PREHEATING OF HOT BRIQUETTED IRON

A. Galvez, R. Dippenaar, G. Brooks BJ:fP Institute for Steel Processing and Products,

University of Wollongong NSW 2522, Australia

Tel: + 61 (02) 4221 4247

Key words: HBI, iron, kinetics, oxidation, reduction, EAF, preheating, off-gases.

ABSTRACT

In an earlier study we have shown that when Hot Briquetted [ron (HBI) is pre-heated in an inerl atmosphere, internal reduction of FeO occurs at tempemtures above 500°C. In a carbon dioxide atmosphere at temperatures above 700°C, oxidation occurs only externally whilst the internal reduction reactions proceed simultaneously. In this investigation these simultaneous reduction and oxidation reactions have been studied in atmospheres resembling more closely the expected composition of the off-gas of an electric-arc furnace. Isothcnnal and non-isothermal thenno­gravirnetry (TG) as well as differential thermal analysis (DT A) techniques were used. The behaviour of HBI in oxidising atmospheres at elevated temperature is discussed and it is shown that simultaneous external and internal reduction occur even when the oxidation potential of the preheating gas is increased significantly. The optimum temperature for preheating is between 750(\C and 800°C.

INTRODUCTION

The result" of a study of the physical-chemical behaviour of HBI in nitrogen and carbon dioxide atmospheres have been reported before 1• It was

shown that when HBI, produced by the FJOR process is exposed to tcmpcnllurcs above 500"C in an inert atmosphere, reduction occurs presumably due to the reaction between FeO and Fe-'C within the HBI1.:?. When specimens of the same geometry were exposed to high tempcmture in a carbon dioxide atmosphere, it was found that oxidation of the iron in the HBl occurred only extcmaJiy with the formation of a thin oxide layer on the external surl'ace1•2• The kinetics of the reduction and oxidation reactions were analysed and will not he repeated here. However, a finding that bears great significance on the practical pre�heating of HBI was that external oxidation only occurred at temperatures up to 600°C but at higher temperatures oxidation of Fe and reduction of FeO occurred simultaneously. More specifically, at temperatures above 800°C, the initial rate of dccarburisation is higher than the rate of oxidation, resulting in a net mass loss in TG analysis.

These fmdings prompted further investigation into the bel1aviour of HBI when it is preheated in atmospheres which may typically be encountered in the off-gas of an ele'ctric-arc furnace (EAF). Such a study is important because HBI can be used to introduce Fc-units into an electric-arc furnace (EAF) but this leads to higher energy consumption mainly due to the presence of unreduced oxide (FeO) and gangue. However, significant energy savings can be realised by preheating the HBI prior to its introductjon into the EAF by the utilisation of the sensible heat of the off-gas1•2•3•4 from the furnace. Moreover, the relatively high carbon content of HBI can enhance energy savings either by the reduction of FeO or by an increase in the calorific value of the off-gas through post­combustion. The main purpose of the present investigation was consequently to assess whether the preheating of HBI feed to an

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EAF using the post-combustcd gases from the process, would result in excessive oxidation of the iron;�.

In this context it is pertinent to refer to some earlier work by other researchers. TriL�ls at preheating of HBl for various steelmaking operations were performed by FIOR5. HBI was preheated to �15"C under an equivalent pressure of 8.2m of HBI without sticking. Midrex also reported preheating of HBJ using a scrap prcheater6• The HBI was preheated to 1 OOO''C in an oxidising atmosphere for 42 minutes, with an overall loss in metallisation of only 3.6%.

The charging of hot DRI pellets into an EAF was practised at the COSIGUA stee1 plant in Brazil, where DRI discharged at approximately H00°C from the PUROFER shaft f1Jmace, DRI was transported by trucks and continuously fed into the EAF7• Hyl and Midrex have proposed a new system, which links the DRI reactor with the EAF·'-)·10• Hot DRI at approximately 650''C is fed continuously in a reducing atmosphere. An energy reduction of 20% and increased productivity were reported.

Improved scrap preheating systems are currently being used in EAF shops including the utilisation of shafts, horizontal continuous conveyors, twin shells and multi stage preheating systems 11• These systems could easily be converted to HBI preheating. This means that preheating of HBI has become a practical possibil ity if excessive oxidation can he prevented.

