energy considerations of classical and new iron- and steel-making technology

Energy Vol. 12, No. lO/ll, pp. 1153-1168, 1987 0360-5442/87 $3.00 +O.OO

Printed in Great Britain Pergamon Journals Ltd

ENERGY CONSIDERATIONS OF CLASSICAL AND NEW IRON- AND STEEL-MAKING TECHNOLOGY

SVEN EKETORP

Royal Institute of Technology, Stockholm, Sweden

Abstract-Energy consumption can be decreased in classical iron- and steel-making processes mainly by using heat in waste gases and in hot sinter, coke, slag, steel, and cooling water. A total consumption of 4000 Meal/tonne (17 GJ/t) of steel can realistically be aimed for, i.e. 20-50% less than present levels. Iron making consumes 70% of the total energy and here new smelting reduction methods, originating from the blast furnace or basic oxygen converter, are being developed. The new methods use different energy sources and are flexible. Direct casting close to final size could also save a lot of energy. Not long ago it was generally believed that an increased technical and material standard inevitably meant increased energy consumption. To be optimistic required believing that energy consumption should increase. Nowadays progressive people strive for lowered energy consumption, total as well as specific (see Ref. 1). The iron and steel industry in Sweden takes 20% of total industrial energy and is energy intensive, as shown by its annual energy use of 500 MWhr (1.8 x 10” J) per employee, 10 times the energy use per employee of the heavy machinery and construction industries. It is therefore understandable that energy questions will always be important in the iron and steel industry.

1. FUTURE ENERGY CONSUMPTION USING CLASSICAL TECHNIQUES

Energy use in a modern integrated iron- and steel-works is extremely complicated. Ninety percent of the ingoing energy is coal, primarily to make coke. Coke is fed to sinter works and blast furnaces. Gas from coke ovens and blast furnaces is used internally, above all for heating in rolling mills. The energy circulation system includes an internal power station.

Energy consumption in the different operating stages of a steel-works is, roughly: iron and steel production, 70%; rolling, 20%, and miscellaneous, 10%. It is therefore understandable that most conservation efforts have been directed towards the primary step. However, as seen in Fig. 1, the blast furnace itself has a relatively high energy efficiency. Losses are low and 28.4% of the energy is recovered as blast-furnace gas.

The total energy efficiency of an integrated steel mill is of the order of 30% (as of some years ago). The main reasons for this are shown in Fig. 2, in which blast-furnace gas is considered a loss as is LD-gas (i.e. the gas given off in basic oxygen, or BOF, steel making). Both of these can be used, yielding a higher overall efficiency. Important losses are radiation losses of metal (downwards in Fig. 2).

Japan has, as we know, made extensive efforts to save energy. The results shown in Fig. 3 were already achieved in actual operations by 1973. Energy consumption per tonne of crude steel was 5091 Meal/t (21.31 GJ/t). The hot metal ratio7 in the basic oxygen converter is 92%, gases from iron and steel making are utilized and electrical energy is accounted for at 2450kcal/kWhr (35% efficiency). In Fig. 3 steel making from scrap is also shown. Figure 4 shows energy use for a model plant as published in 1976. In Fig. 4, the hot metal ratio is 75%. The indirect energy for production of electrodes, molds, refractories, ferroalloys, and so forth is not considered.

By comparing Figs. 3 and 4 it can be seen that great improvements can be made even in today’s most efficient operations while still using classical metallurgical technology, i.e. coking, sintering, blast furnace, and BOF. Some of the many additional ways of saving energy are shown in Table 1. These could mean saving 860 Meal/t (3.3 GJ/t).

One reason for low overall energy efficiency is the energy wasted in non-productive energy. A large portion of this heat is warm waste water, which is left unutilized because, although technology is available, the economics are typically unfavorable. There are, on

tThe metallic charge is essentially molten pig iron and solid scrap, with the fraction of molten iron called the hot metal ratio.

1154 SVEN EKETORP

Other sensble

heats 1.6% Exhaust

Sensible heat

2.3%

Combustion heat of fuel

12.9%

Radiatlan 10 5s , Oh 0.6 % Sensible heat

d coke 76.4%

SemiMe heat of

Sensible heat

dust 0.6.1.

