discharged by tapping.

This melter operates in a counter-current mode with temperatures of about 1500°C in the lower melting part and about 1O00-120O0C in the upper part where the gasification takes place. Thus, the melter reactor operates as both gasifier and smelter. The metal and slag chemistry is the same as for the blast furnace.

The engineering principles of this reactor are very similar to those of the blast furnace from which it borrows its refractory technology and its tapping discharge process.

So far, there is only one commercial process, the Corex direct smelting process, that uses this melter technology. The Corex process has the following characteristics.

(1) It can be used in conjuction with mini-mills, so producing pig iron at a lower cost and a lower installed capacity than is possible with the blast furnace.

(2) It allows large integrated iron/steel making plant to increase pig iron capacity by up to 600 ktonne per year and to supplement the large blast furnaces operating at capacities of up to 10 Mtonne per year. This gives the integrated iron/steel making plant the flexibility to modify production according to market demand.

3.2 Electric arc furnace

Today the most common commercial routes for the production of steel from iron are based either on basic oxygen furnace (BOF) steel-making or on the electric arc furnace ( E M ) (Report, 1987; Open University, 1979) (Table 2). Being an autogenous steel-making process, the basic oxygen furnace can only take a limited quantity of iron scrap and direct reduction iron sponge. The electric arc furnace on the other hand can operate with a charge of 100% iron scrap or 100% direct reduced iron sponge. For this reason, it is an obvious candidate fo the basis of a mini-mill steel plant.

Steel-making is a refining process in which undesirable impurities such as sulphur and phosphorus are removed by oxidation. The electric arc furnace was first used commercially in 1914 to produce stainless and other forms of steel. The consumption of these alloy steels has increased since then and with it has come the need for larger electrical arc furnaces. Initially, both the electric arc furnace and the open

1110 T. ZERVAS. J . T. McMULLAN AND B. C. WILLIAMS

Powder h n

Figure 3. Simplified diagram of the fluidized bed reactor

PREREDUCED DRI REDUCING GAS

COAL

1 MELTER I

PREREDUCED DRI REDUCING GAS

)SLAG

OXYGEN -+I-> Figure 4. Simplified diagram of the fluidized bed melter gasifier

hearth steel-making processes opeated on a cold charge which was composed entirely of either direct reduction iron sponge or iron scrap. Today, the open hearth steel-making process is declining and the electric arc furance is taking over, and is the prefered choice for mini-mill steel plant of capacity up to about 500 ktonne per year.

The electric arc furnace is used for the smelting of iron and ferroalloys as well as for steel-making. Electric energy is fed through carbon electrodes which are either dipped into the slag which acts as the main resistor, or are terminated above the bath causing electric arcs to be produced between the electrode and the bath. Another example is the submerged arc furnace, in which heat is generated partly by arcs and partly by resistance heating in the solid and semimolten iron/steel.

The advantages are that heat is generated in the semimolten iron/steel or slightly above it and with good heat transfer; there are no combustion gases and the effluent furnace gases are limited to those produced by the process, with corespondingly smaller losses.

The electric arc furnace is the most common method of producing steel from direct reduction iron ore. Like the basic oxygen furnace, it is a cylindrical refractory lined vessel, but its diameter-to-height ratio is greater at between 4 and 6. The main components are the shell, where the hearth is lined with basic


Table 2. Crude steel production by process, 1992 (IISI, 1993)

Production Oygen Electric Open Other Total Country (Mtonneperyear) (%) (%) hearth (%) (%) (%)

Belgium Denmark France FR Germany? Greece Ire 1 and Italy Luxembourg Netherlands Portugal Spain United Kingdom EC Total

Austria Finland Norway Sweden Turkey Yugoslavia Other W Europe Total W Europe

Canada United States Japan Australia New Zealand South Africa Total Indust. Cts Argentina Brazil Chile Mexico Peru Venezuela

India Rep. of Korea Taiwan (ROC) Total Market Econ

Bulgaria (E) Czechoslovakia Hungary Poland Romania Russia Ukraine Total EITs $ Total §

EDPt

13.3 0 6

18% 39.7 0 9 0.3

24.9 3.1 5.4 0.7

12.3 16.0

132.2

4.0 3.1 0 4 4.4

10.3 0.7

22.8 155.0

13.9 883 98.1 6.9 0.8 9.1

368 1

2.7 23.9

1 .o 8.4 0.3 3.3 2 5

18.1 28.1 10.7

467.1

1.6 11.1 1.6 9.9 5.4

67.0 41.7

1382 605.3

89.8

69.9 77.1

408 100.0 95.5 55.4 40.6 75.3

66.8

90.9 83.1

664 34.1 74.6

56.9 65.3 67.3 62.0 68.4 92.5 69.7 63.4

65.9

37.7 78.9 95.1 44.4 42.6

35.4 45.5 69.8 50.6

64-6

50.3 53.9 85.9 63.3 527 345 39.8

41.2 592

102 100 100.0 100 30.1 100 25.6 1.3 100

100.0 100 100.0 100 59.2 100

100 4.5 100

44.6 100 59.4 100 24.7 100

32.8 0.4 100

9 1 100 16.9 100 100 100 35.6 100 59.6 6.3 100 20.9 4.5 100

401 3.0 100 33.9 0 8 100 32.7 100 38.0 100 31.6 100 7.4 100

30.3 100 35.6 1.0 100

33.7 0 3 00 100

62.3 100 19.3 1.9 100 4.9 100

55.6 100 57.4 100

100.0 100 58.8 5% 100 28.2 26.3 100 30.2 100 48.8 0.5 100

340 1.3 0 1 100

40.0 9.7 100 11.0 35.0 100 5.1 9 0 100

18.2 18.4 100 30.8 16.6 100 15.4 50.1 100 7.6 52.6 100

13.6 45.2 100 294 11.3 0.1 100

tIncluding former GDR. $Economy in transition. §The countries included account for 84% of total world output.