It is pertinent to note that it is essentially the surface-to-volume ratio of the charge that determines the extent of surface oxidation of scrap or HBJ. Shredded scrap has a much higher surface­to-volume ratio and hence, it is expected that HBI will not oxidise as much as shredded scrap under any given set of preheating conditions.

EXPERIMENTAL

The typical chemical analysis of the HBI used in this study is shown in Table I (FIOR). Isothennal experiments were performed in a thermogravimetric

(TG) fumace as shown in Figu re I; machined cylindrical samples of I 2 mm length and R mm diameter were used. The furnace chamber consisted of an alumina tube with un internal diameter of 60 mm. A sample contained in a platinum ba.<;ket was introduced into the hot zone at the desired reaction temperature. Prior to enlcring the reaction chamber, the gas was passed through a small copper furnace and silica glass beds ior deoxidation and dehydration because previous studies have shown that the presence of moisture can accelerate decarburi[o;ation reactions 1�. A flow rate of 4.06 !/min. was used which is high enough to render the reaction rates independent of flow rate. The duration of the experiment was usually 2 hours. The mass change was recorded every 20 seconds by an analytical the1mobalance processed by a computer. On completion of the experiments the samples were quenched in water.

....... �. ---

Fig. 1. Schematic diagram of the thermogravimetric apparatus.

Non-isothermal thermo-gravimetric ;md differential thermal analyses were also conducted using a SETARAM TG-DTA92 thermal analyser. Less than 250 mg of powder as well as cylindrical samples of 6 mm length and 2-3 mm diameter, were tested at a heating rate of 10°C/minute up to 1200°C. Microstructural observations were made by optical and scanning electron microscopy with analysis by energy dispersive spectroscopy (EDS). Phases were identified by X-ray diffraction. Chemkal analyses were carried out using volumetric analysis, inductively coupled plasma (lCP) and fusion-infrared gas detection (IR). It is

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important to note that the chemical analyses reported in the tables were detenuincd by the dissolution of the whole sample. Hence the FeO content reported includes the residual FeO as well us the surface oxide layer where applicable. In similar fashion, mctallisation refers to that of the specimen as a whole.

Table I Typical chemi<.:al analysis of the HBI as received1•

TEST RANGE,wt% Fe, (V) �9.H - 91.0

Fe0 (V) S0.9 - 83.0 Mo - 90.1 - 91.2

FeO (V) 10.5 - 9.0

CaO (lCP) 0.02 - 0.05

Si02 (ICP) 1.1 - 1.38

MgO (ICP)· 0.40

P�O.s (ICP) 0.20

ZnO (lCP) <0.02

c (lR} 2.1 - 2.6

s (lR) 0.01 - 0.02

RESULTS AND DISCUSSION

Before the details of the relative contribution of internal FeO reduction and external oxidation are discussed, it is pertinent to note that the geometry of the HBI samples in this study was chosen to resemble the shape of individual HBI briquettes13• However, the surface-to-volume ratios are very different. For HBI FIOR briquettes A/V -150. m2/m3, compared to -670 m2/m3 for the samples used in isothermal TG analysis and -2000 m':!./m3

for the samples used in non-isothermal TG/DTA analysis. Consequently oxidation, which is a surface phenomenon, will be relatively more severe in the experimental. samples than in real HBJ briquettes. Moreover, the metallisation of the HBI samples in this investigation was detennined for the specimen as a whole (i.e. including the oxide layer) and is therefore the net gain or loss. Hence, metallisation for indusuially prepared briquettes is expected to be significantly higher than that of the expcrimentaJ samples under the same conditions of exposure.

HBI Behaviour in N1

The results of isothermal TG experiments between 400''C and 900''C in a nitrogen atmosphere arc shown in Figure 2. Although these results have been reported before 1.� it is pertinent to briefly summarise the findings so that the behaviour of HBI in an inert atmosphere may be compared to experiments done in oxidising atmospheres. The mass loss observed has been attributed to internal reactions between feO and carbon present in HBl1·J

as shown by the following equations:

(I)

(2)

The decrease in carbon and FeO content, shown in Table II, for HBI samples after heating in nitrogen provide strong evidence that these reactions have occurred.