Latent hear of tap

BLast gas 204%

furnace ( 3784.4

x103 kcaL/

u Combustion heatof oil 9.0%

Fig. 1. Energy flow through a blast furnace (Ref.2). BF = blast furnace.

Latent of c I” pig

u 9 3%

Heat of oil dmwsl- Sensible tion 0.5% heat of

i:$,O Sensible heat of mauen ptg 8.2 %

06.0 BLAST FURNACE GAS

5.0 WASTE GASES

f

0.6 LO- GASES

26.0 ?N%htL

0.3 METAL TAPPING

v 0.7 SLAG

” 2.0 COOLING WATER

v3.9 COOLING BEDS

Fig. 2. Final energy distribution at a steelworks in GJ.

the other hand, many forms of waste energy that are not effectively recovered for the lack of technology. S. Toyoda’ showed that the total waste heat is 2280 Meal/t (9.54GJ/t) for a typical integrated steel-works. In Fig. 5, the ordinate represents temperature and the area heat value. For each type of energy, recovery technologies, either already in use or under development, are cited. Some 300 Meal/t (1.26GJ/t) of crude steel can be used with present technology and 380Mcal/t (l.S9GJ/t) with technology under development. The total 680 Meal/t (2.85 GJ/t) is 30% of the heat now wasted. This is 12% of total energy consumption. High-temperature energy (to the left in Fig. 5) is easier to recover than the low-grade cooling water energy (to the right).

The second law of thermodynamics deals with the energy “quality.” We can speak of two forms of energy, namely exergy, which is transformable, and anergy which is not transformable.6 In Fig. 6 the energy flow of an arc furnace for scrap melting is shown. The cooling water losses are shown to be 100% anergetic, while 77% of the slag heat should be recoverable. A classification of energy as anergy is not necessarily immutable. In this connection heat pumps should be mentioned. By adding some high quality energy, electrical

Energy considerations of classical and new iron- and steel-making technology 1155

5091

5000 - 335 Indwecl rwrqy

745 \ Oll,.A.

-d Y

4000 - 392 Ektrr power

“0

z

3000

-

1 3619 COIW c 2000 -

1946

8 492 lndlmct amqy

B ‘-3, ofhws

b & Tooo- 1423

Elecfrlc Pww

0

Blast turnoa EAF + +

EOF Scmp

Fig. 3. Energy consumption per ton of crude steel in Japan, 1973 (Ref. 3). 1 cal = 4.186 J.

5000

L

H

4000

P 3000

D g

F 2000

6 &.I

I: 1000

0 , -

* 1 90 others

*2 690rygcn and electrode

4796

71

674

-y; E

1

1370 Norural

*1 90s =*2 D-L ,191

Elecmc PWc(

BMt flmmcc EAF EAF + + +

6OF scrap Ore3 reduced IrOn

Fig. 4. The energy requirement in model plants (Ref. 3). I cal = 4.186 J.

or fossil, it is possible to raise the fraction of exergy in waste heat. That it is possible to lower energy consumption substantially by efficient use of

conventional technology has been admirably shown by Swedish Steel AB (SSAB).’ In 1979 a drive to lower energy consumption was started because of the enormous increases in cost of fuel oil, LPG (liquified petroleum gas), and even coal. The goal was set to reduce energy consumption by 7% in each of the years 1980, 1981 and 1982. As shown in Fig. 7 this goal was achieved: energy consumption was 6422 KWhr/t (23.12GJ/t) in 1979 and 5166 kWhr/t (18.60GJ/t) in 1982, i.e. 20% lower. Conservation is continuing; the target for 1985 is 4575 kWhr/t (16.47 GJ/t).

There are many reasons for such good results in such a short time: closure of small furnaces, more scrap-based steel, increased share of continuous casting (from 65% in 1979 to 100% in 1982), elimination of primary ingot mills, improved blast-furnace operations, increased use of burnable by-product gases (internally and in the community heating system), energy recovered from hot-rolled materials, as well as control systems based on microprocessors.