1112 T. ZERVAS. J. T. McMULLAN AND B. C. WILLIAMS

1

Figure 5. Simplified diagram of electric arc furnace

refractories and holds the charge, the water cooled wall and roof panels to allow for greater power input, an automatic positioning mechanism to hold the electrode and maintain the proper arc length, a mechanism to remove the roof for feeding and another to tilt the furnace of tapping and deslagging. Electric furnace shops are equipped with environmental equipment for water treatment and degreasing, and noise and fume extraction.

3.3 Pneumatic reactors (basic oqgen furnace) There are two kinds of pneumatic reactor (Lankford et al., 1985; Report, 1987). The basic oxygen furnace is oxidizing, as in steel-making converters. Reducing reactors use a reducing gas in place of the oxygen, so that reduction and deoxidation of the bath is obtained. Lime may also be introduced into the smelting bath.

In the basic oxygen furnace, the oxygen reacts with a molten charge and is introduced through tuysres directly into the smelting bath. Depending on the oxygen injection configuration, the furnace is designated as top-blown, bottom-blown or side blown. The oxygen can also be introduced directly through lances which dip into the bath.

In basic oxygen furnace steel-making, the reaction in the smelting bath is normally sufficiently exothermic to supply the required heat. For reducing processes where a reductant gas is blown into the reactor, the required heat may be obtained by introducing oxygen along with the reducing gas, or by feeding a mixture of coal power and oxygen-enriched air.

The basic oxygen furnace is the most common process in the production of steel. It is one unit in the integrated iron/steel making process, which also includes the coking unit, pelletizing plant, sintering plant and blast furnace. In the basic oxygen furnace process, liquid metal, scrap and fluxes are fed into a pressurized refractory lined vessel. First, the oxygen reacts with carbon and silicon to produce the required heat to increase the temperature of the bath from about 1350" C to over 1650" C and melt the scrap. The lime removes the sulphur and phosphorus from the liquid iron.

4. COMPARISON OF STEEL-MAKING PROCESSES When we compare the basic oxygen furnace and the electric arc furnace we must remember that the main function of the electric arc furnace is to supply heat for melting, while that of the basic oxygen

ALTERNATIVE IRON/STEEL PRODUCTION PROCESSES 1113

Figure 6. Simplified diagram of the basic oxygen furnace

furnace is to refine by oxidizing the carbon and silicon and other elements and to increase the temperature of the iron by using the heat of the reactions (Report, 1987). Consequently, they use different charge materials, mainly direct reduced iron or scrap in the electric arc furnace, and molten iron or iron carbide in the basic oxygen furance, to produce a similar product - the molten steel.

The major difference between the direct reduction electric arc furnace and the blast furnace basic oxygen furnace steelmaking routes is the difference in energy consumption. For current efficient integrated blast furnace/basic oxygen furnace plant, the energy requirement is typically 19.24 GJ tonne- ' of molten steel while that for scrap-based arc furnace mini-mill steelmaking is 8.49 GJ tonne-' molten iron. The difference in energy consumption lies in the fact that a large part of the energy needed to produce molten steel is associated with the reduction of iron oxide. With scrap, there is no need for reduction because this stage is already completed.

With the basic oxygen furnace, the charge contains 25-30% scrap and the rest contains liquid iron and flux, usually lime. Oxygen is introducd at a rate of 20 to 30 thousand cubic feet per minute. The oxidation starts first with silicon, and is continued with carbon, then manganese. Through these oxidation reactions, the scrap is melted and the iron temperature increases from about 1350" C. The lime flux merges with the oxidized silicon, iron and manganese to form slag which removes about half of the sulphur and the majority of the phosphorus. The process time is usually 15 to 20miutes. After oxidation, the liquid metal is sampled and then discharged.

Electric arc furnaces for steelmaking have developed significantly over the past 20 years. Usually, scrap is fed into an empty furnace and the arc melts a hole down through it using the scrap burden to protect the walls from the arc flare. When all of the iron scrap is melted, during the refining and superheating steps, the voltage is decreased to reduce the power, so protecting the walls and roof from excessive radiant heat. Foamy slag practices have been used in which oxygen and carbon are introduced into the bath to form a gas which foams the slag around the electrodes, protecting the walls and roof from the arc and allowing higher power to be used. The carbon is removed by introducing oxygen.

Natural gas or oil can be used to supply extra heat to the electric arc furnace during melting, reducing the melting time by about lominutes and the electrical consumption by 30 to 50KW h tonne-' of molten steel.

It is usually not easy to evaluate in detail the advantages and disadvantages of each method of


producing molten steel because they depend on local conditions such as price and quality of the raw materials and the condition of the market in which the steel products will be sold. Consequently, only general conclusions of the advantages and disadvantages can be drawn.

The investment cost of an electric arc furnace steel-making plant is considerably less than that for an integrated coke oven, blast furnace and basic oxygen plant. Also, producing steel from scrap costs less in terms of cost, labour and environmental impact (see Table 3). Additionally, 60% less energy is needed to produce molten steel in an electric arc furnace and relatively small plants of the order of 600 ktonne per year capacity can be erected economically to produce a wide range of steel products competitively.

The disadvantages of electric arc furnace are as follows.

(a) The off-gas contains a dust classified as a hazardous waste. (b) As the nitrogen content in the steel can be higher in the electric arc furnace than in the basic

oxygen furnace, it is difficult to produce certain steels. (c) The main energy supply is electricity, which is expensive.

Despite these disadvantages, developments in the last 20 years have improved the efficiency of electric furnace operation in terms of both its productivity and its ability to produce steel products competitively.

5. SMELTING PROCESSES

5.1 Corex process

The Corex process is the only commercial process that produces molten iron by direct reduction of iron ore (Chatterjee, 1994; Delport, 1991; Feichtner et al., 1989; Scott, 1994; and Steffen, 1990). It was initially developed as the K-R process by Korf engineering and Voest Alpine in West Germany in 1978. After a successful pilot plant operation at Kehl/Rhine, Germany, in 1981, a plant of 300 ktonne per year was constructed at ISCORs Pretoria Works and began successful operation in 1989.