The onset of the endothennic reactions (1) and (2) have also been detected in TG-DT A experiments, as reported earlier and illustrated in Figure 31•

5

� �\

·25 0

l�� ]-•()()•c

soo•c

� 600'C

�� !"---r--

9oo•c

� Io..... ..........

t-- [70tT'C r.-.

['JWC

10 20 30 40 Sit 60 . 70 80 90 100 110 120 nme 1mm)

Fig. 2. TG curves of HBl reacted isothermally in N2 atmosphere at temperatures between 400-900°C 1•

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,,.,. •• .· '·"'J g.,. , .... . .. , . � Jl"'t•t.• , ,, ___

Fig. 3. TG/DT A curves for HBI samples reacted in aN" up to JOOO"C, healing rate IO"C/min'·2. Hf is the DTA-curve.

Table II Chemical analysis in mass %, of the HBI after reaction for 2 hours in N2•

TEST 400°C . 600°C 700"C 800°C Fe, �9.5 90.8 92.3 94.H

Pc') Hl.l 84.3 86.1 X9.3

Ml' 90.6 92.8 93.3 94.2

FeO 10.2 7.7 7.4 6.3

CaO 0.046 0.022 0.027 0.017

K20 <0.01 <0.01 <0.01 <0.01

Si02 1.5 1.3 l.l 1.0

ZnO <0.01 <0.01 <0.01 <0.01

c 2.f5 1.87 l.T6 0.3X

s 0.012 0.009 0.01 0.008

HBI Behaviom· in C02

When HRJ is exposed to a C02 atmosphere at temperatures up to 900°C; oxidation is observed only externally at any given temperature. The thickness . of the oxide layer increases with temperature, as shown in Table Ill.

Table III Thickness of the II HI external ox ide layer in co� <lnd air respectively after -2 hours exposure time.

T, C02, Air, "C _!l_ffi J:!m

400 >0-2.5 4-H

500 >0-7.5 -R 600 >0-12.5 12, small

pcnetrat ions 700 >0-15 smnll -24, small

_Q_enelrations penetrJtions xoo 7-40 small )5-65

penetrations 900 J()-50, small 100-125

pcnctraliuns

At tempcrar.urcs higher than 70(fC reduction of FeO occurs within the HB1, as in an ine11 atmosphere, but oxidation at the extemal surface occmTcd simultaneously by reaction (3) 1' ':

The results of the TG experiments are shown in Figure 4. The conclusion that oxidation occurs only

externally was verified by optical microscopy where the growth of the external oxide film could be followed whilst no visible internal oxidation could

be observed, Table TTl. To provide further proof of this premise, as well as to demonstrate the effecl of

initial carbon content on the reduction of HBI, . specimens of HBI were first reacted isothcnnally in

a nitrogen atmosphere followed by reaction in pure carbon dioxide gas. In these experiments, most of the FeO contained internally was reduced in the inert ahnosphere with a concomitant reduction in carbon content. Hence, during the exposure to C02, external oxidation could be studied essentially independent of intemal reduction. The results of

these experiments are shown in Figure 5. It follows that oxidation is enhanced at increasing temperatures consistent with the observations of previous researchers14'15'1<'·17• It also follows that intemal reduction of FcO will only occur when the e<-trbon content in the HBI is high enough.

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A comparison between Figure 4, which has been reported before, and Figure 5 clearly shows that iC the carbon content of the I-181 is high enough, internal reduction of FcO will occur (in parallel with external oxidation) but if the Co.lrbon has <llreaciy been consumed, only extemul oxidation will proceed. These results suggest that in practice, significant reduction of residual reO can be achieved provided the carhon content of the HBI is

high enough while the external oxidation can be manipulated by the composition of the preheating gas.

0 10 20 30 40 50 00 70 80 90 100 110 120

nme(min)

Fig. 4. TO-curves for HBl samples reacted isothennally in pure co� atmosphcre1•2.

These findings were further verified by reacting HBI non-isothennally in a C02 atmosphere at a heating rate of I O"C/min. The results of these TG/DTA experiments are shown in Figure 6. Oxidation on the external surface only occurred at temperatures between 200"C and 700°C. Between 700°C and R50°C the rate of the reduction reactions (I) and (2) arc evidently higher than that of the

oxidation reaction, hence lhc decrease in mass. Oxidation occurs at a high rate at temperatures above 850°C.