Similar efforts to decrease energy consumption are being made in many countries. The

1156 SVEN EKETORP

Table 1. Possible additional ways of saving energy

Estimated saving per tonne of crude steel processed

Blast furnace-BOF route

Amount

Equivalent in primary energy MJ

Known to be practicable Dry coke quenching 440 Sinter cooling air used for combination 30 kg coke breeze 880

Blast-furnace top-gas turbine 33 kWh 120

More speculative Steam generation from slag 75% of loss 460

Shaping and Treating

Waste-heat boilers at slab furnaces

Speculative Ability to send, say, 75% of slabs through to mill without cooling (If successful this would reduce saving at waste-heat boiler to 50 MJ.)

Source: Ref. 4

50% of stack loss

1000 MJ gas or oil

200

1000

Examples: , OG boiler

nmdium

0 Undmlop.d I16001 This orea rmnda for xx) Meal/f-cruda

I Meal/f -crud# rtnl

Combustion exhaust gas

Fig. 5. The main sources and quantities of waste heat and status of recovery technology (Ref. 5). CDQ = coke dry quenching, LDG = LD gas, COG = coke oven gas, BFG = blast-furnace gas,

OG boiler = gas boiler.

French steel industry has decreased total energy use 3%/yr on the average since 1975.* The small (567m3) blast furnace in Koverhar, Finland, in 1979 already had a fuel consumption of 457 kg/t using sinter (and achieved 98.5% availability).


EWIY -Y in in Exergy

kWhr/t kWhr/t in%

Fuels8chemical reoctians 24.: (240) 30.1 * NaturaCgos (23) 2.9

Electrical wmqy 541 kWlw/t 67.6% 1 Chemical reactions 35 (35) -4.4

w I * ElectroQeatnsumptian 32 (32) 4.0

a CarbonidngmaMrioQ 43 (43) 5.4

107 (107) 13.1

Input energy 796kWhr/lOO%

Unaccounted for 17 (17) 2.1 I -_.-.-. -_-.-.-_-.-

Cooling water tosses 2 (127) 0.3

Off gas losses 100 (134) 12.4

Others 25 (34) 3.1

Slao 62 (60) 7.6

Steel 317 (423) 39.7

Fig. 6. Exergy-anergy flow in a UHP-arc furnace (Ref. 6). 1 kWhr = 3.6 MJ.

4000 - 79 60 61 62 83 64

Year

Fig. 7. Total energy consumption at SSAB, Sweden, in kWhr/t of crude steel (Ref. 7).

2. NEW PROCESSES FOR IRON PRODUCTION FROM ORE

The blast-furnace process, which is about 500 yr old, today comes very close to its theoretical energy limits and, as shown in Fig. 2, is efficient if it is credited for the off- gases. The drawbacks are really of another nature. The blast furnace uses coke, which is expensive and coke ovens present environmental and energy problems; it needs sinter, pellets, or lump ore: it produces a very poor fuel gas (with only 20% CO) out of very high grade fuel; it has a high capital cost and has much auxiliary equipment, it operates economically only in large units; and it operates with a badly defined “softening zone” in the shaft, which makes exact control difficult.

Some of these disadvantages cannot be removed, but efforts are being made to adapt the classical operation, as shown, for instance, in Fig. 8. Here coke consumption is halved by circulating the gas, removing CO* and replacing coke energy with 14OOkWhr/t (5.04 GJ/t) electrical energy. These changes should also offer better possibilities for control.

Due to the disadvantages mentioned, there have been tremendous efforts to find other reduction systems. All these efforts start, in a sense, with the blast furnace concept. In extreme simplification the blast furnace operates as shown in Fig. 9: in the upper zone

1158 SVEN EKETORP

Classical blast furnace

Coke 1 440kg 1

B.FGa5 Power stotlon

985Nm3 425Nm3 T=3YC

560Nm3

7

Stove

n

Stove

* T* 1200’C

A possible trend

of the blast furnace process

Coke 225 kg

B.F Gas

1280 Nm3

T=93’C

Bleed off

Removal CO,

co2

--280Nm3

Plasma dwlce

Rec;kc;te; gas

*

\

w/.co,+n,o

35% N, 1400 kWhr

Fig. 8. Classical and possible future blast-furnace processes (Ref. 9). Nm3 = normal cubic meters (i.e. at 25”C, 1 atmosphere).

reduction by gas occurs (to a maximum concentration CO,/[CO + COJ of 50%). In the high-temperature zone the reducing gas CO is created; but in spite of there being two zones there is just one operating shaft.