The start-up was very successful and about 270 ktonne of hot metal were produced during the first year of operation, representing 90% of the design capacity.

The Corex process combines a gasifier-smelter with a vertical-shaft reduction reactor. Hot reductant gases are generated from a wide variety of untreated coals and oxygen in a smelter/gasifier and are passed through a counter-current reduction shaft charged with lump ore and/or pellets.

The Corex process is designed to operate at a pressure of up to 5 bar. Charging of coal and iron ore is done through a lock hopper feeder. The coal is stored in a feed bin and charged into the melter-basifier by a feed screw conveyer. The coal falls by gravity into the gasifier where it comes into contact with a hot reductant gas atmosphere at a temperature between 900°C and 1300°C and is instantly dried and degasified. Reducing gas is generated by gasification of coal in a fluidized bed in the upper part of the smelter.

The flow velocity is selected to maintain the fluidized bed at a temperature between 1500°C and 1800" C. The reductant gas has a high CO content of about 85% and after leaving the melter gasifier is cooled to between 800" C and 900" C by mixing with cooling gas, and is prereduced in a hot dust cyclone before being introduced to the vertical shaft reductant reactor. A small amount of the cleaned gas is converted to cooling gas in a gas cooler. The fines captured in the hot cyclone are recirculated to the gasifier.

The reductant gas is fed into the reduction furnace, flows counter-current through the descending iron ore and escapes from the top end where it is exported for electricity generation or chemicals production, or is recycled back to the process. Partial reduction takes place in the vertical shaft, the partially reduced iron is continuously charged into the melter-gasifier, where further reduction and smelting takes place. Molten iron and slag are discharged by conventional tapping, using a practice similar to that used in the blast furnace. The tapping time is between 150 and 180 minutes.


Table 3. Breakdown of steel-making costs (Report, 1987)

Item cost US$ Electric furnace Oxygen furnacet

Materials (ore, scrap fluxes etc) Energy (coal, oil, electricity,

10235

oxygen, gas credits) 32.84

Maintenance, materials and services

83-26

31.6

4 9 4

Labour 13.44 1913 Capital (interest and depreciation) 20 47.02 Total liquid steel cost 17263 190.41

?Oxygen furnace costs include components in coke and hot wet.

COAL

~~

G A S I W

I >El.AG

& HOTMETfi

Figure 7. Simplified diagram of the Corex process

The degree of metallization of direct reduction iron in the vertical shaft furnace reactor is about 95% and the carbon content varies from 2% to 5%, depending on the raw material and operating conditions. The energy consumption is about 13.4 GJ tonne-' of molten iron.

The advantages of the Corex process are as follows.

(a) It is the only smelting process in which coke can be replaced by a wide variety of coals. (b) In comparison with the blast furnace, it is more environmentally friendly as it avoids coke

production.


Table 4. Capital cost of a typical 300ktonne-per-year COREX plant (in M$US - 1987 estimates) (Chatterjee, 1994)

Item Corex unit

~~

Power

Engineering Equipment (imported) Other equipment

Civil and structural Utilities

Erection supervision Licence fee

Total cost of Corex + oxygen +power plant Other costs @ 5% on Item 8

Subtotal of 8 + 9 Contingencies @ 15% Total cost

12.6 37.1 7.7

6.3 1.4

9.8 2.1

77.0

2.1 25,2

(cost included in power plant)

(cost included in power plant) 0.7

0.7

28.7

198.8

-

9.8

208.6 31.5

240.1

6.3 75.6 3.5

4.2 1.4

2.1

93.1 -

Table 5. Operating cost of a 300 ktonne-per-year Corex plant (in US$ - 1987 estimates) (Chatterjee, 1994)

Blast furnaces Corex

Raw materials Fuel Labour Stores Transfers Credit gas

Total cost above

Total works cost Credit for slag

Net works cost

74.0 0.1 3 .O 1 .o

17.0 - ( 5 )

16.2

106.3 - (3.0)

103.3

64.3

3.0

18.0

2.1

68.4 - (3.0)

65.4

-

-

-(19.0)

(C)

(d)

(e)

The process generates large volumes of process gases that have a high utility value and can be exported. In the event that the off-gas cannot be exported economically, it can be recirculated - reducing coal and oxygen consumption. Finally, Corex plant is relatively cheap to construct.

5.2 AISI process for direct smelting

Development of this process began in 1987 when AISI undertook a feasibility study of direct steel making. In 1990, a pilot plant for preheating and prereduction of ore pellets was constructed with HYLSA at Monterrey, Mexico, and a smelting pilot plant was build in Pittsburg (Fruehan, 1993; Chatterjee, 1994; Scott, 1994; Aukrust, 1992).


The main part of the project is the 5 tonne h- ' pilot plant at Pittsburgh, Pennsylvania, where liquid iron and semi-steel has been produced. This direct smelting process operates as follows (see Figure 8); coal and prereduced iron are charged to a liquid metal and slag bath (smelting reactor) in which the prereduced iron is reduced and combustion of the coal supplies the necessary heat for reduction and melting of the iron. The important factor in this process is that the heat requirement is met by post-combustion of the reductant off-gases. The off-gas contains a mixture of 40% carbon dioxide and steam and is used for the prereduction of the iron ore pellets from haematite to wustite. This prereduction is done in a conventional shaft furnace. The final reduction takes place in the smelter, where gasification of the coal also occurs.

Although research and development on the AISI process began later than that on most other process development programs, it has reached a more advanced state of development, and a 50 tonne h- ' of hot metal demonstration plant is being designed with the following criteria.

(a) The prereduction of haematite to wustite will be about 255%. (b) The post combustion will be at least 40%. (c) The system with be pressurized. (d) The process will involve continuous smelting. (e) Batch tapping will be used. (f) The production intensity will be 10 tonnes of hot iron per day per cubic metre. (g) The refractory wear will be 0.25 mm h- I .

The AISI feasibility study reports that operation and investment cost will be lower and more favourable than the classical route of coke oven and blast furnace.