·

0 10 20 30 40 50 60 70 RO 90 10() '10 1�()

Time (min)

Fig. 5. TG-curvcs of IIBI samples rcaw:d isothermal ly in N2 atmosphere for I ho ur followed by reaction for 2 hours in pure C02�.

[11ttJ.�t�! . . • . ;w�l·t·�· �� .. ' lltt • •• •

-- ---..... \ ' .,, , •• O.J� .... '

, , 1 '\

•

Fig. 6. TG/DTA curves of HBI samples in CO� atmosphere up to I 000°C, heating rate I O"C/min�. llf refers to the DTA-curvc.

The Behaviour of HBI in 02 Mixtures

The degree of external oxidation achieved by reacting HBI isothermally in air between 400°C nnd 900°C was higher than that observed in C02 at the corresponding temperattlre, as shown in Table Ill. TJtis is in accord with the higher oxidation potential

of air. The thickness of the external oxide layer increased with temperature and duplex scales were

found above 500vC, presumably formed according to the following reactions:

(4)

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Internal oxidation was not observed at any temperature. Above 700"C the oxide layer formed in ;.�ir was more porous than the layer formed in co�. A photomicrograph of an oxide layer formed at XOO''C after I 05 minutes of exposure to air is shown in Figure 7. The metal-oxide interface is composed of a cluster of voids, dispersed over the metal surface, but there is still direct contact hetween mct�1l and oxide. The oxide layer is duplex in nature with two different oxide layers clearly discernible on the extemal surface in Figure 7.

Fig. 7. Photomicrograph of HBI oxidised externally in air at 800°C for 105 minutes, etched in thioglycolic acid. A duplex oxide layer formed on

the external surface. Bar length= 20J.un

Figure 8 shows TG-curves isothermally derived in air between 400°C and 900"C. At temperatures below 600°C, a net mass increase is recorded. The thickness of the external oxide layer increases with temperature as shown in Table ill. The carbon and FeO content decrease with an increase in temperature as shown in Table fV. It is interesting to note that the net mass gain in the TG experiments were lower at 500°C �d 600°C than at 400°C. Nevertheless, the thickness of the oxide layer increased with temperature. One possible explanation is that reactions (1) and/or (2) could have been initiated at a temperature between 400°C and 500°C. A comparison between the net mass gain at the same temperature in air and C02 reveals that the mass gain during reaction in air is much smaller than the mass gain in a C02 atmosphere. This observation suggest that reactions (I) and (2)

occurred at higher rate in air thw1 in CO� between 400''C and 700"C. Galvez� has proposed that the internal FeO reduction reactions (I) and (2) are accelerated through the oxidation of the gaseous CO product, for example:

Some evidence of this premise is found in Tahle TV where it is shown that the FeO content (total) as well as the carbon content is decreased between 400°C and 600°C. However, the net effect and the issue of practical significance, is that thicker oxide layers form at higher oxygen potentials .

10 20 30 40 50 60 70 80 90 100 1 10 120

Time (min)

Fig. 8. TG-curves of HBI samples reacted in air for -105 min. at temperatures between 400-900°C2•

The highest initial rate of mass loss in air was obtained at 800°C, as shown in Figure R. The variation in mass is a net balance between two competitive reactions: • external oxidation of Fe and • internal reduction of residual feO.

Table V and Figures 9 and 10 show the chemical analysis and typical sections of the oxide layer after various exposure times at H00°C. A continuous oxide layer fonned within 10 minutes, with an

average thickness of 16 ).lm, as shown in Figure 9.

A degree of metallisation of 93.6% was obtained after this exposure time. With further exposure, the porous oxide layer grew to a thickness of 24-36 11m

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as shown in Figure 10 and metallisation dencascd to 8X.3%.

Table IV Chemical analysis or HBI, mass %, isothermally reacted in air for- lOS min.

TEST Fct Fe'' M''

FeO

c

",. 1 .... . . :-· ...

400"C 8�.7 73:7

83.1 18.8

2.25

500°C 600"C 700°C

91.1 90.R 92.9

H0.7 80. 1 84.8

88.6 i)8.2 9 1.2

12.5 12.� 9.7

1.99 2.08 1.05

Fig. 9. Photomicrograph of HBI externally oxidised at 800"C aflcr I 0 minutes exposure to air, etched in thioglycol acid2• Bar length= 20).Ull

Fig. 10. Photomicrograph of the HBI extemally oxidised at X00°C after 30 minutes exposure in air, etched in thioglycol acid2. ·Bar Jength = 20r..un

Table V Chemical analysis of HBI rcnctcd isothermally in air at �OO"C for 10 and 30 minutes respeclively.