Fe,& C

Fig. 9. Schematic sketch of the blast furnace process.

A number of proposals have been devoted to improving the conditions in the upper part of the shaft to produce solid iron, i.e. sponge iron, and off-gases close to 100% CO2


(i.e. with high yield from the reducing agent). Martin Wiberg in Sweden showed 70 yr ago that this was possible by using circulation and electrical energy for the endothermic reaction COZ + C = 2C0. Later developments such as Midrex, Hyl III, Plasmared, and Fior are all built on the Wiberg principle, and total world production is now about 10 million t/yr (capacity 20 million t/yr). As shown in Fig. 4, prereduction to over 90% followed by electric melting will not enable low energy consumption, but can be appropriate in local conditions with, for instance, cheap natural gas.

The most efficient sponge iron processes consume 2400 Meal/t (10.0 GJ/t) in the primary reduction, but coupled with electric melting total energy consumption is high. For sponge iron melting, 550 kWhr/t (2.0 GJ/t) is a minimum, which is equal to 1350 Meal/t (5.65 GJ/t) primary energy, if 1 kWh corresponds to 2540 kcal(33.9% conversion efficiency). A modern coke-oven blast furnace complex involves 3231 Meal/t (13.52GJ/t) after credits for off- gases, but it must be remembered that molten blast furnace metal has chemical energy in the form of carbon and silicon, which enables some scrap melting in the steel-making process. The above figures can be compared with the theoretical minimum energy for molten steel production from oxide, 1760 Meal/t (7.37 GJ/t).

During the last 10 yr interest has instead shifted to conditions in the high-temperature region with methods called “smelting reduction.” The aim in all these efforts is to separate the two zones in the blast furnace using two separate reactors and, in many cases, to avoid the use of coke, using ordinary coal directly, and possibly also iron ore concentrate directly.

There are two schools of thought in smelting reduction. In one, final reduction and gas generation is carried out in a coke-filled high-temperature shaft roughly as with a blast furnace, while partial reduction with the gas is done in another reactor. The other idea is to use a reactor similar to a steel-making converter, directly contacting prereduced ore, coal, and oxygen in the iron bath, using the off-gas for prereduction in a similar way to the first approach.?

The first approach is here represented by the KR (Korf-Stahl-Voest Alpine) method, Germany-Austria, the Kawasaki process, Japan, and Plasmasmelt, Sweden. In the KR- process (Fig. 10) the high-temperature reactor is filled with coke, and prereduced pellets from the upper shaft are continuously charged into this hot coke bed. Heat and reduction gas are generated by injection of oxygen in the bottom of the !ower reactor.

The Kawasaki process (Fig. 11) operates with a similar principle, but uses powdered ore reduced in a fluidized bed, thus avoiding sintering, which saves some 6OOMcal/t Fe. The coke used is of low grade as the coke doesn’t have to stand up to a heavy load as in the tall blast furnace. The prereduced material is directly injected into the zone where pulverized coal, air and oxygen are also introduced.

In the Plasmasmelt process, combustion of powdered coal or coke with air or oxygen is replaced by electrical energy delivered by a plasma generator in front of which coal (or other fuel) and prereduced ore is injected (Fig. 12). The off-gas from the coke-filled shaft is cooled to 800°C before entering the prereduction stage. The same off-gas after dust removal is also used for injection of ore and coal.

The other main route of smelting reduction development is to use a liquid iron bath as a reaction medium, injecting prereduced material, coal, and oxygen into it in a way similar to oxygen steel making. Such methods are being worked on in Germany (Krupp with the COIN method and Klockner with KS, KSG, and KG), Japan (Sumitomo Metals) and Sweden (CIG [coal iron gasification]).

The possibilities of these latter processes are demonstrated by Fig. 13, where it is shown how the oxygen converter can be used for many purposes.” There can be many main products as well as by-products, and the converter can be charged with different materials. The first one is the usual bottomblowing steel-making converter, labelled OBM (or Q- BOP). By injecting coal through nozzles in the bottom, KMS, the amount of scrap can be

tin this paper the two Swedish process developments ELRED and INRED are only mentioned as they very often have been described in the literature. Both of them work with two reactors and both transform gas energy to electrical energy with an internal power plant. They are both based on coal and should consume 600-650 kg coal/t iron (4200-4550 Meal/t, or 17X-19.0 GJ/t).