5.3 DIOS process The DIOS process has been developed through a collaboration of Japan Iron and Steel Federation, eight major integrated steel producers and the Coal Mining Research Centre of Japan (Chattejee, 1994; Scott, 1994).

COAL

~ T E D & p p L p u I u ( 3 D ~ m

HUI'BUZALISIAG

Figure 8. Simplified diagram of the AISI process

1118 T. ZERVAS, J . T. McMULLAN AND B. C. WILLIAMS

As shown in Figure 9, the DIOS process consists of three main units, a fluidized bed reactor where prereduction takes place, a smelting reduction vessel where final reduction and smelting take place and a gas reforming furnace. A pilot plant is under construction to examine the following aspects.

(a) When can liquid iron be produced in one step in a single unit in the smelter reduction vessel where gasification, reduction and smelting take place in the same reactor?

(b) When can liquid iron ore be produced in two steps in two units, with prereduction in the prereduction furnace and final reduction and smelting in the smelting reduction vessel?

(c) When can liquid iron ore be produced in three steps in three units, with prereduction in the prereduction furnace, gasification in the gas reforming furnace and smelting in the smelting vessel?

5.4 Hismelt process

The Hismelt process was developed by the Australian company CRA and Klockner in the 1980s. The process consists of two main units, a prereduction system in which prereduction takes place, and a smelt reduction vessel in which final reduction and smelting take place (see Figure 10). The process is somewhat similar to the AISI process (Chatterjee, 1994; Scott, 1994).

The Hismelt process is a two-step process. Iron ore is prereduced to about 22% in the prereduction system and the prereduced iron is introduced at 850°C to the smelter reduction vessel where final reduction and smelting take place. As with the DIOS and AISI processes, Hismelt involves a high post-combustion of gas in the smelter. The Hismelt process differs from the AISI and DIOS processes because, instead of using deep slag formation, it relies on violent agitation of the liquid metal to secure a high level of heat exchange in the smelter. Some characteristics of the process are as follows.

(a) As the smelt reduction reactor is of the horizontal type, it is possible to achieve high injection

(b) Lower end pulverized coal injection maximizes the gasification of the coal and so increases the

(c) Part of the off-gas is utilized by the post combustion step to preheat the air.

rates.

reduction rate.

6. ALTERNATIVE ROUTES IN IRON- AND STEEL-MAKING

6.1 Plasma technology in iron-making

Steel refining and melting has used plasms technology for many years (Fey, 1985; Eriksson et al., 1987; Santen et al., 1987; Barcza, 19861, but commercial application for direct reduction and smelting of iron ore has not been applied (except for the Plasmadust process, which is a variation of the Plasmasmelt system in which iron/steel dust is reduced to iron). Despite a considerable effort in research and development over the last twenty years, no such process has passed the pilot stage.

The three main domains are being developed: the application of plasma in the blast furnace, in direct reduction and in direct reduction/smelting.

In plasma processes, a plasma is generated either from an inert gas such as argon or from a reactive gas such as hydrogen. Iron ore and carbon in the form of fines are then introduced to the system. Plasma provides an intense concentrated source of energy for endothermic reactions and has a high energy efficiency if primary energy is ignored and only net energy is considered for comparison with other processes. Because of the energy density, the reactions are very fast, leading to smaller reactor sizes and lower capital costs for the same plant capacity.

In spite of these advantages, the process has not been commercialized because of the cost of electric energy for plasma generation. The only commercial application is the Plasmadust process of SKF steel. A


Smelter Rduttion V U r d

1119

LIMESTONE

ORE

LIMESTONE

1"

Smoltor G8oMr

Hot k Mot81

Figure 10. Simplified diagram of the Hismelt process

plant of 70ktonnes per year was erected between 1982-1984 at Landskrona, Sweden, to recover iron from various waste oxide dusts (Steinmetz et al., 1986).

In conclusion, research and development on these processes is targeted towards replacing metallurgical coke with less expensive coal, to using lower quality characteristic iron minerals and building plants with smaller capacities.

6.1.1 Plasmasmelt process The plasmasmelt process (Figure 11) (Smith and Corbett, 1987,1988; Eriksson et al., 1987) was developed by SKF in Sweden. The process consists of three main units, two are fluidized beds, one of which is for

Y

Tabl

e 6.

Det

ails

of

larg

e-sc

ale

appl

icat

ions

of p

lasm

a sy

stem

s (B

arcz

a, 1

986)

Com

mod

ity

Com

pany

Pr

oces

s typ

e Pr

oces

s na

me

Dev

ice

or c

ontr

acto

r Pr

oduc

tion

capa

city

Iron

and

ste

el

Ferr

o-al

loys

1. D

RI

2. D

RI

3. P

ig ir

on

4. P

ig ir

on (a

lloye

d w

ith C

r, M

n an

d ot

hers

) 5.

Pig

iron

6. S

teel

7. S

teel

8. S

tain

less

stee

l

1. F

erro

-chr

omiu

m

2. Fe

rro-

chro

miu

m

3. F

erro

-man

gane

se

SKF

Swed

en

USC

O, S

outh

A

fric

a

Coc

keril

l St

eel-

CR

M

and

a U

.S. s

teel

pr

oduc

er

SKF

ASE

A

M.A

.N.