TEST 10 Minutes 30 Minutes fc, 9!.8 90.8

Fe" 85.9 80.2

M'l 93.6 88.3

FeO 7.7 \3.4

c 1.59 0.49

The non- isothermal TG-TDA curves displayed in Figure I l show a net mass decrease between 6 70"C and 860"C, which is similar to the results ohtained in C01. These results are also wnsistent with t.he isothe1mal experiments. Consequently, the optimum temperature range for HBI preheating is between 750° C and 800°C. Preheating temperatures above S50°C should be avoided not only because iron oxidation occurs at a very high rate, Figure II. but also because of the possibility that HBI can soften and stick.

,,

"J t . · t)Ut'h� .CU• .;: ,,. . ... ""''

,.,

--;i"' ••

Fig. 11. TG-DT A cur ves for H131 reacted in air up to 1400°C, heating rate lO"C/min�. HF refers to the DTA-curve.

TG/DT A experiments were also conducted in gas mixtures which simulate more closely the off-gas composition in an EAF after post combustion. Isothennal TG-curvcs from 400°C to 900"C in a gas mixture of 32.9%C02 /61.H%N� /5.3%02 were similar to that in air, as shown in Figure 12. Simultaneous )ron oxidation and FeO reduction by reactions ( 1) and (2) occun·ed above 500°C. At

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700''C the mass decreased. The carbon content on completion of the experiment was 0.56%, compared to 1.05% in air. The highest rate of mass loss w<ls obtained during the first ten minutes of exposure at

HOO"C. as shown in Figure I 2 .

40

3D

� �20 .§. c 10 �

� 0 "' > : -10 ..

:; ·20

o 10 2o lo •o so eo 7o eo go 100 110 120 130

Time {min)

Fig. 12. TO-curves for HBI samples reacted isothermally in a 32.9%COz/ 61.8% N'2 I 5.3% 0"! gas mixture between 400-900°C.

The results of a non-isothennal TG-DTA

experiment perfonned in a 37%C02 I60%N2 12.9%02 gas mixture is shown in Figure I 3. These results confirm that the optimum temperature range for HBI preheating is between 750°C and 800°C. ln this temperature rcmge, the rate. of decarburisation of FeO is higher than that of the oxidation of iron2.

� ...( -':-! I? • 1 l t " �·{I !k•J \. ' ' n · ,u., , ,�

I f

Fig. 13. TG-DTA curves for HBI reacted in a 37% C02 I 60% N2 I 2.90% 02 gas mixture at a heating rate of 30°C/min. HF refers to the DTA-curvc.

At the suggested preheating temperature bet ween 750°C and 800°C, intemal reduction of FeO and

external oxidation of Fe will occur simultaneously. The extent of oxidntion will depend on the surface­to-volume mtio and consequently, relatively less oxidation will occur in large HBI briquettes than in the small samples used in this investigation. It also means that l!BI fines will oxidise extensively and f ines should be avoided in :my HBI charge subject to an oxidising atmosphere during preheating.

CONCUJSJONS

The main conclusions to be dr<.�wn are:

• Isothermal thermogravimetric analyses supported by non-isothermal TG and differential thennal analyses have confirmed that residual iron oxide contained in l mi is reduced by carbon at temperatures higher than 500"C.

• A sufficiently high carbon content in the HBI is required to ensure efficient internal reduction of residual FeO.

• The metallisation of the HBI is significantly increased by the reduction reactions.

• Exposure of HBI to oxidising atmospheres results in the oxidation of the external surface only.

• Exposure of HBI to oxidising atmospheres at temperatures higher than soouc results in external surface oxidation while residual iron oxide contained within the HBI is reduced at the same time. This occurred in gas mixtures with a variety of oxidation potentials, some specifically designed to simulate the most likely composition of the otl'-gas of an electric-arc

furnace.