1160 SVEN EKETORP

IRON ORE

EXPORT

AND SLAG

-=s ‘OLING GAS

GAS

1 MELTER GASiF07R 2 REDUC T/ON

SHAFT FURh!4CE 3 COAL FEE0 BIN L HOT DUST CYCLONE 5 GAS COOLER 6 TOP-GAS COOLER

Fig. 10. The KR process (Korf-Stahl-Voest Alpine).

increased and the off-gas gets richer. With more coal injected, KS, it is possible to melt 100% scrap. If coal as well as iron ore is injected together with the oxygen, KSG, the amount of gas will be large due to the requirements of the endothermic reaction and the fact that only CO and Hz are produced. Some 20-30% of the CO can, however, be combusted in the converter itself with a heat recovery of about 80%. Such after-combustion naturally makes the heat value of the gas lower, but this can in many cases be economically justified. The converter can also be utilized simply as a coal gasifier, KG, in which case the necessary cooling can be done by water vapor producing a high HZ-content in the gas.

The KMS-process has been in use industrially since 1978 and the KS since May 1983 in Germany. The gas is completely used for heating of rolling mill furnaces.

Using the name CIG (Coal Iron Gasification), research and tests in laboratory- and pilot-plant scale have been going on at the Royal Institute of Technology in Stockholm since 1975. This process is very similar to KG. In addition to investigation of operational data, dust formation and sulfur content in the gas have been studied. It has been found that almost all the sulfur reacts with the dust which contains metallic iron, forming FeS, so that a sulfur level of about 1OOppm can be reached.

It can be seen from Fig. 13 that smelting reduction processes can be used in many different ways and thus be very flexible. This is true for the iron raw material as well as for the energy source, which can be coal, coke, natural gas, or electricity. Furthermore the


Pre-reduction furnace

r

I ’

Pre-reduced nre “,._

Two stage tuyeres

Air, oxygen ‘c001’

\ Exhaust gas lme

more-feeder

pre-heater

Feedmg pope for pre reduced oar

furnace

Molten slag

Fig. Il. Kawasaki smelting reduction process

powder

Fig. 12. The Plasmasmelt process, SKF Steel, Sweden. (1) Raw materials; (2)fluidizedbed drier; (3) prereduction two-stage fluidized-bed; (4) feeder prereduced powder; (5)feeder coal powder; (6) heat by plasma generators; (7)shaft filled with coke; (@liquid slag and iron; (9)gas cleaner;

(1O)gas for powder feeding; (1l)gas for drying, preheating.

1162 SVEN EKETORP

carriers : 75% Molmn iron 50% Molmn iron hP/ - 25% Scrap 50 x scrap sponge iron

ST $OQ KMsb Ks&

t t t t t 02 4 c 02 c

Main procluct: Steel

By- product: sag has)

St001 slag/ gas

Steel Slag/

got

250

200

2 ; 150

3 ‘0 0 100 v

50

Iron ore

i’ G KSG

8

ttt 4 C2 Fez03

Molmn ircn Gas/ slag

A...Iron ore M...Molten iron S...Scrap/sponge it0n G...Gos

Fig. 13. The Kbckner-CRA family of processes (Ref. 10).

- 1600

- 1400

- 1200 5 ~ C .I!

- 1000 : u d

600

- 600

t G

KG

8

t t 02 c

Gas

slag

Degree of preduchon %

Fig. 14. The consumption of electric energy, coal, and coke per tonne of hot metal with 4%C, as a function of the oxygen removal in prereduction.

product can be different, i.e. the emphasis can be put on production of iron, steel or gas. An example of variations in operation is shown in Fig. 14, demonstrating the variation

in consumption of materials and energy with the degree of prereduction in the fluidized bed of the Plasmasmelt process. It can be seen that in this process no excess gas is produced at about 50% prereduction and that electricity can be partly replaced by burning coal with oxygen (as in the KR and Kawasaki process).

In the process where oxide, coal, and oxygen are fed into the iron bath, electrical energy can substitute for much of the combustion of coal (or carbon) in the bath. Table 2 shows some possibilities where an ordinary coal is used. In Table 3 other variations in energy inputs are shown.