Gm

bH

Frei

tal-

Voe

st

Alp

ine,

ASE

A-K

rupp

Min

tek-

ASE

A

SFK

Sam

anco

r

Indi

rect

non

-tra

nsfe

rred

fu

rnac

e ga

s re

form

er-s

haft

LPG

or c

oal s

lurr

y In

dire

ct n

on-t

rans

ferr

ed g

as

refo

rmer

-sha

ft fu

rnac

e sy

ngas

In

dire

ct n

on-t

rans

ferr

ed

supe

rhea

ting

of g

as b

last

to

blas

t fur

nace

Smel

ting

in a

cok

e-fil

led

shaf

t fur

nace

-Zn,

Sn

fum

es

prod

uced

d.

c. tr

ansf

erre

d-ar

c op

en

bath

d.

c. tr

ansf

erre

d-ar

c op

en-b

ath

scra

p m

eltin

g d.

c. tr

ansf

erre

d-ar

c op

en-b

ath

scra

p m

eltin

g

d.c.

tran

sfer

red-

arc

open

-bat

h

d.c.

tran

sfer

red-

arc

open

-bat

h co

-mel

ting

and

smel

ting

Dire

ct n

on-t

rans

ferr

ed s

haft

fu

rnac

e (c

oke-

fille

d) -

preh

eate

d or

e fe

ed

d.c.

tran

sfer

red-

arc

open

-bat

h m

eltin

g

Plas

mat

ech

arc

heat

er

lx6

MW

Hul

s arc

hea

ters

3

X8

MW

Wes

tingh

ouse

arc

1

x3

MW

Plas

mat

ech

arc

heat

ers

3X

6M

W

7X

6M

W

Hol

low

gra

phite

el

ectr

ode 4

0 M

W

Solid

gra

phite

el

ectr

ode

6 M

W

Wat

er-c

oole

d pl

asm

atro

ns

3X

7M

W

4X

7M

W

Solid

gra

phite

el

ectr

ode

18 M

VA

Hol

low

gra

phite

el

ectr

ode

16 M

VA

Plas

mat

ech

arc

heat

ers

4X

6M

W

Frei

tal-

Voe

st A

lpin

e pl

asm

atro

n 10

8 MV

A (8

MW

)

Plas

mar

ed p

roce

ss

Fluo

r/D

avy

McK

ee

Piro

gas p

roce

ss

Plas

mad

ust

(Sca

ndus

t)

(Pla

smas

mel

t)

Elre

d pr

oces

s

Una

rc p

roce

ss

SE p

roce

ss (s

ingl

e el

ectr

ode)

d.c.

chr

ome

proc

ess

plas

mac

hrom

e

Frei

tal-

Voe

st

Alp

ine

70 k

tonn

e pe

r ann

um

320

kton

ne

per a

nnum

Indu

stri

al b

last

fu

rnac

e

70 k

tonn

e pe

r ann

um

(250

kto

nne-

per-

annu

m

stud

y on

ly

(600

kto

nne

pe a

nnum

st

udy

only

15

tonn

e fu

rnac

e

35 to

nne

45 to

nne

55 to

nne

50 o

r 20

kton

ne p

er a

nnum

t

78 k

tonn

e pe

r ann

um

50 k

tonn

e pe

r ann

um

t50

Kt/

a M

eltin

g an

d sm

eltin

g 2.

1 20

Kt/

a Sm

eltin

g

ALTERNATIVE IRON/ STEEL PRODUCTION PROCESSES 1121

drying and preheating and the other is for prereduction of the iron ore. The third is a shaft furnace plasma reactor in which final reduction and smelting take place. Iron oxide fines are mixed with lime and charged to the preheat fluidized bed reactor, which uses the off-gas from the shaft smelter reactor or the fluidized bed prereduction reactor. After this, the dried iron ore is passed to the second fluidized prereducer reactor, where prereduction takes place by contact with the off-gas from the shaft smelter reactor. Prereduced iron oxide with 70% reduction is pneumatically injected with coal fines to the shaft smelter reactor. The vertical shaft smelter reactor is fed at the top with coke. Plasma generators are mounted in the lower part of the shaft reactor. Molten iron is collected from the lower end of the shaft smelter by tapping. The majority of the off-gas from the smelter reactor consists of CO and H,, and after cleaning it is recirculated back to the two fluidized bed reactors. A small part of the cleaned off-gas is compressed and used as plasmagas and for pneumatic injection of coal fines and the prereduced haematite. Energy consumption is estimated to be about 10 GJ tonne-' of direct smelted liquid iron. Plasmadust is a variation of this process in which various iron oxide dust wastes are reduced to iron, and Plasmachrome is another in which chromite concentrate is reduced to ferrochrome.

6. I. 2 Elred process The Elred process (Smith and Corbett, 1987,1988; Davis et al., 1982; Chatterjee, 1994) was developed by ASEA in Sweden and consists of two main units, a fluidized bed reactor prereducer and a d.c. plasma arc furnace reducer-smelter. Combustion air and pulverized coal is introduced into the fluidized bed reactor to heat and maintain the bed at 980" C and also to produce the CO and H, for the prereduction. Iron ore is charged and heated to about 200°C in a venturi preheater by off-gas from the fluidized bed. The burden is reduced about 75% in the prereducer fluidized reactor and is fed through a hollow electrode at about 700°C to the arc plasma furnace where final reduction takes place. The excess gas is used to produce electricity for the process. The energy consumption is about 15.5 GJ thm-'.

slag Hot-

Figure 1 1 . Simplified diagram of the Plasmasmelt process


Hot air Prereduction

Prereduced carbonaceous material 'I f Combu8tlon (I.8

/ Electricity :r-

Slag +( ,gee )4-l( Substation )

1 Liquid Iron To I the grid

Figure 12. Outline of the Elred process

6.1.3 Pirogas process The Pirogas process (Smith et al., 1988, 1987; Eriksson et al., 1987) is an application of plasma technology in the blast furnace. It was originally developed at the Centre de Research Metallurgiques (CRM), and the name stands for 'plasma injection of reducing overheated gas'. The intention of the process is to decrease coke consumption by replacing part of the hot blast blown through the blast furnace tuysres by reductant gas heated to 2000" C in a plasma heater. The advantage of the process lies in the increased production of the blast furnace and in the possibility of using off-peak electicity. Depending on the operating conditions, the ratio of electricity to coke can be varied over a wide range. The optimum economic conditions occur with operation at a temperature of about 1300" C. Operation is normally between 1300 and 2200" C, and electrical energy consumption is around 500 KW h tonne-' of liquid metal.