• Extrapolation of the experimental results of this investigation to preheating of HBI in practice suggests that:

a preheating temperature of 750°C-800°C should be used to maximise the reduction of residual FeO contained in HBI. Minimal external oxidation occurs initially at these temperatures, but the oxidation potential of the gas used for

preheating should nevertheless be kept to a minimum. Because oxidation is a surface phenomenon, the surface-to-volume ratio

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of HBI briquettes should be minimised. Therefore the charging of large briqueues is preferred and fines should be elim inated.

ACKNOWLEDGEMENTS

The authors are grateful to Dr. M. Ionescu from the Department of Materials Engineering at the Wollongong University, for his assistance in the experiments. The authors also wish to thank BliP's Electric Arc Fumace Technology Group and BHP Minerals Development, Newcastle for their contribution to this work, parlicularly D. Varcoe, J. Sencviratne and T. Honeyands.

REFERENCES

I. A. Galvez, �.A. Brooks, R. Dippenaar, .. Kinetics of Hot Briquettcd Iron Oxidation­Reduction", Electric Furnace Conference Proc., Chicago, 1SS, 1997, pp. 635-642.

2. A. Galvez, "Preheating of Hot Briquetted Iron for Feeding to Electric Arc Furnace Steelmaking", Masters Thesis (Honours), Department of Materials Eng.. University of Woll ongong, Australia, 1998.

3. G.A. Brooks, R. Reeves, "Thennodynamic modeling of an Electric Arc Furnace". Electric Furnace Conference Proc., Chicago, ISS, 1997, pp. 645-649.

4. G.A.. Brooks, A. Galvez, R. Reeves, "Preheating of HBI for Electric Arc Furnace", Proc. of the Biennial Materials Conf. <Yf the Inst. of Mater. Eng., Australasia, Wollongong, JMEA, 1998, Vol. I, pp. 33-40.

5. J.W. Brown, D.L. Campbell, A.L. Saxton and J.W. Carr jnr "FIOR- The Esso Fluid Iron Ore Direct Reduction Process", JOM, Feb. 1966, pp. 237-242.

6. M.O. Davies and J.A. Lepinski "HBI as a Raw Material for EAF Steelmaking" SEA1SJ Seminar on DRI and Scrap as Raw Materials in Steelmaking, Taiwan, Nov. 1987, pp. 5/1-15.

7. C-H. Queens, "Experiences with PUROFER­Sponge Iron in Cosigua's Steelmaking", Meeting of the Committee on Technology-US!, Rio de Janeiro, April 1979, pp. 8 7 -I 09.

R. P.E. Duarte. R. Lopez, .. llytcmp DRI tran�fer to thin Slah Casting Facility at Hylsu", Iron and Steel Engineer, Nov. 1996, pp. JH-41. ---

9. D. M. Sheedy (Ed.), "Hot transport - Midrex Style", Dire,£LFrom Midrex, 3n1

Quarter 1998, pp. 3-5.

10. A. Chatterjee, Beyond the Blast t:urnace, CRC Press, USA, 1994, pp. 244.

I I. J.A.T. Jones, "New Steel Melting Technologies", Iron and Steelmaker, Vol. 24, No 2, Feb. 1997, ISS, pp. 4 J -42.

12. H.J. Grabkc, "The Carburization and Decarburization of Iron in Gas Mix tures", Proc. gf � Sy_ll1.Q,_ High temperature Gas-Metal Reactions in Mixed Environme nts. Boston, May 1972, AIME, pp. 130 - 142.

13. H.S. Ray, "Chemically Controlled Reactions", Kinetics of Metallurgical Reactions, International Science Publisher, New York, USA, 1993.

14 . . G.J. Billings, W.W. Smeltzer, J.S. Kirkaldy, "Oxidation and Decarburization Kinetics of Iron Carbon Alloys", Electrochemical Socie�. Vol. 117

' 1970, pp. 111-117.

15. W.E. Boggs, <'The role of Structural and Compositional Factors in the Oxidation of Iron

. and Iron Based Alloys'', Proc. of the Symp. High Temperature Gas-Metal Reactions in Mixed Environments, Boston, May 1972, AIME, pp. 84-128.

16. H.J. Kim, R.B. Runk," The Characterization of the Thin Oxide Film Formed over Fe3C at 300°C", Oxidation of Metals, Vol. 2, No 3, 1970, pp. 307-3 I 8.

17. N. Birks, G.H. Meier, lntro"duction to High Temperature Oxidation of Metals, Edward Arnold Ltd., London, 1983.

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