It is not possible to give reliable energy figures for most of these processes, however, as they have not yet been applied industrially. They should, however, be regarded very favorably as they operate much faster than the blast furnace (high throughput), they can produce a rich gas if wanted, and they should have lower investment cost and lower energy requirements because of the direct use of powdered ore and powdered coal.

S. Fukagawa I3 has studied the molten bath final reactor combined with prereduction using the gas generated in the bath. The energy is completely used within the system. He calculates a total coal consumption of 441 kg/t of molten metal (3090 Meal/t or 12.9 GJ/t), if the gas is directly used, and 545 kg/t (3820 Meal, or 16.0GJ/t) if the gas is cooled down between the two reactors for dust removal.

The ideal smelting reduction process would be one where the CO evolved in the high temperature reactor could be oxidized to CO, with oxygen in the same reactor. Efforts in


Table 2. Variations in forms of energy used in smelting reduction

Electrical Coal Oxygen Gas Net energy of iron consumption

energy kWtu/tonne kg/ tonne Nm3/tonne

prog uction Meal/tonne (GJ/tonne) Nm /tonne Net Primary*

0 3282 2194 6142 6750 (28.3) 9100 (38.1;

500 2579 1668 4826 5730 (24.0) 8270 (34.6)

1100 1737 1038 3250 4460 (18.7) 7260 (30.4)

1700 894 407 1673 3190 (13.4) 6230 (26.1)

2090 350 0 655 2390 (10.0) 4930 (20.6)

* Calculated by multiplying electric energy by 2.776 (36.0% efficiency).

Source : S. Eketorp (Ref. 11).

Table 3. Energy use in selected smelting reduction processes

Coal

kg/tonne

Coke

kg/tonn&

Electricity Total primary energy

kWhr/tonne Meal/tonne (GJ/t)

Plasmasmelt, electricity 201 50 1125 4590 (19.2) only

Plasmasmelt, with oxygen 402 67 799 5390 (22.6)

KR (pellets) 1015 _- 398 8050 (33.7)

COIN (pellets) 447 __ 471 4250 (17.8)

Source : J. 0. EdstrGm (Ref. 12)

this direction have been made in such processes as Dored, Eketorp-Vallak, CIP, Rotovert, and Wiberg. If the CO produced could be burned for direct heating of the endothermic reduction according to:

Fez0,+3C=2Fe+3C0 AH + 114 Meal/mole 3co + 1.50, = 3c0, AH - 203 Meal/mole

Fe,O, + 3 C + 1.5 O2 =2Fe+3CO,

AH - 91 Meal/mole

the excess heat would be enough for heat losses, and a minimum carbon consumption of 321 kg/t Fe (2250 Meal/t or 9.42 GJ/t) or about 400 kg coal/t could be reached.14 However, none of the processes mentioned solved the difficult problem of heat transfer by combustion combined with mass transfer from the high oxygen potential. Recent studies of after- combustion in the steel-making converter have shown that it is possible to have 20, 30 and, perhaps, 40% combustion and deliver this heat to the bath.

In Japan the interest in smelting reduction is very intense. H. Katayama et al. at Nippon Steel are working with direct smelting reduction of chromium in an iron bath.15 The experimental converter shown in Fig. 15 is blown from above with oxygen and from the

1164 SVEN EKETORP

bottom with oxygen and argon. With the correct slag composition it has been found possible to combust CO to almost 100% CO, and still obtain very low chromium content in the slag (Fig. 16). The slag layer has to be rather thick. The heat transfer properties are as yet not fully investigated, but if it could be proved that combustion to CO, could be done simultaneously with complete reduction, a great step towards the ideal process will have been made.

II O,- lance

Gas sampler

Fig. 15. Experimental smelting reduction apparatus (550 kg capacity; Ref. 15).

lm& 280kW)

0.8 -

stag 0 3 holes - nozzle

iz-iiz = o.4 A Slnple hole- nozzle

5 0.6 -

8 1

E 04

b G A A0 0 0

E 0 0.2 -

0 1 I 1 I 20 40 60 80 . 100

co 2 %

x 100 I%) co *% + co %

Fig. 16. Effect of the oxidation of CO on the chromium content in final slag

3. STEEL MAKING AND CASTING

If steel making is defined as oxidation and removal of silicon and carbon from the molten blast-furnace iron, it is an exothermic process allowing scrap melting with the excess heat. This was true 30 yr ago when we had few process steps (Fig. 17). In today’s metallurgy, pretreatment of the metal is done before the converter and after-treatment is normally done for C, P, S, 0, N, H, and, in addition, temperature adjustment, slag inclusion removal, and so on. This treatment in ladles or other vessels is metallurgically the most important step. With so many operations it is often required to heat the metal.