6.2 Iron carbide process

The iron carbide process (Dancy, 1990; Munson, 1995; Scott, 1994; Chatterjee, 1994) was invented and developed by F. Stephens with his company Iron Carbide Holding Ltd. U.S.A. A 25 tonne per day pilot plant was erected and began operation in 1989. The first commercial iron carbide process plant has been completed, with start-up in Trinidad during 1995. In this process, iron ore fines are reduced and iron carbide formed in a fluidized bed reactor using a mixture of hydrogen and carbon monoxide. The reduction of the iron ore fines (haematite) takes place at 600" C and 1.8 atm in a fluidized bed reactor. The iron carbide is discharged with a typical composition of 93% Fe,C, 4% Fe,O, and 3% gangue and is transferred to a basic oxygen furnace at a temperature of 500°C. The iron carbide can be a 40% substitute for molten iron produced in a blast furnace and a 100% substitute for scrap in a basic oxygen furnace operation. Energy consumption is about 12.9 GJ tonne-'.

7. DIRECT STEEL-MAKING

The convesion of iron minerals to steel takes place in four different steps: gasification, reduction, smelting and refining. Commercial processes today use at least two reactors. The classical route of


IRON ORE

CONSENTRATE

1123

Y O FLUIDIZED BED UNIT PREHEATER

PLASMAARC HEATER

Fe,C

SCRAP HOT

‘I I I

A co : 440

COOLAND , & MAKEUP CLEAN

Figure 13. Plasma arc blow-pipe and tuyere assembly

* STEEL

Figure 14. Outline of the iron carbide process

coke-making, blast-furnace and oxygen steel-making (refining) uses three reactors; the alternative route of direct reduction (gas based) and electric arc furnace (refining) uses two reactors. Finally, the new alternative routes try to convert iron minerals to steel in only one reactor in which all four reaction steps take place (OTA, 1980; Fruehan, 1990; Nabi, 1993; Tanaka, 1988). The advantages of direct steel-making are as follows.

1124 T. ZERVAS. J. T. McMULLAN AND B. C. WILLIAMS

(a) Steel is produced in one reactor only by replacing the classical route of cokemaking, blast-furnace

(b) Steel is produced in one reactor only by replacing the alternative route of direct reduction/smelt-

(c) The direct steel-making process can generate large quantities of process off-gases which have a

(d) As air is used instead of oxygen, no oxygen production unit is needed and both investment and

(e) Environmental problems are decreased. (f) Any grade of coal or any alternative reductant/fuels and any type of iron minerals with different

(g) Capital costs are low.

and oxygen steelmaking.

ing and electric arc furnace.

high value for electricity generation or chemicals production (e.g. Methanol).

operating costs are reduced.

characteristics may be used, leading to increased flexibility.

In spite of these advantages, no commerical plant has yet been built. An experimental pilop plant (AISI) claims to have succeeded in producing semi-steel directly from iron ore.

8. ADVANTAGES OF IRON DIRECT SMELTING PROCESSES

The advantages of direct smelting processes may be summarized as follows (Tanaka, 1988).

(a) Flexibility in capacity is improved because, as the process is faster than the balst furnace, the reactor size can be reduced. Also, as iron ore lumps or fines can be used directly, there is no need for pretreatment such as pelletizing or sintering.

(b) Iron direct smelting process use coal, so there is no need to construct a coke oven. (c) There is high flexibility in raw materials and reductant fuels. This means that iron ore fines, iron

sand, scrap or any mixture of these materials can all be used. As a result, there is a wide choice of cheap raw materials for the smelting reduction process. Concerning reductants/fuels, non-metallurgical coke, low-grade coal, anthracite and lignite can all be used.

(d) Operation is highly flexible, which means that the direct iron smelting process can be stopped and started with relative freedom.

(e) There is wide flexibility in the use of the off-gas which are generated in large quantities. These process gases have a high utility value as energy or chemical raw materials. Thus, the smelting reduction furnace can also be used as an energy generator. In fact, it can be run in a variety of modes determined by the rate of prereduction in the production furnace and the rate of secondary combustion of the gases produced in the smelting reduction furnace. The lowest mode generates only the minimum energy required for the production of molten pig iron. In the highest operating mode, the furnace operates like a coal gas generator to produce more energy than is required by the steel-making system. The surplus is supplied to outside needs.

9. COMPARISON OF DIRECT REDUCTION WITH DIRECT SMELTING PROCESSES

Direct reduction and direct smelting processes have many similar advantages (Gudenau et al., 19891, most of which have already been discussed. In addition, direct smelting processes allow the use of a wide range of raw materials and reductants/fuels, and the smelting furnace can also act as a gasifier.

Direct smelting processors usually employ a vertical shaft furnace for prereduction of the iron ore (Corex process). The advantage here is that, as the iron ore charge does not need to be agglomerated, investment and operating costs are reduced.

Direct reduction has a major disadvantage (Eketorp, 1987) in the nature of the product itself the solid sponge iron. After reduction and discharge, it has to be protected against reoxidation. Also, melting of


the solid sponge requires electric power, which can be uneconomical in many countries. Sponge iron also has to complete with scrap, which is often cheaper and more easily available. Sticking is also a problem during the production of sponge iron. By contrast, the direct smelting process has resolved the sticking problem as the iron ore is prereduced in the reductant shaft furnace, avoiding metallization as final reduction, metallization and melting all take place in the smelting furnace (Corex process).

Commercial direct reduction process plants use air instead of oxygen, while the Corex direct smelting process uses oxygen.

10. RAW MATERIALS FOR DIRECT REDUCTION AND SMELTING

The designer of a direct reduction and smelting plant must take into consideration the characteristics of the raw materials (size distribution, strength and chemical properties) that it is intended to use because this determines the type of process (vertical shaft reactor, fluidized bed reactor) that can be adopted. The characteristics of the iron ore determine the type of reactor and vice versa (Stephenson and Smaller, 1980).

(a) Fines iron ore is used in fluidized bed reactor processes, but the iron content in the mineral should be at least 64% (e.g. the commercial application of fluidized bed reactor for prereduction in the Plasmasmelt process).

(b) A mixture of pellets and lumps can be used in shaft reactors, but there is a maximum allowable percentage of lumps for efficient operation. Any reduction in the concentration of lumps increases the efficiency of the process, and optimum operation is actually obtained when the charge is 100% pellets (e.g. in the commerical application of a shaft reactor for prereduction in the Corex process). The iron content in the mineral should be at least 65% (Delport, 1991).

The important role played by the propeties of the raw materials in the direct smelting process is obvious and it is important to examine how this is affected by factors such as factory siting, the type of process and the raw materials used (Steffen, 1989).

The iron and steel industries normally pay a price that includes transport (the CIF price) (Hoddinott, 1992, 1993, 1994). If, on the other hand, industry is connected with an iron mining operation and has a local direct reduced iron plant, they will be charged the FOB price (or less, as they include an element of the mining costs). It is very difficult to be precise about the correct variation between FOB price and CIF price as the cost of transportation is different for every location. As an indication, an agreement was made in 1991 between Australia’s Hamersley and Nippon Steel Corporation, Japan, for an FOB price on a long-term basis of 33.49 cents per Fe unit for Hamersley fines and 41-48 cents per Fe unit for Hamersley lump.

Sea freight is an important element in the cost of iron ore (Duisemberg, 1993, 1994; Metals and Minerals Annual Review, 1992). The rates are variable and 1991 the spot freight rates varied from between $US 8 and $US 9.4 per tonne from Brazil to Europe to between $9.5 and $ 11.2 per tonne from Australia to Europe, and between $11.7 per tonne and $13.5 per tonne from Brazil to Japan.

Energy cost is the single most important factor in the total cost of direct smelting of iron. In all processes the energy requirement is met by consuming coal, and in some cases, such as Plasmasmelt and Elred, supplementary energy to meet the heat requirement is also provided by using electricity. In 1991, steam coal prices in Europe were between 37 and $39 per tonne from South Africa and between $45 and $46 per tonne from many U.S. mines. The average price of coking coal was $61 per tonne.

11. CRITERIA FOR EFFICIENT OPERATION OF DIRECT SMELTING PROCESSES

In direct smelting processes, energy efficiency can be increased by the following measures.


(a) The gas should be operated in counter-current flow for high gas utilization (Oeters and Saatci,

(b) The experience of years of blast furnace operation must be applied to direct smelting processes,

(c) The process should operate with minimum off-gas for high productivity. (d) A high degree of post-combustion degree is needed for optimal prereduction, reduction and

(e) Waste heat should be used for electricity generation.

1987).

particularly during reduction.

smelting, and for high energy efficiency (Fruehan et al., 1989; Wright et al., 1991).

The necessary heat requirement for gasification of carbon and smelting can be provided by:

(a) combustion of fuels; (b) electrical energy where cheap electricity is available (Mitsubishi, 1991).

12. ENERGY CONSUMPTION

When comparing direct smelting processes and their alternatives, it is apparent, as shown in Table 7, that the Plasmasmelt process has the lowest energy consumption at about 10 GJ tonne-' of liquid melt. Nonetheless, the process is economically feasibly only where cheap electricity is available.

13. CONCLUSIONS AND RECOMMENDATIONS

Coal represents about 28% of total world energy consumption (Hoddinott, 1992, 1993, 19941, and with estimated reserves of 1040 Btonne has a reserve life of 230 years at a consumption of 4.5 Btonne per year. By contrast, oil has an estimated reserve life of 42 years and natural gas has an estimated 59 years. Additionally, 70% of oil and gas reserves are concentrated in the Middle East and the former Soviet Union while coal has a more even distribution. Nonetheless, the increasing cost and scarcity of metallurgical coal for producing coke for blast furnaces ensure that direct processes for producing iron will continue to be developed. Today, the most widely used direct processes for producing iron are based on natural gas, but this is neither available nor cheap on a world-wide basis. Thus, coal must have a high R&D priority.

The main research and development thrusts needed for the new iron-making direct smelting processes are (Fruehan, 1993) as follows.

(a) Higherproduction intensity: the target is further to reduce the working volume of the prereducer

(b) Increased post-combustion and heat tmnsfer: the majority of the energy for melting must be met by

(c) New rejructories must be developed because of the large amount of energy released.

and the smelter furnace relative to the blast furnace.

post combustion, and this energy must be efficiently transferred to the smelting system.

The following options look promising for the future development of direct smelting processes.

Table 7. Comparison in energy consumptions

Process Iron

Corex Plasmasmelt Elred Carbide

Energy (GJ tonne-' of product) 13.4 10 15.5 129 Reductan t coal coal coal gas


(a) Combination of a smelter reactor with a gas-based shaft reactor. In this way, the waste gas from the

(b) Use of the waste gas from the smelter reactor for producing electricity or marketable chemicals

(c) If the smelter reactor is used as a gasifier, any gas-based process can be incorporated. (d) Use of a combination of coal or peat with biomass to charge the smelter reactor. In this way, the

sulphur content of the reductant gas and the liquid iron product can be reduced. (e) Use of waste material that can be produced in a form and quality similar to peat (in terms of

humidity, density, etc.) to charge the smelter. (f) Combination of a large capacity gasifier with a plasma heater for direct smelting of iron or for

injection to a blast furnace (Pirogas) with electrictric power generation or a chemical plant.

smelter reactor can be used in the direct reduction stage.

such as methanol.