Figure 18 shows the course of temperature from the steel-making furnace to casting, and with or without ladle heating. Such heating is not disadvantageous as the initial steel temperature can be lowered about 60°C which lowers losses, heat time, and refractory wear, and saves 40 kWhr/t. Ladle heating takes 30 kWhr/t.16

With many separate operations it is more important that heat losses be kept low. It is


Lump ore, LOBOF)

Moteriols in blast furnace

Saop -

EAF

Sinter pellets, coke

Fig. 17. The specialization of process metallurgy. EAF = electric arc furnace; OH = open hearth.

Time (mid

Fig. 18. Courses of temperature with and without ladle heating (LF) (Ref. 16).

important that the steel be not tapped from one vessel to another many times. In many plants using after-treatment, the steel is left in the ladle during the successive treatments. Figure 19 shows the system at Aichi Steel, Japan, where the steel is contained in the same ladle during slag removal, heating, vacuum degassing, and continuous casting.

It should be possible to build a complete steel plant according to the same principle.” In Fig. 20 all the operations are carried out in the same tank. With such a scheme it is possible to operate in a closed system using well-fitting lids during the whole operation cycle.

However, the most energy can be saved in casting. Continuous casting has meant sharply decreased energy consumption mostly because of the higher steel yield. Nevertheless, the

1166 SVEN EKETORP

VSC LF RH

NDISH

Fig. 19. System for refining at Aichi Steel, Japan. EF = electric furnace; VSC = vacuum slag cleaning; LF = ladle furnace (heating); RH = vacuum degassing; CC = continuous casting.

\ AGA ix Inspection

Granulation

Fig. 20. Steel making in a series of batch operations (Ref. 17). AGA Infrared [i.r.] inspection,

thick blooms or slabs created in continuous casting demand much heating and reheafing before final dimensions are reached. As a rule of thumb, each heating from the cold state requires energy equal to three times the heat of fusion, i.e. about 1200 kWhr/t. There is, therefore, throughout the whole world a drive for casting closer to final dimensions or even in some cases directly to the final dimension.

Figure 21 illustrates the tremendous energy saving which can be had if thin sections can be cast directly. The figure shows the energy consumption for each stage of rolling and heating from ingot to wire, with a total of almost 5000 kWhr/t in this case. Operating advantages such as fine-grained structure and little segregation could also be obtained by controlling the thermal history of the material. Such control is not possible with thick castings.

4. COMMENTS ON ENERGY STRUCTURE

Energy considerations should include not only different ways of minimizing energy at the steel-works, but also optimal use of different energy sources and possible use of excess energy outside the steelworks.


t

/ 1000

\ B / ~~;ile+ hwhng

.i; open Pll

0 It ’ ’ ’ ’ ’ ’ ’ 105 104 103 102 10 1 0.1 0.01

Area ;m2 , , 1

5.0 I.0 0.5 0.1

Diam. mm

Fig. 21. Energy consumption in conventional rolling and drawing from ingot to wire (stainless 2333)

Since the oil crises there has been a drive for oil-less operation in many countries and Japan has, as usual, had the most success. S. Toyoda’ foresees that in the near future the energy structure will change from 77% coal to 97.1% coal, no oil, and only 2.9% purchased power (the rest produced within the plant by different off-gases).

The great interest in smelting reduction for iron production as well as direct reduction of chromium and other metals in an iron bath is due to the high price of coke, oil, natural gas, and electricity in most countries. It should be possible to replace all these energy sources with ordinary coal, as described in this paper. Furthermore, as shown in Fig. 13, many of the smelting reduction processes produce excess gas, which can be used for prereduction, for heating in steelworks, for power generation, and in the chemical industry. The gas can also be used when injecting oxygen or powders into converters or shafts. It was formerly thought that an endothermic reaction was the mechanism for protecting refractories close to the tuyeres, but it seems as if Fe0 formation is the main mechanism of attack and that introduction of CO around the oxygen gives good results.i8

An iron bath may be an ideal reaction medium for gasification of coal; this is the reason for the great interest in Sweden in the CIG process.” The process compares favorably with other gasification methods. Table 4 shows gas analyses. The CIG process also has good desulfurizing power.