REFERENCES

Aukrust, E. (1992). ‘Results of the AISI/DOE direct steelmaking program’, Int. Symp. on Smelting, Quebec, Canada, pp. 591-610. Barcza, N. (1 986). ‘The development of large-scale thermal-plasma systems’, J. South Afncan Imt. Mining & Metallurgy, August,

Chatterjee, A. (Ed.) (1994). Beyond the Blast Furnace, CRC Press, U.S.A. Dancy, T. (1990). ‘Solid state reduction or iron ores’, Elliot Symp. Proc., Cambridge, Massachusetts, U.S.A., pp. 11-31. Davis, C., McFarlin, J. and Pratt, H. (1982). ‘Direct-reduction technology and economics’, Ironmaking & Steelmaking, 9, 93-129. Delport, H. (1991). ‘The COREX process’, 2nd European Ironmaking Congress, Glasgow, U.K., pp. 289-302. Duisemberg, E. dos S. (1993). ‘Steel industry metals’, Metals & Minemls Ann. Reu., 58-62. Duisemberg, E. dcs S. (1994). ‘Steel industry metals’, Metals & Minemls Ann. Reu., 51-55. Eketorp, S. (1987). ‘Energy considerations of classical and new iron and steel-making technology’, Energy, 12, 1153-1168. Eriksson, S., Johansson, B. and Santen, S. (1987). ‘Applications of plasma technology to steelplant waste treatment’, Plasma

Feichtner, H., Maschlanka, W. and Helten, F. (1989). ‘The COREX process’, Skillings’ Mining Reu., 14, 20-27. Fey, M. (1985). ‘Plasma technology for the steel industry’, Iron & Steel Engr, 38-45. Fmehan, R. (1990). ‘Iron bath smelting-current status and understanding’, Elliot Symp. Proc., Cambridge, Massachusetts, U.S.A.,

Fruehan, R. (1993). ‘Challenges and opportunities in the steel industry’, Iron & Steelmaker, March, 59-64. Fruehan, R., Ito, K. and Ozturk, B. (1989). ‘Analysis of bath smelting processes for producing iron’, Steel Res., 60, 129-137. Gudenau, H., Fang, J., Hirata, T. and Gebel, U. (1989). ‘Fluidized bed reduction as the prestep of smelting reduction’, Steel Res., 60,

Hoddinott, P. (1992). ‘Energy minerals (coal)’, Metals & Minerals Ann. Reu., 118. Hoddinott, P. (1993). ‘Energy minerals (coal)’, Metals & Minemls Ann. Reu., 121. Hoddinott, P. (1994). ‘Energy minerals (coal)’, Metals & Minerals Ann. Reu. 105. IlSl (1993). World Steel in Figures, International Iron and Steel Institute. Lankford, W., Samways, N., Craven, R. and McGannon, H. (Eds) (1985). The Making, Shaping and Treating of Steel, United States

Meyers, R. (1984). Handbook of Synfuels Technology, (Eds), McGraw Hill Book Company, U.S.A., Metals and Minerals Annual

Mitsubishi (1991). ‘Apparatus for manufacturing iron by melting and reduction’, European Patent Publication Number: 0 488 429

Munson, M. (Ed.) (1995). ‘Events and trends in metal and mineral commodities’, JOM, 47, April, 54-58. Nabi, G. (1993). ‘Methods and apparatus for steelmaking’, Iron & Steel Maker, 71-77. Oeters, F. and Saatci, A. (Eds) (1987). Mass and Heat Balances During the Reduction oflron Process, Stahleisen, Germany. Open University (Eds) (1979). Iron and Steel Production, The Open University Press, U.K. OTA Report (1980). Technology and Steel in dust^^ Competitiueness, OTA, U.S.A. Report (1987). ‘Technoeconomic Assessment of Electric Steelmaking Through the Year 2000’. Center for Metals Production,

Rosenqvist, T. (Ed.) (1983). Principles of Ertmctiw Metallurgy, McGraw-Hill, U.S.A. Santen, S., Bentell, L., Johansson, B. and Westerlund, P. (1987). ‘Applications of plasma technology in ironmaking’, Plasma

Scott, D. (Ed.) (1994). Developments Affecting Metallurgical Uses of Coal. IEA Coal Research, England, U. K . Smith, R. and Corbett, M. (1987). ‘Coal-based ironmaking’, Ironmaking & Steelmaking, 14, 49-76. Smith, R. and Corbett, M. (1988). ‘Coal-based ironmaking’, Process Technology Conf. Proc., Toronto, Canada, pp. 147-178. Steffen, R. (1989). ‘Direct reduction and smelting reduction-an overview’, Steel Res.,60, 96-103. Steffen, R. (1990). ‘The COREX process’, Metallurgical Plant Technol., 20-23. Steffen, R. and Lungen, H. (1988). ‘Current state of the direct reduction of iron ores to sponge iron’, MetaNurgical Plant & Technol.

Steinmetz. E., Steffen, R. and Thielmann, R. (1986). ‘Present state and development potential of processes for the direct reduction

317-333.

Technology in Metallurgical Processing, Iron & Steel Society, U.S.A.

pp. 1-10,

138-144.

Steel, U.S.A.

Review (1992). ‘Steel industry metals’, 57-64.

A2.

Pittsburgh, Pennsylvania, U.S.A.

Technology in Metallurgical Processing, Iron & Steel Society, U.S.A.

18-25.

and smelting reduction’, Metallurgical Plant & Technol., 24-39.


Stephenson, R. and Smaller, R. (Ed9 (1980). Direct Reduced Iron, The Iron & Steel Society of AIME. U.S.A. Strassburger, J. (Ed.) (1969). Blast Furnace Theory and Practice. Gordon & Breach, U.S.A. Tanaka. K. (1988). ‘Future trends in the technological evolution of the Japanese steel industry’, Iron & SteelMaker, 11-15. Wright. J. Taylor, 1. and Philip, D. (1991). ‘A review of progress of the development of new ironmaking technologies’, Minerals

Zervas, T., McMullan, J. T. and Williams, B. (1996a). ‘Developments in iron and steel making’, Int. J. Energy Research, 20, 69-91

Zervas, T., McMullan, J. T. and Williams, B. (1996b). ‘Gas-based direct reduction processes for iron and steel production’, Int. J .

Zervas, T., McMullan, J. T. and Williams, B. (1996~). ‘Solid-based direct reduction processes for iron and steel production’, Int. J.

Engng, 4, 983-1001.

( 1 996).

Energy Research, 20, 157-186 (1996).

Energy Research, 20, 255-278 (1996).

Download - Direct smelting and alternative processes for the production of iron and steel

Top Related