Table 4. Gas analyses using different coal gasification methods (% yield)

H2

co

co2

CH4

N2

Lurgi Texaco Koppers-Totzek CIG

40 37 32 30

19 42 55 66

30 19 11 1.5

10 0.1 0.1 __

0.6 0.6 2 2.5

1168 SVEN EKETORP

5. CONCLUSIONS

As shown in Fig. 5, it should be possible to force energy requirements down to 3674 Meal/t (15.4 GJ/t) via ore and 1370 Meal/t (5.7 GJ/t) via scrap melting. With iron coming 50/50 from ore and scrap, which is a probable figure for most countries in the future, this means 2522 Meal/t (10.6 GJ/t) of crude steel. With the very probable developments in casting and rolling mentioned earlier, it should be possible to reach a total energy use of 4000 Meal/t (17 GJ/t). Great improvements in waste heat recovery will be made and excess gas delivered to the nearby users. Most important, all developments in the future will be directed towards precision and control of operation, increased speed of processes, development of high energy-density equipment and reactors and furnaces with low heat capacity, and maintenance of a good environment using closed systems. All these measures will result in lower energy consumption. It should, however, be stressed that suitable technology is not enough. Knowledge and engagement of the people involved is essential.

REFERENCES

1. T. Johansson, P. Steen, E. Bogren and R. Fredricksson, Science 219, 355 (1983). 2. T. Yamamoto and T. Nakagawa, Trans. ISIJ 23, 862-892 (1983). 3. K. Takeda, Trans. ISIJ 19, 455-463 (1979). 4. A. H. Leckie, A. Millar and J. E. Medley, Ironmoking Steelmaking 9, 222-235 (1982). 5. S. Toyoda, Trans. ISIJ 23, 1-13 (1983). 6. F. Fett, H. Pieiffer and H. Siegert, Stahl Eisen 102, 461-465 (1982). [In German] 7. Personal communication from Mr. 8. Westin, Swedish Steel AB (May 1984). 8. C. Lachenait et al.. Reo. Merd. 199-210 (March 1985). fin French1 9. J. A. Michard et ai., Rev. Metall. 851-867‘(November’l983). [In French].

10. L. von Bogdandy and J. Innes, Stahl Eisen 104, 1025-1030 (1984) [In German]; see also L. von Bogdandy, K. Brotzmann, E. Fritz and E. G. Schempp, fron Steel Engr 21 (May 1984).

11. S. Eketorp, “The Injection Process for Smelting Reduction and Coal Gasification”, Jernkontoret research nr. D 325 (1979). [In Swedish]

12. J. 0. Edstrom, “On the Economy of Iron Making Smelting Reduction Processes”, Japan-Sweden Seminar, Tokyo (23-24 April 1985).

13. S. Fukagawa, “Studies on Metallurgical Processes using Coal for Iron- and Gas-Production”, Doctoral thesis, Royal Institute of Technology, Stockholm (1983).

14. S. Eketorp, “Smelting Reduction” Ironmaking Proc. AIME 36-39 (1968). 15. H. Katayama et al., “Smelting Reduction of Chromepellet in Stirred Bath”, Report, Nippon Steel Corp.

(1985). 16. W. Glitscher, K. H. Weimen and H. Zorcher, Stahl Eisen 105, 331-334 (1985). 17. S. Eketoro and U. Holmbere. “A Modulized Flexible Steel nlant” Steel Times 562-564 (November 1983). 18. T. Sakurya et al., “Protection%f Oxygen Bottom Blown Tuyeres by CO Gas”, Report, Research Laboratories,

Kawasaki Steel Corp. (1984). 19. C. L. Axelsson, “Some Phenomena Involved in a Process for Gasification of Coal in an Iron-Carbon Melt”,

Int. Symp. on Coal Processing, Katowice, Poland (November 1983).

energy considerations of classical and new iron- and steel-making technology

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